Stability of finite difference numerical simulations of acoustic logging-while-drilling with different perfectly matched layer schemes

In acoustic logging-while-drilling （ALWD） finite difference in time domain （FDTD） simulations, large drill collar occupies, most of the fluid-filled borehole and divides the borehole fluid into two thin fluid columns （radius -27 mm）. Fine grids and large computational models are required to model the thin fluid region between the tool and the formation. As a result, small time step and more iterations are needed, which increases the cumulative numerical error. Furthermore, due to high impedance contrast between the drill collar and fluid in the borehole （the difference is 〉30 times）, the stability and efficiency of the perfectly matched layer （PML） scheme is critical to simulate complicated wave modes accurately. In this paper, we compared four different PML implementations in a staggered grid finite difference in time domain （FDTD） in the ALWD simulation, including field-splitting PML （SPML）, multiaxial PML（M- PML）, non-splitting PML （NPML）, and complex frequency-shifted PML （CFS-PML）. The comparison indicated that NPML and CFS-PML can absorb the guided wave reflection from the computational boundaries more efficiently than SPML and M-PML. For large simulation time, SPML, M-PML, and NPML are numerically unstable. However, the stability of M-PML can be improved further to some extent. Based on the analysis, we proposed that the CFS-PML method is used in FDTD to eliminate the numerical instability and to improve the efficiency of absorption in the PML layers for LWD modeling. The optimal values of CFS-PML parameters in the LWD simulation were investigated based on thousands of 3D simulations. For typical LWD cases, the best maximum value of the quadratic damping profile was obtained using one do. The optimal parameter space for the maximum value of the linear frequency-shifted factor （a0） and the scaling factor （β0） depended on the thickness of the PML layer. For typical formations, if the PML thickness is 10 grid points, the global error can be reduced to 〈1% using the optimal PML parameters, and the error will decrease as the PML thickness increases.

The Analysis of Human Behaviour and Evacuation in a Fire Situation in a Building for Higher Education

The paper presents the analysis of a human evacuation from a higher education building located in Iassy,Romania,by means of engineering techniques to approach fire safety.Because in Romania(as in other European countries)fire safety design of buildings is prescriptive and not performance-based,a fire safety engineering approach arouse great interest in many countries such as the U.S.A.,Australia,New Zealand,England,Sweden,Finland,etc.This paper is based on the assumption of starting a fire in the space of a hall for festivities,located on the ground floor of the building,near two human evacuation routes;We consider two building evacuation scenarios:two exits and,respectively,just one exit(assuming that the second would be accidentally blocked).

Fire Risk Quantitative Assessment of Offshore Supported Platform

The living area of an offshore platform is the main living place for operators in offshore oil and gas fields.Fire risk assessment plays an important role in the safety of personnel in offshore platforms.In this paper,a fire risk assessment mathematical model for offshore platfoms is proposed based on a comprehensive safety assessment method.The concept of danger time is presented according to the evaluation criteria of safe evacuation.The fire risk of offshore platforms is assessed by combining probability statistics with numerical simulation.The fire risk is quantitatively assessed by using an N500 deep water semi-submersible support platform as an example.According to the FN curve,fire frequency,fire escalation probability,and casualty probability,the rationality of marine general layout is analyzed,and the general layout design could be optimized to reduce the fire risk.

Relative Flow Analysis around Passing-Train in the Single-Bore Subway Tunnel with Vertical Ventilation Using Moving IBM

Moving IBM （immersed boundary method） is applied to analyze the relative motion of railway car flow in the single-bore subway tunnel with vertical ventilation. The tested car body is modeled by cylinder type body. The subway tunnel is assumed to be the single-car-passing straight type （single-bore tunnel）. The modeled car is relatively moved forward. On the other hand, the tunnel and vertical ventilation are fixed. The momentum equations are solved by LES （large eddy simulation） method. The initial condition of fluid in the subway tunnel is stationary. The Reynolds number is 1,500 based on the cylinder radius. The turbulent flow field in the subway tunnel and vertical ventilation shaft are to be qualitatively investigated.

A study on the characterization of smoke movement in shafts with different fire source positions

In this study,experimental and numerical simulation methods were combined to simulate the changing course of the temperature and velocity fields in nine different fire scenes. The characteristics of smoke movement in shafts with different fire source position factors(h/H) were quantitatively investigated,and the non-dimensional fitting function between the fire source position factors and the maximum temperature was deduced. The results showed that the location of the neutral plane moved upward as the fire source rose,and all the generated smoke spread to the upper areas;however,there was barely any smoke in the lower areas. The maximum temperature was inversely proportional to the fire source position factor;the higher the source position is,i.e. the higher the ratio factor is,the lower the maximum temperature is in the shaft. The experimental verification of the fire dynamics simulator(FDS) showed good results.

Agent-Based Evacuation Model Incorporating Fire Scene and Building Geometry

A comprehensive description of the key factors affecting evacuations at fire scenes is necessary for accurate simulations. An agent-based simulation model which incorporates the fire scene and the building geometry is developed using a fire dynamics simulator （FDS） based on the computational fluid dynamics and geographic information system （GIS） data to model the occupant response, The building entities are generated for FDS simulation while the spatial analysis on GIS data represents the occupant＇s knowledge of the building. The influence of the fire is based on a hazard assessment of the combustion products. The agent behavior and decisions are affected by environmental features and the fire field. A case study demonstrates that the evacuation model effectively simulates the coexistence and interactions of the major factors including occupants, building geometry, and fire disaster during the evacuation. The results can be used for the assessments of building designs regarding fire safety.