Numerical analysis on plume temperature properties formed above a harmonically oscillating fire source

Large eddy simulation (LES) of turbulent buoyant plumes formed above an oscillating fire source has been carried out so as to clarify their temperature properties in the offshore environment. For the sake of simplicity, we are concerned here only with a rolling environment among six-degree-of-freedom system of complex ship motion. Furthermore, under the assumption of a small rolling angle, unsteady fire source movement associated with the rolling motion can be approximated by simple harmonic oscillation in the horizontal direction. Numerical results obtained under several sets of oscillation conditions, especially under different amplitude conditions, are compared with temperature variations of conventional axisymmetric and two-dimensional fire plumes above stationary sources. Consequently, it has been clarified that in the case where the amplitude is four times longer than the side length of a square source, the present buoyant plumes exhibit almost the same feature as the two-dimensional fire plume in the lower part of the buoyant plume region. On the other hand, it has been found that for any value of amplitude, the temperature properties are close to those of axisymmetric plumes in the upper part of the buoyant plume region. Based on these findings, a simple theoretical model to estimate the variation in mean excess temperature as a function of elevation has been developed by the use of a well-recognized, semi-analytical solution of the axisymmetric plume formed above a virtual point source. It has been confirmed that this simple model can reasonably reproduce the same plume features as obtained by the LES simulations.