How set experimental points values as wind tunnel boundary conditions

vidade · December 22, 2012, 13:47

Hi everyone, my first thread..
I had to test car performances using a k-epsilon realizable model in a slotted walls WT and I need to impose some experimental point values (pressure) as wind tunnel wall boundary conditions. How can I impose a wall pressure conditions like this? Is it possible?
thank you all, if you need some extra info please ask!

LuckyTran · December 23, 2012, 01:05

Without doing some magic, you cannot specify the pressure on the wall.

You must follow the physical mathematics of what each boundary condition is. For a solid wall, this means that the wall-normal velocity is equal to the wall velocity (kinematics). For the no-slip condition, the tangential velocity is equal to the wall tangential velocity. If there is a slip-condition then tangential velocity is related to the wall tangential velocity by some relation. Note, there is no pressure!

Think of it this way. You cannot specify the pressure at the wall in your wind tunnel either. You can try by various methods to adjust the tunnel conditions to obtain a certain pressure at the wall but by no means are you specifying the pressure at the wall, you are just changing the inlet and outlet conditions of your wind tunnel.

With that said, I have seen cases where variables are "fixed" over an entire zone. Not sure if you can do specific locations (possibly with a UDF). If you set the pressure in this manner, you will end up with inconsistencies and your solution most likely will have trouble converging / converging to an inaccurate result.

vidade · December 23, 2012, 10:56

Quote:

Originally Posted by LuckyTran

Without doing some magic, you cannot specify the pressure on the wall.

You must follow the physical mathematics of what each boundary condition is. For a solid wall, this means that the wall-normal velocity is equal to the wall velocity (kinematics). For the no-slip condition, the tangential velocity is equal to the wall tangential velocity. If there is a slip-condition then tangential velocity is related to the wall tangential velocity by some relation. Note, there is no pressure!

Think of it this way. You cannot specify the pressure at the wall in your wind tunnel either. You can try by various methods to adjust the tunnel conditions to obtain a certain pressure at the wall but by no means are you specifying the pressure at the wall, you are just changing the inlet and outlet conditions of your wind tunnel.

With that said, I have seen cases where variables are "fixed" over an entire zone. Not sure if you can do specific locations (possibly with a UDF). If you set the pressure in this manner, you will end up with inconsistencies and your solution most likely will have trouble converging / converging to an inaccurate result.

thanks, I know about Navier Stokes formulation actually, and that's the problem. It need a magic trick as you said.. I thought to modify the wall function equation imposing not directly the pressure but the tangential stress (with no slip boundary condition) and iteratively impose the desired pressure. Another way could be an energetic way playing with kinetics energy and its dissipation to impose right experimental pressure condition. I am not a fluent expert so these stuff, easy on paper, could be impossible to do on CFD, if anyone has some other ideas i'd be happy to know.

LuckyTran · December 23, 2012, 16:02

I recommend visiting the Fluent Manual section 7.2.4 on Fixing the Values of Variables. I have used it before for fixing y,z velocity in an entire zone. You can specify pressure if you are using the pressure-based solver. Like all the default properties in Fluent, it is designed for you to fix variables throughout an entire zone and not a single point.

You can specify non-constant profiles by specifying the profile or using a UDF. If you construct the correct profile/UDF then I imagine you can get it to specify the pressure at a specific location.

Sorry I can't give a complete walkthrough after that as I avoid these options (complicated CFD is not very useful, IMO). But it is possible and fairly straightforward (if you can get the profile/UDF). You do not need to change the solver, wall functions, or mess with the details of the turbulence modeling.

December 22, 2012, 13:47	How set experimental points values as wind tunnel boundary conditions	#1
vidade New Member Davide Tartarini Join Date: Dec 2012 Posts: 2 Rep Power: 0	Hi everyone, my first thread.. I had to test car performances using a k-epsilon realizable model in a slotted walls WT and I need to impose some experimental point values (pressure) as wind tunnel wall boundary conditions. How can I impose a wall pressure conditions like this? Is it possible? thank you all, if you need some extra info please ask!

December 23, 2012, 01:05		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,668 Rep Power: 65	Without doing some magic, you cannot specify the pressure on the wall. You must follow the physical mathematics of what each boundary condition is. For a solid wall, this means that the wall-normal velocity is equal to the wall velocity (kinematics). For the no-slip condition, the tangential velocity is equal to the wall tangential velocity. If there is a slip-condition then tangential velocity is related to the wall tangential velocity by some relation. Note, there is no pressure! Think of it this way. You cannot specify the pressure at the wall in your wind tunnel either. You can try by various methods to adjust the tunnel conditions to obtain a certain pressure at the wall but by no means are you specifying the pressure at the wall, you are just changing the inlet and outlet conditions of your wind tunnel. With that said, I have seen cases where variables are "fixed" over an entire zone. Not sure if you can do specific locations (possibly with a UDF). If you set the pressure in this manner, you will end up with inconsistencies and your solution most likely will have trouble converging / converging to an inaccurate result. Last edited by LuckyTran; December 23, 2012 at 01:30.

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December 23, 2012, 16:02		#4
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,668 Rep Power: 65	I recommend visiting the Fluent Manual section 7.2.4 on Fixing the Values of Variables. I have used it before for fixing y,z velocity in an entire zone. You can specify pressure if you are using the pressure-based solver. Like all the default properties in Fluent, it is designed for you to fix variables throughout an entire zone and not a single point. You can specify non-constant profiles by specifying the profile or using a UDF. If you construct the correct profile/UDF then I imagine you can get it to specify the pressure at a specific location. Sorry I can't give a complete walkthrough after that as I avoid these options (complicated CFD is not very useful, IMO). But it is possible and fairly straightforward (if you can get the profile/UDF). You do not need to change the solver, wall functions, or mess with the details of the turbulence modeling.