Bernd Wunderlich

Droplet collisions and interaction with the turbulent flow within a two-phase wind tunnel

Experiments in wind tunnels concerning meteorological issues are not very frequent in the literature. However, such experiments might be essential, for instance for a careful investigation of droplet-droplet interactions in turbulent flows. This issue is crucial for many configurations, in particular to understand warm rain initiation. It is clearly impossible to completely reproduce cloud turbulence within a wind tunnel due to the enormous length scales involved. Nevertheless, it is not necessary to recover the whole spectrum in order to quantify droplet interactions. It is sufficient for this purpose to account correctly for the relevant properties only. In the present paper, these properties and a methodology for setting those in a two-phase wind tunnel are first described. In particular, droplet size and number density, velocities, turbulent kinetic energy, k, and its dissipation rate, ɛ, are suitably reproduced, as demonstrated by non-intrusive measurement techniques. A complete experimental characte...

Improved 3-D Particle Tracking Velocimetry with Colored Particles

The present work introduces an extension to three-dimensional Particle Tracking Velocimetry (3-D PTV) in order to investigate small-scale flow patterns. Instead of using monochrome particles the novelty over the prior state of the art is the use of differently dyed tracer particles and the identification of particle color classes directly on Bayer raw images. Especially in the case of a three camera setup it will be shown that the number of ambiguities is dramatically decreased when searching for homologous points in different images. This refers particularly to the determination of spatial parti- cle positions and possibly to the linking of positions into trajectories. The approach allows the handling of tracer parti- cles in high numbers and is therefore perfectly suited for gas flow investigations. Although the idea is simple, difficult- ties may arise particularly in determining the color class of individual particle when its projection on a Bayer sensor is too small. Hence, it is not recommended to extract features from RGB images for color class recognition due to infor- mation loss during the Bayer demosaicing process. This article demonstrates how to classify the color of small sized tracers directly on Bayer raw images.

Examination of a Swimming Dummy’s Flow Field Using Laser Doppler Velocimetry

Motivated by the fact that the evaluation of different swim suit designs produced inconsistent results, this study takes a new approach. In this pilot study, the Laser Doppler Velocimetry Method was applied to examine the flow field around a water rescue dummy, which was mounted in a swimming flume. Laser Doppler Velocimetry allows velocity measurement of particles that move with the water flow. Measurements can be taken from a distance to the object of about 600 mm. Through this, interference of the measurement devices with the water flow can be avoided. It was found that this approach has the potential to analyze a swimmer’s flow field, and the horizontal component of the flow velocity was determined at around 5000 positions. Limitations were the minimum spatial resolution of 1 millimeter and the time-consuming data collection due to the small number of tracer particles inside the water. It is intended to use this test set up in the future to investigate the effects of different swimsuit designs with respect to a swimmer’s flow field.

Use of Coloured Tracers in Gas Flow Experiments for a Lagrangian Flow Analysis with Increased Tracer Density

In this article a 3-d particle tracking velocimetry system (PTV system) is presented which enables the investigation of relatively fast gaseous (air) flows and tiny turbulences in a small scaled wind tunnel. To satisfy the demand of a high spatial and temporal resolution, a sufficiently high tracer particle concentration has to be applied to the gas. Solving the correspondence problem among different cameras becomes extremely difficult due to ambiguities: Each tracer has to be found in all pictures of the different views during many successive time steps. Here, the correspondence problem is facilitated by the use of coloured particles and the application of suitable classifiers for particle classification.

Coloured Tracer Particles Employed for 3-D Particle Tracking Velocimetry (PTV) in Gas Flows

This work describes an improved method for Particle Tracking Velocimetry in three dimensions (3-DPTV), applicable for gaseous flows and based on coloured tracer particles. This method allows a considerable increase of the tracer number density, while maintaining a constant rate of 3-D correspondence ambiguities from the different cameras. It is therefore perfectly suited for gas flows, where a high tracer density is requested to follow small-scale flow features. Furthermore, coloured tracer particles enable a longer next-step searching distance for the lagrangian reconstruction of trajectories. First, theoretical and numerical considerations are presented to check the ability of the employed coloured tracer particles to follow the considered flow. Then, 3-D PTV results obtained by the method described here are shown and appear to be very promising for later investigations of gas flows involving vortical structures and recirculation zones down to small scales, around 1 mm.

3-D Particle Tracking Velocimetry (PTV) in gas flows using coloured tracer particles

Although relatively often used for liquid flows, Particle Tracking Velocimetry (PTV) is still considered as a major challenge in gaseous flows. One of the main objections is the higher tracer density necessary for gaseous measurements [1, 2], resulting from higher characteristic speeds and smaller spaceand time-scales of the important flow structures. Nevertheless, the widely recognized interest of Lagrange-based measurements (such as PTV) for the investigation of turbulence and vortical structures in real flows [3] is a sufficient reason to face all these challenges. The solution proposed in this work is to employ coloured particles and use the associated separation into different colour classes. Considering separately each resulting colour class, the apparent particle density is decreased without restrictions in the measurement accuracy.