OCR Text |
Show particle lies within the laminar boundary film of the particle. The snow is then plastered onto the damming obstruction. The equilibrium of the shear stresses acting on the individual granule as a result of the viscosity of the air fj and the density of the entrained air permits one to assume the following qualitative relationship ( in technical units, t]~ 1.7 x 10" ko- sec/ m2): thickness of the boundary layer 6 ~ ( 2 f] Av/^* ) for A v ~ 50 m/ sec the thickness of the entrained air layer reaches already about 1 cm. Separation of snow begins either at lower air velocities or by damming of air at higher velocities, if this is only lightly loaded with snow, i. e., if the density of the snow- air mixture is small. If the distance between particles is smaller than the thickness of the boundary layer, then the aerosol at the velocity corresponding to this must behave mechanically like a homogeneous gas of corresponding average density. The above indications show that the effect of obstructions on the movement of snow laden air currents still needs clarification. For more accurate investigations a value of v corresponding to the velocity distribution v1 is inserted in Equation ( 25). The investigations of Bazine and Boussinesq lead one to expect a parabolic velocity distribution for the circumstances in question, i. e., the velocity v' at the depth z below the surface is-: ( 26) v' = v [ 4/ 3 - ( z/ h') 2] This kind of velocity distribution generally is in agreement with the measurements of Bagnold [ 12]. As an additional limiting case, for powder avalanches with strong back damming of the front along with consideration of the relative back flowing motion at the surface, the ordinate z to be inserted in the equation above is measured above and below half the flow height. The velocity near the underlying surface amounts to one third the average velocity v. The velocity distribution is of great influence on the pressure effect of powder avalanches, which have a great flow height h*. For the pressure on inclined surfaces for shooting flow the following is true: if a stream of cross section f and velocity v strikes a solid surface at an angle p, then one can think of the stream as being divided into a stream of cross section f cos pparallel to the surface struck and a stream of cross section f sinp perpendicular to this surface. In the second instance the mass m = f y / g flows with the velocity v sinp against the solid surface. The uniform specific damming pressure originates from the transmission of pressure into the deflected material: ( 27) Po = ( f y/ g) ( v2 sin2p/ 2f sinp) = ( yv2/ 2g) sinp The specific resistance of an inclined surface referred to the projection in the flow direction is likewise given by Equation ( 27). This rule 42 |