With gorgeous real-life footage of water falling from the sky and fumes from a car after a fire, there’s no escaping the laws of physics in the movies either! Fluid simulation in the 1950s and 1960s was mathematically modeled before the computer graphics industry arose. In the early 90s, computer graphics (CG) in films such as Waterworld and Titanic were limited to wide-open shots of the ocean and there was not much to experiment with with this technology. Today, using real fluid physics, visual effects that are difficult to practically duplicate have been made possible through the various fluid simulation platforms.
Imitation of fluid flow on a commercial scale is a relatively new technology. Before the development of fluid simulation platforms, that is to say until the end of the 90s, the effects of water were sketched and developed frame by frame on the computer. This was achieved mostly by cel animation, computer generated images, or by overlaying liquid skins on solids. Antz was the first film to use smooth simulations, and the visual effects team found it too long and complex. In 1999, Jos Stam introduced technology that used basic fluid equations for realistic fluid flow simulations. Soon, this technology was incorporated into various animation software. But the flaws of this technology were that it could only simulate water (no other fluids), and fluids could not be controlled to create something unrealistic (at the same time convince the audience to believe). In recent years, control of fluid flow has been possible by introducing invisible suction pumps and vortices as objects in equations that would move fluid. These effects were used in the New York flood scene in the movie The Day After Tomorrow.
Although the use of fluid simulation is growing deep in the world of visual effects, most artists have no idea of the basics of simulation. They fix it by tweaking a few settings until it looks right. Initially, the simulation of fluids in movies focused more on surface properties such as semi-flat oceans, but today the focus is more on the volume of fluids. For example, water rushing into and around rigid bodies showing the height of the water above the ground. The underlying principle of all these fluid simulations is the Navier Stokes equation, which describes the behavior of the fluid with certain key assumptions and the output is often in the form of a velocity field, not a simple number. It is recommended to understand the following fundamentals to better understand fluid dynamics:
- Conservation of mass, energy/momentum and volume
- Connective acceleration (fluid acceleration is controlled by the space around it)
- Main forces that control the fluid – viscosity and gravity
- Boundary conditions (it is important to understand the near edge of the fluid, which can be a wall or another fluid)
In the visual effects industry, there are award-winning designers with and without science and technology degrees. All that matters is the basic understanding of the underlying principles; additional experience is expected to shape your in-depth knowledge. But Eugénie von Tenzelmann, VFX designer at Framestone, has a different take on this, and according to her, if you’re trying to model fluid flow, “you have to have an understanding of thermodynamics.” While making the movie Gravity, physicist turned VFX creative director Tim Webber and his team had to write a simulation of what happened in microgravity. The making of this film is the culmination of his entire career.
Achieving a balance between realism, computing power, human knowledge and available hours decides which fluid simulation technology to use. Although the underlying principle of 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 main methods developed to achieve brilliant fluid animation results in the available time frame, which required years of fluid simulation research, are:
- Smooth Particle Hydrodynamics (SPH)
- Volume Grid Methods
- Marker and cellulo method (MAC)
- Particle in Cels (PIC)
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While we’re talking about movies and how smooth simulations play a big role in visual effects, it would be great to hear how Disney is using CFD in their new desktop design. The Walt Disney Company’s new headquarters are located at 4 Hudson Square, which was once New York’s Printing Street. For a sustainable building design that meets LEED requirements, Wintech consultants use the latest CFD technologies to meet National Ambient Air Quality Standards (NAAQS) and perfect the building design to have minimal impact on wind and thermal stacking at inlets. With this building designed and built, Disney aims to display the best practices in the service of the company, its employees and the city.
From Merida’s curly-haired realism in Brave to the blazing fire of 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’re witnessing in the age of fluid simulations in Hollywood. Fluid simulations have helped and are helping many industries deal with their design implications for the best solution within the constraints of the problem. Future fluid simulation solutions should be smarter for maximum solution accuracy with minimum human effort and that said, we are not too far from this aspiration becoming a reality.
Veena Parthan is a senior CFD writer/editor at Cadence Design Systems.