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Numerical Modeling of Supercavitating and Surface-Piercing Propeller Flows
No full textA 3-D panel method has been extended to model the flow around fully submerged supercavitating propellers and surface-piercing propellers. Overviews of the formulation and solution methodology is presented. Comparisons of the numerical predictions with measurements from experiments are given. Discussion of the numerical results, and initial work on modeling of impact flows are provided
Visualizations of Leading Edge Cavitation in an Inducer at Different Temperatures
No full textVisualizations of the leading edge cavity on a four-bladed inducer working with refrigerant 114 are presented. The evolution of the cavity length with the cavitation number is given for three different temperatures. These data are used to estimate the thermodynamic effect in R114. In addition, the onset of cavitation instabilities (alternate blade cavitation and rotating cavitation) are determined from the analysis of pressure fluctuations. The thermodynamic effect which affects the onset of instabilities is also estimated and compared to the one deduced from visualizations
Numerical Study of the Effect of the Leading Edge Shape on Cavitation around Inducer Blade Sections
No full textA numerical study of the cavitation behaviour of two-dimensional hydrofoils simulating a section of an inducer blade is presented. Two leading edge shapes were chosen to approach rocket engine inducer designs. They was tested with respect to the development of sheet cavitation.
The numerical model of cavitating flows is based on the 3D code FINE/TURBOTM, developed by NUMECA International. The cavitation process is taken into account by using a single fluid model, which considers the liquid vapour mixture as a homogeneous fluid whose density varies with respect to the static pressure.
Numerical results are compared with experimental ones, obtained in the CREMHyG large cavitation tunnel [Reboud et al. 1996]. Pressure distributions along the foil suction side and the tunnel walls were measured for different cavity lengths. Total pressure measurements along the foil suction side allow characterizing the effects of cavitation on the liquid flow.
Influence of the leading edge shape on the cavitation behaviour and comparison between experiments and numerical predictions are discussed
A Modified Isenthalpic Model of Cavitation in Plane Journal Bearings
No full textThis paper presents the development of a quasi-homogeneous isenthalpic cavitation flow model, suitably modified to account for thermal cavitation, and its application to the study of plane journal bearings with constant eccentricity. The proposed model treats the cavitating and noncavitating portions of the fluid in a unified manner with the aim of avoiding the use of matching conditions at the phase interface, whose accuracy is questionable in the presence of significant inertial and/or unsteady effects. A non-linear analysis which accounts for the inertia of the lubricant is used to determine the reaction forces caused by the shaft eccentricity both in the viscosity-dominated regime and at intermediate values of the Reynolds number, where the inertia of the lubricant is no longer negligible. The classical iteration method for the Reynolds lubrication equation (Muster and Sternlicht, 1965; Mori and Mori, 1991; Reinhardt and Lund, 1975) has been extended to the two-phase flow case in order to account for flow acceleration effects in the presence of cavitation. Comparison with available experimental data are shown in a number of representative cases, in order to illustrate the validity and the capabilities of the proposed model for the analysis of cavitating flows in journal bearings, in view of its extension to the case of whirling loads and eccentricitie
Proposal of a Groove Cavitator on a Supercavitation Propeller
No full textA supercavitation propeller (SCP) that has high efficiency under supercavitating condition is expected as one of the most suitable propulsors for a high-speed vessel. To design a SCP with higher efficiency, a thinner cavity is recommended. However, less supercavitation sometimes occur contrary to the designer's expectation, thus the thrust becomes less than the design value and the efficiency becomes lower. In order to obtain the predicted thrust and high efficiency, it is necessary to let stable cavitation occur from the leading edge as predicted by theory. The authors propose a new cavitator that stimulates cavitation by a thin groove near the leading edge on the backside surface. Through the present comparative tests between propellers with and without the cavitator, it was clarified that the proposed cavitator is effective in stimulating and stabilizing a supercavity, and that it increases the propeller efficiency
Cavitation Inception by Almost Spherical Solid Particles in Water
No full textThe tensile strength of water increases when solid particles are filtered out, and it becomes greater the smaller the remaining particles are. Natural particles are of random shape, making parametric studies on the relationship between tensile strength and particle characteristics difficult. In this investigation, using degased tap water from which natural particles larger than about 1 (m were filtered out, the tensile strength was measured before and after seeding with almost spherical solid balls of diameters from 3 (m up to 76 (m. The smallest balls, though hydrophobic and notably larger than the remaining natural nuclei, had no measurable influence on the tensile strength. Seeding with balls at least a factor of ten to forty larger than the largest remaining natural nuclei reduced the tensile strength by only between 1/3 and 2/3 of that measured for the unseeded filtered water. On this basis it is concluded that a greater tensile strength is connected to the almost spherical solid balls than that connected to natural particles of the same size. The critical cavities developed from the larger balls had radii much smaller than those of the balls themselves. This supports the hypothesis that the cavitation nuclei are related to the fine scale surface structures observed on the balls. A model of their development is presented
Hydrodynamic Effects of Phase Relative Motion on Critical Bubbly Cavitating Flows Through a Converging-Diverging Nozzle
No full textOne-dimensional bubbly flows through converging-diverging nozzles are investigated using a two-fluid model. Effects associated with both translational and radial relative motions between bubbles and liquid are incorporated. Calculation of a subsonic case is performed first and shows good agreement with experiments. The model is then applied to critical (or choked) flow situations studied previously by Muir and Eichhorn (1963). In their experiments, Muir and Eichhorn found larger critical pressure ratios (which are defined as the ratios of the throat pressure to the pressure upstream of the nozzle under choked conditions) and mass flow rates than homogeneous flow theory. They measured significant slip between phases which, therefore, was speculated to be responsible for these discrepancies. It is demonstrated in this paper that the phase relative velocity can be predicted reasonably well (within the experimental uncertainty) using the present model. Excellent agreement between the predicted critical mass flow rates and the experimental data is obtained. However, compensation for the critical pressure ratios is not apparent.
Other important natures of the critical flows are also explored, including the formation of compression shock waves present in the divergent part of the nozzle. Our computations show that the pressure ratios across the shocks agree very well with the Hugoniot relation established by Thang and Davis (1981)
Modeling of Unsteady Blade Sheet and Developed Tip Vortex Cavitation
No full textA boundary element method is used for
the numerical modeling of unsteady blade sheet
and developed tip vortex cavitation on propellers.
The objective of this work is to predict more accurately blade sheet and developed
tip vortex cavity in the vicinity of the blade tip
subject to a non-axisymmetric flow-field. The
ultimate goal of this work is to predict more accurately the hull pressures
induced by the unsteady cavities on the blade and tip.
Initially, we assume that the section of the tip vortex cavity shape is
circular and the wake a pure helical surface without contraction and roll-up.
Once the fully wetted problem
is solved by applying the potential based panel method on the assumed tip vortex cavity and wake geometry, the
three-component
velocities on the tip vortex cavity are calculated by numerically
differentiating the velocity potential, and those on the wake surface
are determined from the differentiated Green's formula.
The new wake surface and the trajectory of the tip vortex cavity core
are determined by aligning the wake surface with the flow velocity,
in fully unsteady manner.
Once the aligned wake surface is determined in an iterative way,
the shape of the blade sheet and tip vortex cavity, having a
constant pressure distribution, is determined by applying the
dynamic and the kinematic boundary conditions on the cavity surface.
The method is applied in the case of
simplified 2-D vortex cavity, 3-D elliptic wing, and propeller blades
subject to inclined and non-axisymmetric inflows. Comparisons with experiments
in terms of unsteady cavity shapes, tip vortex cavity trajectories, and unsteady
blade forces, are finally presented
"Ariane 5" TPLOX Inducer Design Strategies To Enhance Cavitating Performance
No full textOptimization process in cavitating conditions of the axial inducer of the ``Ariane~5'' main engine liquid oxygen (LOX) turbopump is described in details. Different inducer configurations were set up and investigated, starting from the reference one to produce an optimized final geometry.
Both 3D fully--viscous computations and experimental tests in water and LOX were exploited during this activity. Computations allowed the designer to identify cavitation inception on the reference inducer, and to compare its
performance to the final one, assessing the effectiveness of the redesign process.
Experiments were carried out to study the development of cavitation, investigate the inducer behavior in cavitating conditions, and suggest suitable modifications to the geometry.
As a result, an optimized blade leading edge shape was set, which revealed a significant trend to reduce cavitation effects with a good smoothing of the head/cavitation curve
Molecular Dynamics Study of the Nucleation of Bubble
No full textIn this paper the effect of internal degrees of freedom on the limit of metastability is analyzed by the
Molecular Dynamics (MD) method. Oxygen is assumed as the liquid and oxygen molecules are assumed as
both monatomic and diatomic molecules. The Lennard{Jones (LJ) potential is used as the intermolecular
potential for monatomic molecules and 2 Center Lennard{Jones (2CLJ) potential is used as the intermolecular
potential for diatomic molecules. These parameters are determined so that the averaged potentials of the
respective molecules are consistent with each other. Simulations are performed at various states and an
Equation of State (EOS) of each liquid is obtained. The spinodal lines obtained from the respective EOSs
are compared with each other and the effect of molecular orientation in diatomic liquid on the thermodynamic
limit of metastability is investigated. Moreover, the kinetic limit of metastability is also investigated and
the difference between the monatomic and diatomic liquid oxygen is analyzed