This paper presents an experimental boundary layer transition investigation of the windward side of a generic hypersonic forebody performed in April 2015 in theBoeing/AFOSR Mach 6 quiet tunnel facility at Purdue university (BAM6QT). At 0 and 4 degrees of angle of attack, =11x10 6 /m, flow was fully laminar in quiet conditions. Under noisy conditions, an early transition front (Re θ~2 00) was observed, even when dividing the unit freestream Reynolds number by 6. In quiet conditions, several diamond-shaped roughness trips were found to efficiently trip the laminar boundary layer when > . Temperature-Sensitive Paint (TSP) enabled a global measurement of the heat flux distribution and detection of the transition front. PCB sensors confirmed the state of the boundary layer : laminar, turbulent or transitional. Transition results with a single continuously blowing sonic air jet was also collected at various pressure ratios, giving the laminar to turbulent threshold. NomenclatureCFD = Computational Fluid Dynamics BAM6QT = Boeing/AFOSR Mach 6 Quiet Tunnel NT = Natural Transition BLT = Boundary Layer Transition WT = Wind-Tunnel JISC = Jet In Supersonic Crossflow PR = Pressure Ratio LT = Laminar to Turbulent transition TSP = Thermal Sensitive Painting Re u = Unit Reynolds number, /m St = Stanton number δ = Laminar boundary layer thickness, mm μ = Dynamic viscosity, m2/s ρ = Density, kg/m3 u = Streamwise velocity, m/s M e = Edge boundary layer mach number P e = Edge boundary layer pressure T w = Wall temperature, K 2 k = Roughness element height, mm h = Mach disk height Re hh = Mach disk height Reynolds number Re kk = Roughness element height Reynolds number Re θ = Momentum thickness Reynolds number P i0 = Stagnation pressure, Pa T i0 = Stagnation temperature, K P ∞ = Freestream pressure, Pa T ∞ = Freestream temperature, K J = Jet momentum flux ratio

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Direct numerical simulations (DNS) of the Navier-Stokes equations have been performed to investigate the receptivity and breakdown mechanisms in a Mach 6 flow over a generic forebody geometry with freestream acoustic disturbances. The simulations are based on transition experiments carried out in April 2015 in the Boeing/AFOSR Mach 6 facility at Purdue University. A three-dimensional model for both fast and slow freestream acoustic waves with multiple frequencies and spanwise wavenumbers has been adopted in the numerical simulations, for which high-amplitude disturbances have been considered in order to simulate noisy wind tunnel conditions. The numerical results reveal similarities in comparison to the experimental observations, especially when slow acoustic waves are considered as freestream disturbances. In particular, slow acoustic waves have been found to induce the breakdown process via crossflow instabilities located in the off-centerline region, with formation of streamwise streaks. Fast acoustic waves, in contrast, appear more efficient in inducing earlier nonlinear growth through destabilisation of the boundary layer along the symmetry plane of the body.Receptivity is the internalization process of the external disturbances into the boundary layer in the form of instability modes, and determines the initial conditions for the downstream linear growth of the unstable modes. In the case of high-amplitude disturbances, nonlinearities can become predominant in the 3 of 31 American Institute of Aeronautics and Astronautics receptivity process, which would lead directly to nonlinear growth and rapid transition to turbulence. In some particular conditions, an intermediate route to transition (between a linear modal and a fully nonlinear process) is possible, i.e. transient growth, consisting of a non-modal growth of linear instabilities, as in the case of lift-up-induced streamwise streaks in conjunction with secondary instabilities and consequent nonlinear breakdown [1,2].The body leading edge is a highly-receptive zone, due to the non-parallel effects and the related shortscale streamwise variations of the mean flow, which, in turn, cause a wavelength-conversion process from the scale of the external forcing to that of the induced boundary-layer disturbances [3]. At hypersonic Mach numbers, however, the small difference in phase speed between the forcing waves and the boundary-layer dominant modes can lead to a direct excitation of these modes via a resonance mechanism at the leading edge [4,5,6,7], without the need of a wavelength-conversion mechanism. By applying Fedorov's notation [5], the internal mode generated at the leading edge through the receptivity to fast acoustic waves is called the fast mode, or mode F, while the mode associated to the slow acoustic waves is known as the slow mode, or mode S, which is the mode pertaining to the class of the unstable boundary-layer modes. In hypersonic boundary layers, different unstable modes can coexist, including the first mode, corresponding to the Tollmien-...

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