Wake Characteristics Research Articles

CFD study on wake characteristics of a dual rotor wind turbine using different turbulence models

CFD study on wake characteristics of a dual rotor wind turbine using different turbulence models

Numerical simulation of wind turbine wake characteristics by flux reconstruction method

In the paper, a CFD method that employs the flux reconstruction (FR) method and the actuator line method (ALM) is proposed, to develop a novel numerical framework to improve the efficiency and accuracy of turbulent wake predictions of horizontal axis wind turbines (HAWT). The PyFR solver, which is an open-source program that uses the FR method is applied. The ALM is for the first time implemented in the FR solver to simulate HAWTs. To increase the fidelity of the wake prediction, the nacelle and tower of the HAWT are also modeled, and the proposed numerical simulation method is named as AL-PyFR(N&T) model. The NTNU “Blind Test 1” wind turbine is used to validate the proposed model. First, the mesh and time step dependence of the AL-PyFR(N&T) model are verified. Second, the accuracy of the proposed model is verified by comparing the velocity deficits and turbulent kinetic energy results at different downstream locations with the measured values and the results from other CFD studies. By comparison of the results with and without the effect of the nacelle and tower, it is found that tower wake plays a significant role in wake asymmetry. Crucially, the AL-PyFR(N&T) model demonstrates a capacity for accurate wake predictions with fewer mesh requirements compared to conventional CFD models. This efficiency marks a significant advancement for high-fidelity numerical simulations of offshore wind farms.

Experimental investigation of the wake replenishment mechanisms of paired counter-rotating vertical-axis wind turbines

The understanding of wake recovery mechanisms is crucial for the design of efficient wind farm layouts and the development of accurate wake models. Recently, placing two vertical-axis wind turbines (VAWTs) in close proximity has demonstrated potential for increased power output. In this study, wind tunnel experiments were conducted to investigate the wake replenishment mechanisms behind paired VAWTs. The experimental campaign included testing an isolated VAWT and paired counter-rotating VAWTs. By combining qualitative observations of key flow field variables with a quantitative analysis based on momentum conservation, this study aims to enhance our understanding of the mixing mechanisms supporting the reintroduction of streamwise momentum into the wake of paired VAWTs. This research also involves a comparison of these mechanisms with those observed in the wake of a standalone VAWT. The results show that the differences between isolated and paired VAWTs in overall wake characteristics are minimal. The increased lateral advection within the wake of the isolated VAWT is offset by the enhanced vertical advection in the paired configuration, as a result of the change in the direction of cross-stream velocity within the gap between the paired VAWTs, which promotes a shift towards vertical flow rather than lateral flow.

Experimental Investigation Of Open Channel Flow Around A Hemisphere At Different Relative Depths Using 3D-PTV

In regulated rivers and small streams, the presence of large roughness elements like boulders and concrete structures significantly impacts the local flow field and creates complex hydraulic conditions important for aquatic habitats. This study focuses on investigating and analyzing the complex three-dimensional flow field around an idealized stone (represented as a hemisphere) to understand how the flow field changes for different relative depths i.e. water depth relative to boulder height, and flow rates. Time-resolved 3D Particle Tracking Velocimetry was employed in laboratory experiments conducted in an open-channel flume, to mimic varying river conditions and their impact on flow characteristics. Results reveal that the relative depth significantly influences the flow structure behind the hemisphere, impacting recirculation zones and wake characteristics. The flow field behind the hemisphere was found to be more sensitive to changes in relative depth than bulk velocity. The study also highlights the larger regions of lower streamwise velocity that occurred downstream of the hemisphere caused by the lower submergences combined with a higher magnitude of the downward vertical velocity. Furthermore, this study demonstrates the capability of the measurement technique and the time-resolved 3D Particle Tracking Velocimetry system used to effectively capture the complex flow dynamics around the boulder. This approach allowed for detailed visualization and quantification of flow parameters such as streamwise and vertical velocities within the wake region, which are important factors for aquatic habitats. Understanding the underlying flow dynamics around an idealized stone at low relative depths could yield insights into ecohydraulic engineering and ecological restoration in shallow streams. This can contribute to advancing our understanding of riverine flow dynamics and their ecological implications.

Unsteady wake and heat transfer characteristics of three tandem circular cylinders in forced and mixed convection flows

In natural convection (high Richardson number Ri), a high Prandtl number (Pr) leads to thinner thermal boundary layers, enlarging the thermal gradient and hence the enhancement of buoyancy effect. In forced convection (low Ri), a high Pr introduces thicker velocity boundary layers. In mixed convection scenarios, where both forced and natural convection are significant, the interaction between Pr and Ri determines the resultant flow pattern and heat transfer characteristic. Three tandem circular cylinders with an identical spacing ratio of 4.0 in both forced and mixed convection flows were numerically investigated by using finite element method. The computations were carried out in the range of Pr = 5–50 and Ri = 0–2 at a low Reynolds number of Re = 150. The results of the squared strain rate and the vorticity shed light on the enstrophy transfer process. Thermal plume structures in the far wake originate from the upper dispersed vortices due to the high superimposed buoyancy at low Pr, while they are suppressed at high Pr. The increase in Pr plays a role as the flow stabilization, while the growth of Ri plays the reverse role. The time-averaged velocity, pressure coefficient, and temperature become more asymmetrical at high Ri. The Nusselt number of the upstream cylinder is approximately equal to the empirical result without the consideration of thermal buoyancy. Due to the thermal buoyancy, the migration of shear layers along the cylinder surface leads to the frequency alteration and harmonic frequency in the drag, lift, and Nusselt coefficients.

The flow interference investigation of multi-square prisms under fluid–structure interaction. I. Proximal wake characteristics of tandem square prisms

The flow interference investigation of multi-square prisms under fluid–structure interaction. I. Proximal wake characteristics of tandem square prisms

Study on the hydrodynamic and wake characteristics of variable speed control of horizontal axis tidal turbine under surge motion

Study on the hydrodynamic and wake characteristics of variable speed control of horizontal axis tidal turbine under surge motion

The groove effect on wake characteristics of rotating cylinders

In the present study, active and passive flow control methods have been implemented to investigate their effects on the wake flow structures of a circular cylinder. Grooves having circular, rectangular, and triangular cross sections have been applied to the cylinders exposed to the rotation rates, α, from 0 to 0.79. The experiments have been conducted by particle image velocimetry at a Reynolds number of Re = 5 × 103. The contour graphics of time-averaged results have been presented. Moreover, the variations in velocity profiles have also been depicted. The experimental results revealed significant variations for flow patterns, wake structures, and turbulence parameters due to the effects of both groove geometries and rotational motion. In the stationary cases, for turbulence intensity, the circular grooved cylinder exhibited a 15% increase, while the triangular grooved cylinder showed a slightly higher increase of around 20% compared to that of the bare cylinder (BC). Conversely, in non-stationary cases, the rectangular grooved cylinder displayed the most prominent reduction in turbulence intensity, decreasing by approximately 10% compared to that of the BC. The groove type has considerably affected the flow structures of the wake regions, especially for the lower rotation rates.

Numerical investigation on the influence of free surface on the hydrodynamic and wake characteristics of submarine

In the process of movement, the submarine inevitably involves free surface navigation, such as floating observation, suction power generation, and rescue. Under various circumstances, the interaction between the submarine and the free surface leads to complex flow fields and wakes, which affects its hydrodynamic performance. In this study, a three-dimensional numerical model of a submarine considering the influence of free surface is established. The volume-of-fluid model with an artificial compression term is used to capture the gas–liquid interface, and the unsteady flow field and hydrodynamics are predicted using the shear stress transport k–ω turbulence model. Based on the analysis of mesh convergence and numerical reliability, the hydrodynamic performance and wake flow field characteristics of submarines under different submergence depths were studied. The results show that the free surface has a significant influence on the resistance, wave wake, wake field, and vortex structure of the submarine, which is closely related to the submergence depth. Compared with the infinite submergence depth, the total resistance of the submarine near the free surface increases by 159.2%, mainly due to the pressure resistance. The surface wave system generated by the interaction between the free surface and the hull will directly affect the distribution of the wave surface wake, the wake flow field, and the vortex structure. As the submergence depth increases, the free surface effect gradually weakens, and it can be ignored when the submergence depth is more than 4 times the diameter of the hull.

Numerical investigation of power-law flow past two side-by-side identical circular cylinders

The non-Newtonian flow past multiple cylinders is widely encountered in engineering applications, such as slurry transport, petroleum drilling, and heat transmission systems using hot kerosene. However, the wake characteristics of non-Newtonian flow past multiple cylinders are far from well understood. This paper reports the numerical results of power-law flow past two side-by-side identical circular cylinders with a various gap ratio (G/D = 1.1–6.0) and a power-law index (n = 0.8–1.5) at a fixed Reynolds number (Re = 100) based on the incoming uniform flow velocity. Six wake patterns are identified, including the single bluff-body regime, deflected regime, in-phase regime, anti-phase regime, and two subclasses of flip-flopping regime (FF1 and FF2 regimes). The hydrodynamic coefficients of two cylinders are sensitive to both the gap ratio and the power-law index. The wake structure evolution is closely related to the wake patterns, and six modes of wake evolution are accordingly observed. Since the apparent viscosity of power-law fluid changes with the shear rate, the distribution of local Reynolds number (ReL) around the cylinder surface varies with the wake pattern. As it goes outward along the normal direction from the cylinder surface, the ReL shows a trend of increasing and then decreasing when n < 1, while the opposite trend is observed when n > 1.

Study on Aerodynamic Performance and Wake Characteristics of a Floating Offshore Wind Turbine in Wind–Wave Coupling Field

Floating offshore wind turbines (FOWTs) exhibit complex motion with multiple degrees of freedom due to the interaction of wind and waves. The aerodynamic performance and wake characteristics of these turbines are highly intricate and challenging to accurately capture. In this study, dynamic fluid body interaction (DFBI) and overset grid technology are employed to investigate the dynamic motion of a 5 MW FOWT. We use the volume of fluid (VOF) method and improved delayed detached eddy simulation (IDDES) model to investigate the aerodynamic performance and wake evolution mechanism for various wave periods and heights. According to the findings, the magnitude of the pitch motion increases with the period and height of the waves, leading to a decrease in both the power output and thrust; the maximum power was reduced by nearly 6.8% compared to a wind turbine without motion. The value of power and thrust reduction varies for different wave periods and heights, and is influenced by the relative speed and pitch angle, which play a crucial role. Wind–wave coupling has a significant impact on the evolution of both wake and vortex structures for FOWT. The wake shape downstream is also dynamically influenced by the waves. In the presence of wind and wave coupling, the interaction between the wind turbine and the wake is heightened, leading to the merger of two unstable vortex rings into a single, larger vortex ring. The research unveils a comprehensive picture of the offshore wind energy dynamics and wake field, which holds immense significance for the design of floating wind turbines and the optimization of wind farm layout.

Toward Scale-Adaptive Subgrid-Scale Model in LES for Turbulent Flow Past a Sphere

This study explores the dynamics of turbulent flow around a sphere at a Reynolds number of Re=103 using large-eddy simulation, focusing on the intricate connection between vortices and strain within the recirculation bubble of the wake. Employing a relatively new subgrid-scale modeling approach based on scale adaptivity, this research implements a functional relation to compute ksgs that encompasses both vortex-stretching and strain rate mechanisms essential for the energy cascade process. The effectiveness of this approach is analyzed in the wake of the sphere, particularly in the recirculation bubble, at the specified Reynolds number. It is also evaluated in comparison with two different subgrid-scale models through detailed analysis of the coherent structures within the recirculation bubble. These models—scale-adaptive, k-Equation, and dynamic k-Equation—are assessed for their ability to capture the complex flow dynamics near the wake. The findings indicate that while all models proficiently simulate key turbulent wake features such as vortex formation and kinetic energy distribution, they exhibit unique strengths and limitations in depicting specific flow characteristics. The scale-adaptive model shows a good ability to dynamically adjust to local flow conditions, thereby enhancing the representation of turbulent structures and eddy viscosity. Similarly, the dKE model exhibits advantages in energy dissipation and vortex dynamics due to its capability to adjust coefficients dynamically based on local conditions. The comparative analysis and statistical evaluation of vortex stretching and strain across models deepen the understanding of turbulence asymmetries and intensities, providing crucial insights for advancing aerodynamic design and analysis in various engineering fields and laying the groundwork for further sophisticated turbulence modeling explorations.

Experimental and numerical study on characteristics of wake and vortex structure of cluster personnel

The purpose of this study is to study the characteristics of unsteady flow field caused by human activities in crowded public space. This study analyses the wake and vortex structure characteristics of cluster personnel through experiments and numerical simulations. The wake velocity field and vortex structure of 2 (columns) × 5 (rows) human walking were analysed by experimental measurement and large eddy simulation (LES). Personnel movement is simulated by using the dynamic mesh model of computational fluid dynamics (CFD). The results show that the wake of the leg is fully developed, and the wake end presents a “swallow-tail shaped”. The rear row personnel will make a secondary disturbance to the wake of the front row personnel in the wake of the cluster personnel. Therefore, there will be a process of mutual transformation of large-scale vortices and small-scale vortices in the wake field. In addition, the wake flow significantly affects the diffusion and propagation of respiratory pollutants.

Analysis of wake and power fluctuation of a tidal current turbine under variable wave periods

In the Marine environment, waves and turbulence usually coexist, affecting the operation performance of the tidal current energy turbine. This paper conducts experiments on a scaled tidal stream turbine to study the wave period effect on the wake and power characteristics. Under a wave-current environment, the wake is found to display Gaussian distribution. Within the 4 times diameter downstream range (4D), the turbulence is significant, and both the wake velocity loss and fluctuation increase first and then decrease with wave period. After 6D, the flow field gradually recovers to the inflow level. The wave period affects the transversal turbulence, more in the near than far wake region, as the significant turbulence intensities at the hub and blade tips gradually merge and become consistent after 6D. The wave period affects little the power coefficient CP owing to the sinusoidal inflow velocity nature. However, the amplitude of the phase average power increases first and then decreases with the wave period. By contrast, the phase average power fluctuations are sensitive to cycle changes only within the low TSR interval; they decrease sharply with TSR. In addition, the energy distribution of power fluctuation in the wave flow coupling environment is mainly concentrated at the sum and difference, and as well multiples, of the rotational and wave frequency. The wave and its multiples energy distribution increases and then decreases with the wave period.

Dynamic wake behavior and aerodynamic characteristics of a custom airfoil under combined pitching and heaving motion

Dynamic wake behavior and aerodynamic characteristics of a custom airfoil under combined pitching and heaving motion

Connections between propulsive efficiency and wake structure via modal decomposition

We present experiments on oscillating hydrofoils undergoing combined heaving and pitching motions, paying particular attention to connections between propulsive efficiency and coherent wake features extracted using modal analysis. Time-averaged forces and particle image velocimetry measurements of the flow field downstream of the foil are presented for a Reynolds number of $Re=11\times 10^3$ and Strouhal numbers in the range $St=0.16\unicode{x2013}0.35$ . These conditions produce 2S and 2P wake patterns, as well as a near-momentumless wake structure. A triple decomposition using the optimized dynamic mode decomposition method is employed to identify dominant modal components (or coherent structures) in the wake. These structures can be connected to wake instabilities predicted using spatial stability analyses. Examining the modal components of the wake provides insightful explanations into the transition from drag to thrust production, and conditions that lead to peak propulsive efficiency. In particular, we find modes that correspond to the primary vortex development in the wakes. Other modal components capture elements of bluff body shedding at Strouhal numbers below the optimum for peak propulsive efficiency and characteristics of separation for Strouhal numbers higher than the optimum.

Rotor Performance and Wake Interaction of Controlled Dual Surging FOWT Rotors in Tandem

Using Large Eddy Simulation (LES) with Actuator Line Model (ALM), this work investigates the system of two surging wind turbine rotors operating under realistic turbulent inflow conditions (TI = 5.3%). The two rotors are placed in tandem with a spacing of 5D and the surging motions are harmonic. A widely used torque controlling strategy, MPPT (Maximum Power Point Tracking), is implemented to ensure a maximium power extraction under all conditions. The rotor performances as well as the field data are surveyed to examine the effectiveness and impacts of the controller. It is found that the power performances of the surging rotors are benefited by the controller with a small margin (∼ 1%) when the surging motions are moderate. The results also show that the controller reacts much slower than the considered surging frequency, making the power performances of the rotors worse than the quasi-steady predictions (targeted values) and complicating the system dynamics. In general, the implementation of the controller has minor impacts on the wake characteristics; however, the strengths of Surging Induced Periodic Coherent Structures (SIPCS) are found to be enhanced.

Impact of rotational direction on wake replenishment mechanisms of counter-rotating paired vertical-axis wind turbines

Recent studies have shown that the placement of paired vertical-axis wind turbines (VAWTs) in close proximity is a promising approach to design efficient VAWT farm layouts, mainly due to the increased power output. This study aims to experimentally investigate the impact of the rotational direction of paired counter-rotating VAWTs on their wake characteristics and replenishment mechanisms through a quantitative analysis using the momentum balance equation. The results of this study enhance our understanding of the effect of rotational direction on wake dynamics of counter-rotating paired VAWTs and offer valuable insights for optimising the design of VAWT farm configurations.

Numerical Investigation of Wake Properties of a Propeller Using Immersed Boundary Method

Abstract A level-set based sharp-interface immersed boundary method (IBM) was applied with large eddy simulation to simulate the complex flow field around a marine propeller. The wake characteristics under the open-water conditions including velocity, pressure and turbulent kinetic energy were analyzed. Additionally, the comparative analysis of the axial Reynolds stress of the propeller operating beneath incident waves and that under the open-water condition indicates that the nonlinear interaction between waves and wake leads to the increase in Reynolds stress within the radial range of r/D=0.35 to r/D=0.55 in the far-wake region. The obtained numerical simulation results confirmed the feasibility of the IBM for the numerical simulation of complex flows through hydraulic machinery such as propellers.

The effect of nacelle-to-rotor size on the wake of a miniature wind turbine

Wind tunnel experiments are performed to investigate the effect of nacelle-to-rotor size on the wake of a wind turbine under different Reynolds numbers. Four different turbine configurations are tested, which vary in the rotor diameter and nacelle length and diameter. The near wake region is observed to be significantly affected by the nacelle-to-rotor size of the turbine. The difference in the averaged streamwise velocity and streamwise turbulence intensity is affected more by the change in the rotor diameter than by the change in the nacelle diameter. This is likely due to the change in the nacelle length to rotor diameter ratio, and the change in the distance between the ground and rotor lower tip. The differences in the near wake characteristics are reduced by the increase in the Reynolds number. The onset of the far wake is unaffected by the nacelle-to-rotor size, and consequently, the far wake modeling can be performed with good accuracy using existing analytical models. The power coefficient values show an improvement with the increase in the Reynolds number due to the higher efficiency of the rotor and motor.