Challenge

Week 3 - External flow simulation over an Ahmed body.

SSuraj Sarkar22 Jan 2023

1.INTRODUCTION

The Ahmed body is simplified car body used in automotive field to study the impact of the flow pattern on the drag. The external aerodynamics of the car defines many major traits of an automobile like stability, comfort and fuel consumption at high speeds. The flow around the vehicle is characterized by high turbulent and three dimensional flow separations as well as there is a growing need for more insight into the physical features of these dynamical flows. The Ahmed Body is a simplified car, used in automotive field to investigate the flow analysis and find the wake flow around the body. Ahmed body is made up of round front part, a moveable z slant plane in the rear of the body to study the detachment phenomena at different angles, and a rectangular box which link the front part and the rear slant plane. The principal objective to study such a simplified car body is to tackle the flow processes involved in drag production. Through perceiving the mechanisms involved in creating drag one can be able to design a car to minimize drag and therefore reducing fuel consumption and maximize vehicle
performance

Geometry

This is the sketch drawing:

In this project, the angle of bend (φ) is 20 degrees. The geometry can be imported from another CAD software or it can be created in ANSYS. I imported a .STEP file for this project in SpaceClaim.

Since this is a complex simulation and the ahmed body is symmetrical along the z axis, the geometry was split into two along the z axis using ANSYS's split tool.

This is the geometry after the spliting:

GENERATING SIMULATION FOR BASE MESH

OUTER ENCLOSER :

Right face is located at a distance of 5m from the right side face of Ahmed body along the X axis.
Left face is located at a distance of 2m from the left side face of Ahmed body along the X axis.
Top face of the encloser is at a distance of 1 m from the top side face of the Ahmed body along the Y axis.
The bottom face of the encloser is attached to the bottom face of the Ahmed body along the YZ plane.
The front and back face of the encloser is at a distance of 0.5 m along the Z axis.
Make sure the original extrusion is suppressed before closing SpaceClaim.

MESH GENERATION :

SOLVER SETUP:

We are using K-E model as viscous property and taking the initial Velocity as 25m/s. Pressure based solver and steady state simulation is to be generated ,We are going to observe the Velocity & Pressure contours to estimate the results and which can be useful in further future modefication.

Velocity contour plot:

Pressure Contour Plot:

Drag Coefficient:

From this run the results are not upto what we want so we make mesh more finer to capture the wake region.

The flow around the Ahmed reference body is complicated. Due to the slant in the rear of the vehicle,flow separation and counter rotating vortices are generated at the slant edges. The drag forces of Ahmed Body reaches at maximum when the slant angle is 30 deg. For slant angles higher than this value, the adverse pressure gradient in between the slant and the roof is so intense that the flow fully detaches over the slant. Below this critical slant angles (30 degree), the flow still separates but the pressure difference between the slant region and the side walls is still sturdy enough to generate substantial stream-wise vortices at the lateral slant edges. These prompt a downward motion over the slant, mainly in the downstream part. As a result, the flow separating at the upstream end of the slant can coupled further to the downstream. The flow around the Ahmed body has several flow separations from the front to rear of the vehicle. The flow recirculation caused by these flow detachment contributes the vehicle’s drag. The location point at which the flow separates determines the size of the separation zone, and accordingly the drag force, thus an exact simulation of the wake flow and of the separation process is essential for the accurate result of drag predictions.

INNER ENCLOUSER :

Right face is located at a distance of 1 m from the right side face of Ahmed body along the X axis.
Left face is located at a distance of 0.5 m from the left side face of Ahmed body along the X axis.
Top face of the encloser is at a distance of 0.5m from the top side face of the Ahmed body along the Y axis.
The bottom face of the encloser is attached to the bottom face of the Ahmed body along the YZ plane.
The front and back face of the encloser is at a distance of 0.26 m along the Z axis.

Initially there was a huge interference between the Outer and Inner encloser so that the mesh gets overlapped which gets not acceptable so to avoid this we made share topology and get deleted the Interference region that by using the interference command after this we get a good interaction between the Inner and Outer enclosers so that we can get the mesh properly and also share the boundaroies for the uid ow

MESHING :

In this meshing we can assign the named selection for Inlet,Outlet,Symmetry wall and Car Wall,We can use the different type of meshing process for this model.

In this we follow three stages,

1. Meshing will be done for the outer enclouser choosen as a method of the Multizone and this will create the Hexa mesh,The element size used for this is 0.08m.

2.Next for inner enclouser we choose body sizing method to make more refinement,the element size we use for this is 0.04m.

3.Next we select the face sizing method to select the legs of the body to capture them with better refinement,we take the element length as 0.005m.

4.Next to capture the flow over the body we make the inflation layer which helps us in caputuring the flow very finer way and the mesh will also be capture along the body perfectly.We take the number of inflation layers as 6, Total thickness value as 0.0036.

SETUP :

Solver type : Pressure based

Check the Energy equation

Turblent Model : K-Omega SST model

Fluid Material : Air

Boundary conditions

Inlet velocity : 25 m/s

Outlet pressure : 0 pa

Symmetry : For side walls

Wall : Bottom ,

Creating a surface plane to observe the contours.

RESULTS :

VELOCITY CONTOUR :

The above figure represents the velocity vector over the Ahmed body.we can see clearly that the because of the rening mesh the wake formation is captured properly at the rear end of the Ahmed body and reined it futher more to get the more wake region.

After the calculations we get the value of the Drag Coeff value as 0.36 ,Drag Force 16 and Lift Coeff value as 0.225,Lift Force 10.

FINAL REFINEMENT :

MESHING: