Swirling wake

Figure 2.3 Typical configurations on discrete/obstacle scale. Normal approach angle 0 small enough (i.e. cos0 > 2w/d) for independent wakes to be formed (approximately parallel to the approach wind). Note the complex forms of obstacle wakes, in which swirl may persist.

A different pattern of diffusion occurs when obstacles are closely packed and tall (i.e. b/d > 1/3) and H > 2b. Then, even though there is an amplified rate of vertical dispersion as a result of vertical swirling and turbulent motion around the obstacles (Figure 2.18), neither the average vertical concentration profiles nor the detrainment from their wakes are uniform over the depth of the average canopy layer H. 

When a fluid flows past a bluff body, the wake downstream will form rows of vortices that shed continuously from each side of the body as illustated in Figure 4.16. These repeating patterns of swirling vorticies are referred to as Karman vortex streets named after the fluid dynamicist Theodore von Karman. Vortex shedding is a common flow phenomenon that causes car antennas to vibrate at certain wind speeds and also lead to the collapse of the famous Tacoma Narrows Bridge in 1940. Each time a vortex is shed from the bluff body it creates a sideways force causing the body to vibrate. The frequency of vibration is linearly proportional to the velocity of the approching fluid stream and is independent of the fluid density. 

A second viscous swirl model that accounts for downstream rotor effects is the Gaussian wake viscous swirl model ... 

In addition to amplified heat transfer effects, understanding microrotorcraft operation at low Reynolds numbers also requires better modeling of viscous swirl effects. There are numerous enhanced viscous swirl models that could be utilized, a review of some of which has been provided by Kunz [5]. The average wake deficit viscous swirl model is based on using a power-law fit of computationally-modeled airfoil wake profiles at low Re (using INS 2d data) ... 

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