On the Digital Twin of The Ocean Cleanup Systems—Part I: Calibration of the Drag Coefficients of a Netted Screen in OrcaFlex Using CFD and Full-Scale Experiments

Abstract

The Ocean Cleanup introduces a Digital Twin (DT) describing the cleanup systems made of netting to extract marine litter from our oceans. It consists of two wings forming a “U-shape” and a retention zone. During operation, the system is towed and drag-driven with a span-to-length ratio of 0.6 ≤SR*≤ 0.8. The twine Reynolds number is Ret*∈[800:1600], making it experience various local drag coefficients. The DT was built with OrcaFlex (OF) aiming at: (i) avoiding over- or under-designing the system; (ii) supporting the scale-up of the system; and (iii) estimating the costs and/or the impact of our offshore operations. Therefore, we present an attempt to build an accurate DT using data from the Great Pacific Garbage Patch (GPGP). We developed a three-cycle validation: (i) initial guess applying Naumov’s semi-empirical drag coefficient to define the OF drag coefficients without the influence of the angles of attack θ of the wings; (ii) adjustment of the OF drag coefficients using AquaSim (AS) with its twine-by-twine drag correlation for various θ; (iii) re-adjustment of the OF drag coefficients from two-dimensional CFD simulations using Direct Numerical Simulation (DNS) for a twine-by-twine establishment of a drag correlation on a 1 m plane net, highlighting the shielding effects for θ<24°. Consequently, an initial underestimation of −3% in the combined towline tension, for a nominal span (SR*=0.6), was corrected to a slight overestimation of +7% compared to the GPGP data. For a wide span (SR*=0.8), the deviation remained between +1% and +15% throughout the validation process. For a narrow span (SR*∼ 0.02), mostly exhibiting low θ, the first cycle showed a +276% deviation, whereas at the end of the third cycle, it showed a +43% deviation.