Wake Interaction Research Articles

Wake interactions of two horizontal axis tidal turbines in tandem

The wake interactions between two horizontal axis model tidal turbines for two different spacings were investigated by flow measurements using acoustic Doppler velocimetry (ADV). The analysis of the velocity time-series data showed that a velocity sampling time period of at least 1000 integral time scales is required for the convergence of turbulence statistics in the wake of the turbines. The velocity measurement showed that a distinct three-layer (core, inner, and outer) structure was observed in the wake of a single turbine, whereas a simple wake pattern without such a structure was observed downstream of the two turbines. The comparison between the measurements for the different turbine spacings showed that the mean velocity fields, turbulent stress fields, velocity spectra plots, and integral length scales behind the second turbine were nearly independent of the turbine spacing. It was concluded that the increased turbulent momentum transfer across the wake boundary, mainly due to the wake meandering motion by the first turbine, overshadows the effect of the first turbine. This suggests that the ambient turbulence intensity induced by the upstream turbine could be a key factor for the wake characteristics within a tidal turbine array.

Propeller Slipstream Effect on Aerodynamic Characteristics of Micro Air Vehicle at Low Reynolds Number

A numerical investigation on propeller-induced flow effects in tractor configurations on a Zimmerman wing-fuselage using the cambered thin airfoil is presented in this paper. The Reynolds number based on the mean aerodynamic chord was 1.3 × 105. Significant aerodynamic performance benefits could be found for a propeller in the tractor configuration. The numerical results showed that the propeller slipstream effect on the wings was highly dependent on the size of the propeller, and the major slipstream effect was working at 60% inboard wingspan, whereas less effects were observed towards the wingtip. The propeller slipstream increased the local angle of attack on the up-going blade side. This effect simultaneously augmented the section lift. The unsteady Reynolds-averaged Navier–Stokes (URANS) simulations helped to improve understanding of the interaction of the propeller wake and the wing-fuselage, which is an important aspect to guide the design of future efficient and controllable micro air vehicles. The results indicated that, in MAV designs, the slipstream from the propeller had a significant effect on the wing aerodynamics, regarding both performance and stability of the vehicle.

Research on the tandem arrangement of the ducted horizontal-axis tidal turbine

Array research of tidal turbine is critical to make it cost-effective and commercializing. There are many studies of turbine arrays, which mostly focus on a conventional horizontal-axis turbine. While a ducted turbine is also available for commercial application, which draws little attention. In this paper, the tandem arrangement of ducted horizontal-axis turbines is studied based on the flume experiment and the large eddy simulation (LES). The turbine's performance changes are investigated under the 5-15D (D represents the turbine diameter) tandem distance. Research results show that the downstream turbine's performance degrades significantly, and its efficiency drops to roughly half of the upstream turbine when the tandem distance is 5-10D. When the distance increases more than 10D, the downstream turbine performance can reach 80% of the upstream turbine. As the distance increases, this performance recovery becomes less significant. In terms of load fluctuation, with the interaction of the upstream wake and the surrounding flow field, a spiral oscillation wake region will be formed about 10D behind the upstream turbine. Therefore, with the increase of the tandem distance, the load fluctuation of the downstream turbine does not gradually decrease as a result. This fluctuation is more than twice that of the upstream turbine, and the maximum can reach 5 times.

Numerical Modeling of Flexible Net Panels under Steady Flow Using a Coupled Fluid–Structure Partitioned Scheme

Fluid–structure interactions of flexible net panels are complex and lack sufficient exploration. To examine the flow characteristics of flexible net panels with large deformation, we propose a partitioned coupling scheme in this paper. The coupled fluid–structure equations are solved separately under finite volume and finite element frameworks. The interface traction from the fluid solver is considered as a Neumann boundary condition for the solid domain, and the interface velocity is applied as a Dirichlet boundary condition for the fluid problem. Then, the forces can be transferred along the interface via Dirichlet-to-Neumann mapping. The results show that both the drag coefficient and the velocity reduction increase alongside the net solidity ratio (Sn), but they decrease as the Reynolds number/attack angle increases. A comparative study of drag coefficients is made between the present numerical simulations and the analytical predictions. This paper also examines the velocity distribution and vortex formation of flexible net panels. A single vortex forms in the shear layers and the wake when Sn = 0.16, and a pair of vortices mostly forms in the wake when Sn = 0.33. The vertical net twines predominantly affect the formation of the vortex behind the net, leading to delayed vortex shedding. The flow exhibits wake interactions due to the interference between the net twines in the high-solidity net panel. No such interference occurs in the low-solidity net panel, but the altered shear layers could cause severe velocity fluctuations in the near field.

Comparative study of decentralized instantaneous and wind-interval-based controls for in-line two scale wind turbines

The existing decentralized wind farm control incorporating the wind turbine wake interaction has almost all concentrated on the condition of fixed wind speed/direction without considering the variability of wind in nature. Meanwhile, most of the current wake models cannot meet the requirement for wind turbine control optimization research with the high demand of wake calculation speed and accuracy. In this paper, a novel decentralized control strategy, named the wind-interval-based (WIB) control which adopts the uniform operation among the same interval of variable wind speeds/directions, is proposed for the control optimization study of in-line two scaling wind turbines for demonstration. To prove the effectiveness of the new control mechanism, the ideal instantaneous control is introduced for the comparative study. At the same time, a wake model incorporating the two directly controllable parameters (i.e., the tip speed ratio λ and pitch angle γ) is established based on artificial neural network (ANN). The comparative results show that the total power output applying the instantaneous control and the new WIB control mechanisms are increased by 0.1%–4.1% and 0.45%–3.9% depending on the tested wind scenarios, respectively. By comparing the optimized power output with the two controls, it is found that the error between them is generally lower than 3%, while the proposed WIB control reduces the operational difficulty to a large extent facilitating its application to the real wind turbine operation in reality. In summary, this paper shows that the new proposed WIB control not only reduces the difficulties of the wind turbine control mechanism, but maintains the control effectiveness by achieving a comparable total power output with respect to the traditional instantaneous control, which is of great significance to the wind farm developer.

Dynamic inflow model for a floating horizontal axis wind turbine in surge motion

Abstract. Floating offshore wind turbines may experience large surge motions, which can cause blade–vortex interaction if they are similar to or faster than the local wind speed. Previous research hypothesized that this blade–vortex interaction phenomenon represented a turbulent wake state or even a vortex ring state, rendering the actuator disc momentum theory and the blade element momentum theory invalid. This hypothesis is challenged, and we show that the actuator disc momentum theory is valid and accurate in predicting the induction at the actuator in surge, even for large and fast motions. To accomplish this, we develop a dynamic inflow model that simulates the vorticity–velocity system and the effect of motion. The model's predictions are compared to other authors' results, a semi-free-wake vortex ring model, other dynamic inflow models, and CFD simulations of an actuator disc in surge. The results show that surge motion and rotor–wake interaction do not result in a turbulent wake or vortex ring state and that the application of actuator disc momentum theory and blade element momentum theory is valid and accurate when applied correctly in an inertial reference frame. In all cases, the results show excellent agreement with the higher-fidelity simulations. The proposed dynamic inflow model includes a modified Glauert correction for highly loaded streamtubes and is accurate and simple enough to be easily implemented in most blade element momentum models.

Real-time parametric estimation of periodic wake-foil interactions using bioinspired pressure sensing and machine learning

Periodic wake-foil interactions occur in the collective swimming of bio-inspired robots. Wake interaction pattern estimation (and control) is crucial to thrust enhancement and propulsive efficiency optimization. In this paper, we study the wake interaction pattern estimation of two flapping foils in tandem configurations. The experiments are conducted at a Reynolds number of 1.41 × 104 in a water channel. A modified wake-foil phase parameter Φ, which unifies the influences of inter-foil distance L x , motion phase difference Δφ and wake convection velocity U v , is introduced to describe the wake interaction patterns parametrically. We use a differential pressure sensor on the downstream foil to capture wake interaction characteristics. Data sets at different tandem configurations are collected. The wake-foil phase Φ is used to label the pressure signals. A one-dimensional convolutional neural networks (1D-CNN) model is used to learn an end-to-end mapping between the raw pressure measurements and the wake-foil phase Φ. The trained 1D-CNN model shows accurate estimations (average error 3.5%) on random wake interaction patterns and is fast enough (within 40 ms). Then the trained 1D-CNN model is applied to online thrust enhancement control of a downstream foil swimming in a periodic wake. Synchronous force monitoring and flow visualization demonstrate the effectiveness of the 1D-CNN model. The limitations of the model are discussed. The proposed approach can be applied to the online estimation and control of wake interactions in the collective swimming and flying of biomimetic robots.

Piloted Simulation of the Rotorcraft Wind Turbine Wake Interaction during Hover and Transit Flights

Helicopters are used for offshore wind farms for maintenance and support flights. The number of helicopter operations is increasing with the expansion of offshore wind energy, which stresses the point that the current German regulations have not yet been validated through scientific analysis. A collaborative research project between DLR, the Technical University of Munich, the University of Stuttgart and the University of Tübingen has been conducted to examine the sizes of the flight corridors on offshore wind farms and the lateral safety clearance for helicopter hoist operations at offshore wind turbines. This paper details the results of piloted helicopter simulations in a realistic offshore wind farm scenario. The far-wake of rotating wind turbines and the near-wake of non-rotating wind turbines have been simulated with high-fidelity computational fluid dynamics under realistic turbulent inflow conditions. The resulting flow fields have been processed by superposition during piloted simulations in the research flight simulator AVES to examine the flight corridors in transit flights and the lateral safety clearance in hovering flights. The results suggest a sufficient size for the flight corridor and sufficient lateral safety clearance at the offshore wind turbines in the considered scenarios.

Effect of porous media on wake-induced vibration (WIV) in tandem circular cylinder

In practical engineering, cylindrical structures are frequently arranged in tandem under different flow environments. In this study, wake-induced vibration (WIV) of a solid circular cylinder in the wake of a stationary porous cylinder is numerically investigated. A numerical model including the porous medium and vibration modules is established by using the Immersed Boundary (IB) method and the calculation accuracy of the model is verified by the experiment. Seven porosities of the upstream cylinder from 0.0 to 1.0 are considered and the downstream cylinder is allowed to oscillate in both streamwise and transverse directions. The effects of the gap between the two cylinders, the Reynolds number (Re) and the reduced velocity (Ur) on the upstream-stationary-downstream-vibrating tandem cylinders are investigated in terms of shear layer reattachment, vortex impingement and wake interference. The wake flow pattern and the vortex shed from the porous cylinder are vulnerable to the porosity of the cylinder. As a result, the vibration of the downstream cylinder caused by the wake changes at different porosities. Besides, the hydrodynamic force and vibration frequency are also tightly associated with the wake interaction. At different reduced velocities (Ur) and Reynolds numbers (Re), the WIV and the force on the cylinder change significantly.

Optimal closed-loop wake steering – Part 2: Diurnal cycle atmospheric boundary layer conditions

Abstract. The magnitude of wake interactions between individual wind turbines depends on the atmospheric stability. We investigate strategies for wake loss mitigation through the use of closed-loop wake steering using large eddy simulations of the diurnal cycle, in which variations in the surface heat flux in time modify the atmospheric stability, wind speed and direction, shear, turbulence, and other atmospheric boundary layer (ABL) flow features. The closed-loop wake steering control methodology developed in Part 1 (Howland et al., 2020c, https://doi.org/10.5194/wes-5-1315-2020) is implemented in an example eight turbine wind farm in large eddy simulations of the diurnal cycle. The optimal yaw misalignment set points depend on the wind direction, which varies in time during the diurnal cycle. To improve the application of wake steering control in transient ABL conditions with an evolving mean flow state, we develop a regression-based wind direction forecast method. We compare the closed-loop wake steering control methodology to baseline yaw-aligned control and open-loop lookup table control for various selections of the yaw misalignment set-point update frequency, which dictates the balance between wind direction tracking and yaw activity. In our diurnal cycle simulations of a representative wind farm geometry, closed-loop wake steering with set-point optimization under uncertainty results in higher collective energy production than both baseline yaw-aligned control and open-loop lookup table control. The increase in energy production for the simulated wind farm design for closed- and open-loop wake steering control, compared to baseline yaw-aligned control, is 4.0 %–4.1 % and 3.4 %–3.8 %, respectively, with the range indicating variations in the energy increase results depending on the set-point update frequency. The primary energy increases through wake steering occur during stable ABL conditions in our present diurnal cycle simulations. Open-loop lookup table control decreases energy production in the example wind farm in the convective ABL conditions simulated, compared to baseline yaw-aligned control, while closed-loop control increases energy production in the convective conditions simulated.

Near wake interactions and drag increase regimes for a square-back bluff body

The three-dimensional near wake of a square-back bluff body in ground proximity is experimentally perturbed by placing a pair of D-shaped obstacles under the body. Five obstacle widths $d$ , from 12 % to 26 % of the height of the body, are used to perform a sensitivity study of the body's pressure drag by varying the relative distance $l$ between the obstacle pair and the base. Two successive drag-sensitive regimes are identified for obstacle-to-base distances $l/d < 2.5$ , where the pressure drag of the body is increased up to 22 %. In different regimes, the flow dynamics measured downstream of the obstacles are found to be very different. When the obstacles are the closest to the base, $l/d<1.5$ , the pressure drag changes of the main body are driven by mean merging between the wakes of the obstacles and of the main body, and scale with $d$ . Contrarily, when the obstacles are located farther from the base, $1.5< l/d<2.5$ , the wakes of the obstacles are isolated from the main body wake. Here the dynamics of the obstacle wake drive the pressure drag changes of the main body, which scale with $d^{2}$ . In both regimes, we measure a mean mass transfer from the wake of the main body to the wakes of the obstacles. This is the main mechanism responsible for the pressure drag changes. Using our results and reference studies describing the effects of base suction on the pressure drag of bluff bodies, a physical model is proposed to explain the contrasting scalings of the pressure drag increase in the different regimes observed in this study.

Improved Evaluation of The Wind Power Potential of a Large Offshore Wind Farm Using Four Analytical Wake Models

The objective of this paper is to investigate the ability of analytical wake models to estimate the wake effects between wind turbines (WTs). The interaction of multiple wakes reduces the total power output produced by a large offshore wind farm (LOFWF). This power loss is due to the effect of turbine spacing (WTS), if the WTs are too close, the power loss is very significant. Therefore, the optimization of turbine positions within the offshore wind farm requires an understanding of the interaction of wakes inside the wind farm. To better understand the wake effect, the Horns Rev 1 offshore wind farm has been studied with four wake models, Jensen, Larsen, Ishihara, and Frandsen. A comparative study of the wake models has been performed in several situations and configurations, single and multiple wakes are taken into consideration. Results from the Horns Rev1 offshore wind farm case have been evaluated and compared to observational data, and also with the previous studies. The power output of a row of WTs is sensitive to the wind direction. For example, if a row of ten turbines is aligned with the 270° wind direction, the full wake condition of WTs is reached and the power deficit limit predicted by Jensen model exceeds 70%. When a wind direction changes only of 10° (260° and 280°), the deficit limit reduces to 30%. The obtained results show that a significant power deficit occurs when the turbines are arranged in an aligned manner. The findings also showed that all four models gave acceptable predictions of the total power output. The comparison between the calculated and reported power output of Horns Revs 1 showed that the differences ranged from - 8.27 MW (12.49%) to 15.27 MW (23.06%) for the Larsen and Frandsen models, respectively.

Numerical analysis of the wing–wake interaction of tandem flapping wings in forward flight

Tandem flapping-wing micro aerial vehicles, which are inspired by dragonflies, are unique in that they have two pairs of wings, and this could increase their maneuverability during flight. In this study, a series of tandem flapping-wing arrangements were designed in polar coordinates, and the polar distance and polar angles of the hindwing were used as design variables. Such a consideration of variations in both the horizontal and vertical positions of the hindwing in the design of tandem-wing arrangements has the potential to elucidate new interaction mechanisms, leading to improvements in the aerodynamic characteristics of tandem flapping-wings. Two-dimensional numerical analyses of these arrangements were conducted. The results of these analyses showed that, compared with a single flapping wing, tandem wings can increase the lift coefficient by 78.1% with an appropriate arrangement, but decrease the lift coefficient by 51.6% with an inappropriate arrangement. The vorticity distributions for typical wing arrangements were examined to establish the effect of the design variables on the aerodynamic characteristics of the tandem wings in forward flight. Two types of wing–wake interaction mechanisms were revealed based on this analysis. Both of these interactions can significantly reduce the negative force during the upstroke, resulting in an increase in the time-averaged lift.

Characterization of coupling between inertial particles and turbulent wakes from porous disk generators

This study presents the findings of a wind tunnel experiment investigating the behaviour of micrometric inertial particles with Stokes numbers around unity in the turbulent wake of a stationary porous disk. Various concentrations $\varPhi _{v}\in ([6-19] \times 10^{-6})$ of poly-disperse water droplets (average diameter 40–50 $\mathrm {\mu }$ m) are compared with sub-inertial tracer particles. Hot-wire anemometry, phase Doppler interferometry and particle image velocimetry were implemented in the near- and far-wake regions to study the complex dynamics of such particles. Quadrant analysis is used to explore the shear effects of the particle wake interaction. Turbulence statistics and particle size distributions reveal distinct differences in the structure of the wake when inertial particles are present in the flow. Additionally, there are different structures in the near and far wake regions and structures change with particle volume fraction.

Decoupling wind–wave–wake interactions in a fixed-bottom offshore wind turbine

The interest and benefits of offshore wind energy have also brought along legitimate challenges. In the golden age of renewables, offshore wind-energy holds the most potential for growth, but the burgeoning benefits of offshore energy are also entangled in dynamics not fully understood, such as the dynamic coupling of the atmospheric boundary layer, the wind-turbine generated wake, and the surface waves. This study establishes the first experimental turbulent-interaction between the traditionally distinct fields of airflow-dynamics above the air–sea interface and the characterization of wind-turbine wakes. The study details a non-trivial experimental setup combining a wave tank, wind tunnel, and scaled fixed-bottom wind turbine. Particle image velocimetry (PIV) was performed on three successive image planes to visualize wind–wake, wind–wave, and wave–wake interaction far downstream of the turbine. The wave phase-dependent dynamics of the turbine wake on the passing ocean-wave profile and location are outlined. The velocity and stress profiles showed a horizontal wake-pumping motion along with vertical wake-lifting of the offshore turbulent wake. The data were decomposed into incremental wave-phases revealing localized and predictable velocity maxima, stress maxima, and wake modulation that is normally obscured by a time-averaged mean. The results quantified wake pumping and meandering, as well as the Reynolds stress and wave-induced phase-averaged fluctuations. There is a phase-dependent oscillation in both the horizontal (streamwise) direction, as well as a vertical displacement of the wake. The shear stress and advection terms show this imbalance along the vertical direction of the turbine. The results illustrate a more complete picture of offshore wind-energy dynamics, which have implications for insidious mechanical issues, design optimization and/or control strategies.

SPTV sheds light on flow dynamics of fractal-induced turbulence over a plate-fin array forced convection

A 3D stationary particle tracking velocimetry (SPTV) with a unique recursive corrective algorithm has been successfully established to detect the instantaneous regional fluid flow characteristics. The veracity of SPTV is corroborated by conducting actual displacement measurement validation, which gives a maximum percentage deviation of about 0.8%. This supports the accuracy of the current SPTV system in 3D position detection. More importantly, the SPTV detected velocity fluctuations are highly repeatable. In this study, SPTV is proven to be able to express the nature of chaotic fractal grid-induced regional turbulence, namely: the high turbulence intensity attributed to multilength-scale wake interactions, the Kolmogorov’s −5/3 law decay, vortex shedding, and the Gaussian flow undulations immediately leeward of the grid followed by non-Gaussian behaviour further downstream. Moreover, by comparing the flow fields between control no-grid and fractal grid-generated turbulence of a plate-fin array, SPTV reveals vigorous turbulence intensity, smaller regional integral-length-scale, and energetic vortex shedding at higher frequency for the latter, particularly between fins. Thereupon, it allows the unravelling of detailed thermofluid interplays of plate-fin heat sink heat transfer augmentation. The novelty of SPTV lies in its simplicity, use of low-cost off-the-shelf components, and most remarkably, low computational complexity in detecting fundamental characteristics of turbulent fluid flow.

On the Interaction of Wake and Power Fluctuation of a Tidal Stream Turbine in Wave-Current Coexistence Environment

On the Interaction of Wake and Power Fluctuation of a Tidal Stream Turbine in Wave-Current Coexistence Environment

Wake interaction studies of flow past tandem circular cylinders for different diameter and gap ratios

Wake interaction studies of flow past tandem circular cylinders for different diameter and gap ratios

Exploiting Hardware and Software Advances for Quadratic Models of Wind Farm Layout Optimization

A key aspect of the design of a wind farm is the wind farm layout optimization (WFLO) problem: given a wind farm site and information about the wind patterns, the problem is to decide the location of individual wind turbines to maximize energy production subject to proximity restrictions and wake-based interference between turbines. Given the pairwise wake interactions, it is natural to model the energy objective as a quadratic function as, indeed, has been done in some existing optimization approaches. However, state-of-the-art solutions often trade-off between speed in producing designs and quality in terms of finding and proving optimal solutions. In this work, we aim to find a balanced approach to obtain WFLO solutions quickly for interactive design and solve the problem to optimality when quality is more important. To this end, we exploit recent advances in optimization hardware and software that target quadratic constraints: commercial mixed integer linear solvers have been extended to address some quadratic problems and nascent specialized hardware, including quantum computing systems, have focused on solving quadratic unconstrained binary optimization (QUBO) problems. We introduce two novel quadratic programming models for WFLO: a quadratic constrained optimization problem (QCOP) with binary decision variables and a QUBO. A thorough numerical evaluation using a commercial solver and specialized QUBO hardware show that our quadratic framework achieves fast, high-quality solutions that improve the state of the art and strike a balance between speed and quality. In particular, the QUBO model delivers high quality solutions in a few seconds while the QCOP model can be used to find better solutions and provide quality guarantees over a longer run-time.

A Low-Order Wake Interaction Modeling Framework for the Performance of Ocean Current Turbines Under Turbulent Conditions

A Low-Order Wake Interaction Modeling Framework for the Performance of Ocean Current Turbines Under Turbulent Conditions