Optimization and Computational Fluid Dynamics - Department of ...

148 René A. Van den Braembussche
Fig. 6.1 2D view of the 3D flow at the exit of a turbine stage
lyze the fluid flows in the same way Finite Element Analysis (FEA) is used
for stress predictions. They provide detailed information about the 3D flow
around existing blade shapes and constitute an attractive alternative for detailed
flow measurements. Complex flow phenomena can now be studied in
what is called “Numerical Laboratories”. Although this has resulted in a
drastic decrease of the number of prototype testing, there are still two problems
that prevent a more efficient use of CFD in the turbomachinery design
process.
The first one results from the difficulty to analyze 3D flows on 2D screens or
drawings. 2D vector plots are only a poor representation of the reality. They
can be very misleading as they may suggest that the flow is penetrating the
solid walls (Fig. 6.1). Synthetic environments, also called virtual reality, are
very promising in this respect. These techniques are not only applicable to
mere computer games but will become part of everyday reality for engineers
in the next decade [16]. Designers will walk inside blade rows and diffusers to
inspect the complex 3D flow structures by tracing the streamlines and to find
out what geometrical changes may be needed to improve the performance.
The second problem relates to the abundance of information provided by
the NS calculation. The output of an NS solver contains all the information
needed to improve the performance. However, it does not provide any information
on what modifications are needed to reach that goal. Three velocity
components, the pressure and the temperature in typically 10,000 points (2D
flows) or in more than 1,000,000 points (3D flows) are more than what the
human brain is able to grasp and fully exploit in new designs. Most of the
available information remains unused as the designer will often calculate a
global parameter to find out if one geometry performs better than another.