What turbulence scale to use for "still" air?
 

I am modeling a racecar wing (w/ endplate) for my undergraduate senior design project. An additional goal is to become proficient with Fluent as I have never used CFD software before this semester.

I have run a few solutions now, both times using the k-epsilon (realizable) turbulence model. I am using intensity and scale for the BC's, 1% turbulence intensity (because I read that <1% is appropriate for the still-air situation). Since the wing is moving through what I am assuming to be undisturbed air, what is the proper scale to use to define the eddies? I have used .25m and .5m so far. (The wing has a .75m half-span-length and is in a 3mx3mx5m volume but I am probably going to increase that volume). And since the k-e model was chosen with rather little knowledge, is that a good choice? It is moving at 70 m/s and I'm also using the pressure-based solver. Any other suggestions to improve the learning process? I have found the Fluent documentation to be extremely informative.

Thanks for any advice! Chris Newport

Re: What turbulence scale to use for "still" air?
 

you should ask your supervisor/search the web/ look at journals to see how others have done it.

Re: What turbulence scale to use for "still" air?
 

A part of that's what he did!!!

Re: What turbulence scale to use for "still" air?
 

ok then smart arse, you suggest a scale

Re: What turbulence scale to use for "still" air?
 

Just Kidding buddy.. Anyway, I too do the same thing when I need to give the inlet/oulte conditions for the still air, I take the turbulent intensity<1%...not experienced problems in solution

Re: What turbulence scale to use for "still" air?
 

If you calculate the Reynolds number you can calculate the required boundary conditions.

The equations are buried inside the fluent documentation close(I think) to the intensity & hydraulic diameter BC definition.

If you only specify the intensity,etc at the inlet then it doesn't affect what happens far away from the boundary!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

This is why they are a matter of personal preference rather than set in stone.

CHRIS- ARE YOU TALKING ABOUT THE CHARACTERISTIC LENGTH OR 'HYDRAULIC DIAMETER' OF THE BOUNDARY CONDITIONS? THIS IS NORMALLY SIMPLY THE PHYSICAL DIAMETER OF THE BC FACE.

EVERYTHING YOU REQUIRE IS BURIED IN THE FLUENT AND GAMBIT DOCUMENTATION, YOU DON'T EVEN NEED TO POST A MESSAGE!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!

Re: What turbulence scale to use for "still" air?
 

Having asked my adviser for this project (yes I do know how to use my resources, I just like to get as many opinions as I can) he suggested using the chord of the wing as the turbulence scale. I'm talking about the scale of the turbulence representing the size of the eddies at the boundary, not hydraulic diameter (which I don't think would be applicable since the flow is not "in" anything.)

I did in fact look in the Fluent documentation and it specifies using a fraction of the diameter of the pipe or conduit (my flow is not in a conduit), or using the size of an obstruction in the flow (if the flow goes through a grating or something upstream, which mine doesn't). So the documentation has several suggestions, but I was looking to see if anyone had something more relevant. As Phil said, it's just a BC, but I did want to make sure I was in the ballpark. So in the 2 solutions I've performed I've used .25 and .5m, and the wing chord is .4m so I was about right, and I don't seem to have had any issues with the solution caused by this.

Thanks.