This video shows a sphere in a small supersonic wind tunnel at Mach 2.7. Once the tunnel starts, a curved bow shock forms in front of the sphere, close to but not touching the model’s surface. Areas of low pressure are visible behind the sphere, as is a weak shock wave caused by overexpansion in those low pressure areas. Contrast this with a sharp cone in the same tunnel at the same Mach number. In the case of the cone, the shock wave is attached at the nose of the model. The attached shock follows the body more closely, resulting in a shock that impacts the walls of the tunnel further downstream than in the sphere’s case.
Tag: fluid dynamics
Swirling Fluids
In this video, researchers investigate swirling fluids by studying the shapes of the free surface between air and the liquid. As parameters like the diameter of the glass, initial (unperturbed) height of the liquid, and angular velocity of the rotation change, the surface of the liquid displays different modal behaviors, seen in the photos on the lower left of the video. By non-dimensionalizing the physical parameters of the system (students: think Buckingham pi theorem), they are able to replicate the shape of the free surface by matching a Froude number and dimensionless depth and offset. Such similitude between fluids under different conditions is key to understanding the underlying physics. (Video credit: M. Reclari et al; submitted by co-author M. Farhat)
Surf’s Up
Diffusion of ink in water + Lego minifigs = an awesome example of fluid mechanics as art. (Photo credit: Alberto Seveso; via io9; thanks to Jennifer for the link!)
Reader Question: Creeping Flow
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David asks:
I’m taking an undergraduate fluid dynamics course, and I’m having trouble understanding what a Creeping Flow exactly is. The only thing I understand about that is that the Re should be 0 or close to 0 for the flow… Could you post an example of a creeping flow please? Thank you!
Absolutely! Creeping flow, also called Stokes flow, is, like you said, a very low Reynolds number flow. It would be hard to say that the Reynolds number is zero because that would seem to imply no flow at all. Think of it instead as a Reynolds number much, much less than one. When the Reynolds number is very low, it means that viscous forces are dominating the flow. The video above shows creeping flow around a cylinder; notice how the streamlines stay attached all the way around the surface of the cylinder. There’s no separation, no turbulent wake, no von Karman vortex street. Viscosity is so dominant here that it’s damped out all of that inertial diffusion of momentum.
We’ve posted some other great examples of creeping flow, as well, though not by that name. There are the reversible laminar flow demos and various experiments in Hele-Shaw cells, all of which qualify as creeping flow because of their highly viscous nature. If you have the time, there’s also a great instructional video from the 1960s called “Low Reynolds Number Flow” (Parts 1, 2, 3, 4) starring G. I. Taylor (a famous fluid dynamicist) that is full of one demo after another.
Reynolds Stress
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From reader jessecaps who hung it on the office door. I expect this joke will make sense to very few but as someone who once dabbled in turbulence, I could not resist.
Particle Patterning
Here a container filled with a suspension of neutrally buoyant polystyrene beads and fluid is rotated. As the container rotates, a thin layer of fluid and bunches of particles get drawn up onto the wall by capillary forces capable of holding the particles in place even if the container stops rotating. The density and patterning of the particles on the wall depends on the container’s rotation speed and the volume fraction of particles. (Video credit: J. Kao and A. Hosoi)
Science off the Sphere: Thin Films
Stuck here on Earth, it’s hard to know sometimes how greatly gravity affects the behavior of fluids. Fortunately, astronaut Don Pettit enjoys spending his free time on the International Space Station playing with physics. In his latest video, he shows some awesome examples of what is possible with a thin film of water–not a soap film like we make here on Earth–in microgravity. He demonstrates vibrational modes, droplet collision and coalescence, and some fascinating examples of Marangoni convection.
Soap Film Breakup
This high-speed video shows a soap film formed across two rings and its deformation and breakup as the two rings are pulled apart. As the rings get further apart, surface tension deforms the soap film until the distance is too great to continue sustaining that shape. The film breaks into two–a sheet of soap film in each ring–and a little satellite bubble. Note the similarities in breakup between this soap film and a thin liquid column or water from a faucet.
Colliding Jets
Two jets colliding can form a chain-like fluid structure. With increasing flow rate, the rim of the chains becomes wavy and unstable, forming a fishbone structure where droplets extend outward from the fluid sheet via tiny ligaments. Eventually, the droplets break off in a pattern as beautiful as it is consistent. (Photo credits: A. Hasha and J. Bush)
Examples of Flutter
Aeroelasticity is the study of the interaction of structural and aerodynamic forces on an object, and its most famous example is flutter, which occurs when the aerodynamic forces on an object couple with its natural structural frequencies in such a way that a violent self-excited oscillation builds. What does that mean? Take a look at the video above. This compilation shows examples of flutter on wind tunnel models, road signs, airplanes, and the Tacoma Narrows Bridge–one of the most famous examples of all time. When air moves over and around an object, like a stop sign, it exerts forces that cause the structure to twist or vibrate. Those vibrations then alter the airflow around the object, which changes the aerodynamic forces on the object. If the motion of the object increases the aerodynamic forces which then increase the oscillation, then a potentially destructive flutter cycle has been created. Flutter is very difficult to simulate computationally, so tests are usually performed experimentally to ensure that any vibrations in the system will damp out rather than grow to the point of structural failure like many of the examples in the film.
