Parametric Studies
For full details of the parametric study see the report in downloads.
Simulation time
Simulations presented below show that taking velocity readings over at least 120s time-window beginning at T1= 120s is a credible and optimal way of establishing the magnitude of in-draft.
Both the percentage error and CPU requirements are within the acceptable limits.
This conclusion is only applicable when measurements are taken in the area of interest stretching 80m, possibly up to 100m, form the fire. When measuring farther downwind the time window has to be modified accordingly.
Both the percentage error and CPU requirements are within the acceptable limits.
This conclusion is only applicable when measurements are taken in the area of interest stretching 80m, possibly up to 100m, form the fire. When measuring farther downwind the time window has to be modified accordingly.
The larger area you are interested in the longer simulation time is required. To start measurements flow in part of the domain you are interested in must be developed. There is no need to wait until the whole of the domain becomes fully developed.
Time convergence in windless conditions suggests that measurements should be started at T1=120s and finished at T2=240s.
Time convergence in windy conditions suggests that measurements should be started at T1=120s and finished no sooner than T2=180s.
Domain length
Length of the domain has a strong effect on the flow characteristics in the domain. If the domain is too small the boundary conditions affect the flow. Additionally, the behaviour of vortices and in-draft distribution shows high ununiformities which are not observed using larger domains. If domain is too long it requires a lot of processing time.
Convergence of domain length in windless conditions suggests that the domain should be at least 140m long
Convergence of domain length in windy conditions also suggests that the domain should be at least 140m long
Domain height
Similar effects as for domain length are observed in domain height. Additionally, short domains do not allow for flow to develop in the whole of the domain.
On the other hand to tall domains become only partially affected by fires suggesting wasted processing time.
On the other hand to tall domains become only partially affected by fires suggesting wasted processing time.
Fire in windy conditions. Domain is too tall as the top part of it is not affected by the fire.
Convergence in windless conditions suggests that the domain should be at least 40m high.
Convergence in windy conditions suggests that the domain should be at least 40m high.
Grid size
Flow is very grid sensitive. There is no convergence observed. Non-conforming with the rest behaviour of the coarsest, 0.40m grid, suggests that only smaller mesh sizes should be used.
No wind convergence in windless conditions
No wind convergence in windy conditions.
Wind profile
It is found that the standard parabolic wind profile applied 20-40m from the fire is inadequate. Instead a wind development zone is suggested. It is important to ensure that flow in the domain is driven by a pressure gradient imposed between the upstream and downstream boundaries.
Standard parabolic wind profile (left) develops to become Developed wind profile (right). No pressure gradient imposed.
Standard parabolic wind profile (left) develops to become Developed wind profile (right). Pressure gradient has been imposed. Results are thought to be realistic.
Mirroring of domain
Mirroring is a process by which a symmetric domain is cut into half and only that one half is simulated. The main reason for doing that is to reduce the processing time. This study has found that mirroring is not appropriate for modelling of wildfires. Different flow regimes are observed. It has also been found that flow development time has been greatly extended which is not offset by the reduction in size of the domain.
Mirroring is used to reduce the size of the domain and thus cut computing time
Flow behaviour is altered when mirroring is applied. Standard domain (magenta); using mirroring (blue) presents significantly underdeveloped flow.
Pressure correction
Pressure correction is an optional command which can be applied in FDS5. It allows to ensure that flow using multiple meshes is adheres to the mass conservation equations. It has been observed that using it in windy conditions makes little difference to the flow behaviour. In windless conditions it significantly increases the time needed for flow to develop but the results obtained using it are thought to be realistic.
In windy conditions pressure correction makes little difference (top and bottom lines); In windless conditions it alters the rate of flow development (line in the middle)
Windless conditions. Domain without pressure correction (top) is affected by the fire; Domain with pressure correction applied (bottom) is little affected by the fire suggesting significantly longer flow development time.
