Tip Vortex Cavitation Research Articles

소산이 고려된 보오텍스 모델과 버블 이론을 이용한 수중익 날개 끝 보오텍스 캐비테이션 거동 및 소음의 수치적 해석

Cavitation is the dominant noise source of the marine vehicle. Of the various types of cavitation , tip vortex cavitation is the first appearance type of marine propeller cavitation and it generates high frequency noise. In this study, tip vortex cavitation behavior and noise are numerically investigated. A numerical scheme using Eulerian flow field computation and Lagrangian particle trace approach is applied to simulate the tip vortex cavitation on the hydrofoil. Vortex flow field is simulated by combined Moore and Saffman`s vortex core radius equation and Sculley vortex model. Tip vortex cavitation behavior is analyzed by coupled Rayleigh-Plesset equation and trajectory equation. The cavitation nuclei are distributed and released in the vortex flow result. Vortex cavitation trajectories and radius variations are computed according to nuclei initial size. Noise is analyzed using time dependent cavitation bubble position and radius data. This study may lay the foundation for future work on vortex cavitation study and it will provide a basis for proper underwater propeller noise control strategies.

Investigation of the Role of Polymer on the Delay of Tip Vortex Cavitation

Cavitation tunnel measurements have shown tip vortex cavitation inception is delayed when polymers are injected from the hydrofoil tip. Experiments with polymers have also shown that the polymers cause the cavitation to become more violent. To better understand the role of polymer in the delay of tip vortex cavitation, a theoretical analysis of tip vortex cavitation inception in pure water and in polymer solution is presented. The analysis shows that while polymer injection causes instability in small bubbles, its main effect is an increase in tip vortex core radius, resulting in the delay of tip vortex cavitation inception.

Noise due to extreme bubble deformation near inception of tip vortex cavitation

A study on the tip vortex cavitation inception based on extreme bubble deformation and jet noise is presented. First, two preliminary experiments involving bubble splitting between two plates in the absence of swirl are performed to provide a correlation between the numerically computed splitting/jet noise and the measured noise. The bubble behavior and pressure signal predicted by the axisymmetric method are compared with those recorded simultaneously by using a high-speed video camera and a hydrophone. Then, numerical studies on the bubble behavior in the tip vortex flow field are conducted. The tip vortex flow near a hydrofoil is provided by a viscous flow computation, and the bubble behavior is simulated by an axisymmetric boundary element method which accounts for the provided vortex flow field. The characteristics of the bubble behavior and jet noise over a range of cavitation numbers are investigated. The effect of initial bubble nucleus size and the Reynolds number effect of the tip vortex flow on the tip vortex cavitation inception, the bubble behavior including its splitting, and jet noise are also discussed.

Application of a Boundary Element Method in the Prediction of Unsteady Blade Sheet and Developed Tip Vortex Cavitation on Marine Propellers

Most marine propellers operate in nonaxisymmetric inflows, and thus their blades are often subject to an unsteady flow field. In recent years, due to increasing demands for faster and larger displacement ships, the presence of blade sheet and tip vortex cavitation has become very common. Developed tip vortex cavitation, which often appears together with blade sheet cavitation, is known to be one of the main sources of propeller-induced pressure fluctuations on the ship hull. The prediction of developed tip vortex cavity as well as blade sheet cavity is thus quite important in the assessment of the propeller performance and the corresponding pressure fluctuations on the ship hull. A boundary element method is employed to model the fully unsteady blade sheet (partial or supercavitating) and developed tip vortex cavitation on propeller blades. The extent and size of the cavity is determined by satisfying both the dynamic and the kinematic boundary conditions on the cavity surface. The numerical behavior of the method is investigated for a two-dimensional tip vortex cavity, a three-dimensional hydrofoil, and a marine propeller subjected to nonaxisymmetric inflow. Comparisons of numerical predictions with experimental measurements are presented.

Scaling Effect on Prediction of Cavitation Inception in a Line Vortex Flow

The current study considers the prediction of tip vortex cavitation inception at a fundamental physics based level. Starting form the observation that cavitation inception detection is based on the “monitoring” of the interaction between bubble nuclei and the flow field, the bubble dynamics is investigated in detail. A spherical model coupled with a bubble motion equation is used to study numerically the dynamics of a nucleus in an imposed flow field. The code provides bubble size and position versus time as well as the resulting pressure at any selected monitoring position. This model is used to conduct a parametric study. Bubble size and emitted sound versus time are presented for various nuclei sizes and flow field scales in the case of an ideal Rankine vortex to which a longitudinal viscous core size diffusion model is imposed. Based on the results, one can deduce cavitation inception with the help of either an “optical inception criterion” (maximum bubble size larger than a given value) or an “acoustical inception criterion” (maximum detected noise higher than a given background value). We use here such criteria and conclude that scaling effects can be inherent to the way in which these criteria are exercised if the bubble dynamics knowledge is not taken into account.

On the mechanism of the bursting phenomena of propeller tip vortex cavitation

The bursting phenomenon of tip vortex cavitation of a propeller sometimes causes severe high-frequency vibration, but its mechanism has not yet been elucidated. In this study, we carried out model experiments by changing the propellers, wake distributions, thrust coefficients, and cavitation numbers parametrically, examined the bursting phenomenon with a high-speed video camera, and measured the pressure fluctuations caused by the phenomenon. We also measured flow distribution around the tip vortex. As a result, we found that in the bursting phenomenon, large pressure fluctuations occurred twice, and that they strongly depended on the wake distribution. Two means were suggested to suppress the bursting phenomenon, other than changing the wake distribution: stabilizing tip vortex cavitation or reducing the cavity volume. Numerical fluid simulations around a propeller in noncavitating, unsteady conditions were also conducted, and the strength of the tip vortex along the circumference and its derivative were examined. As a result, the phenomena were parameterized by the time derivative of the strength of the tip vortex, and if it was higher than a threshold value, the tip vortex cavitation burst. Therefore, it is possible to predict the occurrence of the bursting phenomenon by numerical analysis.

On the importance of the effect of turbulence in cavitation inception tests of marine propellers

Current methods for predicting the inception of cavitation in marine propellers rely heavily on physical model tests in cavitation tunnels and these tests are subject to complex scale effects. The viscous nature of the flow, particularly the free–stream turbulence, is one of the main factors contributing to the scale effects on the inception of cavitation. Research to explore this factor has been grossly neglected and there is an urgent need for systematic experimental data to establish possible trends in this effect to be included in the procedures used to extrapolate small–scale experimental results to predict the inception of cavitation on the full–scale propeller. This paper presents data and reports on some interesting and important findings on the effects of different turbulence generation techniques from recent research into the scale effects on the inception of cavitation of marine propellers. Systematic measurements were taken with a five–bladed model propeller to explore the viscous scale effects on the tip vortex and sheet–type cavitation using two different methods to generate turbulence. These were a wire mesh screen with variable mesh sizes placed in front of the model propeller and several grades of carborundum applied to the leading edges of the model propeller blades. The results of the measurements displayed distinct but similar trends in the effects of the two methods on the resulting inception values, in particular for the tip vortex cavitation. This finding has an important practical implication on the current cavitation testing procedures, especially for the most widely used ‘leading–edge roughness’ technique. This technique, which has various practical disadvantages, can now be replaced by a more effective and controlled way of altering the free–stream turbulence. The paper also presents an important set of systematic data on the effect of free–stream turbulence, which is scarce in the literature, and that of leading–edge roughness and it discusses the use of these data for the development of an extrapolator to predict the cavitation inception of a marine propeller at full scale.

Bursting Phenomenon of Propeller Tip Vortex Cavitation and its Relationship with the Result of Numerical Simulation of Propeller in Unsteady Condition

Bursting phenomenon of tip vortex cavitation causes severe high-frequency vibration, but estimation of it by numerical computation is not yet realized. In this study the authors made model experiments to observe the bursting phenomena and measured the pressure fluctuations caused by that, changing two propellers, wake distributions, thrust coefficients and cavitation numbers parametrically. After analyzing frequency of it we found that there would be two methods to suppress the bursting phenomenon : stabilizing the tip vortex cavitation or reducing the volume of cavitation. We also did numerical fluid simulations around a propeller in non-cavitating, unsteady condition and compared the strength of tip vortex along the circumference and its derivatives. As a result the phenomena were parameterized by the time derivative of the strength of tip vortex, and if it was higher than a certain value, tip vortex cavitation bursted. Therefore there is the possibility to predict the occurrence of bursting phenomenon by numerical analysis.

Performance Characteristics of Controllable Pitch Side Thrusters with Different Skew Distributions

In the previous paper, authors presented experimental and theoretical data on open water characteristics of two side thrusters which impellers have the forward skew (type (A)) and the backward skew (type (B)).In this report, cavitation characteristics and blade stress of these two side thrusters are investigated theoretically and experimentally. Cavity extent on the impeller blades was estimated by the propeller lifting surface theory. Then, cavitation tests were performed by using the impeller models with the duct in the cavitation tunnel. These results show that the cavity extent on the impeller blades is similar comparing these two impellers. But, the tip vortex cavitation of impeller (B) is much stronger than that of impeller (A), which causes more severe noise and vibration. The finite element method was applied for the stress analysis of the impeller blades. The stress level of impeller (B) was normal, but slightly high stress, which was acceptable, was found on the blade of impeller (A) near the leading edge of 0.7 R-0.8 R.

Generation Mechanism of Bursting Phenomena of Propeller Tip Vortex Cavitation

Bursting phenomenon of tip vortex cavitation of a propeller causes high frequency vibration, but its mechanism is not elucidated yet. In this study the authors made the experiment that used two kinds of propellers and changed some experimental conditions such as wake distributions, and examined the burst phenomenon with high speed video camera. We also measured flow distribution around the tip vortex. As a result we found that in the burst phenomenon big pressure fluctuations were measured twice, and it depended strongly on the flow distribution of the wake. With the bursting wake tip vortex moved below downstream and was bigger and slower than that of non-bursting wake so that it interfered in the cavitation of the next blade.

An experimental insight into the effect of confinement on tip vortex cavitation of an elliptical hydrofoil

The present paper is devoted to an analysis of tip vortex cavitation under confined situations. The tip vortex is generated by a three-dimensional foil of elliptical planform, and the confinement is achieved by flat plates set perpendicular to the span, at an adjustable distance from the tip. In the range of variation of the boundary-layer thickness investigated, no significant interaction was observed between the tip vortex and the boundary layer which develops on the confinement plate. In particular, the cavitation inception index for tip vortex cavitation does not depend significantly upon the length of the plate upstream of the foil. On the contrary, tip clearance has a strong influence on the non-cavitating structure of the tip vortex and consequently on the inception of cavitation in its core. The tangential velocity profiles measured by a laser-Doppler velocimetry (LDV) technique through the vortex, between the suction and the pressure sides of the foil, are strongly asymmetric near the tip. They become more and more symmetric downstream and the confinement speeds up the symmetrization process. When the tip clearance is reduced to a few millimetres, the two extrema of the velocity profiles increase. This increase results in a decrease of the minimum pressure in the vortex centre and accounts for the smaller resistance to cavitation observed when tip clearance is reduced. For smaller values of tip clearance, a reduction of tip clearance induces on the contrary a significant reduction in the maxima of the tangential velocity together with a significant increase in the size of the vortex core estimated along the confinement plate. Hence, the resistance to cavitation is much higher for such small values of tip clearance and in practice, no tip vortex cavitation is observed for tip clearances below 1.5 mm. The cavitation number for the inception of tip vortex cavitation does not correlate satisfactorily with the lift coefficient, contrary to classical results obtained without any confinement. Owing to the specificity introduced by the confinement, the usual procedure developed in an infinite medium to estimate the vortex strength from LDV measurements is not applicable here. Hence, a new quantity homogeneous to a circulation had to be defined on the basis of the maximum tangential velocity and the core size, which proved to be better correlated to the cavitation inception data.

A model for tip vortex roll-up in the near field region of three-dimensional foils and the prediction of cavitation onset

A research program known as “Action Concertée Cavitation” initiated in France in 1991 was aimed at investigating Tip Vortex Cavitation (TVC) in various experimental facilities operating over a large range of Reynolds numbers. An original method to analyze the numerous data collected during the program was developed. From this method, a correlation model of the roll-up process close to the tip (near field region) of elliptical loading foils is proposed. It is based on the experimental evidence of a linear relationship, during the roll-up process, between a and (Γt) 0.5 , where a is the local vortex core radius, Γ the local vortex intensity and t the convection time. The core radius and the vortex intensity are modeled with power laws in such a way the linear relationship is satisfied. With the hypothesis of an axisymmetric vortex, the minimum of the pressure coefficient on the vortex path is computed. It agrees well with the experimental critical cavitation number obtained for lift coefficients ranging from 0.2 to 0.6 and for Reynolds numbers ranging from 4×10 5 to 6×10 6 . Moreover, the model shows that the actual vortex diffusion appears to be faster than that predicted by laminar flow hypothesis. This can be explained by an apparent viscosity of about two orders of magnitude larger than the molecular viscosity during the roll-up process. Finally, as the model is based on a limited number of parameters to describe a complex phenomenon, it indicates also some important trends which should be examined by those seeking to mitigate the occurrence of cavitation on lifting surfaces as designers of pumps, propellers and other fluid machinery.

Sea Trials of the Ducted Tip Propeller

Sea trials of a 45 ft seine boat, fitted first with a conventional tip, and subsequently with a ducted tip, 36 in. diameter propeller have been conducted. The trials consisted of careful propeller efficiency measurements at a number of advance ratios, and underwater video imaging of the different propellers under Bollard pull conditions. The trials have shown that flow-through ducts installed at the blade tips suppress the tip vortex roll-up, thus resulting in a substantial delay in the onset of tip vortex cavitation. The cavitation inception index for the ducted tip propeller is 50 percent lower than for the conventional propeller. This cavitation improvement comes without any efficiency penalty. In fact, the efficiency of the ducted tip propeller is up to 6 percent higher (at high advance ratios) than the efficiency of the same propeller fitted with conventional tips. The ducted tip propeller was originally thought to be useful only for boats with heavily loaded propellers, such as tug boats and trawlers. The current research has shown that the ducted tips may have much broader application.

Tip Vortex Cavitation on an Oscillating Hydrofoil

This paper discusses tests conducted in the hydrodynamic tunnel of the University of Grenoble on a 3D oscillating hydrofoil. Visualization of unsteady tip vortex cavitation indicates a strong influence of the water nuclei content. The investigation was focused on the influence of the oscillation frequency on tip vortex cavitation inception. For very low nuclei content, cavitation inception is strongly delayed as compared to the steady-state results at very small oscillation frequencies. This delay is significantly reduced by nuclei seeding. The results can be explained by assuming that the time required for the inception of cavitation in the tip vortex corresponds to the time necessary for a cavitation nucleus to be captured by the vortex core.

Tip Clearance and Tip Vortex Cavitation in an Axial Flow Pump

A combined study of tip clearance and tip vortex cavitations in a pump-type rotating machine is presented. Cavitation patterns are observed and cavitation inception is determined for various gap heights, clearance and blade geometries, and rotor operating conditions. An optimum clearance geometry is seen to eliminate clearance cavitation when the clearance edge is rounded on the blade pressure side. The gap height has a strong effect on clearance cavitation inception, but the trends vary considerably when other parameters are also modified. The gap height and clearance geometry have less influence on tip vortex cavitation but forward and backward blade skew is observed to reduce and increase tip vortex cavitation, respectively, as compared to a blade with no skew.

Tip Vortex Formation and Cavitation

This paper summarizes recent research on the relation between boundary layer flow, tip vortex structure for a finite span wing, and cavitation. Three hydrofoils of elliptic planform of aspect ratio 3 were constructed with different NACA cross sections. Using a sprayed oil droplet technique to visualize the boundary layer flow, each foil was found to have dramatically different flow separation characteristics on both the suction and pressure sides. Careful examination of the tip region suggests that while the initial stages of vortex roll-up from the pressure side are similar for each hydrofoil section, the vortex boundary layer interaction on the suction side differs for each section. The degree of interaction was observed to increase as the lifting efficiency decreased. Over the Reynolds number range tested, tip vortex cavitation inception has been observed to follow an almost universal scaling. Differences in this scaling law are correlated with the degree of vortex/boundary layer interaction.

The Case of the Singing Vortex

A relatively high amplitude, discrete tone is radiated from fully developed tip vortex cavitation under certain conditions. The phenomenon of the “singing vortex” was first reported by Higuchi et al. (1989). This study more closely examines the singing phenomenon by varying the hydrofoil cross-section, scale, angle of attack, water quality, and cavitation number in two different facilities. Noise data were collected for each condition with visual documentation using both still photography and high speed video in an effort to explain the mechanism of vortex singing. The theory of Kelvin (1880) provides a framework for correlating all the data obtained.

Effect of Tripping Laminar-to-Turbulent Boundary Layer Transition on Tip Vortex Cavitation

It is by now well established that, for Reynolds numbers larger than those corresponding to the conditions of laminar-to-turbulent boundary layer transition over a flat plate (≈0.5 × 106) and for a variety of wing shapes and cross sections, desinent cavitation numbers divided by the Reynolds number to the power 0.4 correlate with the square of the lift coefficient. In the case of foils having an NACA 16020 cross section and for Reynolds numbers below or close to those leading to transition over a flat plate, the results are very much different from those obtained for well-developed turbulent boundary layer conditions. Thus, a research program has been conducted in order to investigate the effect of boundary layer manipulation on cavitation occurrence. It consisted in determining the critical cavitation numbers, the lift coefficients, and the velocities in the tip vortex of foils having either a smooth surface or tripping roughness (promoters) near the leading edge. Tests were performed using elliptical foils of NACA 16020 cross section having the promoters extending over 60, 80 and 90 percent of the semi-span. The region near the tip was kept smooth in order to distinguish laminar-to-turbulent transition effects from tip vortex cavitation inhibition effects associated with artificial roughness at the wing tip. Results obtained at very low Reynolds numbers, ≥ 0.24 × 106, with the foil tripped on both the pressure and suction sides collapse rather well with those previously obtained at much larger Reynolds numbers with the smooth foil, and correlate with the square of the lift coefficient. The differences between the tripped and smooth foil results are due to the modification of the lift characteristics through the modification of the wing boundary layer, as shown by flow visualization studies, and as a result of the local tip vortex intensity.

Effect of hydrofoil planform on tip vortex roll-up and cavitation

Effect of hydrofoil planform on tip vortex roll-up and cavitation

The Ducted Tip—A Hydrofoil Tip Geometry With Superior Cavitation Performance

A novel hydrofoil design, consisting of a small diameter flow-through duct affixed to the tip, has been studied. The tip vortex cavitation inception index, σi, of this hydrofoil geometry is about a factor of 2 lower than that of a conventional rounded hydrofoil tip. This inception improvement comes with little associated performance penalty. For angles of attack greater than 8 deg the noncavitating lift-drag ratio is actually superior to that of an unducted hydrofoil of equal span, although with lower wing loadings the hydrofoil performance is diminished by application of the ducted tip. The ducted tip is effective at reducing the tip vortex inception index because, in contrast with the rounded tip, for which vorticity in the Trefftz plane is confined to a line, the ducted tip shed vorticity at the trailing edge is distributed over a line and circle. Distributing the vorticity in this fashion causes the trailing vortex to roll up less tightly, and hence have a higher core pressure and lower σi, than a conventional hydrofoil tip. It is also suspected that the interaction at the microscale level between the flow through the duct, and the flow around it, makes the vortex core size larger, and therefore σi smaller. The ducted tip design has many potential marine applications, including to ship and submarine propellers, submarine control fins, and ship rudders.