FYFD

Month: February 2013

  • Reader Question: Frosty Cars

    Reader Question: Frosty Cars

    Reader Mike L asks:

    Why do I never see frost on my car when I park in a detached garage or under a carport?

    Great question! Frost forms on surfaces when their temperature drops below the freezing point of water and the dew point of the surrounding air. The water vapor in the air gets deposited as a solid directly; this is called deposition. This means that the surface–in this case your car–has to be colder than the nearby air. Neither conduction nor convection of heat between your car and the surrounding air can cause this drop; heat transfer between your car and the surrounding air would tend to make them the same temperature, not make the car colder than the air. The third–and typically least effective–type of heat transfer, radiation, is the answer because it allows heat transfer between two objects that are not in direct contact like the air and car are.

    Frost typically forms on still, clear nights with little clouds or wind. A car sitting beneath a clear night sky will radiate heat out into space. Since space is much, much colder than the air, this radiation cooling to space allows the car’s surface temperature to drop below that of the surrounding air, which is not a good radiator by comparison. On a night with little wind (and thus little convection), this radiation cooling can be quite effective. Frost will tend not to form on one’s car under a carport because the car is sheltered from the night sky, blocking such radiative cooling. Having a tree or house blocking the car from the night sky is also effective at preventing frost formation. (Photo credit: N. Sharp; with thanks to Keri B and Jerry N for the meteorological assistance)

  • Featured Video Play Icon

    Dropping Through Strata

    When a droplet falls through an air/water interface, a vortex ring can form and fall through the liquid. In this video, the researchers investigate the effects of a stratified fluid interface on this falling vortex ring. In this case, a less dense fluid sits atop a denser one. Depending on the density of the initial falling droplet and the distance it travels through the first fluid, the behavior and break-up of the vortex ring when it hits the denser fluid differs. Here four different behaviors are demonstrated, including bouncing and trapping of the vortex ring. (Video credit: R. Camassa et al.)

  • Mixing Physics

    Mixing Physics

    When a dense fluid sits above a lighter fluid in a gravitational field, the interface between the two fluids is unstable. It breaks down via a Rayleigh-Taylor instability, with mushroom-like protrusions of the lighter fluid into the heavier one. The image above comes from a numerical simulation of this effect well after the initial instability; the darker colors represent denser fluids and lighter colors are less dense fluids. The flow here has progressed to turbulence, and the authors of the simulation are exploring the statistical nature of this flow breakdown relative to the classical case of isotropic, homogeneous turbulence. (Photo credit: W. Cabot and Y. Zhou)