On the spatio-temporal dynamics of cavitating turbulent shear flow over a microscale backward-facing step: A numerical study

• Published in: International journal of multiphase flow Vol. 177; p. 104875
• Main Authors: Maleki, Mohammadamin, Rokhsar Talabazar, Farzad, Koşar, Ali, Ghorbani, Morteza
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.07.2024
• Subjects: Cavitation; Compressibility; Microscale backward-facing step; Spatio-temporal dynamics; Spectral proper orthogonal decomposition (SPOD); Turbulent separation bubble (TSB); Spectral proper orthogonal decomposition (SPOD); Spatio-temporal dynamics; Compressibility; Turbulent separation bubble (TSB); Cavitation; Microscale backward-facing step
• ISSN: 0301-9322; 1879-3533
• DOI: 10.1016/j.ijmultiphaseflow.2024.104875

•A sophisticated simulation captures shear cavitation in a backward step, accounting for compressibility, finite mass transfer, and turbulence.•The development of vapor within the flow diminishes the average growth rate of the shear layer and extends its reattachment point further downstream.•Analysis of Reynolds stresses and pressure fluctuations reveals that vapor formation and collapse significantly influence turbulence decay and production within the shear layer.•Spectral analysis identifies two dominant low frequencies linked to reattachment point movement. In cavitating conditions, these frequencies are lower compared to non-cavitating cases.•Cavitation leads to a significant increase in the spectral energy of high-frequency fluctuations within the reattachment region.•Spectral proper orthogonal decomposition (SPOD) analysis unveils the impact of cavitation on the dynamics of coherent structures within the flow. The influence of cavitation on the mean characteristics and unsteady behavior of turbulent separated flows was comprehensively investigated in this study over a microscale backward-facing configuration at the Reynolds number (ReD) of 7440. The computational approach took both compressibility and finite mass transfer (Thermodynamic non-equilibrium) into account, to accurately capture the effects of shock waves, as well as to capture baroclinic phenomena on vortex dynamics within the turbulent separated flow. The compressibility effects were handled by using appropriate equation of states for each phase and for the mixture. Phase-change was considered through a transport equation for the vapor volume fraction, allowing for finite mass transfer contributions. Additionally, a wall adaptive large eddy simulation (LES) approach was utilized for simulating turbulent structures and their effects. The findings reveal that vapor development diminishes the mean growth rate of the shear layer and delays its reattachment to a longer distance from the step. Moreover, analysis of Reynolds normal and shear stresses, as well as the root mean square (RMS) of pressure fluctuations, demonstrates that the formation and collapse of vapor packets significantly influence turbulence decay and production in the second half of the shear layer and reattachment. It was also observed that both mean pressure and pressure fluctuations increased in vicinity of the reattachment region when cavitation was present, which was attributed to the condensation and collapse events. Spectral analysis further indicates the emergence of two dominant low frequency modes, linked to the displacement of the reattachment point. In the presence of cavitation, the frequencies associated with dominant Power Spectral Densities (PSDs) were smaller than those in the absence of cavitation. Additionally, each of these low frequencies corresponded to a specific vapor transport mechanism within the Turbulent Separation Bubble (TSB). Furthermore, it is shown that cavitation leads to a significantly higher spectral energy of high frequency fluctuations within the reattachment region in comparison to the condition where cavitation is absent. This can be attributed to the frequent collapse of bubbles in this region. At the end, we employed Spectral Proper Orthogonal Decomposition (SPOD) for modal analysis. This method offers valuable insights into the coherent structures and associated frequencies that arise in both the presence and absence of cavitation, which provides a deeper understanding of the effect of cavitation on the coherent structures and their dynamics. [Display omitted]