Browsing by Author "Hurst, Charles J."

Now showing 1 - 16 of 16
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    Acoustic performance of a stretched membrane and porous blanket combination
    Thomas, W. A. Jr.; Hurst, Charles J. (Acoustical Society of America, 1976)
    The sound absorption performance of an acoustic absorber consisting of a stretched circular membrane placed a short distance in front of a fiberglass blanket was both measured and predicted. Both theoretical and experimental analyses were restricted to plane acoustic waves. Theoretical predictions indicated that the membrane-blanket combination would have a sound power absorption coefficient nearly equal to the sound power absorption coefficient of the blanket alone if the incident acoustic plane wave drove the membrane at one of its resonance frequencies. Theoretical analysis also predicted that the sound power absorption coefficient would approach zero when the membrane was driven at an antiresonance frequency by the incident acoustic plane wave. Experimental agreement with theoretical predictions was good for several membrane-blanket combinations. The results show that membrane-blanket combinations can be effective acoustic absorbers in frequency ranges which do not include the antiresonance frequencies of the membrane. The equations developed may be used to predict the acoustic performance of any membrane-blanket combination.
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    Analysis of a dynamic pressure measuring system
    Blevins, Roger Allen (Virginia Tech, 1970-05-05)
    A dynamic pressure measuring system composed of long connecting tube, transducer, D-C amplifier and galvanometer oscillograph were used in a wind study conducted by the Department of Agricultural Engineering, Virginia Polytechnic Institute. The instrumentation was used to measure and record dynamic pressures due to wind velocity. Data recorded in that study indicated velocity head pressure oscillations on some surfaces of the test structure in excess of 100 Hz. This investigation was undertaken to see if the recorded oscillations could have been generated in the measuring system or if they were truly windpressure variations. Components of the pressure measuring system were modeled by transfer functions. From these transfer functions, a system operational transfer function was determined and used to define system frequency response. The frequency analysis indicated the system was severely limited in response by the oscillating air column constrained within the interconnecting tube. The usable frequency range of the system (+ 10% tolerable amplitude error allowable) was found to go from zero to 34.2 Hz. By eliminating the air column this frequency range could have been extended to 99.4 Hz. Phase shifts in these frequency ranges were found to be negligible. Transducers were mounted rigidly to exterior walls of the test structure. The measured fundamental natural frequencies of these walls (transducer mountings) were found to be well within the usable frequency range of the instrumentation; and, therefore, a source of vibration pickup. To improve the measuring system reliability, recommendations were made to eliminate the air column and stiffen the transducer mountings.
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    Computer-aided, interactive design routine for the prediction of sound levels in irregularly shaped factory spaces
    Blanding, James M.; Mitchell, Larry D.; Hurst, Charles J. (Acoustical Society of America, 1977)
    An interactive computer algorithm is presented which utilizes modified room acoustics theory to predict dBA sound pressure levels in regularly- and irregularly-shaped factory spaces. Irregularly positioned and nonuniform room absorptions are treated. The user-oriented algorithm, suited to remote interactive terminal operation, employs a conversational format to facilitate input of room dimensions, of absorption data, and of locations and noise levels of machinery. A redesign feature is included which employs three options. These allow the user to change sound source data and acoustic treatments in the process of finding the most economical accommodation of OSHA regulations during any phase of the factory design or redesign. [Work supported by NSF.]
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    Determination of acoustic ray paths in enclosed spaces
    Mitchell, Bruce Wayne (Virginia Tech, 1975)
    Equations were derived to calculate the reflection points of acoustic rays emanating from a source to a receiver. Reflection points for up to three reflections could be determined if the location of the source, receiver and all reflecting surfaces were known. Assuming a point source, the distances along the calculated ray paths were used along with the power level of the source and the absorptive characteristics of all the reflecting surfaces to determine the sound pressure levels at specified receiver locations. A computer program was developed to perform the necessary calculations for reflection points and sound pressure levels. The output was in the form of ray tracing plots which showed the unique reflection paths for up to three reflections and the sound pressure levels at each receiver location. Comparisons were made between predicted and calculated sound pressure levels in a rectangular parallelopiped shaped room and a long narrow hallway. The results of this investigation showed a promising potential in the area of sound pressure level predictions and the use of ray tracing plots to provide a means of reducing the sound pressure levels. A particularly interesting point of the program was its ability to handle very irregularly shaped rooms which include slanted surfaces.
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    Dynamic finite element modeling and analysis of a hermetic reciprocating compressor
    Kelly, Allan D. (Virginia Tech, 1992-01-30)
    Dynamic finite element modeling and analysis of a refrigeration compressor was investigated as part of a noise emission study. Natural frequencies and normal mode shapes were calculated for the major structural components of the compressor. The components were later combined to form a model of the compressor assembly which was subsequently solved for its dynamic properties. Model development included coordination with test data for verification and revision to improve model prediction accuracy. Considerable efforts were made to accurately represent the hermetic shell which presents several inherent modeling difficulties due to its geometry and other characteristics which result from a deep drawn manufacturing process. The importance of physical simplifications such as geometry representation, thickness variation, attachments, the welded seam, and residual stresses were established. In addition, theoretical limitations of the finite element method were addressed as a cause for analysis-test discrepancies. Housing models developed were found to agree within 12% of experimental natural frequencies up to 1100 Hz. Compatibility of analytical normal modes with resonant dwell experimental deflection shapes was considered. Analytical forced vibration response showed situations when the deflected shapes can be a superposition of modes rather than the pure mode shape. Analytical simulation of the test setup improved the agreement of analysis and test data. Additional components modeled include the internal compressor mechanism and its supports. Analysis showed that interactions with the internal components, particularly resonances within the suspension springs, are important for a valid representation of the compressor assembly. Resonances within the internal suspension components more than double or nearly triple the number of resonance frequencies in the compressor assembly.
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    Frictional oscillations in elastomeric sliding
    Rorrer, Ronald Alvin Lee (Virginia Tech, 1991)
    The impetus for this study was the noise generated by an automotive stabilizer bar, rotating in an elastomeric bushing. The focus of the research is to determine the conditions under which noise or stick-slip occurs. Elastomers tested include natural rubber with 0,5,10, 20, and 50 phr (parts per hundred rubber) carbon black, butyl, polydimethyl siloxane, and fluorocarbon. The hard counterfaces include chromium oxide, aluminum, epoxy paint, and glass. Various combinations of these materials were tested in an elastomeric hemisphere-on-flat configuration. Three regimes of sliding have thus far been identified: 1. Steady state sliding where the sliding friction is constant; 2. A high frequency self-excited oscillation of the test structure superimposed on the mean value of friction; 3. Stick-slip, where the elastomeric specimen goes through alternating periods of no relative and relative motion to the counterface. Friction maps which showed the regimes of stick-slip and steady state sliding as a function of load and velocity were experimentally determined. Stick-slip, the dominant mode of unsteady sliding did not occur below a critical velocity. The effects of surface energy, surface roughness and temperature on the critical velocity for stick-slip were also investigated. Stick-slip motion of materials has been attributed to a difference in static and kinetic coefficients of friction (typically for metallic systems) or a negative slope of the friction velocity curve. It has also been related to a maximum of tan delta or the loss modulus. Test results show that it is possible to have stick-slip occur in regions of positive or zero slope of the friction velocity curve. While the mechanism of stick-slip is not known for all of the elastomeric specimens, stick-slip of the butyl specimens appear to be related to Schallamach waves traversing the interface. For the natural rubber elastomers the critical velocity for stick-slip is dependent on the amount of carbon black. Decreasing amounts of carbon black decreases the critical velocity. The carbon black greatly increases the stiffness of the specimen while not appreciably shifting the frequency of the viscoelastic maxima. In addition, it was shown that a reduction in stiffness of the elastomeric specimen in the direction of sliding, by hollowing it out caused the critical velocity to decrease. Thus the critical velocity for stick-slip could be changed without changing the viscoelastic properties.
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    A geometric acoustics approach to the study of sound propagation in ducts containing sheared flows
    Grimm, Denny W.; Hurst, Charles J. (Acoustical Society of America, 1979-12-01)
    Geometric acoustics has been used to study the propagation of sound waves in a homogeneous moving medium with sheared flow bounded by the hard walls of a duct. Differential equations describing the ray trajectories and the spreading losses along each ray were developed and solved numerically for a range of centerline Mach numbers and shear boundary-layer thicknesses. Results were obtained which show the effects of upstream and downstream sound propagation on the ray paths. A method was also developed to allow the calculation of intensity loss profiles at specified downstream cross sections of the duct.
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    Improvements to the kidney dialysis machine
    Hundley, Robert Wynne (Virginia Tech, 1992-02-05)
    Five changes were suggested to improve the kidney dialysis machine, including a conductivity control feedback system, a positive pressure deaeration system, a balancing system for dialysate flow control, a diaphragm-type blood pump, and an ultrafiltration control system using blood pressure measurement. A kidney dialysis machine was constructed to test the changes for possible use in future dialysis machine designs. Tests were made on each of the five system improvements. All five ideas had some merit for a commercial dialysis machine. A description of the investigation and the various factors used in the development and testing of the five systems is included.
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    A methodology to evaluate photvoltaics: storage as a commercial customer initiated demand side management tool
    Jockell, John F. (Virginia Tech, 1992-09-15)
    While photovoltaic research conducted to date has been primarily in the areas of utility peak shaving, residential applications, and stand alone applications, this research examines photovoltaic potential as a commercial customer initiated demand side management tool and develops a methodology which can be used to evaluate the value of a PV storage system. The primary focus is on the effect photovoltaic orientation and utility billing practices have on overall system value. Using a year's worth of 10 minute data collected at the Virginia Tech Solar Experimental Station and an academic building load, the effects of array orientation on clear sky photovoltaic production and the match between that production and the building load profile are discussed. The performance of five photovoltaic array configurations for both building energy and demand reduction were simulated. It was shown that arrays facing south-southwest and southwest have lower performance that a south facing array in spite of the presence of an afternoon building load peak. The impact of battery storage as a supplement to a PV system has been examined. The affect of battery efficiency, array orientation, and battery dispatch setpoint on ovarall system value has been investigated. Based on economic assumptions and billing schedules obtained from a medium sized utility, the present value of the various array performances were evaluated. It was found that O&M costs could significantly effect the value being assigned to array configurations with superior performance. Due to the variability of the available insolation, smaller arrays have a greater value than larger arrays. It was shown that the value of a PV storage system is greater than either of the two component systems taken alone. By adding battery storage to the PV system, the optimum system size was increased.
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    Prediction of sound pressure levels in irregularly shaped factory spaces
    Zinskie, John H.; Mitchell, Larry D.; Hurst, Charles J. (Acoustical Society of America, 1976)
    The prediction of sound pressure levels in irregularly shaped factory spaces is approached through an analysis of acoustically coupled rooms. Previous work has indicated that room surface sound absorption characteristics create a theoretical interdependence between the sound pressure fields in each coupled volume. Whenever a barrier is interposed between a sound source and observer, the effects of diffraction must also be considered. A computer algorithm was developed to predict sound pressure levels in irregularly shaped rooms using a modified statistical acoustic theory. The program is based on a conversational technique and requires the user to enter information regarding room geometry, surface absorptions, receiver placements, and source sound power and placement information. Several output options are provided, among which is a plan view print-plot of sound pressure level contours in the analysis room. [Work supported by NSF.]
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    Prediction of the sound diffracted around barriers in large rooms using an extension of Maekawa's approach
    Johnson, R. A.; Hurst, Charles J.; Mitchell, Larry D. (Acoustical Society of America, 1977)
    A computer program has been developed for the prediction of sound pressure levels in large, irregular rooms utilizing a geometric acoustics approach. Experiments indicated that the program gave good predictive accuracy when receiver positions were in the line of sight of the sound source. However, the accuracy was poor when the receiver was out of the line of sight. The present discussion reports on the further development of the program to include diffraction effects using an extension of Maekawa's approach for thin screens and right angle wedges. The program is capable of finding all valid diffracted rays which undergo three or fewer reflections before diffraction and three or fewer reflections after diffraction. Limited experimental work indicates good predictive accuracy.
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    The role of system dynamics on the behavior of elastomeric friction
    De Togni, Robert S. (Virginia Tech, 1992-05-15)
    Friction induced vibration of an epoxy coated shaft rotating in an elastomeric bushing is investigated. This study investigates the manner in which system dynamics and friction mechanisms are responsible for friction induced vibration and noise generation. A test method was developed to measure the friction torque and the system and acoustic response of the sliding system. Several materials including a fluorocarbon elastomer, a polydimethylsiloxane, and a natural rubber were tested. Three friction regimes were observed which were stick-slip oscillations, quasi-harmonic oscillations, and steady sliding. System stiffness and load were varied to observe changes in the critical velocities bounding each regime. System parameters were varied to determine sliding conditions leading to self-induced vibration, to establish how the character of vibration is affected, and to correlate friction torque with system and acoustic vibration for each elastomeric material. A two degree-of-freedom, lumped parameter model was developed to simulate the effect of system dynamics on the sliding behavior of the elastomeric bushing. The comparison of simulated and experimental response using analyses in the time and frequency domain indicate the predictive model provides an excellent representation of stick-slip behavior at various operating conditions.
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    Sound transmission between absorbing parallel planes
    Hurst, Charles J.; Mitchell, Larry D. (Acoustical Society of America, 1976)
    Partially absorbing parallel surfaces can be dominant acoustical feature of many rooms. The sound fields in such rooms are not diffuse, which causes difficulty in predicting sound pressure levels. A method is developed for predicted sound pressure levels in these rooms. It rests on the prediction of sound pressure levels caused by a nondirectional source of known sound power radiating between absorbing parallel planes. The development proceeds from a geometrical acoustics viewpoint. Good correlation has been found between predicted and measured levels in existing rooms having simple geometries. [Work supported by NSF.]
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    Sound transmission between absorbing parallel planes
    Hurst, Charles J. (Acoustical Society of America, 1980-01-01)
    Partially absorbing parallel surfaces can be the dominant acoustical feature of many rooms. The sound fields in such rooms are not diffuse, which causes difficulty in predicting sound pressure levels. A method is developed for predicting sound pressure levels in these rooms. It rests on the prediction of sound pressure levels caused by a nondirectional source of known sound power radiating between absorbing parallel planes. The development proceeds from a geometrical acoustics viewpoint. Good correlation has been found between predicted and measured levels in existing rooms having simple geometries.
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    A study of active control techniques for noise reduction in an aircraft fuselage model
    Jones, James D. (Virginia Polytechnic Institute and State University, 1987)
    A simplified cylindrical model is used to investigate the elementary mechanisms of control of sound transmission into aircraft cabins by two active control techniques: propeller synchrophasing and active vibration control. Propeller synchrophasing involves controlling the relative rotational phase of the engines to achieve maximum cabin noise reduction. Active vibration control involves structurally controlling the vibrational response of the cabin wall to reduce the important modes which transmit their energy into the cabin. Noise reductions for harmonic excitation at acoustic cavity resonance are shown to be in excess of 20 dB throughout most of the cavity whether synchrophasing or active vibration control is used. Off-resonance reductions are substantially less due to increased modal density requiring a larger number of actuators for effective control of the complex sound field. Additional studies were performed using synchrophasing in conjunction with active vibration control to study their joint capabilities in controlling complex sound fields. The dual control system displayed improved control performance with noise reductions on the order of 25-35 dB and a more uniform sound field. Also, the complementary control characteristics of the system clearly demonstrated effective control of orthogonal acoustic modes of the cavity. However, the improved effectiveness of the control system was dependent upon judiciously positioning the actuators for optimal control of the sound field. An independent study was performed to identify the effects of a complex geometry on sound transmission into an aircraft fuselage model interior. For this study, a geometrically scaled cabin floor was installed in the unstiffened test cylinder to investigate the structural and acoustic influence of the simulated cabin floor. Results indicated that the stiffening of the cylindrical model associated with insertion of the floor strongly influenced the structural response of the cylinder but generally had little effect on the coupled pressure response. Conversely, the modification of the interior acoustic cavity tended to have little influence on the cylinder response but substantially reduced the coupled pressure response. Thus, this investigation identified the fundamental mechanisms of control of sound transmission into simplified models of aircraft fuselages by active control techniques.
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    The vibration of instrument ball bearings in a controlled environment and the effect of the resulting fretting corrosion on bearing torque
    Hite, Gregory Charles (Virginia Tech, 1970-08-05)
    Fretting corrosion is a form of wear which occurs at the interface of two contacting solid materials as the result of small, relative vibratory motion. It is generally identified by the presence of a red oxide, Fe203. Previously, the majority of the investigators studied the fretting corrosion between two flat specimens or between a single ball and a flat plate held in contact by a normal force. There are a number of interrelated influencing factors involved in fretting corrosion including: the vibration frequency and amplitude, the environmental conditions, the characteristics of the material, and the type of lubrication. The present investigation was conducted in order to investigate the effects of frequency and amplitude of axial vibration and the consequent accelerations acting to produce fretting damage within an unlubricated instrument ball bearing. The effect of bearing axial play on the fretting damage was also examined. The reproducibility of the damage resulting from these variables was determined.