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1. TURBULENT FLOW SYMMETRY-BREAKING IN PERIODIC POROUS MEDIA IN THE INTERMEDIATE-POROSITY REGIME

   https://doi.org/10.1615/ICHMT.2024.CHT-24.170  

   Srikanth, Vishal; Kuznetsov, Andrey V (January 2024, Begell house)

   In this paper, we report a novel symmetry-breaking phenomenon that occurs in turbulent convective flow in periodic porous media in the intermediate porosity flow regime for values of porosities in between 0.8 and 0.9. Large eddy simulation is used to numerically simulate the momentum and thermal transport inside the porous medium at the microscale level. The phenomenon is observed to occur for periodically repeating porous media consisting of an in-line arrangement of circular cylinder solid obstacles, such as typically found in heat exchangers. The transition from symmetric to asymmetric flow occurs in between the pore scale Reynolds numbers of 37 (laminar) and 100 (turbulent), and asymmetric flow patterns are reported for Reynolds numbers up to 1,000. A Hopf bifurcation resulting in unsteady oscillatory laminar flow marks the origin of a secondary flow instability arising from the interaction of the shear layers around the solid obstacle. In turbulent flow, stochastic phase difference in the vortex wake oscillations caused by the secondary flow instability results in asymmetrical velocity and temperature distributions in the pore space. Consequently, high and low velocity flow channels are formed in the pore space that leads to the asymmetrical velocity and pressure distributions. At the macroscale level, symmetry-breaking results in residual transverse drag force components acting on the solid obstacle surfaces. The vortex wake oscillations caused by the secondary flow instability promote attached flow on the solid obstacle surface, which improves the surface averaged heat flux from the solid obstacle surface. 

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2. Impact of gaps on the flow statistics in an emergent rigid canopy

   https://doi.org/10.1063/5.0088527  

   Ranjan, Pallav; Mittal, Ketan; Chamorro, Leonardo P.; Tinoco, Rafael O. (June 2022, Physics of Fluids)

   High-resolution large eddy simulations and complementary laboratory experiments using particle image velocimetry were performed to provide a detailed quantitative assessment of flow response to gaps in cylinder arrays. The base canopy consists of a dense array of emergent rigid cylinders placed in a regular staggered pattern. The gaps varied in length from [Formula: see text] to 24, in intervals of 4 d, where d is the diameter of the cylinders. The analysis was performed under subcritical conditions with Froude numbers [Formula: see text] and bulk Reynolds numbers [Formula: see text]. Results show that the gaps affect the flow statistics at the upstream and downstream proximity of the canopy. The affected zone was [Formula: see text] for the mean flow and [Formula: see text] for the second-order statistics. Dimensionless time-averaged streamwise velocity within the gap exhibited minor variability with gap spacing; however, in-plane turbulent kinetic energy, k, showed a consistent decay rate when normalized with that at [Formula: see text] from the beginning of the gap. The emergent canopy acts as a passive turbulence generator for the gap flow for practical purposes. The streamwise dependence of k follows an exponential trend within [Formula: see text] and transitions to a power-law at [Formula: see text]. The substantially lower maximum values of k within the gap compared to k within the canopy evidence a limitation of gap measurements representative of canopy flow statistics. We present a base framework for estimating representative in-canopy statistics from measurements in the gap. 

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3. Numerical Study of Circular-Cylinder Disturbance Generators with Rigid Splitter Plates

   https://doi.org/10.2514/1.J062729  

   Jenkins, Michael; Suresh_Babu, Arun Vishnu; Bryant, Matthew; Gopalarathnam, Ashok (December 2023, AIAA Journal)

   This paper describes the numerical study of oscillating circular cylinders with rigid splitter plates of different lengths. These geometries may be used as disturbance generators for the study of unsteady airfoils and wings operating in highly vortical flowfields. It has been shown that cylinders undergoing forced rotational oscillations at their natural shedding frequency can produce wakes with minimal deviation in cycle-to-cycle vortex strength and position. Adding a splitter plate allows these deviations to be reduced even further. We present cases for oscillating cylinders having splitter-plate lengths up to [Formula: see text] at a Reynolds number of 7600. Frequencies are maintained at the natural shedding frequency, and a rotational amplitude of 45 deg is used. Numerical simulations are performed using a two-dimensional unsteady Reynolds-averaged Navier–Stokes (RANS) code. Results are presented in the form of vorticity contours and cycle-averaged velocity profiles, as well as the dominant frequencies of cylinder lift force and downstream velocity angles. The results show that splitter-plate lengths shorter than [Formula: see text] adversely affect the ability to generate a coherent vortex wake due to shear layer roll-up near the trailing edge of the plate. Splitter plates longer than [Formula: see text] produced a reverse von Kármán wake with consistent cycle-to-cycle vortex shedding. 

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4. EFFECT OF MICROSCALE TURBULENT STRUCTURES DYNAMICS ON FORCED CONVECTION IN TURBULENT POROUS MEDIA FLOW

   https://doi.org/10.1615/TFEC2021.tfl.036761  

   Huang, Ching-Wei; Srikanth, Vishal; Kuznetsov, Andrey V. (January 2021, Proceeding of 5-6th Thermal and Fluids Engineering Conference (TFEC))

   null (Ed.)

   The influence of microscale flow structures (smaller than the pore size) on turbulent heat transfer in porous media has not been yet investigated. The goal of this study is to determine the influence of the micro-vortices on convection heat transfer in turbulent porous media flow. Turbulent flow in a homogeneous porous medium was investigated using Large Eddy Simulation (LES) at a Reynolds number of 300. We observed that the convection heat transfer characteristics are dependent on whether the micro-vortices are attached or detached from the surface of the obstacle. There is a spectral correlation between the Nusselt number and the pressure instabilities due to vortex shedding. A secondary flow instability occurs due to high pressure regions forming periodically near the converging pathway between obstacles. This causes local adverse pressure gradient, affecting the flow velocity and convection heat transfer. This study has been performed for obstacles with shapes of square and circular cylinders at porosities of 0.50 and 0.87. Understanding the dominant modes that affect convection heat transfer can aid in finding an optimum geometry for the porous medium. 

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5. Pressure drop measurements over anisotropic porous substrates in channel flow

   https://doi.org/10.1007/s00348-024-03873-2  

   Vijay, Shilpa; Luhar, Mitul (September 2024, Experiments in Fluids)

   AbstractPrevious theoretical and simulation results indicate that anisotropic porous materials have the potential to reduce turbulent skin friction in wall-bounded flows. This study experimentally investigates the influence of anisotropy on the drag response of porous substrates. A family of anisotropic periodic lattices was manufactured using 3D printing. Rod spacing in different directions was varied systematically to achieve different ratios of streamwise, wall-normal, and spanwise bulk permeabilities ($$\kappa _{xx}$$ ${\kappa }_{\mathrm{xx}}$,$$\kappa _{yy}$$ ${\kappa }_{\mathrm{yy}}$, and$$\kappa _{zz}$$ ${\kappa }_{\mathrm{zz}}$). The 3D printed materials were flush-mounted in a benchtop water channel. Pressure drop measurements were taken in the fully developed region of the flow to systematically characterize drag for materials with anisotropy ratios$$\frac{\kappa _{xx}}{\kappa _{yy}} \in [0.035,28.6]$$ $\frac{{\kappa }_{\mathrm{xx}}}{{\kappa }_{\mathrm{yy}}}\in [0.035,28.6]$. Results show that all materials lead to an increase in drag compared to the reference smooth wall case over the range of bulk Reynolds numbers tested ($$\hbox {Re}_b \in [500,4000]$$ ${\text{Re}}_b\in [500,4000]$). However, the relative increase in drag is lower for streamwise-preferential materials. We estimate that the wall-normal permeability for all tested cases exceeded the threshold identified in previous literature ($$\sqrt{\kappa _{yy}}^+> 0.4$$ ${\sqrt{{\kappa }_{\mathrm{yy}}}}^{+}>0.4$) for the emergence of energetic spanwise rollers similar to Kelvin–Helmholtz vortices, which can increase drag. The results also indicate that porous walls exhibit a departure from laminar behavior at different values for bulk Reynolds numbers depending on the geometry. Graphical abstract 

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