Blown Diffuser: How does it work actually and why it's there

[jian]

Regarding recent protests by the Hispania about the ban on blown diffuser for Monaco GP, makes me wonder, how does blown diffuser work actually?

Recent searches on this forum, or youtube or google have returned me little to no relevant result.

I know it has something to do with hot air coming from the engine, and the exhaust system mountings... but actually what does it for and how it is going to improve aero efficiency in F1 and why Colin Kolles is trying to have it banned?

So, anyone care to enlighten me please? [-o<

[f1motta]

Start with this http://scarbsf1.wordpress.com/2010/07/1 ... s-q3-pace/ to understand what is all about with engine map and off-throttle.

Next just imagine that exhaust gases are directed into diffuser (blowing in to). More gasses, more downforce. Simple...

[horse]

f1motta is right in one way, you do want more gases through the diffuser, however you could argue that providing those gases from the exhaust would not be beneficial as you want to attract these gases from beneath the car in order to further reduce the pressure under the car and, therefore, increase the downforce.

Now exhaust gases are at higher pressure than ambient due to their temperature so, intuitively, this would not be useful for causing a reduction in pressure, but they also have higher energy than the air in the diffuser. An exhaust blown diffuser uses that energy in the exhaust gas to reduce losses in the diffuser allowing a larger diffuser to be used than you would without blowing.

QED.

[Rob01]

What Red Bull do is retard the ignition and maintain some throttle and fuel to allow combustion to continue to take place. However the ignition of the air and fuel mixture now takes place later in the engines revolution, when the exhaust valve has already opened. Rather than driving the piston down, the explosion of the mixture goes into the exhaust, still expanding as it does so. This creates a rush of gas through the exhaust mimicking the effect of running with the throttle open. Thus the diffuser still sees a flow of gas and maintains downforce despite the engine slowing down

Of course this gain doesn’t come for free, the heat of combustion now takes place in the exhaust port, so that the exhaust valve, cylinder head and exhaust pipe all suffer excessive heat. This will affect them, as they cannot withstand this sort of thermal load for long periods. Equally the process burns additional fuel, in the race this is a negative thing as fuel is limited and no refuelling is allowed.
This ignition retard mapping would be controlled via the ECU via the driver selecting a steering wheel control, using quite normal tuning parameters and not some clever workaround.

[rfs]

Does the EBD work in a similar way to the fans on a "fan car"?

[l.etranger]

The exhaust blowing probably only works a little like the fan, in the broadest understanding. Aerodynamically and of course mechanically they're quite different.

The fan works like any diffuser in that it's effectively a pump. An unblown diffuser uses a natural pressure difference to drive the pump (between the dynamic pressure at the front of the car and the wake pressure at the back). A fan and a blown diffuser both augment that natural flow by adding energy from the car's engine (roundaboutly, for the fan).

The fan does this by pulling the flow out from under the car to a secondary exit, lowering the static pressure by mechanical pumping.

The blown diffuser does this by accelerating the airflow under the car - adding, rather than removing gas flow from the rear of the car. Aerodynamically, it's more like a blown flap:
http://en.wikipedia.org/wiki/Blown_flap
or perhaps somwthing like an eductor:
http://en.wikipedia.org/wiki/Aspirator

[hardingfv32]

1) Exactly where do we want to add flow to the diffuser from the exhaust:
A) At or after the flat floor to diffuser transition (throat?) ?
B) on top or below the diffuser?

2) In RB case, are they adding flow to the diffuser OR using the flow to seal the edge of the diffuser?

Brian

[DaveW]

This might be a good starting point for researching the topic.

[godlameroso]

The more exhaust and hot air pass through the rear of the car the lower the pressure will be behind the car, this can form a vortex if controlled properly, and suck the car to the ground far more efficiently than the diffuser could do alone.
[youtube]http://www.youtube.com/watch?v=3Qu9wR03GVA[/youtube] Notice how they comment on the pressure dropping right before a huge gust sweeps by their vehicle at 1:20ish? Mind you this is only caused by pressure differentials, no wings or diffusers involved.
[youtube]http://www.youtube.com/watch?v=DShp7d6yozY[/youtube]

[hardingfv32]

I'm sorry, I did my research but this is still not clear to me....

1) I don't see the full relevance of the blown flap in this situation. How can we get the full benefit of blown flaps without holes on slat in the diffuser roof? We are not working directly with the beam wing are we?

2) To energize the diffuser I assume you want to add flow below the diffuser roof near the throat. Again, I gather this is not possible per the rules.

So what am I missing? It is probably some simple detail or incorrect assumption.

Brian

[DaveW]

I'm sorry, I did my research but this is still not clear to me....

I am not really qualified to respond in detail, Brian, but, for what its worth, here is an attempt. I'm sure other posters will correct me if/when I stray.

The downforce generated by a diffuser is, amongst other things, proportional to its expansion ratio (the ratio of exhaust CSA to throat CSA). Make the expansion ratio too large, and the flow through the diffuser will separate from the diffuser walls (the flow will stall) and much of the potential downforce will disappear. Diffuser stall can be delayed in a number of ways (many of them illegal in F1), but simply blasting high energy air along the upper surface of the diffuser (not in the diffuser itself) will help both to keep flow attached & to increase the downforce generated by the diffuser (proportional to the energy supplied). Having a second element to direct the high energy air will increase the effect &, very probably, the beam element can be arranged to perform that function.

According to aircraft people, the technique can more than double the maximum lift coefficient of a wing, but the actual increase will depend upon the energy supplied. In other words, diverting fuel energy from the wheels to the exhaust will reduce available traction, but will also reduce the need for traction (up to a point), by increasing corner exit speeds by virtue of the increased downforce. Putting it another way, the technique exchanges end of straight speed for corner exit speed. It will, however, be most effective through medium & high speed corners & least effective through low speed corners.

Practically, there are many things to consider when using exhaust gas to re-energize diffuser flow. These include aero balance (probably will require more dowforce from the front wing), power on/power off performance (minimizing the difference makes the technique driver-friendly), engine life, temperatures underneath the rear bodywork, fuel consumption, and "optimizing" the balance between traction & downforce. Incidentally, (& unusually in the over-regulated series that is F1) the balance can be varied at will by a driver during a race simply by changing engine maps.

[PlatinumZealot]

Ok my 2 cents..

I think present day blown diffusers have two generations (types).

1. 2010 blown diffusers.. these work by adding energy to the diffuser as described by Dave above. It seems teams prefered to blow the diffusers from the top and sides. I think they might have some gasses curling under from the sides.

2. 2011 RedBull blown "vortex" difffusers.. work by creating a large vortex using a strake that is placed before the throat of the diffuser. The strake is placed infront of the exhaust and it causes the exhaust gases to curl under and into the diffuser while creating a huge side sealing vortex. This vortex is extrememly faster than a regular free stream air vortex because it is at the speed of the exhaust. Way over 100m/s faster than the free stream air. So a super vortex is created.RedBull has mastered this.
I call these second generation of diffusers exhaust vortex diffusers.

This is the principle of operation.. inspired by strakes on a fighter jet. Notice you need an angle of attack for the vortices to get stronger.. couple this with a high speed exhaust and you have a total sealing off of the diffuser.. this means you can raise the diffuser higher of the ground and increase you expansion ration WHILE still having energy added from the exhaust to prevent turbulence.



Implementation on the Ferrari F-150..

[hardingfv32]

I agree with the 2011 RedBull blown "vortex" diffuser statement.

1) Is there a vortex formed by the exhaust flow as it leaves the exhaust pipe or is an interaction required with a body part?

2) Assuming the RB exhaust outlet location and the high velocity of the exhaust flow, will a vortex form soon enough to help the diffuser? I'm thinking that this exhaust location might be more about an air screen than vortex formation.

Brian

[DaveW]

....I think present day blown diffusers have two generations (types)...

The shielding, heat staining & detailing of the F150 undertray would appear to support your "vortex" thesis.

I would like to try to explain a little my "blown flap" analogy.

It was found that blown flaps (as in the Hunting H126) "re-energised" the upper wing surface boundary layer, delaying flow separation & hence stall, and increased "lift coefficient" by a substantial margin. There is no doubt that maximum lift was increased substantially, & that was caused by an increase in the effective wing plan area (the high energy jets supported a pressure difference across them). Thus blown flaps behaved rather like the large slotted flaps to be found on modern transport aircraft, & increasing the energy in the blown flap was rather like (further) extending a slotted flap to increase the plan area of the wing.

This much is known (& rather old) technology that was researched extensively around 50 years ago. It didn't make a widespread transition to "production" aircraft for various reasons, including a fairly terminal failure mode. Failure modes are not normally high on an F1 designer's list of priorities, however, & I'm sure the possibilities offered by the technology for improving diffuser flow stability & increasing its effective length & expansion ratio have occurred to more than one, along with the thought "if only exhaust energy was independent of throttle position".

Anyway, the solution to that problem has now been supplied (most probably by Renault engine people) & the genie is well & truly out of the bottle. It will be interesting to see if the belated attempt by the FIA to reverse it will be successful, now that the performance benefits of the technology have been demonstrated on track.

[shelly]

Agree with nn smikle, as I wrote here

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