This study presents the scale effect induced by Reynolds number ( \({R}_{e}\) ) on hydrodynamic performance and flow characteristics of a horizontal-axis tidal turbine (HATT) based on both blade element momentum theory (BEMT) and computational fluid dynamics (CFD) techniques. Three different \({R}_{e}\) -based model scales are examined under rotational operating conditions corresponding to cut-in speed, peak power, and experimental highest curve region. The BEMT method evaluates the scale effect from blade spanwise refinement of axial and tangential forces. A systematic increase in the axial induction factor along the blade span reflects axial flow deceleration with rising \({R}_{e}\) . However, the tangential induction factor decreases signifying a reduction in wake swirl. The RANS-based CFD solver incorporating the Reynolds Stress Turbulence model enhances the scale effect investigation. Uncertainty quantification has been performed according to the International Towing Tank Conference recommended grid-independent procedures to verify the CFD model. The CFD simulation results have provided detailed scale effect insights into the scalar flow field around HATT, including velocity contours, pressure distribution on the blades, wake field and vortex visualization at various rotational speeds according to the intensity and development of turbulence, axial convection velocity fields, pressure gradients, and flow separation. The scale effect on cavitation inception at the suction side leading edge extends from tip to middle of the blade with increasing \({R}_{e}\) . The comparative analysis between the BEMT and CFD methods for non-dimensional performance coefficients reveals that the power and torque coefficients decrease with scaling, but the thrust coefficient has less deviation across all the scales. The scale correction factors conclude that a minimal scale effect persists while operating in the peak-power region compared to other operational speeds and the scale effect diminishes as the \({R}_{e}\) increases.
