Particle Image Velocimetry Results Research Articles

Non-Intrusive Measurements in Fire Sprinkler Sprays

The results of phase Doppler interferometry (PDI) and particle image velocimetry (PIV) measurements to characterize the water sprays produced by fire sprinklers are presented. The large size of the water drops produced by fire sprinklers, and the relatively large coverage area of the spray, present significant challenges when attempting to characterize these sprays. These difficulties are especially relevant when using PDI because large drops and large coverage areas may result in attenuation of the transmitting laser beams. For the fire sprinkler investigated, it was determined that attenuation of the laser beam resulted in over-counting of drops due to burst splitting, a situation in which the Doppler signals from single drops are misinterpreted as being due to multiple drops. This effect was minimized by carefully choosing the operating conditions of the PDI processing electronics. PIV measurements provide velocity profiles that can be used as input for fire dynamics simulations to predict the effect of sprinkler sprays on fires. The results from the PIV measurements show good agreement with the velocity measurements obtained from PDI in the core of the spray, but poorer agreement along the sprinkler axis. The discrepancy was attributed to recirculation zones present in the experimental facility and possible biasing of the PIV measurements towards the larger drops.

Turbulent kinetic energy dissipation rate estimation from PIV velocity vector fields

Particle image velocimetry (PIV) offers a new possibility for turbulence kinetic energy dissipation rate measurements. It can be used for the estimation of the turbulent kinetic energy dissipation rate using its definitions. Dissipation rate definitions include spatial derivatives of the velocity fluctuation components. Two different estimates for two-dimensional and isotropic definitions calculated from PIV results have been compared with direct numerical simulations in one case. Results show that the spatial resolution achieved is a critical factor in the accuracy of the computed dissipation rate. An error in velocity measurements can dominate the measured dissipation rate. The optimal result for the estimate will be achieved if the interrogation area is neither too large (under-sampling phenomenon) nor too small (noise in the measurement data/physical limits of the PIV system).

Review of LDA and PIV applied to the measurement of sound and acoustic streaming

A review of laser Doppler anemometry (LDA) and particle image velocimetry (PIV) with their application to the measurement of sound is presented. The fundamental principles behind LDA and PIV are discussed and extended to the application of sound measurement. Special attention is paid to analysis of LDA signals including the Hilbert transform, which enables amplitude information to be obtained about various frequency components of a signal and wavelet analysis, which allows non-stationary signals to be accurately analysed. The influence of the refractive index variations in a medium due to a sound wave on the laser beams of an LDA signal is discussed. Attention is also paid to acoustic streaming which arises due to high-intensity sound, and PIV results are presented to demonstrate the effect.

Chaotic mixing in a bounded three-dimensional flow

Even though the first theoretical example of chaotic advection was a three-dimensional flow (Hénon 1966), the number of theoretical studies addressing chaos and mixing in three-dimensional flows is small. One problem is that an experimentally tractable three-dimensional system that allows detailed experimental and computational investigation had not been available. A prototypical, bounded, three-dimensional, moderate-Reynolds-number flow is presented; this system lends itself to detailed experimental observation and allows high-precision computational inspection of geometrical and dynamical effects. The flow structure, captured by means of cuts with a laser sheet (experimental Poincaré section), is visualized via continuously injected fluorescent dye streams, and reveals detailed chaotic structures and chains of high-period islands. Numerical experiments are performed and compared with particle image velocimetry (PIV) and flow visualization results. Predictions of existing theories for chaotic advection in three-dimensional volume-preserving flows are tested. The ratio of two frequencies of particle motion – the frequency of motion around the vertical axis and the frequency of recirculation in the plane containing the axis – is identified as the crucial parameter. Using this parameter, the number of islands in the chain can be predicted. The same parameter – using as a base-case the integrable motion – allows the identification of operating conditions where small perturbations lead to nearly complete mixing.

A comparative study of the PIV and LDV measurements on a self-induced sloshing flow

Particle Imaging Velocimetry (PIV) and Laser Doppler Velocimetry (LDV) measurements on a self-induced sloshing flow in a rectangular tank had been conducted in the present study. The PIV measurement result was compared with LDV measurement result quantitatively in order to evaluate the accuracy level of the PIV measurement. The comparison results show that the PIV and LDV measurement results agree with each other well in general for both mean velocity and fluctuations of the velocity components. The average disagreement level of the mean velocity between PIV and LDV measurement results was found to be within 3% of the target velocity for the PIV system parameter selection. Bigger disagreements between the PIV and LDV measurement results were found to concentrate at high shear regions. The spatial resolution and temporal resolution differences of the PIV and LDV measurements and the limited frames of the PIV instantaneous results were suggested to be the main reasons for the disagreement.

Inverse Energy Cascade Structure of Turbulence in a Bubbly Flow. PIV Measurement and Results.

The inverse energy cascade, which is one of the important phenomena to enhance the large-scale flow instability in bubbly flow, is investigated by measuring a local two-phase flow structure driven by buoyant bubbles using particle imaging velocimetry (PIV). In PIV, the flow field of liquid phase is measured by separating an original image to respective phase images using a statistical thresholding method for separating image parameters of bubbbles and particles. The present results obtained in the case where the bubble Reynolds number and average void fraction are less than 30 and 1.5%, respectively, confirm the large energy decay with a slope index steeper than -5/3 in the log-log diagram of energy spectrum in a high wavenumber region. An important relationship between the energy spectrum and the bubble-bubble interval distance is also detected.

An experimental investigation of the blowout process of a jet flame

An experimental study was performed to investigate the roles of triple flames and flame front instabilities in the blowout transient process. Two-dimensional laser-induced predissociative fluorescence (LIPF) OH and particle image velocimetry (PIV) diagnostic techniques were used for measurements of instantaneous flame structure and velcotiy data. Initial conditions were aligned by external acoustic excitation and triggering. The blowout transient process can be divided into four regions: the pulsating, onset of receding, receding, and extinction regions, according to the dynamic characteristics of the flame. In the pulsating region, the flame base is basically pulsating at two specific heights with jittering. Flame from instability may play a role in leading to the onset of blowout process. Both LIPF OH image and PIV results show the possible existence of the triple- (or edge-) flame structures in the flame base in the pulsating and onset regions. High strain rate, higher than the extinction strain rate, encountered by the flame base in the onset region should be cousidered as a prominent factor for the blowout process.

133 PIVを用いた開水路キャビティー流れの解析

This paper describes the results of the flow visualization of cavity in open-channel flows by means of a Particle Image Velocimetry(PIV). The comparison of mean values, between the results of flow visualization and the high-accurate turbulence measurements conducted by making use of a two component fiber-optic laser Doppler anemometer (LDA) was carried out intensively, and the comparison of coherent structures, between the PIV results and the results of Large Eddy simulation(LES) was also conducted. The coherent structures were investigated in mixing layer by time-space correlation analysis. As the result, the characteristics and effects of coherent structures were revealed and then discussed.

Examination of vortex deformation during blade-vortex interaction

Results of a blade-vortex interaction (BVI) study incorporating particle image velocimetry (PIV) are presented. Test cases encompass a variety of intersection geometries, including head-on parallel interactions. Results provide quantified flow visualization images of the flows generated during interactions. The deformation and division of the interaction vortex during head-on interactions are documented. In addition, secondary vortical structures generated during interactions are observed and their effect upon the interaction vortex is examined. All PIV results are examined in the light of understanding gained from previous pressure measurements made from the same BVI facility.