Duct Exit Research Articles

Laminar flow of four gases through a helical rectangular duct

AbstractMeasurements and a model of gas flow through a helical duct of rectangular cross section are reported. Observations of the rate of rise of pressure in a known volume located downstream from the duct yielded molar flow rates of helium, nitrogen, argon and sulfur hexafluoride. The model predicts flow rates from the duct's entrance pressure, exit pressure, and temperature. It accounts for the gas's nonideal equation of state, gas expansion along the duct's length, the increase of kinetic energy near duct's entrance, and slip. After adjusting only one parameter (the duct's height), the model agrees with the data to within 0.2% in the range of Reynolds number 0.08<Re<40. To extend the model to Re = 1,000, the centrifugal effects were accounted for by rescaling Targett et al.'s (1995) numerical calculations. The extended model agrees with the data to within 0.2% in the range 40 < Re < 400 and to within 1% in the range 400 < Re < 1,000.

Vacuum arc plasma jet influence on the device neutralatmosphere

The interaction of a vacuum arc plasma jet (VAPJ) with theneutral atmosphere filling the plasma duct in a vacuum arc plasma coatingsystem was modelled. The VAPJ velocity, ion density and neutral gas densitywere calculated using a one-dimensional approximation in steady state, as afunction of distance from the duct entrance. Three cases were considered,with (1) the duct entrance, (2) the duct exit and (3) the averageneutral gas pressure fixed. It was found that, generally, the VAPJ velocitydecreases as it propagates along the duct, while the neutral densityincreases as a function of the distance from the duct entrance. The jetdeceleration is steepest when the pressure is fixed at the duct entrance.When the pressure is fixed at the duct exit, the deceleration of the VAPJis greater for less-dense jets.

Vortex Shedding from Staggered Tube Banks and Flow Induced Acoustic Resonance.

The relation between flow induced acoustic resonance and vortex shedding from the tube banks was experimentally investigated. We measured sound pressure level near the duct exit and the vortex shedding frequency, the intensity of velocity fluctuation and the spectrums of velocity fluctuation in the tube banks by using a tube banks model which has same pitch ratio as that of a boiler heat exchanger. We found three types of vortex shedding with different Strouhal number, 0.29, 0.22 and 0.19. The vortex shedding of St =0.29 and 0.22 are generated inside of the tube banks, other one of 0.19 is in the wake of the last row of the tube banks. The velocity fluctuation and the periodicity of the vortex shedding from the second row of the tube banks are the most intense in the tube banks. The amplitude of velocity fluctuation due to the vortex shedding became large in accordance with the generation of the acoustic resonance which has the fundamental natural frequency of the acoustic resonance in the transverse direction of the duct.

Combustion Oscillations Close to the Lean Flammability Limit

The nature of premixed methane-air flames stabilised on a symmetric, plane sudden-expansion has been examined in terms of wall pressures and the chemiluminescence of the CH radical. Large-amplitude acoustic oscillations at the half-wave frequency of the entire duct were observed at high velocities and with near-stoichiometric mixtures, and instabilities with much lower-frequencies close to the lean and rich extinction limits. The emphasis is on these near-limit instabilities and, due to the asymmetry of the plane flow, the experiments were extended to round ducts in which the flammability and stability limits were similar to those in the plane duct. The branches of flame behind the steps of the plane expansion extinguished non-simultaneously, and gave rise to low-frequency flapping oscillations immediately prior to extinction of the first of these, and the remaining branch gave rise to lateral oscillations prior to its extinction. The oscillations were associated with axial movement of extinction along the shear layer due to the high strain rates close to the step, until the strain rate was sufficiently low and allowed upstream propagation of the flame through the recirculation region. The flames in the round duct gave rise to oscillations of the same nature. The frequency of the oscillations increased with flow velocity and flame speed, and the amplitude with heat release and much more with constriction of the duct exit due to coupling with a bulk-mode of the combustor cavity. These increased amplitudes caused the flammability limits to narrow due to the consequently higher strain rates and have implications for gas turbine operation.

Plasma drift and nonuniformity effects in plasma immersion ion implantation

Measurements of the ion current collected by a substrate biased to high voltage have been carried out in plasma immersion ion implantation with a filtered vacuum arc plasma source. We have found that the ion saturation current increases with applied voltage and that this effect depends upon the angle of the normal to the substrate with respect to the plasma stream and on the distance of the substrate from the plasma duct exit. We also found that the ion current increases with increasing angle of the normal to the substrate with respect to the plasma stream. A model was developed for the sheath expansion in a nonuniform plasma with substantial ion drift velocity. We find that nonuniformity and high drift velocity lead to a decrease in sheath thickness. In a nonuniform plasma, the ion saturation current increases with applied voltage. The predictions of the model were found to be in good agreement with experiment.

3925 千鳥配列の管群における気柱共鳴現象

In the present paper the attention is focused on the relation between the vortex shedding phenomena and the velocity field around the tubes in tube banks. We measured a spectrum of velocity fluctuation, coherence function, gap velocity in the tube banks and sound pressure level at the duct exit in the case of a model of tube banks that has same pitch ratio as that of a boiler heat exchanger. When acoustic resonance has generated at the natural frequencies of the duct, which correspond to Strouhal number of 0.29,there are mainly two vortices which have Strouhal numbers about 0.29 and 0.22,in the tube banks and the wake of tube banks, respectively. These vortices are out of phase like Karman vortex shedding from a circular cylinder.

STUDY OF THE DISCHARGE OF WEAK SHOCKS FROM AN OPEN END OF A DUCT

Shock discharge from a duct exit is often encountered in many engineering practices and usually gives rise to an annoying noise problem similar to a sonic boom. Such a noise can have hazardous effects on human beings, as well as structures in the vicinity, unless proper control strategies are developed. The objective of the current work is to characterize the impulse wave caused by a weak shock discharged from an open end of a duct. Experiments were performed using an open-ended shock tube for shock Mach number of 1·02–1·45. Computational analysis was applied to model the unsteady flow field by application of axisymmetric, inviscid, compressible equations. A total variation diminishing (TVD) numerical scheme was used to solve the conservation equation system. The effect of a baffle plate, installed at the duct exit, on the impulse wave was investigated both experimentally and by numerical calculation. The results of the experiments were in good agreement with the results of numerical calculations. The results showed that the baffle plate affects the strength of the impulse wave only when its diameter is less than 3 times the duct diameter. With the diameter of the baffle plate less than 3 times the duct diameter, the strength of an impulse wave for a given shock Mach number increases when the baffle plate becomes larger. The impulse wave has a directivity to the centerline of the duct. It was found that for prediction of the impulse wave aero-acoustical theory should be used only at distances larger than four times the duct diameter.

Combustion of premixed fuel and air downstream of a plane sudden-expansion

Experiments have been performed to quantify the isothermal and combusting flows downstream of a plane sudden-expansion. The detailed measurements correspond to an area expansion ratio of 2.86 and a Reynolds number of 20000, and the combusting flows comprised premixed methane and air over a range of equivalence ratios with emphasis on values of 0.72 and 0.92 which gave rise to smooth and rough combustion, respectively. The results show that the extent of asymmetry of the isothermal flows was reduced by coupling the pressures between the two recirculation regions, and by imposing oscillations at the half-wave or full-wave frequency of the duct, and by combustion. Periodic variations of flame shape, velocities, acceleration, and temperature were observed in sympathy with the dominant pressure oscillation of rough combustion, and the length of the recirculation zones varied from less than 0.5 to 3 step heights. Rich and lean limits were established for combustion within the duct and, whereas the flame blew off at the lean limit, it detached from the expansion at the rich limit and stabilised on the flange at the duct exit. Within these limits, there were ranges of equivalence ratios over which the flame stabilised on one of the two steps with incomplete combustion. The imposition of oscillations narrowed the range of equivalence ratios over which the flame could be stabilised but reduced the equivalence ratio of the lean limit at which the flame could be stabilised on both steps and the effect increased with amplitude and was greatest when the frequency of the imposed oscillations corresponded to that of the half-wave in the duct. An increase in the amplitude of flow oscillations, natural or imposed, caused the concentrations of NO x measured at the duct exit to decrease. Active control of flows with high amplitude of oscillations produced the expected reductions, but not over the entire measured range of equivalence ratio, and the imposition of pressure oscillations at the second harmonic of the half-wave frequency had a greater effect and over a wider range.

Calculation of the acceleration of a cylindrical body by an unsteady two-phase flow

Calculations are performed for the acceleration of a cylindrical body by the unsteady flow of a two-velocity two-temperature gas-disperse medium taking into account the following two mechanisms of interparticle interaction: collisions of randomly moving dispersed particles in a rarefied state or deformation of a porous skeleton in closepacked particles. It is established that there are regimes of acceleration of the body in which its velocity exceeds the maximum velocity of the particles at the duct exit. The possibility of using simplified calculation schemes in modeling the acceleration of the body is shown. Calculation and experimental results are compared.

Aerodynamic experimentation with ducted models as applied to hypersonic air-breathing vehicles

A methodology of experimentation in high supersonic wind tunnels for studying aerodynamic characteristics of hypersonic flying vehicles powered by air-breathing engines is discussed. Investigations of such total aerodynamic forces as drag, lift and pitching moment at testing the models are implicit when the air flow through the model ducts is accomplished so that to provide the simulation of the external flow around the airplane and flow over the inlets, but the operating engines and, hence, the exhaust jets are not modeled. The methods used for testing such models are based on the measurement of duct stream parameters alongside with the balance measurement of aerodynamic forces acting on the models. In the tests, aerometric tools are used such as narrow metering nozzles (plugs), pitot and static pressure probes, stagnation temperature probes and pressure orifices in walls of the model duct. The aerometric data serve to determine the flow rate and momentum of the stream at the duct exit. The internal non-simulated forces of the model ducts are also determined using the conservation equations for energy, mass flow and momentum, and these forces are eliminated from the aerodynamic test results. The techniques of the said model testing have been well developed as applied to supersonic aircraft, however their application for hypersonic vehicles whose models are tested at high supersonic speeds, Mach number M∞>4, implies some specific features. In the present paper, the results of experimental and theoretical study of these features are discussed. Some experimental data on aerodynamics of hypersonic aircraft models received in methodological tests are also presented. The tunnel experiments have been carried out in the Mach number range M∞=2–6.

The Effect of Inlet Boundary Layer Thickness on the Flow Within an Annular S-Shaped Duct

Experimental and numerical investigations were carried out to gain a better understanding of the flow characteristics within an annular S-shaped duct, including the effect of the inlet boundary layer (IBL) on the flow. A duct with six struts and the same geometry as that used to connect compressor spools on our experimental small two-spool turbofan engine was investigated. A curved downstream annular passage with a similar meridional flow path geometry to that of the centrifugal compressor has been fitted at the exit of S-shaped duct. Two types of the IBL (i.e., thin and thick IBL) were used. Results showed that large differences of flow pattern were observed at the S-shaped duct exit between two types of the IBL, though the value of “net” total pressure loss has not been remarkably changed. According to “overall” total pressure loss, which includes the IBL loss, the total pressure loss was greatly increased near the hub as compared to that for a thin one. For the thick IBL, a vortex pair related to the hub-side horseshoe vortex and the separated flow found at the strut trailing edge has been clearly captured in the form of the total pressure loss contours and secondary flow vectors, experimentally and numerically. The high-pressure loss regions on either side of the strut wake near the hub may act on a downstream compressor as a large inlet distortion, and strongly affect the downstream compressor performance. There is a much-distorted three-dimensional flow pattern at the exit of S-shaped duct. This means that the aerodynamic sensitivity of S-shaped duct to the IBL thickness is very high. Therefore, sufficient care is needed to design not only downstream aerodynamic components (for example, centrifugal impeller) but also upstream aerodynamic components (LPC OGV).

Shock and blast attenuation by aqueous foam barriers: influences of barrier geometry

A Free-Lagrange CFD code is used to simulate the attenuation, by means of barriers of aqueous foam, of shocks and blast waves emerging from the open end of a two-dimensional duct. A range of foam barrier configurations is explored, comprising foam sheets that extend the effective duct length, and foam “caps” that completely obstruct the duct exit. Near-field off-axis overpressure time-histories are presented for each configuration. Two attenuation mechanisms are identified, both of which are influenced by the barrier geometry used: partial transmission at foam/air interfaces due to impedance mismatching, and delay of blast waves within the foam. Attenuation by dissipation within the foam is not simulated.

Fluid flow in a dynamic mechanical model of the vocal folds and tract. I. Measurements and theory

In this study, aerodynamic and acoustic measurements were obtained in a dynamic mechanical model of the larynx and vocal tract. The model consisted of a uniform duct, intersected by a pair of sinusoidally oscillating shutters. A controlled airflow along the duct was periodically disturbed by the action of the shutters and pressure, and flow velocity measurements were obtained in the region downstream. The velocity field in the duct could be decomposed into three distinct components: a mean flow, a fluctuating acoustic particle velocity, and a fluctuating nonacoustic velocity associated with the transport of vortices along the duct at the local mean flow velocity. Two theoretical models for sound radiation from the duct exit were investigated. The first was based on the in-duct acoustic field alone and was unable to provide a realistic prediction of the measured, radiated sound field except at the first formant of the duct. In the second a simple description of sound generation due to the interaction of vortices with the duct exit was added. In this case much closer prediction of the measured values was achieved, leading to the conclusion that the interaction of the nonacoustic velocity fluctuation with the duct boundaries results in a significant additional source of acoustic energy located in the region of the duct exit.

The Influence of Downstream Passage on the Flow Within an Annular S-Shaped Duct

Experimental and numerical investigations were carried out to gain a better understanding of the flow characteristics within an annular S-shaped duct, including the influence of the shape of the downstream passage located at the exit of the duct on the flow. A duct with six struts and the same geometry as that used to connect the compressor spools on our new experimental small two-spool turbofan engine was investigated. Two types of downstream passage were used. One type had a straight annular passage and the other a curved annular passage with a meridional flow path geometry similar to that of the centrifugal compressor. Results showed that the total pressure loss near the hub is large due to instability of the flow, as compared with that near the casing. Also, a vortex related to the horseshoe vortex was observed near the casing. In the case of the curved annular passage, the total pressure loss near the hub was greatly increased compared with the case of the straight annular passage, and the spatial position of this vortex depends on the passage core pressure gradient. Furthermore, results of calculation using an in-house-developed three-dimensional Navier–Stokes code with a low-Reynolds-number k–ε turbulence model were in good qualitative agreement with experimental results. According to the simulation results, a region of very high pressure loss is observed near the hub at the duct exit with the increase of inlet boundary layer thickness. Such regions of high pressure loss may act on the downstream compressor as a large inlet distortion, and strongly affect downstream compressor performance.

Evidence of more efficient whistler-mode transmission during periods of increased magnetic activity

Abstract. In a previous study it was reported that whistler- mode signals received at Faraday, Antarctica (65°S,64°W) and Dunedin, New Zealand (46°S,171°E) with entry regions in Pacific longitudes (typically from the VLF transmitter NLK, Seattle, USA) showed an increase in transmission of wave energy as magnetic activity increased. However, signals with entry regions in Atlantic longitudes (typically from the NSS transmitter, Annapolis, USA) did not appear to show such a relationship. This paper reports the results of a study of the same two longitude ranges but with the opposite transmitter providing additional whistler-mode signal information, with L-values in the range 1.8–2.6. Transmissions from NLK once again indicate a relationship between the transmission of wave energy and magnetic activity even though the signals were propagating in Atlantic longitudes, not Pacific. Any trend in NSS events observed at Dunedin was obscured by a limited range of magnetic activity, and duct exit regions so close to the receiver that small-scale excitation effects appeared to be occurring. However, by combining data from both longitudes, i.e Pacific and Atlantic, and using only ducts with exit regions that were >500km from the receiver, NSS events were found to show the same trend as NLK events. No significant longitude-dependent or transmitter-dependent variations in duct efficiency could be detected. Duct efficiency increases by a factor of about 30 with Kp=2–8 and this result is discussed in terms of changes in wave-particle interactions and duct size.

Distributions of wave normals at the duct exit of a whistler and the morphology of ionospheric exits of low‐latitude whistlers observed by direction finding

Direction‐finding observations at low latitudes reveal that whistlers arrive from narrow regions of the zenith of tens of kilometers in diameter and that their ionospheric exits have a variety of shapes. The distribution of the wave normals of whistlers at the duct exit was studied using a field‐aligned duct model by ray tracing in order to examine the cause of the morphology of the ionospheric exits. The characteristics of the distribution and its relation to the trapping cone have been clarified. When the distribution is represented by plotting the wave normal dip angle against the escape position at the duct exit, two kinds of wave normal distributions are evident at the duct exit: One is a flat distribution of downward wave normals and has a region of almost vertically downward wave normals at the main area of the duct exit, and the other is a flat distribution of more upward wave normals. They can be explained, respectively, as the innermost direction and the outermost direction of a trapping cone. The region of vertical downward wave normals in the downward wave normal distribution occurs because of the vertical downward electron density gradient in the topside ionosphere and also because of the effects of the trapping cone shrinking on the whistler's passage through the ionospheric maximum electron density region and of the expansion of this cone in the lower ionosphere. This region of vertical downward wave normals is responsible for the whistler's penetration to the ground. Because the downgoing whistler's wave normals are oriented to the vertical downward direction uniformly at any point of the duct exit due to the downward wave normal distribution, whistler rays emitted from each point of the duct exit propagate in parallel and reach the bottom of the lower ionosphere. Thus the observed morphology of ionospheric exits can be interpreted as the result of the projection of the section of the duct exit onto the ionospheric lower boundary.

ACTIVE INTENSITY CONTROL FOR THE REDUCTION OF RADIATED DUCT NOISE

Mean intensity based active control for the cancellation of radiated noise out of the duct exit is studied. The active intensity control strategy is derived based on the relation of the exterior sound field radiated out of the duct termination and the interior sound field of the duct. One of the characteristics of this control strategy is that the maximum possible control performance can be maintained regardless of the sensor location, compared with the conventional local pressure control methods at either interior downstream or exterior field positions. This is a simple consequence of the active intensity at the interior downstream being not space-dependent as long as it is plane wave. A time-domain adaptive filtering method for the active intensity control is also suggested and experimental results for an open ended duct based on the adaptive filtering method are presented. For the purpose of practical comparison, experimental results for conventional sound pressure control based on the well known filtered-xLMS algorithm are also presented. The experimental results show the potential of the active intensity control strategy for reducing the emitted noise out of the duct exit.

Evidence of more efficient whistler-mode transmission during periods of increased magnetic activity

In a previous study it was reported that whistler- mode signals received at Faraday, Antarctica (65°S,64°W) and Dunedin, New Zealand (46°S,171°E) with entry regions in Pacific longitudes (typically from the VLF transmitter NLK, Seattle, USA) showed an increase in transmission of wave energy as magnetic activity increased. However, signals with entry regions in Atlantic longitudes (typically from the NSS transmitter, Annapolis, USA) did not appear to show such a relationship. This paper reports the results of a study of the same two longitude ranges but with the opposite transmitter providing additional whistler-mode signal information, with L-values in the range 1.8–2.6. Transmissions from NLK once again indicate a relationship between the transmission of wave energy and magnetic activity even though the signals were propagating in Atlantic longitudes, not Pacific. Any trend in NSS events observed at Dunedin was obscured by a limited range of magnetic activity, and duct exit regions so close to the receiver that small-scale excitation effects appeared to be occurring. However, by combining data from both longitudes, i.e Pacific and Atlantic, and using only ducts with exit regions that were >500km from the receiver, NSS events were found to show the same trend as NLK events. No significant longitude-dependent or transmitter-dependent variations in duct efficiency could be detected. Duct efficiency increases by a factor of about 30 with Kp=2–8 and this result is discussed in terms of changes in wave-particle interactions and duct size.

Finite Difference Time Marching in the Frequency Domain: A Parabolic Formulation for the Convective Wave Equation

An explicit finite difference iteration scheme is developed to study harmonic sound propagation in ducts. To reduce storage requirements for large 3D problems, the time dependent potential form of the acoustic wave equation is used. To insure that the finite difference scheme is both explicit and stable, time is introduced into the Fourier transformed (steady-state) acoustic potential field as a parameter. Under a suitable transformation, the time dependent governing equation in frequency space is simplified to yield a parabolic partial differential equation, which is then marched through time to attain the steady-state solution. The input to the system is the amplitude of an incident harmonic sound source entering a quiescent duct at the input boundary, with standard impedance boundary conditions on the duct walls and duct exit. The introduction of the time parameter eliminates the large matrix storage requirements normally associated with frequency domain solutions, and time marching attains the steady-state quickly enough to make the method favorable when compared to frequency domain methods. For validation, this transient-frequency domain method is applied to sound propagation in a 2D hard wall duct with plug flow.

Sound propagation and radiation in a curved duct

An investigation of sound propagation and radiation in a curved duct using a boundary integral method is presented. The duct of interest is assumed to be hard walled both inside and outside. To avoid interference between the fields radiated from both of its ends, the straight part of the duct is closed with a rigid disk. The acoustic source placed close to this disk simulates a spinning mode. Experiments were also carried out in an anechoic chamber for an identical duct geometry. Comparisons between numerical and experimental results are in good agreement. The modal analysis shows, in particular, that all propagating modes are present at the curved end, for a single pure mode at the source. This result is in agreement with previous analytical studies, which established that several angular modes are required to satisfy the Helmholtz equation in the curved part of the duct. As a result of this modal redistribution, the directivity patterns are not axisymmetrical around the duct exit centerline.