Wake Measurements Research Articles

Lidar measurements of wake around a bridge deck

Remote wind sensing technologies allow for measurements in a spatial domain beyond the one accessible by anemometers fixed to a mast. Remote optical wind sensors installed at an existing bridge site can both provide new information on the wind flow upstream of the bridge and capture the disturbed flow around the structure. These possibilities are explored in the presented measurement campaign with three synchronised continuous-wave Doppler lidars at the Lysefjord bridge in Norway. In particular, the interaction between the oncoming wind flow and the bridge deck is studied in terms of the mean velocity deficit wake profile as well as the spectral turbulence characteristics. The lidar measurements are validated by comparing the lidar observations at locations undisturbed by the structure with corresponding sonic anemometer data recorded 6 m and 10 m above the deck. The consistency between the two sets of data is tested in terms of the turbulence time series, their spectral content and the statistical moments. Within the wake of the bridge, significant values of vertical velocity and inclination angles, as well as turbulence intensities are seen. Moreover, the wake region can be distinguished by its singular spectral content, cross-correlation coefficient, and coherence. The results show that the tailored configuration of the lidars succeeded in providing information on 3D turbulence around the bridge, thereby capturing the bridge aerodynamic characteristics in full-scale.

Experimental investigation of the seabed topography effects on tidal stream turbine behavior and wake characteristics

Physical modelling was performed to study the hydrodynamics and wake interferences of a tidal stream turbine sited in close proximity to different seabed elements. The seabed topography variation had a significant effect on the tidal stream turbine behavior. The average power output of the turbine with the presence of the small seabed topography increased by about 11%, while the power output of the turbine sited in close proximity to the larger seabed topography was reduced by approximately 40% compared to that with a flat bed. Additionally, detailed flow field measurements of the turbine wake revealed the seabed topography variation effects on the wake characteristics. The small seabed topography increased the average turbulence intensity in the near wake region behind the turbine and made the flow velocity recovery slightly faster than the case with flat bed. In contrast to the small seabed topography, the larger topography had a certain inhibitory effect on wake recovery. The results presented in the current study not only offer a validation database for wake models but can also be used to optimize the site selection and rotor placement of tidal turbines for a higher power yield.

Quiet Spacecraft Cabin Ventilation Fan: Wake Measurements Results

A spacecraft cabin ventilation fan suitable for aerodynamic and acoustic ground tests was designed and two copies of the fan assembly were fabricated - designated as Quiet Space Fan. Both fans were tested for aerodynamic performance and acoustic levels in the NASA Glenn Research Center Acoustical Testing Laboratory. A new test rig for small axial flow fans was designed to accommodate the instrumentation and back-pressure adjustments. Measurements acquired were - static pressures for measuring performance, a 72-channel in-duct microphone array, external microphone measurements for acoustics, and inter-stage hot wire measurements of the fan wake. This report documents the wake velocity and turbulence measurements as part of a series of reports.

Quiet Spacecraft Cabin Ventilation Fan: Acoustic Measurements Results

A spacecraft cabin ventilation fan suitable for aerodynamic and acoustic ground tests was designed and two copies of the fan assembly were fabricated - designated as Quiet Space Fan. Both fans were tested for aerodynamic performance and acoustic levels in the NASA Glenn Research Center Acoustical Testing Laboratory. A new test rig for small axial flow fans was designed to accommodate the instrumentation and back-pressure adjustments. Measurements acquired were - static pressures for measuring performance, a 72-channel in-duct microphone array, external microphone measurements for acoustics, and inter-stage hot wire measurements of the fan wake. This report documents the acoustic measurements as part of a series of reports.

Quiet spacecraft cabin ventilation fan: aerodynamic measurements results

A metal spacecraft cabin ventilation fan suitable for aerodynamic and acoustic ground tests was designed and two copies of the fan assembly were fabricated. Both fans were tested for aerodynamic performance and acoustic levels in the NASA Glenn Research Center Acoustical Testing Laboratory. A new test rig for small axial flow fans was designed to accommodate the instrumentation and back-pressure adjustments. Measurements acquired were from: static pressures for measuring performance, a 72-channel in-duct microphone array, external microphone measurements for acoustics, and inter-stage hot wire measurements of the fan wake. This report documents the aerodynamic measurements as part of a series of reports.

Comparing scanning lidar configurations for wake measurements based on the reduction of associated measurement uncertainties

Scanning lidars are increasingly being used for new measurement applications, such as wake measurements. This type of measurement requires an uncertainty quantification, and the setup of such instruments should ideally be planned to minimize the overall measurement uncertainty. Based on a previous experimental study, we quantify the overall uncertainty for a scanning lidar wake measurement campaign by analyzing the pointing accuracy for distinct configuration scenarios. The selected scenarios are based on a single ground-based scanning lidar measuring the downstream flow, a scanning lidar mounted on the turbine’s nacelle and, finally, two scanning lidars measuring dual-Doppler points within the wake. The results for each configuration are presented in terms of the overall uncertainty and measurement outputs provided from these setups. In a case study using the experimental dataset to compare ground- and nacelle-based configurations, we find that – under slightly stable conditions at 5D downstream – the overall radial wind speed uncertainty, normalized by reference wind speed without counting the sample size uncertainty, decreases on average from 2.78% for the ground-based lidar to 1.33% for the nacelle-based setup. Another advantage of the nacelle-based setup is the potentially larger sample size, as it can measure multiple downstream distances at once. The dual-lidar configuration has advantages in providing horizontal wind speed and direction instead of radial wind speed components only, hence reducing uncertainties associated with wind direction. However, this comes with the cost of fewer measured points to be compared with an engineering wake model and less flexibility to position the lidars, since their relative spatial position impacts the uncertainty. In summary, the presented methodology can benefit the best selection of scanning lidar configuration for a particular application, here demonstrated for wake measurements, proving the user with an estimation of the overall measurement uncertainty and, most importantly, the sensitivity of each parameter.

Preface

The eighth Wake Conference, held between 20th and 22nd of June 2023, marks nearly a decade and a half of successful international conferences focused on advancements in the field of wake dynamics. This bi-annual conference, held for the past decade on the island of Gotland, Sweden seeks to aid in this by providing a venue for scientists and PhD students working in the field of wake dynamics to come together and share their research and expertise.To understand the need for this conference one only needs to look to modern wind farms. These massive energy facilities rival the production of a nuclear power plant and consist of dozens or potentially hundreds of individual wind turbines. The numbers of turbines involved make it unavoidable that they will invariably be impacted by the wakes of upstream turbines. This wake interaction results in a decreased power production, caused by the lower kinetic energy in the wind, and an increase in the turbulence intensity. This decrease in power emphases the importance of understanding the physical nature of vortices and their dynamics in the wake of a turbine when designing the layout of these immense wind farms. The clear importance of farm design, and the related interest in the field of wake and wind farm aerodynamics, can be seen in the growing number of scientific articles on the subject.Further, in order to make a substantial impact on one of the most significant challenges of our time - global climate change - the wind industry’s growth must continue. A part of making this growth possible, and to solve some of the scientific problems of doing so, will require research into the different areas of the physics of wind turbine and wind farm wakes. To this end, the conference covers the following subject areas: Wake and vortex dynamics, instabilities in trailing vortices and wakes, simulation and measurements of wakes, analytical approaches for modeling wakes, wake interaction along with other wind farm investigations.We would like to thank all those that assisted in the review process for the 55 papers which constitute the proceedings of the Wake Conference 2023 that were selected to be published. We are also thankful for the help we have received from the editorial team at IOP Publishing during the process.Visby, Sweden June 2023 The Organizers, Professor Stefan Ivanell, Uppsala University Professor Jens Nørkær Sørensen, Technical University of Denmark List of Editorial Committee are available in the pdf.

Proper Orthogonal Decomposition (POD) of the Wake Flow Field of a Model Wind Turbine and a Porous Disc under Different Freestream Turbulence Intensity Conditions

The effects of freestream turbulence intensity on the wake development of a model wind turbine and a porous disc are investigated through Proper Orthogonal Decomposition (POD) analysis. The capability of porous discs for reproducing far-wake characteristics of a model wind turbine is examined through coherent structures both in the near-wake and far-wake regions. Instantaneous velocity fields are obtained through wake measurements using two-dimensional two-component particle image velocimetry (2D2C PIV). These velocity fields are considered snapshots of the spatial domain. Results show inherent differences between coherent structures of a model wind turbine and porous disc in the near-wake region, especially when the freestream turbulent intensity level is low. However, these differences reduce, and coherent structures become more comparable when the freestream turbulence intensity level is higher. It is shown that the first five streamwise components of POD modes are paired for the model wind turbine and the porous disc cases under high freestream turbulence intensity conditions in the far-wake region.

Experimental Study on the Effect of the Blade Tip Distance on the Power and the Wake Recovery with Small Multi-Rotor Wind Turbines

In order to investigate the output power and wake velocity of small multi-rotor wind turbines compared to single-rotor wind turbines, which operate in the same swept area at various blade tip distances, this paper used the wind tunnel test method to examine single-rotor wind turbines with diameter D of 0.4 m and 0.34 m corresponding to the triple-rotor wind turbines and double-rotor wind turbines with a single rotor diameter D of 0.24 m, respectively. The experimental results indicated that, without rotation speed control, the triple-rotor wind turbine produced more power than the single-rotor wind turbine with an equivalent swept area and that the output power tended to rise initially and then fall as the distance between each rotor increased. Moreover, the power increase reached a maximum of 8.4% at the 0.4D blade tip distance. In terms of wake measurement, triple-rotor wind turbines had smaller wake losses and faster recovery rates than single-rotor wind turbines. The smaller the blade tip distance, the earlier the wake merged and fused and the faster the recovery rate. In designing small multi-rotor wind turbines, the above discussion can serve as a guide.

Is obstructive sleep apnea a circadian rhythm disorder?

Obstructive sleep apnea is the most common sleep-related breathing disorder worldwide and remains underdiagnosed. Its multiple associated comorbidities contribute to a decreased quality of life and work performance as well as an increased risk of death. Standard treatment seems to have limited effects on cardiovascular and metabolic aspects of the disease, emphasising the need for early diagnosis and additional therapeutic approaches. Recent evidence suggests that the dysregulation of circadian rhythms, processes with endogenous rhythmicity that are adjusted to the environment through various cues, is involved in the pathogenesis of comorbidities. In patients with obstructive sleep apnea, altered circadian gene expression patterns have been demonstrated. Obstructive respiratory events may promote circadian dysregulation through the effects of sleep disturbance and intermittent hypoxia, with subsequent inflammation and disruption of neural and hormonal homeostasis. In this review, current knowledge on obstructive sleep apnea, circadian rhythm regulation, and circadian rhythm sleep disorders is summarised. Studies that connect obstructive sleep apnea to circadian rhythm abnormalities are critically evaluated. Furthermore, pathogenetic mechanisms that may underlie this association, most notably hypoxia signalling, are presented. A bidirectional relationship between obstructive sleep apnea and circadian rhythm dysregulation is proposed. Approaching obstructive sleep apnea as a circadian rhythm disorder may prove beneficial for the development of new, personalised diagnostic, therapeutic and prognostic tools. However, further studies are needed before the clinical approach to obstructive sleep apnea includes targeting the circadian system.

Volumetric measurements of wake impulse and kinetic energy for evaluating swimming performance

Volumetric flow measurements are a valuable tool for studies of aquatic locomotion. In addition to visualizing complex propulsive behaviors (e.g., highly three-dimensional kinematics or multi-propulsor interactions), volumetric wake measurements can enable direct calculation of metrics for locomotive performance including the hydrodynamic impulse and wake kinetic energy. These metrics are commonly used in PIV and PTV studies of swimming organisms, but derivations from planar data often rely on simplifying assumptions about the wake (e.g., geometry, orientation, or interactions). This study characterizes errors in deriving wake impulse and kinetic energy directly from volumetric data in relation to experimental parameters including the level of noise, the flow feature resolution, processing parameters, and the calculation domain. We consider three vortex ring-like test cases: a synthetic spherical vortex with exact solutions for its impulse and energy, volumetric PIV measurements of a turbulent vortex ring, and volumetric PIV measurements of a turning fish. We find that direct calculations of hydrodynamic impulse are robust when derived from a volumetric experiment. We also show that kinetic energy estimates are feasible at experiment resolutions, but are more sensitive to experiment design and processing parameters, which may limit efficiency estimates or comparisons between studies or organisms.Graphical abstract

Enhancing the noise reduction capability of serrations usinglow-profile vortex generators

This paper presents an experimental investigation on improving the noise reduction capability of the trailing edge serrations using vortex generators. Experiments were conducted on a 150 mm chord flat plate at four angle of attack between 0° and 9° and flow speeds of 20-50 m/s. The Reynolds number based on the chord length ranges from 2.1×105 to 5×105. Vane type vortex generators with various heights of h/δ = 0.16-0.25 were placed along with the serration roots, where δ is the boundary layer thickness at the trailing edge without the vortex generators. The aeroacoustic measurements were made with a phased microphone array, and the aerodynamic measurement near the trailing edge was measured using a hot-wire. Additionally, the lift and drag forces were measured by load cells. The source integrated noise spectra showed a slight reduction in the trailing edge noise over a broadband frequency range. The generation of streamwise vortices by vortex generators can be identified in the wake measurement. These streamwise vortices were further found to counteract the cross-flow generated at serration roots, hence improving the performance of the serration. Moreover, the presence of low-profile vortex generators was found to have an insignificant effect on the lift and drag coefficients.

Multi-fidelity vortex simulations of rotor flows: Validation against detailed wake measurements

Two flow models with different fidelity of the DTU vortex solver MIRAS have been used to simulate the wake generated by a model wind turbine with various levels of asymmetry. Predictions are validated against experimental Particle Image Velocimetry measurements and dye visualizations. The experiments were conducted in a recirculating free-surface water channel with an immersed two-bladed rotor mounted on a shaft. The blades were designed to approximate a Joukowsky rotor. A detailed comparison between the measurements and the simulations is first performed for an unperturbed baseline case at different tip speed ratios. The analysis consists of a tip vortex characterization, including the vortex core profile, and a comparison of the instantaneous and mean velocity and vorticity fields, along with the mean wake profile at different downstream locations. Overall, good agreement is obtained between measurements and simulations, especially with the higher-fidelity particle-mesh model, which is capable of very closely predicting many of the flow features observed in the experiments. Rotor asymmetry triggers a vortex instability, commonly known as leapfrogging, which accelerates the breakdown of tip vortices, enhancing the mixing of wake structures and promoting a faster wake recovery. The prediction accuracy of this instability by the solvers is analyzed for different tip speed ratios and perturbation amplitudes. This work aims at setting the groundwork for future flow instability studies with the MIRAS solver.

Application of proper orthogonal decomposition in particle image velocimetry in ocean engineering

Particle image velocimetry (PIV) plays a vital role in flow measurement. Eliminating the effects of overexposure and shadow in particle images is an urgent problem to be solved in the detailed flow measurement of ocean engineering. Seek a robust denoising method for bad test environments, and apply it to the PIV experiments to achieve dual denoising at the image preprocessing and data postprocessing. This study verified the superiority of the Proper Orthogonal Decomposition (POD) filter in pre- and postprocessing compared with traditional mathematical filters. Subsequently, the POD filter is applied to a practical project: the wake field measurement of bulk carrier. Regarding preprocessing, the POD filter is a valuable tool for objectively filtering typical high-frequency spatial noise from the energy perspective based on preserving the original information of particles to the greatest extent, effectively alleviating the impact of overexposure and shadow areas. Regarding postprocessing, the POD filter can fill the data gap by considering the surrounding flow characteristics. At the same time, the flow's spatial distribution and time evolution characteristics can be obtained by POD. The flow reconstructed by selecting the appropriate order model can clearly show the evolution characteristics of large-scale coherent structures, which may provide a deep understanding of complex flow field mechanisms.

Boundary layer and near wake measurements of a two-dimensional airfoil with background-oriented schlieren method

Boundary layer and near wake measurements of a two-dimensional airfoil with background-oriented schlieren method

Increased Prevalence of Blood Pressure Instability Over Twenty-Four Hours in Chronic Spinal Cord Injury.

Spinal cord injury (SCI) leads to cardiovascular dysregulation, including persistent low blood pressure (BP), orthostatic hypotension, and autonomic dysreflexia, leading to daily BP instability that may not be adequately recognized. We compared mean systolic BP, diastolic BP, and heart rate from awake and asleep measurements over a 24-h period among persons with chronic SCI (n = 33; 30 cervical injuries and three upper thoracic injuries), ambulatory/non-injured (Ambulatory-NI; n = 13), and non-injured (NI) in a wheelchair (n = 9). Stability of awake BP was evaluated by deviation of systolic BP from 115 mmHg and percent of systolic BP measurement within and outside of 90-140 mmHg. Variability over 24 h was compared using coefficient of variation and average real variability. Awake hyper- and hypotensive events (change in systolic BP ±20 mmHg from the median) were compared to symptoms reported by the participants corresponding to BP events. Participants with SCI had a lower percentage of awake systolic BP measurements within 90-140 mmHg than Ambulatory-NI and a greater deviation below 115 mmHg. Coefficient of variation and successive differences of awake systolic and diastolic BP were greater in SCI than Ambulatory-NI. Finally, all SCI participants had hyper- and/or hypotensive events and 88% experienced the BP events asymptomatically. In conclusion, participants with SCI had significantly greater BP instability compared with NI, with many hyper- and hypotensive events occurring without symptoms. Clinical management of BP instability, regardless of symptoms, should be a priority after SCI to reduce the risk of cardiovascular disease and improve quality of life.

Reconstruction of flow around a high-rise building from wake measurements using Machine Learning techniques

This paper investigates the unsteady flow around a high-rise building using OpenFoam. A Vortex Method is developed to generate upstream unsteady fluctuations that are validated considering the numerical simulation of a neutral atmospheric boundary layer around a high-rise building. The Vortex Method parameters are tuned to produce an upstream velocity profile whose turbulence characteristics are comparable to experimental data. Comparison between mean and fluctuating velocity profiles of the numerical data measured in the building’s wake with experimental data shows satisfactory results.The resulting database is used to reconstruct the flow from limited velocity measurements inside the wake and static wall pressure measurements on the building surface. Machine learning techniques such as linear regressions (Ridge, Lasso, ElastikNet, MultiTaskLasso regression) and Artificial Neural Network are tested and compared. The flow reconstruction using velocity measurements inside the wake leads to a better result compared to the flow reconstruction from the wall pressure measurements. At the same time, it was noticed that the Artificial Neural Network regression does not lead to more satisfactory results compared to linear regression techniques.

Sliding discharge plasma jet actuators for circular-cylinder wake modification

A sliding discharge (SD) plasma actuator designed for the control of a circular cylinder wake is examined experimentally in this paper. This kind of discharge demonstrates a thicker and higher maximum speed wall jet than a Dielectric Barrier Discharge (DBD). The plasma actuator mounted strategically on the rear part of the cylinder model can induce either a downward or upward jet into the flow around the circular cylinder by simply adjusting the electrodes’ electrical circuits. Experiments were performed in a low-speed and low-turbulence wind tunnel at Reynolds numbers between 7000 and 24,000 based on the diameter of the circular cylinder. Wake measurements by particle image velocimetry (PIV) showed that both the mean velocity and the turbulence level in the cylinder wake were modified under the plasma actuation. Reducing or increasing the cylinder drag force estimated from the velocity field could be realized by changing plasma actuation directions. They showed that up to 30% drag reduction and 24% drag increase were obtained with the downstream and upstream actuation respectively at the continuous plasma blowing. The efficiency of flow control was found to be about 1.8% for drag reduction. This study suggests that an appropriate arrangement of an SD actuator can practically suppress flow separation or enhance flow mixing.

Wake reduction in horizontal axis wind turbine: A review of advancements in techniques

Renewable energy systems have experienced exponential growth toward producing zero-carbon energy. In this context, wind turbines continue to play a significant role. To extract maximum power, wind turbines are installed in array-like formation in wind farms. This arrangement though beneficial, also leads to detrimental wind-wakes effects. The wakes reduce wind speed and induce undesirable turbulence of flow field in downstream direction. To address this issue, several innovative techniques have been proposed. This paper surveys the best methods by classifying these into passive and active techniques. The passive techniques affect the wake flow by modifying only the geometrical or operating characteristics of wind turbines such as adjustment of forward and backward sweeps, etc. The active techniques use additional surfaces/devices for wake handling such as vortex generators, leading edge protuberances, dual-rotors, and cross-axis wind turbines etc. Additionally, this paper also reviews various wake measurement methods and recommends the best suited technique.

Optimisation of Trailing Edge Flaps on the Base Cavity of a Vehicle for Improved Performance at Yaw

Regulations to reduce greenhouse gas emissions of passenger vehicles are becoming increasingly stringent. The aerodynamic drag is a major contributor to the vehicle’s total energy consumption where a large portion is attributed to the base wake. This paper optimises the angles of small trailing edge flaps on a base cavity of a full-scale sports utility vehicle placed in a wind tunnel. The trailing edge flaps are controlled using servos mounted inside the cavity. The flap angles are optimised using a surrogate model based optimisation algorithm with the objective of reducing the aerodynamic drag at different yaw angles and to create a yaw-insensitive geometry by considering several weighted yaw angles to form the driving cycle averaged drag. Low drag designs are further investigated using base pressures and wake measurements. The results show that the base pressures are symmetrised by reducing the crossflow in the wake. As the model is yawed the wake becomes increasingly downwash dominated by a large rotating windward structure which is reduced by the optimised flaps. The cycle averaged drag optimised design has a smaller increase in drag when yawed compared to a design optimised without considering yaw.