A Survey on Control Techniques to Augment Compressible Jet Mixing

The mixing enhancement of a jet and its characteristics are essential for numerous aerospace applications, for example, reducing the infrared radiation of combat aircraft, mitigating noise in passenger aircraft, improving combustion characteristics in conventional jet, ramjet, and scramjet engines, producing vectored thrust for controlling spacecraft, missiles, and satellite. These applications led to studying the compressible jet mixing processes and strategies for controlling them. The mixing process is severely suppressed in high-speed flows (particularly when the jet Mach number is above 0.3) because of the compressibility effects. Jet mixing requires the development of augmentation strategies due to the short flow residence time (about a tenth of a millisecond). This study provides a comprehensive overview of the mixing improvement methods for compressible jets. It begins with an introduction to the compressible flow mixing layer. It examines several methods for enhancing jet mixing, such as active or passive control and unconventional mixing techniques like fluidic oscillators and mixing induced by shock waves. The passive flow control strategies make the flow more unstable and introduce large-scale vortices in the flow direction. The investigators studied the passive jet control configurations based on the above two approaches to increase mixing efficiency while maintaining a tolerable thrust loss and base drag. Active flow control is achieved by inducing instability but are only effective for appropriately selected values of actuating frequency, duty cycle, mass flow ratio, exit diameter of the actuating jet, location of actuators, etc.