# When are the Navier Stokes equations invalid?

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1. May 9, 2017

### ramzerimar

I'm studying the Navier Stokes equations right now, and I've heard that those set of equations are invalid in some situations (like almost any mathematical formulation for a physics problem). I would like to know in which situations I cannot apply the NS equations, and what is the common procedure when something like that happens (like, there are any other set of PDEs that describe fluid flow in situations where the NS equations are invalid?).

2. May 9, 2017

Presumably you made some assumptions when studying the derivation of those equations. Can you list what those assumptions are? That will give you a pretty big hint about what the equations' limitations are.

3. May 9, 2017

### muscaria

One example it can't be applied in is when the system is not Galilean invariant. This occurs for a fluid which is subject to a nonlinear vector potential which depends on the density $\rho$ of the fluid. In this situation, the kinetic energy density of the fluid is nonlinear in $\rho$ and the pressure of the fluid depends explicitly on the flow.
The way forward in these kind of situations is to start from first principles without assuming anything in particular..In your case Cauchy's equation would do.

4. May 9, 2017

### Staff: Mentor

The Navier Stokes equations apply to fluids described by Newton's law of viscosity (i.e., Newtonian Fluids). If the fluid exhibits more complicated behavior than that of a Newton fluid (a Newtonian fluid is one for which the stress tensor in linearly proportional to the rate of deformation tensor), the Navier Stokes equations will not apply. Such fluids are called non-Newtonian fluids, and include viscoelastic fluids and purely viscous non-Newtonian fluids. Examples of such fluids are polymer melts and solutions, and suspensions. And, of course, the Navier Stokes equations don't apply to solids.

5. May 11, 2017

### ramzerimar

I've read that one of the assumptions is that the NS equations are for a incompressible flow. So, for dealing with compressible flow problems, the NS equations cannot be applied?

6. May 11, 2017

### Staff: Mentor

The NS equations can be applied to incompressible flows.

7. May 11, 2017

### ramzerimar

To analyze compressible flow I would need to rewrite the NS equations or just add the energy equation to the mix? The difference that I see is that, in compressible flow, the density also changes and so becomes a variable, and I would need another equation to describe the flow.

8. May 11, 2017

### Staff: Mentor

Yes, and you would also have to use the compressible form of the continuity equation. And you might have to allow for temperature-dependent viscosity.

9. May 12, 2017

The Navier-Stokes equations are absolutely valid for compressible flows. They are a little more complicated since you can't make so many simplifying assumptions and they require a greater number of supplemental equations since there many more variables in the overall problem (there are a minimum of 6 and can be a dozen or more depending on the conditions).

There are the basic $u$, $v$, $w$, $p$, $\rho$, and $T$ variables. Additionally, you could have $\mu$ (dynamic viscosity), $\lambda$ (second coefficient of viscosity), $c_p$, $c_v$, $\kappa$ (thermal conductivity), or even $X_i$ (mass fractions of various chemical species). The Navier-Stokes equations can technically apply to problems involving all of those variables, both compressible and incompressible.

The two most important limitations on the Navier-Stokes equations is that they only apply to (a) fluids that can adequately be modeled by a continuum and (b) Newtonian fluids. Some examples of when the continuum breaks down are in the upper atmosphere where density is so low that the mean free path is comparable to the flow scales or in microfluidics when the flow scales are so small that they are comparable to the mean free path.

10. Aug 21, 2017

### Idrees Afridi

Respected professor
i just want to know the examples of viscoelastic fluids with the values of Prandtl number . I saw a lot of articles , where viscoelastic fluids are used with Prandtl number 0.7 , 1 and 3 etc . which are shocking for me because these Prandtl numbers are for air and water and air ,water are Newtonian fluids .

11. Aug 21, 2017

### Staff: Mentor

The Prantdl number relates to the heat transfer behavior of the fluid, not the mechanical behavior. The term viscoelastic relates to the mechanical behavior of the fluid, not the heat transfer behavior.

12. Aug 21, 2017

### Idrees Afridi

Professor thanks... but by taking Pr =0.7 means v r talking about air ... is ve take a air as viscoelastic fluid...

13. Aug 21, 2017

### Idrees Afridi

If i want to discuss the thermal behaviour of viscoelastic fluid then kindly suggest me a suitable Prandtl number . For which i have a justification that why i am taking this Prandtl number ?

14. Aug 21, 2017

### Staff: Mentor

This is too far removed from the original theme of the present thread to continue along these lines. Please start a new thread that addresses the topic of how to analyze heat transfer to viscoelastic fluids.