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Journal articles on the topic "Pr-Dns"
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Tenneti, Sudheer, Mohammad Mehrabadi, and Shankar Subramaniam. "Stochastic Lagrangian model for hydrodynamic acceleration of inertial particles in gas–solid suspensions." Journal of Fluid Mechanics 788 (January 12, 2016): 695–729. http://dx.doi.org/10.1017/jfm.2015.693.
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The acceleration of an inertial particle in a gas–solid flow arises from the particle’s interaction with the gas and from interparticle interactions such as collisions. Analytical treatments to derive a particle acceleration model are difficult outside the Stokes flow regime, but for moderate Reynolds numbers (based on the mean slip velocity between gas and particles) particle-resolved direct numerical simulation (PR-DNS) is a viable tool for model development. In this study, PR-DNS of freely-evolving gas–solid suspensions are performed using the particle-resolved uncontaminated-fluid reconcilable immersed-boundary method (PUReIBM) that has been extensively validated in previous studies. Analysis of the particle velocity variance (granular temperature) equation in statistically homogeneous gas–solid flow shows that a straightforward extension of a class of mean particle acceleration models (drag laws) to their corresponding instantaneous versions, by replacing the mean particle velocity with the instantaneous particle velocity, predicts a granular temperature that decays to zero, which is at variance with the steady particle granular temperature that is obtained from PR-DNS. Fluctuations in particle velocity and particle acceleration (and their correlation) are important because the particle acceleration–velocity covariance governs the evolution of the particle velocity variance (characterized by the particle granular temperature), which plays an important role in the prediction of the core annular structure in riser flows. The acceleration–velocity covariance arising from hydrodynamic forces can be decomposed into source and dissipation terms that appear in the granular temperature evolution equation, and these have already been quantified in the Stokes flow regime using a combination of kinetic theory closure and multipole expansion simulations. From PR-DNS data we show that the fluctuations in the particle acceleration that are aligned with fluctuations in the particle velocity give rise to a source term in the granular temperature evolution equation. This approach is used to quantify the hydrodynamic source and dissipation terms of granular temperature from PR-DNS results for freely-evolving gas–solid suspensions that are performed over a wide range of solid volume fraction ($0.1\leqslant {\it\phi}\leqslant 0.4$), Reynolds number based on the slip velocity between the solid and the fluid phase ($10\leqslant \mathit{Re}_{m}\leqslant 100$) and solid-to-fluid density ratio ($100\leqslant {\it\rho}_{p}/{\it\rho}_{f}\leqslant 2000$). The straightforward extension of drag law models does not give rise to any source in the granular temperature due to hydrodynamic effects. This motivates the development of better Lagrangian particle acceleration models that can be used in Lagrangian–Eulerian formulations of gas–solid flow. It is found that a Langevin equation for the increment in the particle velocity reproduces PR-DNS results for the stationary particle velocity autocorrelation in freely-evolving suspensions. Based on the data obtained from the simulations, the functional dependence of the Langevin model coefficients on solid volume fraction, Reynolds number and solid-to-fluid density ratio is obtained. This new Lagrangian particle acceleration model reproduces the correct steady granular temperature and can also be adapted to gas–solid flow computations using Eulerian moment equations.
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KERR, ROBERT M., and JACKSON R. HERRING. "Prandtl number dependence of Nusselt number in direct numerical simulations." Journal of Fluid Mechanics 419 (September 25, 2000): 325–44. http://dx.doi.org/10.1017/s0022112000001464.
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The dependence of the Nusselt number Nu on the Rayleigh Ra and Prandtl Pr number is determined for 104 < Ra < 107 and 0.07 < Pr < 7 using DNS with no-slip upper and lower boundaries and free-slip sidewalls in a 8 × 8 × 2 box. Nusselt numbers, velocity scales and boundary layer thicknesses are calculated. For Nu there are good comparisons with experimental data and scaling laws for all the cases, including Ra2/7 laws at Pr = 0.7 and Pr = 7 and at low Pr, a Ra1/4 regime. Calculations at Pr = 0.3 predict a new Nu ∼ Ra2/7 regime at slightly higher Ra than the Pr = 0.07 calculations reported here and the mercury Pr = 0.025 experiments.
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Song, Jiajun, Panxin Li, Lu Chen, Yuhang Zhao, Fengshi Tian, and Benwen Li. "Scaling Law of Flow and Heat Transfer Characteristics in Turbulent Radiative Rayleigh-Bénard Convection of Optically Thick Media." Energies 17, no. 19 (October 8, 2024): 5009. http://dx.doi.org/10.3390/en17195009.
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Radiative natural convection is of vital importance in the process of energy storage, power generation, and thermal storage technology. As the attenuation coefficients of many heat transfer media in these fields are high enough to be considered as optically thick media, like nanofluids or molten salts in concentrated solar power or phase change thermal storage, Rosseland approximation is commonly used. In this paper, we delve into the impact of thermal radiation on the Rayleigh-Bénard (RB) convection. Theoretical analysis has been conducted by modifying the Grossmann-Lohse (GL) model. Based on turbulent dissipation theory, the corresponding scaling laws in four main regimes are proposed. Direct numerical simulation (DNS) was also performed, revealing that radiation exerts a notable influence on both flow and heat transfer, particularly on the formation of large-scale circulation. By comparing with DNS results, it is found that due to the presence of radiation, the modified Nu scaling law in small Pr range of the GL model is more suitable for predicting the transport characteristics of optical thick media with large Pr. The maximum deviation between the results of DNS and prediction model is about 10%, suggesting the summarized scaling law can effectively predict the Nu of radiative RB convection.
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Fu, Jianhong, Sheng Chen, and Xiaochen Zhou. "Effect of heterogeneity on interphase heat transfer for gas–solid flow: A particle-resolved direct numerical simulation." Physics of Fluids 34, no. 12 (December 2022): 123317. http://dx.doi.org/10.1063/5.0130850.
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Particle-resolved direct numerical simulation (PR-DNS) of flow past a particle cluster is conducted to analyze the influence of heterogeneous particle distribution on the gas–solid heat transfer calculation. Then, the heat transfer rates calculated using Gunn's correlation are systematically compared with the DNS results for virtual computational fluid dynamics-discrete element method (CFD-DEM) grids with different levels of heterogeneity. The results show that, for a grid located at the interface between the dense cluster region and dilute region, Gunn's correlation significantly overestimates the heat transfer rate, especially at small Reynolds numbers. This is caused by the large temperature difference between the dense and dilute regions in the heterogeneous CFD-DEM grid. The value calculated by Gunn's correlation can be up to ten times the DNS result. For a homogeneous grid inside a dense region, the conventional Nusselt correlation fails to capture the rapid increase in the fluid temperature gradient around the near-interface particles when the grid approaches the cluster–fluid interface. Furthermore, even if the size of the CFD-DEM grid is reduced to twice the particle diameter, the heterogeneous particle distribution still leads to a remarkable error in the heat transfer calculation. Finally, modifications to Gunn's correlation are proposed for three typical cross-interface cases, which can well reflect the influence of the heterogeneous distribution of particles and yield a heat transfer rate close to the PR-DNS results. The mean relative deviations of the three fitted correlations are 5.8%, 14.3%, and 22.4%, respectively.
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Cui, Haihang, Qi Chang, Jianhua Chen, and Wei Ge. "PR-DNS verification of the stability condition in the EMMS model." Chemical Engineering Journal 401 (December 2020): 125999. http://dx.doi.org/10.1016/j.cej.2020.125999.
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Luo, Heng, Fengbin Zhang, Haibo Huang, Yong Huang, Zhendong Liu, Jianxi Yan, and Chicheng Yang. "The Effect of Ellipsoidal Particle Surface Roughness on Drag and Heat Transfer Coefficients Using Particle-Resolved Direct Numerical Simulation." Processes 12, no. 11 (November 7, 2024): 2473. http://dx.doi.org/10.3390/pr12112473.
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The purpose of this study is to estimate the effect of roughness layer thickness on the heat transfer and drag coefficients of ellipsoidal particles. Using an OpenFOAM-based particle-resolved direct numerical simulation (PR-DNS) method, we calculated the drag coefficient and Nusselt number for an isolated axisymmetric nonspherical particle with a rough surface in a uniform flow. The PR-DNS results indicate that the drag coefficient varies linearly with the effective roughness Sef at different angles, which can be expressed as CD=kSef−1+CD0. The changes in k are consistent with the Happel and Brenner equation. Furthermore, the influence of roughness on the heat transfer efficiency factor can be represented by Ef=Sef−65. The models for the drag coefficient and Nusselt number are valid within the ranges 1.25≤Ar≤2.5,1≤Sef≤2, and 10≤Rep≤200, thereby extending the applicability of the equations developed for smooth particles. These newly developed correlations for the drag coefficient and Nusselt number can be utilized for non-isothermal flows of particle mixtures containing materials with various rough-surfaced ellipsoids.
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Chilamkurti, Yesaswi N., and Richard D. Gould. "CFD-DEM and PR-DNS studies of low-temperature densely packed beds." International Journal of Heat and Mass Transfer 159 (October 2020): 120056. http://dx.doi.org/10.1016/j.ijheatmasstransfer.2020.120056.
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Wu, X., and P. A. Durbin. "Numerical Simulation of Heat Transfer in a Transitional Boundary Layer With Passing Wakes." Journal of Heat Transfer 122, no. 2 (November 29, 1999): 248–57. http://dx.doi.org/10.1115/1.521485.
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Direct numerical simulation (DNS) has been used to investigate heat transfer and provide thermal statistics in a transitional flow in which turbulent wakes traversing the inlet periodically are swept downstream across a constant-temperature flat-plate. The same heat transfer problem was also computed using unsteady Reynolds-averaged Navier-Stokes (RANS) method with the v2-f turbulence model. During transition, the instantaneous Stanton number field exhibits spotlike structure, which in turn results in a strong streamwise modulation in the phase-averaged Stanton number distribution. At molecular Prandtl number Pr=0.7, the Reynolds analogy factor decreases in the transitional region but remains nearly constant afterwards. After the completion of transition, mean and second-order temperature statistics are in good agreement with previous experimental data from slightly heated turbulent flat-plate boundary layers. Throughout the transitional and turbulent regions the turbulent Prandtl number increases sharply as the wall is asymptotically approached. DNS results at a higher wake passing frequency are also presented to illustrate the effect of freestream turbulence. Unsteady RANS predictions of the time- and phase-averaged Stanton numbers as well as the enthalpy thickness are in reasonable agreement with the DNS. [S0022-1481(00)02002-8]
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Trane, D., M. Grespan, and D. Angeli. "Comparison between DNS and RANS approaches for liquid metal flows around a square rod bundle." Journal of Physics: Conference Series 2766, no. 1 (May 1, 2024): 012009. http://dx.doi.org/10.1088/1742-6596/2766/1/012009.
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Abstract The thermal-hydraulic characteristics of liquid metal flows around rod bundles are of great interest for the research and design of fourth generation nuclear reactors. Currently, a large research effort is aimed at the development of accurate numerical models for low Prandtl number fluid flows, since the data available in the literature are quite scarce. Direct Numerical Simulation (DNS) is undoubtedly the most accurate approach, but its large requirements of computational resources and time make it less practical than other simplified methods such as the Reynolds-Average Navier Stokes (RANS) approach. The present paper provides a comparison between numerical results of a flow of liquid Lead-Bismuth Eutectic (LBE) at Pr=0.031 around four vertical cylindrical rods arranged in a square lattice, obtained by DNS and RANS. Several turbulence models are considered, including the standard k−ε, k−ω SST, and two Reynolds stress models, namely the ones by Launder, Reece and Rodi (LRR), and Speziale, Sarkar and Gatski (SSG). The accuracy of these models is assessed by comparing the mean Nusselt number, the pressure drop, and local field distributions with those obtained by DNS.
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Lakehal, D., M. Fulgosi, G. Yadigaroglu, and S. Banerjee. "Direct Numerical Simulation of Turbulent Heat Transfer Across a Mobile, Sheared Gas-Liquid Interface." Journal of Heat Transfer 125, no. 6 (November 19, 2003): 1129–39. http://dx.doi.org/10.1115/1.1621891.
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The impact of interfacial dynamics on turbulent heat transfer at a deformable, sheared gas-liquid interface is studied using Direct Numerical Simulation (DNS). The flow system comprises a gas and a liquid phase flowing in opposite directions. The governing equations for the two fluids are alternately solved in separate domains and then coupled at the interface by imposing continuity of velocity and stress. The deformations of the interface fall in the range of capillary waves of waveslope ak=0.01 (wave amplitude a times wavenumber k), and very small phase speed-to-friction velocity ratio, c/u*. The influence of low-to-moderate molecular Prandtl numbers Pr on the transport in the immediate vicinity of the interface is examined for the gas phase, and results are compared to existing wall-bounded flow data. The shear-based Reynolds number Re* is 171 and Prandtl numbers of 1, 5, and 10 were studied. The effects induced by changes in Pr in both wall-bounded flow and over a gas-liquid interface were analyzed by comparing the relevant statistical flow properties, including the budgets for the temperature variance and the turbulent heat fluxes. Overall, Pr was found to affect the results in very much the same way as in most of the available wall flow data. The intensity of the averaged normal heat flux at high Prandtl numbers is found to be slightly greater near the interface than at the wall. Similar to what is observed in wall flows, for Pr=1 the turbulent viscosity and diffusivity are found to asymptote with z+3, where z+ is the distance to the interface, and with z+n, where n>3 for Pr=5 and 10. This implies that the gas phase perceives deformable interfaces as impermeable walls for small amplitude waves with wavelengths much larger than the diffusive sublayers. Moreover, high-frequency fluctuating fields are shown to play a minor role in transferring heat across the interface, with a marked filtering effect of Pr. A new scaling law for the normalized heat transfer coefficient, K+ has been derived with the help of the DNS data. This law, which could be used in the range of Pr=1 to 10 for similar flow conditions, suggests an approximate Pr−3/5 relationship, lying between the Pr−1/2 dependence for free surfaces and the Pr−2/3 law for immobile interfaces and much higher Prandtl numbers. A close inspection of the transfer rates reveals a strong and consistent relationship between K+, the frequency of sweeps impacting the interface, the interfacial velocity streaks, and the interfacial shear stress.
Dissertations / Theses on the topic "Pr-Dns"
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Butaye, Edouard. "Modélisation et simulations résolues d'écoulement fluide-particules : du régime de Stokes aux lits fluidisés anisothermes." Electronic Thesis or Diss., Perpignan, 2024. http://www.theses.fr/2024PERP0029.
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Les centrales solaires à tour utilisent le flux solaire concentré pour chauffer un fluide caloporteur et générer de l'électricité grâce à un cycle thermodynamique. Pour augmenter le rendement de conversion thermique/électrique, on cherche à augmenter la température de sortie du récepteur à au moins 800°C. Une alternative aux fluides conventionnels réside dans l'utilisation de particules fluidisées par de l'air pour ainsi augmenter la température de travail et maximiser le transfert de chaleur pariétal. Les particules solides utilisées peuvent supporter des températures dépassant les 1000°C sans dégradation de leurs propriétés physiques et peuvent également stocker efficacement la chaleur. Pour répondre à ces enjeux, il est nécessaire de caractériser l'écoulement au sein du tube récepteur ainsi que les mécanismes physiques de transfert de chaleur dans ces configurations. Ce travail s'intéresse particulièrement à la description locale des écoulements anisothermes fluide-particules à l'aide de simulations numériques directes en particules résolues (PR-DNS) réalisées en calcul hautes performances. Des améliorations de l'outil permettant de réaliser des simulations résolues de ces écoulements sont tout d'abord apportées au code pour calculer des grandeurs d'intérêts et optimiser la méthode. Ensuite, plusieurs configurations de lits fluidisés liquide-solide sont étudiées pour caractériser extensivement la dynamique de l'écoulement. Les transferts thermiques pariétaux sont également capturés ainsi que les transferts thermiques entre le fluide et les particules. Des configurations gaz-solide sont étudiées afin de valider l'outil de simulation numérique pour modéliser ces écoulements. Finalement, une nouvelle échelle de résolution est proposée, en particules résolues avec une correction sous-mailles (PR-SCS). Cette échelle permet de modéliser précisément les efforts hydrodynamiques malgré une résolution grossièreSolar tower power plants harness concentrated solar flux to heat a fluid and generate electricity through a thermodynamic cycle that generates steam and drives a turbo-alternator. To increase thermal/electrical conversion efficiency, it is a required to raise the receiver outlet temperature to at least 800°C. An alternative to conventional fluids is to use air-fluidized particles to raise the working temperature and maximize parietal heat transfer. The solid particles used can withstand temperatures in excess of 1000°C without degrading their physical properties, and store heat efficiently. To meet these challenges, it is necessary to characterize the flow within the receiving tube, as well as the physical mechanisms of heat transfer in these configurations. This work focuses on the local description of anisothermal fluid-particle flows using particle-resolved direct numerical simulations (PR-DNS) with high-performance computing. Improvements are first implemented in the code to compute quantities of interest and optimize the numerical method. Next, several liquid-solid fluidized bed configurations are studied to extensively characterize flow dynamics. Parietal heat transfers are also computed as well as fluid-particle heat transfers. Gas-solid configurations are studied to validate the numerical simulation tool for modeling these flows. Finally, a new scale of resolution is proposed, referred to as Particle Resolved - Subgrid Corrected Simulation (PR-SCS). This scale enables hydrodynamic forces to be accurately modeled despite the coarse resolution
Conference papers on the topic "Pr-Dns"
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Bergant, R., and I. Tiselj. "The Smallest Temperature Scales in a Turbulent Channel Flow at High Prandtl Numbers." In ASME 2005 Summer Heat Transfer Conference collocated with the ASME 2005 Pacific Rim Technical Conference and Exhibition on Integration and Packaging of MEMS, NEMS, and Electronic Systems. ASMEDC, 2005. http://dx.doi.org/10.1115/ht2005-72495.
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The object of this paper is to perform the numerical simulations of the temperature fields at low Reynolds numbers, i.e. Reτ = 150 and Reτ = 170.8, and at Prandtl numbers, Pr = 1 and Pr = 100. The spatial scales of the velocity field can be successfully described with DNS accuracy, meanwhile the scales of temperature fields decreasing approximately with Pr3/2 and cannot be resolved for entire energy scalar spectra due to the computer limitations. To overcome these obstacles, filtering and damping of the highest temperature wave number modes in homogeneous directions are introduced rather than modeling the unresolved subgrid scales. First, numerical simulations at Pr = 1 are performed in order to make comparison of the temperature field described with DNS accuracy and filtered and damped temperature fields described with coarser numerical grids. Obtained results show that at least first and second order statistics are comparable to the DNS ones. Next step is to analyze this approach at two orders of magnitude higher Prandtl numbers, i.e: Pr = 100. Comparison is a little bit difficult because no real DNS of the temperature fields at such high Prandtl number has been performed so far. But we estimate that results are still accurate at least in the proximity of the wall.
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Bergant, Robert, Iztok Tiselj, and Gad Hetsroni. "Near-Wall Turbulent Heat Transfer at Prandtl Numbers 1 to 54." In ASME 2002 International Mechanical Engineering Congress and Exposition. ASMEDC, 2002. http://dx.doi.org/10.1115/imece2002-32006.
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Direct Numerical Simulation (DNS) of fully developed turbulent flow in a flume was used to study the heat transfer near the wall. The Reynolds number has very weak influence on the turbulent heat transfer statistics (mean temperature, RMS-fluctuations, turbulent heat fluxes), therefore our goal was to analyze the influence of the increasing Prandtl number. Three different studies were performed at three different Prandtl numbers (Pr = 1, Pr = 5.4 and Pr = 54) at the same friction Reynolds number Reτ = 171. It should be emphasized that simulation with Pr = 54 cannot be called DNS due to the unresolved smallest thermal scales but results are in expected regions anyway. The obtained results at various Prandtl numbers also allowed us to make some predictions (RMS-fluctuations) for intermediate Prandtl numbers.
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Tiselj, Iztok, and Luka Sˇtrubelj. "Passive Scalar Turbulent Channel Flow at Pr=25: DNS-LES Approach." In ASME/JSME 2007 5th Joint Fluids Engineering Conference. ASMEDC, 2007. http://dx.doi.org/10.1115/fedsm2007-37325.
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DNS-LES numerical simulations of a passive scalar field in the turbulent channel flow were performed at friction Reynolds number Re_Tau = 180 and Prandtl number Pr = 25. Direct numerical simulation is used for description of the velocity field. Temperature field is described with LES-like approach with the smallest resolved temperature scales equal to the smallest scales of the velocity field. The consistency of the applied physical modelling and pseudo-spectral scheme is tested with the grid refinement study (grid refine ∼3 times in each direction) and with comparison of the results with the existing DNS simulations of Schwertfirm and Manhart (2006) at the same conditions. The comparison shows that the proposed approach produces very accurate mean temperature profiles, heat transfer coefficients and other low-order moments of the turbulent thermal field. It is shown that the mean temperature profiles near the wall can be accurately predicted even when the temperature scales between the Batchelor and Kolmogorov scale are not resolved. The key to the success of the proposed approach lies in the fact that the large-scale structures govern the turbulent heat transfer at high Prandtl numbers, while the role of the sub-Kolmogorov temperature scales in the diffusive sublayer and the thermal buffer layer (y+<5) is practically negligible. The contribution of the sub-Kolmogorov thermal scales becomes relevant above the thermal buffer layer (y+>5), where the unresolved temperature scales affect spectra and RMS temperature fluctuations, but not the log-law shape of the mean temperature profile and the mean heat transfer coefficient.
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Lai, Jonathan K., Elia Merzari, Yassin A. Hassan, and Aleksandr Obabko. "Validation and Development of DNS Database for Low Prandtl Numbers in Rod Bundle." In ASME-JSME-KSME 2019 8th Joint Fluids Engineering Conference. American Society of Mechanical Engineers, 2019. http://dx.doi.org/10.1115/ajkfluids2019-5036.
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Abstract Difficulty in capturing heat transfer characteristics for liquid metals is commonplace because of their low molecular Prandtl number (Pr). Since these fluids have very high thermal diffusivity, the Reynolds analogy is not valid and creates modeling difficulties when assuming a turbulent Prandtl number (Prt) of near unity. Baseline problems have used direct numerical simulations (DNS) for the channel flow and backward facing step to aid in developing a correlation for Prt. More complex physics need to be considered, however, since correlation accuracy is limited. A tight lattice square rod bundle has been chosen for DNS benchmarking because of its presence of flow oscillations and coherent structures even with a relatively simple geometry. Calculations of the Kolmogorov length and time scales have been made to ensure that the spatial-temporal discretization is sufficient for DNS. In order to validate the results, Hooper and Wood’s 1984 experiment has been modeled with a pitch-to-diameter (P/D) ratio of 1.107. The present work aims at validating first- and second-order statistics for the velocity field, and then analyzing the heat transfer behavior at different molecular Pr. The effects of low Pr flow are presented to demonstrate how the normalized mean and fluctuating heat transfer characteristics vary with different thermal diffusivity. Progress and future work toward creating a full DNS database for liquid metals are discussed.
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Bergant, Robert, Iztok Tiselj, and Gad Hetsroni. "Resolution Requirements for DNS of Turbulent Heat Transfer Near the Heated Wall at Prandtl Number 5.4." In ASME 2001 International Mechanical Engineering Congress and Exposition. American Society of Mechanical Engineers, 2001. http://dx.doi.org/10.1115/imece2001/htd-24129.
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Abstract Theoretically, the grid spacing for Direct Numerical Simulations of heat transfer at Prandtl numbers higher than one should be inversely proportional to the square root of Prandtl number (Tennekes, H., Lumley, J.L [1]). The grid refinement study of Na and Hanratty (2000) [2] at Pr = 10 showed that finer grid is not required in streamwise and spanwise directions, but is necessary in the wall-normal direction. In the present work three different DNS studies were performed with different resolutions at Pr = 5.4 and friction Reynolds number Re = 171. The first DNS was performed using the resolution, which is known to be sufficient for the velocity field simulation, the second simulation was performed with the refined grid in the wall-normal direction and the third DNS was using finer grid in all three directions. Results have shown that there are no significant differences in the first-order statistics and spectra for all three cases.
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Bergant, R., and I. Tiselj. "Numerical Simulations of Turbulent Flume Heat Transfer at Pr = 5.4: Impact of the Smallest Temperature Scales." In ASME 2005 Fluids Engineering Division Summer Meeting. ASMEDC, 2005. http://dx.doi.org/10.1115/fedsm2005-77144.
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In the present paper a role of the smallest diffusive scales of a passive scalar field in the near-wall turbulent flow was examined with pseudo-spectral numerical simulations. Temperature fields were analyzed at friction Reynolds number Reτ = 170.8 and at Prandtl number, Pr = 5.4. Results of direct numerical simulation (DNS) were compared with the under-resolved simulation where the velocity field was still resolved with the DNS accuracy, while a coarser grid was used to describe the temperature field. Since the smallest temperature scales remained unresolved in this simulation, an appropriate spectral turbulent thermal diffusivity was applied to avoid pileup at higher wave numbers. In spite of coarser numerical grid, the temperature field is still highly correlated with the DNS results, and thus point to practically negligible role of the diffusive temperature scales on the macroscopic behavior of the turbulent heat transfer.
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Jahani, B., M. MacDonald, and Stuart E. Norris. "Modelling turbulent stratified open channel flow for Pr=7 using multiscale DNS." In 10th International Symposium on Turbulence, Heat and Mass Transfer, THMT-23, Rome, Italy, 11-15 September 2023. Connecticut: Begellhouse, 2023. http://dx.doi.org/10.1615/ichmt.thmt-23.1260.
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Jahani, B., M. MacDonald, and Stuart E. Norris. "Modelling turbulent stratified open channel flow for Pr=7 using multiscale DNS." In 10th International Symposium on Turbulence, Heat and Mass Transfer, THMT-23, Rome, Italy, 11-15 September 2023. Connecticut: Begellhouse, 2023. http://dx.doi.org/10.1615/thmt-23.1260.
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Otic´, I., and G. Gro¨tzbach. "Direct Numerical Simulation and RANS Modeling of Turbulent Natural Convection for Low Prandtl Number Fluids." In ASME/JSME 2004 Pressure Vessels and Piping Conference. ASMEDC, 2004. http://dx.doi.org/10.1115/pvp2004-3132.
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Results of direct numerical simulation (DNS) of turbulent Rayleigh-Be´nard convection for a Prandtl number Pr = 0.025 and a Rayleigh number Ra = 105 are used to evaluate the turbulent heat flux and the temperature variance. The DNS evaluated turbulent heat flux is compared with the DNS based results of a standard gradient diffusion turbulent heat flux model and with the DNS based results of a standard algebraic turbulent heat flux model. The influence of the turbulence time scales on the predictions by the standard algebraic heat flux model at these Rayleigh- and Prandtl numbers is investigated. A four equation algebraic turbulent heat flux model based on the transport equations for the turbulent kinetic energy k, for the dissipation of the turbulent kinetic energy ε, for the temperature variance θ2, and for the temperature variance dissipation rate εθ is proposed. This model should be applicable to a wide range of low Prandtl number flows.
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Bhushan, S., M. Elmellouki, W. D. Jock, D. K. Walters, J. K. Lai, Y. A. Hassan, A. Obabko, and E. Merzari. "Numerical Investigation of Flow and Heat Transfer Characteristics for Attached and Separated Low-Pr Flows." In ASME-JSME-KSME 2019 8th Joint Fluids Engineering Conference. American Society of Mechanical Engineers, 2019. http://dx.doi.org/10.1115/ajkfluids2019-5273.
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Abstract This study performs a comprehensive analysis of the effect of flow separation and reattachment, convective conditions and Pr to understand their effect on heat transfer characteristics and the predictive capability of low- and hig-fidelity turbulence models are assessed. To achieve the objective DNS is performed for plane channel flow at Reτ = 640, Pr = 0.71 and 0.025 involving mixed forced and natural convection condition, and RANS, hybrid RANS/LES, and LES calculations are performed for backward backing step with expansion ratio 1.5, Pr = 0.71 and 0.0088 and Ri = 0 and 0.338. Channel flow simulations reveal that the convective conditions affect the near-wall turbulent structures and thermal diffusion more significantly in high-Re flows that in low-Re flows. Thus, the generated DNS database provides a challenging test case for turbulence model validation. For backward facing step case, all the turbulence models predict the overall flow characteristics, and Ri = 0 case is a more challenging validation test case than Ri = 0.338, as the former involves complex turbulent diffusion, whereas the latter is dominated by large scale buoyancy driven convection. Results show that well resolved PANS and LES predictions can help in improve understanding of turbulent diffusion under complex convection and flow separation/ reattachment regimes. RANS results are also quite encouraging and indicates that they may represent a reasonable compromise between computational expense and accuracy for cases in which high resolution simulations are not feasible.