Statistical characteristics of flow in the wake region of a vertical bluff cylinder in waves, currents and combined wave-current flows

Abstract

While the flow kinematics on a cylinder in the presence of combined waves and currents are well documented in literature, the unsteady free stream in the wake region of a bluff circular cylinder under wave and current conditions have yet to be fully understood. The complex kinematic phenomenon that occurs in the wake region can be a combination of vortex shedding, flow reversal and streaming. The kinematics in the wake region is important as many offshore installations today are made up of two or more bluff bodies arranged in various tandem configurations, and the spacing between these members may be such that the downstream member is directly in the unsteady free stream wake region. The forces on these downstream members can be very different to that of the upstream member due to the unsteady free stream, and calculations of the total loading on the structure may not be well estimated using the commonly used Morison's equation. Examples can also be extended to floating structures where two structures may often be placed in close vicinity to each other, one being subjected to the wake loading of the other. There is therefore motivation to elucidate the characteristics of the unsteady kinematics in the wake of the first upstream cylinder in combined wave - current conditions. The objective of the presented study is to investigate experimentally, the characteristics of flow in the wake of an upright bluff cylinder in wave only conditions; currents only conditions; and in combined wave-current conditions. The focus is to elucidate the statistics and spectral content of measured locations of the wake to determine if the actual combined wave current kinematics in the wake region may be represented by the superposition of currents only and wave only kinematics. Different wave periods and a range of current speeds were tested to evaluate the superposition hypothesis for the relative effects of wave to current contributions. The measured kinematics were utilized to predict the motions of an elastically mounted cylinder in the near wake of another cylinder. These experiments were conducted in a 0.9m x 0.9m x 39m wave flume in the Hydraulics Laboratory of the National University of Singapore. An upright fixed cylinder was towed at constant velocity to study the effects of current on the cylinder, while a wave maker generated the oscillatory flow. The ranges of parameters investigated were; wake location to cylinder diameter ratio (L/D) of 0.75 to 2.50, Wave current parameter (H/UcT) of 0.08 to 0.70. The statistical characteristics and the spectral content at each measured location were derived from the velocity time series. The hypothesis of the statistical equivalence of combined wave-current wake kinematics using the superposition of currents only and wave only wake kinematics are discussed in this paper. Copyright © 2010 by ASME.