Multiscale Modal Analysis of a Plasma Jet: Coherent Structures and their Observability

Plasma jets naturally develop hydrodynamic instabilities originating from thermal and momentum exchanges with the cold and quiescent surrounding fluid. This work investigates the hydrodynamic instabilities of a simulated plasma jet using multiscale data-driven modal analysis. The investigated conditions are representative of the flow produced in the Plasmatron facility at the von Karman Institute, designed to simulate the harsh environment encountered during an atmospheric entry. Two aspects were investigated. First, we demonstrate the multiscale Proper Orthogonal Decomposition (mPOD) capabilities to extract the main structures in the plasma jet flow from highly resolved thermal fields of the flow. To this end, we analyzed DNS simulations of the plasma jet with characteristic outlet velocity и = 146 m s^-1, a temperature T = 10 665 K, and diameter d = 0.16 m. Second, we investigate the experimental observability of these patterns using a high-speed recording. To this end, we simulated the image acquisition process of a High-Speed Camera (HSC) and generated synthetic videos by rebuilding the light emission from the DNS temperature field. The analysis of the synthetic grayscale images was performed on two light intensity ranges commonly employed to record the plasma flow of the VKI Plasmatron. The decomposition of the thermal field reveals travelling wave patterns belonging to different frequency ranges and evolving in the shear layer of the jet. The analysis of the synthetic grayscale images shows that much of the relevant information on the frequency content and scales of the leading travelling waves is retained, but the associated spatial structures are severely altered.