# Drag force as a function of shear & normal stresses

I would like to understand the relationship between the drag force of an object in a fluid and the microscopic interactions between the object and the fluid particles.

Specifically, I would like to understand the relation between drag and shear/normal stress. In the following post, a formula for drag $$D$$ was provided: https://www.physicsforums.com/threads/drag-equation-derivation.64110/#post-468849

However, there is no documentation on the assumptions and derivation of the equation. Can someone provide a rigorous reference for this equation (or a more accurate version)?

• What exactly are you asking? The formula from your link is just a surface integration of all the pressures and shear stresses projected to the x-direction (direction that you define drag). – Jimmy Aug 7 at 21:02
• Check out my answer to this related question for a reference to Transport Phenomena by Bird, Steward & Lightfoot where this is discussed in detail – nluigi Aug 8 at 7:45