  The hydrodynamic equation derived by N-particle statistical mechanics is
investigated. This is an attempt to provide additional information concerning
the closure problem of turbulence theory. The equation is interpreted as mean
velocity equation for turbulent fluid flow. The third-order term of the
friction force is calculated. Multilinear mode coupling theory is applied in
order to obtain formulas for the third- und fourth-order equilibrium time
correlation functions appearing in the expression. The force term is obtained
as a convolution integral containing higher order gradients of the velocity
field.
