An Advanced Simulation Tool for Damage Assessment
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Laboratory for Computational Physics & Fluid Dynamics
Introduction: One component of improved ship design is the development of technologies that will reduce a ship's vulnerability to weapon impact. Quick identification of the location of weapon impact and an assessment of the potential damage that may be caused by it can significantly reduce vulnerability by providing protection for onboard personnel, weapons, and systems. The planned reduction in manning of future ships and submarines also requires the development of improved tools for automated response to fires and explosions. Such damage containment decision aids and future automated damage control systems need the input from physics-based models and simulations to be realistic and reliable.
The objective of our research effort is to develop a computational tool that can be used to assess the damage to the interior compartment of a surface ship under different war-fighting and peacetime scenarios. For example, such a tool should be capable of simulating the dispersal of fire suppressants and their interaction with a fire in the complex geometry that is typical of the interior of a surface ship.
Computational Strategy: When this project started in FY99, there was no capability to efficiently simulate the detailed flow field inside the complex geometry that is typical of a ship compartment. Zone models that might provide global answers, such as heat transfer from one compartment to the next, do not have the resolution to capture the fluid dynamics of the dispersal of fire suppressants injected from a nozzle into a compartment. These models also cannot simulate the interactions between the fire suppressant and a fire in the enclosure. Detailed numerical simulation models have the ability to capture the local interactions between a fire suppressant (such as water-mist) and a fire. However, the numerical grid resolution that is required to correctly capture the local interactions with a fire make it prohibitively expensive to extend this approach directly to simulate fire suppression in a large compartment. Furthermore, such models do not have the capability to represent the complex geometrical details that are typical of a ship compartment and cannot cost-effectively compute flows evolving over several minutes. A new approach was needed to compute the complex flows over long durations.
Multidomain computational techniques have been adopted to combine flows with different levels of complexity evolving in different regions (or domains) of the system. By using coarser grids and larger timesteps wherever the flow is evolving slowly and smoothly, significant gains in computational time has been made without loss of detail. In combination with this multidomain technique, an efficient approach has also been developed to represent the geometric complexity of the interior of a ship.
Smoke Spread through a Ship: The developed simulation tool is used to demonstrate smoke spread through multiple compartments of the ex-USS Shadwell, the Navy's damage-control and fire-fighting research ship.1 Figure 4 shows details of the interior of the ship that are represented in the model. There are 13 compartments with 9 doors and 8 hatches. In Fig. 5, the smoke spread from an uncontained fire in the laundry room is shown in terms of temperature and velocity distributions. In Fig. 6, details of the local flow field in the laundry room and the laundry-room passageway are shown to highlight the fact that local details, where needed, have not been sacrificed in the simulation tool.
Parametric studies have also been carried out to determine the effects of varying factors such as droplet diameter, mist-density, injection velocity, and nozzle location on the suppression of a fire in one of the compartments. In addition, the effect of blockages within the compartment on the entrainment of water- mist and fire suppression has also been investigated. 2
Significance: The damage to the interior compartment of a surface ship under different war-fighting and peacetime scenarios can be simulated using the developed tool. Effectively using this tool in the design process will lead to a ship design that is improved in active and passive fire protection. The development of such a tool can also be thought of as a first step toward better-automated response to fire scenarios and the consequent reduction in manning requirements.
Acknowledgments: The authors acknowledge the contributions to this research effort by Dr. Kuldeep Prasad, while working at NRL.
[Sponsored by ONR]References
1 K. Prasad, G. Patnaik, and K. Kailasanath, "Advanced Simulation Tool for Improved Damage Assessment. 1-A Multiblock Technique for Simulating Fire and Smoke Spread in Large Complex Enclosures," NRL/MR/6410-00-8428, February 2000.
2 D.A. Schwer, G. Patnaik, and K. Kailasanath, "Effect of Obstructions on Fire-Suppression for Large Enclosed Fires," Proceedings of the Fall Technical Meeting of the Combustion Institute, December 2001.