This paper describes measurements of noise from two-phase flow over hydrofoils. The experiments were performed in a variable-pressure water tunnel which was acoustically calibrated so that sound power levels could be deduced from the sound measurements. It is partially reverberant in the frequency range of interest.Cavitation was generated on a hydrofoil in the presence of either a separated laminar boundary layer or a fully turbulent attached boundary layer. The turbulent boundary layer was formed downstream of a trip which was positioned near the leading edge. High-speed photographs show the patterns of cavitation which were obtained in each case. The noise is shown to depend on the type of cavitation produced; and for each type, the dependence on speed and cavitation index has been determined. Dimensionless spectral densities of the sound are shown for each type of flow.
Measurements of the spectral density of the wall-pressure fluctuations at low wave number on a rough wall are presented. The data were obtained in air with a linear array of six flush-mounted microphones whose outputs were combined in a manner that provides a direct measure of the spectral density as a function of wave number and frequency. Measurements were made at a number of flow speeds ranging from 30 ft/sec (9.1 m/sec) to 160 ft/sec (48.8 m/sec) and for frequencies up to 20 kHz. Three roughness conditions were employed, two of which were hydraulically rough. Roughness Reynold's numbers based on roughness height, friction velocity, and kinematic viscosity ranged from 20 to 1200. Comparisons are made with smooth-wall data and show that the roughness markedly affected the convective and acoustic components of the pressure field.
The response of a flush-mounted transducer to the pressure field in a turbulent boundary layer is known to depend on the spatial and temporal characteristics of the transducer. This paper presents an experimental study of this dependence. The reduced data are presented in a manner similar to that used by Corcos to present his estimation of the response of transducers to a corresponding pressure field.in the reduced data that occurs as different assumptions and idealizations are violated in the experiments; this attempt awaits further research. In Sec. II, the measurement techniques are briefly considered, and in Sec. III the instruments and the transducers used in the experiment are described. Section IV is devoted to the discussion of the results obtained in the experiment. Additional remarks and conclusions are presented in Sec. V. I. DATA-REDUCTION PROCEDUREThe data obtained in the present experiment are presented in a manner similar to that used by Corcos 2 to present his semiempirical estimation of the response of flush-mounted transducers to the pressure field in a turbulent boundary layer. Since the method employed to reduce the data to this form is contingent on a number of assumptions and idealizations, it is pertinent to consider the salient features of the method in order to make the presentation of the results meaningful.If the turbulent pressure field is assumed to be statistically stationary and homogeneous, an expression describing the frequency spectral response of a flush-_.mounted transducer can be derived in the form 2,a (I)m(fM'l)) = • G.to the same class of flush-mounted transducers as defined previously. It is assumed that the pressure fields to which the pair of transducers is subjected are nominally identical so that the frequency spectral responses of the two transducers can be expressed in the form ß and ß respectively. The subscript a designates quantities associated with the transducer whose typical linear spatial dimension is L• and the subscript • designates quantities associated with the transducer whose typical linear spatial dimension is L•. By choosing a sequence of center frequencies such that •<•< ß ß ß (•< ß ß ß <•, it can be readily derived from Eqs. 17 and 18 that
This paper examines the wavevector-frequency spectrum of the turbulent boundary layer wall pressure in the incompressive, inviscid domain in the intermediate and high frequencies range, i.e, wS'/l]^ > > 0.5. It is shown that the wavevectorfrequency spectrum can be normalized by a factor so that it becomes simply a function of nondimensional Strouhal wavenumber U c k,/ij) and U c k 3 /w, where U c is the convective flow velocity, and ki and k 3 are the wavenumbers in the plane of the wall along the streamwise and the crossflow directions, respectively. The normalization factor is the point pressure frequency spectrum times (U c /w) 2 . It follows that the normalized wavevector-frequency spectrum can be scaled with respect to the Strouhal wavenumbers U c k]/w and U c k 3 /o>. The rationale of using a linear regression model for estimating the normalized wavevector-frequency spectrum with a set of measured response data from a wavevector filter is presented. The contention is that the actual spectrum can be obtained by the multiplication of a trial spectrum with a correction spectrum. The correction spectrum is approximated by a polynomial in U c k,/w with a set of coefficients to be determined. The multiple linear regression model relates the response of a measuring system to these coefficients which are determined by least square minimization of a set of measured response data. The advantages of the regression approach are that it relaxes the requirements of the wavevector filter's ability to discriminate against the spectral elements outside the wavenumber bandwidth of the filter, and this approach is capable of better estimating the entire wavevector spectrum as compared to the existing methods which are limited to measurements of the low-wavenumber spectra. Some preliminary numerical results are presented.
The measured values of the power spectral density are known to decrease with increasing transducer diameter. Measurements of this effect have been made on a wall adjacent to a turbulent boundary layer for a range of transducer diameters. Data were taken in a 3-Hz bandwidth for a Reynolds-number range, based on boundary-layer displacement thickness, from 2100 to 9300. The ratio of transducer radius to boundary-layer displacement thickness ranged from 0.09 to 4.3. Strouhal numbers based on angular frequency, displacement thickness, and wind-tunnel speed ranged from 0.07 to 12. The data are presented in dimensionless form and are compared with the theoretical correction curve of Corcos. General agreement is found.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.