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Forced Airflow
Published in James Jones, Demetri Telionis, Aeroform, 2023
Secondly, turns and bends in the duct create friction and resistance to airflow. As previously introduced, any fluid in motion like air or water has momentum and does not “like” to make sharp turns. Abrupt turns in a duct create turbulence, which adds to the losses, reduces the stream energy, and creates noise. Losses along turns can be expressed in Equation (7.5), and minor loss coefficients are shown in Figure 7.28. Turns in ducts should be as gradual as possible rather than abrupt. For this, turning vanes are often inserted into the duct to smoothly turn the air, thus reducing turbulence and noise (Figure 7.29). It is important that the turning vanes be aligned to direct the flow appropriately.
Redistribution of flow velocity in sharp bends using unsubmerged vanes
Published in International Journal of River Basin Management, 2019
Seyed Mohammad Ghaneeizad, Ehsan Bahrami Jovein, Jalil Abrishami, Joseph F. Atkinson
Submerged vanes as a relatively new technology have applications for a wide range of river problems including restoring river banks, stabilizing rivers, remeandering channelized rivers, increasing flood flow capacity and reducing sediment deposits (Odgaard 2015, 2017). Submerged vanes are designed to balance secondary motion of flow in bends. The secondary flow moves high-velocity surface current toward the outer bank and low-velocity near-bed current toward the inner bank. Secondary flow can produce sedimentation in inner banks and superelevation and erosion in outer banks. Submerged vanes placed in the outer half of a bend with an angle to the flow axis create a counter spiral flow to block the secondary flow and avoid erosion (Odgaard and Spoljaric 1989, Dey et al. 2017). In contrast to this placement, a placement parallel to the main flow along the flow axis was proposed with the intention to block the cross sediment transport instead of blocking the secondary flow, although experimental results did not support this intended result (van Dam 2005). Guide vanes or turning vanes are continuous unsubmerged vanes parallel to and along the flow axis commonly used in generators, turbines, and water and wind tunnels to improve flow distribution, avoid flow separation and reduce pressure loss (Wetzel and Arndt 1994, Sieverding et al. 1996, Barlow et al. 1999, Luo and Razinsky 2009). Application of these vanes for open channels can reduce flow separation, intensity of secondary flows, turbulence energy and energy loss downstream of sharp bends (Han et al. 2011). It is known that secondary flow and separation of flow in bends depend on the ratio R = Rc/W, where Rc is the centreline radius of curvature of the bend and W is the width of the channel. Previous literature suggests that a ‘sharp’ bend is one for which R < 3, and a ratio above 3 should be used for designing open channels (Leopold and Wolman 1960, U.S. Army Corps of Engineers 1994, Ramamurthy et al. 2013). Behaviour of flow in sharp bends is different due to the cumulative effects of amplified secondary flow, relatively larger free-surface variations and flow separation along the inner bend wall (Ramamurthy et al. 2013, Gholami et al. 2016a,b). Sharp bends are not, however, avoidable in practice (e.g. irrigation and flood control channels) and are common in nature. Use of unsubmerged vanes to divide a channel with sharp bend into two or more channels increases R, and thus may reduce secondary flow and separation. In addition, these vanes help to achieve a more uniform flow in the channel downstream of the bend (Han et al. 2011). Han et al. experiment showed that better performance can be obtained with multiple vanes, relative to a single vane. In narrow channels, however, the arrangement is restricted to a single array since using multiple vanes in narrow channels is not always possible.