Stop distance

For dry traction more contact is desired and the stopping distance is directiy related, ie, the more contact area the shorter the stopping distance. A softer, more pHable compound conforms to the road surface topography. Too soft a compound (low mechanical strength) abrades more easily and can therefore acts as a roUer and not allow sufficient contact area to be maintained. This is not readily encountered in nominal tires and conditions but has been encountered in cases of extremely high torque conditions for very fast acceleration and sudden stops. 

Stopping distance The maximum distance a moving particle will travel in still air after all the external forces are removed. In the Stokes region it is the velocity of the particle times the relaxation time. 

Farley K.A., Wolf R.A., and Silver L.T. (1996) The effects of long alpha-stopping distances on (U-Th)/He ages. Geochim. Cosmochim. Acta 60, 4223-4229. 

Impaction is caused by the inertial mass of the traveling aerosol particles that forces them to move in a straight-line direction even when the flow of the inhaled air transporting them is bent around a curvature. Hence the particles tend to deposit on obstacles placed in the path of their travel. The inertial mass depends on particle size, density, and velocity. The stopping distance S of a particle having mass mP and initial velocity v0iP is defined according to... 

Sp is the stopping distance of the particle, which is the distance it would travel in still air if given a velocity uv L is the characteristic dimension of the obstacle (the diameter of a cylinder or sphere). For particles in the 1-50 pm range, and less than 5 m s-1, the approximate stopping distance is given by... 

The increased pavement life can be attributed to higher viscosity and impermeability of rubberized asphalt. These properties have decreased thermal cracking, potholing, deformation, and reflective cracking in most states in which tests were performed. Studies by the Alaska Department of Transportation showed decreased stopping distances as a result of rubberized asphalt being more flexible and preventing ice formation (37). 

Typical cascade impactors consist of a series of nozzle plates, each followed by an impaction plate each set of nozzle plate plus impaction plate is termed a stage. The sizing characteristics of an inertial impactor stage are determined by the efficiency with which the stage collects particles of various sizes. Collection efficiency is a function of three dimensionless parameters the inertial parameter (Stokes number, Stk), the ratio of the jet-to-plate spacing to the jet width, and the jet Reynolds number. The most important of these is the inertial parameter, which is defined by Equation 2) as the ratio of the stopping distance to some characteristic dimension of the impaction stage (10), typically the radius of the nozzle or jet (Dj). 

The significance of si is then clear it is a constant that is the reciprocal of the relaxation time for stopping a particle in a stagnant fluid. Similarly, on can show that /sl represents the time for a particle falling in a gravitational field to achieve its terminal speed. Note that the terminal speed qs = gsi l. As t/sAoo, the distance over which the particle penetrates, or the stopping distance L, is qosA l. 

Notice that the particle travels the greatest distance at the higher Reynolds numbers. The laminar flow contribution to the stop distance is small compared to the intermediate or turbulent flow contribution. 

Compute the value of t for a 15-p.m-diameter sand particle (p = 2.65 g/cm3). Then compute (a) its terminal settling velocity, (6) its Reynolds number at this velocity, and (c) its stop distance. 

That is, the particle will move one stop distance out of its initial streamline while losing all its original velocity parallel to the slot. 

Since aerosol particles are continually undergoing molecular bombardment, their paths are smooth curves rather than segments of straight lines. It still is possible to define an apparent mean free path for the aerosol particles (Fuchs, 1964). This is the distance traveled by an average particle before it changes its direction of motion by 90°. The apparent mean free path represents the distance traveled by an average particle in a given direction before particle velocity in that direction equals zero. But this is just the stop distance. 

At any time, a particle may be considered to be moving in a specific direction with a velocity v - /8kT/-nm. From a definition of the stop distance, the pseudo mean free path lB is... 

Kinetics (Cord.) of centrifugation, 115-117 of cyclones, 117-120 electrostatic controlled (see Electrostatic controlled kinetics) equation of motion, 76-77 without external forces, 77-80 ideal stirred settling, 86-88 impaction (see Impaction) isokinetic sampling, 120-124 limitations on, 84 one-dimensional motion, 84-86 respirable sampling, 124-128 stop distance, 83... 

Example 6.7 Determine the stop distance of a 1.5-pm-diameter unit-density sphere which is projected into still air with an initial velocity of 1000 cm/s. (Neglect gravity.)... 

This dimensionless parameter is used to describe impactor behavior. For impactors with rectangular openings, W is the slit width for circular openings W represents the diameter of the impactor opening. Thus the Stokes number is the ratio of the stop distance to the impactor opening half-width. 

Figure 11.1 Mean stopping distance and stopping profiles for positrons as a function...

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