A prediction method is developed for the self-generated noise of an airfoil blade encountering smooth flow. The prediction methods for the individual self-noise mechanisms are semiempirical and are based on previous theoretical studies and data obtained from tests of two- and three-dimensional airfoil blade sections. The self-noise mechanisms are due to specific boundary-layer phenomena, that is, the boundary-layer turbulence passing the trailing edge, separated-boundary-layer and stalled flow over an airfoil, vortex shedding due to laminar boundary layer instabilities, vortex shedding from blunt trailing edges, and the turbulent vortex flow existing near the tip of lifting blades. The predictions are compared successfully with published data from three self-noise studies of different airfoil shapes. An application of the prediction method is reported for a large scale-model helicopter rotor, and the predictions compared well with experimental broadband noise measurements. A computer code of the method is given.

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Airfoil self-noise and prediction

The Riemann problem for fluid flow of real materials is examined. An arbitrary equation of state is allowed, subject only to the physical requirements of thermodynamics. The properties of the isentropes and the shock Hugoniot loci that follow from conditions imposed on the equation of state are reviewed systematically. Important properties of these wave curves are determined by three dimensionless variables characterizing the equation of state: the adiabatic exponent $\ensuremath{\gamma}$, the Gr\"uneisen coefficient $\ensuremath{\Gamma}$, and the fundamental derivative $\mathcal{G}$. Standard assumptions on these variables break down near phase transitions. The result is an anomalous wave structure: either shock waves split into multiple waves, or composite waves form. Additional questions related to shock stability and nonuniqueness of the solution of the Riemann problem are discussed.

The Riemann problem for fluid flow of real materials

https://www.cs.odu.edu/~mln/ltrs-pdfs/NASA-2003-pas-ksb.pdf

Modeling aerodynamically generated sound of helicopter rotors

Extension of Kirchhoff's formula to radiation from moving surfaces

▪ Abstract Interaction of a vortex, or combinations of them, with a cylinder, blade, or foil may involve both rapid distortion of the incident vorticity field and shedding of vorticity from the surface of the body. This review focuses on the underlying flow physics, with the aim of clarifying the origin of the induced loading. In the case of near or direct encounter of the incident vortex, the relation between three-dimensional features of the flow structure and the local loading poses interesting challenges for further research. With recently developed simulation and laboratory techniques, opportunities now exist to determine the instantaneous quantitative structure of these complex distortions and to interpret them within a theoretical, vorticity-based framework. Effective control of vortex interactions appears to be attainable. It will be necessary, however, to develop creative strategies, distinct from those traditionally employed for control of unstable shear flows.

Vortex-body interactions

There have been many attempts to reduce or eliminate the sonic boom For supersonic aircrafts such attempts fall into two categories: (1) aerodynamic minimums and (2) exotic configurations In the first category, changes in entropy and the Bernoulli constant are neglected, and equivalent body shapes required to minimize the overpressure, the shock pressure rise, and the impulse are deduced These results include the beneficial effects of atmospheric stratification In the second category, the effective length of the aircraft is increased by modifying the Bernoulli constant a significant fraction of the flow past the aircraft A figure of merit is introduced which makes it possible to judge the effectiveness of such schemes Finally, the sonic boom reductions that can be achieved by hypersonic flight at high altitudes are summarized

Sonic‐Boom Minimization

Experimental Study of a Ground Vehicle Body Unsteady Near Wake

Sonic Boom Minimization Including Both Front and Rear Shocks

Broadband and discrete frequency radiation from subsonic rotors

This paper reviews helicopter external noise with particular emphasis on the noise due to helicopter main and tail rotors. The bases for annoyance and audibility are discussed. Sources of rotor noise include steady, periodic, and random loads on the rotor blades, as well as volume displacement and nonlinear aerodynamic effects at high blade Mach numbers. Either main or tail rotors can be dominant noise sources at various frequencies and observer positions.

Albert R. George

Papers

Sonic‐Boom Minimization

Experimental Study of a Ground Vehicle Body Unsteady Near Wake

Sonic Boom Minimization Including Both Front and Rear Shocks

Broadband and discrete frequency radiation from subsonic rotors

Helicopter noise: state-of-the-art