Wake Motion Research Articles

A Simulator Development of Surface Warship Torpedo Defense System considering Bubble-Generating Wake Decoy

The wake-homing underwater guided weapon that detects and tracks wake generated during voyage of a surface ship is impossible to avoid with the present acoustic deception torpedo defense system. Therefore, research on bubble-generating wake decoy is necessary to deceive wake-homing underwater guided weapon. Experiments in various environments are required to verify the effective operation method and performance of the wake decoy, but performance verification through underwater experiment is limited. In this paper, we develop a simulator for an torpedo defense system of surface ship, which is applied bubble-generating wake decoy, against acoustic, wake, and hybrid homing underwater guided weapon attack. The simulator includes surface ship model, acoustic decoy(static, mobile) model, bubble-generating wake decoy model, search and motion model of underwater guided weapon and so on. By integrating various models, MATLAB GUI simulator was developed. Through the simulation results for various environmental variables by this simulator, it is judged that effective operation method and performance verification of the bubble-generating wake decoy can be performed.

Comparison of the dynamic wake meandering model against large eddy simulation for horizontal and vertical steering of wind turbine wakes

Accurately predicting the evolution of wake is crucial for power output and structural load estimation in wind farms. This study aims to validate the dynamic wake meandering (DWM) model, an efficient mid-fidelity wake model, against large eddy simulation (LES). The predictive capabilities of the DWM model for various wake properties, namely time-averaged wake deficit, mean wake center deflection, amplitude, and frequency spectrum of wake meandering, are comprehensively analyzed using the IEA 15MW reference wind turbine under different yaw and tilt misalignment angles. Two turbulent inflows with varying shear and turbulence intensity levels are considered. The comparison highlights the significance of the filter size (Cmeand) in DWM as a key parameter determining simultaneously the time-averaged wake deflection and meandering amplitude, with optimal values differing for horizontal and vertical wake displacements. When the appropriate Cmeand values are selected, the implementation of the DWM model in FAST.Farm demonstrates good agreement with LES data, particularly concerning time-averaged wake deficit, wake centerline deflection, and wake meandering amplitude at eight rotor diameters downstream. However, the DWM model tends to overestimate the energy in the lower frequency region with Strouhal’s number less than 0.1 and underestimate the wake oscillation induced by the shear-layer at higher frequencies, even though the wake motion standard deviation is accurately reproduced if the polar grid size is properly adjusted. Furthermore, the influence of the ground effect on downward wake deflection through tilt control is revealed. These findings clearly demonstrate the strengths and weaknesses of the current DWM model and can serve as a reference for the development of advanced wake models.

Nonlinear inviscid aerodynamics of a wind turbine rotor in surge, sway, and yaw motions using a free-wake panel method

Abstract. We investigate the aerodynamics of a surging, heaving, and yawing wind turbine with numerical simulations based on a free-wake panel method. We focus on the UNAFLOW (UNsteady Aerodynamics of FLOating Wind turbines) case: a surging wind turbine which was modeled experimentally and with various numerical methods. Good agreement with experimental data is observed for amplitude and phase of the thrust with surge motion. We achieve numerical results of a wind turbine wake that accurately reproduce experimentally verified effects of surging motion. We then extend our simulations beyond the frequency range of the UNAFLOW experiments and reach results that do not follow a quasi-steady response for surge. Finally, simulations are done with the turbine in yaw and heave motion, and the impact of the wake motion on the blade thrust is examined. Our work seeks to contribute a different method to the pool of results for the UNAFLOW case while extending the analysis to conditions that have not been simulated before and providing insights into nonlinear aerodynamic effects of wind turbine motion.

Modal decomposition of flow instabilities in a straight turbine diffuser around the best efficiency point

Hydraulic turbines sometimes exhibit a sharp efficiency drop around the best efficiency point. The drop is known to originate from large flow separations in their draft tubes, limiting their ability to recover part of the residual kinetic energy exiting the runner. While the conditions leading to the onset of these separations are not yet understood, the potentially unstable vorticity distribution at the runner exit led to the hypothesis that those separations are the result of an interaction between the flow at the center of the draft tube and the boundary layer at the walls. To study this hypothesis, the turbulent flow inside the draft tube of a bulb turbine was measured with time-resolved particle-image velocimetry (TR-PIV). In this work, coherent structures are identified from spectral proper orthogonal decomposition (SPOD) of the velocity fields to correlate changes in their topology with the efficiency drop. Special attention is given to the periodic vortical motions in the runner's wake, whose shape and energy content are found to be linked to the flow rate. Three-dimensional reconstructions of the underlying structure reveal a shift in its topology that correlates with the efficiency drop and separations at the wall. In addition, comparisons of the SPOD coefficients with the runner position show that the phase angle between the structure and the runner remains the same for each operating condition, suggesting a link with a rotating flow imbalance in the runner blade channels.

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.

The effect of the body wake and operator motion on the containment of nanometer-scale airborne substances using a conventional fume hood and specially designed enclosing hood: a comparison using computational fluid dynamics

The effect of the body wake and operator motion on the containment of nanometer-scale airborne substances using a conventional fume hood and specially designed enclosing hood: a comparison using computational fluid dynamics

Hydrokinetic power scavenging from galloping phenomenon with two juxtaposed bluff bodies

A numerical investigation is performed on the flow-induced vibrations (FIVs) of two coupled cylinders with various cross-sections and their performance on power scavenging. The shear stress transport (SST) k - ω turbulence model is employed to study the turbulent conditions. The center distance ratio (l*) values of cylinders are divided into four cases (1.5D, 2D, 2.5D, and 3D) for comparative analysis. The results demonstrate that the reduced velocity and Reynolds number play key roles in the wake motion and dynamic responses of the coupled system. It has also been proven that the square-based cylinder system exhibits galloping without the presence of any vortex-induced vibration through four benchmarks of computational fluid-structure interaction (FSI) models. Considering the coupled square and circular cylinder system, the results indicate that depending on the distance between the two connected cylinders, the Reynolds number and the reduced velocity have a significant influence on the dynamics and efficiency of the system. Additionally, galloping dominants for all considered distances between the square and circular cylinders.

Wake response downstream of a spanwise-oscillating hemispherical turret

An experimental Pressure Sensitive Paint (PSP) study of the unsteady pressure fields downstream of and on an oscillating hemispherical turret at subsonic freestream Mach numbers between 0.3 and 0.6 and Reynolds numbers of about 2 million is presented. The oscillating turret consisted of a hemispheric shell, mounted on an aluminum rectangular plate and is designed to oscillate in the spanwise direction at a single frequency, which lies within the frequency range of the dominant wake motion. The resonant-based aero-elastic response of the turret to the flow resulted in intermittent turret oscillations. Pressure fluctuations in the wake were found to be less energetic for the oscillating turret and the size of the recirculating region was increased, compared to the stationary turret. POD-based modal analysis of the pressure fields revealed that the wake dynamics is characterized by global anti-symmetric and symmetric shedding and showed that the oscillating turret primarily suppressed the global anti-symmetric shedding. The analysis of the unsteady forces acting on the turret demonstrated that the oscillating turret reduces the spanwise component of the unsteady force, while keeping other force components mostly unchanged. Intermittent nature of the spanwise force, which is responsible for the intermittent motion of the oscillating turret, was observed and discussed.

Drag reduction mechanisms on a generic square-back vehicle using an optimised yaw-insensitive base cavity

Regulations on global greenhouse gas emission are driving the development of more energy-efficient passenger vehicles. One of the key factors influencing energy consumption is the aerodynamic drag where a large portion of the drag is associated with the base wake. Environmental conditions such as wind can increase the drag associated with the separated base flow. This paper investigates an optimised yaw-insensitive base cavity on a square-back vehicle in steady crosswind. The test object is a simplified model scale bluff body, the Windsor geometry, with wheels. The model is tested experimentally with a straight cavity and a tapered cavity. The taper angles have been optimised numerically to improve the robustness to side wind in relation to drag. Base pressures and tomographic Particle Image Velocimetry of the full wake were measured in the wind tunnel. The results indicate that a cavity decreases the crossflow within the wake, increasing base pressure, therefore lowering drag. The additional optimised cavity tapering further reduces crossflow and results in a smaller wake with less losses. The overall wake unsteadiness is reduced by the cavity by minimising mixing in the shear layers as well as dampening wake motion. However, the coherent wake motions, indicative of a balanced wake, are increased by the investigated cavities.Graphical abstract

On the correlation between aerodynamic drag and wake flow for a generic high-speed train

The current research attempts to describe the association between the aerodynamic drag and flow structures induced by train running in time-averaged and time-dependant views. A ICE2 train model is used as the research object, with the flow field solved by Large Eddy Simulation (LES). The validation is carried out through comparison with existing research data, and grid independence study in terms of time-averaged and second-order statistics. Numerical results are firstly presented by description of the time-averaged near wake flow topology based on surface flow visualization. Then, the wake integration method is used both time-averaged and instantaneously to link the aerodynamic force with integrated flow quantities on different wake planes: from the time-averaged perspective, the contribution of different flow terms to total drag is presented and compared; from the instantaneous perspective, the overall wake variation tendency is linked to the fluctuating component of aerodynamic force, based on the wake convection velocity and downstream distance of wake planes. The dynamic features of flow quantities in different locations are presented, with cross-correlation method used to describe the correlation between fluctuating aerodynamic drag and wake motion. Approaches and findings presented in the current study could serve as fundamentals which support future study.

Analysis of V-Gutter Reacting Flow Dynamics Using Proper Orthogonal and Dynamic Mode Decompositions

The current work is focused on investigating the potential of data-driven post-processing techniques, including proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) for flame dynamics. Large-eddy simulation (LES) of a V-gutter premixed flame was performed with two Reynolds numbers. The flame transfer function (FTF) was calculated. The POD and DMD were used for the analysis of the flame structures, wake shedding frequency, etc. The results acquired by different methods were also compared. The FTF results indicate that the flames have proportional, inertial, and delay components. The POD method could capture the shedding wake motion and shear layer motion. The excited DMD modes corresponded to the shear layer flames’ swing and convect motions in certain directions. Both POD and DMD could help to identify the wake shedding frequency. However, this large-scale flame oscillation is not presented in the FTF results. The negative growth rates of the decomposed mode confirm that the shear layer stabilized flame was more stable than the flame possessing a wake instability. The corresponding combustor design could be guided by the above results.

Investigation of the motion characteristics for a spring-mounted rotating cylinder in flow

A spring mounted rotating cylinder placed in flow is investigated using a two-dimensional newly improved discrete vortex method at Reynolds number of 105. In this study, the non-dimensional rotation rate, $$\alpha$$ (rate of the cylinder surface velocity and flow velocity), varies from 0 to 16, and the reduced velocity $${\text{Ur}}$$, increases from 1 to 14. The relationship between the wake formation, hydrodynamics, and motion of the cylinder has been illustrated. Three different motions (VIV, reverse direction whirl, and same direction whirl) have been identified and illustrated. The VIV mainly occurs in the cross-flow direction, and it is generated at $$\alpha 3$$. The path of the same direction whirl looks like a slight round circle, and the moving direction is the same as the rotating direction. The frequency of the whirl is higher than that of the VIV at the same rotation rate. The same direction whirl starts from $$\alpha = 8$$ with $${\text{Ur}} = 13$$, and the frequency of this whirl increases with increasing rotation rate. Unlike that of the same direction whirl, the moving direction of the reverse direction whirl is opposite to the rotating direction, and the path of the whirl similarly seems to be a round circle. The reverse direction whirl is generated at $$2 \le \alpha \le 7.5$$. The frequency of the reverse direction whirl decreases with increasing the rotation rate due to the increased effective added mass. The relationship between the effective added mass coefficient and the rotation rate has been interpreted; the effective added mass coefficient remains steady at $$0 \le \alpha \le 1$$ then gradually increases at $$1 < \alpha < 2$$, and finally dramatically increases with increasing rotation rate at $$\alpha \ge 2$$.

Model-based estimation of vortex shedding in unsteady cylinder wakes

This paper considers single-sensor estimation of vortex shedding in cylinder wakes at $Re=100$ in simulations and at $Re=1036$ in experiments. A model based on harmonic decomposition is developed to capture the periodic dynamics of vortex shedding. Two model-based methods are proposed to estimate time-resolved flow fields. First, Linear Estimation (LE) which implements a Kalman Filter to estimate the flow. Second, Linear-Trigonometric Estimation (LTE), which utilizes the same Kalman Filter together with a nonlinear relationship between harmonics of the vortex shedding frequency. LTE shows good performance and outperforms LE regarding the reconstruction of vortex shedding. Physically this suggests that, at the Reynolds numbers considered, the higher harmonic motions in the cylinder wake are slave to the fundamental frequency.

Unsteady response of airfoils due to small-scale pitching motion with considerations for foil thickness and wake motion

Unsteady pressures, forces, and pitching moments generated by foils experiencing vibratory motion in an incompressible, attached flow configuration are studied within this work. Specifically, two-dimensional, unsteady potential flow and unsteady Reynolds-Averaged Navier–Stokes calculations are performed on various Joukowski foils undergoing sinusoidal, variable amplitude, small-scale pitching motion at a chord-based Reynolds number of 106 over a range of reduced frequencies between 0.01–100. These calculated results from both approaches are compared directly to predictions from implementing the Theodorsen model, which treats foils as infinitely thin, flat plates that shed a planar sheet of vorticity. The effects of relaxing these seemingly strict conditions are explored, and the particular terms which control the unsteady responses are identified and discussed. For increasing pitch amplitudes and reduced frequencies the shed wake is seen to become quite non-planar and to form coherent vortex structures. Despite this wake behavior, the normalized airfoil responses at the disturbance reduced frequency are seen to be largely unaffected. However, non-negligible responses are generated across a wide range of other frequencies. Potential flow calculations for symmetric Joukowski foils show that there is marginal effect of foil thickness at reduced frequencies less than one. For higher reduced frequency conditions however, the unsteady lift response is seen to experience both an amplification of level and a phase shift relative to the Theodorsen model. A specific augmenting expression is developed for this behavior through analysis within the potential flow framework.

A single van der pol wake oscillator model for coupled cross-flow and in-line vortex-induced vibrations

In this study a new wake oscillator model is proposed to describe the coupled cross-flow and in-line vortex-induced vibrations of an elastically supported rigid cylinder. Different from many other studies where two wake oscillators have been applied, the current model uses only one wake oscillator coupled to both cross-flow and in-line motions. The new model is based on the van der Pol oscillator with the classic acceleration coupling between the wake and cross-flow motion, while the in-line motion is coupled with the wake variable in a nonlinear manner. The predictions of this new model are compared with the existing experimental data and shown to be in good agreement. In addition to the conventional lock-in range that corresponds to reduced velocities between 5 and 8, another lock-in is predicted around reduced velocity of 2.5 due to the in-line vibration. Most importantly, the new model is proved to be able to predict the appearance of the ‘super-upper’ branch at small mass ratios without changing the tuning parameters. The limitations of the model associated with unrealistic predictions of free vibrations with very small mass ratios and those of forced in-line vibrations at high frequencies are also discussed along with a possible remedy.

Geometry effects on mean wake topology and large-scale coherent structures of wall-mounted prisms

Turbulent wake behind five wall-mounted rectangular prisms is measured by time-resolved particle-image velocimetry with an aim to generalize the effects of prism geometry (in terms of aspect ratio, AR, and side ratio, SR) on the wake characteristics. For the time-averaged wake, the arch-type topology is well supported. With AR increasing from 1 to 4 for the square prisms, a transition process between the “dipole” and “quadrupole” wake patterns is observed. On the other hand, an increase in SR does not seem to greatly influence the mean wake structure behind rectangular prisms unless reattachment occurs on the side walls. The turbulent wake velocity fields are analyzed with a proper orthogonal decomposition method, considering the intrinsic symmetries of vortex shedding activities. In the extraction of dominant low-order flow structures, the low-frequency shift mode and periodic vortex shedding mode pair are always captured as the most important turbulent kinetic energy contributors but with significant nonlinear cross-superposition. A low-pass Gaussian filter is then introduced to achieve separation between these two types of coherent structures via new spatial modes, which maintain the same mode shape patterns. The size of the shift mode vortex is found closely related to the time-averaged wake structure, while the pattern of the Karman vortex shedding mode pair seems completely independent. The two separated pure modes develop continuously and compete with each other along the model height. The present study appears to be the first one to investigate the effects of prism geometry on large-scale coherent wake motions from this perspective.

A Simplified Numerical Model for the Prediction of Wake Interaction in Multiple Wind Turbines

In the design of wind energy farms, the loss of power should be seriously considered for the second wind turbine located inside the wake region of the first one. The rotation of the first wind-front rotor generates a high-vorticity wake with turbulence, and a suitable model is required in computational fluid dynamics (CFD) to predict the deficit of energy of the second turbine for the given configuration. A simplified numerical model based on the classical momentum theory is proposed in this study for multiple wind turbines, which is proposed with a couple of tuning parameters applied to Reynolds-averaged Navier-Stokes (RANS) analysis, resulting in a remarkable reduction of computational load compared with advanced methods, such as large eddy simulation (LES) where two parameters reflect on axial and rotational wake motion, simply tuned with the wind-tunnel test and its corresponding LES result. As a lumped parameter for the figure of merit, we regard the normalized efficiency on the kinetic power output of computational domain, which should be directed to maximize for the optimization of wind farms. The parameter surface is plotted in a dimensionless form versus intervals between turbines, and a simple correlation is obtained for a given hub height of 70% diameter and a fixed rotational speed tuned from the experimental data in a wide range.

Spatiotemporal Correlations in the Power Output of Wind Farms: On the Impact of Atmospheric Stability

The dependence of temporal correlations in the power output of wind-turbine pairs on atmospheric stability is explored using theoretical arguments and wind-farm large-eddy simulations. For this purpose, a range of five distinct stability regimes, ranging from weakly stable to moderately convective, were investigated with the same aligned wind-farm layout used among simulations. The coherence spectrum between turbine pairs in each simulation was compared to theoretical predictions. We found with high statistical significance (p &lt; 0.01) that higher levels of atmospheric instability lead to higher coherence between turbines, with wake motions reducing correlations up to 40%. This is attributed to higher dominance of atmospheric motions over wakes in strongly unstable flows. Good agreement resulted with the use of an empirical model for wake-added turbulence to predict the variation of turbine power coherence with ambient turbulence intensity (R 2 = 0.82), though other empirical relations may be applicable. It was shown that improperly accounting for turbine–turbine correlations can substantially impact power variance estimates on the order of a factor of 4.

Secondary vortex street in the intermediate wake of a circular cylinder

Particle image velocimetry measurements are carried out in the wake of a circular cylinder at two values of the Reynolds number (3460 and 5800), based on the free stream velocity and the cylinder diameter, to investigate the spatial organization of vortical motions in the intermediate wake. The proper orthogonal decomposition method (POD) is used to extract information from the vorticity data. While the coherent motion associated with the von Karman vortex street is well reflected in the first two POD modes which account for about 8% of the total enstrophy, the motion associated with the secondary vortex street is captured in the third and fourth POD modes which account for only about 2.4% of the total enstrophy. The measurements show that the secondary vortex street only alternates in space. This is significantly different from the far wake where there is continuous switching between symmetric and antisymmetric arrangements about the centreline (e.g. Bisset et al., J Fluid Mech 218:439–461, 1990), the alternative regime being nearly twice as frequent as the opposite regime.

Evolution of the bi-stable wake of a square-back automotive shape

Square-back shapes are popular in the automotive market for their high level of practicality. These geometries, however, are usually characterised by high drag and their wake dynamics present aspects, such as the coexistence of a long-time bi-stable behaviour and short-time global fluctuating modes that are not fully understood. In the present paper, the unsteady behaviour of the wake of a generic square-back car geometry is characterised with an emphasis on identifying the causal relationship between the different dynamic modes in the wake. The study is experimental, consisting of balance, pressure, and stereoscopic PIV measurements. Applying wavelet and cross-wavelet transforms to the balance data, a quasi-steady correlation is demonstrated between the forces and bi-stable modes. This is investigated by applying proper orthogonal decomposition to the pressure and velocity data sets and a new structure is proposed for each bi-stable state, consisting of a hairpin vortex that originates from one of the two model’s vertical trailing edges and bends towards the opposite side as it merges into a single streamwise vortex downstream. The wake pumping motion is also identified and for the first time linked with the motion of the bi-stable vortical structure in the streamwise direction, resulting in out-of-phase pressure variations between the two vertical halves of the model base. A phase-averaged low-order model is also proposed that provides a comprehensive description of the mechanisms of the switch between the bi-stable states. It is demonstrated that, during the switch, the wake becomes laterally symmetric and, at this point, the level of interaction between the recirculating structures and the base reaches a minimum, yielding, for this geometry, a 7% reduction of the base drag compared to the time-averaged result.