Some comments on particle image displacement velocimetry

Laser speckle velocimetry (LSV) or particle image displacement velocimetry, is introduced. This technique provides the simultaneous visualization of the two-dimensional streamline pattern in unsteady flows as well as the quantification of the velocity field over an entire plane. The advantage of this technique is that the velocity field can be measured over an entire plane of the flow field simultaneously, with accuracy and spatial resolution. From this the instantaneous vorticity field can be easily obtained. This constitutes a great asset for the study of a variety of flows that evolve stochastically in both space and time. The basic concept of LSV; methods of data acquisition and reduction, examples of its use, and parameters that affect its utilization are described

A vector scanning processing technique for pulsed laser velocimetry

Pulsed laser sheet velocimetry yields nonintrusive measurements of two-dimensional velocity vectors across an extended planar region of a flow. Current processing techniques offer high precision (1 pct) velocity estimates, but can require several hours of processing time on specialized array processors. Under some circumstances, a simple, fast, less accurate (approx. 5 pct), data reduction technique which also gives unambiguous velocity vector information is acceptable. A direct space domain processing technique was examined. The direct space domain processing technique was found to be far superior to any other techniques known, in achieving the objectives listed above. It employs a new data coding and reduction technique, where the particle time history information is used directly. Further, it has no 180 deg directional ambiguity. A complex convection vortex flow was recorded and completely processed in under 2 minutes on an 80386 based PC, producing a 2-D velocity vector map of the flow field. Hence, using this new space domain vector scanning (VS) technique, pulsed laser velocimetry data can be reduced quickly and reasonably accurately, without specialized array processing hardware

Particle image velocimetry applied to internal combustion engine in-cylinder flows

Particle Image Velocimetry (PIV) is now emerging as a powerful tool for the investigation of unsteady fluid mechanics. At the same time, the study and optimisation of in-cylinder flow processes in automotive Internal Combustion (IC) engines is of increasing importance in the design of improved combustion systems with lower emissions and favourable power and efficiency characteristics. This thesis describes the development and application of PIV as a routine diagnostic tool for the investigation of in-cylinder flows in a production geometry single cylinder research engine exhibiting "barrel swirl" or "tumbling" in-cylinder fluid motion. The work has involved the design and installation of a complete PIV engine facility, based around a four-valve, four-stroke Rover research engine equipped with piston crown optical access and a glass cylinder liner. Novel techniques for the on-line monitoring of important experimental parameters have been developed which permit the reliable acquisition of high spatial resolution PIV data from both horizontal and vertical measurement planes within the engine cylinder. A novel optical correction technique has been developed to control the severe particle image degradation which was experienced when imaging vertical planes within the glass cylinder. A simple means for selection of an appropriate corrective lens for this application is described, together with an experimental evaluation of the lens performance. A representative set of PIV images and data from both horizontal and vertical planes are then presented. These have been selected from a comprehensive set of flow mapping experiments in the motored engine. The data are discussed with reference to the work of others in engines of similar geometry and have shed new light on the detailed processes involved in the formation and breakdown of barrel swirl. Initial PIV measurements ahead of a flame under part load, skip fired conditions have also been made in the engine. This has demonstrated the possibility of investigating incylinder flow behaviour under conditions approaching those in a fully firing, production geometry optical engine. Finally, limitations in the PIV technique employed in this work and methods of overcoming them are described and the prospects for further work are discussed

Defocusing digital particle image velocimetry and the three-dimensional characterization of two-phase flows

Defocusing digital particle image velocimetry (DDPIV) is the natural extension of planar PIV techniques to the third spatial dimension. In this paper we give details of the defocusing optical concept by which scalar and vector information can be retrieved within large volumes. The optical model and computational procedures are presented with the specific purpose of mapping the number density, the size distribution, the associated local void fraction and the velocity of bubbles or particles in two-phase flows. Every particle or bubble is characterized in terms of size and of spatial coordinates, used to compute a true three-component velocity field by spatial three-dimensional cross-correlation. The spatial resolution and uncertainty limits are established through numerical simulations. The performance of the DDPIV technique is established in terms of number density and void fraction. Finally, the velocity evaluation methodology, using the spatial cross-correlation technique, is described and discussed in terms of velocity accuracy

Event-based imaging velocimetry using pulsed illumination

The paper addresses the shortcoming of current event-based vision (EBV) sensors in the context of particle imaging. Latency is introduced both on the pixel level as well as during read-out from the array and results in systemic timing errors when processing the recorded event data. Using pulsed illumination, the overall latency can be quantified and indicates an upper bound on the frequency response on the order of 10-20 kHz for the specific EBV sensor. In particle-based flow measurement applications, particles scattering the light from a pulsed light source operating below this upper frequency can be reliably tracked in time. Through the combination of event-based vision and pulsed illumination, flow field measurements are demonstrated at light pulsing rates up to 10 kHz in both water and air flows by providing turbulence statistics and velocity spectra. The described EBV-based velocimetry system consists of only an EBV camera and a (low-cost) laser that can be directly modulated by the camera, making the system compact, portable and cost effective

High-frequency ultrasonic speckle velocimetry in sheared complex fluids

High-frequency ultrasonic pulses at 36 MHz are used to measure velocity profiles in a complex fluid sheared in the Couette geometry. Our technique is based on time-domain cross-correlation of ultrasonic speckle signals backscattered by the moving medium. Post-processing of acoustic data allows us to record a velocity profile in 0.02--2 s with a spatial resolution of 40 $\mu$m over 1 mm. After a careful calibration using a Newtonian suspension, the technique is applied to a sheared lyotropic lamellar phase seeded with polystyrene spheres of diameter 3--10 $\mu$m. Time-averaged velocity profiles reveal the existence of inhomogeneous flows, with both wall slip and shear bands, in the vicinity of a shear-induced ``layering'' transition. Slow transient regimes and/or temporal fluctuations can also be resolved and exhibit complex spatio-temporal flow behaviors with sometimes more than two shear bands.Comment: 15 pages, 18 figures, submitted to Eur. Phys. J. A