Fluid flow will affect with different diffusivity?

tseo

Hello, I solved advection-convection equation with Navier-stokes equation for several different UDS diffusivity using FLUENT. I expected that the wall shear stress should be same even if the values of the UDS diffusivity are different, but the results are not. The advection-convection equation with fluid flow is one-way coupling, so the diffusivity does not affect the fluid flow. In fluent I set the numnber of user-defined scalar as 1 and the flux function is mass flow rate. Did I miss something? If you know the reason physically, please let me know. Thank you in advance.