Compliant coatings: The simpler alternative

Boundary layer manipulation via reactive control strategies is now in vogue, and this topic dominated the meeting in which the present paper was delivered. Reactive flow control requires sensors, actuators and control algorithms. The payoffs are handsome but the difficulties involved are daunting. There are however much simpler alternatives to this kind of sophisticated flow alteration devices and the present article discusses one such alternative: passive compliant walls. The paper reviews the important developments in the field of compliant coatings that took place in the past 10 years. During this period progress in theoretical and computational methods somewhat outpaced that in experimental efforts. There is no doubt that complaint coatings can be rationally designed to delay transition and to suppress noise on marine vehicles as well as other practical hydrodynamic devices. Transition Reynolds numbers that exceed by an order of magnitude those on rigid-surface boundary layers can be readily achieved. Recent theoretical work indicates that transition to turbulence could be delayed indefinitely, at least in principle, provided that optimized multiple-panel compliant walls are used. There is renewed experimental evidence of favorable interactions of compliant coatings even for air flows and even for turbulent boundary layers, but more research is needed to confirm these latest results.