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---
title: "Navier-Stokes equations"
sort_title: "Navier-Stokes equations"
date: 2021-04-12
categories:
- Physics
- Fluid mechanics
- Fluid dynamics
layout: "concept"
---
While the [Euler equations](/know/concept/euler-equations/) govern *ideal* "dry" fluids,
the **Navier-Stokes equations** govern *nonideal* "wet" fluids,
i.e. fluids with nonzero [viscosity](/know/concept/viscosity/).
## Incompressible fluid
First of all, we can reuse the incompressibility condition for ideal fluids, without modifications:
$$\begin{aligned}
\boxed{
\nabla \cdot \va{v} = 0
}
\end{aligned}$$
Furthermore, from the derivation of the Euler equations,
we know that Newton's second law can be written as follows,
for an infinitesimal particle of the fluid:
$$\begin{aligned}
\rho \frac{\mathrm{D} \va{v}}{\mathrm{D} t}
= \va{f^*}
\end{aligned}$$
$$\mathrm{D}/\mathrm{D}t$$ is the [material derivative](/know/concept/material-derivative/),
$$\rho$$ is the density, and $$\va{f^*}$$ is the effective force density,
expressed in terms of an external body force $$\va{f}$$ (e.g. gravity)
and the [Cauchy stress tensor](/know/concept/cauchy-stress-tensor/) $$\hat{\sigma}$$:
$$\begin{aligned}
\va{f^*}
= \va{f} + \nabla \cdot \hat{\sigma}^\top
\end{aligned}$$
From the definition of viscosity,
the stress tensor's elements are like so for a Newtonian fluid:
$$\begin{aligned}
\sigma_{ij}
= - p \delta_{ij} + \eta (\nabla_i v_j + \nabla_j v_i)
\end{aligned}$$
Where $$\eta$$ is the dynamic viscosity.
Inserting this, we calculate $$\nabla \cdot \hat{\sigma}^\top$$ in index notation:
$$\begin{aligned}
\big( \nabla \cdot \hat{\sigma}^\top \big)_i
= \sum_{j} \nabla_j \sigma_{ij}
&= \sum_{j} \Big( \!-\! \delta_{ij} \nabla_j p + \eta (\nabla_i \nabla_j v_j + \nabla_j^2 v_i) \Big)
\\
&= - \nabla_i p + \eta \nabla_i \sum_{j} \nabla_j v_j + \eta \sum_{j} \nabla_j^2 v_i
\end{aligned}$$
Thanks to incompressibility $$\nabla \cdot \va{v} = 0$$,
the middle term vanishes, leaving us with:
$$\begin{aligned}
\va{f^*}
= \va{f} - \nabla p + \eta \nabla^2 \va{v}
\end{aligned}$$
We assume that the only body force is gravity $$\va{f} = \rho \va{g}$$.
Newton's second law then becomes:
$$\begin{aligned}
\rho \frac{\mathrm{D} \va{v}}{\mathrm{D} t}
= \rho \va{g} - \nabla p + \eta \nabla^2 \va{v}
\end{aligned}$$
Dividing by $$\rho$$, and replacing $$\eta$$
with the kinematic viscosity $$\nu = \eta/\rho$$,
yields the main equation:
$$\begin{aligned}
\boxed{
\frac{\mathrm{D} \va{v}}{\mathrm{D} t}
= \va{g} - \frac{\nabla p}{\rho} + \nu \nabla^2 \va{v}
}
\end{aligned}$$
Finally, we can optionally allow incompressible fluids
with an inhomogeneous "lumpy" density $$\rho$$,
by demanding conservation of mass,
just like for the Euler equations:
$$\begin{aligned}
\boxed{
\frac{\mathrm{D} \rho}{\mathrm{D} t}
= 0
}
\end{aligned}$$
Putting it all together, the Navier-Stokes equations for an incompressible fluid are given by:
$$\begin{aligned}
\boxed{
\frac{\mathrm{D} \va{v}}{\mathrm{D} t}
= \va{g} - \frac{\nabla p}{\rho} + \nu \nabla^2 \va{v}
\qquad
\nabla \cdot \va{v} = 0
\qquad
\frac{\mathrm{D} \rho}{\mathrm{D} t}
= 0
}
\end{aligned}$$
Due to the definition of viscosity $$\nu$$ as the molecular "stickiness",
we have boundary conditions for the velocity field $$\va{v}$$:
at any interface, $$\va{v}$$ must be continuous.
Likewise, Newton's third law demands that the normal component
of stress $$\hat{\sigma} \cdot \vu{n}$$ is continuous there.
## References
1. B. Lautrup,
*Physics of continuous matter: exotic and everyday phenomena in the macroscopic world*, 2nd edition,
CRC Press.
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