Aerodynamic Basics

Aerodynamics in FSAE can be used to describe the interaction of air and the racecar, in particular its bodywork and aerodynamic elements such as wings, diffusers and fins. It can be analysed by the use of Computational Fluid Dynamics. The performance of aerodynamic elements can be described by parameters such as the coefficient of Lift ($C_{l}$), coefficient of Drag ($C_{d}$) and the ratio of $C_{l}/C_{d}$.

Conceptual aspect of Aerodynamics

Aerodynamics is not necessary to make a car go around a racetrack. Check beforehand if your car benefits from aerodynamic elements. Simple laptime simulation usually includes lift and drag coefficents and helps you determine your target goals.

Key Parameters

Coefficient of Lift, $C_{l}$

Coefficient of Drag, $C_{d}$

Lift to Drag ratio, $C_{l}/C_{d}$

Boundary Layers

There are many resources available about what exactly boundary layers are, why they exist, and other useful information. This will focus on FSAE applications, as they are fairly unique.

Beyond "what is a boundary layer" type questions, I highly recommend reading High Lift Aerodynamics, as it contains many of the fundamentals for the limitations of downforce generation. Most of what follows is a summary of the paper.

Laminar, Turbulent, and Transition

• Boundary layers (BL) can be either laminar, or turbulent
  • Neither is "worse" or "better" than the other. They are simply different and have different effects aero systems.
• When a BL initially forms, it is laminar. Gradually fluctuations destabilize the flow and it becomes turbulent.
  • This is known as (BL) transition
• The fluctuations may come naturally (from atmospheric turbulence or surface imperfections) or be induced artificially by "BL trips" or vortex generators
  • The artificial methods force the transition to occur sooner than it would otherwise

• Determining when a boundary layer transitions is a function of it's Reynolds Number, $Re_{x}$, using $x$ (distance along a wall) as its characteristic length scale
  • The higher the Reynolds number the more likely it is to be turbulent (over a certain level, it is more-or-less guaranteed)
  • Therefore, increasing flow velocity decreases the distance at which BL transition occurs

Pressure Gradients and Separation

• Flow separation occurs as a direct result of the existence of boundary layers
  • Hence why "flow separation" and "boundary layer separation" are synonymous
• Pressure gradients describe the force being applied to the fluid as it moves
  • A pressure gradient that pushes with the (bulk) fluid motion is known as a Favorable Pressure Gradient (FPG)
  • A pressure gradient that pushes against the (bulk) fluid motion is known as an Adverse Pressure Gradient (APG)
• APGs are what cause flow separation
  • The APG pushes against the boundary layer such that the flow close to the wall (which is significantly slower than the flow outside the BL) begins to move backwards

• laminar vs turbulent in adverse pressure gradients
• Subsequent low-Re number effects (really important for airfoil selection and significant implications on CFD)