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May 1988

Volume 83, Issue S1, pp. S1-S122

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back to top Session R. Engineering Acoustics III and Noise IV: Measurement of Pressure Fluctuations in Turbulent Boundary Layers
Invited Papers
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Acoustic sources in the turbulent boundary layer (A)

J. C. Hardin

J. Acoust. Soc. Am. Volume 83, Issue S1, pp. S40-S41 (1988); (2 pages)

Online Publication Date: 13 Aug 2005

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This paper will be concerned with sources of sound in the simplest case of a turbulent boundary layer over a flat plate. For the majority of the paper, the driving flow will be considered to be subsonic such that gross compressibility effects, such as shocks, do not occur. Standard terminology, governing equations, and characteristics of the turbulence in the boundary layer and the noise it produces will be described. The paper will then examine the fundamental instability of this flow at high Reynolds number and the transition of a laminar boundary layer to turbulence. Generation of Tollmien—Schlichting waves and the myriad large scale structures (hairpin vortices, streaks, spots, bursts, etc.) that have been observed in boundary layers will also be described. Finally, the theoretical understanding of noise production by this flow, which was initiated by Lighthill, Curle, and Powell and carried on by later research workers, will be described. In particular, the use of different Green's functions, which allows the noise to be determined either through integration of various flow quantities over the volume of the boundary‐layer flow or by integration of the pressure over the flat plate, will be developed and the advantages of each noted. In light of this understanding, mechanisms for boundary‐layer noise production and methods for their calculation are discussed. Additiolaal sources in high‐speed boundary layers with shocks present will also be described.
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A semiempirical model for the wave vector‐frequency spectrum of turbulent wall pressure on a smooth planar boundary (A)

D. M. Chase

J. Acoust. Soc. Am. Volume 83, Issue S1, pp. S41-S41 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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The analytical framework and construction of a model of turbulent boundary‐layer pressure on a planar wall by its relation to velocity‐product fluctuations regarded as its sources are reviewed. Expansion for low (subconvective) wavenumbers and low Mach numbers provides a basis for such a trial model of the wall‐pressure spectrum that is potentially satisfactory at all wavenumbers from the acoustic domain to the convective. The proposed form for source cross spectra incorporates the principle of wall similarity, a kinematic assumption about space‐time correlation, possible wall‐normal profiles of vorticity cross spectra, and assumed non‐negativity. The wall‐pressure model is obtained by analytical integration over the source‐spectral profiles in limiting domains followed by convenient rough interpolation. The experimental basis for determination of parameter values of the model is considered by reference to classical wind‐tunnel measurements dominated by the convective wavenumber domain and to more ambiguous ones directed to the subconvective but superacoustic domain. The inadequate state of validation and determination of parameters characterizing the acoustic domain and the domain of pertinence of the Kraichnan‐Phillips theorem is confronted. Likewise, the open but experimentally resolvable question of the joint dependence on wave vector components in the convective domain is recalled.
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The influence of surface roughness on wall‐pressure fluctuations and the radiated sound from a turbulent boundary layer (A)

M. S. Howe

J. Acoust. Soc. Am. Volume 83, Issue S1, pp. S41-S41 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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The noise and vibration produced during turbulent boundary‐layer flow over a nominally plane flexible surface are governed by the low wavenumber and acoustic regions of the wall‐pressure wavenumber‐frequency spectrum. In the case of a rough surface, the spectrum differs from that on a smooth wall on two counts: (i) the strengths of the turbulent Reynold stress fluctuations, which are ultimately responsible for the pressure field, are increased by the action of surface roughness; (ii) the pressure field produced by those enhanced pressure sources is redistributed in the wavenumber plane by diffraction by the roughness elements. The wall‐pressure spectrum can be expressed in the form P(k,ω)  =  P0(k,ω) + PR(k,ω), where P0 denotes the spectrum that would be associated with the roughness‐enhanced Reynolds stresses if the wall were assumed to be perfectly flat, and PR is the additional component due to the diffraction mechanism (ii). It will be shown that PR is expected to dominate the behavior of the wall pressure in the low‐wavenumber and acoustic regions over a wide range of frequencies. Certain problems associated with experimental studies of rough wall boundary‐layer noise will also be discussed.
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Use of piezoelectric foil for measurement of pressure fluctuations and local shear stress in flight (A)

A. Bertelrud

J. Acoust. Soc. Am. Volume 83, Issue S1, pp. S41-S41 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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The measurement of pressure fluctuations in air flows poses several serious problems to the experimentalist. Generally a very limited frequency response is available, as the cavity above ordinary pressure transducers plays a crucial role for the sensor response. This type of transducer requires extensive work for incorporation into a surface. Recently, piezoelectric foil has become available for various applications. It offers ease of application and distributed measurements without strict frequency response limits. However, although the data acquisition is straightforward, it appears that particular caution is required for interpretation of the data as pressure fluctuations. In the present paper, emphasis is put on frequency response, true root‐mean‐square determination, as well as interpretation into skin friction data. Through analysis of flight data and laboratory calibrations, it is shown that the piezoelectric foil in general will respond to pressure fluctuations, and it is also shown that the piezofoil constitutes a repeatable and rugged source of information.
Contributed Papers
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Bubble path in the wake of a cavitating propeller (A)

Mauro Picrucci

J. Acoust. Soc. Am. Volume 83, Issue S1, pp. S42-S42 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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Propellers that operate underwater at high rpm's cavitate at the tip. The tip cavitation creates air bubbles that are then swept downstream by the motion of the surrounding fluid. In this paper, a theory is presented to predict the local velocity and the path of the bubble. The bubble motion is assumed to be governed by a group of terms due to the acceleration of the displaced fluid, the convective term, and the drag due to the cross‐sectional area of the bubble. At very low and very high Reynolds numbers, the equations have been solved in closed form. Results are presented for the bubble velocity and path for the following flow fields: (a) uniform axial flow field and (b) uniform flow field with an axially decaying swirling component. In all cases presented the bubble axial velocity component asymptotes to the free stream velocity; the manner in which it asymptotes is exponential at very low Reynolds numbers and algebraic at high Reynolds numbers. Bubble helical paths and velocity patterns are shown for different bubble sizes.
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Extended frequency response of a fiber‐optic microphone (A)

Allan J. Zuckerwar and Mathew L. Areford

J. Acoust. Soc. Am. Volume 83, Issue S1, pp. S42-S42 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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A condenser microphone responds to the mean displacement of a stretched membrane excited into an axially symmetric mode of vibration. At some frequency between the first and second resonant frequencies, the mean displacement falls to zero and the microphone response drops sharply after the first resonance. A fiber‐optic microphone, on the other hand, responds to the displacement at the center of the membrane. The center displacement of an axially symmetric mode never falls to zero. Consequently, through judicious backplate design, the membrane motion can be damped to achieve a smooth merger in response between the first and second resonances, and the bandwidth of the microphone can be extended to beyond the second resonant frequency. After a brief review of the construction of the condenser and fiber‐optic microphones, the results of a theoretical analysis will be presented for two cases: (1) a B&K ½‐in. condenser microphone compared with its fiber‐optic equivalent, and (2) ⅛‐in. fiber‐optic microphone of special design to measure pressure fluctuations in a turbulent boundary layer.
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A calibrator for the measurement of the static and dynamic shear stresses along a stretched Mylar membrane (A)

Frank W. Cuomo

J. Acoust. Soc. Am. Volume 83, Issue S1, pp. S42-S42 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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Recent power spectral density measurements of the pressure fluctuations in a turbulent boundary layer obtained with a fiber‐optic lever pressure sensor in a low‐speed wind tunnel have compared favorably with data reported in the literature. Utilizing a similar sensor arrangement, incorporating a flush‐mounted, metallized Mylar membrane, the measurement of shear stresses leading to skin friction has been contemplated. The first step toward the realization of a shear sensor has been to establish the behavior of a stretched Mylar membrane acted upon by forces parallel to the flow direction. A calibrator designed to meet the requirements normally found in a wind‐tunnel environment has been constructed and tested. In this paper, its operation is described and data of Mylar samples are presented. The resolution of the device for static measurements is based on translations on the order of 1 μin. while the dynamic response covers a frequency range of dc to 18 kHz.
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Comparison between design and installed acoustic characteristics of the NASA Lewis 9‐ × 15‐ft low‐speed wind‐tunnel acoustic treatment (A)

Milo D. Dahl and Richard P. Woodward

J. Acoust. Soc. Am. Volume 83, Issue S1, pp. S42-S42 (1988); (1 page)

Online Publication Date: 13 Aug 2005

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The test section of the NASA Lewis 9‐ × 15‐ft low‐speed wind tunnel was acoustically treated to allow the measurement of sound under simulated free‐field conditions. The treatment was designed for high sound absorption at frequencies above 250 Hz and to withstand the environmental conditions that exist in the test section. To achieve the design requirements, a fibrous bulk absorbing material was packed into removable panel sections. Each section was divided into two equal‐depth layers that were packed with material to different bulk densities. The lower density was next to the face of the treatment. The facing consisted of a perforated plate and screening material layered together. Sample tests for normal incidence acoustic absorption were conducted in a small rectangular duct. Tests with no air flow involving the measurement of the absorptive properties of the installed treatment combined the use of time delay spectrometry with a previously established free field measurement method. These measurements were compared to the design model and sample tests of the treatment.
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