CFD Study of Flow Around Ahmed Body

Raphael Lienhart, Vimaldoss Jesudhas, Ram Balachandar, Ron Barron,
Romain Rodriguez, Frédéric Murzyn

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Raphael Lienhart, Vimaldoss Jesudhas, Ram Balachandar, Ron Barron, Romain Rodriguez, et al. CFD Study of Flow Around Ahmed Body. 4th Thermal and Fluids Engineering Conference, Apr 2019, Las Vegas, United States. pp.1865-1868, 10.1615/tfec2019.tfl.027816 . hal-04503142

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# CFD study of flow around Ahmed body

Conference Paper · April 2019

DOI: 10.1615/TFEC2019.tfl.027816

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Raphaël Lienhart

École Supérieure des Techniques Aéronautiques et de Construction Automobile

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Vimaldoss Jesudhas

University of Windsor

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Ram Balachandar

University of Windsor

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Ronald Barron

University of Windsor

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ASTFE

American Society
of Thermal and Fluids Engineers

4th Thermal and Fluids Engineering Conference (TFEC)

April 14–17, 2019

Las Vegas, NV, USA

TFEC-2019-27816

# CFD STUDY OF FLOW AROUND AHMED BODY

Raphael Lienhart<fnref n="1" />,<fnref n="2" />, Vimaldoss Jesudhas<fnref n="1" />*, Ram Balachandar<fnref n="1" />, Ron Barron<fnref n="1" />, Romain Rodriguez<fnref n="2" /> and Frédéric Murzyn<fnref n="2" />

## ABSTRACT

This paper presents a numerical study of flow past an Ahmed body using Improved Delayed Detached Eddy Simulations (IDDES). Ahmed body with straight-back (zero rear slant angle) is used for the analysis. Experimental studies have been conducted in the wake of such car model using two-dimensional optical techniques or at fixed locations. The CFD study can be used to bridge the gap in the experimental data, by simulating the complete three-dimensional flow field. The velocity and turbulence characteristics predicted by the simulation are compared with experimental results and are presented with detailed discussions. The coherent structures in the wake flow are educed using vortex identification techniques. Their effect on the aerodynamics of the Ahmed body is presented with pertinent analysis. Due to the complexity of the flow field, this present study could also be used as a benchmark for assessing the performance of IDDES model in complex flow fields.

**KEY WORDS:** Ahmed Body, IDDES, Turbulent Wake flow, coherent structures, $\lambda_2$ criteria

## 1. INTRODUCTION

Recent studies have revealed that the concentration of pollutants inside the car cabin can be higher compared to the atmospheric level [1, 2]. This is caused by the penetration of exhaust fumes from the lead vehicle into the air intake system of the following vehicle. To interpret this transport phenomena, it is pertinent to evaluate the flow dynamics in the wake of a vehicle. Furthermore, fuel efficiency in the automotive industry is also a major concern for car-manufacturers. Understanding the wake-flow dynamics would aid in the reduction of pressure drag, thereby increasing the fuel efficiency. Researchers often use an Ahmed body [3] to generalize the flow phenomena in the wake region of a car. Several experimental investigations have been performed to evaluate the wake behind an Ahmed body [4, 5]. However, most of these experimental studies measure the flow in specific locations or planes, the complete three-dimensional flow field is not available for analysis. To bridge these gaps, complimentary experimental and numerical investigations of Ahmed bodies are mandatory. To this end, a three-dimensional (3D), unsteady, Improved Detached Eddy Simulation (IDDES) of a straight-back (zero rear slant angle) Ahmed body was performed at University of Windsor. The simulation was based on the experiments of [3] and used a scaled down model of the original Ahmed body as shown in Fig. 1a.

Based on previous studies [4, 6], the typical flow field behind a straight-back Ahmed body is shown in Fig. 1b. The flow that emanates from below the body resembles that of a wall-jet. This region is marked as WJ in Fig. 1b. The flow separates at the sharp-edges behind the straight-back Ahmed body forming the two recirculation zones (R1 & R2) as seen in Fig. 1b. This is a region of negative streamwise velocity. The flow velocity recovers to the free-stream velocity $U_\infty$ in the free-stream region (FS in Fig. 1b) above the body. A shear layer is formed between the wall-jet type flow and R1 starting at the lower sharp edge. Similarly, a shear layer is formed between the free-stream flow and R2, originating from the upper sharp edge. The combination of these flow features makes the

<footnote n="1">¹University of Windsor, Windsor, Ontario, N9G 3P4, Canada.</footnote>

<footnote n="2">²ESTACA West Campus, Laval, 78066, France.</footnote>

<footnote>*Corresponding Author: jesudha@uwindsor.ca</footnote>
Ahmed body wake flow inherently complex and thereby ideal for assessing the performance of the IDDES turbulence model.

<figure><img src="image_7.png" /><figcaption>Fig. 1a Sketch of the Ahmed body (b) Flow field behind the straight-back Ahmed body</figcaption></figure>

## 2. COMPUTATIONAL MODEL

As mentioned earlier, the simulation domain (Fig. 2) was based on the experiments of [3] to enable direct comparison. A structured grid composed of 7.6 million hexahedral cells was used in the simulation, with 15 prism layers near the walls to capture the boundary layer. Block refinements were used in the regions of interest i.e., near the Ahmed body, the wake region, etc. IDDES [7] along with $k-\omega$ Shear Stress Transport (SST) model was used in the present simulation. IDDES is a hybrid Reynolds-Averaged Navier-Stokes (RANS) – Large Eddy Simulation (LES) technique, were RANS models are used near the walls and LES away from it. It is less computationally expensive than the LES model, while still managing to resolve the anisotropic flow structures in the regions of interest [8]. The blending function of the IDDES model was monitored at different timesteps to ensure that LES was performed in the regions of interest. The solution was considered to be converged if the residuals fell below $10^{-6}$. The velocity field was monitored at several locations to ensure that steady-state is achieved. The mean quantities presented herein are averaged for 5 s after the achievement of steady-state condition.

<figure><img src="image_8.png" /><figcaption>Fig. 2 Details of the present computational domain.</figcaption></figure>
## 3. RESULTS AND DISCUSSIONS

Exhaustive validation of the simulation was carried out based on the AIAA guidelines for Validation and Verification of Computational Fluid Dynamics Simulations [9]. As a sample, Fig. 3 presents the velocity and Reynolds shear stresses at two streamwise locations ($x/h = 0.64$ & $1.75$) at a vertical ($x-y$) plane located at $z/h = 0.31$ ($h$ being the height of the Ahmed body). It is apparent that both the velocity and Reynolds shear stresses at the two streamwise locations agree well with the experimental results of [3]. It must be noted here that the vertical zero position is at the lower sharp edge of the Ahmed body, i.e., above the supports to match with the experimental results. It can be seen from Fig. 3 that the velocity is zero at the wall and increases to reach a positive peak within the wall-jet region (WJ). Beyond this, it decreases to become negative in the re-circulation region (R1 & R2). The streamwise velocity recovers in the free-stream region (FS) where $U = U_∞$. The positive and negative peak values of Reynolds stresses occur in the shear layers between WJ and R1; R2 and FS, respectively. It is apparent from Fig. 3 that the simulations accurately predict the different flow features in the wake region of the straight-back Ahmed body. Furthermore, the recirculation length predicted by the simulation was also compared with that of the experiments [3], and the difference was less than 5%.

To evaluate the flow structures in the wake region of the straight-back Ahmed body, the coherent structures were identified using $λ_2$ criteria. The instantaneous $λ_2$ isosurface coloured z-vorticity is presented in Fig. 4. The periodic shedding of counter-rotating vortical structures from the rear straight vertical edges of the body is marked in Fig. 4. These represent the legs of a hairpin like structure which is shed from the back of the straight-back Ahmed body. Similar structures have been observed in the wake of other sharp-edged bluff bodies [10]. The evolution of these complex vortical structures and the interaction with the trailing vehicles must be evaluated further to ascertain their influence in pollutant dispersion.

<figure><img src="image_9.png" /><figcaption>Fig. 3 Comparison between CFD and experiments results</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Fig. 4 Isosurface $\lambda_2$ criteria featuring with vorticity scalar on the wake of the 0° Ahmed body</figcaption></figure>

## 4. CONCLUSIONS

In this paper, we have compared the experimental data and CFD investigations in the wake of a straight-back Ahmed body. The results showed that the IDDES model accurately predicted all the relevant flow features of the wake flow behind the straight-back Ahmed body. Furthermore, the coherent structures in the wake flow of a straight-back Ahmed body was evaluated using $\lambda_2$ criteria. The results showed the presence of the hairpin like vortices being shed from the rear of the straight-back Ahmed body. The dynamics of the hairpin like structures and their role in pollutant dispersion will be further evaluated.

## REFERENCES

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[9] American Institute of Aeronautics and Astronautics. *AIAA guide for the verification and validation of computational fluid dynamics simulations*. American Institute of aeronautics and astronautics, (1998).

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