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At least 127 records · Page 7

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NASA NTRS1975

Wind tunnel blockage tests at Mach 5 of vacuum duct models for two sound radiation shields

Two sound shield models with dummy vacuum exhaust ducts were tested in a Mach 5 pilot quiet tunnel. The first model simulates a new sound shield of 3 in. (7.62 cm) inside diameter and the second model is a shield of 4 in. (10.16 cm) inside diameter. The dummy vacuum exhaust ducts were attached to the external housing of the models. The flow in the first model, which had a by pass mass flow ratio of about 0.6, could not be started except at the two highest test Reynolds numbers where only the central core flow region was started. The flow in the second model with a mass ratio of approximately 0.3 was fully started except at the lowest unit Reynolds number where some unsteadiness and partial flow separation at the wall was observed. Since the external housing and dummy vacuum ducts were the same for both models, these results indicate that the ratio of by pass mass flow to total mass flow for a wind tunnel sound shield of this particular design must be less than about 0.3. Hence, a lower limit is imposed on the inlet diameter of the sound shield in relation to the exit diameter of the wind tunnel nozzle. This lower limit on the inlet diameter may possibly be reduced by improvements in streamlining of the external housing and ducts, by reductions in blockage area, or by the use of external ducting shrouds.

Beckwith, I. E.↗

NASA NTRS1976

A multi-modal solution for the transmission of sound in nonuniform ducts

The method of weighted residuals in the form of a modified Galerkin method with boundary residuals is developed for the study of the transmission of sound in nonuniform ducts carrying a steady compressible flow. In this formulation the steady flow is modeled as essentially one-dimensional but with a kinematic modification to force tangency of the flow at the duct walls. Good agreement has been obtained with known results for transmission and reflection coefficients in hard walled ducts up to near sonic velocities. In ducts with acoustically compliant boundaries good comparisons have been more difficult to achieve except at low Mach numbers. The problem of transmission in a straight, acoustically treated duct with a uniform flow has been formulated and the Galerkin method used with basis functions derived from the case when flow is absent. Results indicate that favorable comparisons with exact computations can be obtained if care is taken in choosing the basis functions.

Eversman, W.↗

NASA NTRS1977

Transmission of sound through nonuniform circular ducts with compressible mean flows

An acoustic theory is developed to determine the sound transmission and attenuation through an infinite, hard-walled or lined, circular duct carrying compressible, sheared, mean flows and having a variable cross section. The theory is applicable to large as well as small axial variations, as long as the mean flow does not separate. Although the theory is described for circular ducts, it is applicable to other duct configurations - annular, two dimensional, and rectangular. The theory is described for the linear problem, but the technique is general and has the advantage of being applicable to the nonlinear case as well as the linear case. The technique is based on solving for the envelopes of the quasi-parallel acoustic modes that exist in the duct instead of solving for the actual wave. A computer program was developed. The mean flow model consists of a one dimensional flow in the core and a quarter-sine profile in the boundary layer. Results are presented for the reflection and transmission coefficients in ducts with varying slopes and carrying different mean flows.

Nayfeh, A. H.↗

NASA NTRS1977

Effect of flow on quasi-one-dimensional acoustic wave propagation in a variable area duct of finite length

The general equation for the velocity potential of quasi-one-dimensional acoustic wave motion in a variable area, finite duct with one-dimensional flow is derived by using a perturbation technique. The nonlinear second-order partial differential equation is linearized and then solved, by either a power series expansion method or the Runge-Kutta fourth-order method, for harmonic time dependence. The boundary condition taken at the duct mouth is that of matching the impedance of the duct sound field to that of the radiation field at the duct opening. Three axial Mach number variations along the duct axis are considered and the results obtained are compared with those for the case of constant Mach number, to determine the influence of the axial velocity gradient on sound propagation. The effect of flow on the radiation impedance is also considered.

Lumsdaine, E.↗

NASA NTRS1978

Sound propagation through a subsonic jet due to a source near the duct exit

Matched asymptotic solutions are constructed for the acoustic potentials of a periodic point source located in a two-dimensional subsonic jet near the exit of the duct with the ratio of the duct thickness to the acoustic wave length as the small parameter. The leading term of the far field solution has the same directionality effect as that for an infinite jet without the duct and that when the plane at the duct exit is considered to be a plane of symmetry. However, the intensity is different because of the wave propagation into the duct and is dependent on the location of the source.

Ting, L.↗

NASA NTRS1979

Modal propagation angles in a cylindrical duct with flow and their relation to sound radiation

The main emphasis is upon the propagation angle with respect to the duct axis and its relation to the far-field acoustic radiation pattern. When the steady flow Mach number is accounted for in the duct, the propagation angle in the duct is shown to be coincident with the angle of the principal lobe of far-field radiation obtained using the Wiener-Hopf technique. Different Mach numbers are allowed within the duct and in the external field. For static tests with a steady flow in an inlet but with no external Mach number the far-field radiation pattern is shifted considerably toward the inlet axis when compared to zero Mach number radiation theory. As the external Mach number is increased the noise radiation pattern is shifted away from the inlet axis. The theory is developed using approximations for sound propagation in circular ducts. An exact analysis using Hankel function solutions for the zero Mach number case is given to provide a check of the simpler approximate theory.

Rice, E. J.↗

NASA NTRS1979

Reciprocity principle in duct acoustics

Various reciprocity relations in duct acoustics have been derived on the basis of the spatial reciprocity principle implied in Green's functions for linear waves. The derivation includes the reciprocity relations between mode conversion coefficients for reflection and transmission in nonuniform ducts, and the relation between the radiation of a mode from an arbitrarily terminated duct and the absorption of an externally incident plane wave by the duct. Such relations are well defined as long as the systems remain linear, regardless of acoustic properties of duct nonuniformities which cause the mode conversions.

Cho, Y. C.↗

NASA NTRS1979

Reciprocity principle in duct acoustics

Cho, Y.-C.↗

NASA NTRS1981

Influence of exit impedance on finite difference solutions of transient acoustic mode propagation in ducts

The time-dependent governing acoustic-difference equations and boundary conditions are developed and solved for sound propagation in an axisymmetric (cylindrical) hard-wall duct without flow and with spinning acoustic modes. The analysis begins with a harmonic sound source radiating into a quiescent duct. This explicit iteration method then calculates stepwise in real time to obtain the steady solutions of the acoustic field. The transient method did not converge to the steady-state solution for cutoff acoustic duct modes. This has implications as to its use in a variable-area duct, where modes may become cutoff in the smal-area portion of the duct. For single cutoff mode propagation the steady-state impedance boundary condition produced acoustic reflections during the initial transient that caused finite instabilities in the numerical calculations. The stability problem is resolved by reformulating the exit boundary condition. Example calculations show good agreement with exact analytical and numerical results for forcing frequencies above, below, and nearly at the cutoff frequency.

Baumeister, K. J.↗

NASA NTRS1981

Note on reflection and transmission coefficients for converging-diverging ducts

Simple formulas for calculating acoustic reflection and transmission coefficients for converging-diverging ducts are derived; they extend the method of Cho and Ingard to arbitrary, slowly varying ducts. These formulas involve two parameters. The first is a function of duct shape and the second is the ratio of the duct radius downstream of the throat to that upstream of the throat to the upstream of the throat. An extension of the method to include mean flow is made for symmetric ducts.

Durbin, P. A.↗

NASA NTRS1983

Sound propagation in segmented exhaust ducts - Theoretical predictions and comparison with measurements

Suppressions due to acoustic treatment in the annular exhaust duct of a model fan have been theoretically predicted and compared with measurements. The predictions are based on the modal analysis of sound propagation in a straight annular flow duct with segmented treatment. Measured values of the mode distribution of the fan noise source (fan-stator interaction source only) and the acoustic impedance of the treatment in the duct were used as input to the prediction program. The predicted suppressions (under the assumption of uniform flow in the duct) compared well with the measured in-duct suppression for all test conditions. The measurements were made in an anechoic chamber and the interaction modes generated in the fan spanned a cut-off ratio range from nearly one to seven.

Joshi, M. C.↗

NASA NTRS1983

Composite engine duct fabrication

The trimonomer polyimide resin reinforced with graphite fibers, PMR15, is lower in weight (by 20 percent) and less expensive than titanium. The duct walls are of solid laminate and are made of woven T300 graphite cloth. The original concept for the duct involved a solid shell of T300/PMR15 made up of seven plies of cloth tapered to eleven plies at each end. The bosses and built-up areas around the cutouts are also of T300/PMR15. The end flages riveted to the shell, however, are of titanium. A buckling test demonstrated the ability of the composite analytical design techniques employed to predict the buckling strength of the composite shell. Engines tests are described, noting that the successful repair of a hole demonstrated an important advantage over titanium. The development work now being done to eliminate the titanium end flanges is described, noting that an all-composite duct is in prospect. The savings estimated with a titanium-flanged composite duct are $16,891,000 (1982 dollars) and $35,504,000 (then-year dollars); the savings possible with an all-composite duct are put at $27,025,000 (1982 dollars) and $56,806,000 (then-year dollars).

Pratt, R. D.↗

NASA NTRS1984

A magnetohydrodynamic model of whistler duct structure in the magnetosphere

In this study, the physical structure for the propagation of whistler waves within a duct in the earth's magnetosphere is investigated by means of magnetohydrodynamic (MHD) theory. Expressions for the current density and induced magnetic field are determined analytically and evaluated in terms of two models for the duct plasma density distribution. It is found that once the duct is formed, forces associated with the current structure will maintain it. MHD instabilities are examined briefly and found to be unlikely to threaten duct maintenance in regions where whistlers are typically observed. Examination of some effects of field-aligned currents suggest that this may be a viable mechanism for duct formation.

Wang, S.↗

NASA NTRS1984

Uniformly spaced field-aligned ionization ducts

A number of interesting cases of combination mode ducted echoes for mid- and low-latitude regions are presented that show nearly uniformly spaced multiple combination mode traces on the ionograms in the frequency range above 1 MHz. These traces suggest that a parallel system of field-aligned ducts is present. Ray tracing studies are made to determine the structure that would explain the observations using the electron density profile derivable from the vertical trace and assuming field-aligned ducts. Spacing perpendicular to the ducts is found to be as much as 70 km. Some of these parallel duct structures are found to extend to the conjugate hemisphere, possibly to the F peak.

Gross, S. H.↗

NASA NTRS1987

Circular-to-rectangular duct flows - A benchmark experimental study

A comprehensive set of total pressure and three-component laser velocimetry (LV) data has been obtained within two-circular-to-rectangular transition ducts at low subsonic speeds. This set of reference data was acquired for use in identifying secondary flow mechanisms and for assessing the accuracy of computational procedures for calculating such flows. Data were obtained at the inlet and exit planes of an aspect ratio three (AR3) duct having a length-to-diameter ratio (L/D) of one (1) and an aspect ratio six (AR6) duct having an L/D of three (3). Each duct was unseparated throughout its transition section. Cross-flow-vector plots and contour plots of axial velocity, total pressure, static pressure, axial turbulence, and axial vorticity in the exit plane of each duct are presented.

Patrick, William P.↗

NASA NTRS1988

Computational simulation of turbulent flow in Space Shuttle Main Engine turnaround ducts

Two axisymmetric 180-deg turnaround ducts are used in the Space Shuttle Main Engine (SSME) to connnect the preburners with the main thrust chamber. The prediction of the flow field and heat transfer within the turnaround ducts is important to minimize the pressure drops, size of recirculation and stagnation zones, local overheating, etc. This paper presents the computational approach and selected results for the turbulent flow in the turnaround ducts of the fuel and oxidizer sides of the SSME. The time-averaged, Navier-Stokes equations for the viscous, compressible, turbulent flow are solved in body-fitted-coordinates by using a finite-volume approach. Two turbulence models, viz: the k-epsilon model and a multiple scale turbulence model, are used to examine the sensitivity of calculated flows. Both models produce almost identical solutions for the fuelside turnaround duct (which has no recirculation region). However, for the oxidizer-side duct which has a large recirculation region, the two models show quite different results.

Przekwas, Andrzej J.↗

NASA NTRS1986

Influence of bulk turbulence and entrance boundary layer thickness on the curved duct flow field

The objective of this investigation was the experimental evaluation of bulk turbulence and boundary thickness influence on the secondary flow development in a square, 90 deg turning duct. A three dimensional laser velocimetry system was utilized to measure the mean and fluctuating components of velocity in the large curved duct facility. The three dimensional development of the viscous shear layers in the curved duct has a strong influence on the complete flow field. Since ducted three dimensional flows are found in many engineering applications, including gas turbine engines, and contain high turbulence levels and high wall heat transfer rates, they present a difficult challenge to computational fluid mechanics codes. Turbulence modeling remains one of constraints to CFD advance due to inadequate physical understanding and experimental definition of turbulent shear flows. The results of this investigation expand the curved duct data base to higher turbulence levels and thicker entrance boundary layers. The experimental results provide a challenging benchmark data base for computational fluid dynamics code development and validation. The variation of inlet bulk turbulence intensity provides additional information to aid in turbulence model evaluation.

Crawford, Roger A.↗

NASA NTRS1988

Acoustical scattering from finite compliant surfaces bounding a duct

Acoustic radiation effects of a finite compliant surface covering a transverse slot in a two-dimensional parallel walled rigid duct are investigated. Radiation from a harmonic source external to the duct is shown to impinge on the slot, reradiating to the half plane and propagating through the duct. Integral equation techniques are employed to study the cases of an open slot, a membrane cover, and a flexible plate cover. The total power radiated to the half plane external to the duct and to the region internal to the duct is determined, taking into account slot-width and fluid-loading variations and the effect of the surface on the power radiated.

Smith, M. E.↗