Analysis of the Near Wake of Bluff Bodies in Ground Proximity

2002 ◽  
Author(s):  
Szabolcs R. Balkanyi ◽  
Luis P. Bernal ◽  
Bahram Khalighi
Keyword(s):  
Vector Fields ◽  
Aerodynamic Drag ◽  
Base Pressure ◽  
Wake Flow ◽  
Vehicle Body ◽  
Recirculation Region ◽  
Bluff Bodies ◽  
Pressure Measurements ◽  
Near Wake ◽  
Mean Velocity

The effect of several drag reducing devices on the near wake of a generic ground vehicle body was investigated. Drag and base pressure measurements were conducted to identify the effects of the devices on the base drag. A Particle Image Velocimetry (PIV) study was conducted to determine changes of the near wake flow field. Averages of more than 200 PIV velocity vector fields were used to compute the mean velocity and turbulent stresses at several cross section planes. The results of the drag and base pressure measurements show that significant reductions of the total aerodynamic drag (as high as 48%) can be achieved with relatively simple devices. The results also indicated that models with base cavity have lower drag than their counter parts without it. The base pressure distributions showed a strong effect of the ground, resulting in decrease of pressure towards the lower half of the base. The PIV study showed that the extent of the recirculation region is not strongly affected by the drag reducing devices. The tested devices however, were found to have a strong effect on the underbody flow. A rapid upward deflection of the underbody flow in the near wake was observed. The devices were also found to reduce the turbulent stresses in the near wake. The turbulent stresses were found to decrease in magnitude with increasing drag reduction.

Flow behind castellated blunt-trailing-edge aerofoils at supersonic speeds

1998 ◽  
Vol 375 ◽  
pp. 85-111 ◽  
Author(s):  
E. C. MAGI ◽  
S. L. GAI
Keyword(s):  
Quantitative Data ◽  
Base Pressure ◽  
Qualitative Description ◽  
Wake Flow ◽  
Spanwise Direction ◽  
Pressure Measurements ◽  
Near Wake ◽  
Trailing Edge ◽  
Blunt Trailing Edge ◽  
The Mean

A study of the near-wake flow of castellated blunt-trailing-edge aerofoils at a Mach number of 2 was conducted to understand the nature of the flow and the mechanisms of base pressure recovery. The investigation has shown that strong gradients exist in the spanwise direction and that the formation of the wake recompression shock occurs further away from the wake axis. Also, the wake neck is broader and diffused. Detailed quantitative data involving pressure measurements, schlieren and holographic interferometry, and laser transit velocimetry, are presented. A theoretical model to predict the mean base pressure on a castellated base is also proposed. Comparison with experimental data shows that the model provides a qualitative description of the flow behind a castellated base at supersonic speeds.

Multi-Time-Delay LSE-POD Complementary Approach Applied to Wake Flow Behind a Bluff Body

2007 ◽  
Author(s):  
Vibhav Durgesh ◽  
Jonathan W. Naughton
Keyword(s):  
Time Delay ◽  
Surface Pressure ◽  
Bluff Body ◽  
Wake Flow ◽  
Time Varying ◽  
Pressure Measurements ◽  
Near Wake ◽  
Time Resolved ◽  
Complementary Approach ◽  
The Time Domain

An understanding of the near wake dynamics of a bluff body is desired to better link base drag reduction observed on these bodies with the coherent structures in the wake. This investigation explores different Linear Stochastic Estimation-Proper Orthogonal Decomposition (LSE-POD) methods that can be employed to estimate the dynamics of the energy containing structure. Statistically independent two-dimensional PIV measurements and time-resolved surface pressure measurements are used to determine spatial POD modes and LSE coefficients for estimating the time-varying POD coefficients using measured surface pressures. These results are used with the time-resolved surface pressure measurements to estimate the time-varying POD coefficients that may be used for a low-order, time-resolved reconstruction of the flow field. The multi-time LSE approach formulated in the time domain (multi-time-delay LSE) is found to be successful in capturing the important near wake dynamics.

A study on flow past a suspended bluff object in an open channel

2001 ◽  
Vol 28 (4) ◽  
pp. 547-554 ◽  
Author(s):  
F N Krampa-Morlu ◽  
R Balachandar
Keyword(s):  
Open Channel ◽  
Velocity Profiles ◽  
The Body ◽  
Wake Flow ◽  
Wall Jet ◽  
Wall Region ◽  
Near Wake ◽  
Mean Velocity ◽  
Scour Holes ◽  
Channel Bottom

Detailed velocity and erosion measurements in the near wake of a flat plate with a gap between the plate and the channel bed in an open channel are presented. A "wall jet" like flow (confined to the wall region) is observed to interact with the wake flow behind the body. Two axial stations downstream of the body were chosen to obtain the velocity profiles in the near wake region. The span-wise velocity profiles across the wake at two distances from the channel bottom were also obtained. This yielded information in the development of the "wall jet" like flow and also provided some sense of the velocity components in the near wake. At mid-section of the gap, the streamwise mean velocity attains its peak value at one body width. The velocity measurements across the wake exhibit the cause of erosion. Erosion measurements were simulated on an erodible sand bed of D50 = 0.59 mm grain size. The erosion patterns obtained show symmetry about the centerline of the plate. A ripple-like pattern was observed with deeper scour holes generated towards the edge of the wake. Flow separations from the lower corners and the sides of the plate account for this typical nature of the erosion pattern.Key words: velocity profile, local scour, wake flow, wall jet flow.

A computational investigation of the instability of the detached shear layers in the wake of a circular cylinder

2010 ◽  
Vol 659 ◽  
pp. 375-404 ◽  
Author(s):  
MAN MOHAN RAI
Keyword(s):  
Shear Layer ◽  
Shear Layers ◽  
Wake Flow ◽  
Recirculation Region ◽  
Computational Investigation ◽  
Near Wake ◽  
Layer Transition ◽  
Flow Phenomena ◽  
Considerable Impact ◽  
Velocity Spectra

Cylinder wakes have been studied extensively over several decades to better understand the basic flow phenomena encountered in such flows. The physics of the very near wake of the cylinder is perhaps the most challenging of them all. This region comprises the two detached shear layers, the recirculation region and wake flow. A study of the instability of the detached shear layers is important because these shear layers have a considerable impact on the dynamics of the very near wake. It has been observed experimentally that during certain periods of time that are randomly distributed, the measured fluctuating velocity component near the shear layers shows considerable amplification and it subsequently returns to its normal level (intermittency). Here, direct numerical simulations are used to accomplish a number of objectives such as confirming the presence of intermittency (computationally) and shedding light on processes that contribute significantly to intermittency and shear-layer transition/breakdown. Velocity time traces together with corresponding instantaneous vorticity contours are used in deciphering the fundamental processes underlying intermittency and shear-layer transition. The computed velocity spectra at three locations along the shear layer are provided. The computed shear-layer frequency agrees well with a power-law fit to experimental data.

Analyze and Control of Near-Wake Flow on a Simplified Car Geometry

2006 ◽  
Author(s):  
M. Roume´as ◽  
P. Gillie´ron ◽  
A. Kourta
Keyword(s):  
Drag Reduction ◽  
Control Strategy ◽  
Pressure Loss ◽  
Total Pressure ◽  
Aerodynamic Drag ◽  
The Body ◽  
Total Pressure Loss ◽  
Wake Flow ◽  
Near Wake ◽  
Aerodynamic Drag Reduction

A 3D numerical simulation, based on the Lattice Boltzmann Method is carried out on a rectangular body (initially proposed by Ahmed [1]) to analyze the influence of blowing devices on the near-wake flow of a generic blunt body model. First, the results obtained without control are compared with experimental data from the literature. An open loop flow control strategy is then applied by setting blowing slots around the periphery of the body base. The blowing velocity is set to 1.5 V0, V0 being the upstream velocity. The resulting aerodynamic drag reduction is then analyzed by studying velocity, vorticity and total pressure loss distributions in the near-wake flow. Typical results show that a 29% drag reduction is obtained at a blowing angle of θ = 45° with respect to the base surface. Blowing jets are analogous to separated longitudinal fluidic elements which permit to reduce the transversal wake section by inclining the streamlines in the near-wake flow. The momentum introduced into the flow leads to a reduction in the wake total pressure loss and an increase in the base static pressure distribution. Finally, a parametric analysis is conducted on the blowing velocities in order to optimize the efficiency of the chosen control strategy, i.e. to minimize the ratio between the energy used to generate the jets and the energy saved through aerodynamic drag reduction.

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