LONG-TERM GOALS We are focused on understanding small-scale processes that influence the ocean's thermodynamic and dynamic properties on the sub-mesoscale (scales less than 10 km). This includes the turbulent evolution of cold wakes caused by typhoons, and the subsequent mixing processes the restore the upper ocean stratification after a storm event. OBJECTIVES I propose to investigate the energy dissipation properties of the mixed layer and mixed-layer base / thermocline transition layer during direct forcing by a typhoon. It is hypothesized that inertial energy loss occurs not only through dissipative processes in the mixed layer, but also through dissipation occurring well into the transition layer between the mixed-layer base and the thermocline, where shear is enhanced. Energy is also lost to the thermocline by conversion of inertial energy into near-inertial wave radiation. The turbulence generated at in the transition layer is tied to shear instability occurring below the mixed-layer base, which appears to be a key mechanism in parameterizations for mixed-layer response to strong wind forcing. Energy dissipation will be measured using a glider equipped with turbulence probes. A Slocum glider system from Webb Research will be adapted for this purpose. This is a novel instrument system, which will allow for turbulence measurements in a manner not previously possible. APPROACH To overcome the limitations of conventional microstructure profiling, we will use the recently developed turbulence glider system developed between the co-PI (St. Laurent), Rockland Scientific, 1

Hot-wire measurements in the boundary layer developing on a concave surface of 2.0 m radius of curvature in the presence of forced wavelength Görtler vortices have been conducted for a free-stream velocity of 3.0 m / s. The wavelengths of vortices were preset by vertical perturbation wires of 0.2 mm diameter located 10 mm upstream of the concave surface leading edge. The velocity contours in the cross-sectional planes at several streamwise locations show the growth and breakdown of the vortices that are similar to those found in the transitional flow field. It shows the occurrence of the second instability mode that is indicated by the formation of small horseshoe eddies generated between the two neighboring vortices traveling in the streamwise direction to form mushroom-like structures as a consequence of the nonlinear growth of the Görtler vortices. The breakdown of these structures before the boundary-layer flow becomes turbulent is also shown to qualitatively predict the start of the transition in the flow. The Görtler number where the start of the transition was predicted is found to be within the range of transitional Görtler numbers previously reported for naturally developed Görtler vortices. The average of the spanwise wavelength after being normalized by / u is comparable with the generally quoted value of 100 for turbulent boundary layers.

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Measurements of the lateral components of the vorticity fluctuation have been made in the self-preserving turbulent wake of a circular cylinder. Each component was obtained separately using two X-wires separated in the appropriate lateral directions. The two velocity derivatives which make up the streamwise vorticity component were also determined but not simultaneously. An approximation to the streamwise vorticity was made from these measurements. Moments, probability density functions and spectra of the three vorticity components across the wake are presented and discussed. The high-wavenumber behaviour of the spectra is compared with calculations, based on local isotropy. Satisfactory agreement with the calculations is obtained for the lateral vorticity components over a significant high-wavenumber range. The approximated streamwise vorticity spectrum tends towards the isotropic calculation at very large wavenumbers.

The linear and nonlinear developments of Görtler vortices were experimentally investigated by means of hot-wire anemometer measurement. The wavelengths of the vortices were preset to be the most amplified using thin perturbation wires. Three different vortex wavelengths of 12, 15, and 20 mm were considered. These wavelengths were preserved downstream which confirm the prediction of the most amplified wavelength of Görtler vortices. The onset of the nonlinear region occurs at about the same Görtler number of 5.0 for all the wavelengths considered. In this nonlinear region, the secondary instability is initiated near the boundary layer edge, and it develops further downstream. The maximum turbulent intensity increases as the secondary instability becomes dominant in the flow. In the transition region, however, it slightly decreases before drastically increasing due to the onset of turbulence. Three maxima of intense turbulence are found in the turbulent intensity contours in the nonlinear region, which indicate the occurrence of the so-called varicose and sinuous modes of the secondary instability. Comparison with the previous available results shows that all data of maximum disturbance amplitude obtained from the same experimental setup seem to lie on a single line when they are plotted against Görtler number, regardless of the values of free-stream velocity and concave surface radius of curvature. Smaller radius of curvature results in higher vortex growth rate in the linear region due to stronger centrifugal effect. However, the vortex growth rate seems to be unaffected by free-stream velocity. The normal position of maximum disturbance amplitude reaches the maximum point exactly at the onset of nonlinear region before it drastically drops as the secondary instability is overtaking the primary instability.