Effects of the Initial Boundary-Layer State on Turbulent Jet Mixing

Abstract

Theme A MAJOR problem in many jet mixing flows is scaling, since research and development tests are usually done at much lower Reynolds numbers than the full scale application. It often occurs that if no special tripping is done, the initial boundary layer in the jet of a subscale test is laminar, while the full scale hardware will have a turbulent boundary layer. One of the primary goals of our investigation of initial boundary layer effects was to evaluate the importance of this difference on the accuracy of full scale predictions deduced from model tests. Secondary objectives included a study of the reasons for the differences in core lengths obtained by different investigators and possible isolation of particular boundary-layer characteristics that enhance turbulent mixing rates. Both axisymmetric and two-dimensional free jet flowfields exhibit a power law dependence for the decay of centerline velocity with distance from nozzle exit at points that are well downstream of the initial mixing region. However, variations in shear layer properties close to the jet exit can cause the centerline velocity profile to shift upstream or downstream under different operating conditions. Measurements made by different investigators in flowfields produced by similar nozzles frequently show significant variations in core length; the far field centerline velocity decay, however, follows a predictable variation with axial distance. Much of this earlier research on turbulent mixing in free jets has addressed cases where the effects of initial conditions were intentionally minimized. The few cases in which data regarding initial conditions are available indicate that the effects of nozzle boundary layers on mixing rates are often significant. While there are many aspects of turbulent mixing that warrant further investigation, it was the consensus of both the attendees and the review committee at the Langley Working Conference on Free Turbulent Shear Flows that the effect of initial conditions is a primary area requiring study. In this paper we describe some effects of nozzle wall boundary-layer conditions on incompressible air jets discharging into quiescent surroundings. This investigation included the influence of the boundary-layer state on the turbulent mixing layer surrounding axisymmetric and two-dimensional free jets and on time averaged measurements obtained in these flows.