Define compressible and incompressible fluids in terms of temperature dependence of viscocity
Answers
Incompressible And Compressible Flow. Incompressible flow refers to the fluid flow in which the fluid's density is constant. ... All the liquids at constant temperature are incompressible. Compressible flow means a flow that undergoes a notable variation in density with trending pressure.
The compressibility of a fluid is a measure of the relative volume change in response to a pressure change. It is defined regardless of whether or not this fluid is flowing since it is a thermodynamic characteristic of that fluid.
On the other hand, the compressibility of a flow is directly related to the value of the Mach number in the flow. One can show that second order taylor decomposition of the momentum equation leads to a term for fluctuating pressure that scales as M2. As a consequence, one can consider that the flow is incompressible as long as the Mach number is lower than 0.3 regardless of the fluid at hand. In the case M=0.3, the "acoustic" fluctuating pressure (which is how is often called this second order term in pressure) correspond to 10% of the convective (or aerodynamic) pressure term. To explore further this link please have a look at the derivation of the Low-Mach Number Navier Stokes equation derivation.