[srv537]

i am trying to validate a paper of air flow in a ractangular channel of height 100mm, in paper it is mentioned that the turbulent intensity of flow is less than 25% so my questions are :

in Inlet boundary condition

1) velocity magnitude i have specified but what should i specify in supersonic/initial gauge pressure (pascal).

2) in turbulence specification method i have seleted intensity and hydraulic diameter so i should input

(a) 25 % as turbulent intensity ?
(b) and what should be the Hydraulic Diameter?

for channel flow we can approx hydraulic dia as 2h=200mm but in fluent manual it is mentioned that l=0.07L so for my case hydraulic diameter should be 0.07*2h = 14?

if i am wrong please correct me.

thanks

[LuckyTran]

You can generally ignore the supersonic / initial gauge pressure. That pressure has two uses. If the inlet is supersonic then you use it to specify the supersonic static pressure, otherwise the problem is under-defined.

If you use the "compute from" feature in the solution initialization pane and choose that particular surface, Fluent will use that value as the initial gauge pressure.

If your inlet is not supersonic and you do not use the "compute from" feature on that particular inlet, then that value is never used.

You should be using the turbulence intensity at the inlet of the paper. If it's not available, then you have to guess. Turbulence decays rapidly however, so putting in the wrong turbulence at the inlet of your domain generally does not do much harm unless your problem is extremely sensitive to inlet turbulence.

I believe it is Schlicting's or Pope's book (too lazy to look it up at the moment) but the l=0.07L result is also accompanied by the I=0.16*Re^(1/8). You should use these two relations together.

Btw, it is valid only for fully developed flows in smooth circular pipes. Although the equations are so handy and ppl normally don't pay enough attention so that it is often erroneously applied to external flows as well.

The big L is the diameter. I intentionally omitted hydraulic because the result is from circular pipes!

The little l (l=0.07L) is an estimate of the turbulent length scale (which is of course much smaller than the geometric diameter). The little l, combined with the turbulence intensity (and also a velocity) to calculate the variables needed by the turbulence model (k and epsilon, or k and omega. If you want to get nit-picky you should always choose the k-epsilon or k-omega.

Very interesting though, is that in Fluent and many software you put in the value for big L even though little l is actually what the solver needs. Also, Fluent actually does calculate little l from big L using l=0.07L.

For flow through extremely wide ducts, the hydraulic diameter tends towards 2H (twice the gap height). So you actually would overestimate if you used the hydraulic diameter in l=0.07L, it would be better to use the gap height H.

Yet again for external flows there is no hydraulic diameter! But you still must specify big L and let Fluent calculate little l! That is, even if you knew the true turbulent length scale l (somehow, magically) you must manually calculate L=l/0.07 and feed that number into Fluent. Somehow that's supposed to make sense!?