Hydrodynamic Optimization of Riverine Current Energy Harvest Turbine and Non-inertial Effect on the Dynamic of Isotropic Turbulence

<p dir="ltr">Riverine energy has become an important source of sustainable and affordable electrical energy for off-grid families. A modular river current energy converter (MRCEC) is designed to harvest riverine energy stably with high efficiency. To maximize the energy production, the understanding of the turbulent flow under rotation conditions is essential.</p><p dir="ltr">In part I, the performance of a hydrodynamic cycloidal turbine is evaluated and optimized. 3D RANS simulation with sliding mesh and k-omega turbulent model is applied. The power coefficient is introduced to evaluate the turbine performance. The optimization consists of the following aspects: (1) an advanced hydrofoil, PU-20, is developed to maintain a high lift coefficient at a high angle of attack; (2) optimized operating conditions include a tip-speed ratio of 2.25, turbine dimensions of 2 m diameter and 4 m span length, solidity of 3 blades with c/D=0.175; (3) an advanced method to determine the pitch motion mechanism by combining the CFD results of the velocity field and analysis based on lift and drag on the hydrofoil; (4) a 3-section design to smooth the output of the turbine; (5) an augmentation duct to increase the flow velocity passing through the turbine and the output power by 12%. A final power coefficient of 0.515 is achieved with all optimizations applied, which is close to the Betz limit. Finally, an on-site performance evaluation is conducted to estimate the annual output of the turbine with river data and river simulation with the DELFT3D package, indicating that an annual power output of 111,630 kWh is achievable.</p><p dir="ltr">In part II, the rotational effect on isotropic turbulence is studied experimentally in terms of energy transfer across different ranges of length scales. The data are obtained using Tomographic Particle Image Velocimetry (Tomo-PIV) in a water box, in which nearly zero mean velocity isotropic turbulence is generated by eight propellers at the corners, capturing the 50 mm by 50 mm by 10 mm volume at the center and resolving the 3-dimensional 3-component velocity measurements. The data are processed to investigate the degree of turbulence, distribution of velocity fluctuation, autocorrelation function, energy spectra, Kolmogorov scale, Taylor microscales, and integral scales, showing close to isotropic turbulence at the center region of the box. By installing the Tomo-PIV system and isotropic box on a well-designed turntable, the off-axis rotation effect on the isotropic turbulence is quantified, showing anisotropy between radial and rotation axis directions. The turbulent Rossby number is defined to quantify the ratio of rotation over the turbulent intensity. The anisotropy of energy transfer is found in the inertial subrange, corresponding to the Taylor microscales. The range of scale affected and the degree of anisotropy are positively related to the turbulent Rossby number. Analysis of the NS equation in the rotation coordinate is performed to show the turbulent kinetic energy transfer between radial and tangential directions, which is partially consistent with the experimental results.</p><p dir="ltr">In part III, a further experimental study was conducted to quantify the off-axis rotational effect on the isotropic turbulence with a focus in r-theta plane, using high-resolution high-speed PIV in the same isotropic turbulent water box as in part II. The turbulence dynamics are near-zero-mean isotropic under non-rotation conditions by examining the unity of the degree of anisotropy and the velocity spectra following the Kolmogorov power law. As the stronger rotational effect is applied, quantifying by the increase of the inverse turbulent Rossby number, stronger anisotropy between the two directions is found in the first-order statistics. Also, the velocity spectra in the two directions increasingly diverge within a wider range of length scales. The lower bound of this anisotropic length scale range follows a linear relationship to the turbulent Rossby number, demonstrating the Coriolis effect plays a more significant role in a larger range of length scales. A generalized Taylor augmentation is applied to the spatial and temporal spectra to determine characteristic convection velocities, quantifying the convection of the coherent structures faster than the velocity fluctuations with linear scales to the flow velocity fluctuation. Directional turbulent rotation numbers are defined to link the integral-scale dynamics to small-scale dynamics with linear relationship to the turbulent Rossby number, serving as a starting point for the modeling of the anisotropy of turbulence under rotation conditions. The linear relation between the normalized Coriolis transport and turbulent Rossby number also establishes a basis for future modeling of the energy redistribution between radial and tangential directions under rotation effects.</p><p dir="ltr"><br></p>