Convective amplification of real simple sources

Abstract

This paper examines the problem of a pulsating compact body moving with constant velocity. This problem is usually incorrectly treated as a convected monopole. The analysis here shows that the motion of such a ‘real’ source introduces additional coupled multipoles, whose combined effects generate previously unexpected convective features. The amplification obtained is not the monopole convective amplification (1 —Mr)−2. It is found to depend on the virtual mass tensor of the body, the minimum effect being (1 —Mr)−3. There is also amplification in the direction perpendicular to the flight path (unless the motion is parallel to a principal axis of the virtual mass tensor).The field produced by oscillation of a convected compact body of constant geometry is also investigated. Again, this problem is often misrepresented as a moving dipole. Here it is shown that the effect of convection on such a real source is surprising and complicated. It cannot be described completely by Doppler factors, and there is amplification in the direction perpendicular to the source motion.These two model problems serve as a warning that the effect of flight on real sources cannot be anticipated until such real sources are correctly modelled, and that also the influence of source motion is likely to be much greater than has so far been anticipated.