Dynamic analysis of wind turbines subjected to Ice loads Sudhakar Gantasala PhD student

How the dynamic behavior of wind turbine blades change due to ice ? March 14, 2017 Luleå University of Technology How the dynamic behavior of wind turbine blades change due to ice ? Depends on operating conditions Varies from turbine to turbine Modifies aerofoil shapes of the blade Ice shapes and mass Increases mass density Reduces lift and increases drag forces Introduces asymmetry in the blade assembly Structural and aerodynamic changes in the blade due to ice Power reduces Aerodynamic loads decrease Structural loads increase Natural frequencies reduce Aeroelastic damping factors decrease Influence of icing on power, loads and vibrations of the turbine

Ice shapes from references March 14, 2017 Luleå University of Technology Ice shapes from references f A x=0 x=l Parameters: 1. Span ( l) 2. Sine curve frequencies (f) 3. Exp.coefficient (k) Different ice shapes are recreated using superposition of different sine curves.

Leading edge ice modeling March 14, 2017 Luleå University of Technology Leading edge ice modeling # # Beaugendre et al, “Development of a second generation in-flight icing simulation code”, Journal of Fluids Engineering, 2006, 128(2), pp. 378-387

Influence of icing on power March 14, 2017 Luleå University of Technology Influence of icing on power

CFD simulations of iced aerofoil March 14, 2017 Luleå University of Technology CFD simulations of iced aerofoil NACA 64618

Luleå University of Technology March 14, 2017 Luleå University of Technology Influence of icing on the modal behavior of blades Clean aerofoil case

Aeroelastic damping with icing March 14, 2017 Luleå University of Technology Aeroelastic damping with icing

Luleå University of Technology March 14, 2017 Luleå University of Technology Influence of icing on blade natural frequencies ## Ice accumulation on wind turbine blades reduces its natural frequencies and each frequency reduce differently based on the location and quantity of ice mass This behavior of natural frequencies can be used to detect and monitor ice growth on the wind turbine blades ## Brenner, D. Determination of the actual ice mass on wind turbine blades Measurements and methods for avoiding excessive icing loads threads, WinterWind, Åre, February 9, 2016.

Experimental modal analysis with added masses Natural frequency (Hz) March 14, 2017 Luleå University of Technology Experimental modal analysis with added masses Vibration mode Natural frequency (Hz) Case 1 Case 2 Case 3 Case 4 1st Flap 20.0 20.0 (-0.00%) 19.7 (-1.50%) 18.4 (-8.00%) 2nd Flap 125.9 124.7 (-0.95%) 118.4 (-5.96%) 122.8 (-2.46%) 3rd Flap 348.8 334.7 (-4.04%) 341.6 (-2.06%) 342.2 (-1.89%) Each frequency reduce differently with the location of added mass.

Neural network for inverse problem March 14, 2017 Luleå University of Technology Neural network for inverse problem f1 f2 f3 m1 m2 m3 An Artificial Neural Network model is trained with a dataset of natural frequencies of the structure calculated with different added masses at different locations

Luleå University of Technology March 14, 2017 Luleå University of Technology Identification of added mass on a cantilever beam in experiments A cantilever beam is divided into three zones and an added mass of zero or some mass value in each zone creates 7 possible cases Cases 1-7 in the below figure considers 0 or 11 g added mass on the cantilever beam Cases 8-14 in the below figure considers 0 or 27 g added mass on the cantilever beam Natural frequencies of the beam in these cases are estimated from experiments and given as an input to the trained neural network model Added masses identified in 14 test cases using natural frequencies estimated from experiments

Luleå University of Technology March 14, 2017 Luleå University of Technology Application of the technique to detect ice mass on a wind turbine blade Ice accumulation on a wind turbine blade can be approximated in terms of three masses that are distributed with constant linear mass density along the blade length Wind turbine blade natural frequencies are calculated using its FEM model considering different values of ice masses in the three different zones defined on the blade An Artificial Neural Network (ANN) model is trained with the dataset of natural frequencies and added masses ANN approximates the nonlinear relation between ice masses and blade natural frequencies Once the network is trained, it can be used to detect ice masses for any given set of natural frequencies of the iced blade The detection technique requires first few natural frequencies of the blade as an input and these can be estimated from the blade’s vibration measurements

Artificial Neural Network model for ice mass detection March 14, 2017 Luleå University of Technology Artificial Neural Network model for ice mass detection An Artificial Neural Network model is trained with a data set of blade natural frequencies calculated using its FEM model considering different ice masses in the three zones defined on the blade Trained neural network model is tested with the blade natural frequencies corresponding to four test cases shown in the below figure and the identified ice masses are compared in below table with the actual ice masses used in the calculations Note: WAPE means weighted average percentage error defined as the ratio of sum of the absolute error in the identified masses to the sum of actual ice masses Table The proposed detection technique identifies ice masses roughly

Luleå University of Technology March 14, 2017 Luleå University of Technology Future plan To analyze modal behavior, ambient vibration response, loads acting on the NREL 5 MW wind turbine blade using the aerodynamic behavior of iced aerofoils simulated using CFD Building a small experimental wind turbine set-up to demonstrate the proposed ice detection technique

Luleå University of Technology March 14, 2017 Luleå University of Technology Thank You