Influence of turbulence length scale and platform surge motion on wake dynamics in tandem floating wind turbines

Wake interaction is a key factor limiting the performance of floating offshore wind turbine (FOWT) arrays, yet the combined influence of inflow turbulence structure and platform motion on wake dynamics remains poorly understood. This study examines how the integral length scale of inflow turbulence and platform surge motion shapes wake development and power performance in a tandem configuration of two aligned FOWTs separated by five rotor diameters. High-fidelity computational fluid dynamics simulations are performed using OpenFOAM, based on large-eddy simulation with an actuator-line model and the wall-adapting local eddy-viscosity subgrid-scale closure. Synthetic turbulent inflows are generated using the divergence-free synthetic eddy method, with prescribed integral length scales spanning 0.25–1.25 times the rotor radius. Over this range, increasing the integral length scale naturally leads to higher freestream turbulence intensity, which increases from approximately 1.9%–7.2% at the first turbine location. The corresponding dominant inflow frequencies are extracted from time-resolved velocity signals, yielding Strouhal numbers in the range St≈0.71–0.12. Larger turbulence scales introduce energetic, low-frequency structures that directly interact with the wake, promote earlier destabilization of tip vortices, enhance entrainment, and accelerate wake mixing. As a result, the inter-turbine velocity deficit is significantly reduced, leading to substantial increases in the downstream turbine power up to 140% relative to uniform inflow conditions. Platform surge motion further enhances wake unsteadiness and mixing, contributing to additional power recovery; however, its effect remains secondary compared to the influence of turbulence scales. The relative phase between turbines shows only a minor impact on the mean wake behavior.