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There are many ways to learn from data. Our first experiment consisted in reproducing the way aerodynamicists work [2] with a genetic optimizer. The data pool was too narrow and asymptotic tendencies were unreliable. Our 2nd Experiment, a simple version of [4], had a virtually unlimited data pool and used neural networks. Results were better, but computationally expensive. Data assimilation approaches used in EO [ 7] could yield better results.., Wind Energy

A methodology for a posteriori uncertainty quantification of pressure data retrieved from particle image velocimetry (PIV) is proposed. It relies upon the Bayesian framework, where the posterior distribution (probability distribution of the true velocity, given the PIV measurements) is obtained from the prior distribution (prior knowledge of properties of the velocity field, e.g., divergence-free) and the statistical model of PIV measurement uncertainty. Once the posterior covariance matrix of the velocity is known, it is propagated through the discretized Poisson equation for pressure. Numerical assessment of the proposed method on a steady Lamb–Oseen vortex shows excellent agreement with Monte Carlo simulations, while linear uncertainty propagation underestimates the uncertainty in the pressure by up to 30 %. The method is finally applied to an experimental test case of a turbulent boundary layer in air, obtained using time-resolved tomographic PIV. Simultaneously with the PIV measurements, microphone measurements were carried out at the wall. The pressure reconstructed from the tomographic PIV data is compared to the microphone measurements. Realizing that the uncertainty of the latter is significantly smaller than the PIV-based pressure, this allows us to obtain an estimate for the true error of the former. The comparison between true error and estimated uncertainty demonstrates the accuracy of the uncertainty estimates on the pressure. In addition, enforcing the divergence-free constraint is found to result in a significantly more accurate reconstructed pressure field. The estimated uncertainty confirms this result.

In this work, we present a simple inviscid point vortex model to study the dynamics of asymmetric vortex rows, as might appear behind misaligned vortex generator vanes. Starting from the existing solution of the in_nite vortex cascade, a numerical model of four base-vortices is chosen to represent two primary counter-rotating vortex pairs and their mirror plane images, introducing the vortex strength ratio as a free parameter. The resulting system of equations is also de_ned in terms of the vortex row separation and the qualitative features of the ensuing motion are mapped. A translating and orbiting regime are identi_ed for di_erent cascade separations. The latter occurs for all unequal strength vortex pairs. Thus, the motion is further classi_ed by studying the cyclic behaviour of the orbiting regime and it is shown that for small mismatches in vortex strength, the orbiting length and time scales are su_ciently large as to appear, in the near wake, as translational (non-orbiting). However, for larger mismatches in vortex strength, the orbiting motion approaches the order of the starting height of the vortex. Comparisons between experimental data and the potential ow model show qualitative agreement whilst viscous e_ects account for the major discrepancies. Despite this, the model captures the orbital mode observed in the measurements and provides an impetus for considering the impact of these complex interactions on vortex generator designs., Wind Energy

The actuator disc (AD) model is commonly used to simplify the simulation of horizontal-axis wind-turbine aerodynamics. The limitations of this approach in reproducing the wake losses in wind farm simulations have been proven by a previous research. The present study is aimed at providing an experimental analysis of the near-wake turbulent flow of a wind turbine (WT) and a porous disc, emulating the actuator disc numerical model. The general purpose is to highlight the similarities and to quantify the differences of the two models in the near-wake region, characterised by the largest discrepancies. The velocity fields in the wake of a wind turbine model and a porous disc (emulation of the actuator disc numerical model) have been measured in a wind tunnel using stereo particle image velocimetry. The study has been conducted at low turbulence intensity in order to separate the problems of the flow mixing caused by the external turbulence and the one caused by the turbulence induced directly by the AD or the WT presence. The analysis, as such, showed the intrinsic differences and similarities between the flows in the two wakes, solely due to the wake-induced flow, with no influence of external flow fluctuations. The data analysis provided the time-average three-component velocity and turbulence intensity fields, pressure fields, rotor and disc loading, vorticity fields, stagnation enthalpy distribution, and mean-flow kinetic-energy fluxes in the shear layer at the border of the wake. The properties have been compared in the wakes of the two models. Even in the absence of turbulence, the results show a good match in the thrust and energy coefficient, velocity, pressure, and enthalpy fields between wind turbine and actuator disc. However, the results show a different turbulence intensity and turbulent mixing. The results suggest the possibility to extend the use of the actuator disc model in numerical simulation until the very near wake, provided that the turbulent mixing, Aerodynamics, Wind Energy & Propulsion, Aerospace Engineering

The article seeks to unify the treatment of conservative force interactions between axi-symmetric bodies and actuators in inviscid ow. Applications include the study of hub interference, di_user augmented wind turbines and boundary layer ingestion propeller con_gurations. The conservation equations are integrated over in_nitesimal streamtubes to obtain an exact momentum model contemplating the interaction between an actuator and a nearby body. No assumptions on the shape or topology of the body are made besides (axi)symmetry. Laws are derived for the thrust coe_cient, power coe_cient and propulsive e_ciency. The proposed methodology is articulated with previous e_orts and validated against the numerical predictions of a planar vorticity equation solver. Very good agreement is obtained between the analytical and numerical methods, Wind Energy

This paper presents an experimental assessment of the working principles of pylon trailing-edge blowing for the mitigation of the interaction between a pusher propeller and its associated pylon. The experiments were performed at the Large Low-Speed Facility of the German–Dutch wind tunnels (DNW–LLF), using a powered propeller model and an upstream pylon equipped with a trailing-edge blowing system. In-flow microphone measurements demonstrated the impact of pylon installation on the propeller’s tonal noise emissions, with increases of up to 16 dB relative to the isolated propeller. Analysis of the unsteady blade pressures showed that this installation effect is caused by the impulsive increase in blade loading during the pylon-wake passage. The efficacy of pylon trailing-edge blowing to reduce the momentum deficit in the pylon wake was confirmed by stereoscopic particle-image-velocimetry measurements between the pylon and the propeller. Consequently, application of the pylon-blowing system alleviated the pylon-installation effects at the source. At an intermediate thrust setting, the root-mean-square of the blade-load fluctuations due to the wake encounter was reduced by up to 60%, resulting in noise emissions approximately equal to those recorded for the isolated propeller., Flight Performance and Propulsion, Wind Energy

Pylon-mounted pusher propellers suffer from installation effects due to the interaction between the pylon and the propeller. The impact of angular inflow on these installation effects was quantified at the Large Low-Speed Facility of the German-Dutch wind tunnels (DNW-LLF). Particle-image-velocimetry measurements showed that the pylon wake's width and velocity deficit were hardly affected by the introduction of a six-degree sideslip angle. Application of pylon trailing-edge blowing reduced the integral velocity deficit in the wake by up to 65%. Evaluations of the surface pressures on the blades confirmed the sinusoidal loading behavior in angular inflow and the impulsive loading peak due to the pylon-wake encounter. The circumferential velocity components induced by the pylon tip vortex strongly affected the steady-state propeller performance by modifying the effective advance ratio sensed by the blades. Increased performance was measured when the rotation direction of the pylon tip vortex was opposite to that of the propeller. Angular inflow affected the propeller noise emissions due to the resulting unsteady blade loads and the circumferential variation of the effective Mach number of the blade sections. The installation of the pylon added a noise source due to the unsteady blade loads caused by the pylon-wake encounter. Depending on the sideslip angle, application of blowing eliminated a large part of the installation noise penalty, despite remaining non-uniformities in the blown wake profiles., Flight Performance and Propulsion, Wind Energy

The flow past a NACA 0018 airfoil with sawtooth trailing edge serrations has been investigated using stereoscopic particle image velocimetry (PIV). The serration flap angle and airfoil incidence are varied in order to study the effect of secondary flow establishing between the suction and pressure sides of the serrations. The flow topology around the serrations is inferred from the analysis of time-averaged streamlines close to the airfoil surface and from the wall-normal flow velocity in between serrations. Additional PIV measurements with a plane in cross-flow highlight the formation of streamwise vortex pairs. The flow behavior is further characterized in terms of its turbulence statistics. Noise emissions are measured with an acoustic phased array in combination with beamforming. The serrations are found to be effective in reducing noise, and their application is studied for different degrees of airfoil incidence and serration flap angle., Aerodynamics, Wind Energy, Aerodynamics, Wind Energy & Propulsion

The aerodynamic interaction e_ects characteristic of pusher propellers were studied at the Large Low-Speed Facility of the German{Dutch wind tunnels (DNW-LLF). A propeller model was positioned downstream of a pylon equipped with a trailing-edge blowing system. Surface-pressure transducers integrated into the propeller blades con_rmed the local impact of the pylon wake on the blade loads. At an intermediate thrust setting, the sectional lift impulsively increased by 30% during the wake passage. The application of pylon blowing decreased the integral velocity de_cit in the pylon wake by up to 77% compared to the unblown case. As a result, the load uctuations during the wake encounter were practically eliminated, thereby mitigating the adverse installation e_ects., Flight Performance and Propulsion, Wind Energy

The three-dimensional flow field over the suction side of a NACA 0018 airfoil with trailing-edge serrations was studied by means of time-resolved tomographic particle image velocimetry. Mean flow results show that the boundary layer thickness decreases along the streamwise direction with a corresponding reduction of the size of the turbulent structures developing over the suction side of the serrations. At a positive angle of attack, streamwise-oriented and counter-rotating vortices aligned with the edge of the serrations are found to be the main features of the mean flow field. Their formation is attributed to the pressure imbalance between the two sides of the airfoil and the mixing layer at the edge. They locally modify the effective angle seen by the turbulent flow approaching the serrated edge. This effect may contribute to the serration underperformance in terms of noise reduction reported in literature. The spatial distribution of the spectra of the source term of the Poisson equation, which relates the velocity field to pressure fluctuations, suggests that the contribution of the serrations to far-field broadband noise is a function of the streamwise location. This observation is congruent with the spectra of the wall-normal and spanwise velocity fluctuations, which typically show low intensity close to the tips of the individual serrations. It follows that analytical models must take into account the local contribution to the far-field noise induced by the streamwise variation of the hydrodynamic pressure on the serration surface., Wind Energy

A methodology for a posteriori uncertainty quantification of pressure data retrieved from particle image velocimetry (PIV) is proposed. It relies upon the Bayesian framework, where the posterior distribution (probability distribution of the true velocity, given the PIV measurements) is obtained from the prior distribution (prior knowledge of properties of the velocity field, e.g., divergence-free) and the statistical model of PIV measurement uncertainty. Once the posterior covariance matrix of the velocity is known, it is propagated through the discretized Poisson equation for pressure. Numerical assessment of the proposed method on a steady Lamb–Oseen vortex shows excellent agreement with Monte Carlo simulations, while linear uncertainty propagation underestimates the uncertainty in the pressure by up to 30 %. The method is finally applied to an experimental test case of a turbulent boundary layer in air, obtained using time-resolved tomographic PIV. Simultaneously with the PIV measurements, microphone measurements were carried out at the wall. The pressure reconstructed from the tomographic PIV data is compared to the microphone measurements. Realizing that the uncertainty of the latter is significantly smaller than the PIV-based pressure, this allows us to obtain an estimate for the true error of the former. The comparison between true error and estimated uncertainty demonstrates the accuracy of the uncertainty estimates on the pressure. In addition, enforcing the divergence-free constraint is found to result in a significantly more accurate reconstructed pressure field. The estimated uncertainty confirms this result., Aerodynamics, Wind Energy

Trailing edge serrations designed to reduce airfoil self-noise are retrofitted on a NACA 0018 airfoil. An investigation of the boundary layer flow statistical properties is performed using time-resolved stereoscopic PIV. Three streamwise locations over the edge of the serrations are compared. An analysis of the results indicates that, while there is no upstream effect, the flow experiences significant changes as it convects over the serrations and toward its edges. Among the most important, a reduced shear stress and modifications of the turbulence spectra suggest beneficial changes in the unsteady surface pressure that would result in a reduction of trailing edge noise. Microphone array measurements are additionally performed to confirm that noise reduction is indeed observed by the application of the chosen serration design over the unmodified airfoil., Aerodynamics, Aircraft Noise and Climate Effects, Wind Energy

A new system for large-scale tomographic particle image velocimetry in low-speed wind tunnels is presented. The system relies upon the use of sub-millimetre helium-filled soap bubbles as flow tracers, which scatter light with intensity several orders of magnitude higher than micron-sized droplets. With respect to a single bubble generator, the system increases the rate of bubbles emission by means of transient accumulation and rapid release. The governing parameters of the system are identified and discussed, namely the bubbles production rate, the accumulation and release times, the size of the bubble injector and its location with respect to the wind tunnel contraction. The relations between the above parameters, the resulting spatial concentration of tracers and measurement of dynamic spatial range are obtained and discussed. Large-scale experiments are carried out in a large low-speed wind tunnel with 2.85 × 2.85 m2 test section, where a vertical axis wind turbine of 1 m diameter is operated. Time-resolved tomographic PIV measurements are taken over a measurement volume of 40 × 20 × 15 cm3, allowing the quantitative analysis of the tip-vortex structure and dynamical evolution., Aerodynamics, Wind Energy, Aerodynamics, Wind Energy & Propulsion

harvest AIAA 2015-2356, Flight Performance and Propulsion, Aerodynamics, Wind Energy

harvest AIAA 2015-2358, Flight Performance and Propulsion, Aerodynamics, Wind Energy

Aerodynamics, Wind Energy, Aerodynamics, Wind Energy & Propulsion

An experimental investigation was performed at the Large Low-speed Facility of the German-Dutch wind tunnels (DNW-LLF) to study the pylon interactions associated with pusher propellers. The aerodynamic and aeroacoustic effects of pylon trailing edge blowing and swirl recovery vanes (SRVs) were evaluated. The pylon installation increased the summed tonal noise levels by up to 13 dB in unblown conditions, while the propulsive performance was affected in asymmetric inflow conditions only. At a sideslip angle of +6°, pre-swirl of the flow due to the pylon tip vortex led to a thrust increase of 10% at high thrust up to 45% at a low thrust setting. Stereoscopic PIV measurements showed the beneficial effect of pylon blowing in compensating the momentum deficit in the pylon wake. At the optimal blowing rate, a recovery effect of about 80% was obtained compared to the unblown case. Application of the blowing system eliminated the pylon installation penalty, recovering the noise levels to those measured for the isolated propeller. Installation of the SRVs downstream of the isolated propeller resulted in a swirl recovery of 50%, at the cost of a 2 dB tonal noise increase., Aerodynamics, Wind Energy & Propulsion, Aerospace Engineering

The present paper is the summary of 3 years of research on the wake aerodynamics of horizontal axis wind turbine at Delft University of Technology, the Netherlands. In particular, the main results and the conclusions of two experimental campaigns are collected. The underlying research question is: how do the near-wake turbulent flow structures affect the re-energising of the far wake and to what extend is the actuator disc assumption valid for the representation of the near wake dynamics? In the first experiments, stereo particle image velocimetry is used for analysing the turbulent velocity field in the near and transition wake of a small two-bladed wind turbine model. The results showed the important role of the tip-vortex helix instability (leapfrogging) in the mixing process and in the re-energising of the wake. The tip-vortex instability and breakdown, in fact, give rise to a more efficient turbulent mixing. In the second campaign, the same measurement technique is used for acquiring data in the near wake of the wind turbine model and in the near wake of a porous disc, emulating the numerical actuator disc. The results show a good match velocity fields between wind turbine and actuator disc, but show a different turbulence intensity and turbulent mixing. The analysis suggest the possibility to extend the use of the actuator disc model in numerical simulation until the very near wake, provided that the turbulent mixing is correctly represented., Aerodynamics, Wind Energy & Propulsion, Aerospace Engineering

The present work proposes a methodology for the a posteriori quantification of the uncertainty of pressure data retrieved from PIV measurements. It relies upon the Bayesian framework, where the posterior distribution (probability distribution of the true velocity, given the PIV measurements) is obtained from the prior distribution (prior knowledge of the velocity, e.g., within a certain bound or divergence-free) and the distribution representing the PIV measurement uncertainty. Once the posterior covariance matrix of the velocity is known, it is propagated through the discretized Poisson equation for pressure. Numerical assessment of the proposed method on a steady Lamb Oseen vortex shows excellent agreement with Monte Carlo simulations, while linear uncertainty propagation underestimates the uncertainty of the pressure by up to 30%. The method is finally applied to an experimental test case of a turbulent boundary layer in air, obtained using time-resolved tomographic PIV. The pressure reconstructed from tomographic PIV data is compared to a microphone measurement conducted simultaneously at the wall to determine the actual error of the former. The comparison between actual error and estimated uncertainty shows the accuracy of the proposed method for uncertainty quantification of pressure data from tomographic PIV experiments: for a 95% confidence level, 93% of the data points fall within the estimated uncertainty bound with the Eulerian approach, and 90% with the Lagrangian approach. When using the prior knowledge that the velocity field should be divergence-free, these values are 98% with the Eulerian approach and 94% with the Lagrangian approach. The Lagrangian approach results in more accurate reconstructed pressure fields than the Eulerian approach. Also, enforcing the divergence-free constraint is found to result in a more accurate reconstructed pressure field. Both observations also follow from the uncertainty quantification, through a decrease in the estima, Aerodynamics, Wind Energy & Propulsion, Aerospace Engineering

In this work, the measured modulated integral boundary layer (IBL) characteristics of low-profile vortex generators (VGs) are used to validate new developments in a viscousinviscid interaction code which is modified to incorporate the effect of the passive mixing devices. The motivations are laid out and sample validation data is presented within this abstract., Aerodynamics, Wind Energy & Propulsion, Aerospace Engineering

In this study, the mean flow organization in the symmetry plane of the recirculation region ahead of a cylindrical roughness element immersed in an incoming laminar boundary layer at Mach number equal to 6.5 is investigated. The flow topology is inspected with planar particle image velocimetry (PIV), InfraRed (IR) Thermography and Schlieren flow visualization. The flow approaching the roughness element forms a main recirculation region ahead of the cylinder leading edge. The reattachment vortex is the responsible of a heat flux local peak in front of the protuberance. Secondary, a more complex system of vortices is observed upstream of the protuberance, which also corresponds to the local maximum of turbulent kinetic energy and wall heat transfer., Aerodynamics, Wind Energy & Propulsion, Aerospace Engineering

The flow around the blade of a horizontal axis wind turbine wind tunnel model, operating at its optimal tip speed ratio in axial flow, has been investigated by means of stereoscopic particle image velocimetry (SPIV). The aim was to assess the possibility of measuring the loads impinged on the blade, using the acquired velocity field and its spatial derivatives. Thus, the three-dimensional (3D) velocity field and the pressure distribution surrounding the blade, as well as the aerodynamic loads, responsible for thrust and torque, were obtained with a non-intrusive method. The SPIV equipment was mounted on a traverse system and provided with phase-locked velocity planes perpendicular to the blade axis, scanning the blade from the root to the tip, at a fixed azimuthal position. The obtained velocity fields were used to estimate the pressure distribution and the aerodynamic loads on each plane, using a 3D formulation. Finally, these results were compared and discussed with similar results obtained computationally with a panel method model, showing good consistency. In the future, the proposed methodology could be used to study relevant topics such as active load control techniques, rotational augmentation, dynamic stall phenomena and the aerodynamics of small wind turbines operating at low Re numbers, among others. The processed and averaged flow fields from the experimental SPIV data are made available online to the reader. See appendix for description of the files., Peer Reviewed, Postprint (published version)

An experimental study of the flow inside a saxophone mouthpiece in playing conditions is carried out by means of particle image velocimetry at high acquisition rate. Planar velocity measurements on the midsection of a Plexiglas tenor saxophone mouthpiece are performed, respectively, in the mouthpiece baffle and in the reed channel. Sequences of velocity fields inside the mouthpiece baffle and around the reed tip are shown for one reed duty cycle. Maxima of the velocity fluctuations are observed at the upper surface of the mouthpiece at a distance between five and ten reed apertures from the tip. The proper orthogonal decomposition analysis reveals that almost 50% of the kinetic energy in the baffle is distributed in the first two modes displaying a periodic behavior at the fundamental frequency, the rest being turbulent flow behavior. The measured dynamical vena contracta coefficient at the inlet is reasonably constant around the value of 0.6 for reed positions far from closure. This is in agreement with existing steady flow analytical models and previous experimental results., Aerodynamica, Aerospace Engineering

The thesis describes the application of particle image velocimetry (PIV) to study the aerodynamic loads of airfoils and aircraft propellers. The experimental work focuses on the development of a measurement procedure to infer the pressure of the flow field from the velocity distribution obtained by PIV velocimetry. The technique offers important advantages in aircraft propellers, since the loads can be locally inspected without the need to install pressure sensors and momentum balances in rotating devices. The applicability of the approach in problems relevant to the aerospace industry requires theoretically defining and experimentally verifying the principles of this method. Modern aircraft propellers operate in the compressible regime, in conditions where the blade profiles are usually experiencing transonic effects, especially at the blade tip. Therefore, the PIV based load determination is firstly applied in airfoils immersed in transonic flows (Mach 0.6 to 0.8) and further compared to conventional pressure measurement techniques to measure the surface pressure distribution, airfoil lift and drag. Three different fields of view have been used at different spatial resolutions, to obtain 2D velocity vector fields from which the surface pressure, as well as the lift and drag coefficients were computed. In the lift computation, the contour integration method is applied, whereas the drag is derived from a wake traverse approach. The surface pressure showed excellent agreement with the data recorded by the pressure orifices in the absence of shocks, although to correctly capture the pressure on the nose region of the airfoil an extrapolation of the PIV data towards the actual surface is needed, in view of the large pressure gradient normal to the surface and to the limited spatial resolution. The lift obtained by PIV can be reliably compared to conventional load determination approaches, showing uncertainties of the order of few percent. On the other side, the drag ne, Aerodynamica, Aerospace Engineering

The flow field at the tip region of a scaled DHC Beaver aircraft propeller, running at transonic speed, has been investigated by means of a multi-plane stereoscopic particle image velocimetry setup. Velocity fields, phase-locked with the blade rotational motion, are acquired across several planes perpendicular to the blade axis and merged to form a 3D measurement volume. Transonic conditions have been reached at the tip region, with a revolution frequency of 19,800 rpm and a relative free-stream Mach number of 0.73 at the tip. The pressure field and the surface pressure distribution are inferred from the 3D velocity data through integration of the momentum Navier-Stokes equation in differential form, allowing for the simultaneous flow visualization and the aerodynamic loads computation, with respect to a reference frame moving with the blade. The momentum and pressure data are further integrated by means of a contour-approach to yield the aerodynamic sectional force components as well as the blade torsional moment. A steady Reynolds averaged Navier-Stokes numerical simulation of the entire propeller model has been used for comparison to the measurement data., Aerodynamics & Wind Energy, Aerospace Engineering

The present investigation assesses a procedure to extract the aerodynamic loads and pressure distribution on an airfoil in the transonic flow regime from particle image velocimetry (PIV) measurements. The wind tunnel model is a two-dimensional NACA-0012 airfoil, and the PIV velocity data are used to evaluate pressure fields, whereas lift and drag coefficients are inferred from the evaluation of momentum contour and wake integrals. The PIV-based results are compared to those derived from conventional loads determination procedures involving surface pressure transducers and a wake rake. The method applied in this investigation is an extension to the compressible flow regime of that considered by van Oudheusden et al (2006 Non-intrusive load characterization of an airfoil using PIV Exp. Fluids 40 988–92) at low speed conditions. The application of a high-speed imaging system allows the acquisition in relatively short time of a sufficient ensemble size to compute converged velocity statistics, further translated in turbulent fluctuations included in the pressure and loads calculation, notwithstanding their verified negligible influence in the computation. Measurements are performed at varying spatial resolution to optimize the loads determination in the wake region and around the airfoil, further allowing us to assess the influence of spatial resolution in the proposed procedure. Specific interest is given to the comparisons between the PIV-based method and the conventional procedures for determining the pressure coefficient on the surface, the drag and lift coefficients at different angles of attack. Results are presented for the experiments at a free-stream Mach number M = 0.6, with the angle of attack ranging from 0? to 8?., Aerospace Engineering

Aerospace Engineering

Aerospace Engineering