Wake Interference Effects Research Articles

Wake interference effects for two surface-mounted cubes in tandem

Wake interference effects for two surface-mounted cubes in tandem

Enhancing stability and performance of flexible membrane structures in wake flows through jet flow control

This study investigates jet flow control on flexible membrane structures placed in the wake of a stationary cylinder to enhance performance and stability. Flexible membranes, found in both nature and engineering, often face nonlinear disturbances where passive deformation is insufficient. Inspired by adaptive fish fins, we explore active flow control strategies using a high-fidelity fluid–structure interaction solver to simulate the dynamics of a flexible membrane downstream of a cylinder. High-momentum jet flows with different momentum coefficients are applied at membrane leading edge to modulate the boundary layer flow features. Numerical simulation results reveal that jet flow control can weaken the wake interference effect on the membrane dynamics, resulting in lift improvement and flow-induced vibration suppression. The flow separation within the boundary layer is suppressed by high-momentum jet flows, thereby enhancing lift performance and stability. The reason of the flow-induced vibration suppression is attributed to the redistribution of the vibration energy in high-order structure modes excited by jet flows. Our results demonstrate that jet control has great potential for optimizing the performance and stability of flexible membrane structures in complex flow environments, providing valuable insights into the design of next-generation intelligent underwater vehicles and ships with flexible membrane propulsion systems.

Wake interference effects on flow-induced vibration of flexible membrane wings

This work investigates the effect of wake interference on the nonlinear coupled dynamics and aerodynamic performance of flexible membrane wings at a moderate Reynolds number. A high-fidelity computational aeroelastic framework is employed to simulate the flow-induced vibration of flexible membrane wings in response to unsteady vortex wake flows produced by an upstream stationary circular cylinder. The coupled dynamics of the downstream membrane are investigated at different gap ratios, aeroelastic numbers, and offset distances. The variations in flow features, membrane responses, and frequency characteristics are analyzed to understand the wake interference effect on membrane aeroelasticity. The results indicate that the aerodynamic performance and flight stability of the downstream membrane are degraded under the wake interference effect. Four distinct flow regimes are classified for the cylinder–membrane configuration, namely (i) single body flow, (ii) co-shedding I, (iii) co-shedding II, and (iv) detached vortex-dominated vibration, respectively. The mode transition is found to build new frequency synchronization between the flexible membrane and its own surrounding flows, or the wake flows of the cylinder, to adjust the aerodynamic performance and membrane vibration. This study sheds new light on membrane aeroelasticity in response to wake flows and enhances understanding of the fluid–membrane coupling mechanism. These findings can facilitate the development of next-generation bio-inspired drones that have high flight efficiency and robust flight stability in gusty flows.

Wake Interactions of Two Tandem Semisubmersible Floating Offshore Wind Turbines Based on FAST.Farm

Wake effects commonly exist in offshore wind farms, which will cause a 10–20% reduction of whole power production as well as a 5–15% increase of fatigue loading on the wind turbine main structures. Obviously wake interaction between floating offshore wind turbine (FOWT) is more complicated, and needs careful assessment which is a prerequisite for active wake control (AWC). The primary objective of the present research is to investigate in detail how the wake inflow condition, streamwise spacing, turbulence intensity, and wind shear influence the power performance, platform motion dynamic and structural loading of FOWT. FAST.Farm, developed by the National Renewable Energy Laboratory (NREL), was used for simulating two tandem FOWTs in different conditions. Comparisons were made between FOWTs in different conditions on power performance and platform motion dynamic, which were presented through both time and frequency domain analysis. Damage equivalent loads change in FOWTs interference under typical working conditions were discussed and summarized. Half wake inflow would pose many challenges to the downstream FOWT. These research studies can be incorporated into further offshore wind farm wake models, providing applicable AWC strategies to reduce wake interference effects for higher energy production and for the longer life of FOWT.

Numerical investigations of coupled aeroelastic performance of wind turbines by elastic actuator line model

With the economic considerations of wind power generation, blades of wind turbines are gradually becoming longer and more flexible. Therefore, it is necessary to research the influence of coupling aerodynamic and elastic structure of blades of wind turbines. Moreover, in a wind farm, the turbulent wind is generated by the irregular movement of the atmosphere, which makes the elastic deformations of blades and the complex wake interference effect between wind turbines. Therefore, the coupled aeroelastic performance of the blade is applied to analyze the interaction of the wake flow field of two tandem wind turbines based on the elastic actuator line model and the Euler-Bernoulli beam theory. Firstly, the correctness of the coupled aeroelastic model is illustrated by comparing the aeroelastic response with OpenFAST software. Secondly, the influence of aeroelastic performances is researched on the structural deformations of blades. It is found that the aerodynamic power and thrust decrease after considering the deformations of blades. Furthermore, the interference effect on the thrust is more evident than aerodynamic power. Thirdly, the far wake region is affected apparently when the deformations of blades are taken into consideration, which causes the tip vortex interference phenomenon to delay. Finally, compared with the uniform inflow condition, the displacement and speed of the deformations of the blades are both increased under the turbulent inflow condition. Meanwhile, the disturbance of the wake vortex in the far wake region by the turbulent wind is stronger. Besides, by comparing the differences between the four vortex identification methods, it is significant that the ΩR method can better capture the vortex system behind the rotor plane without manual intervention threshold selection. The vortex structure also represents the vorticity of the wake vortex evolution.

Construction of offline predictive controller for wind farm based on CNN–GRNN

In order to reduce Levelized Cost of Energy(LCOE), the wind farm development trend includes mainly two points: (1) rotor diameter of wind turbines will be larger, (2) density of offshore facilities will be higher. Consequently, the wake interference effect will be more severe, and predictive control has become one of the main studies for reducing wake disturbance. However, most of these strategies are usually computationally expensive and have limitations for large-scale wind farms since the optimization cost tends to increase exponentially with prediction horizons and number of wind turbines, which is unfavorable for real-time control. Considering control effectiveness and online computational efficiency, an offline predictive controller using convolutional neural network-general regression neural network (CNN–GRNN) is proposed for wind farm power control considering fatigue load. A CNN–GRNN hybrid network is trained to carry out the mapping between system states and optimal control settings calculated by particle swarm optimization (PSO) algorithm under various wind directions and wind speeds, and CNN–GRNN acts as an offline optimizer to provide ideal control laws with negligible online computation workload. The power at Horns Rev 1 can be improved up to 5.6% in comparative experiments with lookup table (LUT) strategy while keeping the fatigue load within the preset limitation, and the extensive simulation results under various wind conditions indicate that this proposed strategy can provide a more desirable performance for wind farm power control with fatigue constraint while retaining the advantage of negligible online computational burden.

An experimental investigation on interfering VIVs of double and triple unequal-diameter flexible cylinders in tandem

Many unequal-diameter risers have been widely applied to satisfy different operational requirements in ocean oil and gas drilling engineering, and adjacent tandem risers can undergo complex vibration responses and even collisions under ocean flow conditions. To investigate the wake interference responses for multiple risers in tandem configuration, an experiment of double and triple unequal-diameter flexible cylinders was conducted under uniform flow with Reynolds numbers Re ranging from 2832 to 11,328 for the larger cylinder and from 1805 to 7220 for the smaller cylinder. The two cylinders were 8 m in length with aspect ratios equal to 282.49 and 443.21, respectively, and the corresponding diameter ratio was 0.64. Two different tandem configurations were arranged with a large cylinder upstream and downstream, and the wall spacing between adjacent cylinders was fixed at 4D, where D is the diameter of small cylinders. Fiber Bragg grating strain sensors were used to measure both in-line (IL) and cross-flow (CF) strain responses. The modal superposition method was applied to reconstruct displacement responses based on measured strains, and drag forces were further inversely identified from the mean IL displacements. Equivalent wake velocities were calculated from drag force reductions. The results show that the downstream large cylinder in a triple-cylinder system endures a stronger wake interference effect than that in a double-cylinder system. Wake interference can decrease the dominant mode of downstream cylinders and increase the vibration amplitudes due to the reduction in the local wake velocity. In addition, a downstream cylinder undergoing wake-induced vibrations has the same dominant frequency in the CF and IL directions, and its CF frequency is identical to that of the upstream cylinder.

Study on the collision probability of vertical risers considering the wake interference effect

This paper develops a model to predict the collision probability of risers. Considering the wake interference effect, a dual-riser model is built based on the vector form intrinsic finite element. The central difference method is employed to obtain the dynamic responses of risers. Then, an analytical geometry-based procedure is established to identify a minimum distance from the displacement response in a duration. A random sample of the minimum distance is obtained based on a series of repeated such processes. Further, the collision probability is predicted from this random sample utilizing the probability paper method and the least-squares fitting method. Finally, the effects of critical parameters (i.e., initial spacing, top tension factor (TTF), reduced velocity) on the collision probability of two vertical risers arranged tandem are investigated. Results show that the collision probability is considerably underestimated without considering the wake interference effect. Moreover, an increase in TTF of the upstream riser (URTTF) or a decrease in that of the downstream riser (DRTTF) is more feasible to reduce the collision probability. For every 0.1 increases in URTTF, the collision probability drops by 2 orders of magnitude. For every 0.1 decreases in DRTTF, the collision probability drops by 1-2 orders of magnitude.

Prediction of Yawing Moment for a Hand-Launched UAV Considering Interference Effect of Propeller Wake

In this paper, three-dimensional unsteady computational fluid dynamic(CFD) analyses based on overset grid technique have been performed for a hand-launched unmanned aerial vehicle(UAV) considering the wake effect generated by a rotating propeller. In addition, the defection of rudder is considered in order to consider to predict the equilibrium condition of yawing moment during cruise flight conditions. It is importantly shown in this paper that the wake interference effect of the propeller is significant to accurately predict the yawing moment of the UAV and the yawing moment coefficient corresponding to a flight speed can be different because of its different amount of wake effect due to the different rotating speed of the propeller.

Flow-induced cross-flow vibrations of long flexible cylinder with an upstream wake interference

Flow-induced vibration (FIV) of a flexible cylinder with an upstream wake interference at a subcritical Reynolds number is numerically investigated in this study. Two cylinders are installed in a tandem arrangement with the tandem separation between the cylinder centers set at 5.0 diameters. The downstream cylinder is flexible and placed in the wake of the stationary rigid upstream cylinder. A quasi-three-dimensional fluid-structure interaction (FSI) numerical methodology that couples the strip theory-based Lagrangian discrete vortex method with the finite-element method (FEM) for structural dynamics is developed to simulate the FIV response of the flexible cylinder with the upstream wake interference. The vortex-induced vibration (VIV) of an identical isolated cylinder is also numerically simulated as a contrast. This numerical study characterizes the dynamic response of the cylinder FIV with the upstream wake interference and sheds light on the FSI mechanisms responsible for the structural dynamic response. With the upstream wake interference, the cylinder FIV response shows two features distinct from the isolated VIV response: the vibration of large amplitude during the modal resonance branch transition and the extension of the modal resonance branch. The hydrodynamic coefficients database is constructed by the rigid cylinder forced vibration experiment to help explain the FSI properties of the FIV dynamic response. The lower added mass coefficient for the FIV with the upstream wake interference than the VIV of the isolated cylinder guarantees the synchronization between the vortex shedding frequency and the “true” natural frequency of the structure persisting to higher reduced velocity in a certain modal resonance response branch. The excitation coefficient distribution indicates that the cylinder FIV with the upstream wake interference reaches higher amplitude at high reduced velocity, instead of ceasing resonance as the isolated cylinder. The numerical wake visualization is shown and used to explain the correlation between the distribution of hydrodynamic coefficients along the cylinder span and the wake vortex mode. It is found that the upstream wake interference effect is strongly correlated with the vortex–structure interaction pattern between the upstream wake vortices and the downstream motion. When the upstream vortex impinges on the downstream cylinder and splits into subvortices, the effect of the upstream wake interference acting on the downstream cylinder reduces. When the downstream cylinder enters the gap between the upstream vortices over the entire vibration process, the upstream wake has a stronger interference effect on the downstream FIV response.

On the wake interference effects for flow around tandem bodies

The present study investigates the wake interference effects for flow around two tandem cylinders under the effect of gap spacing ranging from 0.5 to 10. The flow Reynolds number is fixed at 150, and lattice Boltzmann method is used as a numerical tool for this study. The results show that three different regimes exist depending on gap spacing and flow structure mechanism around cylinders: single cylinder body regime, reattachment regime and co-shedding regime. The results also show that in a particular regime multiple flow patterns may coexist and affect the behavior of fluid forces. This study reveals that there exists a critical value of spacing which alters the fluid flow characteristics abruptly. The drag coefficient of second cylinder is negative until critical spacing and becomes positive after that. It is found that when spacing crosses the critical value, both cylinders start shedding vortices and the drag coefficient oscillates. The wake interference effect is found to be dominant at small spacing values which weakens with increment in spacing between cylinders.

Low-Order Response Surface Model for Simulating Wake Interference in Aerial Refueling

Aerial refueling is a critical element of Navy operations, made difficult by the inherent aerodynamic interference between the tanker and receiver aircraft. Current flight simulation capabilities are unable to accurately reproduce the complex wake interference effects on the receiver aircraft, solely accounting for the inviscid vortex. This research develops an aerodynamic interference module that may be incorporated into the simulation environment to account for incremental changes in the forces and moments on the receiver aircraft as it approaches the tanker during typical refueling scenarios. The interference module is based on low-order response surface models derived from a database of high-fidelity computational fluid dynamics predictions. The low-order model encompasses all of the complex wake effects on the receiver aerodynamic coefficients and may be quickly interrogated for use in real-time flight simulation. Predicted aerodynamic coefficient increments are validated against flight data for an E-2D receiver trailing behind a KC-135 tanker.

Flow over a symmetrically curved circular cylinder with the free stream parallel to the plane of curvature at low Reynolds number

Three-dimensional flow past a symmetrically curved circular cylinder with the free stream parallel to the plane of curvature is numerically investigated by high-order spectral element method. The axial line of the cylinder is described in Cartesian coordinates as y=A[cosh(x∕A)−1] with A being a catenary coefficient used to scale its span ratio. In this geometry configuration, the deformed cylinder can be divided into two half segments, respectively of the convex segment in the upstream and the concave segment in the downstream. Different span ratios are investigated with respect to A=0.5∼10. It is found that four wake topologies, namely of single body, bi-stable, vortex impingement and steady wakes, are successively appeared with the increase of the span ratio. By examining the flow field quantities including mean and fluctuation of velocities and pressure along the axial line, some important characteristics of axial flow dynamics are intensively revealed. Most interestingly, the generation of axial flow along the curved surface of the cylinder is strongly influenced by not only the curvature ratio but also the wake interference effects.

Wake interference effects for two finite cylinders: A brief review and some new measurements

A brief review of the literature for the flow around two surface-mounted finite-height cylinders in a staggered configuration illustrates some of the additional complexity associated with changes in the cylinder aspect ratio (AR), the effects of downwash, the three-dimensional structure of the wake, and the presence of additional vortex structures. To date, however, few extensive systematic studies of centre-to-centre pitch ratio (P/D), incidence angle ( α), aspect ratio, boundary layer thickness on the ground plane (δ/D) and Reynolds number (Re) have been reported for two finite cylinders. Some new measurements of the mean drag and lift coefficients (CD and CL) and the Strouhal numbers (St) for two staggered finite cylinders, for AR = 9, 7, 5, and 3, P/D=1.125–4.5, and α = 0∘–90°, at Re = 6.5×104 and δ∕D=1.4, illustrate the moderating effect of the three-dimensional flow on the wind loading and vortex shedding behaviour compared to the flow around two infinite cylinders. These new results suggest some changes to the flow pattern boundaries and uncover some new trends in the CD, CL and St data with P/D and/or α; there is a need for new flow visualization, flow field measurements, and numerical simulations to advance physical interpretation.

Numerical Modeling of Slip Flow and Heat Transfer Over Microcylinders With Lattice Boltzmann Method

In this paper, a lattice Boltzmann (LB) model is established to simulate the gaseous fluid flow and heat transfer in the slip regime under the curved boundary condition. A novel curved boundary treatment is proposed for the LB modeling, which is a combination of the nonequilibrium extrapolation scheme for the curved boundary and the counter-extrapolation method for the macroscopic variables on the curved gas–solid interface. The established numerical model can accurately predict the velocity slip and temperature jump of the microscale gas flow on the curved surface, which agrees well with the analytical solution for the microcylindrical Couette flow and heat transfer. Then, the slip flow and the heat transfer over the single microcylinder are numerically studied in this work. It shows that the velocity slip and the temperature jump are obviously influenced by the Knudsen number and the Reynolds number, and the local Nusselt number depends on which gas rarefaction effect (velocity slip or temperature jump) is dominant. An increase in the Prandtl number leads to a decrease in the temperature jump, which enhances the heat transfer on the microcylinder surface. The numerical simulation of the flow and heat transfer over two microcylinders in tandem configuration are carried out to investigate the wake interference effect. The results show that the slip flow and heat transfer characteristics of the downstream microcylinder are influenced by the wake region behind the upstream cylinder as the spacing is small.

Proximity Effects on Characteristics of Flow around Three Inline Square Cylinders

This work presents the numerical investigations performed to study the proximity effects on fluid flow characteristics around three inline square cylinders using the lattice Boltzmann method. For this purpose the gap spacing (g) is systematically varied in the range 0.5 to 16 diameters of cylinder by keeping Reynolds number fixed at 200. Five different flow patterns are observed at different values of spacing: bluff body flow, gap trapped flow, irregular flow, alternate shedding, and modulated shedding. These patterns have a significant effect on flow induced forces and vortex shedding frequency. The spacing value g = 2 is found to be critical due to sudden changes in fluid flow characteristics. The flow parameters of first cylinder are found to be closer to single cylinder values but for middle and third cylinder the differences confirm the wake interference effect even at large values of spacing.

Wake and performance interference between adjacent wind farms: Case study of Xinjiang in China by means of mesoscale simulations

To ameliorate the efficiency of wind farms, except assessing the wake effect between the wind turbines, the wake interference between the wind farms must be considered. Based on the Weather Research and Forecasting (WRF) model, the overall performance and power output characteristics, and wake interference effects between the adjacent wind farms in Hami region of Xinjiang province under real terrain and atmospheric conditions were investigated. The wind turbine drag parameterization (WTDP) scheme was elaborated. The results show that the wake of the whole field generally recovers at downstream 16.5 km under prevailing wind direction and annual average wind speed, and the frequency of power output around the rated power is up to 30%. Moreover, the disturbance induced by the wake effect of a large-scale wind farm on its downstream adjacent farm was quantitatively evaluated. Due to the impact of the upstream farm, the wake distance of the downstream wind farm is doubled. The influence on the power output presented a regularity of day-night alternation, with a higher frequency of great loss at night, dawn and evening. The average relative loss ratio reached 5.8%. This study is expected to provide a theoretical basis and engineering guidance for micrositing of wind farms.

Stability analysis of the wake-induced vibration of tandem circular and square cylinders

We present a stability analysis to investigate the underlying fluid–structure modes during transverse wake-induced vibration (WIV) of an elastically mounted downstream cylinder in a tandem arrangement at low Reynolds number. The upstream cylinder with an equal diameter is kept stationary, whereas the downstream cylinder in the tandem arrangement is free to vibrate in the transverse direction. The WIV involves complex interaction dynamics of the upstream wake with the freely vibrating downstream cylinder, which leads to a relatively large transverse force and vibration in the post-lock-in region. We consider a data-driven model reduction approach to construct an eigenvalue representation of WIV system and perform the stability analysis to examine the underlying process of WIV for the tandem circular and square cylinders. The model reduction of the fluid system is constructed by the eigensystem realization algorithm (ERA) and coupled with a transversely vibrating bluff body in a state-space format. Unlike the wake-oscillator model, the ERA-based ROM does not rely on any empirical formulation and captures naturally the essential linear fluid dynamics through the solution of the Navier–Stokes equations. Results show that the WIV region and the onset reduced velocity can be predicted accurately by tracing the eigenvalue trajectories of the ERA-based low-dimensional model for a range of reduced velocities. The stability analysis reveals that there exists a persistently unstable eigenvalue branch that sustains WIV, which also implies that the highly nonlinear behavior of WIV has its linear origin. The sharp corner of the square cylinder is found to have the stabilizing effects, namely (i) the reduced velocity for the WIV onset is larger than its circular cylinder counterpart, (ii) the transverse response amplitude and the lift force are consistently lesser in both lock-in and post-lock-in regimes for the square cylinder configuration. This work has a potential impact on the development of control strategies for reducing undesired vibrations and loads in flexible structures undergoing wake interference effects.

Experimental study of wake-induced instability of coupled parallel hanger ropes for suspension bridges

Hangers in suspension bridges are usually composed of closely-spaced multiple cables, and they may suffer from server wind-induced vibrations due to the wake interference effect. The primary objective of this research was to examine the wake-induced instability of the coupled twin or quadruple cylinders that are tied together to have the same motion, instead of separate cylinders vibrating independently. The elastically mounted twin cylinders free to oscillate in streamwise and transverse directions were tested at various cylinder spacings. The Scruton (Sc) number, Sc = 2mδ/ρD2 (D is cylinder diameter, δ is logarithmic decrement, m is cylinder mass per unit length, and ρ is fluid density), was varied from about 65–420 by changing the damping, and the critical damping necessary to suppress the wake-induced instability was established. The wake-induced instability occurred with an elliptical orbit at cylinder spacing of 3.2D and wind incidence angle of 10°, while no instability was found at other spacings. The results of force measurements were presented and used to discuss the excitation mechanism based on the 1-DoF and 2-DoFs quasi-steady model. Despite some differences between theoretical analysis and measurements, the 2-DoFs model outperforms greatly the 1-DoF model in predicting the critical wind speed. For the coupled quadruple cylinders arranged in rectangular configuration, no large amplitude vibrations were found at all wind incidence angles due to complicated flow interference. Finally, the wake-induced instability of hanger subspans divided by spacers was investigated with a new aeroelastic model, and it was found that four spacers placed at equal intervals along hanger length are sufficient to suppress the wind-induced instability within the subspan for hangers in the Xihoumen Bridge.

Dynamics of tandem cylinders in the vicinity of a plane moving wall

We present dynamics of the flow around two cylinders in a tandem configuration along a moving plane wall. A spectral element method is employed to perform the simulations with high accuracy at Reynolds number Re=200. A moving wall with no-slip boundary is considered rather than a stationary wall to avoid the confusing interaction of the wall boundary layer and thus focus completely on the influence of wall proximity effects on the force and wake dynamics. The influence of the moving wall with gap ratio e/D=0.2–5.0 and longitudinal center-to-center separation L/D=1.5−8.0 on the unsteady force dynamics is examined for the two cylinder configuration. Through detailed analysis of the flow field dynamics, we observe early transition from reattachment to co-shedding behavior. At co-shedding separation distances, the combined wake interference and wall proximity effects lead to a parallel double-row of vortices for the tandem cylinders at Re=200 for e/D=0.5. For a longitudinal separation of 4D, the ratio of the street width h to distance between two adjacent vortices in the same row l is in good agreement with that obtained from inviscid theory. Finally, we provide detailed flow visualizations, Strouhal number and force coefficient trends and investigate recovery of freestream behavior as the tandem cylinder configuration of varying L/D is gradually distanced further from the moving plane wall.