2.1. Instrumentation
The presented experimental investigation includes (a) velocity measurement with an acoustic Doppler velocimeter (ADV), (b) an array of wave gauges (WG) to capture the free surface elevation, as well as (c) the force and moment measurement conducted with the force plate (FP) integrated in the tank floor.
Figure 2 provides an overview of the experimental set-up of the instrumentation. In total, nine different WG were installed.
Table 1 presents the location of the nine WG in relation to the centre of the force plate with the subscript
as well as the circular tank (index
). The spacing between the WG1-5 are based on a Golomb ruler with the marks of (11,9,4,1,0) and a base length of 1 m. WG4 is located above the centre of the FP, and a further five WG are located along the
x direction. As shown in
Figure 2a, WG6 and WG7 are installed with an
offset on both sides of the WG4 using the local coordinate system with the origin at the FP. WG8 has the same
distance from the origin as WG4 but mirrored about the global tank
-axis. The WG are calibrated with five points over a range of ±0.1 m. This process ensured that the accuracy of the WG was less than 1 mm [
31,
32,
33].
The main instrument for this investigation was a six degree-of-freedom (6-DoF) AMTI OR6-7 (specification 1000) force plate [
34]. This instrument has a maximum capacity of 500 N in both horizontal directions, 1000 N for the vertical force measurement and applied moments of 10,000 Nm (
) and 5000 Nm (
). The manufacturer specifies the hysteresis as well as the non-linearity of the force plate to be
% of full-scale output and also specifies a cross-talk of less than 2% for all channels [
34]. It has a top surface that measures 0.464 m by 0.508 m and a total thickness of close to 0.09 m. The latter is not relevant for this investigation, as the FP was installed with a custom-made top plate (can be adapted to specific projects) to ensure the top-face of the FP was in the same plane as the floor.
Figure 3a shows the dry installation made available by the raisable central part of the tank floor. A small gap between the top plate of the FP and the surrounding floor ensures a good measurement of the forces acting on the plate. The FP is rotated by 22.5
to the
x-orientation of the tank, which is necessitated by the available divisions of the floor surface. This angle is included in the processing of the FP, and the reported values are based on the global tank coordinate system (
Figure 2). A cable connects the FP with the instrumentation on the gantry. It was ensured that this cable was outside of the main investigation area and either sideways or downstream of the waves (
Figure 3b). The force plate was fully submerged, and after allowing for temperature adaptation, all values were zeroed. Consequently, the measured values are changes from the hydrostatic pressure. A constant water depth of 2 m was maintained for all the tests.
The primary comparison is intended to be made between the WG and the FP. The current interacts with the waves by changing the wave kinematics, and, hence, knowledge of the current is required. The ADV installed was a Nortek Vectrino Profiler, which was operated in point measurement mode and provided the velocity vector in the main components based on the tank coordinate system. The manufacturer specifies that the accuracy of the instrument is 0.5% of the measured value ±1 mm/s [
35]. The instrument uses four beams, resulting in a redundant measurement, which provides two vertical velocity components
and
in addition to the two horizontal velocity measurements
and
which are parallel to the tank
x and
y axes, respectively. Both vertical velocity values should be identical, which enables quality control of the overall velocity measurement. The measurement volume was located 0.2 m under the still water surface to ensure that the targeted value is representative for the location of the FP. Therefore, it is mirrored along the global
-axis of the tank.
Figure 2 presents a detailed view of the experimental investigation.
All datasets are synchronised based on a digital tank trigger, which is provided by the wave makers. WG measurements were captured directly in the tank software. The ADV started with the rising trigger as well as the National Instrument-based data capturing system, which recorded and digitised the analogue signal of the FP.
Each deployed instrument used a separate measurement frequency. The ADV measurement was limited to 100 Hz; for the WG, the standard frequency was 128 Hz and the FP was captured with the doubled frequency of 256 Hz.
Section 4 provides a description of the provided dataset.
2.2. Investigated Cases
The presented investigations covers conditions with waves, currents and combinations of waves and currents. All current flows were limited to 0
based on the tank coordinate system. The capture time for waves was a constant 80 s. The wave makers were active for 64 s (run time) and the wave repeat time was chosen to be 32 s. The repeat time is part of the wave definition, and it ensures that, at the start and the end of this period, the waves are identical. A full number of wave repeats have to be fit into the repeat time, which can result in a difference between the requested and generated wave frequencies (
Table 2). Current-only investigations were conducted with a longer capture time of 300 s. Each wave or current condition is named based on a prefix starting with ET (empty tank) and an additional letter, which indicates the order of the tests. The conditions were conducted in the following order:
Variation of the wave frequency
between 0.2 and 0.7 Hz with no current in the wave direction
= 0
(ETA), followed by
= 180
(ETB). The conditions for each individual run are provided in
Table 2;
Variation of the wave direction
between 0 and 180
(ETC) with no current and a fixed requested
of 0.4 Hz.
Table 3 provides the investigated directions, which are based on the tank definition (
Figure 2);
After those wave-only investigations, the current speed was investigated, as shown in
Table 4. This table shows that the current condition of 0.2 m/s (RPM) was conducted twice (ETD0008 and ETD0009). In between those two measurements, the following described wave and current combinations were measured;
Variation of
in combination with a current speed of 0.2 m/s and a wave direction of 0
, representing following waves (ETE) and 180
opposing wave conditions (ETF). The specific conditions can also be found in
Table 2;
Variation of
with a constant requested
of 0.4 Hz and a current speed of 0.2 m/s (ETG in
Table 3).
All wave cases were limited to one single requested wave amplitude
of 0.03 m. The quality of the velocity measurements were found to be strongly dependent on the amount of seeding in the water. For the initial waves (ETA and ETB), a local addition of seeding was used, which was not sufficient to provide enough seeding for the initial wave conditions. This was especially evident when the small current velocities were investigated. Consequently, instead of increasing the flow speed incrementally, the procedure was altered and the maximum speed was run first. This is the reason for the change in the speeds presented in
Table 4. The focus of the investigation was on the comparison of the WG to the FP, with adequate knowledge of the mean current velocities required to estimate wave-induced velocities. Hence, the run was not repeated even if the quality control criteria of the velocity measurements were not achieved. The quality control of the velocity measurements is discussed in
Section 3.2.