FYFD

Tag: stagnation

  • Jellyfish Make Their Own Walls

    Jellyfish Make Their Own Walls

    When we walk, the ground’s resistance helps propel us. Similarly, flying or swimming near a surface is easier due to ground effect. Most of the time swimmers don’t get that extra help, but a new study shows that jellyfish create their own walls to get that boost.

    Of course, these walls aren’t literal, but fluid dynamically speaking, they are equivalent. Over the course of its stroke, the jellyfish creates two vortices, each with opposite rotation. One of these, the stopping vortex, lingers beneath the jellyfish until the next stroke’s starting vortex collides with it. When two vortices of equal strength and opposite rotation meet, the flow between them stagnates — it comes to halt — just as if a wall were there.

    In fact, mathematically, this is how scientists represent a wall: as the stagnation line between a real vortex and a virtual one of equal strength and opposite rotation. It just turns out that jellyfish use the same trick to make virtual walls they can push off! (Image and research credit: B. Gemmell et al.; via NYTimes; submitted by Kam-Yung Soh)

  • Drawing With Microfluidic Tweezers

    Drawing With Microfluidic Tweezers

    One of the challenges of dealing with objects at the microscale is finding ways to manipulate them. This is what techniques like optical tweezers or magnetic traps are used for. The downside to these methods is that they often require complex experimental set-ups or place restrictions on the kinds of particles that can be manipulated. Recently, however, researchers have developed a new hydrodynamic alternative: the Stokes trap.

    Using a six-channel microfluidic device like the the ones shown in A) and B) above, scientists can alter the flow in the device in such a way that they trap and manipulate two particles at the same time. The simultaneous inflow and outflow in the device creates streamlines like those shown in C) and D) above. The large white areas where the streamlines converge and diverge are stagnation points–areas of little to no velocity. The scientists trap their particles at the stagnation points and then carefully shift the flow rates into and out of the device to move the stagnation points–with particles in tow–wherever they want them. In the animation, you can see part of a movie where they use the particles to write out a capital I (for University of Illinois). The researchers hope the technique will be used in the future for studying the physics of soft materials and biologically-relevant molecules like DNA. For more, check out the full paper or the group’s website.  (Image credit and submission: C. Schroeder et al.)

  • How Eyelashes Work

    How Eyelashes Work

    New research shows that eyelashes divert airflow around the eye, serving as a passive filter that reduces dust collection and controls evaporation. Mammal hairs in places like the nose act as ram filters that trap the particles that hit them and which require regular cleaning via sneezing. Eyelashes, on the other hand, prevent dust collection by altering airflow at the surface of the eye. At the optimal length of roughly 1/3rd the width of an eye, eyelashes create a stagnation zone near the eye surface that forces air to travel above rather than through the eyelashes. This results in lower shear stress and lower flow speeds at the eye surface, both of which help reduce evaporation and shield the eye from dust. Lashes can get too long, though; the researchers found that longer lashes tended to channel higher flow speeds toward the eye surface, leading to faster evaporation rates. Thus, donning longer fake eyelashes may dry out your eyes. (Image credit: G. Diaz Fornaro; research credit: G. Amador et al.; via skunkbear)

  • Streamlines in Oil

    Streamlines in Oil

    Bernoulli’s principle describes the relationship between pressure and velocity in a fluid: in short, an increase in velocity is accompanied by a drop in pressure and vice versa. This photo shows the results left behind by oil-flow visualization after subsonic flow has passed over a cone (flowing right to left). The orange-pink stripes mark the streamlines of air passing around the Pitot tube sitting near the surface. The streamlines bend around the mouth of probe, leaving behind a clear region. This is a stagnation point of the flow, where the velocity goes to zero and the pressure reaches a maximum. Pitot tubes measure the stagnation pressure, and, when combined with the static pressure (which, counterintuitively, is the pressure measured in the moving fluid), can be used to calculate the velocity or, for supersonic flows, the Mach number of the local flow. (Photo credit: N. Sharp)