Quiz 14 Flashcards
There are two ways for FDS to solve stability problems with HVAC, which 2?
o HVAC_PRES_RELAX
o HVAC_LOCAL_PRESSURE
To reduce the computational time of the HVAC solver, how should the duct be defined?
To reduce the computational cost of the HVAC solver, a duct should be considered as any length of duct that connects two items that must be defined as nodes (i.e., a connection to the FDS domain, a filter, or a location where more than two ducts join). That is, a duct should be considered as any portion of the HVAC system where flow can only be in one direction at given point in time (flow can reverse direction over time).
What are the three ways of simulating wildland fire spread in FDS and how do they work?
o Particle Model: The vegetation is represented by a collection of Lagrangian particles that are heated via convection and radiation
o Boundary Fuel Model: Ground vegetation is modelled like a porous solid with a thickness equal to the height of the vegetation
o Level Set Model: The fire propagates through empirically set rules.
Can FDS account for firebrands accumulated in a wildland fire and if so, does it do it well?
o Yes, as Lagrangian particles
o Difficulties with high densities
Why is it not possible to define a leak directly on the numerical grid?
o Leakage is inherently a sub-grid scale phenomenon because the leakage area is usually very small. In other words, it is not possible to define a leak directly on the numerical mesh. It is sometimes possible to “lump” the leaks into a single mesh-resolvable hole, but this is problematic for two reasons. First, the leakage area rarely corresponds neatly to the area of a single mesh cell-sized hole. Second, the flow speeds through the hole can be large and cause numerical instabilities.
Which methods exist in FDS to approach leakage?
o Pressure zone leakage - the pressure zone leakage approach is intended to capture the bulk leakage that occurs through walls.
o Localized leakage - the local leakage approach is intended to represent leakage through a specific crack.
What theory does FDS use to Model Wind Method 1?
Monin-Obukhov Similarity
How does L determine the temperature?
Depending on whether L is negative or positive the temperature increases or decreases relative to the ground temperature. In other words if the air over the ground
is warmer or colder to the ground. A negative L gives us a warmer air at the ground and the other way around for positive.
Describe the three ventilation parameters VEL, VOLUME_FLOW, and MASS_FLUX_TOTAL in short.
o VEL sets the velocity of the flow from a vent/obst. VOLUME_FLOW sets a volume per time hence calculating the velocity from the required volume and given area. MASS_FLUX calculates a velocity given the area to correspond with the massflux.
Which direction corresponds to positive or negative numbers for VEL, VOLUME_FLOW and MASS_FLUX_TOTAL and which one of these ventilation parameters should only be used in one direction?
A negative VEL, VOLUME_FLOW or MASS_FLUX_TOTAL corresponds to an inflow and vice versa for positive numbers. MASS_FLUX_TOTAL should only be used in an outgoing flow.
When aerosols and soot deposits on solid surfaces the soot concentration in the gas phase reduces, how does this affect fire safety design?
o longer detection time on smoke detectors
o greater visibility
o covering of signs etc
What are the three deposition mechanisms that FDS calculates?
o Gravitational
o Thermophoretic
o Turbulent
