CFD stands for Computational Fluid Dynamics. To put it in short, CFD are numerical models that are used to study the behavior of fluids within a set of boundary conditions.
As I am building a car at 1/3 scale, it will translate to 1,64 m vehicle in which aerodynamics will play an important role, as it does with the real F1 cars. This is why, for first time in all my cars I have decided to use CFD to improve the aerodynamic efficiency of my car. At this stage, only a preliminary study has been conducted but some valuable data has been obtained. This data will be used in the detail design later on in the project.
The process starts with meshing the geometry to study. The model is the same that was made for the DMU in CATIA, with the difference that I only study the front and rear sections, not the full car. This split is made to save modelling and CPU time.
The model was imported in COSMOS FlowWorks 2007 in iges format. After cleaning the geometry and fixing some defects on the surface, these is what is obtained:
The boundary conditions are stablished as 20 or 30 m/s flow, symetry on the XZ plane and symetry on the XY plane. The latter is used to simulate the road, as a big simplification. I did not want to use rolling floors since it is the preliminary stage only. A real F1 car has about 1500 kg downforce, which translate to 18,5 kg at 1/3 scale. This design is therefore not too far away, considering it still has plenty of room for optimization (assuming a maximum speed of 120km/h which would be 360 km/h on the 1:1 car).
The results were as follows:
FRONT
20m/s: 12.1 N
30m/s: 28.2 N
REAR
20 m/s: 22.7 N
30 m/s: 52.4 N
As you can see, there is flow detachment, especially on the diffuser and front wing airfoils. I am aware this is not correct and should be further developed, but it is just the starting point and a preliminary design.
As you can see, there is flow detachment, especially on the diffuser and front wing airfoils. I am aware this is not correct and should be further developed, but it is just the starting point and a preliminary design.
Drag values were really high, and I assume it is due to not having the full body, rolling floor, and main simplifications on the cooling outlets on the rear section. For the dynamic simulation, a 1/3 of the downforce coefficient is used accounting for the drag.
The rear section, was optimized during several iterations using a NACA profile airfoil. This increased the initial downforce values by around 50%. Further refinements were made to remove airflow separation close to the trailing edge of the RUMP (rear upper main plane) and rear flap.
On the other hand, the front section was not refined, and thus a low downforce value is obtained. Additionally, the front wing is closer to the floor. Therefore the influence of not having a non-rolling floor is more acute than for the rear wing.
The Center of Pressure (CoP) is located -223 mm behind the CoG. This is too far behind, but is the result of the poor front downforce at the moment. The downforce value, might seem low but in fact, comparing it to what should be at 1/3 is not that low. Making some numbers, we find that the downforce is not scaled by 3 times, but by 81 times (F=1/2*p*A*Cz*V^2, area is 3^2 and speed is 3^2, hence 3^4=81). Reynolds number is way within the laminar flow and so turbulent flow is not a concern.
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