For 10 points, give this quantity that expresses the rate at which a fluid flows.
The square of this quantity multiplied by density times the Fanning friction factor over two equals the shear stress at the wall of a pipe. Multiplying the square of this quantity by density over the bulk modulus gives the Cauchy number. The divergence of this quantity times density plus the time derivative of density (*) equals zero according to one of Euler's equations, and Bernoulli's equation is quadratic in this quantity. If the divergence of this quantity is zero, the flow is incompressible. In a horizontal pipe, an increase in this quantity will be accompanied by a decrease in pressure. Taking the curl of this quantity gives the vorticity. For 10 points, give this quantity that expresses the rate at which a fluid flows.
The square of this quantity multiplied by density times the Fanning friction factor over two equals the shear stress at the wall of a pipe.
fluid velocity (or flow velocity; or velocity field; prompt on "u"or "v")