# How does a diffuser (in ground effect) accelerate the air in front of it?

A diffuser reduces speed and increases pressure, so how does a diffuser accelerate air in front of it if we have an adverse pressure gradient situation?

Wouldn't be better that we put the lower pressure (instead of higher) on end which will "pull"/accelerate air in front of it?

Example of F1 diffuser in ground effect:

• Where is the diffusor? The question is unclear. – Peter Kämpf Jul 19 '20 at 17:35
• @PeterKämpf, at rear floor on F1 car,youtube.com/watch?v=FMojq8Qj5MY.. – IQ 240 Jul 19 '20 at 17:50
• @PeterKämpf,is it possible that nobody at physics site and aviation site dont know answer? – IQ 240 Jul 19 '20 at 19:52
• Would this question and its answers help? – Peter Kämpf Jul 20 '20 at 9:59

Even though this isn’t strictly an aviation question, it does involve aerodynamic principles.

To put it simply, the diffuser’s main purpose is to gradualize the transition from rapidly flowing air under the car’s body as it transitions back to the speed of the ambient air behind the car.

There is a high pressure zone in front of the car from air particles colliding with the vehicle, and a low pressure zone in the wake behind the car as airflow is attempting to rejoin on the other side. This high to low flow draws air under the car where space is limited. This limited space results in an increase in speed and a decrease pressure, thus increasing downforce as the pressure below the car is less than the pressure on the top and sides of the car. This is Bernoulli’s equation at work.

However, the rapidly moving air experiences adverse pressure gradient at the rear of the car and airflow separation results in significant turbulence. This turbulence not only increases drag, but limits how quickly air can be evacuated from underneath the car. By carefully shaping a diffuser, the airflow can be slowed more gradually, easing its transition back into the ambient flow, minimizing turbulence which increases the flow speed under the car, decreasing the pressure and increasing downforce on the wheels even further.

The diffuser is shaped to increase the airflow speed in front of this funnel to ‘push’ a low pressure zone in front of the funnel, more effectively drawing air into the diffuser.

• ,,,wouldn be better to put wing low pressure side on the end of floor,so low pressure from wing will "suck/accelarate air under floor,so in this case we dont have adverse pressure gradient-situation? – IQ 240 Jul 20 '20 at 4:46
• The low pressure side IS under the car, where the strakes begin and air is accelerated into the front of the diffuser. The diagram you have up there isn’t precise to scale, it just shows the general region that the adverse pressure gradient begins. It can’t be avoided altogether. The airflow is going to separate from the underside surface. It’s just a matter of making that transition as smooth as possible. There are wind tunnel videos you can look up that use smoke to show the difference in turbulence with and without a diffuser. – Aaron Holmes Jul 20 '20 at 5:59
• is pressure in diffuser lower than atmosferic pressure? – IQ 240 Jul 21 '20 at 18:48
• Pressure in a diffuser is in transition from much-lower-than-ambient-atmospheric-pressure to a pressure that matches the low-pressure zone in the wake of the car, which is still lower than ambient, but higher than the flow underneath the vehicle. – Aaron Holmes Jul 21 '20 at 19:41
• ,So diffuser itself produce significiant downforce because his average pressure is lower than ambient pressure? – IQ 240 Jul 21 '20 at 19:57