Active flow control of jet mixing using steady and pulsed fluid tabs

Abstract

Flow control techniques for increasing the rate of jet mixing in axisymmetric nozzle flows have been investigated. A combination of water tunnel and high-speed airflow facilities is used to assess the near-field jet behaviour. Solid tabs, steady fluid tabs (i.e. discrete radially discharged control jets located close to the core jet exit), and pulsed fluid tabs are compared. The effect of fluid tab velocity amplitude, pulse rate, and pulse phase are studied using open-loop control. The measurements indicate that fluid tabs generate a similar streamwise vortex formation process (and hence display increased mixing) as previously observed in solid-tabbed nozzle flows. In incompressible testing the mixing effectiveness with a pair of pulsed fluid tabs 180° out-of-phase was as good as a twin solid tab nozzle for a control jet flowrate of only 0.5 per cent of the primary (core) jet flow. In preliminary high-speed testing similar benefits of fluid tabs over solid tabs were observed. Further study of pulsed fluid tabs is recommended; they have the attractive performance benefit that they can be easily switched off when not needed and offer increased flexibility as the basis of an optimized active control jet mixing device.