EXPERIMENTAL INVESTIGATION OF TURBULENCE NEAR A LARGE-SCALE VORTEX

The influence of a strong large scale vortex on neighbouring turbulenc e is investigated. For this purpose the statistical properties of the tangential velocity fluctuations are systematically investigated as a function of the distance to the vortex core. Far from the vortex, the characteristics usually found in isotropic homogeneous turbulence are approximately obtained. In the central region of the vortex a strong d ecrease of the amplitude of the velocity fluctuations and of the dissi pation is observed. However in this region of space the Bow is not smo oth: the energy spectrum E(k) still exhibits the equivalent of an iner tial range bur with a slope which can range from k(-1.25) to k(-1) ins tead of the classical k(-5/3). Furthermore the intermittency is much w eaker inside the vortex central region: the velocity differences PDFs no longer exhibit long stretched tails and are more self-similar from scale to scale. In between these two regions, at the periphery of the vortex, the intermittency is strongly increased. It is shown that this increase is due to the mixing in the velocity signal of the two types of statistics (the strongly fluctuating bulk turbulence and the weakl y fluctuating vortex flow). This situation is analysed using a variant of extended self similarity that we call LSS (loose self similarity) which lends itself to the analysis of situations where a clear scaling is not obvious. (C) Elsevier, Paris.