Patent GB-881570-A (GB)
Title: Device for stabilizing laminar boundary layer flow
IPC: B63B1/34
Filed: 1959-07-28
Abstract: 881,570. Boundary layer controls. CLAYTON, T. A. (Coleman-Kramer Inc.). July 28, 1959, No. 25901/59. Class 4. [Also in Group XXXII] Means for maintaining a laminar boundary layer at the surface of a body moving relatively to a fluid, e.g. an aeroplane, ship, submarine, propeller, torpedo, or duct, comprises a resilient coating attached to the surface of the body, the coating having an outer flexible membrane exposed to the fluid, means supporting the membrane from the surface of the body for flexural response to local pressure variations in the fluid, and, between the membrane and outer surface of the body, substantially uniformly distributed means for damping displacements of the membrane. According to the Specification, the transition from laminar to turbulent flow is associated with instability in the boundary layer in the form of self-sustaining oscillations of frequency and wavelength depending on the velocity of flow and the kinematic viscosity of the fluid. If the characteristics of the resilient coating, especially the damping means, are suitably chosen, the coating will absorb energy from the oscillations sufficiently to prevent the formation of a turbulent boundary layer, and thus decrease the drag. In Fig. 13, a base layer 90 of rubber or elastomeric material supports an intermediate layer 95 of rubber partitions 91 each formed with a longitudinal right-angled bend, to form similarly shaped spaces 94. These spaces are interrupted by partitions 92, spaced apart by not less than a half wavelength of the incipient boundary layer oscillations. The outer membrane 93 should not be thicker than the average thickness of the boundary layer, and may be selected to provide, by mechanical hysteresis, damping of oscillations in the fluid caused by imperfections in the surface finish, the frequency of these oscillations being substantially higher than those of incipient boundary layer instability. The spaces 94 are filled with fluid of a suitable viscosity, and flow of this fluid within each space, in response to deflections of membrane 93 by local oscillations in the ambient fluid, provides the damping required to prevent formation of a turbulent boundary layer. The partitions 92 prevent flow of the damping fluid in response to an overall pressure gradient along the body. The three layers 90, 95, 93 may be integrally moulded together. The layers 90, 93 may be held spaced apart by uniformly distributed projections integral with either or both layers instead of partitions 91, the space between the layers being filled with damping fluid, see Figs. 2 and 3 (not shown) or by flat walls dividing the space between the layers into cells elongated in the direction of ambient flow and filled with damping fluid, see Figs. 4 and 5 (not shown). Mathematical analysis in the Specification enables the incipient boundary layer oscillation characteristics to be determined, and indicates how the characteristics of the membrane 93 and of the damping fluid are to be chosen. Preferably, the resilient coating is capable of damping oscillations over a broad rather than a narrow band of frequencies. The coating need not extend to the leading edge or bow of the body. Fig. 12 is a partly broken away plan view, and Fig. 11 a cross-section of a further embodiment. An outer membrane 12 is spaced from a metal plate 21 having bores 22, by a porous foam rubber layer 20. Plate 21 is supported from the body 10 by ribs 18. Flexible walls 23, 24, all inclined to the direction of flow, divide the spaces between the ribs into cells, the walls preventing passage of damping fluid between the cells, but transmitting pressure variations from cell to cell. Damping of displacements of the membrane 12 is provided by passage of the damping fluid through bores 22. The foam rubber layer 20 may be replaced by a plurality of parallel wires inclined to the direction of flow, see Fig. 10 (not shown). In a further modification, Figs. 6-8 (not shown), the membrane 12 is spaced from the metal plate by ribs formed on the membrane, and the ribs 18 are replaced by conical projections formed on the metal plate, The membrane 12 may be under tension. The damping fluid may be the surrounding medium, the outer layer being slightly porous. The range of boundary layer oscillation frequencies involved may coincide with the audible or ultrasonic sound range, in which case the resilient coating also serves to prevent the generation of acoustic noise by turbulence, and inhibits the reflection of sounds from external sources, thus preventing detection of the body by acoustic techniques. As described above, in connection with Fig. 13, the outer layer by itself may damp oscillations at a higher frequency than that of boundary layer oscillations, and a reduction in generated or reflected sound may be achieved in this way.