Fuck Yeah Fluid Dynamics

Celebrating the physics of all that flows. Ask a question, submit a post idea or send an email. You can also follow FYFD on Twitter and Google+. FYFD is written by Nicole Sharp, PhD.

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Posts tagged "numerical simulation"
As young stars form, they often produce narrow high-speed jets from their poles. By astronomical standards, these fountains are dense, narrowly collimated, and quickly changing. The jets have been measured at velocities greater than 200 km/s and Mach numbers as high as 20. The animation above (which you should watch in its full and glorious resolution here) is a numerical simulation of a protostellar jet. Every few decades the source star releases a new pulse, which expands, cools, and becomes unstable as it travels away from the star. Models like these, combined with observations from telescopes like Hubble, help astronomers unravel how and why these jets form. (Image credit: J. Stone and M. Norman)ETA: As it happens, the APOD today is also about protostellar jets, so check that out for an image of the real thing. Thanks, jshoer!

As young stars form, they often produce narrow high-speed jets from their poles. By astronomical standards, these fountains are dense, narrowly collimated, and quickly changing. The jets have been measured at velocities greater than 200 km/s and Mach numbers as high as 20. The animation above (which you should watch in its full and glorious resolution here) is a numerical simulation of a protostellar jet. Every few decades the source star releases a new pulse, which expands, cools, and becomes unstable as it travels away from the star. Models like these, combined with observations from telescopes like Hubble, help astronomers unravel how and why these jets form. (Image credit: J. Stone and M. Norman)

ETA: As it happens, the APOD today is also about protostellar jets, so check that out for an image of the real thing. Thanks, jshoer!

Though they may appear random at first glance, turbulent flows do possess structure. The video above shows a numerical simulation of a mixing layer, a flow in which two adjacent regions of fluid move with different velocities. The upper third of the frame shows a top view, and the bottom frame shows a side view, in which the upper fluid layer moves faster than the lower one. The difference in velocities creates shear which quickly drives the mixing layer into turbulence. But watch the chaos carefully, and your eye will pick out vortices rolling clockwise in the largest scales of the mixing layer. These features are known as coherent structures, and they are key to current efforts to understand and model turbulent flows. (Video credit: A. McMullan)

This numerical simulation shows a swimming stingray and the vorticity generated by its motion. Stingrays are undulatory swimmers, meaning that the wavelength of their motion is much shorter than their body length. Manta rays, in contrast, move their fins through a wavelength longer than their body length, making them oscillatory swimmers. Observe the difference in this video. To swim faster, stingrays increase the frequency of their undulation, not the amplitude. This is quite common among swimmers because increasing the amplitude also increases projected frontal area, which causes additional drag. Increasing the frequency of motion does not affect the projected area, making it the more efficient locomotive choice. (Video credit: G. Weymouth; additional research credit: E. Blevins; submitted by L. Buss)

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Here on Earth, placing a dense layer of fluid atop a less dense layer is unstable. Specifically, the situation causes the interface between the two fluids to break down in what is known as the Rayleigh-Taylor instability.The video above shows a 2D numerical simulation of this breakdown, with the darker, denser fluid on top. The waviness of the initial interface provides a perturbation—a small disturbance—which grows in time. The two fluids spiral into one another in a fractal-like mushroom pattern. The continued motion of the dense fluid downward and the lighter fluid upward mixes the entire volume toward a uniform equilibrium. For those interested in the numerical methods used, check out the original video page. (Video credit: Thunabrain)

During a solar flare, magnetic field lines on the sun are often visible due to the flow of plasma—charged particles—along the lines. According to theory, these magnetic lines should remain intact, but they are sometimes observed breaking and reconnecting with other lines. An interdisciplinary team of researchers suggests that turbulence may be the missing link. In their magnetohydrodynamic simulation, they found that the presence of chaotic turbulent motions made the magnetic line motion entirely unpredictable, whereas laminar flows behaved according to conventional flux-freezing theory. (Photo credit: NASA SDO; Research credit: G. Eyink et al.; via SpaceRef; submitted by jshoer)

Motion in the ocean is driven by many factors, including temperature, salinity, geography, and atmospheric interactions. While global currents dictate much of the large-scale motion, it’s sometimes the smaller scales that impact the climate. This visualization shows numerically simulated data from the Southern Ocean over the course of a year. The eddies that swirl off from the main currents are responsible for much of the mixing that occurs between areas of different temperature, which ultimately impacts large-scale temperature distributions, in this case affecting the flux of heat toward Antarctica. (Video credit: I. Rosso, A. Klocker, A. Hogg, S. Ramsden; submitted by S. Ramsden)

When a dense fluid sits above a lighter fluid in a gravitational field, the interface between the two fluids is unstable. It breaks down via a Rayleigh-Taylor instability, with mushroom-like protrusions of the lighter fluid into the heavier one. The image above comes from a numerical simulation of this effect well after the initial instability; the darker colors represent denser fluids and lighter colors are less dense fluids. The flow here has progressed to turbulence, and the authors of the simulation are exploring the statistical nature of this flow breakdown relative to the classical case of isotropic, homogeneous turbulence. (Photo credit: W. Cabot and Y. Zhou)

This numerical simulation from NASA Goddard shows the motion of particulates in Earth’s atmosphere between August 2006 and April 2007. These aerosols come from various sources including smoke, soot, dust, and sea salt. As these fine particles move through atmosphere, they can have significant effects on weather as well as climate. For example, the particles serve as nucleation sites for the condensation and formation of rain drops. (Video credit: NASA Goddard SFC)

This video features simulation of the laminar flow around a plate plunging sinusoidally in a quiescent flow. As the plate moves up and down, it mixes the fluid around it. This is visualized in several ways, beginning with the vorticity. Clockwise and anti-clockwise vortices are shed by the edges of the plate as it moves. The flow is also visualized using particle trajectories, which are classified by their tendency to accumulate (attract) or lose (repel) particles. These trajectories are particularly intriguing to watch develop as they appear to show ornate faces and designs. (Video credit: S. L. Brunton and C. W. Rowley)

Supercomputing has been an enormous boon to fluid dynamics over the past few decades. Many problems, like the interaction between a supersonic shock wave and a bubble, are too complicated for analytical solutions and difficult to measure experimentally. Numerical simulation of the problem, combined with visualization of key variables, adds invaluable understanding. Here a shock wave strikes a helium bubble at Mach 3, and the subsequent interactions in terms of density and vorticity are shown. This situation is relevant to a number of applications, such as supersonic combustion and shockwave lithotripsy—a medical technique in which kidney stones are broken up inside the body using shock waves. After impact, an air jet forms and penetrates the center of the structure while the outer regions mix and form a persistent vortex ring. (Video credit: B. Hejazialhosseini et al.; via Physics Buzz)