Free Shear Layer Research Articles

Modeling of air and particles flow with revolving rotor hovering over the particles layer

The rotation effect by a revolving two-bladed rotor on hydrodynamics of gas and particles over the particles layer in an open chamber has been numerically studied using a three-dimensional transient Euler-Euler two-fluid model (TFM). The collisional interactions of particles are modeled by the kinetic theory of granular flow (KTGF). The results highlight that the entrained particles are generated by a revolving rotor plume and the local free shear layer from the surface of the particle layer. The major cylindrical domain and the perimeter region exist with the change of hovering height of rotor. The air plume changes from the vertical flow in the cylindrical domain to the horizontal flow in the perimeter region along the surface of the particles layer. The entrained particles mass is increased with the decrease of hovering height. CFD predictions are also validated by experimental results of indoor air movement distribution measured in an office.

A collaborative effort towards the accurate prediction of turbulent flow and heat transfer in low-Prandtl number fluids

This article reports the experimental and DNS database that has been generated, within the framework of the EU SESAME and MYRTE projects, for various low-Prandtl flow configurations in different flow regimes. This includes three experiments: confined and unconfined backward facing steps with low-Prandtl fluids, and a forced convection planar jet case with two different Prandtl fluids. In terms of numerical data, seven different flow configurations are considered: a wall-bounded mixed convection flow at low-Prandtl number with varying Richardson number (Ri) values; a wall-bounded mixed and forced convection flow in a bare rod bundle configuration; a forced convection confined backward facing step (BFS) with conjugate heat transfer; a forced convection impinging jet for three different Prandtl fluids corresponding to two different Reynolds numbers of the fully developed planar turbulent jet; a mixed-convection cold-hot–cold triple jet configuration corresponding to Ri = 0.25; an unconfined free shear layer for three different Prandtl fluids; and a forced convection infinite wire-wrapped fuel assembly. This wide range of reference data is used to evaluate, validate and/or further develop different turbulent heat flux modelling approaches, namely simple gradient diffusion hypothesis (SGDH) based on constant and variable turbulent Prandtl number; explicit and implicit algebraic heat flux models; and a second order turbulent heat flux model. Lastly, this article will highlight the current challenges and perspectives of the available turbulence models, in different codes, for the accurate prediction of flow and heat transfer in low-Prandtl fluids.

On the statistical evolution of viscous vortex-gas free shear layers

The temporal vortex-gas free shear layer is a Hamiltonian system of N-point vortices in a singly-periodic domain relevant to the (large-scale) development of plane (3D Navier–Stokes) turbulent shear layers. It has been shown (Suryanarayanan et al., 2014) that there are three distinct temporal regimes in its evolution, including an intermediate self-similar regime (RII) with a universal constant spreading rate, and a final state of relative equilibrium. Here, we study the evolution of vortex-gas free shear layers affected by viscosity as simulated by the addition of a random walk component to the motion of the vortices (Chorin, 1973). We show that while the momentum thickness of the layer, θ, scales as t1∕2 in the first and final regimes for the viscous system, it grows as t1 in the intermediate Regime II (observed here for momentum thickness Reynolds numbers Reθ from 50 to over 5000 across different simulations), with the same universal growth rate dθ∕dtΔU=0.0166±0.0002 as in the inviscid case. This suggests that the non-equilibrium universality of Regime II is robust to small perturbations of the Hamiltonian, and that this result could have general consequences for systems with long-range interactions. We further find that, even under conditions where the viscous shear stress is not negligible, the Reynolds shear stress adjusts itself in such a way that their sum is a constant in Regime II and equal to the value of the latter in the inviscid case. The implications of this observation on the dynamics of turbulent shear flows are elaborated.

Assessment of the SST-IDDES with a shear-layer-adapted subgrid length scale for attached and separated flows

A shear-layer-adapted subgrid length scale is applied to the improved delayed detached eddy simulation using the shear stress transport background model (SST-IDDES). The aim is to assess the combination of the wall-modeled LES (WMLES) branch of the SST-IDDES with the new length scale in computing attached flows, as well as to assess the effect of the new length scale when it is applied to the SST-IDDES for mitigating the “grey area” issue through initiating a dramatic drop of eddy viscosity in the initial region of a free shear layer. The assessment is conducted through simulations of a turbulent boundary layer, a fully developed channel flow, a near-sonic turbulent jet and a backward-facing step flow. The results provide strong evidence for the conclusion that the SST-IDDES combined with the new length scale performs the same as the original SST-IDDES when its WMLES branch is applied to compute the resolved parts of an attached flow, and the combination helps mitigate the “grey area” issue of the SST-IDDES and accurately represent the K-H instability in the initial region of a free shear layer. In addition, the superiority is particularly remarkable for the simulations with coarse grids.

On the turbulence amplification in shock-wave/turbulent boundary layer interaction

The mechanism of turbulence amplification in shock-wave/boundary layer interactions is reviewed, and a new turbulence amplification mechanism is proposed based on the analysis of data from direct numerical simulation of an oblique shock-wave/flat-plate boundary layer interaction at Mach 2.25. In the upstream part of the interaction zone, the amplification of turbulence is not essentially shear driven, but induced by the interaction of the deceleration of mean flow with streamwise velocity fluctuations, which causes a rapid increase of turbulence intensity in the near-wall region. In the downstream part of the interaction zone, the high turbulence intensity is mainly due to the free shear layer generated in the interaction zone. During the initial stage of turbulence amplification, the characteristics of wall turbulence, including compact velocity streaks, streamwise vortices and an anisotropic Reynolds stress, are well preserved. The mechanism proposed explains the high level of turbulence in the near-wall region observed in some experiments and numerical simulations.

Numerical study of passive forcing on the secondary instability of a laminar planar free shear layer

Direct numerical simulations were performed of a free shear layer developing downstream from a no-slip splitter plate. The simulation results show that three-dimensional streaky perturbations developing in the boundary layer on the high-speed side of the splitter plate are convected downstream from the trailing edge and influence the development of three-dimensional perturbations in the planar free shear layer. Simulations are performed in which a spanwise-wavy shape is introduced to the trailing edge of the splitter plate to influence the downstream development of the free shear layer. Introduction of the spanwise-waviness to the trailing edge results in significantly earlier small-scale breakdown of the free shear layer.

Numerical Investigation of the Unsteady Flow Past an Iced Multi-Element Airfoil

Unsteady flow characteristics are investigated using wall-modeled large-eddy simulation for a 30P30N three-element airfoil with streamwise ice and horn ice accreted at its slat leading edge, respectively. The ice accretions cause massive flow separation above the slat, which results in a surface pressure plateau on the slat and further unloads the downstream wing elements. The above mentioned two ice shapes lead to lift decreases of 10.66 and 16.45% and drag increases of 44.58 and 59.42%, respectively. Ice-induced vortices first roll up due to Kelvin–Helmholtz instability above the slat with ice accretions, and the subsequent vortex pairing halves their characteristic motion frequency. The developed vortices, composed of multiscale structures, dominate the slat upper surface and are convected through the main element to aggravate the flow separation over the flap. Moreover, extremely strong acoustic resonance occurs in the iced slat cove, and its tonal frequencies are consistent with theoretical model predictions. These intense tones are believed to result from the elevated pressure fluctuations along the cavity free shear layer, which are in turn related to the propagation of the ice-induced pressure fluctuations from above the slat because their most dominant frequencies identified by proper orthogonal decomposition are nearly equal to the resonant frequencies.

Experimental study on suppressing vortex-induced vibration of a long-span bridge by installing the wavy railings

It is known that vortex shedding behind bluff bodies can be suppressed by introducing some three-dimensional geometric disturbance. Therefore, a new passive control device named wavy railings is proposed to mitigate the vortex-induced-vibration (VIV) of a single box section bridge. The wavy railing is composed of vertical posts and wavy crossbars, which can provide three-dimensional perturbation to the flow field and then lead to the development of a three-dimensional free shear layer which does not roll up into a vortex street and hence suppress the VIV. In this study, the control effects of the wavy railings on mitigating VIV are investigated through wind tunnel tests. The results reveal that when the wavelength of the wavy bar is 0.5H or 1H (H is the height of the bridge deck), both vertical and torsional VIV can be completely suppressed, and the upper surface edge is the optimal installation position.

Comparative Study of different vortex identification methods in a tip-leakage cavitating flow

The tip leakage vortex cavitating flow generated by a straight hydrofoil is computed using Large Eddy Simulation combined with Zwart-Gerber-Belamri cavitation model. Comparisons between the numerical and experimental results show a good agreement in velocity field and complicated vortical structures, including tip leakage vortex, tip separation vortex, the induced secondary vortices and wake vortices shedding off the foil. Spatial evolution of tip vortices is studied with three generations of vortex identification methods, including vorticity, Q-criterion (Hunt et al., 1988), the Ω method (Liu et al., 2016) and the Liutex method (Liu et al., 2018a). Performances of different methods are compared and explained, which indicate the advantages of Liutex method in the investigation of this flow regime. The evolution of tip vortices is observed to be affected by the free shear layer around TLV dramatically, which means that it may play an important role in TLV cavitating flow.

Submesoscale Eddy and Frontal Instabilities in the Kuroshio Interacting With a Cape South of Taiwan

AbstractThe processes underlying the strong Kuroshio encountering a cape at the southernmost tip of Taiwan are examined with satellite‐derived chlorophyll and temperature maps, a drifter trajectory, and realistic model simulations. The interaction spurs the formation of submesoscale cyclonic eddies that trap cold and high‐chlorophyll water and the formation of frontal waves between the free stream and the wake flow. An observed train of eddies, which have relative vorticity about one to four times the planetary vorticity (f), is shed from the recirculation that occurs in the immediate lee of the cape as a result of flow separation. These propagate downstream at a speed of 0.5–0.6 m s−1. Farther downstream, the corotation and merging of two or three adjacent eddies are common owing to the topography‐induced slowdown of eddy propagation farther downstream. It is found that the relative vorticity of a corotating system (1.2f) is 70% weaker than that of a single eddy due to the increase of eddy diameter from ~16 to ~33 km, in agreement with Kelvin's circulation theorem. The shedding period of the submesoscale eddies is strongly modulated by either diurnal or semidiurnal tidal flows, which typically reach 0.2–0.5 m s−1, whereas its intrinsic shedding period is insignificant. The frontal waves predominate in the horizontal free shear layer emitted from the cape, as well as a density front. Energetics analysis suggests that the wavy features result primarily from the growth of barotropic instability in the free shear layer, which may play a secondary process in the headland wake.

Laser-induced fluorescence velocimetry for a hypersonic leading-edge separation

Two-dimensional mapping of the velocity distribution for a hypersonic leading-edge separation flowfield generated by a “tick” shaped geometry is presented for the first time. Discrete measurements of two velocity components were acquired at a flow condition having a total specific enthalpy of 3.8 MJ/kg by imaging nitric oxide fluorescence over numerous runs of the hypersonic tunnel at the Australian Defence Force Academy (T-ADFA). The measured freestream velocity distribution exhibited some non-uniformity, which is hypothesized to originate from images acquired using a set of ultraviolet specific mirrors mounted on the shock tunnel deflecting under load during a run of the facility, slightly changing the laser sheet orientation. The flow separation point was measured to occur at 1.4 ± 0.2 mm from the model leading edge, based on the origin of the free shear layer emanating from the expansion surface. Reattachment of this free shear layer on the compression surface occurred at 59.0 ± 0.2 mm from the model vertex. Recirculating the flow bound by the separation and reattachment points contained supersonic reverse flow and areas of subsonic flow aligned with the location of three identified counter-rotating vortices. A comparison of the recirculation flow streamline plots with those computed using Navier–Stokes and direct simulation Monte Carlo (DSMC) codes showed differences in flow structures. At a flow time close to that produced by the facility, flow structures generated by the DSMC solution were seen to agree more favorably with the experiment than those generated by the Navier–Stokes solver due to its ability to better characterize separation by modeling the strong viscous interactions and rarefaction at the leading edge. The primary reason for this is that the no-slip condition used in the Navier–Stokes solution predicts a closer separation point to the leading edge and structures when compared to the DSMC solution, which affects surface shear stress and heat flux, leading to a difference in flow structures downstream of the separation.

Optimal Disturbances in the Development of the Instability of a Free Shear Layer and a System of Two Counter-Streaming Jet Flows

Optimal Disturbances in the Development of the Instability of a Free Shear Layer and a System of Two Counter-Streaming Jet Flows

Gas–solid two-phase flow and erosion calculation of gate valve based on the CFD-DEM model

Gate valves are important pipeline components in pneumatic conveying systems, and they were eroded severely by the impact of solid particles. In this study, the CFD-DEM simulation method was adopted to study the gas–solid two-phase flow characteristics and erosion characteristics of the gate valve. The accuracy of the simulation was validated by comparing the simulation and experimental data. Results show that a high-speed jet flow appears under the flashboard and induces a free shear layer, and the free shear layer attaches at the rear wall of the cavity. Particle distribution shows that the particle number at the upstream of the flashboard increases with decreasing valve opening. The particle number inside the cavity has a similar trend. The erosion results indicate that the number of particles plays an important role in erosion. The range of the erosion gradient region and the maximum erosion rate decrease with increasing particle size. Erosion of the cavity is more serious than that of the flashboard.

Numerical Analysis of Sweeping Jets for Active Flow Control Application

Abstract It has become apparent recently that the fluidic oscillators, also known as sweeping jets, can be used to create a combination of steady (streamwise vortices) and unsteady (spanwise vortices) forcing mechanisms which have the potential to fulfill many of the promises of active separation control. The fluidic oscillators contain no moving parts, but produce an unsteady component via a natural feedback loop inherent to their geometry. The oscillations are entirely self-induced and self-sustaining. Their simple and robust design and their effectiveness over a wide range of flow conditions make them more attractive than other flow control devices, such as synthetic jets and plasma actuators. Figure 1 shows the instantaneous jet generated in quiescent environment using the Improved Delayed Detached Eddy Simulation (IDDES) model, where the Large Eddy Simulation (LES) branch of the IDDES model is able to capture the turbulence structures properly. An instantaneous iso-surface of vorticity magnitude, colored by streamwise velocity for flow over a wall-mounted hump is depicted in Figure 2. As expected, a massive flow separation occurs behind the hump in the uncontrolled condition (Figure 2 (a)), with a nearly two-dimensional free shear layer at the edge of the separation line. Breakdown of the shear layer by an array of sweeping jets located slightly downstream of the separation line is seen in Figure 2 (b), which is followed by the elimination of the separation region behind hump. The three-dimensional structures generated by the sweeping jets are smaller and closer to the hump wall than those produced by the steady jets shown in Figure 2 (c). Presence of a large region of reversed flow near the hump wall in its aft section is also seen in the case of the steady jet. This study indicates a superior effectiveness of sweeping jets on separated flows.

Zamana Bağlı Kavite Akışı Aeroakustiğinin Büyük Girdap Simülasyonu ile Sayısal Olarak İncelenmesi

Flow along a cavity is of special interest for researchers due to the occurrence of free shear layer flow and related high levels of sound and pressure forces. In this study, turbulent flow along an open cavity at a low inlet Mach number (Ma = 0.034) is modelled by Large Eddy Simulations (LES). The velocity profiles at various stations inside the open cavity are compared to available experimental data. It is found that LES results agree with experimental data and detects the transient pressure change in the flow field satisfactorily. Transient pressure data in the flow field is evaluated in acoustic analogy. The noise generated by the cavity is compared with the established Rossiter modes and is found to be reasonable. To create an effect on the sound pressure levels (SPL), a small obstacle with quadrilateral cross section is immersed in the shear layer at three different locations. This causes that the SPL peaks are reduced compared to the case without any obstacle. Thus, cavity-induced noise form specific frequencies are redistributed to high frequency broadband noise as a result of this passive flow control method.

Numerical investigation of turbulent shear flows using production‐limited delayed detached‐eddy simulation

AbstractIn chemical engineering, turbulent shear flows are often encountered. The grid induced separation (GIS) and the slow RANS‐LES transition issues should be alleviated when the delayed detached‐eddy simulation (DDES) is used to simulate turbulent shear flows. This paper studies the performance of the production‐limited DDES (PL‐DDES) model in improving the GIS and the slow RANS‐LES transition issues. Since the simplified IDDES (S‐IDDES) model is proposed to improve the GIS issue, the S‐IDDES model is chosen as the model for comparison. The simulation results show that the PL‐DDES model with constant C d1 = 14 alleviates the GIS issue better than the S‐IDDES model and the PL‐DDES model with C d1 = 8. The results of the free shear layer show that the PL‐DDES model can switch RANS to LES more rapidly and unlock the Kelvin‐Helmholtz instability more effectively than the S‐DDES model. For the backward‐facing step flow, the S‐IDDES model performs poorly when unlocking the Kelvin‐Helmholtz instability in the separation zone. On the other hand, the PL‐DDES model has a rapid RANS‐LES transition after the step and produces a significant transport of momentum in the shear layer, leading to reasonable separation distance and flow structures.

Non-stationary vortex streets in shear flows

Spatially periodic vortex systems that form due to unstable shear flows are called vortex streets. A number of exact and asymptotic solutions of two-dimensional hydrodynamic equations describing nonstationary vortex streets have been constructed. It is shown that the superposition of the flow with a constant horizontal shear and its perturbations in the form of a nonmodal wave provides an exact solution that describes a nonstationary vortex street with rotating elliptic current lines. The width of the zone occupied by such a vortex street has been determined. The equation of separatrix separating vortex cells with closed current lines from an external meandering flow has been obtained. The influence of the quasi-two-dimensional compressibility and beta effect on the dynamics of vortex streets has been studied based on the potential vorticity transport equation. The solutions describing the formation of vortex streets during the development of an unstable zonal periodic flow and a free shear layer have been constructed using a longwave approximation.

Experimental Investigation of Coherent Vortex Structures in a Backward-Facing Step Flow

Coherent vortex structures (CVS) are discovered for more than half a century, and they are believed to play a significant role in turbulence especially for separated flows. An experimental study is conducted for a pressured backward-facing step flow with Reynolds number (Reh) being 4400 and 9000. A synchronized particle image velocimetry (PIV) system is developed for measurement of a wider range of velocity fields with high resolution. The CVS are proved to exist in the separation-reattachment process. For their temporal evolution, a life cycle is proposed that vortices form in the free shear layer, develop with pairings and divisions and finally shed at the reattachment zone, and sometimes new vortical structures are restructured with recovery of flow pattern. The CVS favor the free shear layer with frequent pairings and divisions particularly at the developing stage around x/h = 2~5 (x: distance from the step in flow direction, h: step height), which may contribute to the high turbulent intensity and shear stress there. A critical distance is believed to exist among CVS, which affects their amalgamation (pairing) and division events. Statistics show that the CVS are well organized in spatial distribution and show specific local features with the flow structures distinguished. The streamwise and vertical diameters (Dx and Dy) and width to height ratio (Dx/Dy) all obey to the lognormal distribution. With increase of Reh from 4400 to 9000, Dx decreases and Dy increases, but the mean diameter (D=0.5 × (Dx + Dy)) keeps around (0.28~0.29) h. As the increase of Reh, the vortical shape change toward a uniform condition, which may be contributed by enhancement of the shear intensity.

Nonlinear evolution and acoustic radiation of coherent structures in subsonic turbulent free shear layers

Large-scale coherent structures are present in compressible free shear flows, where they are known to be a main source of aerodynamic noise. Previous studies showed that these structures may be treated as instability waves or wavepackets supported by the underlying turbulent mean flow. By adopting this viewpoint in the framework of triple decomposition of the instantaneous flow into the mean field, coherent motion and small-scale turbulence, a strongly nonlinear dynamical model was constructed to describe the formation and development of coherent structures in incompressible turbulent free shear layers (Wu & Zhuang, J. Fluid Mech., vol. 787, 2016, pp. 396–439). That model is now extended to compressible flows, for which the coherent structures are extracted through a density-weighted (Favre) phase average. The nonlinear non-equilibrium critical-layer theory for instability waves in a laminar compressible mixing layer is adapted to analyse coherent structures in its turbulent counterpart. The strong non-parallelism associated with the fast spreading of the turbulent mean flow is taken into account and found to be significant. The model also accounts for the effect of fine-scale turbulence on coherent structures via a gradient type of closure model which now allows for a phase lag between the phase-averaged small-scale Reynolds stresses and the strain rates of coherent structures. The analysis results in an evolution system comprising of an amplitude equation, the critical-layer temperature and vorticity equations along with the appropriate initial and boundary conditions. The physical processes of acoustic radiation from the coherent structures are described by examining the far-field asymptote of the hydrodynamic fluctuations. We demonstrate that the nonlinearly generated slowly breathing mean-flow distortion radiates low-frequency sound waves. The true physical sources are identified. Equivalent sources in a Lighthill type of acoustic analogy context also arise, but they cannot be fully determined before the acoustic field is calculated, in which sense the radiated sound waves act back on the source. The numerical solutions to the evolution system show that coherent structures attenuate nonlinearly and their vorticity field rolls up to form the characteristic rollers. A study is also made of coherent structures represented by modulated wavetrains consisting of sideband modes, in which case nonlinear interactions generate components with frequencies that are combinations of those of the dominant modes. These components, especially the difference-frequency one, acquire significant amplitudes. Finally, the directivity and spectrum of the emitted acoustic field are calculated for both cases where the coherent structures consist of discrete, and a continuum of, sideband modes.

Experimental analysis of the effect of local base blowing on three-dimensional wake modes

Wake modes of a three-dimensional blunt-based body near a wall are investigated at a Reynolds number . The targeted modes are the static symmetry-breaking mode and two antisymmetric periodic modes. The static mode orientation is aligned with the horizontal major -axis of the base and randomly switches between a positive and a negative state leading to long-time bistable dynamics of the turbulent wake. The modifications of these modes are studied when continuous blowing is applied at different locations through four slits along the base edges (denoted L for left, R for right, T for top and B for bottom) in either four single asymmetric configurations or two double symmetric configurations (denoted LR and TB). Two regimes, referred to as mass and momentum, are clearly identifiable for all configurations. The mass regime, which is fairly insensitive to blowing momentum and location, is characterized by the growth of the recirculating bubble as the total injected flow rate is increased, and is associated with a base drag reduction and interpreted as resulting from the equilibrium between mass fluxes feeding and emptying the recirculating region. A simple budget model is shown to be in agreement with entrainment velocities measured for isolated turbulent mixing layers. The strength of the static mode is reduced up to 20 % when the bubble length is maximum, whereas no change in the periodic mode frequencies is found. On the other hand, the momentum regime is characterized by the deflating of the recirculating bubble, leading to base drag increase, and it is interpreted by the free shear layer forcing, which increases the entrainment velocity, thus emptying the recirculating bubble. In this regime the static mode orientation is imposed by the blowing symmetry. Lateral L and R (respectively top/bottom T and B) blowing configurations select or states in the horizontal (respectively vertical) direction, while bistable dynamics persists for the symmetric LR and TB configurations. The shape of periodic modes follows the changes in wake static orientation. The transition between the two regimes is governed by both the total injected flow rate and the location of the injection.