Diffuser Pressure Peak

[tomspotley]

The reason could be that the air accelerates when approaching the transition point, like at the nozzle of a vacuum-cleaner.

Perhaps if you had finer resolution, ie more blue shades, this acceleration could be seen from a farther distance?

Could you explain the vacuum-cleaner analogy? Isn't a diffuser the opposite of a nozzle?

You can see in some of the images posted above that the flow definitely converges laterally (and therefore speeds up if incompresible) as it approaches the "kick up point". The flow from the sides seems to be sucked inwards.

I feel like the air is "competing" to get into the diffuser, like people scrambling for the exit at a concert (except because of incompressibility it has to speed up). I just can't work out what exactly is causing the air to rush into the diffuser rather than just go around it. I could have it the wrong way round though and the only reason the air is sucked in is because there is a low pressure there in the first place (caused by some other mechanism).

[xpensive]

Perhaps the vacuum-cleaner anology was not the best of xplanations, but when the diffuser entry is narrower than the floor,
air need to speed up in that transition point. The way I'm reading your post, you see it in a similar way?

[tomspotley]

Perhaps the vacuum-cleaner anology was not the best of xplanations, but when the diffuser entry is narrower than the floor, air need to speed up in that transition point. The way I'm reading your post, you see it in a similar way?

Sort of. It's a bit of a paradox though, as by definition a diffuser should expand and slow down the flow.

[bhallg2k]

www.ku.edu.np/mech/Tutorials/nozzles&diffusers.pdf

[hollus]

This is a tricky question of cause and effect.
Is there low pressure because the flow accelerates or is the flow accelerating because there is low pressure? If you ask me, both, both are cause and effect at the same time. If one of them fails, so does the other.

Now I'll try to explain it in a more mechanical "what goes where why" way. I like masses pushing masses more than I like Bernoulli, even if in the end they describe the same phenomenon.

To start with, there is a motor: The large force driving it all is the vacuum hole that the car creates behind itself as it punches through the air. The diffuser is an elegant way of moving this partial vacuum from behind the car, where it causes drag, to below the car, where it creates downforce. So the vacuum cleaner analogy is not at all out of place here.

Now to your two questions:

Why does air from the sides get attracted to the diffuser?
Air from everywhere rushes in to fill in the hole behind the car. The diffuser makes sure that some of it comes all the way from in front of the floor, but also air from the sides (and from above and from far behind the car) is trying to rush in. Skirts in the 80s were introduced to counter this effect. Today's cars live with it, but it is detrimental to downforce.

Why is there a speed peak / pressure low at the kink or if it be, the convex transition at the beginning of the diffuser?
You can think of it as a Coanda effect. Air is moving below the floor towards the back of the car (because the diffusser is giving it a clean path to the vacuum cleaner engine). Then it reaches the kink. This mass of air, if it were to continue to move straight, as it entrails air around it, would create a vacuum right behind the kink point. That of course won't happen (to any large extent), and any air available will move in to fill that partial vacuum that is being created right past the kink. This is the very same air trying to create the vacuum, which then a) bends upwards to fill this partial vacuum b) accelerates against a partial vacuum to do so (there is less air opposing it there than air pushing behind it to go there). Think of a gas expanding against a perfect vacuum. It accelerates as it goes there because nothing is pushing back.

So again, the Coanda effect is happening because there is low pressure there, and there is low pressure there because there is Coanda going on.

Confusing? To me the Bernoulli version is confusing.

[tomspotley]

Thanks, I will look into the Coanda effect, I don't know very much about it.

[xpensive]

...
To me the Bernoulli version is confusing.

Bernoulli is not really a "version" of anything, it's science, but I admit that I'm always interested in home-made xplanations.

However, by introducing the coanda effect into the mix you lost me.

[hollus]

Gas accelerating against a vacuum does so by transforming some of its internal energy (molecular kinetic energy with no net momentum) into net kinetic energy applying to the whole mass of air (which now has momentum).
The total energy is constant, it gets transferred from "internal" modes to "external" modes. From heat to net kinetic energy if you wish.
Bernoulli says: The total pressure is constant, it gets transferred between the static and dynamic terms.

Exactly the same thing described with two different sets of words. Marekk would be able to explain it better, but he seems to not be around these days.

Why do you think gases under Coanda effect change their trajectory? Why wouldn't the Coanda effect apply inside a diffuser?

[xpensive]

A "vacuum", what in the wide world of sports are you talking about, you should be more careful with your terminology?

[hollus]

Partial vacuum of course. A very small fraction of the atmospheric pressure (or density). It is just easier to visualize the effect by imagining a total vacuum. A partial vacuum will then achieve a fraction of the effect.

Some pictures in this page, http://ngcraft.com/coanda-effect/ what explains it much in the same terms I did, illustrates it more clearly. What is shown in the second picture only happens to a very small extent, because the situation in picture 3 is less energetically costly to achieve. But you can think of picture 2 as the cause of what happens in picture 3.




Every time you have a mass of air accelerating there is either something directly pushing it, or higher pressure in one side than in the opposite side. Bending a trajectory is nothing else than transversal acceleration.

[n smikle]

The kick-up point is at the transition from flat floor to diffuser.

That image shows that the pressure on the flat floor is lower than everywhere in the diffuser except around the kick-up point. My question is why does the air accelerate and lose pressure as it enters the diffuser (creating a low pressure peak at the kick up point).

that is correct. The reason for those pressure points is because as you said, the air passing closest to the kink has to turn sharply around the kink to stay attached. In layman's terms the air not only travels faster near the kink it also "stretches" itself as it takes the sharp corner so the pressure is lower. Any small radius corner that turns away from the air stream you will see this happen.

[tomspotley]

The kick-up point is at the transition from flat floor to diffuser.

That image shows that the pressure on the flat floor is lower than everywhere in the diffuser except around the kick-up point. My question is why does the air accelerate and lose pressure as it enters the diffuser (creating a low pressure peak at the kick up point).

that is correct. The reason for those pressure points is because as you said, the air passing closest to the kink has to turn sharply around the kink to stay attached. In layman's terms the air not only travels faster near the kink it also "stretches" itself as it takes the sharp corner so the pressure is lower. Any small radius corner that turns away from the air stream you will see this happen.

So it's a localised acceleration only at the parts of the flow near the floor? That was my initial explanation, but then I thought that you wouldn't be able to conserve mass flow without either the density decreasing, or there being a corresponding decrease in velocity elsewhere in the flow (which I couldn't really see, but didn't investigate thoroughly).

I guess I shouldn't have been so quick dismiss that as an explanation. Does the Coanda effect play a role in how the flow stays attached?

Edit: One could perhaps find a theoretical solution to this situation assuming a compressible, non viscous 2D flow using the N-S equations. I might try this tomorrow.

[Diff-user]

well i made a low speed wind tunnel (a small one) for my grad project and what i understood about the diffusers is something like this
Imagine the car has no diffuser (and zero ground effect), the pressure below the car is same through out its length and since the pressure at the back is atmospheric, so is the pressure for rest of the flow
Now consider a car with a diffuser, the pressure at the end of the diffuser is more than the pressure at its start.... however the pressure at the end has to be atmospheric because it is in direct contact with the outside air...... and thus the pressure at the start of the diffuser is less than the atmospheric pressure..... in wind tunnels this leads to higher test section speeds for the same amount of fan power.

[tomspotley]

well i made a low speed wind tunnel (a small one) for my grad project and what i understood about the diffusers is something like this
Imagine the car has no diffuser (and zero ground effect), the pressure below the car is same through out its length and since the pressure at the back is atmospheric, so is the pressure for rest of the flow
Now consider a car with a diffuser, the pressure at the end of the diffuser is more than the pressure at its start.... however the pressure at the end has to be atmospheric because it is in direct contact with the outside air...... and thus the pressure at the start of the diffuser is less than the atmospheric pressure..... in wind tunnels this leads to higher test section speeds for the same amount of fan power.

The pressure at the back of a car isn't atmospheric (otherwise it would have no drag!)

But yes, that is how a diffuser works.

[Diff-user]

so was that your doubt or have i just explained something you already knew and wasted a few kilobytes of F1T storage space ? (boy i need to learn some people skills)