Offshore Technology Research Center

 

OTRC Project Summary

Project Title:

An Experimental Study of the Flow Structures and Hydrodynamic Forces of a Cylinder in a Reversing Flow

Prinicipal Investigators:

Richard Miksad

Sponsor:

National Science Foundation

Completion Date:

August, 1995

Final Report ID#

B74(Click to view final report abstract)

The study of forces induced by fluid flow on structural members is a vast topic of particular importance for submerged bodies.  Of current interest is the study of the hydrodynamic forces generated by an unsteady turbulent ocean environment on compliant offshore structures.

The understanding of fundamental flow/structure interaction mechanisms is key to efficient design of submerged structures.  The problem of flow/structure interaction is so prevalent, though, that it concerns many engineering branches.  These branches deal in general with the study of the following problems:

  • Buffeting – a problem of vibration induced on an elastic body due to turbulence in the incident flow.
  • Flutter – a primary problem of aero-hydroelasticity defined as a dynamic instability of an elastic body in a fluid stream as it deflects from its undeformed state under flow forces.
  • Galloping – referred to large amplitude vibration of lightweight, flexible structure exposed to a flow.
  • Vortex-induced loading – a problem of structure vibration caused by vortex shedding.

The present study deals with a few fundamental aspects of vortex-induced loading.  In particular, the relationship of hydrodynamic forces to the nearfield vortex flow structures around a stationary rigid circular cylinder immersed in a planar reversing water flow is investigated experimentally.  The focal point of this project is placed on the association between transverse force (lift) wave forms and the corresponding time-varying nearfield flow structures.

In addition, to improve our understanding of unsteady flows abut bluff bodies, this study is aimed at providing quantitative information on fluid velocity and lift force – with emphasis on the latter – that might assist current efforts dedicated to the development of new or improvement of existing force models for offshore engineering applications.

The objectives of this research can be itemized as follows:

  • Improve our understanding of the hydrodynamic lift force induced by a sinusoidal planar flow on a circular cylinder.
  • Map out the flowfield structures around a cylinder in association with the instantaneously occurring lift and inline forces for the predominant modes of vortex shedding.
  • Develop a lift force periodic model-equation using sound concepts of Fluid Mechanics and test it with experimental data.

This research approach is anchored to a conditional sampling technique that is aimed at distinguishing intermittent flow modes around the cylinder.  The technique uses information in fluctuating lift force to distinguish patterns of flow that might occur symmetrically with respect to the oscillation direction over an array of cycles.  After classifying flow cycles according to whether they refer to an upwash or downwash flow mode, one has to access to velocity/force data sets which belong to a common flow mode.  After mode sampling flow/force measurements, data reduction and analysis follow conventional procedures.  Specifically, fluid velocity acquired not simultaneously across the flowfield are mode-averaged to yield maps of time-varying structures of the flow, and forces on the cylinder are correlated to mode-average fluid properties.  Besides the quest for periodic data out of an intermittent flow regime, comprehensive time histories of data are processed and studied as well.

 

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