Ventilation Fan Aeroelastic Analysis

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Ventilation Fan Aeroelastic Analysis

• Rotating Fan Stability Analysis.

(Courtesy of FläktWoods AB, Sweden)
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Ventilation Fan Modeling in LINFLOW

• Rotating Fan Stability Analysis.

Twin blade fan model Rotating at 987 rpm Inner
radius 0.68 m Outer radius 1.02 m
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Steady Flow Analysis of Rotating Fan
Steady Flow Absolute Velocity Contours.
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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
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Ventilation Fan Aeroelastic Stability Analysis
Aeroelastic Eigenfrequency Diagram
Aeroelastic Damping Diagram
One mode showed increasing damping requirements
with increasing load, Which?
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Ventilation Fan Stability Analysis Evaluation
Unstable Mode Shape
LINFLOW predicted an unstable 3.rd mode with a
frequency of 382.6 Hz Measurements at full load
later show that a maximum amplitude of vibration
existed at 389 Hz
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Ventilation Fan Experimental Response Evaluation
Measurements at full load show that a maximum
amplitude of vibration existed at 389 Hz
Measured frequency response diagram.
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Ventilation Fan Unstable Mode Animation
(Click on Picture for Animation)
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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.

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