Session: 16-01-01: Poster Session: NSF-Funded Research (Grad & Undergrad)

Paper Number: 99418

99418 - Spectral Behavior of Horizontal-Axis Tidal Turbine in Elevated Levels of Homogeneous Anisotropic Turbulent Inflow 

Tidal turbines are subjected to highly turbulent inflow conditions, directly impacting turbine performance and fatigue loading. The inflow conditions are more severe than their equivalents, such as wind energy; therefore, understanding the conversion dynamics from velocity fluctuations to turbine power and thrust fluctuations is crucial for optimal design and control. The current study examines the spectral behavior of power and thrust fluctuations of a 1:20 model scale horizontal axis tidal turbine; subjected to various anisotropic inflow conditions generated in a water tunnel using an active grid turbulence generator. Five inflow conditions are tested for turbine spectral response: a quasi-laminar (QL) with turbulence intensity (Ti) ~1.5%; double random DR1 and DR025 having Ti (16-19%), and an integral length scale (L) of ~0.2D and ~0.8D (D: turbine diameter) respectively; synchronous SY1 and SY025 having Ti and L similar to DR1 but dominant scales at the corresponding grid operating frequencies (1Hz and 0.25Hz). We perform simultaneous upstream velocity (1D from rotor plan) and turbine measurements for different tip-speed ratios (λ).

             The elevated turbulent inflow cases (DR and SY) were found to increase the standard deviation of rotor torque by 4.2 times that of the QL cases, with maximum deviations for DR025 and SY025 at the highest λ studied. We identify three distinct regions: low, intermediate, and high-frequency regimes for turbine power and thrust spectra scaling, similar to findings reported for wind turbines. For the QL case, the power spectra follow  slope dictated by the rotor dynamics while thrust scales as   in the intermediate frequencies, and both follow inflow velocity spectra ( in the low-frequency region. Similarly, the power and thrust spectra follow the inflow in lower frequencies for the elevated turbulent inflow cases, while the corresponding inertial subrange has  and  slope, respectively, for λ>4. However, we find that DR025 and DR1 power spectra cases also show  slope in the lower frequencies of the inertial subrange. For λ<4, power spectra for DR cases scale only as  for the inertial subrange until the turbine frequency () interrupts, and the plateau-like region forms, extending the rotor's low-pass filtering to higher frequencies. This transition in the scaling of power spectra is likely due to the turbine's response to its inertial timescale (; where I is a moment of inertia,  angular velocity, and T is the mean torque). The thrust spectra continue to follow  slope though a similar plateau-like shape is observed for λ<4. Additionally, for synchronous forcing SY1 and SY025, the turbine picks up the inflow peaks (grid frequency) and their harmonics highlighting the strong scale-to-scale interaction between the rotor and inflow fluctuations. We observe that both spectra are strongly impacted by the increase in turbine rotational speed, particularly in the high-frequency region. The larger integral length scale cases (DR025 and SY025) shift the bifurcation between low and intermediate regions to lower frequencies, whereas the bifurcation between intermediate and high frequencies (called a cutoff frequency) is not affected by the different inflows. As a result, tidal turbine power and thrust fluctuations are strongly dictated by the inflow turbulence intensity, tip-speed ratios, and rotor dynamics, all contributing in a nonlinear fashion.

 

Funding Acknowledgment: The authors acknowledge funding support from NSF CBET Fluid Dynamics Program, Award # 170635

Presenting Author: Mohd Hanzla Lehigh University

Presenting Author Biography: I am a 2nd year Ph.D. student at Lehigh University Mechanical Engineering and Mechanics Department with CGPA 4.0/4.0. My masters is in Aerospace Engineering from Indian Institute of Technology, Kanpur India, and prior joining Lehigh I had a 3.5 years of work experience in Oil refinery (Indian Oil Corporation Limited, IOCL, India). Currently, I am working on the performance and loading characteristics of a 1:20 scaled horizontal axis tidal turbine. We use a lab-scale water tunnel equipped with a Makita-style active grid to generate inflow conditions which can mimic actual tidal site conditions. The turbine is subjected to such inflows to analyze blade loading along its span and understanding the conversion dynamics from inflow fluctuations to turbine power and thrust fluctuations.

Authors:

Mohd Hanzla Lehigh University
Arindam Banerjee Lehigh University