Prediction and reduction of rotor broadband noise

Prediction techniques which can be or have been applied to subsonic rotors, and methods for designing helicopter rotors for reduced broadband noise generation are summarized. It is shown how detailed physical models of the noise source can be used to identify approaches to noise control

Sources, paths, and concepts for reduction of noise in the test section of the NASA Langley 4x7m wind tunnel

NASA is investigating the feasibility of modifying the 4x7m Wind Tunnel at the Langley Research Center to make it suitable for a variety of aeroacoustic testing applications, most notably model helicopter rotors. The amount of noise reduction required to meet NASA's goal for test section background noise was determined, the predominant sources and paths causing the background noise were quantified, and trade-off studies between schemes to reduce fan noise at the source and those to attenuate the sound generated in the circuit between the sources and the test section were carried out. An extensive data base is also presented on circuit sources and paths

A study of the variable impedance surface concept as a means for reducing noise from jet interaction with deployed lift-augmenting flaps

The feasibility of quieting the externally-blown-flap (EBF) noise sources which are due to interaction of jet exhaust flow with deployed flaps was demonstrated on a 1/15-scale 3-flap EBF model. Sound field characteristics were measured and noise reduction fundamentals were reviewed in terms of source models. Test of the 1/15-scale model showed broadband noise reductions of up to 20 dB resulting from combination of variable impedance flap treatment and mesh grids placed in the jet flow upstream of the flaps. Steady-state lift, drag, and pitching moment were measured with and without noise reduction treatment

Some factors influencing radiation of sound from flow interaction with edges of finite surfaces

Edges of surfaces which are exposed to unsteady flow cause both strictly acoustic effects and hydrodynamic effects, in the form of generation of new hydrodynamic sources in the immediate vicinity of the edge. An analytical model is presented which develops the explicit sound-generation role of the velocity and Mach number of the eddy convection past the edge, and the importance of relative scale lengths of the turbulence, as well as the relative intensity of pressure fluctuations. The Mach number (velocity) effects show that the important paramater is the convection Mach number of the eddies. The effects of turbulence scale lengths, isotropy, and spatial density (separation) are shown to be important in determining the level and spectrum of edge sound radiated for the edge dipole mechanism. Experimental data is presented which provides support for the dipole edge noise model in terms of Mach number (velocity) scaling, parametric dependence on flow field parameter, directivity, and edge diffraction effects

Shoreline configuration and shoreline dynamics: A mesoscale analysis

The author has identified the following significant results. Atlantic coast barrier island shorelines are seldom straight, but rather sinuous. These shoreline curvatures range in size from cusps to capes. Significant relationships exist between the orientation of shoreline segments within the larger of these sinuous features and shoreline dynamics, with coefficients ranging up to .9. Orientation of the shoreline segments of Assateague Island (60 km) and the Outer Banks of North Carolina (130 km) was measured from LANDSAT 2 imagery (1:80,000) and high altitude aerial photography (1:120,000). Long term trends in shoreline dynamics were established by mapping shoreline and storm-surge penetration changes

LANDSAT application of remote sensing to shoreline-form analysis

The author has identified the following significant results. Orientation of the shoreline segments of Assateague Island (55 km) was measured from LANDSAT 2 imagery enlarged to 1:250,000 and 1:80,000. Long term trends in shoreline dynamics were established by mapping shoreline and storm-surge penetration changes from historical low altitude aerial photography spanning four decades

Sonic boom simulation by means of low-pressure sources

Sonic boom simulation by low pressure source

USB noise reduction by nozzle and flap modifications

The development of concepts for reducing upper surface blown flap noise at the source through flap modifications and special nozzles is reviewed. In particular, recent results obtained on the aerodynamic and acoustic performance of flaps with porous surfaces near the trailing edge and multi-slotted nozzles are reviewed. Considerable reduction (6-10 db) of the characteristic low frequency peak is shown. The aerodynamic performance is compared with conventional systems, and prospects for future improvements are discussed

Exploratory investigation of aeroacoustic optimization of the variable impedance edge concept applied to upper surface blown configurations

The feasibility of using porous surfaces on lift augmentation flaps to reduce the noise at the source, without adversely affecting aerodynamic performance was investigated. Numerous flap configurations were tested on a USB (upper surface blowing) type powered lift model (approximately 1/5 full scale). Significant reductions of far field noise and aeroacoustic pressures were found, and many configurations exhibited aerodynamic characteristics comparable to unmodified configurations of the nozzle/flap system

Comparison of options for reduction of noise in the test section of the NASA Langley 4x7m wind tunnel, including reduction of nozzle area

The acoustically significant features of the NASA 4X7m wind tunnel and the Dutch-German DNW low speed tunnel are compared to illustrate the reasons for large differences in background noise in the open jet test sections of the two tunnels. Also introduced is the concept of reducing test section noise levels through fan and turning vane source reductions which can be brought about by reducing the nozzle cross sectional area, and thus the circuit mass flow for a particular exit velocity. The costs and benefits of treating sources, paths, and changing nozzle geometry are reviewed