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-   -   How to set Boundary in comepute low speed flow (http://www.cfd-online.com/Forums/fluent/37213-how-set-boundary-comepute-low-speed-flow.html)

 zuo July 9, 2005 13:25

How to set Boundary in comepute low speed flow

hi all

when i will use fluent to compute the low speed(Ma=0.2, alpha =18 degree) flow around the air-plane, i make icem to meshing hex grids ,I'd like to know whether i use couple or segerate , and what Y+ value is best(mine Y+ is 30-200)? how to set inlet or outlet?

when i used SA tublence model , the Cd was about double of the winner tunnel's values , so i think it is too large , and Cl is also larger. would you tell me how can i do it better?

 Razvan July 10, 2005 05:48

Re: How to set Boundary in comepute low speed flow

1. You must surely use segregated solver because your flow is dominated by low velocities.

2. If you succedeed in making a structured grid around the airplane with orthogonal boundary layer, there is not so much concerning about Y+, but it must not exceed 100 if you're using Ymin>30, and the medium value is best around 40-50.

3. Your velocity is pretty low, but the compressibility is not negligible, especially if you want to calculate more precisely Cl and Cd (taking into account that these values are oftenly obtained with errors of 10 to 25 percent!!, even with the most complicated turbulence models) so I advise you to try compressible approach.

4. The flow domain could be made in two ways:

-try to simulate the windtunnel environement and in this case you have to use a pressure-inlet+pressure-outlet pair (but it would be a lot more convenient to use a incompressible approach with a velocity-inlet+pressure-outlet pair), which should give you results closer to the tunnel measurements

-use an external domain with a pressure-far-field boundary placed at (at least!!)10 lengths of airplane, that should give results closer to real life situations

5. Your angle of attack is quite high, so the flow over the airplane's wings could be partially separated, which will strongly influence Cl and Cd values. In this case, you better try a k-w model (not sst-kw), but be very careful what turbulence specification method you choose (better try a turb.intensity+lenght-scale pair, these values are easy to calculate in windtunnel environement, or turb.intensity<0.5%>-turb.viscosity-ratio<1> for external flow). The best solution is of course RSM, but it is hard to work with.

Hope this will help you.

Best wishes, Razvan

 zuo July 11, 2005 06:50

Re: How to set Boundary in comepute low speed flow

thank you very very much , i hope to study more from you ! i will try again according to your hint!

 ilkerrior June 4, 2009 16:44

Hello,
i know the topic is old but i need to try my chance :D

firstly ur informations helped me a lot, i found some ways to calculate the intensity and the lenght scale;

I=0,16.Re^(-1/8) and L=0,07.Dh

I: Intensity
Re: Reynolds number
L: Lenght scale
Dh: Hydraulic diameter

i wonder if these equations are useful for a wind tunnel flow,
(i am analysing a car model in a wind tunnel)

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