885096009

THURSDAY MORNING, 2 MAY 1991

LIBERTY A, 8:45 TO 11:50 A.M.

Session 6EA

Engineering Acoustics: Measurements and Instrumentation

Allan J. Zuckerwar, Cochair

NASA Langley Research Center, Mail Stop 238, Hampton, Virginia 23665

Kennth D. Rolt, Cochair

Ocean Engineering Department, Massachusetts lnstitute of Technology, Cambridge, Massachusetts 02139

Chair’s Introduction—8:45

Contributed Papers

8:50

6EA1. Improvements to thc resonance apparatus. Gilbert F. Lee, Brian A. Miller (Naval Surface Warfarc Ctr., Silver Spring, MD 20903), and David H. Hanson (Marc Island Naval Shipyard, Vallejo, CA 94592)

The resonance apparatus has been used to determine the complex Young’s modulus of polymeric matcrials. A bar of materiał is cxcited into resonance. One end of the test bar is adhesively bonded to a mc* chanical shaker via a mounting błock, while thc other end of the test bar is bonded to an acceleromełer. A second accelerometer is bonded to the shaker mounting błock. At resonance, the complex Young’s modulus is determined at four to six discrete frequencies over 1.5 decades of fre-quency. There are two disadvantages to the above technique. The first disadvantage deals with accelerometer leads. Accelerometer leads are fragile and are easily broken off. Also the accelerometer lead on the free end of the test bar is an unknown added end mass. In addition, this lead may also be an unknown source of absorption. The second disadvantage is that the complex modulus is only determined at resonance. The goal of this presentation is to improvc the resonance apparatus by replacing the accelerometers with a noncontacting magnetie pickup and to determine the complc.s modulus off resonance, which extends the usable frequency rangę to about 3 decades.

9:20

6EA3. Estimation of boundary layer transition noise from vclocity measurements. Michacl H. Kranc and Wayne R. Pauley (Dept. of Aerospace Eng., Penn State Univ., 233 Hammond Bldg., University Park, PA 16802)

VcIocity measurements of artificially generated flow structures in the transition region of an incompressible boundary layer with zero pressure gradient are described. These measurements madę in a laminar flow water channel allow calculation of the velocity norma! to the wali in a turbuient spot. This velociiy specifies the lincarized boundary con-dition for the acoustic equation at the wali. The approach relates the radiated noise to fluctuations in the normał vclocity at the piąte through fluctuations in the displacement thickness. Allhough this approach has been previously proposed (H. W. Liepmann, unpublished (1954), J. Laufer, J. E. Ffowcs-Williams, and S. Childress, AGARDograph 90, 39-42 (1964), G. C. Lauchle, J. Acoust. Soc. Am. 69, 665-671 (1981), G. C. Lauchle, ASME NCA 5, 31-38 (1989)1 it has never been applied. The results of these efcperiments will be compared to concurrent expcr-iments run in an anechoic wind tunnel. Ultimately this work will be extended to naiurally occurring structures in thc transition region. [Work supported by ONR under Grant #N00014-90-J-1365.J

9:05

6EA2. Noninvasive measurement of dynamie pressure inside pipes using piezoelectric strip. Gerard P. Carroll (David Taylor Res. Ctr., Bethesda, MD 20084-5000)

This paper presents the use of piezoelectric materiał wrapped around the outside diameter of a pipę wali for measuring the dynamie pressure inside the pipę. Recently available piezoelectric materials such as polyrinylidine fluoride (PVDF) arc flexible enough to wrap around a pipę wali and provide an excellent transducer for measuring the breathing response of the pipę wali to the internal dynamie pressure. The strain induced in the piezoelectric element associatcd with radial deformation of the pipę wali is measured. It is shown that the sensor, sińce it sums the response around the entire circumfercnce, will discrim-inate against strains associatcd with bending of the pipę for all circum-ferential orders which occur below the ring frequency. The relationship between the measured piezoelectric element voltage and pressure, taking into account the pipę wali characteristics, is presented. Various config* urations of the device are discussed and experimental results comparing the sensor output to that of a flush mounted hydrophone are presented.

9:35

6EA4. Radiation control of sound due to evolving instability waves along a curved surface. L. Maestrello (NASA Langley Res. Ctr., M.S. 463, Hampton, VA 23665-5225) and N. El-Hady (Analytical Services and Materials, Inc., Hampton, VA 23665-5225)

Active control of the acoustic pressure in the far field resuiting from Ihe growth and decay of a wave packet com ecting in a boundary layer over a concave-convex surface is investigated numerically using direct computations of the Navier-Stokes equations. The resuiting sound radiation is computed using the lincarized Euler cquations with the pressure from the Navier-Stokes solution as a time-dependent boundary condition. The acoustic far field cxhibits directivity type of behavior that points upstream to the flow direction. A fixed control algorithm is used where the attenuation signai is synthesized by a filter which activeły adapts itself to the amplitudc-time response of the outgoing acoustic wave. This concept of active control may bc applicable to wind tunnel contraction where noise radiated from the concave-convex surface is known to exist.

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J. Acoust. Soc. Am., Vol. 89. No. 4. Pt. 2. April 1991

121st Meeting: Acoustical Society of America

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