Simultaneous temperature and velocity measurements in the plane of symmetry of a transitional turbulent spot

Abstract

Simultaneous measurements of the longitudinal velocity U, normal velocity V, and temperature T have been made in turbulent spots generated in a slightly heated laminar boundary layer. The measurements were made in the plane of symmetry of the spot at several streamwise stations downstream of the spot-producing spark. Ensemble averages of U, V and T, obtained relative to the leading edge of the spot, are presented in terms of disturbances relative to the unperturbed values in the laminar flow. Ensemble-averaged disturbances of U, V and T in the outer part of the spot are consistent with a picture of a relatively large vortical structure with a spanwise vorticity in the same sense as that of the laminar flow. A rather dominant feature of these distributions is the large negative disturbance in V and U near the leading edge in the outer part of the spot; associated with this disturbance is a positive perturbation in T. The terms contributing to the ensemble-averaged values of UV, UT and VT are obtained and discussed. In the case of UV, contributions by the disturbance are found to be of the same order as those by the random turbulence superposed on the disturbance. For VT, the disturbance appears to play a more dominant role in the transfer of heat. The conical property of the spot is tested for disturbance temperature and velocity fields by comparing contours, at three streamwise stations, of the velocity and temperature disturbances using the co-ordinates introduced by Cantwell, Coles & Dimotakis (1978). The results show that the conical transformation, which was found to be a good approximation for the full velocity field, is not an accurate representation for the disturbance fields of velocity and temperature measured relative to laminar values. An alternative viscous transformation represents more accurately the disturbance fields. Features of the turbulence within the spot are compared with those obtained in a fully turbulent boundary layer.