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181.
182.
为了在较弱的条件下研究一类具有内激波层现象的二次Dirichlet问题.用合成展开法构造出该问题的一阶渐近表达式,并利用不动点定理证明了解的存在性及其当ε→0时的渐近性质。在一定程度上,它比起传统的微分不等式方法放宽了对所提问题的条件限制。 相似文献
183.
Experimental results presented in this work enable us to classify the three-dimensional structure of the detonation into
two fundamental types: a rectangular structure and a diagonal structure. The rectangular structure is well documented in the
literature and consists of orthogonal waves travelling independently from each another. The soot record in this case shows
the classical diamond detonation cell exhibiting ‘slapping waves’. The experiments indicate that the diagonal structure is
a structure with the triple point intersections moving along the diagonal line of the tube cross section. The axes of the
transverse waves are canted at 45 degrees to the wall, accounting for the lack of slapping waves. It is possible to reproduce
these diagonal structures by appropriately controlling the experimental ignition procedure. The characteristics of the diagonal
structure show some similarities with detonation structure in round tube. Pressure measurements recorded along the central
axis of the cellular structure show a series of pressure peaks, depending on the type of structure and the position inside
the detonation cell. Pressure profiles measured for the whole length of the two types of detonation cells show that the intensity
of the shock front is higher and the length of the detonation cell is shorter for the diagonal structures.
Received 17 May 2000 / Accepted 29 November 2000 相似文献
184.
Investigations of gas and particle dynamics in first generation needle-free drug delivery devices 总被引:1,自引:0,他引:1
Abstract. Transdermal powdered drug delivery involves the propulsion of solid drug particles into the skin by means of high-speed gas-particle
flow. The fluid dynamics of this technology have been investigated in devices consisting of a convergent-divergent nozzle
located downstream of a bursting membrane, which serves both to initiate gas flow (functioning as the diaphragm of a shock
tube) and to retain the drug particles before actuation. Pressure surveys of flow in devices with contoured nozzles of relatively
low exit-to-throat area ratio and a conical nozzle of higher area ratio have indicated a starting process of approximately
200 s typical duration, followed by a quasi-steady supersonic flow. The velocity of drug particles exiting the contoured nozzles
was measured at up to 1050 m/s, indicating that particle acceleration took place primarily in the quasi-steady flow. In the
conical nozzle, which had larger exit area ratio, the quasi-steady nozzle flow was found to be overexpanded, resulting in
a shock system within the nozzle. Particles were typically delivered by these nozzles at 400 m/s, suggesting that the starting
process and the quasi-steady shock processed flow are both responsible for acceleration of the particle payload. The larger
exit area of the conical nozzle tested enables drug delivery over a larger target disc, which may be advantageous.
Received 12 March 2000 / Accepted 8 June 2000 相似文献
185.
The ability of cavitation bubbles to effectively focus energy is made responsible for cavitation erosion, traumatic brain injury, and even for catalyse chemical reactions. Yet, the mechanism through which material is eroded remains vague, and the extremely fast and localized dynamics that lead to material damage has not been resolved. Here, we reveal the decisive mechanism that leads to energy focusing during the non-spherical collapse of cavitation bubbles and eventually results to the erosion of hardened metals. We show that a single cavitation bubble at ambient pressure close to a metal surface causes erosion only if a non-axisymmetric energy self-focusing is at play. The bubble during its collapse emits shockwaves that under certain conditions converge to a single point where the remaining gas phase is driven to a shockwave-intensified collapse. We resolve the conditions under which this self-focusing enhances the collapse and damages the solid. High-speed imaging of bubble and shock wave dynamics at sub-picosecond exposure times is correlated to the shockwaves recorded with large bandwidth hydrophones. The material damage from several metallic materials is detected in situ and quantified ex-situ via scanning electron microscopy and confocal profilometry. With this knowledge, approaches to mitigate cavitation erosion or to even enhance the energy focusing are within reach. 相似文献
186.
V.V. Shuvalov 《Shock Waves》1999,9(6):381-390
Eulerian, three-dimensional, numerical code, which conserves mass, momentum and energy simultaneously both in the Lagrangian
and remap steps, is developed. The use of special form of linear viscosity provides a weaker time step restriction as compared
with the Courant condition. The code is designed to investigate the multi-material problems, including dusty flows. The performance
of the code is illustrated by the modeling of shock wave interaction with a dusty thermal layer.
Received 15 June 1998/ Accepted 6 January 1999 相似文献
187.
We study a class of traffic flow models with nonlocal look-ahead interactions. The global regularity of solutions depend on the initial data. We obtain sharp critical threshold conditions that distinguish the initial data into a trichotomy: subcritical initial conditions lead to global smooth solutions, while two types of supercritical initial conditions lead to two kinds of finite time shock formations. The existence of non-trivial subcritical initial data indicates that the nonlocal look-ahead interactions can help avoid shock formations, and hence prevent the creation of traffic jams. 相似文献
188.
The broad-band noise has been experimentally used to monitor the cavitation activity in a sonochemical reactor, an ultrasonic cleaning bath, a biological tissue, etc. However, the origin of the broad-band noise is still under debate. In the present review, two models for the mechanism of the broad-band noise are discussed. One is acoustic emissions from chaotically (non-periodically) pulsating bubbles. The other is acoustic emissions from bubbles with temporal fluctuation in the number of bubbles. It is suggested that the latter mechanism is sometimes dominant. Further studies are required on the role for bubble cluster dynamics as well as the bubble–bubble interaction in the broad-band noise especially at relatively low ultrasonic frequencies. 相似文献
189.
An experimental investigation on gaseous mixing zones originated from the Richtmyer-Meshkov instability has been undertaken
in a square cross section shock tube. Mass concentration fields, of one of the two mixing constituents, have been determined
within the mixing zone when the shock wave passes from the heavy gas to the light one, from one gas to an other of close density,
and from the light gas to the heavy one. Results have been obtained before and after the coming back of the reflected shock
wave. The diagnostic method is based on the infrared absorption of one of the two constituents of the mixing zone. It is shown
that the mixing zone is strongly deformed by the wall boundary layer. The consequence is the presence of strong gradients
of concentration in the direction perpendicular to the shock wave propagation. Finally, it is pointed out that the mixing
goes more homogeneous when the Atwood number tends to zero. 相似文献
190.
A Free-Lagrange method for unsteady compressible flow: simulation of a confined cylindrical blast wave 总被引:1,自引:0,他引:1
G. J. Ball 《Shock Waves》1996,5(5):311-325
A Free-Lagrange numerical procedure for the
simulation of two-dimensional inviscid compressible flow
is described in detail. The
unsteady Euler equations are solved on an unstructured
Lagrangian grid based on a density-weighted Voronoi mesh.
The flow solver is of the Godunov type, utilising either the
HLLE (2 wave) approximate Riemann solver or the more recent HLLC
(3 wave) variant, each adapted to the Lagrangian frame.
Within each mesh cell, conserved properties are
treated as piece-wise linear, and a slope limiter of the MUSCL type
is used to
give non-oscillatory behaviour with nominal
second order accuracy in space.
The solver is first order accurate in time.
Modifications to the slope limiter to minimise
grid and coordinate dependent
effects are described.
The performances of the HLLE and HLLC solvers are compared for
two test problems; a one-dimensional
shock tube and a two-dimensional blast wave confined
within a rigid cylinder. The blast wave is initiated by impulsive
heating of a gas column whose centreline is parallel to,
and one half of the cylinder radius from,
the axis of the cylinder.
For the shock tube problem, both solvers predict shock and expansion
waves in good agreement with theory.
For the HLLE solver, contact resolution
is poor, especially in the blast wave problem. The HLLC solver achieves
near-exact contact capture in both problems.
Received May 25, 1995 / Accepted September 11, 1995 相似文献