A control volume is a chosen region over which fluid mass conservation and momentum balance are integrated. It may be fixed or move with a prescribed velocity. Boundary fluxes must use velocity relative to the moving boundary. In Stokes flow, negligible inertia reduces the momentum balance to the sum of body forces and boundary tractions, making a carefully chosen control volume useful for finding drag without integrating directly over a curved body.
A control volume is a specified region in space used in the analysis of fluid flow and thermodynamic processes. It can be fixed in space or can move with the fluid. The boundaries of the control volume can be real or imaginary and are often referred to as control surfaces. In the context of fluid mechanics and thermodynamics, control volumes are essential for applying the principles of mass, momentum, and energy conservation.
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