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Ram pressure is a pressure exerted on a body moving through a fluid medium, caused by relative bulk motion of the fluid rather than random thermal motion. It causes a drag force to be exerted on the body. Ram pressure is given in tensor form as
where ρ {\displaystyle \rho } is the density of the fluid; P ram {\displaystyle P_{\text{ram}}} is the momentum flux per second in the i {\displaystyle i} direction through a surface with normal in the j {\displaystyle j} direction. u i , u j {\displaystyle u_{i},u_{j}} are the components of the fluid velocity in these directions. The total Cauchy stress tensor σ i j {\displaystyle \sigma _{ij}} is the sum of this ram pressure and the isotropic thermal pressure.
In the simple case when the relative velocity is normal to the surface, and momentum is fully transferred to the object, the ram pressure becomes