Mechanical turbulence

There are basically two different causes of turbulent eddies. Eddies set in motion by air moving past objects are the result of mechanical turbulence. Parcels of superheated air rising from the heated earth s surface, and the slower descent of a larger portion of the atmosphere surrounding these more rapidly rising parcels, result in thermal turbulence. The size and, hence, the scale of the eddies caused by thermal turbulence are larger than those of the eddies caused by mechanical turbulence. 

The manifestation of turbulent eddies is gustiness and is displayed in the fluctuations seen on a continuous record of wind or temperature. Figure 19-3 displays wind direction traces during (a) mechanical and (b) thermal turbulence. Fluctuations due to mechanical turbulence tend to be quite regular that is, eddies of nearly constant size are generated. The eddies generated by thermal turbulence are both larger and more variable in size than those due to mechanical turbulence. 

Surface topography can affect the local wind patterns one example is the onshore and offshore breeze, and another example is the heat island over large urban areas. Another manmade effect is the generation of mechanical turbulence caused by the nonuniform height of buildings in a city. We will discuss this effect in more depth later on. 

Mechanical turbulence Any turbulence produced by means other than natural, such as fans. The term is also used incorrectly to define wind currents set up in and around buildings. 

Atmospheric stability and mechanical turbulence (important near to the ground) are used to derive the vertical and horizontal dispersion coefficients. Table 45.2 shows Pasquill s stability categories used to derive the coefficients by reference to standard graphs. 

Spiros n/a 1, 16, or 30 Unit dose blister Mechanical Turbulence, impaction,... 

The third factor affecting dispersion is turbulence. Mechanical turbulence is caused by the roughness of the Earth s surface. Away from the surface, convective turbulence (heated air rising and cooler air falling) becomes increasingly important. The amount of turbulence and the height to which it operates depends on the surface roughness, wind speed and atmospheric stability. 

Furthermore we have to prove the influence of obstacles nearby the emission source. Such obstacles will induce mechanical turbulence to the wind turbulence. In large scale simulation such effects are parameterized the turbulence influence is collected in general diffusion coefficients. In the small scale of the source vicinity this method is still questionable. 

In forests, mechanical turbulence is caused by trees, and temperature inversions by the forest canopy. Ventilation inside a forest is complex and not readily described by existing air flow models (Aylor, 1976). 

Typically, a fire growth model is evaluated by comparing its calculations (predictions) of large-scale behavior to experimental HRR measurements, thermocouple temperatures, or pyrolysis front position. The overall predictive capabilities of fire growth models depend on the pyrolysis model, treatment of gas-phase fluid mechanics, turbulence, combustion chemistry, and convective/radiative heat transfer. Unless simulations are truly blind, some model calibration (adjusting various input parameters to improve agreement between model calculations and experimental data) is usually inherent in published results, so model calculations may not truly be predictions. 

At low wind speed at night, the surfece was cooled by emission of heat radiation, air was cooled upon contact with the surface, and cold dense air pooled on the surface as if a separate fluid. We had an inversion. Where the ground sloped, the cold surface air flowed downhill, a katabatic wind, and mechanical turbulence was induced by this wind. When there was a prevailing wind, its mechanically induced turbulence was reduced by the... 

The fine droplets contained in a ULV spray are not all blown large distances downwind. If ULV application is undertaken such that mechanical turbulence is... 

Turbulent and laminar mixing are quite different. Turbulent mixing takes place by three mechanisms turbulent eddy motion, bulk or convective flow, and molecular diffusion. Laminar flow has no eddies to assist in mixing. Laminar mixing first depends on creating very thin layers between initially unmixed components, followed by molecular diffusion. 

Concepts initially under investigation include those of microjet injection [1] and the use of Bluebell (corrugated) nozzles with tabs/chevrons and/or nozzle lip injection [2]. These concepts embody several types of technologies for jet-noise reduction which include those of conventional vortical mechanisms, turbulent structure alteration, and shock attenuation. 

When a gas (explosive mixture) is released into another gas (air), both inevitably mix under the effect of the following mechanisms turbulence induced by discharge of the gas (turbulence related to the initial inertia of the jet and its possible buoyancy),... 

Stable 0 < Ri < 0.25 Mechanical turbulence weakened by stable stratification... 

The Richardson number is a turbulence indicator and also an index of stability. Meteorologists classify atmospheric stability in the surface layer as unstable, neutral, and stable. Strongly negative Richardson numbers indicate that convection predominates, winds are weak, and there is a strong vertical motion characteristic of an unstable atmosphere. Smoke leaving a source spreads rapidly vertically and horizontally. As mechanical turbulence increases, the Richardson number approaches zero, and the... 

The geometric mean value of z over the layer to be dealt with is then more appropriate. For a windy day with some sunshine L = —100 m, with zq = 20 cm, a value appropriate for grasslands with some trees, z = 100 m and Zm = 10 m, and p = 0.20 with mechanical turbulence and 0.145 when convection is added. It is apparent that con-veetion ean have a signifieant effect on the value of p. 

When z/Z < — 1, buoyant production of turbulence is greater than mechanical production of turbulence when z/Z > —1 mechanical production of turbulence exceeds buoyant production of turbulence and when buoyant production is negUgible, and all the turbulence is created by wind sheer, i.e., mechanical turbulence -0.1 < z/Z <0.1. 

Mechanical turbulence Turbulence driven by (vertical) wind shear. 

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