FYFD

Tag: channel flow

  • Roll Waves in Debris Flows

    Roll Waves in Debris Flows

    When a fluid flows downslope, small disturbances in the underlying surface can trigger roll waves, seen above. Rather than moving downstream at the normal wave speed, roll waves surge forward — much like a shock wave — and gobble up every wave in their way.

    Such roll waves are fairly innocuous when flowing down a drainage ditch but far more problematic in the muddy debris flows of a landslide. Debris flows are harder to predict, too, thanks to their combined ingredients of water, small grains, and large debris.

    A new numerical model has shed some light on such debris flows, after showing good agreement with a documented landslide in Switzerland. The model suggests that roll waves get triggered in muddy flows at a higher flow speed than in a dry granular flow but a lower flow speed than is needed in pure water.

    For a great overview of roll waves, complete with videos, check out this post by Mirjam Glessner. (Image credit: M. Malaska; research credit: X. Meng et al.; see also M. Glessmer; via APS)

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  • Bending in the Stream

    Bending in the Stream

    Nature is full of cilia, hairs, and similar flexible structures. Unsurprisingly, flows interact with these structures very differently than with smooth surfaces. Here, researchers investigate flow in a channel lined with flexible, hair-like plates. Initially, the channel is filled with oil and dark particles that help visualize the flow. Then, they pump water into the setup.

    As the water intrudes, it forms an interface with the oil. That interface is powerful enough to bend individual hairs in the system. When the hair bends far enough, it can touch its neighbor, sealing the oil inside the gap between them. Along the length of the channel, this behavior leads to trapped pockets of oil that never drain, no matter how much water flows by. (Image and research credit: C. Ushay et al.)

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    Hydraulic Jumps

    Chances are that you’ve seen plenty of hydraulic jumps in your life, whether they were in your kitchen sink, the whitewater of a river, or at the bottom of a spillway. Practical Engineering has a great primer on this oddity of open channel flow. 

    When water (or other liquids) flow with a surface open to the air – think like a river rather than a pipe – the flow has three important regimes: subcritical, critical, and supercritical. Which state the flow is in depends on the speed of the flow compared to the speed of a wave traveling in that flow. If the waves are faster than the flow, we call it subcritical. If the flow is faster than the waves, it’s called supercritical. (This is equivalent to subsonic or supersonic flow, where the regime depends on the flow speed compared to the speed of sound.)

    Flows can transition naturally from one state to another, and where they transition from fast, supercritical flow to slower, subcritical flow, we find hydraulic jumps – places where the kinetic energy of the supercritical flow gets changed into turbulence and potential energy through a change in height. Check out the video above to learn how civil engineers use hydraulic jumps to control water and erosion. (Video and image credit: Practical Engineering)

  • Structures in Turbulence

    Structures in Turbulence

    Despite its appearance, there is order in the chaos of turbulence. These snapshots from a turbulent channel flow simulation outline these coherent structures in black. The top photo shows a top view looking down on the channel and the bottom image shows a side view of the channel. It is thought that studying these coherent structures may help shed light on turbulence and its formation, which remains one of the great open questions of classical physics. (Photo credit: M. Green)

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    Vortex Street Sim

    This numerical simulation shows a von Karman vortex street in the wake of a bluff body. As flow moves over the object, vortices are periodically shed off the object’s upper and lower surfaces at a steady frequency related to the velocity of the flow. The simulation takes place in a channel; note how the thickness of the boundary layers on the walls increases with downstream distance, forcing a slight constriction on the vortex street in the freestream.

  • River Jumps

    River Jumps

    Hydraulic jumps occur when a high velocity liquid runs into an area of low velocity liquid. The faster moving liquid decelerates rapidly and increases in height, effectively trading kinetic energy for potential energy. The phenomenon is frequently observed in open channel flow, like in spillways or along rivers, as in the photos above. In nature, the hydraulic jump will often be laminar upstream and turbulent downstream. #