Comment on wes-2025-276

Abstract. The actuator line method (ALM) is a popular method for representing wind turbine blades in computational fluid simulations. It utilizes a model to compute aerodynamic forces that requires an undisturbed flow velocity relative to the blade. However, the input velocity and output forces are sampled and applied locally; within the bound vortex that forms around the blade, and is thus disturbed. This inherent conflict translates to a strong dependency of the outcome of the simulation, and its convergence, on the sampling approach. This work presents a quantitative description of the bound vortex, and thereby the disturbance on the sampled velocity, in time dependent simulations (e.g. large eddy simulations, LES). The properties of the bound vortex were found to be a function of the single parameter Λ = Δs/ε; the relative distance travelled by an ALM node within one time step, divided by the ALM kernel size. We use simulation data to validate this description and demonstrate that convergence of simulations can largely be attributed to the disturbance from sampling within the bound vortex. Consequently, this attribution to an erroneous process implies that convergence of the ALM is only partly related to the degree of representation of the relevant physical phenomena, which convergence is often assumed to imply. We propose that one must either ensure the disturbance is adequately accounted for, or that an alternative convergence criterion is to be used. We further use the description of the bound vortex to explain that a certain class of velocity sampling methods can indeed successfully account for the disturbance; removing the error and bringing simulations closer to convergence, yet that a correction based on the present description of the bound vortex would be more computationally efficient.