Bottom Wall Research Articles

FreeFEM analysis of thermally induced higher-grade Darcy-Forchheimer flow through heated semi-circular cylinders

Abstract In this article, a higher-grade Darcy-Forchheimer porous model is derived by using the concepts of tensor calculus. The model presented is accounting for non-linear flow behavior at highly permeable media where the flow is induced by temperature boundary
conditions. To this end, a square geometry with two semi-circular heating cylinders mounted at its bottom wall is considered for the analysis of thermal flow dynamics. To solve the obtained coupled system of highly nonlinear partial differential equations the
finite element procedure is adopted. Weak formulation of the problem is calculated via the application of variational calculus. The numerical algorithm is implemented through the open source code FreeFEM++. Obtained solutions are validated by reduced model
with exact solutions. Mesh independence of the solution is shown through mesh independence analysis test. Results are computed for varying physical parameters with some interesting new observations. Moreover, streamline plots for the velocities and isotherms
are shown and discussed. It is found that the Nusselt number increases with increasing Grashhoff and Frochheimer numbers, but decreases with increasing medium porosity.

Investigation of asymmetric heating in Poiseuille-Rayleigh-Bénard water flow: A numerical study

In this paper, a numerical investigation of the impact of asymmetric heating on laminar mixed convection in Poiseuille-Rayleigh-Bénard water flow within parallel horizontal channels is presented. The study has been carried out in a rectangular channel with a transverse aspect ratio of 10, and considered both low (Ra = 1.28 × 104) and high (Ra = 1.4 × 105) Rayleigh numbers, with Reynolds numbers of 50 and 100. A uniform heat flux was applied to the top and bottom walls of the heated region to assess its effect on the system's thermoconvective behavior and heat transfer efficiency. Two flux ratio scenarios were considered: qt/qb = 1 and qt/qb = 2.The results indicate that increasing the flux ratio intensifies the destabilizing temperature gradient and significantly enhances buoyancy-induced flow, thereby influencing the patterns of thermoconvective structures. Specifically, flux ratios lead to an increased number of plumes originating from the bottom of the channel, while reducing their height and confining them between the bottom wall and the upper thermal boundary layer. It is also observed that flux ratios do not affect the mechanisms involved in the formation of longitudinal rolls. Furthermore, at low Rayleigh numbers, asymmetric heating has a pronounced impact on the establishment length. In contrast, this effect diminishes and becomes negligible at higher Rayleigh numbers. Numerical computations further reveal that near the bottom wall, the Nusselt number exhibits singular behavior, approaching infinity. Regardless of Reynolds and Rayleigh numbers, flux ratios significantly enhance heat transfer within the system. Additionally, near the top wall, the buoyancy effects from the bottom wall have negligible impact on heat transfer, except in the case where qt/qb = 2, Re = 50 and Ra = 1.4 × 105, where instability in the upper thermal layer was observed.

Exploration of Arrhenius activation energy and thermal radiation on MHD double-diffusive convection of ternary hybrid nanofluid flow over a vertical annulus with discrete heating

The primary objective of this article is to examine the effect of discrete heating on MHD double-diffusive convection and thermal radiation of ternary hybrid nanofluid flow heat and mass transfer in a vertical cylindrical annulus along with Arrhenius activation energy and chemical reaction. In this study, the cavity inner wall has two distinct flush-mounted heat sources, while the outer wall is isothermally cooled at a lower temperature. The top and bottom walls are thermally insulated. The ensuing equations that govern the physical framework are solved using the implicit Crank-Nicholson finite difference technique. As the heater advances upward, the flow intensity decreases, leaving a part of the fluid static at the bottom of the cylinder. Because more heat induces high buoyant flow in the annulus, the absolute value of axial velocity and wall temperature rises as the length of the heat source rises. Enhancing the activation energy parameter values drops the Arrhenius energy function, elevating the pace of the generative chemical process and hence the concentration. Increasing the thermal radiation parameter lowers the surface heat flux while enhancing the nanofluid temperature. The Brownian motion parameter corresponds to the random motion of nanoparticles in a fluid, and this irregular movement augments the collision of nanoparticles with fluid particles, causing the particle's kinetic energy which leads to thermal energy and hence increases temperature, a 10% increment in Brownian motion parameter leads to 26% rise in the temperature. Also, the heat and mass transfer characteristics are forecasted and analyzed by considering the Levenberg–Marquardt backpropagating artificial neural network technique.

NUMERICAL INVESTIGATION ON THE INFLOW MACH NUMBER VARIATIONS IN A STRUT INJECTED CAVITY-BASED SCRAMJET COMBUSTORS

The effect of enhancing the air inflow Mach number of a strut-injected cavity incorporated into a scramjet combustor is numerically studied. The computational investigations are performed using the commercial code Ansys Fluent software. The simulation is carried out using the SST k-ω turbulence model with single-step reaction chemistry and a two-dimensional planar model using the Reynolds-averaged Navier-Stokes (RANS) method. The cavities are positioned downstream of the strut injector that are fixed to the top and bottom walls. This configuration enables the analysis of the shock wave pattern and its correlations with shear layer mixing features. Three inflow Mach numbers are opted for the present investigation, i.e., Mach 2.0, 2.5, and 3.0. The investigations are accomplished on the flow pattern, static pressure, and temperature distributions. Performance analyses for the twin cavity designs that fit into the German Aerospace Center (DLR) scramjet are performed throughout the entire combustor length. A reduction in combustion chamber length by 20% and complete combustion is achieved from the twin cavity configuration as compared to the baseline model. The overall pressure drop is increased around 23% due to the formation of additional shock waves from the cavities. Moreover, the combustion zone prolongs along the flow direction due to the increase in the inflow Mach numbers.

Methylcellulose enhances resolution in gravitational field-flow fractionation: Going beyond viscosity.

Gravitational Field-Flow Fractionation (GrFFF) is an elution-based method designed for the separation of particles ranging from a few micrometers up to approximately 100 μm in diameter. Separation occurs over time, with particles being fractionated based on size and other physico-chemical properties. GrFFF takes advantage of gravitational forces acting perpendicularly to a laminar flow in a thin channel. The fluid exhibits a parabolic velocity profile, with the maximum velocity at the center of the channel and zero velocity at the walls. The exit time of particles depends on their equilibrium position relative to the bottom wall. In hyperlayer mode, larger particles elute faster than smaller ones due to their higher velocities within the channel. This study investigated the effect of adding methylcellulose (MC) to the carrier fluid on the elution behavior - specifically, peak time (tpeak) and resolution (R) - of polystyrene-based (PS) microparticles with sizes of 7, 8, and 10 μm. The results demonstrated that MC not only increases the viscosity of the carrier fluid but also exerts a secondary, predominant effect that improves resolution (R), thereby enhancing the separation of particle populations. This was confirmed by comparing the use of water as the carrier fluid at two different temperatures: 14 °C (high viscosity) and 28 °C (low viscosity). While increasing viscosity by lowering temperature only led to modest reduction in elution time of the fractograms, the addition of MC had a size-dependent effect on the microparticles, significantly improving R without changing other experimental parameters. This suggests the presence of additional phenomena contributing to the improved separation. In conclusion, the addition of MC to the carrier fluid increases the resolving power of GrFFF, enabling the separation of PS microbeads with a size difference of up to 2 μm. This advancement pushes the boundaries of GrFFF and opens up potential new applications. These studies, conducted on PS microbeads, provide a preliminary basis for future work on cells, which have similar density and size. This could pave the way for improved cell separation in diagnostic applications.

Effects of wall perturbations on the stabilized FEM solution of steady MHD flow in a duct

In this study, the effects of curved boundary perturbations on the solution of steady magnetohydrodynamic (MHD) duct flow are investigated. Hartmann (upper and bottom) walls are perturbly curved and perfectly conducting while the side walls are insulated and plane. The velocity of the flow and induced magnetic field are obtained numerically by solving the steady MHD flow coupled equations using the finite element method (FEM with Streamline Upwind Petrov Galerkin (SUPG)) stabilization to inhibit instabilities in the flow. The results are obtained for Hartmann number ($Ha$) values up to $500$, for several definitions of the curved upper and bottom walls, and for several values of perturbation parameters of the curved walls ($0 \le \epsilon_u , \epsilon_b \le 0.3$). The velocity and the induced magnetic field sensitivity to the curved wall shapes are visualized in terms of equivelocity and current lines. It is found that the flow and the induced magnetic field are affected by the curved boundary shapes especially near those boundaries and also, to some extent, in the whole duct. It is also observed that increasing the Hartman number pushes the flow near the upper boundary even if both upper and bottom walls are perturbed since the external magnetic field applies vertically. The increase in the perturbation parameter of the curved upper boundary forces the flow to move through this wall and the induced magnetic field reaches its maximum value near the maximum points of the perturbed curve. Further, an increase in ${\rm Ha}$ delays the effect of the curved boundaries and gives rise to flattened flow with side layers and stagnant fluid at the central part of the duct overwhelming the effects of curved boundaries.

Numerical simulation of a non-Newtonian nanofluid on mixed convection in a rectangular enclosure with two rotating cylinders

Non-Newtonian fluid flows are a present advancement in industrial heat transfer and fluid mechanics, with numerous applications in engineering field as well as in real life such as food industry, petroleum refiners, biological processes, biomedical engineering, chemical engineering, etc. As a result, this numerical investigation explore the heat transfer and fluid flow behaviour of a mixed convective non-Newtonian power-law nanofluid ( Al 2 O 3 - H 2 O ) in a rectangular enclosure. The physical model is considered as a two-dimensional rectangular enclosure consisting two rotating hot cylinders ( T H )in the middle part of the cavity, where one moving clockwise and the other counter-clockwise directions. The left and right walls of the chamber are taken relatively low temperature ( T C ) while the remaining top and bottom walls are insulated. The important thermophysical property, viscosity, depends on the fluid shear rate, and the thermal conductivity depends on the fluid temperature. The related governing equations are solved by using the finite element method, and to describe the response surfaces the response surface methodology (RSM) is applied. The influence of the relevant non-dimensional parameters, Prandtl number ( Pr = 6.2 ), Reynolds number ( Re = 10 , 100, 200 and 400), power-law index (n = 0.6, 0.8, 1.0 and 1.4), Richardson number ( Ri = 1.0 , 2.0, 4.0 and 6.0), and nanoparticle volume fraction ( ϕ = 0.00 , 0.01, 0.02 and 0.04), are explained with physical interpretation by using streamlines, isotherm lines, velocity profile, temperature profile, and average Nusselt number ( Nu av ). To visualise the response surfaces, 2D and 3D contour plots are explained using the RSM. It is concluded that by adding nanoparticle into base fluid, the rate of heat transfer developed significantly. Same phenomena exhibits for larger magnitudes of Ri and Re . And, by increasing the value of n from 0.6 to 1.4, the Nu av would go down by 39.8 percent.

Gravity currents under oscillatory forcing

We investigate the effect of external oscillatory forcing on evolving two-dimensional (2-D) gravity currents, resulting from the well-known lock-exchange set-up, by superimposing a horizontally uniform oscillating pressure gradient. This pressure gradient generates a 2-D horizontally uniform laminar oscillating flow over the flat no-slip bottom that interacts with the evolving gravity current. We explore the effect of the velocity amplitude of the applied oscillating flow and its period of oscillations on the behaviour of the evolving gravity currents. A key element introduced by the external forcing is the Stokes boundary layer near the no-slip bottom wall generating differential advection near the bottom wall when the propagation direction of the gravity current and the oscillating externally imposed flow are in the same direction. This results in a phenomenon that we refer to as lifting of the gravity current, which clearly distinguishes the oscillatory forced gravity current from the freely evolving case. This phenomenon induces fine-scale density structures when the externally imposed flow is opposite to the propagation direction of the gravity current a semi-period later. We have explored the effect of lifting on the current propagation and the density structure of the gravity current front. Three separate regimes are distinguished for the evolution of the density structure in the front of the gravity current depending on the period of forcing, including a regime reminiscent of tidally forced estuarine flows. The present study shows the existence of significant effects of an oscillatory forcing on the dynamics, advection and density distribution of gravity currents.

Multi-agent Reinforcement Learning for the Control of Three-Dimensional Rayleigh–Bénard Convection

AbstractDeep reinforcement learning (DRL) has found application in numerous use-cases pertaining to flow control. Multi-agent RL (MARL), a variant of DRL, has shown to be more effective than single-agent RL in controlling flows exhibiting locality and translational invariance. We present, for the first time, an implementation of MARL-based control of three-dimensional Rayleigh–Bénard convection (RBC). Control is executed by modifying the temperature distribution along the bottom wall divided into multiple control segments, each of which acts as an independent agent. Two regimes of RBC are considered at Rayleigh numbers $$\textrm{Ra}=500$$ Ra = 500 and 750. Evaluation of the learned control policy reveals a reduction in convection intensity by $$23.5\%$$ 23.5 % and $$8.7\%$$ 8.7 % at $$\textrm{Ra}=500$$ Ra = 500 and 750, respectively. The MARL controller converts irregularly shaped convective patterns to regular straight rolls with lower convection that resemble flow in a relatively more stable regime. We draw comparisons with proportional control at both $$\textrm{Ra}$$ Ra and show that MARL is able to outperform the proportional controller. The learned control strategy is complex, featuring different non-linear segment-wise actuator delays and actuation magnitudes. We also perform successful evaluations on a larger domain than used for training, demonstrating that the invariant property of MARL allows direct transfer of the learnt policy.

Enhancing Heat Transfer Efficiency Through Controlled Magnetic Flux in a Partially Heated Circular Cavity Using Multi-Walled Carbon Nanotube Nanofluid and an Internal Square Body

Applications including aircraft systems and electronics cooling depend on effective heat transfer. This study investigates magnetohydrodynamic (MHD) free convection and thermal radiation for heat transfer in a circular cavity filled with multi-walled carbon nanotube (MWCNT) nanofluid and containing a square obstruction. This study examines the impact of the internal geometry on heat transfer and fluid flow dynamics under three distinct boundary conditions, and it presents a comprehensive analysis based on a wide range of Hartmann (Ha) and Rayleigh (Ra) numbers. MWCNT nanofluid with high thermal conductivity was employed to enhance heat transfer efficiency, using a solid volume fraction (SVF) of 4% for MWCNTs and assuming Newtonian behavior for computational simplification. Magnetic properties were imparted to the nanofluid by assuming the dispersion of carbon nanotubes in a base fluid containing magnetic nanoparticles. Other walls were insulated, the bottom wall was heated, and a magnetic field (MF) with Ha ranging from 0 to 100 was applied. It was observed that raising Ra from 103 to 106 improved the Nusselt number (Nu) from 0.08 to 7.1 using the Galerkin finite element method. Ha increased from 0 to 100 and reduced Nu by 35%. Three boundary conditions for the square body showed that the heated conditions provided the largest Nu. By means of an increase in SVF from 0 to 0.04, the MWCNT nanofluid improved heat conductivity by 18%. Radiation effects with the radiation parameter Rd = 0.5 increased heat transmission by 22%. These results underline the importance of considering MHD and nanofluid characteristics in maximizing heat transfer for commercial purposes, and the approaches employed in this study contribute to a deeper understanding of the behavior of thermal systems under the influence of MHD and internal geometry.

Seismic Performance of Shaped Steel Reinforced High‐Strength Concrete Walls: Cyclic Loading Test and Analytical Modeling

ABSTRACTThe thickness of the bottom shear walls in tall and super‐tall structures is often designed too large to meet the limit requirements of axial compression ratios, which increases the weight and cost of the structure. To address this issue, high‐strength concrete (HSC) and shaped steel are combined to form composite shear wall members. This paper focuses on seismic performance and analytical hysteresis model of steel reinforced HSC shear walls (SRHCW) under high axial compressive load. Firstly, the reversed cyclic test was conducted to investigate the influence of concrete strength and the steel ratio on the seismic performance of SRHCWs by comparing the failure mechanism, hysteretic curves, stiffness degradation, ductility, energy dissipation, and steel bar strain variation of each specimen. Then, a hysteresis model for SRHCWs was established, and the model can be used for the nonlinear analysis and seismic performance evaluation of SRHCWs. Finally, the accuracy of the proposed hysteresis model was evaluated by the experimental data. The experimental results show that under high axial compression ratio, the interstory drift ratio capacity of SRHCWs can reach 3.03% showing excellent deformation performance. The wall specimens built with different strength concrete and shaped steel ratios exhibited similar strength, deformation, and initial stiffness, indicating that the steel ratio of the wall can be effectively reduced by upgrading the concrete strength of the wall specimens while ensuring its seismic performance. The analytical model can well predict the hysteresis force–deformation curves of SRHCWs under high axial load ratios.

Intelligent optimization of thermal performance in nanoparticle-enhanced enclosures with sinusoidal heating and magnetic field interaction using Multi Expression Programming

This study advances a comprehensive numerical analysis aimed at enhancing thermal transfer within square enclosures filled with water-based oxide nanoparticle suspensions subjected to central sinusoidal heating. Central to this research is the integration of Multi Expression Programming (MEP) for the predictive optimization of thermal efficiency, taking into account the intricate effects of sinusoidal heating geometry, nanoparticles concentration, and an inclined magnetic field. The analysis maintains the initial setup boundary conditions: no-slip at the enclosure walls, isothermal conditions at the left and right walls, and adiabatic conditions at the top and bottom walls, except where sinusoidal heating is applied. Using MEP, these conditions are explored to identify configurations that significantly enhance thermal performance. This method allows for a detailed examination of the impacts of heating element undulation, magnetic field orientation, and nanoparticle dispersion on flow dynamics and thermal transmission. The results emphasize the significant impact of heating element undulation on the heat transfer rate, with MEP predicting optimal undulations that boost thermal efficiency. Furthermore, the strategic application of magnetic fields, as optimized through MEP, facilitates controlled flow distribution and buoyancy effects, with an increased Rayleigh number leading to enhanced convection patterns. The study also delineates the specific boundary conditions under which the Nusselt number, indicative of thermal performance, increases. These MEP-driven insights are invaluable for designing optimized heat transfer systems and energy-efficient applications, establishing a new benchmark for thermal management strategies in practical engineering contexts, firmly rooted in the precision afforded by computational optimization and predictive modeling.

Experimental research on heat transfer enhancement by a wall-proximity circular cylinder under an axial magnetic field

This work experimentally investigates the flow and heat transfer of liquid metal around a cylinder in a rectangular channel with a heated bottom wall under an axial magnetic field. Wall electrical potential probes measure the streamwise and vertical velocity components, while an immersed array probe measures the temperature distribution in the vertical profile. The coupling effects of the gap ratio (ratio of the distance between the center of the cylinder and the wall to the diameter of the cylinder) and the magnetic field on heat transfer enhancement are studied. The experimental results suggest that the Lorentz force suppresses the wall recirculation zone from shedding secondary vortices and alters the trajectory of the vortex street, affecting the thermal boundary layer. The probability density function of temperature indicates that the magnetohydrodynamics effect causes a bimodal distribution due to a quasi-two-dimensional vortex street and a trimodal distribution due to additional secondary vortices. The vortex street notably reduces the thermal boundary layer thickness and the local temperature of the heated wall. The analysis of the correlation coefficients between velocity and temperature fluctuations and the frequency spectrum reveals the physical mechanism enhancing heat transfer. The wall-proximity effect and buoyancy strengthen flow fluctuations and enhance heat transfer. For Ha (Hartmann number) ranging from 161.6 to 646.4, optimal heat transfer occurs at G/d = 1.0, whereas for 808 ≤ Ha ≤ 1131.2, optimal heat transfer is achieved at G/d = 0.5, which is attributed to the coupling effect of the magnetic field and gap flow on vortex dynamics.

Comparative study to analyze the overall performance of upstream and downstream wedge ribs microchannels using thermal lattice Boltzmann method

The thermal lattice Boltzmann method (TLBM) is used to analyze the overall performance for upstream and downstream wedge ribs microchannels (MC) under slip flow conditions. The thermal–hydraulic enhancement criterion is investigated to evaluate the performance of the channel and compare it for various roughness MC. In order to improve the channel performance, two alternative artificial roughness geometry, upstream and downstream wedge ribs, are taken both on the top and bottom walls of the microchannel with aspect ratio (AR) 7, where AR = L/H; L and H are channel length and height respectively in micrometer (μm). This study focused on simulating temperature profiles, velocity vectors in terms of stream lines, pressure gradients, and friction factor in terms of Poiseuille number as well as heat transfer rate in terms of Nusselt number (Nu). The overall performance of the channel is calculated based on flow friction and heat transfer rate for different Knudsen numbers (Kn) ranging from 0.01 to 0.10 with upstream and downstream wedge ribs height up to 20% of channel height. The results have been compared with previously published work and are found a very good agreement. The analysis revels that, the vortices are formed behind each upstream wedge rib, whereas they are created in front of each downstream wedge rib. The size and shape of vortices are influenced by Kn. As Kn increases from 0.0 to 0.10, the fluid circulation area becomes smaller for upstream wedge ribs MC, while it is changing very slowly for downstream wedge ribs MC; hence, the pressure gradient is also responsible for changing Kn. The flow friction is linearly decreased with increasing Kn but significantly increased with ribs height. But compared to the smooth channel, the friction is significantly increased for upstream and downstream wedge ribs MC. The average rate of heat transfer in terms of Nu is also linearly decreased with increasing Kn, but Nu increased with ε for lower Kn and decreased for higher Kn. Therefore, compared to smooth MC, Nu increased and decreased for the same for upstream and downstream wedge ribs MC. Finally, the performance enhancement (η) is calculated, and it is found that η decreased with increasing Kn for upstream and downstream wedge ribs MC. The higher performances are indicated for lower Kn as well as lower ribs height. For all cases, the better performance is noted for downstream wedge ribs MC compared to upstream MC.

Comparison of hyperbolic and parabolic equations modelling buoyancy driven flow in a square cavity

The effects of a hyperbolic and parabolic heat transfer equation on buoyancy-driven flow in a square cavity are studied. Boundary conditions where the bottom wall is hot and the side and top walls are cold are investigated. The case when the bottom and sidewalls are warm and the top wall is cold is also examined. Simulation of the flow is done using the finite element approach. An equation for the heat function is determined. The finite difference method is used to determine solutions to the heat transfer equation. We find that a hyperbolic heat transfer equation increases the magnitude of vorticity, stream function, and heat function. This suggests that hyperbolic heat transfer equations have stronger circulation and achieve a stable state sooner than parabolic heat transfer equations.

Interaction between vapor bubbles during flow boiling heat transfer in microchannels

Microchannel flow boiling is an efficient cooling solution for high-power-density miniaturized systems. Many studies on microchannel flow boiling focused on the dynamics of single vapor bubbles, while neglecting the interaction between bubbles, which is important in relevant applications. Here, numerical simulations are carried out to study the interaction between multiple vapor bubbles in microchannel flow boiling. The results show that for different numbers of bubbles in the microchannels with the same initial size and position of leading bubbles, the bubble size in a single-bubble microchannel is larger compared to the leading bubble of multiple-bubble cases because of heat absorption by the vaporization at the rear bubbles. As the initial volume ratio between the leading bubble and the rear bubble decreases, the leading bubble size in the downstream becomes smaller because of the reduced contact with the superheated thermal boundary layer. With increasing the Reynolds number, both the leading and the trailing bubbles increase slightly in size in the upstream of the heated region, because the bubbles at higher Reynolds number move faster and firstly get in contact with the superheated fluid. The increase in the bottom wall thickness increases the growth rate of the multiple bubble sizes with earlier bubble coalescence because of the higher upstream wall temperature by heat conduction in the solid wall.

Experimental investigation on identification of heat transfer deterioration and optimization of correlations for supercritical CO2

The existing experimental research on heat transfer deterioration of supercritical CO2 (S-CO2) in horizontal heated tubes and how to improve accuracy of heat transfer correlations by identifying this deterioration is still lacking. This paper analyzes the effects of inlet pressure, heat flux and mass flux on heat transfer across various states, with focus on precise heat flux intervals near state transitions. The results indicate that heat transfer at top wall and bottom wall can exhibit significant differences, necessitating independent analysis for accurate understanding and prediction. As heat flux increases during transition from liquid region to liquid-like region, heat transfer coefficient exhibits an upward trend. Both buoyancy and flow acceleration effects have significant impacts on local heat transfer near the pseudocritical region. An optimization method for heat transfer correlations has been proposed and validated. When an optimal value of 0.8 for heat transfer coefficient ratio is set by referencing Dittus-Boelter equation, newly introduced correlation predicts top wall heat transfer coefficient with 95.76 % of data points falling within a ±15 % error margin. The study identifies key factors that influence the deterioration of heat transfer, proposing correlations for identifying critical heat flux and practical measures to mitigate deterioration in industrial applications.

Impact of spanwise rotation on flow separation and recovery behind a bulge in channel flows

Direct numerical simulations of spanwise-rotating turbulent channel flow with a parabolic bump on the bottom wall are employed to investigate the effects of rotation on flow separation. Four rotation rates, $Ro_b := 2\varOmega H/U_b = \pm 0.42$ , $\pm$ 1.0, are compared with the non-rotating scenario. The mild adverse pressure gradient induced by the lee side of the bump allows for a variable pressure-induced separation. The separation region is reduced (increased) when the bump is on the anti-cyclonic (cyclonic) side of the channel, compared with the non-rotating separation. The total drag is reduced in all rotating cases. Through several mechanisms, rotation alters the onset of separation, reattachment and wake recovery. The mean momentum deficit is found to be the key. A physical interpretation of the ratio between the system rotation and mean shear vorticity, $S:=\varOmega /\varOmega _s$ , provides the mechanisms regarding stability thresholds $S=-0.5$ and $-$ 1. The rotation effects are explained accordingly, with reference to the dynamics of several flow structures. For anti-cyclonic separation, particularly, the interaction between the Taylor–Görtler vortices and hairpin vortices of wall-bounded turbulence is proven to be responsible for the breakdown of the separating shear layer. A generalized argument is made regarding the essential role of near-wall deceleration and resultant ejection of enhanced hairpin vortices in destabilizing an anti-cyclonic flow. This mechanism is anticipated to have broad impacts on other applications in analogy to rotating shear flows, such as thermal convection and boundary layers over concave walls.

Experimental Study on Flow and Thermal Transport in Additively Manufactured Lattices Based on Cube-Shaped Unit Cell

Abstract This paper presents the convective heat transfer coefficient of cubic lattices under both buoyancy-induced and forced convection. Additionally, it examines the effective thermal conductivity, permeability, and inertial coefficient of a cubic unit cell of porosity ∼0.87. The test specimens were additively manufactured using stainless steel 420 (with 40% bronze infiltration) using the binder jetting technique. In the buoyancy-driven convection experiments, three different aspect ratios (width/height) varying from 0.5 to 2 were tested across three different heating orientations, viz., bottom wall (0 deg), side wall (90 deg), and top wall (180 deg). The lattice with the lowest aspect ratio had the highest convective heat transfer coefficient in all three heating orientations. The forced convection heat transfer coefficient was determined for an additively manufactured part comprising 10 × 10 cubic unit cell array in the plane perpendicular to the flow and 20 unit cells in the streamwise direction. Additionally, the flow characteristics of the cubic lattice were characterized through permeability (K) and inertial coefficient (Cf), determined by conducting separate pressure drop experiments over a wide range of flow velocities. The thermal hydraulic performance (THP) of the cubic lattice was assessed by combining the periodic regime convective heat transfer coefficient with the pressure drop data obtained from the experimentally determined values of K and Cf. The comprehensive characterization of flow and thermal transport, including K and Cf, along with hsf, keff, presented in this paper, provides a robust foundation for their application in volume-averaged computations for detailed parametric study.

Influences of submerged V-broken rib geometry on the hydrothermal performance of vortex and engulfment flow regimes within a T-channel

The current research presents a three-dimensional numerical study of detailed hydrothermal performance in a T-channel fitted with staggered V-broken ribs under laminar vortex and engulfment flow regimes (Re = 50 to 250) using CFD software tool FLUENT. Comprehensive parametrical investigations are executed to probe the influences arising from the geometrical parameters alterations of the V-broken ribs on the flow field and convective heat transfer characteristics in T-channel. A set of geometrical parameters involving, the orientation rib angles (θ = 30°, 45°, 60°), the pitch-to-outlet-channel width ratio (p/Wo = 1, 1.5, 2), the length-to-outlet-channel width ratio (l/Wo = 0.5, 0.75, 1), the height-to-outlet-channel height ratio (h/Ho = 0.2, 0.3, 0.4), and the staggered inclined rib arrangements (forward, backward, and mixed), are considered. With regard to all examined geometrical parameters, the presence of staggered V-broken ribs consistently enhances fluid mixing and convective heat transfer performance as compared with that without ribs. In addition, the findings illustrate that the V-broken ribs mounted on the bottom wall of the T-channel have approximately the same PEC at 45° and 60° orientation angles with a PEC of 2.23 at Re = 250. At Re = 250, the PEC improves by 40 % and 53 % in comparison to the standard T-channel for the 2 cm and the 4 cm of longitudinal pitch, respectively. Regarding, V-broken rib lengths of 1.5 cm and 1 cm, PEC shows similar trend and eventually reaches double, while the PEC for a 2 cm rib length is smaller than that of 1 and 1.5 cm. The results also exhibit that the smaller rib height of 2 mm gives higher PEC, approximately 2.15, in comparison to 3 mm and 4 mm. Finally, the backward V-broken rib arrangement provides the best PEC of the T-channel, around 2.42.