Twente WaterChannel
| Organisation: | University of Twente - Twente WaterChannel (TWC) |
|---|---|
| Location: | Enschede, Netherlands |
| Website: | http://pof.tnw.utwente.nl |
| Contact: | Detlef Lohse |
Summary
- strong turbulence in two-phase flows generated by an active grid
- unique bubbly and light particle flow regimes
- advanced measurement techniques: 3D PTV, optical probes, hot-film anemometry
- outstanding expertise in the field of two-phase flow
The Twente WaterChannel is an 8m high facility in which strong turbulence (up to a Taylor-Reynolds number of 200) can be created thanks to an active grid. The average velocity is downwards. Injected large bubbles (with a concentration up to 10%) or light particles (from now on all called particles) rise against the flow and can be observed and followed in the measuring section. Small bubbles or particles can also be injected upflow. The instrumentation includes 3D Particle Tracking Velocimetry, hot-film anemometry, and optical probes.
The control parameter of the channel are the density ratio between particles and water, the particle size and their concentration, and the Reynolds number. The questions which will be addressed with the channel are the particles distribution in the flow and in particular particle clustering, effect of particles on spectra and collision rates, average rise/sink velocity of particles in turbulence, PDFs of relative velocities and accelerations, further Lagrangian aspects, and finally also bubbly drag reduction.
The Twente WaterChannel is a unique facility, due to the large Reynolds numbers, which can be achieved (active grid). The bubble and light particle regime of the phase space relative particle density vs. Stokes number is only accessible in the Twente WaterChannel. For both reasons the Twente WaterChannel should be made accessible to a broader turbulence community. Twente offers adequate support by local technicians and scientists. On the other hand, Twente would greatly benefit from outside experience, in particular on 3D particle tracking velocimetry
Within the Twente WaterChannel up to now mainly hot-film anemometry measurements were performed. Here the challenge to overcome is to disentangle the gaseous part and the liquid part of the hot-wire signal. We employed high-speed imaging and correlate it with the time series from the hot-wire probe, see Rensen et al, IJMF, 2005. We characterize different types of bubble-probe interactions and identify their corresponding signals. We find that hitherto methods have suffered from too simplistic assumptions, which lead to erroneous estimates of the local void fraction and the local fluid velocities. With the knowledge of the typical bubble signatures in the signal, we can identify the bubbles in long time series of turbulent bubbly flow, employing a self-developed pattern recognition algorithm, Rensen et al, JFM 2005. In order to verify the bubble recognition techniques from the hot-wire anemometer, we compared them with those from a four-point optical probe, both for single bubbles and for turbulence bubbly flow. With this technique the velocity structure functions and spectra can be calculated from the hot-film signal. We find a strong energy enhancement on small scales and a weaker energy enhancement on large scales, effectively leading to a less steep spectral slope (Rensen et al, JFM 2005), just as in the numerics, Mazzitelli et al, JFM 2003 and PoF 2003. A less steep spectral slope was also found when micro bubbles were injected upstream, see van der Berg et al., PoF 2005.