Diffuser Confusion

[vonk]

vonk, I think the basic issue that has to be ironed out is that there is no 90° degree surface turn in the picture of the rbr diffuser you posted. Once this misunderstanding is removed, discussion could maybe proceed.

Also note this: a section like the one you sketched would not have upward deflecting streamlines, but rather an horizontal wake boundary line with no pressure recovery (and no downforce on the floor). Remember that in a subsonic field information travels both upstream and downstream.

Shelly,

I now call it a "sharp upturn" to avoid the 90° issue. But you must admit that the picture undeniably shows such an upturn.



I don’t think anybody knows what the flow in the diffuser would really be until some test data and pictures are available. Until that time, we’re all just guessing.

In my earlier post (#p256804) I said: “….. the grey bands represent the interface between the turbulent region denoted by the black vortex and the inflow. What goes on in that interface, and how broad it would be, would depend on the inflow conditions.

As I mentioned before, all of this is highly conjectural on my part. To what degree is the inflow at B laminar? Will an interface actually develop, or will the inflow burst into turbulence the moment it enters the diffuser. Is “diffuser” a misnomer for this device? Since it is the equivalent of “backflow”, the turbulent region would be at ambient pressure, though.”

vonk

[vonk]

Could the flow at the throat/kink be that slow as to allow the steep rake (say 30 deg) at the front of the diffuser without stalling?

Brian

I presume by "stalling" you mean flow separation at the throat/kink. I think the sharp turn is designed to force separation and make the flow ride the corner vortex instead.

vonk

[vonk]

Could this situation be thought of as a Backward-Facing Step flow shape?

Here is a paper on the subject. Does it offer any insight into our concave diffuser discussion? It is beyond my skill set. At least I might have found a description to use for searches.

http://www.flow3d.com/pdfs/tn/FloSci-TN81.pdf

Brian

Brian,

Thanks for that link. The graphics seem to at least confirm the existence of a corner vortex.

[shelly]

vonk, the "sharp upturn" is probably around 30°, so that the flow is attached.
It is sharp to increnment the intensity of the suction peak; there is no separation on purpose
If you look at the fences in the picture you can realize by yourself that the arrow you have drawn are wrong and the turn is sharp, but around 30°

[hardingfv32]

Is it correct to say the the ratio of the diffuser opening to exit opening sets the pressure differential for the system. This assumes proper flow. That said, why have a concave shape when convex would be just as good.

Are they trying to increase the negative pressure under the roof of the diffuser to create a net downforce system. I assume there would need to be a higher pressure zone on top of the diffuser. Is the diffuser creating downforce beyond its normally understood method?

Brian

[horse]

there is no separation on purpose

Agreed, I can't see why you would want to induce separation at the diffuser kink line.

I think for me, the main argument against such a sharp corner would be the space usage. Why take all that useful packaging space for a region of stagnant air?

There is quite a lot of dependence on the side vortices for good performance at lower ride heights. Would such a geometry generate these vortices? I'm not sure. If they did then surely it would simply push the flow to the roof of the diffuser anyway? What do you gain other than moving the centre of pressure?

[hardingfv32]

Wouldn't side vortices be for good performance at all ride heights except very low. Maybe there is an effort to form the vortices at a greater strength sooner.

Brian

[shelly]

You want a sarp turn to have a high suction peak, it is not only the cross section ratio that rules the pressure on the floor.The pressure drop on the kink line is very important.
After the sharp turn, a concave shape could favour pressure recovery or simply be dictated by rule complying issues

[hardingfv32]

Yes, the pressure drop on the kink line is the main goal. The steepest angle you can use, say about 30 deg, will create the greatest pressure drop. The rest of the roof, mostly flat, gets the exit to favorable location in relation the the beam wing, etc.

Brian

[slimjim8201]

I've run a simple but informative study to examine the effects of both diffuser angle and type (straight, convex, concave). I've included data for three different angles (5, 10, and 15 degrees) and for each of the three configurations. There is also a baseline case in which there is no diffuser.

Study Details:

- Generic 2D body
- Velocity = 25 m/s

Enjoy...

[hardingfv32]

Is there any significance to the rate or amount of pressure recovery in the diffuser area?

It seems that there is a compromise with lower pressures under the floor being associated with almost no pressure drop in the diffuser. The diffuser area itself producing less down-force in the 10 deg concave example above.

Brian

[vonk]

Thinking out loud #1: Generic Diffuser




The “Generic F1 Diffuser” shown in this picture has the following properties:

1. It is an Open system that does NOT isolate the flow within its boundaries from the environment. Instead the open space, marked in blue, allows the intrusion of ambient static pressure.

2. Because of the relatively low flow speeds in the diffuser, the ambient static pressure, communicating at the speed of sound, will permeate the entire diffuser volume.

3. Inlet and outlet static pressure will always be ambient static pressure. Therefore, regardless of its roof shape, no “suction” can be generated at the diffuser inlet.

4. Therefore, the flow inside the diffuser will not provide any down-force.

5. With proper placement and geometry, the outside upper surface of the diffuser might be used for upward deflection of slip stream, thereby creating some down-force.



6. Strakes and other means might be use for horizontal flow deflection to aid wake management.

7. The great variety of diffuser designs seen at the races indicates that the final answer on diffusers is still evasive.

Stay tuned for “Thinking out loud #2: Under-Car Flow.

vonk

[slimjim8201]

1. It is an Open system that does NOT isolate the flow within its boundaries from the environment. Instead the open space, marked in blue, allows the intrusion of ambient static pressure.

2. Because of the relatively low flow speeds in the diffuser, the ambient static pressure, communicating at the speed of sound, will permeate the entire diffuser volume.

3. Inlet and outlet static pressure will always be ambient static pressure. Therefore, regardless of its roof shape, no “suction” can be generated at the diffuser inlet.

4. Therefore, the flow inside the diffuser will not provide any down-force.

5. With proper placement and geometry, the outside upper surface of the diffuser might be used for upward deflection of slip stream, thereby creating some down-force.



6. Strakes and other means might be use for horizontal flow deflection to aid wake management.

7. The great variety of diffuser designs seen at the races indicates that the final answer on diffusers is still evasive.

1. The air in this region is typically below ambient pressure. As flow is squeezed under the floor and in between the tires (a basic venturi or orifice), flow speed is generally increased while static pressure decreases.

2. This is untrue. It would be like saying all regions of an incompressible flow domain are iso-baric. Pressure can absolutely vary at lower than compressible speeds.

3. Neither are at ambient static pressure.

4. The diffuser functions to speed up the air under the car. Note the huge increase in diffuser entrance velocity comparing ANY of the diffuser designs/angles to the simulation without a diffuser.

5. True. Changing the direction of flow increases pressure, therefore increasing forces (both drag and downforce)

[n_anirudh]

Thinking out loud #1: Generic Diffuser




The “Generic F1 Diffuser” shown in this picture has the following properties:

1. It is an Open system that does NOT isolate the flow within its boundaries from the environment. Instead the open space, marked in blue, allows the intrusion of ambient static pressure.

2. Because of the relatively low flow speeds in the diffuser, the ambient static pressure, communicating at the speed of sound, will permeate the entire diffuser volume.

3. Inlet and outlet static pressure will always be ambient static pressure. Therefore, regardless of its roof shape, no “suction” can be generated at the diffuser inlet.

4. Therefore, the flow inside the diffuser will not provide any down-force.

5. With proper placement and geometry, the outside upper surface of the diffuser might be used for upward deflection of slip stream, thereby creating some down-force.



6. Strakes and other means might be use for horizontal flow deflection to aid wake management.

7. The great variety of diffuser designs seen at the races indicates that the final answer on diffusers is still evasive.

Stay tuned for “Thinking out loud #2: Under-Car Flow.

vonk

1) ur field of view is 2D..however, the flow in that region is turbulent and also being drawn in by the low pressure of the beam wing and rear wing.
2) It subsonic flow..(i am sure we have heard sonic booms at grand prixs )
3) its true in a large system, but the near wake is of concern, a lower pressure is generated
4) Please tell that to Adrian Newey and co
5) Of course, streamlines are deflected and a low pressure region is above the diffuser as a beam wing exists and other componens of the car

[youtube]http://www.youtube.com/watch?v=YvkFKgBU-8Q[/youtube]

[youtube]http://www.youtube.com/watch?v=BitbhumT ... re=related[/youtube]

[youtube]http://www.youtube.com/watch?v=4rbOtVIa ... re=related[/youtube]


and finally a link by Scarbs


http://www.scarbsf1.com/diffuser.html

[shelly]

So a brief recap of your points, vonk:
1) wrong
2) wrong
3) wrong
4) wrong
5) true but marginal
6) wrong
7) wrong
Explainations are in previous posts