Influence of free-stream turbulence and acoustic noise on laminar-to-turbulent transition in 2-D wing flow is investigated experimentally. The quantitative evaluation of external disturbances on spatial growth rate N is intensively required to predict the transition location by the use of the e{sup}N method. Natural transition on an identical model is examined in five different wind tunnels. Early transition is observed in the wind tunnels with large acoustic noise and/or large residual free-stream turbulence. The contribution of free-stream turbulence Tu and sound pressure level SPL is evaluated separately, and an empirical relation between N and sound pressure level is proposed.
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