Learning Forum

[prajput]

How can I understand displacement and momentum boundary layer thickness for a flat plate?

[Karthik Remella]

The displacement thickness is a direct consequence of the conservation of mass in the boundary layer. It is the distance by which a solid surface would have to be displaced in an inviscid potential flow to obtain the same mass flow rate between the original solid surface and this new reference plane in a real viscous fluid. In other words, it is the distance by which an external potential flow is displaced outwards to accommodate for the decrease in the velocity in the boundary layer. The momentum thickess, on the other hand, falls out of the conservation of fluid momentum in the boundary layer. It is the distance by which a solid surface would have to be moved in an inviscid potential flow to obtain the same total momentum as occurs between the original solid surface and this new reference plane in a real viscous fluid. In other words, it is defined as the loss of momentum in the boundary layer compared to an external potential flow.

I hope this answers your question.

Thanks.