As of writing this post, my bachelor thesis has been submitted and presentation successfully defended. The official title of the thesis was: “Design and Development of Ground-Effect Aerodynamics for the 2023 UCL Formula Student Racing Car”. This post is a summary of my bachelor thesis.
This project consisted of benchmarking the baseline performance, developing the design in a concept and detailed design stage, and wind-tunnel testing of a scaled-down model in UCL Mechanical Engineering’s wind tunnel facility. The delivery of this project was completed under a unique design methodology and new tools for design tracking and decision-making (called the Design Map). The choice to develop ground effect aerodynamics, rather than front & rear wings, was a strategic decision in line with UCL Racing’s design philosophy which focuses on developing a compact, short and narrow vehicle to prioritise the vehicle’s driveability and minimise the chances of hitting cones.
Brief motivation
Litterature review suggests that implementing a full aerodynamic package can reduce the laptime of a Formula Student racing vehicle by up to 5%. For UCLR, this would mean +30 points in the competition, enough to claim 5th place overall rather than 6th. The primary performance diagnostics were determined to be the lift coefficient and aerodynamic balance. Using a laptime simulation software, it was determined that a 10% change in lift coefficient leads to a 90 ms/lap difference, whereas a 10% change in drag coefficient leads to a 30 ms/lap difference. This was in line with the litterature review which suggests that designers of Formula Student racing cars should focus on maximising downforce, rather than minimising drag. Therefore, this project aims to design and develop ground effect aerodynamics that will maximise downforce. Success criteria:
- Simulated laptime reduction must be at least 0.5 seconds (considered as minimum gain for the project to be considered a worthy investment).
- The aerodynamic balance must be between 40% and 45%
Baseline of 2023 vehicle
CFD was used to assess the aerodynamic performance of the 2023 baseline vehicle without any aerodynamic package. In the pre-processing stage, the geometry was simplified from the “manufacturing” CAD model, to a CFD-ready model. The vehicle was simulated as travelling in a straight line at 15 m/s (average velocity around FSUK endurance track), with wheels rotating and only one half of the vehicle was simulated to save on computational cost.
The processing of the simulation was a done by the k-omega SST (RANS) solver with a wall turbulence model as it had proven to be the most effective solver for simulating highly turbulent Ahmed Bodies. In this case, the Reynold’s number was 2.6 million. In post-processing, different parts of the vehicle were analysed to determine their contributions to the lift and drag.
In addition, a mesh indepence of individual parts was carried out in order to determine the necessary amount of refinment of every part:
Concept and Detailed Design Stage
The design was developed using a novel design method. The concepts were developed by an iterative process, where only one design change for each model, and monitored thanks to a “Design Map” tool – this gave a clear indication on whether the design change increased or reduced performance. This details the primary aerodynamic diagnostics, chosen to be the lift coefficient (CL) and aerodynamic balance (AB), were used to assess the performance.
The final model was obtained in the detailed design stage, where the dowforce was first maximised by fine-tuning the angle of the rear diffuser. Secondly, the targetted aerodynamic balance was achieving by adding a pair of front winglets (shown in orange below):
It is also worth noting that the addition of the winglets does not increase the turning radius of the vehicle, and therefore is compatible with the philosphy of the vehicle.
The pressure contour of the underside of the vehicle displays regions of very low pressure, which demonstrates that the ground effect occurs as desired.
Wind Tunnel Experimental Testing
A wind tunnel testing rig was designed, manufactured and assembled. Red pieces of string (tufts) were glued onto the vehicle to allow for a visual analysis of the fluid flow around the vehicle.
The tufts allow for a qualitative comparison between the wind tunnel and CFD case:
Both cases demonstrate a similar flow behaviour arouund the vehicle. More comparisons shown at bottom of the page. Lift and drag forces of the wind tunnel vehicle were recorded by external load cells and analysed in the critical analysis section.
Critical Analysis
Both success criteria of this project were achieved:
It was quickly determined that the Reynold’s number of the wind tunnel (even at maximum velocity) was a magnitude lower compared to the one of the CFD case. Therefore, it was decided to reduce the velocity in the CFD simulations to match the Reynold numbers.
The difference in the results was due to 3 phenomena:
- Important boundary layer thickness with respect to the size of the vehicle: In the wind tunnel, the false floor is immobile with respective to the vehicle – a boundary layer develops from the false floor, which at the rear of the vehicle, has got the same thickness as the ground clearance. In CFD, this is not the case as the ground is moving at the same direction and velocity as the air.
- Important blockage ratio of the wind tunnel: it was calculated that the wind tunnel led to a blockage ratio of 8.2%. This is the ratio between the projected frontal area of the vehicle and the total cross-sectional area of the wind tunnel (the false floor acts as a partition). Research had shown that for any blockage ratios >5%, the drag increases significantly. This requires analytical correction methods to the results, such as the Maskell correction method which reduced the error to 53%.
- Important surface roughness of the 3D printed model: it is possible to observe individual layers in the 3D printed model, particularly arouund the sidepod regions. Research had shown that for a similar Reynold’s number, a rough 3D-printed airfoil develops 30% more drag than a perfectly smooth one – this is in the same order of magnitude as the discrepacy observed in this project.
Conclusion
Implementing the proposed ground-effect device leads to a reduction in laptime of 1.06 seconds. It is estimated that this will allow UCL Racing to score 15 additional points in the endurance and sprint events. As well as engineering skills and critical thinking, this project highlighted the importance of correct project management and communication for the delivery of wind tunnel parts, and strong teamwork with the suspension and chassis team from UCL Racing. The learnings and new methodologies developed in this project will be used by the upcoming UCL Racing team to further develop on their success.
For future work, it is recommended that UCL Racing seeks collaboration with a larger wind tunnel facility in order to reduce the discrepancy in lift and drag due to blockage ratio and boundary layers. In addition, it is also recommended to develop an aerodynamic map to understand the evolution of aerodynamic diagnostics when subjected to pitch, roll, heave etc. This can be coupled to a more advanced laptime simulation software to provide more accurate laptimes. Topological optimisation can also be used in the detailed design stage to faster optimise simple design aspects, such as the angle of the rear diffuser.
I would like to thank my project supervisor, Dr. Chrisitan Klettner, for his thorough support and the UCL Mechanical Engineering department for providing me with the necessary resources to successfully carry out this project.
Picture and Video gallery:
