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The Beauty of Fluid Mechanics In Video and Photos

Posted by ryuzaki0 on from the pretty-things-to-look-at dept.

An anonymous reader writes "F/A-18 Hornet jet fighters just having some fun — and making science interesting at the same time: Video #1, #2, #3, and a photo gallery. The formal name for the cool 'vapor cone' is Prandtl-Glauert Condensation Cloud, which is due the Prandtl-Glauert singularity."

4 of 31 comments (clear)

  1. Re:How does it follow the jet? by jbeaupre · · Score: 3, Informative

    Quick answer is the condensation isn't following the jet. It occurs in air at the edge of the shockwave. As the shockwave passes, the air returns to the same state.

    --
    The world is made by those who show up for the job.
  2. Re:How does it follow the jet? by EnderGT · · Score: 3, Informative

    The first link about prandtl-glauert clouds explains it better than I can, but the summary answer is that the shock wave actually follows the cloud, not precedes it, and it is the shock wave that rapidly reheats the air.

  3. Re:How does it follow the jet? by pclminion · · Score: 1, Informative

    The shock wave doesn't have anything to do with it. The effect occurs because at speeds close to the speed of sound (both above AND below) there is an extreme amplification of the temperature gradient in the air around the wing. At subsonic speeds there is no shock wave, and yet these clouds form at subsonic as well as supersonic speeds. So it cannot be that the shock wave causes the evaporation of the cloud.

    The cloud evaporates simply because when the disturbance passes, the air returns to its equilibrium state, the temperature rises back up, and the water evaporates.

  4. Re:How does it follow the jet? by EnderGT · · Score: 3, Informative
    You are correct that the vehicle does not have to be travelling at supersonic speeds to cause the cloud to form. You are also correct that it is the return of the air to its previous pressure and temperature that causes the cloud to evaporate. You are incorrect in that the shock wave has nothing to do with it - the shock wave greatly accelerates this state change, giving us the clean sharp termination of the cloud.

    If you read the page, you will see that even at subsonic speeds variations in the surface can result in transonic flow, which in turn results in a terminating shock wave.

    I don't want to quote the entire discussion here, but the page clearly states that subsonic clouds may be amplified by the prandtl-glauert singularity, but will not be cleanly terminated (see this image for reference). The sharply defined cone (which is, I believe, what the original question was about) only appears when transonic flow is in play. In this case, it is the shock wave that causes the clean termination of the cloud.