High-speed non-stationary gas flow in shock tube has been studied with four panoramic visualization methods (shadowgraphy recording with digital CCD cameras, Particle Image Velocimetry (PIV) method, discharge visualization method and particle tracing). The flow evolution was recorded for 12 ms after shock wave passed with high speed recording (digital camera Photron High speed camera Photron FASTCAM SA5, exposition 1-2 mu s at recording rate 300-500 frames per second). Digital image processing was used for images and video films - cross-correlation processing with Davis soft, background subtraction, and glow intensity analysis.

We examine the dynamics of a high-speed shock-induced flow near the open end of a shock tube using the particle image velocimetry (PIV) and the background oriented schlieren (BOS) methods along with two- and three-dimensional numerical simulations. In experiments, planar shock waves (\(M=1.3\)–1.6) are discharged from a rectangular (\(24\,\hbox {mm} \times 48\,\hbox {mm}\)) low-pressure section of a shock tube open to the atmosphere. Due to the rectangular exit geometry, the resulting flow is highly three-dimensional and, thus, more complicated, compared to well-studied circular/axisymmetric geometries. The study focuses on the spatio-temporal flow structure up to 1 ms after the shock wave diffraction. PIV and BOS visualization techniques share the same post-processing principle, and the iterative multi-step cross-correlation algorithm applied in the PIV software is adapted here for the calculation of background pattern displacement on the BOS images. Particular attention is given to the resolution of flow regions where sharp gradients are present, such as a diffracted shock front or embedded shocks. Computational fluid dynamic simulations of the problem are also conducted to validate the experimental results and methods and to gain more insight into the three-dimensional flow dynamics. PIV and BOS images are found to be consistent with the corresponding numerical flow visualizations.

Nonstationary velocity fields that arise during the development of flows behind shock (blast) waves initiated by pulsed surface sliding discharge in air at a pressure of (2–4) × 104 Pa have been experimentally studied by the particle image velocimetry (PIV) technique. Plasma sheets (nanosecond discharges slipping over a dielectric surface) were initiated on walls of a rectangular chamber. Spatial analysis of the shape of shock-wave fronts and the distribution of flow velocities behind these waves showed that the pulsed energy deposition is homogeneous along discharge channels of a plasma sheet, while the integral visible plasma glow intensity decreases in the direction of channel propagation.

The present work was aimed at the quantitative particle image velocimetry (PIV) measurement of a velocity field near the front of a propagating shock wave and the study of the dynamics of liquid tracers crossing the shock front. For this goal, a shock tube with a rectangular cross-section (48  ×  24 mm) was used. The flat shock wave with Mach numbers M  =  1.4–2.0 propagating inside the tube channel was studied as well as an expanding shock wave propagating outside the channel with M  =  1.2–1.8 at its main axis. The PIV imaging of the shock fronts was carried out with an aerosol of dioctyl sebacate (DEHS) as tracer particles. The pressures of the gas in front of the shock waves studied ranged from 0.013 Mpa to 0.1 MPa in the series of experiments. The processed PIV data, compared to the 1D normal shock theory, yielded consistent values of wake velocity immediately behind the plain shock wave. Special attention was paid to the blurring of the velocity jump on the shock front due to the inertial particle lag and peculiarities of the PIV technique. A numerical algorithm was developed for analysis and correction of the PIV data on the shock fronts, based on equations of particle-flow interaction. By application of this algorithm, the effective particle diameter of the DEHS aerosol tracers was estimated as 1.03  ±  0.12 μm. A number of different formulations for particle drag were tested with this algorithm, with varying success. The results show consistency with previously reported experimental data obtained for cases of stationary shock waves.

The results of recording gas flow in a shock tube by the schlieren and background oriented schlieren (BOS) methods after initiating a pulsed (surface or volume) discharge are presented. Simultaneous recording of the flow field by the two methods allows a complete qualitative and quantitative analysis of the shock-wave processes resulting from the interaction of a pulse discharge with high-velocity flow. The vector displacement field of the BOS method was determined by the cross-correlation method. The density field was obtained by solving the Poisson equation with special boundary conditions. It was shown that the BOS method yields a good quality map of the flow structure that corresponds to the classical schlieren method and provides reliable quantitative results except in areas of high gradients. A modification of the BOS method was proposed and tested to measure the density jump at the shock-wave front. Recording was performed at an angle to the plane of the wave front. Various Schemes of processing of digital flow images were tested. The proposed method provides a resolution of large density gradients at the shock-wave front. The obtained quantitative results are consistent with the calculated values.

In this paper, flow associated with the shock wave discharged from an open end of a shock tube is studied experimentally. The investigations were carried out by the method of particle image velocimetry (PIV), and focused on the spatial characteristics of the transmitting shock wave, vortex ring, and the trailing jet. Mach numbers of the initial shock wave varied around 1.4 and 1.8. Initial phase of the flow (t < 1 ms) was studied. Results include observations on the propagation of different features of the flow, estimates of width of the shock front as detected by PIV, and brief analysis of the vortex ring structure.

The shock waves arising as a result of the pulse discharges in air at pressure 6−13 kPa have been investigated experimentally. The objective of the present experimental investigation is to study shock dynamics and to determine the density fields for the case of surface discharge and of volume discharge combined with surface discharge. The present experiments were carried out in a shock tube discharge test section using the shadow method and the background oriented schlieren (BOS) method. The density fields of the 2D flows and the velocities of the shock waves were determined by two visualization methods.

ABSTRACT: The paper presents the results of experimental quantitative studies of flat shock waves by means of background oriented schlieren (BOS) method. The results of previous experiments, as well as works by other scientific groups, show that quantitative capturing of the density jump on a shock wave with the BOS in its conventional scheme poses a difficult problem [1]. It is shown that the problem arises from the limitations on the density gradient being captured by the BOS processing. The limitations are defined by the set-up optical scheme and the properties of the cross correlation algorithm used in the processing program. Key problems are connected with the small width of the shock wave. Although the position and shape of the shock is generally well-defined by BOS, the quantitative measurements of the density field do not provide reliable results. In order to solve these problems, a modification of the optical scheme and image processing sequence is proposed. In this scheme, the flat shock wave front is being captured at an angle to the axis of the optical scheme. In this set-up the visible width of the shock becomes significant (Fig.1), and the observed density gradient decreases proportionally. Several variants of the image processing technique can be used with this optical set-up to measure the density jump on the shock wave. Three such options have been tested, and the results were compared with the normal shock theory calculated via the Rankine-Hugoniot shock conditions. The results show acceptable agreement between the BOS data and the theoretical calculations. Accuracy of the proposed experimental technique is discussed.