Highly Accurate Experimental Heave Decay Tests with a Floating Sphere: A Public Benchmark Dataset for Model Validation of Fluid–Structure Interaction

Morten Bech Kramer,Jacob Andersen,Thanh Toan Tran,Sarah Thomas,Flemming Buus Bendixen,Carl-Erik Janson,Claes Eskilsson,Nikolaj Holk,Robert Read,Kim Nielsen,Harry Bingham,Csaba Horvath,Scott Brown,Edward Ransley,Josh Davidson,Yi-Hsiang Yu

doi:10.3390/en14020269

Abstract

Highly accurate and precise heave decay tests on a sphere with a diameter of 300 mm were completed in a meticulously designed test setup in the wave basin in the Ocean and Coastal Engineering Laboratory at Aalborg University, Denmark. The tests were dedicated to providing a rigorous benchmark dataset for numerical model validation. The sphere was ballasted to half submergence, thereby floating with the waterline at the equator when at rest in calm water. Heave decay tests were conducted, wherein the sphere was held stationary and dropped from three drop heights: a small drop height, which can be considered a linear case, a moderately nonlinear case, and a highly nonlinear case with a drop height from a position where the whole sphere was initially above the water. The precision of the heave decay time series was calculated from random and systematic standard uncertainties. At a 95% confidence level, uncertainties were found to be very low—on average only about 0.3% of the respective drop heights. Physical parameters of the test setup and associated uncertainties were quantified. A test case was formulated that closely represents the physical tests, enabling the reader to do his/her own numerical tests. The paper includes a comparison of the physical test results to the results from several independent numerical models based on linear potential flow, fully nonlinear potential flow, and the Reynolds-averaged Navier–Stokes (RANS) equations. A high correlation between physical and numerical test results is shown. The physical test results are very suitable for numerical model validation and are public as a benchmark dataset.

Highlights

The uncertainty of the optical motion capture system increased with larger velocities of the test specimen, and for the largest drop height the uncertainty was less than
Strong correlations were found between the physical test results and the results from independent numerical modelling blind tests for linear potential flow (LPF), fully potential nonlinearflow potential flow (FNPF), and Reynolds-averaged Navier–Stokes (RANS) models, ranged with increasing fidelities
RANS models produce heave decay time series with deviations of 0–4 mm at troughs and crests for the highest drop height, which correspond to 0–3% of the drop height

Summary

Introduction

Numerical models with complex fluid–structure interactions are often developed to simulate motions of floating bodies influid–structure the ocean, which can be applied to assess the perNumerical models with complex interactions are often developed to formances of wave energy devices; [1,2].Despite theapplied complexity of such simulate motions of floating bodies insee, thee.g., ocean, which can be to assess the models, perforthe discretization and assumptions needed formulate numerical model mathematmances of wave energy devices; see, e.g.,[1,2].toDespite the the complexity of such models, the ically inevitably introduce errors, for many of which the influences are unknown.Engidiscretization and assumptions needed to formulate the numerical model mathematically neers mayintroduce struggle errors, to identify whether linearthe wave theory are canunknown.be appliedEngineers with sufficient inevitably for many of which influences may accuracy more whether advanced computational dynamics (CFD)methods shouldorbe struggle to or identify linear wave theoryfluid can be applied with sufficient accuracy used.advancedPhysical tests of high accuracy and reproducibility are paramount forused.validation and more computational fluid dynamics (CFD) methods should be Physical calibration using such advanced methods;for see,validation e.g., [3,4].and calibration tests of highpurposes accuracy when and reproducibility are paramount Thewhen International. Numerical models with complex fluid–structure interactions are often developed to simulate motions of floating bodies influid–structure the ocean, which can be applied to assess the perNumerical models with complex interactions are often developed to formances of wave energy devices; [1,2]. Despite theapplied complexity of such simulate motions of floating bodies insee, thee.g., ocean, which can be to assess the models, perforthe discretization and assumptions needed formulate numerical model mathematmances of wave energy devices; see, e.g.,. Be appliedEngineers with sufficient inevitably for many of which influences may accuracy more whether advanced computational dynamics (CFD). Validation and more computational fluid dynamics (CFD) methods should be Physical calibration using such advanced methods;for see,validation e.g., [3,4].and calibration tests of highpurposes accuracy when and reproducibility are paramount Thewhen International

Methods

Results

Discussion

Conclusion