Wake Strength Research Articles

Does the rotational direction of a wind turbine impact the wake in a stably stratified atmospheric boundary layer?

Abstract. Stably stratified atmospheric boundary layers are often characterized by a veering wind profile, in which the wind direction changes clockwise with height in the Northern Hemisphere. Wind-turbine wakes respond to this veer in the incoming wind by stretching from a circular shape into an ellipsoid. We investigate the relationship between this stretching and the direction of the turbine rotation by means of large-eddy simulations. Clockwise rotating, counterclockwise rotating, and non-rotating actuator disc turbines are embedded in wind fields of a precursor simulation with no wind veer and in wind fields with a Northern Hemispheric Ekman spiral, resulting in six combinations of rotor rotation and inflow wind condition. The wake strength, extension, width, and deflection depend on the interaction of the meridional component of Ekman spiral with the rotational direction of the actuator disc, whereas the direction of the disc rotation only marginally modifies the wake if no veer is present. The differences result from the amplification or weakening/reversion of the spanwise and the vertical wind components due to the effect of the superposed disc rotation. They are also present in the streamwise wind component of the wake and in the total turbulence intensity. In the case of an counterclockwise rotating actuator disc, the spanwise and vertical wind components increase directly behind the rotor, resulting in the same rotational direction in the whole wake while its strength decreases downwind. In the case of a clockwise rotating actuator disc, however, the spanwise and vertical wind components of the near wake are weakened or even reversed in comparison to the inflow. This weakening/reversion results in a downwind increase in the strength of the flow rotation in the wake or even a different rotational direction in the near wake in comparison to the far wake. The physical mechanism responsible for this difference can be explained by a simple linear superposition of a veering inflow with a Rankine vortex.

Experimental investigations on the bubbly wake of a transom stern model using optical laser beam scattering characteristics

In this paper a new procedure for detection and quantification of the main features of ship models bubbly wake is presented. The method is based on light scattering characteristics of laser beams passing through the wake of ship models. An optical light scattering model is developed and validated in a series of steady bubbly flow tests in a water tank, with time-invariant mean multiphase flow properties. Then a series of unsteady tests are conducted in a towing tank so as to study bubbly wake properties of a transom stern model. Temporal wake strength and its final detection time and length are measured using light scattering measurements. Light scattering analysis provides longitudinal, lateral, and vertical distributions of the wake without any flow intrusion. Also by combining the experimental and analytical results, a complementary assay for determination of average bubble size and bubble number density distribution is presented. Effects of model velocity on the bubbly wake features of the transom stern are also investigated using the suggested procedures. Results indicate that increasing the model velocity increases the total bubbly wake length and lifetime. Details of the wake lateral distribution reveal the prominent contribution of transom stern diverging waves on the bubbly wake air entrainment.

Effect of Blade Row Interaction on Rotor Film Cooling

Abstract The mechanisms of blade row interaction affecting rotor film cooling are identified to make recommendations for the design of film cooling in the real, unsteady turbine environment. Present design practice makes the simplifying assumption of steady boundary conditions despite intrinsic unsteadiness due to blade row interaction; we argue that if film cooling responds nonlinearly to unsteadiness, the time-averaged performance will then be in error. Nonlinear behavior is confirmed using experimental measurements of flat-plate cylindrical film cooling holes, mainstream unsteadiness causing a reduction in film effectiveness of up to 31% at constant time-averaged boundary condition. Unsteady computations are used to identify the blade row interaction mechanisms in a high-pressure turbine rotor: a “negative jet” associated with the upstream vane wake, and frozen and propagating vane potential field interactions. A quasi-steady model is used to predict unsteady excursions in momentum flux ratio of rotor cooling holes, with fluctuations of at least ±30% observed for all hole locations. Computations with modified upstream vanes are used to vary the relative strength of wake and potential field interactions. In general, both mechanisms contribute to rotor film cooling unsteadiness. It is recommended that the designer should choose a cooling configuration that behaves linearly over the expected unsteady excursions in momentum flux ratio as predicted by a quasi-steady hole model.

Contributions of very large-scale motions to turbulence statistics in open channel flows

Time-resolved particle image velocimetry measurements were performed in smooth-walled open channels to investigate the contributions of very large-scale motions (VLSMs) to the turbulence characteristics in open channel flows. The focal point is to clarify the free surface effects on the characteristics of VLSMs and the contributions of VLSMs to the unique statistical features in open channel flows (i.e., the turbulent kinetic energy (TKE) redistribution and smaller wake strength of the mean velocity profile). The resulting wavelength of VLSMs in present smooth-walled open channels is approximately $20h$ ( $h$ is water depth), which is comparable to that in pipe and closed channels while smaller than that in rough-walled open channels, and they are shown to make a great contribution to turbulence statistics with over 50 % of streamwise turbulence intensity, Reynolds shear stress and negative net force coming from VLSMs in the outer layer. Compared with other wall-bounded flows, VLSMs maintain higher strength in the outer layer of open channel flows with non-negligible strength even in the near surface region ( $y\sim >0.8h$ ), indicating that the free surface seems to sustain/promote VLSMs. This strength difference of VLSMs closely relates to the TKE redistribution and smaller wake strength of the mean velocity in the outer layer of open channel flows. The higher streamwise turbulence intensity is mainly contributed from the higher strength of VLSMs therein. The decelerating role of VLSMs combining with their higher strength is vital for shaping the mean velocity profile, which therefore is speculated to make a great contribution to the smaller wake strength phenomenon.

Ultracold ions wake in dusty plasmas

Motivated by the recent experimental realization of ultracold dusty plasma (2019 Sci. Rep. 9 3261), we present the results of particle-in-cell simulation with Monte-Carlo-collisions for wake behind a dust particle due to focusing of ions at superfluid helium temperature (∼2 K). Dynamical screening (wakefield) defines structural and dynamical properties of charged dust particles in plasmas such as phase transition, crystal formation, vibration modes (waves) etc. Here, we delineate in detail the dependence of wake strength on the streaming velocity of ions and on the ion-neutral charge exchange collision frequency (neutrals density) in the ultracold dusty plasma. Lowering the temperature to ultracold level leads to a wake pattern behind a dust particle that completely differs from the wake at normal conditions. For wide range of parameters, most remarkable features of the wakefield are (i) the formation of wake pattern with two maxima split in transverse to ion flow direction in the downstream area, (ii) pronounced inverse V shape of the wakefield closely resembling the wake in quark-gluon plasma and dense quantum plasma (warm dense matter), and (iii) the inter-dust attraction region in transverse direction. The latter shows that molecule-like interaction between dust particles is realized in ultracold dusty plasmas. These observations show a fundamental difference of ultracold dusty plasma physics from well studied complex plasmas at normal conditions.

Experiment characterization of the influence of wall wettability and inclination angle on bubble rising process using PIV

Bubble behavior is an important branch of the research field of gas–liquid two-phase flow, and the wall has a significant impact on the rising process of a bubble. In this paper, a visual bubble observation platform with an adjustable wall was built to observe the effect of the wall with different inclination angles and different wettability on bubble rising behavior. During this experiment, particle image velocimetry (PIV) was used to obtain the velocity vector field of water around the rising bubble. Through experiments, the instantaneous and average rising velocity of the bubble as well as the flow field around the bubble under different conditions was obtained, and the morphological change of the bubble during its rising process was found. The experimental results showed that the rising velocity of the bubble under the influence of the hydrophobic wall was obviously faster than that under the influence of the hydrophilic wall. Even when the bubble rose near the hydrophobic wall, it was 34.53% faster than that in the free case. When the bubble contacted the hydrophobic wall, it deformed and adhered to the wall to slip up, but when the bubble contacted the hydrophilic wall, it was bounced off the wall. In addition, when the wall was vertical, the bubble rising close to the hydrophilic wall formed a larger range of wake flow. However, the wake formed by the bubble rising near the hydrophobic wall had a smaller wake range and a weaker wake strength. That is, the hydrophilic wall combined with the rising of bubble would bring better mixing effect to the flow field.

A Numerical Investigation of a Winglet-Propeller Using an LES Model

The generation of tip vortex cavitation (TVC) is a common phenomenon in marine propellers. Therefore, it is important to find a way for the effective suppression of TVC. Based on the enlightenment of bionics, a propeller with winglets was numerically investigated by using a large eddy simulation (LES) model and the commercial software STAR-CCM+. Various variables, such as mesh size, number of prism layers, vapor properties and time step, were analyzed using the benchmark MAU5-80 propeller. The open water characteristics calculated for the benchmark propeller were compared with experimental data. The meshes in the region of the tip vortex wake were refined. The power spectral density (PSD) of the thrust coefficient and axial velocity were investigated. The comparison of TVC between the benchmark propeller and the propeller with winglets was studied with the Q-criterion, helicity and volume fraction of the vapor. The strength of the tip vortex wake is weakened by winglets; therefore, the presence of winglets leads to a reduction in vapor volume, which in turn alleviates TVC.

On the mechanism of symmetric vortex shedding

In this paper, we study the vortex-shedding mechanism of a square cylinder subjected to a mean flow with a superimposed pulsatile flow, under symmetric vortex shedding conditions. The near-body vortical events are interpreted in accordance with the two-dimensional vorticity transport equation, and the interactions between different vortical regions are quantified by circulation balance for a control volume in the near wake. Strong wake counter flow that splits into two branches, with one branch considerably disrupting the boundary layer while other branch cuts off supply of vorticity to the shear layer, is found to be an essential feature of symmetric vortex shedding. To further clarify the roles of correlated terms contributing to the vorticity generation and transport in the wake, the Reynolds number was varied while keeping other excitation parameters constant. This results in the intensity of symmetric vortex shedding increasing with Reynolds number. The increase in wake counter-flow strength with increased Reynolds number cannot be explained either by a relative velocity between the mean and pulsatile flow components or relative acceleration between bluff body and fluid. Based on the results it is inferred that the brief rolling up of the shear layer to form a wake vortex during a part of the pulsation cycle, together with the associated unsteady tangential pressure gradient around the body, contribute to the stronger wake counter flow at higher Reynolds numbers. At lower pulsation amplitudes, the tendency of the fluid in the wake to form strong counter flow can interact significantly with the transverse entrainment flow to alter the natural vortex-shedding mechanism, to give different vortex-shedding modes.

Assessing the Impact of Velocity Dip and Wake Coefficients on Velocity Prediction for Open Channel Flows

The aim of the present study is to assess the impact of the velocity-dip and wake strength on the velocity prediction using the dip modified laws. The dip modified laws, particularly the Dip Modified Log Wake law (DMLW-law), are preferred over the traditional wall laws in the narrow open channels. This is mainly because these analytical-based laws basically rely on parameters for the velocity dip (α) caused by secondary flow and for the wake strength (Π) due to the turbulence and boundary walls. In this study, comprehensive expressions for estimating these two key parameters were proposed and tested for smooth and rough flows. The results indicated that the proposed expressions can noticeably improve the application of the DMLW-law model to both smooth and rough flows.

Rotor boundary layer development with inlet guide vane (IGV) wake impingement

This paper examines the transition process in a boundary layer on a rotor blade under the impingement of an inlet guide vane wake. The effects of wake strengths and the reduced frequency on the unsteady boundary layer development on a low-speed axial compressor were investigated using particle image velocimetry. The measurements were carried out at two reduced frequencies (fr = fIGVS0/U2i, fr = 1.35, and fr = 0.675) with the Reynolds number, based on the blade chord and the isentropic inlet velocity, being 97 500. At fr = 1.35, the flow separated at the trailing edge when the wake strength was weak. However, the separation was almost totally suppressed as the wake strength increased. For the stronger wake, both the wake’s high turbulence and the negative jet behavior of the wake dominated the interaction between the unsteady wake and the separated boundary layer on the suction surface of the airfoil. The boundary layer displacement thickened first due to the negative jet effect. Then, as the disturbances developed underneath the wake, the boundary layer thickness reduced gradually. The high disturbance region convected downstream at a fraction of the free-stream velocity and spread in the streamwise direction. The separation on the suction surface was suppressed until the next wake’s arrival. Because of the long recovery time at fr = 0.675, the boundary layer thickened gradually as the wake convected further downstream and finally separated due to the adverse pressure gradient. The different boundary layer states in turn affected the development of disturbances.

Wakes behind surface-mounted obstacles: Impact of aspect ratio, incident angle, and surface roughness

The dependence of the wake strength behind a windbreak is experimentally investigated as a function of several relevant parameters. A semiempirical model for lateral wake deflection is formulated and found to compare well with experimental data.

Measurements of wake-induced electron beam deflection in a dechirper at the Linac Coherent Light Source

The RadiaBeam/SLAC dechirper, a structure consisting of pairs of flat, metallic, corrugated plates, %a corrugated structure in flat geometry, has been installed just upstream of the undulators in the Linac Coherent Light Source (LCLS). As a dechirper, with the beam passing between the plates on axis, longitudinal wakefields are induced that can remove unwanted energy chirp in the beam. However, with the beam passing off axis, strong transverse wakes are also induced. This mode of operation has already been used for the production of intense, multi-color photon beams using the Fresh-Slice technique, and is being used to develop a diagnostic for attosecond bunch length measurements. Here we measure, as function of offset, the strength of the transverse wakefields that are excited between the two plates, and also for the case of the beam passing near to a single plate. We compare with analytical formulas from the literature, and find good agreement. This report presents the first systematic measurements of the transverse wake strength in a dechirper, one that has been excited by a bunch with the short pulse duration and high energy found in an X-ray free electron laser.

Effect of Tripping on Hypersonic Turbulent Boundary-Layer Statistics

The effect of varying three-dimensional, cylindrical post-type trip size on the mean and turbulent velocity profiles of a Mach 7.6 turbulent boundary layer is examined using particle image velocimetry. It is shown that the effect of under- and overtripping is to amplify the wake component of the mean velocity profile and outer-layer turbulence intensity, confirming trends from incompressible flow. Such results indicate that overly aggressive tripping introduces artificial large-scale turbulence that requires longer downstream distances to decay. For the current experiment, adequate tripping was obtained for trip sizes between 1.7 and 2.3 times the laminar boundary-layer displacement thickness at the trip, , with the optimum size approximately 2.3 . The wake strength for the optimally tripped cases followed the correlation of Fernholz and Finley (AGARDograph 253, Neuilly sur Seine, France, 1980) at the same Reynolds number, providing a good indicator for under- or overtripping. These results confirm that compressible boundary layers mimic incompressible trends but require larger trip sizes and have increased sensitivity, making a boundary layer free from initial conditions harder to achieve.

Mach and Reynolds Number Effects on the Wake Properties of Microramps

A parametric study has been conducted into the effects of device height (), Mach number (), and Reynolds number ( to ) on the effectiveness of microramp vortex generators as flow control devices. The control authority of the microramp comes from the two counter-rotating vortices it introduces into the boundary layer. The vortices transport high-momentum fluid toward the wall, thus creating a fuller velocity profile that is less prone to separation. This momentum transport, however, comes at the price of a low-momentum wake that is formed downstream of the device. The induced flowfield has been studied by means of two-dimensional particle image velocimetry, oil-flow, and schlieren visualizations. Most of the geometric flow features were found to scale linearly with microramp height : the wake height, wake strength, vortex core height, and the momentum flux added to the near-wall region of the flow all increase linearly with . The control effectiveness of the microramp in the symmetry plane is reduced for higher Mach numbers () as the strength of the vortices is reduced, which adds less momentum to the near-wall region of the flow. Also, a stronger wake is recorded for the higher Mach number cases. Only a weak Reynolds number effect is observed, with the microramp becoming slightly more effective at higher Reynolds numbers.

Simulations of turbulent asymptotic suction boundary layers

ABSTRACTA series of large-eddy simulations of a turbulent asymptotic suction boundary layer (TASBL) was performed in a periodic domain, on which uniform suction was applied over a flat plate. Three Reynolds numbers (defined as ratio of free-stream and suction velocity) of Re = 333, 400 and 500 and a variety of domain sizes were considered in temporal simulations in order to investigate the turbulence statistics, the importance of the computational domain size, the arising flow structures as well as temporal development length required to achieve the asymptotic state. The effect of these two important parameters was assessed in terms of their influence on integral quantities, mean velocity, Reynolds stresses, higher order statistics, amplitude modulation and spectral maps. While the near-wall region up to the buffer region appears to scale irrespective of Re and domain size, the parameters of the logarithmic law (i.e. von Kármán and additive coefficient) decrease with increasing Re, while the wake strength decreases with increasing spanwise domain size and vanishes entirely once the spanwise domain size exceeds approximately two boundary-layer thicknesses irrespective of Re. The wake strength also reduces with increasing simulation time. The asymptotic state of the TASBL is characterised by surprisingly large friction Reynolds numbers and inherits features of wall turbulence at numerically high Re. Compared to a turbulent boundary layer (TBL) or a channel flow without suction, the components of the Reynolds-stress tensor are overall reduced, but exhibit a logarithmic increase with decreasing suction rates, i.e. increasing Re. At the same time, the anisotropy is increased compared to canonical wall-bounded flows without suction. The reduced amplitudes in turbulence quantities are discussed in light of the amplitude modulation due to the weakened larger outer structures. The inner peak in the spectral maps is shifted to higher wavelength and the strength of the outer peak is much less than for TBLs. An additional spatial simulation was performed, in order to relate the simulation results to wind tunnel experiments, which – in accordance with the results from the temporal simulation – indicate that a truly TASBL is practically impossible to realise in a wind tunnel. Our unique data set agrees qualitatively with existing literature results for both numerical and experimental studies, and at the same time sheds light on the fact why the asymptotic state could not be established in a wind tunnel experiment, viz. because experimental studies resemble our simulation results from too small simulation boxes or insufficient development times.

The three-dimensional hydrodynamics of thunniform swimming under self-propulsion

This paper presents the hydrodynamics of thunniform swimming under self-propulsion as the body and caudal fin kinematic behaviors are systematically varied. The swimmer is a three-dimensional (3D) tuna-like flexible model with prescribed kinematics of thunniform mode. Simulations are carried out for various body behaviors by varying the wave frequency f and the tail amplitude Ap and for various caudal fin behaviors by varying the maximum angle of attack αmax and the phase difference φ. The results show that the swimming velocity as well as the propulsive efficiency is an increasing function of both f and Ap, but their trend with αmax and φ is found to be not monotonic. Specially, for peak swimming velocity generation, αmax occurs at 20–30°, and φ ranges from 60° to 75°, while for peak efficiency, αmax is 20–30° and φ nears 90°. The wake structure formed by the swimmer is single row wake consisting of a series of disconnected vortex rings that are resembled by sickle-like shape vortices. As φ increases, the tail of the sickle-shape vortices is shortening gradually while the head is stretching. As αmax increases, the aligned sickle-like vortices have almost identical shape with each other, but the wake strength is reducing gradually.

A wind-tunnel investigation of wind-turbine wakes in yawed conditions

Wind-tunnel experiments were performed to study the performance of a model wind turbine and its wake characteristics in a boundary layer under different operating conditions, including different yaw angles and tip speed ratios. High-resolution particle image- velocimetry (PIV) was used to measure the three velocity components in a horizontal plane at hub height covering a broad streamwise range from upstream of the turbine to the far- wake region. Additionally, thrust and power coefficients of the turbine were measured under different conditions. These power and thrust measurements, together with the highly-resolved flow measurements, enabled us to systematically study different wake properties.The near-wake region is found to have a highly complex structure influenced by different factors such as tip speed ratio and wake rotation. In particular, for higher tip speed ratios, a noticeable speed-up region is observed in the central part of near wake, which greatly affects the flow distribution in this region. In this regard, the behavior of the near wake for turbines with similar thrust coefficients but different tip speed ratios can vary widely. In contrast, it is shown that the mean streamwise velocity in the far wake of the turbine with zero yaw angle has a self-similar Gaussian distribution, and the strength of wake in this region is consistent with the magnitude of the thrust coefficient.With increasing yaw angle, as expected, the power and thrust coefficients decrease, and the wake deflection increases. The measurements also reveal that, in addition to turbulent momentum flux, lateral mean momentum flux boosts the flow entrainment in only one side of the wake, which results in a faster wake recovery in that side. It is also found that the induced velocity upstream of a yawed turbine has a non-symmetric distribution, and its distribution is in agreement with the available model in the literature. Moreover, the results suggest that in order to accurately predict the load distribution in yawed conditions, both normal and tangential (with respect to the rotor plane) components of the induced velocity upstream of the turbine should be taken into account.

Stability of Compressible Mixing Layers Modified by Wakes and Jets

Binary free shear layers are frequently used as a model to study the stability of flows relevant in propulsion devices. The stability behaviour of binary, compressible mixing layer have been investigated and reported in the literature by many research groups and detailed description of the stability characteristics and flow topology have been published. The stability of more complex free shear layers, such as mixing layers modified by jets and wakes, has received less attention and a smaller number of references report on the stability characteristics of such flows. The availability of advanced numerical techniques as well as computational power nowadays allow the study of complex flows by direct numerical simulation. The present investigation is an extension of previous work on the stability of complex free shear flows where besides growth rates, wave numbers and frequencies, the flow topology resulting from the interaction of mixing layers, jets and wakes are investigated. The results show that, depending on the jet or wake strength relative to the mixing layer fast and slow stream velocities, the characteristic Kelvin-Helmholtz vortices may be enhanced and a more complex vortex distribution topology results. These vorticity distributions have a significant effect on the mixture between different species on the upper and lower stream and the mixing layer to jet species ratio. The effect of three-dimensional disturbances with spanwise wavenumber β * 0 are also considered and the most amplified modes are determined for a low convective mach number condition. Different combinations of hydrogen and oxygen on the mixing layer and jets are considered. For that study the base flow is determined from a parabolic, variable properties boundary layer code. The results show that oblique disturbances are more stable than two-dimensional disturbances but with different strength depending on the particular binary free shear layer considered.

An Integral Solution for Skin Friction in Turbulent Flow Over Aerodynamically Rough Surfaces With an Arbitrary Pressure Gradient

The combined law of the wall and wake, with the inclusion of the “roughness depression function” for the inner law in the “Log” region, is used as the inner coordinates' velocity profile in the integral form of the x momentum equation to solve for the local skin friction coefficient. The “equivalent sand grain roughness” concept is employed in the roughness depression function in the solution. Calculations are started at the beginning of roughness on a surface, as opposed to starting them using the measured experimental values at the first data point, when making comparisons of predictions with data sets. The dependence of the velocity wake strength on both pressure gradient and momentum thickness Reynolds number are taken into account. Comparisons of the prediction with experimental skin friction data, from the literature, have been made for some adverse, zero, and favorable (accelerating flows) pressure gradients. Predictions of the shape factor, roughness Reynolds number, and momentum thickness Reynolds number and comparisons with data are also made for some cases. In addition, some comparisons with the predictions of earlier investigators have also been made.

Compact bubble clusters in Newtonian and non-Newtonian liquids

We studied the terminal velocity of a packed array of bubbles, a bubble cluster, rising in different fluids: a Newtonian fluid, an elastic fluid with nearly constant viscosity (Boger fluid), and a viscoelastic fluid with a shear dependent viscosity, for small but finite Reynolds numbers (1 × 10−4 < Re < 4). In all three cases, the cluster velocity increased with the total volume, following the same trend as single bubbles. For the case of clusters in elastic fluids, interestingly, the so-called velocity discontinuity was not observed, unlike the single bubble case. In addition to the absence of jump velocity, the clusters did not show the typical teardrop shape of large bubbles in viscoelastic fluids and the strength of the negative wake is much weaker than the one observed behind single bubbles. Dimensional analysis of the volume-velocity plots allowed us to show that, while the equivalent diameter (obtained from the total cluster volume) is the appropriate length to determine buoyancy forces and characteristic shear rates, the individual bubble size is the appropriate scale to account for surface forces.