FYFD

Tag: supersonic

  • Shock Waves in Flight

    Shock Waves in Flight

    Schlieren optical systems have been used to visualize shock waves in labs for more than a century, but the technique did not translate well to photographing shock structures outside the lab. But now NASA’s Armstrong Research Center and Ames Research Center have developed a method that allows them to capture highly-detailed images of the shock waves around airplanes while they are flying. This is incredible stuff. Be sure to check out the high-resolution versions on this page, along with more description of the coordination necessary to pull off the photos.

    The light and dark lines you see emanating from the airplane are places with strong density gradients. The dark lines are mostly shock waves, with the strongest shock waves appearing black due to the large change in air density. Many of the light streaks are expansion fans, areas where the density and pressure drop as air speeds up.

    The goal of this research is to better understand shock wave structures around supersonic planes in order to reduce the noise supersonic aircraft cause when flying overhead. As you can see in the photos, the shock waves at the nose and tail of the aircraft persist far away from the aircraft; these are what cause the twin sonic boom heard when the plane flies by. (Photo credit: NASA; via J. Hertzberg)

  • Testing a Supersonic Car

    Testing a Supersonic Car

    How do you test a supersonic car like the Bloodhound SSC in a wind tunnel? With free-flying objects like airplanes, wind tunnel testing is relatively straightforward. Mounting a stationary model in a supersonic flow gives an equivalent flow-field to that object flying through still air at supersonic speeds. The same does not hold true for the supersonic car, though, because you need to account for the effect of the ground on airflow. One option is to build a moving wall in the wind tunnel. For low-speed applications, this is feasible but incredibly complicated and very expensive. For supersonic speeds, it’s impossible. You could achieve the same moving-wall effect at supersonic speeds with a rocket sled, but that is also expensive and difficult to fit in most experimental facilities. The simplest solution is the one you see above – build two models and mount them belly-to-belly. Reflecting the models makes the plane of symmetry a stagnation plane, which, fluid dynamically speaking, acts like an imaginary ground plane relative to the model. For more on the project and the technique, check out this article.  (Photo credit: B. Evans; via ThinkFLIP; submitted by G. Doig)

  • Krakatoa

    Krakatoa

    Volcanoes seem to be a common topic these days. Yesterday Nautilus published a great piece by Aatish Bhatia on the 1883 eruption of Krakatoa, which tore the island apart and unleashed a sound so loud it was heard more than 4800 km away:

    The British ship Norham Castle was 40 miles from Krakatoa at the time of the explosion. The ship’s captain wrote in his log, “So violent are the explosions that the ear-drums of over half my crew have been shattered. My last thoughts are with my dear wife. I am convinced that the Day of Judgement has come.“

    In general, sounds are caused not by the end of the world but by fluctuations in air pressure. A barometer at the Batavia gasworks (100 miles away from Krakatoa) registered the ensuing spike in pressure at over 2.5 inches of mercury. That converts to over 172 decibels of sound pressure, an unimaginably loud noise. To put that in context, if you were operating a jackhammer you’d be subject to about 100 decibels. The human threshold for pain is near 130 decibels, and if you had the misfortune of standing next to a jet engine, you’d experience a 150 decibel sound. (A 10 decibel increase is perceived by people as sounding roughly twice as loud.) The Krakatoa explosion registered 172 decibels at 100 miles from the source. This is so astonishingly loud, that it’s inching up against the limits of what we mean by “sound.” #

    Those are some mindbogglingly enormous numbers. Aatish does a wonderful job of explaining the science behind an explosion whose effects ricocheted through the atmosphere for days afterward. Check out the full article over at Nautilus.  (Image credit: Parker & Coward, via Wikipedia)

  • The Chelyabinsk Meteor

    The Chelyabinsk Meteor

    In February 2013 a meteor streaked across the Russian sky and burst in midair near Chelyabinsk. A recent Physics Today article summarizes what scientists have pieced together about the meteor, from its origins to its demise. The whole article is well worth reading. Here’s a peek:

    The Chelyabinsk asteroid first felt the presence of Earth’s atmosphere when it was thousands of kilometers above the Pacific Ocean. For the next dozen minutes, the 10 000-ton rock fell swiftly, silently, and unseen, passing at a shallow angle through the rarefied exosphere where the molecular mean free path is much greater than the 20-m diameter of the rock. Collisions with molecules did nothing to slow the gravitational acceleration as it descended over China and Kazakhstan. When it crossed over the border into Russia at 3:20:20 UT and was 100 km above the ground, 99.99997% of the atmosphere was still beneath it.

    Because the asteroid was moving much faster than air molecules could get out of its way, the molecules began to pile up into a compressed layer of high-temperature plasma pushing a shock wave forward. Atmospheric density increases exponentially with depth, so as the asteroid plunged, the plasma layer thickened and its optical opacity rapidly increased. About one second later, at 95 km above the surface, it became bright enough to be seen from the ground. That was the first warning that something big was about to happen. #

    How often are scientific articles that gripping?! Kring and Boslough provide some excellent descriptions of the aerodynamics of the meteor and its airburst. Be sure to check it out. (Photo credit: M. Ahmetvaleev; paper credit: D. Kring and M. Boslough; via io9)

  • Mach Diamonds

    Mach Diamonds

    Rocket engines often feature a distinctive pattern of diamonds in their exhaust. These shock diamonds, also known as Mach diamonds, are formed as result of a pressure imbalance between the exhaust and the surrounding air. Because the exhaust gases are moving at supersonic speeds, changing their pressure requires a shock wave (to increase pressure) or an expansion fan (to decrease the pressure). The diamonds are a series of both shock waves and expansion fans that gradually change the exhaust’s pressure until it matches that of the surrounding air. This effect is not always visible to the naked eye, though. We see the glowing diamonds as a result of ignition of excess fuel in the exhaust. As neat as they are to see, visible shock diamonds are actually an indication of inefficiencies in the rocket: first because the exhaust is over- or under-pressurized, and, second, because combustion inside the engine is incomplete. (Photo credit: Swiss Propulsion Laboratory)

  • Forming a Jet

    Forming a Jet

    Many situations can generate high-speed liquid jets, including droplet impacts, vibrated fluids, and surface charges. In each of these cases, a concave liquid surface is impulsively accelerated, which causes the flow to focus into a jet. The image above shows snapshots of a microjet generated from a 50 micron capillary tube visible at the right. This jet formed when the meniscus inside the capillary tube was disturbed by a laser pulse that vaporized fluid behind the interface. Incredibly, the microjets generated with this method can reach speeds of 850 m/s, nearly 3 times the speed of sound in air. Researchers have found the method produces consistent results and suggest that it could one day form the basis for needle-free drug injection. You can read more in their freely available paper. (Photo credit: K. Tagawa et al.)

  • AEDC 16-ft Supersonic Tunnel

    AEDC 16-ft Supersonic Tunnel

    This 1960 photo shows three men standing inside Arnold Engineering Development Complex’s 16-ft supersonic wind tunnel facility. The wind tunnel was capable of Mach numbers between 1.60 and 4.75 through a test section 4.8 meters wide and 20.2 meters long. It served as a large-scale testing facility for aircraft and propulsion systems. Like many large-scale and high-speed wind tunnel facilities in the United States, it is no longer active. In recent years, many unique wind tunnel facilities at NASA, military bases, and universities have been closed down, depriving researchers and engineers of the ability to include large-scale testing in their design and development of new technologies. These facility closures leave a substantial gap between the speeds and Reynolds numbers achievable in small-scale experiments and computational fluid dynamics and those experienced in flight. (Photo credit: P. Tarver)

  • Protostellar Jets

    Protostellar Jets

    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!

  • Controlling Supersonic Flight

    Controlling Supersonic Flight

    The forces on an object in flight come from the distribution of pressure on the surface. To alter an object’s trajectory, one has to shift the pressure distribution. On subsonic and transonic aircraft, this is usually done with control surfaces like an aileron, but at supersonic speeds this can require a lot of force. The schlieren images above show an alternative approach in which a plasma actuator near the nosetip generates asymmetric forces on the cone. The actuator discharges plasma at t=0, and flow is from left to right. In the first image, the bubble of plasma is expanding on the upper side of the cone, disrupting the nearby shock wave. Over time, it moves downstream, carrying its disruption with it. The asymmetric effect of the plasma causes uneven pressures on either side of the cone that can be triggered in order to turn it in flight.  (Photo credit: P. Gnemmi and C. Rey)

  • Bullet Through a Bubble

    Bullet Through a Bubble

    A bullet passes through a soap bubble in the schlieren photo above. The schlieren optical technique is sensitive to changes in the refractive index and, since a fluid’s refractive index changes with density, permits the visualization of shock waves. A strong curved bow shock is visible in front of the bullet as well as weaker lines marking additional shocks waves around the bullet. Impressively, the bullet’s passage is so fast (and the photo’s timing so perfect) that there are no imperfections or signs of bursting in the soap bubble. The photo’s caption suggests that the bubble may be filled with multiple gases. If they are unmixed and of differing densities, this may be the source of the speckling and plume-like structures inside the bubble. Incidentally, if anyone out there has high-speed schlieren video of a bullet passing through a soap bubble, I would love to see it. (Photo credit: H. Edgerton and K. Vandiver)