How it works-Sports car diffuser

Incredible · May 19, 2016, 09:24

What I know: Nozzle increases the fluid velocity and decreases the pressure. Diffuser (a divergent pipe) in a general sense decreases the fluid velocity and increases the pressure.

Now apply this to car undertray: Diffuser at the end of the undertray should decrease air velocity and increase air pressure. This increase in air pressure below the undertray should create LIFT.

Paradox: Diffuser at the end of car's undertray actually creates Downforce!!

Can anyone solve this paradox? Thanks in advance!

fluid23 · May 19, 2016, 16:21

You are forgetting 2 things:

1) the effect of the undertray as a whole, the nozzle and throat sections will create more downforce than the 'lift' of the diffuser. the diffuser is only there to slow the air back down before it dumps back out the back which will reduce drag penalty of the system.

2) ground effect and the air moving over the top of the car will still result in a net downforce even in the diffuser, I believe.

View Poll Results: What does an undertray diffuser on a sports car do
Creates Lift force	0	0%
Creates Downforce	3	100.00%
Voters: 3. You may not vote on this poll

May 19, 2016, 09:24	How it works-Sports car diffuser	#1
Incredible New Member Anonymous Join Date: Apr 2016 Posts: 1 Rep Power: 0	What I know: Nozzle increases the fluid velocity and decreases the pressure. Diffuser (a divergent pipe) in a general sense decreases the fluid velocity and increases the pressure. Now apply this to car undertray: Diffuser at the end of the undertray should decrease air velocity and increase air pressure. This increase in air pressure below the undertray should create LIFT. Paradox: Diffuser at the end of car's undertray actually creates Downforce!! Can anyone solve this paradox? Thanks in advance!

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May 19, 2016, 16:21		#2
fluid23 Senior Member Matt Join Date: Aug 2014 Posts: 947 Rep Power: 18	You are forgetting 2 things: 1) the effect of the undertray as a whole, the nozzle and throat sections will create more downforce than the 'lift' of the diffuser. the diffuser is only there to slow the air back down before it dumps back out the back which will reduce drag penalty of the system. 2) ground effect and the air moving over the top of the car will still result in a net downforce even in the diffuser, I believe.