Showing papers on "Starting vortex published in 2016"

New omega vortex identification method

Abstract: A new vortex identification criterion called W -method is proposed based on the ideas that vorticity overtakes deformation in vortex. The comparison with other vortex identification methods like Q -criterion and l 2-method is conducted and the advantages of the new method can be summarized as follows: (1) the method is able to capture vortex well and very easy to perform; (2) the physical meaning of W is clear while the interpretations of iso-surface values of Q and l 2 chosen to visualize vortices are obscure; (3) being different from Q and l 2 iso-surface visualization which requires wildly various thresholds to capture the vortex structure properly, W is pretty universal and does not need much adjustment in different cases and the iso-surfaces of W =0.52 can always capture the vortices properly in all the cases at different time steps, which we investigated; (4) both strong and weak vortices can be captured well simultaneously while improper Q and l 2 threshold may lead to strong vortex capture while weak vortices are lost or weak vortices are captured but strong vortices are smeared; (5) W =0.52 is a quantity to approximately define the vortex boundary. Note that, to calculate W , the length and velocity must be used in the non-dimensional form. From our direct numerical simulation, it is found that the vorticity direction is very different from the vortex rotation direction in general 3-D vortical flow, the Helmholtz velocity decomposition is reviewed and vorticity is proposed to be further decomposed to vortical vorticity and non-vortical vorticity.

Three-dimensional direct numerical simulation of wake transitions of a circular cylinder

Abstract: This paper presents three-dimensional (3D) direct numerical simulations (DNS) of flow past a circular cylinder over a range of Reynolds number ( ) up to 300. The gradual wake transition process from mode A* (i.e. mode A with large-scale vortex dislocations) to mode B is well captured over a range of from 230 to 260. The mode swapping process is investigated in detail with the aid of numerical flow visualization. It is found that the mode B structures in the transition process are developed based on the streamwise vortices of mode A or A* which destabilize the braid shear layer region. For each case within the transition range, the transient mode swapping process consists of dislocation and non-dislocation cycles. With the increase of , it becomes more difficult to trigger dislocations from the pure mode A structure and form a dislocation cycle, and each dislocation stage becomes shorter in duration, resulting in a continuous decrease in the probability of occurrence of mode A* and a continuous increase in the probability of occurrence of mode B. The occurrence of mode A* results in a relatively strong flow three-dimensionality. A critical condition is confirmed at approximately , where the weakest flow three-dimensionality is observed, marking a transition from the disappearance of mode A* to the emergence of increasingly disordered mode B structures.

Characterizing a burst leading-edge vortex on a rotating flat plate wing

Abstract: Identifying, characterizing, and tracking incoherent vortices in highly separated flows is of interest for the development of new low-order models for unsteady lift prediction. The current work examines several methods to identify vortex burst and characterize a burst leading-edge vortex. Time-resolved stereoscopic PIV was performed on a rotating flat plate wing at Re = 2500. The burst process was found to occur at mid-span and is characterized by axial flow reversal, the entrainment of opposite-sign vorticity, and a rapid expansion of vortex size. A POD analysis revealed that variations in certain mode coefficients are indicative of the flow state changes characteristics of burst. During burst, the leading-edge vortex evolves to a region of inhomogeneous vorticity distributed over a large area. Several methods of defining the vortex size and circulation are evaluated and a combination of these can be used to characterize the leading-edge vortex both pre- and post-burst.

Design optimization of the aerodynamic passive flow control on NACA 4415 airfoil using vortex generators

Abstract: The present paper provides an experimental optimization of a NACA 4415 airfoil equipped with vortex generators (VGs) to control its flow separation. To build this optimal configuration an experimental parametric study was conducted on five geometrical parameters: thickness and height of vortex generators, position, orientation angle with respect to the mean flow direction, spacing in the spanwise direction. Moreover, a new configuration that includes micro generators behind the conventional ones was also investigated as a potentially interesting solution. For all these cases wind tunnel tests were performed and compared for different angles of attack and various Reynolds numbers up to 2 10 5 . These experiments enabled us to highlight the main trends to get an optimal design, for which quantitative improvement can be achieved by passive means in terms of aerodynamic performances on NACA4415 airfoil. The results reveal that triangular shape vortex generators are best suited to control boundary layer separation. An optimum angle of VGs is obtained for 12°with a 3 mm distance between vortex generators located at 50% of the chord. It was found that micro vortex generators are very effective in controlling the flow with less parasite drag. The maximum lift coefficient for an airfoil with coupled vortex generators increases by 21% and a flow separation is delayed by 17°. However, this very good performance is counterbalanced by the appearance of parasitic drag. Indeed, it creates a counter-rotating array of vortices with the second raw of micro-vortex generators that reinforce the vortexes strength without any increase in device height.

Vortex formation and shedding from a cyber-physical pitching plate

Abstract: We report on the dynamics of the formation and growth of the leading-edge vortex and the corresponding unsteady aerodynamic torque induced by large-scale flow-induced oscillations of an elastically mounted flat plate. All experiments are performed using a high-bandwidth cyber-physical system, which enables the user to access a wide range of structural dynamics using a feedback control system. A series of two-dimensional particle image velocimetry measurements are carried out to characterize the behaviour of the separated flow structures and its relation to the plate kinematics and unsteady aerodynamic torque generation. By modulating the structural properties of the cyber-physical system, we systematically analyse the formation, strength and separation of the leading-edge vortex, and the dependence on kinematic parameters. We demonstrate that the leading-edge vortex growth and strength scale with the characteristic feeding shear-layer velocity and that a potential flow model using the measured vortex circulation and position can, when coupled with the steady moment of the flat plate, accurately predict the net aerodynamic torque on the plate. Connections to previous results on optimal vortex formation time are also discussed.

Aerodynamically shaped vortex generators

Abstract: An aerodynamically shaped vortex generator has been proposed, manufactured and tested in a wind tunnel. The effect on the overall performance when applied on a thick airfoil is an increased lift to drag ratio compared with standard vortex generators. Copyright © 2015 John Wiley & Sons, Ltd.

Coriolis Effect on Dynamic Stall in a Vertical Axis Wind Turbine

Abstract: The immersed boundary method is used to simulate the flow around a two-dimensional cross section of a rotating NACA 0018 airfoil in order to investigate the dynamic stall occurring on a vertical axis wind turbine. The influence of dynamic stall on the force is characterized as a function of tip-speed ratio and Rossby number. The influence of the Coriolis effect is isolated by comparing the rotating airfoil to one undergoing an equivalent planar motion that is composed of surging and pitching motions that produce an equivalent speed and angle-of-attack variation over the cycle. Planar motions consisting of sinusoidally varying pitch and surge are also examined. At lower tip-speed ratios, the Coriolis force leads to the capture of a vortex pair when the angle of attack of a rotating airfoil begins to decrease in the upwind half cycle. This wake-capturing phenomenon leads to a significant decrease in lift during the downstroke phase. The appearance of this feature depends subtly on the tip-speed ratio. On the one hand, it is strengthened due to the intensifying Coriolis force, but on the other hand, it is attenuated because of the comitant decrease in angle of attack. While the present results are restricted to two-dimensional flow at low Reynolds numbers, they compare favorably with experimental observations at much higher Reynolds numbers. Moreover, the wake-capturing is observed only when the combination of surging, pitching, and Coriolis force is present.

On vortex evolution in the wake of axisymmetric and non-axisymmetric low-aspect-ratio accelerating plates

Abstract: Impulsively started, low-aspect-ratio elliptical and rectangular flat plates were investigated to determine the role of geometric asymmetries on vortex evolution. Dye visualizations, force measurements, and particle image velocimetry were used throughout to characterize the variation between shapes. For all the shapes studied, aspect ratio was observed to have the largest influence on force production and vortex evolution. Non-uniform curvature and edge discontinuities characteristic of ellipses (with aspect ratios other than one) and rectangles, respectively, play a secondary role. Furthermore, it was shown that stably attached vortex rings form behind the circular and square flat plates, which reduce the instantaneous drag force of each plate until the vortex rings break down. In contrast, all flat plates with aspect ratios other than one are subjected to fast-modulating elliptical vortex rings in the wake. These vortex rings increase the drag force of each plate until pinch-off occurs. Finally, pinch-off was identified with the streamwise pressure-gradient field and compared with formation numbers calculated using the circulation-based methodology, yielding good agreement for all plates with aspect ratios greater than one.

Single-Particle Motion and Vortex Stretching in Three-Dimensional Turbulent Flows.

Abstract: Three-dimensional turbulent flows are characterized by a flux of energy from large to small scales, which breaks the time reversal symmetry. The motion of tracer particles, which tend to lose energy faster than they gain it, is also irreversible. Here, we connect the time irreversibility in the motion of single tracers with vortex stretching and thus with the generation of the smallest scales.

A vortex force study for a flat plate at high angle of attack

Abstract: The vortex force is studied for a flat plate at arbitrarily large angle of attack. A suitable vortex force approach, adapted from a previous work, is used to study the vortex force and to build a vortex force line map to identify the force effect of any potential vortex. This map can be used exactly for a potential point vortex and approximately for a concentrated leading-edge vortex (LEV) or trailing-edge vortex (TEV); the latter are shown to have a non-potential vortex core. By means of this map, we identify a force-producing critical region, due to pressure suction, above the front and rear parts of the plate for an LEV and a TEV, respectively. The impulsively started flow problem is used as an application, with validation by computational fluid dynamics. The force variation in time is decomposed into four repeatable stages (force release, force enhancement, stall and force recovery) in close relation to the individual and combined effect by an LEV and a TEV. A pressure distribution analysis shows that force enhancement is due to pressure suction by an LEV, while stall and force recovery are respectively due to the upwash effect (which reduces the pressure below the plate) of a new TEV right off the plate and the pressure suction of this TEV having now moved above the plate. A viscous effect causes a small-amplitude oscillation on the force curves by promoting multiple small-scale LEVs.

Leading-edge vortex burst on a low-aspect-ratio rotating flat plate

Abstract: Leading-edge vortex burst on a rotating flat plate wing was investigated experimentally using stereoscopic particle image velocimetry. Vortex burst was found to occur near the point of maximum lift production and was characterized by a concentration of opposite-sign vorticity entrainment and axial flow stagnation within the vortex core.

Enhanced wake vortex decay in ground proximity triggered by plate lines

Abstract: Purpose From pilot reports, field measurements and numerical simulations, it is known that wake vortices may persist within the glide path in ground proximity, leading to an increased encounter risk. This paper aims to investigate wake vortex behaviour during final approach and landing to understand why landings can be safe nevertheless. Further, it is investigated whether and to which extent the installation of plate lines beyond the runway tails may further accelerate wake vortex decay and thus improve safety by reducing the number of wake vortex encounters. Design/methodology/approach A hybrid numerical simulation approach is used to investigate vortex evolution from roll-up until final decay during the landing manoeuvre. The simulations are complemented by field measurement data accomplished at Munich Airport and at Special Airport Oberpfaffenhofen. Findings During touchdown, the so-called end effects trigger pressure disturbances and helical vortex structures that appear to ensure vortex decay rates in ground proximity needed to guarantee the required safety targets of aviation. Light detection and ranging (LIDAR) measurements indicate that vortex decay indeed can be accelerated by a plate line installed on the ground surface. The lifetime of the most safety relevant, long-lived and strongest vortices can be reduced by one-third. Practical implications The installation of plate lines beyond the runway tails may improve safety by reducing the number of wake vortex encounters and increase the efficiency of wake vortex advisory systems. Originality/value The novel numerical simulation technique and the acquired insights into the wake vortex phenomena occurring during landing as well as the demonstration of the functionality of the patented plate line provide high originality and value for both science and operational application.

Vortex reconnection in a swirling flow

Abstract: Processes of vortex reconnection on a helical vortex, which is formed in a swirling flow in a conical diffuser, have been studied experimentally. It has been shown that reconnection can result in the formation of both an isolated vortex ring and a vortex ring linked with the main helical vortex. A number of features of vortex reconnection, including the effects of asymmetry, generation of Kelvin waves, and formation of various bridges, have been described.

On the wake pattern of symmetric airfoils for different incidence angles at Re = 1000:

Abstract: In the current study, numerical simulations are performed in order to investigate effects of incidence angle and airfoil thickness on alternating vortex pattern of symmetric airfoils at Re = 1000. ...

CFD Simulations of Pressure Drop and Velocity Field in a Cyclone Separator with Central Vortex Stabilization Rod

Abstract: A problem of cyclone separators is the low grade efficiency of small particles. Therefore, a high efficiency cyclone separator has been developed and successfully tested in former work. In this cyclone separator, a vortex stabilizer is used to suppress the vortex core precession. In this article, the pressure and flow field in this cyclone separator are calculated by means of computational fluid dynamics using the commercial software Ansys Fluent 13. The position of the vortex core is tracked in these simulations by searching the position of minimal dynamic pressure and the centre of moment of the horizontal velocity components as function of the axial coordinate. The results are compared with experimental data. It is demonstrated that when using a stabilizer, the vortex is kept in position. Furthermore the maximum of the tangential velocity is found to be larger, which is known to have a positive effect on the separation of small particles in the inner solid body rotation vortex.

Three-Dimensional CFD Simulation of Fluid Flow inside a Vortex Tube on Basis of an Experimental Model- The Optimization of Vortex Chamber Radius

Abstract: Vortex-chamber is a main part of vortex tube which the pressured gas is injected into this part tangentially. An appropriate design of vortex-chamber geometry leads to better efficiency and good vortex tube performance. In this study, the computational fluid dynamics (CFD) model is created on basis of an experimental model and is a three-dimensional (3D) steady compressible model that utilizes the k-e turbulent model. In this paper the effect of changing radius of vortex-chamber (R*) on vortex tube performance has been studied for different value of R* and the optimized radius of vortex-chamber has been determined. According to numerical results the cold temperature difference has increased when we take into account the effect of the radius of vortex-chamber in range of 5.7-11 mm and when the radius of vortex chamber has located in range of 11-13 mm, the cold temperature difference has decreased. The highest ΔTc is 46.77 K for R*=11 mm at a cold mass fraction of 0.3, higher than basic model around 6.3% at the same cold flow fraction. Finally, the results obtained, particularly the temperature values, are compared with some available experimental data, which show good agreement.

Evolution of supersonic corner vortex in a hypersonic inlet/isolator model

Abstract: There are complex corner vortex flows in a rectangular hypersonic inlet/isolator. The corner vortex propagates downstream and interacts with the shocks and expansion waves in the isolator repeatedly. The supersonic corner vortex in a generic hypersonic inlet/isolator model is theoretically and numerically analyzed at a freestream Mach number of 4.92. The cross-flow topology of the corner vortex flow is found to obey Zhang’s theory [“Analytical analysis of subsonic and supersonic vortex formation,” Acta Aerodyn. Sin. 13, 259–264 (1995)] strictly, except for the short process with the vortex core situated in a subsonic flow which is surrounded by a supersonic flow. In general, the evolution history of the corner vortex under the influence of the background waves in the hypersonic inlet/isolator model can be classified into two types, namely, from the adverse pressure gradient region to the favorable pressure gradient region and the reversed one. For type 1, the corner vortex is a one-celled vortex with the ...

Effect of Vortex Generators on Transonic Swept Wings

Abstract: This paper examines the effects of corotating blade-type vortex generators on transonic sweptback wings using computational fluid dynamics studies. Infinite-span (two-dimensional) swept wings are first considered to understand the basic physics of vortex generators. Sweep angles are given virtually to the wings by changing the freestream direction. Vortex generators are placed on the wings, and the visualized interactions of their tip vortices with the boundary layer reveal the relationship between the effect of the vortex generators and the wing sweep angle. The physics of vortex-generator tip vortices are then described to explain how vortex generators on swept wings efficiently suppress shock-induced separation by mixing boundary layers. It is also shown that the vortex-generator angle of incidence to the local flow can slightly improve the effect of the vortex generators but that wing sweep angle has a greater influence on their effect. Based on the discussion of infinite-span wings, the computational...

Spinning Effects on a Trapped Vortex Combustor

Abstract: The trapped vortex combustor concept provides a simple design for flame stabilization by trapping a pilot flame inside a cavity instead of exposing it to the mainstream. Under some circumstances, the combustor may operate in a high-spinning motion (for example, when it is installed in a spin-stabilized ramjet projectile), in which the spinning rate can be as high as 30,000 rpm. The objective of this study is to numerically investigate the effects of high-spinning motion on the trapped vortex combustor, including the cavity vortex dynamics, fuel–air mixing, and combustion performance. Numerical computations have been performed with the Reynolds stress model for turbulence and the eddy dissipation model for combustion in a rotating reference frame. The results of the spinning trapped vortex combustor show that the Coriolis effects dominate the flow in the cavity when it is subjected to a high-spinning motion (30,000 rpm). The vortex breakdown in the cavity brings strong three-dimensional flow and promotes f...

Cylindrical vortex wake model: skewed cylinder, application to yawed or tilted rotors

Abstract: A vortex system consisting of a bound vortex disk, a root vortex and a vortex cylinder is presented and applied for skewed wake situations Both the longitudinal and tangential components of vorticity of the cylinder are considered A subset of this system leads to a model, which is commonly used in Blade Element Momentum method codes for yawed conditions Here, all the components of the full vortex system are analyzed in view of extending Blade Element Momentum models The main assumptions of the current study are a constant uniform circulation, an infinite number of blades, an un-expanding wake shape and a finite tip-speed ratio The investigation remains within the context of inviscid potential flow theory The model is derived for horizontal-axis rotors in general, but results are presented for wind-turbine applications For each vortex element, the velocity components in all directions are computed analytically or semi-analytically for the entire domain Simplified engineering models are provided to ease the evaluation of velocities in the rotor plane The predominant velocity components are assessed Copyright © 2015 John Wiley & Sons, Ltd

Pressure evolution in the shear layer of forming vortex rings

Abstract: This study investigated the relationship between the pinch-off of axisymmetric vortex rings and the evolution of pressure in the shear layer being entrained into the vortex rings. A piston-cylinder apparatus was used to generate the vortex rings, and five cases of constant piston acceleration over distances ranging from zero (impulsive start) to eight piston diameters were investigated. It was determined that increasing the distance over which the piston accelerated increased the dimensionless formation time at which the vortex ring pinches off, consistent with previous observations. A limiting value of vortex ring formation number of approximately seven is approached when the piston is accelerated over more than six piston diameters. For each case, the evolution of pressure in the shear layer was calculated based on PIV measurements of the velocity field and spatial integration of the corresponding pressure gradients using a recently developed algorithm. Plots of the shear layer pressure in X-T diagrams aided in identifying key features of the pressure associated with the evolution of vortex rings, including a high-pressure region that forms behind the leading ring. By extrapolating the motion of this high-pressure region back to the nozzle exit plane in the X-T diagram, its time of first appearance can be estimated. It is found that the appearance of the extrapolated local pressure maximum in the shear layer at the nozzle exit plane coincides with vortex ring pinch-off, as conventionally quantified by the vortex ring formation number.

Novel Vortex Generator for Mitigation of Shock-Induced Flow Separation

Abstract: A novel vortex generator, termed as “slotted vortex generator,” is proposed in this study. Computations are conducted for supersonic flow at a freestream Mach number of 2.5 past single vortex generators of three different heights. For each device height h, three values of the slot radius r=0.3h, 0.4h, and 0.6h are used. Comparisons made with a standard wedge-type vortex generator using streamwise-velocity profiles, near-surface streamwise-velocity contours, pitot pressure deficit contours, etc., indicate that the new device has less device drag and produces fuller near-surface streamwise velocities downstream of the device. It is observed that the primary counter-rotating vortex pair formed due to the vortex generator lifts off at a slower rate when a slotted vortex generator is used. Computations of an impinging oblique-shock/boundary-layer interaction at Mach 2.5 for a flow turning angle of 7 deg, with flow control using (separately) an array of slotted and standard vortex generators, indicate that the ...

Flow Control over an Airfoil in Fully Reversed Condition Using Plasma Actuators

Abstract: Flow control experiments were performed using nanosecond dielectric-barrier-discharge plasma actuators on a NACA 0015 airfoil with flow approaching from the geometric trailing-edge side, which is a condition anticipated to occur on the retreating blade side of advanced helicopters such as slowed-rotor compound rotorcraft. This symmetric airfoil, which is not typical of those used in rotorcraft blades, was used for simplification of an otherwise very complex problem. The Reynolds number based on the chord length was fixed at 0.50·106, corresponding to a freestream flow of approximately 38 m/s. An angle of attack of 15 deg was used. Fully separated flow on the suction side extended well beyond the airfoil with naturally shed vortices occurring at a Strouhal number of 0.19. Plasma actuation was evaluated at both the aerodynamic leading and trailing edges of the airfoil. Excitation at very low (impulse excitation) to moderate (∼0.4) Strouhal numbers at the aerodynamic leading edge generated organized coheren...