CCD Recording method for cross-correlation PIV development in unstationary high speed flow

Up to now, the use of CCD cameras, for cross-correlation development in Particle Image Velocimetry (PIV), is reduced to relatively slow flows. An original storage method of two images on the two half frames of a video camera permits now to decrease the interval between exposures (10 s). Therefore, it is possible to study high speed flows. Applications are shown of a jet and a turbulent flame propagation.     

Conditional averaging of PIV plane wake data using a cross-correlation approach

A new approach is investigated for conditional averaging of a series of random snapshots of periodic velocity fields obtained from PIV measurements. This approach departs from conventional conditional averaging approaches that require a reference signal for extracting phase information and uses the cross-correlation between the velocity fields as a phase identifier. The methodology is applied on the natural and forced turbulent wake of a circular cylinder, the latter serving also as a validation/verification case. The results show that the vortex formation and shedding processes in the wake are drastically affected by inflow oscillations and reveal the underlying vortex dynamics. Additionally, detailed velocity measurements are reported for the two cases investigated.     

Evaluation of FFT-based cross-correlation algorithms for PIV in a periodic grooved channel

The design of a pneumatic droplet generator to produce small (~0.2 mm diameter) water droplets on demand is described. It consists of a cylindrical, liquid-filled chamber with a small nozzle set into its bottom surface, connected to a gas cylinder through a solenoid valve. Rapidly opening and closing the valve sends a pressure pulse to the liquid, ejecting a single droplet through the nozzle. Gas in the chamber escapes through a vent hole so that the pressure drops rapidly and more droplets do not emerge. We photographed droplets as they emerged from the nozzle, and recorded pressure fluctuations in the chamber. We determined the duration of the pressure pulse required to generate a single drop; longer pulses produced satellite drops. The length of the water jet when its tip detached and the diameter of the droplet that formed could be predicted using results from linear stability analysis. The peak pressure in the cavity could be increased by raising the supply pressure, increasing the width of the pressure pulse, or by reducing the size of the pressure relief vent.     

有限元应力计算结果改善处理的一种实用方法

用有限元法进行数值模拟时,针对刚度相差较大的材料组成的构造,如何对其应力计算结果进行改善处理,以合理地反映客观实际,本文提出了一种简单实用的处理方法,并通过实例证明了该方法的合理性。     

The effect of a discrete window offset on the accuracy of cross-correlation analysis of digital PIV recordings

 This paper describes how the accuracy for estimating the location of the displacement-correlation peak in (digital) particle image velocimetry (PIV) can be optimized by the use of a window offset equal to the integer-pixel displacement. The method works for both cross-correlation analysis of single-exposure image pairs and multiple-exposure images. The effect is predicted by an analytical model for the statistical properties of estimators for the displacement, and it is observed in the analysis of synthetic PIV images of isotropic turbulence, and in actual measurements of grid-generated turbulence and of fully-developed turbulent pipe flow. Received: 29 April 1996/Accepted: 29 October 1996     

Improving the approximation ability of Volterra series identified with a cross-correlation method

This paper proposes an improvement in cross-correlation methods derived from the Lee–Schetzen method, in order to obtain a lower mean square error in the output for a wider range of the input variances. In particular, each Wiener kernel is identified with a different input variance and new formulas for conversion from Wiener to Volterra representation are presented.     

Evaluation of the performance of high-speed PIV compared to standard PIV in a turbulent jet

In this paper, a comparison between two particle image velocimetry (PIV) systems, one based on a standard cross-correlation charge coupled device (CCD) camera with pulsed laser and another using high-speed complementary metal oxide semiconductor (CMOS) camera with continuous laser is performed. The objective of the paper is to point out advantages and disadvantages of the two systems when computing large and small flow scale statistics. The comparison is performed on velocity measurements in the near and far fields of a circular water jet: on this flow several experimental data and empirical self-similarity laws are available for comparisons. The results show that both systems are suitable for measurements with a preference for the standard one when investigating small-scale statistics. This result depends on the lower number of effectively independent samples acquired by a high-speed system and on the higher noise levels of CMOS sensors in comparison to CCDs.     

Particle response to shock waves in solids: dynamic witness plate/PIV method for detonations

Studies using transparent, polymeric witness plates consisting of polydimethlysiloxane (PDMS) have been conducted to measure the output of exploding bridge wire (EBW) detonators and exploding foil initiators (EFI). Polymeric witness plates are utilized to alleviate particle response issues that arise in gaseous flow fields containing shock waves and to allow measurements of shock-induced material velocities to be made using particle image velocimetry (PIV). Quantitative comparisons of velocity profiles across the shock waves in air and in PDMS demonstrate the improved response achieved by the dynamic witness plate method. Schlieren photographs complement the analysis through direct visualization of detonator-induced shock waves in the witness plates.     

Multiple trenching: a simple self-calibrating method of residual-stress measurement

A simple semidestructive residual-stress-measurement technique, particularly suitable for measurement parallel to edges or on outside radii of components, is presented. The nature of the technique obviates reliance upon calibration constants such as have been obtained for the hole-drilling and ring-cutting methods for residual-stress measurement. Use of such calibration constants might seriously underestimate stress values for relatively shallow surface stresses such as may be produced by, for example, grinding or thermochemical treatment.     

A comparison of tracer and PIV results in visualizing water flow around a cylinder close to the free surface

 Sheridan et al. (1997) have recently published particle-image-velocimetry (PIV) results showing water flow patterns around a cylinder at various values of shallow immersion. These results have been compared with a turbulent-flow tracer study of some of the same flow configurations. In shallow-immersion cylinder comparison tests, the streaklines of the tracer technique offer a virtually identical impression of the principal flow characteristics when compared with the PIV velocity vectors. Received: 30 March 1998/Accepted: 9 April 1999     

PIV with volume lighting in a narrow cell: An efficient method to measure large velocity fields of rapidly varying flows

In this work we test a methodology for PIV measurements when a large field of view is required in planar confined geometries. Using a depth of field larger than the channel width, we intend to measure the in-plane variations of the velocity of the fluid averaged through the width of the channel, and we examine in which operating conditions this becomes possible. Measurements of the flow through a narrow channel by PIV are challenging because of the strong velocity gradients that develop between the walls. In particular, all techniques that use small particles as tracers have to deal with the possible migration of the tracers in the direction perpendicular to the walls. Among the complex mechanisms for migration, we focus on the so called Segré-Silberberg effect which can lead to transverse migration of neutrally buoyant tracers of finite size. We report experimental PIV measurements in a Hele-Shaw cell of 1 mm gap, which have been carried out by using neutrally buoyant tracers of size around 10 μm. By considering steady flows, we have observed, in particular flow regimes, the effect of an accumulation of the tracers at a certain distance to the wall due to the so called Segré-Silberberg effect. The particle migration is expected to occur at any Reynolds numbers but the migration velocity depends on the Reynolds number. A significant migration therefore takes place each time the observation duration is large enough compared to the migration time. For a given observation duration, the tracers remain uniformly distributed at low Reynolds numbers whereas they all accumulate at the equilibrium position at large ones. When using volume lighting, the PIV algorithm provides the average velocity of the flow through the gap at low Reynolds number, while it leads to the velocity of the flow at the equilibrium position of the tracers at large Reynolds numbers. By considering unsteady flows, we have observed that the migration does not occur if the timescale of flow variation is short compared to the time required for the parabolic flow to develop across the gap. In this case, there is no transverse velocity gradient and the PIV algorithm provides the fluid velocity. Altogether, these results allow us to propose guidelines for the interpretation of PIV measurements in confined flow, which are based on the theoretical predictions of the tracer migration derived by Asmolov [1].     

A simple method to compute standard two-fluid models

We describe in this paper, a tool to compute approximate solutions of standard two-fluid models with an equilibrium pressure assumption. The basic approach takes its grounds in the two-fluid two-pressure formalism, and takes advantage of the relaxation techniques. The method may be used to compute either the single- or the two-pressure model, depending on the size of mesh, which is used. It is also shown on the basis of a simple numerical experiment that the local equilibrium assumption may lead to a blow-up of the numerical solution on fine meshes, even if one accounts for drag stabilizing effects.     

A dual-beam dual-camera method for a battery-powered underwater miniature PIV (UWMPIV) system

A battery-powered in situ Underwater Miniature PIV (UWMPIV) has been developed and deployed for field studies. Instead of generating high-energy laser pulses as in a conventional PIV system, the UWMPIV employs a low-power Continuous Wave (CW) laser (class IIIb) and an oscillating mirror (galvanometer) to generate laser sheets. In a previous version of the UWMPIV, the time between exposures of a pair of particle images, $\Updelta t$ , could not be reduced without loss of illumination strength. This limitation makes it unsuitable for high-speed flows. In this paper, we present a technique to solve this problem by adopting two CW lasers with different wavelength and two CCD cameras in a second-generation UWMPIV system. Several issues including optical alignment, non-uniform distribution of $\Updelta t$ due to the varying speed of the scanning beam and local flow velocities are discussed. The timing issue is solved through a simple calibration procedure that involves the reconstruction of maps of laser beam arrival time. Comparison of the performance between the new method and a conventional PIV system is presented. Measurements were performed in a laboratory open-channel flume. Excellent agreement was found between the new method and the standard PIV measurement in terms of the extracted vertical profiles of mean velocity, RMS fluctuation, Reynolds stress and dissipation rate of turbulent kinetic energy.     

PIV investigation of the flow induced by a passive surge control method in a radial compressor

Due to recent emission regulations, the use of turbochargers for force induction of internal combustion engines has increased. Actually, the trend in diesel engines is to downsize the engine by use of turbochargers that operate at higher pressure ratios. Unfortunately, increasing the impeller rotational speed of turbocharger radial compressors tends to reduce their range of operation, which is limited at low mass flow rate by the occurrence of surge. In order to extend the operability of turbochargers, compressor housings can be equipped with a passive surge control device such as a ??ported shroud.?? This specific casing treatment has been demonstrated to enhance the surge margin with minor negative impact on the compressor efficiency. However, the actual working mechanisms of the system remain not well understood. Hence, in order to optimize the design of the ported shroud, it is crucial to identify the dynamic flow changes induced by the implementation of the device to control instabilities. From the full dynamic survey of the compressor performance characteristics obtained with and without ported shroud, specific points of operation were selected to carry out planar flow visualization. At normal working, both standard and stereoscopic particle imaging velocimetry (PIV) measurements were performed to evaluate instantaneous and mean velocity flow fields at the inlet of the compressor. At incipient and full surge, phase-locked PIV measurements were added. As a result, satisfying characterization of the compressor instabilities was provided at different operational speeds. Combining transient pressure data and PIV measurements, the time evolution of the complex flow patterns occurring at surge was reconstructed and a better insight into the bypass mechanism was achieved.     

A simple algorithm to improve the performance of the WENO scheme on non-uniform grids

This paper presents a simple approach for improving the performance of the weighted essentially nonoscillatory(WENO) finite volume scheme on non-uniform grids. This technique relies on the reformulation of the fifthorder WENO-JS(WENO scheme presented by Jiang and Shu in J. Comput. Phys. 126:202–228, 1995) scheme designed on uniform grids in terms of one cell-averaged value and its left and/or right interfacial values of the dependent variable.The effect of grid non-uniformity is taken into consideration by a proper interpolation of the interfacial values. On nonuniform grids, the proposed scheme is much more accurate than the original WENO-JS scheme, which was designed for uniform grids. When the grid is uniform, the resulting scheme reduces to the original WENO-JS scheme. In the meantime,the proposed scheme is computationally much more efficient than the fifth-order WENO scheme designed specifically for the non-uniform grids. A number of numerical test cases are simulated to verify the performance of the present scheme.     

Stereoscopic PIV: validation and application to an isotropic turbulent flow

 A new stereoscopic PIV system to measure the three velocity components is developed and applied to grid turbulence flows. This system uses two CCD cameras coupled with an accurate cross-correlation calculation method. An experimental test (based upon three-dimensional displacements) has been carried out to demonstrate the capability of this process to locate the maximum of correlation, and to detect accurately the 3D displacements. Experiments in a well-established turbulent flow have validated the method for quantitative measurements and a comparison with LDV results showed a good agreement in terms of mean and fluctuating velocities. Combined PIV and stereoscopic PIV measurements on a turbulent flow revealed the need to the stereoscopic systems to measure accurate 2D velocity fields. It has been shown that an error of up to 10% in the velocity fluctuation measured by conventional PIV could be attained due to 3D effects in highly turbulent cases. Finally, the digital cross-correlation technique adapted to the determination of small displacements seems to be the most suitable technique for stereoscopic PIV. Received: 22 July 1997/Accepted: 27 January 1998     

A novel method to improve surface quality in cylindrical grinding

Experimental Techniques -