Unsteady aerodynamic analysis for offshore floating wind turbines under different wind conditions
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A free-vortex wake (FVW) model is developed in this paper to analyse the unsteady aerodynamic performance of offshore floating wind turbines. A time-marching algorithm of third-order accuracy is applied in the FVW model. Owing to the complex floating platform motions, the blade inflow conditions and the positions of initial points of vortex filaments, which are different from the fixed wind turbine, are modified in the implemented model. A three-dimensional rotational effect model and a dynamic stall model are coupled into the FVW model to improve the aerodynamic performance prediction in the unsteady conditions. The effects of floating platform motions in the simulation model are validated by comparison between calculation and experiment for a small-scale rigid test wind turbine coupled with a floating tension leg platform (TLP). The dynamic inflow effect carried by the FVW method itself is confirmed and the results agree well with the experimental data of a pitching transient on another test turbine. Also, the flapping moment at the blade root in yaw on the same test turbine is calculated and compares well with the experimental data. Then, the aerodynamic performance is simulated in a yawed condition of steady wind and in an unyawed condition of turbulent wind, respectively, for a large-scale wind turbine coupled with the floating TLP motions, demonstrating obvious differences in rotor performance and blade loading from the fixed wind turbine. The non-dimensional magnitudes of loading changes due to the floating platform motions decrease from the blade root to the blade tip.Keywords:
Inflow
Tip-speed ratio
Stall (fluid mechanics)
Inflow
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The present experimental study demonstrates the effects of clean and distorted inflow conditions on the performance and nature of stall inception in a single stage axial flow fan. A 90° distortion screen located upstream of the rotor leading edge was rotated up to 40 per cent of the rotor speed in the clockwise (co-rotation) and counterclockwise (counter-rotation) directions to generate dynamic inflow distortion. It was observed that the stall margin deteriorated substantially under co-rotating inflow distortion as compared to counter-rotating inflow distortion. The degradation in stall margin was about 15 per cent and 1.98 per cent under co- and counter-rotating inflow distortions, respectively. Tip injection was used as a means of enhancing the fan performance under distorted inflow. With tip injection under co-rotating inflow distortion, about 2.8 per cent improvement in stall margin was observed. The improvement in stall margin under counter-rotating inflow distortion with tip injection was 2.98 per cent. The unsteady static pressure traces show clear differences in the nature of stall inception under co- and counter-rotating inflow distortions. The stall inception occurs through long-length-scale disturbances under co-rotating inflow distortion, while the mode of stall inception under clean flow and counter-rotating inflow distortions was through short-length-scale disturbances. The nature of stall inception under dynamic inflow distortion with tip injection remains the same as without tip injection.
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Stall (fluid mechanics)
Axial Compressor
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The rate of inflow to a long well can vary along its completion length, e.g. due to frictional pressure losses or reservoir heterogeneity. These variations often negatively affect the oil sweep efficiency and the ultimate oil recovery. Inflow Control Devices (ICDs) represent a mature well completion technology which provides uniformity of the inflow profile by restricting high specific inflow segments while increasing inflow from low productivity segments. This paper introduces a mathematical model for effective reduction of the inflow imbalance caused by reservoir heterogeneity. The model addresses one of the key aspects of the ICD technology application - the trade-off between well productivity and inflow equalisation. Our analytical model relates the specific inflow rate and specific productivity index to well characteristics taking into account the intrinsically stochastic nature of reservoir properties along the well completion interval. A general solution to our model is available in a non-closed, analytical form. We have derived a closed form solution for some particular cases. The practical utility of the model is illustrated by considering a case study with prolific and medium productivity reservoirs. Finally, we identify limitations in using our model.
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The effect of inflow condition and rotor tip clearance size on the stall inception in an axial compressor is experimentally investigated. The rig in this research is a low-speed, single-stage flow compressor, which has two types of tip clearance: design clearance (DC, ε = 0.5 %) and wide clearance (WC, ε = 2.2 %). The experiment is conducted in the different inflow conditions: the clean inflow and the distorted inflow. The rotor configuration is the forward-swept blade (Sweep). The stall inception is detected by applying a discrete special Fourier series analysis to the wall pressure trace data in front of the rotor blade. At the clean inflow condition, the type of the stall inception is modal-spike, regardless of the clearance size. On the other hand, at the distorted inflow condition, the type of stall inception of DC is modal-spike, and that of WC is spike. It seems that the difference of the stall inception between clearance sizes at distorted inflow condition is influenced by the circumferential distribution of the blade loading at tip region.
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Stall (fluid mechanics)
Tip clearance
Axial Compressor
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Abstract The prediction of water inflow in cavern is significant for underground engineering, as the inflow has important influence on construction and drainage system. At present, the empirical formulas for predicting inflow are usually based on a single cavern, but the influence of nearby caverns is not taken into account. In order to consider the influence of spacing and radius on water inflow, the inflow in an underground crude oil storage cavern is analyzed by numerical analysis in this paper. The results show that when the s/r < 20, the spacing has great influence on the inflow; when 20<s/r<60, it has some effect on the inflow; and when s/r>60, it almost has no effect on the inflow (s and r represent spacing between caverns and radius). Based on numerical analysis, empirical formulas for predicting water inflow are modified, and the new formula is proposed considering the influence of the spacing. Finally, field data show that the new formula is more suitable for predicting inflow.
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Open water
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The effect of inflow condition and rotor configuration on the stall inception in an axial flow compressor is experimentally investigated. The rig in this research is a low-speed, single-stage axial flow compressor, which has two types of rotor blades: the radially stacked blade (Radial) and the forward-swept blade (Sweep). The experiment is conducted in the different inflow conditions: the clean inflow and the distorted inflow. The stall inception is detected by applying a discrete spatial Fourier series analysis to the pressure trace data. At the clean inflow condition, the type of the stall inception of Radial is the spike and that of Sweep is the modal-spike. On the other hand, at the distorted inflow condition, the stall inceptions of both blades are modal-spike. It seems that the differences of the stall inception between blade configurations, and at inflow conditions are influenced by the span-wise distribution of the blade loading.
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Stall (fluid mechanics)
Axial Compressor
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Abstract Inflow Control Device, often referred to as equalizer, is a completion hardware that is deployed as a part of well completions aimed at distributing the inflow evenly. Even though the detail structures vary from one design to another, the principle for different inflow control devices is the same - restrict flow by creating additional pressure drop, and therefore balancing or equalizing wellbore pressure drop to achieve an evenly distributed flow profile along a horizontal well. With a more evenly distributed flow profile, one can reduce water or gas coning, sand production and solve other drawdown related production problems. In general, inflow control devices are not adjustable; once installed in the well, the location of the device and the relationship between rate and pressure drop are fixed. This makes the design of a well completion and inflow control devices extremely critical for production. Inflow control devices can be either beneficial or detrimental to production, strongly depending on the reservoir condition, well structure and completion design. Realizing that reservoir conditions will change during the life of a well, the impact of an inflow control device is a function of time. The inflow control devices sometimes can be overlooked if the design is only based on reservoir flow simulation. In this paper, we will investigate how and when an inflow control device should be used. An integrated analysis method of inflow (reservoir) and outflow (wellbore) is used to generate the flow profile of a horizontal well, and additional frictional pressure drop created by inflow control devices will be considered. Two conditions that result in production problems, wellbore pressure drop and breakthrough of unwanted fluids, will be addressed. The focus will be on when and how an inflow control device can optimize production. Examples at field conditions will be used to illustrate that it is critical to understand the reservoir conditions and wellbore dynamics together when designing a well completion with inflow control devices. Since uncertainty of reservoir condition always exists, backup plans and conservative designs are desirable. The observations from this study show that overdesigned inflow control devices will not just increase the cost of well completion, but also impact the well performance negatively.
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Outflow
Pressure Control
Flow Control
Drawdown (hydrology)
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In this paper, a dynamic stall control scheme for vertical-axis wind turbine (VAWT) based on pulsed dielectric-barrier-discharge (DBD) plasma actuation is proposed using computational fluid dynamics (CFD). The trend of the wind turbine power coefficient with the tip speed ratio is verified, and the numerical simulation can describe the typical dynamic stall process of the H-type VAWT. The tangential force coefficient and vorticity contours of the blade are compared, and the regular pattern of the VAWT dynamic stall under different tip speed ratios is obtained. Based on the understanding the dynamic stall phenomenon in flow field, the effect of the azimuth of the plasma actuation on the VAWT power is studied. The results show that the azimuth interval of the dynamic stall is approximately 60° or 80° by the different tip speed ratio. The pulsed plasma actuation can suppress dynamic stall. The actuation is optimally applied for the azimuthal position of 60° to 120°.
Stall (fluid mechanics)
Tip-speed ratio
Plasma actuator
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Vertical axis wind turbines boast several advantages over their horizontal counterparts. Their development and large-scale installation have been limited due to their inherent aerodynamic complexity and in particular due to the occurrence of dynamic stall. Vortices associated with dynamic stall induce aerodynamic load transients on the wind turbine blades that can lead to structural fatigue and premature mechanical failure. Despite these inherent risks, vertical axis wind turbines are often operated under kinematic conditions that lead to dynamic stall because this maximizes their power output. The ability to quantify the trade-off between these undesirable load transients and the benefit of increased power generation is of utmost importance for future optimization studies. Here, we introduce a novel experimental setup to study the aerodynamic performance of a blade in a model-scale H-type Darrieus vertical axis wind turbine and present the results of direct blade load measurements using strain gauges for various tip speed ratios. We compare the measured blade load responses with predictions based on Greenberg's potential flow theory for different tip-speed ratios. The magnitude of the load responses and their fluctuations increase with decreasing tip-speed ratio in both the upwind and the downwind halves of the rotation. In the upwind part of the cycle, stall is more severe than in the downwind half across all tip-speed ratios. The relative stall magnitude decreases exponentially with increasing tip-speed ratio. The measured blade loads deviate more strongly from the predictions by Greenberg's analytical potential flow model in the downwind half of the cycle than in the upwind half.
Stall (fluid mechanics)
Tip-speed ratio
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