Passive control of single and multiple jets using cross wire vortex generators at sonic and supersonic mach numbers

Abstract

The effectiveness of crosswire in controlling the mixing characteristics of a circular and an equivalent elliptic jet is investigated experimentally. While circular jets are conventional, elliptic jets have gained attention due to their better mixing characteristics and faster decay. To further explore and augment the capabilities of elliptic jets for practical utility, it is investigated whether using an elliptic jet with crosswire control gives additional benefit in mixing enhancement over an axisymmetric jet. Experiments are performed for subsonic and choked flow conditions with nozzle pressure ratios ranging from 1.2 to 7.0. Time-averaged pitot pressures and schlieren visualization is used for diagnosis. The jet bifurcation can be seen in controlled elliptical jets at all nozzle pressure ratios (NPRs). Core length is reduced to as much as 70% in the elliptic jet and 84% in the case of the circular jet. The core length values estimated from the present data are compared with the previous investigations. Mean flowfield and the mixing characteristics of free supersonic jets from twin and triple converging-diverging nozzles placed in proximity are also investigated. The nozzles are designed for Mach numbers 1.5 and 2.0, with an inter-nozzle spacing of twice the nozzle exit diameter. The typical interaction process and the evolution of the triple jet are discussed using cross-sectional contour plots. The influence of introducing additional similar jets on the near flowfield characteristics such as jet-spread, supersonic core, and the shock wave structure is studied using pressure measurements along the jet centerline. As the number of jets increases, the spreading rate decreases due to a reduction in the entrainment. This causes the jets to decay at a slow rate, and the core length increases in the order of an increased number of jets. Schlieren's images of single, twin, and triple jets reveal that the supersonic jet core is different in twin and triple when compared with a single plane. A simple yet effective approach is presented in the present work to get a reasonable estimate of the Mach number from the schlieren images for a Mach 2.0 nozzle jet. Results are compared with the numerical simulations for the estimated Mach number from the experimental data. The uncontrolled center line pitot pressure decay results obtained from numerical simulations are compared with the uncontrolled centerline pressure decay results obtained from the experimental. The crosswire tab is used as a passive control tool at the nozzle exit in two orientations to study the control effect. Schlieren's images reveal that the supersonic jet core is different in a controlled jet than the uncontrolled jet. Up to 83% reduction in core length is obtained from Mach 1.5 using vertical orientation of crosswire passive control at the nozzle ext. From the present research, it is evident that the crosswires' performance in multiple jets effectively reduces the supersonic core length at all NPRs of supersonic Mach numbers and higher NPRs of sonic Mach number. The most effective orientation in jet mixing enhancement is the vertical wire (control - 2) among the wire orientations studied.