Physics Pointwise Bounds on Eigenfunctions and Wave Packets in ...
Water Wave Packets
Transcript of Water Wave Packets
Water Wave Packets
Ki-hoon Kim
2017-10-18Computer Graphics @ Korea University
S.Jeschke and C.WojtanSIGGRAPH 2017
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β’ The motion of water surface waves is well-modeled by the two-phase incompressible Navier-Stokes equation. Intractable for detailed water surface geometry. Linearizes the problem. Restricts the waves to a height-field defined over 2D domain. However, still too complex to solve.
Introduction
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β’ Wave Packets Introduce the concept of wave packets to computer graphics Describe their dynamics for dispersive water waves.
β’ Visual Detail Improves previous Lagrangian particle approaches. Incorporate wave behaviors like
dispersion, diffraction, refraction, reflection, dissipation.
β’ Efficient computation Unconditionally stable and inherently parallel.
β’ Novel control Parameters New mechanism which allow artists to directly control wave
spectra and computational complexity.
IntroductionContributions
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β’ 1st surface wave research Long crested wave models
[Bruce Schachter. /1980 Computer Graphics and Image]
β’ Motion of ocean Interactive animation of ocean waves
[D. Hinsinger et al./ SCA 2002]
Related Work
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β’ Novel approaches to water surface wave simulation Water Wave Animation via Wavefront Parameter Interpolation
[STEFAN JESCHKE and CHRIS WOJTAN/SIGGRAPH 2015]β’ Handle complex boundaryβ’ Pre-computation
Related Work
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β’ Gives a linearised description of the propagation of gravity waves on the surface of a homogeneous fluid layer. Often referred to as linear wave theory.
β’ This linear theory is often used to get a quick and rough estimate of wave characteristics and their effects.
β’ Water surface as a height function that varies with time, ππ(π±π±, π‘π‘).
Airy Wave TheoryDefinition
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β’ We can view the surface as an integral of many waves.
ππ is wavenumber. ππ = 2ππ/ππ ππ is wavelength. π₯π₯ is the spatial coordinate. π‘π‘ is time ππ(ππ, β) is angular frequency. ππ(ππ) is amplitude. β is the water depth.
Airy Wave TheoryWave Function
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β’ A special form that gives water waves their distinct characteristics
ππ is gravity. ππ is the surface tension. ππ is the water density.
β’ This relationship between ππ and ππ is known as the dispersion relationship.
Airy Wave TheoryAngular Frequency
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β’ From previous page, argument to the wave in Equation (1) can be factored into ππ π₯π₯ β ππππ ππ,β π‘π‘ , where
is the propagation speed of a given wavelength, known as the phase velocity.
Airy Wave TheoryPropagation Speed
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β’ In 2D, the energy of a water wave with wavenumber ππ over a surface area π΄π΄ is
β’ Wave energy travels with the group velocity ππππ, which is defined as
Red dot: phase speed Green dot: group speed
Airy Wave TheoryWave Energy
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β’ Propagate localized packets of waves. Each packet will represent a collection of similar wavelengths. It will cover a larger region of space than a single wave crest. Waves interact with a dynamically changing environment. Obey the qualitative behaviors described by Airy wave theory.
β’ Free to choose what weβd like these wave packets to look like.
β’ Each packet should be compactly supported in spatial coordinates
β’ Energy act locally in frequency space.
Wave packetsPreview
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β’ Break up the integral in Equation (1) into a summation of individual packets of wavenumbers centered around some representative wavenumber ππππ:
Ξ¦ is wave packet shape ππ is the number of wave packets.
β’ We can approximate with a 1st order Taylor expansion.
ππ is a kernel function
Wave packetsDiscretize(1/2)
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ππππ = πππππ‘π‘ , ππππ = πππππ‘π‘
ππππππ = ππππ β ππππ , οΏ½π₯π₯ = π₯π₯ β ππππ
In two dimensions,
Wave packetsDiscretize(2/2)
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β’ Reflection A wave packet reflects when it collides with an obstacle.
β’ Dispersion The faster waves will push part of the packet ahead of the
average group speed, and the slower waves will pull. The packet spreads out as it traverses space.
β’ Refraction The group speed of the packet may change as it traverses space,
because ππππ change with water depth.
The packet will change direction(Snellβs law).
β’ Diffraction Waves diffract differently depending on the wavenumber. We have not yet worked out the theoretical diffractive spreading
behavior.
Qualitative wave behaviors
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β’ Enforce conservation if energy as a wave packet propagates through space.
β’ From Eq.(4)
Quadratic in amplitude and wavenumber. Linear in area.
β’ Alter the packetβs amplitude to compensate for such changes. Exact amplitude scaling law for a packet with a Gaussian kernel.
Energy of a wave packetConservation
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β’ Often modeled by explicitly reducing the wave amplitude.
β’ In the absence of all other amplitude changes, the amplitude will decay over time:
Energy of a wave packetViscosity
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β’ Surface contamination will have a strong damping effect.
β’ This effect can be modeled as an additional decay rate.
Energy of a wave packet
Surface Contamination
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β’ A rectangular patch of initial dimensions 3ππππ in the traveling direction.
6ππππ in the tangential direction.
β’ Two vertices π©π©1 and π©π©2centered at the front edge of the packet. Track the deformation and rotation of the packet.
Representing wave packetsInitial Packet Modeling
3ππππ
6ππππ
π©π©1 π©π©2
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β’ Two vertices propagate at the group speed ππππ Simple forward Euler method.
β’ More advanced time integration schemes may be useful. But they will not make the method any more stable. Our method is stable regardless of the time step size.
β’ Geometric method for conserving energy.
Representing wave packetsPacket Time Integration-Travel
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β’ Tracking the fastest and slowest wavenumbers in the spectrum.β’ Assign each packet a length parameter ππ
Initially set to 3ππππ
β’ Track the dispersive stretching using a similar integration rule
ππfast and ππslow are the wavenumbersβ’ Correspond to the fastest and slowest group speeds
Representing wave packetsPacket Time Integration-Dispersion
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β’ Compute the area of the packet in each time step
β’ Then conserve energy by scaling the amplitude as described
Amplitude adjustmentCompute Area and Amplitude
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β’ If π©π©1 π‘π‘ππ β π©π©2 π‘π‘ππ > 3ππππor angle between ππππ(ππ1) and ππππ(ππ2) exceeds 18 degrees
β’ New amplitude is equal to the original divided by β2
Packet subdivisionGeometric subdivision
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β’ When the packet length stretches beyond a threshold, replace the packet by two new ones with exactly the same position and other parameters.
Packet subdivisionDispersive subdivision
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β’ We use a more compact and efficient approximation function
π’π’, π£π£ β [0,1] and local coordinates of a packet.
VisualizationCompact Gaussian Function
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β’ We are left with several options for reducing a 2D rectangular packet to 1D. Use piecewise circular coordinates Piecewise constant makes artifacts
Visualization1D-2D Transform from
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β’ βwakeβ shape that we expect to see is quite expensive to simulate Need too many Patch(or particle, etc.)
β’ We would like to emit only visually domain. Wake pattern trick Using Kelvinβs theory
ControlWakes
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β’ We can delete them more aggressively if we want.
β’ πΌπΌ is using to
ControlControl over lifetime
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β’ Most of the parameters map directly to measurable quantities. Surface tension Viscosity Gravity Etc.
β’ Thresholds and some parameter is numerical.
ResultsParameters
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β’ Limitations Oversimplified Linear theory. One-way coupling Fake wake
β’ Efficiency Parallel on the CPU 60fps speed cut-off was about
85k wave packets. Rendering is bottleneck
ResultsLimitation & Efficiency