National Physical Laboratory

About: National Physical Laboratory is a facility organization based out in London, United Kingdom. It is known for research contribution in the topics: Dielectric & Thin film. The organization has 7615 authors who have published 13327 publications receiving 319381 citations.

Distortion of sonic bangs by atmospheric turbulence

Abstract: Recorded pressure signatures of supersonic aircraft often show intense, spiky perturbations superimposed on a basic N -shaped pattern. A first-order scattering theory, incorporating both inertial and thermal interactions, is developed to explain the spikes. Scattering from a weak shock is studied first. The solution of the scattering equation is derived as a sum of three terms: a phase shift corresponding to the singularity found by Lighthill; a small local compression or rarefaction; a surface integral over a paraboloid of dependence, whose focus is the observation point and whose directrix is the shock. The solution is found to degenerate at the shock into the result given by ray acoustics, and the surface integral is identified with the scattered waves that make up the spikes. The solution is generalized for arbitrary wave-forms by means of a superposition integral. Eddies in the Kolmogorov inertial subrange are found to be the main source of spikes, and Kolmogorov's similarity theory is used to show that, for almost all times t after a sonic-bang shock passes an observation point, the mean-square pressure perturbation equals $(\Delta p)^2 (t_c/t)^{\frac{7}{6}}$ , where Δ p is the pressure jump across the shock and t c is a critical time predicted in terms of meteorological conditions. For an idealized model of the atmospheric boundary layer, t c is calculated to be about 1 ms, a figure consistent with the qualitative data currently available. The mean-square pressure perturbation just behind the shock itself is found to be finite but enormous, according to first-order scattering theory. It is conjectured that a second-order theory might explain the shock thickening that actually occurs.

Density of water in SI units over the range 0-40°C

Abstract: A table of water densities over the range 0-40°C, expressed in SI units (kg m-3, i.e. 10-3 g cm-3), is presented. The table has been obtained by applying the relationship 1 litre (1901) = 1.000 028 cm3 to new values derived from an improved combination of the observations of Chappuis and Thiesen on the dilatation of water, no modern determinations being, or likely soon to be, available. The table relates, as is usual, to air-free water, but the effect of air, which is small, is indicated.