Flow visualization of the jet exiting a chevron nozzle

Control of jets has been studied for over a century and even today continues to be a significant research topic. It finds applications in various fields especially in aerospace, where enhancement of mixing characteristics of a jet can improve the performance greatly. Review articles by Gutmark et al.;1 Gutmark & Grinstein;2 Seiner et al.;3 Glezer & Amitay;4 Reynolds et al.;5 Ginevsky Vlasov & Karavosov;6 Knowles & Saddington;7 Henderson,8 provide an excellent survey of mixing enhancement techniques. In general all types of control can be broadly classified into active and passive controls. In active control, an auxiliary power source, for example, fluid tabs9‒17 is used to control the jet characteristics. On the other hand, passive control finds its importance in the research area due to its simple geometric modification, leading the mixing enhancement without requiring any power source, such as non-circular nozzles,2,18 implementation of tooth like tabs19‒24 and chevron nozzles.25‒31 In spite of the extensive study of tabs and non-circular nozzles, they have lost its scope while implementing on practical situations because of its disadvantages. Once tabs are mounted, they are prone to erosion, induce thrust loss, increase in base drag and making the jet to become asymmetry.18 Frequent implementation of non-circular nozzles causes very high cost and requires a very aft-duct. Both tabs and non-circular nozzles cause the jet structure to become non-axisymmetric, thereby increasing the mixing of the jet with the ambient. So, the underlying phenomenon is that the mixing increases if the jet is non-axisymmetric. Hence the alternate passive control technique which overcomes the disadvantage of tabs and non-circular nozzles is the \Chevrons”. Chevrons, a saw tooth pattern on the trailing edge of exhaust nozzles, are being implemented on modern jet engines is one of the most popular passive control device in the recent years.