NFA Simulation of Planing Boat

Principle Investigator: Dr. Douglas Dommermouth
HPC System: Garnet
Sponsor: DTRA (Defense Threat Reduction Agency)


Textured Boat in Breaking Waves

Three Views

Visualization Details:

Isosurfaces for this research were saved every 5 time steps of the simulation, and the entire data field (three velocity components, pressure, and volume fraction) was output every 20 time steps to permit analysis of turbulence, air entrainment, and spray formation. Each save of the entire data field requires 6.2 GBytes of storage. Extracting isosurfaces reduces this by a factor of 23.

These extracted isosurfaces were provided to the Data Analysis and Assessment Center (DAAC) in two sets of Visualization Toolkit (VTK) geometry files. The first set represented the free water surface and had on the order of 5.5 million triangles per isosurface. The second set represented the boat's geometry and was provided with a scalar pressure value per node.

From these two sets of data files, the DAAC produced three types of animations: several realistic perspective views, a bottom view showing normalized pressures experienced by the boat's hull, and a caustics study of light through the turbulent wake.

The realistic animations were accomplished by ray tracing the scene with reflection and transparency attributes applied to the water using Adobe's 3D Studio Max commercial rendering software. A special rendering model of the boat was developed by applying rich texture maps to one of the simulation boat isosurfaces. Each frame was rendered via 3D Studio Max scripts that performed the following sequence:

for each frame {
      import texture mapped boat file;
      import next simulation boat file;
      import next simulation water surface file;
      orient texture mapped boat to the simulation boat position;
      delete simulation boat from scene, (leaving only the correctly oriented textured boat);
      render the scene;

The bottom view was shown simultaneously with the realistic view. Dramatic slamming of the boat from the realistic views could be quantified by the color mapped pressure displayed on the boat hull, as shown in the second video.

Finally, a visualization employing caustics was produced by the DAAC team. Caustics are the collection of light rays that are reflected or refracted through a curved surface. Light rays were ray cast through the wake and projected onto a surface located some distance under the bottom of the boat. This visualization technique results in light and dark bands on the bottom surface which provides more information about the structure of the turbulence of the wake.

Simulation Details:

Simulations of a high-speed planing boat accelerating from rest to a constant forward speed in head seas were performed using Numerical Flow Analysis (NFA). The boat is permitted to have four degrees of freedom, including sway, heave, roll, and pitch. The magnitude and location of pressure on the surface provide a means to understand the shock loads the crew must endure in order to design vessels that will provide a load reduction without adversely affecting craft performance.

The planing boat in this simulation was 62.3ft (19m) and was traveling at 40 knots (20.6 m/s) or a Froude number of 1.5. The domain has a length, width, and depth of 4.3, 2.0, and 1.8 boat lengths, respectively. The number of cells in x, y, and z was 1536, 1024, and 512, resulting in 805 million cells in the total simulation. Spacing near the body was 0.0018 or 1.3 in (3.4 cm), necessitating a nondimensional time step of 6.24x10-4. Simulations were run for 70,000 time steps, or 44 body lengths, which correspond to about 30 wave impacts.

The research was completed on the Cray XE6 located at the Air Force Research Laboratory (AFRL) supercomputing center. The simulation's 805 million grid cells were distributed over 3,072 processors and run for 190 wall-clock hours.