As an revolutionary electric car in the automotive industry since 2014, Model S has marked a symbol of pure electric, long range and luxury car. It has remarkably low drag, which is essential to electric car thanks to its simple drivetrain design to enable more freedom on aerodynamic design.
| Length | 4980 mm |
| Width | 2189 mm |
| Height | 1440 mm |
| Wheel base | 2960 mm |
| Weight distribution (Front:rear) | 48:52 |
Aerodynamic devices
The Model S have two popular design features for reducing drag and lift, ducktail and diffuser. These design features can also been seen on many modern cars.
Ducktail
Ducktail is a small edge extended from the very end of the rear bonnet in both upward and rearward directions, where the airflow from the top leave the car. As a result, it increases air pressure on the rear bonnet region which, in turn, reduces drag and increases downforce at the same time.
Another effect creates from the ducktail is to reduce the size of the low pressure region behind the car and thus helps on reducing drag.
Diffuser
Diffuser is a upward sloping surface on the rear end of the car floor. By slowly expanding the bottom airflow pressure to match the atmospheric pressure, more air is introduced to flow through the bottom of car instead of the top and thus increases airflow beneath the car. With this faster airflow, lower air pressure is created, according to Bernoulli’s principle and so thus reduces lift (increase downforce).
Simulation model
A full scale model is provided from Chaos Studio and put into virtual wind tunnel for testing. Wind speed is 27.7 m/s and with moving floor.
| Results | Value | Coef. | Official Coef. |
|---|---|---|---|
| Drag | 116 N | 0.26 | 0.24 |
| Lift | -5 N | -0.012 | – |
| Pitching (-ve down) | -169 Nm | -0.128 | – |
Drag and lift
We can see from the simulation result, drag is quite low for sedan size car and the drag force is at the same order of magnitude as the drag on Porsche 911 Carrera S. This demonstrates the level of aerodynamic performance an electric car can achieve.
Besides low drag, there is also downforce existed on the car. The pressure plots below shows that a high pressure region exists on the rear bonnet, as discussed on the ducktail section, and also low pressure region at the bottom near the front and rear axle as a result of the diffuser.
The combination of these high and low pressure regions generate signifcant downforce to compensate the lift generates from the low pressure zone on the roof.
Pressure distribution (Red: 0.7, blue: -0.7) 
Pressure distribution (Red: 0.7, blue: -0.7) 
Pressure distribution on the bottom (Red: 0.7, blue: -0.7)
As seen from the centre plane velocity plot below, airflow from the bottom expands after leaving the rear reduces the low pressure zone and reduces drag.
Loss
By the comparing the total pressure isosurface, an indication of loss, with the one for Porsche 911 Carrera S, we can see that there are significantly less loss on Model S, specially on the rear of the wheels.
Isosurface (Total pressure= 0 Pa) 
Isosurface (Total pressure= 0 Pa)
Streamline coloured with velocity (Red: 30 m/s, blue: 0 m/s) 
Streamline coloured with velocity (Red: 30 m/s, blue: 0 m/s)
The streamline plot below illustrates the turbulant air passing by the mirrors is correctly guided to avoid mixing with rear clear air. This could prevent performance deterioration of the ducktail.
Conclusion
Tesla Model S makes a good demostration on the effect of different aerodynamics devices that could largely improve aerodynamic performance. It also shows the large freedom of aerodynamic design on electric car that helps increasing range and improve driving stability.