LES and analyses on the vortex structure behind supersonic MVG with turbulent inflow

In this paper, an implicitly implemented high order large eddy simulation by using the fifth order bandwidth-optimized WENO scheme is applied to make comprehensive studies on ramp flows with and without control at Mach 2.5 and Re_θ = 5760. Flow control in the form of microramp vortex generators (MVG) is applied. The mechanism of vortex ring generation behind MVG has been studied in detail and shear layer instability has been studied and found as the mechanism of K–H vortex ring generation. A series of new observations on the flow around supersonic MVG have been made including inflection points (surface in 3-D), vorticity conservation, interaction of the primary vortex and new generated K–H vortex rings, and the K–H vortex ring structure. The numerical observations have been confirmed by the experimental work.     

Numerical simulation of the interaction between vortex ring and bubble plume

In present paper a three-dimensional Vortex-In-Cell method with two-way coupling effect was developed to study the bubble plume entrainment by a vortex ring. In this method the continuous flow was calculated by the three-dimensional Vortex-In-Cell method and the bubbles are tracked through bubble motion equation. Two-way coupling effect between continuous flow and dispersed bubbles is considered by introducing a vorticity source term, which is induced by the change of void fraction gradient in each computational cell. After validated by the comparison between experimental measurements and simulation results for the motion of vortex rings and the rising velocity of bubble plume, present method is implemented to simulate the interaction between an evolving vortex ring and a rising bubble plume. It was found that there is little effect of the bubble entrainment to the total circulation of vortex ring while the effect of bubble entrainment to the vortex ring structure is quite obvious. The bubble entrainment by the vortex ring not only changed the vorticity distribution in the vortex structure, but also displaced the positions of the vortex cores. The vorticity in the lower vortex core of the vortex ring decreases more than that in the upper vortex core of the vortex ring while the vortex core in the upper part of the vortex ring is displaced to the center of vortex ring by the entrained bubbles. Smaller bubbles are easier to be entrained by the large scale vortex structure and the transportation distance is in inverse proportion to bubble diameter.     

Vortex breakdown in turbulent pipe flows

In many technical applications turbulent flows with embedded slender vortices exist. Depending on the boundary conditions vortex breakdown can occur. The purpose of this work is to develop and implement a solution scheme for large‐eddy simulations of vortex breakdown in turbulent pipe flows. One of the main problems in this simulation is the formulation of the inflow boundary condition for a fully developed turbulent flow with an embedded vortex. For that purpose a rescaling technique is developed in which a solution at a downstream location is inserted at the inflow boundary after an appropriate rescaling. To determine rescaling laws for pipe flows with an embedded vortex, analytical velocity profiles of swirling flows are first prescribed in a laminar flow. From the spatial development of the vortex a scaling law is deduced. In a next step this procedure is to be transferred to turbulent flows.     

Localized instabilities of vortex rings with swirl

Axisymmetric vortex rings with swirl in an inviscid incompressible fluid are considered and it is demonstrated how the geometrical optics method can be used for investigating their stability. The evolution of rapidly oscillating initial data is studied and it is shown that the corresponding rings are unstable if the transport equations associated with the wave which is advected by the flow have unbounded solutions. It is shown that unbounded solutions grow either exponentially or algebraically. By means of the analysis of the corresponding transport equations effective stability conditions for general vortex rings with swirl are obtained. © 1993 John Wiley & Sons, Inc.     

Dynamics and self-propulsion of a spherical body shedding coaxial vortex rings in an ideal fluid

We describe a model for the dynamic interaction of a sphere with uniform density and a system of coaxial circular vortex rings in an ideal fluid of equal density. At regular intervals in time, a constraint is imposed that requires the velocity of the fluid relative to the sphere to have no component transverse to a particular circular contour on the sphere. In order to enforce this constraint, new vortex rings are introduced in a manner that conserves the total momentum in the system. This models the shedding of rings from a sharp physical ridge on the sphere coincident with the circular contour. If the position of the contour is fixed on the sphere, vortex shedding is a source of drag. If the position of the contour varies periodically, propulsive rings may be shed in a manner that mimics the locomotion of certain jellyfish. We present simulations representing both cases.     

Laboratory experiments on vortices colliding with multiple islands

Laboratory experiments have been performed to investigate the physical processes that govern the interaction of a self-propagating barotropic cyclonic vortex with aligned circular cylinders. The motivation was to understand the dynamics which controls the interaction of North Brazil Current (NBC) rings with the Lesser Antilles (LA) in the Eastern Caribbean Sea. The geometrical parameter regulating the flow in the experiments was the ratio of the gap size to the diameter of the incident vortex, G /d. The range 0.03 ≤ G /d ≤ 0.4 was studied. Since the passages of the LA have values of 0.07 ≤ G /d ≤ 0.3 [2], the experimental results might explain recent oceanic observations [3]. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Analysis of Sound Radiated by Axisymmetric Vortex Ring

A theoretical and numerical study using the unsteady, 3D Navier-Stokes equations to generate axisymmetric vortex rings is reported. Increasing the vorticity, the vortex ring transition to a turbulent state are analyzed. After transition to a turbulent stage, the self-similarity of the temporal evolution of the flow is observed. Then we can compare sound radiated by the vortex ring to jet noise, at similar Reynolds number and low Mach number. The agreement between the simulation results and the simplified model is good. (© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Analytical solutions of laminar swirl decay in a straight pipe

In this work, the laminar swirl flow in a straight pipe is revisited and solved analytically by using prescribed axial flow velocity profiles. Based on two axial velocity profiles, namely a slug flow and a developed parabolic velocity profiles, the swirl velocity equation is solved by the separation of variable technique for a rather general inlet swirl velocity distribution, which includes a forced vortex in the core and a free vortex near the wall. The solutions are expressed by the Bessel function for the slug flow and by the generalized Laguerre function for the developed parabolic velocity. Numerical examples are calculated and plotted for different combinations of influential parameters. The effects of the Reynolds number, the pipe axial distance, and the inlet swirl profiles on the swirl velocity distribution and the swirl decay are analyzed. The current results offer analytical equations to estimate the decay rate and the outlet swirl intensity and velocity distribution for the design of swirl flow devices.     

Numerical study of helicopter blade–vortex mechanism of interaction using the potential flow theory

The blade–vortex interaction (BVI) phenomenon plays a key role in the rotorcraft aerodynamics. Numerical investigations of BVI using classical CFD approaches are computationally expensive. In the present research we propose a numerical approach, based on the potential flow theory, for the numerical investigation of helicopter blade–vortex mechanism of interaction. This approach overcomes the computational expenses posed by the CFD techniques. The influence of vertical miss distance, angle of attack, airfoil camber, and vortex strength on the helicopter blade–vortex mechanism of interaction is subject of investigation. The study reveals that the magnitude of the aerodynamic coefficients decreases with the increase of vertical miss distance and angle of attack, and the decrease of vortex strength and core size.     

针对壁面旋转变径管内螺旋流的压力应变项研究

通过壁面旋转变径圆管内螺旋湍流流动特征的分析，确定其切向速度场内涡流区为微团旋转主导的椭圆形流动，外涡流区为微团变形主导且受壁面旋转影响的双曲形流动.进而利用张量的不变量理论，引入旋转率张量与应变率张量的综合不变量作为模型系数，将适用于微团旋转主导的旋转湍流Reynolds应力压力应变项修正模型拓展到了非旋转效应主导的双曲形流动中.将修正压力应变项应用于壁面旋转变径圆管流场的模拟，并将结果与实测结果进行了对比，验证了修正模型的改进效果.     

湍动尺度的模糊聚类分析

湍流运动可看成是大小不同尺度的涡体运动的叠加.定量地确定湍动尺度的分类,对于更好地描述不同尺度的涡体运动,探讨不同尺度涡体之间的相互作用,建立较好的湍流模式都具有重要的意义. 对事物按一定要求进行分类的数学方法,叫做聚类分析.由于湍动尺度的分类具有一定程度的模糊性,因而本文采用模糊聚类的方法,对壁面光滑及租糙两种边界条件下的湍动尺度进行了分类,并对各类结构的特性及其相互作用进行了探讨.     

The dynamics of vortex rings: Leapfrogging, choreographies and the stability problem

We consider the problem of motion of axisymmetric vortex rings in an ideal incompressible fluid. Using the topological approach, we present a method for complete qualitative analysis of the dynamics of a system of two vortex rings. In particular, we completely solve the problem of describing the conditions for the onset of leapfrogging motion of vortex rings. In addition, for the system of two vortex rings we find new families of motions where the relative distances remain finite (we call them pseudo-leapfrogging). We also find solutions for the problem of three vortex rings, which describe both the regular and chaotic leapfrogging motion of vortex rings.     

翼涡干扰前缘开孔被动控制数值研究

开孔方法是一种简单的流动被动控制方法.为找到一种有效降低桨涡干扰效应的被动控制方法,以NACA 0012翼型作为研究对象,建立了4种前缘开孔的模型.在不同来流速度、涡的强度和干扰距离条件下,对4种前缘开孔模型和无孔的基准翼型进行了二维平行桨涡干扰（翼涡干扰）数值模拟,对比了升力系数的变化.结果表明:前缘开孔可以降低翼涡干扰效应,但对翼型升力系数有一定的影响;宽度为2.5%弦长的直孔能在翼型升力系数损失较小的情况下有效地降低翼涡干扰效应,且适用范围较广.     

Ordered and random movement in the dynamics of three coaxial vortex rings

The article presents the results of the theoretical investigation of the movement of a system of three coaxial vortex rings in an ideal liquid. It is shown that when the rings interact with each other, the process may become randomized in time. The conditions of ordered and random movement of three vortex rings are determined. The article presents the paths and Poincaré mappings for a number of characteristic situations.Kiev. Translated from Teoreticheskaya i Prikladnaya Mekhanika, No. 21, pp. 100–104, 1990.     

High performance computation for DNS/LES

The paper is focused on high-order compact schemes for direct numerical simulation (DNS) and large eddy simulation (LES) for flow separation, transition, tip vortex, and flow control. A discussion is given for several fundamental issues such as high quality grid generation, high-order schemes for curvilinear coordinates, the CFL condition for complex geometry, and high-order weighted compact schemes for shock capturing and shock–vortex interaction. The computation examples include DNS for K-type and H-type transition, DNS for flow separation and transition around an airfoil with attack angle, control of flow separation by using pulsed jets, and LES simulation for a tip vortex behind the juncture of a wing and flat plate. The computation also shows an almost linear growth in efficiency obtained by using multiple processors.     

Calculation of the effective speed of sound in corrugated pipes by multiple scales

The flow through corrugated pipes is known to lead to strong whistling tones which may be harmful in many industrial appliances. The mechanism is known to originate from a coupling between vortex shedding at the edges of the cavities forming the wall of the tube and the acoustical modes of the pipe. The latter depend upon the effective velocity of sound c_eff within the corrugated pipe. The purpose of this paper is to compute accurately this effective velocity of sound through an asymptotic calculation valid in the long-wave limit. Results are given for a number of geometries used in previous works, and compared with a simple model in which the effective speed of sound is function of the geometry of the pipe. The latter is found to work best for short cavities but significant disagreement is found for longer cavities. (© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

The structure of fully nonlinear Taylor vortices

The structure of nonlinear short-wavelength Taylor vorticesin the flow between rotating concentric cylinders is considered.In the short-wavelength limit, the nonlinear vortex motion isgoverned by a mean-flow-first-harmonic interaction problem.The initial structure of the nonlinear vortex state is shownto be governed by a multilayer structure in which the vortexis constrained to lie between the inner cylinder and a positioninternal to the flow regime. This position is dependent uponthe Taylor number and it is found that there is a critical valueof the Taylor number at which the vortex first impinges on theouter boundary. The vortex field then develops a double boundarylayer structure at both the inner and outer boundary as theTaylor number is increased past this critical Taylor number.     

Existence and numerical modelling of vortex rings with elliptic boundaries

We revisit in this paper the theory of axisymmetric vortex rings in an ideal fluid. The boundary separating the vortex ring from the external (potential) flow is assumed of elliptic shape. For a given distribution of vorticity in the vortex core, we theoretically put into evidence the critical parameter for the existence of non-trivial solutions, thus confirming the numerical observation of Durst et al. [ZAMP 32 (1981) 156]. A sharp estimation of the critical threshold is analytically derived. Theoretical predictions are confirmed by numerical simulations using finite elements. A new numerical algorithm is presented and shown to display better performances compared to previous published algorithms using finite differences. The convergence of the iterative algorithm is proved using the theory of elliptic partial differential equations with discontinuous nonlinearities.