FYFD

Tag: shock reflection

  • Noisy Jets

    Noisy Jets

    One major problem that has plagued supersonic aircraft is their noise. The Concorde – thus far the only supersonic commercial airliner – was plagued with noise complaints that ultimately restricted its usability. Noise reduction is a major area of inquiry in aerospace, and the video below shows one experiment trying to understand the connections between supersonic flow and noise.

    Above you see a supersonic, Mach 1.5 microjet emanating from a nozzle at the top of the image. The jet is hitting a flat plate at the bottom of the image. Just beyond nozzle’s exit, you can see the X-shape of shock waves inside the jet. The position of that X is oscillating up and down.

    In the background, you can see horizontal light and dark lines traveling up and down. Those horizontal lines in the background are acoustic waves. When they hit the bottom plate, they reflect and travel upward until they hit another surface (outside the picture) and reflect back down. As they travel, they interact with the jet, causing those X-shaped shock waves to move up and down. This coupling between flow and acoustic waves makes the jet much louder – up to 140 dB – than it would be otherwise.

    Researchers hope that unraveling the physics of simpler systems like this one will help them quiet more complicated aircraft. (Image and video credit: F. Zigunov et al.)

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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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    Rocket Engine Testing

    Rocket engine tests usually feature a distinct and steady pattern of Mach diamonds in their exhaust. This series of reflected shock waves and expansion fans forms as a result of the exhaust pressure of the rocket nozzle being lower or higher than ambient pressure. A rocket will be most efficient if its exhaust pressure matches the ambient pressure, but since atmospheric pressure decreases as the rocket gets higher, engines are usually designed with an optimal performance at one altitude.

  • Computational Shock Compression

    [original media no longer available]

    Computational modeling can help verify and visualize experimental results, as in this video of supersonic flow. Oak Ridge National Laboratory produced the work as part of a project using shock compression and turbines to capture carbon dioxide gas. Shock waves and velocity profiles are shown throughout the computational field, and velocity isosurfaces paint a telling portrait of the complicated flow pattern. Wired Science features other award-winning simulation videos, many of which also feature fluid dynamics. #

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    Shock Waves

    Flow visualization really can be considered a form of art. Though we fluid mechanicians are looking for physics, we’re quite aware of the beauty of what we study. The clips in this video mostly show transient shockwave behavior, including lots of shock reflection and even a few instabilities. It’s unclear what the speeds are, aside from faster than sound; the medium is air.

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    Mach Diamonds

    Joe asks:

    Why does this rocket have that repeating pattern in its exhaust? I’m amazed that it’s so stable for so far as distance from the nozzle.

    Excellent question! The diamond-shaped pattern seen in the rocket’s exhaust is actually a series of reflected shock waves and expansion fans. The rocket’s nozzle is designed to be efficient at high altitudes, which means that, at its nominal design altitude, the shape of the nozzle is such that the exhaust gases will be expanded to the same pressure as the ambient atmosphere. At sea level, the nozzle is overexpanded, meaning that the exhaust gases have been expanded to a lower pressure than the ambient. The supersonic exhaust has to reach ambient pressure, and it does so through an oblique shock right at the exit of the nozzle. However, the oblique shock, in addition to raising the pressure, turns the gases toward the exhaust centerline. To ensure flow symmetry, two additional oblique shocks form. But then the exhaust is at a higher pressure than ambient. Expansion fans form to reduce the pressure, but those, too, affect the direction the exhaust gases flow. The pattern, then, is a series of progressively weaker oblique shocks and expansion fans that raise the exhaust gas pressure to that of the ambient atmosphere.