Phase-Averaged Vortex Train Flow Generated by Plasma DBD Actuator

It will be shown how plasma actuator can generate wall-jet-like flow or train of periodical vortices depending on the generator setting. For generation the high-frequency high-voltage AC is used. Low-frequency modulation of the supply voltage is required to generate vortices. Data acquisition will be performed using time-resolved PIV technique. Phase-averaging will be studied from two different perspectives. Firstly, sampling of phases will be ensured using trigger that is contained in the PIV software and, secondly, phase-averaged flow will be computed from two main modes of POD analysis. The generated flow patterns are to be applied for control of a boundary layer. (© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

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)     

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.     

Numerical modelling of residual flow and salinity in the Río de la Plata

The Río de la Plata discharges into the Atlantic Ocean. The particular characteristics of the study area, the variable width and shallowness of the river, the high fluvial discharges and the dynamic processes involving interactions between river discharges, tidal currents and wind, generate complex velocity and salinity fields. We applied the hydrodynamic model RMA-10 to examine the effects of various forcing (tides, flow discharge and winds) on residual currents and salinity fields in the Río de la Plata, focusing on the outer zone of the river. The RMA-10 code, developed by Ian King, is a multiparameter finite element model representing estuarine flow in three dimensions. In this study the model has been applied in a depth-averaged-baroclinic mode and a series of observed data is used for model calibration and verification. The model result shows that it is able to simulate velocity and the salinity fields with a reasonable accuracy. The analysis of residual currents in the river, when forced by freshwater discharge and astronomical tide, shows that the flow discharge takes place mainly over the shallower areas of the river and that the saline water is advected up-river through the deeper channels. The numerical simulations show that the winds from the South-West and North-East quadrants have a great influence over the salinity and velocity fields.     

Strongly coupling of partitioned fluid–solid interaction solvers using reduced-order models

In this work a powerful technique is described which allows the implicit coupling of partitioned solvers in fluid–structure interaction (FSI) problems. The flow under consideration is governed by the Navier–Stokes equations for incompressible viscous fluids and modeled with the finite volume method. The structure is represented by a finite element formulation. The method allows the use of a black box fluid and structural solver because it builds up a reduced order model of the fluid and structural problem during the coupling process. Each solution of the fluid/structural solver in the coupling process can be seen as a sensitivity response of an applied displacement/pressure mode. The applied modes and their responses are used to build up a reduced-order model. The proposed model is used to predict the unsteady flow fields of a particular flow-induced vibrational phenomenon – a fixed cubic rigid body is submerged in an incompressible fluid flow (water), an elastic plate is attached to the rigid body in the centre of the downstream face, and the vortices, which separate from the corners of the rigid body upstream, generate lift forces which excite continuous oscillations of the elastic plate downstream. The computational results show that a fairly good convergence solution is achieved by using the reduced-order model that is based on only a few displacement and stress modes, which largely reduces the computational cost, compared with traditional approaches. At the same time, comparison of the numerical results of the model with available experimental data validates the methodology and assesses its accuracy.     

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.     

RBF Neural Network Based Prediction on Blade Surface Pressure Fields in Compressors北大核心CSCD

The airflow characteristics of the internal flow path of an aero-engine compressor are complex, and the vortex flow field around the blade is characterized by high pressure, high speed, rotation, and unsteadiness. Therefore, there is an urgent need to calculate and predict the aerodynamic characteristics of the complex flow field around the compressor blade efficiently and accurately. The computational fluid dynamics (CFD) method was used to generate the aerodynamic load distribution on the blade surface under different operating conditions for the study of the complex flow fields around aero-engine blades. The radial based function (RBF) neural network was applied to establish the pressure surface aerodynamic load prediction model, and the neural network modeling method was combined with the flow field calculation. The neural network method can learn and train the CFD-based data set to properly compensate the errors from the CFD, which provides a reference for the effective prediction of the complex flow fields around aero-engine compressor blades. © 2023 Editorial Office of Applied Mathematics and Mechanics. All rights reserved.     

Elastoviscoplastic fluid flow in non-circular tubes: Transversal field and interplay of elasticity and plasticity

An analytical study of elastoviscoplastic fluid flow in tubes of non-circular cross section is presented. The constitutive structure of the fluid is described by a linear frame invariant combination of the Phan-Thien−Tanner model of viscoelastic fluids and the Bingham model of plastic fluids. Non-circular tube cross sections are modeled by the shape factor method a one-to-one mapping of the circular base contour into a wide spectrum family of arbitrary tube contours. Field variables are expanded into asymptotic series in terms of the elasticity measure, the Weissenberg number We, coupled with an asymptotic expansion in terms of the geometrical mapping parameter ε leading to a set of hierarchical momentum balance equations which are solved successively up to and including the third order in We when the secondary field appears for the first time. The computational algorithm developed is applied to the study of the non-rectilinear flow in tubes with triangular and square cross sections. We find that the presence of the yield stress dampens the intensity of the purely viscoelastic vortices, the higher the yield stress the lower the intensity of the vortices in the cross-section, and the further away the vortices are from the center of the cross section as compared to the purely viscoelastic vortices. The results also evidence that viscoelasticity increases the axial flow for given viscoplastic conditions and pressure drop, and consequently increases the rate of flow, a phenomenon that may find applications in optimizing material transportation.     

热污染的三维椭圆型数值模拟

过去对复杂流动的三维数值模拟往往要采用许多简化处理方法,使得数值模型的适用性受到很大限制,所得结果也不能全面反应流场的特征.本文用有限容积法直接求解三维椭圆型流动控制方程,紊流模型采用有浮力修正的κ-ε模型.本文首先将该模型用于有横流情况下岸边等密度排放问题,以检验本数值模型和计算程序的正确性,所得结果正确预报了排放口下游的回流区,与文献[7]的计算和实验结果一致.然后进一步将其应用于有横流情况下的温排水、取水问题,所得结果合理,并精细地揭示了流场的内部特征.     

Transient cavitating flow structure and acoustic analysis of a hydrofoil with whalelike wavy leading edge

In this paper, the open source software OpenFOAM is used to perform a numerical investigation of the cavitating flow around a modified NACA63₄-021 hydrofoil with bioinspired, wavy leading edge, with particular emphasis on study of the interactions between the cavitation and the streamwise vortices and the far-field radiation noise. A modified k-ω shear-stress transport (SST) model coupled with the Schnerr-Sauer cavitation model and the Ffowcs Williams-Hawkings (FW-H) acoustic analogy approach are introduced to the simulation. The transient cavitation structure and the streamwise vortices are captured well and the results show significant interactions between the cavitation and the streamwise vortices. Cavitation can promote fragmentation of the streamwise vortices, while the streamwise vortices cause cavitation inception to occur earlier and bind the cavitation within the trough region by changing the pressure distribution on the hydrofoil. The transformation of the FW-H equation's solution indicates that the monopole noise is directly related to the cavitation volume acceleration and the dipole noise is related to the mechanical force of the hydrofoil on fluids and the rate at which this force changes. The collapse of cavitation cloud and the collision of the re-entrant jet and main flow will cause violent fluctuation of the mechanical force thus produce instantaneous extreme dipole noise values, while the monopole noise is relatively strong in the cavitation collapse stage due to significant cavitation volume acceleration. The time domain hydroacoustic characteristics are similar for the modified hydrofoil and the baseline hydrofoil.     

The Existence of Görtler Vortices in Separated Boundary Layers

The linear stability properties of Görtler vortices within a general separated boundary layer flow are addressed. There has been little previous theoretical work directed toward this problem and here we are able to characterize the important features of vortices over the complete wavenumber spectrum. This investigation complements earlier studies of vortices within an attached flow which demonstrated that there are three distinctive wavenumber régimes which together describe the most relevant possibilities for vortex behavior. In the first of these, at relatively small wavenumbers, the mode is inviscid in character; as the vortex wavenumber increases so the spatial amplification rate of the vortices increases until viscous effects become significant and the growth rate begins to diminish. As the wavenumber increases yet further so the vortex is completely stabilized. Here we discuss the corresponding structures which may exist within a separated flow and the most significant result we find is that the maximum growth rate of a mode in this type of flow is actually greater than that which occurs when the flow has not separated. In addition, the inviscid modes are shown to have a far more complicated configuration than within an attached boundary layer and, indeed, their structure can only be completely determined by implementation of numerical procedures.     

Film flow over strongly undulated bottom profiles at low Reynolds numbers – Experimental study

We study gravity driven films of a Newtonian fluid flowing down sinusoidal bottom profiles. We give experimental evidence of vortices in the film flow under creeping flow conditions down to Reynolds numbers of the order of 10^–5. The vortices are created in the valleys of the undulated bottom profile. They are visualized employing a particle image velocimeter with fluorescent tracers. It turns out that the vortices are generated beyond a critical film thickness.     

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.     

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.     

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．     

Reservoir flood control operation based on chaotic particle swarm optimization algorithm

Reservoir flood control operation is a complex engineering optimization problem with a large number of constraints. In order to solve this problem, a chaotic particle swarm optimization (CPSO) algorithm based on the improved logistic map is presented, which uses the discharge flow process as the decision variables combined with the death penalty function. According to the principle of maximum eliminating flood peak, a novel flood control operation model has been established with the goal of minimum standard deviation of the discharge flow process. At the same time, a piecewise linear interpolation function (PLIF) is applied to deal with the constraints for solving objective function. The performance of the proposed model and method is evaluated on two typical floods of Three Gorges reservoir. In comparison with existing models and other algorithms, the proposed model and algorithm can generate better solutions with the minimal flood peak discharge and the maximal peak-clipping rate for reservoir flood control operation.     

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.     

Near-field fluctuations and far-field noise of a three-element airfoil system by a discrete vortex method

A circulation-based discrete vortex method is used on a three-element airfoil system. Kutta conditions and Kelvin’s circulation theorem are additional conditions required for this method to determine the circulation distributions on each element and to determine vortex shedding. Discrete shed vortices are introduced near the four sharp edges to represent the sharp-edge vortex shedding caused by unsteady flow separation. The computational procedure warrants neutrally stable solutions of the self-sustained fluctuating flowfield that can provide broad-band spectral information for far-field noise predictions. The near-field vortex method directly calculates the parameters used in an asymptotic formula for far-field sound computation that attributes the noise sources to vortex interactions among the shed vortices and the surface circulations of the three-element airfoil system. The far-field noise characters are then analyzed and compared to the experimental data in the literature.