AnsweredAssumed Answered

Cylinder Drag... 'the need for speed'..AND accuracy!

Question asked by Karl Mohr on Dec 7, 2011
Latest reply on Dec 7, 2011 by Bill McEachern

Great to see Joe back!


Motion is all around us and inside us!


With motion flow is unavoidable!


In Transportation, even the smallest of people movers, travelling at mere Human Powered speeds has SMALL parts experiencing flows with Re>=1.7e4 and most powered transportation experiences flows above Re=3.8e6!


In an effort to assess accuracy of FlowSim I dropped back to 2D (external) many weeks ago and have been attempting to achieve 'standardaized' results, accurate or otherwise!


Several have suggested that the code's accuracy drops off around Re=6e6!


My objectives:

Prove the degree of accuracy of FlowSim across the Re range 6e6-1.2e7 (for my craft, hump speed to cruise)


My beliefs:

If the accuracy is known and the test method (Re/CD/Mesh/refinement strategy) remain consistent it should be reasonable Engineering practice to apply an 'error correction'!


I understand that there are some changes between the mesher/solver in FlowSim 2012 and 2011! (I am now using 2012!)


On the 2D 'Cylinder Drag' tutorial:


  1. it is clear that the tutorial maintains a 'ratio' for the depth of the extrusion/boundary to the diameter, as the diameter is varied (presumably for cell mesh reasons) is there a ratio that is considered 'optimum'?
  2. in SW2012-64, with default tutorial settings, I have several Re's that will not converge (try 1e4!)
  3. if I compare 1m and 5m CYLINDER, H2O, Re=1e6, tutorial settings, (boundaries maintaining ratios) I get arount 13% difference in result! (my application, when I can learn to crawl, is a Surface Effect craft, with a wing root section of 5m!)
  4. Real testing, as per the tutorial's refference and 'real World' applications would not rescale!
  5. Joe Galliera (invaluable, thanks so much!) has said to use 'solution addaptive' mesh refinement, including for transient/time-dependent, I did believe in this, Bill MacEachern (has been thanked before! ) seemed to advise against, I am leaning towards Bill's advise now (for Transient/time dependent ONLY!) believing that one or two solution adaptive mesh refinements per travel leaves "the mesh too far behind the FlowSim time march/action"
  6. I am now using FOUR "Local Initial Meshes" (in FlowSim tree) defined by three elipses (generally concentric but offset downstream!)  plus the cylinder surface as the fourth!
  7. UPSTREAM water problem: Amazingly, Flow Sim seems to know I am in China and that the water is NEVER clean! (see images!) PLEASE ADVISE!
  8. Has anyone managed to get convergence at 'consistent settings' across the Re range for ANY model??


Help appreciated!