Active control of mixing in turbulent separated shear layers and effects of forcing on the fractal geometry of scalar interfaces

Abstract

This paper explores effects of active control on mixing and the fractal geometry of scalar interfaces. In particular, a pulsed plasma actuator is used to implement active mixing control of turbulent-separated shear layers, with the advantage that this actuator has no moving parts. The effects on mixing are determined based on the scalar power spectrum and the scalar probability density function from scalar images recorded with laser Mie scattering for fog-seeded freestream air and pure ambient air. In addition, the fractal geometry of the scalar interfaces is examined to determine the effects of the forcing. The Reynolds number of the shear layer based on the visual thickness is 3.0 × 104 at the imaging station. The range of Strouhal numbers of the forcing is 0.3 ≤ St ≤ 7.8. Effects of forcing on the scalar power spectrum are found in terms of significant modifications of the spectral scalar energy at the lowest wavenumbers which is observed to increase by as much as 50% and decrease by as much as 40% depending on the forcing frequency. These modifications of scalar fluctuations at the largest scales indicate that the forcing is able to inhibit and enhance, respectively, the formation of large-scale scalar structures, which can be denoted as Large Structure Suppression (LSS) and Large Structure Regularization (LSR). Shifts of the intermediate peak of the scalar probability density are found to occur away from and toward, respectively, the higher speed fluid concentration value for the most pronounced cases of LSS and LSR. Regarding the effects of forcing on the fractal geometry of scalar interfaces, evidence is presented that forcing has a relatively small effect on the fractal dimension value but a significant effect on the range of scales of geometrical self-similarity. The fractal dimension exhibits values in the range of D2∼ 1.56 +/− 0.02 to 1.65 +/− 0.02 depending on the forcing conditions, with the highest dimension value found at the highest forcing frequencies. The range of scales exhibiting geometrical self-similarity is found to be extended by up to 75% or reduced by up to 50% depending on the forcing frequency. Therefore, these results show that although the fractal dimension value is relatively robust, periodic forcing of the turbulent-separated shear layer can substantially alter the extent of the range of scales exhibiting geometrical self-similarity.