Week 10: Modeling and Simulation of flow around an Ahmed Body
Aim
To model and simulate flow around an Ahmed Body using Converge CFD and compare the simulation results with provided experimental data
Answer
Introduction and Theory
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, the Ahmed body allows us to capture characteristic features that are relevant to bodies in the automobile industry. This model is also used to describe the turbulent flow field around a car-like geometry. Once the numerical model is validated, it is used to design new models of the car.
Ahmed body is simplified car model created by Ahmed et in the 1980s to investigate the behaviour of newly developed turbulence models for complex geometry cases. The Ahmed body is made up of around front part, a movable slant plane placed in the rear of the body to study the separation phenomena at different angles, and a rectangular box, which connects the front and rear slant plane as shown. Ahmed body is a simplified model of a car body, but it demonstrates all the flow features involved in an actual case of a moving car. Current vehicle development needs a strong background in aerodynamics to improve flow control by means of active or passive control devices. The complexity involved in the automobile design especially due to the great number of accessories and devices that form its geometry makes the validation tasks unaffordable. The Ahmed model is a simple geometric body that retains the main flow features, especially the vortex wake flow where most part of the drag is
concentrated and it is a good candidate to be used as a benchmark test.
The Ahmed body represents a simplified, ground vehicle geometry of a bluff body type. Its shape is simple enough to allow for accurate flow simulation but retains some important practical features relevant to automobile bodies. Selection of the configuration of this body is governed by the requirement that it should generate the essential features of a real vehicle flow field except for the flow fields that occur due to rotating wheels, engine and passenger compartment, rough underside and surface projections. The model chosen would generate strong 3-D flows in front, relatively uniform flow in the middle and a large structured wake at the rear.
Cause of Negative pressure in the wake region
The path lines showing the exact circular motion or vorticity generation just at the rear end of the car. The separation takes two regions of distribution. One at the upper part of the rear and the other at the lower part of the rear end.
The path lines show about the nature of the fluid interaction with the rear end of the Ahmed body which here shows that the fluid just after leaving the contact from the slope still possess the momentum in it and the fluid particles near to the surface almost show the no-slip region which means the very least possible velocity of the fluid particles is in contact with the slope and base of the rear part. Thus the fluid particles away from the rear region having the higher pressure beside the wake region try to approach from the higher pressure region to the lower pressure region thus creating a circularity exactly in the wake region created.
The body penetrating the fluid creates a region of emptiness just beside its end due to the nature of the geometry of the body here which can be seen least in case of a perfectly streamlined shaped body. This emptiness created is a low-pressure region towards which the fluid column exactly behind the wake region which of atmospheric pressure tries to fill the space thus dragging the fluid column with it thus creating the region of vorticity. This also results in the huge pressure gradient in the region exactly rear of the body creating the –ve pressure at the end. Also, this is the layman’s reason why there is negative gauge pressure in the wake region.
Simulation Prepration
To create a Solid geometry of an Ahmed body using the given dimensions.
I choose the Solid-Works CAD program to do this simulation.
The final geometry of Ahmed Body.
Here is the final geometry of the ahmed body with the slant angle alpha = 35 Degree.
Opening in Converge.
In order to do a smooth simulation, we scaled down the geometry to about 0.01 times, so the geometry became smooth and the simulation will run smoothly.
The distance for the control volume is about L distance in front of the vehicle and 5L at the back of the vehicle. The height and width of the wind tunnel is taken as 2 times the length and width of the ahmed body.
We now cut the geometry into half so the simulation takes only half of the computational time.
The boundaries are assigned to the walls of Ahmed body and the wind tunnel.
Starting the Case Setup
1. Application type: time-based
2. Materials:
a. Gas simulation: air
b. Global transport properties: Default
c. Species: O2 and N2
3. Simulation parameters:
a. Run parameters:
Transient solver, full hydrodynamic, compressible
b. Simulation time parameters:
- End time- 1 sec
Calculating end time:
End time is calculated by analysing the flow time from the inlet to outlet and the inlet velocity at which the flow generates, by knowing these 2 parameters, we can calculate the end time.
Since, length of the pipe is approx 11.24m and velocity at inlet = 40 m/s
Therefore, end time = distance of pipe/ inlet velocity
This implies, that 11.14/40= 0.2785 sec
It takes 0.2785 secs for the flow to complete 1 cycle, we can have 4 cycles which is 4x0.2785= 1.114 ≅≅1 sec
- Time step- 1e-6
- the min time step- 1e-6
- the max time step- 1
- Convection CFL number- 0.5
c. Solver Parameters-
- Solver type- density-based solver
4. Boundary Conditions-
a. Inlet- Inflow
- Total pressure- NE
- Inlet velocity- 40m/s
- Initial temp- 300K
- Species boundary condition- add air
b. Outlet- Outflow
- Pressure- NE
- Backflow - temp= 300k
- add air
c. Wall Symmetry(sides and top)- Symmetry BC
- Symmetry
d. Ahmed body- Wall
- Law of the wall
- Omega wall conditions: Wall model
Regions and Initialization
Fixed Embedding
Grid size
Output files
Processing
The case setup files is then exported and the simulation is run on Cygwin.
RESULTS
Diagram showing Mesh and Fixed Embedding.
The velocity magnitude contour
The contour showing the pressure
The Density Contour
Line plot of Velocity Magnitude.
Total Cell vs Time Graph
Average Velocity vs Time plot
Plot showing Total Pressure vs Time
Youtube Videos
Animation of Pressure Contour
Animation of Velocity Contour
Conclusion
- The total drag coefficient of the Ahmed body is the key measurement for this simulation.
- It is made up of measurements for the pressure coefficients in the front, slant, and base of the body as well as the body’s skin friction.
- In the results of our simulation, the total drag is very well predicted but the individual measurements deviate from the experimental results in varying amounts. For a slant angle of 35-degree the value of drag and lift coefficients are found to be 0.156 and 0.937
- When the body is moving at 40 m/s (although the science of air moving around stationary body or body moving around the stationary air is same) it encounters a resistive foroce of 38.8 N ie,, the drag force.
- Even better simulation results can be achieved with increase computational power and decrease in grid size.
- The plots show that the simulation results are coinciding with some parts of the plot which indicates that out simulation is performed correctly.
- To decrease the Drag, we can decrease our slant angles therefore reduced flow separation is achieved at the rear end of the car. Which gives us better performance efficiency of the vehicle.