Wake Core Research Articles

Influence by the hub vortex on the instability of the tip vortices shed by propellers with and without winglets

Large-eddy simulations on a cylindrical grid consisting of 5 × 109 points are reported on both conventional and winglets propellers with and without a downstream shaft. Comparisons are focused on the influence by the hub vortex on the process of instability of the tip vortices. They demonstrate that in straight ahead conditions, this influence is actually quite limited for both propellers. The presence of the hub vortex at the wake core results in only a slight upstream shift of the instability of the tip vortices. Meanwhile, the development of the instability of the hub vortex is always delayed, compared to that of the tip vortices, and the former keeps coherent further downstream of their breakup. The results of this study highlight that the hub vortex is not a major source of instability of the tip vortices. Therefore, simplified configurations with no hub vortex, often adopted in the literature, can also provide a good approximation of the process of instability of the tip vortices shed by actual propellers. In contrast, the instability of the tip vortices could be the trigger of that of the hub vortex, whose development is slower. Therefore, experimental and computational studies aimed at analyzing the dynamics of the hub vortex should be designed accordingly, extending to further downstream distances.

Analysis of the momentum recovery in the wake of aligned axial-flow hydrokinetic turbines

Large-Eddy Simulations are reported, dealing with an axial-flow hydrokinetic turbine operating in the wake of an upstream one. Computations were conducted on a cylindrical grid consisting of 3.8 × 109 points, using an Immersed-Boundary methodology. The performance of the downstream turbine was negatively affected by the wake of the upstream one and substantially dependent on its distance. Results demonstrated a faster wake development, compared to the case of the same turbine operating in isolated conditions within a uniform flow, due to the faster instability of the tip vortices, induced by the perturbation of the inflow conditions by the wake of the upstream turbine. In contrast with the turbine performance, the process of wake recovery was found rather insensitive to the distance from the upstream turbine. In comparison with the case of the isolated turbine, the role of radial turbulent transport just downstream of the instability of the tip vortices was found especially important in accelerating the process of wake recovery at the outer radii, providing a significant contribution together with radial advection. Further downstream, the contribution by turbulent transport was verified reinforced also within the wake core, where instead momentum replenishment by radial advection was rather limited.

Dominant flow features in the wake of a wind turbine at high Reynolds numbers

Dominant flow features in the near and intermediate wake of a horizontal-axis wind turbine are studied at near field-scale Reynolds numbers. Measurements of the axial velocity component were performed using a nano-scale hot-wire anemometer and analyzed using spectral methods to reveal the extent and evolution of the flow features. Experiments were conducted at a range of Reynolds numbers, of 2.7×106≤ReD≤7.2×106, based on the rotor diameter and freestream velocity. Five different downstream locations were surveyed, between 0.77≤x/D≤5.52, including the near wake, transition to the intermediate wake, and the intermediate wake. Three dominant wake features are identified and studied: the tip vortices, an annular shear layer in the wake core, and wake meandering. The tip vortices are shown to have a broadband influence in the flow in their vicinity, which locally alters the turbulence in that area. It is shown that shedding in the wake core and wake meandering are two distinct and independent low frequency features, and the wake meandering persists into the intermediate wake, whereas the signatures of the core shedding vanish early in the near wake.

Analysis of coherence in turbulent stratified wakes using spectral proper orthogonal decomposition

We use spectral proper orthogonal decomposition (SPOD) to extract and analyse coherent structures in the turbulent wake of a disk at Reynolds number $ {\textit {Re}} = 5 \times 10^{4}$ and Froude numbers $ {\textit {Fr}} = 2$ , 10. We find that the SPOD eigenspectra of both wakes exhibit a low-rank behaviour and the relative contribution of low-rank modes to total fluctuation energy increases with $x/D$ . The vortex shedding (VS) mechanism, which corresponds to $ {\textit {St}} \approx 0.11 - 0.13$ in both wakes, is active and dominant throughout the domain in both wakes. The continual downstream decay of the SPOD eigenspectrum peak at the VS mode, which is a prominent feature of the unstratified wake, is inhibited by buoyancy, particularly for $ {\textit {Fr}} = 2$ . The energy at and near the VS frequency is found to appear in the outer region of the wake when the downstream distance exceeds $Nt = Nx/U = 6 - 8$ . Visualizations show that unsteady internal gravity waves (IGWs) emerge at the same $Nt = 6 - 8$ . A causal link between the VS mechanism and the unsteady IGW generation is also established using the SPOD-based reconstruction and analysis of the pressure transport term. These IGWs are also picked up in SPOD analysis as a structural change in the shape of the leading SPOD eigenmode. The $ {\textit {Fr}} = 2$ wake shows layering in the wake core at $Nt > 15$ which is captured by the leading SPOD eigenmodes of the VS frequency at downstream locations $x/D > 30$ . The VS mode of the $ {\textit {Fr}} = 2$ wake is streamwise coherent, consisting of $V$ -shaped structures at $x/D \gtrsim 30$ . Overall, we find that the coherence of wakes, initiated by the VS mode at the body, is prolonged by buoyancy to far downstream. Also, this coherence is spatially modified by buoyancy into horizontal layers and IGWs. Low-order truncations of SPOD modes are shown to efficiently reconstruct important second-order statistics.

Wake flow patterns and turbulence around naturally deposited and installed trees in a gravel bed river

AbstractLarge wood structures, such as wood fragments, debris jams, or entire trees, create flow and habitat diversity in rivers. A key flow feature associated with such structures is the wake, characterised by a core zone of reduced velocity and shear layers at its margins. Wakes are largely controlled by geometric and structural properties of the wood. In the present study, the flow patterns and turbulence created by different wood structures were compared at two study sites: naturally eroded and fragmented oaks (Site A) and artificial poplar installations (Site B). Flow and turbulence were quantified using pointwise velocity measurements with acoustic Doppler velocimeters (ADVs) and surface particle tracking velocimetry (SPTV). The measured flow patterns exhibited similarities with shallow porous wakes that feature fluid advection through the structure into the wake core downstream. Two additional features of wood structures were identified in the present study: (i) the growth of the shear layers was hindered by bed friction like for shallow mixing layers and (ii) the presence of a tree stem and sediment deposit in the wake centre delayed or even suppressed the interaction of the shear layers and vortex street formation similar to a wake‐splitter plate. Methodologically, the combined ADV/SPTV measurement approach and the use of analytical models for shallow mixing layers proved to be highly valuable to decipher the complex flow patterns around wood structures in the field.

Analysis of momentum recovery within the near wake of a cross-flow turbine using Large Eddy Simulation

Momentum recovery within the near wake of a cross-flow turbine is studied, focusing on the different terms of the momentum balance equation. The flow is computed via Large-Eddy Simulation, coupled with an Immersed-Boundary method. Results demonstrate the dominant role of the spanwise flows (oriented along the axis of the turbine) in contributing to momentum recovery within the near wake, especially via advection in the vicinity of its spanwise boundaries and via turbulent transport at inner locations, closer to the mid span. Large streamwise-oriented vortices on the windward side promote momentum recovery into the wake core from its spanwise boundaries, moving the lower-momentum fluid downstream of the turbine from the leeward side towards the windward side, increasing this way the asymmetry of the wake system. Away from the turbine the importance of the spanwise flows declines, compared to that of the cross-stream ones (orthogonal to the axis of the turbine), due to a faster depletion of the spanwise gradients within the wake. As a consequence, the relative importance of the cross-stream flows on further momentum recovery becomes more significant. Meanwhile, as mean gradients decay away from the turbine, turbulent transport contributes more than advection to complete the process of momentum replenishment within the wake.

Validation of a free vortex filament wake module for the integrated simulation of multi-rotor wind turbines

Multi wind tUrbine Simulation Tool (MUST) is a new CENER's in-house code, based on NREL's OpenFAST code, designed to allow the aero-hydro-servo-elastic modelling of novel multi-rotor wind turbine concepts. MUST also includes a new aerodynamic module, called AeroVIEW (Aerodynamic Vortex fIlamEnt Wake) based on an implementation of the Free Vortex filament Method (FVM) combined with an unsteady Lifting Line (LL). This model allows to represent the influence existing between all the rotors, in each one of them. In this article, a validation of AeroVIEW performance for single and multi-rotor configurations is presented. First, a study of the wake properties (wake geometry, core radius and circulation of tip vortex filament) obtained with AeroVIEW has been successfully compared with several MexNext III cases. Then, an analysis of the rotor wakes interaction, in different arrays of wind turbines, has been made, obtaining power and thrust increases, due to this rotor-wake interaction, for different conditions. The increases obtained in power are very similar to results of high fidelity tools found in the literature. Moreover, this beneficial effect on the power generated, has a counterpart in the average thrust, whose increase is around half of the power production increase.

Modelling Deep Green tidal power plant using large eddy simulations and the actuator line method

The Deep Green technique for tidal power generation is suitable for moderate flows which is attractive since larger areas for tidal energy generation hereby can be used. It operates typically at mid-depth and can be seen as a “flying” kite with a turbine and generator attached underneath. It moves in a lying figure-eight path almost perpendicular to the tidal flow.Large eddy simulations and an adaption of the actuator line method (in order to describe arbitrary paths) are used to study the turbulent flow with and without Deep Green for a specific site. This methodology can in later studies be used for e.g. array analysis that include Deep Green interaction. It is seen that Deep Green creates a unique wake composed of two velocity deficit zones with increased velocity in each wake core. The flow has a tendency to be directed downwards which results in locally increased bottom shear. The persistence of flow disturbances of Deep Green can be scaled with its horizontal path width, Dy, with a velocity deficit of 5% at approximately 8–10Dy downstream of the power plant. The turbulence intensity and power deficit are approximately two times the undisturbed value and 10%, respectively, at 10Dy.

Spatial evolution and kinetic energy restoration in the wake zone behind a tidal turbine: An experimental study

In the case of tidal turbine arrays, the spatial evolution pattern of turbine wakes affects the power performance of other turbines and the hydrodynamic environment of adjacent turbines. However, the evolution mechanism is unclear, particularly in terms of wake energy restoration process. Therefore, we investigated the wake spatial evolution mechanism of a tidal current turbine experimentally. The experiment used a porous disc to simulate the turbine rotor based on the equivalence principle of force. An acoustic Doppler velocimeter and a strain gauge measured the time-varying velocities and flow loads, respectively. The momentum loss generated a wake zone in the downstream with enhanced turbulence originating from the boundary shear layer. Initially, the velocity deficit in the wake core increased as the shear layer spread inwards. Moreover, the disc blockage accelerated the flow, generating high momentum transported into the wake zone. However, the wake expansion and turbulence dissipation were limited until the shear-induced turbulence appeared at the wake centre. The kinetic energy in the wake region recovered significantly in the range x = (2D, 3D). As the wake moved downstream, further energy restoration was observed in the entire wake region. However, the rate of energy transformation decreased beyond 7D downstream.

Momentum recovery downstream of an axial-flow hydrokinetic turbine

Large-Eddy Simulation is adopted to reproduce the wake of an axial-flow hydrokinetic turbine. The process of momentum recovery is correlated with the destabilization of the wake system, starting wake contraction and radial flows from the free-stream towards the wake core. All terms of the momentum balance equation are analyzed. The radial advection dominates the momentum recovery at the outer radii (in the vicinity of the radial boundary of the wake), just downstream of the instability of the system of tip vortices. Further downstream the reduction of the mean radial gradients makes advection less significant and the radial turbulent transport becomes the main source of momentum replenishment at inner radii, near the wake axis. The details of the process of momentum recovery and its correlation with the instability of the wake system, revealed by the present high-fidelity eddy-resolving computations, provide new insight on the development of the wake of axial-flow hydrokinetic turbines.

Characterization of the turbulent wake of an axial-flow hydrokinetic turbine via large-eddy simulation

Large-Eddy Simulation on a grid composed of about two billion points is utilized to characterize the turbulent wake of an axial-flow hydrokinetic turbine. The dominant role of the tip vortices is revealed, both in the near wake, where they are very coherent, and downstream, after development of instability. As long as the tip vortices are stable, sharp peaks of Reynolds stresses populate the outer boundary of the wake, where turbulence is strongly anisotropic and dominated by the fluctuations of the radial velocity component. Such anisotropy is also confirmed by the shear stresses, with the most significant one tied to the fluctuations of the radial and streamwise velocities, originating from the interaction between tip vortices. When the system of tip vortices develops instability, the behavior of turbulence is substantially modified. The outer maxima become more diffused and move gradually towards the wake core. In contrast with the near wake, the fluctuations of the radial velocity become lower than those of the azimuthal and streamwise velocities. Instead, the shear stress associated to the radial and streamwise velocities keeps the most significant one, contributing to momentum recovery via turbulent transport. The present results demonstrate a strong anisotropy of turbulence within the wake, which should be taken properly into account when lower-fidelity methodologies, relying on turbulence modeling, are utilized to simulate this class of flows.

Distinct Turbulent Regions in the Wake of a Wind Turbine and Their Inflow-Dependent Locations: The Creation of a Wake Map

Wind turbines are usually clustered in wind farms which causes the downstream turbines to operate in the turbulent wakes of upstream turbines. As turbulence is directly related to increased fatigue loads, knowledge of the turbulence in the wake and its evolution are important. Therefore, the main objective of this study is a comprehensive exploration of the turbulence evolution in the wind turbine’s wake to identify characteristic turbulence regions. For this, we present an experimental study of three model wind turbine wake scenarios that were scanned with hot-wire anemometry with a very high downstream resolution. The model wind turbine was exposed to three inflows: laminar inflow as a reference case, a central wind turbine wake, and half of the wake of an upstream turbine. A detailed turbulence analysis reveals four downstream turbulence regions by means of the mean velocity, variance, turbulence intensity, energy spectra, integral and Taylor length scales, and the Castaing parameter that indicates the intermittency, or gustiness, of turbulence. In addition, a wake core with features of homogeneous isotropic turbulence and a ring of high intermittency surrounding the wake can be identified. The results are important for turbulence modeling in wakes and optimization of wind farm wake control.

Impact of Tip-Vortex Modeling Uncertainty on Helicopter Rotor Blade–Vortex Interaction Noise Prediction

Free-wake models are routinely used in aeroacoustic analysis of helicopter rotors; however, their semiempiricism is accompanied with uncertainty related to the modeling of physical wake parameters. In some cases, analysts have to resort to empirical adaption of these parameters based on previous experimental evidence. This paper investigates the impact of inherent uncertainty in wake aerodynamic modeling on the robustness of helicopter rotor aeroacoustic analysis. A free-wake aeroelastic rotor model is employed to predict high-resolution unsteady airloads, including blade–vortex interactions. A rotor aeroacoustics model, based on integral solutions of the Ffowcs Williams–Hawkings equation, is utilized to calculate aerodynamic noise in the time domain. The individual analytical models are incorporated into an uncertainty analysis numerical procedure, implemented through nonintrusive Polynomial Chaos expansion. The potential sources of uncertainty in wake tip-vortex core growth modeling are identified and their impact on noise predictions is systematically quantified. When experimental data to adjust the tip-vortex core model are not available the uncertainty in acoustic pressure and noise impact at observers dominated by blade–vortex interaction noise can reach up to 25% and 3.50 dB, respectively. A set of generalized uncertainty maps is derived, for use as modeling guidelines for aeroacoustic analysis in the absence of the robust evidence necessary for calibration of semiempirical vortex core models.

Dependence of the wake recovery downstream of a Vertical Axis Wind Turbine on its dynamic solidity

The momentum recovery downstream of a Vertical Axis Wind Turbine (VAWT) is analyzed as a function of its dynamic solidity, modified changing both the number of blades and the rotational speed of the turbine. Computations were carried out using Large-Eddy Simulations (LES), coupled with an Immersed-Boundary (IB) methodology. The streamwise increase of the momentum deficit in the near wake was found dominated by negative cross-stream advection, due to wake expansion, and especially negative pressure transport. Both of them were found larger at higher values of dynamic solidity. Downstream of the near wake region, the instability of the shear layers at both edges of the wake produced a substantial change of the wake topology and balance across momentum transport terms. The collapse of the shear layers into the wake core promoted wake recovery, via both positive cross-stream advection and turbulent transport. The destabilization of the wake system occurred earlier at larger values of dynamic solidity, producing this way a faster wake recovery, in spite of larger values of momentum deficit within the near wake. The switch to the wake of a bluff body was characterized by an increase of the energy at frequencies lower than that of the blades passage.

Survey of the near wake of an axial-flow hydrokinetic turbine in the presence of waves

Flow field results are presented for the near-wake of an axial-flow hydrokinetic turbine in the presence of surface gravity waves. The turbine is a 1/25 scale, 0.8 m diameter, two bladed turbine based on the U.S. Department of Energy's Reference Model 1 tidal current turbine. Measurements were obtained in the large towing tank facility at the U.S. Naval Academy with the turbine towed at a constant carriage speed and a tip speed ratio selected to provide maximum power. The turbine has been shown to be nearly scale independent for these conditions. The selected wave form was intended to represent oceanic swell encountered off the U.S. eastern seaboard. The resulting model wave is a deep water wave, in terms of relative depth, traveling with the “current”, in the opposite direction of the towing carriage. Velocity measurements were obtained using a submersible, planar particle image velocimetry (PIV) system at streamwise distances of up to two diameters downstream of the rotor plane. PIV ensembles were obtained for phase locked conditions with the reference blade at the horizontal position. Phase averaged results for no-wave and wave conditions are presented for comparison showing further expansion of the wake and shear layer in the presence of waves as compared to the no-wave case. When ensembles are selectively sampled on the wave phase, a high degree of coherency is shown to remain and the wake width is shown to undulate with the passing of the wave, with the vertical displacement range on the same order as that of a particle under similar conditions. The impact of waves on turbine tip vortex helical structure is also examined. Waves are shown to change the location of adjacent helices. In the streamwise direction, this changes the pitch of the helix, shown in previous studies to affect the downstream wake recovery distance. In the vertical direction, depending on the wave-induced flow field at the time they were created, vortices are forced outward, into the mean flow or inward, into the wake core, potentially enhancing kinetic energy transport and accelerating the re-energization process.

Characterization of the wake of a submarine propeller via Large-Eddy simulation

Results of large-eddy simulations of a submarine propeller in open-water (isolated) configuration are presented for three load conditions. An immersed boundary approach is adopted to handle the rotating geometry of the propeller within a stationary cylindrical grid composed of about 840 million nodes. Direct comparisons with Particle Image Velocimetry experiments conducted in parallel demonstrate that the simulations reproduce the wake very accurately. In particular the wake dynamics are mainly dominated by tip and hub vortices. Strong structures are also shed in the near wake from the suction side of the propeller blades, correlating with local maxima of turbulent kinetic energy. However, they are not a long standing feature of the propeller wake. In contrast, helical structures originating form the root of the propeller blades are more persistent and their footprint is still visible few propeller diameters downstream. We verified that load conditions affect substantially both hub vortex and tip vortices. In a similar way, for increasing loads turbulent kinetic energy experiences a faster growth at the wake axis, populated by the hub vortex, compared to the outer radii, dominated by the tip vortices. Also, the evolution of turbulent kinetic energy at the outer edge of the wake is not monotonic, in contrast with that in the wake core, due to mutual interaction and associated shear between tip vortices and the wake of the following blades.

Hydrodynamic effects of the ratio of rotor diameter to water depth: An experimental study

A series of flume experiments were carried out to investigate the influence of tidal turbine rotor diameter to depth ratio on the hydrodynamic process of wake flow using two different diameter rotor discs of the same porosity. Time-varying velocities were measured by an Acoustic Doppler Velocimeter at 8 cross-sections over the distance of 10 diameters downstream, and the three-dimensional structures of wake flow and turbulence fields were obtained. Immediately downstream, the peak of velocity deficit occurred at the wake core and the value was greater for the large diameter-depth ratio. Strong wake turbulence was mainly located in the shear stress layer around wake core. However, the attenuation processes of wake hydrodynamics were different under two diameter-depth ratios. The momentum transfer was caused by Reynolds shear stress in both transverse and vertical directions. The vertical momentum transfer process was much more significant above the wake core than the lateral transfer process, but it decayed rapidly in near wake as the diameter-depth ratio enlarged. The experimental results provide detailed data to better understand the wake propagation processes behind rotor discs.

Experimental study of wake structure behind a horizontal axis tidal stream turbine

A detailed experimental investigation of the wake propagation behind a horizontal axis turbine with three blades was conducted in a recirculating water flume. An Acoustic Doppler Velocimeter was employed to measure the time varying velocities at fifteen depths across the width of the open flume to obtain the three-dimensional velocity and turbulence fields within the length of 20 Rotor Diameters downstream. The experimental results indicated that velocity reduction in the wake was caused by both the kinetic energy extraction and blockage effects of the tidal stream turbine rotor and stanchion. The maximum velocity deficit occurred at the wake core due to a lower tip speed ratio and blockage of the hub. The wake strip gradually enlarged with the distance downstream, with less mixing in the transverse direction. The wake zone shifted towards the water surface in the vertical direction, which mainly resulted from the merging of two wakes induced by turbine rotor and stanchion. The wake rotation was also observed, and the maximum circumfluence velocity was approximately 20% of the stream-wise velocity; thus, it had a significant influence on the process of near wake mixing. Furthermore, the wake turbulence of the turbine was strong and anisotropic, which would have an impact on the behaviour of other turbines located downstream if they were in a turbine array. In addition, the turbine stanchion had a visible influence on the wake structure, especially near the wake, which should not be neglected when studying the wake characteristics.

Study of Burst Wakes in a Multi-Element Airfoil Flowfield

High-lift multi-element airfoils, such as those used on large transport aircraft during takeoff and landing, can generate strong adverse pressure gradients that, while the surface flow is attached, can cause off-the-surface separation in the wake: so-called wake bursting. The sudden expansion and thickening of the separated wakes has been shown to decrease lift and increase drag. Wake bursting was experimentally studied over a three-element high-lift airfoil, and unsteady velocity measurements were taken with a split-film probe. The tests were performed in the University of Illinois at Urbana–Champaign low-speed low-turbulence subsonic wind tunnel on a multi-element airfoil with a chord length of 1.35 ft (0.411 m) and a model span of 2.8 ft (0.85 m). Results for a Reynolds number of indicate that wake bursting was observed for the wake of the main element and the first flap. A methodology was developed to numerically define the core of each wake both upstream and downstream of the burst point. Data show that the local flowfield angle in the wake core does not significantly change relative to the flowfield outside the wake core. Unsteady results indicate that the velocity fluctuations within the burst-wake region are dominated by turbulence in the shear layers between the wakes with less turbulence observed in the wake cores. These turbulent fluctuations were largest in the shear layers and were observed to spread into the wake cores.

Three-dimensional flow visualization in the wake of a miniature axial-flow hydrokinetic turbine

Three-dimensional 3-component velocity measurements were made in the near wake region of a miniature 3-blade axial-flow turbine within a turbulent boundary layer. The model turbine was placed in an open channel flow and operated under subcritical conditions (Fr = 0.13). The spatial distribution of the basic flow statistics was obtained at various locations to render insights into the spatial features of the wake. Instantaneous and phase-averaged vortical structures were analyzed to get insights about their dynamics. The results showed a wake expansion proportional to the one-third power of the streamwise distance, within the first rotor diameter. Wake rotation was clearly identified up to a distance of roughly three rotor diameters. In particular, relatively high tangential velocity was observed near the wake core, but it was found to be nearly negligible at the turbine tip radius. In contrast, the radial velocity showed the opposite distribution, with higher radial velocity near the turbine tip and, due to symmetry, negligible at the rotor axis. Larger turbulence intensity was found above the hub height and near the turbine tip. Strong coherent tip vortices, visualized in terms of the instantaneous vorticity and the λ2 criterion, were observed within the first rotor diameter downstream of the turbine. These structures, influenced by the velocity gradient in the boundary layer, appeared to loose their stability at distances greater than two rotor diameters. Hub vortices were also identified. Measurements did not exhibit significant tip–hub vortex interaction within the first rotor diameter.