Stochastic wave properties are crucial for the design of offshore structures. Short-crested seas are commonly seen at the sites of offshore structures, especially during storm events. A long time duration is required in order to obtain the statistical properties, which is challenging for numerical simulations because of the high demand of computational resources. In this scenario, a potential flow solver is ideal due to its computational efficiency. A procedure of producing accurate representation of short-crested sea states using the open-source fully nonlinear potential flow model REEF3D::FNPF is presented in the paper. The procedure examines the sensitivity of the resolutions in space and time as well as the arrangements of wave gauge arrays. A narrow band power spectrum and a mildly spreading directional spreading function are simulated, and an equal energy method is used to generate input waves to avoid phase-locking. REEF3D::FNPF solves the Laplace equation together with the boundary conditions using a finite difference method. A sigma grid is used in the vertical direction and the vertical grid clustering follows the principle of constant truncation error. High-order discretisation methods are implemented in space and time. Message passing interface is used for high performance computation using multiple processors. Three-hour simulations are performed in full-scale at a hypothetic offshore site with constant water depth. The significant wave height, peak period, kurtosis, skewness and ergodicity are examined in the numerically generated wave field. The stochastic wave properties in the numerical wave tank (NWT) using REEF3D::FNPF match the input wave conditions with high fidelity.
{"title":"High-Fidelity Representation of Three-Hour Offshore Short-Crested Wave Field in the Fully Nonlinear Potential Flow Model REEF3D::FNPF","authors":"Weizhi Wang, Csaba Pákozdi, A. Kamath, H. Bihs","doi":"10.1115/omae2020-18262","DOIUrl":"https://doi.org/10.1115/omae2020-18262","url":null,"abstract":"Stochastic wave properties are crucial for the design of offshore structures. Short-crested seas are commonly seen at the sites of offshore structures, especially during storm events. A long time duration is required in order to obtain the statistical properties, which is challenging for numerical simulations because of the high demand of computational resources. In this scenario, a potential flow solver is ideal due to its computational efficiency. A procedure of producing accurate representation of short-crested sea states using the open-source fully nonlinear potential flow model REEF3D::FNPF is presented in the paper. The procedure examines the sensitivity of the resolutions in space and time as well as the arrangements of wave gauge arrays. A narrow band power spectrum and a mildly spreading directional spreading function are simulated, and an equal energy method is used to generate input waves to avoid phase-locking. REEF3D::FNPF solves the Laplace equation together with the boundary conditions using a finite difference method. A sigma grid is used in the vertical direction and the vertical grid clustering follows the principle of constant truncation error. High-order discretisation methods are implemented in space and time. Message passing interface is used for high performance computation using multiple processors. Three-hour simulations are performed in full-scale at a hypothetic offshore site with constant water depth. The significant wave height, peak period, kurtosis, skewness and ergodicity are examined in the numerically generated wave field. The stochastic wave properties in the numerical wave tank (NWT) using REEF3D::FNPF match the input wave conditions with high fidelity.","PeriodicalId":431910,"journal":{"name":"Volume 6B: Ocean Engineering","volume":"68 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-08-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"132568847","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
This study compares the effect of non-linear free-surface boundary conditions for a high-order non-linear free-surface Rankine-source boundary element method on wave disturbance and hydrodynamic forces acting on an underwater vehicle travelling near a calm free-surface. In particular, simulations for a steady nonaxisymmetric prolate spheroid using different basis flows and linearization techniques were compared to an analytical method achieved by Chatjigeorgiou using a multipole expansion of Green’s functions. It appears that at low Froude numbers, the basis flow used in the formulation contributes significantly to differences in the steady solutions for wave resistance and pitch, whereas for higher Froude numbers the linearization technique becomes a more defining feature. Upon observation of the analytical solution for wave resistance, one can see that it was formed under a Neumann-Kelvin formulation and this is supported by the Neumann-Kelvin simulations converging well to the analytical solution. Further comparisons were made using a wave directional energy spectrum gathered from transverse wave cuts of the free wave pattern. The spectral analysis allows for a higher level of comparison between all of the different cases, establishing a direct relation between the change in wave resistance and the energy content variation of the particular wave spectrum components.
{"title":"On the Effect of Non-Linear Boundary Conditions on the Wave Disturbance and Hydrodynamic Forces of Underwater Vehicles Travelling Near the Free-Surface","authors":"William Lambert, S. Brizzolara","doi":"10.1115/omae2020-18214","DOIUrl":"https://doi.org/10.1115/omae2020-18214","url":null,"abstract":"\u0000 This study compares the effect of non-linear free-surface boundary conditions for a high-order non-linear free-surface Rankine-source boundary element method on wave disturbance and hydrodynamic forces acting on an underwater vehicle travelling near a calm free-surface. In particular, simulations for a steady nonaxisymmetric prolate spheroid using different basis flows and linearization techniques were compared to an analytical method achieved by Chatjigeorgiou using a multipole expansion of Green’s functions. It appears that at low Froude numbers, the basis flow used in the formulation contributes significantly to differences in the steady solutions for wave resistance and pitch, whereas for higher Froude numbers the linearization technique becomes a more defining feature. Upon observation of the analytical solution for wave resistance, one can see that it was formed under a Neumann-Kelvin formulation and this is supported by the Neumann-Kelvin simulations converging well to the analytical solution.\u0000 Further comparisons were made using a wave directional energy spectrum gathered from transverse wave cuts of the free wave pattern. The spectral analysis allows for a higher level of comparison between all of the different cases, establishing a direct relation between the change in wave resistance and the energy content variation of the particular wave spectrum components.","PeriodicalId":431910,"journal":{"name":"Volume 6B: Ocean Engineering","volume":"29 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-08-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"134258721","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The sound of waves breaking on shore, or against an obstruction or jetty, is an immediately recognizable sound pattern which could potentially be employed by a sensor system to identify obstructions. If frequency patterns produced by breaking waves can be reproduced and mapped in a laboratory setting, a foundational understanding of the physics behind this process could be established, which could then be employed in sensor development for navigation. This study explores whether wave-breaking frequencies correlate with the physics behind the collapsing of the wave, and whether frequencies of breaking waves recorded in a laboratory tank will follow the same pattern as frequencies produced by ocean waves breaking on a beach. An artificial “beach” was engineered to replicate breaking waves inside a laboratory wave tank. Video and audio recordings of waves breaking in the tank were obtained, and audio of ocean waves breaking on the shoreline was recorded. The audio data was analysed in frequency charts. The video data was evaluated to correlate bubble sizes to frequencies produced by the waves. The results supported the hypothesis that frequencies produced by breaking waves in the wave tank followed the same pattern as those produced by ocean waves. Analysis utilizing a solution to the Rayleigh-Plesset equation showed that the bubble sizes produced by breaking waves were inversely related to the pattern of frequencies. This pattern can be reproduced in a controlled laboratory environment and extrapolated for use in developing navigational sensors for potential applications in marine navigation such as for use with autonomous ocean vehicles.
{"title":"Acoustic Study of Wave-Breaking to Enhance the Understanding of Wave Physics","authors":"Michael N. Odzer, Kristina Francke","doi":"10.1115/omae2020-19352","DOIUrl":"https://doi.org/10.1115/omae2020-19352","url":null,"abstract":"\u0000 The sound of waves breaking on shore, or against an obstruction or jetty, is an immediately recognizable sound pattern which could potentially be employed by a sensor system to identify obstructions. If frequency patterns produced by breaking waves can be reproduced and mapped in a laboratory setting, a foundational understanding of the physics behind this process could be established, which could then be employed in sensor development for navigation.\u0000 This study explores whether wave-breaking frequencies correlate with the physics behind the collapsing of the wave, and whether frequencies of breaking waves recorded in a laboratory tank will follow the same pattern as frequencies produced by ocean waves breaking on a beach.\u0000 An artificial “beach” was engineered to replicate breaking waves inside a laboratory wave tank. Video and audio recordings of waves breaking in the tank were obtained, and audio of ocean waves breaking on the shoreline was recorded. The audio data was analysed in frequency charts. The video data was evaluated to correlate bubble sizes to frequencies produced by the waves.\u0000 The results supported the hypothesis that frequencies produced by breaking waves in the wave tank followed the same pattern as those produced by ocean waves. Analysis utilizing a solution to the Rayleigh-Plesset equation showed that the bubble sizes produced by breaking waves were inversely related to the pattern of frequencies. This pattern can be reproduced in a controlled laboratory environment and extrapolated for use in developing navigational sensors for potential applications in marine navigation such as for use with autonomous ocean vehicles.","PeriodicalId":431910,"journal":{"name":"Volume 6B: Ocean Engineering","volume":"26 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-08-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"117024797","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Bores propagating in shallow water transform into undular bores and, finally, into trains of solitons. The observed number and height of these undulations, and later discrete solitons, is strongly dependent on the propagation length of the bore. Empirical results show that the final height of the leading soliton in the far-field is twice the initial mean bore height. The complete disintegration of the initial bore into a train of solitons requires very long propagation lengths, but unfortunately these required distances are usually not available in experimental tests or nature. Therefore, the analysis of the bore decomposition for experimental data into solitons is difficult and requires further approaches. Previous studies have shown that by application of the nonlinear Fourier transform based on the Korteweg–de Vries equation (KdV-NFT) to bores and long-period waves propagating in constant depth, the number and height of all solitons can be reliably predicted already based on the initial bore-shaped free surface. Against this background, this study presents the systematic analysis of the leading-soliton amplitudes for non-breaking and breaking bores with different strengths in different water depths in order to validate the KdV-NFT results for non-breaking bores, and to show the limitations of wave breaking on the spectral results. The analytical results are compared with data from experimental tests, numerical simulations and other approaches from literature.
{"title":"Analysis of Bore Characteristics Using KdV-Based Nonlinear Fourier Transform","authors":"M. Bruehl, S. Wahls, I. B. Granged, P. Liu","doi":"10.1115/omae2020-19074","DOIUrl":"https://doi.org/10.1115/omae2020-19074","url":null,"abstract":"\u0000 Bores propagating in shallow water transform into undular bores and, finally, into trains of solitons. The observed number and height of these undulations, and later discrete solitons, is strongly dependent on the propagation length of the bore. Empirical results show that the final height of the leading soliton in the far-field is twice the initial mean bore height. The complete disintegration of the initial bore into a train of solitons requires very long propagation lengths, but unfortunately these required distances are usually not available in experimental tests or nature. Therefore, the analysis of the bore decomposition for experimental data into solitons is difficult and requires further approaches. Previous studies have shown that by application of the nonlinear Fourier transform based on the Korteweg–de Vries equation (KdV-NFT) to bores and long-period waves propagating in constant depth, the number and height of all solitons can be reliably predicted already based on the initial bore-shaped free surface.\u0000 Against this background, this study presents the systematic analysis of the leading-soliton amplitudes for non-breaking and breaking bores with different strengths in different water depths in order to validate the KdV-NFT results for non-breaking bores, and to show the limitations of wave breaking on the spectral results. The analytical results are compared with data from experimental tests, numerical simulations and other approaches from literature.","PeriodicalId":431910,"journal":{"name":"Volume 6B: Ocean Engineering","volume":"26 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-08-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"116755725","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Sea-keeping model tests of ships based on transient waves have been widely applied over the past several decades. In order to obtain response amplitude operators (RAOs) of a ship, most of the post-processing of the experimental data uses the fast Fourier transform (FFT) to obtain the wave spectrum and the corresponding response spectrum. However, for transient waves related model tests, FFT may produce larger errors due to its characteristics. Hilbert-Huang transform (HHT) is a newly developed signal analysis tool which is suitable for nonlinear and non-stationary data. The application of HHT to the post-processing of the experimental data of sea-keeping model tests of ships has not yet been investigated. In this study, the transient wave packets satisfying a Gaussian wave spectrum were generated in a large towing tank to conduct the sea-keeping model tests of a drilling ship under the condition of head waves, oblique waves and beam waves, respectively. Then the marginal Hilbert spectrum (MHS) in the framework of HHT is introduced to obtain the motion and the acceleration RAOs the drilling ship. In order to demonstrate the effectiveness of the approach, the results based on FFT and regular waves are also presented. It is found that in most cases, in comparison to that by means of FFT, the RAOs of the ship based on the transient Gaussian wave packets by means of MHS agree better with the results based on regular waves, especially for roll motion with significant nonlinear characteristics. Due to the advantages of HHT, the MHS approach employed in this study is expected to play a vital role in more sea-keeping related model tests of ships.
{"title":"Comparative Study on RAOs of a Ship Under Transient Gaussian Wave Packets by Marginal Hilbert Spectrum and Fourier Spectrum","authors":"Zhen Liu, She-ming Fan, L. Tao","doi":"10.1115/omae2020-18343","DOIUrl":"https://doi.org/10.1115/omae2020-18343","url":null,"abstract":"\u0000 Sea-keeping model tests of ships based on transient waves have been widely applied over the past several decades. In order to obtain response amplitude operators (RAOs) of a ship, most of the post-processing of the experimental data uses the fast Fourier transform (FFT) to obtain the wave spectrum and the corresponding response spectrum. However, for transient waves related model tests, FFT may produce larger errors due to its characteristics. Hilbert-Huang transform (HHT) is a newly developed signal analysis tool which is suitable for nonlinear and non-stationary data. The application of HHT to the post-processing of the experimental data of sea-keeping model tests of ships has not yet been investigated. In this study, the transient wave packets satisfying a Gaussian wave spectrum were generated in a large towing tank to conduct the sea-keeping model tests of a drilling ship under the condition of head waves, oblique waves and beam waves, respectively. Then the marginal Hilbert spectrum (MHS) in the framework of HHT is introduced to obtain the motion and the acceleration RAOs the drilling ship. In order to demonstrate the effectiveness of the approach, the results based on FFT and regular waves are also presented. It is found that in most cases, in comparison to that by means of FFT, the RAOs of the ship based on the transient Gaussian wave packets by means of MHS agree better with the results based on regular waves, especially for roll motion with significant nonlinear characteristics. Due to the advantages of HHT, the MHS approach employed in this study is expected to play a vital role in more sea-keeping related model tests of ships.","PeriodicalId":431910,"journal":{"name":"Volume 6B: Ocean Engineering","volume":"56 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-08-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"134564221","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
In the ice breaking condition, on account of the low speed and heavy propeller load, the ship resistance is large, which will aggravate the propeller cavitation and the propeller-induced pressure. In this paper, the cavitation performance of the ice-classed propeller is analyzed by numerical simulation and model experiment. Commercial CFD software was used for the numerical simulations, in which the cavitation flow is solved by Schneer & Sauer cavitaiton model based on a single-fluid multiphase mixture flow approach. Model tests to measure cavitation flow on an ice-classed propeller were carried out in SSSRI K15 Cavitation Tunnel. The size of the test section of SSSRI K15 Cavitation Tunnel is 600mm*600mm. The propeller performances in uniform flow over a range of advance coefficients were carried out in open water test in a towing tank. The diameter (D) of the model propeller was 248mm in this research. Firstly, the open water performance of propeller is numerically studied. Near the design conditions, the numerical results are almost consistent with the test results, with an error of less than 1%. In the case of ice breaking, the blocking effect of ice in front of a propeller is studied. The experiment results show that with the ice block close to the propeller, one or more vortex tube structures are generated between the propeller blade and the ship bottom while the vortex cavitation occurs. Such phenomenon is also found between the propeller and the ice block. When the blocking effect is significant, the stable vortex tube structure will appear and significantly change the cavity shape near the blade. When the distance between the ice and the blade disc exceeds 0.72D, the vortex tube structure will disappear.
{"title":"Cavitation Performance of Low Speed Ice-Classed Propeller","authors":"Chu-rui Wan, Zhenghao Liu","doi":"10.1115/omae2020-18128","DOIUrl":"https://doi.org/10.1115/omae2020-18128","url":null,"abstract":"\u0000 In the ice breaking condition, on account of the low speed and heavy propeller load, the ship resistance is large, which will aggravate the propeller cavitation and the propeller-induced pressure. In this paper, the cavitation performance of the ice-classed propeller is analyzed by numerical simulation and model experiment. Commercial CFD software was used for the numerical simulations, in which the cavitation flow is solved by Schneer & Sauer cavitaiton model based on a single-fluid multiphase mixture flow approach. Model tests to measure cavitation flow on an ice-classed propeller were carried out in SSSRI K15 Cavitation Tunnel. The size of the test section of SSSRI K15 Cavitation Tunnel is 600mm*600mm. The propeller performances in uniform flow over a range of advance coefficients were carried out in open water test in a towing tank. The diameter (D) of the model propeller was 248mm in this research. Firstly, the open water performance of propeller is numerically studied. Near the design conditions, the numerical results are almost consistent with the test results, with an error of less than 1%. In the case of ice breaking, the blocking effect of ice in front of a propeller is studied. The experiment results show that with the ice block close to the propeller, one or more vortex tube structures are generated between the propeller blade and the ship bottom while the vortex cavitation occurs. Such phenomenon is also found between the propeller and the ice block. When the blocking effect is significant, the stable vortex tube structure will appear and significantly change the cavity shape near the blade. When the distance between the ice and the blade disc exceeds 0.72D, the vortex tube structure will disappear.","PeriodicalId":431910,"journal":{"name":"Volume 6B: Ocean Engineering","volume":"35 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-08-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"125237889","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Daniel de Oliveira Costa, Joel Sena Sales Junior, A. C. Fernandes
When under influence of an incident wave system, any freely floating body presents a general motion with all six degrees of freedom. The Instantaneous Center of Rotation, as defined in classical mechanics, is a concept that allows the description of a general motion in 6 degrees of freedom as a pure rotation around such point. This approach, although not widely used in ocean engineering, might be an alternative tool that allows fast and precise analysis in many cases. Recent studies have shown that under specific conditions, such as a heading wave condition, the ICR varies in time but it is always located along a line for one wave frequency. Similar results were presented regarding beam waves as well. The present work continues with the investigation regarding the behavior of ICR under more generic conditions, assuming oblique waves exciting a vessel with typical geometry of a FPSO platform. The study extends the knowledge derived based on 2D approaches from previous works, comparing the results obtained from the different methods. An analytical model is presented, assuming only harmonic motion to all 6 degrees of freedom and showing that, similar to what was observed in the simplified 2D cases, the ICR tends to present dependence on the frequency of motion. Numerical data acquired from commercial codes based on potential theory is also presented.
{"title":"Instantaneous Center of Rotation of a Vessel Submitted to Oblique Waves","authors":"Daniel de Oliveira Costa, Joel Sena Sales Junior, A. C. Fernandes","doi":"10.1115/omae2020-18860","DOIUrl":"https://doi.org/10.1115/omae2020-18860","url":null,"abstract":"\u0000 When under influence of an incident wave system, any freely floating body presents a general motion with all six degrees of freedom. The Instantaneous Center of Rotation, as defined in classical mechanics, is a concept that allows the description of a general motion in 6 degrees of freedom as a pure rotation around such point. This approach, although not widely used in ocean engineering, might be an alternative tool that allows fast and precise analysis in many cases. Recent studies have shown that under specific conditions, such as a heading wave condition, the ICR varies in time but it is always located along a line for one wave frequency. Similar results were presented regarding beam waves as well.\u0000 The present work continues with the investigation regarding the behavior of ICR under more generic conditions, assuming oblique waves exciting a vessel with typical geometry of a FPSO platform. The study extends the knowledge derived based on 2D approaches from previous works, comparing the results obtained from the different methods. An analytical model is presented, assuming only harmonic motion to all 6 degrees of freedom and showing that, similar to what was observed in the simplified 2D cases, the ICR tends to present dependence on the frequency of motion. Numerical data acquired from commercial codes based on potential theory is also presented.","PeriodicalId":431910,"journal":{"name":"Volume 6B: Ocean Engineering","volume":"18 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-08-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"115880079","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
In this paper, a boundary element method (BEM) is applied to a tip loaded propeller (TLP) to predict its open water characteristics and induced hull pressures under fully-wetted and uniform inflow. Tip of a TLP blade has a winglet-like tip plate on the pressure side to improve the overall propeller efficiency over the traditional open tip propellers by preventing circulation loss toward the tip region. TLPs are also used to reduce the tip vortex strength and thus free from the trade off the propeller efficiency against the cavitation performance; therefore, predicting their performance early in the designing stage via numerical applications can provide the initial knowledge on the loading distributions and cavitation performance. In the present method, the trailing wake is first aligned using the full wake alignment (FWA) scheme by aligning the wake surface to the local stream in order to satisfy the force free condition. The FWA is shown to improve the open water characteristics of the TLPs compared to the simplified alignment scheme that ignores the details of the flow behind the trailing edge due to the simplicity of the method. Afterwards, a pressure-BEM solver is used to solve for the diffraction potentials on the hull and estimate the propeller-induced hull pressures. In this case, both the FWA and the unsteady wake alignment scheme (UWA), which considers the time dependency of the problem, produce the same results as the testing flow is assumed to be uniform. This paper briefly introduces the model TLP, proper ways to consider the viscous effect on the blade surface, wake alignment scheme, and the pressure-BEM solver. Then, the predicted open water characteristics of the benchmark TLP and its induced hull pressures are compared to the experimental data, as well as the results from unsteady full-blown Reynolds-Averaged Navier-Stokes simulations for validations of the numerical predictions.
本文将边界元法应用于顶载螺旋桨(TLP)在全湿均匀入流条件下的开放水域特性和诱导船体压力预测。TLP叶片的叶尖压力侧有一个小翼状的叶尖板，通过防止流向叶尖区域的循环损失，提高了传统开叶尖螺旋桨的整体效率。TLPs还用于降低叶顶涡强度，从而免于权衡螺旋桨效率与空化性能;因此，在设计阶段早期通过数值应用预测其性能可以提供有关载荷分布和空化性能的初步知识。在该方法中，为了满足无力条件，首先采用全尾迹对准(FWA)方案，将尾迹面对准局部流。与简化的对准方案相比，FWA可以改善张力腿平台的开放水域特性，由于方法简单，简化的对准方案忽略了尾缘后面的流动细节。然后，利用压力边界元求解器求解了船体上的衍射势，并估计了螺旋桨诱导的船体压力。在这种情况下，无论是FWA还是考虑了问题的时间依赖性的非定常尾迹对准方案(UWA)，其结果都与假设测试流为均匀流的情况相同。本文简要介绍了TLP模型、考虑叶片表面粘性影响的方法、尾迹对准方案以及压力-边界元法求解。然后，将预测的基准张力腿平台的开放水域特性及其诱导船体压力与实验数据以及非定常成熟reynolds - average Navier-Stokes模拟结果进行了比较，以验证数值预测的有效性。
{"title":"Prediction of Performance of Tip Loaded Propeller and its Induced Pressures on the Hull","authors":"Seungnam Kim, S. Kinnas","doi":"10.1115/omae2020-18999","DOIUrl":"https://doi.org/10.1115/omae2020-18999","url":null,"abstract":"\u0000 In this paper, a boundary element method (BEM) is applied to a tip loaded propeller (TLP) to predict its open water characteristics and induced hull pressures under fully-wetted and uniform inflow. Tip of a TLP blade has a winglet-like tip plate on the pressure side to improve the overall propeller efficiency over the traditional open tip propellers by preventing circulation loss toward the tip region. TLPs are also used to reduce the tip vortex strength and thus free from the trade off the propeller efficiency against the cavitation performance; therefore, predicting their performance early in the designing stage via numerical applications can provide the initial knowledge on the loading distributions and cavitation performance. In the present method, the trailing wake is first aligned using the full wake alignment (FWA) scheme by aligning the wake surface to the local stream in order to satisfy the force free condition. The FWA is shown to improve the open water characteristics of the TLPs compared to the simplified alignment scheme that ignores the details of the flow behind the trailing edge due to the simplicity of the method. Afterwards, a pressure-BEM solver is used to solve for the diffraction potentials on the hull and estimate the propeller-induced hull pressures. In this case, both the FWA and the unsteady wake alignment scheme (UWA), which considers the time dependency of the problem, produce the same results as the testing flow is assumed to be uniform. This paper briefly introduces the model TLP, proper ways to consider the viscous effect on the blade surface, wake alignment scheme, and the pressure-BEM solver. Then, the predicted open water characteristics of the benchmark TLP and its induced hull pressures are compared to the experimental data, as well as the results from unsteady full-blown Reynolds-Averaged Navier-Stokes simulations for validations of the numerical predictions.","PeriodicalId":431910,"journal":{"name":"Volume 6B: Ocean Engineering","volume":"46 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-08-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"131082545","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Hongwei Wang, Zizhao Zhang, Gang Ma, Rong-Hua Ma, Jie Yang
Select the common mooring system-soft yoke mooring system as the research object. The soft yoke mooring system is regarded as a structure composed of multiple rigid bodies, and the theoretical analysis of multi-body dynamics is used to discuss the interaction of multi-rigid bodies. The classical HYSY113 FPSO is selected as an example, for the soft yoke mooring system, the stiffness characteristics and static restoring force curved compared with those of software OrcaFlex, and they are in good agreement, which verify the reliability of the formula derived, and it is a prerequisite for the accurate simulations in further steps. Coupled analysis to the whole system in time domain is also carried out both in OrcaFlex and AQWA, and the representative response of the FPSO under different environmental conditions is compared, the results are consistent well with each other. It is a good reference for the future study in this field. Good static characteristics are a prerequisite for accurate analysis of time-domain motion. By comparing the results in the time domain, it is found that under the same working conditions, the analysis results calculated by different commercial software (AQWA and OrcaFlex) may be different. We need to perform design analysis based on the characteristics of the software.
{"title":"Research on the Static and Dynamics Characteristics of Soft Yoke Mooring System Based on Multi-Rigid Body Interaction","authors":"Hongwei Wang, Zizhao Zhang, Gang Ma, Rong-Hua Ma, Jie Yang","doi":"10.1115/omae2020-19061","DOIUrl":"https://doi.org/10.1115/omae2020-19061","url":null,"abstract":"\u0000 Select the common mooring system-soft yoke mooring system as the research object. The soft yoke mooring system is regarded as a structure composed of multiple rigid bodies, and the theoretical analysis of multi-body dynamics is used to discuss the interaction of multi-rigid bodies.\u0000 The classical HYSY113 FPSO is selected as an example, for the soft yoke mooring system, the stiffness characteristics and static restoring force curved compared with those of software OrcaFlex, and they are in good agreement, which verify the reliability of the formula derived, and it is a prerequisite for the accurate simulations in further steps. Coupled analysis to the whole system in time domain is also carried out both in OrcaFlex and AQWA, and the representative response of the FPSO under different environmental conditions is compared, the results are consistent well with each other. It is a good reference for the future study in this field. Good static characteristics are a prerequisite for accurate analysis of time-domain motion. By comparing the results in the time domain, it is found that under the same working conditions, the analysis results calculated by different commercial software (AQWA and OrcaFlex) may be different. We need to perform design analysis based on the characteristics of the software.","PeriodicalId":431910,"journal":{"name":"Volume 6B: Ocean Engineering","volume":"53 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-08-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"124273759","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Wind energy is the most promising clean, renewable energies to the power industry in the world. More and more wind turbine structures equipped with the larger capacity, taller towers, and longer blades were installed at the offshore/onshore wind farms. But these structures face many harsh environmental conditions, and structural damage and foundation scour are continuously accumulated. It could alter the modal parameter and dynamic response and further reduce the safety of structures. It is a significant challenge on how to accurately estimate the structural states if there is structural damage or foundation scour. For addressing these limitations, a One Dimensional Convolutional Neural Network (1D CNN) method is developed to estimate the structural state. After the Fast Fourier Transform of the acceleration signals, these frequency responses are used as the input to train the 1D CNN, while these states are estimated as the output. A simplified spring-beam model is introduced to simulate the pile-soil interaction, and the effects of the damage and scour on natural frequencies are investigated and compared. The effectiveness and robustness of the proposed 1D CNN method have been numerically investigated by several scenarios associated with the wind turbine structure. Results demonstrate that the 1D CNN method can accurately estimate the structural states, even under a noisy environment. Further, the 1D CNN method can identify the location of damage and scour depth with very high accuracy. This approach may be useful in the on-site structural health monitoring in the wind turbine structure.
{"title":"Neural Network-Based Method for Structural Damage and Scour Estimation Using Modal Parameters and Dynamic Responses","authors":"Shuqing Wang, Yufeng Jiang","doi":"10.1115/omae2020-18461","DOIUrl":"https://doi.org/10.1115/omae2020-18461","url":null,"abstract":"\u0000 Wind energy is the most promising clean, renewable energies to the power industry in the world. More and more wind turbine structures equipped with the larger capacity, taller towers, and longer blades were installed at the offshore/onshore wind farms. But these structures face many harsh environmental conditions, and structural damage and foundation scour are continuously accumulated. It could alter the modal parameter and dynamic response and further reduce the safety of structures. It is a significant challenge on how to accurately estimate the structural states if there is structural damage or foundation scour.\u0000 For addressing these limitations, a One Dimensional Convolutional Neural Network (1D CNN) method is developed to estimate the structural state. After the Fast Fourier Transform of the acceleration signals, these frequency responses are used as the input to train the 1D CNN, while these states are estimated as the output. A simplified spring-beam model is introduced to simulate the pile-soil interaction, and the effects of the damage and scour on natural frequencies are investigated and compared. The effectiveness and robustness of the proposed 1D CNN method have been numerically investigated by several scenarios associated with the wind turbine structure. Results demonstrate that the 1D CNN method can accurately estimate the structural states, even under a noisy environment. Further, the 1D CNN method can identify the location of damage and scour depth with very high accuracy. This approach may be useful in the on-site structural health monitoring in the wind turbine structure.","PeriodicalId":431910,"journal":{"name":"Volume 6B: Ocean Engineering","volume":"1 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2020-08-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"114170232","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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