Unsteady force measurements in sphere flow from subcritical to supercritical Reynolds numbers |
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Authors: | A?K?Norman Email author" target="_blank">B?J?McKeonEmail author |
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Institution: | (1) Graduate Aerospace Laboratories, California Institute of Technology, Pasadena, CA 91125, USA; |
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Abstract: | The flow over a smooth sphere is examined in the Reynolds number range of 5.0 × 104 < Re < 5.0 × 105 via measurements of the fluctuating forces and particle image velocimetry measurements in a planar cut of the velocity field.
Comprehensive studies of the statistics and spectra of the forces are presented for a range of subcritical and supercritical
Reynolds numbers. While the subcritical lateral force spectra are dominated by activity corresponding to the large-scale vortex
shedding frequency at a Strouhal number of approximately 0.18, there is no such peak apparent in the supercritical spectra,
although resolution effects may become important in this region. Nor does the large-scale vortex shedding appear to have a
significant effect on the drag force fluctuations at either sub- or super-critical Reynolds numbers. A simple double spring
model is shown to capture the main features of the lateral force spectra. The low-frequency force fluctuations observed in
earlier computational studies are shown to have important implications for statistical convergence, and in particular, the
apparent mean side force observed in earlier studies. At least one thousand dimensionless time units are required for reasonable
estimates of the second and higher moments below the critical Reynolds number and even more for supercritical flow, stringent
conditions for computational studies. Lastly, investigation of the relationship between the motion of the instantaneous wake
shape, defined via the local position where the streamwise velocity is equal to half the freestream value, and the in-plane
lateral force for subcritical flow reveals a significant negative correlation throughout the near wake, which is shown to
be related to a structure inferred to arise from the large-scale vortex shedding convecting downstream at 61% of the freestream
velocity. In addition to its utility in understanding basic sphere flow, the apparatus is also a testbed that will be used
in future studies, examining the effect of both static and dynamic changes to the surface morphology. |
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