Musing on tracers, two-fluids and stuff

Cipherflak · April 13, 2015, 08:58

Hi, everyone. Just a little physics philosophy from me today; working on solving a problem of "diffusion of sewage in water", and suddenly became a really unsure about the workings of the universe.

When I use a passive tracer or scalar, it is presented by only one number, the concentration fraction. This number is advected and diffused throughout the domain, but it strictly follows the current of the medium.
But isn't it so that this is only valid if the concentration of the tracer (in my case, sewage..) is much lower than the concentration of water? At some point of concentration I will need to address the physical properties of the tracer itself, right? How do I know when to it is reasonable to use a passive tracer? Am I thinking correctly about the problem at all? To proceed, if I were to start using physical properties of the tracer as its own fluid, would this be what is called a two-fluid model?

Sorry if i'm out on the fields here. Thank you for any enlightenment.

H0T_S0UP · April 13, 2015, 09:32

You are correct. A crappy rule of thumb is a concentration less than 5% does not effect the viscosity of the fluid. Otherwise, you don't treat it as a 2-fluid solution, but instead a non Newtonian fluid, as the concentration of your tracers will change the stress tensor term. The advanced fluids class I took closed with a good discussion on non Newtonian flows, and we used a power law model to approximate their behavior.

April 13, 2015, 08:58	Musing on tracers, two-fluids and stuff	#1
Cipherflak New Member John E Join Date: Oct 2011 Posts: 11 Rep Power: 14	Hi, everyone. Just a little physics philosophy from me today; working on solving a problem of "diffusion of sewage in water", and suddenly became a really unsure about the workings of the universe. When I use a passive tracer or scalar, it is presented by only one number, the concentration fraction. This number is advected and diffused throughout the domain, but it strictly follows the current of the medium. But isn't it so that this is only valid if the concentration of the tracer (in my case, sewage..) is much lower than the concentration of water? At some point of concentration I will need to address the physical properties of the tracer itself, right? How do I know when to it is reasonable to use a passive tracer? Am I thinking correctly about the problem at all? To proceed, if I were to start using physical properties of the tracer as its own fluid, would this be what is called a two-fluid model? Sorry if i'm out on the fields here. Thank you for any enlightenment.

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April 13, 2015, 09:32		#2
H0T_S0UP Member Alex Join Date: Jan 2014 Posts: 54 Rep Power: 12	You are correct. A crappy rule of thumb is a concentration less than 5% does not effect the viscosity of the fluid. Otherwise, you don't treat it as a 2-fluid solution, but instead a non Newtonian fluid, as the concentration of your tracers will change the stress tensor term. The advanced fluids class I took closed with a good discussion on non Newtonian flows, and we used a power law model to approximate their behavior.