Predicting the laminar to turbulent transition is an important aspect of computational fluid dynamics because of its impact on skin friction. Traditional methods of transition prediction do not make it possible to consider configurations where the boundary layer develops in presence of surface defects (bumps, steps, gaps, etc.). A neural network approach is used in this paper, based on an extensive database of boundary layer stability computations in presence of gap-like surface defects. These computations consists on linearized Navier-Stokes calculations and provide informations on the effect of surface irregularity geometry and aerodynamic conditions on the transition to turbulence. Physical and geometrical parameters characterizing the defect and the flow are provided to a neural network whose outputs inform about the effect of a given gap on the transition through the Δ𝑁 method.

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Predicting the laminar to turbulent transition is an important aspect of computational fluid dynamics because of its impact on skin friction. Traditional transition prediction methods such as local stability theory or the parabolized stability equation method do not allow for the consideration of strongly non-parallel boundary layer flows, as in the presence of surface defects (bumps, steps, gaps, etc.). A neural network approach, based on an extensive database of two-dimensional incompressible boundary layer stability studies in the presence of gap-like surface defects, is used. These studies consist of linearized Navier–Stokes calculations and provide information on the effect of surface irregularity geometry and aerodynamic conditions on the transition to turbulence. The physical and geometrical parameters characterizing the defect and the flow are then provided to a neural network whose outputs inform about the effect of a given gap on the transition through the ${\rm \Delta} N$ method (where N represents the amplification of the boundary layer instabilities).

In the present study, the effect of various twodimensional surface defects (forward-facing steps and ramps, backward-facing steps and ramps, gaps and steps-and-gaps) on boundary-layer transition was experimentally investigated in the transonic regime. A laminar profile was specifically designed and manufactured by ONERA so as to allow for a maximum of defects to be tested simultaneously, and to include resin pockets to accurately monitor laminar-turbulent transition using infra-red thermography. Transition was also characterised using the ∆N model based on linear stability calculations. Relatively good agreement with existing ∆N models for forward and backward facing steps as well as gaps was found, indicating that these models, which were mostly developed for incompressible flows, can still be used as a conservative estimate for compressible flows. More practical defects referred as step-and-gap were also considered.

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