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Surface Irregularity Effects on Laminar-Turbulent Transition - Understanding the Surface Quality Requirements for Laminar Flow on Wings
The effect of gaps and bumps on transition to turbulence in boundary layers was investigated. Linear stability theory and direct numerical simulations revealed that gaps can behave like large cavities and induce transition. Results explained experiments recently developed and published by Boeing. The effect of Re, Ma, ratio between boundary layer thickness and gap dimensional and gap aspect ratio was investigated and explained physically. Transition is caused by the interaction of Rossiter and centrifugal modes. Centrifugal modes nonlinearly affect the Rossiter instability, but the linear instability provides a lower safe bound for laminar flow. The interaction of TS waves and bumps were studied via very accurate direct numerical simulations. A boundary for a linear effect of the bump height on TS waves was found to be about 10% boundary layer displacement thickness. The effect of Mach and frequency was also investigated. A very precise and efficient model based on a Taylor expansion combined with a Stokes layer was developed to be used in numerical simulations. Theoretical equations were deduced for the optimal choice of computational parameters for use of matrix-free methods of global instability analysis.
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