Experimental study of the turbulent field behind a perforated vortex generator

International audience

Large eddy simulation of shock boundary layer interaction control using micro-vortex generators

The performance of supersonic engine inlets and external aerodynamic surfaces can be critically affected by shock wave / boundary layer interactions (SBLIs), whose severe adverse pressure gradients can cause boundary layer separation. Currently such problems are avoided primarily through the use of boundary layer bleed/suction which can be a source of significant performance degradation. This study investigates a novel type of flow control device called micro-vortex generators (µVGs) which may offer similar control benefits without the bleed penalties. µVGs have the ability to alter the near-wall structure of compressible turbulent boundary layers to provide increased mixing of high speed fluid which improves the boundary layer health when subjected to flow disturbance. Due to their small size,µVGs are embedded in the boundary layer which provide reduced drag compared to the traditional vortex generators while they are cost-effective, physically robust and do not require a power source. To examine the potential of µVGs, a detailed experimental and computational study of micro-ramps in a supersonic boundary layer at Mach 3 subjected to an oblique shock was undertaken. The experiments employed a flat plate boundary layer with an impinging oblique shock with downstream total pressure measurements. The moderate Reynolds number of 3,800 based on displacement thickness allowed the computations to use Large Eddy Simulations without the subgrid stress model (LES-nSGS). The LES predictions indicated that the shock changes the structure of the turbulent eddies and the primary vortices generated from the micro-ramp. Furthermore, they generally reproduced the experimentally obtained mean velocity profiles, unlike similarly-resolved RANS computations. The experiments and the LES results indicate that the micro-ramps, whose height is h≈0.5δ, can significantly reduce boundary layer thickness and improve downstream boundary layer health as measured by the incompressible shape factor, H. Regions directly behind the ramp centerline tended to have increased boundary layer thickness indicating the significant three-dimensionality of the flow field. Compared to baseline sizes, smaller micro-ramps yielded improved total pressure recovery. Moving the smaller ramps closer to the shock interaction also reduced the displacement thickness and the separated area. This effect is attributed to decreased wave drag and the closer proximity of the vortex pairs to the wall. In the second part of the study, various types of µVGs are investigated including micro-ramps and micro-vanes. The results showed that vortices generated from µVGs can partially eliminate shock induced flow separation and can continue to entrain high momentum flux for boundary layer recovery downstream. The micro-ramps resulted in thinner downstream displacement thickness in comparison to the micro-vanes. However, the strength of the streamwise vorticity for the micro-ramps decayed faster due to dissipation especially after the shock interaction. In addition, the close spanwise distance between each vortex for the ramp geometry causes the vortex cores to move upwards from the wall due to induced upwash effects. Micro-vanes, on the other hand, yielded an increased spanwise spacing of the streamwise vortices at the point of formation. This resulted in streamwise vortices staying closer to the wall with less circulation decay, and the reduction in overall flow separation is attributed to these effects. Two hybrid concepts, named “thick-vane” and “split-ramp”, were also studied where the former is a vane with side supports and the latter has a uniform spacing along the centerline of the baseline ramp. These geometries behaved similar to the micro-vanes in terms of the streamwise vorticity and the ability to reduce flow separation, but are more physically robust than the thin vanes. Next, Mach number effect on flow past the micro-ramps (h~0.5δ) are examined in a supersonic boundary layer at M=1.4, 2.2 and 3.0, but with no shock waves present. The LES results indicate that micro-ramps have a greater impact at lower Mach number near the device but its influence decays faster than that for the higher Mach number cases. This may be due to the additional dissipation caused by the primary vortices with smaller effective diameter at the lower Mach number such that their coherency is easily lost causing the streamwise vorticity and the turbulent kinetic energy to decay quickly. The normal distance between the vortex core and the wall had similar growth indicating weak correlation with the Mach number; however, the spanwise distance between the two counter-rotating cores further increases with lower Mach number. Finally, various µVGs which include micro-ramp, split-ramp and a new hybrid concept “ramped-vane” are investigated under normal shock conditions at Mach number of 1.3. In particular, the ramped-vane was studied extensively by varying its size, interior spacing of the device and streamwise position respect to the shock. The ramped-vane provided increased vorticity compared to the micro-ramp and the split-ramp. This significantly reduced the separation length downstream of the device centerline where a larger ramped-vane with increased trailing edge gap yielded a fully attached flow at the centerline of separation region. The results from coarse-resolution LES studies show that the larger ramped-vane provided the most reductions in the turbulent kinetic energy and pressure fluctuation compared to other devices downstream of the shock. Additional benefits include negligible drag while the reductions in displacement thickness and shape factor were seen compared to other devices. Increased wall shear stress and pressure recovery were found with the larger ramped-vane in the baseline resolution LES studies which also gave decreased amplitudes of the pressure fluctuations downstream of the shock.

Techniques for Enhancing Wind Energy Generation - A CFD Based Multibody Dynamics Approach in Horizontal Axis Wind Turbines

Wind energy has emerged as a major sustainable source of energy.The efficiency of wind power generation by wind mills has improved a lot during the last three decades.There is still further scope for maximising the conversion of wind energy into mechanical energy.In this context,the wind turbine rotor dynamics has great significance.The present work aims at a comprehensive study of the Horizontal Axis Wind Turbine (HAWT) aerodynamics by numerically solving the fluid dynamic equations with the help of a finite-volume Navier-Stokes CFD solver.As a more general goal,the study aims at providing the capabilities of modern numerical techniques for the complex fluid dynamic problems of HAWT.The main purpose is hence to maximize the physics of power extraction by wind turbines.This research demonstrates the potential of an incompressible Navier-Stokes CFD method for the aerodynamic power performance analysis of horizontal axis wind turbine.The National Renewable Energy Laboratory USA-NREL (Technical Report NREL/Cp-500-28589) had carried out an experimental work aimed at the real time performance prediction of horizontal axis wind turbine.In addition to a comparison between the results reported by NREL made and CFD simulations,comparisons are made for the local flow angle at several stations ahead of the wind turbine blades.The comparison has shown that fairly good predictions can be made for pressure distribution and torque.Subsequently, the wind-field effects on the blade aerodynamics,as well as the blade/tower interaction,were investigated.The selected case corresponded to a 12.5 m/s up-wind HAWT at zero degree of yaw angle and a rotational speed of 25 rpm.The results obtained suggest that the present can cope well with the flows encountered around wind turbines.The areodynamic performance of the turbine and the flow details near and off the turbine blades and tower can be analysed using theses results.The aerodynamic performance of airfoils differs from one another.The performance mainly depends on co-efficient of performnace,co-efficient of lift,co-efficient of drag, velocity of fluid and angle of attack.This study shows that the velocity is not constant for all angles of attack of different airfoils.The performance parameters are calculated analytically and are compared with the standardized performance tests.For different angles of ,the velocity stall is determined for the better performance of a system with respect to velocity.The research addresses the effect of surface roughness factor on the blade surface at various sections.The numerical results were found to be in agreement with the experimental data.A relative advantage of the theoretical aerofoil design method is that it allows many different concepts to be explored economically.Such efforts are generally impractical in wind tunnels because of time and money constraints.Thus, the need for a theoretical aerofoil design method is threefold:first for the design of aerofoil that fall outside the range of applicability of existing calalogs:second,for the design of aerofoil that more exactly match the requirements of the intended application:and third,for the economic exploration of many aerofoil concepts.From the results obtained for the different aerofoils,the velocity is not constant for all angles of attack.The results obtained for the aerofoil mainly depend on angle of attack and velocity.The vortex generator technique was meticulously studies with the formulation of the specification for the right angle shaped vortex generators-VG.The results were validated in accordance with the primary analysis phase.The results were found to be in good agreement with the power curve.The introduction of correct size VGs at appropriate locations over the blades of the selected HAWT was found to increase the power generation by about 4%

Studio e ottimizzazione della ipersostentazione del velivolo V-173

Studio della ipersostentazione di un velivolo non convenzionale degli anni ‘40, il Vought V-173 Flying Pancake. Per realizzare l’intero lavoro si sono adoperati dei software di fluidodinamica computazionale come Flow Simulation di SolidWorks e il programma JavaFoil, inoltre, si è scelto l’ambiente Matlab di supporto ai calcoli e per rappresentare i risultati ottenuti a seguito delle simulazioni, realizzando script e funzioni per l’approssimazione di questi e la loro successiva raffigurazione grafica. In particolar modo, a partire dal modello tridimensionale in SolidWorks del V-173 si sono ricreate le curve CL-α e CD-α a partire dai punti ottenuti dalle simulazioni per diverse configurazioni del velivolo. In una prima fase si è valutata l’aerodinamica del velivolo ‘pulito’ senza la presenza delle eliche e di ipersostentatori, successivamente si sono seguite due strade diverse per valutare il comportamento del velivolo: nel primo caso si sono eseguiti studi dell’aerodinamica del Pancake in presenza degli ipersostentatori già presenti sul velivolo (i plain flap), proponendo soluzioni alternative per migliorare l’ipersostentazione del V-173 tramite spillamenti (slots) e diverse configurazioni di vortex generator che energizzassero lo strato limite e ottimizzassero le prestazioni del velivolo con particolare attenzione alla fase di atterraggio. In secondo luogo, tenendo in considerazione che il Pancake è un aeroplano bielica, si è voluta studiare l’influenza delle due eliche sulla sua aerodinamica: dopo aver riprodotto nel modo più verosimile entrambe le eliche utilizzando SolidWorks si è fatto uno studio di massima ricavando risultati che potessero indicare la compatibilità tra elica e velivolo a seguito dei risultati sperimentali ottenuti con Flow Simulation.

Some (3+1)-dimensional vortex solutions of the CP(N) model

We present a class of solutions of the CP(N) model in (3 + 1) dimensions. We suggest that they represent vortexlike configurations. We also discuss some of their properties. We show that some configurations of vortices have a divergent energy per unit length while for the others such an energy has a minimum for a very special orientation of vortices. We also discuss the Noether charge densities of these vortices.

Three-vortex configurations in trapped Bose-Einstein condensates

We report on the creation of three-vortex clusters in a (87)Rb Bose-Einstein condensate by oscillatory excitation of the condensate. This procedure can create vortices of both circulations, so that we are able to create several types of vortex clusters using the same mechanism. The three-vortex configurations are dominated by two types, namely, an equilateral-triangle arrangement and a linear arrangement. We interpret these most stable configurations respectively as three vortices with the same circulation and as a vortex-antivortex-vortex cluster. The linear configurations are very likely experimental signatures of predicted stationary vortex clusters.

Excitation energies from ground-state density-functionals by means of generator coordinates

The generator-coordinate method is a flexible and powerful reformulation of the variational principle. Here we show that by introducing a generator coordinate in the Kohn-Sham equation of density-functional theory, excitation energies can be obtained from ground-state density functionals. As a viability test, the method is applied to ground-state energies and various types of excited-state energies of atoms and ions from the He and the Li isoelectronic series. Results are compared to a variety of alternative DFT-based approaches to excited states, in particular time-dependent density-functional theory with exact and approximate potentials.

Power imbalance application region method for distributed synchronous generator anti-islanding protection design and evaluation

This paper presents a novel graphical approach to adjust and evaluate frequency-based relays employed in anti-islanding protection schemes of distributed synchronous generators, in order to meet the anti-islanding and abnormal frequency variation requirements, simultaneously. The proposed method defines a region in the power mismatch space, inside which the relay non-detection zone should be located, if the above-mentioned requirements must be met. Such region is called power imbalance application region. Results show that this method can help protection engineers to adjust frequency-based relays to improve the anti-islanding capability and to minimize false operation occurrences, keeping the abnormal frequency variation utility requirements satisfied. Moreover, the proposed method can be employed to coordinate different types of frequency-based relays, aiming at improving overall performance of the distributed generator frequency protection scheme. (C) 2011 Elsevier B.V. All rights reserved.

Piezoaeroelastic Modeling and Analysis of a Generator Wing with Continuous and Segmented Electrodes

Unmanned air vehicles (UAVs) and micro air vehicles (MAVs) constitute unique application platforms for vibration-based energy harvesting. Generating usable electrical energy during their mission has the important practical value of providing an additional energy source to run small electronic components. Electrical energy can be harvested from aeroelastic vibrations of lifting surfaces of UAVs and MAVs as they tend to have relatively flexible wings compared to their larger counterparts. In this work, an electromechanically coupled finite element model is combined with an unsteady aerodynamic model to develop a piezoaeroelastic model for airflow excitation of cantilevered plates representing wing-like structures. The electrical power output and the displacement of the wing tip are investigated for several airflow speeds and two different electrode configurations (continuous and segmented). Cancelation of electrical output occurs for typical coupled bending-torsion aeroelastic modes of a cantilevered generator wing when continuous electrodes are used. Torsional motions of the coupled modes become relatively significant when segmented electrodes are used, improving the broadband performance and altering the flutter speed. Although the focus is placed on the electrical power that can be harvested for a given airflow speed, shunt damping effect of piezoelectric power generation is also investigated for both electrode configurations.

Digital control and integration of a 192 MW wind farm with doubly fed induction generator into the Brazilian power system

This paper reports on design of digital control for wind turbines and its relation to the quality of power fed into the Brazilian grid on connecting to it a 192 MW wind farm equipped with doubly fed induction generators. PWM converters are deployed as vector controlled regulated current voltage sources for their rotors, for independent control of both active and reactive power of those generators. Both speed control and active power control strategies are analyzed, in the search for maximum efficiency of conversion of wind kinetic energy into electric power and enhanced quality of delivered power. (C) 2009 Elsevier B.V. All rights reserved.

On the wake-induced vibration of tandem circular cylinders: the vortex interaction excitation mechanism

The mechanism of wake-induced vibrations (WIV) of a pair of cylinders in a tandem arrangement is investigated by experiments. A typical WIV response is characterized by a build-up of amplitude persisting to high reduced velocities; this is different from a typical vortex-induced vibration (VIV) response, which occurs in a limited resonance range. We suggest that WIV of the downstream cylinder is excited by the unsteady vortex-structure interactions between the body and the upstream wake. Coherent vortices interfering with the downstream cylinder induce fluctuations in the fluid force that are not synchronized with the motion. A favourable phase lag between the displacement and the fluid force guarantees that a positive energy transfer from the flow to the structure sustains the oscillations. If the unsteady vortices are removed from the wake of the upstream body then WIV will not be excited. An experiment performed in a steady shear flow turned out to be central to the understanding of the origin of the fluid forces acting on the downstream cylinder.

3D CFD Simulation of Vortex-induced Vibration of Cylinder

The 3D flow around a circular cylinder free to oscillate transversely to the free stream was simulated using Computational Fluid Dynamics (CFD) and the Spalart-Allmaras Detached Eddy Simulation (DES) turbulence model for a Reynolds number Re = 10(4). Simulations were carried out for a small mass-damping parameter m*zeta = 0.00858, where m* = 3.3 and zeta = 0.0026. We found good agreement between the numerical results and experimental data. The simulations predicted the high observed amplitudes of the upper branch of vortex-induced vibrations for low mass-damping parameters.

An air-standard cycle and a thermodynamic perspective on operational limits of Ranque-Hilsh or vortex tubes

A Thermodynamic air-standard cycle was envisaged for Ranque-Hilsh (R-H) or Vortex Tubes to provide relevant Thermodynamic analysis and tools for setting operating limits according to the conservation laws of mass and energy, as well as the constraint of the Second Law of Thermodynamics. The study used an integral or control volume approach and resulted in establishing working equations for evaluating the performance of an R-H tube. The work proved that the coefficient of performance does not depend on the R-H tube operating mode, i.e., the same value is obtained independently if the R-H tube operates either as a heat pump or as a refrigeration device. It was also shown that the isentropic coefficient of performance displays optima values of cold and hot mass fractions for a given operating pressure ratio. Finally, the study was concluded by comparing the present analysis with some experimental data available in the literature for operating pressures ranging 2-11 atm. (C) 2010 Elsevier Ltd and IIR. All rights reserved.

Experimental investigation of Vortex-Induced Vibration on rigid, smooth and inclined cylinders

This paper presents new experimental results of Vortex-Induced Vibration (VIV) on inclined cylinders. Models are mounted on a low damping air-bearing elastic base with one degree-of-freedom, constrained to oscillate only in the transverse direction to a free stream. The Reynolds number varied in the range 2000 less than or similar to Re less than or similar to 8000. New measurements on the dynamic response oscillations of inclined cylinders, due to VIV, are compared with previous experiments of a vertical cylinder. Models with circular and elliptical cross sections have been tested. The purpose of this work is to check the validity of the normal velocity correction of VIV studies of inclined structures. The results show that the reduced velocity range, in which the upper and lower branches of VIV occurs, is similar to the vertical cylinder case if the proper projected velocity is considered. Tests have been conducted to support this observation with inclinations up to 45 degrees. We have also observed that the amplitudes of oscillation of the inclined circular cylinder are comparable, but slightly lower than, to the amplitudes observed in the vertical cylinder experiments. Measured forces and added mass also show similar behaviour. However, for cases with an elliptical cylinder, the amplitudes of oscillation are considerably lower than those observed for a circular cylinder. This difference is explained by the higher added mass of the elliptical cylinder. (C) 2009 Elsevier Ltd. All rights reserved.

Mitigation of Vortex-Induced Motion, (VIM) on a Monocolumn Platform: Forces and Movements

A great deal of works has been developed on the spar vortex-induced motion (VIM) issue. There are, however, very few published works concerning VIM of monocolumn platforms, partly due to the fact that the concept is fairly recent and the first unit was only installed last year. In this context, a meticulous study on VIM for this type of platform concept is presented here. Model test experiments were performed to check the influence of many factors on VIM, such as different headings, wave/current coexistence, different drafts, suppression elements, and the presence of risers. The results of the experiments presented here are motion amplitudes in both in-line and transverse directions, forces and added-mass coefficients, ratios of actual oscillation and natural periods, and motions in the XY plane. This is, therefore, a very extensive and important data set for comparisons and validations of theoretical and numerical models for VIM prediction. [DOI: 10.1115/1.4001440]