15 results
Reactive control of velocity fluctuations using an active deformable surface and real-time PIV
- Findlay McCormick, Bradley Gibeau, Sina Ghaemi
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- Journal:
- Journal of Fluid Mechanics / Volume 985 / 25 April 2024
- Published online by Cambridge University Press:
- 16 April 2024, A9
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This study demonstrates an experimental realization of turbulence control strategies previously explored by Choi et al. (J. Fluid Mech., vol. 262, 1994, pp. 75–110) through numerical simulations. To conduct the experiments, a deformable surface with a streamwise array of 16 independently controlled actuators was developed. A real-time particle image velocimetry (RT-PIV) system was also created for flow measurements. The objective of the control strategy was to target the sweep and ejection motions of the vortex shedding from a spherical cap placed in a laminar boundary layer. Reactive control strategies consisted of wall-normal surface deformations that opposed or complied with the wall-normal (v) or streamwise (u) velocity fluctuations obtained from the RT-PIV. The results showed two primary outcomes of the control approach. Firstly, it effectively hindered the advancement of sweep motions towards the wall. Secondly, it disrupted the periodic shedding of vortices. The v-control with opposing wall motions and u-control with compliant wall motions exhibited strong inhibition of sweep motions, while the v-control with compliant and u-control with opposing wall motions showed weaker inhibition. All reactive control cases resulted in the disruption of vortex shedding. In some instances, this disruption was accompanied by increased turbulent kinetic energy due to the generation of additional flow motions. However, the v-control with opposing wall motions significantly reduced the vortex-shedding energy while maintaining total turbulent kinetic energy close to or below that of the unforced flow. Overall, the experiments show the effectiveness of reactive control strategies in mitigating sweep motions and disrupting vortical structures, offering insights for developing reactive control strategies.
Local flow topology of a polymer-laden turbulent boundary layer
- Lucas Warwaruk, Sina Ghaemi
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- Journal:
- Journal of Fluid Mechanics / Volume 983 / 25 March 2024
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- 18 March 2024, A22
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Fine-scale flow motions are measured in a Newtonian and polymer drag-reduced turbulent boundary layer (TBL) at a common momentum thickness Reynolds number $Re_{\theta }$ of 2300. Relative to the Newtonian TBL, the polymer-laden flow has a 33 % lower skin-friction coefficient. Three-dimensional (3-D) particle tracking velocimetry is used to measure the components of the velocity gradient tensor (VGT), rate of deformation tensor (RDT) and rate of rotation tensor (RRT). The invariants in these tensors are then used to distinguish the different types of fine-scale flow motions – a method called the $\varDelta$-criterion. Joint probability density functions (j.p.d.f.s) of the VGT invariants, $Q$ and $R$, for the Newtonian TBL produce the familiar tear-drop pattern, commonly seen in direct numerical simulations of Newtonian turbulence. Relative to the Newtonian TBL, the polymer-laden flow has significantly attenuated values of $R$, implying an overall reduction in fluid stretching. The invariants in the RDT, $Q_D$ and $R_D$, imply that straining motions of the polymeric flow are more two dimensional compared with the Newtonian flow. Moreover, j.p.d.f.s of $Q_D$ and the invariant in the RRT $Q_W$, suggest that the flow consists of fewer biaxial extensional events and more shear-dominated flow. Few, if any, experimental investigations have measured the 3-D structure of fine-scale motions in a Newtonian and polymer drag-reduced TBL using the $\varDelta$-criterion. We provide the first experimental evidence that supports the notion that an attenuation of fluid stretching, particularly biaxial straining motions, is central to the mechanism of polymer drag reduction.
Manipulation of a turbulent boundary layer using active surface deformations
- Bradley Gibeau, Sina Ghaemi
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- Journal:
- Journal of Fluid Mechanics / Volume 966 / 10 July 2023
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- 27 June 2023, A6
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We experimentally evaluate whether active wall-normal surface deformations are suitable for the targeted control of very-large-scale motions (VLSMs) in a turbulent boundary layer at a friction Reynolds number of $Re_\tau =2600$. Circular surface deformations with a diameter $D$ roughly equal to the boundary layer thickness $\delta$ are generated periodically at a constant amplitude of $0.03\delta$ and at actuation frequencies of $St=0.05$ to 0.20, where $St$ is the Strouhal number based on $D$ and the free stream velocity $U_\infty$. The resulting impact on the flow was captured using high-speed particle image velocimetry and analysed using a triple decomposition. We find that the active surface deformations produce high- and low-speed streamwise velocity fluctuations that are concentrated along the centreline of the actuator. These motions have a negligible impact on the mean velocity profile downstream, i.e. they are truly high and low speed with respect to the unactuated base flow. The motions produced at $St\lesssim 0.1$ are comparable to synthetic VLSMs in terms of their lengths and widths but with a reduced wall-normal extent and rapidly decaying strength. These synthetic motions produce a strong modulation of the turbulence similar to that of the naturally occurring VLSMs. Most notably, we observe that synthetic high-speed motions with magnitudes of the order of $0.05U_\infty$ cause up to a 30 % reduction in turbulence production within the logarithmic layer. The strength and turbulence-modulating characteristics of the synthetic motions appear well suited for targeting the naturally occurring VLSMs locally using a control scheme.
Polymer and surfactant flows through a periodically constricted tube
- Lucas Warwaruk, Sina Ghaemi
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- Journal:
- Journal of Fluid Mechanics / Volume 960 / 10 April 2023
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- 31 March 2023, A19
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The flow of three non-Newtonian fluids, comprising polymer and surfactant additives, in a periodically constricted tube (PCT) are experimentally compared. The radius of the tube walls is sinusoidal with respect to the streamwise direction. The three fluids are aqueous solutions of flexible polymers, rigid biopolymers and surfactants, which are typically used for drag-reduction in turbulent flows. Steady shear viscosity measurements demonstrate that rigid and flexible polymer solutions are shear-thinning, while surfactant solutions have a Newtonian and water-like shear viscosity. Capillary driven extensional rheology demonstrates that only flexible polymer solutions produce elastocapillary thinning. Particle shadow velocimetry is used to measure the velocity of each flow within the PCT at five Reynolds numbers spanning roughly 0.5 to 300. Relative to the Newtonian flows, rigid polymer solutions exhibit a blunt velocity profile. Flexible polymer solutions demonstrate a distinct chevron-shaped velocity contour and zones of opposing vorticity when the Deborah number exceeds 0.1. Using the vorticity transport equation, it is revealed that the opposing vorticity zones are coupled with a non-Newtonian torque. The PCT reveals that the surfactant solutions have similar non-Newtonian features as flexible polymer solutions – those being a chevron velocity pattern, opposing vorticity and a finite non-Newtonian torque. This observation is of practical importance since conventional shear and extensional rheometric measurements are not capable of demonstrating non-Newtonian features of the surfactant solutions. The investigation demonstrates that the PCT serves as a viable geometry for showing the non-Newtonian traits of dilute surfactant solutions.
Unsteady motions in the turbulent separation bubble of a two-dimensional wing
- Sen Wang, Sina Ghaemi
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- Journal:
- Journal of Fluid Mechanics / Volume 948 / 10 October 2022
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- 02 September 2022, A3
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The unsteadiness of a turbulent separation bubble (TSB) formed close to the trailing edge of a two-dimensional wing was investigated using time-resolved particle image velocimetry. The angle of attack was set to 9.7° and the chord-based Reynolds number was 720 000. The TSB consisted of two shear layers and formed a triangular shape in the streamwise–wall-normal plane. The vertices of this triangle consisted of an intermittent detachment point, a fixed corner close to the airfoil trailing edge and an intermittent endpoint in the wake region. The velocity field had three energetic regions each with different Strouhal numbers (Stl): (a) an upstream turbulent boundary layer (TBL) with Stl = 0.1 to 4, (b) a TSB with Stl = 0.03 to 0.08 and (c) two shear layers with Stl = 0.4 to 0.8. The low-frequency motions in the TSB consisted of large zones of positive and negative streamwise velocity fluctuation that were several times wider than the large-scale structures of the upstream TBL. These zones forced an undulation of the separation line and were attributed to Görtler structures. They were also correlated with the velocity fluctuations between the two shear layers. The breathing motion of the TSB occurred at Stl = 0.05. This breathing correlated with the location of the TSB endpoint and the flapping of the upper shear layer. The detachment point of the TSB featured broad fluctuations and did not demonstrate a strong correlation with the breathing motion.
Low- and mid-frequency wall-pressure sources in a turbulent boundary layer
- Bradley Gibeau, Sina Ghaemi
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- Journal:
- Journal of Fluid Mechanics / Volume 918 / 10 July 2021
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- 07 May 2021, A18
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Simultaneous wall-pressure and high-speed particle image velocimetry measurements were used to identify the coherent structures that generate low- and mid-frequency wall-pressure fluctuations in a turbulent boundary layer at a friction Reynolds number of $Re_\tau =2600$. The coherence function between wall pressure and velocity at a range of wall-normal locations revealed two distinct frequency bands of high coherence that span the low- and mid-frequency regions of the wall-pressure spectrum. Pressure was filtered to isolate the frequencies associated with each region of high coherence, and space–time pressure-velocity correlations were computed using the filtered signals to expose the motions responsible for the observed pressure-velocity coupling. The resulting correlation patterns were attributed to very-large-scale motions (VLSMs) and hairpin packets, revealing that these two types of coherent motions are the dominant sources of wall-pressure fluctuations at the low and mid frequencies. Although the VLSMs and hairpin packets are closely related, the mechanisms by which these motions affect wall pressure were found to be different. The VLSMs were found to cause positive and negative wall-pressure fluctuations via splatting and lifting of fluid at the wall, respectively. In contrast, hairpin packets affected wall pressure because of their low-pressure vortex cores and regions of high-pressure stagnation. The frequency at which the wall-pressure source changes from the VLSMs to the hairpin packets coincided with the peak of the wall-pressure spectrum, suggesting that the peak may be a result of the transition between pressure sources that occurs at the same point in the frequency domain.
A direct comparison of turbulence in drag-reduced flows of polymers and surfactants
- Lucas Warwaruk, Sina Ghaemi
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- Journal:
- Journal of Fluid Mechanics / Volume 917 / 25 June 2021
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- 21 April 2021, A7
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We experimentally compared the drag-reduced turbulent channel flow of three different additives: a flexible polymer, a rigid polymer and a surfactant. A high drag reduction (HDR) of approximately 58 % was achieved using the flexible polymer, the rigid polymer and the surfactant. A maximum drag reduction (MDR) of approximately 70 % was also achieved using the flexible polymer and the surfactant. Solutions of flexible polymer and surfactant had a small shear viscosity, while the rigid polymer solution had a large shear viscosity with a considerable shear-thinning behaviour. The flexible polymer solution was the only fluid to exhibit a large extensional relaxation time. At HDR, the wall-normal distribution of mean velocity and the turbulent statistics of the drag-reduced flows were a function of the additive type and Reynolds number, Re. At MDR, the wall-normal distribution of mean velocity and turbulent statistics of the drag-reduced flows were similar, and not contingent on the additive type or Re. Due to its larger shear viscosity, the rigid polymer solution did not reach the MDR state in terms of drag reduction and mean velocity profile. However, the Reynolds stress profiles and turbulent length scale of the rigid polymer solution at HDR were similar to those of the flexible polymer and surfactant solutions at MDR. Our investigation demonstrated that different additives generate drag-reduced flows with similar turbulent statistics; however, no common rheological feature has been identified as of yet.
Long non-axisymmetric fibres in turbulent channel flow
- Mobin Alipour, Marco De Paoli, Sina Ghaemi, Alfredo Soldati
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- Journal:
- Journal of Fluid Mechanics / Volume 916 / 10 June 2021
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- 06 April 2021, A3
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In this work, we investigate the dynamics of long non-axisymmetric fibres in turbulent channel flow. The experimental facility is the TU Wien Turbulent Water Channel, consisting of a closed water channel (aspect ratio of 10), and the experiments are performed at a shear Reynolds number of 360. Fibres are neutrally buoyant rods that are curved and characterised by a length-to-diameter ratio of 120. Illumination is provided by a laser sheet and the motion of fibres is recorded by four high-speed cameras in a fully developed flow section. We apply multiplicative algebraic reconstruction techniques to the recorded images from four high-speed cameras to identify the three-dimensional location, shape and orientation of the fibres. The fibres are also tracked in time to obtain their three-dimensional vectors of velocity and rotation rate. We investigate the behaviour of the fibres, from the near-wall region to the channel centre, and we produce original statistics on the effect of curvature of the fibres on their orientation and rotation rate. Specifically, we measured the orientation and rotation rate of the fibres, and we can confirm that in the centre, the most homogeneous part of the channel, statistics, although influenced by the curvature, bear similarities to those obtained in previous investigations in homogeneous isotropic turbulence. In addition, we have been able to compare the tumbling rate of our long non-axisymmetric fibres with previous solutions for curved ellipsoids in simple shear flow.
Time-resolved topology of turbulent boundary layer separation over the trailing edge of an airfoil
- Austin Ma, Bradley Gibeau, Sina Ghaemi
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- Journal:
- Journal of Fluid Mechanics / Volume 891 / 25 May 2020
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- 18 March 2020, A1
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The unsteady organization of a separated turbulent boundary layer was investigated upstream from the trailing edge of a NACA 4418 airfoil. The angle of attack was $9^{\circ }$ in the pre-stall regime. Two particle image velocimetry fields of view were of interest: a streamwise–wall-normal plane at midspan of the airfoil and a streamwise–spanwise plane parallel to and near the surface of the airfoil. In the near-surface streamwise–spanwise plane, the mean velocity field revealed a saddle point near midspan and a pair of counter-rotating foci at the sides. This pattern is reminiscent of a stall cell, which has been traditionally associated with flow separation on thick airfoils at and slightly beyond the angle of attack of maximum lift. Isolating the low frequencies showed that the instantaneous separation front consisted of several smaller structures that also resembled a stall cell pattern, but they were an order of magnitude smaller than the one found in the mean pattern. These instantaneous stall cells were of two types: forward and backward. The forward stall cells were formed by strong high-speed streaks from upstream, while backward stall cells formed as a result of strong backflow just downstream from the separation front, resulting in a foci pair and a saddle point on their upstream side. In both cases, the foci pairs acted to mobilize high-speed momentum of the associated streak into a rotational motion, causing these streaks to dissipate. Finally, proper orthogonal decomposition revealed that low-order modes were associated with the movement and distortion of the separation front.
Negative skin friction during transition in a zero-pressure-gradient flat-plate boundary layer and in pipe flows with slip and no-slip boundary conditions
- Xiaohua Wu, Mike Cruickshank, Sina Ghaemi
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- Journal:
- Journal of Fluid Mechanics / Volume 887 / 25 March 2020
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- 28 January 2020, A26
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In searching for the mechanisms of boundary layer bypass transition, Schubauer & Klebanoff (NACA-TR-1289, 1956) conducted an extensive search for negative skin-friction events during the laminar-to-turbulent transition in a zero-pressure-gradient, smooth flat-plate boundary layer (ZPGSFPBL) under a wide range of free-stream turbulence intensity levels. They concluded that no evidence of incipient flow separation could be found in any part of the transition region. Although it has been known that extremely rare backflow events can occur in a fully turbulent ZPGSFPBL and in fully developed turbulent pipe flow, the Schubauer–Klebanoff conclusion regarding the total absence of negative skin friction in ZPGSFPBL transition has remained unchallenged over the past six decades. Here we report our discovery of negative skin-friction events during the bypass transition in an extensively researched representative ZPGSFPBL case, and in pipe flows with no-slip and slip boundary conditions under circumferential mode inlet disturbance, respectively. The peak probability of such events is located in the late stage of transition between the streamwise stations where the minimum and maximum mean skin friction are attained. The magnitude of the peak probability is substantially larger than the corresponding probability in the downstream fully turbulent region: in the case of no-slip pipe flow, the difference is two orders of magnitude and is primarily due to an enhanced number density of the backflow events coupled with a notable increase in the wall footprint size of such events. For both the boundary layer and pipe flows considered here, and in both the transitional and fully turbulent regions, the incipient negative skin friction is found, through instantaneous flow visualization and conditional averaging, to be induced by the head element of a reverse hairpin vortex.
The mode B structure of streamwise vortices in the wake of a two-dimensional blunt trailing edge
- Bradley Gibeau, Sina Ghaemi
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- Journal:
- Journal of Fluid Mechanics / Volume 884 / 10 February 2020
- Published online by Cambridge University Press:
- 05 December 2019, A12
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The structure of streamwise vortices that arise due to secondary instabilities in the wake of a two-dimensional blunt body with a chord-to-thickness ratio of 12.5 was investigated using high-speed stereoscopic particle image velocimetry. Reynolds numbers spanning an order of magnitude from $Re(h)=2600$ to 25 800 were considered, where $h$ is the height of the blunt trailing edge. A modified two-dimensional $Q$-criterion ($Q^{\prime }=\unicode[STIX]{x1D714}_{x}Q/|\unicode[STIX]{x1D714}_{x}|$) was applied to identify the streamwise vortices. The wavelength of the streamwise vortices, defined as the spanwise distance between adjacent streamwise vortex pairs in the wake, was investigated by applying an autocorrelation algorithm to snapshots of $Q^{\prime }$. The most probable wavelength was found to range from $0.67h$ to $0.85h$ with increasing $Re$, and the mean wavelengths increased from $0.77h$ to $0.96h$. These wavelength values appeared to increase asymptotically. Visual inspection and cross-correlation analyses based on $Q^{\prime }$ showed that the streamwise vortices maintain their directions of rotation during primary shedding cycles. The latter analysis was carried out at low $Re$ because of a large amount of wake distortion and the absence of time-resolved data at high $Re$. The characteristics of the streamwise vortex structure found here match those of mode B, which, at similar $Re$, dominates the wakes of circular and square cylinders and has also recently been shown to exist in the wake of an elongated blunt body with a larger chord-to-thickness ratio of 46.5 (Gibeau et al., J. Fluid Mech., vol. 846, 2018, pp. 578–604).
Dynamics and wall collision of inertial particles in a solid–liquid turbulent channel flow
- Masoud Ebrahimian, R. Sean Sanders, Sina Ghaemi
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- Journal:
- Journal of Fluid Mechanics / Volume 881 / 25 December 2019
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- 25 October 2019, pp. 872-905
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The dynamics and wall collision of inertial particles were investigated in non-isotropic turbulence of a horizontal liquid channel flow. The inertial particles were $125~\unicode[STIX]{x03BC}\text{m}$ glass beads at a volumetric concentration of 0.03 %. The bead-laden flow and the unladen base case had the same volumetric flow rates, with a shear Reynolds number, $Re_{\unicode[STIX]{x1D70F}}$, of the unladen flow equal to 410 based on the half-channel height and friction velocity. Lagrangian measurements of three-dimensional trajectories of both fluid tracers and glass beads were obtained using time-resolved particle tracking velocimetry based on the shake-the-box algorithm of Schanz et al. (Exp. Fluids, vol. 57, no. 5, 2016, pp. 1–27). The analysis showed that on average the near-wall glass beads decelerate in the streamwise direction, while farther away from the wall, the streamwise acceleration of the glass beads became positive. The ejection motions provided a local maximum streamwise acceleration above the buffer layer by transporting glass beads to high velocity layers and exposing them to a high drag force in the streamwise direction. Conversely, the sweep motion made the maximum contribution to the average streamwise deceleration of glass beads in the near-wall region. The wall-normal acceleration of the beads was positive in the vicinity of the wall, and it became negative farther from the wall. The investigation showed that the glass beads with sweeping motion had the maximum momentum, streamwise deceleration, and wall-normal acceleration among all the beads close to the wall and these values increased with increasing their trajectory angle. The investigation of the beads that collided with the wall showed that those with shallow impact angles (less than $1.5^{\circ }$) typically slide along the wall. The sliding beads had a small streamwise momentum exchange of ${\sim}5\,\%$ during these events. The duration of their sliding motion could be as much as five times the inner time scale of the unladen flow. The wall-normal velocity of these beads after sliding was greater than their wall-normal velocity before sliding, and was associated with the rotation induced lift force. Beads with impact angles greater than $1.5^{\circ }$ had shorter interaction times with the wall and smaller streamwise and wall-normal restitution ratios.
Streamwise and spanwise slip over a superhydrophobic surface
- Wagih Abu Rowin, Sina Ghaemi
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- Journal:
- Journal of Fluid Mechanics / Volume 870 / 10 July 2019
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- 15 May 2019, pp. 1127-1157
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The near-wall turbulent flow over a superhydrophobic surface (SHS) with random texture was studied using three-dimensional Lagrangian particle tracking velocimetry (3D-PTV). The channel was operated at a constant mass flow rate over the SHS and a smooth surface at a Reynolds number of 7000 based on the bulk velocity of $0.93~\text{m}~\text{s}^{-1}$ and the full channel height. The friction Reynolds number was 217, based on the friction velocity and half channel height. The 3D-PTV processing was based on the shake-the-box algorithm applied to images of fluorescent tracers recorded using four high-speed cameras. The SHS was obtained by spray coating, resulting in a root-mean-square roughness of $0.29\unicode[STIX]{x1D706}$ and an average texture width of $5.0\unicode[STIX]{x1D706}$, where $\unicode[STIX]{x1D706}=17~\unicode[STIX]{x03BC}\text{m}$ is the inner flow scale over the SHS. The 3D-PTV measurements confirmed an isotropic slip with a streamwise slip length of $5.9\unicode[STIX]{x1D706}$ and a spanwise slip length of $5.9\unicode[STIX]{x1D706}$. As a result, both the near-wall mean streamwise and spanwise velocity profiles over the SHS were higher than the smooth surface. The streamwise and spanwise slip velocities over the SHS were $0.27~\text{m}~\text{s}^{-1}$ and $0.018~\text{m}~\text{s}^{-1}$, respectively. The near-wall Reynolds stresses over the SHS were larger and shifted towards the wall when normalized by the corresponding inner scaling, despite the smaller friction Reynolds number of 180 over the SHS. The near-wall measurement of streamwise velocity showed that the shear-free pattern consists of streamwise-elongated regions with a length of $800\unicode[STIX]{x1D706}$ and a spanwise width of $300\unicode[STIX]{x1D706}$. The plastron dimensions correspond to the mean distance of the largest roughness peaks $(20~\unicode[STIX]{x03BC}\text{m})$ obtained from profilometry of the SHS. The drag reduction over the SHS was 30 %–38 % as estimated from pressure measurement and the flow field using the 3D-PTV.
Experimental investigation of coherent structures of a three-dimensional separated turbulent boundary layer
- Mohammad Elyasi, Sina Ghaemi
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- Journal:
- Journal of Fluid Mechanics / Volume 859 / 25 January 2019
- Published online by Cambridge University Press:
- 15 November 2018, pp. 1-32
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Coherent structures of a three-dimensional (3D) separation due to an adverse pressure gradient are investigated experimentally. The flow set-up consists of a flat plate to develop a turbulent boundary layer upstream of an asymmetric two-dimensional diffuser with one diverging surface. The diffuser surface has an initial mild curvature followed by a flat section where flow separation occurs. The top and the two sidewalls of the diffuser are not equipped with any flow control mechanism to form a 3D separation. Planar particle image velocimetry (PIV) using four side-by-side cameras is applied to characterize the flow with high spatial resolution over a large streamwise-wall-normal field of view (FOV). Tomographic PIV (tomo-PIV) is also applied for volumetric measurement in a domain flush with the flat surface of the diffuser. The mean flow obtained from averaging instantaneous velocity fields of this intermittent unsteady flow appears as a vortex with an elliptical cross-section. The major axis of the ellipse is tilted with respect to the streamwise direction. As a result, the average velocity in the mid-span of the diffuser has an upstream forward flow and a downstream backward flow, separated by a point of zero wall shear stress. Sweep motions mainly carry out transport of turbulent kinetic energy upstream of this point, while ejections dominate at the downstream region. In the instantaneous flow fields, forward and backward flows have equivalent strength, and the separation front is extended in the spanwise direction. The conditional average of the separation instants forms a saddle-point structure with streamlines converging in the spanwise direction. Proper orthogonal decomposition (POD) of the tomo-PIV data demonstrates that about 42 % of the turbulent kinetic energy is present in the first pair of modes, with a strong spanwise component. The spatial modes of POD also show focus, node and saddle-point structures. The average of the coefficients of the dominant POD modes during the separation events is used to develop a reduced-order model (ROM). Based on the ROM, the instantaneous 3D separation over the diffuser is a saddle-point structure interacting with focus-type structures.
Counter-hairpin vortices in the turbulent wake of a sharp trailing edge
- Sina Ghaemi, Fulvio Scarano
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- Journal:
- Journal of Fluid Mechanics / Volume 689 / 25 December 2011
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- 28 November 2011, pp. 317-356
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The unsteady organization and evolution of coherent structures within the turbulent boundary layer and subsequent wake of the sharp symmetric trailing edge of a NACA0012 aerofoil are investigated. The experiments are conducted in an open test-section wind tunnel at based on the aerofoil chord and based on the boundary layer momentum thickness. An initial characterization of the flow field using two-component particle image velocimetry (PIV) is followed by the investigation of the unsteady organization and evolution of coherent structures by time-resolved three-dimensional PIV based on a tomographic approach (Tomo-PIV). The inspection of the turbulent boundary layer prior to the trailing edge in the region between 0.15 and demonstrated streaks of low- and high-speed flow, while the low-speed streaks are observed to be more coherent along with strong interaction with hairpin-type vortical structures similar to a turbulent boundary layer at zero pressure gradient. The wake region demonstrated gradual deterioration of both the low- and the high-speed streaks with downstream progress. However, the low-speed streaks are observed to lose their coherence at a faster rate relative to the high-speed streaks as the turbulent flow develops towards the far wake. The weakening of the low-speed streaks is due to the disappearance of the viscous sublayer after the trailing edge and gradual mixing through the transport of the remaining low-speed flow towards the free stream. This transport of low-speed flow is performed by the ejection events induced by the hairpin vortices as they also persist into the developing wake. The higher persistence of the high-speed streaks is associated with counter-hairpin vortical activities as they oppose the deterioration of the high-speed streaks by frequently sweeping the high-speed flow towards the wake centreline. These vortical structures are regarded as counter-hairpin vortices as they exhibit opposite characteristics relative to the hairpin vortices of a turbulent boundary layer. They are topologically similar to the hairpins as they appear to be U-shaped but with inverted orientation, as the spanwise portion is in the vicinity of the wake centreline and the legs are inclined at an approximately to the wake axis in the downstream direction demonstrating a strain-dominated topology. The counter-hairpin vortices are partially wrapped around the high-speed streaks and contribute to the wake development by transporting high-speed flow towards the wake centreline. Similar to the hairpin vortices of a turbulent boundary layer, the occurrence of a complete counter-hairpin vortex is occasional while its derivatives (portions of spanwise or quasi-streamwise vortices) are more frequently observed. Therefore, a pattern recognition algorithm is applied to establish characterization based on an ensemble-averaged counter-hairpin vortex. The formation of the counter-hairpin vortices is due to an additional degree of interaction between the low- and high-speed streaks after the trailing edge across the wake centreline. The shear layer produced along the wake centreline by neighbouring low- and high-speed streaks promotes the formation of spanwise vortices that form the counter-hairpin vortices by connection to quasi-streamwise vortices. Finally, a conceptual model is proposed to depict the three-dimensional unsteady organization and evolution of coherent structures in the wake region based on the hairpin and counter-hairpin vortex signatures.