This stunning National Geographic photo contest winner shows an F-15 banking at an airshow and a array of great fluid dynamics. A vapor cloud has formed over the wings of the plane due to the acceleration of air over the top of the plane. The acceleration has dropped the local pressure enough that the moisture of the air condenses. Some of this condensation has been caught by the wingtip vortices, highlighting those as well. Finally, the twin exhausts have a wake full of shock diamonds, formed by a series of shock waves and expansion fans that adjust the exhaust’s pressure to match that of the ambient atmosphere. (Photo credit: Darryl Skinner/National Geographic; via In Focus; submitted by jshoer)
Search results for: “shock wave”
The Veil Nebula
There is no grander scale for the observation of fluid dynamics than that of the astronomical. Here Hubble astronomers discuss the formation of the Veil Nebula, a supernova remnant formed some 5,000-10,000 years ago. Wisps of gas and plasma remain, creating stunning astronomical landscapes that are the result of shock waves, turbulence, diffusion, and other processes familiar to us here on Earth. (Video credit: ESA/Hubble)
Martian Landing Physics
A little over a week ago, NASA’s Curiosity rover landed on Mars, the culmination of years of engineering. The mission’s landing, in particular, was the subject of intense scrutiny as Curiosity’s size necessitated some new techniques in the final segments of the landing sequence. As it hit the Martian atmosphere at 13,000 mph, the compression of the carbon dioxide behind the capsule’s shock wave slowed the descent. At roughly 1,000 mph–speeds still large enough to be supersonic–Curiosity deployed its parachute. Shown above are the parachute in numerical simulation (from Karagiozis et al. 2011), wind tunnel testing at NASA Ames, and during descent thanks to the Mars Reconnaissance Orbiter. The simulation shows contours of streamwise velocity at different configurations; note the bow shock off the capsule and the additional shocks off the parachute. These help generate the drag needed to slow the capsule. For an interesting behind-the-scenes look at the wind tunnel testing for Curiosity’s parachute check out JPL’s four–part video series. Congratulations to all the scientists and engineers who’ve made the rover a success. We look forward to your discoveries! (Photo credits: K. Karagiozis et al., NASA JPL, NASA MRO)
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)
Volcanic Clouds
The volcano Tungurahua erupts in a cloud of ash while molten lava flows down the mountain’s sides. Overhead a wispy lenticular cloud has formed where moist air flowing over the volcano dropped below its dew point. Volcanic eruptions have been known to produce shock waves and vortex rings as well as their distinctive turbulent plumes. (submitted by A. Jones III)
How Dams Affect Rivers
This video shows how the installation of a dam can affect river flow and sediment transport. Before the dam is added, the flow is shallow and the sediment transport is uniform. The installation of the dam creates deep subcritical flow upstream and supercritical flow downstream. This means that wave information–like ripples–can propagate upstream on the subcritical side; on the supercritical side, the wave velocity is lower than the flow velocity and ripples cannot propagate upstream. This is analogous to sub- and supersonic flow in air. The critical flow over the dam is analogous to a shock wave. The lower velocity upstream of the dam is unable to carry sediment downstream and transport essentially ceases until the sediment builds up to a height where the depth of the water above the dam is roughly equal to that below the dam and sediment transport resumes, scouring the downstream supercritical section. Around 0:40, a gate is closed on the downstream side (off frame), creating a hydraulic jump. In the final section of the video, after sediment has built up on both sides of the dam, the downstream gate is re-opened and the jump reforms as sediment is blown out below the dam. (Video credit: Little River Research and Design, with funding from the Missouri Department of Natural Resources)
Supersonic Flow
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.
Supersonic Flow Around a Cylinder
This numerical simulation shows unsteady supersonic flow (Mach 2) around a circular cylinder. On the right are contours of density, and on the left is entropy viscosity, used for stability in the computations. After the flow starts, the bow shock in front of the cylinder and its reflections off the walls and the shock waves in the cylinder’s wake relax into a steady-state condition. About halfway through the video, you will notice the von Karman vortex street of alternating vortices shed from the cylinder, much like one sees at low speeds. The simulation is inviscid to simplify the equations, which are solved using tools from the FEniCS project. (Video credit: M. Nazarov)