Dust suspensions accelerated by shock waves

The motion of dust suspensions accelerated by shock waves has been experimentally investigated in a vertical shock tube, in which a completely developed plane shock wave of moderate strength propagates into a homogeneously distributed dust suspension with a planar interface. Trajectories of the accelerated interfaces as well as transmitted and reflected shock waves are recorded by using a shadowgraph system with a Cranz-Schardin camera. Two kinds of particle samples, i.e. porous lycopodium particles 30 μm in diameter and corn starch particles with a mean diameter of 10 μm, are employed. The effects of shock wave strength and particle loading ratio are also examined. Experimental data are compared with theoretical results, and the agreement is good. Received: 7 October 1998/Accepted: 1 June 1999     

激波绕射碰撞加速诱导爆轰的数值模拟

基于单步化学反应的Euler方程和对激波(爆轰波)、接触间断具有良好捕捉效果的Roe/HLL混合格式以及自适应网格技术,模拟了激波在方形管中与方块障碍物相互作用,并发生绕射碰撞来诱导爆轰的过程.结果表明,弱激波在绕经方块时,形成上、下绕射激波并在方块尾部发生碰撞,生成局部高温高压点,可加快爆轰的形成;而当管内阻塞比超过...     

Experimental study on a heavy-gas cylinder accelerated by cylindrical converging shock waves

The Richtmyer–Meshkov instability behavior of a heavy-gas $(\text{ SF }_6)$ cylinder accelerated by a cylindrical converging shock wave is studied experimentally. A curved wall profile is well-designed based on the shock dynamics theory [Phys. Fluids, 22: 041701 (2010)] with an incident planar shock Mach number of 1.2 and a converging angle of $15^\circ $ in a $95\,\text{ mm }\times 95$ mm square cross-section shock tube. The $\text{ SF }_6$ cylinder mixed with the glycol droplets flows vertically through the test section and is illuminated horizontally by a laser sheet. The images obtained only one per run by an ICCD (intensified charge coupled device) combined with a pulsed Nd:YAG laser are first presented and the complete evolution process of the $\text{ SF }_6$ cylinder is then captured in a single test shot by a high-speed video camera combined with a high-power continuous laser. In this way, both the developments of the first counter-rotating vortex pair and the second counter-rotating vortex pair with an opposite rotating direction from the first one are observed. The experimental results indicate that the phenomena induced by the converging shock wave and the reflected shock formed from the center of convergence are distinct from those found in the planar shock case.     

Atomization of liquid droplets on surfaces exposed to moving shock waves

Many engineering applications involve the stripping of liquid droplets from surfaces, one example being the entrainment of surface fuel from the inlet valves, ports, cylinder walls and piston crowns of internal combustion engines during the induction process. This configuration is likely to exhibit differences from the more commonly studied case of suspended droplets. In order to study the atomization of liquids from surfaces, shock waves at low Mach numbers (M = 1.05 and 1.12) have been used in the present work to initiate the flow over water droplets with visualization obtained from shadowgraph photographs, high-intensity flash photography and a CCD camera. Visualization paths both normal and angled at ±45° to the flow were used in order to obtain improved examination of the atomization details. Surface wave formation and a specific pattern of droplet distortion followed by stripping, was observed. There are similarities in the processes to those of suspended droplets that are modified by the boundary layer effects. At the Weber numbers considered, a cave-like formation occurs near the wall due to surface flow around the droplet with a major liquid flow directed tangentially across the air flow towards the cave peak where bag or chaotic type break-up and stripping takes place.     

Full-field spreading velocity measurement inside droplets impinging on a dry solid surface

Liquid droplet impacts onto solid surfaces have attracted enormous amount of attention from wide range of research fields including experimental and numerical investigations. Unlike experimental efforts, numerical and analytical studies generated various sets of data. In this study, we investigated the spreading velocities inside the water droplets impinging onto a dry glass substrate using time-resolved PIV. The method, together with the high spatiotemporal resolution and the additional treatments improving the robustness, allowed us to resolve the radial velocity profiles efficiently in the spreading phase. Several impact velocity cases ranging from 0.40 to 0.96 m/s were studied. They correspond to low and moderate level Weber numbers (4.9–27.6). We observed that instantaneous radial velocity distributions exhibit linear and nonlinear modes. The nonlinearity is caused by the vortical flows formed at outer regions of the spreading liquid lamella. We demonstrated that even at low impact velocities the linear parts of the profiles obey a quasi-one-dimensional theory proposed in the literature. The comparison of obtained results with a literature-based numerical study, performed for high range of Weber numbers, confirmed the simultaneous existence of linear and nonlinear parts in the radial velocity profiles. In spite of the scale differences in terms of Weber number, the agreements in the tendencies of the profiles imply the validity of the mechanism considered in the numerical study even at low and moderate level range of Weber numbers.     

Unsteady drag on a sphere by shock wave loading

The dynamic drag coefficient of a sphere by shock wave loading is investigated numerically and experimentally. The diameter of the sphere is varied from 8 m to 80 mm in numerical simulation. The axisymmetric Navier-Stokes equations are solved on a fine grid, and the grid convergence of the drag coefficient is achieved. The numerical result is validated by comparing the experimental data of a 80 mm sphere, measured by the accelerometer in a vertical shock tube. It is found that the sphere experiences in the early interaction one order higher drag than in the steady state. A transient negative drag, mainly resulting from the focusing of shock wave on the rear side of the sphere, is observed only for high Reynolds number flows, and the drag becomes positive because of increased skin friction for low Reynolds number flows.Received: 10 March 2004, Accepted: 24 May 2004, Published online: 20 August 2004[/PUBLISHED]M. Sun: Send offprints requests to     

On the drag coefficient of triangular profiles

Summary Results of a laboratory study on the determination of the drag coefficient of triangular profiles simulating a train of sedimentary dunes are here presented. The study is carried out in a low speed wind tunnel by performing a direct measurement of the total drag force acting on one of the profiles and then subtracting the skin shear force to obtain the net pressure drag force. Drag coefficient behavior is studied as a function of the profile Reynolds number. Results are then discussed using some recent criteria available in literature.

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Sommario Si espongono i risultati di un lavoro sperimentale volto allo studio del coefficiente di resistenza di un profilo triangulare con geometria analoga a quella delle dune fluviali. Gli esperimenti sono condotti in una galleria del vento a bassa velocità, misurando direttamente la forza totale esercitata dalla corrente su uno dei profili e provvedendo poi a sottrarre la forza di attrito superficiale stimata. L'andamento del coefficiente di attrito è studiato in funzione del numero di Reynolds. I risultati sono po discussi alla luce di alcuni recenti contributi bibliografici.

     

The effect of dust and water droplets on the relaxation zone developed behind strong normal shock waves

The propagation of strong normal shock waves into a quiescent suspension composed of argon gas, water droplets and solid dust particles is studied. For evaluating the flow behaviour in the relaxation zone, the conservation equations for a steady, one-dimensional suspension flow are formulated and solved numerically. The solution indicates that the presence of water droplets and dust particles has a significant effect on the fiow inside the relaxation zone and on the eventually reached post-shock suspension equilibrium state. Higher pressures and temperatures are obtained in the suspension as compared with a similar pure pure argon case. Changes in the physical properties of the dust have noticeable effect on the dust behaviour in the relaxation zone. However, these changes have only small effect on either the behaviour of the gaseous phase or the eventually reached post-shock suspension equilibrium state.     

Propagation characteristics of shock waves driven by gaseous detonation waves

We experimentally investigated propagation characteristics of the shock wave driven by a gaseous detonation wave emerging from the open end of a cylindrical detonation tube. In the present study, we visualized the shock wave and exhaust flowfields using a shadowgraph optical system and we obtained peak overpressure in the tube axial direction and the continuous shape transformation of shock waves around the tube open end. We also obtained overpressure histories of the shock wave using piezo-pressure transducers within 201 m from the open end of the tube. We normalized and classified these results by four regions using non-dimensional pressure and distance which are independent of variety of mixture and tube diameter. In the vicinity of the open end of the tube, the shock wave is nearly planar and does not significantly attenuate, and the peak overpressure maintains approximately C–J pressure. Subsequently, the shock wave attenuates rapidly, transforming from quasi-spherical to spherical. Farther from the tube open end, the shock wave propagates with approximately sound characteristic so that the peak overpressure decreases proportional to 1/r. Eventually, the shock wave begins to attenuate more rapidly than ideal sound attenuation, which may be due to the viscous effect.     

Investigations involving shock waves generation and shock pressure measurement in direct ablation regime and confined ablation regime

Recent investigations involving shock waves generation and shock pressure measurement in direct ablation regime and confined ablation regime for aluminium, copper, titanium and steel (40C130) materials are reported. Experimental measurements demonstrated that in direct ablation regime the peak pressures typically are less than 10 Pa when the incident laser intensity is about 10 W/cm and the time duration of the applied pressure is roughly equal to the laser pulse duration. It is shown that confinement of the surface with a transparent overlayer provided an effective method of enhancing laser-induced shock waves pressure in the target material with an order of magnitude for same laser intensity. Also, in this second regime, the pressure is applied over a period much longer than the laser-pulse duration. As an application measurements of the hardness of target surface before and after laser irradiation in direct ablation regime and confined ablation regime are given, and it is shown that the maximum value of surface hardness is obtained in confined ablation regime. Received 10 March 2001 / Accepted 13 April 2001     

Comparison of different methods of measurement of pressure of underwater shock waves generated by electrical discharge

Different methods for measurement of strong underwater shock waves pressure pulses with peak pressures of up to 200 MPa and rise time of tens to hundreds of nanoseconds are described and compared. The experimental techniques include direct methods of pressure measurement using various electromechanical gauges such as quartz, carbon-based, and commercially available PCB gauges, and nondirect methods based on measurement of the velocity of the shock wave such as time-of-flight and fast-streak photography. Advantages and disadvantages of the used gauges and methods are discussed. The shock waves were produced by underwater electrical discharge (discharge current amplitude ≤100 kA, pulse duration ≤5 μs) initiated by an exploding wire. A good correspondence between the pressure amplitudes measured by the various gauges and methods was observed. The obtained dependence of the shock wave pressure on the distance from the discharge channel was found to be best fitted by a r ^−0.7 law. It is also shown that none of these methods can be used to determine the time evolution of the pressure behind the front of the shock wave.     

Optical measurement of liquid film thickness and wave velocity in liquid film flows

Two optical techniques are described for measurement of a liquid film's surface. Both techniques make use of the total internal reflection which occurs at a liquid-vapor interface due to the refractive index difference between a liquid and a vapor. The first technique is used for film thickness determination. A video camera records the distance between a light source and the rays which are reflected back from the liquid-vapor interface. This distance can be shown to be linearly proportional to film thickness. The second technique measures surface wave velocities. Two photo sensors, spaced a fixed distance apart, are used to record the time varying intensity of light reflected from the liquid-vapor interface. The velocity is then deduced from the time lag between the two signals.The authors appreciate the support of the Air Conditioning and Refrigeration Center at the University of Illinois at Urbana-Champaign under project 45.     

Attenuation of shock waves by granular filters

Attenuation of shock waves in granular filters has been studied. Both pressurized air and solid explosives have been used for generating shock waves in a shock tube. The shock tube had a total length of m, and an internal diameter of 355 mm. Two large scale experiments have also been carried out in a tunnel with a cross-sectional area of 6.5 m². The results are compared with results found in the literature (Zloch, 1976; Medvedev et al., 1990; Britan et al., 2001) and previous experiments in a smaller scale by Slungaard (2002). A simple correlation based on the work of Zloch (1976) for shock wave attenuation in tube bundles and an extensive amount of experiments, is proposed. The correlation can be used to estimate the attenuation of the shock wave through a granular filter with filter characteristic . Setting B=6 will give a conservative estimate of the attenuation, while setting B=3 will give the best fit to all the results from this study and the results found in the literature. The correlation is independent of the type of driver (pressurised air or solid explosives) and upstream shock strength.Received: 30 September 2002, Accepted: 19 December 2002, Published online: 6 March 2003     

Measurement of shock waves velocity in the air plasma of capacitively coupled RF discharge

The velocity of shock wave propagation in the air plasma of stationary capacitively coupled RF discharge at different gas pressure and charged particles concentration has been measured. It is shown, that the velocity of the shock wave increases at the increase of the concentration. Measurement results are brought to the universal dependence. Received 17 August 1998 / Accepted 10 December 1999     

Spectral intensity measurement for soot formation of benzene behind reflected shock waves

Soot formation of benzene was observed using He-Ne laser extinction at 632.8 nm as well as the visible and infrared thermal radiation at 0.63, 0.80, 1.12, 1.52, 2.3, 3.4, 3.9, 4.3 m. From the light extinction we determined the temperature-dependent soot formation rate. Also from the visible and infrared radiation we calculated the temperature of soot particles and kcl-values which were dependent on the soot volume concentration. The temperature dependence of soot formation rates below 2500 K was obtained in the early part of soot formation and that above 1900 K was obtained in the later part of soot formation for the cases of the laser extinction and the visible-infrared radiation intensities, respectively. Received 5 February 2002 / Accepted 13 February 2002 Published online 11 June 2002     

New formula for drag coefficient of cylindrical particles

The drag force on a cylindrical particle is calculated using lattice Boltzmann method.The results show that the drag coefficient of a particle with different orientation angles decreases with increasing Reynolds number.When the principal axis of the particle is parallel to flow,the drag coefficient is much larger than that of others and decreases fastest with increasing Reynolds number,which becomes more obvious with increasing particle aspect ratio.When the principal axis of the particle is inclined to flo...