The spatial development of planar incompressible countercurrent shear layers was investigated experimentally. A facility was constructed to establish countercurrent shear layers without the formation of global stagnation in the flow. Particle image velocimetry was employed to obtain detailed measurements within the region of self-preservation for velocity ratios $U_{2}/U_{1}$ between 0 and −0.3. The spatial growth rate of countercurrent shear layers was found to agree generally with simple analytical theory. At 30% counterflow, the growth rate was approximately twice as large as the case with no counterflow. Peak turbulence quantities, when normalized by the applied shear magnitude, $\Delta U$, were found to be nominally constant for low levels of counterflow, but at counterflow velocities above 13% of the primary stream velocity, peak turbulence levels increased. The observed transition is accompanied by the development of mean flow three-dimensionality. The deviation occurs at a counterflow level that is in agree- ment with theoretical predictions for transition from convective to absolute instability.
During the last decade, countercurrent shear has been established as an effective flow control technique for increasing turbulent mixing in a variety of flow configurations and operating regimes. Based on the robust mixing enhancement observed for jets and shear layers, the technique appears to have many potential benefits for enhancement and control for turbulent combustion flows. Countercurrent shear flow control has been applied to a planar asymmetric rearward-facing step dump combustor. A nonreacting flow study on the implementation of suction-based countercurrent shear at the dump plane provided insight into the flow control mechanisms. Control of turbulence velocity and length scales occurs through two mechanisms, the development of a countercurrent shear layer near the dump plane, and enhanced global recirculation caused by the removal of mass at the dump plane. Parametric studies on the geometry of the suction slot indicate that the enhancement of the global recirculation zone is the primary mechanism for increasing global turbulence levels within the combustor. Turbulence energy and length scales both increase in a manner such that the spatially-filtered strain rates as measured with particle image velocimetry remain nominally constant, a desirable characteristic for premixed turbulent combustion. Connections will be made to a recent study on fully-developed turbulent countercurrent shear layers showing additional attractive features of countercurrent shear including enhanced turbulent energy production, entrainment, and three dimensionality. Preliminary reacting flow results for the dump combustor operating while burning premixed/prevaporized JP-10 illustrate qualitative changes in the turbulent combustion process within the combustor. The companion paper will describe the quantitative effects of countercurrent shear on the global heat release rates within the combustor.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.