Diffuser Confusion

[hollowBallistix]

hollowBallistix and vonk

You should both read:

Experimental study of multiple-channel automotive underbody diffusers

http://pid.sagepub.com/content/224/7/865.full.pdf+html

hollowBallistix, at the start of the study it states how/why the diffuser functions. It differs from your ideas.
vonk, in the main body of the report there are actual pressure tap test results that show, that in fact, the diffuser functions as/in an open system.

Brian

Cheers for the Link Brian, but does it really differ from my understanding ?

Surely the diffuser section at the rear does not speed up the air flow as was stated in this thread earlier ?

Ground interaction occurs on the flat section of the floor, is this not where the pressure becomes the lowest because the flow is the fastest ? without the diffuser section at the rear, then high drag would be the penalty for the downforce generated by the low pressure area under the car.

Now to overcome the drag penalty, the diffuser is added, which if i'm reading this paper correctly created an up-wash of vortices that keep the air flow attached to the diffuser, so wouldn't there be a increase in pressure & drop in velocity of the air within this diffuser section ?

Taken from another online resource is this...

"A diffuser is a specially shaped aerodynamic device attached to the rear of many racecars. A moving car causes a low pressure region to develop just ahead of the diffuser. This decrease in pressure (relative to the higher pressure on top of the car) causes an increase in downforce, which allows a car to corner faster. The diffuser actually decelerates the airflow after the air reaches its maximum speed (which according to Bernoulli's principle equates to its minimum pressure), and attempts to return the air to its pre-accelerated condition in order to minimize drag."

Isn't that what I said in really basic terms in my original post

[godlameroso]

Exactly the air moving under a flat floor is faster than the air on top of the chassis, the diffuser tries to get the air to the same speed so that some of the flow underneath can re-attach to the flow above.

[PlatinumZealot]

You see this suction peak thing... it is only at one line across the throat of the diffuser. Doesn't mean the whole floor of the car is working good.

The diffuser doesn't pull anything. It cannot do mechanical work. zero work... please.. you break the laws of physics when you say that. The car and the atmosphere is what is doing the work.. the diffuser only makes the work easier. Like oiling a bearing. Diffuser keeps energy loss to a minimum. That is the key to a diffuser. The kink is not a part of the diffuser btw.. the kink is just and edge.. a diffuser is not an edge... the effect of high peak velocities around that edge is also facilitated by the diffuser.

The concave diffuser will work. The major difference between it and regular diffuser is that your rigid diffuser upper wall is now replaced by a wall made of the fluid (air). Since this wall is not rigid, there will be a difference in behaviour which I personally have not looked into as yet. .

One has to explain the behviour of the eddy currents or whatever is inside there that makes up the boundary between the smooth stream before you can make some conclusion.

[vonk]

You see this suction peak thing... it is only at one line across the throat of the diffuser. Doesn't mean the whole floor of the car is working good.

The diffuser doesn't pull anything. It cannot do mechanical work. zero work... please.. you break the laws of physics when you say that. The car and the atmosphere is what is doing the work.. the diffuser only makes the work easier. Like oiling a bearing. Diffuser keeps energy loss to a minimum. That is the key to a diffuser. The kink is not a part of the diffuser btw.. the kink is just and edge.. a diffuser is not an edge... the effect of high peak velocities around that edge is also facilitated by the diffuser.

The concave diffuser will work. The major difference between it and regular diffuser is that your rigid diffuser upper wall is now replaced by a wall made of the fluid (air). Since this wall is not rigid, there will be a difference in behaviour which I personally have not looked into as yet. .

One has to explain the behviour of the eddy currents or whatever is inside there that makes up the boundary between the smooth stream before you can make some conclusion.

Great minds think alike, n smikle!

Let’s hear it for the “Kamm Diffuser”! =D>

I like this concept because, like the Kamm Tail, it would be self adaptable to speed, unlike a fixed geometry diffuser. If this comes at some cost of efficiency, so be it.

As to why it works? Under the pressures of racing, intuition fathers more inventions than theory. “Letts just try it”.

vonk

[godlameroso]

I should have said the interaction between the shape of the diffuser and the air stream causes a siphoning effect with the airflow at the tea tray and the leading edge of the floor.

[xpensive]

Exactly, the air moving under a flat floor is faster than the air on top of the chassis, the diffuser tries to get the air to the same speed so that some of the flow underneath can re-attach to the flow above.

I would try to describe the same this way, the air xiting the diffuser will have the same speed as the air on top of the car,
which forces the air under the floor to speed up in order to maintain the same volumetric flow all the way.

The latter is in a perfect world of course, without "leakage" of air coming in from the sides, which is why sliding skirts are banned since 30 years.

A flat floor all the way, without diffuser, would still accellerate the air under the car due to the wake behind it, which Mauro Forghieri discovered at Ferrari almost 40 years ago when comparing downforce of the F1 car with the wider sportscar.

But by far not as efficiently.

Wonder what happened to the novel diffuser theory which prompted this thread in the first place?

[PlatinumZealot]

Great minds think alike, n smikle!

Let’s hear it for the “Kamm Diffuser”! =D>

I like this concept because, like the Kamm Tail, it would be self adaptable to speed, unlike a fixed geometry diffuser. If this comes at some cost of efficiency, so be it.

As to why it works? Under the pressures of racing, intuition fathers more inventions than theory. “Letts just try it”.

vonk

Hey it will work.. but it might not work as well as a rigid wall... i do not know. I was setting up an ultrasonic flowmeter to measuere some drainage flows the other day.. and the closer the sensor got to the abrubt opening at the end of the pipe the more turbulence was experienced (seen as an inaccuaracy in the reading). The end of this pipe was bounded by the same fluid itself. So a non-rigid boundary in most cases cause energy losses because the fluid in the smooth part of the stream can turn into it. I think you have to play some tricks to get that concave diffuser to work as well if not better than a rigid wall diffuser. I think the main advantage of it would be having a "variable wall" almost like the f-duct of diffusers.

[hardingfv32]

What would be the benefit of this variability with the diffuser?

I assume we are talking drag reduction of some sort? Reducing the exit area?

Brian

[godlameroso]

Great minds think alike, n smikle!

Let’s hear it for the “Kamm Diffuser”! =D>

I like this concept because, like the Kamm Tail, it would be self adaptable to speed, unlike a fixed geometry diffuser. If this comes at some cost of efficiency, so be it.

As to why it works? Under the pressures of racing, intuition fathers more inventions than theory. “Letts just try it”.

vonk

Hey it will work.. but it might not work as well as a rigid wall... i do not know. I was setting up an ultrasonic flowmeter to measuere some drainage flows the other day.. and the closer the sensor got to the abrubt opening at the end of the pipe the more turbulence was experienced (seen as an inaccuaracy in the reading). The end of this pipe was bounded by the same fluid itself. So a non-rigid boundary in most cases cause energy losses because the fluid in the smooth part of the stream can turn into it. I think you have to play some tricks to get that concave diffuser to work as well if not better than a rigid wall diffuser. I think the main advantage of it would be having a "variable wall" almost like the f-duct of diffusers.

I'm curious to see it as well, maybe it could be done with the rear crash structure, to reduce some drag, I don't recall anything in the rules about a concave end to the crash structure, I don't think it would impede the rear light any.

[vonk]

Hey it will work.. but it might not work as well as a rigid wall... i do not know. I was setting up an ultrasonic flowmeter to measuere some drainage flows the other day.. and the closer the sensor got to the abrubt opening at the end of the pipe the more turbulence was experienced (seen as an inaccuaracy in the reading). The end of this pipe was bounded by the same fluid itself. So a non-rigid boundary in most cases cause energy losses because the fluid in the smooth part of the stream can turn into it. I think you have to play some tricks to get that concave diffuser to work as well if not better than a rigid wall diffuser. I think the main advantage of it would be having a "variable wall" almost like the f-duct of diffusers.

I think there’s general agreement that the air entering the diffuser will do so at speeds/pressures that vary with the speed of the car. But the exit condition is always the ambient static pressure in the car’s wake.

So, regardless of the philosophy by which one arrives at the inflow conditions, the diffuser must always have a matching area ratio to avoid back flow. A fixed wall diffuser can’t do that.

The following drawings depict three expansion conditions that might be expected in a concave diffuser (if it works ).



But it’s late and I’m going to bed. Good night, I’ll talk to you tomorrow.

[shelly]

Diffuser is not just about smooth transition of fast flow under the flat floor.

Suction peak is on the kink line; after that, a concave shape will give you a more aggressive pressure recovery.
The two dirvers of underbody performance aree the two suction peaks on the underbody leading edge and on the kink line.
One must take into account th 3dimensionality of flow and the presence of strakes which generate vortices at thier edges. The fences start at the kinkline and enhance the low pressure in the channels, abit upstream and a bit downstream of their leading edge

Shelly,

Could you be more specific?

What are “suction peaks”? And, why are they where you say?

Please define “aggressive pressure recovery”.

What do you mean by “enhance the low pressure in the channels”?

What’s “a bit”?

How do you know all this?

vonk

As somebody has already pointed out, concave diffusers are not a novelty: they have been around for at least 10 years in f1.
As far as diffuser function and suction peaks: vonk, i suggest you get these and others ideas from the previous thread "diffuser functions". The name "suction peaks"is self explanatory, and they are located in the two zones where canges in direction in the flow occur, i.e. floor leading edge and diffuser kink line.
They are very important in determining the overall downforce level of the floor, and their importance shoud not be underestimated like n_smikle does.
If tuo have a low pressure zone -line, its effect will not vanish immedialtely, but it will be felt a bit upstream and a bit downstream; how much is this bit? It dependns on how strong your peak is: it could be some 30cm upstream and downstream

Of course if your suction peak on the kink line is vey intense, you will need an aggressive pressure recovery to turn the flow back to ambient pressure in the diffuser, and in this a concave shape will do the job better.

The strakes act as turning vanes, and on of their functions is producing vortexes whoase low pressure cores help in increasing the general downforce level of the floor and diffuser system.

It is more 3d then 2d.

Your idea about different inlet conditions for the diffuser with varying speed is wrong: except for reynolds effects the flow is in dynamic similarity at different speeds

[vonk]

Diffuser is not just about smooth transition of fast flow under the flat floor.

Suction peak is on the kink line; after that, a concave shape will give you a more aggressive pressure recovery.
The two dirvers of underbody performance aree the two suction peaks on the underbody leading edge and on the kink line.
One must take into account th 3dimensionality of flow and the presence of strakes which generate vortices at thier edges. The fences start at the kinkline and enhance the low pressure in the channels, abit upstream and a bit downstream of their leading edge

Shelly,

Could you be more specific?

What are “suction peaks”? And, why are they where you say?

Please define “aggressive pressure recovery”.

What do you mean by “enhance the low pressure in the channels”?

What’s “a bit”?

How do you know all this?

vonk

As somebody has already pointed out, concave diffusers are not a novelty: they have been around for at least 10 years in f1.
As far as diffuser function and suction peaks: vonk, i suggest you get these and others ideas from the previous thread "diffuser functions". The name "suction peaks"is self explanatory, and they are located in the two zones where canges in direction in the flow occur, i.e. floor leading edge and diffuser kink line.
They are very important in determining the overall downforce level of the floor, and their importance shoud not be underestimated like n_smikle does.
If tuo have a low pressure zone -line, its effect will not vanish immedialtely, but it will be felt a bit upstream and a bit downstream; how much is this bit? It dependns on how strong your peak is: it could be some 30cm upstream and downstream

Of course if your suction peak on the kink line is vey intense, you will need an aggressive pressure recovery to turn the flow back to ambient pressure in the diffuser, and in this a concave shape will do the job better.

The strakes act as turning vanes, and on of their functions is producing vortexes whoase low pressure cores help in increasing the general downforce level of the floor and diffuser system.

It is more 3d then 2d.

Your idea about different inlet conditions for the diffuser with varying speed is wrong: except for reynolds effects the flow is in dynamic similarity at different speeds

Shelly, in “Diffuser Functions” n smikle wrote on Apr 03, 2011 5:54 pm:

Diffuser do not do work. Thermodynamics.

Suction is the opposite of pumping. which are both forms of work.
To suck something has to do work.


It only looks like the diffuser is suction but in reality the ORIGINAL stream of air front the front of the car is expanding at high speed. Diffuser is just facilitating it.


OK.. i know people say I like to bring up "unrelated stuff" but Air conditioning systems, and turbines use diffusers to make expansion to atmospheric more efficient. Less ducting losses.


Inside is lower than atmospheric, so it is easy to say it is sucking, (if you tap the diffuser throat air will be sucked in from outside.) But since suction is work what is doing the work? In fact doing this will cause the machine that is doing work to lose power. Your turbine will loose power and your AC system will experience more ducting loses, so the fans will need to do more work. So in these cases something is doing more work to make up for tapping the throat of the diffuser to get suction.


butRemember the diffuser itself is not doing anything. Take the formula 1 car, what is doing the work? The car. The car is "pushing" the fluid under the floor.
If you follow the original streamline of the fluid from beginning of the front splitter you will see that it is not being sucked, because the air is actually moving from low pressure from the throat of the front splitter, to a slightly higher pressure in the diffuser throat to higher pressure at the end of the diffuser. (expansion to atmospheric) Suction is not low pressure to high pressure is it? That is why you have to be very careful with this.


The trick of pointing the exhaust near the throat of the diffuser might make it seem that the diffuser is doing the sucking as well (like the tapping I mentioned before). And just like the tapping example I mentioned before something has to do the work to make the suction happen. The diffuser cannot do it, so what is left is the car or the exhaust gasses and the atmosphere. You know what this means? Your car will work harder when you tap the diffuser. It will be seen as more drag under the floor. Or if you wisely use the exhaust your exhaust will loose some of its energy, which is actually not bad at all, since it's not going to be used for anything else anyway.


That is just my take though. I always try to work from the fundamentals.

I’m sorry, but I’m with n smikle.

[godlameroso]

I have been under the impression that Reynolds number was to calculate conditions for scale models in a wind tunnel(please correct me if I'm mistaken).

Anyway I found this article while researching something, and though it would be interesting for this discussion.

http://www.nasa.gov/centers/dryden/pdf/ ... H-2255.pdf

[shelly]

I’m sorry, but I’m with n smikle.

I am sorry for you both.

The basic premise in n smikle post, that it is the car doing the work, is correct though. But implying from that that there are not suction peaks on the car is wrong. If the use of the word "suction" gets you confused, try substituting it with "low pressure" and then rethink the whole reasoning.

@godlameroso: Re comes into play about when scale and speed are vaied. Keeping the same scale, Re will change with changing speed and this will influence the flow (boundary layer thickness, turbulence microscale dimension). Reynolds effects can be neglected sometimes. For examples in f1 wind tunnel they assume it to be negligible when they test a 60% scale model at 60 m/s, because they assume turbolence is fully developed, transition point are not significantly offset etc.

But if they wanted to keep Re similarity, they would have to double the speed each time they half the scale.

[PNSD]

They use rather large transition strips on the front wing dont they?

Shelly, I guess you are from an aerospace background?