| 17 Feb 2026
Experimental investigation of harmonic surge motions on the far wake of a wind turbine model and analysis of a resulting subharmonic wake response
Abstract. As the number of floating offshore wind farm projects grows across the world, several questions remain unanswered in the domain of floating wind turbine wakes. Previous studies observed that the motions impact the wake under laminar inflow conditions, and that the modifications are observable even under high turbulent inflow conditions, that are more realistic. However, these spatiotemporal behavior modifications still require precise characterization, especially under turbulent conditions. Thus, the present article aims to extend the previous work by studying the wake dynamics induced by surge motion under turbulent inflow, and to find the frequency and the spatiotemporal characteristics related to this motion. For this purpose, a porous disk is placed in a wind tunnel with a high level of turbulent inflow, and some experiments are performed at 8.125D (D the turbine diameter) downwind of the disk. First, hot-wire measurements are used to identify a 'critical' frequency at which wake dynamics are the greatest, and, then, stereo-PIV measurements are conducted for this frequency configuration to characterize the spatiotemporal wake dynamics. The results indicate a critical Strouhal number of approximately St = 0.24, for which the surge motion induces periodic contraction and expansion of the wake at the same frequency as the imposed motion. A quasi-steady-state analysis reveals that these modifications are larger than the predictions of basic wake models, highlighting the dynamic nature of the wake response. Moreover, inclined surge-generated periodic coherent structures are observed during this work, which are supposed to be related to shear effects. Additionally, an unexpected subharmonic frequency peak appears in the far wake velocity spectra. POD and coherence analyses suggest that this is related to a far wake meandering phenomenon, locked onto this subharmonic frequency.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Wind Energy Science.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.
General comments
The manuscript addresses a relevant and still open research topic concerning the effects of surge motion of floating offshore wind turbines (FOWTs) on wake dynamics. The study is timely and contributes to the ongoing effort to better characterize wake behavior under realistic turbulent inflow conditions.
The literature review is generally well structured and provides an appropriate overview of previous studies. However, the discussion would benefit from a clearer differentiation of results obtained across different Strouhal number ranges and across different types of imposed motions (surge, pitch, heave). This would help to better highlight the specific contribution and novelty of the present work.
The experimental methodology is robust. In particular, the combined use of hot-wire measurements and stereo-PIV provides a solid and comprehensive dataset. The application of coherence analysis together with Proper Orthogonal Decomposition (POD) represents an original and valuable approach, offering meaningful insight into the underlying wake physics.
Finally, the manuscript should more explicitly discuss how the use of a porous disk instead of a rotating turbine model may influence the results and their applicability to real wind turbines. A clearer distinction between porous-disk-based studies and rotor-based studies is also recommended to improve the interpretation of the findings.
Specific comments
Technical corrections