Organized structures in a turbulent plane jet: topology and contribution to momentum and heat transport

Antonia, R. A.; Chambers, A. J.; Britz, Dieter; Browne, L. W. B.

doi:10.1017/s0022112086001714

Cited by 86 publications

(48 citation statements)

References 16 publications

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“…Although the turbulence intensity could intuitively be associated with the flow mixing, it has been demonstrated that the Reynolds shear stresses uv, uw, and vw are responsible of the flow kinetic energy transport (Antonia et al, 1986;Cal et al, 2010;Cantwell and Coles, 1983;Escudie and Line, 2003;Hussain, 1983;and Reynolds and Hussain, 1972) and that as such they must be taken into consideration for evaluating the turbulent mixing in a particular region of the flow. For this reason, the last term of Equation (2), which represents the spatial gradients of the flux of mean-flow kinetic energy, has been evaluated in the x-y plane as U ¼ Àuðu 0 v 0 Þ (streamwise mean-flow kinetic energy flux in the radial direction).…”

Section: F Wake Mixingmentioning

confidence: 99%

Experimental comparison of a wind-turbine and of an actuator-disc near wake

Lignarolo

Ragni

Ferreira

et al. 2016

Journal of Renewable and Sustainable Energy

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The actuator disc (AD) model is commonly used to simplify the simulation of horizontal-axis wind-turbine aerodynamics. The limitations of this approach in reproducing the wake losses in wind farm simulations have been proven by a previous research. The present study is aimed at providing an experimental analysis of the near-wake turbulent flow of a wind turbine (WT) and a porous disc, emulating the actuator disc numerical model. The general purpose is to highlight the similarities and to quantify the differences of the two models in the near-wake region, characterised by the largest discrepancies. The velocity fields in the wake of a wind turbine model and a porous disc (emulation of the actuator disc numerical model) have been measured in a wind tunnel using stereo particle image velocimetry. The study has been conducted at low turbulence intensity in order to separate the problems of the flow mixing caused by the external turbulence and the one caused by the turbulence induced directly by the AD or the WT presence. The analysis, as such, showed the intrinsic differences and similarities between the flows in the two wakes, solely due to the wake-induced flow, with no influence of external flow fluctuations. The data analysis provided the time-average three-component velocity and turbulence intensity fields, pressure fields, rotor and disc loading, vorticity fields, stagnation enthalpy distribution, and mean-flow kinetic-energy fluxes in the shear layer at the border of the wake. The properties have been compared in the wakes of the two models. Even in the absence of turbulence, the results show a good match in the thrust and energy coefficient, velocity, pressure, and enthalpy fields between wind turbine and actuator disc. However, the results show a different turbulence intensity and turbulent mixing. The results suggest the possibility to extend the use of the actuator disc model in numerical simulation until the very near wake, provided that the turbulent mixing is correctly represented.

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Section: F Wake Mixingmentioning

confidence: 99%

Experimental comparison of a wind-turbine and of an actuator-disc near wake

Lignarolo

Ragni

Ferreira

et al. 2016

Journal of Renewable and Sustainable Energy

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“…Experimental studies in the past 40 years acquired abundant evidence and data supporting the dynamical role of CS. CS control entrainment and mixing of species (Dimotakis & Brown 1976, Dutton & Lucht 2006, and contribute comparable amount to the Reynolds stresses and heat flux as does random turbulence (Hussain & Zaman 1985, Antonia et al 1986). The Reynolds stresses may take negative values in the region of positive mean-flow gradient, indicating that closure models of the mean-flow gradient type are inappropriate.…”

Section: Introductionmentioning

confidence: 99%

Nonlinear dynamics of large-scale coherent structures in turbulent free shear layers

Zhuang

2015

J. Fluid Mech.

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Fully developed turbulent free shear layers exhibit a high degree of order, characterized by large-scale coherent structures in the form of spanwise vortex rollers. Extensive experimental investigations show that such organised motions bear remarkable resemblance to instability waves, and their main characteristics, including the length scales, propagation speeds and transverse structures, are reasonably well predicted by linear stability analysis of the mean flow. In this paper, we present a mathematical theory to describe the nonlinear dynamics of coherent structures. The formulation is based on the triple decomposition of the instantaneous flow into a mean field, coherent fluctuations and small-scale turbulence but with the mean-flow distortion induced by nonlinear interactions of coherent fluctuations being treated as part of the organised motion. The system is closed by employing gradient type of models for the time-and phase-averaged Reynolds stresses of fine-scale turbulence. In the high-Reynolds-number limit, the nonlinear non-equilibrium critical-layer theory for laminar-flow instabilities is adapted to turbulent shear layers by accounting for (a) the enhanced non-parallelism associated with fast spreading of the mean flow, and (b) the influence of small-scale turbulence on coherent structures. The combination of these factors with nonlinearity leads to an interesting evolution system, consisting of the coupled amplitude and vorticity equations, in which non-parallelism contributes the so-called translating critical-layer effect. Numerical solutions of the evolution system capture vortex roll-up, which is the hallmark of turbulent mixing layer, and the predicted amplitude development mimics the qualitative feature of oscillatory saturation that has been observed in a number of experiments. A fair degree of quantitative agreement is obtained with one set of experimental data.

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“…Of course upstream events doubtless play a role: indeed Didden & Ho (1985) show that the ring vortices periodically generated in the shear layer of an axisymmetric impinging jet induce unsteady separation in the impingment region, a feature further explored by Fox et al (1993). But the evolution of coherent structures in a plane jet (Antonia et al 1986), not to mention plane impinging jet, are not nearly so well documented as say those in an axisymmetric impinging jet or in a plane mixing layer. The object of the present work therefore was to determine not only the phase evolution of the three-dimensional vorticity field in the impingment region, but also the structure of the plane jet.…”

Section: Introductionmentioning

confidence: 99%

On the vortical structure in a plane impinging jet

2001

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The vortical structure of a plane impinging jet is considered. The jet was locked both in phase and laterally in space, and time series digital particle image velocimetry measurements were made both of the jet exiting the nozzle and as it impinged on a perpendicular wall. Iso-vorticity and iso-λ 2 surfaces coupled with critical point theory were used to identify and clarify structure. The flow near the nozzle was much as observed in mixing layers, where the shear layer evolves into spanwise rollers, only here the rollers occurred symmetrically about the jet midplane. Accordingly the rollers were seen to depict spanwise perturbations with the wavelength of flutes at the nozzle edge and were connected, on the same side of the jet, with streamwise 'successive ribs' of the same wavelength. This wavelength was 0.71 of the distance between rollers and, contrary to some experiments in mixing layers, did not double when the rollers paired. Structures not reported previously but evident here with iso-vorticity, λ 2 and critical point theory are 'cross ribs', which extend from the downstream side of each roller to its counterpart across the symmetry plane; their spanwise periodic spacing exceeds that of successive ribs by a factor of three. Cross ribs stretch because of the diverging flow as the rollers approach the wall and move apart, causing the vorticity within them to intensify. This process continues until the cross ribs reach the wall and merge with 'wall ribs'. Wall ribs remain near the wall throughout the cycle and are composed of vorticity of the same sign as the cross ribs, but the intensity level of the vorticity within them is cyclic. Details of the expansion of fluid elements, evaluated from the rate of strain tensor, revealed that both cross and successive ribs align with the principal axis and that the vorticity comprising them is continuously amplified by stretching. It is further shown, by appeal to the production terms of the phase-averaged vorticity equation, that wall ribs are sustained by merging and stretching rather than reorientation of vorticity. Moreover production of vorticity is a maximum when cross and wall ribs merge and is greatest near the symmetry plane of the jet. The demise of successive ribs on the other hand occurs away from the symmetry plane and would appear to be less important dynamically than cross ribs merging with wall ribs.

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Organized structures in a turbulent plane jet: topology and contribution to momentum and heat transport

Cited by 86 publications

References 16 publications

Experimental comparison of a wind-turbine and of an actuator-disc near wake

Experimental comparison of a wind-turbine and of an actuator-disc near wake

Nonlinear dynamics of large-scale coherent structures in turbulent free shear layers

On the vortical structure in a plane impinging jet

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