Week 3 - External flow simulation over an Ahmed body.
External flow simulation over an Ahmed body
Aim: Run the simulation for the velocity of 25 m/sec with the default air properties in fluent.
Objectives:
1. Velocity and pressure contours.
2. The drag coefficient plot for a refined case.
3. The vector plot clearly showing the wake region.
4. Perform the grid independency test and provide the values of drag and lift with each case.
Theory:
Ahmed body: The Ahmed body is a generic car body (a simplified vehicle model). The airflow around the Ahmed body captures the essential flow features around an automobile and was first defined and characterized in the experimental work of Ahmed . Although it has a very simple shape, Ahmed body allows us to capture characteristic features that are relevant to bodies in the automobile industry.
Significance of the ahmed body:
- It helps in validating CFD codes and results of different turbulent models since flow over ahmed body is studied and has lot of experimental data with which we can compare our results and CFD codes to know if they are correct.
Flow seperation:
Flow separation or boundary layer separation is the detachment of a boundary layer from a surface into a wake. Separation occurs in flow that is slowing down, with pressure increasing, after passing the thickest part of a streamline body or passing through a widening passage, for example.
Flowing against an increasing pressure is known as flowing in an adverse pressure gradient. The boundary layer separates when it has travelled far enough in an adverse pressure gradient that the speed of the boundary layer relative to the surface has stopped and reversed direction. The flow becomes detached from the surface, and instead takes the forms of eddies and vortices. The fluid exerts a constant pressure on the surface once it has separated instead of a continually increasing pressure if still attached. In aerodynamics, flow separation results in reduced lift and increased pressure drag, caused by the pressure differential between the front and rear surfaces of the object.
Effect of flow seperation:
- Creates adverse pressure gradiend behind the flow seperation, Creates wake region
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When the boundary layer separates, its remnants form a shear layer and the presence of a separated flow region between the shear layer and surface modifies the outside potential flow and pressure field. In the case of airfoils, the pressure field modification results in an increase in pressure drag, and if severe enough will also result in stall and loss of lift, all of which are undesirable.
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For internal flows, flow separation produces an increase in the flow losses, and stall-type phenomena such as compressor surge, both undesirable phenomena.
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Another effect of boundary layer separation is regular shedding vortices, known as a Kármán vortex street. Vortices shed from the bluff downstream surface of a structure at a frequency depending on the speed of the flow.
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Vortex shedding produces an alternating force which can lead to vibrations in the structure. If the shedding frequency coincides with a resonance frequency of the structure, it can cause structural failure.
Base Setup:
As the body is perfectly symmetric, we can run the simulation by considering the only half body. We can save the number of cells and reuce the simulation time.
1.Geometry:
- Import ahmed body into space claim
- Create enclosure around the ahmed body by selecting enclosure option in prepare toolabar
2. Meshing :
1.Create named selections
2. create base mesh: As we can see the mesh near the ahmed body is coarse mesh, we refine it in further cases.
Setup:
Solver,material and turbulent model:
Find the projected area of car wall normal to the flow and subtitue the value into reference values to get correct values of cd and cl.
Hybrid initialize the setup and create velocity contour and solution animation, Create force reports to get the values of cd and cl
Plots and Values:
Residuals:
Drag coefficient:
Lift coefficient:
Values of Cd and Cl:
Velocity contour:
Velocity contour animation:
Vector plot of velocity:
Refining the mesh: Creating enclosure
1.Geometry:
Create enclosure around the ahmed body, and create second enclosure close to the ahmed body so that we can create finer mesh close to the ahmed body.
select interference from prepare toolbar and fix the mesh entanglment between two enclosures we have created.
Create plain on the origin along the xy plane normal to z axis, use split by plane option and split the ahmed body along with the enclosures.
Select share option, this enables the information to be shared between two enclosures.
Meshing:
- Insert method in meshing and select multizone to create hexahedral mesh.
- Insert sizing for enclosure around the ahmed body and for two small bottom supports
- Create inflation layer around the ahmed body wall
Mesh:
Setup:
Solver,method,material and model is set same as for base model.
Plots and Values:
Residuals:
Drag coefficient:
Lift coefficient:
Values of Cd and Cl:
Velocity contour:
Pressure contour:
Velocity contour animation:
Vector plot of velocity:
Further refinement with Y+ value:
We can refine it further using y+ and insert exact wall spacing values for accurate values of coefficient of lift and coefficient of drag.
Since we have used k-epsioln turbulent model, Y+ values need to be between 30 - 300, in our case we will use y+ value as 200
Calculation of wall spacing:
`Deltas = (Y^+mu)/(U_(Fric)rho`
Where:
`U_(Fric) = sqrt(tau_(wall)/(rho))`
`tau_(wall) = (C_(f)rhoU^2)/(2)`
`C_f = 0.026/(Re^(1/7))`
`Re = (rhoUL)/(mu)`
Where:
`U` = Free stream velocity
`Re`= Renolds number
`rho`= Density of air
`L`= length of boundary layer
`tau_(Fric)`= Shear stress
`mu`= viscocity
`C_f`= skin friction coefficient
Calculation:
Given:
`rho = 1.225 ((kg)/(m^3))`
`mu = 0.000017894 ((kg)/(ms))`
`U = 25 (m/s)`
`L = 1.004 m`
`Y^+ = 200`
We will use online grid spacing calculator tool to find accurate value of wall spacing
We get wall spacing value as:
`Deltas = 0.0028579 m`
Use this value of wall spacing in the inflation layer around the ahmed body and use it as first layer thickness.
Further refine the mesh by decreasing eleemnt size in sizing method
Mesh detail:
Mesh:
Setup:
Solver,method,material and model is set same as for base model.
Plots and Values:
Residuals:
Drag coefficient:
Lift coefficient:
Values of Cd and Cl:
Velocity contour:
Pressure contour:
Velocity contour animation:
Vector plot of velocity:
Grid dependence test:
we have calculated Cd and Cl values for different mesh sizes, we tabulate the values below:
Conclusion:
Effect of grid size and refinement on value of Cd and wake region:
- In base setup we see that the mesh is coarse, because of this we dont get proper wake region as we can see in the velocity contour.
- We refine this case further by adding second enclosure around the ahmed body, in this case we can clearly see the wake region behind the ahmed body but the value of Cd varies from the litterature availble.
- We refine the case further by using Y+ value to calculate the value of Cd, and refine the mesh further by decreasing the element size and using wall spacing value we got from y+ as first layer thickness for inflation layer.
- The value of Cd we get from this case is accurate
Nagetive pressure in wake region:
The flow behind an object separates from the surface and creates a highly turbulent region behind the object, called the wake. The pressure inside the wake region remains low as the flow separates and a net pressure force (pressure drag) is produced. In other words, due to the production of pressure drag being produced from flow separation, the pressure in the wake region is low. This is what is causing the vacuum.
Since wake region creates a vacum behind the vehicle the number of particles in wake region are too low compared to front side of the vehicle, this creates a nagetive pressure gradient in the wake region and this results in pressure drag and it reduces the performance of the vehicle.
Pressure contour showing nagetive pressure in wake region:
Reference Research papers:
Flow and Turbulence Structures in the Wake of a Simplified Car Model
Experiments and numerical simulations on the aerodynamics of the Ahmed body