Diffuser Profile - Convex or Concave?

[shelly]

That's what I am talking about!

Could you also make some more section without streamlines, to better see the colors?

[DRCorsa]

I think that these vortices are generated by the entrance of outsie air on the diffuser volume due to low pressure.

Here's the pressure contour and velocity contour on the plane we are looking at.

[shelly]

Yes exactly,they are the tip vortices of the diffuser. You can see in this picture of yours


The strong blue low pressure zone they produce on the downfacing flat surface of the footplate. EBD is about accelerating d enhancing these vortices in my opinion.

[DRCorsa]

shelly, what if the footplates were rounded, like the channels of the front wing endplates?

[shelly]

I think rebdull rb5 had those type of channels, and also tioro rosso str5 last year.
I suppose you could house a small vortex in the arch and makethe flow look a bit differen. Then some flow could get inside the diffuser going unde the small cirular vortex - I am not sure about it.

It is interesting to see how those arches were on the original rb5 withour ddd, and progressively got eliminated with ddd and ebd.

[shelly]

See for example here:
http://www.formula1.com/news/technical/ ... 9/663.html

[wesley123]

also what is coming back(as well being used as far back as the early 90ies) is the rounded channels, if you take a look at the red bull you see that the strakes arent just placed inside of the diffuser, no they have a rounded edge. Early 90ies this was done even further, having fully circular channels

[hardingfv32]

if you take a look at the red bull you see that the strakes arent just placed inside of the diffuser, no they have a rounded edge. Early 90ies this was done even further, having fully circular channels

What is meant by round edge? A fillet where mount to the diffuser floor?

Brian

[hardingfv32]

Here is the image from "Investigation of Turbulence Created by Formula One™ Cars with the Aid of Numerical Fluid Dynamics and OptimizationofOvertakingPotential"



1) What is meant by "shows clearly the breaking at the diffuser and the turbulences at the backlash"? Where and what is the backlash?

2) What is meant by "steadying current setting back down"?

Can these be restated in layman terms?

3) Why is the different pressure value (color?) above the bodies in the two images? Does that make for a valid comparison?



Brian

[marekk]

I yet to see a concave shape ANYWHERE in nature or industry in the context we are speaking. That is a bold statement I hope someone will take the time to disprove.

Brian

Well, think this one is not that uncommon:




If you imagine ground effect as mirrored (second half under the ground) picture of the aero shape - which is basically correct, it would be quite similiar.

It's the best shape for pressure recovery efficiency in supersonic flows, but works for lower speeds to.

The basic idea behind is the same in both cases - optimal expansion from higher (at floor's leading edge or rockets combustion chamber) to lower (behind the nozzle/diffuser) pressure.

The difference between both cases is rocket nozzle is driven by high pressure in combustion chamber, diffuser by low pressure behind the car, but physics works almost always in symmetrical way, so no big deal at all.

I'm quite sure there will be separation bubble just behind kink line with such extreme design (air molecules do have a mass and just can't change the direction 90 degree at the sudden), but the flow has a chance to re-attach later.

As far as strakes goes, my very uneducated guess is that beside obvious roles of separation of regions with different pressure, they are slightly cambered, vertical airfoils, and as any lift generating airfoil they produce tip vortices, one being shedded near the roof of the diffuser (which helps to energize boundary layers), and the other on the ground facing side, probably co-rotating and merging with vorticies entering the diffuser. Nothing is free, so they add some drag (both form and induced) to the system.

[hardingfv32]

I did consider the engine nozzle as there is many studies on the subject. I just did not think that they were concerned about generating low pressure. But I guess they are concerned with energy conservation. Maybe the Reynolds numbers are in a different ball park, because I did see a study showing that they effect the diffuser's performance.

Brian

[KeithYoung]

Be careful when comparing this to the diverging section of a rocket. F1 cars operate basically in the incompressible region, whereas rockets have supersonic flow at the throat, which means during the diverging area (physically like the diffuser of the car) is actually accelerating the air.

A diffuser slows the flow and increases the pressure.

[DRCorsa]

Here is the image from "Investigation of Turbulence Created by Formula One™ Cars with the Aid of Numerical Fluid Dynamics and OptimizationofOvertakingPotential"



1) What is meant by "shows clearly the breaking at the diffuser and the turbulences at the backlash"? Where and what is the backlash?

2) What is meant by "steadying current setting back down"?

Can these be restated in layman terms?

3) Why is the different pressure value (color?) above the bodies in the two images? Does that make for a valid comparison?



Brian

The two images have different color scale, so not directly comparable.
If we make a quick try to "translate" the colours we will see that on Fig.8 there is high pressure on the concave surface (yellow-light green colour which is actually positive pressure, which is indicating that the diffuser has stalled), while on Fig.7 the whole space under the concave surface has negative pressure (light blue-light green).
The prolem is that this is very different to the situation on an actual F1 car, where the beam wing is situated before and a lot higher to the diffuser throat (see picture of the RB7 below) and not directly after it. The author of the paper just wanted to make the concave diffuser work, not to simulate the actual conditions of an F1 car.

[wesley123]

if you take a look at the red bull you see that the strakes arent just placed inside of the diffuser, no they have a rounded edge. Early 90ies this was done even further, having fully circular channels

What is meant by round edge? A fillet where mount to the diffuser floor?

Brian

Take a look at the strakes, normally these are mount directly to the floor with a sharp edge, here it is more rounded.


And here the Nissan P35, you can see how rounded the tunnels are.

[shelly]

if you take a look at the red bull you see that the strakes arent just placed inside of the diffuser, no they have a rounded edge. Early 90ies this was done even further, having fully circular channels

What is meant by round edge? A fillet where mount to the diffuser floor?

Brian

Arches like in the fornt wing and in the picture I linked