| Abstract: | A new method for obtaining instantaneous vertical profiles of two components of velocity and temperature in thermally stratified turbulent shear flows is presented. In this report, the design and construction of the traversing system will be discussed and results to date will be presented. The method is based on rapid vertical sampling whereby probe sensors are moved vertically at a high speed such that the measurement is approximately instantaneous. The system is designed to collect many measurements for the calculation of statistics such as vertical wave number spectra, mean square vertical gradients, and Thorpe scales. Results are presented for vertical profiles of temperature and compared to vertical profiles measured by single-point Eulerian time averages. The quality of the vertical profiles is found to be good over many profiles. Some comparisons are made between vertical measurements and standard single-point Eulerian measurements for three cases of stably stratified turbulent shear flow in which the initial microscale Reynolds number, Reλ≈30. In case 1, the mean conditions are characterized by a gradient Richardson number, Rig=0.015, for which the flow is “unstable”, meaning the spatially evolving turbulent kinetic energy (Ek) grows. In case 2, Rig=0.095, for which the evolving turbulent kinetic energy is almost constant. In case 3, the flow is highly stable, where Rig=0.25 and Ek decays with spatial evolution. The measurements indicate anisotropy in the small scales for all cases. In particular, it is found that the ratio
grows initially to a maximum and then decays with further evolution. Maximum Thorpe displacements are measured and compared to single-point measures of the vertical scales. It is found that vertical length scales derived from single-point measurements, such as the Ozmidov scale, LO=(ε/N3)1/2 and the overturn scale, Lt=θ′/(dT/dz), do not represent well the wide range of overturning scales which are actually present in the turbulence. |
