Handbook of Turbomachinery Second Edition Revised - Ventech!

To satisfy these requirements, it is desirable to utilize the geometric
modeling capabilities of a computer-aided design (CAD) system, if possible.
These systems offer the ability to efficiently model complex shapes, using
representations such as Beziers and NURBS (Non-Uniform Rational B-
Splines) to precisely define surfaces. Use of such surface-description devices
allows for a relatively automated scheme for the description and
modification of component geometry. CAD tools also permit easy access
by different user groups via the concept of neutral files, typically written in a
standard IGES format. This facilitates the concept of a master model, used
by both design and manufacturing functional organizations within a
concurrent engineering environment.
Grid Generation. Once the geometric model of the turbomachinery
component has been established, the next step in the process of
communicating this configuration to the CFD analysis program is to
define the computational grid within the physical domain.
The boundaries of this region are typically defined by the flow-path
surfaces (end walls, blades, etc.) and by the periodic boundaries between
blade passages, where appropriate. Inlet and exit boundaries are established
at points upstream and downstream, where the necessary flow conditions
are assumed to be known. Within this region, a three-dimensional
computational grid is applied, such that the governing equations will be
solved at every point on the grid, or within every cell formed by the grid.
The grid imposed on the physical domain must conform to the boundaries
of that domain and must provide adequate resolution in all areas of the flow
field to permit accurate prediction of the flow behavior.
Computational grids may be classified by their type of construction. A
structured grid has a fixed number of points in each of three directions, and
the grid lines are arranged according to some scheme that is related to the
geometric configuration of the physical domain. Common types of
structured grids include H-grids, C-grids, and O-grids. In contrast,
unstructured grids are not oriented in predetermined directions, nor do
they have a fixed number of points in any particular direction. Unstructured
grids typically utilize tetrahedral elements, or volumes, as opposed to the
hexahedral elements of structured grids. Because the unstructured grid is not
restricted in its formation, it is well suited to complex geometric regions,
where structured grids would be difficult to construct.
Grid generation for structured grids may be performed using either
algebraic methods or differential equation methods. In the algebraic
approach, grid coordinates are interpolated between boundary surfaces
using algebraic functions. In contrast, the differential equation method uses
partial differential equations, such as Poisson’s equation, to determine the
Copyright © 2003 Marcel Dekker, Inc.