The translational and rotational equations of motion are solved using a modified velocity- Verlet algorithm, where the translational motive force part is solved using a self-consistent dissipative velocity-Verlet algorithm [35] and the other parts are solved using the stan- dard velocity-Verlet algorithm, which can be found in any basic textbooks on molecular dynamics simulations. The time step used in the algorithm is adjusted during the simula- tions by the maximum forces exerted on agents. The minimum time step varies between 0.01 and 0.001 seconds, by default.
The estimated evacuation time used in the exit door selection algorithm is approximated
in the present version of FDS+Evac. The walking time to the exit door is simply approx- imated by dividing the distance to the door by the unimpeded walking velocity of the person. The distance to a door is calculated along a bee line for the visible doors (L2 dis- tance) and for non-visible doors the L1 metric is used. The queueing time is calculated for the visible doors by counting how many agents are closer to the door than the present one and by dividing this number by an estimated flow through the door. The estimated human flow is given by the door width times the specific flow value given by the user (default 1.3 1/m/s). The presently chosen door is favoured so that 10 % longer time is tolerated by default. For the non-visible doors distance should perhaps be calculated along the exit path and also some kind of an estimated queueing time at the door should be added to the estimated evacuation time, but this is not yet implemented in the model. The queueing time is only estimated for those visible doors, which do not have disturbing conditions due to the fire. If there is disturbing conditions then the agents try to choose a door where the conditions are “best”. The default method to rank the “best” conditions is to choose the door, which is most visible through the smoke. The agents are updating their target exit doors on every second, on the average.
The smoke density calculated by the FDS fire simulation can be used to detect a fire.
By default, smoke density is not used for detection. User gives as inputs a detection time (distribution) and a reaction time (distribution) for the evacuating agents. If smoke is used to detect fire then user should give the detection threshold concentration (mg/m3).
An agent detects a fire when the smoke concentration reaches its threshold value at the position of the agent or if the user given detection time has been reached.
The smoke concentration affects the exit door selection algorithm. The user gives a threshold visibility value for a door to be considered as a “smoke free” door. A door is usable as long as the visibility is larger than half the distance to the door, where local visibility = 3/extinction coefficient. Similarly as for the “smoke free” case, the presently chosen door is favoured so that 10 % more smoke is tolerated by default. This makes the door selection algorithm more stable to a small changes in the smoke concentration. If there is no line-of-sight to the door, then the local concentrations at the position of the agent are used and the distance that is used is the L1 metric distance to the door.
4. Testing and Verification
4.1 Introduction
In the verification test cases the default parameter values of FDS+Evac for the different predefined agent types were used unless otherwise stated. In many cases the simulation runs were also done using a value ofλi = 0.5for the anisotropy parameter of the social force instead of the default value ofλi = 0.3, see Eq. (3). An archive of the verification tests of FDS+Evac are at the FDS+Evac web pages1. This manual contains only a short summary of these test and it might not be up to date, so more interested reader should visit the web pages to get the most recent information.
Note, that most of the results reported below are just based on one FDS+Evac simu- lation per each scenario. FDS+Evac is a stochastic modelling programme, i.e., it uses stochastic distributions to generate the initial positions of the agents and their properties.
There are also small random forces and torques in the equations of motion and random numbers are also used in the door selection and counterflow algorithms. For the qualita- tive verification, however, it is enough just to run the model once for each scenario. Same is true for the numerical verification of some of the sub-models.
Some of the qualitative verification cases of the agent movement algorithm are based on the International Maritime Organization (IMO) document “Guidelines for Evacuation Analyses for New and Existing Passenger Ships” [33], where eleven different test cases are listed. These tests are referred below as “IMO 1”,etc. Note, that the IMO document specifies the test persons to be 30–50 years old males defined in the table 3.4 of the IMO document [33]. This person group is similar to the default “Male” of FDS+Evac, but the unimpeded walking velocities are generated from an uniform distribution between 0.97–
1.62 m/s. If the test case in question is a IMO test case, then the reference to a “Male”
person type is the default “Male” of FDS+Evac, but with different walking speeds.
The tests suggested by IMO do not include any quantitative verification, because IMO sees that “At this stage of development there is insufficient reliable experimental data to allow a through quantitative verification of egress models. Until such data becomes available the first three components of the verification process are considered sufficient.”,
1http://www.vtt.fi/fdsevac/
where the first three components are component testing, functional verification, and qual- itative verification.