The Era Of Fluid Simulations In Hollywood

With the magnificent real images of water pouring from the sky and the after-fire fumes from a car, there is no escaping from the laws of physics in movies either! Fluid simulation in the ’50s and ’60s was modeled mathematically before the computer graphics industry made its appearance. In the early ’90s, computer graphics (CG) in movies such as Waterworld and Titanic were restricted to wide-open shots of the ocean and there wasn’t much that could be experimented with using this technology. Today, using real fluid physics, the visual effects which were difficult to duplicate practically have been made possible using the various fluid simulation platforms.

Imitating fluid flow on a commercial scale is a fairly new technology. Before the development of fluid simulation platforms, i.e. until the late 90s, the water effects were sketched and developed one frame at a time on computers. This was carried out mostly by cel animation, computer-generated frames, or by covering liquid-like skins over solids. Antz was the first movie to use fluid simulations and the visual effects team found it too time-consuming and complex. In 1999, Jos Stam introduced a technology that used basic fluid equations for realistic fluid flow simulations. Soon this technology was incorporated into various animation software. But the shortcomings of this technology were that it could only simulate water (no other fluids), and the fluids could not be controlled for creating something unrealistic (at the same time convincing the audience to believe it). In recent years, controlling fluid flow has been possible by introducing invisible suction pumps and whirlpools as objects into the equations that would move the fluid. These effects have been used in the flooding scene of New York City in the movie The Day After Tomorrow.

Although the use of fluid simulation is profoundly growing in the VFX world, most artists have no clue about the basics behind the simulation. They work it out by tweaking a few parameters until it looks right. Initially, the fluid simulation in movies was more focused on surface properties such as semi-flat oceans, but today there is a shift in interest toward the volume of fluids. For example, water rushing in and around rigid bodies showcasing the height of the water from the ground. The underlying principle for all these fluid simulations is the Navier Stokes equation, which describes the behavior of fluid with some key assumptions and the output is often in the form of a velocity field, not a mere number. It is recommended to understand the following fundamentals to get a better grasp of fluid dynamics:

• Conservation of mass, energy/momentum, and volume
• Connective acceleration (fluid acceleration is controlled by the space around)
• Key forces that control fluid – viscosity and gravity
• Boundary conditions (understanding the nearing edge of fluid is important, which could be a wall or another fluid)

In the visual effects industry, there are award-winning designers with and without degrees in science and technology. All that matters is the basic understanding of the underlying principles; further experience is expected to mold your in-depth knowledge. But Eugenie von Tenzelmann, VFX designer at Framestone, has a different view on this, and according to her, if you were trying to model fluid flow, “you’d need to have an understanding of thermodynamics.” During the making of the movie Gravity, Tim Webber, physicist turned VFX chief creative officer, and his team had to write a simulation for what happened in microgravity. The making of that movie is the highlight of his entire career.

Achieving a balance between realism, computational power, human knowledge, and available hours decides on what fluid simulation technology to use. Although the underlying principle behind fluid animation and CFD is the same, fluid simulations are primarily focused on the visual behavior of the fluid and are not used for the scientific study of fluid flow. The key methods developed for brilliant fluid animation results in the available time frame, which took years of fluid simulation research, are as follows:

1. Smooth Particle Hydrodynamics (SPH)
2. Volume Grid Methods
3. Marker and Cel Method (MAC)
4. Particle in Cels (PIC)
5. FLIP

As we are talking about movies and how fluid simulations is playing an avid role in the visual effects, it would be nice to know how Disney is using CFD in designing their new office. The new headquarters for the Walt Disney Company is set out at 4 Hudson Square, which was once the printing street of New York. For a sustainable building design that satisfies the LEED requirements, Wintech Consultants are using the latest technologies in CFD to meet the National Ambient Air Quality Standards (NAAQS) and are perfecting the building design to have minimal impact on wind and thermal stacking at entryways. With this building design and construction, Disney aims to display the best practices to serve the company, its employees, and the city.

From bringing realism to the curly hair of Merida in Brave to blazing fire from the dragon’s mouth in Harry Potter and the Goblet of Fire, fluid simulations have helped expand the horizon of visual effects in movies, and today, we witness the era of fluid simulations in Hollywood. Fluid simulations have and are helping numerous industries address their design implications for the best solution within the problem constraints. Future fluid simulation solutions are expected to be more intelligent for maximum solution accuracy with minimum human efforts and having said that, we are not too far from that aspiration becoming a reality.