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Considered is a cylinder channel with a single row of ten aligned impinging jets, with exit flow in the axial direction at one end of the channel. For the present predictions, an unsteady Reynolds-Averaged Navier-Stokes (RANS) sol...
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Considered is a cylinder channel with a single row of ten aligned impinging jets, with exit flow in the axial direction at one end of the channel. For the present predictions, an unsteady Reynolds-Averaged Navier-Stokes (RANS) solver is employed for predictions of flow characteristics within and nearby the ten impingement jets, where the jet Reynolds number is 15,000. Spectrum analysis of different flow quantities is also utilized to provide data on associated frequency content. Visualizations of three-dimensional, unsteady flow structural characteristics are also included, including instantaneous distributions of Y-component vorticity, three-dimensional streamlines, shear layer parameter psi, and local static pressure. Kelvin-Helmholtz vortex development is then related to local, instantaneous variations of these quantities. Of particular importance are the cumulative influences of cross flows, which result in locally increased shear stress magnitudes, enhanced Kelvin-Helmholtz vortex generation instabilities, and increased magnitudes and frequencies of local flow unsteadiness, as subsequent jets are encountered with streamwise development.
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Laminar incompressible flow over a semi-infinite flat plate, whose surface moves counter to the oncoming stream, is considered. The asymptotic flow structure is investigated and a numerical solution of the time-dependent Navier-St...
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Laminar incompressible flow over a semi-infinite flat plate, whose surface moves counter to the oncoming stream, is considered. The asymptotic flow structure is investigated and a numerical solution of the time-dependent Navier-Stokes equations is obtained.
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This paper addresses by means of high-resolution numerical simulations and experimental quantitative imaging the three-dimensional unsteady separation process induced by large-amplitude heaving oscillations of a low-aspect-ratio w...
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This paper addresses by means of high-resolution numerical simulations and experimental quantitative imaging the three-dimensional unsteady separation process induced by large-amplitude heaving oscillations of a low-aspect-ratio wing under low-Reynolds-number conditions. Computed results are found to be in good agreement with experimental flow visualizations and PIV measurements on selected cross-flow planes. The complex unsteady three-dimensional flow structure generated during dynamic stall of the low-aspect-ratio wing is elucidated. The process is characterized by the generation of a leading-edge vortex system which is pinned at the front corners of the plate and which exhibits intense transverse flow toward the wing centerline during its initial stages of development. This vortex detaches from the corners and evolves into an newly found arch-type structure. The legs of the arch vortex move along the surface toward the wing centerline and reconnect forming a ring-like structure which is shed as the next plunging cycle begins. Vortex breakdown, total collapse and reformation of the wing tip vortices are also observed at various stages of the heaving motion. At the relatively high value of reduced frequency considered, these basic flow elements of the complex three-dimensional dynamic stall process are found to persist over a range of Reynolds numbers.
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Recently, several papers have appeared in the CFD literature, proposing an idealized instability problem as a benchmark for discriminating among numerical algorithms for two-dimensional Navier-Stokes flows. The problem is a double...
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Recently, several papers have appeared in the CFD literature, proposing an idealized instability problem as a benchmark for discriminating among numerical algorithms for two-dimensional Navier-Stokes flows. The problem is a double shear layer simulated at coarse resolution and with a prescribed interface perturbation. A variety of second-order accurate schemes have been tested, with all results falling into one of two solution patterns - one pattern with two eddies and the other with three eddies. In the literature, there is no fast-and-firm rule to predict the results of any particular algorithm. However, it is asserted that the two-eddy solution is correct. Our own research has led to two conclusions. First, the appearance of the third eddy is tied up with small details of the truncation error; we illustrate this point by prescribing small changes that lead to reversal of the appearance/disappearance of the third eddy in several schemes. Second, we discuss the realizability of the two solutions and suggest that the three-eddy solution is the more physical. Overall, we conclude that this problem is a poor choice of benchmark to discriminate among numerical algorithms.
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The experiments performed in this article led to the identification of the characteristic flow patterns generated in a rectangular bubble column when a partially aerated plate is used.Considering the traditional time-averaged appr...
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The experiments performed in this article led to the identification of the characteristic flow patterns generated in a rectangular bubble column when a partially aerated plate is used.Considering the traditional time-averaged approach and based on the flow regimes identified by several authors,it is concluded that the flow regime existing at all values of the superficial gas velocity (U_G from 1.2 mm/s to 21.3 mm/s) is the coalesced bubble regime.Furthermore,homogeneous and turbulent regimes are not observed and the prevailing flow regime is the 3-region vortical regime.However,focusing on both the local and instantaneous values of the most characteristic flow variables,it is possible to identify different flow regimes from those previously described as a function of U_G.Based on this approach,it is proposed for the first time the existence of three new flow patterns included in the generic vortical flow regime: the dispersed bubble vortical flow (DBVF),the transition vortical flow (TVF) and the fully developed vortical flow (FDVF).The main characteristics of these new flow regimes and the hydrodynarnic variables that determine the transition between them are presented.
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We use two different dye injection approaches, in two different water tunnels, to visualize the formation and subsequent evolution of leading-edge vortices and related separated structures, for a pitching low aspect ratio plate. T...
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We use two different dye injection approaches, in two different water tunnels, to visualize the formation and subsequent evolution of leading-edge vortices and related separated structures, for a pitching low aspect ratio plate. The motion is a smoothed linear pitch ramp from 0° to 40° incidence, brief hold, and return to 0°, executed at reduced pitch rates ranging from 0.1 to 0.35 and about various pivot locations. All cases evince a leading edge vortex with pronounced axial flow, which leads to formation of large-scale, three-dimensional flow structures, culminating in a large vortical structure centered at the wing symmetry plane. Pitch is also compared to plunge, where the plunge-induced angle of attack is taken as the geometric pitch incidence angle, ignoring pitch-rate effects. At successively increasing values of convective time C/U, the three-dimensional patterns of the flow structure are remarkably similar for the pitching and plunging motions. The similarity of these patterns persists, though they are shifted in time, for variation of either the location of the pitching axis or the dimensionless pitch rate.
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A conservative vorticity method for solving the 2-D incompressible unsteady viscous flow in the eulerian frame is proposed. The numerical method implements the vorticity-stream function formulation of the Navier-Stokes (N-S) equat...
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A conservative vorticity method for solving the 2-D incompressible unsteady viscous flow in the eulerian frame is proposed. The numerical method implements the vorticity-stream function formulation of the Navier-Stokes (N-S) equations, solved by the cell-centered Finite Volume Method (FVM). A convection limiter, coupling the first order upwind scheme and the Quadratic Upstream Interpolation for Convective Kinematics (QUICK) scheme, is introduced to maintain the accuracy and keep the monotonicity of vorticity interpolations. The vorticity induced stream functions are recovered by a closed-form expression, which also accurately predicts the far-field boundary conditions. The convective flux is evaluated by the difference of stream functions on the neighboring two nodes, which spontaneously eliminates the net mass flux out of a control cell. The conservative FVM and QUICK scheme and the definite evaluations of stream functions and convective flux guarantee the conservation of circulation and mass. The impulsively started flow past a circular cylinder at Reynolds numbers ranging from 100 to 9500 and the inline oscillatory flow around a circular cylinder at Reynolds numbers 100 and 200 are numerically simulated using this method. The current results under the inertial and non-inertial translational frame are well validated by comparisons with other numerical and experimental benchmark tests.
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A numerical technique is developed on the solution of the Navier-Stokes equations of unsteady axisymmetric flow of a viscous incompressible fluid in any orthogonal system of coordinates. Using a finite difference scheme for time a...
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A numerical technique is developed on the solution of the Navier-Stokes equations of unsteady axisymmetric flow of a viscous incompressible fluid in any orthogonal system of coordinates. Using a finite difference scheme for time and spatial derivatives the discretization of equations of motion leads to a convergent and efficient algorithm for the un-kown values of the functions of the flow at the (n + l)th time level in terms of the known values at the nth time level for any nodal point. Accordingly this technique is used for the numerical simulation of Taylor vortices stable or time dependent in spherical annular gaps at large aspect ratios σ = 0.4, 0.44, 0.48 and 0.5 for the first time. The method also applies to more composite problems such as MHD flows, flows with heat transfer etc. Our computations are compared with available numerical results and experimental measurements with very good agreement in all cases.
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In this paper, time dependent vortex structures are numerically analyzed for both non-cavitating and cloud cavitating flows around a Clark-Y hydrofoil with angle of attack a = 8 degrees at a moderate Reynolds number, Re = 7 x 10(5...
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In this paper, time dependent vortex structures are numerically analyzed for both non-cavitating and cloud cavitating flows around a Clark-Y hydrofoil with angle of attack a = 8 degrees at a moderate Reynolds number, Re = 7 x 10(5). The numerical simulations are performed using a transport equation-based cavitation model and the large eddy simulation (LES) approach with a classical eddy viscosity subgrid scale (SGS) model. Compared with experimental results, present numerical predictions are capable of capturing the initiation of cavity, growth toward the trailing edge and subsequent shedding process. Results indicate that in noncavitating conditions, the trailing edge vortex and induced positive vortex shed periodically into the wake region to form the vortex street. In cloud cavitating conditions, interrelations between cavity and vortex induce different vortex dynamics at different cavity developing stages. (i) As attached cavity grows, vorticity production is greatly enhanced by the favorable pressure gradient at the leading edge. The trailing edge flow does not have a direct impact on the attached cavity expansion process. Furthermore, the liquid vapor interface that moves toward the trailing edge enhances the vorticity in the attached cavity closure region. (ii) When the stable attached sheet cavity grows to its maximum length, the accumulation process of vorticity is eventually interrupted by the formation of the re-entrant jet. Re-entrant jet's moving upstream leads to a higher spreading rate of the attached cavity and the formation of a large coherent structure inside the attached cavity. Moreover, the wavy/bubbly cavity interface enhances the vorticity near the trailing edge. (iii) As the attached sheet cavity breaks up, this large vortex structure converts toward the trailing edge region, which will eventually couple with a trailing edge vortex shedding from the lower surface to form the cloud cavity. The breakup of the stable attached cavity is the main reason for the vorticity enhancement near the suction surface.
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A non-inertial vorticity-streamfunction algorithm of the Reynolds-averaged Navier-Stokes equations is developed for studying unsteady and turbulent flows past airfoils. The equations are solved on a nonorthogonal C-grid using a co...
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A non-inertial vorticity-streamfunction algorithm of the Reynolds-averaged Navier-Stokes equations is developed for studying unsteady and turbulent flows past airfoils. The equations are solved on a nonorthogonal C-grid using a combined finite-difference/finite-volume technique. Comparative computations are carried out employing three different turbulence models. These are the algebraic zero-equation model of Baldwin and Lomax [1], and the one-equation models of Baldwin and Barth [2] and Spalart and Allmaras [3]. The performance of the models is evaluated against experiments of the Aerospatiale-A airfoil at incidences up to 40° and against two cases of an oscillating NACA 0015 airfoil experiencing light and deep stall. The computations demonstrate that the vorticity-streamfunction formulation combined with standard eddy-viscosity turbulence models is capable of predicting accurately unsteady and turbulent flows past airfoils.
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