16,970 research outputs found

    Transonic rotor noise: Theoretical and experimental comparisons

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    Two complementary methods of describing the high speed rotor noise problem are discussed. The first method uses the second order transonic potential equation to define and characterize the nature of the aerodynamic and acoustic fields and to explain the appearance of radiating shock waves. The second employs the Ffowcs Williams and Hawkings equation to successfully calculate the acoustic far field. Good agreement between theoretical and experimental waveforms is shown for transonic hover tip Mach numbers from 0.8 to 0.9

    Aeroacoustic research: An Army perspective

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    A short perspective of the Army aeroacoustic research program is presented that emphasizes rotary wing, aerodynamically generated noise. Exciting breakthroughs in experimental techniques and facilities are reviewed which are helping build a detailed understanding of helicopter external noise. Army and joint Army/NASA supported research programs in acoustics which promise to reduce the noise of future helicopters without severe performance penalties are included

    Helicopter impulsive noise: Theoretical and experimental status

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    The theoretical and experimental status of helicopter impulsive noise is reviewed. The two major source mechanisms of helicopter impulsive noise are addressed: high-speed impulsive noise and blade-vortex interaction impulsive noise. A thorough physical explanation of both generating mechanism is presented together with model and full-scale measurements of the phenomena. Current theoretical prediction methods are compared with experimental findings of isolated rotor tests. The noise generating mechanism of high speed impulsive noise are fairly well understood - theory and experiment compare nicely over Mach number ranges typical of today's helicopters. For the case of blade-vortex interaction noise, understanding of noise generating mechanisms and theoretical comparison with experiment are less satisfactory. Several methods for improving theory-experiment are suggested

    Acoustically swept rotor

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    Impulsive noise reduction is provided in a rotor blade by acoustically sweeping the chord line from root to tip so that the acoustic radiation resulting from the summation of potential singularities used to model the flow about the blade tend to cancel for all times at an observation point in the acoustic far field

    Kinetic Vlasov Simulations of collisionless magnetic Reconnection

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    A fully kinetic Vlasov simulation of the Geospace Environment Modeling (GEM) Magnetic Reconnection Challenge is presented. Good agreement is found with previous kinetic simulations using particle in cell (PIC) codes, confirming both the PIC and the Vlasov code. In the latter the complete distribution functions fkf_k (k=i,ek=i,e) are discretised on a numerical grid in phase space. In contrast to PIC simulations, the Vlasov code does not suffer from numerical noise and allows a more detailed investigation of the distribution functions. The role of the different contributions of Ohm's law are compared by calculating each of the terms from the moments of the fkf_k. The important role of the off--diagonal elements of the electron pressure tensor could be confirmed. The inductive electric field at the X--Line is found to be dominated by the non--gyrotropic electron pressure, while the bulk electron inertia is of minor importance. Detailed analysis of the electron distribution function within the diffusion region reveals the kinetic origin of the non--gyrotropic terms

    Hovering impulsive noise: Some measured and calculated results

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    In-plane impulsive noise radiating from a hovering model rotor was measured in an anechoic environment. The hover acoustic signature was compared with existing theoretical prediction models with previous forward flight experiments using the same model rotor. These hover tests showed good experimental consistency with forward flight measurements, both in pressure level, and waveform character, over the range of Mach numbers tested (0.8 to 1.0). Generally poor correlation, however, was confirmed with current linear theory prediction efforts. Failure to predict both the peak pressure levels and the shape was reported, especially with increasing tip Mach number

    A comparison of optimal and noise-abatement trajectories of a tilt-rotor aircraft

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    The potential benefits of flight path control to optimize performance and/or reduce the noise of a tilt-rotor aircraft operating in the takeoff and landing phases of flight are investigated. A theoretical performance-acoustic model is developed and then mathematically flown to yield representative takeoff and landing profiles. Minimum-time and minimum-fuel trajectories are compared to proposed noise-abatement profiles to assess the reductions in annoyance possible through flight path control. Significant reductions are feasible if a nearly vertical-takeoff flight profile is flown near the landing site; however, the time expended and fuel consumed increase

    A Limnological Study of Ricks Pond and the Gulpha Creek Drainage in Garland County, Arkansas

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    A limnological investigation of Ricks Pond and the Gulpha Creek drainage of Garland County, Arkansas was conducted between 1 June 1978, and 21 August 1978. Water samples taken from ten stations on three different dates indicated that the stream and pond systems were typical in water quality characteristics of other small, high gradient streams and impoundments in the Ouachita Mountains of Arkansas. In Ricks Pond, thermal stratification occurred along with the development of an oxygen deficient zone below a depth of one meter. Other water quality parameters indicated that Ricks Pond is a moderately productive ecosystem, with the productivity limited by the nitrogen species. The fecal coliform bacterial counts were very low, indicating no direct input of excessive amounts of fecal matter into the system during the present study. However, a Hot Springs city sewer line runs through the pond, and two manholes emerge from the pond\u27s surface. The possibility exists that this sewer line could discharge raw sewage into Ricks Pond during periods of high water. A biological investigation was also conducted in the study area, and lists of the phytoplankton, periphyton, higher aquatic vegetation, zooplankton, benthic macroinvertebrates, and fishes are presented. Twenty-seven species of fishes were collected from the Gulpha Creek drainage, and no rare or endangered forms were found. Ricks Pond is best-suited for the establishment of a put- and-take fishery for channel catfish, Ictalurus punctatus. The following recommendations were made for the establishment of such a fishery: (1) Renovation of the pond by draining and deepening it; (2) Removal of the sewer line from the pond; (3) Stocking of catchable size channel catfish at the rate of approximately 300-400 pounds per acre; (4) Periodic monitoring of the water quality

    Helicopter model rotor-blade vortex interaction impulsive noise: Scalability and parametric variations

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    Acoustic data taken in the anechoic Deutsch-Niederlaendischer Windkanal (DNW) have documented the blade vortex interaction (BVI) impulsive noise radiated from a 1/7-scale model main rotor of the AH-1 series helicopter. Averaged model scale data were compared with averaged full scale, inflight acoustic data under similar nondimensional test conditions. At low advance ratios (mu = 0.164 to 0.194), the data scale remarkable well in level and waveform shape, and also duplicate the directivity pattern of BVI impulsive noise. At moderate advance ratios (mu = 0.224 to 0.270), the scaling deteriorates, suggesting that the model scale rotor is not adequately simulating the full scale BVI noise; presently, no proved explanation of this discrepancy exists. Carefully performed parametric variations over a complete matrix of testing conditions have shown that all of the four governing nondimensional parameters - tip Mach number at hover, advance ratio, local inflow ratio, and thrust coefficient - are highly sensitive to BVI noise radiation

    Model helicopter rotor high-speed impulsive noise: Measured acoustics and blade pressures

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    A 1/17-scale research model of the AH-1 series helicopter main rotor was tested. Model-rotor acoustic and simultaneous blade pressure data were recorded at high speeds where full-scale helicopter high-speed impulsive noise levels are known to be dominant. Model-rotor measurements of the peak acoustic pressure levels, waveform shapes, and directively patterns are directly compared with full-scale investigations, using an equivalent in-flight technique. Model acoustic data are shown to scale remarkably well in shape and in amplitude with full-scale results. Model rotor-blade pressures are presented for rotor operating conditions both with and without shock-like discontinuities in the radiated acoustic waveform. Acoustically, both model and full-scale measurements support current evidence that above certain high subsonic advancing-tip Mach numbers, local shock waves that exist on the rotor blades ""delocalize'' and radiate to the acoustic far-field
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