Paul,

I need you to clear something up for me please.  The F1 diffusers
diverge both vertically and laterally aft of the rear axle centerline,
which would slow the airflow.  The undercar airflow speed would also
decrease (since it flows into the diffuser), raising pressure--the
opposite of what is needed.  What am I missing here?

Ron

Well the divergence in the duct cross sectional area causes a low pressure
area back there. Air wants to flow from a high pressure area (just behind
the front wheels under the tub) where the diffuser starts to the back of the
diffuser. The area divergences causes a drop in pressure which causes a speed up
of the air flowing under the diffuser. That causes a low pressure area on the
under surface of the diffuser. This must be done carefully as a diffuser can
"stall". Perhaps. I think. It must be tested full scale on the car or
in a wind tunnel with a moving ground plane as the air adjacent to the ground
is stationary while the air immediately adjacent to the front under surface
of the diffuser is moving at vehicle speed.

BTW The first time I tried a crude one on the Chaparral 2F in the late 1960's it
had so much down force it ripped it self off. I had to beef it up. That gave me
a clue to the potential. Of course we did not exploit that because we had high
aspect ratio movable wings. I suspected a full width high mounted  wing would
have a better down force to drag ratio but I could be wrong. I asked a Ferrari
engineer at the 2008 Oxford  WMSS if he thought the down force to drag ratio of
the diffuser was better than the current dumb and silly narrow span wing and he
choose the diffuser. There is a reason gliders have long wings.


Paul Lamar


I still don't get it.  The divergence of the diffuser cross section causes the air flow to slow, which you say causes a pressure drop.  But in a venturi, the pressure drop is due to the airflow speeding up.  This conflict is what's confusing me.

Ron

Yes the air relative to the car does slow in certain areas.
However the pressure behind the car is always below ambient pressure.
That is where a lot of the drag comes from.
However this low pressure area draws more air volume under neath the car and through the diffuser.

It is not the same as a venturi. The walls of a Ventura move together.
A diffuser is moving over a stationary ground plane.

What is important is the pressure distribution on the ceiling of the diffuser.
That is the bottom line. The vertical component of the air pressure vector is down force and the horizontal component is drag. The pressure vectors are always perpendicular to the surface.

The red line in this diagram is the air velocity profile underneath
the diffuser. The air velocity is zero near the ground and at the speed
of the car next to the ceiling of the diffuser. Think of air as having viscosity and stickiness. The arrows are the pressure vectors.

Paul Lamar