Cavitation Tunnel Research Articles

Experimental investigation on tip vortex cavity deformation and flow dynamics using high-speed imaging and laser Doppler velocimetry measurements

Cavitation affects engineering applications relating to aquatic operations. Tip vortex cavitation (TVC) leads to several technical problems, such as efficiency loss and noise. Hence, there is a need for a comprehensive understanding of the formation mechanism, shape distortions, and underlying physical phenomena of TVC. The dispersion relation of inertial waves on inviscid cavitating vortices is a valuable tool for predicting various TVC shapes. This study explains the patterns of flow around the tip of an elliptical foil and the cavity shape deformations under different flow conditions through experimental analysis. Experiments are conducted using a National Advisory Committee for Aeronautics hydrofoil in the cavitation tunnel at Chungnam National University. The appearance, development, and shapes of vortex cavitation are closely examined using high-speed imaging technology. There is good agreement between the vortex cavity shapes captured by this high-speed imaging and those derived in previous studies. Using laser Doppler velocimetry (LDV) measurements and analysis through image processing of high-speed images, we compare the vortex core trajectories in cavitating and non-cavitating conditions. There is a good match between the two, demonstrating the feasibility of predicting the flow behaviors around vortex cavities using LDV data. As the noise from the TVC is considered a significant source of underwater radiated noise we also measured sound pressure level, which exhibits abrupt changes at specific cavitation numbers, supporting the occurrence of the singing vortex phenomenon and highlighting its sensitivity to small variations in flow conditions. Furthermore, we obtain precise measurements of the instantaneous TVC diameter under different cavitation numbers, allowing the physical functionality of distinct TVC shapes to be determined. Our results significantly extend the scientific understanding of the flow around TVC and the fundamental causes of TVC distortions.

Deep learning approach to generate propeller cavitation noise

In underwater acoustic simulators, propeller cavitation noise has traditionally been modeled as a modulated broadband signal. This study aims to enhance the realism of these simulators by employing deep learning to generate propeller cavitation noise. The training data were collected from the modeled propeller under various pressure conditions in the Samsung Cavitation Tunnel. We have utilized a variant of Generative Adversarial Networks (GANs), wherein both the generator and discriminator are designed with a recursive structure. To assess the advantages of our data-based approach, we analyze the characteristics of the signals generated and compare them with the real signals or the modeled signal based on physics. The results demonstrate the effectiveness of our proposed model in generating realistic propeller cavitation signals. [Work supported by the Korea Research Institute for Defense Technology Planning and Advancement (20-106-B00-003).]

Experiment investigation of the tip vortex cavitation around a pitching hydrofoil

Experimental research was carried out to investigate the characteristics of tip vortex cavitation (TVC) caused by tip vortex in an oscillating hydrofoil. Shanghai Shipping and Research Institute's looped cavitation tunnel served as the site of the experiments. The hydrofoil used as the test was NACA (National Advisory Committee for Aeronautics) 63820. Two high-speed cameras, one torque sensor, and six hydrophones were employed to capture the cavitation patterns, the torque exerted on the hydrofoil, and the cavitation noise, respectively. The cavitation flow under static angle of attack (AoA) and the dynamic AoA have all been investigated in order to have a better knowledge of the differences between the TVC surrounding these two conditions. The AoA of 8.5°, 10°, and 14.5° and the AoA range of 1.8°–16° were selected for the static condition research and the dynamic condition study, respectively. A tip vortex flow was created between the tip of the stationary hydrofoil and the ceiling of the tunnel test section. To investigate the impact of the cavitation number on the growth of TVC surrounding the pitching hydrofoil, various pressure environments were built up. As for the experiment environment, cavitation numbers 2.47, 2.00, and 1.66 were used. The results indicate: (1) under static conditions, decreasing cavitation and increasing AoA lead to increased stability and topological complexity of TVC. (2) Comparing and analyzing cavitation patterns during the ascending, static, and descending processes reveal that the strongest cavitation intensity during the descending process, followed by static conditions, with the least intensity during the ascending process. (3) The frequency properties under pitching motion include those of each static AoA condition, although the acoustic characteristic of higher AoA has a stronger influence on that of dynamic condition.

DESIGN AND ANALYSIS OF CAVITATION ON 22 M SUBMARINE PROPELLERS

Many methods approach in designing a propeller for a 22 m submarine, which is based on the literature of propeller design that has been widely publicized, specifically the type B-series propeller. From the B-series propeller design, it can be known that the performance diagram in the form of the propeller coefficient is finally correlated with the 22m submarine resistance value that has been tested at BTH-BPPT. This study's objective is to produce a propeller design with good hydrodynamic aspects, especially propeller with high efficiency and cavitation phenomena that do not occur on the propeller. The implementation of this research can be done by modification on the skew angle on the B-series propeller. It is expected to choose the right B-series propeller and skew angle propeller modification, capable of providing high-efficiency values and loss of cavitation in the leaf area of the propeller. The production of propeller models is needed for conducting an open water test in the Towing Tank and the cavitation test in the cavitation tunnel at the Hydrodynamics Technology Center for the Agency for Assessment and Application of Technology (BTH – BPPT).

PIV wake survey of a drone propeller for amphibious applications

Aerial and underwater drones are one of the most attractive technologies to a large number of applications, extending from surveillance of natural reserves to the monitoring of structures. For these reasons, innovative and uncommon applications are constantly appearing. Within this context, drones capable to operate continuously and efficiently in air and water can extend the standard aerial capabilities allowing underwater inspections of harbour structures, pipelines or marine reserves. For this reason, an attractive characteristic for drones is the capability to manoeuvre in different fluids as air and water. This article presents an experimental campaign, carried out both in a towing tank and in a cavitation tunnel, aimed at study the fluid-dynamics of an aeronautic drone-propeller in off-design conditions. Dynamometric, PIV and Stereo-PIV tests are carried out to evaluate the aeronautic propeller functioning in water and optimize it for amphibious missions. In particular, the forces are measured for different advance ratios J provided by varying both rotational regime and free stream velocity, in open water conditions (Towing tank tests) and in confined water (Cavitation tunnel tests). PIV and Stereo-PIV fields, acquired in the Cavitation tunnel, allow us to investigate the behaviour of the propeller in the water in a number of selected, operative conditions.

Experimental investigation of the effects of cavitation control on the dynamics of cavitating flows around a circular cylinder

In this work, we experimentally investigated effects of cavitation control on cavitating flows around a circular cylinder using mesoscale surface structuring. We considered two kinds of scalloped riblets, machined on the surface of a cylinder, to control cavitation. First, we tested a smooth cylinder placed in a cavitation tunnel at various cavitation numbers and Reynolds numbers. Then, we tested a cylinder with horizontal riblets and a cylinder with vertical riblets under the same conditions. We analyzed the effects of these scalloped riblets on cavitation dynamics behind the cylinder and on the cavitation-induced vibration. A high-speed camera captured the cavity structures behind the cylinders, and a load cell measured the associated pulsating lift force. Results showed that the riblets reduced the amplitude of a cylinder’s cavitation-induced vibration significantly compared to the amplitude of a smooth cylinder. In addition, the riblets mitigated tip-vortex cavitation and cavity structures in most cavitating regimes. At Reynolds numbers 1.0×105 and 1.5×105 and cavitation number 1.0, the vibration was decreased up to 60% for the cylinder with horizontal riblets.

Visualization of Turbulent Flow Fields Around a Circular Cylinder at Reynolds Number 1.4×105 Using PIV

This study investigates the experimental parameters of particle image velocimetry (PIV) to enhance the measurement technique for turbulent flow fields around a circular cylinder at a Reynolds number (Re) of 1.4×10<sup>5</sup>. At the Korea Research Institute of Ships & Ocean Engineering (KRISO), we utilized the cavitation tunnel and PIV system to capture the instantaneous flow fields and statistically obtained the mean flow fields. An aspect ratio and blockage ratio of 16.7% and 6.0%, respectively, were considered to minimize the tunnel wall effect on the cylinder wakes. The optimal values of the pulse time and the number of flow fields were determined by comparing the contours of mean streamlines, velocities, Reynolds shear stresses, and turbulent kinetic energy under their different values to ensure accurate and converged results. Based on the findings, we recommend a pulse time of 45 μs corresponding to a particle moving time of 3–4 pixels, and at least 3,000 instantaneous flow fields to accurately obtain the mean flow fields. The results of the present study agree well with those of previous studies that examined the end of the subcritical flow regime.

Experimental analysis of the flow field around a propeller with inclined shaft

The objective of the present study is to investigate, by means of velocimetric measurements, the flow field generated by a marine propeller running with inclined shaft.Measurements have been carried out on a scaled model of a propeller in the cavitation tunnel of UNIGE using Laser Doppler Velocimetry (LDV) and the acquired data have been post-processed applying the ensemble average technique. The propeller configuration includes shaft appendages as in a typical twin screw vessel configuration. The tests have been performed also without propeller, considering different velocities, in order to characterize the incoming flow field and analyse possible effects on propeller flow. The complete 3-D flow field has been measured considering four transverse sections at different longitudinal locations, from upstream to downstream of the propeller, focusing on the near field wake. A longitudinal plane containing the propeller shaft has been considered as well. A dedicated interpolation technique is presented to improve the spatial resolution of data and allow advanced analyses. Results allow analysing the main characteristics of the flow field, including the evolution of its main structures. The effects of non uniform inflow conditions caused by the oblique flow are analysed focusing on the dynamics of vortexes and in particular on the trajectories and strength of the vortexes.

A systematic study into the cavitation erosion test for marine propeller materials by cavitating jet technique

This paper presents an experimental investigation of cavitation erosion on widely used marine propeller materials by the cavitating jet technique as an alternative to cavitation tunnel tests, following the American Society for Testing and Materials (ASTM) standards. The cavitation erosion tests on aluminium samples were, initially, conducted to investigate the effect of stand-off distance and to compare the results with the available data in the literature. The comparative study confirmed that the erosion on the materials has practically the same pattern even if the nozzle diameter and sample size are different for the same cavitation number and at the optimum stand-off distance condition. Following this validation, the samples made of copper-based alloy marine propeller materials, specifically Cu1, Cu3, and Cu4, were tested for the first time in the literature to assess their erosion resistance at various cavitation numbers and test durations by considering various characteristics, such as erosion area, rate and intensity through comprehensive examinations. Among the materials examined, the Cu1, which has the lowest mechanical strength, was found to be the most susceptible to cavitation erosion. Interestingly, despite the Cu4 having the highest mechanical strength, the Cu3 demonstrated better erosion resistance, suggesting that the nickel and copper contents play a crucial role in the material's resistance to erosion. In order to evaluate and assess the cavitation effect more accurately the erosion damage on the material should be examined not only on the surface, but also in the depth. Based on the findings of this study, the cavitating jet technique may be a good alternative to investigate the cavitation erosion characteristics of propeller materials, if a relationship or scaling law between the water jet test technique and full-scale propeller loading, surface conditions, turbulence intensity, and bubble dynamics in operation can be established.

Experimental and numerical study on the effects of sweep angle on cavitation around a wedge-section hydrofoil

The influence of sweep angles on cavitation characteristics and mechanisms of a wedge-section hydrofoil is investigated experimentally and numerically. Four hydrofoils with sweep angles of 0° (straight), 30°, 45°, and 60° are considered across a range of cavitation numbers from 2.00 to 0.48 and angle of attack (AOA) of 0°, 5°, 10°, and 15°. Two high-speed cameras are used to visualize the cavitation flow in a high-speed cavitation tunnel. The numerical simulation is conducted using unsteady Reynolds-averaged Navier–Stokes equations through OpenFOAM. At an AOA of 0° and 5°, vortex cavitation first appears in the wake region of all models at a cavitation number of 0.98. However, at higher AOA values of 10° and 15°, tip-vortex cavitation (TVC) begins first for the straight foil, and this is followed by sheet and wake cavitation. In contrast, the swept foil does not succumb to TVC. Instead, as the sweep angle increases, sheet cavitation develops into root leading-edge vortex cavitation (LEVC). The inclination angle of the LEVC from the leading edge is observed to be between 6° and 15°, depending on the sweep angle, and it is independent of the cavitation number and AOA. The wake vortex changes from eddy vortex shedding at the wake region of the straight hydrofoil into two root trailing-edge vortices as the sweep angle increases. The swept hydrofoil reduces the average cavity volume by more than 45% compared with the straight foil. The lift coefficient of the straight hydrofoil increases as sheet cavitation is generated and reaches a maximum value of 0.6 when cavitation covers the suction side of the hydrofoil before dropping sharply when it extends to the wake region. However, there is only minor deterioration in the lift coefficient of the 60° swept-angle foil when the cavitation occurs. The drag coefficient reduces when cavitation forms for both foils. However, the drag coefficient of the swept hydrofoil is lower than that of the straight foil. These findings offer valuable insight into the design and optimization of foils for various applications where cavitation affects their performance and stability.

Micro/Bubble Drag Reduction Focused on New Applications

Bubble drag reduction has been shown to be a promising technique for reducing the drag in ships, thus reducing the emission of pollutants and allowing the compliance with the new requirements imposed recently in this respect. Different searches have been conducted in the publications related to this technique, and an increase in interest has been shown, especially in the last decade. In this context, a review of the experimental work related to bubble drag reduction published in the last decade is presented in the present article. The works were classified according to the facility used (towing tank, cavitation tunnel, water channel, Taylor–Couette…), and the main finding are presented. It was found that two new trends in research have arisen, while there are still contradictions in the fundamental basis, which needs further study.

Localization of acoustic sources in water tunnel with a demodulation technique

The prediction of the underwater radiated noise of a vessel is still an important issue during the design process of a ship. Due to the influence of shipping noise on marine life and for acoustic stealth in military applications, this problem has seen increased interest for decades. The preferred way to estimate the future radiated noise level is to perform measurements at the model scale and extrapolate the results obtained to full scale. DGA Hydrodynamics owns its cavitation tunnel with low background noise that allows such measurements. The test section of this cavitation tunnel looks like a rectangular corridor filled with demineralized water. Among all the parameters of the experimental set-up that influence the accuracy of the acoustic measurements, the position of the acoustic source could be difficult to estimate, especially at low frequencies. In this frequency domain, acoustic propagation could be described advantageously by the modal theory. In this communication, we propose to use the modal theory and the associated demodulation techniques to estimate the position of the source inside the test section. The first part of the paper presents the modal theory and the developed method. Simulations have been carried out to estimate the performances of such methods. The results of this simulation and analysis are then presented. The method appears as an efficient way to estimate the position of the source with, nevertheless, some limitations in frequency and number of measured modes in the demodulation technique.

Применение современных конструкций винтов регулируемого шага и особенности их проектирования

Object and purpose of research. This paper discusses controllable-pitch propellers (CPPs) of various designs. The purpose of this study was to summarize the findings of CPP studies in Russia and abroad, as well as to identify current challenges in Russian CPP design and manufacturing. Materials and methods. This paper relies on the test data obtained at Deepwater Tank and cavitation tunnels of Krylov State Research Centre. It analyses the results of computational studies for CPP hydrodynamics and strength obtained as per the latest techniques of CFD and FE analysis. Main results. The history of CPP design and manufacturing in Russia clearly shows that our country has considerable expertise in design of controllable-pitch propellers that can operate in the harshest conditions. Analysis of global shipbuilding experience has shown that CPPs not only ensure optimal operational conditions for “engine-propeller” system in the conditions of variable hydrodynamic load on the propeller, but also feature better cavitation and vibroacoustic performance because the skew of CPP blades can be moderate and high: actually, it can be significantly higher than for fixed-pitch propellers (FPPs) because FPP blades in reversal conditions do not have to operate their trailing edges forward, which reduces the risk of high stresses and damage of skewed blades in crash-stop scenarios. Analysis of CPP test results obtained at Krylov State Research Centre hydrodynamic and cavitation laboratories, as well as the findings of foreign computational studies enabled justified assessment of CPP advantages. It also highlighted the challenges that require dedicated studies and special consideration in design of skewed CPPs, so as to improve their performance in partialcavitation and cavitation-inception conditions. Conclusion. Currently, Russia has all the pre-requisites for increased production and wide application of modern CPPs. Controllable blades with moderate and high skew have a certain potential in terms of improving their vibroacoustic performance as compared to FPPs. However, to design these blades correctly, with consideration of their possible operational risks, it is necessary to use state-of-the art computational and experimental methods, as well as to rely on the experience of full-scale trials.

Behaviour of nickel coatings made by brush plating technology in conditions of cavitation erosion and corrosion

The basic hypothesis of the research was that the structure of the coating, i.e. the type of defects, affects the mechanical and electrochemical properties, as well as the resistance to cavitation erosion and corrosion. For this purpose, nickel coatings produced by brush-plating technology using two different electrolytes and deposited on X20Cr13 steel were tested. The electrolytes affected the structure and properties of the coatings: one coating had a network of cracks, and the second one had a “cauliflower-like” structure. The hardness and Young's modulus of both coatings were investigated under multi-cycle loading modes. Cavitation erosion resistance was tested in a cavitation tunnel equipped with a barricade system at an inlet pressure of 600 kPa and outlet pressure of 120 kPa. Both nickel coatings protected the substrate against cavitation erosion. The lowest rate of cavitation erosion had a coating with a network of cracks and high hardness. Microscopic examinations showed no influence of the net of cracks on the degradation process. Electrochemical tests confirmed the better corrosion resistance of the compact coating. The compact coating had about an order of magnitude lower corrosion potential and about 100 mV higher corrosion current density than that of the agglomerate coating. The agglomerate coating had electrochemical parameters comparable to the substrate.

Cavitation Hydrodynamic Performance of 3-D Printed Highly Skewed Stainless Steel Tidal Turbine Rotors

Hydraulic turbines contribute to 60% of renewable energy in the world; however, they also entail some adverse effects on the aquatic ecology system. One such effect is their excessive noise and vibration. To minimize this effect, one of the most effective and feasible solutions is to modify the design of the turbine rotor blade by introducing a skew. In this study, two 0.3-meter tidal turbines with 0-degree (no-skewness) and positive 90-degree skewness made of stainless steel 316L were designed and printed using a 3-D printing powder bed fusion technique. These rotors were then tested at the Emerson Cavitation Tunnel (ECT) at Newcastle University, UK, and the variation in the skewness of the blades of the turbines as a function of the power coefficient on a given tip speed ratio (TSR) value was ascertained. Results showed that the highly skewed rotor had significantly lower drag and torque fluctuations, with a slight decrease in efficiency compared to the non-skewed one, which warrants further investigation on the effect of added skew to reduce vibration and noise. Numerical simulations were also performed for verification and validation of the experimental tests, using the H45 dynamometer at the ECT. A comprehensive software code for propellers and tidal turbines, ROTORYSICS, was used to examine the cavitation effect of the two rotors; a comparison was made for both, with and without cavitation. The results indicate that for a high immersion depth of tidal turbine rotors, cavitation rarely occurs, but for hydrokinetic turbines that are installed on dams in rivers and falls, cavitation could be a serious concern. It was concluded that the 0-degree skewed rotor is more hydrodynamically efficient than the 90-degree skewed rotor.

Computational fluid dynamics and hydroacoustics analyses of underwater radiated noise of an ice breaker ship

Flow and acoustics analyses have been carried out for an ice breaker ship. The numerical acoustic solution relies on the computational fluid dynamics simulations with regard to the sound generation. The numerical method for acoustics is based on a wave equation for the density perturbations which is solved using an acoustic finite element method. The simulations are compared to full-scale underwater noise measurements, as well as model-scale measurements conducted in a cavitation tunnel. The hydrodynamic performance of the thruster unit was well captured in the numerical simulations. The flow around the unit was visualized, and different hydrodynamic noise sources caused by the propulsion system were investigated. The hydrodynamic noise of the ship was decently captured in the simulations.

Multiphase Flow Simulation of ITTC Standard Cavitator for Underwater Radiated Noise Prediction

This work focuses on the main issues related to noise measurements in cavitation tunnels. The scope of the paper is to twofold: to obtain a better understanding on the main phenomena underlying experiments and to define consistent cavitation tunnel measurement corrections for background noise, wall reflections, and distance normalisation. To this aim, the acoustic field generated by the ITTC standard cavitator model inside a cavitation tunnel is predicted by Lighthill’s acoustic analogy and solved through a finite element method that inherently accounts for the presence of the walls. Sources of sound detection relies on two multiphase CFD solvers, namely, the homogeneous mixture model—Volume of Fluid method and the Euler–Euler formulations. Starting from the computation of the sound pressure level in the free field with the assumption of spherical spreading without absorption, corrections from losses and spreading are detected by the above approach. Background-corrected sound pressure levels are identified and then compared with the source levels measured in the cavitation tunnel of the Potsdam Model Basin (SVA). It is found that free-field computations corrected by tunnel-induced effects match well with experiments up to 100 Hz (in the one-third octave band), whereas relevant discrepancies arise out of this range that need further investigations.

Application of a Deep Neural Network for Acoustic Source Localization Inside a Cavitation Tunnel

Navigating with low noise is the key capability in the submarine design considerations, and noise reduction is also one of the most critical issues in the related fields. Therefore, it is necessary to identify the source of noise during design stage to improve the survivability of the submarines. The main objective of this research is using the supervised neural network to construct the system of noise localization to identify noise source in the large acoustic tunnel. Firstly, we started our proposed method by improving the Yangzhou’s method and Shunsuke’s method. In the test results, we find that the errors of the both can be reduced by using the min-max normalization to highlight the data characteristics of the low amplitude in some frequency. And Yangzhou’s method has higher accuracy than Shunsuke’s method. Then, we reset the diagonal numbers of the cross spectral matrix in Yangzhou’s method to zero and replace mean absolute error to be the loss function for improving the stability of training, and get the most suitable neural network construction for our research. After our optimization, the error decreases from 0.315 m to 0.008 m in cuboid model test. Finally, we apply our method to the cavitation tunnel model. A total of 100 data sets were used for training, 10 sets for verification, and 5 for testing. The average error of the test result is 0.13 m. For the model test in cavitation tunnel in National Taiwan Ocean University, the length of ship model is around 7 m. And the average error is sufficient to determine the noise source position.

Change in Cavitation Regime on NACA0015 Hydrofoil by Heating the Hydrofoil Surface

AbstractAn experimental study of cavitating flow on a heated NACA0015 hydrofoil was conducted in a cavitation tunnel to investigate the influence of the hydrofoil surface temperature on the cavitating flow. The cavitation behavior under different heating conditions was examined using high-speed video, and an image processing method was used to obtain the periodic characteristics of the cavitating flow. The results revealed that attached sheet cavitation and supercavitation occurred on both heated and unheated hydrofoils. However, sheet-cloud cavitation was observed only on the unheated hydrofoil, whereas transient cavitation was observed only on the heated hydrofoil. Transient cavitation also exhibited periodic growth/collapse behavior; however, there was no shedding of a large vapor cloud. Moreover, with a further increase in the hydrofoil surface temperature, transient cavitation turned into open-type cavitation. The cavitating flow exhibited a quasi-steady cavity length with an open cavity closure. It was considered that the surface temperature promoted vapor generation at the cavity leading edge, which enlarged the vapor-filled fore part of the sheet cavity. This enlarged sheet cavity prevented the reentrant flow from moving upstream and thus turned the cavity closure into an open type. Once the cavity closure turned into an open type, the local disturbance led to a smaller adverse pressure gradient, which was not sufficiently strong to create a reentrant flow. In this case, if the vapor generation at the cavity leading edge was sufficiently large to reach a balance with vapor condensation at the open cavity closure, the cavity would be steady.

선미 부가물 수정에 따른 프로펠러 캐비테이션 성능 향상 연구

In order to improve the propeller cavitation performance composed of Cavitation Inception Speed (CIS), cavitation extent and pressure fluctuation, it needs to improve the wake distribution that flows into the propeller. The warship propeller cavitation is strongly influenced by the wake created at the V-strut of various appendages. The inflow characteristics of the V-strut were investigated using Computational Fluid Dynamics (CFD) and the twisted angles of the V-strut were aligned with upstream flow. The resistance and self-propulsion tests for the model ship with the existing and modified V-struts were conducted in Towing Tank (TT), and wake distribution, CIS, cavitation observation and pressure fluctuation tests were conducted in Large Cavitation Tunnel (LCT). The propeller behind the modified V-strut showed better cavitation characteristics than that behind the existing V-strut. Another model test was conducted to investigate rudder cavitation performance by the change of the V-strut. The rudder cavitation characteristics were not improved by the change of the operating conditions. On the basis of the present study, it is thought that the stern appendages for better propeller cavitation performance would be developed.