Current Drag Force Research Articles

Design and Dynamic Analysis of a Novel Large-Scale Barge-Type Floating Offshore Wind Turbine with Aquaculture Cage

In this study, a novel large-scale barge-type floating offshore wind turbine with an aquaculture cage (LSBT-FOWT-AC) in a water depth of 100 m is designed through fully coupled analysis using the SESAM tool to support the Technical University of Denmark (DTU) 10 MW wind turbine. The intact stability and natural period of motion of the newly designed LSBT-FOWT-AC are evaluated based on the DNV rules and standards. Then, the dynamic responses of the LSBT-FOWT-AC under various sea conditions are studied. The motion of the LSBT-FOWT-AC platform is considerably affected by waves, and its motion response is within a reasonable range even under the extreme sea conditions of the 100-year return period. By analyzing the results of the out-of-plane bending moment of root of blade 1 (RootMyc1), it can be seen that the rotor frequency (1P) has a visible influence on the wind turbine. Through the analysis of dynamic response statistics of the LSBT-FOWT-AC structure by the single variable method of environmental loads, it is found that wind force exerts the greatest impact on the dynamic response compared to the wave-excitation force and current drag force.

Multi-objective Optimization of the Buoy Shape of an Ocean Wave Energy Converter using Neural Network and Genetic Algorithm

The point absorber is one of the most popular types of ocean wave energy converter (WEC) that harvests energy from the ocean. Often such a WEC is deployed in an ocean location with tidal currents or ocean streams, or serves as a mobile platform to power the blue economy. The shape of the floating body, or buoy, of the point absorber type WEC is important for the wave energy capture ratio and for the current drag force. In this paper, a new approach to optimize the shape of the point absorber buoy is developed to reduce the ocean current drag force on the buoy while capturing more energy from ocean waves. A specific parametric modelling is constructed to define the shape of the buoy with 12 parameters. The implantation of neural networks significantly reduces the computational time compared with solving hydrodynamics equations for each iteration. And the optimal shape of the buoy is solved using a genetic algorithm with multiple self-defined functions. The final optimal shape of the buoy in a case study reduces 68.7% of current drag force compared to a cylinder-shaped buoy, while maintaining the same level of energy capture ratio from ocean waves. The method presented in this paper has the capability to define and optimize a complex buoy shape, and solve for a multi-objective optimization problem.

On the effect of drag forces in mooring system restoring forces

Mooring line is a major source of stability and plays a key role in the global response of offshore floating wind turbine. In the current state of the research, a formulation based on the analytical catenary equation is most commonly used for the analysis of mooring lines. However, due to the inability of catenary equations to consider the ocean current drag forces on mooring lines, the effect of drag forces on fairlead restoring forces has not been investigated yet. In this study, we have investigated the influence of drag forces on fairlead forces using discrete catenary formulation for modeling mooring line. The discrete catenary formulation has the ability to incorporate ocean current drag forces. Three types of elements; fully suspended, touchdown and seabed element are formulated to model the suspended, touchdown and seabed portion of a slack mooring line, respectively. The influence of viscous drag on the fairlead restoring forces is demonstrated through the analysis of OC3-Hywind mooring system subjected to ocean currents. It was found that the viscous drag significantly influences the fairlead forces.

Mooring Control of Semi-submersible Structures for Wind Turbines

This paper deals with the mooring control of tri-floater support structures for wind turbines by means of a numerical parametric study; both catenary and vertical tensioned mooring lines are considered. Operational and extreme climate conditions are examined, in particular wave, wind and current parameters with return periods of 5 and 50 years are determined for three different locations in Southern Mediterranean Sea. Mooring system is dimensioned in such a way that the offset due to environmental actions is restrained within allowable limits. The response of the platform is assessed in terms of: static forces due to turbine thrusts, wind and current drag forces and steady drift forces; the wave frequency motion RAO curves; the low frequency second order wave motions. Finally, assuming the Dutch tri-floater structure as a test case, a parametric study is carried out to identify the most suitable mooring configurations, varying the water depth in the range between 50 and 200 m.

Hydrodynamic characteristics of an oscillating circular disk under steady in-plane current conditions

The heave motion performance of a Spar platform could be improved significantly by installing heave plates. The heave plates not only increase the vertical added mass of the platform but also provide extra damping in heave. This paper presents an investigation of the hydrodynamic characteristics of a heave plate under steady current conditions using three-dimensional direct numerical simulations. A circular disk of diameter D is forced to oscillate along its axis sinusoidally. Three aspect ratios of the disk, i.e., td/D=0.1,0.05 and 0.02, are considered, respectively. The current is introduced in the plane of the disk. The influences of three parameters, the Keulegan–Carpenter number (KC), the reduced current velocity (Vr) and the aspect ratio of the disk, on the flow structures, the current drag force, the added mass and the damping of the heave plate are examined both qualitatively and quantitatively. The ranges of the KC and Vr numbers considered in this study are 0≤KC≤1 and 0≤Vr≤0.25, respectively. The frequency number β for the oscillating disk is fixed at 200.

Ice Thrust in Reservoirs

We undertook a three-year program to measure the magnitude of static ice forces in four reservoirs located in central and northern Quebec. These static forces may be generated by a temperature change or may arise from such other mechanisms as water level variations, wind, and current drag force. Field observations have revealed two important facts: (1) ice covers have circumferential cracks caused either by water level variations or thermal contraction, and (2) the static ice forces are, in some instances, sufficient to trigger an instability of the broken ice covers by buckling. Noting that an ice cover cannot transmit a force to a structure larger than its own resistance, we derive an upper bound for static forces by determining the in-plane compression force at which a fragmented ice cover collapses. Empirical formulas are presented for three typical structure shapes: retaining walls, sluice gates, and piers. These formulas correlate well with the field data collected from the four dam sites, and sugge...

Steady-State Configuration of an Underwater Suspended Bipod Cable System

Underwater suspended bipod cable systems are utilized to moor oceanographic instrumentation. An analysis to determine the steady-state configuration of an underwater-suspended bipod cable system subject to buoyant lift forces as well as current drag forces is presented. It is assumed that the cables are neutrally buoyant or can be made neutrally buoyant by the addition of buoyancy and that the effects of tangential drag on each cable are negligible. The equations used to find the equilibrium position of the buoyed joint and also the tensions in each mooring cable are of two kinds: three force equilibrium equations at the buoyed joint and two compatibility equations for the displaced bipod system. These equations are sufficient to solve for the five unknowns, namely, the displacement of the buoyed end in three directions and the two cable tensions. A computer program was developed to solve the system of equations. Iterations are performed until a convergence is obtained on an equilibrium position and cable tensions. As an example, a bipod system 10,000 ft high with a 20,000 Ib buoyant force under installed conditions was considered. The flow direction was taken at an angle of 30 deg with the normal of the plane containing the system. A linear velocity profile ranging from 1 ft/s at the top to 0 at the bottom was found to cause a 670 ft horizontal and 153 ft vertical deflection of the buoyed joint.

Performance Characteristics of a Translatory, Magnetic Suspension System

An experimental study was conducted of the performance of a translatory, magnetic suspension utilizing ceramic permanent magnets for vertical support and servo-controlled electromagnets for lateral stability. Drag measurements were confined to low velocities ([less than double sim] 40 cm/sec); a typical measured value of drag coefficient is drag force/weight = 1.8 × 10−4 at 18 cm/sec. Use of laminated electrical sheet rather than mild steel should provide an order of magnitude reduction in this value. For low amplitude oscillations in a magnetic valley, drag decreases further by about two orders of magnitude; free oscillations have been observed to continue for 12 min. In contrast, instrument quality ball bearings have a friction coefficient in the 10 to 40 × 10−4 range. The total power required to maintain support is about two orders of magnitude lower than for a dc electromagnet suspension system. Assuming eddy current drag force proportional to velocity and hysteretic drag force independent of velocity, measured drag values extrapolated to high speed indicate that with laminated magnetic elements, the magnetic drag force would be low compared to air drag for a similarly suspended vehicle traveling at 270 m/sec (600 mph) in a tube evacuated to 10−3 atm. The vehicle coasting at this velocity would lose little more than 1% of its velocity in 93 km (53 miles).