Designing a Whirling Arm for Water Droplet Erosion Testing Apparatus

Erosion on wind turbine blades due to falling water droplets cannot be quantitatively measured with modern methods of analysis. Initially, erosion caused by water droplets was ignored in wind turbine designs because the rotational speed did not reach the erosion threshold; with larger blades rotating at faster speeds, water droplet erosion generates concern in lowering the wind turbine’s overall efficiency, so the need for quantitative data is prevalent. In order to obtain this data, an experimental facility is being developed at the University of Tulsa. This facility allows for investigation into the phenomenon of water droplet erosion by allowing multiple parameters to be tested. Thus, a model can be developed to predict the erosion resistance of wind turbine blade materials under different conditions. An important part of this facility is the whirling arm apparatus which simulates the movement of wind turbine blades and is capable of reaching tip velocities up to 100 m/s. This paper considers different aspects of designing and developing the whirling arm, including the durability, corrosion resistance, and properties of the material, dynamic movement of the arm, beam deflection, vibration and resonance of the beam, and force of the water droplets. It is especially important to study the effect of water droplet impacts on the natural vibrations occurring within the system in multiple planes. The rotational motion of the apparatus causes the arm to deflect proportional to the moment generated by the motor, and the water droplets apply additional force in the same plane as the deflection. To explore the mode shapes of the system, computer simulations are being used to determine the theoretical range of speeds for operation. This range will then be tested with shakers installed along the arm of the apparatus to determine the mode shapes experimentally from the theoretical predictions. Through exploring the vibrations of the system both when water droplets are exciting the system and when they are not, the operating speeds of the apparatus to avoid resonance will be determined. Resonance occurs when the natural frequency of the system is reached, and a large amplitude is produced, making the system unstable. Exploring the mode shapes of this system determines if counterweights are needed to oppose the weight of the arm and coupon. Though, the effect of the counterweights on the drag force and the possible reduction of the arm speed should be considered as well. These considerations ensure the designed apparatus accurately and safely measures erosion on wind turbine blade material induced by water droplets.