Unsteady flow structures around a high-drag Ahmed body

2015 ◽  
Vol 777 ◽  
pp. 291-326 ◽  
Author(s):  
B. F. Zhang ◽  
Y. Zhou ◽  
S. To
Keyword(s):  
Coherent Structures ◽  
Leading Edge ◽  
Side Surface ◽  
The Body ◽  
Flow Structures ◽  
Rear Window ◽  
Hairpin Vortices ◽  
Recirculation Bubble ◽  
Slant Angle ◽  
Ahmed Body

This work aims to gain a relatively thorough understanding of unsteady predominant coherent structures around an Ahmed body with a slant angle of $25^{\circ }$, corresponding to the high-drag regime. Extensive hot-wire, flow visualization and particle image velocimetry measurements were conducted in a wind tunnel at $\mathit{Re}=(0.45{-}2.4)\times 10^{5}$ around the Ahmed body. A number of distinct Strouhal numbers (St) have been found, two over the rear window, three behind the vertical base and two above the roof. The origin of every St has been identified. The two detected above the roof are ascribed to the hairpin vortices that emanate from the recirculation bubble formed near the leading edge and to the oscillation of the core of longitudinal vortices that originate from bubble pulsation, respectively. The two captured over the window originate from the hairpin vortices and the shear layers over the roof and side surface, respectively. One measured in the wake results from the structures emanating alternately from the upper and lower recirculation bubbles. The remaining two detected behind the lower edge of the base are connected to the cylindrical struts, respectively, which simulate wheels. These unsteady structures and corresponding St reconcile the widely scattered St data in the literature. The dependence on Re of these Strouhal numbers is also addressed, along with the effect of the turbulent intensity of oncoming flow on the flow structures. A conceptual model is proposed for the first time, which embraces both steady and unsteady coherent structures around the body.

Active drag reduction of a high-drag Ahmed body based on steady blowing

2018 ◽  
Vol 856 ◽  
pp. 351-396 ◽  
Author(s):  
B. F. Zhang ◽  
K. Liu ◽  
Y. Zhou ◽  
S. To ◽  
J. Y. Tu
Keyword(s):  
Drag Reduction ◽  
The Body ◽  
Force Balance ◽  
Effective Control ◽  
Rear Window ◽  
Flow Measurements ◽  
Slant Angle ◽  
Vertical Base ◽  
Active Drag ◽  
Ahmed Body

Active drag reduction of an Ahmed body with a slant angle of $25^{\circ }$, corresponding to the high-drag regime, has been experimentally investigated at Reynolds number $Re=1.7\times 10^{5}$, based on the square root of the model cross-sectional area. Four individual actuations, produced by steady blowing, are applied separately around the edges of the rear window and vertical base, producing a drag reduction of up to 6–14 %. However, the combination of the individual actuations results in a drag reduction 29 %, higher than any previous drag reductions achieved experimentally and very close to the target (30 %) set by automotive industries. Extensive flow measurements are performed, with and without control, using force balance, pressure scanner, hot-wire, flow visualization and particle image velocimetry techniques. A marked change in the flow structure is captured in the wake of the body under control, including the flow separation bubbles, over the rear window or behind the vertical base, and the pair of C-pillar vortices at the two side edges of the rear window. The change is linked to the pressure rise on the slanted surface and the base. The mechanisms behind the effective control are proposed. The control efficiency is also estimated.

Active Flow Control of the 3D Separation on a Ahmed Body Using Steady Microjet Arrays

2010 ◽  
Author(s):  
S. Aubrun ◽  
F. Alvi ◽  
A. Leroy ◽  
A. Kourta
Keyword(s):  
Flow Control ◽  
Aerodynamic Drag ◽  
Separation Bubble ◽  
Active Flow Control ◽  
Aerodynamic Load ◽  
Flow Topology ◽  
Rear Window ◽  
Slant Angle ◽  
Control Approach ◽  
Ahmed Body

A model of a generic vehicle shape, the Ahmed body with a slant angle of 25°, is equipped with an array of blowing steady microjets 6mm downstream of the separation line between the roof and the slanted rear window. The goal of the present study is to evaluate the effectiveness of this actuation method in reducing the aerodynamic drag, by reducing or suppressing the 3D closed separation bubble located on the slanted surface. The efficiency of this control approach is quantified with the help of aerodynamic load measurements. The changes in the flow field when control is applied are examined using PIV measurements and skin friction visualizations. By activating the steady microjet array, the drag coefficient was reduced by 9 to 11%, depending on the Reynolds number. The modification of the flow topology under progressive flow control is particularly studied.

Recent Advances in Wake Dynamics and Active Drag Reduction of Simple Automotive Bodies

2021 ◽  
Author(s):  
Yu Zhou ◽  
Bingfu Zhang
Keyword(s):  
Machine Learning ◽  
Drag Reduction ◽  
Closed Loop ◽  
Three Dimensional ◽  
The Body ◽  
Flow Structures ◽  
Slant Angle ◽  
Reduction Mechanisms ◽  
Reduction Methods ◽  
Active Drag

Abstract This is a compendium of recent progresses in the development of wake dynamics and active drag reduction of three-dimensional simple automotive models, largely focused on the generic Ahmed body. It covers our new understanding of involved instabilities, predominant frequencies, pressure distribution and unsteady flow structures in the high- (12.5° < f < 30°) and low-drag (f > 30°) bodies and the square-back body (f = 0°), where f is the rear slant angle of the body. Various drag reduction methods and their performances are reviewed, including open- and closed-loop controls along with machine-learning control. The involving drag reduction mechanisms, net saving and efficiencies are discussed. Comments are made for the areas that deserve more attention and future investigation.

scholarly journals The nature of the vortical structures in the near wake of the Ahmed body

2017 ◽  
Vol 231 (9) ◽  
pp. 1239-1244 ◽  
Author(s):  
James Venning ◽  
David Lo Jacono ◽  
David Burton ◽  
Mark C Thompson ◽  
John Sheridan
Keyword(s):  
Water Channel ◽  
Field Measurements ◽  
The Body ◽  
Wake Flow ◽  
Generic Model ◽  
Toroidal Vortex ◽  
Near Wake ◽  
Slant Angle ◽  
Vortical Structures ◽  
Ahmed Body

This study presents the results from high-spatial-resolution water-channel velocity-field measurements behind an Ahmed body with 25° rear slant angle. The Ahmed body represents a simplified generic model of a hatchback automobile that has been widely used to study near-wake flow dynamics. The results help clarify the unresolved question of whether the time-mean near-wake flow structure is topologically equivalent to a toroidal vortex or better described by a pair of horizontally aligned horseshoe vortices, with their legs pointing downstream. The velocimetry data presented allows the tracking of the vortical structures throughout the near wake through a set of orthogonal planes, as well as the measurement of their circulation. The spanwise vortices that form as the flow separates from the top and bottom rear edges are shown to tilt downstream at the sides of the body, while no evidence is found of a time-mean attached toroidal vortex, at least for the Reynolds number (based on the square root of the frontal area) of [Formula: see text] under consideration.

Numerical Simulation of Unsteady Flow around Ahmed Body with Active Flow Control

2013 ◽  
Vol 275-277 ◽  
pp. 402-408
Author(s):  
Bing Xin Wang ◽  
Zhu Hui ◽  
Zhi Gang Yang
Keyword(s):  
Flow Control ◽  
Recirculation Zone ◽  
Active Flow Control ◽  
The Body ◽  
Slant Angle ◽  
Control Approach ◽  
Pressure Drag ◽  
Active Flow ◽  
Velocity Pressure ◽  
Ahmed Body

The numerical investigations presented in this paper deal with active flow control approach at the rear end of the Ahmed body model with the slant angle of 25°.Results of the velocity, pressure and vorticity field demonstrate the main reasons that cause the pressure drag. The influence of the spanwise and streamwise vortices rolling up from the slant and the edges on the recirculation zone behind the body is examined. A control slot is set on the separated line at the conjunction of the roof and the slant. Two different actuation concepts by blowing and suction steady jets through the slot lead to a drug increase of 5.61% and a drug reduction of 13.20% with the efficiency of 12.53% respectively.

scholarly journals Aerodynamic Study of an Ahmed Body with the Help of CFD Simulation

2020 ◽  
Author(s):  
Indrashis Saha ◽  
Tathagatha Mukherjee ◽  
Ankit Saha ◽  
Richa Pandey
Keyword(s):  
Cfd Simulation ◽  
Fuel Efficiency ◽  
Turbulence Models ◽  
Numerical Data ◽  
Primary Objective ◽  
The Body ◽  
Stream Velocity ◽  
Noise Emission ◽  
Slant Angle ◽  
Ahmed Body

Automotive aerodynamics comprises of the study of aerodynamics of road vehicles. Its main goals are reducing drag, minimizing noise emission, improving fuel economy, preventing undesired lift forces and minimizing other causes of aerodynamic instability at high speeds. The Ahmed body has the form of a highly simplified car, consisting of a blunt nose with rounded edges fixed onto a box-like middle section and a rear end that has an upper slanted surface, the angle of which can be varied. It retains vital features of real vehicles in order to study the flow fields around it and the related turbulence models which characterizes the actual flow at elevated Reynolds number. In the present study, the aerodynamic behavior of this body is investigated numerically by the aid of commercial CFD tool: Ansys Fluent. The results of the simulation are validated with available experimental data and results of the simulations from other literatures. The numerical data were obtained for a fixed free stream velocity of 25 m/s at the inlet. The simulations were performed at a fixed slant angle of 25 degree and zero yaw angle. The present study focuses on how local refinement of mesh inside the concerned body and the outside, helps affect the results and for which grid dependency test is the primary objective of this paper. The present study also helps demonstrate how the drag of the body behaves, which is mainly the effect of pressure drag force generated at the rear portion of the body. The study also focuses on important properties like the velocity magnitude at different locations for different meshing cases, and to capture the flow pattern in the front or near the wake region. The study can be further helpful to future researchers in determining resistance, fuel efficiency etc. helping designers to optimize in specialized areas for better efficiency.

Experimental Study of Flow Control Over a Standard Road Vehicle Using Plasma Actuator

2020 ◽  
Vol 22 (4) ◽  
pp. 1047-1060
Author(s):  
S. Shadmani ◽  
S. M. Mousavi Nainiyan ◽  
R. Ghasemiasl ◽  
M. Mirzaei ◽  
S. G. Pouryoussefi
Keyword(s):  
Flow Control ◽  
Pressure Distribution ◽  
Drag Force ◽  
Separated Flow ◽  
Plasma Actuator ◽  
Road Vehicle ◽  
Slant Angle ◽  
Total Drag ◽  
Slant Surface ◽  
Ahmed Body

AbstractAhmed Body is a standard and simplified shape of a road vehicle that's rear part has an important role in flow structure and it's drag force. In this paper flow control around the Ahmed body with the rear slant angle of 25° studied by using the plasma actuator system situated in middle of the rear slant surface. Experiments conducted in a wind tunnel in two free stream velocities of U = 10m/s and U = 20m/s using steady and unsteady excitations. Pressure distribution and total drag force were measured and smoke flow visualization carried out in this study. The results showed that at U = 10m/s using plasma actuator suppress the separated flow over the rear slant slightly and be effective on pressure distribution. Also, total drag force reduces in steady and unsteady excitations for 3.65% and 2.44%, respectively. At U = 20m/s, using plasma actuator had no serious effect on the pressure distribution and total drag force.

scholarly journals Experimental Study on Coherent Structures by Particles Suspended in Half-Zone Thermocapillary Liquid Bridges: Review

Fluids ◽  
2021 ◽  
Vol 6 (3) ◽  
pp. 105
Author(s):  
Ichiro Ueno
Keyword(s):  
Coherent Structures ◽  
Three Dimensional ◽  
Wave Number ◽  
Flow Structures ◽  
Liquid Bridges ◽  
Thermocapillary Effect ◽  
Particle Accumulation ◽  
Azimuthal Wave Number ◽  
Experimental Approaches ◽  
Particle Behaviour

Coherent structures by the particles suspended in the half-zone thermocapillary liquid bridges via experimental approaches are introduced. General knowledge on the particle accumulation structures (PAS) is described, and then the spatial–temporal behaviours of the particles forming the PAS are illustrated with the results of the two- and three-dimensional particle tracking. Variations of the coherent structures as functions of the intensity of the thermocapillary effect and the particle size are introduced by focusing on the PAS of the azimuthal wave number m=3. Correlation between the particle behaviour and the ordered flow structures known as the Kolmogorov–Arnold—Moser tori is discussed. Recent works on the PAS of m=1 are briefly introduced.

Maximizing the efficiency of a flexible propulsor using experimental optimization

2015 ◽  
Vol 767 ◽  
pp. 430-448 ◽  
Author(s):  
Daniel B. Quinn ◽  
George V. Lauder ◽  
Alexander J. Smits
Keyword(s):  
Leading Edge ◽  
The Body ◽  
Phase Lag ◽  
Force Measurements ◽  
Experimental Optimization ◽  
Image Velocimetry ◽  
Flexible Panel ◽  
Gradient Based ◽  
Power Scale ◽  
Effective Angle

AbstractExperimental gradient-based optimization is used to maximize the propulsive efficiency of a heaving and pitching flexible panel. Optimum and near-optimum conditions are studied via direct force measurements and particle image velocimetry (PIV). The net thrust and power scale predictably with the frequency and amplitude of the leading edge, but the efficiency shows a complex multimodal response. Optimum pitch and heave motions are found to produce nearly twice the efficiencies of optimum heave-only motions. Efficiency is globally optimized when (i) the Strouhal number is within an optimal range that varies weakly with amplitude and boundary conditions; (ii) the panel is actuated at a resonant frequency of the fluid–panel system; (iii) heave amplitude is tuned such that trailing-edge amplitude is maximized while the flow along the body remains attached; and (iv) the maximum pitch angle and phase lag are chosen so that the effective angle of attack is minimized. The multi-dimensionality and multi-modality of the efficiency response demonstrate that experimental optimization is well-suited for the design of flexible underwater propulsors.

Active procedures to control the flow past the Ahmed body with a 25° rear window

2011 ◽  
Vol 1 (3/4) ◽  
pp. 299 ◽  
Author(s):  
Charles Henri Bruneau ◽  
Emmanuel Creuse ◽  
Delphine Depeyras ◽  
Patrick Gillieron ◽  
Iraj Mortazavi
Keyword(s):  
Rear Window ◽  
Flow Past ◽  
Ahmed Body
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