# Re: airfoil in OpenLB

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Hi robin,

i’m back on this post.

I found out some instability problems. I’vs set Smagorinsky model to investigate high Reynolds number, then the boundery condition are set as inlet velocity (left one), pressure outlet (right one) and periodic (top and bottom). If i delate the Poiseuille inlet profile, the problem crash at the top and bottom left corner where periodic boundery meets the inlet. Leaving the Poiseuille gives more stability, but after a certain time the instability strikes out and i really don’t know how to solve this.

I’ve set latticeL=x and Delta_T=x^2 to avoid the compressibility problem. What i don’t understand is why latticeU is defined in the guide like delta_t/delta_x (page 26). I know that delta parameters are adimensional, but velocity is space frac time not time frac space. Anyway like delta_x=latticeL/charL, maby also delta_u=latticeU/charU then if charU=1 the result become delta_u=latticeU, but the problem is still that delta_u=delta_t/delta_x (time/space) and not delta_x/delta_t (space/time).

Just using the guide, if i want delta_t=delta_x^2 it should be latticeU=latticeL because latticeL=delta_x=x, latticeU=delta_u=delta_t/delta_x, but delta_t=delta_x^2=x^2 so delta_u=x^2/x –> delta_u=x that is latticeU=x.

Am I doing something wrong?

Thanks for help!