Query: PIV ；smoke ；shock tube

title: A comparative three dimensional study of impulsive flow emanating from a shock tube for shock Mach number 1.6
abstract: A three dimensional transient impulsive flow emanating from an open end of a 165 mm driver section shock tube is investigated numerically in the present study for a shock Mach number of 1.6. Here, the main objective was to find out an appropriate model to simulate the early evolution of the high Mach number transient flows by comparing the flow field obtained from two numerical models with both qualitative and quantitative experimental observations. The 3 D numerical simulations were performed by solving RANS equations using the shear stress transport K o model and large eddy simulation of the flow with the help of ANSYS CFX software. The experiments were performed with the simultaneous 2 D and 3 D particle image velocimetry systems and high resolution smoke flow visualizations. The 2 D PIV results were used for comparing the numerical results obtained on a plane, and the 3 D PIV results were used to compare the azimuthal variations across the vortex ring. It is observed that the velocity field of the transient jet, Kelvin Helmholtz (K H) vortices at the trailing jet and the interaction of K H vortices with the primary vortex ring were resolved well in LES quite similar to the experiments. However, the SST K o model resolved only the velocity field of the transient jet in the axial region similar to the experiments and dissipated the K H vortices at the jet boundary. Though both models resolved the shear layer originated from the triple point, they did not predict the rollup and formation of vortices and their interactions observed in a 24 MP camera. This demands the use of a higher spatial resolution. It is also noticed that a substantial adverse pressure gradient experienced by the vortex ring and trailing jet during the early stage of evolution due to the presence of precursor/incident shock was responsible for the pairing up and merging of the shear layer vortices to form a stronger counter rotating vortex ring. This adverse gradient also plays a dominant role in the azimuthal expansion (rapid increase in diameter) of the vortex ring.

title: On the late time growth of the two dimensional Richtmyer Meshkov instability in shock tube experiments
abstract: In the present study, shock tube experiments are used to study the very late time development of the Richtmyer Meshkov instability from a diffuse, nearly sinusoidal, initial perturbation into a fully turbulent flow. The interface is generated by two opposing gas flows and a perturbation is formed on the interface by transversely oscillating the shock tube to create a standing wave. The puncturing of a diaphragm generates a Mach $1. 2$ shock wave that then impacts a density gradient composed of air and SF6, causing the Richtmyer Meshkov instability to develop in the 2.0 m long test section. The instability is visualized with planar Mie scattering in which smoke particles in the air are illuminated by a Nd:YLF laser sheet, and images are recorded using four high speed video cameras operating at 6 kHz that allow the recording of the time history of the instability. In addition, particle image velocimetry (PIV) is implemented using a double pulsed Nd:YAG laser with images recorded using a single CCD camera. Initial modal content, amplitude, and growth rates are reported from the Mie scattering experiments while vorticity and circulation measurements are made using PIV. Amplitude measurements show good early time agreement but relatively poor late time agreement with existing nonlinear models. The model of Goncharov ( 2002, 134502) agrees with growth rate measurements at intermediate times but fails at late experimental times. Measured background acceleration present in the experiment suggests that the late time growth rate may be influenced by Rayleigh Taylor instability induced by the interfacial acceleration. Numerical simulations conducted using the LLNL codes Ares and Miranda show that this acceleration may be caused by the growth of boundary layers, and must be accounted for to produce good agreement with models and simulations. Adding acceleration to the Richtmyer Meshkov buoyancy drag model produces improved agreement. It is found that the growth rate and amplitude trends are also modelled well by the Likhachev Jacobs vortex model (Likhachev Jacobs, Phys. Fluids, vol. 17, 2005, 031704) Circulation measurements also show good agreement with the circulation value extracted by fitting the vortex model to the experimental data.

title: High speed tomographic PIV of cylinder wakes in a shock tube using a pulse burst laser
abstract: High speed tomographic particle image velocimetry measurements describing flow development in cylinder wakes at Reynolds numbers from 8,200 to 53,000 is presented. Experiments are conducted in a shock tube, which creates an impulsive start followed by uniform flow conditions for 8.0 ms. A pulse burst laser is used for illumination, allowing a larger measurement volume compared to typical high speed setups (approximately 10x larger) with traditional oil droplet particle seeding in air. Measurements are acquired at 10 kHz, allowing 90 snapshots to be acquired per shot. Additional advances are made in the data processing: single image self calibration accounts for shock tube recoil and distributed cluster processing allows compilation of a large dataset for statistical analysis. The initial wake development and transition to regular von Karman shedding in the cylinder wake is shown in terms of the vortex topology, and statistics of vorticity components and the recirculation region are shown as a function of time and Reynolds number. Analysis of multiple runs and conditions shows an initially rapid development of spanwise vorticity, with a slow development in streamwise/transverse vorticity that peaks at the time of the first shedding cycle and assumes periodic shedding afterwards.

title: High resolution, time resolved PIV measurements on the Richtmyer Meshkov instability in a dual driver vertical shock tube.

title: Pulse burst PIV of an impulsively started cylinder in a shock tube for Re 105
abstract: The impulsive start of a circular cylinder in a shock tube was characterized with time resolved particle image velocimetry measurements (TR PIV) at 50 kHz using a pulse burst laser. Three Reynolds numbers Re of 1.07, 1.63 and 2.46 x 105 were studied adding insight into the transient process near the drag crisis. In all cases, vorticity was maximum in the first pair of vortices formed. In a fashion analogous to previous studies at Re 104, a single symmetric vortex pair was first shed from the cylinder at Re 1.07 x 105 prior to the eventual transition to a von Karman vortex street. In contrast, at Re 1.63 x 105, two or more symmetric vortex pairs were first shed. The non dimensional time for the wake to begin to exhibit asymmetry was also found to be lower at the two higher Re. The time required to reach a fully antisymmetric wake (peak von Karman shedding) was roughly five times the asymmetric onset time. Altogether, the study indicates a transformation in the impulsive wake structure and associated time scales to occur at Re near 1.6 x 105.

title: Time resolved pulse burst tomographic PIV of impulsively started cylinder wakes in a shock tube

title: Pulse Burst PIV Measurements of Transient Phenomena in a Shock Tube
abstract: Time resolved particle image velocimetry (TR PIV) measurements were made in a shock tube using a pulse burst laser. Two transient flowfields were investigated including the baseline flow in the empty shock tube and the wake growth downstream of a cylinder spanning the width of the test section. Boundary layer growth was observed following the passage of the incident shock in the baseline flow, while the core flow velocity increased with time. The measured core flow acceleration was compared to that predicted using a classical unsteady boundary layer growth model. The model typically provided good estimates of core flow acceleration at early times, but then typically underestimated the acceleration. As a result of wall boundary layers, a significant amount of spatial non uniformity remained in the flow following the passage of the end wall reflected shock, which could be an important factor in combustion chemistry experiments. In the transient wake growth measurements, the wake downstream of the cylinder was symmetric immediately following the passage of the incident shock. At later times 0.5 ms) the wake transitioned to a von Karman vortex street. The TR PIV data were bandpass filtered about the vortex shedding frequency to reveal additional details on the transient wake growth.

title: Time Resolved PIV in a Shock Tube using a Pulse Burst Laser.

title: Interaction of pulsed laser plasma with normal shock In a shock tube
abstract: With the prominent advantage of high peak power density and easy to break down air to form plasma, nanosecond pulsed laser has an important application value in reducing supersonic wave drag. To reveal the mechanisms of drag reduction by nanosecond pulsed laser, bow shock is simply considered as a normal shock in this paper. The phenomenon of the interaction of laser plasma and normal shock is studied by experiment. A high resolved schlieren system is applied to reveal the complex wave structures. A high speed PIV system is built to measure the velocity and vorticity. Time resolution of the PIV system reaches up to 500ns. Firstly, a nanosecond pulsed laser is focused in the experiment section of the shock tube to form laser plasma. Secondly, laser plasma is impacted by a normal shock in the shock tube. The flow features and evolution of the laser plasma during the interaction are revealed. Research results show that: Under the impact of the normal shock, the high temperature and low density region evolves into an upper and lower symmetric double vortex ring structure, and the size of them increases with the laser energy. The entrainment and contra flow of the vortex can cause the reduction of the surface pressure of the aircraft, and reform the shape of the bow shock. It is the key flow structure that causes the drag reduction of supersonic vehicle.

title: Measurements of Transient Phenomena in a Shock Tube using Pulse Burst PIV