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  • X-51A Scramjet Test Flight

    X-51A Scramjet Test Flight

    The X-51A Waverider hypersonic aircraft had its second test flight earlier this week. Unfortunately, its supersonic combustion ramjet (scramjet) engine failed to transition from its start-up fuel to its primary fuel. According to the US Air Force Research Laboratory:

    A US Air Force B-52H Stratofortress released the experimental vehicle from an altitude of approximately 50,000 feet. After release the X-51A was initially accelerated by a solid rocket booster to a speed just over Mach 5. The experimental aircraft’s air breathing scramjet engine lit on ethylene and attempted to transition to JP7 fuel operation when the vehicle experienced an inlet un-start. The hypersonic vehicle attempted to restart and oriented itself to optimize engine start conditions, but was unsuccessful. The vehicle continued in a controlled flight orientation until it flew into the ocean within the test range. #

    Un-starting is the term used when supersonic flow is lost in an engine or wind tunnel. If the pressure or temperature in the engine deviates too far from the ideal conditions, the upstream mass flow through the engine will be greater than the downstream mass flow and the engine will choke (video). A shock wave forms and travels upstream, leaving subsonic flow in its wake. Loss of supersonic flow inside the engine would likely also result in losing ignition of the fuel/air mixture, resulting in flameout. #

    If you haven’t guessed already, engineers like to make up words.

  • Jet Breakup

    Jet Breakup

    A non-cylindrical stream falling through a slit nozzle exhibits the Plateau-Rayleigh instability, which drives a falling jet of fluid to break into droplets due to surface tension. The fingers formed off the falling stream may be a form of Rayleigh-Taylor instability. #

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    Jets from Waves

    When vibrated, fluid surfaces can exhibit standing waves known as Faraday waves. In this experiment, increased forcing of these standing waves causes the formation of a jet. Under the right conditions, as the standing wave collapses, a singularity forms on the fluid surface when velocity and surface curvature diverge. The narrow jet column forms as a result of the fluid’s kinetic energy getting focused by the collapse. For more, see this letter to Nature. #

  • Instability in a Jet

    Instability in a Jet

    This photo shows the development of a flow instability in an axisymmetric jet. On the left, the jet is smooth and fully laminar, but, by the center of the photo, disturbances in the jet have grown large enough to distort the laminar profile. The jet is then in transition; by the right side of the frame, it has reached a turbulent state, as evidenced by the increased mixing (which causes the smoke to disperse more quickly) and intermittency of the flow. #

  • Jet-Based Control

    Jet-Based Control

    Researchers have flown the first aircraft designed to maneuver without conventional control surfaces like ailerons and flaps. Instead of changing the wing geometry to alter the lift on different parts of the craft, the UAV uses strategically placed jets of air along the wing to control its flight. The plane can also alter the direction of its thrust, not by turning the nozzle as is conventionally done, but by modifying the thrust vector by directing and firing a secondary jet into the exhaust. #

  • The Bouncing Jet

    The Bouncing Jet

    Under some circumstances, a thin stream of a Newtonian fluid impacting a deep pool of the same fluid can produce a bouncing jet. The effect is observed in common liquids like canola oil and can be replicated at home. Be sure to check the research page for a video of the effect. #

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    Blowing Droplets Apart

    In raindrops, ink-jet printing, and spray painting, droplets get broken apart by the air flowing around them. Here, researchers investigate how a jet of fluid can push a droplet from its perch, or–with enough momentum–shatter the droplet. There are some lovely flow visualizations here; it’s especially cool to see the starting vortex ring of the jet push and deform the droplet. (Video and image credit: S. Jain et al.)

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    The Subtleties of Pipe Flow

    If you put a thumb over your hose’s outlet, you get a faster jet, but how does that affect the flow rate? That’s the question Grady starts from in this Practical Engineering video exploring pipe flow physics. From continuity and control volumes to practical losses, he touches on both the theory and practice behind this all-important aspect of engineering. Pipe flow is often where engineering students begin their studies of fluids, but that doesn’t mean it’s easy! (Video and image credit: Practical Engineering)

  • Making Quieter Shock Waves

    Making Quieter Shock Waves

    NASA’s X-59 aircraft is intended to demonstrate supersonic flight without the boom. Although it’s broken into supersonic speeds, we haven’t yet heard its “sonic thump” because, so far, it’s been accompanied by conventional supersonic aircraft, which are louder.

    The idea behind the long, skinny X-59 is to create weaker, widely-spaced shocks along the aircraft body. Weaker shocks are easier for atmospheric effects to damp out before they reach the ground, and spacing them out makes it harder for them to “pile up” at the nose and tail to create the strong double shocks that merge into a sonic boom.

    Schlieren image of a model X-59 tested at supersonic conditions in a wind tunnel.
    Schlieren image of a model X-59 tested at supersonic conditions in a wind tunnel. The model is mounted upside-down. The three dark diagonal lines are shock waves originating from the wind tunnel and can be ignored. The fainter lines coming off parts of the aircraft model are the plane’s shock waves.

    NASA is preparing to test the X-59’s in-flight shocks soon, including with in-air schlieren photography like they’ve done in the past for other aircraft. But we’ve already have a glimpse of what to expect, thanks to wind tunnel testing, shown above. Ignore the three dark diagonal lines in the middle of the image (those are from the wind tunnel, not the model aircraft), and focus instead on the thin bright lines coming off the plane. Those are some impressively subtle shock waves!

    Hear more from the aircraft’s designer and test pilots in this Ars Technica article. (Image credit: NASA; see also: Ars Technica, M. Ahaus et al.)

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  • NASA Testing Supersonic Rotors for Mars

    NASA Testing Supersonic Rotors for Mars

    NASA’s Ingenuity helicopter was the first aircraft humanity has flown on another planet, and engineers are looking to make the next generation of Martian helicopters bigger and more capable. That’s challenging in Mars‘ thin atmosphere, which is only 1% as dense as Earth’s. To get adequate lift, the rotors need to spin faster there.

    During Ingenuity’s mission, the team intentionally designed the craft to keep the rotor tips below supersonic speeds. But for the next mission–SkyFall–they’re looking to push the rotorcraft further. In recent tests in a Mars simulator chamber, they successfully spun the new rotors to tip speeds as high as Mach 1.08, significantly increasing the loads SkyFall could carry. (Image and video credit: NASA/JPL-Caltech; via Ars Technica)

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