AN EXPERIMENTAL STUDY OF THE TRANSITION OF GRID TURBULENCE TO INTERNAL WAVES IN A SALT-STRATIFIED WATER CHANNEL

The evolution of grid generated turbulence in a stably stratified fluid is investigated in a closed loop salt stratified water channel. Both uniform and linear sheared velocity profiles are used to produce low Reynolds number turbulent flows (Re(,m) = 2500 to 4700). Simultaneous single point measurements of the horizontal and vertical velocity and density fields were obtained including turbulent mass fluxes central in understanding the energetics of the fluctuating motion.;It is observed that buoyancy restoring forces cause anisotropy in the largest scales first, prevent them from overturning, and that smaller isotropic turbulent motions remain embedded within the larger scale wave motions. These small motions exhibit classical turbulent behavior and scale universally with Kolmogorov length and velocity scales. Eventually even the smallest scales of the decaying turbulence are affected by buoyancy, all isotropic motions disappear, and Kolmogorov scaling fails. The turbulent vertical mass flux is observed to decrease to zero for this condition and thus the original turbulent field has been completely converted to random interval wave motions.;The minimum energy flux or dissipation rate for transition from a wave field to one having minimal turbulent motion is found to be.;(epsilon)(,t) = 24.5 (nu)N('2).;Results from neutral and stratified experiments reveal that energetic stratified turbulent flows show initial decay patterns as in nonstratified turbulent flows. Deviations from neutral behavior occur only after the high inertial forces associated with the initial motion decay and buoyancy forces begin to dominate.;where (nu) is the kinematic viscosity and N the Brunt-Vaisala frequency of the fluid.;Results show that during transition the buoyancy field is never capable of extracting large amounts of turbulent kinetic energy from the motion. Maximum efficiencies of 15-20% of the energy being dissipated by viscosity are observed.;Scalar salinity spectra exhibit slopes of -1 in the viscous-convective subrange is described by Batchelor (1959), but only for heavily stratified cases. Uniform rates of strain necessary to obtain the -1 region may be achieved due to the heavily damped internal wave field.;It is determined from the present experiments that active overturning turbulence can exist at scales L in a stably stratified fluid only when.;1.4 L(,R) > L > 15.4 L(,K).;where L(,R) = ((epsilon)/N('3))('1/2) and L(,K) = ((nu)('3)/(epsilon))('1/4) represent buoyant and viscous length scales. This description successfully predicts the transitional behavior of the present experiments and also a high Reynolds number stratified grid turbulence study performance by Dickey (1977). The dynamics observed support the model for turbulent behavior in stratified fluids proposed by Ozmidov (1965) and Gibson (1980).