Ahmed Body Challenge

Aim:

The aim of this challenge is to perform steady state simulation on an Ahmed body with incoming air at 25m/s velocity.

Introduction:

The Ahmed Body was first created by S.R. Ahmed in his research “Some Salient Features of the Time-Averaged Ground Vehicle Wake” in 1984. Since then, it has become a benchmark for aerodynamic simulation tools. The simple geometrical shape has a length of 1.044 meters, height of 0.288 meters, and a width of 0.389 meters. It also has 0.5-meter cylindrical legs attached to the bottom of the body

Ahmed body is a generic car body (a simplified car model) i.e the flow of air around Ahmed body captures the essential flow features around an automobile.

Case Setup:

Base Mesh Case 

1. Mesh Description

Base mesh size was used as default, of size 0.435 m size

Triangular Mesh,

Number of Nodes: 17710

Number of elements: 90167

Skewness: majorly in 0-0.6

2. Case Setup

Steady solver was used. As velocity is 25m/s, less than 0.3M, so pressure-based solver was selected.

k - ɛ turbulence model was used with Coupled method and under relaxation factors with Pressure as 0.1 and Momentum as 0.3

3. Solution

Improved Mesh Case

1. Mesh Description

Multizone hexahedral mesh was created for outer enclosure with 100mm size. Inner enclosure was improved by body sizing and face sizing method.

Number of Nodes: 77506

Number of elements: 151316

Skewness: majorly in 0-0.6

2. Case setup

Steady solver was used. As velocity is 25m/s, less than 0.3M, so pressure-based solver was selected.

k - ꞷ SST turbulence model was used with Coupled method and under relaxation factors with Pressure as 0.1 and Momentum as 0.3

Desired y+ value taken as 50, so first layer thickness was calculated to be around 6mm,  5 inflation layers with 1.2 increasing rate.

3. Solution

Conclusion:

Numerical analysis of flow over Ahmed body is understood using both k-ɛ and k-ꞷ models. It was observed that, on the basis of boundary layer formation over the body, appropriate viscous and turbulence model, wake region was captured.

Pressure drag occurs when air flowing past an object pushes harder against the front than the back. This difference creates a backward force. When separation of flow occurs, the turbulent air behind the Ahmed body decreases in pressure, causing an increase in pressure drag.

Flow separation or boundary layer separation is the detachment of a boundary layer from a surface into a wake. Separation occurs in flow that is slowing down, with pressure increasing.

Grid dependency tests were performed to compare better capturing of wake region.