FYFD

Tag: supersonic

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    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)

  • Rocket Engine Test

    [original media no longer available]

    In this static test of XCOR Aerospace’s Lynx rocket engine, Mach diamonds (shown at the top of the frame) are visible in the rocket exhaust. The distinctive pattern is a result of the over- or under-expansion of the exhaust jet with respect to the ambient air; in other words, the gases exiting the rocket are either too high or too low in pressure relative to the surrounding air. A series of shock waves and expansion fans forms in the exhaust jet until the pressure is equalized to ambient. It is these compressions and expansions that form the diamond pattern. (Video credit: XCOR Aerospace)

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    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.

  • Seeing Shock Waves

    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.)

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    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)

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    Supersonic Stellar Jets

    Astronomers studying stellar jets–massive outflows of gases and particles pouring from the poles of newborn stars–are finding reasons to turn to fluid dynamicists to understand the timelapse videos they’ve stitched together from multiple exposures from the Hubble telescope. Usually astronomical events unfold on such a slow timescale that our only view of them is as a snapshot frozen in time.  Stellar jets can move relatively quickly, though, with portions of the jet flowing at supersonic speeds. Over the course of Hubble’s lifetime, these jets have been imaged multiple times, allowing astronomers to create movies that reveal swirling eddies and shock wave motion previously unseen. (submitted by sakalgirl)

  • How Shock Waves Form

    How Shock Waves Form

    Most people are familiar with the Doppler effect–in which the frequency of a wave changes depending on the motion of the observer relative to the wave source–from the shifting pitch of sirens as they pass.  But the effect is important for pressure waves in addition to acoustic waves. When an object moves through air, its motion disturbs the surrounding air via pressure waves, which travel at the speed of sound. If an object moves slower than the speed of sound (top right), then those pressure waves extend in front of the object, carrying information about the object and allowing the air to shift and move smoothly around it.

    If the object is moving at the speed of sound (bottom left), then it arrives at the same time as the pressure waves. In essence, the object is striking a stationary wall of air–this is what was meant by “breaking the sound barrier”. At Mach 1, the physics of the problem have fundamentally shifted. Now the only way for air to deflect to allow the object’s passing is by the sudden compression of a shock wave.

    Moving even faster than the speed of sound (bottom right) the pressure and sound waves created by the object’s motion stretch in a cone behind it. The cone, known as a Mach cone, is the shock wave that deflects air around the moving object. The result is that the object will actually pass an observer before the observer will hear it. This is because no information can travel forward of the Mach cone’s leading edge. That’s why the area outside of the Mach cone is sometimes called the Zone of Silence. When the Mach cone passes an observer, the shock wave will register as a boom, like when the space shuttle passes overhead while landing. (via fyeahchemistry)

  • Bow Shock over a Perforated Plate

    Bow Shock over a Perforated Plate

    This schlieren image shows a sphere traveling at Mach 3 over a perforated plate. The bow shock in front of the sphere is clearly visible, as is its reflection off the plate. The pressure caused by the bow shock produces a series of spherical acoustic waves below the plate. A tiny vortex ring moves downward from each hole, followed at the right by a secondary ring moving upward from the holes in the plate. (Photo credit: U.S. Army Ballistic Research Laboratory; reprinted in Van Dyke’s An Album of Fluid Motion)

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    Voyager Explores the Edge of the Solar System

    Though unconventional by our terrestrial concepts of fluids, the solar wind and its interaction with objects in and around our solar system can be considered a form of fluid dynamics. This NASA video discusses discoveries made by the Voyager spacecrafts as they leave our solar system and pass into interstellar space. The solar wind, a rarefied stream of charged particles, streams outward from the Sun at supersonic speeds. Eventually, the pressure from the interstellar medium surrounding the solar system is sufficient to slow the solar wind to subsonic speeds, causing a termination shock much like the hydraulic jump that forms in a kitchen sink when you turn the faucet on.

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    How Scramjets Work

    The scramjet–supersonic combustion ramjet–engine has been a holy grail of aerospace engineering for 50 years. It is an air-breathing engine with no moving parts capable of propelling crafts at hypersonic speeds beyond Mach 5. As indicated in the name, combustion in the scramjet occurs at supersonic speeds, where the heating due to air compression is sufficient to ignite fuel when injected into the engine. At present the record for the highest speed attained in scramjet flight is held by the NASA X-43A, which reached Mach 9.8 in 2004 after about 10 seconds of scramjet free-flight. The longest scramjet flight belongs to the Boeing X-51 Waverider with 140 seconds of burn time in a 2010 test flight. Few tests of these experimental hypersonic crafts have been completely successful; they represent the frontier of aerospace technology.