Ahmed Body Challenge

As we mentioned, Ahmed body is the prototype of the automotive car and we run the simulation for calculating the drag and lift force on the Ahmed body. The resistance of an object in a fluid environment is quantified by the drag force. It varies with speed and direction of the flow, Object shape, and size and the density and viscosity of the fluid. The lower the drag coefficient of an object, The less aerodynamics or hydrodynamic drag occurs. In terms of a car, the lower the drag coefficient, the more efficient the car is. The wake flow behind the car is the location at which the flow separates determines the size of the separation zone, and consequently the drag force. So we need to do more exact simulation of the wake flow and of the separation process is essential for the correctness of drag predictions. However, a real-life automobile is a very complex shape to model or to study experimentally. Therefore the Ahmed body is best to use. This also helps us to achieve efficient vehicles with minimized fuel consumption, CO2 emissions, and noise, while keeping good driving stability.

Also, Car with low drag is better but decreasing the drag drastically can reduce the downforce and lead to a loss in road traction and a higher and a higher chance of car accidents.

Wake region:

Consider a fluid particle flows withing the boundary layer around the circular cylinder. From pressure distribution measured, the pressure is a maximum at the stagnation point and gradually decreases along the front half of the cylinder. The flow stays attached in this favorable pressure region as expected. However, the pressure starts to increase in the rear half of the cylinder and the particle now experiences as an adverse pressure gradient. Consequently, the flow separates from the surface and creating a highly turbulent region behind the cylinder called 'WAKE'. The pressure inside the wake region remaining low as the flow separates and a net pressure force (pressure drag) is produced.

Flow Separation:

The presence of the fluid viscosity slows down the fluid particles very close to the solid surface and forms a thin slow-moving fluid layer called a boundary layer. The flow velocity is zero at the surface to satisfy the no-slip boundary condition. Inside the boundary layer, flow momentum is quite low since it experiences a strong viscous flow resistance. Therefore, the boundary layer flow is sensitive to the external pressure gradient (as the form of a pressure force acting upon fluid particles). If the pressure decreases in the direction of the flow, the pressure gradient is said to be favorable. In this case, the pressure force can assist the fluid movement and there is no flow retardation. However, if the pressure is increasing in the direction of the flow, an adverse pressure gradient condition as so it is called exist. In addition to the presence of a strong viscous force, the fluid particles now have to move against the increasing pressure force. Therefore, the fluid particles could be stopped or reversed, causing the neighboring particles to move away from the surface. This phenomenon is called the boundary layer separation.

The pressure gradient is an is one of the factors that influence a flow immensely. As discussed above, A negative pressure gradient is termed a Favourable pressure gradient. Such a gradient enables the flow. A positive pressure gradient has the opposite effect and is termed the Adverse Pressure Gradient. Fluid might find it difficult to negotiate an adverse pressure gradient. Sometimes, we say the fluid has to climb the pressure hill. One of the severe effects of an adverse pressure gradient is to separate the flow.
Consider flow past a curved surface as shown in Fig. The geometry of the surface is such that we have a favorable gradient in pressure to start with and up to a point P. The negative pressure gradient will counteract the retarding effect of the shear stress (which is due to viscosity) in the boundary layer. For geometry considered we have an adverse pressure gradient downstream of P.
Now the adverse pressure gradient begins to retard. This effect is felt more strongly in the regions close to the wall where the momentum is lower than in the regions near the free stream. As indicated in the figure, the velocity near the wall reduces and the boundary layer thickens. Continuous retardation of flow brings the wall shear stress at the point S on the wall to zero. From this point onwards the shear stress becomes negative and the flow reverses and a region of recirculating flow develops. We see that the flow no longer follows the contour of the body. We say that the flow has separated. The point S where the shear stress is zero is called the Point of Separation.

Modeling of Ahmed-Body:

While calculating area: (Keep Ahmed Body in front-facing direction)

A=(0.288*0.389)+2*(0.05*0.03).. (2 times because two legs visible at front side)

2.Import the geometry to Ansys SpaceClaim:

3. From a bigger computational domain using the Enclosure option in Spaceclaim as per the below dimension.

The bigger enclosure will be:

4. Our geometry of Ahmed body is perfectly symmetric so using Split Body option in Spaceclaim allows us to reduce nos. of cells and brings out an accurate result using lesser nos. of cells at a lower computational time. Here, we are simulating only half of the complete domain.

Domain after Split Body: After unclicking on Ahmed-Body:

Step2: Mesh

Start with BaseLine Mesh to checkup the setup

(After Section-Plane)

Also, Provide Boundary Names:

Step3: Fluent-Setup

check the mesh first and after that define the solver. Here we used the density-based solver in steady-state simulation because mach is greater than 0.3 and for this condition, density-based solvers give a much accurate result as compared to the pressure-based solvers.

Viscous Model-
We are here working on velocity i.e. 25m/s. Reynold's number also will be higher hence adding the Turbulence model is our necessity.

Checked Energy equation.

Model - K-epsilon (2 equations)
K-epsilon model- Standard
Near-wall Treatment- Standard wall functions

Material- Air with default properties.

Boundary Conditions-

Inlet- Type= Velocity inlet
Velocity Magnitude= 25 m/s
Temperature= 300 K

Outlet- Type= Pressure outlet
Gauge Pressure= 0 Pa

Symmetry- Type= Symmetry

Walls- Type= Wall
Wall Motion= Stationary wall
Shear conditon= No slip
Roughness model= Standard

Results:

Residual plot:

Velocity-contour plot:

Pressure-contour plot:

CoEfficient of Drag: 0.39

CoEfficient of Lift: 0.29

Step-4: Post-Processing

Pressure-contour:

Velocity-contour:

Vector representation of velocity:

Velocity plots for different positions:

Since we run the set-up for a baseline mesh we get the expected results as shown above but as we study on the velocity contour and velocity vector, we get some diffused solution because of the unstructured mesh also get the velocity vectors in some unexpected regions. In computational fluid dynamics, we are very sensitive to selecting the correct meshing which mainly depends on the element size. Normally we did a mesh sensitivity study and after getting the appropriate residual graph for convergence we take it for the problem.Also, there is 62.5% error in the drag coefficient.