[TurbulenceTaxes]

Hello, turbulence geniuses,

What should be the appropriate assumption for the boundary condition for the integral length scale at the wall? And are there any physical explanations for that boundary condition?

It is an urgent question and thanks a lot for answering.

[aero_head]

Originally Posted by TurbulenceTaxes

Hello, turbulence geniuses,

What should be the appropriate assumption for the boundary condition for the integral length scale at the wall? And are there any physical explanations for that boundary condition?

It is an urgent question and thanks a lot for answering.

Hello,

It depends on your slip length.

The no-slip boundary condition or no-velocity-offset boundary condition assumes that the speed of the fluid layer in direct contact with the boundary is identical to the velocity of this boundary. There is no relative movement between the boundary and this fluid layer, therefore there is no slip. The slip boundary condition or velocity-offset boundary condition assumes a discontinuity in the velocity function, i.e., a relative movement between the fluid and the boundary, therefore there is slip. The hypothetical distance inside of the boundary at which the velocity function would effectively reach the velocity of the boundary is referred to as slip length Ls. Ls is the ratio of the coefficient of viscosity (m) to the coefficient of slip (Cs).

The Knudsen number allows a rough numerical approximation of when to use which condition (i.e. no slip or slip). In general, the no-slip boundary condition can be applied in most applications in microfluidics when dealing with solid boundaries and incompressible fluids at characteristic length scales above 300 nm.