Experiments were carried out in a new type of stratified flow facility
to study the evolution of turbulence in a mean flow possessing both uniform
stable stratification and uniform mean shear.
The new facility is a thermally stratified wind tunnel consisting of ten
independent supply layers, each with its own blower and heaters, and is
of producing arbitrary temperature and velocity profiles in the test section.
In the experiments, four
different sized turbulence-generating grids were used to study the effect
different initial conditions. All three components of the velocity were
along with the temperature. Root-mean-square quantities and correlations
measured, along with their corresponding power and cross-spectra.
As the gradient Richardson number Ri = N2/(dU/dz)2 was increased, the downstream spatial evolution of the turbulent kinetic energy changed from increasing, to stationary, to decreasing. The stationary value of the Richardson number, Ricr, was found to be an increasing function of the dimensionless shear parameter Sq2/∈ (where S = dU/dz is the mean velocity shear, q2 is the turbulent kinetic energy, and ∈ is the viscous dissipation).
The turbulence was found to be highly anisotropic, both at the small scales
the large scales, and anisotropy was found to increase with increasing
Ri. The evolution of the velocity power spectra for
Ri [les ] Ricr, in which the
energy of the large scales
increases while the energy in the small scales decreases, suggests that
small-scale anisotropy is caused, or at least amplified, by buoyancy forces
which reduce the amount of spectral energy transfer from large to small
scales. For the largest values of Ri, countergradient buoyancy flux occurred for the small scales of the turbulence, an effect noted earlier in the numerical results of Holt et al. (1992), Gerz et al. (1989), and Gerz & Schumann (1991).