Mechanism Investigation Of Interaction Of A Single Or A Double Spherical Model With Shock Waves

The interactions of shock wave and solid particles are common phenomena existing insupersonic gas-solid two-phase flows, involving health care, clean energy, material surfaceprocessing, safety prevention and control, aeronautics and astronautics etc. The specificapplications include needle-free injection of granular drugs, supersonic separation in gas turbine,impulse-type fire extinguisher, supersonic cold spraying, solid fuel propellant booster nozzle,and so on.The main purpose of this thesis is to investigate the generation mechanism and evolutionrule of unsteady drag caused by shock loadings on a single or double spherical model throughexperiments and numerical simulations. The research results of this thesis are indispensible forthe understanding of compressible gas-solid two-phase flow.The experimental set-up system is mainly composed of a force measuring system, whichconsists of accelerometers, force transducers, a high-speed data acquisition system and acomputer with a set of dedicated analysis software, a high-speed photography system, ahorizontal shock tube with the inner diameter of200mm, and a visible testing section. Bothsingle and double spherical models with the same diameter of40mm were adopted in theexperiments. Meanwhile, high-pressure nitrogen gas was employed as the driving gas. In thesingle spherical model cases, the only changeable parameter incident shock Mach number Msranged from1.07to1.26by using aluminum diaphragams with different thicknesses from0.05to0.2mm. In the double spherical model cases, except for Ms with the same changeable rangeas in the single spherical model cases, dimensionless interval distance H, defined as the ratio ofinterval distance of centers of double spheres to sphere diameter, was the other variableparameter from1.0to3.0. Band-pass filtering technology was applied to process thehigh-frequency acceleration signals derived from the measurements. The upper and lowerfiltering limits were determined based on the analyses aiming at different frequency levels ofsignals.The numerical simulations were conducted based on the CFD platform composed ofGAMBIT and FLUENT and by using post-processing softwares including Tecplot and Origin.The numerical calculations for shock-induced flows around sphere(s) were carried out under the experimental conditions of single and double spherical models. The CFD calculation model wasvalidated in comparison to measured values of unsteady drag. The dependency of flow fieldaround sphere(s) on unsteady drag was analyzed.Some creative achievements obtained in this study include:(1) The first to directlymeasure the unsteady drag exerted on double spheres by shock loading;(2) The design for thespherical model made up of a larger and a smaller hemispheres by screw pair connection,capable of the installation of a high-frequency accelerometer inside, and with a smooth outersurface, in combination with the use of a fine aluminum alloy wire with the diameter only o.5mm to connect and fix the spherical models, these two aspects are helpful to effectivelyovercome the un-expected turbulence problem that was faced in previous experimentalinvestigations, and to ensure the accuracy of measured data;(3) The evolution mechanism andinfluence rule were revealed by the analyses of the dependency of parameters (pressure andvelocity) in flow field around sphere(s) and stress distribution on sphere wall(s) on unsteadydrag.The main conclusions can be drawn as follows:(1) For single spherical model cases, at first, the reflection of incident shock wave by aspherical model at the position near to frontal stagnation point, and next, the diffractions ofincident shock and following wavelets after traversing the equator, and then, the focusing ofshock waves at the position close to the rear stagnation point, and at last, the tendency of flowat the vicinity around a sphere to be steady lead to that the drag increases drastically to amaximum peak firstly, and decreases in manner of fluctuations secondly, and would probablydrop down to minus values thirdly, and finally tends to a stable value, respectively. Dragincreases as shock Mach number increases, however, drag coefficient changes in the reverseway with Mach number. Besides, the peak value of an unsteady drag (coefficient) is muchlarger than that of a steady one.(2) The interferences of shock waves and wake vortexes occur in double spherical modelcases, resulting in the differences of flow around double spheres and corresponding pressureand shear stress distributions comparing with a single spherical model situation. Furthermore,the extent of interference depends on the relative distance. As the result, with dimensionlessinterval distance H increasing, drag coefficient first increases then decreases, the minimum drag coefficient occurs at H=1.0, and the Maximum does at H abound2.5. Additionally, the smallerthe shock Mach number, the larger is the peak value of drag coefficient and the more difficultthe drag coefficient trends to be stable.(3) Facing a gas-solid two-phase flow companied with moving shock waves, we can notchoose a single sphere drag coefficient model simply, instead, it is necessary to analyze theinfluence extend of mutual interferences between different shock wave, possible wake vortexand turbulence structures belonging to different solid spheres or particles.