Fire-induced Winds In The 20 October 1991 Oakland Hills Fire

Abstract

The winds generated at 11:45 a.m. by 38 fires and at 12:OO p.m. by 259 fires for the 20 October 1991 Oakland Hills Fire are simulated using the Baum and McCaffrey mass fire model. Each house is modeled as having a 50 MW heat release rate at the time of the computation. For each single fire, the flame tip is 15 m high, the maximum vertical velocity is 16 m/s and the maximum radial velocity is 2.4 m/s. At 11:45 a.m., for the mass fire the maximum induced wind is 2.6 m/s. The maximum vertical velocity is 14 m/s. At 12:OO p.m., the maximum horizontal velocity is 13 m / ~. The maximum vertical velocity is 26 m / ~. Results indicate that the strong fire-induced winds at and after 12:OO p.m. contributed to the post-noon decrease in the fire spread rate in the ambient windward direction. KEY WORDS: urban/wildland interface or intermix, fire-induced winds, mass fire modeling. NOTATION Roman c,, Specific heat (J1kg.K) D/Dr Scalar operator a/at + u. v (s-'1 i., Unit vector in the j-direction 8 Acceleration of gravity (m/s2) h Hypotenuse (m) k Thermal conductivity (W1m.K) LC Characteristic length (m) N Number of fireslsources P Pressure (Pa) Q, Net total chemical heat release rate (W) Gfeek O Dimensionless temperature difference P Density (kg/m3) 4, Potential function (m2/s) Y Stokes stream function (m3/s) w Vorticity (,-I) V Gradient (del) operator, 2,3/ax + i., J/ay + z,a/az (m-') Superscripts * Nondimensional quantity Quantity per unit time , , , Quantity per unit volume 1. Present address: Building and Total chemical power output (w/m3) Radius of Gaussian profile (m) Fit for the width of the Gaussian profile (m) Radial direction (m) Temperature (K) Time (s) Velocity (mls) Solenoidal and expansion velocity vectors lSt, 2"d, and 3 1 ~ cartesian coordinates (m) Axial direction (m) Subscripts c Characteristic quantity cl Center-line quantity i, j Indices o Ambient condition r Radial component X X-direction component Y Y-direction component Z Z-direction component z Axial component