An experimental investigation of the effect of a density gradient on shear layer instability

Abstract

Measurements of mass flow rate and mean density have been made in separated laminar boundary layers having large transverse density gradients. A 3/8 in. by 1 1/2 in. rectangular half-jet was used to generate a two-dimensional shear layer and the density heterogeneity was produced in an incompressible flow by exhausting one gas into a reservoir of another gas having a different molecular weight. Two Freons were used having a density ratio of 1.98 and unique properties which permitted the measurement of the mass flow rate in all mixtures of the gases with a single hot wire. Mean density and mass flow rate profiles were compared to the Holmboe model used in theoretical hydrodynamic stability analyses. Fluctuations in the mass flow rate were analyzed and the frequency, wave number and amplification rate of the most unstable oscillation were measured and compared to theoretically predicted values. The oscillations were found to have a higher amplification rate, lower wave number and lower frequency than homogeneous flow when the lighter gas flowed into the heavier one, in agreement with the theoretical predictions. Opposite trends were observed with the heavy gas flowing. The development of harmonic frequency oscillations in the non-linear region is discussed briefly.