Week 3 - External flow simulation over an Ahmed body.
EXTERNAL FLOW SIMULATION OVER AN AHMED BODY
OBJECTIVE:
To determine the aerodynamic forces on the Ahmed body such as drag and lift coefficient and to perform the grid independence test. The expected results are,
1. Velocity and pressure contours.
2. The drag coefficient plot for a refined case. ( For a velocity of 25m/sec, the drag coefficient should be around 0.33).
3. The vector plot that clearly shows the wake region.
4. Perform the grid independence test and provide the values of drag and lift with each case.
THEORY:
Ahmed body:
The Ahmed body was first created by S.R. Ahmed. The Ahmed body is a simplified vehicle body used for capturing basic yet characteristic features seen in objects used in the automobile industry. It is used for testing new CFD solvers or testing one's ability to perform simulations as there are a lot of experimental data available about the Ahmed body to compare simulation data to. The drag force is an important factor in considering fuel efficiency. As the drag force increases the fuel consumption also increases which in turn becomes expensive for the car owners. One of the main factors considering whether the car is fuel-efficient or not is the car's geometry.
Wake Region:
When the air moving over the vehicle is separated at the rear end, it leaves a large low-pressure turbulent region behind the vehicle which is known as the wake. This contributes to the formation of pressure drag which affects the vehicle's performance.
The negative pressure can be observed in this wake region because at the rear end, the vicious fluid gets detached from the body of the vehicle causing the pressure to drop below zero.
Point of Separation:
The point of separation will occur when there is a discontinuity in the surface. In Ahmed body at the top rear end of the body where pressure drop is observed, the point of separation occurs. The recirculation region happens beyond the point of separation.
SIMULATION PROCEDURES:
a) BASELINE MESH(0.08 million cells):
Geometry - SpaceClaim:
- The Ahmed body .STEP file is imported to SpaceClaim which has the angle of bend = 20 degrees,
- For baseline simulation, a single wind tunnel is created around the body. The Enclosure dimensions are shown below,
- Create a plane section at the origin along the x,y, and z axes,
- The Ahmed body is suppressed and the cross-section view is shown below,
Mesh - Meshing:
- The named selections are created as follows,
- For the baseline simulation, we use the default mesh size. Generate Mesh,
- The geometry is cut along the z-axis to observe the meshing around the body,
- The number of elements = 84053
Setup - Fluent:
- The meshed geometry is loaded in ANSYS Fluent,
- For baseline simulation, the Density-Based solver is used. The steady state is considered,
- The reference value is updated,
- The frontal area for the reference value is found in Fluent using the Project Surface Area tool,
- The reference length is the length of the body along the y-axis,
- Although K-Epsilon is not preferred for external flow, we use it for baseline simulation. The energy equations are included,
- The default air is used for fluid material.
- The inlet(velocity boundary) velocity = 25 m/s and temperature = 300 K,
- The outlet is a pressure boundary.
- The wind tunnel walls are symmetry boundaries.
- The car wall is a wall boundary.
- A cut plane along z is created to observe the velocity and pressure contours,
- The lift coefficient and drag coefficient plots are defined,
- The hybrid initialization is performed to enable create a contours tool.
- The velocity and pressure contours are created for the cut plane.
Simulation - Fluent:
- The simulation is performed for 500 iterations.
Results - PostProcessing:
- The velocity and pressure contours are generated here.
RESULTS for Baseline simulation(a):
Residual plot:
Drag coefficient plot:
- The fore report tool can be used to obtain the exact value of drag coefficient which is a force along x direction,
Lift coefficient plot:
- The fore report tool can be used to obtain the exact value of the lift coefficient which is a force along y direction,
Velocity vector plot:
Pressure plot:
b) FINE MESH ( 0.2 million cells):
Geometry - Spaceclaim:
- The geometry is the same as the baseline simulation. the only change here is, there are 2 enclosures in this simulation. The 2nd enclosure dimension are given below,
- The geometry after adding the plane section,
- The interference is removed using the interference tool in SpaceClaim to avoid mesh overlapping,
Mesh - Meshing:
- Method- The highest quality of the mesh, the hexahedral mesh is used for the external enclosure. the element size = 100 mm.
- Sizing- For the inner enclosure sizing is performed. The element size = 50 mm.
- Face sizing- The four stands(wheels) are face sized to retain the cylindrical shape. The element size = 5 mm.
- Inflation layer- The car_wall is selected for inflation layers. the number of divisions = 6, maximum layer thickness = 12.5 mm.
- The number of cells = 202644 (0.2 million),
Setup - Fluent:
- The meshed geometry is loaded in ANSYS Fluent,
- The pressure-based solver is used here,
- The K-omega SST model which is best for external flow is used here,
- In solution methods, coupled scheme and Rhow-Chow: distance-based flux type is used,
- The above are the only changes from the baseline simulation to be made.
RESULTS for fine mesh simulation(b):
Residual plot:
Drag coefficient plot:
Lift coefficient plot:
Velocity vector plot:
Velocity plane contour:
Pressure plane plot:
c) FINER MESH ( 0.35 million cells):
- The only changes to be made are as follows,
Mesh - Mesher:
- Face sizing- After hiding the outer planes, using the box select tool the car_wall is selected for face sizing. The element size = 10 mm,
- The number of cells = 356695 (0.35 million).
RESULTS for finer mesh simulation(c):
Residual plot:
Drag coefficient plot:
Lift coefficient plot:
Velocity contour plot:
Pressure car_wall plot:
Velocity vector at the wake region:
d) SPLIT GEOMETRY:
Geometry - SpaceClaim:
- As the Ahmed body is symmetrical along the z-axis, half of the geometry can be removed to reduce the number of mesh cells.
- Use the split body tool to cut the geometry along the z-axis.
Mesh - Mesher:
- To calculate the boundary values accurately, the mesh needs to be finner at the boundaries. The y+ value can be used to determine the mesh size to be used at the boundaries.
- Y+ value: It is a non-dimensional distance that is often used to describe how coarse or fine a volumetric mesh is for a flow pattern. The y+ is applicable only for turbulent flow and mesh cells directly in contact with a no-slip wall (here, Ahmed body walls).
- The y+ values less than 5 are preferred for boundary layer problems, as the solver in the post-process uses viscous sublayer equations for those values.
- The formulae to calculate y+ are as follows,
- As default fluid air is used, the material properties of the fluid are available in Fluent,
- The values we need for the generation of inflation layers in meshing are as follows,
- Using an open-source inflation layer calculator by fluidmechanics101,
- Inflation layers,
- To capture the wake region clearly, the mesh is enhanced at the rear end of the body using face sizing- sphere of influence (element size = 15mm),
- The resulting mesh is shown below,
- The total cell count = 470256 (0.47 million).
Setup and solution generation - Fluent:
- As the geometry is sliced, the reference area is changed accordingly,
RESULTS for Sliced Geometry(d):
Residual plot:
Drag coefficient plot:
Lift coefficient plot:
Pressure plots:
Car-Body:
Fluid flow:
Velocity plots:
Fluid flow (with Ahmed body):
Fluid flow (without Ahmed body):
Vector plot:
Streamline plot:
Flow animation:
OBSERVATION:
Pressure plots:
- High pressure can be observed in the frontal area of the body due to the direct impact of the high-velocity fluid.
- In the rear top end of the body, the point of separation can be observed. This occurred as there is a discontinuity of surface in the path of fluid flow.
- Due to this discontinuity of surface, a low-pressure region is created behind the body thereby pushing the high-pressure air towards it changing the direction of the flow. This region is called the wake region.
- The wake region is formed beyond the point of separation.
- The negative pressure in the wake region is primarily because there is no flow in that region.
Velocity plots:
- As Ahmed body is static, the velocity of the body is zero.
- Due to no-slip walls, the velocity of fluid flowing near the car walls is low.
- The fluid circulation in the wake region can clearly be observed in the vector and streamline plot.
- The fluid can't escape the wake region as the velocity of flow around this region is high resulting in recirculation.
Grid independence test:
- From performing the grid independence test, it is inferred that the sliced geometry method was best suited for this problem.
- Even though the cell count is high, the solution converged in less than 70 iterations.
- The Drag coefficient value of sliced geometry mesh seems closer to the experimental value of 0.33
- Also, since the geometry is sliced, the number of cells decreased almost by a factor of 2 paving the way for very high mesh refinement.
y+ value:
- Due to the proper usage of the y+ to calculate boundary layer inflation values, accurate results can be observed at the boundaries in sliced geometry plots than other mesh plots.
CONCLUSION:
- The external flow simulation over the Ahmed body was conducted and the wake region was clearly visualized using Vector, streamline, and Contour plots.
- The coefficient of drag and coefficient of lift were estimated.
- The point of separation and its influence was studied.
- The grid Independence test was performed and an optimal grid condition was found.
REFERENCES:
1) Experiments and numerical simulations on the aerodynamics of the Ahmed body, W. Meile, G. Brenn, A. Reppenhagen, B. Lechner, A. Fuchs.
2) Flow and Turbulence Structures in the Wake of a Simplified Car Model (Ahmed Model), H. Lienhart, C. Stoots, S. Becker.
3) https://www.cadence.com/ko_KR/home/tools/system-analysis/computational-fluid-dynamics/y-plus.html