Turbulent Wake Research Articles

An investigation of drone propeller noise generation in a turbulent wake

Unmanned aerial vehicles (UAVs) are becoming popular as they can be used in a range of applications including surveillance, delivery, agriculture, aerial photography, and search and rescue. This paper presents an experimental study to characterise the effect of turbulent inflow disturbance on the noise generated by a drone propeller using a low-noise, open-jet anechoic wind tunnel facility at UNSW. Advanced precision composites (APC) two-bladed, 12-inch rotors were tested at rotational speeds between 4000 and 8000 RPM at a freestream velocity of 15 m/s. The inflow disturbance upstream of the propeller is the wake of a NACA0012 profile wing to which the propeller is mounted. Hot-wire anemometry has been used to characterise the wake of the strut. The turbulence intensity, mean velocity, and turbulence length scale is calculated to characterise the turbulence ingested by the propeller. Acoustic measurements have been taken with multiple microphones to study the sound directivity and noise radiation of the propeller. A load cell is used to measure the torque and thrust produced by the propeller. This experimental program aims to characterise the wake profile ingested by the propeller and examine the associated noise generation.

The Effects of Flexible Cylinder Structural Dynamics to the near Wake Turbulence

The utilization of a rigid and projecting surface, coupled with an agitator and vortex generator, frequently results in the dissipation of more energy than the production of turbulence that meets the required criteria. By contrast, a passively oscillating flexible protruding surface can generate a greater turbulence level. In the current study, a circular finite cylinder (cantilever) was used as the geometry of the rigid and protruding surface. Both the material and the aspect ratio were varied. Also, a local Reynolds number within the subcritical flow range (102 < ReD < 105) was considered. The results from the rigid protruding surface (finite cylinder) serve as a validation of the published results and a benchmark for the improvement of the turbulence generated by the flexible protruding surface. The results obtained via an ultrasonic velocity profiler have further demonstrated that the flexible cylinder is capable of generating greater turbulence by examining the turbulence intensity, the turbulence production term and the Reynolds stress. All the flexible cylinders that oscillate show an increase in turbulence production but at different percentages. The cylinders studied in this work ranged from the least structural stiffness (EVA) to moderate (aluminum) and the highest structural stiffness (carbon steel). Through studying the normalized amplitude responses graph for the flexible cylinders, it is found that the oscillating motion does indeed contribute to the increment. A further examination of the results shows that the increase is due to the structural velocity instead of just the oscillating motion.

A production term study of delayed detached eddy simulation for turbulent near wake based on proper orthogonal decomposition

In this study, we investigate the distribution of turbulent kinetic energy (TKE) in the near wake of a circular cylinder in a turbulent flow. Numerical calculations were performed using the delayed detached eddy simulation method, incorporating two different production terms for TKE: one in its original form and the other with the Kato–Launder correction term. Our results demonstrate that the turbulence model utilizing the Kato–Launder correction term exhibits a strong correlation between TKE and the vorticity field, which is related to the calculations of mean velocity, velocity fluctuation, and other parameters that are in closer agreement with direct numerical simulation and experimental values. By employing the proper orthogonal decomposition technique, we extract and reconstruct three significant modes within the flow: the shear layer mode, vortex shedding mode, and near-wake bubble mode. The findings reveal that the Kato–Launder correction term offers a more detailed portrayal closer to the real flow physics. Conversely, the original form of the TKE production term exhibits an uneven energy distribution among the three modes and affects the role of the diffusion term within the flow. This leads to a less accurate representation of the vortex shedding mode and an excessive diffusion effect in the near-wake bubble mode. Finally, possible modifications of the turbulence model in this problem are given to enhance the portrayal of these characteristics. This work presents an analytical framework that enables a comprehensive analysis of turbulence models, providing valuable physical insights and guidance for improvement.

Stochastic Dynamical Modeling of Wind Farm Turbulence

Low-fidelity engineering wake models are often combined with linear superposition laws to predict wake velocities across wind farms under steady atmospheric conditions. While convenient for wind farm planning and long-term performance evaluation, such models are unable to capture the time-varying nature of the waked velocity field, as they are agnostic to the complex aerodynamic interactions among wind turbines and the effects of atmospheric boundary layer turbulence. To account for such effects while remaining amenable to conventional system-theoretic tools for flow estimation and control, we propose a new class of data-enhanced physics-based models for the dynamics of wind farm flow fluctuations. Our approach relies on the predictive capability of the stochastically forced linearized Navier–Stokes equations around static base flow profiles provided by conventional engineering wake models. We identify the stochastic forcing into the linearized dynamics via convex optimization to ensure statistical consistency with higher-fidelity models or experimental measurements while preserving model parsimony. We demonstrate the utility of our approach in completing the statistical signature of wake turbulence in accordance with large-eddy simulations of turbulent flow over a cascade of yawed wind turbines. Our numerical experiments provide insight into the significance of spatially distributed field measurements in recovering the statistical signature of wind farm turbulence and training stochastic linear models for short-term wind forecasting.

Characterization of energy transfer and triadic interactions of coherent structures in turbulent wakes

Turbulent wakes are often characterized by dominant coherent structures over disparate scales. Dynamics of their behaviour can be attributed to interscale energy dynamics and triadic interactions. We develop a methodology to quantify the dynamics of kinetic energy of specific scales. Coherent motions are characterized by the triple decomposition and used to define mean, coherent and random velocity. Specific scales of coherent structures are identified through dynamic mode decomposition, whereby the total coherent velocity is separated into a set of velocities classified by frequency. The coherent kinetic energy of a specific scale is defined by a frequency triad of scale-specific velocities. Equations for the balance of scale-specific coherent kinetic energy are derived to interpret interscale dynamics. The methodology is demonstrated on three wake flows: (i) ${Re}=175$ flow over a cylinder; (ii) a direct numerical simulation of ${Re}=3900$ flow over a cylinder; and (iii) a large-eddy simulation of a utility-scale wind turbine. The cylinder wake cases show that energy transfer starts with vortex shedding and redistributes energy through resonance of higher harmonics. The scale-specific coherent kinetic energy balance quantifies the distribution of transport and transfer among coherent, mean and random scales. The coherent kinetic energy in the rotor scales and the hub vortex scale in the wind turbine interact to produce new scales. The analysis reveals that vortices at the blade root interact with the hub vortex formed behind the nacelle, which has implications for the proliferation of scales in the downwind near wake.

Investigating the impact of multi-rotor structure shadowing on tidal stream turbine performance

As the tidal stream energy sector develops, reducing the Levelised Cost of Energy (LCOE) is essential to sustain commercialisation. Modular multi-rotor foundations, with bi-directional turbines, reduce offshore operational complexity through smaller turbine diameters and lift weight, in turn reducing the device Operational Expenditure (OpEx). With the introduction of modular, multi-rotor foundations, the wake-induced impacts that these structures have on turbine performance must be investigated to better estimate energy yield, loading, and fatigue life. This study sets the scene for investigating the relationship between the turbulent wake generated by a modular ballast weighted foundation and 2-bladed Horizontal Axis Tidal Turbine (HATT) motivated by the HydroWing multi-rotor device concept. The presented work aims to determine the broader magnitude and severity of the loads and establish a robust methodology to be followed with further high-fidelity modelling. Initially, a transient RANS Computational Fluid Dynamics (CFD) simulations environment with a sliding mesh is configured and validated against experimental data. A turbine in free-stream isolation is simulated as a benchmark case with the modular foundation sequentially introduced to analyse the impact of the structure. Key findings suggest that operating turbines downstream of the multi-rotor foundation could cause a 20% fluctuation in thrust loading at a 1.82 Hz frequency resulting in a mean Cp reduction of 8% over a revolution.

Turbulent far wake of a body of arbitrary shape

Using a recently proposed closure for the Boussinesq-Reynolds turbulent stress we re-derive Prandtl's result for the turbulent wake width behind blunt bodies. We show that it is valid for a body of arbitrary shape, but only for wakes due to a drag oriented parallel to the incoming flow. If the geometry of the body resisting the flow lacks symmetries forbidding it, the force on the body has also a component orthogonal to the incoming velocity, called lift, that allows, for instance, planes to fly. Using our closure theory we show that this lift generates a wake wider than the drag, with a width growing like the one-half power of the distance to the body, instead of the one-third in the case of drag only. This has important consequences as it leads to a change of structure of the wake that remains turbulent all the way to infinity.

Using hydrodynamics to modify fishing performance of a demersal fishing gear

This study demonstrates how hydrodynamics can be used to influence the fishing performance of towed demersal fishing gears. Sea trials are performed in the sea star fishery to show how modifications to the gear design alter the hydrodynamics of the turbulent wake and consequently affect the catching performance of trawl gears. The trials are performed on cockle and mussel beds and the effect of the gap between the towing beam and the seabed, and the effect of the size of the beam are investigated. Two different beam sizes are utilized where a large beam reduces the mussel bycatch and increases the sea star catch rate if positioned a suitable distance over the seabed. The catch rate is independent of the distance between the beam and the fishing line within the investigated interval.

Effect of crossflow oscillation Strouhal number on circular cylinder wake

The wake flow features and aerodynamic characteristics of a circular cylinder in an oscillating crossflow were experimentally investigated. The study focused on crossflow oscillation Strouhal numbers that were smaller than the natural wake vortex-shedding Strouhal numbers. Crossflow oscillations were generated using a downstream rotating plate method. The flow evolution processes were observed using laser-light sheet assisted smoke flow visualization technique. Wake instability and time-averaged velocities were assessed using a one-component hot-wire anemometer and particle image velocimetry. Pressure distributions on the cylinder's surface were quantified using a linear pressure scanner, obtaining pressure coefficient distributions and drag forces. The wake instability Strouhal number, velocity vectors, streamline patterns, and recirculation bubble geometries in the wake region of the circular cylinder were determined. The wake turbulence properties were analyzed using the triple-decomposition method, including turbulence intensities and Lagrangian integral length and time scales. The results of natural and oscillating crossflows were compared. The wake vortex-shedding Strouhal number was lower than the natural Strouhal number but higher than the crossflow oscillation Strouhal number. It was primarily influenced by the oscillation Strouhal number and the Reynolds number of the crossflow. Critical crossflow Reynolds and oscillation Strouhal numbers were identified, beyond which the wake vortex-shedding Strouhal number reached a constant value. The crossflow oscillation intensity did not significantly affect the wake vortex-shedding behavior. The study provided quantitative descriptions and discussions of recirculation bubble geometries and statistical turbulence properties. Furthermore, the crossflow oscillations led to a substantial reduction in the drag coefficient experienced by the circular cylinder.

Estimating Maximum Power from Wind Turbines with a Simple Newtonian Approach

Recent findings in action mechanics showing torques result from rates of variation in impulsive action motivated this more fundamental approach to estimate maximum power from wind turbines. Newton’s third law of equality of action and reaction provides a strictly causal mechanism of wind power from the deflection of wind momentum by twice its angle (θ) of incidence on rotor blades. The lateral reaction needed to conserve wind momentum provides the turning moment for the turbine rotors. This direct approach challenges the current continuum mechanism for generating power from flows of kinetic energy in wind passing through the areas swept by rotating blades. Action mechanics integrates the rates of impulsive wind action on turbine blades as torques (∫mrvdθ/dt ≡ mv2) exerted on rotor surfaces at decreasing radii. Windward torque (Tw) is estimated from rotor dimensions, the angle of wind incidence and radial action of wind impulses on the blade surfaces (also ∫mrvdθ/dt ≡ mv2). A leeward torque (Tb) for back reaction of turbine blades on air mimics drag exerted parallel to the plane of rotation of the blade. Net torque is then converted to potential power (Tw - Tb)Ω by the angular velocity (Ω) of the turbine rotors, a function of tip speed ratio to wind speed. New contributions from action mechanics for managing wind power include larger estimates of its possible magnitude by including vortical energy, much larger than the kinetic energy. Better predictions of limits to wind power can be made, by including control of optimal wind angle and blade length. An analysis of the equivalence of deflected air momentum on turbine blades or air foils for aircraft reveals that even the lifting action on air foils can be explained by the normal reaction to the momentum in an air stream, also deflected by an angle twice that (2θ) of incidence, validating application of action mechanics to airflight. A mechanism for release of vortical field energy from laminar flow of air in anticyclones is predicted in turbulent downstream wakes, possibly assisting achievement of maximum power output by wind farms. Significant heat release by downwind turbulence from vortical energy requires care for their location. Diligence demands that use of windfarms as major sources of renewable energy should minimize any environmental impacts, such as drying of landscapes.

Spatial distribution of coherent structures in a self-similar axisymmetric turbulent wake

Spatial distribution of coherent structures in a self-similar axisymmetric turbulent wake

Spatial evolution of the turbulent/turbulent interface geometry in a cylinder wake

This study aims to examine the spatial evolution of the geometrical features of the turbulent/turbulent interface (TTI) in a cylinder wake. The wake is exposed to various turbulent backgrounds in which the turbulence intensity and the integral length scale are varied independently, and comparisons to a turbulent/non-turbulent interface (TNTI) are drawn. The turbulent wake was marked with a high Schmidt number ( $Sc$ ) scalar, and a planar laser induced fluorescence experiment was carried out to capture the interface between the wake and the ambient flow from $x/d = 5$ to 40, where $x$ is the streamwise coordinate from the centre of the cylinder, and $d$ is the cylinder's diameter. It is found that the TTI generally spreads faster towards the ambient flow than the TNTI. A transition region of the interfaces’ spreading is found at $x/d \approx 15$ , after which the interfaces propagate at a slower rate than previously (upstream), and the mean interface positions of both the TNTI and TTI scale with the local wake half-width. The locations of both the TNTI and TTI have non-Gaussian probability density functions (PDFs) in the near wake because of the influence of the large-scale coherent motions present within the flow. Further downstream, after the large-scale coherent motions have dissipated, the TNTI position PDF does become Gaussian. For the first time, we explore the spatial variation of the ‘roughness’ of the TTI, quantified via the fractal dimension, from near field to far field. The length scale in the background flow has a profound effect on the TTI fractal dimension in the near wake, whilst the turbulence intensity becomes important only for the fractal dimension farther downstream.

Non-equilibrium dissipation laws in a minimal two-scale wake model

The existence of a large-scale non-equilibrium dissipation law (Cε∼Reλ−15/14) has been well documented through various experiments conducted on grid-generated turbulent wake flows. In our previous studies, we have successfully demonstrated the coexistence of a rapid non-equilibrium dissipation law (Cε∼Reλ−2) before the large-scale counterpart in homogeneous isotropic turbulence with inversed or perturbed initial conditions and accordingly developed the closure models. In this paper, we introduce a simplified minimal two-scale wake model, which serves as a simplification of grid-generated turbulence experiments. It is evident that both the large-scale and rapid non-equilibrium dissipation laws coexist in this flow, similar to our previous studies in homogeneous isotropic turbulence. Moreover, we show that the utilization or exclusion of spanwise averages does not significantly affect the qualitative characteristics of non-equilibrium behaviors. Furthermore, by investigating the turbulent/non-turbulent interface, we argue that the presence of coherent structures is not necessarily required to generate the scaling associated with non-equilibrium behavior.

Evolution of high-density submarine turbidity current and its interaction with a pair of parallel suspended pipes

The method of large-eddy simulation (LES) coupled with the density transport equation is employed to simulate the evolution of a gravity-driven high-density turbidity current and its interaction with a pair of parallel suspended pipes. The LES method is validated first using data of a non-Boussinesq lock-exchange experiment and satisfying agreement between LES and experiment is achieved. The simulations reveal that a shear region forms between high- and low-density fluids each moving in opposite directions which lead to the generation of a series of vortices and a substantial mixing region. Close to the bottom boundary, low-density fluid is entrained near the head of the high-density turbidity current, forming a thin water cushion that separates the turbidity current's head from the seabed, the so-called hydroplaning effect, thereby reducing the density of the head and bottom friction. The current study suggests that the effect of hydroplaning phenomena leads to high speed and long distance of the turbidity current. Further, LES simulations of a turbidity current impacting a pair of parallel suspended pipes with different streamwise spacings are performed and impact forces are quantified. The turbulent wake generated by high-density fluid bypassing pipe 1 promotes velocity fluctuations leading to increased impact forces on pipe 2 with increasing streamwise spacing up to 8 times the pipeline diameter (8D). The results suggest that the streamwise spacing between two parallel pipes should be less than 2D to minimize hydrodynamic loads on pipe 2.

A wind tunnel study on cyclic yaw control: Power performance and wake characteristics

In this study, wind-tunnel experiments are used to investigate cyclic yaw control as a wake mitigation strategy aiming to improve wind farm power production. The control strategy is applied to a single wind turbine and a wind farm comprising of a column of three wind turbines in full wake state. The power performance of the wind farm is optimized by adjusting the control parameters, which include the yaw angle amplitude and yaw Strouhal number, and the wake flows are investigated using particle-image velocimetry measurements. The yaw angle of the controlled turbine is varied dynamically in a sinusoidal manner. A power gain of more than 10 % compared to an uncontrolled wind farm is achieved when the yaw angle amplitude and yaw Strouhal number are in the range of 10°–35° and 0.09-0.22, respectively, with a peak power gain of 15.2 %. The power consumed to generate the control is found to be small and predictable. Results from wake measurements reveal that by deflecting the wake cyclically, this control strategy can lead to relatively flattened mean wake velocity deficit profiles, faster wake recovery and very different wake turbulence intensity distributions, compared to the uncontrolled case. The results show that cyclic yaw control has a great potential in improving the power production of wind farms. A scaling of the control parameters based on the kinematic similarity between the wind-tunnel scale and utility-scale wind farms is also proposed.

Numerical analysis on the transom-stern wake with a horizontal plunging jet

The turbulent flow behind a dry-transom stern with a plunging water jet is numerically studied. High-resolution large eddy simulation (LES) is performed to predict the wave-elevation topology and vortex structure in the wake with varying jet height. The theoretical impact position between jet flow and stern wave is proposed. The turbulent wake under the stern-jet configuration can be divided as converging-corner-wave region (CCW), mixing region (MR), transition region (TR) and developed region (DR). High-speed plunging jet speeds up the averaged-axial velocity and enhances the shear effect near the free surface. The upward movement of the rooster tail is suppressed by the parabolic jet flow, and periodical large scale air pockets are clearly observed following the wake angle along the downstream direction. Air pocket vortices and jet vortices are merging in the near wake and forming U-shaped vortex structures. Reducing jet height accelerates the instability of coherent vortex structure and energy dissipation in the developed region. The main contributions in the power spectra density (PSD) spectrum of Turbulent Kinetic Energy appear in the frequency range of 10–20 Hz with the evolution of wake vortices.

Numerical investigation of wake-induced lifetime fatigue load of two floating wind turbines in tandem with different spacings

Floating wind turbine (FWT) is a promising technology to utilize offshore wind resources. For offshore wind farms, wakes can result in 5%–15% increase in fatigue load on the downstream turbine rotor. Current offshore wind turbine design standard lacks accounting for the different wake characteristics and wake-induced fatigue load of the FWT from that of their fixed counterparts. Considering a scenario of two tandem FWTs, this study investigates the wake characteristics of a semi-submersible FWT accounting for multi-degree-of-freedom (DOF) dynamic motion characteristics and its wake effect on fatigue load of downstream unit with different spacing configurations. A comparison with the effective turbulence intensity method suggested in the current IEC 61400-1 standard is also carried out. With an interactive analysis framework composed of computational fluid dynamics (CFD) wake simulation and time domain aero-hydro-servo-elastic simulation for FWT, lifetime wake-induced fatigue loads are analyzed through a full range of turbine normal operational conditions. Results show that the wake characteristics of the FWT show a distinct difference between below-rated and above-rated conditions due to the feature of FWT's controller. Moreover, the Frandsen model adopted in the current IEC 61400-1 standard highly underestimates the FWT's wake turbulence, causing the predicted lifetime fatigue damage equivalent load at the tower base decreases up to about 10∼15% for a spacing of 8D∼10D under combined wind and wave conditions.

Interstellar Bow Shocks around Fast Stars Passing through the Local Interstellar Medium

Bow shocks are produced in the local interstellar medium by the passage of fast stars from the Galactic thin-disk and thick-disk populations with velocities V * = 40–80 km s−1. Stellar transits of local H i clouds occur every 3500–7000 yr on average and last between 104 and 105 yr. There could be 10–20 active bow shocks around low-mass stars inside clouds within 15 pc of the Sun. At local cloud distances of 3–10 pc, their turbulent wakes have transverse radial extents R wake ≈ 100–300 au, angular sizes 10″–100″, and Lyα surface brightnesses of 2–8 R in gas with total hydrogen density n H ≈ 0.1 cm−3 and V * = 40–80 km s−1. These transit wakes may cover an area fraction f A ≈ (R wake/R cl) ≈ 10−3 of local H i clouds and be detectable in IR (dust), UV (Lyα, two-photon), or nonthermal radio emission. Turbulent heating in these wakes could produce the observed elevated rotational populations of H2 (J ≥ 2) and influence the endothermic formation of CH+ in diffuse interstellar gas at T > 103 K.

Optimized Escape Path Planning for Commercial Aircraft Formations

There is growing interest in commercial aircraft formation flight as a means of reducing both airspace congestion and the carbon footprint of air transportation. Wake vortex surfing has been proven to have significant fuel-saving benefits; however, commercial air transportation has yet to take advantage of these formation benefits due to understandable safety concerns. The realization of these formations requires serious consideration of safety contingencies during closer-in maneuvering of large commercial aircraft. Formation contingency scenarios are much more complex than those of individual aircraft and have not yet been studied in depth. This paper investigates the utility of an optimization modeling approach in generating maneuvering and escape paths for commercial formation aircraft by comparing model-generated paths with pilot-generated escape plans. Three high-altitude commercial aircraft formation scenarios are presented: formation join, formation emergency exit, and formation escape. The model-generated paths are evaluated and compared with pilot-generated escape plans using the author’s pilot expertise. Model results compare well with pilot intuition and are useful in presenting solutions not previously considered, in evaluating separation requirements for improvement of escape path planning, and in confirming the viability of the pilot-generated plans. The novel optimization model formulation presented in this paper is the first model capable of generating exit and escape paths comparable to pilot-generated plans. It is also the first model to incorporate path avoidance of persistent and drifting wake turbulence.

Wake tail plane interactions for a tandem wing configuration in high-speed stall conditions

In this work, wake-tail plane interactions are investigated for a tandem wing configuration in buffet conditions, consisting of two untapered and unswept wing segments, using hybrid Reynolds-Averaged Navier–Stokes / Large Eddy Simulations (RANS/LES) with the Automated Zonal Detached Eddy Simulation (AZDES) method. The buffet on the front wing and the development of its turbulent wake are characterized, including a spectral analysis of the fluctuations in the wake and a modal analysis of the flow. The impact of the wake on the aerodynamics and loads of the rear wing is then studied, with a spectral analysis of its lift and surface pressure oscillations. Finally, the influence of the position and the incidence angle of the rear wing is investigated. For the considered flow conditions, 2D buffet is present on the front wing. During the downstream movement of the shock, the amount of separation reaches its minimum and small vortices are present in the wake. During the upstream movement of the shock, the amount of separation is at its maximum and large turbulent structures are present accompanied by high fluctuation levels. A distinct peak in the corresponding spectra can be associated with vortex shedding behind the wing. The impingement of the wake leads to a strong variation of the loading of the rear wing. A low-frequent oscillation of the lift, attributed to the change of the intensity of the downwash generated by the front segment, can be distinguished from high-frequent fluctuations that are caused by the impingement of the wake’s turbulent structures.