ISRN Mathematical Physics

Volume 2012 (2012), Article ID 869070, 19 pages

http://dx.doi.org/10.5402/2012/869070

## Generalizing the Multimodal Method for the Levitating Drop Dynamics

^{1}Institute of Mathematics, National Academy of Sciences of Ukraine, Tereschenkivska 3 St., 01601 Kiev, Ukraine^{2}CeSOS, Norwegian University of Science and Technology, 7491 Trondheim, Norway

Received 23 April 2012; Accepted 30 July 2012

Academic Editors: S. Ansoldi, K. Netocny, K.-E. Thylwe, and G. F. Torres del Castillo

Copyright © 2012 M. O. Chernova et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

The present paper extends the multimodal method, which is well known for liquid sloshing problems, to the free-surface problem modeling the levitating drop dynamics. The generalized Lukovsky-Miles modal equations are derived. Based on these equations an approximate modal theory is constructed to describe weakly-nonlinear axisymmetric drop motions. Whereas the drop performs almost-periodic oscillations with the frequency close to the lowest natural frequency, the theory takes a finite-dimensional form. Periodic solutions of the corresponding finite-dimensional modal system are compared with experimental and numerical results obtained by other authors. A good agreement is shown.

#### 1. Introduction

Drops levitating in an ullage gas appear in chemical industry [1–3] and space technology [4–6]. The levitation is provided by weightless conditions or/and acoustic and/or electromagnetic fields created in the gas. When these external fields do not affect the static spherical drop shape, namely, there is no flattening caused by the fields as described, for example, in [7–9], one can assume that the drop dynamics is primary determined by the surface tension as if the drop levitates in zero-gravity conditions.

The relative drop dynamics with respect to the static spherical shape driven by the surface tension has been studied, experimentally [10–12] and theoretically [13]. Small-amplitude (linear) drop motions were *analytically* described by Lord Rayleigh [14, 15] in 1879. He found the corresponding natural (linear eigen) modes and frequencies in terms of the spherical harmonics. Theoretical studies of the nonlinear drop dynamics were mainly done *numerically* by employing different discretization schemes [16–18]. An alternative approach to numerical simulations could be theoretical methods developed, for example, in [19–21] where a Fourier approximation by the Rayleigh natural modes was combined with variational and asymptotic methods. This approach looks similar to nonlinear multimodal methods elaborated in the 70 s for liquid sloshing dynamics.

The multimodal methods took their canonical form in the pioneering papers by Lukovsky [22] and Miles [23] and, furthermore, were generalized by others. An extensive review on the multimodal methods can be found in the book by Lukovsky [24] and Faltinsen and Timokha [25]. The Lukovsky-Miles version of the multimodal methods makes it possible to derive a well-structured infinite-dimensional (modal) system of nonlinear ordinary differential (modal) equations with respect to generalized coordinates and velocities which, under certain circumstances, is fully equivalent to the original free-surface problem. Naturally, the generalized coordinates in sloshing correspond to the natural sloshing modes. Being “asymptotically-detuned” to a class of liquid sloshing phenomena the Lukovsky-Miles modal equations reduce to a rather compact, finite-dimensional form. The detuning suggests postulating a series of asymptotic relations between the generalized coordinates. Representative examples of such “asymptotic modal equations” can be found in the aforementioned books [24, 25] as well as in [26–30] and references therein.

The present paper generalizes the Lukovsky-Miles multimodal method for the free-surface problem describing the nonlinear dynamics of a levitating drop. This generalization includes derivation of general modal equations of Lukovsky-Miles’ type as well as examples of asymptotic modal equations. The paper plan is as follows. In Section 2,we present both differential and variational formulations of the problem. Following Lukovsky and Miles as well as recalling [19], the variational formulation is based on the Bateman-Luke principle (see, also, [25], Ch. 7). In Section 3, we rederive the Rayleigh-type eigensolution to show that, from a mathematical point of view, the set of natural modes is not complete and extra four spherical harmonics should be included into the modal solution. In Section 4, we derive the general modal equations analogous to those in [24, 25, 31]. An approximate form of these equations describing the weakly-nonlinear axisymmetric drop dynamics is constructed in Section 5. These equations keep up to third-order polynomial terms as it has been in [27, 32] for sloshing problems. Based on these approximate equations, we derive in Section 6 a finite-dimensional system of “asymptotic” modal equations modeling the weakly-nonlinear almost-periodic drop motions with the frequency close to the lowest natural frequency. Periodic solutions of these equations are compared with experimental [12] and numerical [19, 33, 34] results. A good agreement is shown.

#### 2. Statement of the Problem

We consider a levitating drop of an ideal incompressible liquid that performs oscillatory motions as illustrated in Figure 1. Due to the surface tension, the drop takes spherical shape in its hydrostatic state. We choose the radius of the sphere as the characteristic length and introduce the characteristic time ( is the surface tension coefficient). The nondimensional drop dynamics is considered in the spherical coordinate system , , , (, , ) so that the free surface is described by the equation

According to (2.1), perturbations of relative to the static spherical shape are subject to the volume conservation condition
playing the role of a *holonomic constraint*.

The free-surface problem describing the nonlinear drop dynamics couples the function and the velocity potential (see, e.g., [19]):

subject to the volume conservation condition (2.2). Here, the Laplace equation (2.3a) and the Neumann boundary condition (2.3b) constitute together the *kinematic* subproblem and (2.3c) is the so-called *dynamic* boundary condition in which the square bracket term is the sum of the principal curvatures . The dynamic boundary condition expresses the pressure balance on the free surface assuming that the ullage pressure is a constant value and using the Bernoulli equation written for an incompressible inviscid liquid with irrotational flow. The time-dependent function implies the difference of the mean pressure between liquid and gas domains caused by the surface tension.

The free-surface problem (2.3a), (2.3b), and (2.3c) requires either the initial conditions defining initial drop shape and velocity field or the periodicity condition , , where is a fixed period.

Following Lukovsky and Miles [24, 25], we employ the Bateman-Luke variational formulation which states that the free-surface problem (2.3a), (2.3b), and (2.3c) follows from the necessary extrema condition of the action within arbitrary instants and () and independent variables and restricted to where the Lagrangian reads as

Here, defines the area and is the Lagrange multiplier (a time-dependent function) caused by the holonomic constraint (2.2).

The Bateman-Luke variational principle is based on the sum of the pressure-integral and potential energy associated with the surface tension. In addition, there is the Lagrange multiplier which is the same as the mean pressure difference. Equivalence of the Bateman-Luke variational formulation and free-surface problems in fluid dynamics is, for instance, proven in [24, 25] and the book by Berdichevsky [35].

#### 3. Linear Eigensolution and Natural Modes

Let us consider small-amplitude drop oscillations with respect to its static spherical shape by linearizing the volume conservation as well as kinematic (2.3b) and dynamic (2.3c) boundary conditions in terms of and . The linearized volume conservation condition (2.2) takes the form but the linearized boundary conditions can be combined to exclude as follows:

Postulating where is the so-called *natural (linear eigen) frequency* leads to the spectral boundary problem
with respect to spectral parameter and eigenfunction .

The spectral boundary problem (3.4) can be solved by separating the spatial variables , which leads to the equation

The analytical *eigensolution* follows from (3.5) and consists of the eigenfrequencies
and the eigenfunctions
where are the associated Legendre polynomials.

Four eigenfunctions with and imply the zero eigenfrequencies and, from the physical point of view, these eigenfunctions do not belong to the set of *natural modes* by Lord Rayleigh [14, 15]. The case with gives that describes a translatory drop motion (as a solid body) along , but and yield and describing the same translatory motions but along and , respectively. The case corresponds to . Excluding these four eigenfunctions, that is, concentrating on the Rayleigh solution, makes the functional basis (3.7) incomplete from the mathematical point of view [36, 37].

#### 4. Nonlinear Modal Equations

The Lukovsky-Miles multimodal method suggests the modal solution of the free-surface problem (2.3a), (2.3b), and (2.3c) as follows: where and are the complete sets of functions to define admissible shapes satisfying the volume conservation condition and approximating the velocity field, respectively. Dealing with the star-shaped domains , the solid harmonics (3.7) provide the completeness [36, 37] so that we can write down

This transforms the modal solution (4.1) to the form

Accounting for (4.3a) in (2.2) gives the holonomic constraint where implies the cubic, fourth, and so forth polynomial terms. Using the implicit function theorem resolves as follows:

( also denotes the cubic and other higher-order polynomial terms in ) and transforms (4.3a) to the form
defining the free surface as a function of , , , , . Representation (4.6) *automatically satisfies* the volume conservation condition and, as a consequence, the Lagrange multiplier in (2.7) should equal to zero, that is, .

The generalized velocity can also be excluded from consideration due to the identity provided by (2.6) or, more precisely, by

Substituting (4.3b) into (2.7) yields the Lagrangian as a function of generalized coordinates and velocities where

Performing a variation of independent generalized velocities , , , , in the action (2.5) within the Lagrangian (4.9) leads to the equations

(, ). Derivations leading to (4.13a), (4.13b), and (4.13c) are quite tedious but, under certain circumstances, these are similar to those in [24, 25] for sloshing problems.

The differentiation rule

shows that (4.13a), (4.13b), and (4.13c) is a system of nonlinear ordinary differential equations with respect to generalized coordinates where the mass-matrix depends on . On the other hand, relations (4.13a), (4.13b), and (4.13c) can be considered as a system of algebraic equations with respect to generalized velocities , , , , , where are nonlinear functions of generalized coordinates , , , , but the left-hand side , , implies expressions with respect to generalized coordinates , , , , and their first derivative. Equations (4.13a), (4.13b), and (4.13c) are interpreted as *kinematic equations* or a nonholonomic constraint.

The Euler-Lagrange equations follow from the extrema condition of the action with respect to generalized coordinates , , , , . They are often called the *dynamic* modal equations and take the form

The derivative by is done assuming that (4.6) accounts for the volume conservation condition so that, for instance,

In summary, the Lukovsky-Miles modal equations (4.13a), (4.13b), (4.13c), (4.15a), (4.15b), and (4.15c) constitute an infinite-dimensional system of nonlinear ordinary differential equations with respect to generalized coordinates and velocities. A direct Runge-Kutta simulation with this system (Perko-type method [25, 26]) is possible adopting appropriate initial conditions following from (2.4). However, when the goal consists of analytical studies and/or description of almost-periodic motions, it would be better to reduce the system to a simpler approximate form by postulating asymptotic relationships between generalized coordinates and velocities and neglecting the higher-order terms. The reduced (asymptotic) modal system may in particular cases possess a finite-dimensional form.

#### 5. Weakly-Nonlinear Modal Equations for Axisymmetric Drop Motions

For the axisymmetric drop dynamics, the velocity potential takes the form and the free-surface equation is as follows: where and are the Clebsch-Gordan coefficients [38].

Henceforth, adopting ideas from [27, 32], we will construct a weakly-nonlinear, third-order modal equations by postulating the relationships and neglecting the -terms in the Lukovsky-Miles modal equations (4.13a), (4.13b), (4.13c), (4.15a), (4.15b), and (4.15c).

Accounting for (4.14a), kinematic modal equations (4.13a) read as where neglecting the -terms implies that and keep only the second-order polynomial quantities, that is,

Here, is defined by (5.3) and is also expressed via the Clebsch-Gordan coefficients

Furthermore,

Kinematic equations (5.5) can be considered as linear algebraic equations with respect to whose asymptotic solution (neglecting ) should admit the form

Substituting (5.10) into (5.5) and gathering all the similar polynomial terms give

For the axisymmetric drop dynamics, the dynamic modal equations (4.15a), (4.15b), and (4.15c) take the form

Pursuing the announced weakly-nonlinear theory, we should employ (5.6), (5.7) and expand (4.16a) up to the third-order polynomial terms, that is, where

Substituting (5.6), (5.7), (5.10), and (5.13) into (5.12) and neglecting the -terms yield the following weakly-nonlinear modal equations: where are the nondimensional frequencies (3.6) and

We see that the weakly-nonlinear modal equations (5.15) constitute an infinite-dimensional system of ordinary differential equations (with respect to generalized coordinates ) that are not resolved relative to the highest derivative. Again, one can use (5.15) for direct numerical simulations (see, e.g., [32]). However, as in [27] and other analytical papers on sloshing, the derived weakly-nonlinear modal equations can reduce to a finite-dimensional form by employing a Duffing-type asymptotics. Using this asymptotics in sloshing problems implicitly suggests that we look for almost-periodic solutions with the frequency close to the lowest natural frequency of the mechanical system. For the studied case within no external excitations, these periodic solutions mean the nonlinear eigenoscillations of a freely-levitating drop.

#### 6. Nonlinear Axisymmetric Eigenoscillation

We consider almost-periodic oscillations of an axisymmetric drop with the frequency close to the lowest linear eigenfrequency (natural frequency) subject to the Duffing-type third-order asymptotics implying the dominant character of the primary generalized coordinate . Analyzing the nonzero coefficients in (5.15) shows that this asymptotics yields so that neglecting the -terms in (5.15) leads to the finite-dimensional system of nonlinear modal equations

where and

Other modal equations in (5.15) do not include nonlinear terms.

To construct a periodic asymptotic solution of (6.2a), (6.2b), and (6.2c) we assume, as usually [26], the asymptotic closeness condition between and : (the nonlinear eigenfrequency is unknown). The wanted periodic solution takes then the form where is the unknown dominant amplitude and substituting (6.5) into (6.2a) and (6.2b) and accounting for (6.4) give

Gathering the -order terms at the first harmonics in (6.2a) and using (6.4) lead to the secular equation to find the dependence between the normalized nonlinear eigenfrequency and the nondimensional amplitude parameter

Secular equation (6.7) determines the so-called “soft-type” spring behavior suggesting that amplitude increases with decreasing frequency .

Figure 2 compares our asymptotic result (6.7) with experimental data from [12] (see, also [19]) where dependence between and the maximum ratio (*H*/*W*) between the instant drop height and width were reported. The experimental data for 0.49 cm are denoted by , but marks measurements made for 0.62 cm. In the lowest-order approximation, the Legendre polynomials properties deduce that
where is defined by (6.7).

Figure 2 shows that (6.8) provides a good agreement with the numerical values from [19] (dashed line), [33] (∆), and [34] (▲) in which the studies were also based on the incompressible ideal liquid model (the results do not depend on ). Our theoretical values are in good agreement with experimental measurements for 0.63 cm. For lower drop radius, an qualitative agreement with experimental measurements is established. The discrepancy can be related to viscous effects discussed, for example, in [19].

#### 7. Concluding Remarks

The present paper gives a generalization of Lukovsky-Miles’ multimodal method and derives the corresponding general nonlinear modal equations describing the nonlinear dynamics of a levitating drop. The modal equations are a full analogy of the original free-surface problem and look similar to those known for the nonlinear liquid sloshing problem [24, 25, 31].

The derived nonlinear modal equations are used to construct weakly-nonlinear modal equations for axisymmetric drop motions. The weakly-nonlinear equations possess a finite-dimensional form for the case of almost-periodic drop motions with the nonlinear eigenfrequency close to the lowest linear eigenfrequency. The latter case was studied by other authors, experimentally and numerically. To compare our analytical asymptotic results with earlier experimental [11, 12] and numerical [19, 33, 34] results, we constructed periodic solutions of the finite-dimensional modal system. All the results on periodic solutions from different sources are in a good agreement.

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