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Experimental investigation of the wave-induced motion of and force distribution along a flexible stem

Published online by Cambridge University Press:  18 October 2019

Niels G. Jacobsen Open the ORCID record for Niels G. Jacobsen [Opens in a new window] ,
Wout Bakker ,
Wim S. J. Uijttewaal  and
Rob Uittenbogaard
Niels G. Jacobsen*
Affiliation:
Harbour, Coastal and Offshore, Deltares, 2629HV Delft, The Netherlands
Wout Bakker
Affiliation:
Numerical Simulation Software, Deltares, 2629HV Delft, The Netherlands EcoFlows, 1015BL Amsterdam, The Netherlands
Wim S. J. Uijttewaal
Affiliation:
Environmental Fluid Mechanics, Hydraulic Engineering, Delft University of Technology, 2628CN Delft, The Netherlands
Rob Uittenbogaard
Affiliation:
Numerical Simulation Software, Deltares, 2629HV Delft, The Netherlands
*
†Email address for correspondence: [email protected]
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Abstract

The work presents an experimental investigation into the motion of and hydrodynamic forces along a single flexible stem in regular waves. The experiment covers a large range in relevant non-dimensional parameters: the drag-to-stiffness ratio $CaL\in [0.003,3.8]$, the inertia-to-stiffness ratio $CaL/KC\in [4\times 10^{-5},14.8]$, the Keulegan–Carpenter number $KC\in [3.8,145]$ and the Reynolds number $Re\in [230,2900]$. The two first parameters relate to the response of the stem in waves and thus account for material properties, while the two last parameters are relevant for hydrodynamic forces on the stem. The displacement of the stem was captured with a digital video camera and the displacement along the stem was captured for every 2.5 mm at 25 Hz. This unique laboratory data set allowed for the following analyses: (i) Determination of the relevant non-dimensional parameter to predict the stem motion and shape. (ii) A direct comparison between the measured force for mimics of two lengths (0.15 m and 0.30 m) illustrating the force reduction potential for flexible mimics. (iii) Direct evaluation of the average force coefficients $C_{D}$ (drag) and $C_{M}$ (inertia) for the flexible stems. (iv) The distributed external hydrodynamic loading and the internal shear forces were estimated from the laboratory experiments. The distribution of the shear force helped to understand the breakage mechanisms of flexible stems. (v) A linkage between phase lags and internal shear forces was suggested. The data set is considered valuable as validation material for numerical models of stem motion in waves.

JFM classification

Geophysical and Geological Flows: Coastal engineering
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 880 , 10 December 2019 , pp. 1036 - 1069
Copyright
© 2019 Cambridge University Press 

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Jacobsen et al. supplementary movie 1

Movie of the stem motion for mimic 2 with $H = 0.04$ m, $T = 2.0$ s and $l = 0.30$ m. The waves propagate from left to right. Top-left panel: Stem and relative velocities; 1.0 m corresponds to 1.0 m/s. Top-right panel: The distributed hydrodynamic force. Bottom-left panel: Stem and relative accelerations; 0.2 m corresponds to 1.0 m/s${}^2$. Bottom-right panel: The internal shear force.

Download Jacobsen et al. supplementary movie 1(Video)
Video 16.6 MB

Jacobsen et al. supplementary movie 2

Movie of the stem motion for mimic 2 with $H = 0.11$ m, $T = 3.0$ s and $l = 0.30$ m. The waves propagate from left to right. Top-left panel: Stem and relative velocities; 0.4 m corresponds to 1.0 m/s. Top-right panel: The distributed hydrodynamic force. Bottom-left panel: Stem and relative accelerations; 0.1 m corresponds to 1.0 m/s${}^2$. Bottom-right panel: The internal shear force.

Download Jacobsen et al. supplementary movie 2(Video)
Video 24.4 MB

Jacobsen et al. supplementary movie 3

Movie of the stem motion for mimic 3 with $H = 0.04$ m, $T = 2.0$ s and $l = 0.30$ m. The waves propagate from left to right. Top-left panel: Stem and relative velocities; 1.0 m corresponds to 1.0 m/s. Top-right panel: The distributed hydrodynamic force. Bottom-left panel: Stem and relative accelerations; 0.2 m corresponds to 1.0 m/s${}^2$. Bottom-right panel: The internal shear force.

Download Jacobsen et al. supplementary movie 3(Video)
Video 15.8 MB

Jacobsen et al. supplementary movie 4

Movie of the stem motion for mimic 3 with $H = 0.11$ m, $T = 3.0$ s and $l = 0.30$ m. The waves propagate from left to right. Top-left panel: Stem and relative velocities; 0.5 m corresponds to 1.0 m/s. Top-right panel: The distributed hydrodynamic force. Bottom-left panel: Stem and relative accelerations; 0.1 m corresponds to 1.0 m/s${}^2$. Bottom-right panel: The internal shear force.

Download Jacobsen et al. supplementary movie 4(Video)
Video 25.9 MB