Axial ventilation by use of axial fans was the most common cooling concept for 
hydropower generators in Sweden during the 50-70’s when the main part of the 
Swedish hydropower plants where built. It is still used for new produced 
machines especially for high speed machines where the small diameter limits the 
choice of ventilation concept. Axial fans are also common after renovation of 
old machines when the rotating parts are reused. The design of the fans which is 
used in the main part of hydropower generators are made by bended plates which 
are welded on a fan ring or segments.

The working principle of an axial fan blade is very similar as an airplane wing 
profile where a high pressure side occurs below the fan blade and a low pressure 
side occurs above the fan blade. Vortex separations may occur at the low 
pressure side which under some circumstances can results in vibration problem of 
the fan blades (stalling). Such effects are most likely more severe under some 
conditions that will more frequently be passed due to an increase in regulation 
of the electric grid. Cracks and broken fan blades due to fatigue can be a 
consequence of the vibration which may cause serious damages of the machine due 
the fact that the fans are located close to the rotating poles and the stator 
winding.

In order to prevent this kind of problems a frequency analysis of the fan blades 
is carried out during the design phase. It is very important that the flow-
induced frequencies of the forces acting on the fan blades, due to e.g. vortex 
separation, do not coincide with the eigenfrequency of the blades. That requires 
a detailed understanding of the flow around fan blades with designs particularly 
used in axially cooled hydropower generators.

Both numerical and experimental studies will be conducted in the present 
project. CFD with highly resolved Large-Eddy Simulation (LES), or possibly 
Detached Eddy Simulations (DES), turbulence modeling is the most suitable 
technique for detailed studies of vortex separation and other unsteady 
behaviour. The numerical results will primarily be validated with experimental 
results in our linear cascade rig, for which the present post-doc position will 
design appropriate experiments and analyse the results. The actual measurements 
will be conducted in close collaboration with our experimental specialists. The 
possible impact of rotation should be evaluated numerically, and possibly used 
to design complementary experiments. For the structural analysis employed in the 
electric generator design stage it is important to determine the time-averaged 
and unsteady pressure distributions and the frequency distribution over the fan 
blades. The experimental and numerical results will complement each other in the 
final analysis of those and other aspects. Fluid-structure-interaction analysis 
may be employed if it is considered necessary to get accurate results.

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