ISOSURFACE-BASED POPULATION AND SHAPE ANALYSIS OF 3D VORTICES

Vortex is the elementary structure in turbulence and is known as the 'sinews' of fluid motion. Vortex research is of great significance for natural exploration and engineering applications. The swirling strength method is used to extract vortices embedded inside the direct numerical simulation flow field, and the isosurface of 3D vortices was constructed at a certain isovalue. Based on the fast triangle-triangle intersection method, lines of intersection between the triangular mesh of the isosurface of 3D vortex structures and a triangular cutting plane were extracted, and among which vortices were identified as polygons by the property of closed cutting edge. Under the condition of different isovalues in the plane of the log layer and different wall distances from the cutting plane to the bed, variation trends of population and shape factors (circularity, radius, convexity and aspect ratio) of vortices in the cutting plane were studied. Results show that in the cutting plane of the log layer as the isovalue increases, the extracted vortices gradually become fewer, more circular, smaller, and rounder: the vortex population rapidly decreases in a logarithmic law; PDF curves of circularity and radius become taller and thinner, and the mean value of circularity rapidly increases to a stable stage while that of radius decreases monotonously; PDF curves of aspect ratio do not have significant change and its mean value remains essentially unchanged; PDF curves of convexity approximate the impulse function. As the wall distance increases vortices at certain swirling strength threshold continuously vanish while their shapes are relatively stable: vortex population in the outer region except the region near the water surface decreases in a logarithmic law; mean values of shape factors (circularity, aspect ratio, and convexity) first rapidly increase and then remain constant or increase slowly; mean value of radius rapidly decreases and then remains constant.