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High Reynolds number modeling

Question asked by John Bankert on Jul 18, 2018
Latest reply on Jul 19, 2018 by Bill McEachern



I am doing validation modeling of drag in immersed flow of air over a streamlined body, however, I am interested in high Reynolds number flow (i.e. 20e6).  As I understand it, the turbulence model flow sim uses is validated either to 6e6 (Bill McEachern's comment in this thread or 8e6 (Bill McEachern's comment in this thread  Further, in the validation white paper, I only saw data up to a Reynolds number of 1e6 ( ).


My questions are the following:


1) Is it completely useless to model drag for flows in the 20e6 Re realm?  Alternatively, if exact answers are not valid at that Re value, is there any validity in comparing two designs relative to each other, to indicate which design is "best."

2) From a physics standpoint, for a streamlined body in air, would design decisions made on a body modeled at a lower Re like Re=8e6 (or whatever the limit is) give any useful information on the behavior of that body at higher Re values? I'm imagining that the boundary layer behavior is different at Re=8e6 compared to Re=20e6, and so the behavior at the lower Reynolds number would be non-representative at higher Re.  Am I better off modeling it at 20e6, or 8e6, if I decide I'm interested more in comparing designs than absolute results?

3) The items I am modeling do show boundary layer separation and this appears to be important to the drag; would modeling in the time domain provide improved results at Re=20e6 vs a steady state model @ Re=20e6 (see comments in this thread Or is this another case of "there's no use in modeling at this Reynolds number, no matter what you do?"  It also looks like in the validation white paper (re-linked:  ), Figure 17 "Drag over a cylinder" uses time-averaged results, implying time-domain study. 


I am less familiar with fluid mechanics than I would like, but given what I've read about flow sim, I am worried about my results if I don't understand the numerical effects of modeling outside of the validated range.


Thanks all!