Ahmed Body Challenge

• AIM: Simulation of Ahmed body and with different slant angles.

We can divide the methodology into different parts

1. Ahmed body introduction
2. Point of extraction
3. Negative pressure behind wake region
4. Baseline study
5. Mesh independent study
6. Comparion of results
7. Result for slant angle 30 degree
8. Result for slant angle for 40 degree.
9. conclusion

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• Ahmed body:
• Ahmed bodyis a generic car body first created by S.R. Ahmed in his research “Some Salient Features of the Time-Averaged Ground Vehicle Wake” in 1984. (a simplified car model) the flow of air around Ahmed body captures the essential flow features around an automobile. 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 20 degrees in this condition

The Drag coefficient quantifies the resistance of an object in a fluid environment. It is not an absolute constant for a body’s shape because it varies with the speed and direction of flow, object shape and size, and the density and viscosity of the fluid. The lower the drag coefficient of an object, the less aerodynamic or hydrodynamic drag occurs.

• Point of separation
• The position at which the boundary layer leaves the surface of a solid body. It may occur with the laminar flow (laminar separation) or when the boundary layer is turbulent (turbulent separation).

At the point of separation, shear stress is zero. Adverse pressure gradient plays an important role for the separation of flow. 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.This adverse pressure gradient (i.e. flow of fluid inside the boundary layer flowing opposite to the flow of fluid directed over the body because of the motion of the body) then creates a difference in pressure ahead of the body and behind the body which increases the overall drag.

in order to Decrease the drag force The separation layer should be at late stage, If the point of separation is happening at an early stage The drag force will be more.

• How wake region occurred behind the flow?
• When the pressure drag is dominating it is called Bluff body, Here Ahmed's body can be called a bluff body. At the boundary layer separation happened around the body Pressure is maximum at the stagnation point. After that favorable pressure, the gradient starts decreasing and the adverse pressure gradient starts increasing due to the separation of the boundary layer. Consequently, the flow separates from the surface and creating a highly turbulent region behind the cylinder called the wake. Eventually, net force will be negative at that zone due to the adverse pressure gradient.
• Due to more burning of fossil fuels and pollution issues, the study of the reducing drag is a necessity. By reducing the drag flow patterns can be changed and increase environment safety.
• From one of the result, we can conclude that after the point of separation around rear body pressure is decreasing behind the body , where net pressure is becoming negative.Therefore wake region taking place.

Here velocity is taken as 25 m/s

calculating Reynolds number first If the flow is turbulent or laminar

`Re=(rho. v .l)mu`

`mu`For air at 300k is`18.45*10^-6(kg)/(m.s)`

Commonly an external flow said to be turbulent flow when its reynold's number is higher than 500000.Hence it is a case of turbulent flow.

• Before going to the analysis stage Understanding the concept of Y+ and first layer thickness inflation layer.
• While understanding the concept of boundary layer theory boundary layer is composed in 3 parts , nearest one is the viscous sublayer , next is the log layer and the third one is called as outer layer or turbulent region.
• Y+ is nondimensional number , is directly related with first-layer thickness of your mesh. Choosing the value of Y+ can depend on the choice of turbulence model.
• For accurate result y=Y+ value must be within viscous layer. For this layer Y+ value must be within 30. which is also called near-wall approaching.
• The k-ε models, the RSM, and the LES model are primarily valid for turbulent core flows (i.e., regions somewhat far from walls). Consideration therefore needs to be given as to how to make these models suitable for wall-bounded flows. The Spalart-Allmaras and models were designed to be applied throughout the boundary layer, provided that the near-wall mesh resolution is sufficient.
  

• Wall function approach saves computational resources, because the viscosity-affected near-wall region, in which the solution variables change most rapidly, does not need to be resolved. The wall function approach is popular because it is economical, robust, and reasonably accurate. It is a practical option for the near-wall treatments for industrial flow simulations.

As we are doing wall approach Y+ value will be above 30.

• Baseline setup
• Fluent methodology is given below. First initiating with a baseline setup.
• Upload IGES or step file in space claim.

• We will split the body and use only half for the simulation, as the Body is symmetric along the length.

• Make body enclosure around Ahmed body for boundary conditions with given dimensions.

• Final geometry will look like below image.

• we will do unstructured mesh and generate the result. Generating the unstructured mesh with suppressed Ahmed body as we are only interested in external flow.element size is taken by default.

• Boundary conditions are given below.

• Setup for the domain:

• Solver : pressure based
• Model : k-e realizable with scalable wall functions and curvature corrections.
• Inlet velocity : 25 m/s
• Inlet temp : 300K
• Standard initialization is used with 500 iterations, time scale factor is considered is 1
• Reference values are as given below.

Drag force and lift force value is given below.

residuals

• Drag force and lift force graph

• Velocity contour

• Pressure contour

• Mesh independency test
• CASE 1
• For the mesh independency test, we will go for coarse finer and finer mesh and compare the results.
• For the geometry, purpose adds small enclosure in geometry which will be defining the geometry environment at the simulation time. share the topology and cut down the common body using split command.

• Space claim model

• Meshing of the domain
• First shell mesh thickness calculations given below. As we are going for wall approach. assuming Y+80. the first shell thickness will be 1.17 mm

• We have done structured mesh for outer enclosure and instructed mesh for the inner enclosure. The inflation layer is used on the car wall for better data capturing.

• for Outer closure multizone method is used with element size 50 mm.
• For inner closure, the slightly finer unstructured mesh is used with size of 30mm.
• Inflation layer is used growth rate is 1.1 and the first layer height is 1.1 with 10 inflation layer.

• Upload the geometry in setup scenario

• Solver , BC and turbulent setup will be same as base line seup.

Drag force and lift force values are given below

• residuals

• Drag coefficient and lift coefficient

• Velocity contour

• Pressure contour

• Case 2 (Coarse mesh)

Outer core element size: 70mm

Inner enclosure size: 50mm

Y+=160

Inflation layer 10

First mesh size : 2.3mm

Solver k-e realizable

• Drag and lift coefficient value

• Residuals

• Drag coefficient graph

• Lift coefficient graph

• Velocity contour

• Pressure contour

• CASE 3 Finer mesh

Outer closure element size: 30mm

Inner enclosure:15mm

Inflation layer

: 10

Y+=70

First mesh size: 1.02mm

Solver k-e realizable

• Drag and lift the coefficient value

• Residuals

• Drag coefficient

• Velocity contour

• Pressure contour

• Post processing

Here the results of one of the coarser mesh showing wake region of pressure contour and vector os velocity and pressure.

The slant angle plays primary role for generation wake region as you can see at trailing edge pressure is increasing in compare to leading edge while going downside. the total net force due to adverse pressure gradient is negative.

• Pressure vector contour

• Velocity vector contour

• Velocity vector contour for case 1

• Comparison of the cases

• For slant angle 30

• Drag and lift coefficient

• Lift and drag coefficient graph

• Velocity contour

• Wake region

• For slant angle 40

• Drag and lift coefficient

• Velocity contour

• Comparison of velocity contour

• As per the point of separation, Drag value is changing for different slant angles.
• Comparison of results

By increasing the angle there is very little difference in drag. But from experimental data above 40 degrees, there will be a drastic change in the drag coefficient.

• Conclusion:
• Simulation of the Ahmed body is calculated with some remarkable results from experimental data.

• Reference research paper
• http://www.kdm.p.lodz.pl/articles/2018/1/22_1_21MOST
• https://www.researchgate.net/publication/330383775_Experiments_and_numerical_simulations_on_the_aerodynamics_of_the_Ahmed_body
• https://www.researchgate.net/publication/266883948_Flow_and_Turbulence_Structures_in_the_Wake_of_a_Simplified_Car_Model_Ahmed_Model