Nonparallel Effects on the Stability of Jet Flows

1978 ◽  
Vol 45 (4) ◽  
pp. 717-722 ◽  
Author(s):  
V. K. Garg ◽  
G. F. Round
Keyword(s):  
Transverse Velocity ◽  
Outer Region ◽  
Wave Number ◽  
Jet Flows ◽  
Frequency Range ◽  
Governing Equations ◽  
Low Frequencies ◽  
High Frequencies ◽  
The Stability ◽  
Asymptotic Forms

A theoretical analysis of the linear, spatial stability of Bickley’s jet is presented. The analysis takes into account the effects of transverse velocity component and the axial variations of the basic flow and of the disturbance amplitude, wavenumber and spatial growth rate. The integration of stability equations is started from the outer region of the jet toward the jet axis using the solution of the asymptotic forms of the governing equations. Results are compared with those for the parallel-flow stability analysis. It is found that the nonparallel effects decrease the wave number at low frequencies but increase it at high frequencies. Thus the nonparallel effects make Bickley’s jet unstable over a wider frequency range.

Linear Stability of Jet Flows

1977 ◽  
Vol 44 (3) ◽  
pp. 378-384 ◽  
Author(s):  
A. K. Bajaj ◽  
V. K. Garg
Keyword(s):  
Stability Theory ◽  
Outer Region ◽  
Parallel Flow ◽  
Jet Flows ◽  
Complex Eigenvalue ◽  
Spatial Stability ◽  
Laminar Jets ◽  
Flow Approximation ◽  
The Stability ◽  
Asymptotic Forms

A theoretical investigation into the linear, spatial stability of plane laminar jets is presented. The three cases studied are: 1. Inviscid stability of Sato’s velocity profile. 2. Viscous stability of the Bickley’s jet using parallel-flow stability theory. 3. Viscous stability of the Bickley’s jet using a theory modified to account for the inflow terms. The integration of stability equations is started from the outer region of the jet toward the jet axis using the solution of the asymptotic forms of the governing equations. An eigenvalue search technique is employed to find the number of eigenvalues and their approximate location in a closed region of the complex eigenvalue plane. The accurate eigenvalues are obtained using secant method. The inviscid spatial stability theory is found to give results that are in better agreement with Sato’s experimental results than those obtained by him after transformation of the temporal theory results. For the viscous case the critical Reynolds number found by using the theory accounting for inflow is in better agreement with the experimental value than that given by the parallel-flow theory, implying thereby that the parallel-flow approximation for a jet is erroneous for the stability analysis.

scholarly journals THE PECULIARITIES OF THE SOUND FIELD ENERGY DECAY IN A ROOM WITH THE USE OF DIFFERENT PULSED SOUND SOURCES/GARSO LAUKO ENERGIJOS SLOPIMO PATALPOJE YPATUMAI, NAUDOJANT SKIRTINGUS IMPULSINIUS GARSO ŠALTINIUS

1999 ◽  
Vol 5 (2) ◽  
pp. 135-140
Author(s):  
Vytautas Stauskis
Keyword(s):  
Noise Level ◽  
Sound Field ◽  
Maximum Energy ◽  
Reverberation Time ◽  
Frequency Range ◽  
Sound Sources ◽  
Low Frequencies ◽  
High Frequencies ◽  
The Difference ◽  
Field Decay

The paper deals with the differences between the energy created by four different pulsed sound sources, ie a sound gun, a start gun, a toy gun, and a hunting gun. A knowledge of the differences between the maximum energy and the minimum energy, or the signal-noise ratio, is necessary to correctly calculate the frequency dependence of reverberation time. It has been established by investigations that the maximum energy excited by the sound gun is within the frequency range of 250 to 2000 Hz. It decreases by about 28 dB at the low frequencies. The character of change in the energy created by the hunting gun differs from that of the sound gun. There is no change in the maximum energy within the frequency range of 63–100 Hz, whereas afterwards it increases with the increase in frequency but only to the limit of 2000 Hz. In the frequency range of 63–500 Hz, the energy excited by the hunting gun is lower by 15–30 dB than that of the sound gun. As frequency increases the difference is reduced and amounts to 5–10 dB. The maximum energy of the start gun is lower by 4–5 dB than that of the hunting gun in the frequency range of up to 1000 Hz, while afterwards the difference is insignificant. In the frequency range of 125–250 Hz, the maximum energy generated by the sound gun exceeds that generated by the hunting gun by 20 dB, that by the start gun by 25 dB, and that by the toy gun—by as much as 35 dB. The maximum energy emitted by it occupies a wide frequency range of 250 to 2000 Hz. Thus, the sound gun has an advantage over the other three sound sources from the point of view of maximum energy. Up until 500 Hz the character of change in the direct sound energy is similar for all types of sources. The maximum energy of direct sound is also created by the sound gun and it increases along with frequency, the maximum values being reached at 500 Hz and 1000 Hz. The maximum energy of the hunting gun in the frequency range of 125—500 Hz is lower by about 20 dB than that of the sound gun, while the maximum energy of the toy gun is lower by about 25 dB. The maximum of the direct sound energy generated by the hunting gun, the start gun and the toy gun is found at high frequencies, ie at 1000 Hz and 2000 Hz, while the sound gun generates the maximum energy at 500 Hz and 1000 Hz. Thus, the best results are obtained when the energy is emitted by the sound gun. When the sound field is generated by the sound gun, the difference between the maximum energy and the noise level is about 35 dB at 63 Hz, while the use of the hunting gun reduces the difference to about 20–22 dB. The start gun emits only small quantities of low frequencies and is not suitable for room's acoustical analysis at 63 Hz. At the frequency of 80 Hz, the difference between the maximum energy and the noise level makes up about 50 dB, when the sound field is generated by the sound gun, and about 27 dB, when it is generated by the hunting gun. When the start gun is used, the difference between the maximum signal and the noise level is as small as 20 dB, which is not sufficient to make a reverberation time analysis correctly. At the frequency of 100 Hz, the difference of about 55 dB between the maximum energy and the noise level is only achieved by the sound gun. The hunting gun, the start gun and the toy gun create the decrease of about 25 dB, which is not sufficient for the calculation of the reverberation time. At the frequency of 125 Hz, a sufficiently large difference in the sound field decay amounting to about 40 dB is created by the sound gun, the hunting gun and the start gun, though the character of the sound field curve decay of the latter is different from the former two. At 250 Hz, the sound gun produces a field decay difference of almost 60 dB, the hunting gun almost 50 dB, the start gun almost 40 dB, and the toy gun about 45 dB. At 500 Hz, the sound field decay is sufficient when any of the four sound sources is used. The energy difference created by the sound gun is as large as 70 dB, by the hunting gun 50 dB, by the start gun 52 dB, and by the toy gun 48 dB. Such energy differences are sufficient for the analysis of acoustic indicators. At the high frequencies of 1000 to 4000 Hz, all the four sound sources used, even the toy gun, produce a good difference of the sound field decay and in all cases it is possible to analyse the reverberation process at varied intervals of the sound level decay.

scholarly journals Is the Frequency Content of the Calls in North American Treefrogs Limited by Their Larynges?

2014 ◽  
Vol 2014 ◽  
pp. 1-11 ◽  
Author(s):  
Marcos Gridi-Papp
Keyword(s):  
Frequency Modulation ◽  
Sound Intensity ◽  
Frequency Content ◽  
Frequency Range ◽  
Low Frequencies ◽  
High Frequencies ◽  
Mating Calls ◽  
Limited Frequency ◽  
Positive Direct ◽  
High Diversity

A high diversity of mating calls is found among frogs. The calls of most species, however, are simple, in comparison to those of mammals and birds. In order to determine if the mechanics of the larynx could explain the simplicity of treefrog calls, the larynges of euthanized males were activated with airflow. Laryngeal airflow, sound frequency, and sound intensity showed a positive direct relationship with the driving air pressure. While the natural calls of the studied species exhibit minimal frequency modulation, their larynges produced about an octave of frequency modulation in response to varying pulmonary pressure. Natural advertisement calls are produced near the higher extreme of frequency obtained in the laboratory and at a slightly higher intensity (6 dB). Natural calls also exhibit fewer harmonics than artificial ones, because the larynges were activated with the mouth of the animal open. The results revealed that treefrog larynges allow them to produce calls spanning a much greater range of frequencies than observed in nature; therefore, the simplicity of the calls is not due to a limited frequency range of laryngeal output. Low frequencies are produced at low intensities, however, and this could explain why treefrogs concentrate their calling at the high frequencies.

scholarly journals Beam Patterns of the Five-hundred-metre Aperture Spherical Telescope

2013 ◽  
Vol 30 ◽  
Author(s):  
B. Dong ◽  
J. L. Han
Keyword(s):  
Structural Parameters ◽  
Power Level ◽  
Three Dimensional ◽  
High Gain ◽  
Main Beam ◽  
Frequency Range ◽  
Ray Method ◽  
Low Frequencies ◽  
High Frequencies ◽  
Beam Patterns

AbstractThe Five-hundred-metre Aperture Spherical Telescope (FAST) is being constructed in China. With an illuminated aperture of 300-m diameter, it will be the most sensitive single-dish radio telescope in the world. We calculate the beam patterns, gains, and efficiencies of the FAST at 200 MHz, 1.4 GHz, and 3 GHz. A program is developed to calculate the structural parameters and construct the FAST models. The three-dimensional beam patterns are calculated by utilising the shooting and bouncing ray method. We show that, with a coaxial horn feed, the FAST has a beam pattern of high gain and reasonably low first sidelobe over the frequency range of 200 MHz to 3 GHz. Compared with an ideal 300-m parabolic reflector, the un-illuminated spherical part of the FAST would make the power level near both sides of the main beam rise by at least 20 dB and the efficiency tends to decrease at high frequencies. At a zenith angle of 0°, its efficiencies at 200 MHz, 1.4 GHz, and 3 GHz are 71.72%, 66.94%, and 57.55%, respectively. We conclude that the FAST is an excellent telescope at low frequencies. At high frequencies, the triangular spherical panels and the gaps between panels are important factors that affect the performance of the FAST.

The behaviour of ferromagnetic sheets in alternating electric and magnetic fields II. Measurements of the electrical resistance of iron strips from 0 to 30 Mc/s

1963 ◽  
Vol 276 (1364) ◽  
pp. 112-124 ◽  
Keyword(s):  
Domain Walls ◽  
Current Flow ◽  
Preceding Paper ◽  
Domain Theory ◽  
Frequency Range ◽  
Low Frequencies ◽  
High Frequencies ◽  
Electric And Magnetic Fields ◽  
Restoring Forces ◽  
Broad Outline

Resistance measurements made on iron strips of thicknesses ranging from 7.1 to 92-9pm over the frequency range 15 kc/s to 30 Mc/s are presented together with supplementary measurements of the permeability and loss angle on rings cut from the same strip. Both strips and rings had been subjected to the same thorough anneal. A special specimen geometry (Bishop 1962) was employed to ensure current uniformity across the strip width at all frequencies. These measurements were designed for comparison with the predictions of the classical uniform permeability theory and the domain eddy current theories developed in the preceding paper. The measured losses are found to be much higher at low frequencies and somewhat lower at high frequencies than those calculated by the classical theory, especially with the thinner specimens. In this respect these observations agree in broad outline with the predictions of the simplest domain theory (which assumes identical equally spaced domain walls normal to the current flow) but the transition from square law to square-root law dependence of the losses on frequency is found to take place over a much wider frequency range than either theory predicts. The breadth of this transition is however satisfactorily interpreted in terms of a more refined domain theory which takes into account the very wide spread of domain wall restoring forces which is to be expected in the light of contemporary knowledge of the domain structure of iron.

Numerical Computation of the Stability of Plane Poiseuille Flow

1978 ◽  
Vol 45 (1) ◽  
pp. 13-18 ◽  
Author(s):  
L. Wolf ◽  
Z. Lavan ◽  
H. J. Nielsen
Keyword(s):  
Stream Function ◽  
Poiseuille Flow ◽  
Numerical Technique ◽  
Finite Amplitude ◽  
Wave Number ◽  
Plane Poiseuille Flow ◽  
Governing Equations ◽  
The Stability ◽  
Successive Over Relaxation ◽  
Small Disturbances

The hydrodynamic stability of plane Poiseuille flow to infinitesimal and finite amplitude disturbances is investigated using a direct numerical technique. The governing equations are cast in terms of vorticity and stream function using second-order central differences in space. The vorticity equation is used to advance the vorticity values in time and successive over-relaxation is used to solve the stream function equation. Two programs were prepared, one for the linearized and the other for the complete disturbance equations. Results obtained by solving the linearized equations agree well with existing solutions for small disturbances. The nonlinear calculations reveal that the behavior of a disturbance depends on the amplitude and on the wave number. The behavior at wave numbers below and above the linear critical wave number is drastically different.

scholarly journals A Unified Methodology to Evaluate the Radiated Noise Due to Turbulent Boundary Layer Flows

2018 ◽  
Vol 141 (6) ◽  
Author(s):  
Sylvain Morilhat ◽  
François Chedevergne ◽  
Frank Simon
Keyword(s):  
Boundary Layer ◽  
Wave Number ◽  
Frequency Range ◽  
Boundary Layer Flows ◽  
Frequency Wave ◽  
High Frequencies ◽  
Radiated Noise ◽  
Computational Methodology ◽  
Turbulent Wall ◽  
Large Frequency

For vibro-acoustic applications, a turbulent wall pressure (TWP) fluctuations model was derived. The model is based on the resolution of Poisson's equation. The pressure is characterized in time and space through its spectrum in the frequency wave-number domain. The developed model follows trends commonly observed using Corcos model in a large frequency range but also shows new behaviors for low and high frequencies. The radiated noise due to TWP fluctuations is then computed in accordance with the form of the TWP spectrum. A specific computational methodology is proposed to perform the calculation without introducing limiting hypothesis on the radiated impedance.

Effects of the Substrate on the Response of P(VDF-TrFE) Pyroelectric Detector

1994 ◽  
Vol 299 ◽  
Author(s):  
A. Mahrane ◽  
M. Djafari-Rouhani ◽  
A. Najmi ◽  
D. Esteve
Keyword(s):  
Thermal Analysis ◽  
Response Time ◽  
Diffusion Length ◽  
Pyroelectric Detector ◽  
Frequency Range ◽  
Optimal Thickness ◽  
Low Frequencies ◽  
Trade Off ◽  
High Frequencies ◽  
Thermal Insulator

AbstractThe thermal analysis on the response of pyroelectric detector revealed that an optimal configuration structure could be found for a certain frequency range. Thus, it appeared that the pyroelectric layer should be thick at low frequencies but it should be very thin at high frequencies. The introduction of a thermal insulator is done trying to find out a trade off between sensitivity and response time. It has been found that its influence is restricted to a certain frequency range. The optimal thickness is related to the thermal diffusion length. We noticed also that it depends very little on the configuration structure.

Transmission of Airborne Sound from 50-20,000 Hz into the Abdomen of Sheep

1993 ◽  
Vol 12 (1) ◽  
pp. 16-24 ◽  
Author(s):  
Aemil J.M. Peters ◽  
Robert M. Abrams ◽  
Kenneth J. Gerhardt ◽  
Scott K. Griffiths
Keyword(s):  
Pregnant Woman ◽  
Occupational Health ◽  
Health Professionals ◽  
Sound Pressure ◽  
Stimulus Level ◽  
Pressure Level ◽  
Sound Attenuation ◽  
Frequency Range ◽  
Low Frequencies ◽  
High Frequencies

The transmission of audible sounds from the environment of the pregnant woman to the foetus is of growing interest to obstetricians who utilize foetal vibracoustic stimulation in their examinations, and to occupational health professionals who believe that high-intensity sound in the workplace is potentially damaging to the foetus. Earlier reports on transmission of sound into the abdomen and uterus of sheep revealed a significant amount of sound attenuation at frequencies above 2,000 Hz. and some enhancement at frequencies below 250 Hz. However, frequencies above 10,000 Hz, and stimulus levels as possible variables, were not studied. In this report, the effects of frequency from 50-20,000 Hz. and stimulus levels (90 to 110 dB sound pressure level), were studied in five sheep. Sound attenuation varied as a function of frequency (p<0.001). Sound attenuation varied inversely as a function of stimulus level for low frequencies (50-125 Hz) and for high frequencies (7,000–20,000 Hz) (p<0.001). In the mid frequency range (200-4,000 Hz), no effect of stimulus level (p=0.96) was found. Additionally, in the 800-2,000 Hz range there was enhancement of sound pressure of up to 10 dB.

Metachronal Co-ordination of the Comb Plates of the Ctenophore Pleurobrachia

1968 ◽  
Vol 48 (1) ◽  
pp. 111-125
Author(s):  
M. A. SLEIGH
Keyword(s):  
Wave Velocity ◽  
Sea Water ◽  
Wide Frequency Range ◽  
Relative Importance ◽  
Frequency Range ◽  
Mechanical Contact ◽  
Low Frequencies ◽  
High Frequencies ◽  
Common Features ◽  
Rapid Transmission

1. The activity of comb plates of Pleurobrachia was analysed from cine films. 2. The interval between successive beats of a comb plate varied from several seconds to about 50 msec. in normal sea water; this variation in frequency was accompanied by a variation wave velocity. There was an almost linear relationship between wave velocity and frequency, ranging from a minimal wave velocity of about 20 mm./sec. at low frequencies to a maximal wave velocity of about 80 mm./sec. at the highest frequencies. 3. The wave velocity was accelerated at low frequencies by increased Mg2+ concentrations, by ouabain and by curare, and the same substances decreased the wave velocity at high frequencies. 4. The frequency of beat was accelerated by certain concentrations of adrenaline, serotonin, Mg2+, ouabain and curare. Decreases of frequency were found in acetylcholine (and eserine) and strychnine. These substances act on the excitability of the pacemaker. 5. It is concluded that metachronal transmission is by a flow of electric current from one comb plate to the next. Depolarization, either by a recent excitation of the comb plate or by agents like ouabain causes increased excitability and a more rapid transmission. The minimum phase difference between adjacent comb plates was about 5 msec. 6. The comb plates are almost entirely dependent on a conducted impulse for their excitation; occasionally a spontaneous excitation occurs. The membranelles of Stentor depend more on excitation within the ciliary base and less on the conducted impulse. The two systems are believed to share common features, but differ in the relative importance of these features. 7. Reverse waves involving the beating of comb plates in the normal direction were observed. The wave velocity of these waves increased only slightly over a wide frequency range. Reverse waves of this type are believed to be transmitted by mechanical contact between the comb plates.

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