Begell House Inc.
Multiphase Science and Technology
MST
0276-1459
10
4
1998
CLOUD CAVITATION: OBSERVATIONS, CALCULATIONS AND SHOCK WAVES
303-321
10.1615/MultScienTechn.v10.i4.10
Christopher E.
Brennen
California Institute of Technology, Pasadena, California 91125, USA
A recent significant advance in our understanding of cavitating flows is the importance of the interactions between bubbles in determining the coherent motions, dynamic and acoustic, of the bubbly cavitating fluid. Recent experimental and computational findings show that the collapse of clouds of cavitating bubbles involve the formation of bubbly shock waves and that the focusing of these shock waves is responsible for the enhanced noise and damage in cloud cavitation. The recent experiments of Reisman et al. (1998) complement the work begun by Morch and Kedrinskii and their co-workers and demonstrate that the very large impulsive pressures generated in bubbly cloud cavitation are caused by shock waves generated by the collapse mechanics of the bubbly cavitating mixture. Two particular types of shocks were observed: large ubiquitous global pressure pulses caused by the separation and collapse of individual clouds from the downstream end of the cavitation and much more localized local pressure pulses which occur much more randomly within the bubbly cloud.
This paper describes experiments and calculations conducted to investigate these phenomena in greater detail as part of an attempt to find ways of ameliorating the most destructive effects associated with cloud cavitation.
TURBULENCE STRUCTURE OF DISPERSED TWO-PHASE FLOWS (MEASUREMENTS BY LASER TECHNIQUES AND MODELING)
323-347
10.1615/MultScienTechn.v10.i4.20
Koichi
Hishida
Department of System Design Engineering, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, 223-8522, Japan
Yohei
Sato
Department of System Design Engineering, Keio University, 3−14−1 Hiyoshi, Kohoku-ku, Yokohama, 223−8522, Japan
Development of measurement techniques and numerical simulations gives further insights into understanding turbulence structure of dispersed two-phase flows. The present paper describes investigation of Eulerian particle-turbulence interactions preformed by using laser Doppler velocimetry (LDV) and digital particle image velocimetry (DPIV)- Langrangian DPIV measurements are also described. The results are compared with turbulence models based on the multiple time scale concept.
Laser Doppler velocimetry (LDV) has been the instrument of choice for measuring dispersed- and gas-phase velocities in particle-laden flows. LDV was used for simultaneous measurements of continuous- and dispersed-phase velocities, which permits the particle size discrimination based on light scattered intensity.
Digital particle image velocimetry (DIPIV) is increasingly considered a proven technique for detecting both phases simultaneously. The author's group developed measurement systems to distinguish a dispersed-phase particle from a tracer in fluid flow. Experiments in a water channel showed that fluid turbulence was augmented by particles, which are comparable to or slightly larger than the Kolmogorov lengthscale of the flow.
The above experimental results support the idea that several time scales are needed for modeling. The present model (multiple-time-scale) divides the energy containing part of the spectrum into two regions, "production" and "transfer" regions. This model has succeeded in predicting channels flows and wall and confined jets laden with particles.
The author's group has developed Lagrangian measurement methods by DPIV and a CCD camera mounted on a moving shuttle with the mean streamwise velocity of the particles in a downflow water channel. This allows the particle dynamics to be investigated and the exact forces acting on a particle to be determined.
STEADY AND UNSTEADY CONDENSATE FORMATION IN TURBOMACHINERY — BLADE TO BLADE FLOW AND ROTOR/STATOR INTERACTION
349-368
10.1615/MultScienTechn.v10.i4.30
G. H.
Schnerr
Fachgebiet Strömungsmaschinen, Universität Karlsruhe (TH) Kaiserstrasse 12, D-76128 Karlsruhe, Germany
M.
Heiler
Fachgebiet Strömungsmaschinen, Universität Karlsruhe (TH) Kaiserstrasse 12, D-76128 Karlsruhe, Germany
G.
Winkler
Fachgebiet Strömungsmaschinen, Universität Karlsruhe (TH) Kaiserstrasse 12, D-76128 Karlsruhe, Germany
Worldwide, most of the electrical power plants have been in service for more than 25 years and continuous development of CFD codes has made possible the detailed calculation of complex single phase flows in turbine stages. However, commercially available codes do neither contain adequate representation of the steam phase transition mechanism, nor the corresponding condensation loss (Smith, 1991). Therefore, we concentrate on the dynamics of instabilities typical for condensing steam or steam/carrier gas mixtures in the dominating transonic flow regime. Neglecting viscosity effects of the fluid numerical simulations based on the Euler equations confirmed the existence of self-excited flow oscillations in realistic blade configurations of low pressure steam turbines. In linear cascades higher order bifurcations with sudden frequency increase or decrease can develop. As a first approach the forced excitation mechanism of the rotor/stator interaction is modelled by unsteady temperature wake effects which alter significantly the two-phase flow characteristics.
STRUGGLE WITH COMPUTATIONAL BUBBLE DYNAMICS
369-405
10.1615/MultScienTechn.v10.i4.40
Works on computational bubble dynamics having carried out since 1990 in Laboratory of Multiphase Flow Engineering in Kobe University are reviewed in this report. The topics discussed are a feasibility study on interface tracking simulation, modeling of interfacial forces acting on a bubble such as drag and transverse lift forces, a 3D two-way bubble tracking method which can cover a wide range of length scales in a bubbly flow, a 3D one-way bubble tracking method for the prediction of time-spatial evolution of a developing bubbly upflow in a vertical pipe, an averaging method based on a 3D two-fluid model in general curvilinear coordinates, and an averaging method based on a 3D multi-fluid model that can account for a bubble size distribution in a given flow.