Computational study of the front-end downforce enhancing aerodynamic elements in sports cars

While designing sports cars, getting the aerodynamic balance of the vehicle is very important. Aerodynamic elements to create rear downforce are studied extensively while, this research tries to recognize the various ways of developing effective front end down force producing devices. The most widely used add-on devices include the front splitter, canards and underbody vortex generators. CFD simulations are employed to study these devices on the widely acknowledged MIRA fastback model. ANSYS Fluent CFD software was used with the efficient k- ε model to get accurate results while avoiding expensive experimentation costs. Reduction in lift force with the help of these elements generally leads to high drag force. This study aims to find out the most suitable and versatile devices. The average lift decrement on the addition of front aerodynamic elements was Δ C l = 0.0778 while the increase in drag was Δ C d = 0.0393.


INTRODUCTION
Most of the research on improving down force of sports cars focuses on the rear aerodynamic elements such as rear wings and rear diffusers. But the literature concerning the front aerodynamic elements is not available readily. Front end aerodynamic elements such as front splitter (or front diffuser), canards and front-end underbody vortex generators are used in motorsports and expensive sports cars to provide front axle down force. The front splitter (fitted below the front bumper) reduces lift by producing low pressure under the vehicle's front end by acting as barrier for the incoming air.
Furthermore, the wind is collected on the splitter making high pressure zone above it to increase the down force effect. Canards are attached to both sides of the front bumper and designed to direct the air upwards. Canards are also designed specially to direct air flow over the front wheels on the side of the vehicle which helps reduce drag. Optimizing the canards design is a tedious operation and motorsports teams spend a lot of time and effort perfecting it [1,2]. Under-body vortex generators are designed in a way to direct the air through the side of the car in the form of vortices which creates low pressure under the vehicle. Vortex generators are underbody add-on devices and therefore don't affect the aesthetics of the vehicle. Creating a suitable aerodynamic balance of sports cars is very important especially if different race tracks are concerned. Nowadays, adjustable devices are used to optimize performance in different driving conditions [3,4]. For example, front splitters can be adjusted by altering the lip reach and height. Ahmed et al. [5] studied the flow structure of a basic 3-D bluff body known as the Ahmed body and found the flow to be highly unsteady and three-dimensional. Another acknowledged model is the MIRA fastback model which is used in this study. Zhang et al. [6] investigated this model's aerodynamic properties both experimentally and with the help of Computational Fluid Dynamics (CFD) simulation method. This study is an extension of the MIRA fastback model with the addition of front aerodynamic elements. When the vehicle is travelling at high speeds, the ground effect significantly affects the car's aerodynamics, especially with front diffuser and underbody vortex generators. The aim is to study only the direct pressure changes caused by the add-on devices to understand the main reasons for the increase in down force and mimic wind tunnel testing.

Methodology
Numerical simulations are a great way to avoid experimental costs but still provide accurate and reliable results. Steady-state Reynold's Averaged Navier-Stokes (RANS) was used for the current simulation since the road cars are low Mach number transportation and steady aerodynamic characteristics are needed to be studied. The realizable kε model was selected as it produces accurate results.
Here the turbulence kinetic energy is given by k and the dissipation rate of turbulence energy is given by ε. S is the modulus of the mean rate-of-strain tensor, Pk is the shear production of turbulent kinetic energy, νt is the turbulent eddy viscosity and ν is the kinetic viscosity.
In this study, aerodynamic force coefficients namely drag and lift coefficients are calculated to describe the aerodynamic characteristics of different models.

Simulation setup
Simulation is performed with the help of ANSYS Fluent 19.2 CFD software. To mimic wind tunnel results from Zhang et al. [8], the enclosure is constructed keeping in mind the distance between the vehicle and ground (of 50 mm). The respective models are enclosed in suitable enclosures and named selections are applied to the sides (fig 9).

Validation
The simulation results should be validated with experimental results. Drag coefficient of MIRA fastback simulated in this research was obtained to be 0.295. Zhang et al. [8] and Wang et al. [9] carried out wind tunnel tests on MIRA fastback model and achieved 0.286 drag coefficient which corresponds to a 3.2 % error (within experimental engineering error of 5 %).

CONCLUSIONS
The results show that canards increase the most down force with the least increment in drag. The problem with road legal sport cars is the ground clearance. Big speed bumps and uneven road surfaces force. The average lift decrement on the addition of front aerodynamic elements was ΔCl = 0.0778, while the increase in drag was ΔCd = 0.0393. The numerical results also revealed that the k-ε model is a reliable method for simulating the flow field. Future scope of improvements may include experimental and numerical analysis of elements with improved designs and inclusion of the ground effect.