The mean shear has a major influence on near-wall turbulence but
there
are also
other important physical processes at work in the turbulence/wall interaction.
In
order to isolate these, a shear-free boundary layer was studied experimentally.
The
desired flow conditions were realized by generating decaying grid turbulence
with a
uniform mean velocity and passing it over a wall moving with the stream
speed. It
is shown that the initial response of the turbulence field can be well
described by
the theory of Hunt & Graham (1978). Later, where this theory ceases
to give an
accurate description, terms of the Reynolds stress transport (RST) equations
were
measured or estimated by balancing the equations. An important finding
is that two
different length scales are associated with the near-wall damping of the
Reynolds
stresses. The wall-normal velocity component is damped over a region extending
roughly one macroscale out from the wall. The pressure–strain
redistribution that
normally would result from the Reynolds stress anisotropy in this region
was found
to be completely inhibited by the near-wall influence. In a thin region
close
to the wall
the pressure–reflection effects were found to give a pressure–strain
that has an effect
opposite to the normally expected isotropization. This behaviour is not
captured by
current models.