In this video two droplets of oil fall through a bath of isopropyl alcohol. The oil is denser than alcohol, and the two fluids are miscible. The velocity and density gradients where the two fluids meet generate hydrodynamic instabilities that create the distinctive patterns seen in the falling drops. (Video credit: BYU Splash Lab)
Search results for: “density”
Mussels
In this video, schlieren imaging is used to make visible the flow field around a mussel. Mussels are filter-feeders, drawing nearby water in to obtain their food and expelling the unneeded fluid once they’ve gathered the plankton they eat. Normally this process is invisible to the naked eye, but schlieren imaging reveals changes in density (and thus refractive index) that make it possible to visualize the outflow from the mussel. The technique is also commonly used in supersonic flows to reveal shock waves. (Video credit: Stephen Allen)
Traffic Fluid Dynamics
What does traffic have to do with fluid dynamics? Rather a lot, actually! Many parallels exist between traffic and compressible fluid flow. One such example, the concept of a shock wave, is demonstrated in the video above. As the traffic jam develops, the cars experience sudden changes in their velocity and relative distance (in a fluid, this would be density). This change travels backward through the traffic in the form of a shockwave, just the same as discontinuous changes in a fluid.
Road construction provides another common example of compressible-flow-like behavior in cars. For an incompressible fluid like water, reducing the area of a pipe would increase the velocity, but just the opposite happens when a road is reduced from two lanes to one. Traffic slows down and clumps together. When the road opens back up from one lane to two, suddenly the speed and the distance between cars increases. This is exactly what happens in a rocket nozzle–it’s the expanding bell-like shape that causes air to accelerate supersonically. (Video credit: New Scientist)
Schlieren Montage
Dr. Gary Settles, a world-reknown expert in schlieren photography, shows here a montage of some of his lab’s results, including shockwaves from musical instruments, dogs sniffing, guns firing (both sub- and supersonic), and even snapping a wet towel going supersonic. As Settles jokes, schlieren is all mirrors and hot air. Mirrors are used to shine collimated light on the object to be imaged; then the light focused with a lens. By placing a knife-edge at the focal point, part of the light is blocked and the density variations in the final image become visible, thanks to their differing refractive indices. (Video credit: G. Settles et al.)
Fractal Fluids
These images from a numerical simulation of a mixing layer between fluids of different density show the development and breakdown to Kelvin-Helmholtz waves. The black fluid is 3 times denser than the white fluid, and, as the two layers shear past one another, billow-like waves form (Fig 1(a)). Inside those billows, secondary and even tertiary billows form (Fig 1(a) and (b)). Fig 1 (c)-(e) show successive closeups on these waves, showing their beautiful fractal-like structure. (Photo credit: J. Fontane et al, 2008 Gallery of Fluid Motion) #
Salinity Near the Amazon
This numerical simulation shows the variation of salinity in the Atlantic Ocean near the mouth of the Amazon River over the course of 36 months. The turbulent mixing of the fresh river water and salty ocean shifts with the ebb and flooding of the river. Salt content causes variations in ocean water density, which can strongly affect mixing and transport properties between different depths in the ocean due to buoyancy. Understanding this kind of flow helps predict climate forecasts, rain predictions, ice melting and much more. (Video credit: Mercator Ocean)
Convective Cells
Convective cells form as fluid is heated from below. As the fluid near the bottom warms, its density decreases and buoyancy causes it to rise while cooler fluid descends to replace it. This fluid motion due to temperature gradients is called Rayleigh-Benard convection and the cells in which the motion occurs are called Benard cells. This particular type of convection is essentially what happens when a pot is placed on a hot stove, so the shapes are familiar. Similar shapes also form on the sun’s photosphere, where they are called granules.
Reader Question: Fire as a Fluid?
Reader David L asks:
I understand that fire is a form of energy rather than a fluid in the physical/tangible sense. However, is it possible for fire to exhibit fluid-like behaviours to a certain extent.
In other words, could the dynamic properties of fire be described with pseudo-variables analogical to variables that describe a physical fluid (i.e. viscosity, density, Re, etc.)?
Actually, combustion is a major topic of research among fluid dynamicists. Since the part of fire that we identify as visible flame is a reacting mixture of gas and some solid particles, it moves according to the same equations of motion as any other gas. However, when studying combustion thermodynamical equations and chemical reactions must also be tracked in addition to mass and momentum, which makes modeling fire very difficult. Combustion plays a major role in internal flows like those in car, jet, and rocket engines. (Photo credit: master.blitzy)
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)
Seeing Shock Waves
In this still image from a video of a 2008 demonstration of a U.S. Navy railgun, the shock waves in front of the projectile are momentarily visible. When travelling faster than the speed of sound in air, information (in the form of pressure waves) is unable to travel ahead of the projectile, meaning that the air cannot deform around the object as it does at low speeds. Instead, a front known as a shock wave forms on or in front of the object, depending on its speed and shape. Across this shock wave, thermodynamic properties of the gas are discontinuous; the pressure, temperature, and density of the air rise drastically, but the air is also deformed so that it passes around the object. (See also: bullet from a gun.)
