The existence of an organized streamwise vortical structure, which is superimposed on the well known coherent spanwise vorticity in nominally two-dimensional free shear layers, has been studied extensively. In the presence of stratification, however, buoyancy forces contribute to an additional mechanism for the generation of streamwise vorticity. As the spanwise vorticity layer rolls up and pulls high-density fluid above low-density fluid, a local instability results. The purpose of the current investigation is to force the three-dimensional instability in the stratified shear layer. In this manner, we experimentally observe the effect of buoyancy on the streamwise vortex tube evolution, the evolution of the buoyancy-induced instability, and the interaction between these two vortical structures. A simple numerical model is proposed which captures the relevant physics of the flow evolution. It is found that, depending on the location, streamwise vortices resulting from vortex stretching may be weakened or enhanced by the stratification. Buoyancy-induced vortex structures are shown to form where the unstable part of the interface is tilted by the streamwise vortex tubes. These vortices strengthen initially, then weaken downstream, the timescale for this process depending upon the degree of stratification. For initial Richardson numbers larger than about 0.03, the baroclinically weakened vortex tubes eventually disappear as the flow evolves downstream and the baroclinically generated vortices dominate the three-dimensional flow structure.
A bispectral analysis of high Reynolds number turbulent velocity-derivative data is carried out. The computations suggest that contributions of wavenumber triplets to the rate of vorticity production and spectral transfer are non-local in wavenumber space and comparable over the whole range of wavenumbers studied. Statistical resolvability of the bispectral estimates is obtained. An appendix on the asymptotic behaviour of bispectral estimates is given.
Nearly steady state mixing efficiencies, as characterized by the flux Richardson number R , have been measured in stably stratified, turbulent, uniform gradient shear flows. The turbulent dissipation rate e, the vertical mass and momentum fluxes fi-• and •-•, and the mean and rms density and velocity fluctuations were obtained from simultaneous single point measurements of velocity and density. Rf and the gradient Richardson number Ri were calculated at a number of positions in the spatially evolving shear flow. Within the investigated range of stabilities 0.02< Ri < 0.08, Rf was observed to increase with Ri. It is found that the length scale arguments successfully applied by Rohr et al. (1984) to the case of decaying, stably stratified, unsheared grid turbulence can also account for the overall behavior of the mixing efficiency (Rf) as a function of stability (Ri) in the present case of growing, stably stratified, uniform mean shear turbulence. consistency with the more recent literature [e.g., Itsweire et al., [1986]] the present definition of Lt is 1/2 of that used 1 Now at Naval Ocean System Center,
The following nine papers were pre,.;cntc<.l at the AIAA ..-\erothermochemistry of Turbulent F lows Conference in Sa n Diego, Calif., December 13-15, 1965. The Organizing Committee of the conference con:-:istecl of H. Yoshihara, Chairman, R. vanclen B erg, . C. Lin, W. X achbar, T . Y. Tong, and F. A . William;;. The re,·ie,,· procedure for these paper,.; w a..; ha ndled by P aul A. Libb~·.
Progress Report on a Digital Experiment in Spiral TurbulenceDoxALD CoLEti* A~D CHARLES VA~ ATTA t Calijomia I nslilute of T echnology, Pasadena, Calif.Dig ital hot-wi1·e l echn i
Measurements at moderately large Reynolds numbers in a finite laminar circular Couette flow show that the tangential motion near the axial plane of symmetry is two-dimensional, within experimental accuracy, but is nevertheless strongly modified by end conditions. During the course of an experimental investigation of transition in circular Couette flow (Van Atta 1966) it became necessary to measure very accurately the laminar velocity profile midway between the ends of a finite rotating-cylinder apparatus. The formal object of the measurements was to establish a known flow for calibration of a hot-wire probe array for response to changes in pitch angle, yaw angle, and speed. The simplest laminar motion which could conveniently be set up in the apparatus at hand was a flow with the inner cylinder a t rest and with the outer cylinder and end plates rotating together. The measurements were made at relatively high laminar Reynolds numbers, and they showed the influence of the end plates on the tangential flow field to be unexpectedly large. The experiment is therefore reported here for its intrinsic interest as an example of a three-dimensional secondary flow in a rotating fluid.The experimental technique was to carry out a simultaneous flow survey and probe calibration by holding the cylinder speed constant and traversing a hot-wire probe radially in the initially unknown flow. Electrical signals from the probe were then extrapolated to the far wall, where the fluid velocity with respect to the probe was accurately known. Several such extrapolations at different fixed cylinder speeds provided a relationship between velocity and voltage for use away from the wall. To keep the relative velocity at the probe as large as possible, the outer half of the flow was surveyed from the stationary inner cylinder, and the inner half of the flow was surveyed from the rotating outer cylinder. In both cases, however, the wires were supplied with current through slip rings. The same hot wires, slip rings, cables, and instrumentation were used through the measurements, which extended over a period of several weeks.The platinum-rhodium hot wires used had a diameter of 0.0001in. and a length of about 0.04 in, and were operated in the constant-temperature mode (hot resistance/cold resistance about 1.15). All wires were mounted parallel to the axis of rotation of the cylinders. Two wires on a single probe were traversed t Now at University of California at San Diego. 33Fluid Mech. 25
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