Kelvin - Helmholtz and Streamwise Vortices in the Near Wake of a Single-Lobe Forced Mixer

Abstract

Abstract

In the present experimental investigation, the characteristics of two types of vortices, namely the normal Kelvin-Helmholtz (K-H) and streamwise vortices, generated in the near wake of a lobed forced mixer were examined and evaluated at two velocity ratios (r = U₁:U₂ = 1:1 and 0.4:1). The lobed forced mixer has only one lobe and was designed to eliminate any possible interactions between the neighbouring vortices generated by the adjacent lobes. Hot-wire anemometer was used to study the K-H vortices via the spectrum analysis. Laser-Doppler anemometer was employed to examine the streamwise vortices. It was found that there were two main frequencies for the K-H vortex in the wake of the forced mixer (f_{LM_C} and f_{LM_L}); both were higher than the case of plane-free shear layer (f_FP), i.e., f_{LM_C}f_{LM_L}f_FP. where f_{LM_C} represents the K-H vortex shedding frequency in the behind the semi-circle part of the trailing edge, whereas f_{LM_L} represents the frequency in the near wake behind the parallel side-walls of the trailing edge. The velocity ratio appeared to have more important effects at point C than at point L. The Strouhal numbers (St_{LM_C} and St_{LM_L}) reached constant values at the Reynolds number greater than 5 × 10⁵. The K-H vorticities for the velocity ratio r= 0.4 was ∼30 and 110 per cent stronger than for case r= 1 for points L and C, respectively. The K-H vortices were also found to be more contributive to the production of the turbulent kinetic energy in the near wake of the single-lobe forced mixer than the streamwise vortices. The streamwise vortices at r= 0.4 accelerated the breakdown of the K-H vortices, which happened at about one wavelength earlier than r= 1. Finally, the effects of geometry parameters, including lobe height (or penetration angle) and wavelength, were examined.