Handbook of Turbomachinery Second Edition Revised - Ventech!

streamwise diffusion terms are considered negligible. This assumption is
valid for thin viscous layers, but is not appropriate for flows that contain
substantial recirculation zones, or other significant viscous structures that
would produce measurable streamwise diffusion.
Other levels of approximation offer even simpler representations of the
governing equations; however, such flow models are not general enough to
encompass many of the flows encountered in high-performance turbomachinery
components. Although the use of approximations to the governing
equations offers benefits in terms of equation simplicity and, therefore,
solution speed, the design tool that employs such a simplified model
sacrifices generality. Often, the benefits of having a more general CFD
analysis tool will far outweigh the benefits of a faster tool that is only
appropriate over a subset of the expected flow conditions.
The solution of the governing equations is performed over a finite
space, on which a computational grid has been imposed. In order to
properly model the flow physics within this space, a set of appropriate
boundary conditions must be specified for the governing equations. Three
different types of spatial boundaries may be identified for typical
turbomachinery components: (1) wall boundaries, (2) inlet and exit
boundaries, and (3) periodic boundaries.
Wall boundaries consist of blade surfaces, passage end walls, or the
solid surfaces of other objects in the flow path. These boundaries may be
rotating, nonrotating, or a combination of both. For solid walls, a zerorelative-velocity
or no-slip condition is appropriate. Thermal boundary
conditions must also be considered, through the specification of a wall
temperature or a normal temperature gradient.
The most natural form of the inlet and exit boundary conditions is used
to set the mass flow rate, by specifying both total pressure and temperature,
along with flow angle or tangential velocity, upstream, and static pressure at
the downstream boundary. Depending on the type of turbulence model
selected, distributions of turbulent kinetic energy or some similar property at
the inlet would also be required. The inlet and exit boundaries should be
placed far enough away from the blade row that they are not influenced by
its presence. Typically, this is achieved by positioning the boundaries
anywhere from 50% to 100% of the blade chord upstream and downstream
of the blade row. The distribution of inlet conditions may include spanwise
variations, or possibly both spanwise and tangential variations, if the
tangential distribution has the same period as the pitch of the solution space.
Modeling of nonperiodic inlet conditions, as for inlet distortion, would
essentially require that the solution space extend over the entire annulus.
Periodic boundaries upstream and downstream of the blade row are
used to model the repetitive nature of the flow conditions from one blade
Copyright © 2003 Marcel Dekker, Inc.