Mechanism of formation of coherent structures in under-expanded supersonic impinging jets

POSTER

Abstract

Large-eddy simulations of under-expanded impinging jets are performed to study the mechanism by which the initial high-frequency instabilities change to low-frequency coherent structures within a short distance. The spectral characteristics of the Mach energy norm is utilised to obtain the spatial growth of instabilities. Linear spatial instability analysis with streamwise varying mean flow profiles is also performed. Cross-correlation of velocity and pressure show that hydrodynamic wavepackets form approximately one jet diameter downstream of the nozzle lip. No evidence has been found to support the 'collective interactive' mechanism of Ho {\&} Nosseir (JFM, Vol. 105, p. 119-142, 1981). The 'vortex pairing' of Winant {\&} Browand (JFM, Vol. 63, p. 237-255, 1974) is observed near the nozzle; however, it has an insignificant role in the sharp reduction of the most unstable frequency of disturbances. Nonetheless, both Mach energy norm and linear spatial instability analyses show that the most unstable frequency of disturbances decreases rapidly in a very short distance from the nozzle lip in the near-nozzle region through the spatial growth of instabilities where linear instability analysis over-predicts the frequency of the most unstable instabilities.

*This work was supported by the Australian Research Council.The computational facilities supporting this project included the Australian NCI Facility, Pawsey Supercomputing Centre and MASSIVE.

Authors

  • Shahram Karami

    • Monash University
    • LTRAC, Department of Mechanical and Aerospace Engineering , Monash University, Clayton Campus, Melbourne, VIC3800, Australia

  • Daniel Edgington-Mitchell

    • Monash Univ
    • Monash University
    • Department of Mechanical and Aerospace Engineering, Monash University, Clayton, Australia

  • Vassilis Theofilis

    • School of Engineering, University of Liverpool The Quadrangle, Brownlow Hill, L69 3GH, UK
    • University of Liverpool

  • Julio Soria

    • Department of Mechanical and Aerospace Engineering, Monash University (Clayton Campus), VIC 3800, Australia
    • LTRAC, Department of Mechanical and Aerospace Engineering, Monash University, Clayton 3800, Australia.
    • Monash University
    • Laboratory for Turbulence Research in Aerospace and Combustion, Department of Mechanical and Aerospace Engineering, Monash University
    • LTRAC, Department of Mechanical and Aerospace Engineering , Monash University, Clayton Campus, Melbourne, VIC3800, Australia
    • Lab. for Turbulence Research in Aerospace & Combustion, Department of Mechanical and Aerospace Engineering, Monash University, Melbourne, Australia
    • LTRAC, DMAE, Monash University