Ahmed Body Challenge
Objective: The objective of this project is to determine the Aerodynamic forces on the Ahmed body such as Drag and Lift coefficients and also to perform Grid Dependency test.
What is an Ahmed Body?
The Ahmed Body was first created by S.R. Ahmed in his research “Some Salient Features of the Time-Averaged Ground Vehicle Wake” in 1984. Since then, it has become a benchmark for aerodynamic simulation tools. The simple geometrical shape has a length of 1.044 meters, the height of 0.381 meters, and a width of 0.389 meters. It also has 50-millimetre cylindrical legs attached to the bottom of the body and the rear surface has a slant that falls off at 35 degrees.
As the burning of fossil fuels becomes a more pressing issue, manufacturers are introducing more fuel-efficient cars to the market. One main contributor to fuel burn is the car’s aerodynamic drag. Complexly shaped, cars are very challenging to model and it’s difficult to quantify the aerodynamic drag computationally. The Ahmed body is a benchmark model widely used in the automotive industry for validating simulation tools. The Ahmed body shape is simple enough to model while maintaining car-like geometry features.
Three main features were seen in the wake:
1. The recirculation region that is formed as the flow separates at the top of the vertical back surface of the model
2. The recirculation region is formed due to the separation at the base of the model.
What does flow separation or a boundary layer separation mean?
Flow separation or boundary layer separation is the detachment of a boundary layer from a surface into a wake region. Separation occurs in the flow that is slowing down, with pressure increasing, after passing the thickest part of a streamlined body or passing through a widening passage, for example. Flowing against an increasing pressure is known as flowing in an adverse pressure gradient
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 favourable. 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 neighbouring particles to move away from the surface. This phenomenon is called the boundary layer separation.
The fluid exerts 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.
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. For internal flows, flow separation produces an increase in the flow losses, and stall-type phenomena such as compressor surge, both undesirable phenomena.
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. Vortex shedding produces an alternating force which can lead to vibrations in the structure.
Enclosure tool is used to construct a virtual wind tunnel around the Ahmed Body.
As the body is perfectly symmetric, we can simulate by considering the only half body. This is the best practice where you can save on the number of cells and get the results faster as well. Using the 'split body' command in SpaceClaim to operate and then using the symmetry boundary condition in fluent to perform the simulation. The split tool was done along the XY plane to perfectly cut the body from the middle.
The simulation was conducted for the velocity of 25 m/sec with the default air properties in fluent. K-`omega` and K-`omega`SST models were used to model the flow to accurately predict the Aerodynamic forces such as Drag and Lift bar the Ahmed body.
Grid dependency test was carried out to obtain the accurate values of drag and lift coefficients and obtain a grid-independent mesh where the results do not change according to changing mesh.
- BASELINE Simulation:
A baseline mesh was generated using the default setting in Ansys Mesh.
Mesh:
The number of cells generated is 42,693. The mesh is coarse as the setting used are default with 409.2 mm as the element size. Baseline mesh is used to run the simulation and based on the results computed by them, further refinement can be done.
SETUP:
- Density-based Steady-state method opted for the simulation.
- Standard K-`omega` turbulence model was used.
- Energy equation enabled.
- Inlet velocity as 25 m/sec.
- The model was initialized using Hybrid Initialization with 2000 iterations to run the simulation.
RESULTS:
Residuals:
The simulation converged at around 1500 iterations.
Drag Coefficient Plot:
Lift Coefficient Plot:
Velocity Contour:
As seen in the snaps, velocity profile and the wake region with this baseline is very distorted and does not capture the turbulent flow behind the body perfectly. But this result gives us a basic representation of the total wake length and other high-velocity and low-velocity areas, which can be used to further refine locally around the Ahmed Body.
- 1st Refinement: The setup remains the same as the baseline simulation. Reference values were introduced here onwards, to predict Aerodynamic Forces accurately.
Reference Values:
Mesh: The following method was used for all further simulations.
For the local refinement, various mesh methods were used such as Multizone, Body Sizing, Face sizing and Inflation layer. the total element count was 1,69,045.
Multizone: The mesh element size used for this refinement is 80 mm.
Body Sizing: Body sizing was used for the local area refinement around the Ahmed Body and thus the mesh element size used was 40 mm.
Face Sizing: The 2 faces of the pillar of Ahmed body which represents the tyre/wheels (for a real automobile body) was selected for face sizing. The mesh element size used was 5 mm.
Inflation Layer: The inflation layer is used to refine the local region just above the wall/body to capture the boundary layer precisely. The total number of layer allotted was 5 and the total thickness was allowed as 15 mm. The first cell thickness predicted was 5 mm.
Results:
Residuals:
The simulation converged at 867 iterations.
Drag Coefficient Plot:
Lift Coefficient Plot:
Drag and Lift Coefficient:
Velocity Contour:
Pressure Contour:
Wake region: Streamline tool and vector plots were implemented to demonstrate the wake region behind the Ahmed body.
- 2nd Refinement:
Mesh: The Multizone method was used for the outer area and the element size was 70 mm. The body sizing method was implemented for the inner area around the Ahmed body and element size was 30 mm. Face sizing was implemented for the pillars of Ahmed body with 5 mm element size and lastly, the Inflation Layer with the total number of layer 10 and first cell thickness as 3 mm. The total cell count was 3,62,236.
The updated mesh corresponds to the Y+ in the range of 30-50.
Residuals:
The simulation converged 1367 at iterations.
Drag Coefficient plot:
Lift coefficient plot:
Drag and Lift Coefficient:
Velocity Contour:
Pressure contour:
Wake region: Streamline tool and vector plots were implemented to demonstrate the wake region behind the Ahmed body.
- Final refinement:
Mesh: The Multizone method was used for the outer area and the element size was 70 mm. The body sizing method was implemented for the inner area around the Ahmed body and element size was 30 mm. Face sizing was implemented for the pillars of Ahmed body with 5 mm element size and lastly, the Inflation Layer with the total number of layer 10 and first cell thickness as 0.15 mm. The total cell count was 3,75,808.
The updated mesh corresponds to Y+ of 5 and below.
Residuals:
The simulation converged around 1400-1600 iterations.
Drag Coefficient plot:
Lift coefficient plot:
Drag and lift Coefficient:
Velocity Contour:
Pressure Contour:
Wake:
Discussion:
- As seen in all the above snaps of velocity contour, at the frontal area, the velocity gets stagnated causing a zero velocity region at the nose. Then the velocity sweeps through the curve or fillet part of the nose causing a high-velocity area near the fillets reaching its maximum magnitude. As the flow sweeps through the fillets the detachment takes flow between surface and airflow causing a sudden decrease at the edge of the fillet. This air is then carried downstream to the back of the car as per the Coanda effect. As the flow reaches back of the car, the slant edge increases the velocity of the air and departs them to the downward direction. The sudden change of geometry results in boundary layer separation causing a turbulent wake region at the back of the car, resulting in the backflow of the air. The slant of the car results in changing the air direction and eventually giving the downforce.
- From the above pressure contours, the front area of the car, as seen shows the high-pressure region due to direct impact and stagnation of velocity. As the air flows through the fillets of the car, the velocity increases eventually decreasing the air pressure around the edges. As the pressure stables over the car top surface, there is a sudden drop in the slant area at the back of the car.
- This eventually drops the pressure, which can be seen as a sharp line at the edge of the slant. Pressure drag is created when the shape of the surface changes abruptly, as at the point where the roof of an automobile ends. The drop from the roof increases the space through which the airstream flows. This slows down the flow and, by Bernoulli's principle, increases the static pressure. The air stream is unable to flow against this sudden increase in pressure due to adverse pressure gradient in the boundary layer and the boundary layer gets detached from the surface creating an area of low-pressure turbulent wake or flow. Since the pressure in the wake is much lower than the pressure in front of the car, a net backward drag or force is exerted on the car.
- The pressure in the wake remains close to separation-point-pressure since the eddies (formed as a consequence of the retarded layers being carried together with the upper layer through the action of shear) cannot convert rotational kinetic energy into pressure head.
- Since the wake zone pressure is less than that of the forward stagnation point (pressure at front of body) the cylinder experiences a drag force which is basically attributed to the pressure difference.
The drag force, brought about by the pressure difference is known as form drag whereas the shear stress at the wall gives rise to skin friction drag. Generally, these two drag forces together are responsible for resultant drag on a body.
| Refinement | Cd (drag Coeff) | Cl (lift coeff) |
| 1st refinement | 0.3803 | 0.331 |
| 2nd refinement | 0.328 | 0.288 |
| Final refinement | 0.321 | 0.237 |
- For Slant angle of `35^o`
MESH: The mesh remains the same for this simulation as the final refinement.
Residual: The simulation converged around 1300 iterations.
Drag Coefficient Plot:
Lift Coefficient Plot:
Drag and lift Coefficient:
Velocity Contour:
Pressure Contour:
Wake:
Differences in the Separation for different slant edges: