The transformation of internal waves on a stepwise underwater obstacle is studied in the linear approximation. The transmission and reflection coefficients are derived for a two-layer fluid. The results are obtained and presented as functions of incident wave wavenumber, density ratio of layers, pycnocline position, and height of the bottom step. Excitation coefficients of evanescent modes are also calculated, and their importance is demonstrated. This allows one to estimate the number of evanescent modes necessary to take into account to attain the required accuracy for the transformation coefficients.

When atmospheric storms pass over the ocean, they resonantly force near-inertial waves (NIWs), internal waves with a frequency close to the local Coriolis frequency $f$. It has long been recognised that the evolution of NIWs is modulated by the ocean's mesoscale eddy field. This can result in NIWs being concentrated into anticyclones which provide an efficient pathway for NIW propagation to depth. Here we analyse the eigenmodes of NIWs in the presence of mesoscale eddies and heavily draw on parallels with quantum mechanics. Whether the eddies are effective at modulating the behaviour of NIWs depends on the wave dispersiveness $\varepsilon ^2 = f\lambda ^2/\varPsi$, where $\lambda$ is the deformation radius and $\varPsi$ is a scaling for the eddy streamfunction. If $\varepsilon \gg 1$, NIWs are strongly dispersive, and the waves are only weakly affected by the eddies. We calculate the perturbations away from a uniform wave field and the frequency shift away from $f$. If $\varepsilon \ll 1$, NIWs are weakly dispersive, and the wave evolution is strongly modulated by the eddy field. In this weakly dispersive limit, the Wentzel–Kramers–Brillouin approximation, from which ray tracing emerges, is a valid description of the NIW evolution even if the large-scale atmospheric forcing apparently violates the requisite assumption of a scale separation between the waves and the eddies. The large-scale forcing excites many wave modes, each of which varies on a short spatial scale and is amenable to asymptotic analysis analogous to the semi-classical analysis of quantum systems. The strong modulation of weakly dispersive NIWs by eddies has the potential to modulate the energy input into NIWs from the wind, but we find that this effect should be small under oceanic conditions.

We investigate experimentally the planar paths displayed by cylinders falling freely in a thin-gap cell containing liquid at rest, by varying the elongation ratio and the Archimedes number of the cylinders, and the solid-to-fluid density ratio. In the investigated conditions, the oscillatory falling motion features two main characteristics: the mean fall velocity $\overline {u_v}$ does not scale with the gravitational velocity, which overestimates $\overline {u_v}$ and is unable to capture the influence of the density ratio on it; and high-amplitude oscillations of the order of $\overline {u_v}$ are observed for both translational and rotational velocities. To model the body behaviour, we propose a force balance, including proper and added inertia terms, the buoyancy force and vortical contributions accounting for the production of vorticity at the body surface and its interaction with the cell walls. Averaging the equations over a temporal period provides a mean force balance that governs the mean fall velocity of the cylinder, revealing that the coupling between the translational and rotational velocity components induces a mean upward inertial force responsible for the decrease of $\overline {u_v}$. This mean force balance also provides a normalization for the frequency of oscillation of the cylinder in agreement with experimental measurements. We then consider the instantaneous force balance experienced by the body, and propose three contributions for the modelling of the vortical force. These can be interpreted as drag, lift and history forces, and their dependence on the control parameters is adjusted on the basis of the experimental measurements.

The added mass force resulting from the acceleration of a body in a fluid is of fundamental and practical interest in dispersed multiphase flows. Euler–Lagrange (EL) and Euler–Euler (EE) simulations require closure terms for the added mass force in order to accurately couple the conserved variables between phases. Presently, a more thorough understanding of the added mass force in a multi-particle system is developed based on potential flow resulting in a resistance matrix formulation analogous to Stokesian dynamics. This formulation is then used to generate a dataset of added mass resistance matrices for large systems of randomly generated particles. This methodology is used to create a volume fraction corrected binary model for predicting the added mass force in large systems as well as generate statistics of the added mass force in such systems. This work provides clarification to the theory of the added mass force for particle clouds, and modelling options that may be implemented in existing EL and EE codes.

This work presents an experimental investigation of the effects of vortex shedding suppression on the properties and recovery of turbulent wakes. Four plates, properly modified so that they produce different vortex shedding strengths, are tested using high speed particle image velocimetry and hot-wire anemometry, and analysed using spectral proper orthogonal decomposition, mean-flow linear stability analysis and various turbulence statistics. When present, vortex shedding is found to exhibit a characteristic frequency that scales with the mean shear, providing a link between the mean flow and the main turbulent motion. To achieve full suppression of shedding, we combine the effects of porosity and fractal perimeter. The mean shear is then decreased to the point where the flow becomes convectively unstable and shedding vanishes. In that case, the onset of self-similarity is delayed, compared with the case with vortex shedding, and appears after another large-scale structure, the secondary vortex street, emerges. It is also found that both large- and small-scale intermittency are starkly reduced when shedding is absent. A simple theoretical representation of the wake dynamics explains the evolution of the wake properties and its connection to the coherent structures in the flow.

In freely decaying stably stratified turbulent flows, numerical evidence shows that the horizontal displacement of Lagrangian tracers is diffusive while the vertical displacement converges towards a stationary distribution, as shown numerically by Kimura & Herring (J. Fluid Mech., vol. 328, 1996, pp. 253–269). Here, we develop a stochastic model for the vertical dispersion of Lagrangian tracers in stably stratified turbulent flows that aims to replicate and explain the emergence of a stationary probability distribution for the vertical displacement of such tracers. More precisely, our model is based on the assumption that the dynamical evolution of the tracers results from the competing effects of buoyancy forces that tend to bring a vertically perturbed fluid parcel (carrying tracers) to its equilibrium position and turbulent fluctuations that tend to disperse tracers. When the density of a fluid parcel is allowed to change due to molecular diffusion, a third effect needs to be taken into account: irreversible mixing. Indeed, ‘mixing’ dynamically and irreversibly changes the equilibrium position of the parcel and affects the buoyancy force that ‘stirs’ it on larger scales. These intricate couplings are modelled using a stochastic resetting process (Evans & Majumdar, Phys. Rev. Lett., vol. 106, issue 16, 2011, 160601) with memory. More precisely, Lagrangian tracers in stratified turbulent flows are assumed to follow random trajectories that obey a Brownian process. In addition, their stochastic paths can be reset to a given position (corresponding to the dynamically changing equilibrium position of a density structure containing the tracers) at a given rate. Scalings for the model parameters as functions of the molecular properties of the fluid and the turbulent characteristics of the flow are obtained by analysing the dynamics of an idealised density structure. Even though highly idealised, the model has the advantage of being analytically solvable. In particular, we show the emergence of a stationary distribution for the vertical displacement of Lagrangian tracers. We compare the predictions of this model with direct numerical simulation data at various Prandtl numbers $Pr$, the ratio of kinematic viscosity to molecular diffusion.