Toward Defining Objective Criteria for Assessing the Adequacy of Assumed Axisymmetry and Steadiness of Flows in Rotating Cavities

[+] Author and Article Information
G. D. Snowsill, C. Young

 Rolls-Royce plc, P. O. Box 31, Derby, DE24 8BJ, UK

J. Turbomach 128(4), 708-716 (Feb 01, 2005) (9 pages) doi:10.1115/1.2185124 History: Received October 01, 2004; Revised February 01, 2005

The need to make a priori decisions about the level of approximation that can be accepted—and subsequently justified—in flows of industrial complexity is a perennial problem for computational fluid dynamics (CFD) analysts. This problem is particularly acute in the simulation of rotating cavity flows, where the stiffness of the equation set results in protracted convergence times, making any simplification extremely attractive. For example, it is common practice, in applications where the geometry and boundary conditions are axisymmetric, to assume that the flow solution will also be axisymmetric. It is known, however, that inappropriate imposition of this assumption can lead to significant errors. Similarly, where the geometry or boundary conditions exhibit cyclic symmetry, it is quite common for analysts to constrain the solutions to satisfy this symmetry through boundary condition definition. Examples of inappropriate use of these approximating assumptions are frequently encountered in rotating machinery applications, such as the ventilation of rotating cavities within aero-engines. Objective criteria are required to provide guidance regarding the level of approximation that is appropriate in such applications. In the present work, a study has been carried out into: (i) The extent to which local three-dimensional features influence solutions in a generally two-dimensional (2D) problem. Criteria are proposed to aid in decisions about when a 2D axisymmetric model is likely to deliver an acceptable solution; (ii) the influence of flow features which may have a cyclic symmetry that differs from the bounding geometry or imposed boundary conditions (or indeed have no cyclic symmetry); and (iii) the influence of unsteady flow features and the extent to which their effects can be represented by mixing plane or multiple reference frame approximations.

Copyright © 2006 by American Society of Mechanical Engineers
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Figure 1

CFD model of an HPT preswirl/receiver hole system: three-repeat sectors shown for clarity

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Figure 2

Schematic arrangement of burning candle in a cylindrical vessel

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Figure 3

Nonaxisymmetric and unsteady buoyant flow driven by a burning candle

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Figure 4

Stream function predictions of axisymmetric buoyancy-driven flow

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Figure 5

HP compressor rear cone model

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Figure 6

Comparison of 2D and 3D rotor wall temperatures

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Figure 7

Comparison of 2D and 3D stator wall temperatures

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Figure 8

Prediction of strong radial velocities in the 3D drive cone flow solution

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Figure 9

Simple rotor-stator cavity geometry

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Figure 10

Contours of stream function in the axisymmetric flow simulations

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Figure 11

Pathline predictions created using a 3D sector model of the simple cavity

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Figure 12

Secondary flow behavior in a square-sectioned duct

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Figure 13

Typical aero-engine drive cone cavity geometry



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