Visualization study and quantitative velocity measurements in turbulent taylor-couette flow by phantomm flow tagging: a description of the transition to turbulence
Abstract:A visualization study and Quantitative velocity measurements have been performed in Taylor-Couette flow with a medium-gap (ç= 0.356), over a large range of Taylor numbers (2.1x10(4) < Ta < 1.1x10(11)), with the outer cylinder fixed and the inner cylinder rotating about its axis. Quantitative velocity measurements were carried out using the PHANTOMM flow tagging technique. Two techniques were used for visualization study: the PHANTOMM technique that allowed flow structure visualization from small to moderate Ta… Show more
“…persist along the inner cylinder, with a structure similar to the toroidal vortices observed by Biage and Campos [4] for Re = 2.04 × 10 5 , η = 0.38 and Γ = 10. At Re = 10 5 , the iso-values of the Q-criterion obtained by DNS ( Fig.2d) show that the flow is clearly turbulent with even thinner turbulent structures.…”
Section: Resultssupporting
confidence: 68%
“…The radius ratio remains in the range 0.33 < η < 0.67, so that the cavity is considered as a middle gap cavity [4]. For enclosed systems characterized by a low aspect ratio Γ, such as in the experiment of Burin et al [5], the choice of the boundary conditions on the endcap disks is primordial and can favour the development of large Ekman recirculations…”
Section: Control Parametersmentioning
confidence: 99%
“…where R 1 and R 2 are the radii of the inner and outer cylinders respectively and H is the cavity height. Biage and Campos [4] proposed a classification of the systems depending on the value of their radius ratio η as follows: narrow-gap cavities for η > 0.67, middle-gap cavities for 0.33 < η < 0.67 and wide-gap cavities for η < 0.33. Most of the experiments up to now including the seminal one of Taylor [26] have considered cavities with an aspect ratio larger than 100.…”
The accurate prediction of fluid flow within rotating systems has a primary role for the reliability and performance of rotating machineries. The selection of a suitable model to account for the effects of turbulence on such complex flows remains an open issue in the literature. This paper reports a numerical benchmark of different approaches available within commercial CFD solvers together with results obtained by means of in-house developed or open-source available research codes exploiting an innovative Reynolds Stress Model (RSM) closure, Large Eddy Simulation (LES) and a direct numerical simulation (DNS). The predictions are compared to the experimental data of Burin et al. [5] in an original enclosed Couette-Taylor apparatus with endcap rings. The results are discussed in details for both the mean and turbulent fields. A particular attention has been turned to the scaling of the turbulent angular momentum G with the Reynolds number Re. By DNS, G is found to be proportional to Re α , the exponent α = 1.9 being constant in our case for the whole range of Reynolds numbers. Most of the approaches predict quite well the good trends apart from the k-ω SST model, which provides relatively poor agreement with the experiments even for the mean tangential velocity profile. Even though no approach appears to be fully satisfactory, the innovative RSM closure offers the best overall agreement.
“…persist along the inner cylinder, with a structure similar to the toroidal vortices observed by Biage and Campos [4] for Re = 2.04 × 10 5 , η = 0.38 and Γ = 10. At Re = 10 5 , the iso-values of the Q-criterion obtained by DNS ( Fig.2d) show that the flow is clearly turbulent with even thinner turbulent structures.…”
Section: Resultssupporting
confidence: 68%
“…The radius ratio remains in the range 0.33 < η < 0.67, so that the cavity is considered as a middle gap cavity [4]. For enclosed systems characterized by a low aspect ratio Γ, such as in the experiment of Burin et al [5], the choice of the boundary conditions on the endcap disks is primordial and can favour the development of large Ekman recirculations…”
Section: Control Parametersmentioning
confidence: 99%
“…where R 1 and R 2 are the radii of the inner and outer cylinders respectively and H is the cavity height. Biage and Campos [4] proposed a classification of the systems depending on the value of their radius ratio η as follows: narrow-gap cavities for η > 0.67, middle-gap cavities for 0.33 < η < 0.67 and wide-gap cavities for η < 0.33. Most of the experiments up to now including the seminal one of Taylor [26] have considered cavities with an aspect ratio larger than 100.…”
The accurate prediction of fluid flow within rotating systems has a primary role for the reliability and performance of rotating machineries. The selection of a suitable model to account for the effects of turbulence on such complex flows remains an open issue in the literature. This paper reports a numerical benchmark of different approaches available within commercial CFD solvers together with results obtained by means of in-house developed or open-source available research codes exploiting an innovative Reynolds Stress Model (RSM) closure, Large Eddy Simulation (LES) and a direct numerical simulation (DNS). The predictions are compared to the experimental data of Burin et al. [5] in an original enclosed Couette-Taylor apparatus with endcap rings. The results are discussed in details for both the mean and turbulent fields. A particular attention has been turned to the scaling of the turbulent angular momentum G with the Reynolds number Re. By DNS, G is found to be proportional to Re α , the exponent α = 1.9 being constant in our case for the whole range of Reynolds numbers. Most of the approaches predict quite well the good trends apart from the k-ω SST model, which provides relatively poor agreement with the experiments even for the mean tangential velocity profile. Even though no approach appears to be fully satisfactory, the innovative RSM closure offers the best overall agreement.
“…The outer Plexiglas cylinder of radius b ¼ 9 cm is stationary. The cavity is thus defined by an aspect ratio equal to G ¼ h/(bÀa) ¼ 50 and a radius ratio equal to h ¼ a/b ¼ 0.89 (narrow gap cavity [19]). The outer cylinder is made of PMMA to allow optical velocimetry measurements.…”
Section: General Description and Flow Parametersmentioning
“…Considering the full operative range for both pressure and velocity, the maximum torque, shown in Figure 5, is about 200 times the minimum value, highlighting once more how crucial it is to model such losses, already in the preliminary design stage. For the particular configuration considered in this work, where the radii ratio is R 1 /R 2 = 0.94 and the gap between the two cylinders is classified as small [30], the torque at atmospheric pressure (p i = 100 kPa) is about 20 times greater than the torque at vacuum conditions (p i = 1 kPa), for the lowest rotational speed (200 rpm), and about 30 times greater, for the maximum rotational speed (800 rpm). On the other hand, Figure 6 shows that the total torque at a given pressure is about linearly proportional to the flywheel velocity, with a ratio between the lowest and the highest torques (M high /M low ) equal to 12.…”
The wave energy sector is experiencing lively years of conceptual innovation and technological advances. Among the great variety of candidates, only a few are going to be able to reach maturity and, eventually, industrial feasibility and competitiveness. The essential requisite for success is the continuous innovation in response to the incremental experience gained during the design and prototyping stages. In particular, the ability to generate detailed mathematical models, representative of every phenomenon involved in the system, is crucial for informing the design and control stages, allowing to maximize productivity while minimizing costs, and inspiring technological breakthrough and innovation. This papers considers the case of the ISWEC (Inertial Sea Wave Energy Converter), where a technological leap is tightly linked with the modelling of aerodynamic losses around its spinning flywheel, the core of the energy conversion chain. Two mathematical models of increasing complexity are considered, one semi-empiric and one based on computational fluid dynamics, which are successfully validated against experimental data. Such models are used to quantify the benefits of a technological innovation consisting of enclosing the flywheel in a sealed container, allowing pressure regulation to reduce aerodynamic friction. Compared to the free configuration, power losses with the enclosed configuration are about half already at atmospheric pressure, and about one third at half the atmospheric pressure.
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