How does an EBD create downforce?

[jekenev]

Hi all, sorry if this question was asked in another thread already. I was hoping someone could tell me how an EBD that blows the top of the diffuser(ex. Merc, mclaren, STR) creates downforce. From what little I know about aerodynamics it seems like blowing the top of the diffuser would create lift.

[MIKEY_!]

This is a fair question and to begin with it seems you are right, blowing the top of the diffuser should create lift but it seems the fast air running across the diffuser sucks air from under the car through the diffuser. When the faster exhaust gas meets the slower air from under the diffuser at the diffusers rear edge the speeds like to equalize and this speeds up the underbody air and gives downforce.

Hope i haven't made that to confusing (I sometimes do) and hope it helps.

[burchenal]

If you look carefully at how the team are integrating EBD you will see that they don’t blow over the top of the diffuser but rather down the sides (between the tyre and diffuser):





This creates a two-fold effect; firstly it energises the air running through the diffuser thus decreasing the underbody pressure which equals more downforce, secondly it also creates a shear jet that forms a pseudo-seal between the diffuser side and the road, which prevents the air from expanding in the wrong directions which again gives a little more downforce (this phenomenon is used in open-jet wind tunnels).

Of course none of this is new, blowing into the diffuser was developed after ground effect was banned and vortex shedding devices are frequently used to seal the floor.

[rjsa]

Check the video on the home page here:

http://www.dyson.com/fans/

Around 25"

And here:

http://www.dyson.com/technology/airMultiplier.asp

The exhaust gas will pull more air from the undertray, so speed will increase & pressure will drop, generating more downforce.

[munudeges]

I don't like all of these descriptions of it 'energising' the airflow because it tells you nothing.

If you stand in a hot bathroom with the door closed and the room next door is full of cooler air you will feel a rush of cooler air coming under the gap in the door. That's the effect that's being created at the back of the car. However, the trick is keeping the effect going when the cooler air has displaced the hotter exhaust gas so it doesn't go hot and cold, so to speak, and to maintain the integrity of that gas flow in another moving flow.

Red Bull cleverly realised that to draw cooler air through the diffuser at a greater rate and get a greater effect they had to get their exhaust gas around to the rear and more importantly the sides of the diffuser.

[hardingfv32]

James Allison, the Technical Director of Lotus Renault, said that in the case of FEE and rear blown diffusers, that the teams were forming a vortex. The goal was to utilize the low pressure in the center of the vortex to reduce the pressure under the floor for FEE or the diffuser in the case of RBD.

Brian

[hollus]

Yes, all that is at play this year.
But the question was how blowing on top of the diffuser helps downforce, an that was the situation for most teams last year, and it was still working to increase downforce, albeit less than this year.
I think the correct explanation is in the second post of the thread. All the rest would go towards explaining how teams are refining the trick this year.

[hardingfv32]

For blowing the top of the diffuser, what aerodynamic principle is being used?

Brian

[jekenev]

Check the video on the home page here:

http://www.dyson.com/fans/

Around 25"

And here:

http://www.dyson.com/technology/airMultiplier.asp

The exhaust gas will pull more air from the undertray, so speed will increase & pressure will drop, generating more downforce.

Ha! I had no idea F1 and household appliances had so much in common.

And thanks for all the responses guys this has been really helpful!

[Greenish]

I don't like all of these descriptions of it 'energising' the airflow because it tells you nothing.

Energizing the flow has a reasonably specific meaning in aerodynamics. It refers to introducing high-energy (velocity, temperature, pressure, or some combo) air into a spot that otherwise would be low energy (i.e. stagnant) and causing drag, boundary layer separation, or other bad things. The usual mechanism is then shear mixing to transfer this energy to the rest of the flow.

So in this case, the exhaust jet would impinge on the otherwise sluggish air in the wake of the body/sidepods and speed it up. (By aiming that jet properly, additional effects, i.e. the vortex mentioned by Allison can also be created, but that's beyond the basic meaning of "energized.")

Often vortex generators are referred to as "energizing" flow (especially on the suction side of a wing element or other strongly curved area), and in that case it's by mixing speedier air from outside a boundary layer into the area near the surface to keep it from separating.

[PlatinumZealot]

three threads on this already.

[riff_raff]

I don't like all of these descriptions of it 'energising' the airflow because it tells you nothing.

If you stand in a hot bathroom with the door closed and the room next door is full of cooler air you will feel a rush of cooler air coming under the gap in the door. That's the effect that's being created at the back of the car. However, the trick is keeping the effect going when the cooler air has displaced the hotter exhaust gas so it doesn't go hot and cold, so to speak, and to maintain the integrity of that gas flow in another moving flow.

Red Bull cleverly realised that to draw cooler air through the diffuser at a greater rate and get a greater effect they had to get their exhaust gas around to the rear and more importantly the sides of the diffuser.

munudeges,

"energizing" the air is of great importance. Extracting work from an airflow first requires that the airflow has some work (or energy) to contribute. The diffuser on an F1 car has a finite length, and where that diffuser flow surface abruptly ends there will be flow separation, turbulence, and drag. The reason there is flow separation is that the upper and lower diffuser surface trailing flows are at different local dynamic pressures. This basic condition is what makes the diffuser create underbody downforce. And air being air, the diffuser trailing flow naturally migrates from a region of high pressure (the upper flow) to one of lower pressure (the diffuser flow). Of course, this tumbling, turbulent trailing airflow also reduces the diffuser effectiveness.

The "hot blown" concept improves diffuser function by "energizing", or imparting momentum to, the upper surface diffuser air flow. In essence, hot blowing the upper diffuser airflow mass imparts momentum to it, which increases its velocity. And the increased velocity of this airflow as it passes over the trailing edge of the diffuser creates a relative dynamic pressure gradient that minimizes turbulence and underbody flow separation.

Of course, take my explanation for what it's worth. I'm a mechanical engineer, not an aero engineer.

riff_raff