Current and wave effects around windfarm monopile foundations

Abstract

Laboratory measurements were undertaken to investigate wave and current velocities in the vicinity of a wind turbine monopile foundation, in order to inform environmental impact assessments and to quantify flow variability in the region of the power take off cable. Flow measurements were made up to 15.5 pile diameters (D) downstream of the pile. Measurements were also taken around the perimeter of the pile (~0.75 D from the pile centre) at the approximate representative height of the power cable. In current-only tests, the mean flow was reduced immediately downstream of the pile, but returned to within 5% of background levels by 8.3 D downstream of the pile centre in representative conditions. A new parameterisation of the velocity recovery is given. The turbulent eddy shedding frequency was well predicted by the Strouhal number. Turbulence peaked at 1.5 D from the pile, and the subsequent decay was parameterised. Velocity magnitudes at the side of the pile were up to 1.35 times greater than background flow rates, in line with potential flow theory. Velocities in the wake region were much less than predicted by potential flow theory, corresponding with increased turbulence. Tests with waves indicated that oscillatory velocities reduced immediately down-wave of the pile, but returned quickly to background levels (by 1.65 to 3.5 D of the pile centre). The general near-pile distribution of the orbital velocity maximum was well represented by potential flow theory. Orbital velocities were reduced immediately up-wave and down-wave of the pile. At the side of the pile in wind sea conditions, the velocity increased up to 1.66 times the background level. This increased to 1.85 times in swell conditions. For orthogonal currents and waves, a velocity parameter was calculated as the mean current plus wave orbital velocity, resolved. With the mean current direction as a reference, the maximum flow was observed at the side of the pile. At 0.75 D from the pile centre, the flow was enhanced by up to 1.2 times the no-pile case. Spectral peaks in the velocity were evident at both wave frequency and at the Strouhal frequency, immediately down current from the pile.

Numerical Study of the Hydrodynamics of Waves and Currents and Their Effects in Pier Scouring

“…Subsequently, for each of the planes traced around the cylindrical pile (see Figure 2), the vorticity's average performance was determined and the associated streamlines were traced to study and analyze the average performance of the horseshoe vortex. Additionally, eight monitoring stations were defined to obtain the vertical distribution of the flow velocity Figure 1 indicates the distribution of these stations, which is concordant with the methodological approach of Miles et al [51].…”

“…This performance detected in station P4 may be caused by the combined wake effect due to currents and waves, since geometrically this location is completely downstream of both forcings (associates with case C04). The effects on the velocity fields caused by the cylindrical pile in a flow field with waves perpendicular to the currents obtained from the numerical model were compared with data provided by Miles et al [51]. The results for which waves and currents come from perpendicular directions are shown in Figure 7 (associates with case C04).…”

“…The numerical results obtained by the simulation for cases C01 to C03 (codirectional) contrasted with the experimental data provided by Qi and Gao [26], where the flow velocity was the comparison variable. For the case of the perpendicular waves and currents (C04), the experimental information provided by Miles et al [51] was used to verify the numerical results, where the contrasted variable was the average vertical profile of the total flow velocity.…”

“…For case C04, the comparison of numerical and experimental data published by Miles et al [51] was conducted via the vertical distribution of the longitudinal velocity averaged in eight points around the pile, which were denominated P1 to P8 and distributed as shown in Figure 1. The monitoring stations were located 0.75D from the center of the pile, while their angular separation was 45 • .…”

“…= . The velocities were nondimensionalized according to the characteristic velocity (C) described in Equation (22) for all compared cases between the modeled and experimental data (Umeyama [3], Qi and Gao [20], and Miles et al [51]).…”