Category: Fluids

Nadir Akinci, Gizem Akinci, Matthias Teschner

Realistic handling of fluid-air and fluid-solid interfaces in SPH is a challenging problem. The main reason is that some important physical phenomena such as surface tension and adhesion emerge as a result of inter-molecular forces in a microscopic scale. This is different from scalar fields such as fluid pressure, which can be plausibly evaluated on a macroscopic scale using particles. Although there exist techniques to address this problem for some specific simulation scenarios, there does not yet exist a general approach to reproduce the variety of effects that emerge in reality from fluid air and fluid-solid interactions. In order to address this problem, we present a new surface tension force and a new adhesion force. Different from the existing work, our surface tension force can handle large surface tensions in a realistic way. This property lets our approach handle challenging real scenarios, such as water crown formation, various types of fluid-solid interactions, and even droplet simulations. Furthermore, it prevents particle clustering at the free surface where inter-particle pressure forces are incorrect. Our adhesion force allows plausible two-way attraction of fluids and solids and can be used to model different wetting conditions. By using our forces, modeling surface tension and adhesion effects do not require involved techniques such as generating a ghost air phase or surface tracking. The forces are applied to the neighboring fluidfluid and fluid-boundary particle pairs in a symmetric way, which satisfies momentum conservation. We demonstrate that combining both forces allows simulating a variety of interesting effects in a plausible way.

Versatile Surface Tension and Adhesion for SPH Fluids

Zherong Pan, Jin Huang, Yiying Tong, Changxi Zheng, and Hujun Bao

Animation techniques for controlling liquid simulation are challenging: they commonly require carefully setting initial and boundary conditions or performing a costly numerical optimization scheme against user-provided keyframes or animation sequences. Either way, the whole process is laborious and computationally expensive.

We introduce a novel method to provide intuitive and interactive control of liquid simulation. Our method enables a user to locally edit selected keyframes and automatically propagates the editing in a nearby temporal region using geometric deformation. We formulate our local editing techniques as a small-scale nonlinear optimization problem which can be solved interactively. With this uniformed formulation, we propose three editing metaphors, including (i) sketching local fluid features using a few user strokes, (ii) dragging a local fluid region, and (iii) controlling a local shape with a small mesh patch. Finally, we use the edited liquid animation to guide an of offline high-resolution simulation to recover more surface details. We demonstrate the intuitiveness and efficacy of our method in various practical scenarios.

Interactive Localized Liquid Motion Editing

Yubo Zhang, Kwan-Liu Ma

We introduce a novel spatio-temporal extrapolation technique for fluid simulation designed to improve the results without using higher resolution simulation grids. In general, there are rigid demands associated with pushing fluid animations to higher resolutions given limited computational capabilities. This results in tradeoffs between implementing high-order numerical methods and increasing the resolution of the simulation in space and time. For 3D problems, such challenges rapidly become cost-ineffective. The extrapolation method we present improves the flow features without using higher resolution simulation grids. In this paper, we show that simulation results from our extrapolation are comparable to those from higher resolution simulations. In addition, our method differs from high-order numerical methods because it does not depend on the equation or specific solver. We demonstrate that it is easy to implement and can significantly improve the fluid animation results.

Spatio-temporal Extrapolation for Fluid Animation

Dan Gerzewski, Adam Bargteil

Fluid simulation has been one of the greatest successes of physics-based animation, generating hundreds of research papers and a great many special effects over the last fifteen years. However, the animation of large-scale, splashing liquids remains challenging. In this paper, we show that a novel combination of unilateral incompressibility, mass-full FLIP, and blurred boundaries is extremely well-suited to the animation of large-scale, violent, splashing liquids.

Physics-Based Animation of Large-scale Splashing Liquids

Tobias Pfaff, Nils Thuerey, Theodore Kim

Over the last decade, the special effects industry has embraced physics simulations as a highly useful tool for creating realistic scenes ranging from a small camp fire to the large scale destruction of whole cities. While fluid simulations are now widely used in the industry, it remains inherently difficult to control large scale simulations, and there is an constant struggle for increasing visual detail.

In this course, we will tackle these problems using turbulence methods. Turbulent detail is what makes typical fluid simulations look impressive, and the underlying physics motivate a powerful approach for control: they allow for an elegant split of large scale motion and small scale turbulent detail. This results in a two-stage work flow that is highly convenient for artists: first, a rough, and fast initial simulation is performed, which is then turned into a more detailed one by adding turbulent effects.

This course aims at giving an overview and providing practical guide to employing turbulence modeling techniques for fluid simulations in computer graphics. After reviewing the basics of fluid solvers, and the popular wavelet turbulence approach, we will present several powerful methods to capture advanced effects such as boundary layers, and turbulence with directional preferences. In addition, the difficulties of liquid simulations will be explained, and an approach for liquid turbulence that is based on wave dynamics will be presented.

Turbulent Fluids

Bo Zhu, Wenlong Lu, Matthew Cong, Byungmoon Kim, Ronald Fedkiw

We present an efficient grid structure that extends a uniform grid to create a significantly larger far-field grid by dynamically extending the cells surrounding a fine uniform grid while still maintaining fine resolution about the regions of interest. The far-field grid preserves almost every computational advantage of uniform grids including cache coherency, regular subdivisions for parallelization, simple data layout, the existence of efficient numerical discretizations and algorithms for solving partial differential equations, etc. This allows fluid simulations to cover large domains that are often infeasible to enclose with sufficient resolution using a uniform grid, while still effectively capturing fine scale details in regions of interest using dynamic adaptivity.

A New Grid Structure for Domain Extension

Saket Patkar, Mridul Aanjaneya, Dimitriy Karpman, Ronald Fedkiw

A Hybrid Lagrangian-Eulerian Formulation for Bubble Generation and Dynamics

R. Elliot English, Linhai Qiu, Yue Yu, Ronald Fedkiw

We introduce a new method for large scale water simulation using Chimera grid embedding, which discretizes space with overlapping Cartesian grids that translate and rotate in order to decompose the domain into different regions of interest with varying spatial resolutions. Grids can track both fluid features and solid objects, allowing for dynamic spatial adaptivity without remeshing or repartitioning the domain. We solve the inviscid incompressible NavierStokes equations with an arbitrary-Lagrangian-Eulerian style semiLagrangian advection scheme and a monolithic SPD Poisson solver. We modify the particle level set method in order to adapt it to Chimera grids including particle treatment across grid boundaries with disparate cell sizes, and strategies to deal with locality in the implementation of the level set and fast marching algorithms. We use a local Voronoi mesh construction to solve for pressure and address a number of issues that arise with the treatment of the velocity near the interface. The resulting method is highly scalable on distributed parallel architectures with minimal communication costs.

Chimera Grids for Water Simulation

Theodore Kim, John Delaney

We present a new subspace integration method that is capable of efficiently adding and subtracting dynamics from an existing high-resolution fluid simulation. We show how to analyze the results of an existing high-resolution simulation, discover an efficient reduced approximation, and use it to quickly “re-simulate” novel variations of the original dynamics. Prior subspace methods have had difficulty re-simulating the original input dynamics because they lack efficient means of handling semi-Lagrangian advection methods. We show that multi-dimensional cubature schemes can be applied to this and other advection methods, such as MacCormack advection. The remaining pressure and diffusion stages can be written as a single matrix-vector multiply, so as with previous subspace methods, no matrix inversion is needed at runtime. We additionally propose a novel importance sampling-based fitting algorithm that asymptotically accelerates the precomputation stage, and show that the Iterated Orthogonal Projection method can be used to elegantly incorporate moving internal boundaries into a subspace simulation. In addition to efficiently producing variations of the original input, our method can produce novel, abstract fluid motions that we have not seen from any other solver.

Subspace Fluid Re-Simulation