PLANAR VELOCITY MEASUREMENTS IN AN INCOMPRESSIBLE AND A WEAKLY COMPRESSIBLE MIXING LAYER.

다국어 초록 (Multilingual Abstract)

High-vector density planar velocity fields were obtained for an incompressible mixing layer and a weakly compressible mixing layer using particle image velocimetry (PIV). For the incompressible case the velocity ratio was 0.58, and the velocity fields were obtained at a location where Re_x = 1.8 × 10<super> 5</super>, <math> <f> <rm>Re</rm><inf><g>d</g><inf><g>w</g></inf></inf></f> </math> = 1.1 × 10<super>4</super>, and the pairing parameter was Rx/λ = 31. The velocity vector fields indicate the existence of large, two-dimensional Brown-Roshko roller structures with a variety of shapes, orientations, and interactions. Spatial correlations of velocity fluctuations were determined, and these were used to find linear stochastic estimates of roller structures and braids. The linear stochastic estimate of a roller is slightly elliptical with the major axis oriented in the streamwise direction, and the linear stochastic estimate of a braid is obliquely oriented.
For the weakly compressible mixing layer, the velocity ratio was 0.53, the density ratio was 0.67, and the convective Mach number was 0.38. At the location where the PIV images were obtained, Re_x = 3.7 × 10<super>6</super>, <math> <f> <rm>Re</rm><inf><g>d</g><inf><g>w</g></inf></inf></f> </math> = 1.8 × 10<super>5</super>, and Rx/λ = 18. The planar velocity fields obtained in this study fall into three regimes characterized by the size and number of large-scale structures present. The large-scale rollers are either circular or elliptical, with the elliptical rollers having, in general, horizontal major axes. The transverse velocity fluctuations and Reynolds shear stress are suppressed for the weakly compressible mixing layer as compared to the incompressible case. The spatial correlations of velocity fluctuations are also smaller than those for the incompressible case. The linear stochastic estimate of a roller structure in the weakly compressible mixing layer is elliptical with the major axis oriented in the streamwise direction and with an eccentricity greater than for the incompressible case. The linear stochastic estimate of a braid suggests that the braids are vertically oriented, as opposed to the oblique orientation seen in the incompressible mixing layer. In addition, the braids in the weakly compressible case have a vertically oriented stagnation line, as opposed to the braids in the incompressible mixing layer where stagnation occurs at a point.