3.1.1. Convergent models

The differential equation provided by Epstein (Reference Epstein2004) for the amplitude growth rate in spherical geometry is

(3.1) \begin{align} \left (-\gamma _\rho - \gamma _R + \frac {\rm d}{{\rm d}t}\right ) \frac {\rm d}{{\rm d}t}\left (a_l \rho R^2\right ) &= \gamma _0^2 \left (a_l \rho R^2\right )\! ,\end{align}

where $a_l$ is the spatial amplitude of the spherical harmonic perturbation of degree $l$ , $Y_l^m (\theta ,\varphi )$ . The driving term $\gamma _0^2$ in spherical geometry is given by

(3.2) \begin{align} \gamma _0^2 = \frac {l(l+1)}{R} \frac {\rho ^+ - \rho ^-}{l\rho ^+ + (l+1)\rho ^-} g_p , \end{align}

where $\rho ^+$ and $\rho ^-$ are densities above and below the interface, respectively, and $g_p$ is the linearised pressure acceleration at the interface, $g_p = -(1/\rho )[\partial p/\partial x]_{r=R}$ .

To obtain a strain rate formulation, the compression rate and convergence rate are converted to the strain rate equivalents

(3.3a) \begin{align} \gamma _\rho &= -\bar {S}_{11} - 2\bar {S}_{22}, \end{align}

(3.3b) \begin{align} \gamma _R &= \bar {S}_{22}, \end{align}

where $\bar {S}_{11}$ is the mean radial/axial strain rate and $\bar {S}_{22}$ is the symmetric circumferential/transverse strain rate. This assumes a spherically symmetric mean velocity profile, with a singular transverse strain rate, $\bar {S}_{22} = \bar {S}_{\theta \theta } = \bar {S}_{\varphi \varphi }$ . Expanding the derivatives and simplifying gives the solution

(3.4) \begin{align} \ddot {a}_l + \dot {a}_l \left (\bar {S}_{22} - \bar {S}_{11}\right ) + a_l \left (-\bar {S}_{11} \bar {S}_{22} - \dot {\bar {S}}_{11}\right ) = \gamma _0^2 a_l . \end{align}

The same process can be applied to the solution for cylindrical geometry given by Epstein (Reference Epstein2004) with

(3.5) \begin{align} \left (-\gamma _\rho + \frac {\rm d}{{\rm d}t}\right ) \frac {\rm d}{{\rm d}t}\left (a_l \rho R\right ) &= \gamma _0^2 \left (a_l \rho R\right )\!. \end{align}

The cylindrical model neglects activity in the longitudinal direction, such that the perturbations are only a function of the polar coordinate, $\cos (l\theta )$ . The compression rate is given by $\gamma _\rho = -\bar {S}_{11}-\bar {S}_{22}$ , which is different from the spherical model, which has an extra $\bar {S}_{22}$ component. The resulting equation as a function of the strain rates is identical to equation (3.4), with the adjustment for the driving term

(3.6) \begin{align} \gamma _0^2 = \frac {l}{R} \frac {\rho ^+-\rho ^-}{\rho ^+ + \rho ^-} g_p . \end{align}

This similarity shows the strain rate formulation provides a more standardised and universal method to describe the effects of the convergent geometry on the amplitude growth for instabilities. The strain rate formulation is limited as the knowledge of the strain rates in a given experiment is not clear without the velocity field. Whilst the transverse velocity can be obtained by tracking the mean interface velocity and (2.5), the radial velocity gradient for the radial strain rate is not clear. This same problem is faced when trying to calculate the compression rate $\gamma _\rho$ , as it depends upon the strain rates or velocity field. Brasseur et al. (Reference Brasseur, Jourdan, Mariani, Barros, Vandenboomgaerde and Souffland2025) assumes the strain rates to be isotropic, whilst Wu, Liu & Xiao (Reference Wu, Liu and Xiao2021) assumes the compression rate to be constant in time, based on the initial and final volumes enclosed by the mean interface radius.

3.1.2. Planar model

To reproduce the differential equation for the growth rate in convergent geometry, both axial and transverse strain rates need to be applied in the planar geometry. The background fluid velocities, denoted with an overbar, are then given by

(3.7) \begin{align} \bar {u}_1 (x,t) &= \dot {x}_0(t) + \bar {S}_{11} (x-x_0(t)), \end{align}

(3.8) \begin{align} \bar {u}_2 (y,t) &= \bar {S}_{22} y, \end{align}

where $x_0$ is the mean interface position, $\dot {x}_0$ is the mean interface velocity and the transverse velocity is stationary at $y=0$ . The velocity potential, $\bar {u}_i = \partial \varPhi /\partial x_i$ , for the background flow is then

(3.9) \begin{align} \varPhi (x,y,t) = \varPhi _0 (t) + \dot {x}_0(x-x_0) + \frac {1}{2} \bar {S}_{11} (x-x_0(t))^2 + \frac {1}{2} \bar {S}_{22} y^2. \end{align}

The potential field is linearised about the mean interface

(3.10) \begin{align} U(x,y,t) = U_0 + g_{U_x} (x-x_0) + g_{U_y} y, \end{align}

where $g_{U_x} = [\partial U/\partial x]_{x=x_0}$ and $g_{U_y} = [\partial U/\partial y]_{y=0}$ . The pressure field is likewise linearised, only allowing for a pressure gradient in the $x$ -direction

(3.11) \begin{align} p(x,t) = p_0 - \rho g_p (x-x_0), \end{align}

where $g_p=-(1/\rho )[\partial p/\partial x]_{x=x_0}$ . The acceleration of the interface is the sum of the potential and pressure acceleration

(3.12) \begin{align} \ddot {x}_0 = g_{U_x} + g_p. \end{align}

To take into account the transverse expansion or compression, the perturbed interface uses a time-varying wavenumber

(3.13) \begin{align} x_{int}(y,t) = x_0(t) + a_k(t) \cos (k(t) y) ,\end{align}

where $k(t) = 2\pi /\lambda (t)$ . The amplitude $a_k(t)$ corresponds to a specific wavenumber $k(t)$ . Under a transverse strain rate of $S_{22}$ , the wavelength scales with the expansion factor, and therefore the wavenumber evolves as

(3.14) \begin{align} k(t) = k_0 \exp \left [-\int _{t_0}^t S_{22}(t') {\rm d}t'\right ], \end{align}

where $k_0=2\pi /\lambda _0$ is the reference wavenumber at some time $t_0$ . For simplicity, the dependence of $k$ , $a$ , $x_{int}$ and the strain rates will not be written further. The corresponding incompressible velocity potential is given by

(3.15) \begin{align} \phi ^+_k &= b^+_k \cos (k y) \exp \left [-kx\right ], \end{align}

(3.16) \begin{align} \phi ^-_k &= b^-_k \cos (k y) \exp \left [kx\right ], \end{align}

where the $+$ superscript denotes the fluid above the interface ( $x\gt x_0$ ), and the $-$ superscript denotes for below the interface ( $x\lt x_0$ ). Both terms decay to zero as $x$ heads towards the relative infinity. The total velocity potential is then given by

(3.17) \begin{align} \phi ^\pm = \varPhi + \phi ^\pm _k . \end{align}

The $b_k^\pm$ terms can be removed by equating the interface velocity (total time derivative of (3.13)) with the fluid’s $x$ -velocity at the interface ( $x$ -derivative of (3.17))

(3.18) \begin{align} \frac {{\rm d}x_{int}}{{\rm d}t} &= \frac {\partial \phi ^\pm }{\partial x} \bigg {|}_{x=x_{int}}, \end{align}

(3.19) \begin{align} b^\pm _k &= \pm \frac {a_k\bar {S}_{11} - \dot {a}_k}{k} \exp \left [\pm k x_{int}\right ], \end{align}

(3.20) \begin{align} \phi ^\pm _k &= \pm \frac {a_k\bar {S}_{11}-\dot {a}_k}{k} \cos (ky) \exp \left [\mp k(x-x_{int})\right ]. \end{align}

The Bernoulli equation for unsteady, irrotational flow is given by

(3.21) \begin{align} \frac {\partial \phi }{\partial t} + \frac {1}{2} u_i u_i + U + \frac {p}{\rho } = 0. \end{align}

The equation is evaluated at the perturbed interface, equating the pressure on each side of the interface, $p^+ = p^-$ . The cosine harmonic of the solution provides the equation

(3.22) \begin{align} \ddot {a}_k + \dot {a}_k \left (\bar {S}_{22} -\bar {S}_{11}\right ) + a_k \left (-\bar {S}_{11}\bar {S}_{22} - \dot {\bar {S}}_{11}\right ) = a_k k g_p \mathcal{A}_t, \end{align}

where the Atwood number is given by $\mathcal{A}_t = (\rho ^+-\rho ^-)/(\rho ^++\rho ^-)$ . The differential equation is identical to the solution obtained from the spherical model in (3.4), except for the different driving term on the right-hand side. The wavenumber in the driving term is slightly different between the models due to the geometry. The cylindrical wavenumber in the model is $l/R$ , which can be derived by taking the wavelength as $2\pi R/l$ . For the spherical harmonic, the effective wavelength can be calculated using Jeans’ relation, giving $\lambda = 2\pi R/\sqrt {l(l+1)}$ (Jeans Reference Jeans1997; Wieczorek & Meschede Reference Wieczorek and Meschede2018). The cylindrical model uses the standard Atwood number definition, whilst the spherical model uses the definition, $\mathcal{A}_t = \sqrt {l(l+1)} (\rho ^+-\rho ^-)/(l\rho ^+ + (l+1)\rho ^-)$ . For large mode numbers $l$ , the spherical Atwood number will approach the value from the standard definition.

The planar model was derived for a 2-D flow, however, it produced the same differential equation as the 3-D spherical model. A third dimension could be added to the planar model, such that the interface is a function of $y$ and $z$ , however, the same solution is obtained for a uniform transverse strain rate in both directions, such that all wavelengths are affected uniformly.

For the case with only axial strain rate, $\bar {S}_{22}=0$ , the differential equation collapses down to

(3.23) \begin{align} \ddot {a}_k - \frac {\rm d}{{\rm d}t}\left (a_k \bar {S}_{11}\right ) = a_k k g_p \mathcal{A}_t. \end{align}

This model was investigated in the work of Pascoe et al. (Reference Pascoe, Groom, Youngs and Thornber2024) for RMI, which showed the model was accurate within the linear regime. For only a transverse strain rate, $\bar {S}_{11}=0$ , the equation becomes

(3.24) \begin{align} \ddot {a}_k + \dot {a}_k \bar {S}_{22} = a_k k g_p \mathcal{A}_t. \end{align}

For a single-mode RMI where the shock provides the acceleration $g_p = \Delta u \delta (t)$ , the amplitude growth rate is

(3.25) \begin{align} \dot {a}(t) = U_0 \exp \left [-\int _0^t \bar {S}_{22}(t') {\rm d}t'\right ], \end{align}

where $U_0$ is the impulsive velocity as specified by Richtmyer (Reference Richtmyer1960)

(3.26) \begin{align} U_0 = a_0 k_0 \Delta u \mathcal{A}_t. \end{align}

This solution shows an amplification of the growth rate $\dot {a}$ as the system compresses, or a reduction as the system expands. The solution can also be written as

(3.27) \begin{align} \dot {a}(t) = a_0 k(t) \Delta u \mathcal{A}_t , \end{align}

showing the linear growth rate scales as if the impulse were applied at the current wavenumber, as opposed to the initial wavenumber.

3.2.1. Initial conditions

The cases were simulated in a 2-D domain with a 3 : 1 density ratio for the two fluids. The single-mode instability is initialised with a velocity perturbation instead of an amplitude perturbation and shock interaction. The velocity perturbation is designed to produce an initial linear growth rate, as described in Thornber et al. (Reference Thornber, Drikakis, Youngs and Williams2010) and Pascoe et al. (Reference Pascoe, Groom, Youngs and Thornber2024). Starting from a flat interface, the velocity perturbation is designed to grow at a velocity of $U_0=1$ m s^–1 for the initial wavelength of $\lambda _0=0.2$ m. The fluid properties for the initial conditions are given in table 2. As the initial amplitude of the instability starts from zero, there is no directly equivalent shock-induced RMI, but a comparison can be made if a small, finite amplitude is assumed. For example, a shock strength of Mach number Ma = 1.8439 impacting the ideal gases with initial Atwood number of 0.52 at pressure $p=36\,\rm kPa$ will give approximately the same parameters for an initial linearity of $ak=0.015$ . Smaller shock strengths can be used, requiring larger initial amplitudes to compensate. As the viscous and diffusive five-equation model (see (2.14)) is in use, the initial interface is diffuse, defined by an error function for the volume-fraction profile

(3.28) \begin{align} f_1 = \frac {1}{2} \left ( 1 - \text{erf}\left ( \frac {\sqrt {\pi } (x_1-x_0)}{H_0} \right )\right )\!, \end{align}

where $H_0$ is the initial diffusion width set to $\lambda /64$ . The initial Reynolds number of the system is given by

(3.29) \begin{align} Re = \bar {\rho } U_0 \lambda _0/\bar {\mu } = 2048, \end{align}

where the initial average density, prescribed linear amplitude growth rate, initial wavelength and average viscosity have been used. This Reynolds number is sufficiently high that the amplitude growth rate is not significantly affected by the viscosity, as seen in Walchli & Thornber (Reference Walchli and Thornber2017) for unstrained RMI.

Table 2. Fluid properties for the linear regime cases.

3.2.2. Results

Visualisations of the volume-fraction contour for the unstrained case and the high-magnitude strain rate cases are shown in figure 2. Each plot is scaled to the final wavelength of the simulation, where the compression case has a wavelength that is 16 times smaller than the expansion cases. Relative to the final wavelengths, the compression cases have a much larger value of $a/\lambda (t)$ , displaying the formation of penetrating bubbles and spikes that are beginning to roll-up. The unstrained case is around the limit of the linear regime ( $a\leqslant 0.1 \lambda$ ), and the interface appears to be well described by a cosine function. For comparison, the expansion cases show a very small $a/\lambda (t)$ value, showing it is still within the linear regime by standard definitions.

Figure 2. Interface at $\tau =0.1$ for the 2-D single-mode simulations. Heavy fluid ( $f_1=1$ ) is red, light fluid ( $f_1=0$ ) is blue. Major ticks indicate a distance of $\lambda (t)/4$ , with the final wavelength marked below the plot, and the cropping of the domain in the $x$ -direction marked on the right. Panels show (a) $\hat {S} = -14$ ; (b) $\hat {S}=0$ ; (c) $\hat {S} = 14$ .

As the interface is yet to roll-up and remains smoothly connected, the mean interface position is taken along the $f_1=0.5$ volume-fraction isocontour line. The amplitude of the interface is taken to be half of the distance between the maximum and minimum of the isocontour, representing the peak and trough of the perturbation, given by

(3.30) \begin{equation} a = 0.5\left (\max (x_{f_1=0.5}) - \min (x_{f_1=0.5}) \right )\!. \end{equation}

A second-order interpolation scheme is used to locate the minimum and maximum isocontour positions from the simulation’s cell average values. The amplitudes non-dimensionalised by the initial wavelength are plotted in figure 3, along with the theoretical model given in (3.25). The simulation results show that the expansion cases grow the slowest and the compression cases grow the fastest. The model is able to accurately predict the growth rate of the expansion cases, with a small final error for the unstrained case, which can be attributed to saturation as the mode becomes nonlinear. The compression cases have a larger error, with the high-magnitude compression (negative) strain rate cases the least accurate at the final simulation time. These cases have the largest amplitudes, such that they can be expected to be developing secondary instabilities and be saturating by the final simulation time, as observed in the volume-fraction contour plots of figure 2. Longer simulation times may show that the weaker compression cases, $\hat {S}=-7$ , will surpass the amplitude of the stronger compression case. Experiments of converging RMI have shown that the amplitude can continue to grow approximately linearly past the standard linear threshold (Fincke et al. Reference Fincke, Lanier, Batha, Hueckstaedt, Magelssen, Rothman, Parker and Horsfield2004; Yager-Elorriaga et al. Reference Yager-Elorriaga2022).

Figure 3. Amplitude of the single-mode linear regime, non-dimensionalised by (a) the initial wavelength, and (b) the time-varying wavelength. Solid lines indicate numerical results, dashed lines indicate the linearised potential model.

The performance of the model when plotted as a function of the amplitude non-dimensionalised by the time-varying wavelength is shown in figure 3. The same trends can be observed in this plot, with the expansion cases growing the slowest and the compression cases growing the fastest. For the highest-magnitude expansion cases, the growth of $a/\lambda (t)$ goes negative, a result of the wavelength growing faster than the amplitude. For highly strained expansion cases, requiring at least $\hat {S}\geqslant 2.5$ , the perturbation will remain in the linear regime permanently.

The performance of the model confirms that the perturbation becomes nonlinear depending upon the time-varying wavelength and not the initial wavelength. Figure 4 reinforces this by plotting the error between the model and simulation as a function of $a/\lambda (t)$ . After some initial noise from the interpolation approximating the peak/trough of the initially flat interface, the error profile collapses to a rather straight line for the unstrained and compression cases. The expansion cases fall within the general trend, however, the highly expanded cases are not monotonically increasing for $a/\lambda (t)$ . The case of $\hat {S}=14$ shows a final trajectory towards the origin, decreasing in error magnitude and $a/\lambda (t)$ . Other investigations into modelling converging RMI produce amplitude data that are aligned with the model for a longer duration than the results presented. Wu et al. (Reference Wu, Liu and Xiao2021) maintain reasonable agreement with a modified Bell model until $a/\lambda (t) \sim 0.425$ , whilst Fincke et al. (Reference Fincke, Lanier, Batha, Hueckstaedt, Magelssen, Rothman, Parker and Horsfield2004) maintain agreement past $a/\lambda (0) = 1$ . The inclusion of axial strain and Rayleigh–Taylor effects in the converging geometry may delay the development of secondary instabilities, allowing for longer alignment with the linear theory.

Figure 4. Error in the amplitude for the linear regime as a function of the amplitude non-dimensionalised by the time-varying wavelength.

4.2.1. Visualisations

Isosurfaces of the volume fraction at $f_1=0.01$ and $f_1=0.99$ are shown in figure 5 for the compression cases at $\varLambda \approx 0.51$ , and in figure 6 for the expansion cases at $\varLambda \approx 1.97$ . The compression cases are presented at a larger magnification level than the expansion cases to allow for better visibility of the scales within the mixing layer. For the lower-magnitude strain rate cases presented in figures 5(b ) and 6(b ), the specified expansion factor is achieved at a later non-dimensional time. These plots show a thicker mixing layer, and greater penetration of a vortex ring into the heavy fluid (red), located on the bottom-left surface of the plots. For the $\hat {S}=0.020$ expansion case in figure 6(b ), the vortex ring remains intact, but for the $\hat {S}=-0.020$ compression case in figure 5(b ) the structure has bifurcated, splitting the vortex ring into two smaller vortex rings travelling in opposite directions along the $z$ -axis, and has begun deteriorating. The bifurcation is the result of vortex tilting and the vortex ring being pinched by the compressive strain. Vortex pinching has been investigated by Marshall & Grant (Reference Marshall and Grant1994) for planar strain configurations, where expansive and compressive strain rates are applied in perpendicular directions in the plane of the vortex ring. For sufficiently high strain rates, this can cause the vortex ring to deform into an elliptical shape and curve into the third dimension, eventually pinching at the centre and producing two smaller vortex rings, as is observed in figure 5(b ).

Figure 5. Contour of volume fraction $f_1$ for the compression mixing layers at $\varLambda \approx 0.51$ , bounded by $f_1=0.99$ and $f_1=0.01$ iso-surfaces. Panels show (a) $\hat {S}=-0.081$ , $\tau = 9.4$ , (b) $\hat {S}_0=-0.020$ , $\tau = 34.5$ .

Figure 6. Contour of volume fraction $f_1$ for the expansion mixing layers at $\varLambda \approx 1.97$ , bounded by $f_1=0.99$ and $f_1=0.01$ isosurfaces. Panels show (a) $\hat {S}_0=0.081$ , $\tau = 9.4$ , (b) $\hat {S}=0.020$ , $\tau = 34.5$ .

4.2.2. Width and mix measures

The integral width of the mixing layer, defined in (4.5), and normalised by the initial mean wavelength of the interface, is plotted in figure 7(a ). The cases with applied expansion transverse strain rates show a slight increase in the integral width, whilst the compression cases show a decrease. This behaviour is the opposite of what was observed in the linear regime, where transverse compression increased the growth and was well captured by the Bell–Plesset model. El Rafei et al. (Reference El Rafei, Flaig, Youngs and Thornber2019) also noted that, once the modes begin to saturate, Bell–Plesset models fail to accurately predict the growth of the mixing layer for the spherical implosion. This observed trend demonstrates that the influence of the transverse strain rate on the transitional and turbulent mixing layer is fundamentally different from the linear regime and cannot be modelled by the same approach. The impact of the transverse strain on the integral width is not as pronounced as the impact of axial strain, as observed in Pascoe et al. (Reference Pascoe, Groom, Youngs and Thornber2024). Axial strain corresponds to a velocity difference across the mixing layer, which causes the mixing layer to stretch/compress directly from the background velocity difference (Li et al. Reference Li, Tian, He and Zhang2021a ; Ge et al. Reference Ge, Li, Zhang and Tian2022). The less impactful effect of the transverse strain is a similar observation to the one made by Lombardini et al. (Reference Lombardini, Pullin and Meiron2014a ), which noted that compression effects were larger than the convergence effects for the analysed implosion profile. As the quarter-scale $\theta$ -group case is initialised with a narrowband perturbation and a uniform power spectrum, all the perturbation modes are nonlinear at the time of strain application. Different behaviour could be observed for a broadband spectrum, where the high wavenumber modes will saturate much earlier than the low wavenumber modes, which can remain linear for a longer time (Groom & Thornber Reference Groom and Thornber2020). The high wavenumber modes will transition to a turbulent state which observes the reduced growth for compression, meanwhile the growth rate of the low wavenumber modes would be amplified by transverse compression causing an increased mixing layer growth rate.

Figure 7. Temporal evolution of (a) integral width and (b) mixed mass.

The mixed mass is an alternative measure of the mixing layer, and measures how much of one fluids has mixed with another. An attractive feature of the mixed mass is the ability to derive the evolution equation for the mixed mass from the mass fraction transport equation (Zhou, Cabot & Thornber Reference Zhou, Cabot and Thornber2016)

(4.6) \begin{align} \mathcal{M} = \int 4 \rho Y_1 Y_2 \,\text{d}V .\end{align}

The evolution of the mixed mass is plotted in figure 7(b ), and shows a different trend compared with the integral width. For the mixed mass, the compression cases achieve slightly higher growth and the expansion cases achieve less growth. As the name mixed mass suggests, it is not purely a measure of the width of the mixing layer but also has dependence upon mixedness of the mixing layer, whereas the integral width depends on the mean volume-fraction profile. Therefore, depending upon the choice of measurement metric for the mixing layer development, the influence of the transverse strain rate will change signs. The increased mixedness of the compression cases represents a more mixed or homogeneous composition in the mixing layer as compared with the unstrained case, and likewise a less mixed layer for the expansion cases. Without a comparison of the different strain rate effects, the scaling of the two width measures are quite similar. This is also observed by Liang et al. (Reference Liang, Liu, Luo and Wen2023) for converging dual-mode RMI, which shows similar scaling between width measures until the Rayleigh–Taylor stabilisation occurs due to the deceleration of the interface. The mixedness of the mixing layer is further explored using the molecular mixing fraction and normalised mixed mass below.

Due to the different densities of the fluids (i.e. non-zero Atwood number), asymmetries form in the development of the mixing layer. The penetration structures of the heavy fluid into the light fluid (spikes) tends to be larger than the penetration structures of the light fluid into the heavy fluid (bubbles). The spike-to-bubble heights will tend toward a constant ratio if the growth rates of the spikes and bubbles both follow the same power-law growth rate, i.e. $\theta _s=\theta _b$ (Thornber et al. Reference Thornber2017; Mikaelian & Olson Reference Mikaelian and Olson2020). The bubble and spike heights of the mixing layer are defined relative to the mixing layer centre $x_C$ , taken to be the planar location where there is an equal volume of penetrating fluid on either side

(4.7) \begin{align} \int _{-\infty }^{x_C} \bar {f}_2 \,\text{d}x = \int _{x_C}^{\infty } \bar {f}_1 \,\text{d}x .\end{align}

To reduce the impact of statistical fluctuations, the bubble and spike heights used are based on the integral measure proposed by Youngs & Thornber (Reference Youngs and Thornber2020a )

(4.8a) \begin{align} \bar {h}_b^{(m)} &= \left [\frac {(m+1)(m+2)}{2} \frac {\int ^0_{-\infty } |x'|^m (1-\bar {f}_1) {\rm d}x'}{\int ^0_{-\infty } (1-\bar {f}_1) {\rm d}x'}\right ]^{1/m} , \end{align}

(4.8b) \begin{align} \bar {h}_s^{(m)} &= \left [\frac {(m+1)(m+2)}{2} \frac {\int _0^{\infty } |x'|^m \bar {f}_1 {\rm d}x'}{\int _0^{\infty } \bar {f}_1 {\rm d}x'}\right ]^{1/m}. \end{align}

The integrals are taken with respect to the mixing layer centre, $x' = x-x_C$ , and the integral terms of the denominator are equal to the volume of the penetrating fluid on either side of the mixing layer centre. These definitions provided assume that fluid 1 is located below fluid 2 in the $x$ -direction and that $\rho _1\gt \rho _2$ . The bubble and spike heights plotted in figure 8 use the heights $h = 1.1\bar {h}^{(2)}$ which were found to be well aligned with the heights measured by the $1\,\%$ and $99\,\%$ mean volume-fraction cutoff but are less sensitive to statistical fluctuations (Youngs & Thornber Reference Youngs and Thornber2020a ).

Figure 8. Temporal evolution of the bubble and spike heights. (a) Individual heights: spike (dashed lines) and bubble (solid lines); (b) spike-to-bubble height ratio.

The bubble and spike heights show the same behaviour as observed for the integral width, with the expansion cases growing slightly compared with the unstrained simulation. The compression cases tend to have smaller heights than the unstrained cases, however, the difference is not as large as observed for the transverse expansion, such that the compression results are closer to the unstrained simulation results. The ratio of the spike height to bubble height shows a common trend with the unstrained case, as shown in figure 8. The effect of convergence does not appear to change the growth rate of the bubble and spike heights disproportionately, displaying the same self-similar ratio as seen for the unstrained simulations.

To better quantify the mixedness of the mixing layer, which is suggested to change by the discrepancy in the behaviour between the mixed mass and integral width, the molecular mixing fraction and the non-dimensionalised mixed mass are used. The molecular mixing fraction measures how well the species in the mixing layer are mixed, as measured by the volume fraction. A value of $\varTheta =0$ means complete segregation, whilst $\varTheta =1$ corresponds to perfect homogeneity in the plane. The molecular mixing fraction is calculated by

(4.9) \begin{align} \varTheta (t) = \frac {\int \overline {f_1 f_2}{\rm d}x}{\int \bar {f}_1 \bar {f}_2 {\rm d}x}, \end{align}

with the denominator corresponding to the integral width. At late time, a steady $\varTheta$ indicates that a mixing layer has become self-similar. Included in figure 9 is the final value of $\varTheta$ from the quarter-scale case using FLAMENCO in the $\theta$ -group collaboration at a time of $\tau =246$ . The unstrained quarter-scale case simulation can be observed to be approaching the self-similar value marked by the black, dotted line. The strained cases do not appear to be approaching the same asymptote. Instead, the compression cases show an increasing mixedness and the expansion cases are decreasing. These results agree with the observed increase in the mixed mass for transverse compression, and show the transverse strain affects the self-similarity of the evolving mixing layer, either converging to a different self-similar state or not converging at all.

Figure 9. Temporal evolution of the mixing measures. Solid lines indicate $\varTheta$ , dashed lines indicate $\varPsi$ , dotted line is FLAMENCO’s final $\varTheta$ value at $\tau =246$ (Thornber et al. Reference Thornber2017).

The normalised mixed mass is also plotted in figure 9, using the definition

(4.10) \begin{align} \varPsi = \frac {\int \rho Y_1 Y_2 {\rm d}V}{\int \bar {\rho }\bar {Y}_1\bar {Y}_2 {\rm d}V}, \end{align}

with the numerator corresponding to the mixed mass. The results for $\varTheta$ and $\varPsi$ are well aligned, with the values of $\varPsi$ attaining slightly smaller values compared with $\varTheta$ . This behaviour has previously been observed in several other studies when comparing the two mixedness metrics (Zhou et al. Reference Zhou, Cabot and Thornber2016, Reference Zhou, Groom and Thornber2020; El Rafei & Thornber Reference El Rafei and Thornber2020).

Figure 10. Planar-averaged volume-fraction profiles. Panels show (a,b) $\tau =9.84$ ; (c,d) $\tau =34.45$ .

4.2.3. Self-similarity

To further investigate the self-similarity of the simulation, the two contributions of the molecular mixing fraction may be analysed: the mean volume-fraction profile $\bar {f}_1$ , and the mean volume-fraction product $\overline {f_1 f_2}$ . These terms correspond to the denominator and numerator of the molecular mixing fraction, and for a self-similar mixing layer these profiles will collapse under non-dimensionalisation. The profiles of $\bar {f}_1$ and $\overline {f_1 f_2}$ are plotted in figure 10 for two different times, corresponding to the end times of the high strain rate cases, $\tau = 9.84$ , and the end time of the low strain rate cases, $\tau =34.45$ . All cases are visible for the early time, whilst at the late time only information for the low strain rate cases is available. The mean profile of $f_1$ is plotted in the subplots (a) and (c). The mean volume-fraction profile does not appear to vary from the unstrained solution at both times presented. The evolution of the mean volume-fraction profile for the unstrained quarter-scale $\theta$ -group case almost collapsed to a single profile, however, the profile slightly smoothed out with time around the inflection points at $(x-x_C)/W=-2$ and $2.5$ . The agreement between the strain cases and the unstrained case is further evidence that the bubble and spike heights grow in the same proportion as in the unstrained case, reinforcing the results in figure 8.

The mean volume-fraction product shown in subplots (b) and (d) of figure 10 shows a larger deviation from the unstrained case. Whilst the unstrained quarter-scale $\theta$ -group case was self-similar in the centre of the mixing layer for the mean volume-fraction product, the strained cases do not match the profile, explaining the different $\varTheta$ values obtained. The compression cases which possessed the largest $\varTheta$ values have a larger peak value of $\overline {f_1 f_2}$ , representing greater mixing at the centre of the mixing layer. This is due to the changes in the anisotropy of the turbulent kinetic energy, discussed in § 4.2.4, which is in part due to the shear production increasing the mixing within the mixing layer, with the larger-magnitude strain rates showing larger changes in the mean product. The low-magnitude strain rates can be observed to deviate further away from the unstrained profile as the simulation progresses, showing that the strained cases have not achieved a self-similar state.

4.2.4. Turbulent kinetic energy

Unstrained RMI evolves towards an inhomogeneous, decaying turbulent mixing layer. Whilst at the late time it can achieve a self-similar decay rate, current experimental and numerical evidence shows that the distribution of the turbulent kinetic energy does not become isotropic, and instead tends towards a constant value of anisotropy (Tritschler et al. Reference Tritschler, Olson, Lele, Hickel, Hu and Adams2014; Mohaghar et al. Reference Mohaghar, Carter, Musci, Reilly, McFarland and Ranjan2017; Thornber et al. Reference Thornber2017). Under the application of transverse strain rates, the turbulence is no longer purely dominated by dissipation. Instead, shear production will produce/destroy turbulent kinetic energy for compression/expansion. To observe how the shear production alters the turbulent kinetic energy in the domain, the domain-integrated turbulent kinetic energy can be evaluated. The directional turbulent kinetic energy component for the $x$ -direction is given by

(4.11) \begin{align} \mathrm{TKX} = \iiint \frac {1}{2} \rho u_1^{\prime\prime} u_1^{\prime\prime} \,\mathrm{d}V \end{align}

using the Favre velocity fluctuation, $u_i^{\prime\prime} = u_i - \tilde {u}$ , where $\tilde {u}$ is the density weighted average of the $y$ – $z$ plane, $\tilde {u} = \overline {\rho u_i}/\bar {\rho }$ . For the strain cases, the velocity due to the strain rates is also removed for the fluctuation calculation. Similar definitions are applied for $\mathrm{TKY}$ and $\mathrm{TKZ}$ , with the total turbulent kinetic energy being the sum of the three components

(4.12) \begin{align} \mathrm{TKE} = \mathrm{TKX}+\mathrm{TKY}+\mathrm{TKZ} = \iiint \frac {1}{2} \rho u_i^{\prime\prime} u_i^{\prime\prime} \,\mathrm{d}V .\end{align}

The statistical homogeneity of the $y$ – $z$ plane means the $\mathrm{TKY}$ and $\mathrm{TKZ}$ components should be identical, so they are averaged to create $\mathrm{TKYZ}=(\mathrm{TKY}+\mathrm{TKZ})/2$ . Finally, the anisotropy of the mixing layer is measured by comparing the energy in the inhomogeneous $x$ -direction with the energy in the direction of the homogenous plane

(4.13) \begin{align} \mathrm{TKR} = \frac {2\,\mathrm{TKX}}{\mathrm{TKY}+\mathrm{TKZ}}. \end{align}

As the simulations are conducted as ILES, the measured quantities represent the resolved flow, omitting subgrid contributions. The grid convergence of the total turbulent kinetic energy for ILES indicates that the unresolved turbulent kinetic energy is not significant compared with the total amount of energy in the high-Reynolds-number limit. For the Kolmogorov velocity power spectrum scaling of $\kappa ^{-5/3}$ in the inertial range, successive decades will have 21.5 % of the energy of the previous decade. For the mesh resolutions employed, in conjunction with the resolved energy of the large scales, it is estimated that at least 90 % of the energy is resolved as compared with the infinitely resolved high-Reynolds-number limit.

Figure 11. Domain-integrated measurements of the turbulent kinetic energy. (a) Total turbulent kinetic energy; (b) $x$ -component; (c) averaged $y$ – $z$ component; (d) anisotropy.

The results in figure 11 shows the expected behaviour for the unstrained case at late time: the total turbulent kinetic energy and its components all decay at a steady rate, and the anisotropic $\mathrm{TKR}$ has plateaued to a nearly constant value. The strain cases vary from this profile; the shear production from the mean strain rate has caused a slight increase/decrease in the $\mathrm{TKYZ}$ component for the compression/expansion cases, which is also visible in the $\mathrm{TKE}$ component. The $\mathrm{TKX}$ component, however, shows the opposite trend, instead decreasing/increasing for compression/expansion. This is contrary to what would be expected, as the effect of shear production is expected to get redistributed. This difference in behaviour further exacerbates the change in the anisotropy of the turbulent kinetic energy. The strained cases diverge from the asymptotic value, and it is observed that the expansion cases obtain larger values of $\mathrm{TKR}$ , becoming more anisotropic due to the shear production removing turbulent kinetic energy in the transverse directions. The compression cases head towards isotropy, with the compression amplifying the transverse turbulent kinetic energy. Such behaviour is also observable in spherical simulations during convergence between shock interactions (El Rafei et al. Reference El Rafei, Flaig, Youngs and Thornber2019; Li et al. Reference Li, Fu, Yu and Li2021b ; Wang et al. Reference Wang, Zhong, Wang, Li and Bai2023). The changes in the turbulent kinetic energy components explain the differences in the mixing layer growth and mixedness. The axial turbulent kinetic energy trends correlate with the changes in the integral width in figure 7(a ). Where $\mathrm{TKX}$ corresponds to the energy available to entrain new fluid into the mixing layer, and $\mathrm{TKYZ}$ corresponds to the energy available to mix within a plane, the mixedness of the mixing layer would depend on the inverse of $\mathrm{TKR}$ . This is observed in the results, with the increased values of $\mathrm{TKR}$ corresponding to the transverse expansion cases which achieve a lower mixedness value.

4.2.5. Turbulent kinetic energy model

The accurate prediction of the turbulent kinetic energy components is important for calculating the mixing layer width and mixedness. To explain the observed results, a simple Reynolds stress model is derived to evolve the domain-integrated turbulent kinetic energy components. The transport equation for the Favre-averaged, compressible Reynolds Stress tensor $\bar {\rho }R_{\textit{ij}} = \overline {\rho u_i^{\prime\prime} u_{\!j}^{\prime\prime}}$ is

(4.14) \begin{align} \bar {\rho } \frac {\tilde {D} R_{\textit{ij}}}{\tilde {D} t} &= -\underbrace {\bar {\rho } \left (R_{ik} \frac {\partial \tilde {u}_{\!j}}{\partial x_k}+ R_{jk} \frac {\partial \tilde {u}_i}{\partial x_k} \right )}_{\text{Shear production}} + \underbrace {\overline {u_i^{\prime\prime}} \frac {\partial \bar {p}}{\partial x_{\!j}}+\overline {u_{\!j}^{\prime\prime}} \frac {\partial \bar {p}}{\partial x_i}}_{\text{Buoyancy production}} - \underbrace {\bar {\rho }\,\overline {p'\left (\frac {\partial {u_i^{\prime\prime}}}{\partial x_{\!j}} + \frac {\partial {u_{\!j}^{\prime\prime}}}{\partial x_i}\right )}}_{\text{Pressure-rate-of-strain}} \notag \\&\quad + \underbrace {\frac {\partial }{\partial x_{\!j}}\left ( \overline {\rho u_i^{\prime\prime} u_{\!j}^{\prime\prime} u_k^{\prime\prime}} + \overline {p'u_i^{\prime\prime}}\delta _{jk} + \overline {p'u_{\!j}^{\prime\prime}}\delta _{ik} - \overline {\tau _{kj} u_i^{\prime\prime} + \tau _{ki} u_{\!j}^{\prime\prime}}\right )}_{\text{Transport terms}} + \underbrace {\bar {\rho } \varepsilon _{\textit{ij}}}_{\text{Dissipation}}, \end{align}

which has dependence upon the viscous stress tensor $\tau _{\textit{ij}}$ which is not explicitly defined for the conducted ILES, as the physical viscosity is zero. To obtain the transport equation for the domain-integrated quantities, the equation is integrated over the domain. This transforms the diagonal entries of the Reynolds stress tensor into the properties under consideration ( $\mathrm{TKX}$ , $\mathrm{TKY}$ and $\mathrm{TKZ}$ ).

The model is simplified by assuming there is no flux across the boundaries of the domain. This makes the contribution of the transport terms zero to the domain-integrated quantities. For the cases under consideration, the buoyancy production term can also be neglected, as the regime of interest is after the shock transmission and the two fluids are roughly in pressure equilibrium. Whilst small pressure gradients exist, previous analysis of the transport budgets for the unstrained case has shown that the buoyancy production is negligible compared with the other terms (Thornber et al. Reference Thornber, Griffond, Bigdelou, Boureima, Ramaprabhu, Schilling and Williams2019). Further, the addition of the applied strain rates does not cause a pressure gradient. For RTI flows, the buoyancy term needs to be modelled, as a persistent pressure gradient drives the development of the flow, thereby introducing an additional production term which would act in the $x$ -direction.

The simplified model consists of shear production (denoted by $\mathcal{P}$ ), pressure-rate-of-strain (denoted by $\mathcal{R}$ ) and dissipation (denoted by $\varepsilon$ )

(4.15) \begin{align} \frac {\rm d}{{\rm d}t} \begin{pmatrix} \mathrm{TKX} \\ \mathrm{TKYZ} \\ \mathrm{TKE} \end{pmatrix} = \begin{pmatrix} \mathcal{P}_x \\ \mathcal{P}_{yz} \\ \mathcal{P} \end{pmatrix} + \begin{pmatrix} \mathcal{R}_{x} \\ \mathcal{R}_{yz} \\ \mathcal{R} \end{pmatrix} - \begin{pmatrix} \varepsilon /3 \\ \varepsilon /3 \\ \varepsilon \end{pmatrix}. \end{align}

As $\mathrm{TKE} = \mathrm{TKX}+2\,\mathrm{TKYZ}$ , it is not necessary to evolve the $\mathrm{TKE}$ component due to the linear dependence, but it is included for completeness.

4.2.5.1. Shear production

The shear production in the simulation cases primarily acts in the transverse direction due to the applied mean transverse strain rates. By assuming only the mean transverse strain rate is non-zero, the shear productions terms are expressed by

(4.16) \begin{align} \mathcal{P}_{x} = 0,\qquad \mathcal{P}_{yz} = -2\,(\mathrm{TKYZ})\bar {S}, \qquad \mathcal{P} = 2 \mathcal{P}_{yz}. \end{align}

4.2.5.2. Dissipation

An isotropic dissipation rate is utilised for the modelling of the pure dissipation term, although this will be adjusted by the pressure-rate-of-strain model. Where the total turbulent kinetic energy has a dissipation rate of $\varepsilon$ , each component will observe the uniform $\varepsilon /3$ dissipation rate. This model does not evolve the dissipation rate itself, and instead uses a length scale to calculate the dissipation rate. For standard turbulent kinetic energy and Reynolds stress components, $\overline {u_i^{\prime\prime}u_{\!j}^{\prime\prime}}$ , dimensional analysis gives a dissipation rate of the form $\sim u^3/l$ (Groom & Thornber Reference Groom and Thornber2020; Thornber et al. Reference Thornber, Drikakis, Youngs and Williams2010). For the choice of length scale, a common choice would be the mixing layer width. The K-L turbulence models, which evolve the turbulent kinetic energy K and turbulent length scale L, are frequently calibrated for late-time behaviour under the assumption that the turbulent length scale, which is used to calculate the dissipation rate, has constant proportionality to the mixing layer width (Dimonte & Tipton Reference Dimonte and Tipton2006; Morgan & Wickett Reference Morgan and Wickett2015; Zhang et al. Reference Zhang, He, Xie, Xiao and Tian2020). The proposed dissipation rate is therefore given by

(4.17) \begin{align} \varepsilon = C_\epsilon \frac {\mathrm{TKE}^{3/2}}{W\sqrt {M}} ,\end{align}

where the integral width $W$ is interpolated from the simulation data and used as the reference length scale, and $M$ is the mass within the mixing layer, $M = 4\pi ^2 \bar {\rho }_0 W$ . As the turbulent kinetic energies are domain integrated with density, the mass factor is needed to compensate for the modified dimensionality. An alternative approach to interpolating the integral width data would be to couple this model with a buoyancy-drag model, which is presented in § 4.2.7, however, the models here are kept separate. An additional coefficient of $C_\epsilon = 1/3$ is included to calibrate the dissipation rate to match the unstrained simulation. The dissipation rate is assumed to be isotropic in all directions due to the assumption of local isotropy for high-Reynolds-number flows. This dissipation model is not suitable for anisotropic turbulence, for which it causes the flow to become increasingly anisotropic. This is corrected through the return-to-isotropy models in the pressure-rate-of-strain tensor.

4.2.5.3. Pressure rate of strain

For simplicity, it is assumed that the pressure-rate-of-strain tensor has no net effect on the turbulent kinetic energy, and only redistributes energy amongst the components. Analysis of the turbulent kinetic energy budgets shows that the pressure rate of strain makes a net contribution to the turbulent kinetic energy for the unstrained case, however, the significance of this term decreases with time (Thornber et al. Reference Thornber, Griffond, Bigdelou, Boureima, Ramaprabhu, Schilling and Williams2019). A compressible model for the pressure-rate-of-strain tensor will include contributions to the total turbulent kinetic energy, whereas incompressible models do not. The compressible model of Sarkar (Reference Sarkar1992) includes pressure-dilation contributions which are weighted by the turbulent Mach number, however, the mean turbulent Mach number in the strained simulations is below 0.03, calculated using the planar-averaged turbulent kinetic energy. Instead, the pressure rate of strain is calculated using the Launder-Reece-Rodi - Isotropisation of Production (LRR-IP) model by Launder, Reece & Rodi (Reference Launder, Reece and Rodi1975), and is used for Reynolds stress transport models for compressible turbulent mixing (Grégoire et al. Reference Grégoire, Souffland and Gauthier2005; Schwarzkopf et al. Reference Schwarzkopf, Livescu, Gore, Rauenzahn and Ristorcelli2011). The LRR-IP model is the combination of two other models. The return-to-isotropy model by Rotta (Reference Rotta1951) represents the effects of the slow pressure, where decaying turbulence tends towards isotropy. The Rotta constant $C_R$ effectively modifies the dissipation rate of each component and will be active for the unstrained and strained cases. For $C_R$ values greater than unity, the turbulent kinetic energy components will eventually achieve isotropy, with the time required to do so decreasing as the value of $C_R$ increases. A value of $C_R=1$ will cause the system to maintain the initialised isotropy. Whilst Launder (Reference Launder1996) recommends a value of $C_R=1.8$ , RMI-induced mixing layers remain anisotropic, so the value was modified to improve the agreement with the unstrained simulation, using a value of unity here in order to accurately predict the $\mathrm{TKX}$ and $\mathrm{TKYZ}$ components for the unstrained case. The second component of the LRR-IP model is the isotropisation of production model by Naot, Shavit & Wolfshtein (Reference Naot, Shavit and Wolfshtein1970), which acts to redistribute the effects of shear production from one direction onto the others. This model depends upon the term $C_2$ and will only activate for the strain cases where shear production is present. To align with the observed relationship between $C_R$ and $C_2$ in Launder (Reference Launder1996), the value of $C_2$ was set to 0.77, which should allow the model to still describe free shear flows. The simplified expression for the pressure-rate-of-strain components are

(4.18a) \begin{align} \mathcal{R}_{x} &= -C_R \frac {\varepsilon }{\mathrm{TKE}} \left (\mathrm{TKX} - \frac {1}{3} \mathrm{TKE} \right ) - C_2 \left ( -\frac {1}{3} \mathcal{P}\right )\!, \end{align}

(4.18b) \begin{align} \mathcal{R}_{yz} &= -C_R \frac {\varepsilon }{\mathrm{TKE}} \left (\mathrm{TKYZ} - \frac {1}{3} \mathrm{TKE} \right ) - C_2 \left ( \mathcal{P}_{yz} - \frac {1}{3} \mathcal{P}\right ), \end{align}

(4.18c) \begin{align} \mathcal{R} &= 0. \end{align}

4.2.5.4. Application and revised dissipation model

The application of the model for $\mathrm{TKE}$ is shown in figure 12 alongside the simulation results. Whilst both the simulation and the model observe an increase in $\mathrm{TKE}$ from the shear production, the model appears to overestimate the influence of the strain rate on the turbulent kinetic energy. This is not a failing of the shear production approximation, but instead is due to the dissipation rate estimation. The effect of transverse strain means the mixing layer no longer becomes self-similar, and the integral width is no longer proportional to the turbulent length scale. This discrepancy has been highlighted in turbulence models which transport two length scales to avoid this assumption (Schwarzkopf et al. Reference Schwarzkopf, Livescu, Baltzer, Gore and Ristorcelli2016; Morgan, Schilling & Hartland Reference Morgan, Schilling and Hartland2018). Increased dissipation for the compression cases also explains the cause of the observed decrease in $\mathrm{TKX}$ . A turbulent length scale which adjusts with the transverse expansion as well as the integral width provides improved accuracy. It is commonly assumed in turbulence models that bulk compression or expansion will scale the turbulent length scale (Besnard et al. Reference Besnard, Harlow, Rauenzahn and Zemach1992; Dimonte & Tipton Reference Dimonte and Tipton2006). Accounting for this, the transverse model uses a dissipation rate with a transverse expansion modification and is calculated according to

(4.19) \begin{align} \varepsilon = C_\epsilon \frac {\mathrm{TKE}^{3/2}}{W \sqrt {M} \varLambda ^{2/3}}. \end{align}

The transverse modification does not affect the unstrained case, but increases/decreases the dissipation for the compression/expansion cases. A comparison of the percentage error for the original and corrected models is presented in figure 12. There is some inherent inaccuracy in the models for even the unstrained case due to the transitional regime and transient behaviours which do not subside until a later time. The corrected model shows improvement over the base model, maintaining an error below 20 %. Whilst the closure approximations used are common in turbulence models, the model is not perfect and is limited in many aspects. Firstly, the model relies on the domain-integrated quantities, inherently losing the information regarding the spatial distribution of the flow. The pressure-dilatation modelling is simplified in its assumptions of zero total production and the treatment of the return to isotropy modelling, which is calibrated for the unstrained behaviour. For RMI, a more accurate model may involve a return to anisotropy approximation, causing the system to decay towards a specified anisotropy ratio of turbulent kinetic energy.

Figure 12. Performance of the turbulent kinetic energy models. (a) Comparison of the ILES results (solid coloured lines) against the standard model (black dashed lines). (b–d) Error comparison for the standard model (black dashed lines) and the transverse model (coloured dot-dash lines).

Figure 13. Turbulent mass flux profiles. Solid lines indicate results at $\tau =9.84$ , dashed lines indicate results at $\tau =34.45$ .

4.2.6. Turbulent mass flux

The turbulent mass flux is a feature of variable-density flows and is included in many compressible Reynolds-averaged Navier–Stokes models, helping to improve the performance of the turbulence model in calculating the conversion of potential energy to turbulence. The turbulent mass flux, $a_i=\tilde {u_i}-\bar {u}_i = -\overline {u_i^{\prime\prime}}$ , represents the difference between the mean and density weighted velocities. With the transverse velocity gradients removed, the homogeneity of the $y$ – $z$ plane means that $a_2$ and $a_3$ should statistically be zero for the plane. The axial component, $a_1$ , is not zero and is plotted in figure 13 for all cases at $\tau =9.84$ (solid lines), and for the unstrained and low-magnitude strain rate cases at $\tau =34.45$ (dashed lines). The results show the compression cases have a decreased amount of axial turbulent mass flux compared with the unstrained case. The turbulent mass flux can be generated by a term equivalent to shear production

(4.20) \begin{align} \frac { \partial \bar {\rho } a_i}{\partial t} \propto -\bar {\rho } a_{\!j} \frac {\partial \bar {u}_i}{\partial x_{\!j}}. \end{align}

The applied strain rates are in the transverse direction, so this term will act on components $a_2$ and $a_3$ , not $a_1$ . The components $a_2$ and $a_3$ will remain statistically zero, however, the influence of the strain rate will affect the standard deviation/second moment of the $u_i^{\prime\prime}$ distribution, which is the turbulent kinetic energy and investigated in § 4.2.4. The decrease in $a_1$ with transverse compression matches the behaviour observed for $\mathrm{TKX}$ which decreased under compression due to the change in the dissipation rate, showing that the effect of the modified turbulent length scale and dissipation rate is observable for the turbulent mass flux as well. The variation in $a_1$ explains the changes in the growth of the mixing layer, with the increased values of $a_1$ for the expansion cases causing greater entrainment of new fluid into the mixing layer.

4.2.7. Buoyancy-drag model

The computational cost of fully simulating and resolving turbulent mixing layers drives the necessity for simpler models that provide swift estimates of the mixing layer growth. One such model is the buoyancy-drag model, which uses coupled ordinary differential equations (ODEs) that can be solved with greater ease than partial differential equations. This methodology was inspired by the modelling of bubble penetration in the $\mathcal{A}_t=1$ case for RTI by Layzer (Reference Layzer1955), however, there have been many works trying to derive and calibrate the buoyancy-drag model to accurately represent the RMI and RTI for all Atwood numbers (Baker & Freeman Reference Baker and Freeman1981; Hansom et al. Reference Hansom, Rosen, Goldack, Oades, Fieldhouse, Cowperthwaite, Youngs, Mawhinney and Baxter1990; Ramshaw Reference Ramshaw1998; Dimonte Reference Dimonte2000; Oron et al. Reference Oron, Arazi, Kartoon, Rikanati, Alon and Shvarts2001). The simplicity of the buoyancy-drag model has also inspired other models such as the K-L turbulence model (Dimonte & Tipton Reference Dimonte and Tipton2006) which collapses to the buoyancy-drag model under a self-similar analysis. The most relevant buoyancy-drag models to the cases investigated here are the models calibrated to the quarter-scale $\theta$ -group case by Youngs & Thornber (Reference Youngs and Thornber2020a , Reference Youngs and Thornberb ). The buoyancy-drag model for the integral width evolves both the integral width ( $W$ ) and the velocity at which the integral width grows ( $V$ ). For the unstrained RMI case, the model takes the form

(4.21a) \begin{align} \frac {\mathrm{d}W}{\mathrm{d}t} = V, \quad \frac {\mathrm{d}V}{\mathrm{d}t} = - \frac {V^2}{l^{\textit{eff}}(\bar {\lambda }_0,W)}. \end{align}

The effective drag length scale $l^{\textit{eff}}(\bar {\lambda }_0,W)$ is specified as a function of the initial perturbation wavelength, which does not change for the unstrained case, and the integral width. The growth of the integral width is equal to the calculated velocity. As this model is calibrated for the RMI-induced mixing layer, the velocity ODE only contains a drag term, however, for RTI flows a buoyancy term is also included. Youngs & Thornber (Reference Youngs and Thornber2020b ) calibrated the effective drag length scale used in the drag term for the integral width to accurately predict the early-time growth. This analysis was extended to the bubble and spike heights in Youngs & Thornber (Reference Youngs and Thornber2020a ). The drag length scale for the integral width is given by

(4.22a) \begin{align} l^{\textit{eff}} =\bar {\lambda }_0 \max \left \{a-b\left (1-e^{-cW/\bar {\lambda }_0}\right )\!, \frac {\theta }{1-\theta } \left (\frac {W}{\bar {\lambda }_0}-d\right ) \right \}. \end{align}

The effective length scale introduces dependence on the late-time power-law growth rate $\theta$ , the late-time ratio of spike-to-bubble height $R=h_s/h_b$ and utilises several coefficients for calibration. The effective drag length scale for each equation is a piece-wise function that transitions between a length scale dependent on the spectrum perturbation to a drag length scale that scales linearly with the outer length scale. For the late-time growth, a theoretical power-law value of $\theta =1/3$ was used, based upon the work of Elbaz & Shvarts (Reference Elbaz and Shvarts2018). The remaining coefficients of the drag length scale equations are listed in table 4, based off the values in the original works (Youngs & Thornber Reference Youngs and Thornber2020a , Reference Youngs and Thornberb ).

Table 4. Buoyancy-drag coefficients for $\mathcal{A}_t=0.5$ , narrowband RMI (Youngs & Thornber Reference Youngs and Thornber2020a , Reference Youngs and Thornberb ).

Figure 14. Buoyancy-drag model for integral width: (a) $l^{\textit{eff}} = \bar {\lambda }(t) f(\bar {\lambda }(t),W)$ (4.23) and (b) $l^{\textit{eff}} = \bar {\lambda }(t) f(\bar {\lambda }_0,W)$ (4.25). Solid lines indicate ILES results, dashed lines indicate the buoyancy-drag model.

In the unstrained Cartesian configuration, there is no variation in the mean wavelength used to calculate the effective drag length scale. For a spherical geometry, Miles (Reference Miles2004, Reference Miles2009) utilised the time-varying wavelength as the drag length scale, whilst El Rafei & Thornber (Reference El Rafei and Thornber2020) calibrated a buoyancy-drag model by fitting the effective drag length scale as a function of the time-varying wavelength, $l^{\textit{eff}} = f(\bar {\lambda }(t),W)$ . Using this modification, the drag length scale utilises the time-varying wavelength in place of the original wavelength, resulting in the form

(4.23) \begin{align} l^{\textit{eff}} =\bar {\lambda }(t) \max \left \{a-b\left (1-e^{-cW/\bar {\lambda }(t)}\right )\!, \frac {\theta }{1-\theta } \left (\frac {W}{\bar {\lambda }(t)}-d\right ) \right \}. \end{align}

To validate this model, the equations are integrated in time using the initial conditions provided in Youngs & Thornber (Reference Youngs and Thornber2020a ). An offset of $\tau =0.08$ is used to align the buoyancy-drag model with the simulations, as the buoyancy-drag model is fitted to the post-shock behaviour and does not describe the shock transition. The initial heights and velocities are given by

(4.24) \begin{align} W_0 = 0.5642 C \sigma _0, \quad &\quad V_0 = 0.5642 C \bar {k} \sigma _0 \Delta u \mathcal{A}_t F_W^{nl}, \end{align}

with compression factor $C=0.576$ , and nonlinearity factor $F_W^{nl} = 0.85$ . The solutions for all cases are identical until $\tau =1$ , after which the transverse strain rates will begin to change $\bar {\lambda }(t)$ and the effective drag length scale. The results of this model are presented in figure 14(a ), focusing on the domain of interest. The buoyancy-drag model incorrectly predicts the influence of the strain rates, showing an increase in the integral width for the compression cases, as opposed to the ILES which shows a decrease. The addition of strain rate dependent terms to the velocity equation to improve the model is possible, but it would not correctly correspond to the physical shear production. Under the transverse compression, the shear production terms should be adding turbulent kinetic energy and increasing the velocity, which is the opposite of what is needed to improve the model performance. Therefore, it is not the implementation of shear production terms but instead primarily the modification of the drag length scale which is needed. An alternative drag length scale is proposed that scales the original drag length scale by the transverse expansion factor, and otherwise retains the original formulation. This scaling modification is similar to the correction performed for the dissipation rate in § 4.2.5. This can be simplified to scaling linearly with the time-varying wavelength, as $\bar {\lambda }_0 \varLambda (t) = \bar {\lambda }(t)$ . This drag length scale

(4.25) \begin{align} l^{\textit{eff}} =\bar {\lambda }(t) \max \left \{a-b\left (1-e^{-cW/\bar {\lambda }_0}\right )\!, \frac {\theta }{1-\theta } \left (\frac {W}{\bar {\lambda }_0}-d\right ) \right \}, \end{align}

is presented in figure 14(b ), and shows greater alignment with ILES results, accurately predicting the direction in which the integral width is modified. There is a small discrepancy between the model and simulation results, primarily showing a slight overestimation of the influence of the strain rate. The addition of shear production terms may help further reduce this residual error. The inclusion of shear production would be physically accurate when implemented with the corrected drag length scale of (4.25), serving to add/remove energy to increase/decrease the growth of the compression/expansion cases.

The drag length scale which scales with the transverse expansion factor implies that the self-similar growth rate is permanently modified after the period of strain. For a constant value of $\varLambda$ , using (4.25) permits a power-law growth rate of the form $W\propto t^{\hat {\theta }}$ for RMI, where the new growth rate is given by

(4.26) \begin{align} \hat {\theta } = \theta \frac {\varLambda }{\theta (\varLambda -1)+1}, \end{align}

whereby $\hat {\theta }$ decreases for transverse compression ( $\varLambda \lt 1$ ), and increases for expansion ( $\varLambda \gt 1$ ). For an unstrained growth rate of $\theta =1/3$ , a compression by a factor of 2 gives $\hat {\theta } = 0.20$ and a compression factor of 30 for ICF gives $\hat {\theta }=0.016$ . However, this does not account for the presence of any axial/radial strain rates during compression which deposit energy and can increase the growth rate.