On the dynamics of the vortex structures generated by plasma DBD actuator

The plasma actuator is used to generate pseudo-periodical vortices moving almost parallel to the wall under various settings of high-voltage high-frequency power AC. Low-frequency amplitude modulation is desired to generate vortices, otherwise wall-jet-like flow is present. It will be shown how the dynamics of generated coherent vortices alternates depending on generator setting, frequency spectrum will be introduced in dependency on frequency of amplitude modulation and duty cycle. Measurements will be performed using TR-PIV technique or HW anemometry across wall-jet flow. The generated flow patterns are to be applied for control of boundary layers. (© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Generation of the vortex train using plasma barrier discharge actuator

The plasma actuator is used to generate periodical train of vortices moving along the surface. For generation the high-frequency high-voltage AC is used forming more-or-less steady wall-jet-like flow by the dielectric barrier discharge or corona discharge. Low-frequency modulation of the supply voltage is applied to generate vortices. Parameters of the vortex train are studied as function of the generator setting using TR-PIV technique. The generated flow patterns are to be applied for control of a boundary layer. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Turbulent Flow with Embedded Vortical Structures Induced by Vortex Generators in a Cascade

Presented work is the next step after several experimental examinations of vortex generator influence on a flow separation occurring on a model of the NACA 63A421 airfoil with deflected simple flap. In this stage of research the vortices produced by vortex generators (VGs) were studied using Particle Image Velocimetry technique (PIV) and numerical simulations. Vane type VGs with two spacings among VGs pairs in straight channel with turbulent flow were tested. The average velocity flow field, peak of vorticity and circulation decay downstream of VGs were evaluated. (© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the flow around Glauert-Goldschmied body in the narrow channel and separation control strategy

Previously, there was tested a wire dielectric barrier discharge (DBD) plasma actuator to generate sufficiently strong ionic wind to affect freely developed boundary layer in the narrow channel. This paper will report about next step – to installation of actuator inside a streamlined body. The experiment will take place inside perspex rectangular (250 × 100 mm) channel and main task is to find the appropriate place for actuator fitting for three different flow regimes. Hence, the separation point and recirculation area will be investigated via PIV anemometry for base case and for active flow control methods (e.g. plasma actuator) as well as frequency spectrum of the flow will be evaluated to describe the nature of the flow. The measurement plane will be perpendicular to the bottom of the channel and in longitudinal level. These essential information will be used for actuator design, actuator embedding and to tune actuator frequency in order to suppress the recirculation area as much as possible. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

A molecular mechanics-type approach to turbulence

A molecular mechanics-type formulation is applied to the cavity problem to generate primary vortices, secondary vortices, and turbulent flow. The fluid considered is water. Turbulence is defined in terms of the absence of a primary vortex and the rapid appearance and disappearance of many small vortices. The mechanism for generating turbulent flow lies in the generation of large repulsive forces between the particles of the model. This results from the increase in particle speeds due to the increase in wall speed.     

Large eddy simulation and PIV experiments of air–water mixing tanks

The simulations and experiments of a turbulent bubbly flow are carried out in a cylindrical mixing vessel. Dynamics of the turbulent bubbly flow is visualized using a novel two-phase particle image velocimetry (PIV) with a combination of back lighting, digital masking and fluorescent tracer particles. Using an advanced technique, Mie’s scattering at surfaces of bubbles is totally filtered out and, henceforth, images of tracer particles and of bubbles are obtained with high quality. In parallel to the comprehensive experimental studies, numerical results are obtained from large eddy simulations (LES) of the two-phase air–water mixer. The impeller-induced flow at the blade tip radius is modeled by using sliding mesh method. The results demonstrate the existence of large structures such as tip-vortex tips, and also some finer details. In addition, the stability of the jet is found to be connected with the fluctuations of the tip vortices whose dynamics are affected by the presence of bubbles. Numerical results are used to interpret the measurement data and to guide the refinement of consistent theoretical analyses. Such information is invaluable in the development of advanced theories capable of describing bubbly flows in the presence of complex liquid flow. This detailed information is of real significance in facilitating the design and scale-up of practical stirred tanks.     

Occurrence of Taylor vortices in the flow between two rotating conical cylinders

The behavior of the flow between two coaxial conical cylinders with the inner one rotating and the outer one stationary is studied numerically. Special attention is paid to the occurrence of Taylor vortices in basic flow and unsteady helical vortices. It is found that, in basic flow, the vortices occur in the direction toward smaller radius, while toward bigger radius in unsteady helical vortices; moreover, the unsteady helical vortices can coexist with unstable steady Taylor vortices. The results suggest that the behavior of conical flow is dominated by a competition between the meridional flow and radial flow. The effect of meridional flow is most significant at small apex angle or in basic flow and helical vortices, while the radial flow dominates the structure at larger apex angle or in steady vortical flow. In order to get better understanding the competition and the transition of Taylor–Couette flow to conical flow, a velocity angle related to velocity components is defined, and the pattern evolution of velocity, streamlines and the velocity angle are examined with respect to apex angle, as well as Reynolds number. Finally, the statistical properties of turbulent conical flow are investigated.     

Dynamical structure of the turbulent boundary layer on rough surface

The experiments on structure of turbulent boundary layer on the plane rough wall without pressure gradient are presented. Sand roughness of the wall is considered. Measurements are carried out using Time-Resolved PIV technique in planes parallel and perpendicular to the wall. The results on rough wall are compared with the base case of boundary layer on smooth wall. Hairpin vortices have been detected. Topology and typical size of those structures substantially differ in the cases in question. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Low-Reynolds boundary driven cavity flows in a thin liquid shell

Low-Reynolds recirculating cavity flows are traditionally generated from lid-driven boundary motion at a solid-fluid interface or result from shear flow over an opening. Such flows are typically described by the equations of creeping motion, where viscous forces are dominant. We illustrate using Particle Image Velocimetry (PIV) an original family of boundary-driven cavity flows occurring, in contrast to classic configurations, at a liquid-gas interface: thermally-induced Marangoni flows in a thin liquid shell generate forced, steady-state recirculating flows inside the cavity. Forcing relies on viscous mechanisms at the boundary but resulting flow patterns are, however, inviscid. Here, the inviscid equations of fluid motion are not used as an approximation, but rather come as a result from the solution of the creeping motion equations in the region inside the sphere. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

A further note on the force discrepancy for wing theory in Euler flow

Uniform steady potential flow past a wing aligned at a small angle to the flow direction is considered. The standard approach is to model this by a vortex sheet, approximated by a finite distribution of horseshoe vortices. In the limit as the span of the horseshoe vortices tends to zero, an integral distribution of infinitesimal horseshoe vortices over the vortex sheet is obtained. The contribution to the force on the wing due to the presence of one of the infinitesimal horseshoe vortices in the distribution is focused upon. Most of the algebra in the force calculation is evaluated using Maple software and is given in the appendices. As in the two previous papers by the authors on wing theory in Euler flow [E Chadwick, A slender-wing theory in potential flow, Proc. R. Soc. A461 (2005) 415–432, and E Chadwick and A Hatam, The physical interpretation of the lift discrepancy in Lanchester-Prandtl lifting wing theory for Euler flow, leading to the proposal of an alternative model in Oseen flow, Proc. R. Soc. A463 (2007) 2257–2275], it is shown that the normal force is half that expected. In this further note, in addition it is demonstrated that the axial force is infinite. The implications and reasons for these results are discussed.     

Stationary corner vortex configurations

Given a stable configuration of point vortices for steady two dimensional inviscid, incompressible fluid flow in a domainD, it is shown that there is another stable configuration of stationary point vortices inD with vortices near the original vortices plus additional vortices near any of the convex corners ofD. It follows that there are steady flows which have a finite sequence, of arbitrary length, of vortices of alternating sign descending into any convex corner ofD. Several computed examples are given.     

细长体截面绕流中的一种临界状态

应用拓扑分析的方法研究了细长体截面绕流拓扑结构的演变过程．指出随着细长体背涡的发展,导致截面流场的拓扑结构发生变化,会出现一种临界流动状态．在这种临界流态下,流场中会出现一种高阶奇点．这种高阶奇点的指数为-3/2．这种高阶奇点是结构不稳定的,稍有扰动就会产生分叉,使流场的拓扑结构发生变化．     

The critical cross-section of a vortex

Summary This paper discusses the problem of critical-flow cross-sections in vortex flows. It is shown that there are two different types of vortex flows, A-type and B-type vortices (say). An A-type vortex approaches its critical flow state as its cross-sectional area increases and departs from the critical state as the cross-sectional area is decreased. This property is associated with the particular dependence of total pressure and circulation on the stream function, and it holds for both subcritical and supercritical A-type vortices. On the other hand, both subcritical and supercritical B-type vortices approach their critical flow states as their cross-sectional areas are decreased and depart from their critical states for increasing cross-sectional area. As was shown by Benjamin, setting the first variation of the flow force with respect to the stream function equal to zero leads to Euler's equation of motion. The second variation also vanishes if the corresponding flow state is critical. In this case the sign of the third variation decides whether the flow is an A-type or a B-type vortex. Within the framework of inviscid-fluid flow theory the type of a vortex is preserved unless vortex breakdown occurs. Making use of the knowledge that vortex flows are controlled by two different types of critical-flow cross-sections a variety of vortex flow phenomena are investigated, including the two types of inlet vortices that are observed upstream of jet engines, the behavior of vortex valves, the flow characteristics of liquid-fuel atomizers and the bath tub vortex.     

On the influence of plasma DBD actuator on the flow in a rectangular channel

Previously, the vortical structures generated by plasma DBD actuator working in unsteady regime were investigated in detail. The generalized model describing the behaviour of these vortices in dependency on input power parameters was introduced. This paper should reveal how the wall-jet-like-flow generated by that actuator will affect the developed flow in a rectangular channel with cross-section dimension of 250 × 100 mm. The actuator is considered in spanwise configuration where the induced flow has the same or opposite orientation as the main flow. The flow control will be tested both for steady regime and for unsteady regime. The dynamic of that complex phenomenon will be studied and results in qualitatively and quantitatively meaning will be presented. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

细长体截面绕流的稳定性及扰动的影响

应用拓扑结构的稳定性理论,分析了细长旋成体截面绕流的结构稳定性．在分析时取极限流线作为流场的内边界,并证明极限流线的鞍点-鞍点连接是拓扑结构稳定的A·D2通过分析发现,由于旋成体背涡的发展,导致截面流场拓扑结构变化,由稳定对称旋涡流态变成不稳定对称旋涡流态．此时流场中存在空间的鞍点-鞍点连接的不稳定拓扑结构,在小扰动下出现分叉,变成稳定非对称旋涡流态,形成非对称背涡．并应用开折理论分析了扰动对流场结构的影响．     

Numerical study of multiple periodic flow states in spherical Couette flow

The supercritical flow states of the spherical Couette flow between two concentric spheres with the inner sphere rotating are investigated via direct numerical simulation using a three-dimensional finite difference method. For comparison with experiments of Nakabayashi et al. and Wimmer, a narrow gap and a medium gap with clearance ratio β=0.06 and 0.18 respectively are considered for the Reynolds number range covering the first Hopf bifurcation point. With adequate initial conditions and temporary imposition of small wave-type perturbation, multiple periodic flow states with three different pair numbers of spiral Taylor-Görtler (TG) vortices have been simulated successfully for β=0.06, of which the 1-pair and 2-pair of spiral TG vortices are newly obtained. Three different periodic flow states with shear waves, Stuart vortices or wavy outflow boundary, have been obtained for β=0.18. Analysis of the numerical results reveals these higher flow modes in terms of fundamental frequency, wave number and spatial structure.     

On the resistanceless statement of the two-dimensional Neumann-Kelvin problem for a surface-piercing tandem

A new set of supplementary conditions is proposed for the two-dimensionalNeumann-Kelvin problem describing the steady-state forward motionof a surface-piercing tandem in an infinite-depth fluid. Thisproblem is shown to be uniquely solvable for almost every valueof the forward speed U. The velocity potential solving the problemcorresponds to a flow about the tandem providing no resistance(wave and spray resistance vanish simultaneously). On the otherhand, for exceptional values of U examples of non-uniqueness(trapped modes) are constructed using the inverse procedurerecently applied by McIver (J. Fluid Mech. 1996) to the problemof time-harmonic water waves. For the proposed statement ofthe Neumann-Kelvin problem the inverse method involves the investigationof streamlines generated by two vortices placed in the freesurface. The spacing of vortices delivering trapped modes dependson U.     

Generalized Contour Dynamics: A Review

Contour dynamics is a computational technique to solve for the motion of vortices in incompressible inviscid flow. It is a Lagrangian technique in which the motion of contours is followed, and the velocity field moving the contours can be computed as integrals along the contours. Its best-known examples are in two dimensions, for which the vorticity between contours is taken to be constant and the vortices are vortex patches, and in axisymmetric flow for which the vorticity varies linearly with distance from the axis of symmetry. This review discusses generalizations that incorporate additional physics, in particular, buoyancy effects and magnetic fields, that take specific forms inside the vortices and preserve the contour dynamics structure. The extra physics can lead to time-dependent vortex sheets on the boundaries, whose evolution must be computed as part of the problem. The non-Boussinesq case, in which density differences can be important, leads to a coupled system for the evolution of both mean interfacial velocity and vortex sheet strength. Helical geometry is also discussed, in which two quantities are materially conserved and whose evolution governs the flow.     

A comparison of the flow around a flexible and a non-flexible airfoil for flapping wing propulsion

The flow around flapping airfoils is evaluated using the Stereoscopic Particle Image Velocimetry (PIV) technique. Resulting from the measurements is the distribution of the turbulent shear stress. This serves as an indicator for the position of transition from laminar to turbulent flow. The distributions were compared for both a flapping non–flexible airfoil and a flexible airfoil. It was determined that there are differences between the flow around the flexible and the non–flexible airfoils. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

The Clebsch transformation and its capabilities towards fluid and solid mechanics

In fluid dynamics, Clebsch made use of the representation for the velocity field in terms of three potentials Φ, α, β in order to construct a first integral of the equations of motion in case of an inviscid flow with vortices. Apart from this, he received a self-adjoint form of the equations allowing for deriving them from a variational formulation. In latter times the Clebsch transformation has been applied to different physical problems, for instance to baroclinic flow, Maxwell equations in classical electrodynamics [1], in Magnetohydrodynamics and even quantum theory within the context of a quantization of vortex tubes. Viscous flow, however, has not yet been formulated in terms of Clebsch variables to our best knowledge. It is the aim of this paper to demonstrate how Clebsch variables can be applied to viscous flow on the one hand, leading to a first integral of Navier-Stokes equations as a first example. As a second example, solid mechanics is considered: by making use of an analogy between vortices in fluid flow on the one hand and dislocations in crystals on the other hand, a dynamic theory of dislocations can be established by using a certain modification of the Clebsch transformation. (© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)