A technique is described which yields an accurate measurement of the temperature of density maximum of fluids which exhibit such anomalous behaviour. The method relies on the detection of changes in ...convective flow in a rectangular cavity containing the test fluid. The normal single-cell convection which occurs in the presence of a horizontal temperature gradient changes to a double cell configuration in the vicinity of the density maximum, and this transition manifests itself in changes in the horizontal temperature profile across the cavity. The method is applied to a variety of water solutions.
Free convection in a vertical cylinder of water is studied in the vicinity of the density maximum at 4 °C. Results are presented both from cooling curve measurements (using a series of thermistors ...along the central axis of the cylinder) and from flow visualization experiments (using particle image velocimetry), and a comparison is made with simulations. The reversal of the overall flow direction as the temperature crosses the maximum density region gives rise to a plateau feature in the cooling curve, which may be explained by the formation of a rising toroidal structure along the column boundary. This feature effectively diverts flow away from the inner region of the cylinder, resulting in a temporary cessation of convective cooling within this region.
A device is described through which heat flows at different rates depending on the orientation of the temperature gradient. The device consists of two cubic enclosures side by side, one of which is ...filled with pure water with a temperature of maximum density of 4
°C, and the other which contains a saline solution with a temperature of maximum density of 2
°C. A temperature gradient, which spans both of these temperatures of maximum density, is applied horizontally across the composite system, resulting in different rates of heat transfer through the device depending on the gradient direction. Experiments performed with a 12
×
6×
6
cm container yield heat transfer rates of 0.55
W and 0.19
W depending on the direction of the temperature gradient, resulting in a rectification factor of 65.4%. Asymmetrical heat transfer rates are also found in composite systems of water and solids when the temperature gradient spans the temperature of maximum density of the water. Results from computational fluid dynamics confirm the experimental results, and are used to investigate the influence of such parameters as temperature gradient and container aspect ratio on the rectification factor.
Mrk 421 was observed for about 2 days with BeppoSAX in 1998 April as part of a worldwide multiwavelength campaign. A large, well-defined flare was observed in X-rays. The same flare was observed ...simultaneously at TeV energies by the Whipple Observatory gamma-ray telescope. These data provide (1) the first evidence that the X-ray and TeV intensities are well correlated on timescales of hours and (2) the first exactly simultaneous X-ray and TeV spectra. The results imply that the X-ray and TeV photons derive from the same region and from the same population of relativistic electrons. The physical parameters deduced from a homogeneous synchrotron self-Compton model for the spectral energy distribution yield electron cooling times close to the observed variability timescales.
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