accurate prediction of the separation process and the wake flow through ahmed body CFD analysis

The Ahmed body is made up of a round front part, a movable slant plane placed in the rear of the body to study the separation phenomena at 250, 350 angles, and a rectangular box, which connects the front and rear slant plane. The most significant feature of the body is the 250 , 350 angle of rear slant. Air is used as a working fluid. The inlet velocity of fluid is 50 m/s. k-epsilon turbulent model used as a standard model in this case..

With The help of Ahmed body investigation has obtained the behaviour of newly developed turbulence models for complex geometry cases, a simplified car model, known as the Ahmed body, The Ahmed body is made up of a round front part, a moveable slant plane placed at back part of the body to study the separation phenomena and coefficient of drag at different angles, and a rectangular box, which connects the front part and the rear slant plane. As the wake separation flow behind the Ahmed body is the main contributor to the drag force, accurate prediction of the separation process and the wake flow are the key to the successful modelling all cases.

Objective= HEre in this challenge we have to perform a external flow simulation over a ahmed body,by making enclousers over it and meshing is done in prescribed way as per the area of intrest, Also we have to carry out velocity profile at MID-CUT plane, The results are carried out for 3 cases by performing grid dependency test and the results are compared after the test for all three cases.

Set-up and pre-processing settings for the simulation:-

After fluent flow drag , import the model into the geometry model, and open space claim, here we have to create two enclousers ,One is for outer coarse meshing and second is smaller one for refined meshing as that was this our area of intrest, During enclouser creation make sure that there is no intresection between the two enclouser, After that supress the solid body of Ahmed body from physics so that it could\'nt get meshed in the meshing ,module. By clicking the object tree selection ,Make sure shre topology is ON , so that both the enclouser can share the information.

After all the above procedure meshing was open-up and wait for the geometry to load-up, here i created a mesh for both the enclouser in a prescribed way so that it help to get good results,

1. outer enclouser is set to multizone meshing method with 100mm element sizing.Also the MAx size is defined as 100 MM.

2. for internal enclouser , body sizing is set 50mm.

3. and the last the lower hole is provide a spacing of 5MM.

4. For meshing ahmed body surface Infaltion layer was specified with total thickness of 12.6 mm.

After That fluent was open-up for setting up some boundary conditions, After sucessfully display of mesing Check for mesh.

The initial setting is set for absolute velocity formulation and density based solution because here the mach no. is 0.3 or lesser so to capture velocity and pressure distribution correctly density based solver is best, also the energy equation is turned ON .And viscous (k-epsilon turbulent model is choosen).

material is air, Inlet velocity of air is 50m/s and the outlet pressure is set to 0 gauge pressure as the absolute pressure is 1ATM.

After this hybrid initilaization is done , then i created acut plane normal to axis Z so that i could plot a velocity countours on that.

Calculation settings=

In this way all the setting and boundary conditions are set-up and the analysis was ran for 500 itteration ,Then the pressure plots and the velocity profile and vectors along with the graphical results are obtained from the CFX-post.for all three cases, The Three cases are only having the mesh size difference.

case 1 results.

Pressure across ahmed body, mid plane velocity,velocity vector, streamlines, lines for the graph plot,and the chart is listed above in graphical format,

The pressure is going negative Across the side wall of ahmed body because when we observe the velocity profile care fully,It seems that the velocity is more across that region,and

as the velocity is going approx 47-20 it means there is high pressure region created across that area and due to countineous flow the pressure is become negative.

Also the plots are prepared depending on the velocity at diffrent location in the enclouser. The data is exported on excel sheet.

Case=2 Results

enclouser 2 body sizing is set to 45MM

The results for the secondcase is listed above , Here there is difference in pressure distrubution and velocity distribution also the graph are varrying the velocity plot is getting smoother across the same field line as we go on decreasing the mesh size across the second enclouser. Also it is observed that there is negative velocity at the side ends of enclouser 1 ,It means that there is a negative pressure .

Case=3 Results

enclouser 2 body sizing is set to 42MM

here the results obtained isn aproximtely similar to second case as there is very less diffrence between the mesh sizing.

Conclusion:- By performing the grid dependency test it was clear that as we go on reducing the mesh size we re getting the good results and smooth curve for velocity at both the base lines created. Also the velocity variation and pressure variation are changing across the refined meshed region.