CFD validation of wind tunnel data.

FrancoisStevens · February 27, 2020, 08:31

Hello.

I am in the process of validating a 2D airfoil CFD simulation against given wind tunnel data. I get good polars for comparison, but I know I am not using the correct setup.

The paper gives Re = 1e6, Ma = 0.1 and turbulence intensity of the wind tunnel = 0.05% at 46m/s.

For my simulation I just used the Re number and the default density and viscosity to calculate the flow velocity as 15.67[m/s].

My questions are:
- How do I calculate the turbulence intensity for my simulation? I have an equation I = sqrt{(3/2)*(Vt^2)/(U^2)} can I use this to calculate the turbulent velocity scale (Vt) at velocity (U) of 46[m/s], and turbulence intensity (I) of 0.0005. Then v=with the velocity scale known calculate the turbulence intensity at simulation velocity? Or does the velocity scale vary with the velocity?

- Is the Ma number important when I calculate my flow conditions in the simulation?

I am having trouble finding references for these questions I have.

LuckyTran · February 27, 2020, 13:16

It's better to use the same turbulence intensity in your simulation as from the experiment.

Vt is a property of the flow and depends on pretty much everything. Anything that can affect the flow will affect Vt.

The inlet Mach number tells you how much compressibility is already in the inlet flow. It's rather minor compared to the Reynolds number.

The purpose of validating is to find answers to these questions. If the answers were already available, you wouldn't be validating in the first place.

FrancoisStevens · February 27, 2020, 14:05

Thank you. So I can use 0.05%, even though I will be running the simulation at a much lower velocity as the TI reference velocity of 46[m/s]?

FMDenaro · February 27, 2020, 14:52

"- Is the Ma number important when I calculate my flow conditions in the simulation?"

Your inlet Ma number is enough low to assume a full incompressible flow model. I would exclude that around the airfoil the increasing in the local Mach number (I think it can't reach more than 0.3) would produce relevant compressibility effects.

LuckyTran · February 27, 2020, 15:02

Quote:

Originally Posted by FrancoisStevens

Thank you. So I can use 0.05%, even though I will be running the simulation at a much lower velocity as the TI reference velocity of 46[m/s]?

Turbulence intensity is like Reynolds number and Mach number. If you understand why it is important to match Reynolds number and Mach number, the same holds for turbulence intensity. You can run a completely different velocity, density, and viscosity as long as Re is the same. Turbulence intensity is kind of the same way.

FrancoisStevens · February 27, 2020, 15:07

Thank you for the replies. I now have a much better understanding of how to proceed with my study. I really appreciate it.

February 27, 2020, 08:31	CFD validation of wind tunnel data.	#1
FrancoisStevens New Member Francois Join Date: Feb 2020 Posts: 3 Rep Power: 6	Hello. I am in the process of validating a 2D airfoil CFD simulation against given wind tunnel data. I get good polars for comparison, but I know I am not using the correct setup. The paper gives Re = 1e6, Ma = 0.1 and turbulence intensity of the wind tunnel = 0.05% at 46m/s. For my simulation I just used the Re number and the default density and viscosity to calculate the flow velocity as 15.67[m/s]. My questions are: - How do I calculate the turbulence intensity for my simulation? I have an equation I = sqrt{(3/2)*(Vt^2)/(U^2)} can I use this to calculate the turbulent velocity scale (Vt) at velocity (U) of 46[m/s], and turbulence intensity (I) of 0.0005. Then v=with the velocity scale known calculate the turbulence intensity at simulation velocity? Or does the velocity scale vary with the velocity? - Is the Ma number important when I calculate my flow conditions in the simulation? I am having trouble finding references for these questions I have.

February 27, 2020, 14:52		#4
FMDenaro Senior Member Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,897 Rep Power: 73	"- Is the Ma number important when I calculate my flow conditions in the simulation?" Your inlet Ma number is enough low to assume a full incompressible flow model. I would exclude that around the airfoil the increasing in the local Mach number (I think it can't reach more than 0.3) would produce relevant compressibility effects. FrancoisStevens likes this.

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February 27, 2020, 13:16		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,763 Rep Power: 66	It's better to use the same turbulence intensity in your simulation as from the experiment. Vt is a property of the flow and depends on pretty much everything. Anything that can affect the flow will affect Vt. The inlet Mach number tells you how much compressibility is already in the inlet flow. It's rather minor compared to the Reynolds number. The purpose of validating is to find answers to these questions. If the answers were already available, you wouldn't be validating in the first place.

February 27, 2020, 14:05		#3
FrancoisStevens New Member Francois Join Date: Feb 2020 Posts: 3 Rep Power: 6	Thank you. So I can use 0.05%, even though I will be running the simulation at a much lower velocity as the TI reference velocity of 46[m/s]?

February 27, 2020, 15:07		#6
FrancoisStevens New Member Francois Join Date: Feb 2020 Posts: 3 Rep Power: 6	Thank you for the replies. I now have a much better understanding of how to proceed with my study. I really appreciate it.