Near-wall Flow Structures Research Articles

Correlation between skin friction and enstrophy convection velocity in near-wall turbulence

The enstrophy convection velocity (denoted by uΩ) in the evolution equation of the boundary enstrophy is proportional to skin friction (denoted by τ) in an incompressible viscous flow, which can be determined globally as an optical flow problem from a time sequence of the boundary enstrophy fields. The correlation coefficient between |uΩ| and |τ| is evaluated in a turbulent channel flow at the friction Reynolds number Reτ=180, which is about 0.73 in the region of interest containing near-wall strong wall-normal velocity event associated with a sweep-ejection pair) and is approximately independent of the wall-normal coordinate in the viscous sublayer. This correlation coefficient is contributed mainly by strong near-wall coherent structures with high boundary enstrophy. The statistics of the difference between the normalized |uΩ| and |τ| are discussed. The statistical correlation between uΩ and τ elucidates the connection between near-wall flow structures and skin friction.

Direct numerical simulations of the drag degradation mechanism in channel flow over trapezoidal riblets

Direct numerical simulations of turbulent channel flows with trapezoidal riblets were conducted to analyze the characteristics of the near-wall flow structures in the drag reduction, drag neutral, and drag increase regimes. The trapezoidal riblets possess a tip angle of α =15°and a height–to-width ratio of k/s = 0.335 for cross-section-area based sizes lg+=Ag+= 11.7, 17, 23.2, and 29 at a friction Reynolds number of Reτ=395. The Reynolds stresses are compared with the smooth wall case, and it is observed that the intensity of the turbulent activities in the boundary layer is diminished for drag reduction riblet and enhanced for drag increase riblets. The roughness sublayer indicates that riblet only directly affect the near-wall region of the flow. By applying the Fukagata, Iwamoto, and Kasagi (FIK) identity in Phys. Fluids, vol. 14, 2002, L73, it is found that the modification of the overall drag is mainly due to the turbulent shear stress. The variation of the dispersive stress is only evident within the roughness sublayer for the drag increasing riblets. The enhanced mean wall-shear stress in the valley is responsible for the drag increase for oversized riblets. The production and convection terms of the turbulent stress reveal that although the total drag contribution is trivial, the convection inside the riblet valleys caused by secondary flow is crucial for the redistribution of the turbulent stress and drag increase of oversized riblets.

Turbulent superstructure statistics in a turbulent boundary layer with pressure gradients

In turbulent boundary layers, streamwise elongated regions of high- and low-momentum in the log-law layer that can extent up to several boundary layer thicknesses are often referred to as turbulent superstructures. These structures contain a relatively large portion of the layer’s turbulent kinetic energy and have been shown to interact with the near-wall flow structures. In the last few decades extensive research on zero-pressure gradient (ZPG) turbulent boundary layers has been done, however by comparison, the structural characteristics for adverse pressure gradient turbulent (APG) boundary layer flows are much less studied despite their strong significance aero-hydrodynamic vehicle design. Therefore, the three-dimensional dynamics of turbulent superstructures in a turbulent boundary layer flow are investigated in the Atmospheric Wind Tunnel Munich (AWM) using a multi-camera 3D time-resolved Lagrangian particle tracking approach. In this study, Lagrangian and Eulerian statistics will be used to characterize the dynamics and interaction of turbulent superstructures within a zero pressure gradient (ZPG) turbulent boundary layer at Reτ = 5000 or Reθ = 14 000 that then flows over a curved plate subjected to a favorable (FPG) and strong adverse (APG) pressure gradient, which eventually separates. An Eulerian analysis, using multi-point correlations of 3D velocity fields, found that the average superstructure topology is modulated by decelerating flow in the APG region when compared to the ZPG region, however the basic shape and spanwise pattern is preserved. Looking into the behavior of individual trajectories, it was found that the dispersion of single particles along trajectories in the log-law layer are capable of moving more than the average Eulerian superstructure spacing in the spanwise direction. Furthermore, the mean square of the single particle dispersion indicates that the maximum dispersion in the spanwise direction comes from particles released at the wall-normal location corresponding to the so-called “second-peak/plateau” region in the streamwise normal Reynolds stress.

Heat transfer investigation on under-expanded supersonic impinging jets

In this study, wall-resolved large-eddy simulations are performed to investigate the underpinning physics of impingement heat transfer in under-expanded supersonic impinging jets. The jet is generated with an infinite-lipped nozzle of diameter D, has a nozzle-to-wall distance of 5D, a nozzle pressure ratio (NPR) of 3.4 and a Reynolds number of 6×104 based on the ideally-expanded jet velocity. Two thermal boundary conditions of a heated isothermal wall and an adiabatic wall for the impingement plate are considered.Both impinging jets in the current study feature a strong stand-off shock, which oscillates periodically in front of the impingement plate at two dominant frequencies. These oscillations result in two local peaks of the mean heat transfer coefficients with the first peak located in the stagnation region but shifted away from the stagnation point (i.e. jet axis), whilst the second peak is located in the wall jet region. The instantaneous fields of pressure, vorticity and Nusselt number are first discussed to study qualitatively the connection between these two peaks of the mean heat transfer coefficient and the near-wall flow structures. The characteristics of these two peaks are then analysed using the near-wall statistics and spectra of the pressure, axial velocity and Nusselt number distribution.It is found that the first peak of the Nusselt number forms as a consequence of the jet impingement of the non-periodic turbulent structures, whilst the second peak is linked to the reattachment of the recirculation bubble in the wall jet. It is shown that the unsteady separation and reattachment processes are strongly related to the oscillatory motion of the stand-off shock at the dominant frequencies. Last but not the least, the first local minimum close to the stagnation point is also found to be associated with the strong oscillatory behaviour of the stand-off shock.

Drag increase and turbulence augmentation in two-way coupled particle-laden wall-bounded flows

The exact regularized point particle method is used to characterize the turbulence modulation in two-way momentum-coupled direct numerical simulations of a turbulent pipe flow. The turbulence modification is parametrized by the particle Stokes number, the mass loading, and the particle-to-fluid density ratio. The data show that in the wide region of the parameter space addressed in the present paper, the overall friction drag is either increased or unaltered by the particles with respect to the uncoupled case. In the cases where the wall friction is enhanced, the fluid velocity fluctuations show a substantial modification in the viscous sub-layer and in the buffer layer. These effects are associated with a modified turbulent momentum flux toward the wall. The particles suppress the turbulent fluctuations in the buffer region and concurrently provide extra stress in the viscous sub-layer. The sum of the turbulent stress and the extra stress is larger than the Newtonian turbulent stress, thus explaining the drag increase. The non-trivial turbulence/particles interaction turns out in a clear alteration of the near-wall flow structures. The streamwise velocity streaks lose their spatial coherence when two-way coupling effects are predominant. This is associated with a shift of the streamwise vortices toward the center of the pipe and with the concurrent presence of small-scale and relatively more intense vortical structures near the wall.

Lamb dilatation and its hydrodynamic viscous flux in near-wall incompressible flows

In this paper, we introduce a new physical concept referred to as the wall-normal Lamb dilatation flux (WNLDF), which is defined as the wall-normal derivative of the Lamb dilatation (namely, the divergence of the Lamb vector) multiplied by the dynamic viscosity for incompressible viscous flows. It is proved that the boundary Lamb dilatation flux (BLDF, namely the wall-normal Lamb dilatation flux at the wall) is determined by the boundary enstrophy flux (BEF) and the surface curvature-induced contribution. As the first step to explore this new concept, the present study only considers flow past a stationary no-slip flat wall without curvature-fluid dynamic coupling effects. It is found that the temporal–spatial evolution rate of the WNLDF is contributed by four source terms, which can be explicitly expressed by using the fundamental surface quantities including skin friction (or surface vorticity) and surface pressure. Therefore, the instantaneous near-wall flow structures are directly related to the WNLDF in both laminar and turbulent flows. As an example, for the turbulent channel flow at Reτ=180, the intensification of the temporal–spatial evolution rate of the WNLDF is caused by the strong wall-normal velocity event (SWNVE) associated with quasi-streamwise vortices and high intermittency of the viscous sublayer. In addition, near the SWNVE, this evolution rate is mainly contributed by the dot product of skin friction and surface enstrophy gradient, as well as the coupling between the skin friction divergence and the boundary enstrophy. The exact results presented in this paper could provide new physical insights into complex near-wall flows.

Turbulence suppression by streamwise-varying wall rotation in pipe flow

Direct numerical simulations of turbulent pipe flow subjected to streamwise-varying wall rotation are performed. This control method is able to achieve drag reduction and even relaminarize the flow under certain control parameters at friction Reynolds number $Re_\tau =180$ . Two control parameters, which are velocity amplitude and wavelength, are considered. It is found that increasing the wavelength rather than increasing the amplitude seems to be a better choice to improve the control efficiency. An annular boundary layer, called the spatial Stokes layer (SSL), is formed by the wall rotation. Based on the thickness of the SSL, two types of drag-reduction scenarios can be identified roughly. When the thickness is low, the SSL acts as a spacer layer, inhibiting the formation of streamwise vortices and thereby reducing the shear stress. The flow structures outside the SSL are stretched in the streamwise direction due to the increased velocity gradient. Within the SSL, the turbulence intensity diminishes dramatically. When the thickness is large, a streamwise wavy pattern of near-wall streaks is formed. The streak orientation is dominated by the mean shear-strain vector outside the viscous sublayer, and there is a phase difference between the streak orientation and local mean velocity vector. The streamwise scales of near-wall flow structures are reduced significantly, resulting in the disruption of downstream development of flow structures and hence leading to the drag reduction. Furthermore, it is found that it requires both large enough thickness of the SSL and velocity amplitude to relaminarize the turbulence. The relaminarization mechanism is that the annular SSL can absorb energy continuously from wall-normal stress due to the rotational effect, thereby the turbulence self-sustaining process cannot be maintained. For the relaminarization cases, the laminar state is stable to even extremely large perturbations, which possibly makes the laminar state the only fixed point for the whole system.

Numerical study of wall shear stress fluctuations and near-wall structures in a square duct at low Reynolds numbers

Direct numerical simulations (DNS) of turbulent flow through a square duct at two different low Reynolds numbers is performed using OpenFOAM to study the instantaneous wall shear stress fluctuations and near-wall structures. We first benchmarked our simulation results against other DNS studies which used higher-order spectral methods, thereby confirming the accuracy of our results. The mean profile of streamwise wall shear stress is well studied in the literature, however its higher order statistics has not been yet explored. There is also a lack of information on the spanwise component of wall shear stress and its higher-order statistics. Here, we provide an extensive study on streamwise and spanwise wall shear stresses, and their higher-order statistics. The intensities of both components of wall shear stress fluctuations are close to zero near the corner, and monotonically increase while moving towards the duct center. This is in contrast with their mean profiles which do not exhibit similar monotonic trends. However, the third-order moment, i.e. skewness of the streamwise wall shear stress again shows a non-monotonic behavior from the corner to the center. The profiles of the mean as well as the higher-order statistics of wall shear stress follow a better scaling between the two Reynolds number studied, when the spanwise location is normalized using inner scaling. Streamwise velocity correlations near the wall show that the flow structures in a square duct are similar to that of a channel flow near the center, whereas there is a cross-talk of the flow structures between the neighboring walls near the corner. An investigation into the effect of corner on the probability of back flow events is made at a Reynolds number lower than previously reported. The probability follows a non-monotonic behavior as we move from the corner towards the center, where it reaches a minimum at around 50 wall units from the corner. Similar to the wall shear stress profiles, the behavior of the near-wall flow structures as well as the spanwise variation of the probability of backflow events also show a qualitatively similar trend for the two Reynolds numbers, when the spanwise location is normalized using inner scaling.

Near-wall vortices and thermal simulation of coupled-domain transpiration cooling by a recursive regularized lattice Boltzmann method

The present study aims to reveal the near-wall vortices and the effects of flow states in coupled-domain transpiration cooling using a recursive regularized thermal lattice Boltzmann method (RR-TLBM). Large-eddy simulations of turbulent flow and heat transfer have been conducted on high-resolution computational grids using a desktop-level computer with CUDA 11.6. Results indicate that the near-wall flow structures present spatial characteristics along the streamwise direction. The vortex evolution promotes the downstream heat dissipation, even though turbulence impairs the effective cooling area. The spanwise evolvement of vortices strengthens the mixing of coolant and hot gas, and small-scale structures are beneficial for turbulent heat transfer. Moreover, the transition onset occurs earlier at higher Reynolds numbers, and it weakens the downstream cooling. The cooling performance of the derived coolant film is improved as the Reynolds number varies from 5 × 103 to 3 × 104 with a blowing ratio of F = 10%, whereas the local cooling is impaired at the high Reynolds numbers exceeding 5 × 104. The variation in flow states has little influence on the cooling performance at the Reynolds numbers larger than 3 × 106. On the other hand, our in-house RR-TLBM solver is highly stable and efficient for the simulation of flow and heat transfer with high Reynolds numbers. Simultaneously, a high computational performance of 1127 million lattices updated per second is achieved for our simulation of a coupled-domain turbulent flow and heat transfer, using the desktop-level computer with three Tesla V100 graphics processing units.

Mechanism of hysteresis and uncontrolled deflection in jet vectoring control based on Coanda effect

We investigated the mechanism of hysteresis and uncontrolled deflection in jet vectoring control based on the Coanda effect. Thrust vectoring control based on the Coanda effect is highly applicable in the field of fluid dynamics because they can achieve jet deflection control with a simple geometric structure and low energy consumption. However, the hysteresis and uncontrolled deflection phenomena considerably hinder the practical application of these technologies. We designed a new passive fluidic thrust vectoring control model to analyze the mechanism of these disadvantages. We developed a synchronous pressure, force, and flow field measurement system to investigate the evolutions of flow structures and their effects on wall pressure distribution and vectoring force. The characteristics of the key flow structures, including the separation bubble, the secondary flow, and the backflow, were investigated. The transient features of jet attachment and jet detachment were researched to identify the transition of near-wall flow structures and wall pressure distribution during jet uncontrolled deflection. The mechanism of uncontrolled deflection is that the formation/breakdown of the separation bubble changes the flow state of the backflow, and the near-wall mass flux balance is collapsed. This causes a dramatic change in the jet vectoring angle, which is out of the valve's control. During attachment, the shear layer first touches the wall and forms a separation bubble, and then, the bubble shrinks. During detachment, the bubble first enlarges and then breaks down. Therefore, the formation and breakdown of the separation bubble correspond to different valve positions, which is the mechanism of hysteresis.

The passive control on flow and heat transfer with streamwise micro grooves

The flow behavior of smooth channel and channel with streamwise micro grooves are numerically compared at Reynolds number 3000–6000. The numerical prediction results show that the streamwise micro grooves can effectively promote the heat transport in boundary layer. In special cases, the streamwise micro grooves can suppress the ejection and sweep motions of turbulent coherent structure to a certain extent, which is conducive to the improvement of the stability of near-wall flow structure and the reduction in wall shear stress. In addition, 2.16% of drag reduction can be obtained. Furthermore, the streamwise micro grooves increase the number of large-scale vortices and destroy the thermal boundary layer. With the help of the streamwise micro grooves, Nusselt number increases by 8.2% ~ 10.7% in the range of considered Reynolds number.

DNS of compressible turbulent flow in convergent-divergent nozzles with isothermal wall

Compressible turbulent flow through canonical convergent-divergent nozzles with circular cross-section, isothermal walls, with and without bell-shaped divergent portion is investigated by means of Direct numerical simulations. The goal of the present study is to demonstrate the effects of flow expansion (and local flow compression) in these nozzles, which have substantial transonic regions, on the mean flow and the turbulence structure. The efficacy of the kinematic or incompressible displacement thickness as a possible lengthscale for computing nondimensional pressure gradient parameters which are indicators of self-similar regions in these flows is shown. Elongation of near-wall flow structures and the dominance of vortex stretching as local flow topology are found in regions of expansion. This is accompanied by a decrease in Reynolds stresses. Regions of local flow compression are found in the divergent portion of the bell-shaped nozzle where vortex compression is the dominant flow topology. A local increase in the streamwise Reynolds stress is found in this region.

Experimental Study of Boundary Layer flow over a Zero Pressure Gradient Convergent Riblet Plate

Reducing the friction force in turbulence flows is an important research topic in fluid mechanics. Shark skin has less drag than smooth skins, therefore shark skin surfaces have been widely used. However, some turbulence mechanisms are not completely understood and the drag reduction mechanism of real shark skin has not been thoroughly understood. Many researchers have experimentally studied bio-inspired riblet surfaces to explain some specified phenomena for the sharkskin drag reduction effect. The objectives of this study were to investigate the effect of the converging-diverging riblet-type surface roughness on a zero pressure gradient turbulent boundary layers, to investigate the effect of the surface roughness on the large-scale features and to confirm that converging-diverging riblet generate large-scale counter-rotating vortices. The wind tunnel experiments involved laminar and turbulent flow at various Re were performed. Hot-wire anemometry was used to investigate near-wall flow structure over smooth and riblet surfaces. The results showed that the convergent riblet plate significantly impacted the boundary layer properties. Furthermore, the turbulent skin friction drag over the convergent riblet was higher than on a smooth surface. The results agree well with previous studies.

On the Underlying Drag-Reduction Mechanisms of Flow-Control Strategies in a Transitional Channel Flow: Temporal Approach

The underlying mechanisms of three different flow-control strategies on drag reduction in a channel flow are investigated by direct numerical simulations at friction Reynolds numbers ranging from 65 to 85. These strategies include the addition of long-chain polymers, the incorporation of slip surfaces, and the application of an external body force. While it has been believed that such methods lead to a skin-friction reduction by controlling near-wall flow structures, the underlying mechanisms at play are still not as clear. In this study, a temporal analysis is employed to elucidate underlying drag-reduction mechanisms among these methods. The analysis is based on the lifetime of intermittent phases represented by the active and hibernating phases of a minimal turbulent channel flow (Xi and Graham, Phys Rev Lett 2010). At a similar amount of drag reduction, the polymer and slip methods show a similar mechanism, while the body force method is different. The polymers and slip surfaces cause hibernating phases to happen more frequently, while the duration of active phases is decreased. However, the body forces cause hibernating phases to happen less frequently but prolong its duration to achieve a comparable amount of drag reduction. A possible mechanism behind the body force method is associated with its unique roller-like vortical structures formed near the wall. These structures appear to prevent interactions between inner and outer regions by which hibernating phases are prolonged. It should motivate adaptive flow-control strategies to exploit the distinct underlying mechanisms for robust control of turbulent drag at low Reynolds numbers.

Numerical investigation of the wake bi-stability behind a notchback Ahmed body

Large-eddy simulations are used to investigate the origin of the wake asymmetry and symmetry behind notchback Ahmed bodies. Two different effective backlight angles, ${\beta _1} = 17.8\mathrm{^\circ }$ and ${\beta _2} = 21.0\mathrm{^\circ }$ , are simulated resulting in wake asymmetry and symmetry in flows without external perturbations, in agreement with previous experimental observations. In particular, the asymmetric case presents a bi-stable nature showing, in a random fashion, two stable mirrored states characterized by a left or right asymmetry for long periods. A random switch and several attempts to switch between the bi-stability are observed. The asymmetry of the flow is ascribed to the asymmetric separations and reattachments in the wake. The deflection of the near-wall flow structures behind the slant counteracting the asymmetry drives the wake to be temporarily symmetric, triggering the switching process of the bi-stable wake. The consequence of deflection that forces the flow structure to form on the opposite side of the slant is the decisive factor for a successful switch. Modal analysis applying proper orthogonal decomposition is used for the exploration of the wake dynamics of the bi-stable nature observed.

Flow structure and heat transfer of transpiration cooling by using a LBM: The effects of wall blowing and spatially nonuniform injection

Considering the effects of wall bowing and spatially nonuniform injection, the near-wall flow structures and heat transfer characteristics in transpiration cooling are studied through numerical simulations by an in-house thermal lattice Boltzmann solver accelerated by GPGPUs. Results reveal that the development of coolant layer is influenced by local velocity gradients, and a wall blowing ratio of larger than 2%–3% induces the flow separation and unstable coverage of low-temperature layer, and that the coolant injection weakens the surface friction coefficient, while the flow separation facilitates a local increase of the skin friction coefficient downstream the wall. For nonuniform injection, the local increases of mass and momentum with the peak velocity of nonuniform injection is beneficial to the coolant-layer development but the spatial temperature control is unstable, indicating the stable development and coverage of coolant layer are the key factors of the flow and heat transfer in transpiration cooling process.

Near-wall flow structures and related surface quantities in wall-bounded turbulence

By applying the Taylor-series expansion solution of the Navier–Stokes equations, an analysis is given to elucidate the relationships between near-wall flow structures and the fundamental surface quantities (skin friction and surface pressure). The derived results are used to understand the physical features of near-wall flow structures around a typical strong wall-normal velocity event (SWNVE) in a turbulent channel flow based on the direct numerical simulation data at Reτ=180. The simulation is carefully done using a multiple-relaxation-time lattice Boltzmann method combined with an improved on-wall bounce-back implementation. It is found that both the skin friction divergence and the Laplacian of surface pressure have good correspondence with sweep and ejection motions induced by the quasi-streamwise vortex above the viscous sublayer. Interestingly, the surface pressure variation induced by a quasi-streamwise vortex tends to attenuate the wall-normal velocity magnitude in both the sweep and ejection sides through the Laplacian of surface pressure. Similar physical effects of surface-pressure-related terms are also observed for the near-wall Reynolds stress. The concentrated enstrophy and dissipation are associated with the SWNVE and high skin friction magnitude. It is found that the SWNVE is dynamically important in generating the boundary enstrophy flux, greatly enhancing the intermittency of turbulence inside the viscous sublayer. In addition, by applying the methods of differential geometry, the near-wall Taylor-series expansions are generalized for a stationary curved surface in a general curvilinear coordinate system. The generalized results could be useful in evaluating the curvature effect in the near-wall region for complex flows.

Influence of flow patterns on the performance of straight-through labyrinth seals. Numerical study and validation against experimental data

When numerically simulating the performance of a complete Francis turbine, it is common practice to omit the seals to save computing power. Volumetric and friction losses are usually taken into account using empirical values. There is abundant literature on the topic, but all analytic formulations rely on widely variable empirical coefficients. To overcome the uncertainty in determining volumetric losses, a detailed analysis of the sealing is necessary. Focusing on straight seals and configurations with cavities, in-house experimental data and numerical simulations have been used to show that few geometric parameters play a fundamental role in defining the flow patterns inside the clearance and even more so inside and between cavities. As is to be expected, the seals effectiveness is heavily influenced by these flow structures. It is also shown that certain methods of dealing with near-wall flow structures are better suited than others for the estimation of the discharge behaviour when compared to experimental results. Even though in the framework of the RANS equations CFD has proven to be a reliable tool for a wide range of geometries, further investigations are necessary on deep cavities. For aspect ratios above 0.75, the flow patterns inside the cavity change drastically, leading to bad convergence rates of the solver and, more important, to a considerable misalignment between numerical and experimental results. Lid-driven cavity flow is known to be a complex problem in 2D domain, and in this case, the flow becomes three-dimensional due to the tangential velocity induced by rotation. However, the reasons behind the problems in CFD analysis are not entirely clear. Given the observed variations of the flow structures inside sealings with different geometric parameters, it becomes clear why a simplified, generic formulation is very difficult to obtain, if not by adopting empirically defined coefficients which are needed to tune the analytical model.

Adaptive-Fidelity Design Automation Framework to Explore Bioinspired Surface Riblets for Drag Reduction

Bioinspired surface riblets have been known to improve drag performance by altering the near-wall flow structures, especially in the transitional flow regime. Unlike conventional riblet geometries (for example, sawtooth and scalloped shapes) overlaid on flat plates, the use of a new Gaussian-shaped abstraction overlaid on three-dimensional (3-D) airfoil surfaces is conceived in this paper. Given the prohibitive computational burden of simulating the surface flow at high fidelity around the submillimeter-scale riblets, an adaptive-fidelity design approach (namely, surrogate-based optimization with adaptive in situ model refinement) is adopted to enable tractable exploration of the hypothesized benefits of the new riblet concept. The optimization process is integrated with open-source computer-aided design modeling, computational fluid dynamics, and postprocessing tools, with the provision for distributed batch processing, leading to a fully automated bioinspired design framework with potential for wider adoption. Substantial drag reduction of up to 17.2%, compared to the bare 3-D airfoil (without riblets), is achieved. The drag coefficient of ribleted airfoils is found to be mostly distributed around that of the bare airfoil, with the optimum being a favorable extreme point, thereby demonstrating the important role that adaptive-fidelity optimization plays in quickly identifying the (narrow) beneficial range of abstracted shape parameters. Further insights are derived into the flow physics driving the aerodynamic efficiency benefits of smooth riblets, with riblets of height-to-spacing ratios of 0.5–0.8 identified as particularly promising in the case studies.

A study of inner-outer interactions in turbulent channel flows by interactive POD

The amplitude and frequency modulation of near-wall flow structures by the large-scale motions in outer regions is studied in turbulent channel flows. The proper orthogonal decomposition (POD) method is applied to investigate the interactions between the near-wall motions and the large-scale flow modes of the outer regions based on two datasets from direct numerical simulation of turbulent channel flows at Reynolds numbers of 550–1000. The fluctuations in the fields u+, v+, w+ and Reynolds shear stress −(uv)+ are studied to understand the mechanism of amplitude and frequency modulation of the near-wall structures by the outer large-scale motions. The amplitude modulation coefficient of the Reynolds shear stress is larger than that of the velocity components. The frequency modulation effect has an opposite influence in the spanwise direction compared to the streamwise direction. The streamwise characteristic frequency increases with increasing large-scale velocity. However, the spanwise characteristic frequency exhibits a decreasing trend with increasing large-scale velocity in the near-wall region.