Turbulent Wake Research Articles

Turbulent wakes in a non-uniformly stratified environment

This study explores the behavior of turbulent wakes generated by a sphere propagating with constant speed in a non-uniformly stratified fluid. The investigation is based on a series of high-resolution direct numerical simulations in which the background stratification is systematically varied. We consider one linear and three nonlinear density profiles and discover that even modest, spatially localized non-uniformities of stratification can profoundly influence the wake dynamics, structure, and evolution. The analysis of microstructure signatures shows that wakes in non-uniformly stratified fluids tend to be more spread horizontally, and internal waves are much stronger than in linear stratification. Simulations performed with Gaussian perturbations are characterized by a vertically asymmetric energy distribution, which is attributed to internal wave reflections from low-gradient regions. Using microstructure decay rates, we estimate the effective persistence period of wakes, showing that it substantially increases with the increasing Froude number. We also find that wakes persist much longer in high-gradient profiles, whereas weak local gradients can substantially reduce the wake longevity.

Fluid-structure interaction analysis of the rudder vibrations in propeller wake

Severe vibrations on the rudder worsen the vibrations and noise of the vessel. This study examines the rudder vibrations induced by propeller wake considering the fluid-structure interaction. The propeller's turbulent wake is solved with large eddy simulation. The lock-in regime under the different rotational speeds of propeller is discussed. The results show that rudder suffers from span-wise bending under the load induced by the propeller wake. This span-wise bending strengthens when the rotational speed increases. The rudder's vibrations are excited at the excitation frequencies and the natural frequencies. The vibrations of the rudder are locked at the first natural frequency fn1 when the blade passage frequency (BPF) is lower than 1.83 fn1. The vibration mode is similar to the first natural vibration mode in this lock-in regime. Intense vibrations are observed at BPF when the BPF exceeds 1.83 fn1. The vibration at the first natural frequency strengthens again when the shaft frequency exceeds 0.715 fn1. The vibrations at high-order natural frequencies strengthen as the excitation frequencies increase. The vibration at secondary natural frequency fn2 strengthens when the BPF approaches 0.532 fn2.

Full-Scale Reynolds Number Testing of Rotor Hub Drag and Wake Turbulence

A 1:4.25-scale model of a generic helicopter rotor hub was tested at Reynolds numbers ranging from 1. 75× 106 to 7×106 at advance ratio of 0.2 in The Pennsylvania State University Applied Research Laboratory Garfield Thomas 48-inch diameter water tunnel. Measurements including drag and wake characteristics were performed up to full-scale Reynolds number with respect to an industry-representative helicopter rotor hub. In particular, the variation of drag and flow field with Reynolds number was characterized. Load measurements were conducted using an improved load cell design, with greater accuracy than in previous experiments. Wake velocity was measured using laser Doppler velocimetry at two downstream planes, yielding velocity statistics to the second order. Improved load measurement accuracy and wake velocity spatial resolution, at full-scale Reynolds number, provide a unique dataset for computational fluid dynamics validation as part of the Penn State Rotor Hub Flow Prediction Workshops and physical insight into rotor hub flows.

Unstated impacts of the green energy industry on the habitat of a coastal delphinid: Turbid‐turbulent wakes induced by offshore wind turbine foundations

Abstract Offshore wind farms (OWFs) are a green energy solution to reducing CO2 emissions, but often encroach on major habitats of coastal delphinids. Clustered OWF construction in shallow waters may have cumulative negative impacts on coastal delphinids and marine ecosystems. Although the impacts of piling noise have been discussed extensively, the potential impacts of oceanographic alterations induced by wind turbine foundations are rarely discussed. Two OWFs were constructed near the core habitat of a critically endangered subspecies of humpback dolphin (Sousa chinensis taiwanensis) in the eastern Taiwan Strait. Turbid‐turbulent wakes induced by wind turbine foundations were inspected using Landsat images. Sighting rates of the humpback dolphin that were obtained from boat‐based line‐transect surveys near the northern OWF (OWFn) since the late 2000s were examined. Indicators of surface turbidity were used to highlight turbid‐turbulent wakes extending in the direction of the main current. Sighting rates of the humpback dolphins near the OWFn have been declining since the 2000s, but decreased significantly after the OWFn was constructed and began operating. The long‐term decline in sighting rates implies an alarming decline in the population size of humpback dolphins. Decreased sighting rates after the OWFn installation indicate the reduced utilization of a once‐important habitat. Offshore wind farm construction may influence prey abundance and composition and cause acute stress from piling noise; operational noise may mask the chorus of prey fish and turbid‐turbulent wakes may disturb the benthic ecosystem. Site selection committees for future OWFs should avoid the habitats of coastal delphinids and areas subject to strong ocean currents. Ecological impact assessments for OWF projects should examine the impacts of turbulent wakes on benthic ecosystems and of acoustic disturbance on marine mammals. Long‐term environmental monitoring programmes are recommended, which can be funded by wind energy companies as a compensatory measure.

Influence of Different Flow Solvers and Off-Design Conditions On the Determination of Fan-Rotor Wakes for Broadband Noise Prediction

Abstract The acoustic interaction of fan-rotor wakes with the downstream stator vanes is considered as an important noise source of an aircraft engine. The turbulence induced by the rotor generates a stochastic acoustic source that appears as broadband noise in the acoustic spectrum. During the preliminary design phase of an engine, established meanline and throughflow solvers usually do not resolve turbulence and associated unsteady flow parameters. But such solvers provide rotor pressure losses that can be used to estimate the mean and turbulent rotor wakes. A crucial step is the deduction of turbulence parameters from the mean wakes. A semi-empirical model for rotor-wake turbulence estimation is presented in this paper. The meanline method and the throughflow solver are compared to three-dimensional computational flow simulations investigating the capabilities of the different solvers to provide flow data for broadband wake interaction noise prediction. The methods are applied to a representative modern fan stage at a comprehensive number of operating points, comprising several speed lines from surge to choking conditions. Microphone measurements are consulted to assess the noise predictions. The evaluation confirms the applicability of the meanline and throughflow method in combination with the turbulence model for broadband noise estimation during the preliminary design phase. The underestimated turbulence in the tip region of the fan is found to be negligible even during off-design conditions.

A hybrid two-dimensional orthogonal wavelet multiresolution and proper orthogonal decomposition technique for the analysis of turbulent wake flow

A hybrid two-dimensional orthogonal wavelet multiresolution and proper orthogonal decomposition technique is developed to analyze the wake flow behind a semi cylinder. The two-dimensional orthogonal wavelet analysis is first applied to decompose the measured velocity fields into four wavelet components based on their spatial scales. Then the extracted multi-scale structures are further investigated based on proper orthogonal decomposition. The modal energy distributions of multi-scale flow structures suggest that the dominance of first two POD modes is reduced and the relative energy becomes more distributed as the flow structure changes from large-scale to small-scale. The first two POD modes of intermediate-scale structure also appear in a mode pair and they are related to secondary resonance accompanied with large-scale flow oscillation. The POD mode 3 of intermediate-scale structure suggest the existence of symmetric vortex shedding pattern. The first two POD modes of small-scale structure also display a symmetrically distributed vortex pair and the distance between adjacent vortices is about half of the case of mode 3 of intermediate-scale structure. The PSD distributions of POD modes of multi-scale structures suggest that the bandwidth of PSD becomes less concentrated as the flow structure varies from large-scale to small-scale. Distinctive flow patterns associated with first and second order harmonics of fundamental flow oscillation can be successfully extracted from the dominating POD modes of intermediate-scale and small-scale structures. The proposed hybrid technique provides a tool for relate the most energetic flow events over a period of time with multi-scale structures cascaded in the turbulence flow.

An empirical model accounting for added turbulence in the wake of a full-scale turbine in realistic tidal stream conditions

The increased turbulence behind the turbine induces a fatigue load on the downstream turbine blades. An accurate estimate of turbulent intensity in turbine wake is paramount to optimizing the placement of turbines in tidal turbine parks. A simple empirical model is developed using data fitting from numerical simulation of a non-rotational actuator disc model to estimate the added turbulence in realistic tidal stream conditions similar to the Alderney Race, with a rotor diameter to depth ratio of 40%. The study shows a self-similar Gaussian shape streamwise turbulent intensity in a lateral direction similar to the velocity deficit profile. The turbulent wake radius expands according to a power-law depending on the ambient turbulent intensity. The added turbulence model justifies that the major turbulence source in the near wake is attributed to the rotor as it is weakly dependent on the ambient turbulence in the flow. As no known existing empirical model for tidal turbine wake added turbulence, the model is compared with existing models of unbounded wind turbines. The proposed model estimates the average turbulence in the far wake can assist turbine placement in a tidal farm.

Support algorithm for air traffic controllers’ arrival spacing: Improvement of trajectory estimation using Gaussian Process Regression

A flying aircraft generates wake turbulence behind it from its wingtips. To avoid the effects of wake turbulence, successive arrival aircraft at a runway must maintain appropriate longitudinal separation between the leader and follower aircraft. The wake turbulence separation minima are applied based on an air traffic controller’s instructions. This task, arrival spacing, place a high workload on air traffic controllers. To reduce the workload on air traffic controllers, this study proposes a support algorithm for arrival spacing. The support algorithm provides an indicator for each aircraft. The indicator is calculated to maintain an arrival separation that matches the separation minima; thus, if the air traffic controller adjusts the aircraft’s position to match the indicator, the arrival aircraft can maintain proper separation. The accuracy of the trajectory estimation in the support algorithm is crucial for the support algorithm to provide an accurate indicator. To configure an accurate trajectory estimation, an aircraft’s speed profile is generated by applying Gaussian Process Regression (GPR). Actual operational data obtained from radar data is used as input data for the regression. To evaluate the capabilities of the support algorithm, case studies are conducted using radar data. The results show that the support algorithm can calculate the appropriate indicator near the runway threshold. However, the indicator has errors in the initial phase of arrival spacing. By applying the speed profile generated by GPR to the support algorithm, the average of the indicator error reduces to about one-fifth when applying the speed profile based on an airline procedure model.

LES/DNS fluid-structure interaction simulation of non-linear slender structures in Nektar++ framework

Nektar++ is a spectral/hp element open-source framework written in C++ for the construction of classical low-order h-type as well as higher-order p-type finite element solvers. It seeks to overcome the implementation challenges of the complex data structures associated with high-order finite element methods; hence, providing an efficient, flexible and HPC scalable platform for the development of solvers for partial differential equations using the spectral/hp element method. In the present work, capabilities of Nektar++ is leveraged for development of two fluid-structure interaction (FSI) solvers for simulations of highly deformable nonlinear slender structures. The FSI solver uses the incompressible Navier-Stokes (NS) solvers of Nektar++ for fluid flow while the structural dynamics is modelled using Geometrically-Exact Composite Beams (GECB). The open-source SHARPy framework is linked to Nektar++ and used for structural simulation. Aiming at high-fidelity (LES/DNS) FSI simulations, the thick-strip approach is used to reduce computational costs. In this approach, the full 3D fluid domain is represented with series of smaller 3D domains normal to the local axis of the structure and having a finite thickness in the spanwise direction where periodicity is also assumed. Hence, while reducing the computational costs by avoiding the discretization of equations over the entire slender structure, the strip thickness allows capturing the local 3D turbulent wake and accurately predict the fluid forces on the structure. Two approaches are adopted to avoid the dynamic remeshing due to the large and non-linear deformation of the structure. In the first approach, the transformed the NS equations are solved in the non-inertial body-fitted coordinates while in the second approach the NS equations are formulated in the moving frame of reference and solved with the spectral/hp element method. A hybrid parallelisation approach of Nektar++ is extended for the thick-strip method which allows having non-constant cross-section along the structural span as well as efficient and flexible use of computational resources, and excellent HPC performance for the FSI simulations. The capability of the FSI solver is demonstrated via several examples.

Effects of crosswinds and tip configurations on the initial phase of wingtip vortex evolution

Wingtip vortex is the initial phase of aircraft wake flow that jeopardizes flight safety. Previous studies have focused on the effect of incidence angle on tip vortex flow; thus, investigations on tip vortex in the roll-up stage under crosswind conditions are limited. In the present study, two NACA0012 wings with different tip shapes (round and square) were numerically analyzed to explore the effects of wingtip configurations on tip vortex. In addition, the effects of crosswinds on wingtip vortex evolution were also investigated. It was found that sharp edges of the square wingtip generated a multi-vortex system, resulting in a larger turbulent wake vortex core radius. The lift and vortex strength of the square wingtip were 23% higher and only 12% stronger than those of the round wingtip, respectively. Furthermore, crosswinds changed both the pressure distribution and the vortex structure. Crosswinds made the flow field unstable; hence, upwind lower edge vortices of the square wingtip burst earlier and merged faster. Consequently, the tightness of the upwind vortex of the square wingtip increased, and the corresponding vortex strength was enhanced. The downwind vortex of the square wingtip was weakened by 2.7% under a crosswind velocity of 4 m/s, whereas the downwind tip vortex of the round wingtip was strengthened under the same conditions.

A novel 2D analytical model for predicting single and multiple‐interacting wakes of horizontal‐axis wind turbines

AbstractA 2D analytical model has been proposed for predicting the wind velocity distribution inside the wake by coupling the Jensen wake model and the Gaussian function using the conservation of mass and momentum. More realistic approaches were adopted in choosing the control volume and calculation of the wake initial radius at the formation and a new modified expression was employed for the wake turbulence intensity to be used for computation of the revised wake decay. Validation of the model was performed using a body of experimental and numerical data, where the model was thoroughly compared with five analytical models of the literature. The model was successful in predicting double‐interacting wakes, and gave more accurate single wake velocity profiles at the far wake region, especially at downwind distances of ( is the rotor diameter), which are the economically‐ and power harvesting‐optimized spacing between the turbines in wind farms.

The impact of swirl and wake strength on turbulent axisymmetric wake evolution

An experimental investigation of swirl and wake strength influence on axisymmetric turbulent wake evolution was conducted. A novel wake generator design wire mounted in a wind tunnel test section with low free-stream turbulence produced wake Reynolds numbers based on momentum thickness and free-stream velocity in excess of 14 000 and swirl numbers up to 0.4 with minimal blockage. Steady-state blade element momentum simulations of reference wind turbine designs indicated that wind turbines operate in the flow regimes studied, indicating the practical aspects of this work. Stereoscopic particle image velocimetry was used to acquire three components of velocity in the swirling wake at locations up to approximately ten diameters downstream. Quantitative measures of wake growth and decay were deduced using available equilibrium similarity scaling for the swirling wake. The results show an increase above 50% in growth and axial velocity decay rate constants over the range of swirl strength studied compared to those of the non-swirling wake. Tangential velocity decay constants were shown to decrease with swirl strength over the range of conditions studied. Notably, changes in wake strength have little influence on growth and decay rates when compared to changes in swirl strength for the flow regimes studied in this work.

Large-eddy simulation of helical- and straight-bladed vertical-axis wind turbines in boundary layer turbulence

Turbulent wake flows behind helical- and straight-bladed vertical axis wind turbines (VAWTs) in boundary layer turbulence are numerically studied using the large-eddy simulation (LES) method combined with the actuator line model. Based on the LES data, systematic statistical analyses are performed to explore the effects of blade geometry on the characteristics of the turbine wake. The time-averaged velocity fields show that the helical-bladed VAWT generates a mean vertical velocity along the center of the turbine wake, which causes a vertical inclination of the turbine wake and alters the vertical gradient of the mean streamwise velocity. Consequently, the intensities of the turbulent fluctuations and Reynolds shear stresses are also affected by the helical-shaped blades when compared with those in the straight-bladed VAWT case. The LES results also show that reversing the twist direction of the helical-bladed VAWT causes the spatial patterns of the turbulent wake flow statistics to be reversed in the vertical direction. Moreover, the mass and kinetic energy transports in the turbine wakes are directly visualized using the transport tube method, and the comparison between the helical- and straight-bladed VAWT cases show significant differences in the downstream evolution of the transport tubes.

Experimental and Numerical Investigation of Stall on the NACA Airfoil

In this paper, measurements of the stall of a NACA airfoil section are presented and compared to a simulation with a hybrid Reynolds-averaged Navier–Stokes (RANS)/large-eddy simulation (LES) model with the goal of gaining deeper insight into the flow physics of a stall cell and the turbulent wake in such conditions. Detailed measurements of the evolution of the velocity deficit and turbulence within the wake along with surface pressure measurements and flow visualization provided a comprehensive database for code validation and flow physics studies. The measurements in the Laminar Wind Tunnel of the Institute of Aerodynamics and Gas Dynamics were performed at a Reynolds number of and comprised surface pressures, oil flow surface visualizations, and hot-wire measurements of the wake. The results at were used as validation data for a hybrid RANS/LES run at the same flow conditions. It was shown that the Bernoulli-based detached eddy simulation (BDES) model, which uses a shielding function based on a localized Bernoulli formulation, combined with effective gray area mitigation can capture the stall cell pattern and the point of flow separation at an angle of attack of . Furthermore, the velocity deficit and the Reynolds stresses in the wake show good agreement between simulation and experiment under these conditions.

Numerical prediction of underwater noise on a flat hull induced by twin or podded propeller systems

In this article, the numerical prediction of underwater noise induced by multi-propulsor systems is investigated on a flat hull by solving the Ffowcs Williams–Hawkings (FW–H) equation in the form proposed by Farassat. To get the noise source, the hydrodynamic analysis is first performed using twin propellers and contra-rotating propellers via a boundary element method (BEM) in non-cavitating flow. In order to fully consider viscous interactions between neighboring propellers, incoming flow, and non-axisymmetric pod, a BEM/Reynolds-averaged Navier–Stokes (RANS) interactive method is utilized, in which two coaxial propellers are handled by a separate BEM model, and their mutual interactions are considered via a RANS solver. The predicted hydroacoustic and hydrodynamic performance from various configurations is validated with experimental data and results from viscous RANS simulations. Along with the noise spectrum in the flow field, the time history of acoustic pressures on the top wall is investigated to see how the mutual interactions among neighboring propellers and the pod affect the acoustic characteristics. A proper way of considering scattered/reflected noise from underwater geometry in a half-space domain is discussed, and importantly, the limited influence of the linear noise terms of the FW–H equation is pointed out, especially when the flow non-linearities, such as wake turbulence and structure-induced vortices, start to dominate the downstream flow.

Impact of reconfiguration on the flow downstream of a flexible foliated plant

Abstract This paper explores the impacts of reconfiguration and leaf morphology on the flow downstream of a flexible foliated plant. 3D acoustic Doppler velocimetry and particle image velocimetry were used to experimentally investigate the hydrodynamic interaction between a foliated plant and the flow, testing two plants with different leaves morphology under different bulk flow velocities. The model vegetation was representative of riparian vegetation species in terms of plants hydrodynamic behavior and leaf to stem area ratio. To explore the effects of the seasonal variability of vegetation on the flow structure, leafless conditions were tested. Reconfiguration resulted in a decrease of the frontal projected area of the plants up to the 80% relative to the undeformed value. Such changes in plant frontal area markedly affected the spatial distributions of mean velocity and turbulence intensities, altering the local exchanges of momentum. At increasing reconfiguration, the different plant morphology influenced the mean and turbulent wake width. The leafless stem exhibited a rigid behavior, with the flow in the wake being comparable to that downstream of a rigid cylinder. The study revealed that the flexibility-induced reconfiguration of plants can markedly affect the local distribution of flow properties in the wake, potentially affecting transport processes at the scale of the plant and its subparts.

Nonlinear feedback control of bimodality in the wake of a three-dimensional bluff body

The turbulent wake behind a square-back Ahmed body in close proximity to the ground exhibits bimodal switching. This manifests as the center of the wake randomly switching side-to-side between one of two asymmetric positions. This work is the first attempt to employ nonlinear model-based feedback control to suppress this wake bimodality. High fidelity simulations are used to develop and implement the control strategy and investigate the resulting effect on the wake and aerodynamic drag.

Coupled BEM and two-phase mixture model for surrounding flow of horizontal axial turbine over sediment seabed

Horizontal axial turbine (HAT) is widely used to extract kinetic energy from tidal current. The power absorption of HAT is largely decided by the surrounding flow. A two-phase, sediment-laden mixture model is coupled with the blade element momentum method (BEM) for the impact of HAT on surrounding flow over sediment seabed. The mixture model incorporates theoretical relative velocity, turbulence enhancement from particle wake flow and turbulence damping from high concentration sediment. A blade-induced turbulence (BIT) term is introduced into the mixture model to represent the turbulence production at blade edge. The computed velocity and turbulence are improved by the BIT term in the wake region of HAT. Three turbine hub heights are adopted to investigate the impact of the turbine on sediment seabed and the surrounding flow. Results show that with low turbine hub height, long wake region is generated after the turbine, force exerted on the turbine is asymmetric vertically and thrust and power coefficients decrease. The pattern of bed shear stress is highly dependent on the turbine hub height. The bed shear stress is enhanced under the turbine and strong scour and sediment transport tend to occur on the sediment seabed with low turbine hub height.

Turbulent planar wakes of viscoelastic fluids analysed by direct numerical simulations

Direct numerical simulations employing the finitely extensible nonlinear elastic constitutive model closed with Peterlin's approximation (FENE-P) are used to investigate the far-field region of turbulent planar wakes of viscoelastic fluids and to develop the theory describing these flows. The theoretical results display excellent agreement with the simulations and provide new scaling laws for the evolution of the shear layer thickness $\delta (x) \sim x^{1/2}$ , mean velocity deficit ${\rm \Delta} U(x) \sim x^{-1/2}$ and, for very high Deborah numbers, of the maximum polymer shear stresses $\sigma ^{[p]}_c(x) \sim x^{-2}$ and averaged polymer chain extension $\textrm {tr}(\bar {C}(x) - \boldsymbol{\mathsf{I}}) \sim x^{-2}$ , where $x$ is the streamwise distance from the solid body generating the wake. The theory is able to show the existence of self-similarity for the profiles of mean velocity, mean polymer shear stress, averaged polymer chain extension and the conditions for similarity of the turbulent shear stress, and is very well supported by the numerical simulations. Similarly to the case of viscoelastic turbulent planar jets (Guimarães et al., J. Fluid Mech., vol. 899, 2020, p. A11), when the inlet Weissenberg and Deborah numbers are sufficiently large, turbulent viscoelastic wakes exhibit a considerable reduction of the spreading rate and of the normalised Reynolds stresses. However, for very large downstream locations the turbulent viscoelastic wake recovers the classical evolution laws observed for Newtonian turbulent planar wakes.

A study on measuring wind turbine wake based on UAV anemometry system

Unmanned aerial vehicle (UAV) wind measurement is a newly developed wind measurement method that can improve the efficiency of conventional wind measurements. A method for measuring the wake characteristics of wind turbines based on UAV is proposed. The research results show that when the downstream distance was 1D, the wake wind speed and turbulence intensity at the hub height are about 60–70% and 130–150% of the incoming flow, respectively. When the downstream distance was 10D, the wake wind speed and turbulence intensity at the hub height gradually recovered to about 90% and 110% of the incoming flow, respectively, and the wake width in the area was about 2.5 times the diameter of the turbine. The wind speed measured by UAV was in good agreement with that predicted by the AV model and the 3DJG model, indicating that the Gaussian-shape curve could better describe the wake wind speed distribution. The UAV anemometry system provides a new research approach for wind turbine wake wind field measurement and the assessment of in-service wind farm resources. Moreover, the results from this study provide a reference for the determination of wind turbine wake, generation power prediction, and structure design.