A function u is a “subsolution” named for this quantity at point x if, for any smooth function, phi, such that u minus phi is locally maximized at x, a PDE is non-positive on a combination of u and phi. Crandall and Lions introduced that type of generalized PDE solution named for this quantity in reference to a method of solving the Hamilton–Jacobi equations named for the “vanishing” of this quantity. A form of this quantity that equals another form of this quantity plus two-thirds times a third form of this quantity is assumed to equal zero in the (*) Stokes hypothesis. In a linear stress constitutive relation derived from the Cauchy momentum equation, two forms of this quantity each multiply a function of the velocity to give the deviatoric stress tensor. When this quantity is zero, the Navier–Stokes equations reduce to the Euler equations. For 10 points, name this quantity that is zero for a superfluid. ■END■
ANSWER: viscosity [accept viscosity solution, method of vanishing viscosity, volume viscosity, bulk viscosity, kinematic viscosity, shear viscosity coefficient, or second viscosity coefficient] (The Stokes hypothesis is the assumption that bulk viscosity, which equals the shear viscosity coefficient plus two-thirds times the second viscosity coefficient, equals zero.)
<Tim Morrison, Physics>
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