Measurements on the Growth of Small Disturbances in a Stratified Shear Layer

Abstract

A statically stable, stratified free shear layer was formed within the test section of a wind tunnel by merging two uniform streams of air of different temperature at the entrance to the contraction section. The resulting diffusion‐thickened interface was characterized by nearly self‐similar temperature (odd), velocity (odd), and Richardson number J (even) profiles that were adjustable over the ranges 0.07 ≤ J ≤ 0.76 and 50 ≤ Re ≤ 150, where Re is the Reynolds number. Small periodic disturbances were imposed on the interface by oscillating a taut, 0.004‐inch‐diameter steel wire within the interface in the contraction section, and subsequent development was deduced from measurements of the fluctuating temperature downstream. Results indicate complete infinitesimal stability for J > 1/4 and instability over a range of non‐dimensional wave numbers of the order of unity for smaller values of J. Growth rates are presented that agree reasonably well with theoretical predictions based on the assumption of inviscid parallel flow.