Air Entrainment in Ventilated Cavities: Case of the Fully Developed “Half-Cavity”

1984 ◽  
Vol 106 (3) ◽  
pp. 327-335 ◽  
Author(s):  
A. R. Laali ◽  
J. M. Michel
Keyword(s):  
Cavity Length ◽  
Dominant Role ◽  
Air Entrainment ◽  
Cavitation Number ◽  
Pulsation Frequency ◽  
Cavity Pressure ◽  
Air Flow Rate ◽  
Small Step ◽  
Flow Geometry ◽  
Theoretical Considerations

Experimental results are given concerning the behavior of the so called “half-cavity” which is formed by a plane water jet, initially horizontal, projected over a small step at the bottom of a channel. The relation between the air flow rate and the cavity pressure is given particular consideration: the influence of geometrical or dynamic parameters on this relation is studied and it is found that the dominant role is played by the Froude number and the cavitation number. Other results concern cavity pulsation frequency and cavity length. Some theoretical considerations concerning the flow geometry are necessary to identify the gravity effect for the case where the cavity is long compared to the height of the step.

Base-Vented Hydrofoils of Finite Span Under a Free Surface: An Experimental Investigation

1984 ◽  
Vol 28 (02) ◽  
pp. 90-106
Author(s):  
Jacques Verron ◽  
Jean-Marie Michel
Keyword(s):  
Experimental Investigation ◽  
Free Surface ◽  
Flow Rate ◽  
Cavity Length ◽  
Three Dimensional ◽  
Leading Edge ◽  
Pulsation Frequency ◽  
Cavity Pressure ◽  
Air Flow Rate ◽  
Cavity Configuration

Experimental results are given concerning the behavior of the flow around three-dimensional base-vented hydrofoils with wetted upper side. The influence of planform is given particular consideration so that the sections of the foils are simple wedges with rounded noses. Results concern cavity configuration, the relation between the air flow rate and cavity pressure, leading-edge cavitation, cavity length, pulsation frequency, and force coefficients.

Thermodynamic Effects on Developed Cavitation

1975 ◽  
Vol 97 (4) ◽  
pp. 507-513 ◽  
Author(s):  
J. W. Holl ◽  
M. L. Billet ◽  
D. S. Weir
Keyword(s):  
Cavity Length ◽  
Cavitation Number ◽  
Measured Temperature ◽  
Cavity Pressure ◽  
Dimensionless Form ◽  
Dimensionless Numbers ◽  
Thermodynamic Effect ◽  
Thermodynamic Effects ◽  
Temperature And Pressure ◽  
Temperature Depression

The results of an investigation of thermodynamic effects are presented. Distributions of temperature and pressure in a developed cavity were measured for zero- and quarter-caliber ogives. A semiempirical entrainment theory was developed to correlate the measured temperature depression, ΔT, in the cavity. This theory correlates ΔTmax expressed in dimensionless form as the Jakob number in terms of the dimensionless numbers of Nusselt, Reynolds, Froude, and Pe´cle´t, and dimensionless cavity length, L/D. The results show that in general ΔT increases with L/D and temperature and the cavitation number based on measured cavity pressure is a function of L/D for a given model contour, independent of the thermodynamic effect.

scholarly journals Influence of Thermodynamic Effect on Blade Load in a Cavitating Inducer

2010 ◽  
Vol 2010 ◽  
pp. 1-7 ◽  
Author(s):  
Kengo Kikuta ◽  
Noriyuki Shimiya ◽  
Tomoyuki Hashimoto ◽  
Mitsuru Shimagaki ◽  
Hideaki Nanri ◽  
...  
Keyword(s):  
Pressure Distribution ◽  
Liquid Nitrogen ◽  
Cavity Length ◽  
Cold Water ◽  
Pressure Rise ◽  
Cavitation Number ◽  
Design Parameters ◽  
Trailing Edge ◽  
Thermodynamic Effect ◽  
Blade Tip

Distribution of the blade load is one of the design parameters for a cavitating inducer. For experimental investigation of the thermodynamic effect on the blade load, we conducted experiments in both cold water and liquid nitrogen. The thermodynamic effect on cavitation notably appears in this cryogenic fluid although it can be disregarded in cold water. In these experiments, the pressure rise along the blade tip was measured. In water, the pressure increased almost linearly from the leading edge to the trailing edge at higher cavitation number. After that, with a decrease of cavitation number, pressure rise occurred only near the trailing edge. On the other hand, in liquid nitrogen, the pressure distribution was similar to that in water at a higher cavitation number, even if the cavitation number as a cavitation parameter decreased. Because the cavitation growth is suppressed by the thermodynamic effect, the distribution of the blade load does not change even at lower cavitation number. By contrast, the pressure distribution in liquid nitrogen has the same tendency as that in water if the cavity length at the blade tip is taken as a cavitation indication. From these results, it was found that the shift of the blade load to the trailing edge depended on the increase of cavity length, and that the distribution of blade load was indicated only by the cavity length independent of the thermodynamic effect.

A Numerical Optimization Technique Applied to the Design of Two-Dimensional Cavitating Hydrofoil Sections

1996 ◽  
Vol 40 (01) ◽  
pp. 28-38
Author(s):  
Shigenori Mishima ◽  
Spyros A. Kinnas
Keyword(s):  
Numerical Optimization ◽  
Cavity Length ◽  
Optimization Technique ◽  
Cavitation Number ◽  
Quadratic Functions ◽  
Two Dimensional ◽  
Systematic Design ◽  
Hydrodynamic Analysis ◽  
Total Drag ◽  
Cavitating Hydrofoil

A numerical nonlinear optimization technique is applied to the systematic design of two-dimensional partially or supercavitating hydrofoil sections. The design objective is to minimize the hydrofoil drag for given lift and cavitation number. The hydrodynamic analysis of the cavitating hydrofoil is performed in nonlinear theory, via a low-order potential-based panel method. The effects of viscosity are taken into account via a uniform friction coefficient applied on the wetted foil surface. The total drag, lift, cavitation number, and other quantities involved in the imposed constraints, are expressed in terms of quadratic functions of the main parameters of the hydrofoil geometry, angle of attack, and the cavity length. The optimization is based on the method of multipliers by coupling the Lagrange multiplier terms and the penalty function terms. The robustness and convergence of the method are extensively investigated, and the results are compared with those from applying other design methods.

Steady Performance of a Flexible Hydrofoil Near a Free Surface

1990 ◽  
Vol 34 (04) ◽  
pp. 302-310
Author(s):  
Salwa M. Rashad ◽  
Theodore Green
Keyword(s):  
Mathematical Model ◽  
Free Surface ◽  
Cavity Length ◽  
Leading Edge ◽  
Cavity Flow ◽  
Cavitation Number ◽  
Deformation Characteristics ◽  
Flow Depth ◽  
Lift And Drag ◽  
The Galerkin Method

A linearized cavity-flow theory is used to develop a mathematical model to study the steady characteristics of a flexible hydrofoil near a free surface. The Galerkin method is employed to account for the mutual interaction between the fluid and structure forces. Cheng and Rott's method [1]2 is used to derive general expressions for the deformation characteristics in steady flow of an arbitrarily shaped hydrofoil, with a clamped trailing edge and free leading edge. From the analysis it is possible to determine the lift and drag coefficients, cavity length, and the foil steady deformation for any given specific foil shape, cavitation number, angle of attack, flow depth/chord ratio and rigidity. Sample numerical results are given, and the effects of flexibility and the proximity of the free surface are discussed. Chordwise flexibility tends to increase drag and decrease lift coefficients. This effect is more serious near the free surface. A slight increase of the thickness near the leading edge diminishes the flexibility effects.

Numerical Analysis of Cavitation Inception and Desinence Behind Orifices

2020 ◽  
Vol 143 (3) ◽  
Author(s):  
E. L. Amromin
Keyword(s):  
Experimental Data ◽  
Numerical Analysis ◽  
Separated Flow ◽  
Computational Procedure ◽  
Cavitation Number ◽  
Laminar Flows ◽  
Cavity Pressure ◽  
Cavitation Inception ◽  
Separation Zones

Abstract Experimental results and trends for cavitation inception and desinence behind orifices in microchannels are quite different from the data obtained during previous experiments in much larger facilities. The objective of this paper is to explain these differences via a numerical analysis. The employed computational procedure is divided into two parts. The first part is computation of an axisymmetric separated flow around the orifice. The second part is determination of characteristics of cavities appearing within separation zones. The provided analysis of the experimental data of other researchers pointed out two sources of the above-mentioned differences. First, for larger orifices, the cavities appear in the cores of drifting vortices. For such a situation, cavitation inception and desinence number increases with the inflow speed due to an impact of turbulence, but there is no such an increase for microbubbles with laminar flows. Second, because of the difficulty to measure the cavity pressure in microbubbles, cavitation number is usually defined with employment of the vapor pressure, and this leads to misinterpretation of the measurements and their trends.

Air Entrainment and Bubble Generation by a Hydrofoil in a Turbulent Channel Flow

2019 ◽  
Author(s):  
Ichiro Kumagai ◽  
Kakeru Taguchi ◽  
Chiharu Kawakita ◽  
Tatsuya Hamada ◽  
Yuichi Murai
Keyword(s):  
Channel Flow ◽  
Flow Rate ◽  
High Speed ◽  
Air Flow ◽  
Air Entrainment ◽  
Air Bubbles ◽  
Air Flow Rate ◽  
Bubble Generation ◽  
Gas Liquid Flow ◽  
Entrained Air

Abstract Air entrainment and bubble generation by a hydrofoil bubble generator for ship drag reduction have been investigated using a small high-speed channel tunnel with the gap of 20 mm in National Maritime Research Institute (NMRI). A hydrofoil (NACA4412, chord length = 40 mm) was installed in the channel and an air induction pipe was placed above the hydrofoil. The flow rate of the entrained air was quantitatively measured by thermal air flow sensors at the inlet of the air induction pipe. The gas-liquid flow around the hydrofoil was visualized by a backlight method and recorded by a high-speed video camera. As the flow velocity in the channel increased, the negative pressure generated above the suction side of the hydrofoil lowered the hydrostatic pressure in the channel, then the atmospheric air was entrained into the channel flow. The entrained air was broken into small air bubbles by the turbulent flow in the channel. The threshold of air entrainment, the air flow rate, and gas-liquid flow pattern depends on Reynolds number, angle of attack (AOA), and hydrofoil type. We identified at least three modes of air entrainment behavior: intermittent air entrainment, stable air entrainment, and air entrainment with a ventilated cavity. At high flow speed in our experimental condition (9 m/s), a large volume of air bubbles was generated by this hydrofoil system (e.g. air flow rate was 50 l/min for NACA4412 at AOA 16 degrees), which has a high potential to reduce ship drag.

scholarly journals Investigation on the Impact of Protrusion Parameter on the Efficiency of Converting Additional Windage Loss for Ingress Alleviation in Rotor–Stator System

2016 ◽  
Vol 138 (11) ◽  
Author(s):  
Dongdong Liu ◽  
Zhi Tao ◽  
Xiang Luo ◽  
Wenwu Kang ◽  
Hongwei Wu ◽  
...  
Keyword(s):  
Reynolds Number ◽  
Flow Rate ◽  
High Efficiency ◽  
Co2 Concentration ◽  
Experimental Result ◽  
Cavity Pressure ◽  
Air Flow Rate ◽  
System Experiment ◽  
The Impact ◽  
Proper Setting

This paper presents a detailed investigation on the impact of protrusion parameter including both radial position and amount on the efficiency of cavity with protrusion converting additional windage loss for ingress alleviation in rotor–stator system. Experiment is conducted to explore the effect of protrusion parameter on ingress, and the corresponding additional windage loss is also calculated. During the experiment, rotor-mounted protrusions are circumferentially assembled at three different radial positions (0.9b, 0.8b, and 0.7b) each with four different amounts (32, 24, 16, and 8). Measurements of CO2 concentration and pressure inside turbine cavity are conducted. In the experiment, the annulus Reynolds number and rotating Reynolds number are set at 1.77 × 105 and 7.42 × 105, respectively, while the dimensionless sealing air flow rate ranges from 3047 to 8310. Experimental result shows that the cases of protrusion set at 0.8b achieve higher sealing efficiency than other cases as the cavity pressure is enhanced. The effect of protrusion amount on ingress could be obviously seen when CW is small or protrusion set in 0.7b. Furthermore, a parameter to evaluate which case obtains higher efficiency of converting additional windage loss for ingress alleviation, or alleviates ingress more efficiently for short, is applied for discussion. It is found that the case “C, N = 8” alleviates ingress most efficiently among all the cases. Therefore, proper setting of the protrusion could lead to high efficiency of converting additional windage loss for ingress alleviation in rotor–stator system.

Paper 8: Some Hydraulic Modelling Techniques

1967 ◽  
Vol 182 (13) ◽  
pp. 54-63
Author(s):  
K. J. Zanker
Keyword(s):  
Dimensional Analysis ◽  
Scaling Laws ◽  
Air Entrainment ◽  
Model Tests ◽  
Cavitation Number ◽  
Bubbly Flows ◽  
Suitable Model ◽  
Hydraulic Modelling ◽  
Two Phase ◽  
Modelling Techniques

A general discussion is given of modelling techniques based on dimensional analysis and similarity. Particular attention is focused on the modelling of two-phase effects, where it is neither always obvious which are the predominant forces nor whether considering only two forces will give reasonable similarity. Three examples—air-entraining flows, high, siphons, and bubbly flows—are considered and some suggestions of suitable model scaling laws are made. These include We/Re for air entrainment and the cavitation number for high siphons. The results of model tests on air-entraining vortices at the Kariba Hydroelectric Scheme are presented as an application of the discussion.

Thermodynamic Effect on a Cavitating Inducer—Part I: Geometrical Similarity of Leading Edge Cavities and Cavitation Instabilities

2010 ◽  
Vol 132 (2) ◽  
Author(s):  
Jean-Pierre Franc ◽  
Guillaume Boitel ◽  
Michel Riondet ◽  
Éric Janson ◽  
Pierre Ramina ◽  
...  
Keyword(s):  
Transit Time ◽  
Cavity Length ◽  
Cold Water ◽  
Rotation Speed ◽  
Leading Edge ◽  
Cavitation Number ◽  
Thermodynamic Effect ◽  
Cavity Closure ◽  
Cavity Development ◽  
Onset Of Cavitation

The thermodynamic effect on a cavitating inducer is investigated from joint experiments in cold water and Refrigerant 114. The analysis is focused on leading edge cavitation and cavitation instabilities, especially on alternate blade cavitation and supersynchronous rotating cavitation. The cavity length along cylindrical cuts at different radii between the hub and casing is analyzed with respect to the local cavitation number and angle of attack. The similarity in shape of the cavity closure line between water and R114 is examined and deviation caused by thermodynamic effect is clarified. The influence of rotation speed on cavity length is investigated in both fluids and analyzed on the basis of a comparison of characteristic times, namely, the transit time and a thermal time. Thermodynamic delay in the development of leading edge cavities is determined and temperature depressions within the cavities are estimated. Thresholds for the onset of cavitation instabilities are determined for both fluids. The occurrence of cavitation instabilities is discussed with respect to the extent of leading edge cavitation. The thermodynamic delay affecting the occurrence of cavitation instabilities is estimated and compared with the delay on cavity development.

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