Saturday, September 22, 2012

Tutorial: Solving Transonic Flow over a Turbine Blade with Turbo-Specific NRBCs

The standard pressure boundary conditions for compressible flow fix speci fic flow variables at the boundary (e.g., static pressure at an outlet boundary).
As a result, pressure waves incident on the boundary will reflect in an unphysical manner, leading to local errors. The effect are more pronounced for internal flow problems where boundaries are usually close to geometry inside the domain, such as compressor or turbine blade rows.

The turbo-specfiic non-reflecting boundary conditions (NRBCs) permit waves to pass through the boundaries without spurious reflections. The method used in FLUENT is based on the Fourier transformation of solution variables at the non-reflecting boundary.

This tutorial demonstrates how to do the following:
  • Set up and solve the turbine blade flow field using the standard pressure outlet boundary treatment -
  • Activate the turbo-specific NRBCs and solve the problem again
  • Compare the results for the standard and non-reflecting pressure boundaries

Problem Description
This tutorial considers the transonic flow around a turbine blade cascade with a shortened exit boundary. This con gfiuration is frequently encountered in stage analyses where the spacing between adjacent blade rows is small, and hence, the exit boundary of the upstream row must be placed very close to the trailing edge of the blade. Using the traditional pressure outlet boundary treatment can lead to spurious pressure distributions on the blade surface since the exit pressure is typically being assumed to be uniform in the blade-to-blade direction. NRBCs can eliminate this problem by permitting pressure waves to pass through the boundary without reflection, thereby leading to a more accurate solution.

Note: Non-reflecting boundary conditions can only be used with the density-based solver

No comments:

Post a Comment