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How to select frequency range and Length Scales for Periodic Random Flow Generator

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Old   November 8, 2020, 07:55
Default How to select frequency range and Length Scales for Periodic Random Flow Generator
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GDD
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Hello, I'm writing this question with great curiosity and despair. I hope someone can help me. Within the scope of my master's thesis, I wrote a code for proper inflow boundary condition generation method CDRFG as described in the article given below.
Consistent inflow turbulence generator for LES evaluation of wind-induced responses for tall buildings (https://doi.org/10.1016/j.jweia.2015.04.004)
When I check my code after generation, it provides target mean velocity, standard deviation, and spectrum. Required values for generation turbulent inflow with CDRFG technique are turbulent length scales, intensities, mean velocity values, sampling time, and frequencies. Using the variables provided with the article LES evaluation of-wind-induced responses for an isolated and a surrounded tall building (DOI: 10.1016/j.engstruct.2016.02.026), I obtained very high RMS pressure coefficients after several runs. One of the main problems is that these articles use a 1:400 geometric scale of the CAARC building model but I use a 1:45.72 geometric scale. Therefore, for the generation of inflow turbulence for my study, I scaled turbulent length scales which has unit (m) for my problem length scale.Moreover, I use DES code that solves the problem with normalized velocities therefore reference velocity of mine at the building height is also normalized. Therefore they use 10m/s at the building height but I use 0.8638m/s. Since length scales are scaled and reference velocity is normalized, my frequencies are also should be changed to obtain the same normalized target von-Karman spectrum. Therefore while they are using 1Hz to 100Hz for generation, I used 0.01Hz to 1 Hz. The second one is they use 0.0005s for their LES solver but I used 0.1 sec for DES code. However, under these variables, even I obtained target deviation, spectrum, my RMS pressure values are almost five times higher.

So my question is how I can select frequencies for a generation? In the literature, the frequency domain is used between 1Hz to 100Hz. My second question is that turbulent length scales are directly related to eddy sizes so is it correct for them to scale since they have unit m? My computational domain has a 1/45.72 geometric scale and has approximately 7.5 million cells with a maximum 1.3 expansion ratio which is recommended by several articles.

Similar to these questions, I learned that similar to the CDRFG, ANSYS used another generation method called a spectral synthesizer. So for Ansys users how they determine frequencies or length scales?

Last edited by GDD; November 8, 2020 at 19:16.
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Old   February 26, 2021, 20:35
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Hi, GDD. Did you use ANSYS or openfoam to write the CDRFG code? As far as i know, the spectral synthesizer method in ansys don't have the pressure problem, however, the method in ANSYS cannot give the spectrum as you wanted. To avoid this problem, you can set your probe higher than your CAARC model. This problem may be caused by the pressure corection since the changing velocity.
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Old   May 4, 2022, 23:07
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Hi Friends:
when we use OpenFOAM to do a work about architecture engineering,we can get a good mean profile and other information about the wind. However,if there exists high buildings, the results of the wind pressure on the building may be worse which we use it in the fluent.
I want to know if you ever do a similar work like this in the openfoam. I am confused about that.
Best wishes!
Raphael
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Old   May 10, 2022, 04:48
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Your question is very interesting. I am also wondering about this. Thank you for posting it!
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