Particle Image Velocity Technique Research Articles

Experimental study on the failure behavior of rockburst induced by a dynamic disturbance in the circular chamber for deep underground engineering

Rockbursts induced by disturbance loads have become a significant engineering issue. Block specimens featuring a circular chamber structure were designed to investigate the effects of the disturbance frequency on rockbursts. The experiment simulated a series of rockburst failure processes through a disturbance load with four different frequencies by maintaining a constant preset confining pressure. The experimental results showed that an increased disturbance frequency intensifies the shear failure response and enhances the rockburst intensity. Furthermore, the particle image velocity technique was used to calculate the ejection velocity. The results suggested that the initial ejection velocity of fragments during rockburst surged by 253 % as the disturbance frequency was increased from 0.005 Hz to 0.625 Hz. The assessment level of rockburst failure transitions ranged from slight to moderate. These results indicated that an increase in disturbance frequency exacerbated rockburst failures. Two mechanisms were proposed to explain this effect. (i) The increase in disturbance frequency elevates the internal friction angle of rock units, enhancing their ultimate energy storage capacity. (ii) Increased energy input causes the energy storage in rock units to surpass the minimum energy required for rock failure, endowing the fragmented rock mass with greater velocity and leading to more intense rockburst failures. Ultimately, recommendations for early warning of rockburst failure are also provided by discussing the microscopic failure mechanisms of rockburst experiments and the rockburst in situ.

Experimental investigation on flow characteristics of vertical and oblique circular impinging jet

The present study experimentally investigates the flow characteristics of a fully developed circular water jet (vertical and oblique) over a wide range of Reynolds numbers, impinging heights, and impinging angles using particle image velocity technique. This study focuses on the velocity distribution along the jet centerline and the flow structure in the uphill and downhill regions. The results revealed that the velocity profiles of the impinging jet maintain self-similarity before impingement on the bottom plate (at y/H ≤ 0.979). Depending on the impinging height, the development of the jet centerline velocity can exhibit two, three, or four distinct regions. A semi-empirical equation has been developed for the jet centerline velocity based on the obtained experimental data and theoretical analysis. For the oblique impinging jet, the position of stagnation point highly depends on the jet height and impinging angle, but it is insensitive to the Reynolds number. There exists a recirculation zone in the uphill direction induced by pressure gradients and shear forces, whose size and position depend on the impinging height, impinging angle, and Reynolds number. Different flow states are observed for relatively small impinging heights near the geometric center in the downhill region. The flow patterns for various Reynolds numbers and impinging heights are self-similar in the downhill region at a/d ≥ 6.

Strain integration-based soil shear displacement measurement using high-resolution strain sensing technology

The distributed fiber optic sensing technology has emerged as a promising tool for monitoring soil shear deformation. However, the question remains whether the strain measurements obtained by the soil-embedded optical fiber cables are reliable due to cable-soil slippage. In this paper, a direct shear model test was conducted in laboratory to characterize the cable-soil deformation compatibility considering different anchorage conditions. The optical frequency domain reflectometer was employed to measure cable strains. The measurements were compared with those from the particle image velocity technique. When the strain sensing cable couples well with soil, the cable elongation exhibits a linear relationship with the soil shear displacement. A strain integration method is then proposed to convert strain measurements into shear displacements. Furthermore, the effect of block and tube anchors on the cable-soil coupling behavior is investigated. The conclusions drawn in this study provide a reference for establishing new geotechnical deformation monitoring systems.

Observation of Ground Movement with Existing Pile Groups Due to Tunneling in Sand Using Centrifuge Modelling

In developing urban areas, there are unavoidable interactions between existing structures and new construction such as tunnelling through pre-existing pile foundations. An accurate prediction of the ground deformation due to tunnelling is necessary to assess potential damage to those existing structures. Two dimensional tunnelling model tests were carried out to study the movement of both soil surface and subsurface layers of dry Toyoura sand with two pile groups of different length at both sides of the tunnel. The model pile group has four piles with spacing and diameter ratio of 5. The tunnelling process was simulated by reducing the diameter of the model tunnel with initial diameter of 70 mm for various ground loss values at 100g centrifugal acceleration. The induced soil movement and displacement of pile foundation were observed using the particle image velocity technique and displacement transducers. The test cases were conducted at two different tunnel cover depth and diameter ratios (C/D = 1.5 and 2.5) and three different horizontal distances between the pile groups and the tunnel. Tunnel machine showed good agreement of soil movements with the results from real site construction at small ground loss ratios. The cases with the pile group induced larger maximum settlement in the vertical direction than the cases without piles. There was an effect of the pile groups on the ground movement at the location more than five times pile diameter away from the piles in the longitudinal direction.

Interaction of Synthetic Jet with Boundary Layer Using Microscopic Particle Image Velocimetry

The aerodynamic interaction between an unsteady, inclined synthetic jet and a crossflow boundary layer was studied as a precursor toward applying active flow control concepts for rotor applications, such as dynamic stall control and fuselage drag reduction. Because the flowfield offered numerous challenges from a measurement perspective, several experiments were carried out using a phase-locked, two-dimensional microscopic particle image velocity technique in a building block approach, by adding one complexity after another. The procedure began with boundary layer measurements made on a simple flat plate using the microscopic particle image velocity technique. Velocity measurements were made deep in the viscous sublayer, as close as 20μm from the surface. Following this, the synthetic jet actuator was characterized while operating in quiescent air as well as in crossflow. The results showed that the evolution of the synthetic jet in crossflow was substantially different from its evolution in quiescent air, suggesting that any flow physics or performance prediction (for example, the depth of penetration of the jet into the boundary layer) made based on the quiescent flow conditions may not be applicable in crossflow. All the momentum added to the boundary layer had its source from the synthetic jet actuator, and the penetration of the jet was limited to the viscous sublayer and log layer; the outer layer was unaffected, despite using a jet to freestream velocity ratio of four. Significant effort was also made to validate the microscopic particle image velocity technique and evaluate its capability to accurately resolve such a complex flowfield. To this end, microscopic particle image velocity measurements were compared with hot-wire measurements made on a simple steady jet, as well as an unsteady, periodic synthetic jet. Excellent correlation was found between the two techniques, validating microscopic particle image velocity measurements.

Multiphase flow with unequal granular temperatures

Multiphase kinetic theory has been generalized to include rotation of particles with unequal masses and diameters. Inelastic binary collisions of particles with normal and tangential restitution coefficients are considered. New expressions for number of binary collisions, viscosities and conductivities were developed. Collision integrals produced new expressions for energy dissipation involving tangential and normal restitution coefficients. Computed radial profiles of granular temperatures of 530 and 156 μm glass beads flowing in a two story riser matched the experimentally measured profiles of granular temperatures using a particle image velocity technique without rotation. The measured and the computed particle concentration profiles were nearly flat in the central portion of the riser without rotation. Computations show that rotation can alter these profiles. Sufficient particle spin can drive the particles to concentrate near the center of the tube.