4,497 research outputs found
Numerical aeroacoustic analysis of propeller designs
As propeller-driven aircraft are the best choice for short/middle-haul flights but their acoustic emissions may require improvements to comply with future noise certification standards, this work aims to numerically evaluate the acoustics of different modern propeller designs. Overall sound pressure level and noise spectra of various blade geometries and hub configurations are compared on a surface representing the exterior fuselage of a typical large turboprop aircraft. Interior cabin noise is also evaluated using the transfer function of a Fokker 50 aircraft. A blade design operating at lower RPM and with the span-wise loading moved inboard is shown to be significantly quieter without severe performance penalties. The employed Computational Fluid Dynamics (CFD) method is able to reproduce the tonal content of all blades and its dependence on hub and blade design features
An integral formulation for wave propagation on weakly non-uniform potential flows
An integral formulation for acoustic radiation in moving flows is presented.
It is based on a potential formulation for acoustic radiation on weakly
non-uniform subsonic mean flows. This work is motivated by the absence of
suitable kernels for wave propagation on non-uniform flow. The integral
solution is formulated using a Green's function obtained by combining the
Taylor and Lorentz transformations. Although most conventional approaches based
on either transform solve the Helmholtz problem in a transformed domain, the
current Green's function and associated integral equation are derived in the
physical space. A dimensional error analysis is developed to identify the
limitations of the current formulation. Numerical applications are performed to
assess the accuracy of the integral solution. It is tested as a means of
extrapolating a numerical solution available on the outer boundary of a domain
to the far field, and as a means of solving scattering problems by rigid
surfaces in non-uniform flows. The results show that the error associated with
the physical model deteriorates with increasing frequency and mean flow Mach
number. However, the error is generated only in the domain where mean flow
non-uniformities are significant and is constant in regions where the flow is
uniform
Simulated flight effects on noise characteristics of a fan inlet with high throat Mach number
An anechoic wind tunnel experiment was conducted to determine the effects of simulated flight on the noise characteristics of a high throat Mach number fan inlet. Comparisons were made with the performance of a conventional low throat Mach number inlet with the same 50.8 cm fan noise source. Simulated forward velocity of 41 m/sec reduced perceived noise levels for both inlets, the largest effect being more than 3 db for the high throat Mach number inlet. The high throat Mach number inlet was as much as 7.5 db quieter than the low throat Mach number inlet with tunnel airflow and about 6 db quieter without tunnel airflow. Effects of inlet flow angles up to 30 deg were seemingly irregular and difficult to characterize because of the complex flow fields and generally small noise variations. Some modifications of tones and directivity at blade passage harmonics resulting from inlet flow angle variation were noted
Research on combustion instability and application to solid propellant rocket motors. II.
Unstable motions must be anticipated as a possible
problem in solid-propellant rocket motors;
the characteristics of an instability depend primarily
on the geometry of the motor and composition
of the propellant. It is the purpose of this
paper to review mainly the current state of analyses
of combustion instability in solid-propellant
rocket motors, but appropriate measurements and
observations are cited. The work discussed has
become increasingly important, both for the interpretation
of laboratory data and for predicting the
transient behavior of disturbances in full-scale
motors. Two central questions are addressed:
linear stability and nonlinear behavior. Several
classes of problems are discussed as special cases
of a general approach to the analysis of combustion
instability. Application to motors, and particularly
the limitations presently understood, are stressed
Integrated multidisciplinary optimization of rotorcraft: A plan for development
This paper describes a joint NASA/Army initiative at the Langley Research Center to develop optimization procedures aimed at improving the rotor blade design process by integrating appropriate disciplines and accounting for important interactions among the disciplines. The paper describes the optimization formulation in terms of the objective function, design variables, and constraints. Additionally, some of the analysis aspects are discussed, validation strategies are described, and an initial attempt at defining the interdisciplinary couplings is summarized. At this writing, significant progress has been made, principally in the areas of single discipline optimization. Accomplishments are described in areas of rotor aerodynamic performance optimization for minimum hover horsepower, rotor dynamic optimization for vibration reduction, and rotor structural optimization for minimum weight
A Comparison between Deep Neural Nets and Kernel Acoustic Models for Speech Recognition
We study large-scale kernel methods for acoustic modeling and compare to DNNs
on performance metrics related to both acoustic modeling and recognition.
Measuring perplexity and frame-level classification accuracy, kernel-based
acoustic models are as effective as their DNN counterparts. However, on
token-error-rates DNN models can be significantly better. We have discovered
that this might be attributed to DNN's unique strength in reducing both the
perplexity and the entropy of the predicted posterior probabilities. Motivated
by our findings, we propose a new technique, entropy regularized perplexity,
for model selection. This technique can noticeably improve the recognition
performance of both types of models, and reduces the gap between them. While
effective on Broadcast News, this technique could be also applicable to other
tasks.Comment: arXiv admin note: text overlap with arXiv:1411.400
Acoustic radiation from lifting airfoils in compressible subsonic flow
The far field acoustic radiation from a lifting airfoil in a three-dimensional gust is studied. The acoustic pressure is calculated using the Kirchhoff method, instead of using the classical acoustic analogy approach due to Lighthill. The pressure on the Kirchhoff surface is calculated using an existing numerical solution of the unsteady flow field. The far field acoustic pressure is calculated in terms of these values using Kirchhoff's formula. The method is validated against existing semi-analytical results for a flat plate. The method is then used to study the problem of an airfoil in a harmonic three-dimensional gust, for a wide range of Mach numbers. The effect of variation of the airfoil thickness and angle of attack on the acoustic far field is studied. The changes in the mechanism of sound generation and propagation due to the presence of steady loading and nonuniform mean flow are also studied
Boundary Element and Finite Element Coupling for Aeroacoustics Simulations
We consider the scattering of acoustic perturbations in a presence of a flow.
We suppose that the space can be split into a zone where the flow is uniform
and a zone where the flow is potential. In the first zone, we apply a
Prandtl-Glauert transformation to recover the Helmholtz equation. The
well-known setting of boundary element method for the Helmholtz equation is
available. In the second zone, the flow quantities are space dependent, we have
to consider a local resolution, namely the finite element method. Herein, we
carry out the coupling of these two methods and present various applications
and validation test cases. The source term is given through the decomposition
of an incident acoustic field on a section of the computational domain's
boundary.Comment: 25 page
Acoustic design considerations: Review of rotor acoustic sources
It is not sufficient to optimize a rotor design in terms of a single noise level calculated for a single flight condition and a single measurement location. The various noise sources, their frequency content, amplitude, and directivity as a function of operating condition must be considered. A summary of the frequency ranges, directivity patterns and the most important operational and design parameters for major rotor noise sources is presented. It is difficult to generalize design requirements for rotor noise because the acoustic output varies so widely depending on the noise source, flight condition, measurement location, and frequency range. However, assuming the rotor must lift a fixed nominal payload and operate over a wide range of flight conditions, three general design guidelines can be stated: (1) minimize tip Mach number; (2) minimize blade thickness in the tip region; and (3) minimize gradients in the spanwise lift distribution in the tip region. Constraints on blade thickness, maximum values for hover tip Mach number, advancing tip Mach number and spanwise lift coefficient gradient will be specified during the aerodynamic, dynamic and structural optimization process. The rotor noise sources to be considered include the low frequency loading and thickness noise, and the higher frequency noise due to blade-vortex interactions (BVI). The analyses to be employed will include the comprehensive rotor analysis and design program CAMRAD and the rotor noise prediction program WOPWOP
Helicopter external noise prediction and correlation with flight test
Mathematical analysis procedures for predicting the main and tail rotor rotational and broadband noise are presented. The aerodynamic and acoustical data from Operational Loads Survey (OLS) flight program are used for validating the analysis and noise prediction methodology. For the long method of rotational noise prediction, the spanwise, chordwise, and azimuthwise airloading is used. In the short method, the airloads are assumed to be concentrated at a single spanwise station and for higher harmonics an airloading harmonic exponent of 2.0 is assumed. For the same flight condition, the predictions from long and short methods of rotational noise prediction are compared with the flight test results. The short method correlates as well or better than the long method
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