A distortion similitude method for scaled experiments of flexible marine risers undergoing CF VIV

The high aspect ratio of flexible risers has posed great challenges for scaled vortex-induced vibration (VIV) experiments using conventional complete-similarity methods, and available schemes utilizing distortion models generally lack a sound theoretical basis. This study has proposed a distortion similitude method for the scaled experiments of flexible risers undergoing cross-flow (CF) VIV. At first, the governing equation of CF VIV for a typical flexible riser is simplified by introducing key non-dimensional parameters. Then the similarity analysis on the simplified equations yields the constraints on the key non-dimensional parameters of the riser system. These constraints can serve as the scaling laws and thus the distortion similitude method is developed. Then CF VIVs of a flexible riser are simulated employing two distortion models built by the new method and satisfying predictions are witnessed. Furthermore, a distortion model is built and scaled experiments are conducted in a water channel. The predictions (e.g. frequencies, strains) agree well with the simulations of the prototype. The developed method can be provided as a reference for the design of scaled riser models for CF VIV experiments. Moreover, the similarity study can be simplified to a parameter analysis on the key non-dimensional variables of the riser system.