Fluids

Fluids

How to model roughness in FLUENT

    • xing
      Subscriber
      We are modeling open channel flows. We need to investigate the effect of bed roughness height on flow patterns. What is the maximum value of roughness height (m) FLUENT can simulate by adding the height in the wall boundary conditions? We found that the y+ values with rough wall become much greater than 5 when we use near-wall turbulence models whereas y+ for smooth wall with the same turbulence model is less than 5. We assume that when the roughness height is large, we will have to resolve it in our geometry instead of adding it as a modeling parameter. Is that right?n
    • Karthik R
      Administrator
      Hello,nHow large is your roughness height?nWhich model are you using to model the wall roughness?nGenerally, there is a downward shift in the log-law region of your velocity profile when one goes from smooth to rough (which is contrary to what you are observing). And there are two ways Fluent deals with this downward shift:nreducing the wall roughness as y+ decreases.nvirtually shift the wallnVirtually shifting the wall is the default treatment in the recent version. Here are some details you can read about (section 7.4.15.2.8 - Wall Roughness Effect in Turbulent Wall-Bounded Flows)nI hope this helps you get a better understanding of what might be going on.nThank you.nKarthikn
    • xing
      Subscriber
      How large is your roughness height?nIt varies from mm to up to 1 cm. (range is between 0.2 to 1 cm).nWhich model are you using to model the wall roughness?nFor both smooth and rough bed simulation, we have been using k-Epsilon (realizable) model and standard wall function for Near wall treatment.nGenerally, there is a downward shift in the log-law region of your velocity profile when one goes from smooth to rough (which is contrary to what you are observing). And there are two ways Fluent deals with this downward shift:n1. reducing the wall roughness as y+ decreases.n2. virtually shift the wallnVirtually shifting the wall is the default treatment in the recent version. Here are some details you can read about (section 7.4.15.2.8 - Wall Roughness Effect in Turbulent Wall-Bounded Flows)nThe link you attached direct me to ansyshelp.ansys.com. I cannot login to it although I can login to ansys customer portal without problem (https://support.ansys.com/AnsysCustomerPortal/en_us). Please help! Can you just email me that section (xing@uidaho.edu)? What document is that section from? FLUENT users guide, theory, or any document I can download after I login to ANSYS customer portal?n
    • Karthik R
      Administrator
      Hello,nIf you are looking for instructions on how to access the link I shared with you, please take a look into the following posts.nHello,nIf you are looking for instructions on how to access the link I shared with you, please take a look into the following posts.nPlease let me know if you are able to access.nThanks.nKarthikn
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