Aircraft laminar ow control LFC from the 1930's through the 1990's is reviewed and the current status of the technology is assessed. Examples are provided to demonstrate the bene ts of LFC for subsonic and supersonic aircraft. Early studies related to the laminar boundary-layer ow p h ysics, manufacturing tolerances for laminar ow, and insect-contamination avoidance are discussed. LFC concept studies in wind-tunnel and ight experiments are the major focus of the paper. LFC design tools are brie y outlined for completeness.

Direct numerical simulation was used to study the formation and growth of a hairpin vortex in a at-plate boundary layer and its later development i n to a young turbulent spot. Fluid injection through a slit in the wall triggered the initial vortex. The legs of the vortex were stretched into a hairpin shape as it traveled downstream. Multiple hairpin vortex heads developed between the stretched legs. New vortices formed beneath the streamwise-elongated vortex legs. The continued development of additional vortices resulted in the formation of a traveling region of highly disturbed ow with an arrowhead shape similar to that of a turbulent spot.

This paper describes a self-contained, automated methodology for active o w control which couples the time-dependent N a vier-Stokes system with an adjoint N a vier-Stokes system and optimality conditions from which optimal states, i.e., unsteady ow elds and controls e.g., actuators, may be determined. The problem of boundary layer instability suppression through wave cancellation is used as the initial validation case to test the methodology. Here, the objective of control is to match the stress vector along a portion of the boundary to a given vector; instability suppression is achieved by c hoosing the given vector to be that of a steady base ow. Control is e ected through the injection or suction of uid through a single ori ce on the boundary. The results demonstrate that instability suppression can be achieved without any a priori knowledge of the disturbance, which is signi cant because other control techniques have required some knowledge of the ow unsteadiness such a s frequencies, instability t ype, etc. The present methodology has been extended to three dimensions and may potentially be applied to separation control, re-laminarization, and turbulence control applications using one to many sensors and actuators.

Large-eddy simulation results for laminar-to-turbulent transition in a spatially developing boundary layer are presented. The disturbances are ingested into a laminar ow through an unsteady suction-and-blowing strip. The ltered, three-dimensional timedependent N a vier-Stokes equations are integrated numerically using spectral, high-order nite-di erence, and three-stage low-storage Runge-Kutta methods. The bu er-domain technique is used for the out ow boundary condition. The localized dynamic model used to parameterize the subgrid-scale stresses begins to have a signi cant impact at the beginning of the nonlinear transition or intermittency region. The ow structures commonly found in experiments are also observed in the present simulation; the computed linear instability modes and secondary instability lambda-vortex structures are in agreement with the experiments, and the streak-like-structures and turbulent statistics compare with both the experiments and the theory. The physics captured in the present LES are consistent with the experiments and the full Navier-Stokes simulation DNS , at a signi cant fraction of the DNS cost. A comparison of the results obtained with several SGS models shows that the localized model gives accurate results both in a statistical sense and in terms of predicting the dynamics of the energy-carrying eddies, without ad hoc adjustments.

This paper (the first in a series) focuses on using active-control methods to maintain laminar flow in a region of the flow in which the natural instabilities, if left unattended, lead to turbulent flow. The authors review previous studies that examine wave cancellation (currently the most prominent method) and solve the unsteady, nonlinear Navier-Stokes equations to evaluate this method of controlling instabilities. It is definitively shown that instabilities are controlled by the linear summation of waves (i.e., wave cancellation). Although a mathematically complete method for controlling arbitrary instabilities has been developed, the review, duplication, and physical explanation of previous studies are important steps for providing an independent verification of those studies, for establishing a framework for the work which will involve automated transition control, and for detailing the phenomena by-which the automated studies can be used to expand knowledge of flow control.