One of the primary sources of wind-turbine blade damage is erosion of the blade surface at the leading edge from the impact of particles such as sand, rail, or hail during operation (Shohag et al., 2017). High rotational speeds and a high impact count make the leading edge in the blade tip region the most susceptible to severe damage (Corsini et al., 2015). Besides posing structural concerns, leading-edge erosion notably increases profile drag coefficients as much as 200%, which results in annual energy production (AEP) losses of up to 12% for utility-scale wind turbines (Sareen et al., 2014).

To avoid these losses and protect the blades, leading-edge protection (LEP) tapes have proven to be a reliable and affordable solution. There are, however, aerodynamic losses associated with standard LEP tape application to rotor blades, where early transition of the boundary layer is observed at the backward-facing LEP tape edge. Depending on the rotor configuration and operating conditions, drag coefficient increases anywhere from 8% to 15%, resulting in up to a 2% AEP loss due to standard LEP tape application (Schramm et al., 2017).

The goal of this work is to investigate and reduce the aerodynamic impact of leading-edge protection (LEP) tapes on utility-scale wind turbine performance. The proposed novel LEP tape design is a tapered profile with maximum thickness in the middle for erosion protection and minimum thickness at the edge to suppress boundary-layer transition at this location. Two-dimensional CFD models are developed to estimate the effect of both standard and tapered LEP tape designs on lift and drag coefficients of an NACA 64-618 airfoil – a representative wind turbine tip-section airfoil. Numerical predictions of boundary-layer transition behavior are experimentally verified in a wind tunnel on a full-chord tip section of a utility-scale wind turbine blade. A wind turbine design and analysis code, XTurb-PSU (Schmitz, 2012), is used to predict the power output of a representative utility-scale 1.5 MW wind turbine to verify whether the proposed tapered LEP tape design reduces AEP losses compared to standard LEP tapes currently in production.

Numerical predictions of the flow field show distinct boundary-layer transition at the backward-facing step for standard LEP tapes due to a locally high adverse pressure gradient. By tapering the LEP tape, the adverse pressure gradient at the tape step is significantly reduced and boundary-layer transition occurs further downstream. Experiments verify numerical estimates that the change in drag coefficient compared to a clean model notably improves for tapered LEP tapes. Oil visualization of the flow over the full-chord tip section verifies numerical predictions that standard LEP tapes transition the boundary layer at the tape step. For tapered LEP tapes, where cleanly applied, oil visualization reveals that the boundary layer remains undisturbed by the presence of the LEP tape step.

By reducing the backward-facing step height of LEP tapes below the critical height at which boundary-layer transition occurs at the tape edge, numerical and experimental data indicate that the aerodynamic performance of an NACA 64-618 airfoil with a tapered LEP tape applied is comparable to that of the clean airfoil. Using CFD estimates of lift and drag coefficients as inputs for XTurb-PSU, there is negligible change in AEP when a tapered LEP tape is applied to the outboard 40% of the rotor blades of a utility-scale 1.5 MW wind turbine. Preliminary numerical and experimental results indicate tapered LEP tapes can be designed to suppress boundary-layer transition for backward-facing step heights below a critical value, such that the associated impact on wind turbine aerodynamics and AEP is minimal. This is an important finding for future LEP tape applications and wind farm production.