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Jens Norkaer Sorensen - One of the best experts on this subject based on the ideXlab platform.
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stability of helical tip Vortices in a rotor far wake
Journal of Fluid Mechanics, 2007Co-Authors: Valery Okulov, Jens Norkaer SorensenAbstract:As a means of analysing the stability of the wake behind a multi-bladed rotor the stability of a multiplicity of helical Vortices embedded in an assigned flow field is addressed. In the model the tip Vortices in the far wake are approximated by infinitely long helical Vortices with constant pitch and radius. The work is a further development of a model developed in Okulov ( J. Fluid Mech. , vol. 521, p. 319) in which the linear stability of N equally azimuthally spaced helical Vortices was considered. In the present work the analysis is extended to include an assigned vorticity field due to root Vortices and the hub of the rotor. Thus the tip Vortices are assumed to be embedded in an axisymmetric helical vortex field formed from the circulation of the inner part of the rotor blades and the hub. As examples of inner vortex fields we consider three generic axial columnar helical Vortices, corresponding to Rankine, Gaussian and Scully Vortices, at radial extents ranging from the core radius of a tip vortex to several rotor radii. The analysis shows that the stability of tip Vortices largely depends on the radial extent of the hub vorticity as well as on the type of vorticity distribution. As part of the analysis it is shown that a model in which the vortex system is replaced by N tip Vortices of strength Γ and a root vortex of strength − N /Γ is unconditionally unstable.
T. J. Hanratty - One of the best experts on this subject based on the ideXlab platform.
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Observations of the formation of streamwise Vortices by rotation of arch Vortices
Physics of Fluids, 2000Co-Authors: D. K. Heist, T. J. HanrattyAbstract:Vortices, oriented in the streamwise direction, play an important role in generating Reynolds shear stresses (and turbulence) for flow over a smooth wall. Many of these have been observed to originate from tiny streamwise Vortices, located in the immediate vicinity of the wall. In this paper we identify a different process that forms about 30 percent of the streamwise Vortices at locations, away from the wall, in the middle of the viscous wall layer. This is accomplished by examining the changes, with time, of the turbulent field obtained from a direct numerical simulation of turbulent flow in a channel. Streamwise Vortices create a shear layer by pumping low momentum fluid from the wall. One or more small spanwise Vortices are formed at the top of this layer. These grow in size and rotate in the direction of flow. Previous investigators have suggested that spanwise Vortices could have a direct role in the formation of streamwise Vortices. This paper describes, in detail, a process by which this is accomplished. Of particular interest is the need to recognize that the shear layer is asymmetric since it is formed by a single streamwise vortex rather than a pair of counter-rotating Vortices.
Valery Okulov - One of the best experts on this subject based on the ideXlab platform.
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stability of helical tip Vortices in a rotor far wake
Journal of Fluid Mechanics, 2007Co-Authors: Valery Okulov, Jens Norkaer SorensenAbstract:As a means of analysing the stability of the wake behind a multi-bladed rotor the stability of a multiplicity of helical Vortices embedded in an assigned flow field is addressed. In the model the tip Vortices in the far wake are approximated by infinitely long helical Vortices with constant pitch and radius. The work is a further development of a model developed in Okulov ( J. Fluid Mech. , vol. 521, p. 319) in which the linear stability of N equally azimuthally spaced helical Vortices was considered. In the present work the analysis is extended to include an assigned vorticity field due to root Vortices and the hub of the rotor. Thus the tip Vortices are assumed to be embedded in an axisymmetric helical vortex field formed from the circulation of the inner part of the rotor blades and the hub. As examples of inner vortex fields we consider three generic axial columnar helical Vortices, corresponding to Rankine, Gaussian and Scully Vortices, at radial extents ranging from the core radius of a tip vortex to several rotor radii. The analysis shows that the stability of tip Vortices largely depends on the radial extent of the hub vorticity as well as on the type of vorticity distribution. As part of the analysis it is shown that a model in which the vortex system is replaced by N tip Vortices of strength Γ and a root vortex of strength − N /Γ is unconditionally unstable.
Samuel Ohring - One of the best experts on this subject based on the ideXlab platform.
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The oblique rise of a viscous vortex ring toward a deformable free surface
Meccanica, 1994Co-Authors: Hans J. Lugt, Samuel OhringAbstract:Un vortice ad anello in un fluido incomprimibile e viscoso si avvicina obliquamente, con un angolo di 45°, ad una superficie libera deformabile. La descrizione matematica di questa situazione di flusso è un problema di superficie libera non-lineare e dipendente dal tempo, che è stato risolto numericamente per un flusso laminare tridimensionale con l'ausilio delle equazioni di Navier-Stokes. A tal fine sono state usate opportune coordinate al contorno che includono la possibilità di adattare il reticolo ed il raffinamento del reticolo stesso. Per due differenti numeri di Froude e per un numero di Reynolds pari a 100, vengono presentati i risultati sull'incontro del vortice ad anello con la superficie libera, la deformazione di quest'ultima, il decadimento dei vortici primari, la generazione di una vorticità superficiale e di vortici secondari e la riconnessione con la superficie libera. Questi risultati sono presentati sotto forma di linee di corrente e linee di equivorticità nel piano di simmetria, linee di contorno della proiezione verticale della superficie, linee di corrente e linee di equivorticità sulla superficie libera. I due casi rivelano una nuova riconnessione ad anello sulla superficie libera nella forma di un foglio cilindrico. A vortex ring in a viscous, incompressible fluid rises obliquely at an angle of 45° toward a deformable free surface. The mathematical description of this flow situation is a time-dependent nonlinear free-surface problem that has been solved numerically for a three-dimensional laminar flow with the aid of the Navier-Stokes equations. Boundary-fitted coordinates were used that include adaptive gridding and grid refinement. For two different Froude numbers at Reynolds number 100, results are presented on the encounter of the vortex ring with the free surface, the deformation of the free surface, the decay of the primary Vortices, the generation of surface vorticity and secondary Vortices, and reconnection with the free surface. These results are presented in the form of streamlines and equivorticity lines in the plane of symmetry, contour lines of surface elevation, and streamlines and equivorticity lines on the free surface. The two cases reveal a novel ring reconnection at the free surface in the form of a cylindrical sheet.
Haecheon Choi - One of the best experts on this subject based on the ideXlab platform.
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Control of streamwise Vortices with uniform magnetic fluxes
Physics of Fluids, 1998Co-Authors: Haecheon ChoiAbstract:Numerical experiments are conducted to investigate the effect of uniform magnetic fluxes on streamwise Vortices in conducting fluids. A simple flow configuration, in which a pair of two-dimensional Vortices interact with a wall, is used to study the effect of magnetic forcing on the Vortices. Effects of a uniform magnetic flux applied in three directions—streamwise, wall-normal, and spanwise—are investigated. The electromagnetic force induced from either the wall-normal or spanwise magnetic flux inhibits the induced motion by the streamwise Vortices and reduces their strength, while the streamwise magnetic flux does not affect the flow in the present flow configuration. It is also shown that, in the case of a closely interacting pair of Vortices, the spanwise magnetic flux is more effective than the wall-normal magnetic flux in reducing the strength of the streamwise Vortices.
