Free Surface Hydrodynamic Motion Revealed

Students in the lab are working to explain what's so special about a fish's hydrodynamics that gives it what's called "high propulsive efficiency." If the mechanics of fish swimming and maneuvering can be understood, then those principles can be applied in designing a submarine-type vehicle.

Some of the students in the VFR lab also study hydrodynamics in the immediate vicinity of a ship and the wakes it produces. Others look at the waves behind ships, their wakes and their fluid mechanics. Another topic of investigation in the department is accurate determination of the forces acting on an object in the water. What are the velocity distributions, pressure distributions, wave resistance and wave loads?

A comprehensive, computerized visualisation tool is a useful aid analysing the above fluid dynamics data. For some of its visualisation tasks, the VFR lab has chosen IRIS Explorer. In the lab, IRIS Explorer runs on a Silicon Graphics R8000 Power Indigo2. In visualising various aspects of their computational fluid dynamics datasets, velocity fields, pressure contours and vorticity isosurfaces, for example, the VFR lab's staff has found IRIS Explorer very useful and important.

The visualisation process begins with CFD code that models or simulates how a fish swims or a ship moves through the ocean. These simulations usually produce very large, three-dimensional datasets that average between 100 and 400 megabytes in size. Such a dataset may contain information about the velocity and pressure fields around the fish as well as the actual time-dependent geometry of the fish.

With IRIS Explorer, the team can visualise these quantities: the velocity field around the body is portrayed by small arrows of differing colors that signify the changing speed of the fluid. Contours and isosurfaces are used to depict pressure and vorticity in the flow around the body. IRIS Explorer allows the users to load an entire dataset - large or small - and then use a handy module to take chunks of the data and render only those portions necessary. Thus the staff can do data slicing to see contours of just one variable at any particular point.

Among the VFR Lab's preferred visualisation routines are Vectors, Streakline, IsosurfaceLat and Orthoslice. Also, while a variety of sample data input modules come with IRIS Explorer, the group prefers to create their own modules with the software's unique application-building abilities.

Leonard Imas of the VFR lab has found that IRIS Explorer is not one of the more commonly found visualisation packages written with a certain application in mind, such as to see fluid flow around an aircraft or to visualise seismic data. To look at flow involving free surfaces like the ocean, many packages won't do what's needed. "I'm responsible for trying out different packages", he says. "We've evaluated a number of other fairly common 3D visualisation packages. We didn't like one because it's a NASA product geared primarily to observing flows around aircraft. If your work is not in aircraft flow visualisation, you must do extra work to make your datasets work with this package. Another one was good but way too expensive - even at the educational price level - and its rendering quality is not as good as IRIS Explorer's. Others are also not as intuitive as is IRIS Explorer."

As for output, whenever the department reports its findings in technical publications and at conferences, IRIS Explorer is used to generate images for illustrative purposes.

"IRIS Explorer is very easy for us to use because it's intuitive", says Imas. "It helps a user find what he or she needs to do to look at data. This is because IRIS Explorer is a highly flexible visualisation package consisting of a wide variety of building blocks users can link together any way they want - whatever arrangement gives them the output they desire."

"There is very little constraint on what users can and cannot do," states Imas. "We really like the flexibility of IRIS Explorer."