Estimating Time-averaged Turbulent Fluid
Forces From An Ensemble-averaged Flow
Field For Engineering Applications
Author(s)
T. H. Yip, Q. D. Zhang, E. H. Ong & C. K. Tan
Abstract
Solid structures are subjected to mechanical forces imposed upon them by a
moving fluid. If the flow is turbulent, the mechanical forces would fluctuate
randomly with time. For engineering applications, the statistical approach is
normally preferred whenever turbulent flows are encountered. As a consequence,
the measured or numerically computed flow fields are ensemble-averaged. The
objective of this investigation is to develop an engineering methodology to
estimate these mechanical forces from the ensemble-averaged flow field. Special
attention would be paid to those forces that contribute to mechanical vibrations
of the solid structures. In our study of airflow induced vibrations on the working
parts of computer hard disk drives, the turbulent flow fields of an actual 2.5-inch
computer hard drive had been measured in-situ. From the ensemble-averaged
flow field, various types of mechanical forces like pressure, shear (drag) forces
and vortex (lift) forces could be obtained. By neglecting the constant forces, we
focused our attention on the ensemble averaged fluctuating velocity-vorticity
interactions. The consequence of the interaction is the production of
ensemble-averaged fluctuating vortex forces that act on the actuator. The
fluctuating components of vortex forces can be calculated from the gradients of
Reynolds shear stresses measured directly from the flow field experimentally.
The magnitudes of fluctuating vortex forces increases dramatically with the disk
rotational speed, which matches the behaviour of positional error signals (PES)
measured in-situ from an actual operating hard disk drive. The corresponding
increase in PES and fluctuating vortex forces with disk rotational speed
suggested that flow induced vibration in the hard disk drive may be due to an
increase in flow turbulence in the hard disk drive. Some suggestion on flow
turbulence identification and attenuation are discussed.
Keywords: 10,000 rpm, hard disk drive, Reynolds shear stress, flow induced
vibration.
Keywords
10,000 rpm, hard disk drive, Reynolds shear stress, flow induced
vibration.
Related Book
Other papers in this volume
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