Computational Test Design for High-Speed Liquid Impact and Dispersal

Abstract

Transportation accidents frequently involve liquids dispersing in the atmosphere. An example is that of aircraft impacts, which often result in spreading fuel and a subsequent fire. Predicting the resulting environment is of interest for design, safety, and forensic applications. This environment is challenging for many reasons, one among them being the disparate time and length scales that must be resolved for an accurate physical representation of the problem. A recent computational method appropriate for this class of problems has been developed for modeling the impact and subsequent liquid spread. This involves coupling a structural dynamics code to a turbulent computational fluid mechanics reacting flow code. Because the environment intended to be simulated with this capability is difficult to instrument and costly to test, the existing validation data are of limited scope, relevance, and quality. A rocket sled test is being performed where a scoop moving through a water channel is being used to brake a pusher sled. We plan to instrument this test to provide appropriate scale data for validating the new modeling capability. The intent is to get high fidelity data on the break-up and evaporation of the water that is ejected from the channel as the sled is braking. These two elements are critical to fireball formation for this type of event involving fuel in the place of water. We demonstrate our capability in this paper by describing the pre-test predictions which are used to locate instrumentation for the actual test. We also present a sensitivity analysis to understand the implications of length scale assumptions on the prediction results.

Predictions of Transport Accident Fires Using Coupled Structural Dynamics and Computational Fluid Dynamics

“…Initially, the Presto structural dynamics simulations were coupled to the Vulcan fire code, an off-shoot of the ComputIT Kameleon code that is maintained at Sandia National Labs [5]. Subsequent work [4,6,7] has been performed with Fuego, a massively parallel CFD code for predicting fire environments that was built under the Sierra architecture. Fuego is a low Mach number flow solver that uses a control-volume finite element to solve the Navier-Stokes equations together with reacting species transport equations.…”

“…When large-scale tests are conducted at the Sandia National Labs rocket sled track, water is used to brake the sled to preserve the hardware for future tests. Recognizing this as a potential source for validation data, we are planning on collecting data for the spread of liquid caused by the sled braking system [7]. Water is added to a channel between the tracks, and begins to brake the sled when a customdesigned scoop ruptures the barrier of the channel at one end.…”

“…However, they do not directly couple the impact dynamics to the longer-term fire prediction codes. Several reports have described the one-way coupling methods used in this paper in the context of individual scenarios [4][5][6][7]. The method involves initializing the fluid mechanic based on solid mechanics calculations.…”