My son takes a daily medication prescribed by a doctor who thought outside the box. This medication is exceptionally helpful for him, but his issue is not the intended use for this medication. We frequently have to explain to doctors and pharmacists why he takes it and how it has benefitted him. This idea of “off-label” use is also applicable to CFD analysis.
CFD started as a way to solve unsolvable problems and its use continues to grow, though few of us need to solve hypersonic flow over a re-entry vehicle. Just about any three-dimensional flow or heat transfer application becomes very difficult to calculate without making, sometimes detrimental, assumptions. Applications of large areas, like building ventilation and industrial manifolds, are frequently reduced to volumetric calculations, (e.g. air changes per hour) which do nothing to identify flow uniformity issues and regions of low flow. Enter CFD.
One such application, which initially seems an unlikely candidate for CFD, is the humble lazy river. Most of us have had an opportunity to float around one of these narrow, winding pools propelled by flowing water. Historically, river paths have been simple, the jets well defined and flow rate targets low. As with all things, if a little is good, more must be better. Why float around a river at 0.5 feet per second when you can be zipped around bends at 2 feet per second? Why have only one path when the same river can have multiple paths, which increases the amount of time occupants want to stay in the river? These increased requirements have led to a breakdown in how these new “adventure rivers” are designed. Without attention to details, such as: the river path, nozzle location and direction, and entrance and exit regions, the design of the pool can have problems not seen in lower-velocity, single path rivers.
Figure 1: Surface Velocity of a River
In addition to accurately predicting the surface and bulk velocity of the river, CFD analysis shows how the river profile is driving flow, if propulsion nozzles are well located and directed, and many other areas. Tight turns in the river can create slow and recirculating flows, trapping occupants and reducing their enjoyment of the river. A CFD analysis will identify these regions and show how small path changes or nozzle re-location can improve flow. Figure 2 shows the tight bend from the lower left corner of the river.
Figure 2: Surface Velocity Map and Velocity Iso-Surface Showing Flow from Propulsion Nozzles
This is a region where flow must change direction quickly. Without proper propulsion nozzle placement a large region of recirculating flow can restrict river flow and decrease surface velocity throughout the river.
Figure 3: Surface Velocity through a Zero-Depth Entrance and Exit Region
Flow in and around entrance and exit regions, especially large, zero-depth regions can make entrance and exit either easy or challenging for pool occupants. The entrance region in Figure 3 shows the river flow splitting and a large channel of higher-velocity water flowing into the shallow area with a recirculating region between them. This could make both ingress and egress more difficult and possibly dangerous. An inconvenienced occupant could find their tube swept into the river without them, whereas a parent could see a young child swept in. A small change to the upstream wall angle and radius could keep more high velocity flow in the river and out of the entrance area.
CFD is a powerful and flexible tool, able to predict safe re-entry of astronauts and keep vacationing tourists from being caught in recirculating traps. If you have an application where hand calculation fails to show the full picture, contact Rand Simulation to discuss how an out of the box, or off label, approach to CFD can help.
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