Week 11: FSAE Car Project

I. Aim: Aerodynamic flow simulation around a prototype FSAE car using Converge CFD.

II. Introduction:

The Formula SAE® (FASE) series competitions challenge teams of university undergraduate and graduate students to conceive, design, fabricate, develop, and compete with small, formula-style vehicles. The competitions give teams the chance to demonstrate and prove both their creativity and engineering skills in comparison to teams from other universities around the world [1].

The Formula SAE competition is designed to test each team’s knowledge of various aspects of vehicle design and manufacturing. Each year, student-built cars are presented to members of the industry who judge their engineering decisions and designs. These critiques help teams learn about and improve upon designs. The cars are also rigorously tested during dynamic events which evaluate drivability, reliability, and the overall performance of the car. Both static and dynamic events are used to determine a winner[2].

The concept behind Formula SAE is that a fictional manufacturing company has contracted a student design team to develop a small Formula-style race car. The prototype race car is to be evaluated for its potential as a production item. The target marketing group for the race car is the non-professional weekend autocross racer. Each student team designs builds and tests a prototype based on a series of rules, whose purpose is both ensuring on-track safety (the cars are driven by the students themselves) and promoting clever problem solving [3].

The major tests include engine performance, suspension, Aerodynamics, and weight of the car. In particular, the aerodynamics of the car has greater importance in the competition and the aerodynamic performance should be tested and validated by wind tunnel testing or by computational fluid dynamics analysis, and even race track testing[3].

III Problem Statement/Objectives:

Phase1: 

• Run a virtual wind tunnel coarse simulation test on a  prototype FSAE car by cleaning up the geometry and setting up the correct boundary conditions, solver, and extracting the results.

Phase 2 - Detailed Setup

ABCD Racing company is looking to perform Aero Simulations for their FSAE vehicle. The suspension team wants a detailed report on the total downforce on individual components. They have two races in this upcoming season. 

Race details are as follows

Race-1 

• Racing track conditions - Lot of turns
• Average lap speeds - 45kmph
• 70% turns at 45 degrees
• 20% turns at 80 degrees
• 10% turns at 20 degrees

Race-2

• Racing track conditions - Straights
• Average lap speed - 75kmph

Perform aerodynamic simulation for the above situations and provide the drag and downforce on the car components.

 

IV: Solving & Modelling approach:

3D steady-state CFD analysis case of flow around a prototype FSAE car model is set up in ConvergeCFD and then running the solution in the CYGWIN terminal. Atmospheric air is used as the working fluid. Postprocessing is done using ConvergeCFD and ParaView. 

Phase 1:

1. Geometry/Domain:

A prototype FSAE car model in .stl format is imported into Converge CFD with appropriate dimensions. The geometry is then cleaned up to remove missing and duplicate surfaces, nonmanifold edges, intersecting triangles, and normal orientation errors. A total of 48 errors are resolved. A virtual wind tunnel is set up around the car with the bottom wall (ground plane reference) exactly aligned with the bottom surface of the tyres. Boundary zones are flagged for different components of the car and the virtual wind tunnel. The bounding dimensions of the imported car geometry, cleaned up geometry as well as the generated virtual wind tunnel are shown in the below images:

Imported Geometry	Cleaned up Geometry

Virtual Wind Tunnel/Flow domain

2. Boundary Flagging:

The boundary zones in ConvergeCFD are created by flagging boundaries. The created boundary zones using the boundary flags in the boundary tab are as shown below:

3. Case setup:

After assigning boundary zones as mentioned above, the coarse grid solution case setup is to be done. ConvergeCFD has a dedicated menu to completely set up the solution. Details are as follows:

In the application type option, Time-based is selected.

- The materials option are selected as shown in the below images:

Air at atmospheric conditions is chosen as the working fluid. The following are properties of air used:

Density = 1.2 kg/m^3

Dynamic Viscosity = 1.86E-07 Pa-s.

- Simulation Parameters:

 For the phase 1 (coarse grid), flow over the FSAE Car is simulated using a steady-state solver and the respective setting for the solver are shown below:

- Boundary Conditions:

The boundary zones are applied with boundary conditions in this step. The inlet flow velocity is taken as 45 kmph (12.5) and the air properties at the atmospheric condition of 1 Bar pressure and 300 K temperature.

The applied boundary conditions are as follows:

Boundary Zone	Boundary condition
Body	Wall -Stationary, fixed, Law of Wall, Neuman conditions
Rear_Wing	Wall -Stationary, fixed, Law of Wall, Neuman conditions
Front_Wheels	Wall -Stationary, fixed, Law of Wall, Neuman conditions
Body	Wall -Stationary, fixed, Law of Wall, Neuman conditions
Rear_Wing	Wall -Stationary, fixed, Law of Wall, Neuman conditions
Front_Wheels	Wall -Stationary, fixed, Law of Wall, Neuman conditions
Front_Suspension_left	Wall -Stationary, fixed, Law of Wall, Neuman conditions
Front_Wing	Wall -Stationary, fixed, Law of Wall, Neuman conditions
Front_suspension_right	Wall -Stationary, fixed, Law of Wall, Neuman conditions
Rear_Suspension_left	Wall -Stationary, fixed, Law of Wall, Neuman conditions
Rear_Wheels	Wall -Stationary, fixed, Law of Wall, Neuman conditions
Rear_suspension_right	Wall -Stationary, fixed, Law of Wall, Neuman conditions
Human	Wall -Stationary, fixed, Law of Wall, Neuman conditions
Underbody	Wall -Stationary, fixed, Law of Wall, Neuman conditions
Inlet	Inflow (Dirichlet BC's)  -Velocity = 40 m/s along x-direction -Temperature = 300 K - Species - O2 = 0.77, N2 - 0.23
Outlet	Outflow (Dirichlet BC's) - Pressure = 101325 Pa Neuman BCs - zero normal gradient Velocity -Backflow temperature =300 K
Symmetry	Symmetry
Bottom_wall	Wall -Stationary, fixed, Law of Wall, Neuman conditions

The above boundary conditions can be defined only after defining a region in the Initial conditions and Events section of the case set up with velocity in the region the same as inlet velocity.

The turbulence is modeled by selecting the appropriate model in the turbulence option of the physical model.  RNG k-epsilon with standard wall function is used for modeling the turbulence.

- Grid control:

The base mesh size for the domain is defined in this tab as shown below. Along with it, box-type fixed embedding is used around the FSAE car for refining the mesh surrounding it. Since the phase1 simulation is to check the case setup and not the results a very coarse mesh is used.

Grid controls

- Output and post-processing:

The flow variables of interest, which are to be post-processed after the solution are selected in this option, and also at how many intervals these are written to the output file. Also, wall output (BOUNDARIES ONLY) is enabled for the FSAE Car wall boundaries to post-process the wall physics.

After all the above setup is completed final validation option is selected to check for the case setup is correct. After validation, the case setup files are exported using the export option in the File menu to the working folder. These files will be used by the CGYWIN terminal to run the solution. 

VI. Solution in CYGWIN :

The solution is run in the CYGWIN terminal. First, we have to navigate to the respective working folder in the CYGWIN terminal, where case setup files are present.  Then by executing the following command, the solution can be run:

Running the solution:

The solution will run up to the total number of cycles/time or convergence criteria set in the run setup and the output files are saved to the respective folders.

VII. Post-Processing Results:

After the solution is complete, post-processing of the results is done using CONVERGE CFD and ParaView.

The plots are plotted in CONVERGE CFD, whereas to visualize the flow variables using contour plots the output files need to be converted to ParaView compatible files. This can be done in the CONVERGE CFD Post-Processing 3D tab.

Results Discussion:

1. Steady-state solution with coarse mesh:

A steady-state analysis with the coarse grid is run only to check for the analysis setup faults. Also, the coarse mesh will aid in less computational time.

I. Grid:

The above figure shows the coarse mesh used for the steady-state solution. 

II. Pressure and Velocity Contours:

The below figures represent the steady-state Pressure and Velocity contours on the FSAE Car body.

Pressure

 

Velocity

Drag force in cars:

The resistance experienced by a fluid environment to an object in motion is quantified in terms of drag force. Generally, the forces acting on an object in motion or a stationary object in a flowing fluid are classified into drag and lift forces. The component parallel to the fluid flow direction is called the drag force and the forces perpendicular to the fluid flow direction are called lift forces.

The drag force acting on a body is given by the formula:

where, `F_d=Cd*1/2ρAV2`

           Fd = Drag force in Newtons

           Cd= drag coefficient 

            $\rho$ = Density of the fluid in kg/m^3

              A =  frontal area of the object in m^2

              V = fluid velocity/object velocity in m/s.

Downforce in Cars:

Downforce is a downwards lift force created by the aerodynamic features of a vehicle. If the vehicle is a car, the purpose of downforce is to allow the car to travel faster by increasing the vertical force on the tires, thus creating more grip. 

The downforce acting on a body is given by the formula:

`D_F=Cl*1/2ρAV2`

           Df = downforce in Newton

           Cl= lift coefficient 

            $\rho$ = Density of the fluid in kg/m^3

              A =  frontal area of the object in m^2

              V = fluid velocity/object velocity in m/s.

The following histograms show the drag force and downforce acting on the FSAE Car body components for the coarse mesh simulation.

It can be seen from the above histograms that the drag force is highest on the body of the car since it has the major frontal area facing the flow. Also, the downforce is more on the bottom wall.

From the above results, it can be seen that the simulation is running fine with the coarse mesh and the analysis setup. Hence phase 2 problems can be solved with a finer mesh with an appropriate analysis setup.

Phase II:

The analysis settings were fine-tuned by reducing the mesh size for the following cases and solved on a high-end computer:

Race-1 

• Racing track conditions - Lot of turns
• Average lap speeds - 45kmph
• 70% turns at 45 degrees
• 20% turns at 80 degrees
• 10% turns at 20 degrees

Geometry:

Since, we have 70% of turns in this track at 45 degrees, to simulate the scenario the car geometry is rotated to 45 degrees to the wind flow direction as shown below:

Geometry rotated to 45 degrees

The boundary conditions are similar to the coarse grid case except for using a transient solver up to 3.13 sec. Also, the grid is refined with the following settings:

The results are discussed below:

Grid:

 The above figure shows the level of refinement around the car along with the velocity contours at the midsection of the domain. The total cell count for this case is 1.35 million cells.

Pressure

 

Velocity

The above figure shows the velocity contours and formation of the wake region behind the car (low-velocity regions) due to the drag experienced by the car.

The following animation shows the wake formation behind the car:

Drag Force:

Down force: 

Race-2

• Racing track conditions - Straights
• Average lap speed - 75kmph

Geometry:

Since, we have a straight track with a speed of 75 kmph, to simulate the scenario the flow domain is similar to the coarse grid setup except the inlet flow velocity is increased from 45 kmph to 75 kmph(20.83 m/s):

The analysis is carried out using a  transient solver up to 1.932 sec. Also, the grid is refined with the following settings:

The results are discussed below:

Grid:

 The above figure shows the level of refinement around the car along with the velocity contours at the midsection of the domain. The total cell count for this case is 1.18 million cells.

Pressure

 

Velocity

 

The above figure shows the velocity contours and formation of the wake region behind the car (low-velocity regions) due to the drag experienced by the car.

Velocity Streamlines with Pressure contours

The above figure shows the flow at the midsection of the domain with wake formation behind the car in terms of velocity and the corresponding pressure contours on the car.

The following animation shows the wake formation behind the car:

Drag Force:

Down force: 

The above calculations can be found in detail in the following folder:

https://drive.google.com/drive/u/0/folders/1y38lQMbvb9NWzx88KEzdFz0oI9gnxbPV

Conclusion:

In this project, flow over an FSAE Car is simulated in Converge to find the drag and down forces experienced by the car. In the first phase of the project, the crude geometry is cleaned in converge and a base analysis is run with a coarse mesh to check for any analysis setup errors. Then a fine mesh is used for two race conditions and the drag and down forces are determined.

References:

1. https://www.sae.org/students

2. https://engineering.purdue.edu/fsae/wordpress/

3. https://en.wikipedia.org/wiki/Formula_SAE