Thermal fluctuations in low-Prandtl number fluid flows over a backward facing step

Oder, Jure; Tiselj, Iztok; Jäger, Wadim; Schaub, T.; Hering, W.; Otić, Ivan; Shams, A.

doi:10.1016/j.nucengdes.2019.110460

Cited by 11 publications

(4 citation statements)

References 16 publications

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“…In this project, the transition from the heat collecting pipes or subassemblies to a collector or upper plenum is investigated. Pre-test simulations with CFD, DNS and RANS are used to optimize position and range of instrumentation [30]. To simplify maintenance both test sections (Fig.…”

Section: Accepted Manuscript N O T C O P Y E D I T E Dmentioning

confidence: 99%

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Experiment and Codes to Support Safety Assessments for Sodium Fast Reactors (KASOLA, SOLTEC and KARIFA)

Hering

Fuchs

Onea

et al. 2021

Journal of Nuclear Engineering and Radiation Science

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The experimental liquid metal (LM) loops hosted within the Karlsruhe Sodium laboratory (KASOLA) com-prise a set of facilities to study Liquid Metal (LM) flows for various types of energy applications ranging from room temperature conditions used for education and training and fundamental research up to challenges posed by multi-physics problems such as material-fluid interactions at high temperatures. Extreme conditions, such as sodium boiling, relevant to thermo-electric conversion or fast reactor safety are covered in a dedicated facility small -scale (KARIFA). The complete experimental range is comple-mented by system code support and CFD simulation. The outcome is used for validation and develop-ment allowing application not only on component but also on system scale.

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Section: Accepted Manuscript N O T C O P Y E D I T E Dmentioning

confidence: 99%

“…The facility consists mainly of four sections: a pumping, a flow conditioning, a test and a return section [35]. Here only a broad overview is given, a detailed description of each section can be found in [30].…”

Section: Overviewmentioning

confidence: 99%

Experiment and Codes to Support Safety Assessments for Sodium Fast Reactors (KASOLA, SOLTEC and KARIFA)

Hering

Fuchs

Onea

et al. 2021

Journal of Nuclear Engineering and Radiation Science

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“…Therefore, we also compared the accuracy of the Nek5000 solver in the channel upstream of the step with the same distribution of elements to the results obtained with a conservative pseudospectral method that was used in the channel simulation. If the reader is interested in a comparison of Reynolds Averaged Navier Stokes (RANS) simulation and LES results, they can be found in [10]. In the present paper, we rather focus on the statistical uncertainties found in the channel flow with two homogeneous directions, and on the BFS flow with no homogeneous directions.…”

Section: Introductionmentioning

confidence: 99%

Statistical Uncertainty of DNS in Geometries without Homogeneous Directions

2021

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In this paper, we present uncertainties of statistical quantities of direct numerical simulations (DNS) with small numerical errors. The uncertainties are analysed for channel flow and a flow separation case in a confined backward facing step (BFS) geometry. The infinite channel flow case has two homogeneous directions and this is usually exploited to speed-up the convergence of the results. As we show, such a procedure reduces statistical uncertainties of the results by up to an order of magnitude. This effect is strongest in the near wall regions. In the case of flow over a confined BFS, there are no such directions and thus very long integration times are required. The individual statistical quantities converge with the square root of time integration so, in order to improve the uncertainty by a factor of two, the simulation has to be prolonged by a factor of four. We provide an estimator that can be used to evaluate a priori the DNS relative statistical uncertainties from results obtained with a Reynolds Averaged Navier Stokes simulation. In the DNS, the estimator can be used to predict the averaging time and with it the simulation time required to achieve a certain relative statistical uncertainty of results. For accurate evaluation of averages and their uncertainties, it is not required to use every time step of the DNS. We observe that statistical uncertainty of the results is uninfluenced by reducing the number of samples to the point where the period between two consecutive samples measured in Courant–Friedrichss–Levy (CFL) condition units is below one. Nevertheless, crossing this limit, the estimates of uncertainties start to exhibit significant growth.

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“…Many of the previous studies were focused on the heat transfer behind the BFS expansion that might include natural or mixed convection cases [16]. Especially in nuclear engineering, studies are focused on heat transfer of liquid metal flows [12,17]. An extensive review of separated flows, including the BFS flows, has been recently performed by Terekhov [1].…”

Section: Introductionmentioning

confidence: 99%

Turbulent Flow over Confined Backward-Facing Step: PIV vs. DNS

et al. 2021

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Particle Image Velocimetry measurements of the liquid velocity fields in the flow over the backward-facing step were performed in the same flow configuration as in the existing Direct Numerical Simulation (DNS). The experiment and the simulation were performed in an identical cross-section geometry with step expansion rate 2.25 and the square shape of the outlet duct at the Reynolds number in an inlet part of the section 7100. The experiment was performed in transparent test section, 1.2 m long, with 20 × 45 mm2 cross-section upstream and 45 × 45 mm2 downstream, while a domain that was three times shorter was used in the DNS. A 2D-2C PIV system with a single high-speed camera and a pulse laser was used for a series of two-dimensional measurements of the velocity field at several cross-sections from two different perspectives. Variables analyzed in the experiment are time-averaged fluid velocities, velocity RMS fluctuations and two components of the Reynolds stress tensor. The key novelty is the comparison of two very accurate approaches, PIV and DNS, in the same cross-section geometry. Comparison of the similarities, and especially the differences between the two approaches, elucidates uncertainties of both studies and answers the question on what kind of agreement is expected when two very accurate approaches are compared.

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Thermal fluctuations in low-Prandtl number fluid flows over a backward facing step

Cited by 11 publications

References 16 publications

Experiment and Codes to Support Safety Assessments for Sodium Fast Reactors (KASOLA, SOLTEC and KARIFA)

Experiment and Codes to Support Safety Assessments for Sodium Fast Reactors (KASOLA, SOLTEC and KARIFA)

Statistical Uncertainty of DNS in Geometries without Homogeneous Directions

Turbulent Flow over Confined Backward-Facing Step: PIV vs. DNS

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