EFFECTS OF TURBULENT WAKE FLOW FORMED BEHIND A SQUARE CYLINDER ON MASS DIFFUSION PROCESS IN A TURBULENT BOUNDARY LAYER

2018 ◽  
Author(s):  
Tomoya Houra ◽  
Takashi Watanabe ◽  
F. Suzuki ◽  
Masato Tagawa
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Diffusion Process ◽  
Turbulent Wake ◽  
Mass Diffusion ◽  
Wake Flow ◽  
Square Cylinder

The Wake Dynamics Behind a Near-Wall Square Cylinder

2021 ◽  
Author(s):  
Samuel Addai ◽  
Xingjun Fang ◽  
Afua A Mante ◽  
Mark F. Tachie
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Free Stream ◽  
Free Stream Velocity ◽  
Wake Flow ◽  
Stream Velocity ◽  
Reynolds Stresses ◽  
Square Cylinder ◽  
Gap Ratio ◽  
The Mean

Abstract Particle image velocimetry is used to experimentally study the wake dynamics behind a near-wall square cylinder subjected to a thick oncoming turbulent boundary layer. The turbulent boundary layer thickness was 3.6 times the cylinder height (h) while the Reynolds number based on the free-stream velocity and the cylinder height was 12750. The gap distance (G) between the bottom face of the cylinder and the wall was varied, resulting in gap ratios (G/h) of 0, 0.3, 0.5, 1.0, 2.0, 4.0 and 8.0. The effects of varying the gap ratio on the mean flow, Reynolds stresses, triple velocity correlation, two-point autocorrelation and the unsteady wake characteristics were examined. The results indicate that as gap ratio decreases, asymmetry in the wake flow becomes more pronounced and the size of the mean separation bubbles increases. The magnitudes of the Reynolds stresses and triple velocity correlations generally decrease with decreasing gap ratio. Moreover, the size of the large-scale structures increases with decreasing gap ratio and the critical gap ratio, below which Kármán vortex shedding suppression occurs, is found to be 0.3. The dominant Strouhal number in the wake flow expressed in terms of the streamwise mean velocity at the cylinder vertical midpoint increases as gap ratio decreases while that based on the free-stream velocity is less sensitive to gap ratio for the offset cases (G/h > 0).

Wake Interaction of a Square Cylinder With a Splitter Plate Boundary Layer at a Low Reynolds Number

2015 ◽  
Author(s):  
Smriti Srivastava ◽  
Sudipto Sarkar
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Bluff Body ◽  
Acoustic Noise ◽  
Vortex Formation ◽  
Plate Boundary ◽  
Wake Flow ◽  
Stream Velocity ◽  
Square Cylinder ◽  
Plate Length

One of the most important researches in bluff body aerodynamics is to control the shear layer evolution leading to vortex formation. This kind of research is closely associated with reduction of aerodynamics forces and acoustic noise. Passive and active control of wake-flow from bluff bodies have received a great deal of attention in the last few decades [1–4]. Keeping this in mind, authors investigate the interaction of a square cylinder (side of the square = a) wake with a flat plate (length L = a, width w = 0.1a) boundary layer positioned at various downstream locations close to the cylinder. The gap-to-side ratios are maintained at G/a = 0, 0.5, 1 and 2 (where G is the gap between square cylinder and plate), and the simulation is performed at a Reynolds number, Re = 100 (Re = U∞a/v, where U∞ is free stream velocity and v is kinematic viscosity). Instantaneous flow visualization, aerodynamic forces and vortex shedding frequencies for all cases are described to gain insight about the changes associated with wake of the cylinder when a short plate is kept in its downstream.

Phase Locked Analysis of a Simplified Car Geometry Wake Flow Control Using Synthetic Jet

2006 ◽  
Author(s):  
Ce´dric Leclerc ◽  
Euge´nie Levallois ◽  
Quentin Gallas ◽  
Patrick Gillie´ron ◽  
Azeddine Kourta
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Drag Reduction ◽  
Vortex Shedding ◽  
Aerodynamic Drag ◽  
Streamwise Velocity ◽  
Local Effect ◽  
Synthetic Jet ◽  
Wake Flow ◽  
Synthetic Jet Actuator

This paper presents a numerical unsteady analysis of a SJA impact on a car wake flow. First, for the optimal reduced frequency F+, the influence of the Cμ on the mean aerodynamic drag reduction 〈Cd〉 is observed. A spectral analysis of the vortex shedding coming from the upper and the lower part of the car and of the drag coefficient is then presented for different Cμ values. Preliminary results suggest that maximum drag reduction is obtained when most energy in the wake comes from the actuator forcing frequency rather than the natural vortex shedding frequencies of the two contributions. This work is completed by a phase locked analysis of the synthetic jet actuator local effect on the turbulent boundary layer just before the flow separation. For the fixed optimal F+, different Cμ values are compared. The streamwise velocity profiles seem to show that maximal efficiency of the control is obtained when the synthetic jet injected momentum is introduced in the logarithmic sub-layer part of the turbulent boundary layer.

Wakes behind two-dimensional surface obstacles in turbulent boundary layers

1974 ◽  
Vol 64 (3) ◽  
pp. 529-564 ◽  
Author(s):  
J. Counihan ◽  
J. C. R. Hunt ◽  
P. S. Jackson
Keyword(s):  
Boundary Layer ◽  
Wind Tunnel ◽  
Turbulent Boundary Layer ◽  
The Body ◽  
Wake Flow ◽  
Two Dimensional ◽  
Mean Velocity ◽  
Velocity Deficit ◽  
Wind Tunnel Measurements ◽  
The Mean

By making simple assumptions, an analytical theory is deduced for the mean velocity behind a two-dimensional obstacle (of heighth) placed on a rigid plane over which flows a turbulent boundary layer (of thickness δ). It is assumed thath[Gt ] δ, and that the wake can be divided into three regions. The velocity deficit −uis greatest in the two regions in which the change in shear stress is important, a wall region (W) close to the wall and a mixing region (M) spreading from the top of the obstacle. Above these is the external region (E) in which the velocity field is an inviscid perturbation on the incident boundary-layer velocity, which is taken to have a power-law profileU(y) =U∞(y−y1)n/δn, wheren[Gt ] 1. In (M), assuming that an eddy viscosity (=KhU(h)) can be defined for the perturbed flow in terms of the incident boundary-layer flow and that the velocity is self-preserving, it is found thatu(x,y) has the form$\frac{u}{U(h)} = \frac{ C }{Kh^2U^2(h)} \frac{f(n)}{x/h},\;\;\;\; {\rm where}\;\;\;\; \eta = (y/h)/[Kx/h]^{1/(n+2)}$, and the constant which defines the strength of the wake is$C = \int^\infty_0 y^U(y)(u-u_E)dy$, whereu=uE(x, y) asy→ 0 in region (E).In region (W),u(y) is proportional to Iny.By considering a large control surface enclosing the obstacle it is shown that the constant of the wake flow is not simply related to the drag of the obstacle, but is equal to the sum of the couple on the obstacle and an integral of the pressure field on the surface near the body.New wind-tunnel measurements of mean and turbulent velocities and Reynolds stresses in the wake behind a two-dimensional rectangular block on a roughened surface are presented. The turbulent boundary layer is artificially developed by well-established methods (Counihan 1969) in such a way that δ = 8h. These measurements are compared with the theory, with other wind-tunnel measurements and also with full-scale measurements of the wind behind windbreaks.It is found that the theory describes the distribution of mean velocity reasonably well, in particular the (x/h)−1decay law is well confirmed. The theory gives the correct self-preserving form for the distribution of Reynolds stress and the maximum increase of the mean-square turbulent velocity is found to decay downstream approximately as$ (\frac{x}{h})^{- \frac{3}{2}} $in accordance with the theory. The theory also suggests that the velocity deficit is affected by the roughness of the terrain (as measured by the roughness lengthy0) in proportion to In (h/y0), and there seems to be some experimental support for this hypothesis.

On the Response of Planar Wakes to Asymmetric Initial Conditions

2014 ◽  
Author(s):  
Ladan Momayez ◽  
Marouen Dghim ◽  
Mohsen Ferchichi ◽  
Sylvain Graveline
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Flat Plate ◽  
Laminar Boundary Layer ◽  
Initial Conditions ◽  
Leading Edge ◽  
Turbulent Wake ◽  
Lower Side ◽  
Near Wake ◽  
Laminar Wake

This work reports an experimental investigation on the response of a planar wake generated by a profiled flat plate to various upstream flow conditions. A tripping wire was placed on the upper side of the flat plate just downstream of the leading edge of the plate that resulted in asymmetric separating shear layers at the trailing edge. The near wake asymmetry is compared to the symmetrical case at two different Reynolds numbers. Two asymmetric initial conditions resulted, namely, laminar boundary layer on the lower side and a turbulent boundary layer on the upper side, and a turbulent boundary layer on the lower side and tripped turbulent boundary layer on the upper surface. The near wake dynamics were investigated under the effects of the degree of asymmetry using hot-wire anemometry and flow visualizations. The measurements showed when one of the two boundary layers was tripped, the wake shifted towards the tripped side and wake spreading was found to be larger than in the case of the symmetrical wake with the effect being more pronounced in the asymmetric laminar wake. Self-similarity of the asymmetrical wakes was established by properly selecting appropriate similarity variables however, the similarity of the wake was less evident in the tripped laminar boundary layer case. Convection velocity, Uc, of the Von Karman large eddies, estimated using processed flow visualization images seemed to increase with increased Reynolds number and with increased upstream momentum thickness. In the symmetric laminar wake, Uc/U∞ increases from 0.2 and reached an asymptotic value of about 0.85 further downstream. In the presence of perturbation, Uc/U∞ attained a constant value of about 0.83 further downstream compared to the symmetric case. For the turbulent wake, however, asymmetry of the turbulence levels was found to increase the convection speed compared to both the laminar wake and the symmetric turbulent wake reaching a constant value nearly at the same downstream position for both the symmetric and asymmetric turbulent wake.

scholarly journals Improving the performance of aeroacoustic measurements beneath a turbulent boundary layer in a wake flow

2014 ◽  
Author(s):  
Stefan Haxter ◽  
Carsten Spehr ◽  
Michael Hartmann ◽  
Joerg Ocker ◽  
Hironori Tokuno ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Wake Flow
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