Wake Generator Research Articles

A POD-Based Procedure for the Split of Unsteady Losses of an LPT Cascade

A time-resolved particle image velocimetry (TR-PIV) system has been employed to investigate the unsteady propagation of upstream wakes in a low-pressure turbine cascade. Data are obtained in the steady state condition and for two passing wake reduced frequencies. The study is focused on the identification and split of the different dynamics responsible for deterministic and random oscillations, thus loss generation by means of a new procedure based on proper orthogonal decomposition (POD). The paper takes advantage of the properties of POD that reduce the data set to a low number of modes that represent the most energetic dynamics of the system. It is clearly shown that the phase averaged flow field can be represented by a few number of POD modes related to the wake passing event for the unsteady cases. Proper orthogonal decomposition is also able to capture flow features affecting the instantaneous flow field not directly related to the wake passage (i.e., the vortex shedding phenomenon induced by the intermittent separation developing between adjacent wakes), which are smeared out in the phase averaged results. A procedure exploiting the biorthogonality condition of the POD modes, and the related temporal coefficients, has been developed for the quantification of the contribution due to the different POD modes to the overall turbulence kinetic energy production, or, equivalently, the mean flow energy dissipation rate. Results into the paper clearly show that losses due to wake migration, boundary layer and vortex shedding related phenomena can be distinguished and separately quantified for the different tested conditions.

On the wind noise reduction mechanism of porous microphone windscreens.

This paper investigates the wind noise reduction mechanism of porous microphone windscreens. The pressure fluctuations inside the porous windscreens with various viscous and inertial coefficients are studied with numerical simulations. The viscous and inertial coefficients represent the viscous forces resulting from the fluid-solid interaction along the surface of the pores and the inertial forces imposed on the fluid flow by the solid structure of the porous medium, respectively. Simulation results indicate that the wind noise reduction first increases and then decreases with both viscous and inertial coefficients after reaching a maximum. Experimental results conducted on five porous microphone windscreens with porosity from 20 to 60 pores per inch (PPI) show that the 40 PPI windscreen has the highest wind noise reduction performance, and this supports the simulation results. The existence of the optimal values for the viscous and inertial coefficients is explained qualitatively and it is shown that the design of the porous microphone windscreens should take into account both the turbulence suppression inside and the wake generation behind the windscreen to achieve optimal performance.

Optimal active control of laminar flow over a circular cylinder using Taguchi and ANN

Increasing the lifetime of engineering structures is dependent on the unsteady forces exerted by vortex shedding. Flow controllers can be used for suppression of vortex shedding behind the cylindrical structures. The main objective of this paper is to minimize the wake generated behind a circular cylinder in laminar flow, Re=150. Two counter rotating controllers inject the momentum in the wake region or actively control the vortex shedding. The important parameters for defining the effectiveness of flow control are the lift and drag fluctuations besides the drag coefficient. The computational methodology is based on the finite volume method using the SIMPLE algorithm. The parameters that affect on the wake control are the rotation rate, diameter of controllers, radial and the angular distances. Determining the optimum governing parameters is important. A neural network is used for finding these optimum parameters. The input data for training this neural network were provided by the Taguchi method. Moreover the Taguchi method clarified the influence of each parameter on the wake generation. At the optimal obtained conditions the drag coefficient reduced up to 99.99% and the vortex shedding suppressed appropriately.

An influence of the different incoming wake-like flows on the rotor vibrations

The aim of the current investigation is the rotor vibrations generated by the disturbances caused the different types of incoming wake-like flows. Those wakes arriving at the tested rotor were created by two ways: a passive wake generator (immobile disk) and an upstream rotating rotor as an active wake generator. The influence of both wakes on the tested rotor was studied in a water flume. A model of the tested three-bladed rotor designed using Glauert’s optimum theory at an optimal tip speed ratio λ = 5 was placed in both “passive” and “active” wakes to recognize dissimilarities on the vibrations of the tested rotor. The distance from the wake generators to the tested rotor was varied from 4 to 8 rotor diameters. Also, the shift between the rotor axis and axis of the incoming wakes was changed to 0, 0.5 and 1 rotor diameters. The flow condition before rotor was measured with high temporal accuracy using LDA. The turbulent intensity of the incoming wake flows changed from 3 to 16% due to the types of the wake generators. Power and thrust characteristics and their pulsations of the tested rotor were measured by strain gauges. The dependences of power coefficients from tip speed ratios and positions of the wake generators were documented. The present study showed a strong influence of the initial flow from the two different wake generators on the rotor vibrations.

0138 DIFFERENTIAL C-FOS EXPRESSION IN GABAERGIC NEURONS OF THE NUCLEUS PONTIS ORALIS DURING WAKEFULNESS AND CARBACHOL-INDUCED ACTIVE SLEEP

Previous studies have demonstrated that GABAergic processes in the nucleus pontis oralis (NPO) play an important role in the generation and maintenance of active sleep and wakefulness. However, the location of cell bodies and neural activity of this pontine GABAergic system during wakefulness and active sleep are unknown. Accordingly, the present study was undertaken to determine the distribution of activated GABAergic neurons within the NPO during wakefulness and carbachol-induced active sleep (active sleep-carbachol) using c-Fos immunocytochemistry as a functional marker of neuronal activity. Adult cats that were prepared for monitoring behavioral states were used in this experiment. In the awake group, animals were kept awake for 2 hours following the microinjection of saline into the NPO before being euthanized. In the active sleep-carbachol group, animals were euthanized after prolonged episodes of active sleep that was induced by the microinjection of carbachol into the NPO. Subsequently, brainstem sections were immunostained with a GABA antiserum in addition to a Fos polyclonal antiserum in order to double-label neurons that were both GABAergic and Fos immunoreactive. In awake cats, there was a large number of GABAedrgic neurons that expressed c-Fos in the NPO. These GABA+/c-Fos+ neurons were small to medium-sized and were located mainly within the lateral part of the NPO. In contrast, in active sleep-carbachol cats, the numbers of c-Fos expressing GABAergic neurons were decreased in the NPO. Specifically, there were only few GABAergic neurons in the lateral part of the NPO that expressed c-Fos during active sleep-carbachol. The present results demonstrate that there exists a discrete group of GABAergic neurons in the NPO that are activated during wakefulness and inactivated during active sleep. We therefore suggest that these local GABAergic cells may be the neurons that comprise the pontine GABAergic system which functions to promote wakefulness and suppress active sleep by inhibiting active sleep-generator neurons in the NPO during wakefulness and disinhibiting them during active sleep. NS092383.

3D modelling of impacts from waves on tidal turbine wake characteristics and energy output

A Virtual Blade Model is coupled with a CFD model to simulate impacts from a Horizontal Axis Tidal Turbine under combined surface waves and a steady current. A two-equation model is used to represent the turbulence generation and dissipation due to turbine rotation and background wave-current flows. The model is validated against experimental measurements, showing good agreement in both surface elevation and fluid hydrodynamics. It is then scaled up to investigate a steady current with large stream-wise surface waves in the presence of a turbine. A strong interaction is found between surface wave-induced flows and that around the turbine, which clearly impacts on both hydrodynamics within the wake and wave propagation, and produces large fluctuations in power production. Model results show that the wave-period-averaged velocities are similar to those in the steady-current-only condition. However, the wave enhances the turbulence immediately behind the turbine and reduces the length of the flow transition. The wave height reduces by about 10% and the wavelength extends by 12% when propagating over the turbine region in comparison with the no-turbine condition. The wave shape also becomes asymmetric. Compared with the current-alone situation, the model results suggest that the power production is similar. However, wave oscillation produces noticeably larger fluctuations.

Experimental study of wake structure behind a horizontal axis tidal stream turbine

A detailed experimental investigation of the wake propagation behind a horizontal axis turbine with three blades was conducted in a recirculating water flume. An Acoustic Doppler Velocimeter was employed to measure the time varying velocities at fifteen depths across the width of the open flume to obtain the three-dimensional velocity and turbulence fields within the length of 20 Rotor Diameters downstream. The experimental results indicated that velocity reduction in the wake was caused by both the kinetic energy extraction and blockage effects of the tidal stream turbine rotor and stanchion. The maximum velocity deficit occurred at the wake core due to a lower tip speed ratio and blockage of the hub. The wake strip gradually enlarged with the distance downstream, with less mixing in the transverse direction. The wake zone shifted towards the water surface in the vertical direction, which mainly resulted from the merging of two wakes induced by turbine rotor and stanchion. The wake rotation was also observed, and the maximum circumfluence velocity was approximately 20% of the stream-wise velocity; thus, it had a significant influence on the process of near wake mixing. Furthermore, the wake turbulence of the turbine was strong and anisotropic, which would have an impact on the behaviour of other turbines located downstream if they were in a turbine array. In addition, the turbine stanchion had a visible influence on the wake structure, especially near the wake, which should not be neglected when studying the wake characteristics.

Aerodynamic Analysis of Grand Prix Cars Operating in Wake Flows

The effect of the upstream wake of a Formula 1 car on a following vehicle has been investigated using experimental and computational methods. Multiple vehicle studies in conventional length wind tunnels pose challenges in achieving a realistic vehicle separation and the use of a short axial length wake generator provides an advantage here. Aerodynamic downforce and drag were seen to reduce, with greater force reductions experienced at shorter axial spacings. With lateral offsets, downforce recovers at a greater rate than drag, returning to the level for a vehicle in isolation for offsets greater than half a car width.The effect of the wake was investigated in CFD using multiple vehicle simulations and non-uniform inlet boundary conditions to recreate the wake. Results closely matched those for a full two-vehicle simulation provided the inlet condition included unsteady components of the onset wake. Creating a nonuniform inlet condition allowed the wake parameters to be modified to test sensitivity to different wake features. Dynamic pressure deficit in the wake is shown to have the greatest impact on the following vehicle, reducing loading on the downforce producing surfaces. Wake up-wash and vortex flows are shown to have a smaller effect on downforce generated by the following car, but have an important role in diverting the dynamic pressure deficit upwards and over the following car.Future regulation changes, aimed at reducing the downforce loss experienced when following another car, should aim to reduce the velocity deficit onset to the following car; either by reducing wheel and underbody wakes, or by extracting the wake using up-wash from the rear wing.

Interaction of a swept shock wave and a supersonic wake

The interaction of a swept shock wave and a supersonic wake has been studied. The swept shock wave is generated by a swept compression sidewall, and the supersonic wake is generated by a wake generator. The flow field is visualized with the nanoparticle-based planar laser scattering method, and a supplementary numerical simulation is conducted by solving the Reynolds-averaged Navier–Stokes equations. The results show that the pressure rise induced by the swept shock wave can propagate upstream in the wake, which makes the location where vortices are generated move upstream, thickens the laminar section of the wake, and enlarges the generated vortices. The wake is swept away from the swept compression sidewall by the pressure gradient of the swept shock wave. This pressure gradient is not aligned with the density gradient of the supersonic wake, so the baroclinic torque generates streamwise vorticity and changes the distribution of the spanwise vorticity. The wake shock is curved, so the flow downstream of it is non-uniform, leaving the swept shock wave being distorted. A three-dimensional Mach disk structure is generated when the wake shock interacts with the swept shock wave.

An improved apparatus for a wake generator using a cam mechanism and unsteady velocity measurement

An improved apparatus for a wake generator using a cam mechanism and unsteady velocity measurement

Statistical analyses of a screen cylinder wake

The evolution of a screen cylinder wake was studied by analysing its statistical properties over a streamwise range of The screen cylinder was made of a stainless steel screen mesh of 67% porosity. The experiments were conducted in a wind tunnel at a Reynolds number of using an X-probe. The results were compared with those obtained in the wake generated by a solid cylinder. It was observed that the evolution of the statistics in the wake of the screen cylinder was different from that of a solid cylinder, reflecting the differences in the formation of the organized large-scale vortices in both wakes. The streamwise evolution of the Reynolds stresses, energy spectra and cross-correlation coefficients indicated that there exists a critical location that differentiates the screen cylinder wake into two regions over the measured streamwise range. The formation of the fully formed large-scale vortices was delayed until this critical location. Comparison with existing results for screen strips showed that although the near-wake characteristics and the vortex formation mechanism were similar between the two wake generators, variation in the Strouhal frequencies was observed and the self-preservation states were non-universal, reconfirming the dependence of a wake on its initial condition.

Numerical modeling of the free rise of an air bubble

The results of a numerical investigation of the dynamics of a single air bubble rising in water are presented. The bubbles, 1, 2.5, 3, 5, 8, and 10 mm in diameter, are considered. An analysis is based on the numerical solution of the complete three-dimensional system of Navier–Stokes equations for a two-phase medium using an implicit approach with the automatic tracking of the gas-water interface by means of separating the volume fractions. Emphasis is placed on an examination of the local physical characteristics of the motion. The calculated mean rise velocities are compared with experimental data. The rising bubble trajectories are shown to be periodic, zigzag or helical in shape, which is due to the variation in their form and the generation of a characteristic turbulent wake behind them. The bubble rise velocities are correlated with the forces acting on the bubbles.

Vortex flow and lift generation of a non-slender reverse delta wing

The vortex flow and lift force generated by a 50°-sweep non-slender reverse delta wing were investigated via particle image velocimetry, together with flow visualization and force balance measurement, at Re = 11,000. The non-slender reverse delta wing produced a delayed stall but a lower lift compared to its delta wing counterpart. The stalling mechanism was also found to be triggered by the disruption of the multiple spanwise vortex filaments developed over the upper wing surface. The vortex flowfield was, however, characterized by the co-existence of reverse delta wing vortices and multiple shear-layer vortices. The outboard location of the reverse delta wing vortex further implies that the lift force is mainly generated by the wing lower surface while the upper surface acts as a wake generator. The spatial progression of the flow parameters of the vortex generated by the non-slender reverse delta wing as a function of α was also discussed.

VTA dopaminergic neurons regulate ethologically relevant sleep-wake behaviors.

Dopaminergic ventral tegmental area (VTA) neurons are critically involved in a variety of behaviors that rely on heightened arousal, but whether they directly and causally control the generation and maintenance of wakefulness is unknown. We recorded calcium activity using fiber photometry in freely behaving mice and found arousal-state-dependent alterations in VTA dopaminergic neurons. We used chemogenetic and optogenetic manipulations together with polysomnographic recordings to demonstrate that VTA dopaminergic neurons are necessary for arousal and that their inhibition suppresses wakefulness, even in the face of ethologically relevant salient stimuli. Nevertheless, before inducing sleep, inhibition of VTA dopaminergic neurons promoted goal-directed and sleep-related nesting behavior. Optogenetic stimulation, in contrast, initiated and maintained wakefulness and suppressed sleep and sleep-related nesting behavior. We further found that different projections of VTA dopaminergic neurons differentially modulate arousal. Collectively, our findings uncover a fundamental role for VTA dopaminergic circuitry in the maintenance of the awake state and ethologically relevant sleep-related behaviors.

Predictive wind turbine simulation with an adaptive lattice Boltzmann method for moving boundaries

Operating horizontal axis wind turbines create large-scale turbulent wake structures that affect the power output of downwind turbines considerably. The computational prediction of this phenomenon is challenging as efficient low dissipation schemes are necessary that represent the vorticity production by the moving structures accurately and that are able to transport wakes without significant artificial decay over distances of several rotor diameters. We have developed a parallel adaptive lattice Boltzmann method for large eddy simulation of turbulent weakly compressible flows with embedded moving structures that considers these requirements rather naturally and enables first principle simulations of wake-turbine interaction phenomena at reasonable computational costs. The paper describes the employed computational techniques and presents validation simulations for the Mexnext benchmark experiments as well as simulations of the wake propagation in the Scaled Wind Farm Technology (SWIFT) array consisting of three Vestas V27 turbines in triangular arrangement.

Direct numerical simulation of a turbulent wake: The non-equilibrium dissipation law

A Direct Numerical Simulation (DNS) study of an axisymmetric turbulent wake generated by a square plate placed normal to the incoming flow is presented. It is shown that the new axisymmetric turbulent wake scalings obtained recently for a fractal-like wake generator (Dairay et al., 2015), specifically a plate with irregular multiscale periphery placed normal to the incoming flow, are also present in an axisymmetric turbulent wake generated by a regular square plate. These new scalings are therefore not caused by the multiscale nature of the wake generator but have more general validity.

Hypocretinergic system in the medial preoptic area promotes maternal behavior in lactating rats

Hypocretin-1 and 2 (HCRT-1 and HCRT-2, respectively) are neuropeptides synthesized by neurons located in the postero-lateral hypothalamus, whose projections are widely distributed throughout the brain. The hypocretinergic (HCRTergic) system has been associated with the generation and maintenance of wakefulness, as well as with the promotion of motivated behaviors. In lactating rats, intra-cerebroventricular HCRT-1 administration stimulates maternal behavior, whilst lactation per se increases the expression of HCRT type 1 receptor (HCRT-R1). Due to the fact that HCRTergic receptors are expressed in the medial preoptic area (mPOA), a region critically involved in maternal behavior, we hypothesize that HCRT-1 promotes maternal behavior acting on this region. In order to evaluate this hypothesis, we assessed the maternal behavior of lactating rats following microinjections of HCRT-1 (10 or 100μM) and the selective HCRT-R1 antagonist SB-334867 (250μM) into the mPOA, during the first and second postpartum weeks. While intra-mPOA microinjections of HCRT-1 (100μM) increased corporal pup licking during the second postpartum week, the blockade of HCRT-R1 significantly decreased active components of maternal behavior, such as retrievals, corporal and ano-genital lickings, and increased the time spent in nursing postures in both postpartum periods. We conclude that HCRTergic system in the mPOA may stimulate maternal behavior, suggesting that endogenous HCRT-1 is necessary for the natural display of this behavior.

Estimation of wake propagation behind the rotors of wind-powered generators

The objectives of this work are to develop the experimental model of wake behind the wind-power generator rotor to estimate its propagation distance and the impact on the average and pulsation characteristics of incident flow with the possibility of further use of these data in the calculation models of wind and climate changes in the regions and to determine the optimal operation of wind turbines. For experimental modeling, the laboratory model of wind-powered generator with a horizontal axis was used that operated as wind turbine in optimal mode. The kinematic characteristics of flow and changes in the wake structure in the distance of more than 40 rotor diameters downstream with a slight level of turbulent pulsations (less than 2%) of free flow were investigated. A significant impact of external intense pulsations typical for natural atmospheric conditions was purposely excluded in the experimental research in order to define the degree of self-damping of perturbations generated by oneself wind-powered generator. The obtained experimental data for the wake dynamics behind the model of wind-powered generator allowed ascertaining its impact on slowing down of incident vortex flow and determining the distance at which its impact on the stream disappears, and the deceleration values are comparable to the level of pulsations of incident flow. This experimental model with the same degree of damping its velocity and pulsations can be used to adjust the theoretical approximation of the far wake. It is shown that the recovery of velocity of incident flow is faster than has been previously defined in the models of calculating the impact of wind electric power plants on the regional climate changes. Thus, existing wind loss calculated on the model of wake behind the wind-powered generator, adjusted in this study can be even less significant.

Atmospheric Wake of Madeira: First Aerial Observations and Numerical Simulations

Abstract The island of Madeira is well known for giving rise to atmospheric wakes. Strong and unsteady atmospheric wakes, resembling a von Kármán vortex street, are frequently observed in satellite images leeward of Madeira, especially during summer months, when conditions favoring the formation of atmospheric wakes occur frequently under the influence of the Azores high. Reported here is the analysis of the first airborne measurements of Madeira’s wake collected during the 2010 Island-induced Wake (I-WAKE) campaign. High-resolution in situ and remote sensing data were collected in the I-WAKE by a research aircraft. The measurements reveal distinctive wake signatures, including strong lateral wind shear zones and warm and dry eddies downwind of the island. A strong anticorrelation of the horizontal wind speed and sea surface temperature (SST) was found within the wake. High-resolution numerical simulations with the Weather Research and Forecasting (WRF) Model were used to study the dynamics of the wake generation and its temporal evolution. The comparison of the model results and observations reveals a remarkable fidelity of the simulated wake features within the marine boundary layer (MBL). Strong potential vorticity (PV) anomalies were found in the simulated MBL wake, emanating from the flanks of the island. The response of the wake formation within the MBL to surface friction and enhanced thermal forcing is explored through the model sensitivity analyses.

Non-equilibrium scaling laws in axisymmetric turbulent wakes

We present a combined direct numerical simulation and hot-wire anemometry study of an axisymmetric turbulent wake. The data lead to a revised theory of axisymmetric turbulent wakes which relies on the mean streamwise momentum and turbulent kinetic energy equations, self-similarity of the mean flow, turbulent kinetic energy, Reynolds shear stress and turbulent dissipation profiles, non-equilibrium dissipation scalings and an assumption of constant anisotropy. This theory is supported by the present data up to a distance of 100 times the wake generator’s size, which is as far as these data extend.