Calculation of coupled added mass of multibody system immersed in fluids

이진행,Jae-Hun Cho,김성균 대한기계학회 2021 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.35 No.9

The natural frequencies of structures submerged in a fluid are significantly influenced by fluid properties and adjacent structures, which can be expressed by added mass. Some finite element software can effectively determine the added mass matrix of arbitrary structures immersed in viscous fluids, provided the structure lengths are relatively long;however, the added mass specified in a second-order tensor cannot be used as is. Herein, we propose methods to calculate the coupled added mass as a single value for each structure using the MassInFlu code. When the x- and y-directional components of the coupled added mass are unequal, an effective added mass can be determined by rotating the structures or coordinate axes. Various examples considering different scenarios show that the coupled natural frequencies obtained using the coupled or effective added mass coincide with those obtained from fluid-structure interaction analyses.

Added masses computation for unconventional airships and aerostats through geometric shape evaluation and meshing

Marco Tuveri,Alessandro Ceruti,Pier Marzocca 한국항공우주학회 2014 International Journal of Aeronautical and Space Sc Vol.15 No.3

The modern development in design of airships and aerostats has led to unconventional configurations quite different from the classical ellipsoidal and spherical ones. This new class of air-vehicles presents a mass-to-volume ratio that can be considered very similar to the density of the fluid displaced by the vehicle itself, and as a consequence, modeling and simulation should consider the added masses in the equations of motion. The concept of added masses deals with the inertia added to a system, since an accelerating or decelerating body moving into a fluid displaces a volume of the neighboring fluid. The aim of this paper is to provide designers with the added masses matrix for more than twenty Lighter Than Air vehicles with unconventional shapes. Starting from a CAD model of a given shape, by applying a panel-like method, its external surface is properly meshed, using triangular elements. The methodology has been validated by comparing results obtained with data available in literature for a known benchmark shape, and the inaccuracies of predictions agree with the typical precision required in conceptual design. For each configuration, a CAD model and a related added masses matrix are provided, with the purpose of assisting the practitioner in the design and flight simulation of modern airships and scientific balloons.

유한 수심에서 반원형 부유체의 부가질량계수 약산식 개발

구원철(Weoncheol Koo),김준동(Jun-Dong Kim) 한국해양공학회 2013 韓國海洋工學會誌 Vol.27 No.1

This study is to develop the simplified formulae for added mass coefficient of a 20D floating body with a semi-circle section in a finite water depth. The semi-circle floating body may represent a simplified midship section transformed by Lewis form, which can be used for the ship motion analysis by strip theory. Since the added mass coefficient varies with motion frequencies and sea bottom effect, the correction factor representing the effect of water depth and frequencies is developed for accurate prediction of added mass. Using a two-dimensional numerical wave tank (NWT) technique based on the boundary element method(BEM) including sea bottom boundary the reference values of added mass are calculated to develop the correction factor. For verification and effectiveness of the formulae, the predicted added mass coefficients for various frequencies and water depth ratios are compared with the calculated values from NWT technique.

Added masses computation for unconventional airships and aerostats through geometric shape evaluation and meshing

Tuveri, Marco,Ceruti, Alessandro,Marzocca, Pier The Korean Society for Aeronautical and Space Scie 2014 International Journal of Aeronautical and Space Sc Vol.15 No.3

The modern development in design of airships and aerostats has led to unconventional configurations quite different from the classical ellipsoidal and spherical ones. This new class of air-vehicles presents a mass-to-volume ratio that can be considered very similar to the density of the fluid displaced by the vehicle itself, and as a consequence, modeling and simulation should consider the added masses in the equations of motion. The concept of added masses deals with the inertia added to a system, since an accelerating or decelerating body moving into a fluid displaces a volume of the neighboring fluid. The aim of this paper is to provide designers with the added masses matrix for more than twenty Lighter Than Air vehicles with unconventional shapes. Starting from a CAD model of a given shape, by applying a panel-like method, its external surface is properly meshed, using triangular elements. The methodology has been validated by comparing results obtained with data available in literature for a known benchmark shape, and the inaccuracies of predictions agree with the typical precision required in conceptual design. For each configuration, a CAD model and a related added masses matrix are provided, with the purpose of assisting the practitioner in the design and flight simulation of modern airships and scientific balloons.

Relationship between Fusion Mass Shift and Postoperative Distal Adding-on in Lenke 1 Adolescent Idiopathic Scoliosis after Selective Thoracic Fusion

Li Yang,Li Jianlong,Luk Keith D. K.,Zhang Chenggui,Sun Jianmin,Wang Guodong 대한척추외과학회 2023 Asian Spine Journal Vol.17 No.6

Study Design: This is a retrospective cohort study.Purpose: This study aims to investigate the risk factors for postoperative distal adding-on in Lenke 1 adolescent idiopathic scoliosis (AIS) and validate the relationship between fusion mass shift (FMS) and postoperative distal adding-on.Overview of Literature: Postoperative distal curve adding-on is one of the complications in AIS. FMS has been proposed to prevent postoperative distal adding-on, which requires further validation from different institutions.Methods: This study included 60 patients with Lenke 1 AIS who underwent selective thoracic fusion surgery. Coronal spinal alignment parameters were analyzed preoperatively, postoperatively, and at the final follow-up. The postoperative FMS was divided into two groups: the balanced group (FMS ≤20 mm) and the unbalanced group (FMS >20 mm). An independent <i>t</i>-test was used to compare quantitative data between groups, and a chi-square test was used for qualitative data. Furthermore, binary logistic regression and receiver operating characteristics curve analyses were used to identify the risk factors for postoperative distal adding-on in AIS.Results: At 2-year follow-up, the unbalanced group was more likely to have adding-on (17 of 24 patients) than the balanced group (six of 36 patients; <i>p</i><0.001). Twenty-three patients with distal adding-on had significantly greater preoperative and postoperative lower instrumented vertebrae (LIV) rotation, FMS, and FMS angle (FMSA) than those without postoperative distal adding-on. Binary logistic regression analysis selected three independent risk factors for adding-on incidence after surgery: FMS (odds ratio [OR], 1.115; 95% confidence interval [CI], 1.049–1.185; <i>p</i><0.001), FMSA (OR, 1.590; 95% CI, 1.225–2.064; <i>p</i><0.001), and postoperative LIV rotation (OR, 6.581; 95% CI, 2.280–19.000; <i>p</i><0.001).Conclusions: Achieving a balanced fusion mass intraoperatively is important to avoid postoperative distal adding-on, with FMS of <20 mm and FMS angle of <4.5°. Furthermore, correcting LIV rotation helps to decrease the incidence of postoperative distal addingon.

Estimation of Beam Mode Frequencies of Co-axial Cylinders Immersed in Fluid by Equivalent Mass Approach

Kim, Tae-Wan,Park, Suhn,Park, Keun-Bae Korean Nuclear Society 2003 Nuclear Engineering and Technology Vol.35 No.1

In this study, an effective method to estimate the fundamental frequencies of co-axial cylinders immersed in fluid is proposed. The proposed method makes use of the equivalent mass or density that is derived from the added mass matrix caused by the fluid-structure interaction (FSI) phenomenon. The equivalent mass is defined from the added mass matrix based on a 2-D potential flow theory. The theory on two co-axial cylinders extended to the case of three cylinders. To prove the validity of the proposed method, the eigenvalue analyses upon coaxial cylinders coupled with fluid gaps are peformed using the equivalent mass. The analyses results upon various fluid gap is conditions reveal that the present method could provide accurate frequencies and be suitable for expecting the fundamental frequencies of fluid coupled cylinders in beam mode vibration.

Performance of rotational mode based indices in identification of added mass in beams

Prakash Rajendran,Sivakumar M Srinivasan 국제구조공학회 2015 Structural Engineering and Mechanics, An Int'l Jou Vol.54 No.4

This study investigates the identification of added mass and its location in the glass fiber reinforced polymer (GFRP) beam structures. The main emphasis of this paper is to ascertain the importance of inclusion of rotational degrees of freedom (dofs) in the introduction of added mass or damage identification. Two identification indices that include the rotational dofs have been introduced in this paper: the modal force index (MFI) and the modal rotational curvature index (MRCI). The MFI amplifies damage signature using undamaged numerical stiffness matrix which is related to changes in the altered mode shapes from the original mode shapes. The MRCI is obtained by using a higher derivative of rotational mode shapes. Experimental and numerical results are compared with the existing methods leading to a conclusion that the contributions of the rotational modes play a key role in the identification of added mass. The authors believe that the similar results are likely in the case of damage identification also.

Performance of rotational mode based indices in identification of added mass in beams

Rajendrana, Prakash,Srinivasan, Sivakumar M. Techno-Press 2015 Structural Engineering and Mechanics, An Int'l Jou Vol.54 No.4

This study investigates the identification of added mass and its location in the glass fiber reinforced polymer (GFRP) beam structures. The main emphasis of this paper is to ascertain the importance of inclusion of rotational degrees of freedom (dofs) in the introduction of added mass or damage identification. Two identification indices that include the rotational dofs have been introduced in this paper: the modal force index (MFI) and the modal rotational curvature index (MRCI). The MFI amplifies damage signature using undamaged numerical stiffness matrix which is related to changes in the altered mode shapes from the original mode shapes. The MRCI is obtained by using a higher derivative of rotational mode shapes. Experimental and numerical results are compared with the existing methods leading to a conclusion that the contributions of the rotational modes play a key role in the identification of added mass. The authors believe that the similar results are likely in the case of damage identification also.

2 상 횡 유동장에 놓인 관군의 유체탄성불안정성

심우건(Woo Gun Sim),박미연(Mi Yeon Park) 대한기계학회 2009 大韓機械學會論文集B Vol.33 No.2

Experiments have been performed to investigate fluid-elastic instability of tube bundles, subjected to twophase cross flow. Fluid-elastic is the most important vibration excitation mechanism for heat exchanger tube bundles subjected to the cross flow. The test section consists of cantilevered flexible cylinder(s) and rigid cylinders of normal square array. From a practical design point of view, fluid-elastic instability may be expressed simply in terms of dimensionless flow velocity and dimensionless mass-damping parameter. For dynamic instability of cylinder rows, added mass, damping and the threshold flow velocity are evaluated. The Fluid-elastic instability coefficient is calculated and then compared to existing results given for tube bundles in normal square array.

고유진동수 현장계측과 수치해석을 이용한 수문의 부가질량 보정법

김호승(Kim Ho Seung),배정주(Bea Jung Ju),김용곤(Kim Yong Gon),이지호(Lee Jee Ho) 대한토목학회 2009 대한토목학회논문집 A Vol.29 No.6A

본 논문에서는 댐 수문의 지진해석과 안전성 평가에 필수적인 지진시 동수압을 정확하고 효율적으로 산정하는 방법을 제시하였다. 제안된 방법은 비교적 간단한 현장 진동계측과 유한요소해석을 통하여 동수압과 등가인 부가질량을 계산하는 방법으로 정확성과 아울러 실용성을 고려하여 개발되어 실제 수문의 내진성능평가에 적용이 가능하다. In this paper, a method is proposed to accurately and efficiently estimate the equivalent added mass of hydro-dynamic pressure on dam floodgates subjected to earthquake loading. The present method is based on a relatively-simple procedure using on-site vibration measurement and finite element frequency analysis, which is sufficiently practical to be used in the earthquake resistance performance evaluation of dam floodgates.