Passive and Active Control of Boundary Layer Transition
<p>It is well known that laminar-turbulent boundary layer transition is initiated by the formation of Tollmien-Schlichting laminar instability waves. The amplification rates of these waves are strongly dependent on the shape of the boundary layer velocity profile. Consequently, the transition...
| Summary: | <p>It is well known that laminar-turbulent boundary layer transition is initiated by the formation of Tollmien-Schlichting laminar instability waves. The amplification rates of these waves are strongly dependent on the shape of the boundary layer velocity profile. Consequently, the transition process can be controlled by modifying the velocity profile. This can be accomplished by controlling the pressure gradient (dp/dx), using boundary layer suction, installing surface roughness elements, or by surface heating or cooling. Methods used to modify the transition process through changes in the mean velocity profile are called "passive" in this paper. There exists a large set of experiments and theory on the application of passive methods for boundary layer control. In the present work only surface heating will be addressed.</p>
<p>Transition measurements were made on a heated flat plate in water. Results are presented for several plate wall temperature distributions. An increase by a factor of 2.5 in transition Reynolds number was observed for a 5°C isothermal wall overheat. Buoyancy effects on transition were minimal due to the small Richardson and Grashof numbers encountered in the experiments.</p>
<p>The amplification of laminar instability waves is comparatively to process, taking place over many boundary layer thicknesses. After the slow amplification of the laminar instability waves, transition occurs by a strong three dimensional dynamic instability. It appears possible to attenuate (or reinforce) the instability waves by introducing amplitude-and phase-controlled perturbations into the laminar boundary layer using feedback control system. This method is called "active" control and forms the larger part of the research reported in this thesis.</p>
<p>A combination of sensors, activators and feedback control electronics is required for active control. The sensors used in the experiments are flush-mounted hot film wall shear robes. A new type of activator was developed using thin, flush-mounted surface heating elements to excite instability waves in the laminar boundary layer by periodic (active) heating.</p>
<p>Experimental evidence is presented illustrating the effects of periodically heated flush mounted strips in perturbing a flat plate boundary layer in water. The results of superposition of forced laminar instability waves are also given. Finally, an active feedback-control system using a single hot film probe and strip heater was developed to control natural laminar instability waves in real time. It is shown that when the natural waves were attenuated, the transition length was increased by 25%, requiring only 10 watts of strip heater power. To accomplish the same transition delay using passive heating, the internal heating pads had to supply 1900 watts of power.</p> |
|---|