Need help on boundary layer meteorology

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Discussion Overview

The discussion revolves around the application of boundary layer meteorology principles in computational fluid dynamics (CFD) simulations of wind flow in urban areas, specifically focusing on the use of logarithmic velocity profiles for inlet boundary conditions in FLUENT software.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions the appropriateness of using a logarithmic inlet condition, suggesting that it may not accurately reflect the actual velocity profile.
  • Another participant proposes expanding the simulation domain to include space outside the physical domain and using a uniform constant flow instead of a logarithmic profile.
  • A participant mentions the need to create a user-defined function (UDF) for boundary conditions to save computational time and addresses criticism regarding the velocity profiles used in the simulation.
  • There is a suggestion to modify the logarithmic velocity profile by substituting height with height above the surface to avoid negative velocities, indicating a potential adjustment to the model.
  • Concerns are raised about the challenges associated with boundary conditions in CFD codes and the importance of careful analysis of results to avoid issues such as reflections.

Areas of Agreement / Disagreement

Participants express differing views on the use of logarithmic velocity profiles, with some advocating for alternative approaches. The discussion remains unresolved regarding the best method for defining inlet boundary conditions in this context.

Contextual Notes

Participants highlight limitations related to the assumptions made in applying logarithmic profiles and the potential for negative velocities, as well as the need for careful consideration of boundary condition implementations in CFD simulations.

lalula2
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Hi, i am mech undergrad student, currently i am doing my thesis for CFD simulation of wind flow in urban area by using FLUENT. I am trying to apply logarithmic velocity profiles for my inlet boundary condition.
My question is that from the formula of logarithmic wind profile, the velocity will become negative when it less than certain height due to the ln function. Do i need to use power law profiles to describe the velocity profile under certain height?
Thanks.

Best regards.
 
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This is a question you should find the answer to through journal papers.
 
Why are you applying a logarithmic inlet condition? It doesn't make much sense to me. If you are trying to get the inlet condition to be as close as the actual velocity profile, then don't worry about.

Expand your domain to include some space outside of your physical domain. Send in a uniform constant flow and let it do what i wants.
 
I trying so save my computational time by create my own udf for the boundary condition and based on those journals i read.
Beside that, my moderator criticized my velocity profiles and ask me to rectify the problem as he said my velocity above my study area are not logic.
For actual urban area, the building height is relatively low compare to the atmosphere boundary layer.
 
Well, having said that, you could just kind of substitute height for height above the surface, e.g.
Rather than
[tex]u(i,j,k) = log(z(i,j,k))[/tex]
Have something like:
[tex]u(i,j,k) = log( z(i,j,k) - z(i,j,1) )[/tex]

Do remember that boundary conditions are quite a problem in most computational codes, and many many problems can arise from them. Having said that, be very careful when analyzing results such that you ensure you're getting what you think you should.

I image there could be problems with reflections depending on the actual boundary condition implementation.
 

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