# Kármán–Howarth equation

In isotropic turbulence the Kármán–Howarth equation (after Theodore von Kármán and Leslie Howarth 1938[1]), which is derived from the Navier–Stokes equations, is used to describe the evolution of non-dimensional longitudinal autocorrelation[2][3][4].

## Mathematical description

Consider a two-point correlation for homogeneous turbulence

${\displaystyle R_{ij}(\mathbf {r} ,t)={\overline {u_{i}(\mathbf {x} ,t)u_{j}(\mathbf {x} +\mathbf {r} ,t)}}.}$

For isotropic turbulence, this correlation tensor can be expressed in terms of two scalar functions, using the invariant theory of full rotation group, first derived by Howard P. Robertson in 1940[5],

${\displaystyle R_{ij}(\mathbf {r} ,t)=u'^{2}\left\{[f(r,t)-g(r,t)]{\frac {r_{i}r_{j}}{r^{2}}}+g(r,t)\delta _{ij}\right\},\quad f(r,t)={\frac {R_{11}}{u'^{2}}},\quad g(r,t)={\frac {R_{22}}{u'^{2}}}}$

where ${\displaystyle u'}$ is the root mean square turbulent velocity and ${\displaystyle u_{1},\ u_{2},\ u_{3}}$ are turbulent velocity in all three directions. Here, ${\displaystyle f(r)}$ is the longitudinal correlation and ${\displaystyle g(r)}$ is the lateral correlation of velocity at two different points. From continuity equation, we have

${\displaystyle {\frac {\partial R_{ij}}{\partial r_{j}}}=0\quad \Rightarrow \quad g(r,t)=f(r,t)+{\frac {r}{2}}{\frac {\partial }{\partial r}}f(r,t)}$

Thus ${\displaystyle f(r,t)}$ uniquely determines the two-point correlation function. Theodore von Kármán and Leslie Howarth derived the evolution equation for ${\displaystyle f(r,t)}$ from Navier–Stokes equation as

${\displaystyle {\frac {\partial }{\partial t}}(u'^{2}f)-{\frac {u'^{3}}{r^{4}}}{\frac {\partial }{\partial r}}(r^{4}h)={\frac {2\nu u'^{2}}{r^{4}}}{\frac {\partial }{\partial r}}\left(r^{4}{\frac {\partial f}{\partial r}}\right)}$

where ${\displaystyle h(r,t)}$ uniquely determines the triple correlation tensor

${\displaystyle S_{ijk}={\overline {u_{i}(\mathbf {x} ,t)u_{j}(\mathbf {x} ,t)u_{k}(\mathbf {x} +\mathbf {r} ,t)}}}$

i.e.,

${\displaystyle h(r,t)={\frac {S_{111}}{u'^{3}}}.}$