Hot blowing, a theory?

[olefud]

After thinking about it rather too much, I’ve had a crisis of faith regarding hot blowing, as well as a possible revelation. Given that hot gases are less dense than ambient temp gases, it would seem that they should yield less aero effect (altitude density). There’s also talk of “higher energy”, and it’s true that higher exhaust stream velocity would be a positive; but the much greater portion of the energy is thermal energy that drives individual exhaust molecules to higher random internal velocities which would self-null, i.e. for each molecule moving in a given direction, there’s another molecule moving in the opposite direction.

Poking around in fluid dynamic theory suggest a –for me- counterintuitive theory. Air at exhaust temps is three or four time more viscous than at room temp. Boundary layer thickness is largely a direct function of viscosity and an inverse function of velocity. The point being that the hot exhaust flow forms a thick, sticky boundary layer or plume that contains and/or coheres with the lower temp flow. The hot plume will be more easily influence by aero operators than lower temp gas flow. Thus it’s not so much the high energy flow as the viscous heated plum that that allows for better direction and control of the overall diffuser air stream.

Of course, this may only be what the fluid-dynamics modeling has been saying. In any event, comments are invited.

[Tommy Cookers]

without thinking ....

sounds good to me !

also .....

blown surfaces have been around in military aircraft for over 50 years, using pretty hot air

exhaust gas is a mixture of nitrogen, water vapour, and carbon dioxide (quite a lot of the last two), and in this way quite different to air at the same temp & pressure ?

exhaust gas is still around ambient pressure ?


good luck with the thinking, though (sincerely)

[amouzouris]

without thinking ....

sounds good to me !

also .....

blown surfaces have been around in military aircraft for over 50 years, using pretty hot air

exhaust gas is a mixture of nitrogen, water vapour, and carbon dioxide (quite a lot of the last two), and in this way quite different to air at the same temp & pressure ?

exhaust gas is still around ambient pressure ?

good luck with the thinking, though (sincerely)

well...lift is calculated like this:
L=1/2 ρ v^2 A Cl

where:
ρ=density
v=velocity of air
A=planform area

and Cl=the coefficient of lift at the desired AoA...

the difference between air and exhaust gases when compared at the same temp & velocity...is their density....the density of air is 32g/mol and after some calculations i did...the density of the exhaust gases would be around 30g/mol.....so exhaust gases would create less lift (or downforce in our case) than air would...about 6% less...

[Lycoming]

There has been TONS of research on blown wings using hot and cold gasses. The effects it induces go far beyond what that formula may convey.

[amouzouris]

There has been TONS of research on blown wings using hot and cold gasses. The effects it induces go far beyond what that formula may convey.

yes i do know that!! what i posted was just some thoights i had on the subject!

[olefud]

Yeah, this should be old stuff. Forty years ago I was supporting Otto “Pete” Bartoe, a F1-grade engineer who was designing a blown-wing aircraft. Part of my job was to look at the road not taken. He was using bleed air from the engine compressor section –which is itself pretty toasty- for simplicity and safety. I did a brief look-see at combustion section bleed and didn’t see an advantage. Yet there’s something that makes hot exhaust gases very effective in enhancing car aero.

[nuconcept]

Don't they simply use it in f1 as a seal for the diffusor? If so, your segment of sticky air may be right.

[Tommy Cookers]

3000+ Western aircraft have been using flap blowing for up to 55 years (e.g. Starfighter, Buccaneer, and C-17), and some 'Russian' stuff also using Upper Surface Blowing (ie wing+ flap),(it is not certifiable for civil use).

The blowing enables greater AofA/ lift Coeff. Thus C-17s can use reverse thrust in flight for descents at 18000'/min ?. Loss of blowing requires operation at reduced AofA, ie a Starfighter needs to be about 2 miles directly above a suitable airfield if the engine fails (to maintain around 300 mph).

Such blowing is all about entrainment (ie using viscous effects) to get air flowing where it would not naturally go. I assume the same in F1.

Len Terry predicted this in or before 1975.

[olefud]

Such blowing is all about entrainment (ie using viscous effects) to get air flowing where it would not naturally go. I assume the same in F1.

I’m thinking it may be a bit different in F-1. The exhaust flow should be close to ambient pressure as it exits. And the slipstream is at a high relative velocity –not matched but closes enough that the exhaust can merge with minimum shear to maintain a coherent exhaust plume. Rather as the involved car wings don’t much resemble an aircraft wing, I believe (operative word) that the F-1 exhaust flow and functions differently than aircraft wing/flap blowing. Laminar in the former and turbulent in the latter.

[sknguy]

Last year's high velocity exhaust gas (blown diffuser and crazy engine mapping) was more of a sledge hammer approach to creating hot gas boundary and laminar layers. But this year, with the exhaust being much further forward, it's more akin to a boundary and laminar layers approach. More subtle and not as effective as last year's sledge hammer approaches, but beneficial none-the-less.

At Monza last year Red Bull introduced the wrap-around gurney flap on the trailing edge of their diffuser. This season the teams seem to be using the gurney flap more as a spoiler with the development of a small gap between the gurney flap and the body of the diffuser exit. I suspect to create a turbulent boundary layer for the diffuser's low pressure area. Perhaps this is augmented by the hot exhaust gases the way they are positioned now... contouring and flowing rearward more closely to the bodywork and diffuser deck.

You'd think that being more forward exhausting, that the hot gases should be more effectively used as a boundary layer for air flowing over the diffuser now, for this year at least. As opposed to the hot gas blown diffuser from last year which I think were driven more by their velocity/displacement than "strictly" as a laminar layer approach, so-to-speak.

[olefud]

Last year's high velocity exhaust gas (blown diffuser and crazy engine mapping) was more of a sledge hammer approach to creating hot gas boundary and laminar layers.

Agreed. The exhaust has less energy (heat) after the turbine energy harvest and is pointed in a less than advantageous direction. And the big jolts of raw fuel in the exhaust flow are gone -I think, but only the teams know. Still, exhaust management still yields results.