Linearly varying ambient flow past a sphere at finite reynolds number-part 1: wake structure and forces in steady straining flow

Bagchi, Prosenjit; Balachandar, S.

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https://hdl.handle.net/2142/112648 Copy

Description

Title

Linearly varying ambient flow past a sphere at finite reynolds number-part 1: wake structure and forces in steady straining flow

Author(s)

• Bagchi, Prosenjit
• Balachandar, S.

Issue Date

2000-04

Keyword(s)

• Reynolds Number
• Wake Structure
• Flow

Abstract

This study focuses on the effect of spatial non-uniformity in the ambient flow on the forces acting on a spherical particle at moderate particle Reynolds numbers. A scaling analysis is performed to obtain conditions under which such effects are important. A direct numerical simulation, based on spectral methods, is used to compute three-dimensional time-dependent flow past a stationary sphere subject to a uniform flow plus a planar straining flow . The particle Reynolds number, Re, in the range 10 to 300 covering different flow regimes, from unseparated flow to unsteady vortex shedding is considered. A variety of strain magnitudes and orientations are investigated. A systematic comparison with the potential flow results and axisymmetric strain results is given. Under elogational strain, both planar and axisymmetric cases are found to stabilize the sphere wake and delay onset of unsteadiness, while compressional strain leads to instability. In terms of separation angles, length of the recirculation eddy and topology of the surface streamlines, planar and axisymmetric strains yield nearly the same results. The drag force appears to have a linear relation with strain magnitude in both cases, as predicted by the potential flow . However, contrary to the potential flow results, the drag in planar strain is higher than that in axisymmetric strain. Generation of higher drag is explained using surface pressure and vorticity distribution. Planar strain oriented at an angle with the oncoming unform flow is observed to break the symmetry of the wake and results in a lift or side force. Variation of the drag and lift forces may be quite complex and unlike the potential flow results, they may not be monotonic with strain magnitude. The direction of the lift force may be opposite to that predicted by the inviscid and low Reynolds number (Re « 1) theories. This behavior is dictated by the presence or absence of a recirculation eddy. In the absence of a recirculation region at low Reynolds numbers (Re < 20), or, at a very high strain magnitude when the recirculation region is suppressed, results somewhat follow the pattern observed in potential flow. However, with the presence of a recirculation region, results opposite to those predicted by the potential theory are observed.

Publisher

Department of Theoretical and Applied Mechanics. College of Engineering. University of Illinois at Urbana-Champaign

Series/Report Name or Number

• TAM R 938
• 2000-6013

ISSN

0073-5264

Type of Resource

text

Language

eng

Permalink

http://hdl.handle.net/2142/112648

Sponsor(s)/Grant Number(s)

ASCI Center for Simulated Rockets; US Department of Energy

Copyright and License Information

Copyright 2000 Board of Trustees of the University of Illinois

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