  Wall turbulence has a sublayer where the mean wall-normal flux of the
streamwise momentum is constant. Via the law of the wall, this mean flux is
related to the wall-normal profile of the mean streamwise velocity. However,
the momentum flux has large fluctuations, for which the corresponding law is
yet unknown. To formulate such a law, we decompose fluctuations of the
streamwise and the wall-normal velocities. These are smoothed to single out a
component that would dominate the momentum flux fluctuations. It is dependent
on the wall-normal distance. We relate this dependence to the wall-normal
profile of the streamwise velocity variance. The resultant law is consistent
with laboratory and field data across a wide range of that distance and applies
readily to wall modeling of a numerical simulation.
