15 Wind speed

Various studies have documented a decline in the communication distance at increasing wind speeds. Nakamura (1976) reported that the active space of S. litura, as defined by male wing-fanning responses, decreased at higher wind speeds but also at the lowest wind speeds as well. He attributed the latter trend to increased pheromone deposition at the lowest speeds. 

The effect of wind speed on the size of the active space is compounded by possible effects on the rate of odor release. The rate of odor release from many synthetic or natural sources is governed by the rate of transport to the surface of the releasing substrate and would therefore be independent of wind speed. Hirooka and Suwani (1976) argued on theoretical grounds that the release rate from any small dispenser would be unaffected by wind speed. However, Elkin-ton et al. (1984) demonstrated that release rates of dispar lure from cotton wick dispensers increase with increasing wind speed. 

The dispersion models discussed in this chapter are all expressed in terms of odor concentration in space (g cm ). Odor receptors, however, respond to the rate of odor adsorption per unit time which is more directly related to the flux (gcm s ) of odor molecules over the receptor organ than to odor concentration. The rate of flux through a stationary receptor increases in direct proportion to the wind speed offsetting the inverse decline in concentration. The efficiency of adsorption, however, may decrease at higher wind speeds. 

Wind speed may also directly affect the response and pheromone release behavior of organisms. For example, Kaae and Shorey (1972) reported that the persistence of the calling behavior of T. ni was greatest at intermediate wind 

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Cup anemometers have shaped cups mounted on the spokes of a wheel. The cups, under the action of the fluid forces, spin in a horizontal plane about a vertical shaft mounted in bearings. Vane or propeller types use a multibladed rotor, the axis of which is parallel to the flow direction as the rotating member. Both designs are commonly used for wind speed measurement or similar appHcations such as the velocity in ventilation ducts. Because of inertia, anemometers are most accurate under steady conditions. Velocity fluctuations cause readings that are too high. 

Factors considered to affect pond performance are air temperature, relative humidity, wind speed, and solar radiation. Items appearing to have only a minor effect include heat transfer between the earth and the pond, changing temperature and humidity of the air as it traverses the water, and rain. 

The effec t of wind speed is twofold (1) Wind speed will determine the travel time from a source to a given receptor and (2) wind speed... 

Wind speed has velocity components in all directions so that there are vertical motions as well as horizontal ones. These random motions of widely different scales and periods are essentially responsible for the movement and diffusion of pollutants about the mean downwind path. These motions can be considered atmospheric turbulence. If the scale of a turbulent motion (i.e., the size of an eddy) is larger than the size of the pollutant plume in its vicinity, the eddy will move that portion of the plume. If an eddy is smaller than the plume, its effect will be to difhise or spread out the plume. This diffusion caused by eddy motion is widely variable in the atmosphere, blit even when the effect of this diffusion is least, it is in the vicinity of three orders of magnitude greater than diffusion by molecular action alone. 

Aerodynamic Downwash Should the stack exit velocity be too low as compared with the speed of the crosswind, some of the effluent can be pulled downward by the low pressure on the lee side of the stack. This phenomenon, known as stack-tip downwash, can be minimized by keeping the exit velocity greater than the mean wind speed (i.e., typically twice the mean wind speed). Another way to minimize stack-tip downwash is to fit the top of the stack with a flat disc that extends for at least one stack diameter outward from the stack. 

In assessing the hazard of a UVCE or in investigating a UVCE it is often necessary to (1) estimate the maximum distance to the lower flammable hmit (LFL) and (2) determine the amount of gas in a vapor cloud above the LFL. Figure 26-31 shows the maximum distance to the lower flammable limit, i.e., in the centerline of the cloud, based on the previous method from Bodurtha (1980) for wind speeds of 1 iti/s (2.2 mi/h) and 5 m/s (11 mi/h). Maximum concentrations probably occur near 1 m/s. The volume of fuel from the LFL up to 100 percent may be estimated by... 

Q = continuous dense vapor emission rate, mVs at 25°C Xl = distance to momentaiy LFL in centerhne of cloud, m u = wind speed, iti/s... 

FIG. 26-31 Estimated maximum downwind distance to lower flammable limit L, percent by volume at ground level in centerline of vapor cloud, vs. continuous dense vapor release rate at ground level. E atmospheric stability. Level terrain. Momentary concentrations for L. Moles are gram moles u is wind speed. (From Bodmtha, 1980, p. 105, by permission.)... 

Assume a continuous release of pressurized, hquefied cyclohexane with a vapor emission rate of 130 g moLs, 3.18 mVs at 25°C (86,644 Ib/h). (See Discharge Rates from Punctured Lines and Vessels in this sec tion for release rates of vapor.) The LFL of cyclohexane is 1.3 percent by vol., and so the maximum distance to the LFL for a wind speed of 1 iti/s (2.2 mi/h) is 260 m (853 ft), from Fig. 26-31. Thus, from Eq. (26-48), Vj 529 m 1817 kg. The volume of fuel from the LFL up to 100 percent at the moment of ignition for a continuous emission is not equal to the total quantity of vapor released that Vr volume stays the same even if the emission lasts for an extended period with the same values of meteorological variables, e.g., wind speed. For instance, in this case 9825 kg (21,661 lb) will havebeen emitted during a 15-min period, which is considerablv more than the 1817 kg (4005 lb) of cyclohexane in the vapor cloud above LFL. (A different approach is required for an instantaneous release, i.e., when a vapor cloud is explosively dispersed.) The equivalent weight of TNT may be estimated by... 

Introduction Gas dispersion (or vapor dispersion) is used to determine the consequences of a release of a toxic or flammable material. Typically, the calculations provide an estimate of the area affected and the average vapor concentrations expected. In order to make this determination, one must know the release rate of the gas (or the total quantity released) and the atmospheric conditions (wind speed, time of day, cloud cover). 

Parameters Affeeting Gas Dispersion A wide variety of parameters affect the dispersion of gases. These include (1) wind speed, (2) atmospheric stability, (3) local terrain characteristics, (4) height of the release above the ground, (5) release geometry, i.e. from a point, line, or area source, ( momentum of the material released, and (7) buoyancy of the material released. 

As the wind speed is increased, the material is carried downwind faster, but the material is also diluted faster by a larger quantity of air. 

Sutton Micrometeorology, McGraw-Hill, 1953, p, 286) developed a solution to the above difficulty by defining dispersion coefficients, O, Gy, and O, defined as the standard deviation of the concentrations in the downwind, crosswind, and vertical x, y, z) directions, respectively, The dispersion coefficients are a function of atmospheric conditions and the distance downwind from the release. The atmospheric conditions are classified into six stability classes (A through F) for continuous releases and three stability classes (unstable, neutral, and stable) for instantaneous releases. The stability classes depend on wind speed and the amount of sunlight, as shown in Table 26-28,... 

Wind speed, m/s Day radiation intensity Night cloud cover ... 

Notice that the wind speed does not appear exphcitly in Eq, (26-61), It is implicit through the dispersion coefficients since these are a function of distance downwind From the initial release and the atmospheric stabihty conditions,... 

Worst-case atmospheric conditions occur to maximize (C). This occurs with minimum dispersion coefficients and minimum wind speed u within a stability class. By inspection of Figs. 26-54 and 26-55 and Table 26-28, this occurs with F-stability and u = 2 m/s. At 300 m = 0.3 km, from Figs. 26-54 and 26-55, <3 = 11m and <3 = 5 m. The concentration in ppm is converted to kg/m by application of the ideal gas law. A pressure of 1 atm and temperature of 298 K are assumed. 

The model requires a specification of the initial cloud volume, the initial plume volume flux, the duration of release, and the initial gas density. Also required is the wind speed at a height of 10 m, the distance downwind, and the ambient gas density. 

Figure 2 Variation of the gas transfer veloeity with wind speed. The units of transfer veloeity are equivalent to the number of em of the overlying air eolumn entering the water per hour (Taken from Bigg," with permission of Cambridge University Press)...

The ideal condition would be when the rotor output is a cubic function of wind speed. But in practice this may not be so. It is found to be linear or a near quadratic (square) function of the wind speed, as shown in Figure 6.64... 

Figure 6.64 Typical power curve for an induction generator of 400 kW at 11.5 m/s wind speed...

Note Generally, the ratio of rated to mean wind speed may be quite high due to long lean periods, when the maehine may slay idle, redueing the value of the mean speed. 

Shutdown speed = 20 m/s Rotor diameter including hub = 39.35 m Rotational speed of the rotor at the rated wind speed = 38 r.p.m. 

Tower This may be tubular or lattice type to mount the mill s mechanism. The structural design is based on the cutout wind speed. 

Cut-in wind speed is the minimum wind speed at which the generator commences the generation of power. At this speed the brakes release and the prime mover (blades) starts rotating. 

Cut-oul wind speed is the maximum w ind speed beyond which the prime mover may overspeed above its permissible limits. As the structure and the blades are designed for a particular maximum speed, a wind speed higher than this may exceed their mechanical endurance and become unsafe. At this speed the brakes apply and the machine is disconnected from the grid. The cut-in and cut-out speeds define the wind speed limits within which the turbine will work safely through the generator. Rated wind speed is the speed at which the prime mover rotates at the rated negative slip and generates the rated power. 

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