Blown undertray

Formula 1 cars of the past have used blown diffusers to help create more down force at lower cornering speeds in order to achieve faster lap times. I was wondering if it would be possible to use an impeller compressor (like an industrial leaf blower) to blow the entire length of the under tray thus creating extremely high air speeds and volumetric flow underneath the car regardless to the vehicles actual speed. The particular unit I am interested in using as a theoretical benchmark to determine effectiveness creates 190+ MPH wind speed and flows 2600 CFM, while this looks extremely impressive in it's original role, is this enough to achieve noticeable aerodynamic advantages?

Eager to hear peoples opinions on this concept.

In reply to TURB1NE :

what car?

In reply to sleepyhead the buffalo :

EJ2 Civic. I'm looking to create low drag down force that is also effective at low speed. As an alternative to a centrifugal compressor I am considering two of these: http://www.vasyfan.com/vf-390-special-application.html

Use the 2600 CFM number and the width/height of the slot you intend to blow thru to work out what airspeed you will be effectively creating. That will be your cross-over from blower amplified to passive, as a first approximation. The idea of using the blower to keep a continuously blown surface will help you run a steeper diffuser as well. another item to keep in mind is the inflow for your blower, try to make use of that as well. Perhaps pull hte source air from a high pressure/forward facing location like the base of the windshield. That way you get the downforce with a possibility of drag reduction..

Keep in mind the diffuser is primarily controlled by two things: The mass flow of air entering (generally the ground clearance at the diffuser inlet/and or the ground clearance at the nose, AND the base pressure behind the car (the low pressure at the bumper above the diffuser exit). If the inlet is too small the diffuser won't be capable of expanding properly and may stall (your blower will help this) {This is considered stall or choking}. The base pressure is what drives the flow along the diffuser and a lower pressure (suction - below atmospheric pressure) will help the diffuser stay attached longer and at higher angles.

You can make use of the low pressure behind the tire to expand a diffuser left&right as well at up, so consider this. The F1 cars use the exhaust to blow the diffuser between mid-way and the back edge. This allowed the shape to expand beyond the generally accepted 7degrees. The best solution is to blow the trailing edge as much as possible. Putting the blow slot in the middle of the diffuser will have effects that vary with speed (as the normal mass flow changes with car speed).

In reply to stafford1500 :

Very helpful, thank you!

Based off some approximate calculations to get 150 mph worth of airspeed underneath the car I need to move around 30000 CFM with as high of a S.P rating as possible.

If optimized the under tray could in theory develop around 500 kg of down force while stationary all the way to 150 mph.

I think the best option for this application would be an industrial grade hvac blower.

In reply to TURB1NE :

How many cfm is your exhaust at say 4500 rpm?

Maybe it's a big chunk of 30k?

Also, what sort of calcs are you using to predict diffuser performance?

In reply to Robbie :

Just a simple calculation of CFM and duct (undertray) cross section. This obviously is just an estimation since the actual figure is dependent on variables like ride height, road quality, temperature, humidity and more.

While I'm still interested in a fan assisted downforce concept I have decided it will be far more efficient to create a constant vacuum underneath the car like the Chapparral 2J, Brabham BT46B, and the corvette that was built for GRM $2007.

I have ideas on how to maintain a constant seal regardless of suspension travel, I just need to find a suitable electric fan. I need to flow around 20000 CFM in this configuration (more is even better) and have as much In. Wg. pressure output as possible.

The only fan I have found that would be effective for this application is a military howitzer cooling fan, I am still looking for a commercially available option.

stafford1500 said:

Keep in mind the diffuser is primarily controlled by two things: The mass flow of air entering (generally the ground clearance at the diffuser inlet/and or the ground clearance at the nose, AND the base pressure behind the car (the low pressure at the bumper above the diffuser exit). If the inlet is too small the diffuser won't be capable of expanding properly and may stall (your blower will help this) {This is considered stall or choking}. The base pressure is what drives the flow along the diffuser and a lower pressure (suction - below atmospheric pressure) will help the diffuser stay attached longer and at higher angles.

For whatever reason, this thread, and that comment finally clicked something in my head about the Nissan GT-R LM Nismo... and why the internal tunnels make both less drag and more downforce.  Like I knew the whole low pressure feeds the diffuser, and just didn't make that final jump between the two... that you're getting some downforce from the tunnels being raised... but part of it is supplying high speed air at the back of the car to create a stronger low pressure zone at the exit of the diffuser to make it more then draw more air out if it... synergistically.

re: mechanical blowing
Something to keep in mind, is that blowing is usually discussed in "aero" terms in much more limited flow volumes.  Most of the study of it is putting a wide and 'short' blast of air over the 'low pressure' side of a foil to put high-speed flow into the boundary layer, which due to friction tends to have slower flow.  When an airfoil curves around on the low pressure side, the slowing effect of that curve in addition to the slowing of the flow rubbing on the surface can combine to cause the flow to not stay attach.

It sounds like TURB1NE wants is something a bit different... and yeah, you're gonna need some honking fans to pull that kind of flow... especially since the shape of a civic is going to limit where you can suck out/through.  Although, you might consider less "fan" and more "turbo"?

Would a few leaf blowers make a meaningful difference? Didn't someone do that at the challenge? 

A little searching found this.  Certainly not leaf blowers. 

https://grassrootsmotorsports.com/articles/how-turn-corvette-2000-sucker-car/