The Effects of Longitudinal Curvature on Turbulent Two-Stream Mixing Layers.
Abstract
Two-stream turbulent mixing layers subjected to mild (delta/R < 0.06) stabilizing and destabilizing curvature were investigated and compared to a plane mixing layer base case. The experiments were performed in a low-speed water channel facility. The velocity ratio was nominally 2:1, with the faster stream flowing at 18 cm/s. Various flow visualization techniques were used to examine the effects of curvature qualitatively. Quantitative velocity-field measurements were made using three-component laser Doppler velocimetry (LDV). The flow visualization studies illustrated the role of curvature in the turbulence transport and mixing in the shear layers. Secondary streamwise vortical structures which have been studied in plane mixing layers were observed in the curved mixing layers. The streamwise vortices were most vigorous in the unstable case. These results, in conjunction with spanwise mean velocity surveys, showed that destabilizing curvature enhances the development of these streamwise structures. Secondary Reynolds shear stress profiles confirmed the presence of stronger streamwise vortical structures in the unstable layer. Velocity profiles across the mixing layer were acquired at several streamwise locations. For the three cases, the inlet and boundary conditions were as identical as experimentally possible in an unforced flow. The magnitudes of the normal and transverse components of Reynolds stresses in the unstable layer were twice those in the stable layer. The Reynolds shear stress maxima were also twice as great in the unstable layer compared to the stable layer. A linear growth rate was established in all three layers in a region far enough downstream to be considered fully developed. The unstable mixing layer exhibited a growth rate twice that of the stable layer. Higher order statistics were computed to further characterize the nature of the mixing layer turbulence. These included triple correlations, skewness and flatness of the velocity fluctuations. The triple products (dominant terms in the transport equations for turbulent kinetic energy and primary Reynolds shear stress) were two to four times greater in the unstable layer compared to the stable layer. In summary, it was found that moderate streamline curvature has a pronounced effect on the development of turbulent two-stream mixing layers. The stabilizing and destabilizing effects of the Taylor-Gortler instability mechanism coupled with the extra rates of strain due to streamline curvature manifest themselves in the suppression or enhancement of turbulence transport and shear layer growth depending on the sense of the curvature.
- Publication:
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Ph.D. Thesis
- Pub Date:
- 1990
- Bibcode:
- 1990PhDT........56P
- Keywords:
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- Engineering: Mechanical; Physics: Fluid and Plasma