Experiments on the stability of the flat-plate boundary layer with suction

The effects of boundary-layer suction in stabilizing the laminar boundary layer were considered in this experimental work. The experiments were conducted on a flat-plate wind-tunnel model in the V.P.I. & S.U. Stability Tunnel. The capabilities of the wind tunnel in terms of mean flow uniformity and low turbulence level were established and the basic flat-plate experiment was verified by comparison with the well known stability results for the Blasius boundary-layer. The two-dimensional Tollmein-Schlichting waves were introduced into the boundary layer using a vibrating ribbon.

Suction was applied through the model surface vra two porous panels which could be located at various streamwise positions on the model. The porous Dynapore surface material of these panels was adapted by Douglas Aircraft Co. for possible use as a wing surface material on aircraft equipped with Laminar Flow Control. Detailed mean-flow measurements without suction showed that the Dynapore had no destabilizing effects in the laminar boundary layer.

Using the porous panels, suction could be applied either continuously or in a discrete fashion through spanwise suction strips. With suction applied through a single spanwise strip, the mean-flow effects were determined in terms of the boundary-layer shape factor. Measurements of the disturbance behavior with suction were conducted to determine the relative merits of spatially continuous suction versus suction applied through various discrete suction strip configurations. These measurements were conducted in terms of the integral across the boundary layer of the streamwise disturbance amplitude, that is A = ∫ u' I dy. These results were compared with the theory of Reed & Nayfeh (1981). A method was also proposed by this theory for optimization of suction strip placement, and measurements were made which provided partial confirmation as to the validity of this optimization scheme.

Experiments were also conducted without suction to investigate the weak nonlinear two- and three-wave interactions which occur at higher wave amplitudes. In these experiments, two-frequency Tollmein Schlichting waves were introduced simultaneously using a single vibrating ribbon, and initial disturbance amplitudes were controlled. In particular, these experiments established appropriate initial conditions for comparison with analytical models and considered the role of the difference frequencies which were generated through the nonlinear interaction of the two fundamental waves. This work also considered the behavior of the harmonic waves in the nonlinear regime.