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1.
Results of the experimental studies of the commutation characteristics of an electroexplosive breaker upon heating of a copper
foil by a current pulse of duration τ≈1msec are given. The dependences of the electric-field strength and the specific power of the circuit breaker during an electroexplosion
of the foil on the specific power of the source Ps are found and it is shown that the transition of the plasma-free regime of electroexplosion of the foil to a plasma regime
occurs for Ps=30–40GW/g.
Institute of Experimental Physics, Sarov 607190. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 41, No.
1, pp. 14–20, January–February, 2000. 相似文献
2.
An Australian hard wheat flour–water dough has been characterised using parallel plate and capillary rheometers over an extensive
range of apparent shear rates (10 − 3–103 s − 1) relevant to process conditions. Torsional measurements showed that the shear viscosity of the dough increased with strain
to a maximum value and then decreased, suggesting a breakdown of the dough structure. Both torsional and capillary experiments
revealed the shear-thinning behaviour of the dough. The wall slip phenomenon in capillary rheometry was investigated and found
to be diameter dependent and occurred at a critical shear stress of approximately 5–10 kPa. A two-regime power law behaviour
was observed, with the power law index approximately 0.3 in the low shear rate range increasing to 0.67 in the high shear
rate range. Pressure fluctuation was observed in the capillary data and increased with shear rate, in particular, at shear
rates approaching 104 s − 1. The results demonstrate that capillary rheometry is a viable means of rheologically testing dough at high shear rates provided
pressure fluctuation is carefully monitored and capillary rheometry corrections, including wall slip, are accounted for. 相似文献
3.
The results of experimental investigations of bed flow hydrodynamics in spouted beds are compared with CFD simulations (Eulerian–Eulerian
approach) for two different column geometries. The experimental results of bed porosity and fluctuation frequency of mass
flow rate of grain in the fountain region are compared with the corresponding results of simulations. The simulation results
confirmed the observations of Muir et al. (1990, Chem. Eng. Comm. 88: 153–171) and Yang and Keairns (1978, AlchE Symp. Ser. No. 176 74: 218) that fluctuations of bed flow in DTSB are caused by particle cluster formation in the loading region at the bottom
of column. The solids cross into the jet and cover the column inlet and are carried upward periodically through a draft tube.
Subsequent figures obtained from simulations, which show stages of particle cluster formation at the entrance of column, exactly
match visual observations. The frequency of fluctuations of grain mass flow rate predicted in simulations (~5–6 Hz) is in
the range of that experimentally determined. The fluctuating inflow of solids results in slug formation and explains the vertical
variations of height and porosity of the fountain. 相似文献
4.
Measurements of two-components of velocity in the wake of a square cylinder using a hot-wire anemometer are reported. Two
Reynolds numbers, namely 8700 and 17,625, have been considered. The measurements were carried out in a low-speed, low-turbulence
wind tunnel. Benchmark experiments at much lower Reynolds numbers show good agreement between the present experiments and
published results. At higher Reynolds numbers, the experimental data reveal anticipated trends in terms of wake recovery and
turbulence decay. Both velocity and velocity fluctuations show symmetry about the wake axis. The experimental data have been
compared with the large eddy simulation (LES) calculation reported by Wang et al. [University of Illinois at Urbana – Champaign
(1996) Report CFD 96-03] and LDV measurements of Lyn et al. [J Fluid Mech (1995) 304: 285–319]. The agreement among the three
sets is generally acceptable in terms of the time-averaged velocity components, but not the velocity fluctuations. The turbulence
fluctuations in the present experiments are seen to be lower than in the referred work. The differences have been traced to
factors such as the aspect ratio, blockage ratio and upstream turbulence. Experiments with increased upstream turbulence did
show a reduction in the discrepancy between the present experiments and the published data. An assessment of the experimental
data in terms of physical mechanisms revealed that (a) streamwise normal stresses were correlated with the vortex centers,
and (b) the turbulence kinetic energy profiles are similar to the turbulence shear stress. Spectral analysis of the velocity
signals was carried out in the present work. Energy transfer from the mean flow to the streamwise velocity fluctuation was
confirmed in the near wake. A redistribution of the kinetic energy between the streamwise and transverse components of velocity
over a longer distance downstream was subsequently observed.
Received: 17 May 1999/Accepted: 29 December 1999 相似文献
5.
The characteristics of natural fluctuations of density in a laminar near wake behind a sharp cone in a hypersonic flow of
nitrogen at zero incidence are studied by the method of electronbeam fluorescence at Mach numberM=21 and unit Reynolds numberRe
1=6·105 m−1. The distributions of the mean density, integral fluctuations, and spectra of density fluctuations are obtained, the longitudinal
and azimuthal phase velocities of perturbations are determined, and the growth rates of perturbations in the wake are found.
The results are compared with the measurement data in the shock layer on a flat plate.
Institute of Theoretical and Applied Mechanics, Siberian Division, Russian Academy of Sciences, Novosibirsk 630090. Translated
from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 41, No. 3, pp. 111–117, May–June, 2000 相似文献
6.
Delayed detached eddy simulation of the end-effect regime and side-loads in an overexpanded nozzle flow 总被引:2,自引:1,他引:1
Sébastien Deck 《Shock Waves》2009,19(3):239-249
The separated flow in an overexpanded nozzle featuring a restricted shock separation is investigated numerically using delayed
detached eddy simulation and compared with the experimental data of Nguyen et al. (Int J Flow Turbul Combust 71(1):161–181,
2003). First, the enormous cost of a Large Eddy Simulation for such a nozzle flow is assessed before being performed to motivate
the practical need for using an hybrid RANS/LES method. The calculation is then used to investigate the “end-effect” regime
which involves a strong global unsteadiness with very large amplitude fluctuations of about 15–20% of nozzle divergent length.
The flow regime is characterized by high wall pressure fluctuations which are hopefully nearly axisymmetric. The main properties
(rms levels, amplitude of displacement of the separation) of the motion are rather well reproduced by DDES compared to the
experiment. However, a major difference lies in the frequency of the computed motion which is higher than in the experiment.
This major discrepancy is currently not explained by the author. The properties of the side-loads are also briefly discussed.
相似文献
7.
Large-Eddy simulations (LES) of spatially evolving turbulent buoyant round jets have been carried out with two different density
ratios. The numerical method used is based on a low-Mach-number version of the Navier–Stokes equations for weakly compressible
flow using a second-order centre-difference scheme for spatial discretization in Cartesian coordinates and an Adams–Bashforth
scheme for temporal discretization. The simulations reproduce the typical temporal and spatial development of turbulent buoyant
jets. The near-field dynamic phenomenon of puffing associated with the formation of large vortex structures near the plume
base with a varicose mode of instability and the far-field random motions of small-scale eddies are well captured. The pulsation
frequencies of the buoyant plumes compare reasonably well with the experimental results of Cetegen (1997) under different
density ratios, and the underlying mechanism of the pulsation instability is analysed by examining the vorticity transport
equation where it is found that the baroclinic torque, buoyancy force and volumetric expansion are the dominant terms. The
roll-up of the vortices is broken down by a secondary instability mechanism which leads to strong turbulent mixing and a subsequent
jet spreading. The transition from laminar to turbulence occurs at around four diameters when random disturbances with a 5%
level of forcing are imposed to a top-hat velocity profile at the inflow plane and the transition from jet-like to plume-like
behaviour occurs further downstream. The energy-spectrum for the temperature fluctuations show both −5/3 and −3 power laws,
characteristic of buoyancy-dominated flows. Comparisons are conducted between LES results and experimental measurements, and
good agreement has been achieved for the mean and turbulence quantities. The decay of the centreline mean velocity is proportional
to x
−1/3 in the plume-like region consistent with the experimental observation, but is different from the x
−1 law for a non-buoyant jet, where x is the streamwise location. The distributions of the mean velocity, temperature and their fluctuations in the near-field
strongly depend upon the ratio of the ambient density to plume density ρa/ρ0. The increase of ρa/ρ0 under buoyancy forcing causes an increase in the self-similar turbulent intensities and turbulent fluxes and an increase
in the spatial growth rate. Budgets of the mean momentum, energy, temperature variance and turbulent kinetic energy are analysed
and it is found that the production of turbulence kinetic energy by buoyancy relative to the production by shear is increased
with the increase of ρa/ρ0.
Received 16 June 2000 and accepted 26 June 2001 相似文献
8.
A method for observing near-surface fluctuations in pH caused by a water–air flux of carbon dioxide under conditions of ambient
atmospheric carbon dioxide levels is developed and tested. Peaks in fluorescence intensity measured as a function of pH and
turbulence are shown to be consistent with predictions from a chemical kinetics model of CO2 exchange. The square root of the frequency of the pH fluctuations scale linearly with independently measured bulk air–water
gas transfer velocities in agreement with surface divergence models for air–water gas transfer. These data indicate that the
method proposed here is tracking changes in near-surface CO2 concentrations. This laser-induced fluorescence method can be used to study the air–water exchange of CO2 in wind-wave tunnels without the need for elevated CO2 concentrations in the gas phase. 相似文献
9.
N. I. Bolonov V. U. Bondarchuk I. A. Vatutin B. A. Kolovandin V. G. Lobachev 《Journal of Applied Mechanics and Technical Physics》1998,39(4):536-545
The results of experimental and numerical studies of the dynamics of the parameters of uniform turbulence of a stably stratified
fluid for different molecular Prandtl-Schmidt numbers over a wide range of buoyancy times Nτ are given. The tank, the measurement
apparatus used, and the experimental procedure are briefly described. The numerical modeling used a second-order model of
uniform turbulence of a stratified medium. The influence of fluctuations of the turbulent mass (heat) flux q(Nτ) on the evolution
of the statistical parameters of the velocity and temperature fields is analyzed, and an invariant equation is found for the
parameters of the strong turbulence of the stratified fluid. It is shown that the statistical parameters of the turbulence,
being smoothed with respect to the amplitude of the fluctuations, vary self-similarly with time after the collapse point.
Donetsk State University, Donetsk 340055.1Institute of Heat and Mas Transfer, National Academy of Sciences of Belarusia, Minsk 220072. Translated from Prikladnaya Mekhanika
i Tekhnicheskaya Fizika, Vol. 39, No. 4, pp. 64–75, July–August, 1998. 相似文献
10.
A study of some aspects of tracer particle responses to step changes in fluid velocity is presented. The effect of size distribution
within a seed material on measured relaxation time is examined, with polydisperse particles of the same median diameter shown
to possess a significantly higher relaxation time than their monodisperse counterparts when measured via a particle image
velocimetry algorithm. The influence of a shock wave–induced velocity gradient within a PIV interrogation window on the correlation
function is also examined using the noiseless cross-correlation function of Soria (Turbulence and coherent structures in fluids,
plasmas and nonlinear media. World Scientific, Singapore, 2006). The presence of a shock is shown to introduce an artificial fluctuation into the measurement of velocity. This fluctuation
is a function of the shock position, shock strength, spatial ratio and particle distribution. When the shock is located at
the middle of the window, the magnitude of the fluctuation increases monotonically with increasing spatial ratio, increases
asymptotically with shock strength, and decreases for increasing particle polydispersity. When the shock is located at the
left-hand edge of the window, the magnitude of the artificial fluctuation is highest for intermediate spatial ratios, going
to zero at infinitely high and low values. In this instance, particle polydispersity acts to increase the magnitude of fluctuations
in measured velocity. In both cases, particle polydispersity serves to broaden the PDF of measured velocity. For the cases
presented herein, with a shock located within the interrogation window, the root mean square of the artificial velocity fluctuations
reaches values in excess of 30% of the freestream velocity. 相似文献
11.
The propagation mechanism of high speed turbulent deflagrations 总被引:2,自引:0,他引:2
The propagation regimes of combustion waves in a 30 cm by 30 cm square cross–sectioned tube with an obstacle array of staggered
vertical cylindrical rods (with BR=0.41 and BR=0.19) are investigated. Mixtures of hydrogen, ethylene, propane, and methane with air at ambient conditions over a range
of equivalence ratios are used. In contrast to the previous results obtained in circular cross–sectioned tubes, it is found
that only the quasi–detonation regime and the slow turbulent deflagration regimes are observed for ethylene–air and for propane–air.
The transition from the quasi–detonation regime to the slow turbulent deflagration regime occurs at (where D is the tube “diameter” and is the detonation cell size). When , the quasi–detonation velocities that are observed are similar to those in unobstructed smooth tubes. For hydrogen–air mixtures,
it is found that there is a gradual transition from the quasi–detonation regime to the high speed turbulent deflagration regime.
The high speed turbulent deflagration regime is also observed for methane–air mixtures near stoichiometric composition. This
regime was previously interpreted as the “choking” regime in circular tubes with orifice plate obstacles. Presently, it is
proposed that the propagation mechanism of these high speed turbulent deflagrations is similar to that of Chapman–Jouguet
detonations and quasi-detonations. As well, it is observed that there exists unstable flame propagation at the lean limit
where . The local velocity fluctuates significantly about an averaged velocity for hydrogen–air, ethylene–air, and propane–air mixtures.
Unstable flame propagation is also observed for the entire range of high speed turbulent deflagrations in methane–air mixtures.
It is proposed that these fluctuations are due to quenching of the combustion front due to turbulent mixing. Quenched pockets
of unburned reactants are swept downstream, and the subsequent explosion serves to overdrive the combustion front. The present
study indicates that the dependence on the propagation mechanisms on obstacle geometry can be exploited to elucidate the different
complex mechanisms of supersonic combustion waves.
Received 5 November 2001 / Accepted 12 June 2002 / Published online 4 November 2002
Correspondence to: J. Chao (e-mail: jenny.chao@mail.mcgill.ca)
An abridged version of this paper was presented at the 18th Int. Colloquium on the Dynamics of Explosions and Reactive Systems
at Seattle, USA, from July 29 to August 3, 2001. 相似文献
12.
V. A. Vishnyakov V. G. Zasetskii R. K. Karavosov A. G. Prozorov L. I. Sokolinskii 《Fluid Dynamics》1998,33(2):238-244
The initiation of narrow-band pressure fluctuations in the duct of a pipeline gas-compression station when the flow is turned
near a blind-ended cavity was studied in a wind tunnel. The flow pattern under consideration was estimated from visualization
of the flow in a water channel using the hydraulic analogy technique. It is believed that the high-intensity narrow-band pressure
fluctuations observed in the flow result from the instability and regular rearrangement of large-scale vortex structures in
the blind-ended cavity.
Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 2, pp. 104–111, March–April, 1998. 相似文献
13.
We establish a Navier–Stokes–Fourier limit for solutions of the Boltzmann equation considered over any periodic spatial domain
of dimension two or more. We do this for a broad class of collision kernels that relaxes the Grad small deflection cutoff
condition for hard potentials and includes for the first time the case of soft potentials. Appropriately scaled families of
DiPerna–Lions renormalized solutions are shown to have fluctuations that are compact. Every limit point is governed by a weak
solution of a Navier–Stokes–Fourier system for all time. 相似文献
14.
The effect of rarefaction on self-excited oscillations in impact supersonic jets (underexpanded jets interacting with a normally
located, planar, bounded obstacle) is investigated. Amplitude-frequency characteristics of pressure fluctuations at the obstacle
are obtained.
Baltiisk State Engineering University, St.-Petersburg 198005. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika,
Vol. 40, No. 3, pp. 109–115, May–June, 1999. 相似文献
15.
Two-point density measurements by laser induced Rayleigh scattering are used in this study to fully characterise the scalar
field in a Bunsen type turbulent premixed flame. The two points are separated within the flame brush in the axial or radial
directions. Correlation coefficients are obtained by comparing the evolution of one-point density fluctuations in time or
the two-point density fluctuations in both space and time. Time and length scales of the scalar field, and the mean convection
velocity of the turbulent scalar structures are deduced from these correlation coefficients. Time scales are calculated from
the auto-correlation coefficients, length scales are determined from the space correlation coefficients and the mean convection
velocity of the scalar structures in the axial direction is deduced from the space–time correlation coefficients. The relevance
of these results for analysing and modelling the structure of turbulent premixed flames is discussed.
Received: 30 April 1996 / Accepted: 2 September 1997 相似文献
16.
Shock tube study of n-decane ignition at low pressures 总被引:1,自引:0,他引:1
Xiao-Fei Nie Ping Li Chang-Hua Zhang Wei Xie Cong-Shan Li Xiang-Yuan Li 《Acta Mechanica Sinica》2012,28(1):79-82
Ignition delay times for n-decane/O 2 /Ar mixtures were measured behind reflected shock waves using endwall pressure and CH* emission measurements in a heated shock tube. The initial postshock conditions cover pressures of 0.09-0.26 MPa, temperatures of 1 227-1 536 K, and oxygen mole fractions of 3.9%-20.7% with an equivalence ratio of 1.0. The correlation formula of ignition delay dependence on pressure, temperature, and oxygen mole fraction was obtained. The current data are in good agreement with available low-pressure experimental data, and they are then compared with the prediction of a kinetic mechanism. The current measurements extend the kinetic modeling targets for the n-decane combustion at low pressures. 相似文献
17.
A formally possible mechanism is considered for the development of longwave perturbations of flow of a thin layer of a heat-conducting
viscous liquid with a free boundary, whose characteristic feature is that the Marangoni stresses occurring at the boundary
are induced by variations in the internal energy of the interface. The effect of surface internal energy fluctuations on the
layer flow in the approximation considered is of a dispersive character, and, in particular, it can facilitate regularization
of wave regimes.
Institute of Computer Simulation, Siberian Division, Russian Academy of Sciences, Krasnoyarsk 660036. Translated from Prikladnaya
Mekhanika i Tekhnicheskaya Fizika, Vol. 40, No. 1, pp. 10–21, January–February, 1999. 相似文献
18.
A numerical model of the dynamics of turbulence and temperature fluctuations behind a heated grid located in a wind tunnel
is constructed on the basis of closed Kármán-Howarth and Corrsin equations. Results calculated by this model are in reasonable
agreement with available experimental data.
__________
Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 50, No. 3, pp. 118–126, May–June, 2009. 相似文献
19.
Velocity statistics along the stagnation line of an axi-symmetric wall stagnating turbulent flow are studied experimentally.
A low turbulence, uniform air flow from a nozzle type air supply with an exit diameter of 50 mm stagnates at a wall located
50 mm downstream. A flow velocity is set to 3 m/s, 10 mm downstream from the exit of the air supply. Instantaneous values
of streamwise and radial velocities are measured by laser-Doppler velocimetry. The turbulence level in the air flow is changed
by use of turbulence generator. When the turbulence generator is not installed in the air supply, the mean velocity profile
in the streamwise direction fits well with that of a laminar viscous flow with the rms value of velocity fluctuations low
near the wall. With the turbulence generator installed, a significant turbulence structure appears near the wall. When the
wall is approached, the rms value of velocity fluctuations in the streamwise direction decreases monotonically while the profile
of the rms value in the radial direction reaches a maximum near the wall. The increase in the rms value of velocity fluctuations
in the radial direction near the wall is attributed to the bi-modal histogram of the fluctuating velocity in the radial direction.
Near the wall, the instantaneous stagnation streamline fluctuates and the probability of the mean location of the stagnation
point reaches a maximum not at the stagnation line but on a circle around the stagnation line, resulting in the bi-modal histogram.
Turbulence statistics, the rms value of velocity fluctuation and the turbulent kinetic energy, can be normalized successfully
by similarity parameters based on the strain rate and the reference turbulent kinetic energy introduced by Champion and Libby.
Received: 7 April 1995/Accepted: 27 September 1996 相似文献
20.
A coarse-grained model developed for entangled polymeric systems and calibrated to represent melts in equilibrium (Rakshit,
Picu, J Chem Phys 125:164907(1)–(10), 2006) is used to model shear flows. The model is a hybrid between multimode and mean-field representations: chain inner blobs
are constrained to move along the chain backbone and the end blobs are free to move in 3D and continuously redefine the diffusion
path for the inner blobs. Therefore, contour length fluctuations and reptation are captured. Constraint release is implemented
by tracing the position of chain ends and performing a local relaxation of the chain backbones once end retraction is detected.
This algorithm takes advantage of the multi-body nature of the model and requires no phenomenological parameters other than
the length of an entanglement segment. The model is used to study start-up and step strain shear flows and reproduces features
observed experimentally such as the overshoot during start-up shear flow, the Lodge–Meissner law, the monotonicity of the
steady state shear stress with the strain rate, and shear thinning at large . These simulations are performed in conditions in which using a fully refined model of the same system would have been extremely
computationally demanding or simply impossible with the current methods. 相似文献