Week 3 - External flow simulation over an Ahmed body.
Ahmed Body & Its importance
It is a simplified geometry for performing simulations considering flow over car. This generic model was developed to test road vehicles in wind tunnel . It was developed by sir S. R Ahmed and that is how it got its name.
Dimensions of an ahmed body:
Because we have the validated experimental results of the ahmed body and because it is not feasible to test each and every model in a wind tunnel. We check the accuracy of our simulation tools by using flow over ahmed body and validating the results .It is a benchmark for aerodynamic simulating tools.
Explain the reason for the negative pressure in the wake region.
As the flow of air travels from the front of the body to its slanted back, depending on the slant angle (if it is more than sufficient to keep the flow attached to it), the flow detaches itself from the body. As the flow proceeds , suddenly at the rear end of vehicle , the area for the flow increases, the velocity of the flow decreases . As the kinetic energy of the flow decreases, it results in a positive pressure gradient and hence resulting in an increase in pressure in the direction of flow. Because of this pressure gradient, flow starts moving backwards and detaches itself.
Explain the significance of the point of separation.
Point of separation forms at the rear end of vehicle where drag is generated in the wake region.30 degree is the critical angle of slant where separation occurs .Point of separation is highly undersirable phenomenon and can be delayed in turbulent flows. Point of separation also depends on the shape and orientation of the body .Because of flow separation, low pressure region forms where at some specified conditions where even vortex shedding can occur.
Step1: Geometry:
First prepare an enclosure around the ahmed body to create a domain where fluid will flow on the body . Enclosure is just all the domain minus the ahmed body . Secondly, we prepare the second enclosure around ahmed body which contains the ahmed body more closely.
Step2: Generating Mesh:
Method used-Multizone (it creates a hexahedral mesh)
Sizing of 100mm for the bigger enclosure and a smaller mesh size for the enclosure near the car.
6 inflation layers are inserted around the car wall.First layer thickness=5mm
Step3: Setup : Pressure based steady state solver and turbulent type flow is used is setup. Refrence values are given from inlet and area calculated is the frontal area of the car wall in the x direction. Length of body is 1044mm . Calculation is done for 500 iterations and results are observed.
Observations:
Mesh size for inner Enclosure: 100mm, Mesh size for car wall-10mm
Elements:283926, Nodes:89530
Cd=0.300288
Cl=0.24730
Coeffecient of lift:
Coeffecient of drag:
Residual:
The solution converges after 200 iterations.
Velocity contour:
Pressure contour:
From the pressure contour we can see that at the front of the body, the flowing fluid stops and this is the stagnation point where high pressure develops. As the flow travels forward from the front towards the back, the curves provide a favourable pressure gradient and fluid flow converges which leads to increased velocity and low pressure. As the fluid reaches back of the car , the fluid diverges , becomes low in velocity as it decelerates and hence pressure increases at the back of the car wall.
Velocity contour:
Vector chart:
Chart :
Blue is for velocity at x=3 and red for velocity at x=2m . The red line is the one closer to the ahmed body and hence , it shows a low velocity region for a greater portion of y axis as compared to blue line.
Mesh size for enclosure = 50mm, mesh size for car wall=20mm
Elements=115521, 36873
Cd=0.33
Cl=0.302
Coeffecient of drag:
Coeffecient of lift:
Residual:
Velocity contour:
Pressure contour:
Velocity contour:
Vector chart:
Velocity on y axis
Mesh Size-50mm, car wall mesh size=30mm
Elements-81007, Nodes-26297
Cd=0.3
Cl=0.3
Residual:
Coeffecient of lift:
Coeffecient of drag:
Velocity contour:
Pressure contour:
Velocity contour:
Vector chart:
Velocity in y direction
Conclusion: It can be concluded that grid size does not affect the value of Cd which remains 0.3 constantly. The drag coffecient for an ahmed body is very less which means the drag force is less and helps with lesser fuel consumption for vehicle.