Numerical study of turbulent flow over an S-shaped hydrofoil
01 Sep 2008-Vol. 222, Iss: 9, pp 1717-1734
TL;DR: In this paper, a numerical study of turbulent flow over the S-shaped hydrofoil at 0° angle of attack has been reported, where the flow takes place over concave and convex surfaces and is accompanied by the favorable and adverse pressure gradients and flow separation.
Abstract: In this paper, a numerical study of turbulent flow over the S-shaped hydrofoil at 0° angle of attack has been reported. Here, the flow takes place over concave and convex surfaces and is accompanied by the favourable and adverse pressure gradients and flow separation. Modelling such a flow poses a formidable challenge. In the present work four turbulence models, namely, k–∊ realizable, k–ω shear stress transport
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TL;DR: In this paper, the effect of cambered airfoil blades on self-starting of VAWT was investigated using CFD analysis by studying the flow field over the vertical axis wind turbine blades.
Abstract: This is Part-1 of the two-part paper in considering the effect of cambered airfoil blades on self-starting of vertical axis wind turbine. Part 1 reports the numerical studies on self-starting of vertical axis wind turbine with comparative studies involving NACA 0012 and cambered airfoil NACA 4415. Part 2 of the paper deals with numerical studies of NACA 0018 and cambered air foil NACA 63415. Darrieus type VAWT is attracting many researchers attention for its inherent advantages and its diversified applications. However, a disadvantage is when the rotor is stationary, no net rotational forces arises, even at high-wind speed. The principal advantage of the vertical axis format is their ability to accept wind from any direction without yawing mechanism. However, self-starting capability is the major drawbacks. Moreover, literatures based on computational analysis involving the cambered airfoil are few only. The objective of this present study is to select the suitable airfoil blades on self-starting of VAWT at low-Reynolds number. The numerical studies are carried out to identify self-starting capability of the airfoil using CFD analysis by studying the flow field over the vertical axis wind turbine blades. The commercial CFD code, ANSYS CFX 13.0© was used for the present studies. Initially, the flow over NACA 0012 was simulated and analyzed for different angles of attacks and similarly carried out for NACA 4415. The contours of static pressure distribution and velocity as well as the force and torque were obtained. Even though the lift force for cambered airfoil NACA 4415 is higher, based on the torque values of the above blade profiles, asymmetrical airfoil NACA 0012 is found to be appropriate for self-starring of VAWT.
4 citations
TL;DR: In this article, numerical simulation is carried out using commercially available tool Fluent® to predict the performance of a Wells turbine in an oscillating water column wave energy convertor, the main aim of this investigation is to predict complex flow mechanism like separation and recirculation around the turbine blades and subsequently reduction in torque coefficient at higher flow rate.
Abstract: In this paper numerical simulation is carried out using commercially available tool Fluent® to predict the performance of a Wells turbine in an oscillating water column wave energy convertor A wells turbine is the turbo machinery that rotates in same direction as the air flow through the turbine in either axial direction The main aim of this investigation is to predict complex flow mechanism like separation and recirculation around the turbine blades and subsequently reduction in torque coefficient at higher flow rate Numerical simulations have been executed by solving the RANS equations together with k-w SST turbulence model Then a detailed description of flow and overall performance analysis at different flow rate is presented in this paper
4 citations
TL;DR: In this article , high-speed photography is used to visualize the flow field characteristics of the bidirectional pump under different cavitation numbers, and quantitatively analyzed in combination with the pressure pulsation sensor.
Abstract: A bi-directional pump is designed by using S-shaped hydrofoil, is the most convenient way to achieve bi-directional operation. In this paper, high-speed photography is used to visualize the flow field characteristics of the bidirectional pump under different cavitation numbers, and the flow field changes caused by cavitation are quantitatively analyzed in combination with the pressure pulsation sensor. The results show that the operation efficiency of the bidirectional pump in reverse operation is lower than that in forward operation. Tip clearance cavitation occurs on both suction and pressure surfaces of the impeller under reverse operation and large flow. In reverse operation, the influence of guide vane on the main frequency of pressure pulsation in the impeller is obvious. The quasi-periodic vertical cavitation flow phenomenon increases the amplitude of pressure pulsation in the impeller and becomes the main component of the internal flow in the bidirectional axial flow pump.
2 citations
TL;DR: In this paper, a current SLV has been inspected as far as land and water capable abilities are concerned and it was shown that the body shape of the SLV can impact and influence the strength of the vehicle during underwater fording.
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References
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TL;DR: In this paper, two new two-equation eddy-viscosity turbulence models are presented, which combine different elements of existing models that are considered superior to their alternatives.
Abstract: Two new two-equation eddy-viscosity turbulence models will be presented. They combine different elements of existing models that are considered superior to their alternatives. The first model, referred to as the baseline (BSL) model, utilizes the original k-ω model of Wilcox in the inner region of the boundary layer and switches to the standard k-e model in the outer region and in free shear flows. It has a performance similar to the Wilcox model, but avoids that model's strong freestream sensitivity
15,459 citations
TL;DR: In this paper, the authors examined the modeling of the pressure-strain correlation of turbulent flows from a basic theoretical standpoint with a view toward developing improved second-order closure models and proved that for plane homogeneous turbulent flows the equilibrium structure of this hierarchy of models is encapsulated by a relatively simple model which is only quadratically nonlinear in the anisotropy tensor.
Abstract: The modeling of the pressure-strain correlation of turbulence is examined from a basic theoretical standpoint with a view toward developing improved second-order closure models. Invariance considerations along with elementary dynamical systems theory are used in the analysis of the standard hierarchy of closure models. In these commonly used models, the pressure-strain correlation is assumed to be a linear function of the mean velocity gradients with coefficients that depend algebraically on the anisotropy tensor. It is proven that for plane homogeneous turbulent flows the equilibrium structure of this hierarchy of models is encapsulated by a relatively simple model which is only quadratically nonlinear in the anisotropy tensor. This new quadratic model - the SSG model - is shown to outperform the Launder, Reece, and Rodi model (as well as more recent models that have a considerably more complex nonlinear structure) in a variety of homogeneous turbulent flows. Some deficiencies still remain for the description of rotating turbulent shear flows that are intrinsic to this general hierarchy of models and, hence, cannot be overcome by the mere introduction of more complex nonlinearities. It is thus argued that the recent trend of adding substantially more complex nonlinear terms containing the anisotropy tensor may be of questionable value in the modeling of the pressure-strain correlation. Possible alternative approaches are discussed briefly.
1,556 citations
TL;DR: In this article, the k-e-v2 model is used to predict the time-averaged properties of bluff body flow, which is necessary to resolve the coherent vortex shedding.
Abstract: Tlirbulent separated flows over a backstep, in a plane diffuser and around a triangular cylinder, are computed with the k-e-v2 model. These provide examples of massive separation, of smooth separation, and of unsteady vortex shedding. It is shown that to accurately predict the time-averaged properties of bluff body flow, it is necessary to resolve the coherent vortex shedding. The near-wall treatment of the v2-/22 system of equations is able to cope with both the massive and smooth separations. Good agreement between experiment and prediction is found in all
592 citations
TL;DR: In this paper, a two-dimensional, separating turbulent boundary layer for an airfoil-type flow in which the flow was accelerated and then decelerated until separation is presented.
Abstract: The problem of turbulent-boundary-layer separation due to an adverse pressure gradient is an old but still important problem in many fluid flow devices. Until recent years little quantitative experimental information was available on the flow structure downstream of separation because of the lack of proper instrumentation. The directionally sensitive laser anemometer provides the ability to measure the instantaneous flow direction and magnitude accurately. The experimental results described here are concerned with a nominally two-dimensional, separating turbulent boundary layer for an airfoil-type flow in which the flow was accelerated and then decelerated until separation. Upstream of separation single and cross-wire hot-wire anemometer measurements are also presented. Measurements in the separated zone with a directionally sensitive laser-anemometer system were obtained for U, V , $\overline{u^2}, \overline{v^2}, - \overline{uv}$ , the fraction of time that the flow moves downstream, and the fraction of time that the flow moves away from the wall. In addition to confirming the earlier conclusions of Simpson, Strickland & Barr (1977) regarding a separating airfoil-type turbulent boundary layer, much new information about the separated region has been gathered. (1) The backflow mean velocity profile scales on the maximum negative mean velocity U N and its distance from the wall N . A U + vs. y + law-of-the-wall velocity profile is not consistent with this result. (2) The turbulent velocities are comparable with the mean velocity in the backflow, although low turbulent shearing stresses are present. (3) Mixing length and eddy viscosity models are physically meaningless in the backflow and have reduced values in the outer region of the separated flow. Downstream of fully developed separation, the mean backflow appears to be divided into three layers: a viscous layer nearest the wall that is dominated by the turbulent flow unsteadiness but with little Reynolds shearing stress effects; a rather flat intermediate layer that seems to act as an overlap region between the viscous wall and outer regions; and the outer backflow region that is really part of the large-scaled outer region flow. The Reynolds shearing stress must be modelled by relating it to the turbulence structure and not to local mean velocity gradients. The mean velocities in the backflow are the results of time averaging the large turbulent fluctuations and are not related to the source of the turbulence.
273 citations
TL;DR: In this paper, direct numerical simulations of turbulent flows seeded with millions of passive inertial particles are presented, where the maximum Reynolds number is Re λ∼ 200 and the acceleration fluctuations as a function of the Stokes number in the range St ∈ [0.16:3].
Abstract: We present the results of direct numerical simulations (DNS) of turbulent flows seeded with millions of passive inertial particles. The maximum Reynolds number is Re λ∼ 200. We consider particles much heavier than the carrier flow in the limit when the Stokes drag force dominates their dynamical evolution. We discuss both the transient and the stationary regimes. In the transient regime, we study the growth of inhomogeneities in the particle spatial distribution driven by the preferential concentration out of intense vortex filaments. In the stationary regime, we study the acceleration fluctuations as a function of the Stokes number in the range St ∈ [0.16:3.3]. We also compare our results with those of pure fluid tracers (St = 0) and we find a critical behavior of inertia for small Stokes values. Starting from the pure monodisperse statistics we also characterize polydisperse suspensions with a given mean Stokes, .
215 citations