function fall_speed,t,p,pden,r ;----------------------------------------------------------- ; purpose: ; calculate aerosol fall velocity using Stokes Law ; dynamic viscosity of air is only a function of temperature, ; Rogers and Yau (R+Y) pg 103. ; ; The fall speed (v1) is from Pruppacher + Klett (P+K), pg 362 ; ; Another approach from R+Y for fall speed (pg 97) ; v2 = pden2*(2.0*r*1.e-6)^2 * g / (18.0 * dvisc) ; m/s ; ; input: ; t = temperature (K) ; p = pressure (mb) ; pden = particle density (g/cm3) ; r = particle radius (microns) ; ; output: ; v = aerosol fall speed (m/s) ; ; source: Mark Hervig ;----------------------------------------------------------- pden2 = pden * 1000. ; particle density kg/m3 g = 9.8 ; gravity (m/s2) Rd = 287.04 ; gas constant (J/kg/K) aden = 100.0 * p / (Rd * t) ; air density (kg/m3) mfp = 6.6e-2 * (1013.25/p) * (t/293.15) ; P+K pg 323, mean free path (um) KN = mfp / r ; Knudson number a = 1.257 + 0.4*exp(-1.1/KN) ; P+K pg. 261, eqn 12-16 dvisc = 1.72e-5*(393.0/(t+120.0))*(t/273.0)^(1.5) ; dynamic viscosity air (kg/m/s) v1 = 2.*(1.+a*KN)*(pden2-aden)*g*(r*1.e-6)^2 /(9.*dvisc) ; fall velocity, m/s return,v1 end