Wake Flow Measurements Research Articles

Effect of trailing-edge blowing on the acoustic and aerodynamic characteristics of a flatback airfoil

This study investigated the impact of trailing-edge uniform air blowing on the acoustic and aerodynamic characteristics of a flatback airfoil. Near- and far-field pressure fluctuations, surface static pressure distribution, as well as boundary layer and wake flow measurements were conducted to comprehensively understand the effects of the method on both the noise generation mechanism and the aerodynamic characteristics of the airfoil. It was revealed that tonal noise originates from surface pressure fluctuations induced by upstream flow disturbances due to vortex shedding. The application of blowing was found to shift large-scale vortices generated during vortex shedding further downstream, resulting in the suppression of surface pressure fluctuations on both the pressure and suction sides of the airfoil, consequently reducing far-field noise. Additionally, blowing enhanced spanwise coherence at the vortex shedding frequency. In terms of aerodynamic behavior, blowing was shown to increase base pressure, leading to drag reduction without affecting lift. Interestingly, the significant drag reduction was found to occur at the same blowing parameter associated with maximum tonal noise reduction.

Wake oscillator modeling the combined instability of vortex induced vibration and galloping for a 2:1 rectangular cylinder

Reasonable success has been obtained in the literature using Tamura’s nonlinear wake-oscillator model to predict the combined instability arising from the interference of vortex induced vibration and galloping. However, the physical arguments concerning the near-wake behavior, which is a strong point of this model, have never been carefully examined for sharp-edged bluff bodies. This is tackled here, referring to a rectangular cylinder with a side ratio of two, which is known to be very prone to this type of instability, and for which new static and aeroelastic wind tunnel tests are carried out. Therefore, a few physically sound modifications of the equations are proposed and, based on flow measurements, the new version of the model is found to perform considerably better than previous formulations with respect to the local-effect equation, which links the Strouhal number and the characteristic length scales of the wake. Moreover, after a successful attempt to relate the near-wake geometric parameters of the model to specific and measurable wake length scales, a novel method is presented for a direct estimation from wake flow measurements of the key parameter f relating the unsteady force and the wake lamina rotation. This allows the derivation of all the aerodynamic parameters of the model from measurements on the stationary cylinder based on their physical meaning. Nevertheless, f is also calibrated through a set of aeroelastic tests for a high Scruton number. The two approaches provide consistent results, although the calibration procedure yields a somewhat larger value of f, slightly enhancing the prediction capabilities of the model, which are carefully assessed comparing the results with wind tunnel data for a wide range of Scruton numbers. This discrepancy can be explained based on some unconsidered effects in the wake-oscillator model, which are also discussed as a basis for a future development of Tamura’s model.

Aerodynamic noise characteristics of airfoils with morphed trailing edges

Numerical investigation of the aerodynamic and aeroacoustic characteristics of a symmetrical NACA 0012 airfoil developed by National Advisory Committee for Aeronautics (NACA) with a conventional hinged flap and a morphed flap has been carried out. The unsteady flow characteristics over the airfoil surface and at the airfoil wake are captured using high-fidelity Large Eddy Simulations. The tests were carried out for a freestream velocity of U∞=20 m/s, corresponding to a chord-based Reynolds number of Rc=2×105, at angles of attack α=0° and 4°. The results of the airfoil static pressure distribution show that the airfoil with morphed trailing edge produces higher values of lift coefficient than the airfoil with hinged trailing edge due to its higher suction peak resulting in larger pressure difference between the suction and pressure sides. Furthermore, from the wake-flow measurements, boundary-layer analysis, wall-pressure spectra, space–time correlation, it is concluded that the Morphed airfoil produces higher values of surface pressure fluctuations near the trailing edge which extend farther into the wake of the trailing edge compared to the Hinged airfoil. Consequently, the increased pressure fluctuations at the vicinity of the trailing edge results in a higher values of radiated far-field noise for the airfoil with morphed trailing edge flap.

Operational fluid modal analysis of full-scale pressure and wake flow measurements on the Gjemnessund Bridge

The present paper is dealing with observations of vortex shedding on the wedge-shaped Gjemnessund Bridge Box girder in the period from August 2019 to February 2020. Wake flow oscillations are identified using operational fluid modal analysis with correlation functions of surface point pressures near the trailing edge, flow velocities upstream and in the wake measured in full-scale. Even though vortex shedding is barely observed in the lift and response spectra, operational fluid modal analysis is able to identify evidence from vortex shedding. A characteristic vortex shedding fluid mode shape is recognized for 179 ten-minutes records at Reynolds numbers ranging from 500,000 to 2,000,000. The Strouhal numbers from the full-scale measurements are identified simultaneously with the vortex shedding fluid mode shapes. A comparison with wind tunnel tests of a 1:30 scaled section model indicates a Reynolds number dependency of the Strouhal number.

Application of Actuator Line Model for Large Eddy Simulation of Rotor Noise Control

A hybrid Large Eddy Simulation (LES)/Actuator Line Model (ALM) computational approach is employed to simulate the turbulent wake of a two-bladed rotor in hover. Validations are performed for the Knight and Hefner two-bladed rotor case with comparisons to previous LES predictions using the immersed boundary method, as well as hybrid RANS-LES using the Helios code. The Ffowcs-Williams Hawkings (FWH) acoustic analogy is then used for predicting rotor noise. Predictions from the combined LES/ALM/FWH model are next validated against recent PIV wake flow and acoustic measurements for a two-bladed rotor. Finally, the LES/ALM/FWH approach is exploited to demonstrate a novel way to explore active flow control strategies to reduce rotor noise by delaying flow separation and enabling a lower rate of rotation, resulting in lower noise while maintaining thrust.

Flow Interactions Between Low Aspect Ratio Hydrofoils in In-line and Staggered Arrangements.

Many species of fish gather in dense collectives or schools where there are significant flow interactions from their shed wakes. Commonly, these swimmers shed a classic reverse von Kármán wake, however, schooling eels produce a bifurcated wake topology with two vortex rings shed per oscillation cycle. To examine the schooling interactions of a hydrofoil with a bifurcated wake topology, we present tomographic particle image velocimetry (tomo PIV) measurements of the flow interactions and direct force measurements of the performance of two low-aspect-ratio hydrofoils () in an in-line and a staggered arrangement. Surprisingly, when the leader and follower are interacting in either arrangement there are only minor alterations to the flowfields beyond the superposition of the flowfields produced by the isolated leader and follower. Motivated by this finding, Garrick’s linear theory, a linear unsteady hydrofoil theory based on a potential flow assumption, was adapted to predict the lift and thrust performance of the follower. Here, the follower hydrofoil interacting with the leader’s wake is considered as the superposition of an isolated pitching foil with a time-varying cross-stream velocity derived from the wake flow measurements of the isolated leader. Linear theory predictions accurately capture the time-averaged lift force and some of the major peaks in thrust derived from the follower interacting with the leader’s wake in a staggered arrangement. The thrust peaks that are not predicted by linear theory are likely driven by spatial variations in the flowfield acting on the follower or nonlinear flow interactions; neither of which are accounted for in the simple theory. This suggests that unsteady potential flow theory that does account for spatial variations in the flowfield acting on a hydrofoil can provide a relatively simple framework to understand and model the flow interactions that occur in schooling fish. Additionally, schooling eels can derive thrust and efficiency increases of 63-80% in either a in-line or a staggered arrangement where the follower is between two branched momentum jets or with one momentum jet branch directly impinging on it, respectively.

Trailing edge bluntness noise reduction using porous treatments

This paper provides an experimental investigation of the noise generated from a blunt flat plate and the possibility of the control of noise at source using porous trailing edges. The noise generation characteristics have been examined using a flat plate equipped with surface pressure transducers and far-field microphones and was performed in an anechoic open jet wind tunnel. Two types of porous materials with different porosities and permeability constants were tested. Simultaneous near-field and far-field noise measurements, as well as the boundary layer and wake flow measurements, have been carried out to better understand the effects of the porous treatment to the flow-field and the noise generation mechanism of the plate. Results have shown that the use of porous trailing edges can generally lead to the effective suppression of both the tonal noise, due to the vortex shedding from the blunt trailing edge and also the broadband noise of the plate. The level of the noise reduction was also found to be dependent on the porosity and permeability of the porous materials. The directivity patterns of the radiated noise have shown significant noise reduction at the vortex shedding frequencies. The flow measurement results have shown that the use of porous trailing edges leads to the reduction of flow acceleration over the blunt edges, and subsequently the delay and weakening of the vortex shedding. The results presented in this paper provide an impetus for further experimental and numerical studies on the use of porous treatments for the suppression of aerodynamically generated noise at source.

Stereo three-dimensional particle image velocimetry measurement and aerodynamic force analysis of non-spinning soccer balls

A knuckle shot, resulting from non-spinning kicking, is an essential technique in soccer. The irregular flight path of the knuckle shot is caused by the aerodynamic force from the three-dimensional twin vortices generated in the wake behind the ball. However, the detailed behavior of the twin vortices and relation between the jet flow and the acting forces on the balls is still not understood. In addition, a more thorough understanding of the effect of ball panels on the formation of twin vortices and jet flow is important to develop balls with high controllability. To study the effect of the ball panel shape on the flight path, stereo three-dimensional particle image velocimetry wake flow measurements and synchronized force measurements were performed on various soccer balls. It was confirmed that the aerodynamic force on the ball is produced by the jet flow generated by the vortices in the wake flow. The directions of the force followed the changes of the jet flow, and the magnitude of the force was strongly associated with the flow rate of the jet. Moreover, the shape of the ball panels, especially the groove volume, determines the critical Reynolds number and the fluttering of the balls.

Measurements of wake flow behind a flat plate with fractal edge geometries

Measurements of wake flow behind a flat plate with fractal edge geometries

Measurements of wake flow behind a flat plate with fractal edge geometries

Measurements of wake flow behind a flat plate with fractal edge geometries

The design of a small lab-scale wind turbine model with high performance similarity to its utility-scale prototype

This paper discussed the procedure of an optimization design method involving a small lab-scale wind turbine rotor. To simulate the aerodynamic performance of a prototype wind turbine at utility-scale by scaled model experiments (especially, the visualization measurements of wake flow in small wind tunnels), an appropriate similar design of the small lab-scale rotor is needed. However, the similar design of blade performance from a prototype wind turbine for small scaled model is rare. In this paper, using a 2.5 MW utility-scale wind turbine as the prototype, we set up a new optimization process of similar design for a model wind turbine with a 320 mm rotor based on the Lifting-Line Theory (LLT) with wake-induced corrections. Results show a dramatic deviation in performance of the geometric scaled model compared to the objective values, which demonstrates the significance of the Reynolds number effects. With the optimized blade, the distribution of normal thrust force is very similar to the objective values. Geometric characteristics variations along the blade span are similar to the prototype. With the tip speed ratio runs at a roughly matched rated condition, the optimized rotor gives a better performance than other models used in previous wind tunnel studies.

Aerodynamic forces acting on birds during flight: A comparative study of a shorebird, songbird and a strigiform

The flapping wing is one of the most widespread propulsion methods found in nature. The current study focuses on the estimation of unsteady aerodynamic forces on freely flying birds through analysis of wingbeat kinematics and near wake flow measurements using long duration time-resolved particle image velocimetry. Three distinct bird species have been investigated: a shorebird (Western sandpiper, Calidris mauri), a songbird (European starling, Sturnus vulgaris) and a boobook owl (Australian boobook, Ninox boobook), corresponding to different flight strategies. Using long-time sampling data, several wingbeat cycles have been analyzed. Drag and lift were estimated using the momentum equation for viscous flows, revealing a highly unsteady behavior. The owl experienced the highest total drag over the cycle whilst the sandpiper was shown to exert minimal drag compared to the other two birds. The unsteady drag term was found to have a crucial role in the balance of drag (or thrust), particularly during the transition phases. The lift estimation presents a similar distribution over the wingbeat cycle, where both the owl and the sandpiper feature high values of unsteady lift compared to the starling. The large lift variation generated by the sandpiper is presumably used to achieve its high-performance migratory flight. For owls, the large variation of the lift appears to be essential for weight support while flying at slow speeds. The aerodynamic performance of the starling appears to be relatively low which matches their limited migratory behavior. These findings may shed light on the flight efficiency of birds by providing a partial answer to how they minimize drag and maximize lift during flapping flight by incorporating unsteady motion that interacts with the wake flow dynamics.

Validation of the real-time-response ProCap measurement system for full field wake scans behind a yawed model-scale wind turbine

For an accurate prediction of the complex flow conditions in wind farms, model scale wake flow measurements represent important references in well-defined boundary conditions. State-of-the art flow measurement techniques are often time-expensive and require elaborate post-processing to assess the data quality. In this wind tunnel study we demonstrate the advantages of the real-time-response system Probe Capture (ProCap) for measurements of a complex three-dimensional wind turbine wake flow. The complex wake flow behind a yawed model wind turbine is measured with both a Laser-Doppler Anemometer (LDA) and the ProCap system. Both the streamwise and vertical flow component show an accurate agreement with the LDA reference experiment for various measured downstream distances and turbine yaw angles. Areas of strong rotation in the wake flow are accurately resolved by the ProCap measurement system confirming its applicability for wind turbine wake measurement. The system appears to greatly facilitate and speed up larger wake surveys by a factor of 30 and thus has the potential to enhance the understanding of the complex flow topology in wind turbine wakes.

Field investigation on the influence of yaw misalignment on the propagation of wind turbine wakes

AbstractA comprehensive understanding of the wake development of wind turbines is essential for improving the power yield of wind farms and for reducing the structural loading of the turbines. Reducing the overall negative impact of wake flows on individual turbines in a farm is one goal of wind farm control. We aim to demonstrate the applicability of yaw control for deflecting wind turbine wakes in a full‐scale field experiment. For this purpose, we conducted a measurement campaign at a multimegawatt onshore wind turbine including inflow and wake flow measurements using ground‐ and nacelle‐based long‐range light detection and ranging devices. Yaw misalignments of the turbine with respect to the inflow direction of up to 20° were investigated. We were able to show that under neutral atmospheric conditions, these turbine misalignments cause lateral deflections of its wake. Larger yaw misalignments resulted in greater wake deflection. Because of the inherent struggle in capturing complex and highly dynamic ambient conditions in the field using a limited number of sensors, we particularly focused on providing a comprehensive and comprehensible description of the measurement setup, including the identification of potential uncertainties.

An experimental study on the aerodynamic performances and wake characteristics of an innovative dual-rotor wind turbine

A novel dual-rotor wind turbine (DRWT) concept with an auxiliary upwind rotor and a bigger downwind rotor was introduced for improved turbine performance. An experimental study was performed in an Atmospheric Boundary Layer (ABL) wind tunnel with scaled turbine models to investigate the aerodynamic performances and wake characteristics of DRWTs in either co-rotating or counter-rotating configuration (i.e., CR-DRWT or CO-DRWT), in comparison to those of a conventional single-rotor wind turbine (SRWT). In additional to measuring the aerodynamic forces and power outputs, the wake flow characteristics behind DRWTs and SRWT were also quantified by using a high-resolution Particle Image Velocimetry (PIV) system. In comparison to those of SRWT, while the aerodynamic force acting on the CR-DRWT was found to increase 13.3%, the power coefficient of the CR-DRWT design was improved by 7.2%. Faster wake recovery behind the DRWT designs was confirmed from both the measured power outputs of the same turbine model sited in the turbine wakes and the measured flow velocity at various downstream locations. The wake flow measurements were correlated with the aerodynamic force and power output data to elucidate underlying physics for the higher efficiency of DRWT designs in either isolation or wind farm settings.

Experimental study on power curtailment of three in-line turbines

A dataset of wind tunnel power and wake flow measurements on a setup of three aligned model wind turbines is presented. The power outputs of the three turbines are in good agreement with measurements from a full-scale wind farm of similar inter-turbine spacing. A comparison of the wake flow behind the first row and the second row shows a significantly higher mean velocity loss behind the second row justifying a further power drop from the second to the third row turbine. Curtailing the front row turbine to smaller than rated tip speed ratios resulted in insignificant total power gains below 1%. Curtailments of both the first and second row turbine indicate that the best combined array power results are achieved for slightly lower than rated tip speed ratios. Although power curtailment is observed to have a rather small potential for power optimization of a wind farm, it could be an effective method for load distribution at constant farm power.

Experimental study on kinetic energy conversion of horizontal axis tidal stream turbine

The present study aims to understand the energy conversion mechanism of a 100 kW horizontal axis tidal stream turbine by analyzing thrust, torque, and wake flow measurements. The scale ratio of the turbine model was 1/20 and model tests for power and wake measurements were conducted in a towing tank facility. Wake fields were measured by a towed underwater stereoscopic particle image velocimetry (SPIV) system. The chord-length based Reynolds number at 40% of the radius of the turbine ranged from 53,000 to 63,000 in the test conditions. The turbine model showed the highest power coefficient at a tip speed ratio (TSR) of 3.5, and the magnitude of power coefficient was 0.278. Three TSR conditions were selected for SPIV measurement after power measurement tests, representing heavy loading, highest efficiency, and light loading, respectively. In the wake field measurement results, conversion of kinetic energy of the turbine wake was investigated, decomposing it into effectively extracted work, loss due to the drag on the turbine system, kinetic energy of the time-mean axial flow, local flow structures, turbulence, and secondary flow loss. In high TSR conditions with a small angle of attack onto the turbine blade, the secondary flow loss was minimized.

VELOCITY MEASUREMENTS OF TURBULENT WAKE FLOW OVER A CIRCULAR CYLINDER

There are two general concerns in the velocity measurements of turbulence. One is the temporal characteristics which governs the turbulent mixing process. Turbulence is rotational and is characterized by high levels of fluctuating vorticity. In order to obtain the information of vorticity dynamics, the spatial characteristics is the other concern. These varying needs can be satisfied by using a variety of diagnostic techniques such as invasive physical probes and non-invasive optical instruments. Probe techniques for the turbulent measurements are inherently simple and less expensive than optical methods. However, the presence of a physical probe may alter the flow field, and velocity measurements usually become questionable when probing recirculation zones. The non-invasive optical methods are mostly made of the foreign particles (or seeding) instead of the fluid flow and are, thus, of indirect method. The difference between the velocities of fluid and foreign particles is always an issue to be discussed particularly in the measurements of complicated turbulent flows. Velocity measurements of the turbulent wake flow over a circular cylinder will be made by using two invasive instruments, namely, a cross-type hot-wire anemometry (HWA) and a split-fiber hot-film anemometry (HFA), and a non-invasive optical instrument, namely, particle image velocimetry (PIV) in this study. Comparison results show that all three employed diagnostic techniques yield similar measurements in the mean velocity while somewhat deviated results in the root-mean-squared velocity, particularly for the PIV measurements. It is demonstrated that HFA possesses more capability than HWA in the flow measurements of wake flow. Wake width is determined in terms of either the flatness factor or shear-induced vorticity. It is demonstrated that flow data obtained with the three employed diagnostic techniques are capable of yielding accurate determination of wake width.

Experimental Investigation on the Wake Characteristics and Aeromechanics of Dual-Rotor Wind Turbines

An experimental study was carried out to investigate the aeromechanics and wake characteristics of dual-rotor wind turbines (DRWTs) in either co-rotating or counter-rotating configuration, in comparison to those of a conventional single-rotor wind turbine (SRWT). The experiments were performed in a large-scale aerodynamic/atmospheric boundary layer (AABL) wind tunnel, available at Iowa State University with the oncoming atmospheric boundary-layer (ABL) airflows under neutral stability conditions. In addition to measuring the power output performance of DRWT and SRWT models, static and dynamic wind loads acting on those turbine models were also investigated. Furthermore, a high-resolution digital particle image velocimetry (PIV) system was used to quantify the flow characteristics in the near wakes of the DRWT and SRWT models. The detailed wake-flow measurements were correlated with the power outputs and wind-load measurement results of the wind-turbine models to elucidate the underlying physics to explore/optimize design of wind turbines for higher power yield and better durability.

On the Estimation of Time Dependent Lift of a European Starling (Sturnus vulgaris) during Flapping Flight.

We study the role of unsteady lift in the context of flapping wing bird flight. Both aerodynamicists and biologists have attempted to address this subject, yet it seems that the contribution of unsteady lift still holds many open questions. The current study deals with the estimation of unsteady aerodynamic forces on a freely flying bird through analysis of wingbeat kinematics and near wake flow measurements using time resolved particle image velocimetry. The aerodynamic forces are obtained through two approaches, the unsteady thin airfoil theory and using the momentum equation for viscous flows. The unsteady lift is comprised of circulatory and non-circulatory components. Both approaches are presented over the duration of wingbeat cycles. Using long-time sampling data, several wingbeat cycles have been analyzed in order to cover both the downstroke and upstroke phases. It appears that the unsteady lift varies over the wingbeat cycle emphasizing its contribution to the total lift and its role in power estimations. It is suggested that the circulatory lift component cannot assumed to be negligible and should be considered when estimating lift or power of birds in flapping motion.