Control of shock wave oscillation in transonic internal flow around an airfoil using cavity

Abstract

Self sustained shock wave oscillation on airfoils at transonic flow conditions are associated with the phenomenon of buffeting, high speed impulsive noise, non synchronous vibration, high cycle fatigue failure and so on. Though experiments demonstrate the presence of periodic shock oscillation in flow over airfoil in some transonic speeds, the physical mechanism of periodic shock motion is not fully understood yet. The present numerical study investigates the intricate flow phenomenon associated with self sustained shock oscillation and the passive control of shock oscillation using cavity over a symmetrical circular arc airfoil at zero incidence. The effect of incorporating both open and perforated cavity on the airfoil surfaces are demonstrated systematically considering pressure rise across the shock as an indicator of shock strength. Three different cases of open cavity control and nine different cases of perforated cavity control is studied along with the clean (base airfoil) airfoil. Results for base airfoil and airfoil with cavities (open and perforated) are compared to understand the effects of cavity installation. Shock strength, total pressure loss and surface pressure fluctuation are investigated by interpreting various shock characteristics like shock location, shock Mach number, boundary layer thickness and shock boundary layer interaction zone for all the cases. Results show that both open and perforated cavity can reduce the shock strength, surface pressure fluctuation near the airfoil and total pressure loss. The change in both flow characteristics and shock wave characteristics is studied and their dependency on the configuration of cavity and percentage of perforation are observed and proper suggestions are made. The results suggest that open cavity can be a better choice to control shock oscillation for external high speed flow over a single airfoil or internal flows through a series of airfoils separated by sufficient distance. But open cavities have a major difficulty for internal high speed flow application through a series of airfoils placed close to one another. On the other hand perforated cavities can also reduce surface pressure fluctuation for both internal and external high speed flows though their performance is not as good as open cavities. A comparative assessment is performed to ensure the economic feasibility of both open and perforated cavity control by calculating the integrated total pressure loss (ITPL).