Tip Vortex Cavitation Inception Research Articles

Experimental Study On Pressure-Velocity Relation Of Near-Field Tip Vortex Using Tomographic PIV

The non-cavitating tip vortex in the near field of an elliptical hydrofoil is studied using tomographic particle image velocimetry (TPIV). By investigating the distributions of the axial velocity difference and streamwise vorticity, the formation and development of the near-field tip vortex are clearly revealed. In the near field, the axial flow within the tip vortex manifests a jet-like profile, and the majority of the vorticity is contained within the vortex core. A special position is identified during the streamwise evolution of the tip vortex, where the vortex core circulation reaches its local maximum for the first time and tip vortex cavitation (TVC) might be more prone to incept. In the vicinity of this crucial position, a concise relation between the static pressure along the tip vortex center line and the local velocity is derived by combining the three-dimensional measured velocity fields with the governing equations. Based on the Reynolds-averaged Navier-Stokes equation, it is revealed that the mean static pressure along the vortex center line is directly related to the local mean axial velocity, whereas the local velocity fluctuation has little impact on the mean static pressure. It is the first attempt to combine the three-dimensional measurement with the governing equations to investigate the near-field tip vortex flow, which might provide valuable insights for understanding and predicting tip vortex cavitation inception.

Numerical Method to Determine the Inception of Propeller Tip Vortex Cavitation Based on Bubble Dynamics

Due to the scale effect, tip vortex cavitation (TVC) is the earliest type of cavitation that occurs on real ship propellers. As a result, experts in the ship field have been paying close attention to the accurate prediction of propeller TVC inception for a long time. The motion and growth of the microscopic nuclei in the water have a significant influence on TVC inception. However, the minimum pressure coefficient method—a common method at present—based on the traditional Eulerian framework, neglects the influence of microscopic nuclei and therefore cannot accurately predict the cavitation inception. Moreover, the numerical prediction method for cavitation inception, which is based on bubble dynamics models and considers the influence of nuclei, has not established a set of unified and specific discrimination criteria applicable to propeller cavitation inception. In order to make up for the shortcomings of traditional prediction models and the existing methods based on bubble dynamics in the prediction of TVC inception, we propose a new discrimination method for propeller TVC inception based on bubble dynamics in this paper. The comparison with experimental results demonstrates that our proposed method allows us to predict propeller TVC inception more accurately. In addition, the effect mechanism of tip vortex flow characteristics on nuclei evolution is further investigated, and it is found that when approaching the low-pressure region at a vortex core under the influence of tip vortex suction, nuclei grow explosively under the continuous action of the low pressure at the vortex core until they reach their maximum sizes and then collapse rapidly.

Experimental characterization of the flowfield and cavitation physics of a tip-loaded hydrofoil

This paper reports an experimental study of tip vortex flowfield and cavitation inception of a tip-loaded hydrofoil. Vortex strength, wandering, and turbulence statistics are characterized using stereo particle image velocimetry (SPIV) in a water tunnel facility, at a chord Reynolds number of 1.3×106. Cavitation physics are characterized using high-speed videography and dual-hydrophone acoustic cavitation measurements. The loading of the rectangular planform hydrofoil has a maximum at 65% span, 56% greater than that at the root, i.e., the hydrofoil loading is representative of non-elliptically loaded open propellers. Acoustic cavitation inception is quantified and is observed to precede visual cavitation onset using unaided and high-speed imaging. Measurements reported here show that vorticity fluctuations are nearly of the same magnitude as the ensemble vorticity. Instantaneous measurements of vorticity at the trailing edge, 12-chord downstream, and one-chord downstream positions are reported. Their peak magnitudes are located adjacent to the ensemble vortex center and are between four and five times the ensemble mean. The fluctuating vorticity measurements, taken in conjunction with high-speed video observations, provide insight into the hydrodynamic conditions responsible for intermittent cavitation events. The reported measurements elucidate instantaneous and mean turbulence physics associated with vortex cavitation and can provide a validation basis for numerical simulations.

Numerical study on prediction and discrimination methods of propeller tip vortex cavitation inception

Tip vortex cavitation is typically the first type of cavitation in real ship propellers due to the scale effect. To lessen or eliminate propeller cavitation noise, it is necessary to effectively identify and predict the tip vortex cavitation initiation of the propeller and run the propeller as far as possible in the “non-cavitation area.” However, for the current numerical computation, it is impossible to determine the minimum pressure at the vortex core directly and correctly, making it difficult to anticipate the cavitation initiation of the propeller tip vortex. In this paper, based on computational fluid dynamics (CFD) numerical calculations and the tip vortex model proposed by Xin [Proceedings of 2013 Ship Hydrodynamics Conference (2013), pp. 211–218], we propose a new prediction method for propeller tip vortex cavitation initiation—“the tip vortex model method.” The propeller tip vortex is solved by CFD calculation in this model, and the minimum pressure at the downstream vortex core is examined based on the tip vortex model to determine the initiation of propeller cavitation. We examine the cavitation inception using the tip vortex model method and compare it to the results obtained using the minimum pressure coefficient method and the minimum vapor volume method. It is discovered that the cavitation inception number obtained by the tip vortex model method is closer to the experimental results. Furthermore, the effects of different turbulence models and grid settings on the prediction of tip vortex cavitation initiation are investigated, providing an effective reference for the prediction of propeller tip vortex cavitation initiation.

Numerical Study on Tip Vortex Cavitation Inception on a Foil

In this paper, the inception of tip vortex cavitation in weak water has been predicted using a numerical simulation, and a new scaling concept with variable exponent has also been suggested for cavitation inception index. The numerical simulations of the cavitating flows over an elliptic planform hydrofoil were performed by using the RANS approach with a Eulerian cavitation model. To ensure the accuracy of the present simulations, the effects of the turbulence model and grid resolution on the tip vortex flows were investigated. The turbulence models behaved differently in the boundary layer of the tip region where the tip vortex is developed, which resulted in different pressure and velocity fields in the vortex region. Furthermore, the Reynolds stress model for the finest grid showed a better agreement with the experimental data. The tip vortex cavitation inception numbers for the foil, predicted by using both wetted and cavitating flow simulation approaches, were compared with the measured cavitation index values, showing a good correlation. The current cavitation scaling study also suggested new empirical relations as a function of the Reynolds number substitutable for the two classic constant scaling exponents. This scaling concept showed how the scaling law changes with the Reynolds number and provided a proper scaling value for any given Reynolds numbers under turbulent flow conditions.

Numerical Investigation of Tip Vortex Cavitation Inception and Noise of Underwater Propellers of Submarine Using Sequential Eulerian–Lagrangian Approaches

In this study, the high-fidelity numerical methods are developed to investigate the tip vortex cavitation (TVC) inception and noise of underwater propellers, namely, Model-A and Model-B, which are designed to investigate the effects of sweep angle on cavitation inception and noise. In addition, the entire body of the DARPA Suboff submarine is included to consider the effects of the inflow distortion originating from the boundary layer flow of the submarine body on the cavitating flow of the propellers. The Eulerian approach consisting of Reynolds-averaged Navier–Stokes (RANS) solver and the vortex model is coupled with the Lagrangian approach using the bubble dynamics equations and the acoustic analogy for nuclei initially distributed in inlet flow. First, three-dimensional incompressible unsteady RANS simulations are performed to predict the hydrodynamic flow field driven by underwater propellers installed on a DARPA Suboff submarine body. The Scully vortex model and dissipation vortex model (DVM) are used to regenerate the tip vortex dissipated by artificial numerical damping and low grid resolution around the vortex core center, which is identified by using minimum λ2-criterion in the swirling flow field originating from the propeller blade tip. Then, tip vortex cavitation inception is simulated by applying the bubble dynamics equations to nuclei initially distributed in the inflow region. The volume and location of each nucleus are obtained by solving the bubble dynamics equations on the flow field obtained using the Eulerian method. Finally, the cavitation noise is predicted by modeling each bubble with a point monopole source whose strength is proportional to its volume acceleration. The validity of the present numerical methods is confirmed by comparing the predicted acoustic pressure spectrum with the measured ones.

Experimental analysis of tip vortex cavitation mitigation by controlled surface roughness

This study presents results of experiments where roughness applications are evaluated in delaying the tip vortex cavitation inception of an elliptical foil. High-speed video recordings and laser doppler velocimetry (LDV) measurements are employed to provide further details on the cavitation behavior and tip vortex flow properties in different roughness pattern configurations. The angular momentum measurements of the vortex core region at one chord length downstream of the tip indicate that roughness leads to a lower angular momentum compared with the smooth foil condition while the vortex core radius remains similar in the smooth and roughened conditions. The observations show that the cavitation number for tip vortex cavitation inception is reduced by 33% in the optimized roughness pattern compared with the smooth foil condition where the drag force increase is observed to be around 2%. During the tests, no obvious differences in the cavitation inception properties of uniform and non-uniform roughness distributions are observed. However, the drag force is found to be higher with a non-uniform roughness distribution.

Numerical Investigation of Tip-Vortex Cavitation Noise of an Elliptic Wing Using Coupled Eulerian-Lagrangian Approaches

In this study, a numerical methodology is developed to investigate the tip-vortex cavitation of NACA16-020 wings and their flow noise. The numerical method consists of a sequential one-way coupled application of Eulerian and Lagrangian approaches. First, the Eulerian method based on Reynolds-averaged Navier–Stokes equation is applied to predict the single-phase flow field around the wing, with particular emphasis on capturing high-resolution tip-vortex flow structures. Subsequently, the tip-vortex flow field is regenerated by applying the Scully vortex model. Secondly, the Lagrangian approach is applied to predict the tip-vortex cavitation inception and noise of the wing. The initial nuclei are distributed upstream of the wing. The subsequent time-varying size and position of each nucleus are traced by solving spherically symmetric bubble dynamics equations for the nuclei in combination with the flow field predicted from the Eulerian approach. The acoustic pressure at the observer position is computed by modelling each bubble as a point source. The numerical results of the acoustic pressure spectrum are best matched to the measured results when the nuclei number density of freshwater is used. Finally, the current numerical method is applied to the flows of various cavitation numbers. The results reveal that the cavitation inception determined by the predicted acoustic pressure spectrum well matched the experimental result.

Experimental and numerical investigations of tip vortex cavitation for the propeller of a research vessel, “The Princess Royal”

The present study includes experimental and numerical investigations on propeller cavitation phenomena, particularly the tip vortex cavitation, for the test cases conducted with the model propeller of the Newcastle University research vessel, “The Princess Royal”. These test cases were recommended recently as the benchmark data for cavitation observations and noise measurements by the ITTC, and have been tested in a major round-robin campaign by eight cavitation tunnel/basin facilities around the world. This round-robin test campaign included the Newcastle Emerson Cavitation Tunnel where the first set of tests was conducted.This study presents further systematic measurements conducted in the Shanghai Jiao Tong University (SJTU) cavitation tunnel with this benchmark propeller in the open water conditions, including the cavitation observations, and tests for the tip vortex cavitation inception and desinence. Also, the study includes computational fluid dynamics (CFD) investigation to discuss the results of the experiments, especially the tip vortex cavitation, in a comparative manner. To simulate tip vortex cavitation, the newly developed adaptive mesh approach MARCS (Mesh Adaption Refinement Approach for Cavitation Simulations) was further enhanced. The results of the CFD computations using this approach showed good agreement with the results of the tunnel tests for the open water propeller performance characteristics, cavitation observations and tip vortex cavitation pattern, including its extent.

Statistical aspects of tip vortex cavitation inception and desinence in a nuclei deplete flow

Tip vortex cavitation (TVC) inception and desinence behaviour of a NACA 0012 cross section, elliptical hydrofoil is investigated from a statistical perspective in a cavitation tunnel. Measurements were made for incidences from 4° to 16° and Reynolds numbers from 1.0 × 106 to 2.1 × 106. The statistics of TVC inception were quantified by taking repeated measurements of the time until the appearance of a tip vortex cavity for a range of fixed incidences. In other experiments, the angle of attack was continuously increased until inception and then decreased until desinence for a range of fixed cavitation numbers. The data were primarily acquired via an automated process using a laser and photodiode to detect the presence of a cavity. Measurements show that TVC inception in a nuclei deplete flow is a probabilistic process for which a large dataset is required for accurate characterisation. The probability of ingesting and activating a nucleus increases with time at a given test condition due to the increased volume of water exposed to low pressures. TVC desinence exhibits far less statistical variation than inception and is largely independent of the natural nuclei population. It does, however, exhibit hysteresis which is dependent on the topology of the cavitating flow. For the desinence of unattached cavitation, there is a small hysteresis between the inception and desinence indices. However, desinence is delayed for attached cavitation.

Investigations of tip vortex mitigation by using roughness

The application of artificial roughness to mitigate tip vortex cavitation inception is analyzed through numerical and experimental investigations carried out on an elliptical foil. Different roughness configurations and sizes are tested, and effects on cavitation inception, drag, and lift are studied. Implicit Large Eddy Simulation is employed to conduct the simulation on a proper grid resolution having the tip vortex spatial resolution as fine as 0.062 mm. Two different approaches including using a rough wall function and resolving the flow around roughness elements are evaluated. New experiments, performed in the cavitation tunnel at Kongsberg Hydrodynamic Research Center, for the rough foil are presented. The vortical structures and vorticity magnitude distributions are employed to demonstrate how different roughness patterns and configurations contribute to the vortex roll-up and consequently on the tip vortex strength. It is found that the application of roughness on the leading edge, tip region, and trailing edge of the suction side is acceptable to mitigate the tip vortex and also to limit the performance degradation. This is regarded to be in close relation with the way that the tip vortex forms in the studied operating condition. The boundary layer characteristics show that roughness separation line is the reason for a more even distribution of vorticity over the tip compared to the smooth foil condition, leading to a reduction in vortex strength. For the optimum roughness pattern, both the numerical results and experimental measurements show a decrease in the tip vortex cavitation inception as large as 33% compared to the smooth foil condition with a drag force increase observed to be less than 2%.

Study of tip vortex cavitation inception and vortex singing

Tip vortex cavitation (TVC) is an important cavitation phenomenon in marine propeller. The formation and evolution of tip vortex cavitation are hot topics consistently both in engineering application and mechanism research. In this paper some recent studies on tip vortex cavitation inception and the noise of tip vortex cavitation evolution are presented. The effects of both flow field and water qualities on tip vortex cavitation inception are considered by experiments and numerical simulations. The results show that besides the average minimum pressure in the vortex core the turbulence fluctuation and water qualities including air content and nuclei distribution have great influence on tip vortex cavitation inception. Based on the idea of first nucleus cavitating in tip vortex core new prediction formula for tip vortex cavitation inception is proposed. The synchronous technique of high speed video observation and noise measurement are adopted to study the development of tip vortex cavitation. S-type total noise characteristics are obtained when cavitation number from low to high. Vortex singing is found in the case where the tip vortex cavitation just before leaves the tip region. The excited mechanism of vortex singing is proposed by analyzing the wave propagation on the interface of vortex cavity.

Suppressing tip vortex cavitation by winglets

Despite the numerous remedies prescribed so far, tip vortex cavitation (TVC) remains a major issue in design and operation of diverse applications. In this paper, we experimentally investigate the effectiveness of winglets in suppressing TVC. An elliptical hydrofoil is selected as the baseline geometry and various winglets are realized by bending the last 5 or 10% of the span at ± 45° and ± 90° dihedral angles. To better focus on the physics of the problem, we have intentionally avoided any optimization on the geometries and our winglets are only smooth non-planar extensions of the original cross-section. Modifying no more than 3.7% of the lifting surface, lift-and-drag force measurements demonstrate that the hydrodynamic performances of the winglet-equipped hydrofoils are not substantially different from the baseline. Nevertheless, cavitation inception–desinence tests reveal that undeniable advantages are achieved by the winglets in TVC alleviation. It is found that the 10%-bent 90° winglets are more effective than the 45° cases, with − 90° (bent down toward the pressure side) performing superior to + 90°. For instance, the 90°-bent-downward winglet reduces the TVC inception index from 2.5 for the baseline down to 0.8 (a reduction of 68%) at 15 m/s freestream velocity and 14° incidence angle. In addition, the study on the bending length effect conducted for the 90° configurations shows that the 5%-bent winglets are not as striking as the 10% ones. Employing Stereo-PIV technique, the influence of winglets on non-cavitating flow structures is examined. For the most effective winglet (10%-bent 90°-downward), we observe that the maximum tangential velocity of the tip vortex falls to almost half of the baseline and the vortex core size increases significantly (by almost 70%). These effects are accompanied by a tangible reduction in the axial velocity at the vortex core leading to further mitigation of TVC.

On the physical mechanism of tip vortex cavitation hysteresis

Inception and desinence thresholds of tip vortex cavitation (TVC), generated by an elliptical NACA 16-020 hydrofoil, are measured at different flow conditions for various gas contents. It is observed that TVC often disappears at cavitation indices significantly higher than the inception thresholds introducing large hystereses. Our measurements reveal that TVC desinence pressure increases with gas content and, under specific flow conditions, may reach to atmospheric pressure. When the pressure of the cavitating core is below the initial saturation pressure of the dissolved gases, water flowing adjacent to the interface becomes supersaturated, which leads to the diffusion of air molecules into TVC. To estimate the outgassing rate, a simple diffusion model is proposed and analytically solved. In addition, we demonstrate that the extent of the delay in desinence due to outgassing is also dictated by the bulk flow parameters, i.e., the incidence angle and freestream velocity. Owing to flow visualizations, we assert that formation of a laminar separation bubble of appropriate size and shape at the hydrofoil tip is a necessary condition for a delayed desinence. The separation bubble acts like a shelter and creates a relatively calm area at the vortex core by forcing the incoming flow to wrap around the axis. By roughening the hydrofoil tip, we demonstrate that the hysteresis is completely suppressed once the laminar separation bubble is destroyed. Moreover, our velocity measurements show that at near-wake, the incidence angle associated with delayed desinence is accompanied by a jet-like axial velocity profile while a wake-like profile is observed for the low-hysteresis case.

Influence of water quality on the tip vortex cavitation inception

Tip vortex cavitation (TVC) is an important type of cavitation. There is an urgent need for the accurate prediction of its inception condition. The water quality, i.e., the distribution of nuclei, is one of the main contributors to the TVC inception. Since there is no available cavitation model involving the effect of water quality, a new model suitable for TVC prediction is investigated based on the bubble dynamics. The migration and growth of nuclei are simulated in a tip vortex flow. An analytic method is developed to calculate the capture time of nuclei so that we can determine the critical nucleus in a vortex flow according to a filtering principle. The comparisons of the effect of water quality obtained in simulations and analytics are performed. It validates that the embodiment of water quality is rational and the analytical method is applicable for engineering purpose. Finally, the prediction model is quantitatively improved by considering the effect of water quality, and it is a new model for the prediction of tip vortex cavitation inception.

Numerical and experimental analysis of cavitation inception behaviour for high-skewed low-noise propellers

Numerical and experimental investigations of two highly skewed low-noise propellers with a slight alteration in the tip geometry are conducted in both wetted and cavitating flows. The performance of the propellers is measured in open water and inclined setups at different operating conditions and high speed video recordings are used to determine tip vortex cavitation inception behaviour. The numerical simulations are conducted by employing the Implicit Large Eddy Simulation on appropriate grid resolutions for tip vortex propagation, at least 32 cells per vortex diameter according to our previous studies. In order to investigate cavitation inception characteristics of the propellers, different inception prediction methods are employed and evaluated at different advance ratios.It is shown that in addition to the well-captured difference in e.g. the amount of cavitation, the simulations are capable of correctly predicting the small but crucial differences in flow features and cavitation inception characteristics of the two propeller designs. Numerical predictions of the cavitation inception charts are also compared successfully with the measured data where three different types of cavitation patterns are investigated in details. Supported by the experimental videos, the interaction between the tip vortex and trailing vortices and their impact on the pressure field and the cavitation inception are analyzed. It is shown that the numerical simulation can provide further details about the vortical flow structures, and their contributions to cavitation, and is a powerful tool in advanced propeller design stages.

The effect of water quality on tip vortex cavitation inception

The inception of tip vortex cavitation is very sensitive to water quality. In order to quantify the effect of water quality on the inception of tip vortex cavitation, we develop a motion model to describe the migration and growth of nuclei in water. An analytical solution of migration of nuclei in a vortex flow is obtained so that the capture times of various nuclei can be given out directly. A criterion is built to determine the critical nucleus in a certain nuclei spectra distribution. Tensile strength of the critical nucleus is used to quantify the effect of water quality and correct the tip vortex cavitation inception number. Finally this change of cavitation inception number is compared with experimental results to validate our model.

Experimental measurement of tip vortex flow field with/without cavitation in an elliptic hydrofoil

In this paper, recent measurements of tip vortex flow with and without cavitation carried out in Cavitation Mechanism Tunnel of China Ship Scientific Research Center (CSSRC) are presented. The elliptic hydrofoil with section NACA 662-415 was adopted as test model. High-speed video (HSV) camera was used to visualize the trajectory of tip vortex core and the form of tip vortex cavitation (TVC) in different cavitation situations. Laser Doppler velocimetry (LDV) was employed to measure the tip vortex flow field in some typical sections along the vortex trajectory with the case of cavitation free. Stereo particle image velocimetry (SPIV) system was used to measure the velocity and vorticity distributions with and without cavitation. Series measurement results such as velocity and vorticity distributions, the trajectory of tip vortex core, the vortex core radius, cavity size and cavitation inception number were obtained. The results demonstrated that the minimum pressure coefficient in the vortex core obtained by flow field measurement was quite coincident with the tip vortex cavitation inception number obtained under the condition of high incoming velocity and low air content. And TVC would decrease the vortex strength comparing with the case without cavitation.

An acoustic approach to determine tip vortex cavitation inception for an elliptical hydrofoil considering nuclei-seeding

Tip vortex cavitation is usually the first type of propeller cavitation to appear with intense noise. The precise prediction of tip vortex cavitation inception is of great importance. In this paper, via an acoustic approach an experimental investigation on tip vortex cavitation inception for an elliptical hydrofoil has been done in a cavitation mechanism tunnel. For non-nuclei-seeding conditions, the sound level “collapses” when the tip vortex cavitation approaches desinence and there exists an inverse N-shape curve between the sound pressure level and cavitation number, drastically different from the generally known S-shape curve. Three nuclei-seeding conditions are then investigated to study the nuclei effects on the tip vortex cavitation inception. We propose an acoustic criterion to determine tip vortex cavitation inception applicable to both non-nuclei-seeding and nuclei-seeding conditions. The results confirm that the nuclei content and distribution in water indeed play an important role in the cavitation inception process. Supplemental observations from the high-speed video camera validate the proposed acoustic method.

The migration and growth of nuclei in an ideal vortex flow

Tip vortex cavitation occurs on ship propellers which can cause significant noise compared to the wet flow. In order to predict the inception of tip vortex cavitation, numerous researches have been investigated about the detailed flow field around the tip. According to informed studies, the inception of tip vortex cavitation is affected by many factors. To understand the effect of water quality on cavitation inception, the motion of nuclei in an ideal vortex flow, i.e., the Rankine vortex flow, was investigated. The one-way coupling point-particle tracking model was employed to simulate the trajectory of nuclei. Meanwhile, Rayleigh-Plesset equation was introduced to describe the growth of nuclei. The results show that the nucleus size has a significant effect on nucleus’ trajectory. The capture time of a nucleus is approximately inversely proportional to its radius. The growth of nucleus accelerates its migration in the vortex flow and shortens its capture time, especially for the case of explosive growth.