Wake Generator Research Articles

3D Printed Graphene Piezoresistive Microelectromechanical System Sensors to Explain the Ultrasensitive Wake Tracking of Wavy Seal Whiskers

AbstractMany marine animals perform fascinating survival hydrodynamics and perceive their surroundings through optimally evolved sensory systems. For instance, phocid seal whiskers have undulations that allow them to resist noisy self‐induced vortex‐induced vibrations (VIV) while locking their vibration frequencies to wakes generated by swimming fishes. In this study, fully 3D‐printed microelectromechanical systems (MEMS) sensors with high gauge factor graphene nanoplatelets piezoresistors are developed to explain the exquisite sensitivity of whisker‐inspired structures to upstream wakes. The sensors are also used to measure natural frequencies of excised harbor (Phoca vitulina) and grey (Halichoerus grypus) seal whiskers and determine the effect of whisker orientation on the VIV, which can explain the possible natural orientation of whiskers during active hunting. Experimental investigations conducted in a recirculating water flume show that whisker‐inspired sensors successfully sense an upstream wake located up to 10× the whisker diameter by locking to the frequency of the wake generator, thus mimicking the sensing mechanism of the seal whisker. The combination of VIV reduction and frequency‐locking with the upstream wake generator demonstrates the whisker‐inspired sensor's high signal‐to‐noise ratio, indicating its efficiency in long‐distance wake sensing as well as its potential as an alternative to visual and acoustic sensors in underwater robots.

The effects of the cold expansion degree on the fatigue crack growth rate in rail steel

This study investigates the effects of the cold-expansion degree on the fatigue strength of rail steels. Two degrees of cold-expansion, 2% and 4%, and two stress levels were investigated. Results indicate that cold-expansion postpones crack initiation and extends fatigue life, especially with higher degrees of cold-expansion and lower stress levels. Linear superposition principle coupled with the weight function method results in an overestimated fatigue life of cold-expanded specimens, suggesting that reliable predictions can be achieved if both the residual stresses redistribution due to propagating crack and the crack closure effects due to plastic wake generation are considered in the simulations.

Stochastic model predictive controller for wind farm frequency regulation in waked conditions

As renewable energy penetrations increase, wind farms may be required to provide ancillary services such as frequency regulation. For wind farms, two issues complicate frequency regulation: turbine-wake interactions alter the available wind power, and the future power reference signal is uncertain. This paper presents a stochastic model predictive controller (MPC) for wind farms to track an uncertain frequency regulation signal while accounting for wakes. The controller uses a simplified Park model with wake propagation delay to predict turbine-wake interactions. The controller operates without any knowledge of the future power regulation signal or wind speed. The controller solves a nonconvex stochastic optimization program using scenario optimization. Our stochastic MPC tracks with 7%–10% less error than our deterministic MPC controllers.

Wake Patterns Around Front Tyre And Wheel On Vehicle Dynamics Clarified By On-Board PIV Application

Temporal and spatial wake patterns and behaviors around rotating tires and wheels are some of the most critical flow phenomena on aerodynamic performance and vehicle dynamics. In particular, aerodynamic drag is one of the most important factors on greenhouse effect gas and fuel efficiencies, in these days remarkably focused on the shift from conventional fossil fuel power plants to electric ones to prevent the global warming. From the point of view of aerodynamics, OEMs' wind tunnel facilities with moving ground and high fidelity CFD tools are employed to optimize the exterior body shape and aerodynamic device in order to reduce the aerodynamic drag. Recently, high fidelity CFD tools which include tire rotations are easily employed by users, not only specialists, to analyze the flow around the vehicle and to optimize the exterior design at the development phase. However, more accurate and detailed understanding of complicated flow phenomena around vehicle during motion are required to predict these flow phenomena accurately by CFD tools at early development phase. Moreover, the more detailed experimental investigation is essential for the aerodynamicists, not only to develop the prediction tool but also to understand the flow phenomena around vehicle in its essence. Therefore, in this study, the on-board 2D-3C PIV measurement system in wind tunnel facility has been developed to investigate the wake patterns around rotating front tire and wheel. As result, the critical wake patterns have been clarified by POD analysis of the mentioned on-board 2D-3C PIV measurement data. These front wheelhouse wake patterns were classified in terms of the wake generation locations that produced the aerodynamic drag. These results indicate that there is the potential for an improvement in not only aerodynamic drag and but also vehicle dynamics by flow control to suppress the wake generations around front wheelhouse and the flow fluctuation around bodyside, moreover, this system can be applied to real world running condition in near future.

Diurnal impact of atmospheric stability on inter-farm wake and power generation efficiency at neighboring onshore wind farms in complex terrain

Enhancing the power production efficiency of large-scale wind farms is challenging for onshore wind power due to the variability of atmospheric conditions. This work aims to study the impact of atmospheric stability on the inter-farm wake pattern, topographic effect, and power production changes at two neighboring wind farms in Hami City, Xinjiang of China over complex terrain. The results show a discernible diurnal cycle property. At night in a stable layer with hindered mixing, the inter-farm wake persists around 20 km downstream, and the greatest relative wind deficit increases by 8%. However, during the daytime unstable layer with elevated turbulence, shows a limited deficit and recovers about 6 km downstream. Accordingly, a mean relative power production reduction of 17% is subjected to the downwind farm at night, about a factor of 3 greater than that at daytime, when the mean value is only 6%. Interestingly, the largest power production losses are experienced with a larger range between maximum and minimum before dawn and after dusk due to the notable inter-farm wake with a streaky structure related to the large coherent buoyant caused by the tendency of heat flux over the complex terrain. Moreover, the topographic speed-up in the stable layer outperforms that in unstable conditions.

Utjecaj velikih brzina brodova na uske i plitke vodene putove s osvrtom na Jadransko more

Sudden increase of demand of high speed craft, especially fast passenger-cargo ferries and catamarans in the last few decades, has made it necessary to respond in a timely manner to the set of negative impacts that such vessels cause with high speeds near coastal areas. All relevant literature was investigated in this paper, as well as the reports, scientific articles by different databases, and collected data was evaluated by analyzing the content and mitigation measures by specific categories in order to emphasise the importance of potential negative effects of wakes. In addition to the mechanism of wake generation and physical characteristics of waves generated by vessels in coastal areas, a critical overview of the investigated impacts is presented, which includes: impact on safety (lives, vessels, structures), and environmental impact (hydro-morphological impact, impact on flora, fauna and cultural heritage). Particular attention is paid to the analysis and review of applied methods of risk assessment and adoption of measures in various countries where such research has already been conducted to a certain extent, in order to compile a set of effective recommendations that can be applied to reduce negative impacts. Since the previous researches mostly cover some impacts individually, it is also pointed out, among other recommendations, the importance of conducting concrete comprehensive coordinated research on this topic with an emphasis on the Adriatic Sea area, which is unique in multiple locations in terms of safety and environmental aspect, as well as in terms of the aspect of preserving the cultural heritage.

Novel scaling laws in the nonequilibrium turbulent wake of a rotor and a fractal plate

This paper describes an experimental study in which a novel nonequilibrium self-similarity/self-preservation region is found to exist in the wakes of both a rotor and a fractal plate. In this novel nonequilibrium self-similarity region, the ratio of the mean flow length scale to the turbulence length scale and the ratio of the mean flow velocity scale to the turbulence velocity scale are not constant in the streamwise direction. However, in this region, the ratio of the mean flow time scale to the turbulence time scale is approximately constant in the streamwise direction. Using the simplified Reynolds-averaged equation of motion, new scaling laws are derived for this novel nonequilibrium self-similarity region, and these are confirmed by experimental measurements. Significant differences are observed between the rotor wake and fractal plate wake. The rotor wake reaches a self-similarity state much earlier than the fractal plate wake. A transition in the self-similarity state and scaling laws is found to occur in the rotor wake within the measurement region considered in the present study (3D–20D, where D is the effective diameter of the wake generator).

Wake Propagation and Characteristics of a Multi-Rotor Unmanned Vehicle in Forward Flight

In this study, experimental investigations are used to explore the wake propagation and characteristics of a multi-rotor unmanned air vehicle (UAV) in a forward flight mission. Qualitative smoke visualization is used first to gain a qualitative understanding of wake characteristics above and below the body of the multi-rotor UAV which is used as guidance for quantitative particle image velocimetry (PIV) experiments which better resolve the region in the vicinity of the multi-rotor UAV body. The experimental results over a wide range of advance ratios show that as the advance ratio increases, achieved by either lower rotor speeds or higher flight speeds, the distance by which the wake propagates below the UAV is reduced. While above the UAV, the flow returns to the freestream flow closer to the body as the advance ratio increases. Therefore, this study concludes that proximity effects are reduced as the advance ratio increases. Findings from this study can be used to inform in situ sensor placement so that sensor readings are minimally affected by the wake from the multi-rotor UAV. Velocity measurement corrections are provided for sensors mounted above the UAV which can be used to improve sensor data reliability in forward flight. These results can advance autonomous sensing and increase the utility of multi-rotor UAV observations while providing designers and users further guidance to avoid proximity effects.

Wake rotation impacts on wake decay

This study considers the behavior of a well-conditioned swirling turbulent wake. A custom swirling wake generator was used to produce a swirling wake absent of a tower that complicates the flow regime. Four swirling wakes were studied with swirl numbers ranging from 0.19 to 0.37 and compared to a non-swirling counterpart. Two-component Laser-Doppler Anemometry was used to measure mean and turbulent quantities in the wake. Measured profiles were analyzed based on similarity theory applied to a swirling wake. The results show that wake behavior was impacted by the initial swirl strength. Generally, wake growth rate and axial deficit decay rate increased with higher levels of swirl, and tangential velocity decay rate increased with lower levels of swirl. However, the wake that diffused fastest was the case with a swirl number of 0.27.

Relativistic intraction of laser pulse with thermal electron plasma

The interaction of a short laser pulse (λp~L) with thermal fluid plasma in relativistic case is investigated in hydrodynamical approach. The one dimensional description of this process is developed and the fundamental equations describing the plasma quantities during the interaction are obtained. The wake generation by laser pulse is, also, investigated and the laser wake field in thermal plasma by a laser pulse is handled numerically. In weakly relativistic case, the wake potential propagating in snoidal wave form is obtained. While the wavelength of laser pulse is increased, the wavelength of plasma potential wave in thermal plasma is decreased compared to that in cold plasma. Also, due to the thermal motion, the amplitude of plasma wave is decreased.

Analysis of Energy Utilization and Losses for Jet-Propelled Vehicles

The global control volume-based energy utilization balance for an aerospace vehicle is extended to allow for the analysis of jet-propelled vehicles. The methodology is first developed for analyzing the energy utilization and entropy generation characteristics of jet engines without airframe considerations. This methodology, when combined with separate energy utilization analysis for an unpowered airframe, allows for the assessment of a powered vehicle. Wake entropy generation for a powered vehicle is shown to be the summation of the wake entropy generation associated with the propulsion system (no airframe) and the unpowered airframe. The fundamental relationship between overall entropy generation and the flight conditions required for maximum range and endurance of a powered vehicle are also derived. Example energy utilization results obtained for a modeled turbojet engine in off-design operation are provided; wake and engine component entropy generation characteristics are directly related to engine operation and flight conditions. This engine model is then integrated with a legacy (twin-engine) Northrop F-5E Tiger II airframe. The overall entropy generation temporal rate for the vehicle is minimized, as predicted by our analysis, at flight conditions corresponding to maximum endurance. For flight conditions corresponding to maximum range, the overall entropy spatial rate is minimized.

Modified Beetle Annealing Search (BAS) Optimization Strategy for Maxing Wind Farm Power through an Adaptive Wake Digraph Clustering Approach

Owing to scale-up and complex wake effects, the centralized control that processes the command from turbines may be unsuitable, as it incurs high communication overhead and computational complexity for a large offshore wind farm (OWF). This paper proposes a novel decentralized non-convex optimization strategy for maxing power conversion of a large OWF based on a modified beetle antennae search (BAS) algorithm. First, an adaptive threshold algorithm which to establish a pruned wake direction graph while preserving the most critical wake propagation relationship among wind turbines are presented. The adaptive graph constraints were used to create wake sub-digraphs that split the wind farm into nearly uncoupled clustering communication subsets. On this basis, a Monte Carlo-based beetle annealing search (MC-BAS) nonlinear optimization strategy was secondly designed to adjust the yaw angles and axial factors for the maximum power conversion of each turbine subgroup. Finally, the simulation results demonstrated that a similar gain could be achieved as a centralized control method at power conversion and reduces the computational cost, allowing it to solve the nonlinear problem and real-time operations of the OWF.

Wind power production from very large offshore wind farms

<h2>Summary</h2> We provide the first quantitative assessment of power production and wake generation from offshore wind energy lease areas along the U.S. east coast. Deploying 15-MW wind turbines, with spacing equal to the European average, yields electricity production of 116 TWh/year or 3% of current national supply. However, power production is reduced by one-third due to wakes caused by upwind wind turbines and wind farms. Under some flow conditions whole wind-farm wakes can extend up to 90 km downwind of the largest lease areas, and the frequency-weighted average area with a 5% velocity deficit is 2.6 times the footprint of the lease areas. Simulations including maritime corridors demonstrate reduction in the wake effects leading to power-efficiency gains and may offer contingent benefits. First-order scaling rules are developed that describe how "wake shadows" from large offshore wind farms scale with prevailing meteorology and wind turbine installed densities.

Time-Resolved Measurements of the Unsteady Boundary Layer in an Annular Low-Pressure Turbine Configuration With Perturbed Inlet

AbstractIn this study, the unsteady behavior of the boundary layers developing on a low-pressure turbine (LPT) stator profile and their effect on secondary flow patterns in a 1.5-stage turbine configuration are investigated under the influence of periodic inflow perturbations. The experimental setup previously employed to analyze the unsteady secondary flow in the stator wake has been enhanced by hot-film sensor arrays placed on the stator profiles at different blade height positions to provide time-resolved data from within the passage. The stator inflow is perturbed by a rotating wake generator, and a modified T106 profile has been considered for the blading. The modified profile, labeled as T106RUB, was developed for matching the transition and separation characteristics of the popular original T106 profile at low flow speeds, thus facilitating measurements to be taken in a large-scale test rig with its improved accessibility. The transition phenomena occurring in the profile boundary layers are investigated under both unperturbed and periodically perturbed inflow by means of spectral analysis, the semi-quantitative characterization of the wall-stress system, and an evaluation of the statistic quantities. In particular, the periodic changes of the suction-side boundary layer flow region toward the trailing edge are studied in detail. Furthermore, time-resolved hot-film measurements at different blade height positions facilitate a detailed comparison of the quasi two-dimensional mid-span profile flow and the near end-wall profile flow, which is subject to the influence of secondary flow structures. These information are employed to assess to which extent the additional turbulence originating from the wakes affects the blade boundary layers and thus the secondary flow structures. Furthermore, the role of the perturbation frequency on the coupled system of boundary layers and secondary flow structures is evaluated.

Large-Eddy Simulation of Low-Pressure Turbine Cascade with Unsteady Wakes

To better understand the wake effects at low Reynolds numbers, large-eddy simulations of a 50% reaction low-pressure turbine stage and a linear cascade with two different bar wake generators were carried out for a chord Reynolds number of 50,000. For the chosen front-loaded high-lift airfoil, the endwall structures are stronger than for more traditional mid-loaded moderate-lift airfoils. By comparing the 50% reaction stage results with the bar wake generator results, insight is gained into the effect of the three-dimensional wake components on the downstream flow field.For the cases with bar wake generator, the endwall boundary layer is growing faster because of the relative motion of the endwall with respect to the freestream. The half-width of the wake is approximately matched for the larger one of the two considered bar wake generators. To improve the quality of the phase-averaged flow fields, the proper orthogonal decomposition was employed as a filter to remove the low-energy unsteady flow field content. Both the mean flow and filtered phase-averaged flow fields were analyzed in detail. Visualizations of the phase-averaged flow field reveal a periodic suppression of the laminar suction side separation from the downstream airfoil even for the smaller bar wake generator. The passage vortex is entirely suppressed for the 50% reaction stage and for the larger bar wake generator. Furthermore, the phase-averaged data for the 50% reaction stage reveal a new longitudinal flow structure that is traced back to near-wall wake vorticity. This flow structure is missing for the bar wake generator cases.

Investigation of Tidal Turbine Array Performance Using Computational Fluid Dynamics in the Presence of Waves

Tidal turbine arrays have been widely investigated to find the optimum spacing for the efficient performance and less wake generation. Small scaled laboratory tests are usually performed to analyze the structure of wakes prior to the deployment. These tests result in extreme blockage conditions due to the narrowing of channels compared to size of turbines. The main aim is to investigate the behavior of flow around the turbines in blockage conditions and the performance of turbines closed to the free surface in current-only and wave-and-current scenarios. The methodology was based on blade element momentum and computational fluid dynamics approach with the integration of virtual blade model code. The results of this work indicate that the performance of tidal turbine array can be enhanced by up to 7% in blockage conditions in lateral arrangement and 11% in stream wise arrangement. The wake is highly influenced by the surface waves that also increase the performance of downstream turbines.

Investigation of the wake propagation behind wind turbines over hilly terrain with different slope gradients

In this study, a Reynolds-averaged Navier-Stokes (RANS) numerical simulation with wind turbine simulated through actuator disk model was conducted to quantify the characteristics of wind turbine wake over two-dimensional Gaussian hills with different slope gradients. The simulated flow characteristics in the wind turbine wake, such as velocity deficit, wake expansion and wake centerline, were compared with those given by the Jensen wake model to quantitatively evaluate the accuracy of the current engineering wake model for the optimal design of wind turbine layout over complex terrain. For the gentle slope hill, the topography behind the wind turbine has a significant effect on the wake characteristics, which is not taken into account in the Jensen wake model. A new method for calculating wake velocity is proposed based on the local speed-up factor and the simulated velocity in the wake of wind turbine sited on flat terrain that can more reasonably predict the wake velocity over complex terrain. For the steep slope hill, the propagation of wind turbine wake is not related to the hill shape in the region behind the hilltop. The assumption that the wake centerline follows the surface of the hill in the Jensen wake model is no longer applicable. The recovery of wind turbine wake in the far wake region was always faster for the steep slope hill compared with the gentle slope hill case. This acceleration of wake recovery is thought to be closely related to the separation flow at the lee side of the hill, which highly restricts the downward deflection and expansion of the wind turbine wake.

High-Reynolds-number wake of a slender body

Abstract

027 GABAergic Neurons in the Dorsal Raphe Nucleus Are under the Influence of GABAergic Inputs from the Nucleus Pontis Oralis

Abstract Introduction Our previous study has shown that there is a direct connection between GABAergic neurons in the nucleus pontis oralis (NPO) and neurons of the dorsal raphe nucleus (DR), providing a morphological basis for the hypothesis that GABAergic inhibitory processes in NPO play an important role in the generation and maintenance of wakefulness as well as active (REM) sleep through the interaction with neurons in the DR. However, the target of such a GABAergic projection from the NPO within the DR is unknown. In the present study, a double-fluorescent labeling technique was employed to examine the target of GABAergic inputs to the DR. Methods Adult cats were deeply anesthetized and perfused transcardially. Subsequently, the brainstem containing the DR was removed, postfixed and cut into 15 μm coronal sections with a Reichert-Jung cryostat. The sections were immunostained with antibodies against GABA-A or GABA-B receptors and GABA following the procedure of double fluorescence immunohistochemistry. Results Under fluorescence microscopy, a large number of neurons were labeled with antibodies against either GABA-A receptor or GABA-B receptor. In addition, neurons labeled with antibody against GABA were observed in the DR. With double fluorescence immunohistochemical techniques, some neurons labeled by anti-GABA antibody were also stained with antibodies against GABA-A or GABA-B receptors. Conclusion The expression of GABA-A or GABA-B receptors by GABAergic neurons in the DR indicates that GABAergic neurons in the DR receive GABAergic inputs. Our previous study has demonstrated that these GABAergic inputs are from the NPO. These data provide a morphological foundation to support our hypothesis that, during wakefulness, NPO GABAergic “Executive” neurons suppress “Second-Order” GABAergic neurons in the DR, which, in turn, activate (disinhibit) serotonergic wake-on neurons in this nucleus. Support (if any) NS092383

Experimental Study on Film Cooling Effectiveness of Blade With Chevron-Shaped Holes Under Wake Influence

Abstract Gas turbines have been widely used. With the continuous improvement of the performance of gas turbines, the turbine inlet temperature has greatly exceeded the heat-resistance limit of the turbine blade material, so advanced cooling technology is required. The film cooling effectiveness distribution over the blade under the effect of wake was obtained by the pressure-sensitive paint (PSP) technique. The test blade has five rows of chevron film holes on the pressure side, three rows of cylindrical film holes on the leading edge, and three rows of chevron film holes on the suction side. The mainstream Reynolds number is 130,000 based on the blade chord length, and the mainstream turbulence intensity is 2.7%. The upstream wake was simulated by the spoken-wheel type wake generator. The film cooling effectiveness was measured at three wake Strouhal numbers (0, 0.12, and 0.36) and three mass flux ratios (MFR1, MFR2, and MFR3). The results show that the increase of mass flux ratio leads to a decrease of the film cooling effectiveness on the suction surface. In the wake condition, the effect of mass flux ratio is weakened. Wake leads to a marked decrease of the film cooling effectiveness over most blade surface except for the surface near leading edge on the pressure surface. In the high mass flux ratio condition, the effect of wake on the film cooling effectiveness is weakened on the suction surface and strengthened on the pressure surface.