11/12/2023 0 Comments Kinematic viscosityIf the density is variable then mu matters. Who is "people"? If you're dealing with a constant density flow, then nu is the parameter that matters. I am just trying to understand why people favor the constant mu over constant nu assumption How is that relevant to the question? Obviously if you are considering a problem with two totally different fluid properties then assuming constant viscosity or constant density are both very wrong. Consider two fluids in a container with low miscibility, the viscosity clearly non-uniform and dependent on local composition or density, while the temperature can be assumed constant I can come up with an example for the case I described. Off hand, you're probably okay with constant viscosity if you're operating in lower mach numbers. If that's "small" then assuming constant viscosity probably won't negatively impact you, particularly for high Reynolds number flows. If you're talking standard air, then look at Sutherland's law and compute the viscosity difference for your expected temperature differences. I'm wondering if these assumptions has any physical basis. I've seem derivations taking mu out of the spatial derivatives in compressible N-S equation. Maybe that's equivalent to the weakly compressible formulations? If you have density variations that's going to need some equation of state and coupling to the first law of thermodynamics, which makes assuming constant temperature a dubious idea. I'm sure someone somewhere formulates their governing equations like this, but it's not common, as far as I'm aware. If the temperature is kept constant, but density may vary (compressible)
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