Blended-wing-body (BWB) configurations with a significant portion of laminar boundary-layer flow are a promising design concept for future, sustainable and energy-efficient long-range aircraft. The blended three-dimensional (3D) shapes of such aircraft (A/C) are very much different from those of conventional A/C wings, however. On conventional wings, the spanwise gradients of the 3D boundary-layer flow typically are much smaller than the gradients in chordwise direction. Therefore,an infinite swept-wing (ISW) assumption is usually made for laminar-turbulent transition prediction either based on local stability theory (LST) or standard parabolized stability equations (PSE). The standard PSE approach for ISW conditions proved to model the propagation and growth of the different instability modes that trigger laminar-turbulent boundary-layer transition in a physically correct manner taking into account also the growth of the boundary layer, effects of streamline and surface curvature, and the upstream history of the instability modes. In both approaches the overall linear disturbance growth is monitored and measured by the so-called N-factor. Transition is assumed to take place at the downstream position where a previously calibrated critical N-factor value is reached for the first time.
On the 3D shape of blended wing bodies the ISW assumption becomes invalid, due to the fully 3D nature of the boundary-layer flow field. Therefore, the standard PSE approach is no longer appropriate and should be replaced by more advanced PSE-3D concepts which also take spanwise gradients into account. Adjoint parabolized stability equations can be derived which then have to be solved by a marching procedure with reversed marching direction. The adjoint PSE provide information about the sensitivity of the boundary-layer flow and have been used successfully to model certain linear receptivity mechanisms.
A tool chain based on direct and adjoint Navier-Stokes solvers coupled with direct and adjoint PSE-3D solvers would allow an iterative gradient-based shape and suction optimization and thus provide the required design capabilities for hybrid laminar flow control on BWB.