Cam Tropea

Correlation estimators for two-point laser Doppler anemometry

Using two-point LDA systems, estimates of the integral length scales of turbulent flowfields can be performed by forming the cross-correlation function of the two velocity records for various measurement volume separations. Several estimators for the cross-correlation function are reviewed and their bias and variance are examined as a function of particle density and of chosen processing parameters. The investigations were performed using numerical simulations.

Measurement of Particle Accelerations with the Laser Doppler Technique

An extension of the laser Doppler technique for measuring particle acceleration is presented. The basic principles of the technique follow closely those introduced in [11], although numerous improvements have been implemented in the signal processing for increasing the reliability of individual estimates of particle acceleration. The main contribution of this study is to identify and quantify the errors due to optical fringe divergence in the detection volume of the present laser Doppler system, to introduce an appropriate experiment involving a falling wire and to compare the acceleration measurements of the laser Doppler system to the results of a particle tracking system with high-speed cameras in a highly turbulent flow. Noteworthy is the fact that all measurements were performed with a commercial off-the-shelf laser Doppler system.

Point Laser Measurements for Particle Characterisation

In this contribution two novel techniques for particle characterisation in dispersed, multiphase flows will be discussed: the time-shift technique and the rainbow technique using femtosecond laser pulses. Both methods are rather new and each offers some very distinct advantages for specific applications. The purpose of this contribution is to briefly outline the physical working principles of each technique and to outline their specific features. This information is meant to stimulate use of these techniques for hitherto inaccessible applications and measurement quantities.

Low Speed Drop Impact Onto Dry Solid Surfaces

The orthogonal impact of a Newtonian liquid drop impact onto dry solid surfaces is studied. The focus was placed on finding the possible outcomes of the drop impact and the time evolution of the impact process. A new phenomenological classification differentiates between six possible outcomes. In order to be able to identify the influencing parameters, changes of drop diameter, impact velocity, surface tension, viscosity, advancing and receding contact angles, roughness amplitude and wavelength have been made. The influences of these impact parameters on the various phases of the impact process has been investigated.

Editorial: Outgoing Honorary Editor

As a reader, author or referee, it is difficult not to notice the dramatic changes currently taking place in scientific publishing. Authors submit manuscripts electronically and track their progress through the review process using the internet. Readers rarely make the journey to a library any more but simply download articles to their desk, even to the extent of linking directly to references cited therein. And several weeks are removed from the review exercise, as mailed responses become a thing of the past. Forthcoming and potential changes may be even more encompassing, as multimedia contributions begin appearing or portions of the archival literature no longer appear in print. Even simple issues such as pagination then deserve re-examination. My point is that there are other sides to scientific literature, namely the editorial and publishing sides. In these times of rapid change, editors and editorial boards must take decisions about the extent to which new possibilities will be exploited and how they can best serve the authorship and readership. But even the best of intentions on the part of the editors are nothing without a progressive and dynamic publisher. Having been associated with Measurement Science and Technology and Institute of Physics Publishing since 1993, the last three years as Honorary Editor, I am in a very good position to assure all of our readers and authors that you have been, and will continue to be, in excellent hands. It is difficult to express how infectious the motivation of the MST Editorial Board and the IOPP staff is - it is simply most convincing to examine the result - our journal. With our program of review articles, special issues and feature sections, we have attempted to respond to the needs and requirements of the scientific community in this field. This has led to greatly increased readership, an increased number of pages published each year and a steadily increasing impact factor. IOP Publishing has not only encouraged and supported all these initiatives of the Editorial Board, they have also worked systematically on reducing turnaround times of manuscripts, resulting in one of the most efficient journal operations in the field, as many of our authors are already well aware. As outgoing Honorary Editor of MST, I can say that I have found my task fascinating and I have enjoyed it very much. I have been fortunate to be involved in a period of ambitious geographical expansion of our Editorial Board, bringing me into contact with a large number of scientists from a vast spectrum of technical areas and parts of our world. My thanks go to this large number of colleagues directly and indirectly associated with MST and of course to the IOPP staff and management. Furthermore, I would like to pass on to my successor, Professor Peter Hauptmann from the University of Magdeburg, my best wishes for an enjoyable and successful tenure as Honorary Editor.

Spray Impact Onto a Rigid Wall: Modelling Strategy

The common approach to the modelling of spray impact is to treat the phenomenon as a simple superposition of single drop impact events [1]. The main input for such model formulation is obtained either from experimental [2,3] or theoretical [4,5,6] studies of the impact of a single drop onto a dry wall, onto a uniform, undisturbed liquid film or into a deep pool [7]. However, in [8] it was shown that this conventional approach is not universal in the description of the spray impact and that in the case of relatively dense sprays, the interaction of crowns (Fig. 1) and the oscillations of the liquid-wall film must be taken into account. For example, these interactions result in the emerging of uprising jets during spray impingement of the diesel spray (see Fig. 2). In the study of spray impact we have chosen the following strategy of the modelling: 1. Description (experimental and theoretical) of single dropimpact. Determining of the parameters influencing the splash. 2. Description of the interaction of two drops on the wall surface. 3. Determining of the parameters of the single drop impacts influencing the dynamics of the film formed on the wall. Characterization of the film: the time averaged thickness, the time averaged velocity and its fluctuations. 4. Description of the influence of the oscillating motion of the film on the outcome from a single drop impact. Single drop impact onto a wetted wall—The motion of a kinematic discontinuity in the liquid film on the wall due to the drop impact, the formation of the uprising jet at this kinematic discontinuity and its elevation are analyzed. The theory [4] for the propagation of the kinematic discontinuity is generalized for the case of arbitrary velocity vectors in the inner and outer liquid films on the wall. Next, the mass, momentum balance and Bernoulli equations at the base of the crown are considered to obtain the velocity and the thickness of the jet on the wall. An analytical solution for the crown shape is obtained in the asymptotic case of such high impact velocities that the surface tension and the viscosity effects can be neglected in comparison to inertial effects. The edge of the crown is described by the motion of a rim, formed due to the surface tension. The theoretical predictions of the height of the crown are compared with experiments. The agreement is rather good in spite of the fact that no adjustable parameters are used (see Fig. 3). Three different cases are considered: normal axisymmetric impact of a single drop, oblique impact of a single drop, and impact and interaction of two drops. Next, two new parameters of single drop impact influencing the dynamics of the film formed due to the polydisperse spray impact are identified. The first one is associated with the relative presence of the crown on the film surface and allowing one to estimate the probability of crown interactions. The second parameter is associated with the axial momentum in the plane of the wall. Time-averaged film motion—The theory of the creation of the film by spray can be subdivided into three main parts: 1. The characterization of the spray, particularly definition of the flux vectors of scalar properties (number flux vector, volume flux vector, etc.) and the momentum flux tensor. 2. Boundary conditions at the time-averaged spray/film boundary. 3. Dynamics of the film motion on the wall. The mass and momentum equations of the film are formulated accounting for the volume flux of the spray, the dynamic pressure, and the time-averaged stress vector at the film “free” surface caused by the inertia of the spray. The inertial terms of the liquid in the film contains of the inertia of the time-averaged motion and the inertia of film oscillations. These oscillations are modelled as an ensemble of the radial flows in the film associated with the single drop impacts. The probability of the crown interactions is also taken into account. Jetting at the film surface due to impingement of a dense spray—Here we consider impact of such dense sprays that the probability of single crown to propagate without interaction with another crown is very small. The non-uniformities in the dynamic pressure in such sprays yields the significant fluctuations in the film velocity leading to the shocks and jetting (as in the case of the diesel spray impact shown in Fig. 2). We describe the statistically averaged distribution of drop impacts around a given drop assuming that all the impacting drops are distributed randomly in space and in time. The statistically averaged dynamic pressure around given drop is not uniform either in the time or in the radial direction. The self-similar solution for the statistically averaged radial velocity in the film and its thickness (Fig. 4) is obtained. The characteristic time of the instant of shock is estimated. The theoretical predictions of the jets diameter agree with the experimental data in the order of the magnitude.Copyright