Jerry N. Hefner

Viscous drag reduction in boundary layers

The present volume discusses the development status of stability theory for laminar flow control design, applied aspects of laminar-flow technology, transition delays using compliant walls, the application of CFD to skin friction drag-reduction, active-wave control of boundary-layer transitions, and such passive turbulent-drag reduction methods as outer-layer manipulators and complex-curvature concepts. Also treated are such active turbulent drag-reduction technique applications as those pertinent to MHD flow drag reduction, as well as drag reduction in liquid boundary layers by gas injection, drag reduction by means of polymers and surfactants, drag reduction by particle addition, viscous drag reduction via surface mass injection, and interactive wall-turbulence control.

Effect of compliant wall motion on turbulent boundary layers

A critical analysis of available compliant wall data which indicated drag reduction under turbulent boundary layers is presented. Detailed structural dynamic calculations suggest that the surfaces responded in a resonant, rather than a compliant, manner. Alternate explanations are given for drag reductions observed in two classes of experiments: (1) flexible pipe flows and (2) water−backed membranes in air. Analysis indicates that the wall motion for the remaining data is typified by short wavelengths in agreement with the requirements of a possible compliant wall drag reduction mechanism recently suggested by Langley.

Laminar flow control - Introduction and overview

Research in the area of laminar flow control (LFC) dates back to the 1930’s when early applications of stability theory led to the observation that laminar boundary layers can be stabilized by either favorable pressure gradients or small amounts of wall suction. (An excellent summary of this work is presented in References 1 and 2.) Research was performed in many countries to explore approaches for achieving extensive laminar flow with these concepts. Stabilization of boundary-layer disturbances and instabilities by pressure gradient and shaping became known as natural laminar flow (NLF), and NACA research led to the development of the six-series NLF airfoil. International research on stabilization by suction, referred to as LFC with suction, was intensive at the same time and culminated in the United States in the 1960’s with flight tests of a relatively unswept suction glove on an F-94 aircraft (Reference 3) and X-21 flight tests (References 4–7) of a totally new swept LFC wing on a reconfigured WB-66 aircraft.

Current and Future Needs in Turbulence Modeling

The environment for conducting definitive turbulence modeling research has changed drastically over the past several years. With downsizing and reduced budgets in both industry and government, there is obviously reduced funding available for turbulence modeling research even though better turbulence modeling is still critical to computational fluid dynamics becoming more efficient, accurate, and useful. Within NASA, the funding reductions are compounded by the transition processes resulting from a restructuring and reorganization of the research and technology base program. The NASA RT therefore, fundamental research to develop the needed turbulence modeling will have to be advocated and conducted in a manner to explicitly support these goals.