Turbulent wakes

Ejfective Height of an Emission The effective height of an emission rarely corresponds to the physical height of the stack. If the plume is caught in the turbulent wake of the stack or of buildings in... 

When the wind encounters objects in its path such as an isolated structure, the flow usually is strongly perturbed and a turbulent wake is formed in the vicinity of the structure, especially downwind of it. If the structure is semistreamlined in shape, the flow may move around it with little disturbance. Since most structures have edges and corners, generation of a turbulent wake is quite common. Figure 17-23 shows schematically the flow in the vicinity of a cubic structure. The disturbed flow consists of a cavity... 

The effective height of an emission rarely corresponds to tlie physical height of tlie source or the stack. If tlie plume is caught in tlie turbulent wake of tlie stack or of buildings in the vicinity of tlie source or stack, tlie effluent will be mixed rapidl) downward toward the ground. If the plume is emitted free of these turbulent zones, a number of emission factors and meteorological factors influence tlie rise of the plume. 

The stack gas e.xit velocity should be greater tluiti 60 ft/s so tliat stack gases will escape the turbulent wake of the stack. In many cases, it is good practice to liave the gas e.xit velocity on the order of 90 or 100 ft/s. 3. A stack located on a building should be located in a position tliat will assure tliat the exliaust escapes tlie wakes of nearby structures. 

Pfeifer, Turbulent Wake Gas Analyzer Program , Rept No ESD-TR-69-152, Contract AF19(628)-5167, ARPA order-600, MIT, Lexington (1969) 30) H.R. Griem R.H. 

FIGURE 4.54 Recirculation zone and flame-spreading region for a fully developed turbulent wake behind a bluff body (after Williams [57]). 

UBOd/a) > 104, which was found experimentally to be the range in which a fully developed turbulent wake exists. The correlation in this region should be compared to the correlation developed from the work of Zukoski and Marble. 

Roshko, A. 1953. On the development of turbulent wakes from vertex streets. NACA TN 2913, March. 

The rate of mass-transfer, unlike the terminal velocity, may reach its lower limit only when the whole surface of the drop or bubble is covered by the adsorbed film. In the absence of surface-active material, the freshly exposed interface at the front of the moving drop (due to circulation here) could well be responsible for as much mass transfer as occurs in the turbulent wake of the drop. The results of Baird and Davidson 67a) on mass transfer from spherical-cap bubbles are not inconsistent with this idea, and further experiments on smaller drops are in progress in the author s laboratory. In general, if these ideas are correct, while the rear half of the drop is noncirculating (and the terminal velocity has reached the limit of that for a solid sphere), the mass transfer at the front half of the drop may still be much higher, due to the circulation, than for a stagnant drop. Only when sufficient surface-active material is present to cover the whole of the surface and eliminate all circulation will the rate of mass-transfer approach its lower limit. 

At low Re, wakes behind large bubbles and drops are closed (B3, H5, S5, W2, W6), whereas at high Re open turbulent wakes are formed (F15, Ml, W6). The value of Re for transition between these two types of wake has been determined as 110 + 2 (B3) for skirtless bubbles. There is some evidence (H5) that the transition Reynolds number may be increased if skirts are present. 

There has been considerably less work on open turbulent wakes, although some excellent photographs have been published (B3, Ml, W5, W6). Wake... 

Wind tunnel test methods were developed to determine wind induced stresses in cooling towers using aeroelastic models as part of a detailed model of a power station site. The turbulence and shear in the atmospheric wind are simulated. Tests on a model of Ferrybridge C Power Station show that resonant stresses are significant at the design wind speed. These increase as the fourth power of wind speed and can be greatly enhanced by turbulent wakes of upstream structures. 6 refs, cited. 

A characteristic transverse dimension d of the flame holder can be measured more easily than the length / of the recirculation zone. The ratio l/d experimentally has a practically constant value between 5 and 10, independent of flow conditions for hot turbulent wakes. Hence, I d in equation (65), so that d. Of greater interest than the dependence... 

External flow over a tennis ball, and the turbulent wake region behind. 

FIGURE 716 Laminar boundary layer separation witli a turbulent wake flow over a circular cylinder at Re = 7.000. 

In the moderate range of 10 < Re < 1 O . Ihe drag coefficient remains relatively constant. This behavior is characteristic of blunt bodies. Tlie flow in the boundary layer is laminar in this range, but the flow in the separated region past the cylinder or sphere is highly turbulent with a wide turbulent wake. 

B 90° is due to the transition from laminar to turbulent flow. The later decrease ill Nu is again due to the thickening of the boundary layer. Nug reaches its second minimum at about 6 140°, which is the flow separation point in turbulent flow, and increases with 0 as a result of die intense mixing in the turbulent wake region. 

The bubbles occurring in the present electrochemical system are small enough to fulfil the criterion of Stokes flow. Thus, turbulent wakes arising behind bubbles can be neglected [26], Furthermore, we consider all bubbles in the present simulations as small, non-deformable and rigid spheres. This hypothesis holds for bubbles of low Eotvos numbers Eo ... 

Downwind of the canopy where the average drag of any obstacles effectively decreases to zero, the mean air flow descends and accelerates. Typically in neutral conditions the mean velocity adjusts to within 10% of its ultimate (rural) value within about 30 canopy heights (e.g. Counihan et al., 1974 [132]). But if the air flow is stably stratified the well mixed turbulent wake experiences low friction and continues further downwind, with a jet tending to form in the mean velocity profile (Owinoh et al., 2003 [477]). 

For wide obstacles the separation distance d for no interaction is considerably greater, i.e. d/b < 7. Even when the separation distance is large enough not to affect the structure of the flow around downwind obstacles, the turbulent wakes from upwind obstacles can affect the flow around those downwind, for example enhancing or diminishing the mixing. 

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