4 4 £ &l. A & HYDROMECHANICS NEW RESEARCH RESOURCES ATTHE DAVID TAYLOR MODEL BASIN o by AERODYNAMICS Captain E.A. Wright, USN o STRUCTURAL MECHAN ICS o RESEARCH AND DEVELOPMENT REPORT APPLIED MATHEMATICS January 1959 Report 1292 NEW RESEARCH RESOURCES AT THE DAVID TAYLOR MODEL BASIN by Captain E.A. Wright, USN Reprint of paper presented at Spring Meeting of The Society of Naval Architects and Marine Engineers Old Point Comfort, Virginia, June 2-3 1958 January 1959 Report 1292 New Research Resources at the David Taylor Model Basin By Capt. E. A. Wright, USN,'Member This paper describes briefly many of the new laboratory facilities and instruments in the field of ship model research.A planar-motion mechanism now provides hydrodynamic coefficients for the differential equations of motion, a heaving tow- point simulates ship pitching for bodies towed over the stern, a boundary-layer research tunnel reveals the effects of pressure gradients, differential transformers permit miniaturized transducers and remote digital recording, a pneumatic wave- maker generates a programmed frequency spectrum, a large transonic tunnel provides high Reynolds numbers in air, a submarine test tank extends the scope of structural research, a flutter dynamometer explores the phenomenon on control surfaces in water, a large variable-pressure water tunnel provides for testing con- tra-rotating propellers, and seakeeping and rotating-arm basins add new dimen- sions to research in naval architecture at the David Taylor Model Basin.The gamut in size runs from a 6-knot towing carriage for a 57-ft model basin to a 60-knot towing carriage for a 2968-ft basin, and from a transient-thrust dynamometer that serves as the strut barrel of a ship model to a 40,000-lb vibration generator that excites full-scale ship structures.Developments like these suggest to the author several trends in ship research. Nw frontiers of research in naval architecturethat higher speeds and higher performance control are often inaccessible without inspired develop-surfaces are already touching the fringes of this ments in laboratory instruments and facilities.phenomenon, for long well known and explored in Because technical knowledge is exploding radially,aeronautics. laboratories are in contact with these frontiers on The considerable body of research in air is not a rapidly increasing perimeter.The purposes offully applicable to naval design because of differ- of this paper are (a) to describe briefly recent andences in the Strouhal number, Mach number, forthcoming resources for research at the Davidviscosity effects, cavitation, virtual mass, and Taylor Model Basin, and (b) to indicate therebystructuraldamping.Hence a comprehensive some of the current pressure points on the un-study is being made theoretically and experi- known. mentally [1].2 Apparatus has been developed for exploratory Rudder Flutter rudder flutter investigations under the 60-knot Except for singing propellers, hydrodynam- towing carriage.A spade rudder supported by a ically-excited flutter has fortunately been rare instiff shaft, Fig. 1, is free to rotateina cage, in naval architecture.Indicationsare,however,which torsion springs determine the natural fre- quency in rotation.The cage is supported by I Commanding Officer and Director, David Taylor Model Basin, Washington, D. C. horizontal flexures from a frame bolted directly For presentation at the Spring Meeting, Old Point Com- fort, Va., June 2-3, 1958, of THE SOCIETY OF NAVAL I Numbers in brackets designate the References at the ARCHITECTS AND MARINE ENGINEERS. end of the paper. Positioning is accomplished through a range of ± 45 deg at preselected rates from 8 to 16 deg per sec.The rudder may be positioned either as a direct continuous control by the operator, by steps in increments of 5 deg, or by homing to a preselected fixed position. Lift, drag, and torque on each rudder are measured through strain-gaged flexures in the rud- der stock, constituting a 3-component balance. Present capacities are 60 lb lift, 10 lb drag, and 40 in-lb torque. This equipment has direct wire connections, via a fish-pole to avoid forces on the model, to a con- trol console on the towing carriage.Signals for rudder position and forces, as well as heading, heel, and trim from gyros mounted in the model, are brought up to strip-chart recorders on the carriage. Radio-Controlled Models In the new 240-ft by 360-ft maneuvering basin, it is planned to self-propel battery-operated ship models up to 30 ft or more in length.A system is being provided for complete remote control of shaft speeds and rudder angles.This control will be of the continuous-function type rather than in steps, and will be exercised from a console ashore, Fig. 3. Fig. IControl-surface flutter apparatus.3 Command signals from the console are trans- mitted to the model as subcarriers modulating a very high-frequency(vhf)radio carrier.Cir- cuitry in the model demodulates the vhf carrier, to the structure of the towing carriage.Notseparates the various command signals, and dis- shown in the figure is a large surface plate whichtributes them to the proper propulsion and rud- simulates the ship hull over the rudder. der channels.Simultaneously,atelemetering As an essential condition for flutter is that thetransmitter in the model is sending back data energy dissipated by the system must be equal towhich are displayed at the console as dial readings. or less than the energy extracted from the flow,Thus the operator has continuous direct indica- the former has been kept low in the design of thistions before him of the actual values of the quan- apparatus.The mass balance of the rotatabletities he is controlling by the radio link, and the system can be shifted relative to the rudder axis.feel of the over-all system is the same as though he A controllable eddy-current damper is providedwere controlling through direct wire connections. for both translational and rotational motion. The propulsion system in the model will con- Metalectric strain gages transmit amplitude sig-sist of 1 to 4 series d-c propulsion motors, powered nals to recorders on the carriage.This equipmentby I or 2 variable-voltage d-c generators in motor- was designed by Reed Research, Incorporatedgenerator sets, the m-g sets in turn being driven [2], to DTMB basic specifications and built in theby high-capacity nickel-cadmium batteries.Such Model Basin shops. a system, though seemingly elaborate, is necessary to obtain the required fineness of speed control Rudder-Measuring System and constancy of speed during a run, both to A twin-rudder positioning control and force-better than i per cent. measuring system is now in use for maneuvering The rudder control and force-measuring dy- tests of surface ship models, Fig. 2. namometer will be used in the model.Addi- tional radio control and data-handling channels from model to shore are provided for tailoring to All photographs are Official, U.S. Navy, unless other- wise credited. the particular experiment. 2 Fig. 2Rudder dynamometer for positioning and force measurements Free-Running Submarine Models tion will be 12 ft diam, more than 30 ft long, and Turning and maneuvering tests employing free-capable of applying pressures of over 1500 psi [3]. running dynamically-scaled models have becomeThe new facility will be the largest tank in the an essential part of the procedure for predictingUnited States for liquid pressure tests of this the handling qualities of new submarines duringmagnitude. the early design stages.This is especially true Submarine structural models weighing up to for submerged maneuvers in the horizontal plane25 tons are secured vertically in the tank to avoid where, because of coupled motions and other corn-gravity bending forces, and hydrostatic pressure plex effects, it may be difficult or cumbersome tois applied to the exterior of the model, Fig. 5. evaluate performance by analytical methods. Actually, oil is used as the pressure medium to The model isequipped for remote controleliminate the need for waterproofing the 250 to through drop cords extending from the carriage300 wire resistance strain gages used in each test. boom down through a tube into the model, Fig. 4.The deflections of the model as the pressure is The cable tube also carries lights which are pho- increased are scanned by a probe, amplified me- tographed to record the model path.The modelchanically, and plotted automatically on a turn- is run in a flooded condition and carries a pro-table.The deflector neter is removed prior to pulsion motor, rudder-actuator mechanism, stern-collapse of the cylinder. plane control mechanism, vertical gyro, horizontal The 12-ft pressure tank was fabricated by the and rate gyros, and ballast and trim tanks.AllNorfolk Naval Shipyard of steel with a yield recording is done on the towing carriage. strength of 100,000 psi.The main tank body and The helmsman and stern-plane operators findlower ellipsoidal head weigh 56 tons and there- they must be highly alert and practiced, becausemovable head 16 tons.The site for the Sub- the time scale is compressed by the square root ofmarine Structures Facilityis adjacent to the the linear ratio of ship to model.Good correla-Underwater Explosions Test Pond and Air Blast tions have been obtained between predictions byPits for dynamic investigations of hull structures, this technique and full-scale behavior. Fig. 6. 12-Ft Submarine Pressure Tank 40,000-Pound Vibration Generator The increase in size, structural complexity, and Through yearsof development and appli-