TRANSIENT, OSCILLATORY AND STEADY CHARACTERISTICS OF DEAN VORTEX PAIRS IN A CURVED RECTANGULAR CHANNEL INTRODUCTION Phillip M. Ligrani Department of Mechanical Engineering, Code MElLi Naval Postgraduate School Monterey, California 93943-5000 When Dean numbers are high enough, the flow in a curved channel is unstable to centrifugal instabilities, and secondary flows develop which eventually form into pairs of counter-rotating vortices. This flow is referred to as Dean vortex flow, and the accompanying pairs of streamwise- oriented vortices are referred to as Dean vortices. The present paper describes some transient, oscillatory, and steady characteristics of Dean vortex pairs in a curved rectangular channel with 40 to 1 aspect ratio and an inner to outer radius ratio of 0.979. In particular, attention is focussed on secondary instabilities observed in the form of oscillatory motions called twisting and undulating, and in the form of transient events consisting of splitting and merging of vortex pairs. Splitting and merging events are described from visualizations of flow in spanwiselradial planes at Dean numbers of 75 and 100 in the form of two distinctly different types of splitting events, and four distinctly different types of merging events. Even though these transient events produce fairly large amplitudes of unsteadiness, the Dean vortex pairs have preferred positions across the span of the channel about which the motion occurs. These preferred positions are evident from time-averaged distributions of streamwise velocity, which show a variety of steady vortex pair characteristics. EXPERIMENTAL APPARATUS AND PROCEDURES The present experimental results are obtained using the same 40 to 1 aspect ratio curved channel described by Ligrani and Niver (1988). A schematic drawing of the curved channel is shown in Figure 1. The facility is an open-circuit suction facility designed for low-speed transition studies. At the inlet, a honeycomb, screens and nozzle reduce spatial non- uniformities in the flow. These are followed by a 2.44 m long straight duct which provides fully developed laminar flow at the inlet of the curved section for Dean numbers up to 640 (Ligrani and Niver, 1988). The radii of the concentric convex and concave walls in the curved section are 596.9 mm and 609.6 mm, respectively. The 180 degree curved section is then followed by another 2.44 m long straight duct, additional flow management devices, and an outlet plenum. From the inlet of the first straight section to the outlet of the second straight section, the interior dimensions of the facility are 1.27 cm for the height and 50.80 cm for the width. The cylindrical coordinate system is aligned such that (r, x, z ) are the normal, streamwise, and spanwise directions, respectively. y is measured from the concave wall, radially inwards. In the discussion which follows, the Dean number is defined as De = 2 Re (d/ ri )1/2, where Reynolds number Ordered and Turbulent Patterns in Taylor-Couette Flow Edited by C.D. Andereck and F. Hayot, Plenum Press, New York, 1992 281