Asymptotic nonlinear multimodal method for liquid sloshing in an upright circular cylindrical tank. Part 1: Modal equations

Combining the variational method by Lukovsky – Miles and the Narimanov – Moiseev asymptotics, a nonlinear modal system describing the resonant liquid sloshing in an upright circular cylindrical tank is derived. Sloshing occurs due to a small-amplitude periodic or almost-periodic excitation with the...

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spelling Lukovsky, I.
Ovchynnykov, D.
Timokha, A.
2021-02-01T15:44:55Z
2021-02-01T15:44:55Z
2011
Asymptotic nonlinear multimodal method for liquid sloshing in an upright circular cylindrical tank. Part 1: Modal equations / I. Lukovsky, D. Ovchynnykov, A. Timokha // Нелінійні коливання. — 2011. — Т. 14, № 4. — С. 482-495. — Бібліогр.: 41 назв. — англ.
1562-3076
https://nasplib.isofts.kiev.ua/handle/123456789/175504
517.9
Combining the variational method by Lukovsky – Miles and the Narimanov – Moiseev asymptotics, a nonlinear modal system describing the resonant liquid sloshing in an upright circular cylindrical tank is derived. Sloshing occurs due to a small-amplitude periodic or almost-periodic excitation with the forcing frequency close to the lowest natural sloshing frequency. In contrast to the existing nonlinear modal systems based on the Narimanov – Moiseev asymptotic intermodal relationships, the derived modal equations: (i) contain all the necessary (infinitely many) generalized coordinates of the second and the third orders, (ii) include exclusively nonzero hydrodynamic coefficients for which (iii) rather simple computational formulas are found. As a consequence, the modal equations can be used in analytical studies of nonlinear sloshing phenomena that will be demonstrated in the forthcoming Part 2.
Комбiнуючи варiацiйний метод Луковського – Майлса та асимптотику Нарiманова – Моiсеєва, побудовано нелiнiйну модальну систему, що описує резонанснi коливання рiдини у вертикальному круговому цилiндричному резервуарi. Коливання вiдбуваються завдяки перiодичному чи майже перiодичному збуренню з частотою, близькою до першої власної частоти. На вiдмiну вiд iснуючих нелiнiйних модальних систем, якi базуються на асимптотичних спiввiдношеннях Нарiманова – Моiсеєва, побудованi модальнi рiвняння: (i) включають всi необхiднi (нескiнченну кiлькiсть) узагальненi координати другого та третього порядку, (ii) утримують винятково ненульовi гiдродинамiчнi коефiцiєнти, для яких (iii) знайдено достатньо простi обчислювальнi формули. Як наслiдок, модальнi рiвняння можна використати в аналiтичних дослiдженнях нелiнiйних явищ, що буде продемонстровано в наступнiй частинi 2.
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Інститут математики НАН України
Нелінійні коливання
Asymptotic nonlinear multimodal method for liquid sloshing in an upright circular cylindrical tank. Part 1: Modal equations
Асимптотичне нелiнiйне мультимодальне моделювання сплескiв рiдини у вертикальному круговому цилiндричному резервуарi. Ч. 1. Модельнi рiвняння
Асимптотическое нелинейное мультимодальное моделирование всплесков жидкости в вертикальном круговом цилиндрической резервуаре. Ч. 1. Модельные уравнения
Article
published earlier
institution Digital Library of Periodicals of National Academy of Sciences of Ukraine
collection DSpace DC
title Asymptotic nonlinear multimodal method for liquid sloshing in an upright circular cylindrical tank. Part 1: Modal equations
spellingShingle Asymptotic nonlinear multimodal method for liquid sloshing in an upright circular cylindrical tank. Part 1: Modal equations
Lukovsky, I.
Ovchynnykov, D.
Timokha, A.
title_short Asymptotic nonlinear multimodal method for liquid sloshing in an upright circular cylindrical tank. Part 1: Modal equations
title_full Asymptotic nonlinear multimodal method for liquid sloshing in an upright circular cylindrical tank. Part 1: Modal equations
title_fullStr Asymptotic nonlinear multimodal method for liquid sloshing in an upright circular cylindrical tank. Part 1: Modal equations
title_full_unstemmed Asymptotic nonlinear multimodal method for liquid sloshing in an upright circular cylindrical tank. Part 1: Modal equations
title_sort asymptotic nonlinear multimodal method for liquid sloshing in an upright circular cylindrical tank. part 1: modal equations
author Lukovsky, I.
Ovchynnykov, D.
Timokha, A.
author_facet Lukovsky, I.
Ovchynnykov, D.
Timokha, A.
publishDate 2011
language English
container_title Нелінійні коливання
publisher Інститут математики НАН України
format Article
title_alt Асимптотичне нелiнiйне мультимодальне моделювання сплескiв рiдини у вертикальному круговому цилiндричному резервуарi. Ч. 1. Модельнi рiвняння
Асимптотическое нелинейное мультимодальное моделирование всплесков жидкости в вертикальном круговом цилиндрической резервуаре. Ч. 1. Модельные уравнения
description Combining the variational method by Lukovsky – Miles and the Narimanov – Moiseev asymptotics, a nonlinear modal system describing the resonant liquid sloshing in an upright circular cylindrical tank is derived. Sloshing occurs due to a small-amplitude periodic or almost-periodic excitation with the forcing frequency close to the lowest natural sloshing frequency. In contrast to the existing nonlinear modal systems based on the Narimanov – Moiseev asymptotic intermodal relationships, the derived modal equations: (i) contain all the necessary (infinitely many) generalized coordinates of the second and the third orders, (ii) include exclusively nonzero hydrodynamic coefficients for which (iii) rather simple computational formulas are found. As a consequence, the modal equations can be used in analytical studies of nonlinear sloshing phenomena that will be demonstrated in the forthcoming Part 2. Комбiнуючи варiацiйний метод Луковського – Майлса та асимптотику Нарiманова – Моiсеєва, побудовано нелiнiйну модальну систему, що описує резонанснi коливання рiдини у вертикальному круговому цилiндричному резервуарi. Коливання вiдбуваються завдяки перiодичному чи майже перiодичному збуренню з частотою, близькою до першої власної частоти. На вiдмiну вiд iснуючих нелiнiйних модальних систем, якi базуються на асимптотичних спiввiдношеннях Нарiманова – Моiсеєва, побудованi модальнi рiвняння: (i) включають всi необхiднi (нескiнченну кiлькiсть) узагальненi координати другого та третього порядку, (ii) утримують винятково ненульовi гiдродинамiчнi коефiцiєнти, для яких (iii) знайдено достатньо простi обчислювальнi формули. Як наслiдок, модальнi рiвняння можна використати в аналiтичних дослiдженнях нелiнiйних явищ, що буде продемонстровано в наступнiй частинi 2.
issn 1562-3076
url https://nasplib.isofts.kiev.ua/handle/123456789/175504
citation_txt Asymptotic nonlinear multimodal method for liquid sloshing in an upright circular cylindrical tank. Part 1: Modal equations / I. Lukovsky, D. Ovchynnykov, A. Timokha // Нелінійні коливання. — 2011. — Т. 14, № 4. — С. 482-495. — Бібліогр.: 41 назв. — англ.
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fulltext UDC 517.9 ASYMPTOTIC NONLINEAR MULTIMODAL MODELING OF LIQUID SLOSHING IN AN UPRIGHT CIRCULAR CYLINDRICAL TANK. PART 1: MODAL EQUATIONS АСИМПТОТИЧНЕ НЕЛIНIЙНЕ МУЛЬТИМОДАЛЬНЕ МОДЕЛЮВАННЯ ХЛЮПАННЯ РIДИНИ У ВЕРТИКАЛЬНОМУ КРУГОВОМУ ЦИЛIНДРИЧНОМУ РЕЗЕРВУАРI. Ч. 1. МОДАЛЬНI РIВНЯННЯ I. Lukovsky, D. Ovchynnykov, A. Timokha Inst. Math. Nat. Acad. Sci. Ukraine Ukraine, 01601, Kyiv, Tereshchenkivska str., 3 Combining the variational method by Lukovsky – Miles and the Narimanov – Moiseev asymptotics, a nonlinear modal system describing the resonant liquid sloshing in an upright circular cylindrical tank is derived. Sloshing occurs due to a small-amplitude periodic or almost-periodic excitation with the for- cing frequency close to the lowest natural sloshing frequency. In contrast to the existing nonlinear modal systems based on the Narimanov – Moiseev asymptotic intermodal relationships, the derived modal equati- ons: (i) contain all the necessary (infinitely many) generalized coordinates of the second and the third orders, (ii) include exclusively nonzero hydrodynamic coefficients for which (iii) rather simple computati- onal formulas are found. As a consequence, the modal equations can be used in analytical studies of nonlinear sloshing phenomena that will be demonstrated in the forthcoming Part 2. Комбiнуючи варiацiйний метод Луковського – Майлса та асимптотику Нарiманова – Моiсеєва, побудовано нелiнiйну модальну систему, що описує резонанснi коливання рiдини у вертикаль- ному круговому цилiндричному резервуарi. Коливання вiдбуваються завдяки перiодичному чи майже перiодичному збуренню з частотою, близькою до першої власної частоти. На вiдмiну вiд iснуючих нелiнiйних модальних систем, якi базуються на асимптотичних спiввiдношеннях Нарiманова – Моiсеєва, побудованi модальнi рiвняння: (i) включають всi необхiднi (нескiнченну кiлькiсть) узагальненi координати другого та третього порядку, (ii) утримують винятково ненульовi гiдродинамiчнi коефiцiєнти, для яких (iii) знайдено достатньо простi обчислюваль- нi формули. Як наслiдок, модальнi рiвняння можна використати в аналiтичних дослiдженнях нелiнiйних явищ, що буде продемонстровано в наступнiй частинi 2. 1. Introduction. Accounting for liquid sloshing loads is of importance for designing the engi- neering constructions carrying a liquid cargo. Safety, reliability, stability, and control analysis of the liquid containing structures have been extensively studied in the context of aircraft and spacecraft applications, for cargo tanks of automotive vehicles, offshore platforms, and sei- smic analysis of the elevated water tanks. The studies require comprehensive quantitative and qualitative examining the coupled fluid-structure dynamics, its modeling and simulation on the real-time scale. Liquid sloshing response becomes most severe in resonance conditions when the carrying structure oscillates with a frequency close to the lowest natural sloshing frequency. Those resonant free-surface motions are strongly nonlinear and must be described by solving an evolution free-boundary problem in which both instant shapes of the free surface Σ(t) and the velocity field in the liquid domain Q(t) are the unknowns. c© I. Lukovsky, D. Ovchynnykov, A. Timokha, 2011 482 ISSN 1562-3076. Нелiнiйнi коливання, 2011, т . 14, N◦ 4 ASYMPTOTIC NONLINEAR MULTIMODAL MODELING OF LIQUID . . . 483 Under certain circumstances, one can distinguish three different approaches to solving the nonlinear liquid sloshing problem. The first approach is the Computational Fluid Dynamics (CFD). A broad variety of numerical methods exists which could be divided into two sub- classes comprising potential flow, the Navier – Stokes method, and, sometimes, their hybrids typically based on the domain decomposition method [8, 39]. The CFD methods are universal, accurate and efficient, especially on the short-time scale when focus is on transient waves. Their drawback is that they are, generally speaking, computational time demanding, especially for three-dimensional problems. Furthermore, their applicability can be rather limited when the tasks consist in simulation and classification of the so-called steady-state wave regimes occur- ring on the long-time scale and, therefore, requiring long-time simulations with different initial scenarios. The second approach is purely analytical. It is developed for studying the steady-state (peri- odic) solution expected for prescribed small-amplitude harmonic tank excitations. The analyti- cal approach employs asymptotic methods which have been created by great mathematicians of the XIX century in the theory of nonlinear ocean waves [3]. An extension of these methods to nonlinear resonant sloshing in closed basins is often referred to the pioneering paper by Moiseev [32]. More mathematical details on constructing the steady-state asymptotic solution by solving a series of recurrence boundary value problems and deriving the so-called secularity condition (the necessary solvability condition) coupling the forcing frequency and the dominant response amplitude can be found in [5, 36, 37, 11, 12, 8]. The asymptotic steady-state solution technique changes with the mean liquid depth. For finite liquid depths, the Taylor expansion of the nonlinear free-surface conditions with respect to the mean (unperturbed) free surface leads to cubic algebraic secularity equations and yields the so-called third-order Moiseev asymptotics causing the dominant response amplitude be of the orderO(ε1/3) where ε is the nondimensional forcing amplitude. The asymptotic solution methods are generally not applicable to transient waves, and for modeling the fluid-structure interaction. Furthermore, the asymptotic steady- state solution is only valid in a matching forcing-frequency range and for a relatively small forcing amplitude. Forcing frequencies away from this range and increasing the forcing ampli- tude can lead to the so-called internal (secondary, combinatory) resonance and, thereby, cause a failure of the Moiseev intermodal asymptotic relationships (Moiseev’s asymptotics). The third approach is associated with nonlinear multimodal methods whose application assumes an ideal liquid with irrotational flow and no overturning and breaking waves allowed. In this paper, we follow this approach to derive an infinite-dimensional system of nonlinear asymptotic-type modal equations for sloshing in an upright circular cylindrical tank by combi- ning the variational multimodal method by Lukovsky – Miles [19, 20, 29] and the Narimanov – Moiseev intermodal asymptotic relationships [34, 35, 32] which can follow from the second approach or, simply, be postulated. Distinguished details for such a combined variational-and- asymptotic version of multimodal methods, its difference from others are outlined in revi- ews [27, 11]. Readers interested in employing other versions of multimodal methods for liquid sloshing in an upright cylindrical tank are referred to [34, 4, 35, 20, 10] (Narimanov’s modal-type perturbation method), [33, 38, 15] (fully-nonlinear [non-asymptotic] multimodal [Perko-type] method), [16, 17, 18] (combining the Lagrange variational principle and perturbation method), [20, 7, 8] (combining the Bateman – Luke variational principle and perturbation method), and references therein. Sloshing of an ideal liquid with irrotational flow introduces a nonlinear free boundary pro- blem with the two unknowns that are the instant free-surface shapes and the velocity potential. ISSN 1562-3076. Нелiнiйнi коливання, 2011, т . 14, N◦ 4 484 I. LUKOVSKY, D. OVCHYNNYKOV, A. TIMOKHA According to the multimodal method concept for liquid sloshing in tanks with upright walls, the instant free-surface shapes should be defined by the Fourier-type solution with unknown time- dependent coefficients qi(t) (furthermore, generalized coordinates) in the front of fi(y, z) = = ϕi(0, y, z), i.e., x = f(y, z, t) = ∑ i qi(t)fi(y, z), (1) where ϕi(x, y, z) are the so-called natural sloshing modes. Analogous Fourier-type solution involving ϕi(x, y, z) is used for the velocity potential. Even though there exist different versions of nonlinear multimodal methods, all of them are developed to transform the original problem to an infinite-dimensional system of nonlinear ordinary differential (modal) equations coupling the generalized coordinates qi. However, since derivation of nonlinear multimodal equations is a difficult mathematical task, each a version proposes a proper analytical way pursuing the modal equations of desirable structure. Except for the Perko-type methods, the derivations require a postulation of asymptotic relationships between the generalized coordinates qi assuming a small set of dominant generali- zed coordinates. Neglecting the nonlinear terms in qi which have the order higher than the for- cing input signal associated with the highest-order term, O(ε), leads to the so-called asymptotic nonlinear modal equations. Using the asymptotic modal equations helps to avoid physically- unrealistic higher harmonics which give negligible contribution to liquid response, but may cause the stiffness of the differential (modal) equations as it was observed for the Perko-type simulations [15]. The Narimanov – Moiseev asymptotics [34, 35, 32] is the most-often accepted asymptotic relationships used for deriving the asymptotic nonlinear modal systems. They follow from Moi- seev’s asymptotic solution (the second approach) or can be postulated as it has been done by Narimanov in his classical works [34, 35]. Adopting the asymptotic relationships reduces the problem to calculation of the non-zero hydrodynamic coefficients at the polynomial-type quantities in the asymptotic modal equations. Usually, the number of the nonzero coefficients is quite limited. Bearing in mind analytical studies based on the asymptotic nonlinear modal equations, i.e., considering the nonlinear sloshing as an object of either applied mathematics or theoretical mechanics, strongly requires to exclude the zeros from the modal equations as well as to provide simple and compact formulas for the nonzero hydrodynamic coefficients. Of course, using the asymptotic nonlinear multimodal equations as a computational tool [18, 7], i.e., considering the multimodal method as a CFD approach, does not need the analytical extraction of the zeros. For rectangular cross-section, the Narimanov – Moiseev asymptotic relationships lead, due to trigonometric relations between the natural sloshing modes fi, to a nine-dimensional nonli- near asymptotic modal system. This system is explicitly derived and analytically studied in [6, 8]. Other cylindrical tank shapes yield, generally speaking, infinite-dimensional asymptotic multimodal systems. The latter is also true for the case of circular cross-section. The literature presents various analytically-given finite-dimensional asymptotic modal systems [20 – 22] but these systems couple only a few of second and third-order generalized coordinates. To the best authors’ knowledge, the present paper firstly derives in an analytical form the infinite- dimensional Narimanov – Moiseev’ asymptotic modal system for the circle-based tank provi- ding that the modal equations (i) contain all the necessary generalized coordinates of the ISSN 1562-3076. Нелiнiйнi коливання, 2011, т . 14, N◦ 4 ASYMPTOTIC NONLINEAR MULTIMODAL MODELING OF LIQUID . . . 485 Fig. 1. Sketch of an upright circular cylindrical tank and the adopted nomenc- lature. The geometrical and physical characteristics are scaled in our analysis by the dimensional tank radius R0 so that, e.g., h in the figure is the ratio between the mean liquid depth and R0. second and third order following from the Narimanov – Moiseev asymptotic intermodal relati- onships, (ii) include exclusively nonzero hydrodynamic coefficients for which (iii) rather simple computational formulas are found. In the forthcoming Part 2, we will present analytical studies of the nonlinear resonant sloshing based on the derived modal system and compare the analyti- cal results with experiments. 2. Statement of the problem. 2.1. Free boundary problem. An upright circular cylindrical tank is considered which is partly filled with an inviscid incompressible liquid with irrotati- onal flow. Fig. 1 introduces the basic notations. No overturning waves are assumed. The time- dependent liquid domain Q(t) is bounded by the free surface Σ(t) and the wetted tank surface S(t). The mean liquid depth is equal to h. Henceforth in all the mathematical expressions, we suggest that the liquid characteristics, including h and the gravity acceleration g, are scaled by the tank radius R0 so that, everywhere in our forthcoming analysis the theoretical radius of the tank is nondimensional and equal to 1. The liquid motions are considered in the tank-fixed coordinate system Oxyz whose ori- gin is situated in the center of the mean free-surface Σ0. The Ox-axis is superposed with the tank symmetry axis. For brevity, we concentrate on the case when the tank moves translatory with the velocity v0(t) relative to an absolute Earth-fixed coordinate system Ox′y′z′. Small- magnitude angular forcing terms can also be accounted for by assuming that these terms are of the highest order in the Narimanov – Moiseev asymptotic ordering. The latter procedure is extensively discussed in [8]. The absolute velocity potential Φ(x, y, z, t), and the free surface Σ(t) are the two unknowns which should be found from the following nonlinear free-boundary problem ∇2Φ = 0, r ∈ Q(t), (2) ∂Φ ∂ν = v0 · ν + ft√ 1 + |∇f |2 , r ∈ Σ, (3) ∂Φ ∂ν = v0 · ν, r ∈ S(t), (4) ISSN 1562-3076. Нелiнiйнi коливання, 2011, т . 14, N◦ 4 486 I. LUKOVSKY, D. OVCHYNNYKOV, A. TIMOKHA ∂Φ ∂t + 1 2 |∇Φ|2 −∇Φ · v0 + U = 0, r ∈ Σ(t). (5) Here ν is the outer normal vector, U = (g · r) is the gravity potential with r = (x, y, z), g = (−g, 0, 0) is the gravity acceleration vector, and x = f(y, z, t) is the free-surface equation. For the free-boundary problem (2), typical initial conditions (at t = 0) define the initial liquid shape and velocity field and take the form f(y, z, 0) = ξ0(y, z), Φ(x, y, z, 0) = Φ0(x, y, z), r ∈ Q(0). (6) 2.2. Variational formulation. In 1976, Miles [29] and Lukovsky [19] independently proposed to use the Bateman – Luke variational principle for derivation of nonlinear modal systems. Hi- story of the Bateman – Luke principle starts from 1908, when R. Hargneaves [13] has noted that the pressure integral can play the role of the Lagrangian in variational formulations of diverse hydrodynamic problems. The canonical formulation of this principle for an incompressible ideal liquid is given by Bateman [1]. Furthermore, this formulation was generalized by Luke [28] for ocean waves and by Lukovsky [20] for liquid sloshing in a tank performing arbitrary spatial motions. The Bateman – Luke principle for a compressible fluid can be found in [24, 25, 26, 2]. According to Lukovsky [20], the Bateman – Luke principle for (2) – (5) can be formulated as follows: The free-boundary problem (2) – (5) is associated with the necessary extrema of the action W = t2∫ t1 Ldt, (7) where the Lagrangian L is defined by the pressure integral L = ∫ Q(t) (p− po) dQ = −ρ ∫ Q(t) [ ∂Φ ∂t + 1 2 |∇Φ|2 −∇Φ · v0 + U ] dQ (8) and trial functions satisfy the conditions δΦ(x, y, z, t1) = δΦ(x, y, z, t2) = δf(y, z, t1) = δf(y, z, t2) = 0. (9) 3. Nonlinear multimodal modeling. The nonlinear multimodal modeling is based on the Fourier-type solution (1) in which qi(t) are treated as generalized coordinates of the considered hydromechanical system. Here fi(x, y) is a complete orthogonal system of functions satisfying the volume conservation condition ∫ Σ0 fi(x, y) dx dy = 0. In addition, one should introduce the Fourier-type representation of the velocity potential Φ(x, y, z, t) = v0 · r + ∑ n Qn(t)ϕn(x, y, z), (10) where the complete set of harmonic functions ϕn(x, y, z) satisfies both the Laplace equation in the whole tank domain and the zero Neumann boundary conditions on the wetted body surface. ISSN 1562-3076. Нелiнiйнi коливання, 2011, т . 14, N◦ 4 ASYMPTOTIC NONLINEAR MULTIMODAL MODELING OF LIQUID . . . 487 Normally, ϕn and fn(x, y) = ϕn(x, y, 0) are the eigenfunctions (natural sloshing modes) of the spectral boundary problem ∇2ϕn = 0, ~r ∈ Q0; ∂ϕn ∂ν = κnϕn, ~r ∈ Σ0; ∂ϕn ∂ν = 0, ~r ∈ S0, (11) whereQ0 is the mean liquid domain and S0 is the mean wetted tank surface. The natural sloshi- ng frequencies are defined by the eigenvalues κn via σn = √ gκn. The aim of the multimodal modeling is to derive a system of ordinary differential equations (modal equations) with respect to generalized coordinates qi(t). There are different analyti- cal schemes (multimodal methods) how to do that; these are shortly outlined in Introduction. According to [19, 20, 29], the derivation can employ the Bateman – Luke principle instead of the free-boundary problem (2). 3.1. Lukovsky – Miles’ variational method. Lukovsky [20] showed how to use the Bateman – Luke principle for deriving the nonlinear modal equations coupling qi(t) and Qn(t). The result for translatory tank excitations is the following infinite-dimensional system of nonlinear ordi- nary differential equations:∑ i ∂An ∂qi q̇i − ∑ k AnkQk = 0, n = 1, 2, . . . , (12) ∑ n ∂An ∂qi Q̇n + 1 2 ∑ n,k ∂Ank ∂qi QnQk + 3∑ j=1 (v̇Oj − gj) ∂lj ∂qi = 0, i = 1, 2, . . . , (13) where ∂l1 ∂qi = ∫ Σ0 f2 i dS qi, ∂l2 ∂qi = ∫ Σ0 yfi dS, ∂l3 ∂qi = ∫ Σ0 zfi dS, (14) g = (g1, g2, g3) = (−g, 0, 0), and An = ∫ Q(t) ϕn dQ, Ank = ∫ Q(t) ∇ϕn · ∇ϕk dQ. (15) The nonlinear modal equations (12), (13) are a full analogy of the original free-boundary problem. Direct simulations by the modal equations (12), (13) imply the so-called Perko’s numerical method (see Introduction). Lukovsky & Timokha [27] pointed out that these si- mulations can be stiff for resonant sloshing and, therefore, a certain numerical time-integration becomes numerically unstable. This physically-unrealistic stiffness is caused by amplification of higher harmonics which, in the reality, are highly damped due to different dissipative mecha- nisms. An alternative is to introduce asymptotic relationship between generalized coordinates and, thereby, exclude (”filter”) the unrealistically high harmonics. 3.2. Narimanov – Moiseev’ asymptotic intermodal relationships for circular-base tank. For the circular-base tank, the modal solution (1) can be rewritten in the cylindrical coordinate system as follows: x = f(ξ, η, t) = ∞∑ m=0 ∞∑ n=1 (rm,n(t) sin(mη) + pm,n(t) cos(mη))fmn(ξ), (16) ISSN 1562-3076. Нелiнiйнi коливання, 2011, т . 14, N◦ 4 488 I. LUKOVSKY, D. OVCHYNNYKOV, A. TIMOKHA where fmn = Jm(km,nξ) Jm(km,n) (Jm(·) is the Bessel function) and J ′m(km,n) = 0. The zeros of the last equation defines the eigenvalues κm,n and the natural sloshing frequencies σm,n by the formulas κm,n = km,ntanh (km,nh) and σ2 m,n = gκm,n. (17) The generalized coordinates qi as well as rm,n and pm,n are nondimensional (scaled by the tank radius) and one can introduce asymptotic relations between them. When the forci- ng frequency σ is close to the lowest natural frequency σ1,1 associated with the two generalized coordinates p1,1(t) and r1,1(t), the Narimanov – Moiseev asymptotics [32, 20, 30, 31, 27] requires the asymptotic relation p1,1 ∼ r1,1 = O(ε1/3), (18) where ε � 1 implies the nondimensional forcing magnitude. Postulating (18) and using the trigonometric algebra with respect to the angular coordinate η, one can establish the second- and third-order generalized coordinates p0,n ∼ p2,n ∼ r2,n = O(ε2/3); p3,n ∼ r3,n = O(ε), n = 1, 2, . . . , (19) p1,m ∼ r1,m = O(ε), m = 2, 3, . . . . Remaining generalized coordinates are of the order o(ε) and can be neglected in our nonlinear asymptotic multimodal analysis. 4. Nonlinear asymptotic multimodal equations. The most general analytical scheme for combining the Lukovsky – Miles variational method and the Narimanov – Moiseev asymptotics is described in [27]. Accounting for (18) – (19), the scheme suggests the following steps: 1. Using the Taylor expansion, one should find polynomial expressions (in terms of nondi- mensional generalized coordinates qi) for ∂An/∂qk and Ank keeping up to the O(ε2/3)-order and ∂Ank/∂qi keeping the O(ε1/3)-terms. 2. We should find the asymptotic solution Qi = F (qk, q̇k) from modal equations (12) by substituting previously-found asymptotic expressions for ∂An/∂qk andAnk.This solution should neglect the o(ε)-terms. 3. We should substitute expressions Qi = F (qk, q̇k) from the previous step into modal equations (13) and keep up to the O(ε)-terms. This will give the desirable asymptotic modal equations. The scheme was fully realized only for upright cylindrical tanks of rectangular shape. By generalizing [20], the paper [23] showed that the scheme can also be applied for a circular cylindrical tank. It is implemented in the present paper to obtain the required analytically-given asymptotic nonlinear modal equations. Implementing the analytical scheme with 3N (N → ∞) generalized coordinates of the second order and 4N generalized coordinates of the third order leads to the nonlinear asympto- tic modal equations which include the following two differential equations for the lowest- ISSN 1562-3076. Нелiнiйнi коливання, 2011, т . 14, N◦ 4 ASYMPTOTIC NONLINEAR MULTIMODAL MODELING OF LIQUID . . . 489 order generalized coordinates p1,1 and r1,1 : µ1,1 [ p̈1,1 + σ2 1,1p1,1 ] + p1,1 N∑ n=1 d (2) 0,np̈0,n + N∑ n=1 d (3) 0,n (p̈1,1p0,n + ṗ1,1ṗ0,n) + + d1 ( p2 1,1p̈1,1 + p1,1ṗ 2 1,1 + r1,1p1,1r̈1,1 + p1,1ṙ 2 1,1 ) + + d2 ( r2 1,1p̈1,1 + 2r1,1ṙ1,1ṗ1,1 − r1,1p1,1r̈1,1 − 2p1,1ṙ 2 1,1 ) + + N∑ n=1 d (3) 2,n (p̈1,1p2,n + r̈1,1r2,n + ṗ1,1ṗ2,n + ṙ1,1ṙ2,n) + + N∑ n=1 d (2) 2,n (p1,1p̈2,n + r1,1r̈2,n) = −µ1,1κ1,1 k2 1,1 − 1 v̇01, (20a) µ1,1 [ r̈1,1 + σ2 1,1r1,1 ] + r1,1 N∑ n=1 d (2) 0,np̈0,n + N∑ n=1 d (3) 0,n (r̈1,1p0,n + ṙ1,1ṗ0,n) + + d1 ( r2 1,1r̈1,1 + r1,1ṙ 2 1,1 + r1,1p1,1p̈1,1 + r1,1ṗ 2 1,1 ) + + d2 ( p2 1,1r̈1,1 + 2p1,1ṙ1,1ṗ1,1r1,1p1,1p̈1,1 − 2r1,1ṗ 2 1,1 ) + + N∑ n=1 d (3) 2,n (p̈1,1r2,n − r̈1,1p2,n + ṗ1,1ṙ2,n − ṙ1,1ṗ2,n) + + N∑ n=1 d (2) 2,n (p1,1r̈2,n − r1,1p̈2,n) = −µ1,1κ1,1 k2 1,1 − 1 v̇02. (20b) These equations include both the lowest- and second-order generalized coordinates, but the third-order generalized coordinates are absent here. Notations for km,n (the roots of the equati- on J ′m(km,n) = 0), κm,n (see eq. (17)), σm,n (natural sloshing frequency) as well as the translatory velocity components v̇01(t) and v̇02(t) were explained before. The nondimensional hydrodynamic coefficients at the nonlinear terms are defined by the end of this section. The differential equations for finding the second-order generalized coordinates p0,n, p2,n and r2,n take the form 2µ0,n [ p̈0,n + σ2 0,np0,n ] + d (1) 0,n ( ṗ2 1,1 + ṙ2 1,1 ) + d (2) 0,n (p̈1,1p1,1 + r̈1,1r1,1) = 0, (21a) µ2,n [ p̈2,n + σ2 2,np2,n ] + d (1) 2,n ( ṗ2 1,1 − ṙ2 1,1 ) + d (2) 2,n (p̈1,1p1,1 − r̈1,1r1,1) = 0, (21b) µ2,n [ r̈2,n + σ2 2,nr2,n ] + 2d (1) 2,nṙ1,1ṗ1,1 + d (2) 2,n (p̈1,1r1,1 + r̈1,1p1,1) = 0. (21c) Here n = 1, . . . , N, i.e., there is 3N ordinary differential equations for these generalized coordi- nates. Note that equations (21) contain p1,1 and r1,1 defined by (21) and, therefore, one can say ISSN 1562-3076. Нелiнiйнi коливання, 2011, т . 14, N◦ 4 490 I. LUKOVSKY, D. OVCHYNNYKOV, A. TIMOKHA that the first and second-order generalized coordinates are nonlinearly coupled by our modal equations. However, the third-order generalized coordinates p3,n and r3,n are not presented in (21). Equations for these generalized coordinates take the form µ3,n [ r̈3,n + σ2 3,nr3,n ] + d3 ( r1,1ṗ 2 1,1 + 2p1,1ṗ1,1ṙ1,1 − r1,1ṙ 2 1,1 ) + + d4 ( p2 1,1r̈1,1 + 2r1,1p1,1p̈1,1 − r2 1,1r̈1,1 ) + N∑ n=1 d (1) 3,n (ṗ1,1ṙ2,n + ṙ1,1ṗ2,n) + + N∑ n=1 d (2) 3,n (p1,1r̈2,n + r1,1p̈2,n) + N∑ n=1 d (3) 3,n (p̈1,1r2,n + r̈1,1p2,n) = 0, (22a) µ3,n [ p̈3,n + σ2 3,np3,n ] + d3 ( p1,1ṗ 2 1,1 − 2r1,1ṗ1,1ṙ1,1 − p1,1ṙ 2 1,1 ) + + d4 ( p2 1,1p̈1,1 − 2p1,1r1,1r̈1,1 − r2 1,1p̈1,1 ) + N∑ n=1 d (1) 3,n (ṗ1,1ṗ2,n − ṙ1,1ṙ2,n) + + N∑ n=1 d (2) 3,n (p1,1p̈2,n − r1,1r̈2,n) + N∑ n=1 d (3) 3,n (p̈1,1p2,n − r̈1,1r2,n) = 0, (22b) µ1,n [ r̈1,n + σ2 1,nr1,n ] + d5 ( r̈1,1r 2 1,1 + r1,1p1,1p̈1,1 ) + + d6 ( r1,1ṙ 2 1,1 + r1,1ṗ 2 1,1 ) + d7 ( r̈1,1p 2 1,1 − r1,1p1,1p̈1,1 ) + + d8 ( ṙ1,1ṗ1,1p1,1 − r1,1ṗ 2 1,1 ) + N∑ n=1 d (1) 4,n (ṗ1,1ṙ2,n − ṙ1,1ṗ2,n) + + N∑ n=1 d (2) 4,n (p1,1r̈2,n − r1,1p̈2,n) + N∑ n=1 d (3) 4,n (p̈1,1r2,n − r̈1,1p2,n) + + ṙ1,1 N∑ n=1 d (1) 5,nṗ0,n + r1,1 N∑ n=1 d (2) 5,np̈0,n + r̈1,1 N∑ n=1 d (3) 5,np0,n = −µ1,nκ1,n k2 1,n − 1 v̇02, (22c) ISSN 1562-3076. Нелiнiйнi коливання, 2011, т . 14, N◦ 4 ASYMPTOTIC NONLINEAR MULTIMODAL MODELING OF LIQUID . . . 491 µ1,n [ p̈1,n + σ2 1,np1,n ] + d5 ( p̈1,1p 2 1,1 + r1,1p1,1r̈1,1 ) + + d6 ( p1,1ṗ 2 1,1 + p1,1ṙ 2 1,1 ) + d7 ( p̈1,1r 2 1,1 − r1,1p1,1r̈1,1 ) + + d8 ( ṙ1,1ṗ1,1p1,1 − p1,1ṙ 2 1,1 ) + N∑ n=1 d (1) 4,n (ṗ1,1ṗ2,n + ṙ1,1ṙ2,n) + + N∑ n=1 d (2) 4,n (r1,1r̈2,n + p1,1p̈2,n) + N∑ n=1 d (3) 4,n (p̈1,1p2,n + r̈1,1r2,n) + + ṗ1,1 N∑ n=1 d (1) 5,nṗ0,n + p1,1 N∑ n=1 d (2) 5,np̈0,n + p̈1,1 N∑ n=1 d (3) 5,np0,n = −µ1,nκ1,n k2 1,n − 1 v̇01, (22d) where n = 1, . . . , N. Equations (22) are linear in p3,n and r3,n and contain nonlinear quantities in terms of the first- and second-order generalized coordinates. The most important result of the present paper is that nonzero hydrodynamic coefficients in (20) – (22) can be effectively calculated by the following quite simple formulas: d (1) 0,n = d (2) 0,n − d (3) 0,n 2 , d (2) 0,n = π 2 [ 2− k2 0,n κ0,nκ1,1 ] j(0,n)(1,1)2 , d (3) 0,n = π [ j(0,n)(1,1)2 − 1 κ2 1,1 ( j (1,1)2 (0,n) + i(0,n)(1,1)2 )] , d (1) 2,n = d (2) 2,n − d (3) 2,n 2 , d (2) 2,n = π 2 [ j(2,n)(1,1)2 − 1 κ2,nκ1,1 ( j (2,n)(1,1) (1,1) + 2i(2,n)(1,1)2 )] , d (3) 2,n = π 2 [ j(2,n)(1,1)2 − 1 κ2 1,1 ( j (1,1)2 (2,n) − i(2,n)(1,1)2 )] , d1 = π 2κ1,1 [ k4 0,1 ( j(0,1)(1,1)2 )2 4κ01κ1,1 1 j(0,1)2 + i(1,1)4 − j (1,1)2 (1,1)2 ] + d2, d2 = π 4κ1,1  ( j (1,1)(2,1) (1,1) + 2i(2,1)(1,1)2 )2 κ1,1κ2,1 1 j(2,1)2 − 3i(1,1)4 − j (1,1)2 (1,1)2  , d3 = π 4κ1,1 ( j (1,1)(2,1) (1,1) + 2i(2,1)(1,1)2 ) κ1,1κ2,1 ( 2i(1,1)(2,1)(3,1) − j (1,1)(2,1) (3,1) ) j(2,1)2 + + π 4κ1,1 [ j (1,1)2 (1,1)(3,1) − i(1,1)3(3,1) ] + 2d4, ISSN 1562-3076. Нелiнiйнi коливання, 2011, т . 14, N◦ 4 492 I. LUKOVSKY, D. OVCHYNNYKOV, A. TIMOKHA d4 = π 4κ1,1 ( j (1,1)(2,1) (1,1) + 2i(2,1)(1,1)2 ) κ2,1κ3,1 ( 6i(1,1)(2,1)(3,1) + j (2,1)(3,1) (1,1) ) j(2,1)2 − − π 4κ1,1 [ (κ1,1 + κ3,1) 2κ3,1 ( 3i(1,1)3(3,1) + j (1,1)(3,1) (1,1)2 )] , d (1) 3,n = d (2) 3,n + d (3) 3,n − π 2 j(1,1)(2,n)(3,1) − j (1,1)(2,n) (3,1) − 2i(1,1)(2,n)(3,1) κ1,1κ2,n  , d (2) 3,n = π 2 j(1,1)(2,n)(3,1) − j (2,n)(3,1) (1,1) + 6i(1,1)(2,n)(3,1) κ2,nκ3,1  , d (3) 3,n = π 2 j(1,1)(2,n)(3,1) − j (1,1)(3,1) (2,n) + 3i(1,1)(2,n)(3,1) κ1,1κ3,1  , d (1) 4,n = d (2) 4,n − d (3) 3,n − π 2 j(1,1)(2,n)(1,2) − j (1,1)(2,n) (1,2) + 2i(1,1)(2,n)(1,2) κ1,1κ2,n  , d (2) 4,n = π 2 j(1,1)(2,n)(1,2) − j (2,n)(1,1) (1,1) + 2i(1,1)(2,n)(1,2) κ2,nκ1,2  , d (3) 4,n = π 2 j(1,1)(2,n)(1,2) − j (1,1)(1,2) (2,n) − i(1,1)(2,n)(1,2) κ1,1κ1,2  , d (1) 5,n = d (2) 5,n + d (3) 5,n − π j(0,n)(1,1)(1,2) − j (0,n)(1,1) (1,2) κ1,1κ0,n  , d (2) 5,n = π j(0,n)(1,1)(1,2) − j (0,n)(1,2) (1,1) κ0,nκ1,2  , d (3) 5,n = π j(0,n)(1,1)(1,2) − j (1,1)(1,2) (0,n) + i(0,n)(1,1)(1,2) κ1,1κ1,2  in which, by definition, j (c,d) (a,b) = ∫ ξ (∏ fa,b(ka,bξ) )(∏ d dξ fc,d(kc,dξ) ) dξ, i (c,d) (a,b) = ∫ 1 ξ (∏ fa,b(ka,bξ) )(∏ d dξ fc,d(kc,dξ) ) dξ ISSN 1562-3076. Нелiнiйнi коливання, 2011, т . 14, N◦ 4 ASYMPTOTIC NONLINEAR MULTIMODAL MODELING OF LIQUID . . . 493 and there are special indexing rules for i and j exemplified by the formula j (1,2)(0,1)(1,2) (0,2)(2,2)(1,1) = j (0,1)(1,2)2 (0,2)(1,1)(2,2) = = 1∫ 0 ξ(f0,2(k0,2ξ)f1,1(k1,1ξ)f2,2(k2,2ξ)) ( d dξ f0,1(k0,1ξ) ( d dξ f1,2(k1,2ξ) )2 ) dξ. Eqs. (22c) and (22d) contain coefficients d5, d6, d7, and d8 which are computed by the formulas: d5 = −0, 51201 h1,1 − 0, 16879 h1,2 + 0, 50224 h1,2h1,1h0,1 + 0, 17969 h1,2h2,1h1,1 , d6 = −1, 34899 h1,1 − 0, 3376 h1,2 + 1, 00448 h1,2h1,1h0,1 + 0, 37908 h2 1,1h0,1 + 0, 35938 h1,2h2,1h1,1 + 0, 23782 h2 1,1h2,1 , d7 = −0, 11748 h1,1 − 0, 00307 h1,2 + 0, 17969 h1,2h2,1h1,1 , d8 = −0, 68799 h1,1 − 0, 17186 h1,2 + 0, 37908 h2 1,2h0,1 + 0, 35938 h1,2h2,1h1,1 + 0, 50224 h1,2h1,1h0,1 , where hm,n = tanh(km,nh) depends on the nondimensional depth. It may be important for applications that the modal equations (20) – (22) can be rewritten in the following matrix form: Q(~q)~̈q + C~q + ~Ψ(~q; ~̇q) = V, (23) where ~q = (q1,1; q1,2; . . . ; q1,n; q2,1; q2,2; . . . ; q2,n; . . . ; q7,1; q7,2; . . . ; q7,n)T . 5. Conclusions. Bearing in mind analytical studies of nonlinear resonant sloshing in an upright circular-base tank, the present paper analytically derives a system of nonlinear ordi- nary differential equations (modal system) facilitating an approximate modeling of sloshing phenomena. The derivation uses the Narimanov – Moiseev intermodal asymptotic relationships which cause for this tank shape an infinite number of the generalized coordinates coupled by the system. In contrast to the existing analytically-given modal equations, the derived system (i) contains all the necessary generalized coordinates, (ii) includes exclusively nonzero hydrody- namic coefficients for which (iii) rather simple computational formulas are found. 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