A study of the physics of separating and reattaching flows around bodies with sharp edges is reported. Data from direct numerical simulations of the flow around a rectangular cylinder with aspect ...ratio 5 at different Reynolds numbers are used. The flow is decomposed into multiple interacting flow phenomena such as the laminar boundary layer in the front face, the separated shear layer, the flow impingement at reattachment, the reverse boundary layer within the recirculating bubble and the near- and far-wake flow. A detailed analysis of the physics of these phenomena is provided, including the slow modulation induced by large-scale instabilities related with vortex shedding. The entrainment phenomena acting along the separated shear layer and their unbalance between its inner and outer sides are recognised as fundamental mechanisms determining the tendency of the flow to reattach and the overall fluxes of momentum and heat. The behaviour of entrainment is found to be strictly related with the shear-layer velocity difference that in turn is determined by the behaviour of the reverse boundary layer and by its strength in counteract adverse pressure gradients. The physical understanding of the compound role played by these and all the other mechanisms composing the flow, poses the basis for the formulation of theoretical frameworks able to unify all these interacting phenomena. Finally, the present work provides access to high-fidelity flow statistics of relevance for benchmark activities on bluff bodies with sharp edges.
After decades of research efforts, wind–wave interaction mechanisms have been recognized as extremely elusive. The reason is the complex nature of the problem, which combines complex coupling ...mechanisms between turbulent wind and water waves with the presence of multiple governing parameters, such as the friction Reynolds number of the wind, the water depth and the wind fetch. As shown unequivocally here, the use of suitable flow settings allows us to reduce the complex problem of wind–wave interaction to its essential features, mainly as a function of the sole friction Reynolds number of the wind. The resulting numerical solution allows us to study the interactions between water and air layers with their own fluid properties, and to unveil very interesting features, such as an oblique wave pattern travelling upstream and a wave-induced Stokes sublayer. The latter is responsible for a drag reduction mechanism in the turbulent wind. Despite the simulated flow conditions being far from the intense events occurring at the ocean–atmosphere interface, the basic flow phenomena unveiled here may explain some experimental evidence in wind–wave problems. Among other things, the wave-induced Stokes sublayer may shed light on the large scatter of the drag coefficient data in field measurements where swell waves of arbitrary directions are often present. Hence the present results and the developed approach pave the way for the understanding and modelling of the surface fluxes at the ocean–atmosphere interface, which are of overwhelming importance for climate science.
Direct numerical simulations of channel flow and temporal boundary layer at a Reynolds number $Re_{\tau } = 1500$ are used to assess the scale-by-scale mechanisms of wall turbulence. From the peak of ...turbulence production embedded at the small scales of the near-wall region, spatially ascending reverse cascades are generated that move through self-similar eddies growing in size with the wall distance. These fluxes are followed by spatially ascending forward cascades through detached eddies thus reaching sufficiently small scales where eventually scale energy is dissipated. This phenomenology is shared by both boundary layer and channel flow and is recognized as a robust physical feature characterizing wall turbulence in general. Specific features related to the flow configuration are indeed identified in the outer region. In particular, the central region of channels is characterized by a generalized Richardson energy cascade where large scales are in equilibrium with small scales at different wall distances through a combined forward cascade and spatial flux. On the contrary, the interface region of boundary layers is characterized by an almost two-dimensional physics where spatially ascending reverse cascades sustain long and wide interface structures with a forward cascade that survives only in the wall-normal scales. The overall scenario consists in a variety of scale motions that while protruding from the turbulent core towards the external region, squeeze at the interface thus sustaining vertical shear in a thin layer. The observed multidimensional physics sheds light on the complex interactions between outer entrainment and near-wall self-sustaining mechanisms with possible repercussions for theories.
•A technique is presented, for the DNS of turbulent convection in complex domains.•Special near-wall stencils are mapped to the principal mesh for immersed boundaries.•Second-order spatial accuracy ...is demonstrated to hold also on curved boundaries.•Validation is provided for convection about a tube bundle and turbulent pipe flow.
A parallel algorithm is presented for the Direct Numerical Simulation of convection flows in open or partially confined periodic domains, containing immersed cylindrical bodies of arbitrary cross-section. The governing equations are discretized by means of the Finite Volume method on Cartesian grids.
The method presented includes a triperiodic Poisson solver employed irrespective of the actual boundary shape and a second order accuracy for the computational domain, including the near wall regions, when walls are defined as immersed boundaries.
The numerical solution of the set of linear equations resulting from discretization is carried out by means of efficient and highly parallel direct solvers. Verification and validation of the numerical procedure is reported in the paper, for laminar and turbulent pipe flow, and for the case of flow around an array of heated cylindrical rods arranged in a triangular lattice. The formal accuracy of the method is demonstrated in laminar flow conditions, and DNS results in turbulent conditions are compared to available literature data, thus confirming the favorable qualities of the method.
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•The three jets undergo intense mixing within the entrance region (x < 10H).•Far from the inlet a single, non-buoyant stream is observed.•It displays typical self-preserving characteristics of purely ...mechanical jets.•Despite the low-Prandtl number heat transfer is mainly due to turbulent transport.
Mixing of buoyant streams is a phenomenon of relevance in many practical cases like pollutant emission in the atmosphere, discharges from marine outfalls and cooling of fuel rods in nuclear reactors to name a few. A canonical configuration for this class of flows consists in three buoyant jets at different temperatures vertically entering a pool from the bottom. This work reports a Direct Numerical Simulation study performed on the triple jet configuration. The Reynolds number based on the average jet centerline velocity and jet width is set to Re=5000 and mixed convection regime is established at a Richardson number, Ri=0.25. In order to represent flows occurring inside liquid metal fast reactors, the selected Prandtl number is Pr=0.031.
Statistics computed show that in the first stages of development, the three jets undergo a strong interaction. In that same region the shedding of large-scale vortices is originated accompanied by low-frequency undulations. Further from the inlet, the three jets are observed to coalesce in a single, isothermal stream. The analysis of momentum fluxes clarifies the mutual entrainment mechanism behind coalescence, which is commonly known as Coandă effect. At distances larger than ten times the jet width the self-similar characteristics of single and isothermal planar jets are recovered. The flow configuration presented includes several peculiar features, namely buoyancy effects at low Prandtl number, interaction between jets and the presence of multiple shear layers. This leads to an irregular behaviour of the turbulent diffusivity of momentum and heat as well as the misalignment between the temperature gradient and turbulent heat flux. Therefore the flow can be considered very complex and might constitute a demanding test bench for the development and validation of turbulence models.
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Turbulence is investigated in the lee of an open-cell metal foam layer. In contrast to canonical grids, metal foams are locally irregular but statistically isotropic. The solid matrix is ...characterised by two lengths, the ligament thickness $d_f$ and the pore diameter $d_p$. A direct numerical simulation is conducted on a realistic metal foam geometry for which $d_f/d_p = 0.14$ and the porous layer thickness is five times the pore diameter. The Reynolds number based on the pore size is ${\textit {Re}}_{d_p} = 4000$, corresponding to a Taylor-scale Reynolds number ${\textit {Re}}_\lambda \approx 80$. Closer to the foam than two pore diameters, the pressure and turbulent transports of turbulent kinetic energy are non-negligible. In the same region, ${\textit {Re}}_\lambda$ undergoes a steep decrease whereas the dissipation coefficient $C_{\epsilon }$ increases like ${\textit {Re}}_\lambda ^{-1}$. At larger distances from the porous layer, the classical grid turbulence situation is recovered, where the mean advection of turbulent kinetic energy equals dissipation. This entails a power-law decay of turbulent quantities and characteristic lengths. The decaying exponents of integral, Taylor and Kolmogorov scales are close to one-half, indicating that the turbulence simulated here differs from Saffman turbulence. Analysis of the scaling exponents of structure functions and the decorrelation length of dissipation reveals that small-scale fluctuations are weakly intermittent.
Abstract
This paper reports results of Direct Numerical Simulations of fully developed, forced and mixed convection of Liquid Lead Bismuth Eutectic (LBE) around four vertical cylindrical rods ...arranged in a square lattice at Pr = 0.031. The solutions provided here are placed in a context where very few data are available for low-Prandtl flows, especially in mixed convection conditions, and can hence be useful for the development and validation of advanced turbulent heat transfer models. The equations are discretized using a Finite Volume implementation of a second order projection method. The irregular cylindrical boundaries are handled with an original Immersed Boundary technique. A single friction Reynolds number value is set (Re
τ
= 180) and buoyancy effects are accounted for by imposing the Rayleigh number, under the Boussinesq approximation. A Ra-value Ra = 2.5 ×10
4
is selected in order to investigate the main differences between forced and mixed convection at low Prandtl number values. Time-averaged velocity and temperature fields are shown, in order to discuss the main features of the flow and thermal fields. First order statistics are also presented, highlighting the effect of aiding buoyancy on turbulent heat and momentum transport.
•DNS data of a low-Pr flow in a loosely-spaced triangular rod bundle are presented.•Forced and mixed convection conditions are analyzed.•Aiding buoyancy determines an impairment of overall heat ...transfer.•Presented data might be used as benchmark for advanced turbulence modeling.
In this work, reference data obtained by means of Direct Numerical Simulations of fully-developed flow and heat transfer around a vertical rod bundle are presented. Finite-Volume computations are performed by an original discretization technique based on the representation of arbitrarily-shaped cylindrical boundaries on a non-uniform Cartesian grid. A periodic domain consisting of four subchannels of a triangular lattice of rods with a pitch-to-diameter ratio P/D=1.4 is considered as the reference geometry. A Prandtl number Pr = 0.031 is chosen to represent Liquid Lead-Bismuth Eutectic (LBE) as the working fluid. A single friction Reynolds number value is simulated, namely Reτ=550. Both forced and mixed convection regimes are investigated, buoyancy effects being introduced by imposing a Rayleigh number Ra = 5×105, corresponding to a Richardson number Ri = 0.22. Instantaneous snapshots and relevant statistics of the velocity and thermal fields are reported and discussed for the considered cases, focusing on the effect of aiding buoyancy on turbulent flow and heat transfer. Integral results are also compared against available literature data.
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The physical understanding of two-phase flow characteristics in horizontal pipes is of importance in the petroleum industry since significant savings in pumping power can be derived from the ...water-lubricated transportation of crude oil.
An experimental study of water continuous oil–water flow in horizontal pipes is performed using mineral oil and tap water of viscosity ratio about 900 and density ratio 0.9. A set of seven different pipes of Pyrex and Plexiglas where used, with diameters ranging between 21 and 40
mm. Pressure drop measurements, flow pattern maps and clear pictures of the oil–water flow are reported in this article together with comprehensive comments. The results obtained are compared to empirical laws, theoretical findings and experimental results by different authors in the literature.
In order to identify the regions with operational conditions that are suitable for applications, a novel criterion for the location of the annular/stratified transition is proposed which is based only on experimental observations.
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•Numerical simulations of a Heavy Liquid Metal cooled bundle.•Direct Numerical Simulation of an experimental test.•Detailed comparison of DNS and RANS approaches to the same test case.•General ...comparison of numerical results and experimental data.
One of the requirements for achieving high levels of safety in fourth generation nuclear power plants, is that core thermal-hydraulics can be simulated numerically at a good level of accuracy. To this aim, detailed validation of turbulent heat transfer models needs to be carried out and discussed, in relevant flow and heat transfer configurations, and for low Prandtl-number fluids.
The focus of this research is the turbulent, fully developed convection in a heated rod bundle with a triangular arrangement, and a pitch-to-diameter ratio P/D=1.4. Statistics derived from a set of Direct Numerical Simulations (DNS) at the moderate Reynolds number of Re=8290 are compared to Reynolds Averaged Navier-Stokes (RANS) solutions where the closure is provided by the two-equation model SST k-ω and by the Reynolds-Stress Model. Comparisons are drawn for forced flow and in mixed convection conditions (Ri≈0.25). Global quantities extracted from experiments performed in very similar conditions are also compared against the numerical results. Profiles of the turbulent Prandtl number about the unit flow cell are also displayed and discussed.
This work clarifies through detailed comparison that if, on the one hand, only the Reynolds-Stress model is able to reasonably capture important flow features like secondary flow components, on the other hand also the SST k-ω two-equation model considered is acceptably accurate in predicting the integral quantities of interest.
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