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Challenge

Week 3 - External flow simulation over an Ahmed body.

RRaghavendra Reddy17 Aug 2021

External flow simulation over an Ahmed body

Aim: Simulation of external flow over Ahmed Body.

Introduction:

In this challenge, we will be simulating flow a special body, called Ahmed Body.

Ahmed body was developed by scientist named Ahmed, hence the name. The reason for using this particular shape is, some of the main characteristics of the flow field can be obtained from simulating Ahmed's body like velocity, pressure, flow separation, wake region, drag , lift etc

Ahmed Body and its importance:

The Ahmed body is a generic car shaped body (a simplified vehicle model). The airflow around the Ahmed body captures the essential flow features around an automobile and was first defined and characterized in the experimental work of Ahmed. Although it has a very simple shape, Ahmed body allows us to capture characteristic features that are relevant to bodies in the automobile industry.

This body is used to understand the flow field arround a vehicle, to understand flow separation and wake etc. The dimensions of the Ahmed body are standardised and a lot of data has been generated through experiments for the flow field arround this body. Hence this is used to validate any software or solver. The Ahmed body is a benchmark model widely used in the automotive industry for validating simulation tools. The Ahmed body shape is simple enough to model, while maintaining car-like geometry features.

The Ahmed body looks like

In detailed manner, we will be describing the procedure followed in Ansys Fluent.

Procedure followed:
a) Creating Geometry
b) Meshing
c) Setup
d) Results
a) Creating Geometry:
In Space Claim, we import the geometry which was made in a CAD modeling softwares. We can use the Space Claim to make geometries as well. The model in the Space Claim after importing looks like
We want the flow field to be solved arround the Ahmed body. It means the mesh is required arround the Ahmed body. So, we need volume arround the Ahmed body which represents the wind tunnel. To do so, we create an enclosure arround the Ahmed body by going to Prepare in the tools and selecting Enclosure option. Then selecting the Ahmed body and click green tick mark. Choose appropriate dimensions for the enclosure.
The window looks like
As the body is symmetric, to reduce the computational effort we will split the body into two and delete on part. This is done using the split command in the design in the tool bar.
The model looks like
b) Meshing:
In Meshing window, we will be giving names for the boundaries for the model. This is done using named selections option.
Case:1:
Basic Mesh:
Case:2:
In this case enclosure, with in the enclosure was created to improve the mesh quality.
We will be using the options available in improving the mesh quality. Some of them used are sizing, inflation etc.
Basic Mesh:
Mesh after refinement:
Case:3:
Mesh obtained after furthur refinement:
c) Setup:
In the setup, we choose,
In setting up physics, we select pressure based, steady state, k-epsilon standard.
No energy equation is selected as there is no need to calculate temperature.
Material is by default air.
Input boundary conditions.
In post processing, we create planes and contours.
In solving, we create animations.
d) Results:
The following results were obtained for each case, ran with different mesh.
Case-1:
Case-2:
Case-3:
Conclusion:
The following results were obtained after running the simulation for all the three cases.
No of elements No of nodes Lift Coefficients
Coarse Mesh 42803 8629 0.386627381
Medium fine Mesh 291190 83444 0.2904454
Refined Mesh 380855 110185 0.2861473

The results got better after refining of the mesh. But still there is scope for the furthur improvement, as we can clearly see the results are not converged at.

The negative pressure in the wake region is because the body is bluff body(not a stream lined body). The stream line cannot follow the surface of the body due to sudden changes in the cross-section of the body.As there is in-sufficient air to fill the back of the Ahmed body the pressure is low. This difference in the pressure on front and back side contribute to pressure drag.

Flow separation occur, when the flow takes place in adverse pressure gradient. The point where the boundary layer moves away from the body is called Separation point. In order to have low co-efficient of drag, we need to avoid flow separation and sudden changes in cross-section.

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