1. Ventilation Fan Aeroelastic Analysis
Rotating Fan Stability Analysis.
(Courtesy of FläktWoods AB, Sweden)
LINFLOW 1.4

2. Ventilation Fan Modeling in LINFLOW
Rotating Fan Stability Analysis.
Twin blade fan model:
Rotating at 987 rpm
Inner radius m
Outer radius 1.02 m
LINFLOW 1.4

3. Steady Flow Analysis of Rotating Fan
Absolute Velocity Contours.
LINFLOW 1.4

4. Structural Dynamic Analysis Rotating Fan
Eigenmode table
Structural model included fan blades + hub.
Stability analysis was performed using the 8-10 first structural modes with the lowest natural frequencies.
Structural model used
LINFLOW 1.4

5. Ventilation Fan Aeroelastic Stability Analysis
Aeroelastic Eigenfrequency Diagram
Aeroelastic Damping Diagram
One mode showed increasing damping
requirements with increasing load, Which?
LINFLOW 1.4

6. Ventilation Fan Stability Analysis Evaluation
Unstable Mode Shape
LINFLOW predicted an unstable 3.rd mode with a frequency of Hz
Measurements at full load later show that a maximum amplitude of vibration existed at 389 Hz
LINFLOW 1.4

7. Ventilation Fan Experimental Response Evaluation
Measurements at full load show that a maximum amplitude of vibration existed at 389 Hz 2 1 8 3
Measured frequency response diagram.
LINFLOW 1.4

8. Ventilation Fan Unstable Mode Animation
(Click on Picture for Animation)
LINFLOW 1.4

9. Aeroelastic Instability Alleviation
Can aeroelastic instabilities be eliminated or is it possible to raise the critical velocity? Yes,… … eliminate the source of elastic, inertial, and/or aerodynamic coupling.
How?
“Mass balance”-- add or redistributed mass to move the c.g. forward.
Modify the vibration characteristics
Increase spread between frequencies
Avoid frequency rations of one or integer multiples
Increase structural damping
Eliminate sources of fluid-dynamic “forcing” such as vortex shedding or turbulence from wakes etc.
LINFLOW 1.4

10. New Improved Ventilation Fan Geometry
Aeroelastic improvements of the fan design
Damping requirements for all aeroelastic modes
now drop with increasing loading
LINFLOW 1.4

11. New Ventilation Fan, Experimental Evaluation
Measurements at full load on the aeroelasticly improved design show more then one order of magnitude lower stress levels on the fan surface.
Measured frequency response diagram.
(Return)
LINFLOW 1.4