An experimental study of diabatic spiral vortex flow

In spiral vortex flow, between concentric cylinders with the inner cylinder rotating and the outer stationary, the addition of a thermal gradient across the gap is a known complicating factor. The present diabatic study for narrow and wide gaps (radius ratios N=0.955 and N=0.8), with a heated outer and adiabatic inner cylinder, was undertaken to investigate this problem. The heat transfer characteristics and the modes of transition have been investigated together with the relationship between them. Using standard on-line digital computer techniques, the onset of vortex flow and its higher transitions have been shown to cause a sharp increase in Nusselt number. At higher Taylor numbers, of the order of 10⁶, a marked change in the Nusselt number occurs with the onset of the transition to periodic turbulent vortex flow. Outer wall heating is seen to affect the modes of transition. Diabatic critical Taylor numbers are much higher than those for adiabatic conditions and are found to depend on the close approach of the vortices to the outer wall     

Study on multiple ring-like vortex formation and small vortex generation in late flow transition on a flat plate

A high-order direct numerical simulation (DNS) of flow transition over a flat plate at a free stream Mach number 0.5 has been carried out. During the simulation, we cannot find, according to the classical theory of boundary layer transition, the “hairpin vortex breakdown to smaller structures” in the last stage of flow transition on a flat plate. However, we did discover the so-called spikes as a result of a multibridge or multiring formation. This indicated a large and stable vortex structure which can travel for a long distance. We believe that this is a result of ring heads that are located in an inviscid region. These heads of the “turbulence spot” never seem to break down and persist as a stable structure. In addition, we discovered that the U-shaped vortex is a part of an existing coherent structure instead of a newly generated one. The U-shaped vortex also provides an additional channel to transfer vorticity to the ring from the wall. During travel, the leading primary ring in the front of the spot is sloped and skewed, causing disappearance of the second sweep. As a consequence, no energy is brought down to the lower boundary layer near the vortex ring head of the spot. Thus, the small length scale structures become damped and the existing U-shaped vortex structure becomes distinguishable. So, the question is where do the turbulent small length scale vortices come from? We will address this with a new theory which states that all of small length scales (turbulence) are generated by high-shear (HS) layers rather than being produced by “vortex breakdown.” The new DNS shows that the HS layers are produced by strong positive spikes surrounded by low-speed fluids and by the interaction between the secondary and higher-level vortices and the wall surface especially near the ring neck. This multiple ring-like vortex generation also follows the first Helmholtz vortex conservation law. Furthermore, the Λ-shaped vortex is formed and rolling up, and the Λ-vortex is stretched and narrowed, and a new bridge is generated after the neck. The bridge will further become a second ring and so on. Besides the original vortex legs, there are also U-shaped vortex tubes. Finally, the multiple ring vortex structure is formed. From this process, no evidence is found to support that two consequent multiring circles are mixed to generate small vortices. The connection of downdraft/updraft motions is also studied.     

高密度B炸药的燃烧转爆轰实验研究

采用电探针及压力传感器测试技术对密度为1.597 g/cm3的固体B炸药（TNT/RDX=40/60）的燃烧转爆轰性能进行研究。实验结果表明,在较强的约束条件下（45号钢管,内径20 mm,外径64 mm,长500 mm）,B炸药形成了DDT现象,诱导爆轰距离为295~310 mm。     

An investigation of adiabatic spiral vortex flow by means of cross-wire probes

Flow transitions occurring with increase in the Taylor number in an annular gap of radius ratio 0.8, having an imposed axial flow of air of Reynolds number 500 have been studies using the output from a cross-wire probe in a complex digital analysis. Cross and phase spectra, together with auto and cross correlograms, are presented for four Taylor numbers from 10 620 to 12.2 × 10⁶, covering the onset of vortex flow, chaotic flow and turbulent vortex flow. As the Taylor number increases, there is little alteration in the spiral vortex flow in the axial and tangential directions, which oscillates in phase in these two directions. The tangential velocity gradient was seen to become increasingly dominant, with increase in the Taylor number     

Tube diameter effect on deflagration-to-detonation transition of propane–oxygen mixtures

An experimental study was conducted to study the tube diameter effect on deflagration- to-detonation run-up distance. The tube diameter effect is associated with the amplification factor, flame acceleration and heat loss. A simplified correlation of the run-up distance and tube diameter is proposed for the fuel-lean, stoichiometric and slight fuel-rich mixtures. The amplification factor, which is evaluated from the initial conditions of the propane–oxygen mixtures, might also be used to get a quick estimation of the run-up distance in tubes of larger diameter.

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Transition modes in adiabatic spiral vortex flow in narrow and wide annular gaps

The stability of flow in a concentric annulus formed by rotating cylinders is well understood, but there is little information concerning the transitions which occur after the onset of spiral vortex flow. For both the arrow and wide gap cases (N = 0·8 and N = 0·955), it has been shown that the initial spiral flow breaks down from a quasiperiodic to a chaotic mode. Three axial Teynolds numbers were studied, viz., Re_a = 500, 1500, 2500. The introduction of digital techniques provided qualitative and quantitative information regarding the flow behaviour. The effects reported for Re_a > 0 and recent results of other workers for Re_a = 0 have been compared. The phenomenon of re-emergent flows has been observed at very high Taylor numbers and high axial Reynolds numbers.     

Radial variation of adiabatic and diabatic spiral vortex flow in a wide annular gap

Transitions occurring after the onset of spiral vortex flow in a wide concentric annular gap of radius ratio 0.8, formed by a stationary outer cylinder and a rotatable inner cylinder, have been studied experimentally. By isothermal heating of the annular surface, it was possible to consider diabatic as well as adiabatic conditions. At an axial Reynolds number of 500 and for a range of Taylor numbers up to 10⁷, power spectra and auto-correlograms were taken at two radial positions near to the inner and outer annular surfaces; these are compared with previous results taken at mid-gap under adiabatic conditions. Measurements of turbulence intensity across the gap were made also. Probability histograms and signal traces for diabatic flow near to the outer annular surface are presented. It has been shown that the vortex transitions affect the thermal boundary layer and, consequently, the heat transfer rates at the outer surface. A positive radial thermal gradient was seen to have little effect on the flow. The imposed axial flow was shown to be stabilising under adiabatic conditions but destabilising under diabatic conditions.     

Effects of dimensions on the fluid flow and mass transfer characteristics in wavy-walled tubes for steady flow

Flow and mass transfer characteristics in axisymmetric sinusoidal wavy-walled tubes with different dimensions are investigated experimentally. The overall pressure drops are measured with U-manometer, and the measurements of mass transfer rate are performed by the electrochemical method, while the flow patterns are visualized by the aluminum dust method. The results showed that the wavelength and amplitude of the wavy-walled tubes obviously affect the fluid flow and mass transfer characteristics. With the increment of the wave factor F _w , the overall pressure drop increase and thus leads to an earlier shift of transitional flow to turbulence as well as a better mass transfer enhancement. Furthermore, mass transfer enhancement is compared with the corresponding straight-walled tube under equal pumping power condition. It is found that the greater mass transfer enhancement appears at the moderate Reynolds number, and the optimum mass transfer enhancement could be obtained in the wavy-walled tube with F _w  = 0.5. Based on the flow visualization results, the characteristics of the flow structures in wavy-walled tube with different dimensions are displayed clearly.     

Free-surface deformation due to spiral flow owing to a source/sink and a vortex in Stokes flow

The free-surface shape and cusp formation are analyzed by considering a viscous flow arising from the superposition of a source/sink and vortex below the free surface where the strength of the source and vortex are arbitrary. In the analysis, Stokes’ approximation is used and surface tension effects are included, but gravity is neglected. The solution is obtained analytically by using conformal mapping and complex function theory. From the solution, shapes of the free surface are obtained, and the formation of a cusp on the free surface is discussed. Above some critical capillary number with a sink, the free-surface shape becomes singular and an apparent cusp should form on the free surface below a real fluid. On the other hand, no cusp would occur for sources of zero or positive strength. Typical streamline patterns are also shown for some capillary numbers. As the capillary number vanishes, the solution is reduced to a linearized potential flow solution.     

Analytical and experimental insights into fast deflagrations, detonations, and the deflagration-to-detonation transition process

 The present paper reports on experimentally as well as analytically obtained results of fast propagating deflagrations, the deflagration-to-detonation transition process, and detonations. Due to the high propagation velocity of these highly transient combustion waves, experimental data were mainly achieved by means of conventional measuring devices in past. These data were used to develop analytical models describing the physical processes of these fast combustion waves. Due to the progress in the development of new laser-optical measuring-techniques, detailed insight into these combustion waves can nowadays be achieved. These measuring-techniques facilitate the validation of analytical models and, furthermore, it is now possible to understand the physical processes of these fast combustion waves. This paper describes the analytical models proposed by Mayinger et al. and new experimental results obtained by means of the color-schlieren technique and the laser-induced predissociation fluorescence. Received on 12 April 2001     

An investigation of adiabatic spiral vortex flow in wide annular gaps by visualisation and digital analysis

This paper, the third in a series describing experimental investigations into spiral vortex flow, presents visual evidence illustrating adiabatic transition modes in a wide gap of radius ratio 0.848. Also, power spectra, relating to velocity fluctuations, are found to be comparable for two working fluids, oil and air. Good agreement has been found between these results and those published by the authors in which diabatic transition modes were related to the heat transfer characteristics of the flow at various axial Reynolds numbers     

Experimental study of the induced flow birefringence of a suspension of rigid particles at the transition from Couette flow to Taylor vortex flow

The flow birefringence induced in solutions of rigid particles is studied experimentally in the region of the axisymmetrical Taylor vortex flow which arises once the velocity gradient G in the annular gap of a conventional Couette cell reaches a critical value G _c .The measurements are performed for several values of G > G _c and for 10 radial observation points in the annular gap. Solutions of two types of rigid particles are investigated: the first is a suspension of flattened clay particles like bentonite, while the second contains rod-like particles of tobacco mosaic virus (TMV). The variations of the birefringence intensity n and of the extinction angle measured in the domain of the axisymmetrical flow show a different behavior according to the shape of the particle in solution. This fact is confirmed theoretically with a good agreement for the measurements performed with solutions of flat particles.     

Experimental investigation of the flow characteristics within a vortex tube with different configurations

The energy separation in a vortex tube is a combined result of different factors and its explanation remains debatable. As a classical fluid mechanics phenomenon, understanding of the complex helical flow mechanism within a vortex tube is a necessary foundation. The small scale of an industrial vortex tube and the extremely complex flow conditions are the two main challenges in obtaining the internal flow properties. This paper reports the results of an experimental investigation on the flow behaviour within a confined cylindrical system having different configurations corresponding to the actual flow field in a vortex tube at different conditions. Transparent devices were used to enable flow visualisation and Particle Image Velocimetry (PIV) measurement. The results of the flow visualisation and PIV experiments show that a precessing vortex core is significant only in a specific range of swirling strength. A good agreement between the observed flow characteristics and previously published results was observed.     

Compressible and confined vortex flow

The internal compressible flow of a thin vortex chamber was investigated experimentally by measuring the radial distribution of temperature and pressure, from which the velocity field was calculated. The bulk of the internal vortex was found to be described by u_θr^0.69 = constant. The total resistance of the vortex chamber to the flow was also investigated in the context of fluidic vortex diode behavior under conditions of compressible and choked flow. It was found that the vortex chamber choked at an upstream-to-downstream pressure ratio of about 6 and in doing so passed a mass flow rate of 28% of the equivalent one-dimensional ideal nozzle. The resistance of vortex chambers is known to be strongly influenced by the presence of reversed flow in the exit due to vortex breakdown. Schlieren photography of the swirling exhaust flow was used to show that, while vortex breakdown does occur, it can only do so after the flow has become subsonic downstream of the exit and cannot therefore influence the vortex chamber resistance.     

A two-phase mixture theory for the deflagration-to-detonation transition (ddt) in reactive granular materials

In this paper, a two-phase mixture theory is presented which describes the deflagration-to-detonation transition (DDT) in reactive granular materials. The theory is based on the continuum theory of mixtures formulated to include the compressibility of all phases and the compaction behavior of the granular material. By requiring the model to satisfy an entropy inequality, specific expressions for the exchange of mass, momentum and energy are proposed which are consistent with known empirical models. The model is applied to describe the combustion processes associated with DDT in a pressed column of HMX. Numerical results, using the method-of-lines, are obtained for a representative column of length 10 cm packed to a 70% density with an average grain size of 100 μm. The results are found to predict the transition to detonation in run distances commensurate with experimental observations. Additional calculations have been carried out to demonstrate the effect of particle size and porosity and to study bed compaction by varying the compaction viscosity of the granular explosive.     

Buoyancy effects on flow transition in low-density inertial gas jets

Effects of buoyancy on transition from laminar to turbulent flow are presented for a momentum-dominated helium jet injected into ambient air. The buoyancy was varied in a 2.2 s drop tower facility without affecting the remaining operating parameters. The jet flow in Earth gravity and microgravity was visualized using the rainbow schlieren deflectometry apparatus. Results show significant changes in the flow structure and transition behavior with change of buoyancy.     

Effect of acceleration on the wavy Taylor vortex flow

In this paper, we use a laser optical technique to investigate the characteristics of a wavy Taylor vortex flow between two concentric cylinders, with the inner cylinder subjected to a wide range of predetermined acceleration and the outer one at rest. We focus on the inner/outer radius ratio of 0.894, with an acceleration (dRe/dt*) from 0.1123 to 2,247, and Reynolds number from Re/Re _c =1.0 to 36. The results show that, with increasing Reynolds number, there is an initial increase in the wavelength of the wavy vortex flow (λ), and a decrease in the wave speed (c) before they asymptote to a constant value, which is a function of the acceleration. As for the wave amplitude (A), it is found that the effect of acceleration is significant only in a very narrow range of Reynolds numbers. Received: 21 August 2001 / Accepted: 22 November 2001     

旋流阻塞与旋流扩散复合内消能泄洪洞的壁面压强特性

结合雅砻江两河口水电站3#导流洞改建的水工模型试验,研究了新型旋流阻塞和旋流扩散复合内消能泄洪洞的壁面压强特性。研究结果表明:在淹没堰流流态时,壁面压强在进口的堰面、竖井段和旋流洞段因阻塞的顶托作用整体增大;在起旋器出口下游侧的旋流洞的顶部仍然存在局部范围的低压区或负压区;在旋流阻塞段,流速增大,壁面压强减小,水流空化数降低;在旋流扩散段,壁面压强略大于或小于大气压。     

Some open questions on the flow of circular vortex rings

Ring vortices generally go through four phases in their development: generation, laminar, ‘wavy’ and turbulent. For the generation phase, neither the ring growth-rate, not the ring velocity nor the circulation generation process are understood. The rate of entrainment is another interesting problem. The effect of the braids, which occur too at this early stage, on both core growth and stability, remains to be evaluated too. At the end of the generation phase several well-established relationships exist between generating parameters and those of the completed ring, whose interpretation, however, is unclear. The equality of the distance taken to the end of the laminar and the end of the wavy phases, independent of the generating parameters, is also unexplained. Finally, the effect of the geometry (tube, hole) seems to be more significant than might theoretically be expected.