CFD Online Discussion Forums - Velocity fluctuation decrease sharply near inlet in LES

Dear all,

When I tried to simulate urban boundary layer, I found that the imposed velocity fluctuations decrease sharply near the inlet. At the inlet, groovyBC was used:

Code:

inlet

    {

        type            groovyBC;

        variables  

                        (

                          "Uref=3.654;"

                          "zref=0.2;"

                          "alpha=0.31;"

                          "zh=pos().z/zref;"

                          "Uxm=Uref*pow(zh,alpha);"

                          "a1=-0.09365;"

                          "b1=0.1448;"

                          "c1=0.06564;"

                          "a2=0.8404;"

                          "b2=0.4765;"

                          "c2=0.5241;"

                          "a3=0.3152;"

                          "b3=0.1186;"

                          "c3=0.1319;"

                          "deltau=a1*exp(-pow((pos().z-b1)/c1,2)) + a2*exp(-pow((pos().z-b2)/c2,2)) + a3*exp(-pow((pos().z-b3)/c3,2));"

                        );

        valueExpression "vector(Uxm+randNormal()*deltau, 0, 0)";



    }

Please check the attached figures for velocity fluctuations at the inlet (x = -1.1, figure 1.1), and two locations near the inlet (x=-1.08, figure 1.08), (x=-1.02, figure 1.02). The x range of the block is from -1.1 to 1.9.

Does anyone have some ideas about this?

Thanks,

Leo