I want to evalute the value of lamda where lamda=viscosity/(sqrt(density*pressure)) which I will use to find out mean free path. Here inlet pressure is 202650 and outlet pressure 101325 pa. Flow is considered compressible. Density is evaluated by Ideal gas law. Temperature 297k. I want to use nitrogen gas. It is a 3D channel flow. Can anyone help me writing this UDF for fluent. I will be grateful. Thanks.

Hello!

Can you please provide, where do you need to add up this, lambda that you are calculating, by which I mean, where do you want this lamba to be used by fluent?

Hi Lamda is the mean free path i.e. distance between two consecutive molecular collisions. I want to use it to find out the knudsen number to apply for slip condition on the wall boundary condition. I am working with micro flow. There I need to use slip condition on the micro channel walls. For conventional flow we use noslip condition you know. In case of noslip condition the velocity on the wall is 0 but for slip flow velocity on the wall is u=kn*(delu/deln), where kn=lamda/H, is the knudsen number. H is the height of the channel. n is the normal coordinate pointing inward from the channel wall.Thanks Tajul