Influence of Reynolds number on the natural transition of boundary layers over underwater axisymmetric bodies

Abstract

The influence of the Reynolds number on the natural transition of boundary layers over underwater axisymmetric bodies is studied using numerical approaches. This is a fundamental problem in fluid mechanics, and is of great significance in practical engineering problems. The transition locations are predicted over diameter Reynolds numbers ranging from 1.79×10⁵ to 2.32×10⁸ for eight different forebody shapes. The transition onsets are predicted using the semi-empirical eN method based on the linear stability theory (LST), and the wall pressure fluctuation spectra are estimated. The effects of the forebody shapes and the Reynolds numbers on the transition location are studied. At the same Reynolds number, the forebody shape has great influence on transition. As the Reynolds number increases, the changes in the dimensionless transition location are qualitatively similar for the different forebody shapes. The dimensionless transition location shifts closer to the leading edge as the Reynolds number increases, and is more sensitive at lower Reynolds numbers. However, the quantitative changes in transition location for different forebody shapes are distinctly different. Consequently, the sequential order of the transition locations for the eight forebody shapes is not fixed, but changes dramatically with increasing Reynolds number. This irregularity in the sequential order of the transition locations is called the "Reynolds number effect." Finally, the fundamental causes of this effect are analyzed.