Simulation of A Pliable Jellyfish in Fluids

Junsei Hirato∗

The University of TokyoYoichiro Kawaguchi†

The University of Tokyo

Figure 1: Rendering of a variety of kinds of jellyfish (left: Moonjelly, center: Brown jellyfish, right: Box jellyfish)

1 Introduction

Physical simulation of molluscous is hard task. Recently motion ofmolluscous has been represented in various fields such as moviesand computer games, however physical simulation techniques is of-ten ignored due to huge computational cost, therefore simple mo-tion – completely regular, cyclic or symmetrical motion – is fre-quently used. In this paper, we propose the method for asymmet-rical motion of molluscous, especially jellyfish, by an easy mecha-nism with few computational complexity.

This paper shows that the method of the calculation; firstly mod-eling, secondly implementation, thirdly generating motion, and fi-naly the result. Applying this method, a variety kind of jellyfishesand other molluscous can be represented by changing the parame-ters and the texture (Fig 1).

2 Modeling and implementation

Figure 2: Left: jellyfish body and shrink of the circular muscle,right: the control points in the range of a constant distance areconnected with springs. The range is shown as the red circle, andthe connection with springs expressed by the blue lines.

We assumed that the virtual jellyfish consists of three parts for sim-plicity; an umbrella-shaped body called a bell, four oral arms, andthirty-six legs. Both a bell and oral arms are assumed to be 0-thickmembranes, and the tentacles are assumed as curves. All the threeparts are formed by sets of control points. This paper shows thatjellyfish-like motion can be easily realized by a virtual mass-springmodel. In practice, all the control points in the range of a constantdistance are connected with the springs (Fig 2 right). Note that, thisassumption differs from other mass-spring model.

Jellyfish pliable motion was simulated considering influence ofsurrounding fluid. We used the MPS (i.e. Moving Particle Semi-implicit) for fluid simulation. This method is suitable for calcula-tion large boundary deformation of the jellyfish and the fluid. Inthis paper we deal with MPS method and the virtual mass-springmodel together. We generate triangle mesh for curved surfaces of a

∗e-mail: hirato.junsei@iii.u-tokyo.ac.jp†e-mail:yoichiro@iii.u-tokyo.ac.jp

bell and oral arms by connecting their control points, therefore in-teraction between fluid and the membrane is calculated as collisionof the triangle elements and fluid particles. Mass of the triangle ele-ments is required when calculating the collision, hence only at thattime the membrane is assumed to have thickness equivalent to thediameter of the fluid particle and mass of the triangle elements arecalculated. Control points of tentacles calculate fluid pressure aswell as fluid particles, therefore tentacles move influenced by fluidpressure and spring power.

Not only modeling but also motion is also represented simply.The largest motion that jellyfish voluntarily generates is shrinkingof the circular muscle of the bell (see Fig 2 left), thus the motioncompulsorily given to this model is only periodical shrinks of it.

3 Result and Conclusion

Figure 3: Simulation using MPS Method (Top row: a bell, Middlerow: oral arms, Bottom row: tentacles)All results were calculated in the tank filled with fluid as is shownat the top row.

The result of the bell simulation in the tank filled with fluid is shown(see Fig 3 top row), and the result of the oral arms and the tentaclessimulation in the same tank is shown on (Fig 3). It is observed thatthe parts of the jellyfish move softly while maintaining their shape.

Some features similar to real jellyfish motion can be obtained,for example, the simulated jellyfish is driven forward getting buoy-ancy force in then fluid only by shrinks of the circular muscle. Webelieve that this paper is a clue to generate lively motion of mollus-cous with few computational complexities.

AcknowledgmentThis research is supported by CREST of JST (Japan Science andTechnology).

ReferencesS.KOSHIZUKA, AND Y.OKA. 1996. Moving-particle semi-implicit

method for fragmentation of incompressible fluid. Nucl.Sci.Eng.123, 3, 421–434.