Vortex Dissipation Research Articles

Effects of incoming surface wind conditions on the wake characteristics and dynamic wind loads acting on a wind turbine model

An experimental investigation was conducted to examine the effects of incoming surface wind conditions on the wake characteristics and dynamic wind loads acting on a wind turbine model. The experimental study was performed in a large-scale wind tunnel with a scaled three-blade Horizontal Axial Wind Turbine model placed in two different types of Atmospheric Boundary Layer (ABL) winds with distinct mean and turbulence characteristics. In addition to measuring dynamic wind loads acting on the model turbine by using a force-moment sensor, a high-resolution Particle Image Velocimetry system was used to achieve detailed flow field measurements to characterize the turbulent wake flows behind the model turbine. The measurement results reveal clearly that the discrepancies in the incoming surface winds would affect the wake characteristics and dynamic wind loads acting on the model turbine dramatically. The dynamic wind loads acting on the model turbine were found to fluctuate much more significantly, thereby, much larger fatigue loads, for the case with the wind turbine model sited in the incoming ABL wind with higher turbulence intensity levels. The turbulent kinetic energy and Reynolds stress levels in the wake behind the model turbine were also found to be significantly higher for the high turbulence inflow case, in comparison to those of the low turbulence inflow case. The flow characteristics in the turbine wake were found to be dominated by the formation, shedding, and breakdown of various unsteady wake vortices. In comparison with the case with relatively low turbulence intensities in the incoming ABL wind, much more turbulent and randomly shedding, faster dissipation, and earlier breakdown of the wake vortices were observed for the high turbulence inflow case, which would promote the vertical transport of kinetic energy by entraining more high-speed airflow from above to re-charge the wake flow and result in a much faster recovery of the velocity deficits in the turbine wake.

Wave reflection and vortex evolution in Bragg scattering in real fluids

The vortex generation and dissipation accompanying Bragg scattering of water waves propagating over a series of submerged rectangular breakwaters are investigated both numerically and experimentally. The present model, which takes effects of non-linearity, viscosity and turbulence into consideration, is applied to simulate the entire vortex evolution process as water waves pass over a series of artificial rectangular bars. Particle image velocimetry is used to measure the velocity field in the vicinity of the bars. The numerical model is validated via a comparison with measured water surface elevations and velocity fields; the results show good agreement. The mechanism of vortex evolution and its influence on the interaction of water waves with submerged structures for cases with and without resonance are studied. The corresponding wave reflection coefficients for both cases are calculated and compared with experimental data and solutions based on linear wave theory. Examination of the turbulence properties shows that the turbulent intensity decreases on the weather side but increases on the lee side under the Bragg scattering conditions.

Sill effects on physical dynamics in eastern Long Island Sound

This study investigates how Mattituck Sill influences circulation patterns and physical dynamics in eastern Long Island Sound, a major estuary on the U.S. east coast. Observations show there is pronounced across-estuary transport in the area and suggest there may be subtidal anticyclonic flow around the sill. Model runs, with and without sill bathymetry, exhibit this across-estuary transport and anticyclonic circulation. Comparison between these runs indicates that the sill intensifies the anticyclonic circulation. This study finds the sill does not exert internal hydraulic control during neap, mean, or spring tidal conditions. Nevertheless, along-estuary exchange is reduced over the sill and across-estuary fluxes are increased. The Connecticut River plume enters close to the estuary mouth. The sill deflects more of the plume waters towards the mouth, causing less freshwater to take the long looping route through the estuary. The subtidal circulation balance around the sill indicates a barotropic balance between the tidal advection of tidal vorticity and friction. The subtidal vorticity balance indicates the net effect of tidal advection of relative vorticity is balanced with frictional curl associated with lateral speed gradients and vorticity dissipation. Previously developed scalings based on the circulation balance (Nature 290:549–555, 1981), frictional vorticity generation mechanisms (Deep-Sea Res 28:195–212, 1981), and tidal diffusion of potential vorticity (J Phys Oceanogr 29:821–827, 1999) are applicable to Mattituck Sill and predict circulation with a similar magnitudes to model results.

Hydrodynamic Instabilities in Gaseous Detonations: Comparison of Euler, Navier–Stokes, and Large-Eddy Simulation

A large-eddy simulation is conducted to investigate the transient structure of an unstable detonation wave in two dimensions and the evolution of intrinsic hydrodynamic instabilities. The dependency of the detonation structure on the grid resolution is investigated, and the structures obtained by large-eddy simulation are compared with the predictions from solving the Euler and Navier–Stokes equations directly. The results indicate that to predict irregular detonation structures in agreement with experimental observations the vorticity generation and dissipation in small scale structures should be taken into account. Thus, large-eddy simulation with high grid resolution is required. In a low grid resolution scenario, in which numerical diffusion dominates, the structures obtained by solving the Euler or Navier–Stokes equations and large-eddy simulation are qualitatively similar. When high grid resolution is employed, the detonation structures obtained by solving the Euler or Navier–Stokes equations directly are roughly similar yet equally in disagreement with the experimental results. For high grid resolution, only the large-eddy simulation predicts detonation substructures correctly, a fact that is attributed to the increased dissipation provided by the subgrid scale model. Specific to the investigated configuration, major differences are observed in the occurrence of unreacted gas pockets in the high-resolution Euler and Navier–Stokes computations, which appear to be fully combusted when large-eddy simulation is employed.

On the Study of Magneto-Hydrodynamic Non-Newtonian Fluid Flow throughout Curvilinear Channel with Corrugated Walls

This article aims to numerically investigate the flow pattern for Newtonian and power law non-Newtonian fluid in a semi-half circular channel with corrugated walls under the influence of a magnetic field. The results indicate that, presence of a magnetic field affects the flow field in several aspects, especially in the vortex creation and dissipation. In addition, the analysis is carried out for different Reynolds numbers to ascertain the influence of magnetic field on each flow regime. Eventually, the analysis is carried out for a range of power indices including pseudo plastic (shear-thinning) to dilatants (shear-thickening) fluids. The results show that by increasing the power-index, the vortices begin to form and grow gradually so that in the shear-thickening fluid an extra vortex is formed and created nearby the corrugated part of the channel.

Energy dissipation of moved magnetic vortices

A two-dimensional easy-plane ferromagnetic substrate, interacting with a dipolar tip which is magnetised perpendicularly with respect to the easy plane is studied numerically by solving the Landau-Lifshitz Gilbert equation. The dipolar tip stabilises a vortex structure which is dragged through the system and dissipates energy. An analytical expression for the friction force in the limit based on the Thiele equation is presented. The limitations of this result which predicts a diverging friction force in the thermodynamic limit, are demonstrated by a study of the size dependence of the friction force. While for small system sizes the dissipation depends logarithmically on the system size, it saturates at a specific velocity-dependent value. This size can be regarded as an effective vortex size and it is shown how this effective vortex size agrees with the infinite extension of a vortex in the thermodynamic limit. A magnetic friction number is defined which represents a general criterion for the validity of the Thiele equation and quantifies the degree of nonlinearity in the response of a driven spin configuration.

Wake Vortex Control Using Modified Flaps

The present paper is on the experimental investigation of a potentially effective method to alleviate the strength of aircraft wake vortex. The research work was carried out in a water-towing tank which equipped with a 2-D PIV system. By using modified flaps attached to the baseline-airfoil, the Rayleigh-Ludwieg instability of the wake vortex was activated, resulting in a premature dissipation of the wake vortex. The experimental results of PIV measurement exhibited that the circulation of the wake vortex was reduced to over 50% for the optimal case at 45 wingspans downstream with suitable settings, which proved the effectiveness of the modified-flap treatment in alleviating the wake vortex under labor conditions.

On the Mechanism Underlying the Spontaneous Emergence of Barotropic Zonal Jets

Abstract Zonal jets are commonly observed to spontaneously emerge in a β-plane channel from a background of turbulence that is sustained in a statistical steady state by homogeneous stochastic excitation and dissipation of vorticity. The mechanism for jet formation is examined in this work within the statistical wave–mean flow interaction framework of stochastic structural stability theory (SSST) that makes predictions for the emergence of zonal jets in β-plane turbulence. Using the coupled dynamical SSST system that governs the joint evolution of the second-order statistics and the mean flow, the structural stability of the spatially homogeneous statistical equilibrium with no mean zonal jets is studied. It is shown that close to the structural stability boundary, the eddy–mean flow dynamics can be split into two competing processes. The first, which is shearing of the eddies by the local shear described by Orr dynamics in a β plane, is shown in the limit of infinitesimal shear to lead to the formation of jets. The second, which is momentum flux divergence resulting from lateral wave propagation on the nonuniform local mean vorticity gradient, is shown to oppose jet formation. The upgradient momentum fluxes due to shearing of the eddies are shown to act exactly as negative viscosity for an anisotropic forcing and as negative hyperviscosity for isotropic forcing. The downgradient fluxes due to wave flux divergence are shown to act hyperdiffusively.

PIV Experiment on Near Wake Flow of Horizontal Axis Wind Turbine

A three-dimensional horizontal axis wind turbine model was experimentally studied. The experiment was carried out in a laboratory wind tunnel. With PIV measurement, details about flow fields in the near wakeof the turbine blade were obtained. The result shows vortices generateon the tailing edge of the blade, and propagatedownstream then dissipate into small vortices. Vortices also generate at the tip of the blade, propagate downstream and along the radial direction then dissipate. The dissipation of the tip vortex is slower than the former. We also find that the wake of turbine blade rotates in the opposite direction of the blade.

Reviews of high-order methods on unstructured and hybrid grid

While 2nd order methods are dominant in most compressible flow simulations, many types of problems, such as computational aeroacoustics (CAA), vortex-dominant flows and large eddy simulation (LES) of turbulent flows, call for higher order accuracy (third order and more). The main deficiency of widely available, second-order methods for the accurate simulations of the above-mentioned flows is the numerical diffusion and dissipation of vorticity to unacceptable level. Applications of high-order accu- rate, low-diffusion and low dissipation numerical methods can significantly alleviate this deficiency of the traditional second order methods, improve predictions of vortical and other complex, separated, unsteady flows. On the other hand, for complex configurations, the structured/unstructured hybrid grid technique presents the trend of grid generation technique. Therefore the high-order methods on unstructured and hybrid (or mixed) grids are paid much more attention in recent years. In this paper, the high-order methods on unstructured and hybrid grids are reviewed comprehensively, including the k-exact finite volume (FV) methods, the FV methods based on ENO and WENO reconstruction, the discontinuous Galerkin (DG) finite element method, the finite spectral volume (SV) methods, the finite spectral dif- ference (SD) method, DG/FV hybrid methods, the unified method based on correction procedure via reconstruction (CPR). The main ideas of these high-order methods are introduced, and the advantages and disadvantages of these methods are discussed. In addition, some important issues for simulations of complex geometry are discussed, including the treatment of curved boundary, the detector of discontinu-ity and high-order limiters, implicit iteration methods, and hp-multigrid approaches. We believe that the high-order methods on unstructured and hybrid grids will play more and more important role on more accurate simulations of realistic airspace vehicles and study of complex flow mechanism in the future.

Flow Stucture of Supersonic Underexpanded Jet With Microjets Injection

Experimental and numerical study of transversal microjets injection influence on the supersonic underexpanded jet flow structure has been performed. Data of measurements and calculation have acceptable agreement. Interaction of microjets with main supersonic jet sets to a decrease of an initial gasdynamic region. Microjets lead to a longitudinal streamwise vortices generation and a mushroom-like flow structures create on an external jet mixing layer. Dissipation of longitudinal streamwise vortices was observed at the second jet cell. Complex gasdynamic flow structure of the supersonic underexpanded jet interacting with supersonic microjets has been studied for the first time. This structure contains system of complex chock waves and expansion waves spreading from the position of the impact microjets/main jet localization place. Future of interaction process a chock-wave structure of main jet with additional shock waves has been studied

Effect of vortex hotspots on the radio-frequency surface resistance of superconductors

We present detailed experimental and theoretical investigations of hotspots produced by trapped vortex bundles and their effect on the radio-frequency (rf) surface resistance ${R}_{s}$ of superconductors at low temperatures. Our measurements of ${R}_{s}$, combined with the temperature mapping and laser scanning of a 2.36-mm-thick Nb plate incorporated into a 3.3-GHz Nb resonator cavity cooled by the superfluid He at 2 K, revealed spatial scales and temperature distributions of hotspots and showed that they can be moved or split by thermal gradients produced by the scanning laser beam. These results, along with the observed hysteretic field dependence of ${R}_{s}$ which can be tuned by the scanning laser beam, show that the hotspots in our Nb sample are due to trapped vortex bundles which contain $\ensuremath{\sim}{10}^{6}$ vortices spread over regions $\ensuremath{\sim}0.1--1$ cm. We calculated the frequency dependence of the rf power dissipated by oscillating vortex segments trapped between nanoscale pinning centers, taking into account all bending modes and the nonlocal line tension of the vortex driven by rf Meissner currents. We also calculated the temperature distributions caused by trapped vortex hotspots, and suggested a method of reconstructing the spatial distribution of vortex dissipation sources from the observed temperature maps. Vortex hotspots can dominate the residual surface resistance at low temperatures and give rise to a significant dependence of ${R}_{s}$ on the rf field amplitude ${H}_{p}$, which can have important implications for the rf resonating cavities used in particle accelerators and for thin-film structures used in quantum computing and photon detectors.

몰수형 피치댐핑포일 주위 유동의 PIV 해석

An experimental study is carried out to investigate the near-wake characteristics of a NACA 0018 foil with a flat plate. Two-frame grey-level cross correlation PIV method is used to measure the local flow characteristic around a pitch damping foil to control the vertical motion of high speed crafts in a circulating water channel. The analysis also includes angles of attack 10 and 20 degrees respectively. Reynolds number based on the chord length(C=100mm) of NACA0018 has been applied during the whole experiments. The distance between the foil and the flat plate is D/C=0.5, 1.0 and 1.5 respectively. The channel effect according as the distance between the foil and the flat plate has a close relation with the velocity distributions around the foil. In the wake of 20-degree of attack, the complex turbulent flow and a thick boundary layer are formed due to the processes of vortex generation and dissipation.

Local mapping of dissipative vortex motion

We explore, with unprecedented single vortex resolution, the dissipation and motion of vortices in a superconducting ribbon under the influence of an external alternating magnetic field. This is achieved by combining the phase sensitive character of ac susceptibility, allowing us to distinguish between the inductive and dissipative responses, with the local power of scanning Hall probe microscopy. Whereas the induced reversible screening currents contribute only inductively, the vortices do leave a fingerprint in the out-of-phase component. The observed large phase-lag demonstrates the dissipation of vortices at time scales comparable to the period of the driving force (i.e., 13 ms). These results indicate the presence of slow microscopic loss mechanisms mediated by thermally activated hopping transport of vortices between metastable states.

Microwave surface-impedance measurements of the electronic state and dissipation of magnetic vortices in superconducting LiFeAs single crystals

LiFeAs is one of the iron-based superconductors having multiple gaps with the possible sign reversal. To clarify how those novel natures affect the energy dissipation of magnetic vortices, we investigated the microwave surface impedance of LiFeAs single crystals under finite magnetic fields. The flux-flow resistivity enhanced rapidly at low magnetic fields, which is similar to the case of MgB$_{2}$. This is probably the consequence of the multiple-gap nature and the gap anisotropy. This suggest that the sign-reversal is not important for the flux-flow even for multiple-gap superconductors. As for the electronic state, the vortex core of LiFeAs turned out to be "moderately clean". Furthermore, the mean free path inside the vortex core was much shorter than that outside, and was close to the core radius. These results strongly suggest a process specific to the core boundary is important for a scattering mechanism inside the vortex core.

Eddy-Preserving Limiter for Unsteady Subsonic Flows

A slope-limiting algorithm for second-order monotone upstream-centered schemes for conservation laws is introduced to reduce the dissipation of vortices in flow simulations. The algorithm is based on the reconstruction of velocity components along the principle axes of the vortex and the augmentation of the central gradients’ weight for the interpolation of velocity components on the swirl plane of the vortex. The performance of the scheme in different vortical flow problems is investigated. The proposed limiting algorithm has been able to considerably reduce the dissipation of vortices provided that the spatial and temporal discretization of the problem have been fine enough to resolve the length and time scales of the corresponding vortical motion. In particular, the scheme has significantly outperformedthe conventionalvan Albadalimiter to resolvethesecond peak inthe lift coefficient spectrainthe case of a NACA0021 airfoil at a poststall condition.

Writing Vortex Memory Bits Using Electric Field

Magnetic inhomogeneities in dielectrics can be associated with certain electric polarization distribution since they locally reduce the symmetry of the magnetic crystal. This phenomenon known as spin flexoelectricity provides a way to electrically control micromagnetic structure. Our numerical simulations show that magnetic vortexes can be nucleated and stabilized in a magnetic nanodot by static electric field. The process of nucleation and dissipation of vortices is hysteretic. A material can be found, where both vortex and homogeneous states are stable in zero electric field, which provides a basis for an electrically controlled magnetic memory device.

302 曲がりせん断層での渦の合体における渦度の消失(流体工学I)

302 曲がりせん断層での渦の合体における渦度の消失(流体工学I)

Large-eddy simulation and acoustic analysis of a turbulent flow field in a swirl-stabilized combustor

To conduct a comprehensive study on the flow characteristics and acoustic oscillation in a gas turbine combustor, a 3D large-eddy simulation (LES) was implemented. The formulation consists of the Favre-filtered conservation equations of mass, momentum, and energy. The subgrid-scale dynamics are modeled using a compressible flow version of the Smagorinsky model. To investigate the dominant coherent structure, the proper orthogonal decomposition (POD) method was used for post-processing. The combustor of concern is the LM6000, lean-premixed dry low-NOx annular combustor, developed by General Electric Aircraft Engines (GEAE). Four important characteristics of swirl flow are visualized: vortex breakdown, procession and dissipation of vortical structures, recirculation zones, and helical waves immediately downstream of the swirl injector. It is shown that the turbulent motion of swirl flow directly affects acoustic oscillation through the cycle and spectral analysis. The four most dominant acoustic modes are extracted from the flow field by the POD analysis. The transverse modes in the y and z directions are dominant in all four modes, since the pressure fields are significantly affected by swirl flow.

Energy dissipation mechanisms in wave breaking processes: Spilling and highly aerated plunging breaking events

The breaking of free surface waves is investigated numerically via a Navier‐Stokes model for the two‐fluids flow of air and water. Third order Stokes waves in a periodic domain are simulated. The fundamental wavelength is 27 cm, whereas the initial steepness varies from low values, leading to regular wave trains, up to artificially steep wave trains yielding plunging breaking events. Attention is focused on the early stage of the breaking, when most of the energy is dissipated. The energy content in air, the fraction associated to surface tension effects, the viscous dissipation in water, and the work done against the pressure field are analyzed in order to distinguish the different contributions to the dissipation. Vorticity fields and dissipation contours are also presented. In the spilling case, the extra energy content with respect to the steepest nonbreaking case focusses into the breaking region and is gradually dissipated. Once the extra energy has been dissipated, the resulting wave matches the steepest nonbreaking solution. In the plunging case, an important role is played by the air entrainment. A fraction between 10 to 35% of the energy dissipated by the breaking is spent in entraining the air cavity, and most of it is dissipated by viscous effects when the cavity collapses. The phenomenon is clearly highlighted by sequences of vorticity and dissipation contours. The circulation and the area of the cavity generated by the plunging of the jet are provided, and parametric dependencies are proposed.