hi all, i am modeling a flow over a cylinder at Re=20000, now my doubt is to place the first cell centroid height at y+=1 which empirical formula i should use since i dont know the value of Ut=friction velocity beforehand. i would be obliged if anyone can help me to resolve this regards samik

Use one of the tools on http://www.cfd-online.com/Links/tools.html#yplus

thanx for the response jonas. by the way those calculators are specifically for flat plates. but what formula should i use for a flow over cylinder

hi pal,

see as u dont know many variables in the formulae in yplus its quite tough to calculate... ie all these values are flow properties not fliud properties.. so all these vary according to the flow region and behavior... all the softwares have some ranges of y plus .. if u use fluent check for the values of yplus in REPORT SURFACE> INTEGRALS on the required surface.. if it lies between 30 to 300...u can say that u r capturing the physics properly... otherwise change u mesh height by refining and check again ... its a trial and error pros. dont think if u refine the mesh to half the yplus reduces by half.. it depedns on many things... so do as a trial and error.. do the tutorial of flow over airfol in fluent..that will help u.. regards, nag