Ahmed Body
Ahmed Body
PART A-
AHMED BODY
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 Ahmed body is a generic car body (a simplified vehicle model). The airflow around the Ahmed body captures the essential flow features around an automobile. This model is also used to describe the turbulent flow field around a car-like geometry. Once the numerical model in validated, it is used to design new models of the car.
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 and the rear surface has a slant that falls off at 40 degrees.
Modeling Airflow over an Ahmed Body
In airflow over an ahmed body verification model, our Ahmed body has a 25-degree slant and is placed in the following domain, measuring 8.352-by-2.088-by-2.088 meters, to compute the flow field.
The front of the body is placed at a distance of 2 car lengths (2L) from the flow inlet. To reduce the computational cost, a symmetry plane is introduced to model half of the model.
The flow for this model is turbulent, which is based on the Reynolds number determined by the body length and inlet velocity. The simulation solves for the turbulent kinetic energy in addition to the velocity field. For this simulation, we need a larger mesh size than what is usually common to resolve the turbulent flow. More specifically, we use a finer mesh downstream of the model to capture the wake zone.
The flow for this model is turbulent, which is based on the Reynolds number determined by the body length and inlet velocity. The simulation solves for the turbulent kinetic energy and dissipation in addition to the velocity and pressure fields.
PART B-
REFINE THE MESH AROUND AREA OF INTEREST
Step 1
Model is first upload in space claim where using enclosure the model is tunneled as shown in figure and then model is opened in mesh tool
Step 2
And mesh size is define at 100 mm and multi zone used in method used and inflation layer is applied in different cases
Step 3
In CFD operation with respect to boundary condition velocity is 50m/s and density based solver is used.
CASE STUDY
Case 1- (default mesh)
As first case, meshing is done with default parameter (413.6mm) without inflation layer.
Velocity Contour
Pressure Contour
From above images we observe that awake region are not clearly visible so refine of mesh required and inflation layers required for better result analysis. So below there are some case studies with different layer and more and more refined meshes.
MESH PROFILE -
Case 2-(enclosed body)
Velocity Contour
Pressure Contour
Case 3-(enclosed body)
Velocity Contour
Pressure Contour
Case 4-(enclosed body)
Velocity Contour
From Above Velocity Contour and Pressure Contour obtain under academic license it’s clearly evident that results are similar and there is no much different, hence further chart study is done on the cases.
VELOCITY CHART
For theses plots we observe that velocity and pressure chart significant different
In velocity profile
Case1 default mesh velocity value is lower than other cases while the refined velocity profile are similar while y=0.2 velocity profile gets similar to all cases and y=0.4,the velocity becomes constant throughout.
PRESSURE CHART
In pressure profile
For default mesh pressure value as well vary much with refined profiles when y=0.2 pressure drop dramatic and awake region are created near y=0.4,further drop in pressure after arise happens due to vortex formation since flow is turbulence and its student edition proper study of result can’t be obtain.
For Grid dependency test, enclosure of much smaller dimension are used and value are compared with other case studies
VELOCITY CHART WITH MODIFIED GRID MODEL
PRESSURE CHART WITH MODIFIED GRID MODEL
Velocity chart not much changed observe but pressure chart obtain have different, reason due sizer enclosure the element size further reduces which give higher pressure value.
Conclusion from above result are
Mesh become finer when we increase the cell counts and formation of wake region become clearer which in turn help us to get more accurate result.
PART C
The Variation in Pressure why it goes to Negative-
Negative value of static pressure indicates that pressure value are lower than gauge pressure which is at zero, this happens due to following reason-
- Due to recirculation.
- To counter shear stress effects.
- When Velocity is increased over body to maintain the flow, pressure becomes negative.