Mean Recirculation Zone Research Articles

Near-wake structures of a finite square cylinder with a flapping film at its free end

As a follow-up study of Wang et al. [“Control of the flow around a finite square cylinder with a flexible plate attached at the free end,” Phys. Fluids 34(2), 027109 (2022)], this paper presents an experimental study of flow around a wall-mounted finite square cylinder with a vertically clamped flapping film at its free end. The width (d) of the square cylinder was 40 mm, and the aspect ratio (H/d) was 5, where the height H was 200 mm. The flexible film was made of low-density polyethylene, with a thickness of 0.04 mm and the width and length (l) each of d. Flow visualization and particle image velocimetry were conducted in the central lateral plane and several horizontal planes to reveal the 3D structure of the flapping induced vortex (FIV) and its effects on the cylinder near wake. All measurements were done in a low-speed wind tunnel at a flow speed of U∞ = 5 m/s with a Reynolds number of 13 700 based on U∞ and d. Previous study suggests that the flapping film reduces aerodynamic forces of the cylinder significantly and that the fluctuating lateral force is reduced by 60% [Wang et al., “Control of the flow around a finite square cylinder with a flexible plate attached at the free end,” Phys. Fluids 34(2), 027109 (2022)]. Vortices that shed from the trailing edge of the flapping film connect those from the side edges, forming n-shape FIVs downstream. FIVs induce more high-speed flow downwards into the wake, which suppresses the mean recirculation zone near the free end but enlarges it in the lower part of the wake. The two legs of n-shape FIVs are symmetrically arranged near the cylinder free end, whose effects diminish gradually as approaching the bottom wall, where alternating Karman vortex shedding still prevails.

Effect of inclined angle on a wall-mounted finite-height square cylinder with a passive vertical suction

Drilling a vertical cavity from the bottom to the free-end surface has been a passive method of controlling wake flow around a wall-mounted square cylinder. The mean flow field around a wall-mounted finite-height square cylinder with a vertical rectangular cavity was studied experimentally in a water tunnel to explore the effects of inclined angle α. Measurements were conducted using an acoustic Doppler velocimeter at the Reynolds number of Reh = 17,525, in which the incidence angles of the cylinder were α = 0, 10°, 30°, and 45°. Results depict the presence of a recirculation zone in the cavity and a near-wake zone. As α increases from 0 to 10°, the mean wake behind the cylinder shifts in the opposite position, and a single peak is observed. When α changes from 10° to 45°, the mean wake regains its symmetry, and two peaks are observed at the wake. Self-similarity in terms of the streamwise velocities and Reynolds shear stress can be achieved by properly selecting the velocity and length scales irrespective of the angle of incidence. For the wake flow with a higher value of α, the length of the mean recirculation zone, momentum exchange, and turbulence kinetic energy increase, whereas the mean and variance values of the vortex shedding frequency reach a smaller one. In this study, the cavity flow and wake flow are also observed in terms of velocity distribution, turbulent kinetic energy budget, and turbulent events.

Effect of the aspect ratio on the velocity field of a straight open-channel flow

This study investigates numerically the effect of the aspect ratio (AR) on the velocity field characteristics of the turbulent flow of a straight open-channel flow. Five aspect ratio cases (AR = channel width/flow depth) are investigated ranging from a narrow case of AR = 1 to a wide case of AR = 9. The transient three-dimensional Navier–Stokes equations were numerically solved using a finite-volume approach with an improved–delayed detached-eddy simulation turbulence model. The objective of this study is to enhance our understanding of the effect of AR on the formation of secondary currents in a channel flow. The results revealed the formation of a pair of counter-rotating recirculation zones near the bottom corners of the channel, whose axes are aligned with the main flow direction. Each pair consists of a mean recirculation zone that resides near the bed (bed recirculation zone), and another one that resides near the sidewall (side recirculation zone). The AR appears to significantly influence the size and strength of the recirculation zones. With decreasing the AR, the momentum transport by the streamwise-vertical Reynolds shear stress in the horizontal direction appears to be inhibited at higher vertical locations, the momentum transport by streamwise-transverse Reynolds shear stress is considerably enhanced, and the momentum transport by vertical-transverse Reynolds shear stress is slightly enhanced.

Characteristics of the free-end mean pressure distribution for a surface-mounted finite-height cylinder

Wind tunnel experiments at a Reynolds number of Re = 6.5 × 104 were used to study the effect of aspect ratio and boundary layer thickness on the mean static pressure distribution on the free end of a surface-mounted finite-height cylinder. The cylinder's aspect ratio was changed in small increments from AR = 0.5 to AR = 11. Two different boundary layer thicknesses (relative to the cylinder diameter) were employed, δ/D = 0.6 and δ/D = 1.9. From analysis of the mean pressure contour plots, it was found that the sizes and locations of regions of lower pressure, adverse pressure gradient, and higher pressure, and the appearance of “eye-like spots”, are sensitive to both AR and δ/D. The adverse pressure gradient occurs just ahead of the mean reattachment line while the eye-like spots are related to termination points of the legs of the arch vortex within the free-end mean recirculation zone. The total normal force coefficient experienced by the cylinder is strongly influenced by the contribution of the wall shear stress on the sides of the cylinder, with a change in direction of the net vertical shear stress contribution occurring between AR = 7 and AR = 8 for both boundary layers.

Heat transfer in flow around a rotary oscillating cylinder at a high subcritical Reynolds number: A computational study

We studied numerically the heat transfer in flow over a rotationally oscillating cylinder at a subcritical Reynolds number (Re=1.4×105) that is an order of magnitude higher than previously reported in the literature. This paper is a follow-up of the earlier study of hydrodynamics and drag force in a range of forcing frequencies and amplitudes (Palkin et al., 2018). This time we focus on heat transfer and its correlation with the observed flow field and vortical patterns. Four forcing frequencies f=fe/f0=0,1,2.5,4 for two forcing amplitudes Ω=ΩeD/2U∞=1 and 2 are considered, where f0 is the natural vortex-shedding frequency, U∞ the free-stream velocity and D the cylinder diameter. The parametric study was performed by solving three-dimensional unsteady Reynolds-averaged Navier–Stokes (URANS) equations closed by a wall-integrated second-moment (Re-stress) model, verified earlier by Large-eddy simulations and experiments in several reference cases including flows over a stagnant, as well as rotary oscillating cylinders at the same Re number. The thermal field, treated as a passive scalar, was obtained from the simultaneous solution of the energy equation, closed by the standard (GGDH) anisotropic eddy-diffusivity model. The computations showed that for the unforced cylinder heat transfer is characterized by very high local rates due to a strong thinning of the thermal boundary layer as a result of the impact and interactions of large coherent structures with the wall. The overall average Nusselt number does not change much for the forced cylinder but its time-averaged, phase-averaged and instantaneous circumferential profiles show some profound differences compared to the stationary cylinder. The distribution of Nu on the back surface becomes more uniform with less frequent occurrence of high values, especially for the higher frequencies f=2.5 and f=4. This is attributed to diminishing of the mean-recirculation zone as well as to the overall suppression of turbulent fluctuations. The rotary oscillation of the cylinder appears potentially efficient in achieving a more uniform circumferential distribution of Nu and avoiding local overheats and hot spots.

Flow separation control over a rounded ramp with spanwise alternating wall actuation

An implicit large-eddy simulation is carried out to study turbulent boundary-layer separation from a backward-facing rounded ramp with active wall actuation control. This method, called spanwise alternating distributed strips control, is imposed onto the flat plate surface upstream of a rounded ramp by alternatively applying out-of-phase control and in-phase control to the wall-normal velocity component in the spanwise direction. As a result, the local turbulence intensity is alternatively suppressed and enhanced, leading to the creation of vertical shear-layers, which is responsible for the presence of large-scale streamwise vortices. These vortices exert a predominant influence on the suppression of the flow separation. The interaction between the large-scale vortices and the downstream recirculation zone and free shear-layer is studied by examining flow statistics. It is found that in comparison with the non-controlled case, the flow separation is delayed, the reattachment point is shifted upstream, and the length of the mean recirculation zone is reduced up to 8.49%. The optimal control case is achieved with narrow in-phase control strips. An in-depth analysis shows that the delay of the flow separation is attributed to the activation of the near-wall turbulence by the in-phase control strips and the improvement of the reattachment location is mainly due to the large-scale streamwise vortices, which enhance the momentum transport between the main flow and separated region.

The effects of nozzle geometry and equivalence ratio on a premixed reacting jet in vitiated cross-flow

The effects of nozzle geometry, jet equivalence ratio (ϕj), and momentum flux ratio (J) on the flow field, stability characteristics, and flame topology of a premixed ethylene–air jet injected transverse to a vitiated cross-flow are investigated experimentally. Cross-flow conditions of 900 K and 100 m/s were chosen to simulate the environment of a secondary combustor in a staged combustion system. The dependence of the flame liftoff height on J suggests that at these conditions the flame stabilization process is flame-propagation controlled rather than autoignition assisted. A circular nozzle and high aspect ratio slotted nozzle of identical exit area were investigated for jet to cross-flow momentum flux ratios ranging from 5 to 65 for jet equivalence ratios of up to ϕj = 5.0. High-speed particle image velocimetry was utilized to study the time-averaged flow field and OH* chemiluminescence was used to capture time-averaged and instantaneous features of the flame behavior. The nozzle geometry was determined to have a significant effect on RJICF flame stability, with substantially expanded blow-out limits for the slotted nozzle. Enhanced operability of the high aspect ratio slotted nozzle was shown to be attributable to the substantially larger and stronger recirculation zone on the leeward side of the jet when compared to the circular nozzle. This area is characterized by a more disperse region of elevated vorticity levels, resulting in the entrainment of more hot combustion products with a longer residence time in the recirculation zone, which in turn provides a stronger and more stable ignition source to the oncoming, unburned reactants. A correlation for the isothermal JICF trajectory was modified to account for gas expansion effects and found to satisfactorily capture the jet trajectory for both the non-reacting and reacting slotted nozzle. The jet trajectory was demonstrated to be independent of ϕj, whereas the jet flame penetration decreases as ϕj increases, indicating that where the flame situates itself is determined by the local mixture concentration rather than changes in the flow field. Nevertheless, ϕj was also found to have an effect on the mean flow field, with slightly higher magnitudes of reversed flow velocity and an increase in mean recirculation zone length observed as the fuel content of the jet is increased.

The effect of incidence angle on the mean wake of surface-mounted finite-height square prisms

The mean wake of a surface-mounted finite-height square prism was studied experimentally in a low-speed wind tunnel to explore the combined effects of incidence angle (α) and aspect ratio (AR). Measurements of the mean wake velocity field were made with a seven-hole pressure probe for finite square prisms of AR=9, 7, 5 and 3, at a Reynolds number of Re=3.7×104, for incidence angles from α=0° to 45°. The relative thickness of the boundary layer on the ground plane, compared to the prism width, was δ/D=1.5. As the incidence angle increases from α=0° to 15°, the mean recirculation zone shortens and the mean wake shifts in the direction opposite to that of the mean lift force. The downwash is also deflected to this side of the wake and the mean streamwise vortex structures in the upper part of the wake become strongly asymmetric. The shortest mean recirculation zone, and the greatest asymmetry in the mean wake, is found at the critical incidence angle of αcritical≈15°. As the incidence angle increases from α=15° to 45°, the mean recirculation zone lengthens and the mean streamwise vortex structures regain their symmetry. These vortices also elongate in the wall-normal direction and become contiguous with the horseshoe vortex trailing arms. The mean wake of the prism of AR=3 has some differences, such as an absence of induced streamwise vorticity near the ground plane, which support its classification as lying below the critical aspect ratio for the present flow conditions.

Impact of fuel composition on the recirculation zone structure and its role in lean premixed flame anchoring

We investigate the dependence of the recirculation zone (RZ) size and structure on the fuel composition using high-speed particle image velocimetry (PIV) and chemiluminescence measurements for C3H8/H2/air lean premixed flames stabilized in a backward-facing step combustor. Results show an intricate coupling between the flame anchoring and the RZ structure and length. For a fixed fuel composition, at relatively low equivalence ratios, the time-averaged RZ is comprised of two counter rotating eddies: a primary eddy (PE) between the shear layer and the bottom wall; and a secondary eddy (SE) between the vertical step wall and the PE. The flame stabilizes downstream of the saddle point of the dividing streamline between the two eddies. As equivalence ratio is raised, the flame moves upstream, pushing the saddle point with it and reducing the size of the SE. Higher temperature of the products reduces the velocity gradient in the shear layer and thus the reattachment length. As equivalence ratio approaches a critical value, the saddle point reaches the step and the SE collapses while the flame starts to exhibit periodic flapping motions, suggesting a correlation between the RZ structure and flame anchoring. The overall trend in the flow field is the same as we add hydrogen to the fuel at a fixed equivalence ratio, demonstrating the impact of fuel composition on the flow field. We show that the reattachment lengths (LR), which are shown to encapsulate the mean RZ structure, measured over a range of fuel composition and equivalence ratio collapse if plotted against the strained consumption speed (Sc). Results indicate that for the flame to remain anchored, the RZ structure should satisfy LR,isothermalLR,reacting·ScU∞∼0.1. If this criterion cannot be met, the flame blows off, flashes back or becomes thermoacoustically unstable, suggesting a Damköhler-like criterion for aerodynamic flame stabilization in backward-facing step flows.

Flow above the free end of a surface-mounted finite-height circular cylinder: A review

The wake of a surface-mounted finite-height circular cylinder and the associated vortex patterns are strongly dependent on the cylinder aspect ratio and the thickness of the boundary layer on the ground plane relative to the dimensions of the cylinder. Above a critical aspect ratio, the mean wake is characterized by streamwise tip vortex structures and Kármán vortex shedding from the sides of the cylinder. Below a critical aspect ratio, a unique mean wake structure is observed. Recent experimental studies in the literature that used phase-averaged techniques, as well as recent numerical simulations, have led to an improved physical understanding of the near-wake vortex flow patterns. However, the flow above the free end of the finite circular cylinder, and its relationship to the near wake, has not been systematically studied. The effects of aspect ratio and boundary layer thickness on the free-end flow field are also not completely understood, nor has the influence of Reynolds number on the free-end flow field been fully explored. Common features associated with the free end include separation from the leading edge, a mean recirculation zone containing a prominent cross-stream arch (or mushroom) vortex, and reattachment onto the free-surface. Other flow features that remain to be clarified include a separation bubble near the leading edge, one or two cross-stream vortices within this separation bubble, the origins of the streamwise tip or trailing vortices, and various critical points in the near-surface flow topology. This paper reviews the current understanding of the flow above the free end of a surface-mounted finite-height circular cylinder, with a focus on models of the flow field, surface oil flow visualization studies, pressure and heat flux distributions on the free-end surface, measurements of the local velocity field, and numerical simulations, found in the literature.

Local flow field of a surface-mounted finite circular cylinder

The local flow field and near-wake region of a surface-mounted finite circular cylinder were studied experimentally in a low-speed wind tunnel using particle image velocimetry (PIV). The cylinder was mounted normal to a ground plane and was partially immersed in a flat-plate turbulent boundary layer. Four finite circular cylinders of aspect ratios AR=9, 7, 5 and 3 were tested at a Reynolds number of ReD=4.2×104. At the location of the cylinder, the boundary layer thickness relative to the cylinder diameter was δ/D=1.6. PIV velocity field measurements were made in a vertical plane parallel to the mean flow direction on the flow centerline (the symmetry plane), within five diameters upstream and downstream of the cylinder, and also above the free end. Above the free end of the cylinder, flow separation from the leading edge leads to the formation of a mean recirculation zone on the free-end surface. The point of reattachment of the flow onto the free-end surface moves towards the trailing edge as the cylinder aspect ratio is decreased. Large regions of elevated turbulence intensity and Reynolds shear stress are found above the free end. In the near-wake region, the large recirculation zone contains a vortex immediately behind and below the free end; this vortex was found for all four aspect ratios. A second vortex is found behind the cylinder near the cylinder-wall junction; this vortex was not observed for the cylinder of AR=3, indicating a distinct wake structure for this cylinder.

The effect of slot jet size on the confined transverse slot jet

The current study explored the flowfield characteristics of an induced recirculation zone resulting from a transverse slot jet issuing in a cross flow. The slot jet spanned 95% of the full channel spanwise dimension which resulted in a highly three-dimensional mean flowfield. Experimental investigations were carried out at various jet momentums for two different transverse slot jet thicknesses. Velocity field data were gathered using digital particle image velocimetry and hot-wire anemometry. The momentum ratio of the jet to the channel was found to be the leading parameter for scaling the dimensions of the mean recirculation zone. The jet thickness demonstrated an influence on the level of three-dimensionality, turbulence levels, and integral length scales.

The Effects of Canopy Leaf Area Index on Airflow Across Forest Edges: Large-eddy Simulation and Analytical Results

The structure of turbulent flows along a transition between tall-forested canopies and forest clearings continues to be an active research topic in canopy turbulence. The difficulties in describing the turbulent flow along these transitions stem from the fact that the vertical structure of the canopy and its leaf area distribution cannot be ignored or represented by an effective roughness length. Large-eddy simulation (LES) runs were performed to explore the effect of a homogeneous variation in the forest leaf area index (LAI) on the turbulent flow across forest edges. A nested grid numerical method was used to ensure the development of a deep boundary layer above the forest while maintaining a sufficiently high resolution in the region close to the ground. It was demonstrated that the LES here predicted first-order and second-order mean velocity statistics within the canopy that agree with reported Reynolds-Averaged Navier–Stokes (RANS) model results, field and laboratory experiments. In the simulations reported here, the LAI was varied between 2 and 8 spanning a broad range of observed LAI in terrestrial ecosystems. By increasing the forest LAI, the mean flow properties both within the forest and in the clearing near the forest edge were altered in two fundamental ways: near the forest edge and into the clearing, the flow statistical properties resembled the so-called back-facing step (BFS) flow with a mean recirculation zone near the edge. Another recirculation zone sets up downstream of the clearing as the flow enters the tall forest canopy. The genesis of this within-forest recirculation zone can be primarily described using the interplay between the mean pressure gradients (forcing the flow) and the drag force (opposing the flow). Using the LES results, a simplified analytical model was also proposed to explain the location of the recirculation zone inside the canopy and its dependence on the forest LAI. Furthermore, a simplified scaling argument that decomposes the mean velocity at the outflow edge into a superposition of ‘exit flow’ and BFS-like flow with their relative importance determined by LAI was explored.

Computational Study of the Wake and Contaminant Transport of a Walking Human

The unsteady aerodynamic wake of a human is studied using a time-accurate computational fluid dynamics simulation. Transport of a scalar contaminant, which originates on the body, is also considered. An existing Reynolds-averaged Navier-Stokes solver is modified to include energy, scalar-transport, and thermal buoyancy effects. Structured overset grids are used to discretize the geometry and the flow field. Results indicate two distinct wake regions: an unsteady bluff-body wake behind the torso which is characterized by a mean recirculation zone, and a region of unsteady vortex shedding behind the legs which is dominated by a “jet” of air formed between the legs. A significant downwash occurs behind the body which has the effect of laterally spreading the lower portions of the wake. The magnitude of the scalar contaminant is shown to decay exponentially within the wake and is found to be highly dependent upon the source location.