High Reynolds Number Research Articles

Exploring the potential of hybrid nanofluids for enhanced heat transfer in a duct: A comprehensive study utilizing a phases-interaction driven multiphase mixture model

This study investigates the influence of phases interactions in hybrid nanofluids using a multiphase mixture model. Neglecting phase interaction for Al2O3-Cu leads to Nusselt number difference of 0.05 (volume fraction 0.1%), increasing to 0.6 at volume fraction 2%. Among investigated nanofluids (Al2O3-Au NPs, Al2O3-CuO, Al2O3-TiO2, CuO-Au NPs, CuO-TiO2, TiO2-Au NPs), those with gold nanoparticles exhibit exceptional heat transfer enhancements: Al2O3-Au NPs (63.85%), CuO-Au NPs (64.65%), and TiO2-Au NPs (62.10%) at 3% total solid phase volume fraction. CuO-Au NPs perform best, except at ∼5% volume fraction where Al2O3-Au NPs excel. Optimization shows increasing gold particle volume fractions benefit particularly at higher Reynolds numbers. For CuO-Au NPs, highest average Nusselt numbers are obtained at gold volume fractions of 10%, 20%, 40%, and 60% of total solid phases volume fractions for Reynolds numbers 300, 500, 1000, and 2000, respectively. Moreover, the investigation showed that the performance index generally increases with volume fraction up to 5%, except for hybrid nanofluids with gold particles. Notably, in the presence of gold particles, the performance index rises distinctly up to a volume fraction of 3%, followed by a subsequent decline.

Plastron restoration for underwater superhydrophobic surface by porous material and gas injection

Restoring and maintaining the gas layer (plastron) on underwater superhydrophobic surface (SHS) is critical for the real-world application of SHS, such as reducing friction drag in high-Reynolds number turbulent flows. In this work, we experimentally investigated the capability of a technology based on porous material and gas injection to restore the plastron on an underwater SHS from a fully wetted state. The SHS was created by sprayed coating a commercial superhydrophobic coating on a porous steel plate. In the experiments, the SHS was immersed in stationary liquid, the gas injection pressure and gas injection duration were independently controlled. The status of gas layer on SHS was examined by a high-speed camera. We found that the surface area being restored with a plastron increased with increasing gas injection pressure and gas injection duration, and that the plastron restoration process involved bubble formation, merging and detachment. A layer of gas was left on the surface after bubble detachment. The size of the detached bubble increased with time due to bubble merging, and became stable when there was no more bubble merging. Increasing gas injection pressure led to higher gas flow rates, larger detached bubble sizes and faster plastron restorations. A plastron restoration within 0.3 s was achieved at the highest pressure, faster than the in-situ gas generation methods. Furthermore, we found that the gas flow rate through the underwater SHS can be described by a modified Darcy’s law. Our results highlighted the potential of using porous material and gas injection to restore the plastron and made possible the real-world implementation of SHS.

Turbulent Drag Reduction by Streamwise Traveling Waves of Wall-Normal Forcing

We review some fundamentals of turbulent drag reduction and the turbulent drag reduction techniques using streamwise traveling waves of blowing/suction from the wall and wall deformation. For both types of streamwise traveling wave controls, their significant drag reduction capabilities have been well confirmed by direct numerical simulation at relatively low Reynolds numbers. The drag reduction mechanisms by these streamwise traveling waves are considered to be the combination of direct effects due to pumping and indirect effects of the attenuation of velocity fluctuations due to reduced receptivity. Prediction of their drag reduction capabilities at higher Reynolds numbers and attempts at experimental validation are also intensively ongoing toward their practical implementation.

Experiment study on heat transfer enhancement of micro heat pipe PV/T by Reynolds number improvement

The rapid performance improvement of photovoltaic/thermal (PV/T) implies great potential application to various aspects of society. The previously developed micro heat pipe PV/T by our team has shown favorable cogeneration of electricity and heat in cold regions, while it is badly needed to improve its comprehensive performance by enhancement of heat transfer. For same working fluid as HCFC141b in micro heat pipe, higher Reynolds Number (Re) theoretically means better heat transfer. Therefore, two available micro heat pipes were developed with trapezoidal cross section and rectangular cross section respectively while with same cross section area and applied in two micro heat pipe PV/T systems. Subsequent experiment study testifies that the rectangular micro heat pipe PV/T of higher theoretical Re verily shows more favorable performance than the trapezoidal micro heat pipe PV/T when they are at 45° inclination angle. Deducting power consumption of circulating pump, the average electrical efficiency of rectangular micro heat pipe PV/T system is 12.4%, while that of trapezoidal micro heat pipe PV/T system is 11.9%. Moreover, the thermal efficiency, total energy power, and total energy efficiency of rectangular micro heat pipe PV/T system are respectively 28.1%, 472.3 W, and 40.5%, which are 10.1%, 119.8 W, and 10.8% higher than those of trapezoidal micro heat pipe PV/T system. The results are significant and valuable for the application of micro heat pipe PV/T in cold regions.

Miniaturized metachronal magnetic artificial cilia

Biological cilia, hairlike organelles on cell surfaces, often exhibit collective wavelike motion known as metachrony, which helps generating fluid flow. Inspired by nature, researchers have developed artificial cilia as microfluidic actuators, exploring several methods to mimic the metachrony. However, reported methods are difficult to miniaturize because they require either control of individual cilia properties or the generation of a complex external magnetic field. We introduce a concept that generates metachronal motion of magnetic artificial cilia (MAC), even though the MAC are all identical, and the applied external magnetic field is uniform. This is achieved by integrating a paramagnetic substructure in the substrate underneath the MAC. Uniquely, we can create both symplectic and antiplectic metachrony by changing the relative positions of MAC and substructure. We demonstrate the flow generation of the two metachronal motions in both high and low Reynolds number conditions. Our research marks a significant milestone by breaking the size limitation barrier in metachronal artificial cilia. This achievement not only showcases the potential of nature-inspired engineering but also opens up a host of exciting opportunities for designing and optimizing microsystems with enhanced fluid manipulation capabilities.

A study of effects of H–H quartic auto catalytic reaction with equal diffusivity in the swirling flow exterior to a rotating cylinder

In this study, we investigate the influence of quartic auto catalysis with equal diffusivity in the flow past a rotating cylinder. The flow-thermal fields are modeled using partial differential equations (PDEs), assuming a homogeneous–heterogeneous reaction. The resulting flow-thermal equations are appropriately ordered and solved using the BVP4c function in MATLAB. Our analysis focuses on examining the effects of varying the Reynolds number, magnetic field, radiation, and homogeneous–heterogeneous reaction parameters on the velocity, temperature, and concentration of the bulk fluid. Through extensive simulations, we find that the concentration, temperature, and velocity of the homogeneous bulk fluid are predominantly determined by the Reynolds number. We observe that higher Reynolds numbers lead to increased mixing and enhanced fluid motion, resulting in higher concentrations and temperatures. Moreover, the strength parameters associated with the homogeneous–heterogeneous reaction significantly favor the auto catalysis reaction at the surface of the rotating cylinder, indicating a self-catalytic process. To validate our findings, we compare our results with earlier studies that investigated reduced cases. Remarkably, our obtained results exhibit excellent agreement with these prior studies, further bolstering the reliability and accuracy of our research.

Investigating the performance of conventional and hydrophobic surface heat sink in managing thermal challenges of high heat generating components

In the pursuit of optimizing heat dissipation and improving performance in electronic devices, liquid-cooled heat sinks have garnered significant interest due to their potential to improve heat transfer efficiency. Presented study set out to assess the effectiveness of straight channel heat sinks as a thermal solution for high heat generating electronics components, while also investigating the impact of applying a hydrophobic coating on their performance. A series of tests were conducted at different heating powers (50W-250 W) and flow rates (200–950 ml/min), while also considering different orientations and directions of flow. The investigation focused on analysing critical aspects such as heat transfer performance, wall temperature variations, thermal resistance, and pressure drop characteristics of straight channel heat sinks with conventional and hydrophobic-coated surfaces. The results showed that the Nusselt number increased with higher Reynolds numbers and heating powers, indicating improved heat transfer efficiency. However, the hydrophobic coating on the heat sink surface led to a reduction in the Nusselt number due to the formation and retention of bubbles, hindering heat transfer. The maximum average reduction of 20.85% in Nusselt number was noticed at 100 W for the vertically positioned hydrophobic-coated heat sink with downward flow compared to conventional surface (CS) heat sink. Additionally, the findings revealed that pressure drop is influenced by Reynolds number and the presence of bubbles, with gravity playing a role in reducing pressure drop in certain orientations. A hydrophobic coating shows a small decrease in pressure drop, but bubble formation limits its effectiveness to some extent. Comparisons also indicated that the highest average decrease in pressure drop about 11.92% was observed for the same configuration, vertically positioned hydrophobic heat sink with downward flow, at 50 W. Moreover, the study's findings have broader implications beyond electronics. Heat sinks are widely employed in various industries, including automotive, aerospace, renewable energy systems, and industrial processes.

Large eddy simulation investigation of S-shaped intake distortion and swirl characteristics

Curvature change of an S-duct wall naturally causes total pressure and swirl distortions in a compressor inlet. In this study, the large eddy simulation (LES) method is used to investigate the internal flow of a continuous variable curvature S-duct without a downstream compressor. A comparison between the numerical results and the experimental data verifies the effectiveness of the LES method in analyzing the internal flow mechanism of an S-duct. Based on the SAE standard, the transient distortion descriptors of the research cross-section are quantitatively analyzed, and the distortion characteristics of the total pressure and swirl in the aerodynamic interface plane (AIP), downstream outlet plane, and their spatial–temporal variation processes are revealed. Furthermore, the proper orthogonal decomposition and sparsity-promoting dynamic mode decomposition methods are used to decompose the flow structures of the AIP velocity fields, and the main coherent structures affecting the flow and their corresponding frequency/attenuation characteristics are obtained. The results indicate that the twin two swirl pattern near the wall annulus of the S-duct is dominant at high Reynolds numbers, and turbulent mixing downstream inhibits the development of distortion and swirl. In addition, some stable low-frequency mode structures exist near the Starboard, exhibiting multi-spiking frequency properties. Thus, this study reveals the generation, development, and evolution mechanisms of distortion and swirl at the upstream of a compressor inlet, and the analysis results of distortion and swirl characteristics can provide important input conditions for subsequent compressor distortion research and influence analysis of mass injection pre-compressor cooling technology.

Eddy-resolving Reynolds-stress modelling of thermal mixing in cross-stream type T-junction configurations

The present work is concerned with the numerical simulation study of turbulent thermal mixing involving hot main and cold branch streams at high Reynolds numbers in two T-junction configurations that differ in flow rate ratio. This results in two topologically very different flow scenarios, classified as “wall-jet” and “impinging-jet” mixing processes. The underlying reference database originates from the well-known experimental WATLON study[16], dealing with the phenomena of thermal stripping in piping systems, and provides details on the configuration geometry and the associated boundary and operating conditions. The turbulence model used, implemented in the finite-volume-based numerical code OpenFOAM® [30], is a near-wall Reynolds stress model formulation appropriately sensitized by applying the scale-adaptive modeling strategy to enable its eddy-resolving capability [14]. The flow analysis includes the first- and second-order statistics, including both the flow and thermal fields, with the model results closely following the experimental reference. This refers to the global flow topology in terms of the flow separation pattern, both in terms of its shape and size, as well as to the individual mean velocity and temperature profiles and their fluctuating components. In addition, the spectral features of the flow are studied in detail using the proper orthogonal decomposition (POD) method to identify the low-rank structures whose frequency signatures contribute most to the phenomena of thermal stripping phenomena.

Development and evolution mechanism of streamwise vortex in an inward-turning inlet

The three-dimensional flows in an inward-turning inlet were numerically investigated at different incoming flow conditions. When the incoming flow conditions change, the shock angle and the shock interaction form of the external compression wave change, and the development of the near-wall low-energy fluid and the streamwise vortex is also affected. The impingement of the shock wave leads to a sharp increase in the vorticity of the low kinetic energy fluid. Under the pressure gradient caused by the shock wave, the high-vorticity fluid migrates from the cowl to the ramp and entrains the mainstream fluid to form a streamwise vortex, for which the velocity gradient ( ∂v/ ∂y + ∂w/ ∂z) along the vortex axis can accurately determine the rotation direction and the Hopf bifurcation position. By considering high Reynolds number flows, the pressure gradient along the vortex axis is developed to estimate the simplified dilation term (velocity gradient) due to its ease of measurement. However, the pressure gradient ( ∂p/ ∂x) along the vortex axis can lead to bias when evaluating the cross-flow state of the streamwise vortex, with the shock wave structure and high-vorticity fluid leading to under- and overestimation, respectively. This study provides a theoretical basis for an accurate determination of the flow state of a streamwise vortex in an inward-turning inlet and thus lays the foundation for effective vortex control.

Lagrangian large eddy simulations via physics-informed machine learning

High-Reynolds number homogeneous isotropic turbulence (HIT) is fully described within the Navier-Stokes (NS) equations, which are notoriously difficult to solve numerically. Engineers, interested primarily in describing turbulence at a reduced range of resolved scales, have designed heuristics, known as large eddy simulation (LES). LES is described in terms of the temporally evolving Eulerian velocity field defined over a spatial grid with the mean-spacing correspondent to the resolved scale. This classic Eulerian LES depends on assumptions about effects of subgrid scales on the resolved scales. Here, we take an alternative approach and design LES heuristics stated in terms of Lagrangian particles moving with the flow. Our Lagrangian LES, thus L-LES, is described by equations generalizing the weakly compressible smoothed particle hydrodynamics formulation with extended parametric and functional freedom, which is then resolved via Machine Learning training on Lagrangian data from direct numerical simulations of the NS equations. The L-LES model includes physics-informed parameterization and functional form, by combining physics-based parameters and physics-inspired Neural Networks to describe the evolution of turbulence within the resolved range of scales. The subgrid-scale contributions are modeled separately with physical constraints to account for the effects from unresolved scales. We build the resulting model under the differentiable programming framework to facilitate efficient training. We experiment with loss functions of different types, including physics-informed ones accounting for statistics of Lagrangian particles. We show that our L-LES model is capable of reproducing Eulerian and unique Lagrangian turbulence structures and statistics over a range of turbulent Mach numbers.

Topology-Optimized Micromixer Design with Enhanced Reverse Flow to Increase Mixing Efficiency.

In this work, a serpentine mixing unit model based on topology optimization is proposed to enhance the reverse flow in both horizontal and vertical directions. The increase in reverse flow in both directions can enhance the chaotic advection phenomenon, leading to a rapid increase in the mixing index. The proposed mixing unit model is applied in a T-shaped micromixer to create a new micromixer design, named TOD. Numerical simulations of TOD are performed using Comsol Multiphysics software to analyze the characteristics of the liquid flow, mixing surface, and pressure drop. The simulation results confirm that TOD has an outstanding mixing performance. By widening the surface area of contact and enhancing the chaotic advection phenomenon, TOD shows an excellent mixing performance at both a high and low Reynolds number, making it a promising micromixer design. For Re > 5, the mixing indexes of TOD are all beyond 90%.

Simulation of Drag Crisis in Flow Past a Circular Cylinder using 2D Computations

The unsteady incompressible 2D Navier-Stokes equations are solved using a finite element method with Streamline-Upwind/Petrov-Galerkin (SUPG) and Pressure-Stabilizing/Petrov- Galerkin (PSPG) stabilization terms for high Reynolds number (105 to I06) flow past a cylinder to stimulate drag-crisis. The mean drag coefficient for various Re are compared with experimental data. It is observed that the low at Re =106 separates much later, from the cylinder surface as compared to that at Re =105. The wake at Re =106 is narrower than that at Re = 105. The time-averaged flow field shows a standing bubble just downstream of the flow separation point for the Re = 105 flow. The bubble is not observed for the flow at Re = 106. The energy spectrum of the flow exhibits the characteristics of 2D isotropic turbulence.

Prediction of the phase difference between large-scale velocity and Reynolds stress fluctuations in wall turbulence

A resolvent-based model was used to predict the phase-difference profile between velocity and stress coherent motions measured in a high Reynolds number channel flow as a proxy for predicting large- and small-scale turbulent interactions. The resolvent model is based on the transfer-function approach for scale interactions in wall turbulence proposed in Jacobiet al.(J. Fluid Mech., vol 914, 2021, pp. 1–27), but incorporates a quasi-empirical weighting scheme to construct composite mode shapes that represent the realistic dispersion of convection velocities associated with the large scales of turbulence. The weighting scheme was derived from the observed similarity between the spectral region where the resolvent operator is low rank and the streamwise spectral energy density of wall-bounded turbulence, and was found to be superior to both single-convection velocity models and models based on linearly weighted modes, when compared with cross-spectral phase calculations from a channel flow computation. The ability to predict the phase relationship between large-scale coherent motions and their associated stress fluctuations allows for refining and extending resolvent-based models of turbulence to describe small-scale features of wall-bounded flows.

The investigation of a likely scenario for natural tornado genesis and evolution from an initial instability profile

A likely mechanism for the little-understood tornado genesis is proposed and its numerical implementation is presented. The Burgers-Rott vortex with its axis in the vertical direction is introduced as an instability mechanism, and the flow field then evolves under the influence of the atmospheric pressure, temperature and density variations with altitude. Buoyancy effects are implemented using the Boussinesq model. Results are presented and discussed for a set of conditions including mesh type and size, different turbulence models, and a few different boundary conditions. Post-processed results of the transient simulations including animations contain a wealth of information to help analyze tornado behavior. Velocity contours, pressure contours, vorticity contours, streamlines, and iso-surfaces show the evolution of a complex flow field possessing many characteristics of a tornado. At longer times from the start, the flow field becomes more asymmetric with the vortex core becoming more twisted, and the eye of the vortex drifting away from the axis of the computational domain. The single initial vortex then transitions into multiple vortices of varying size and orientation. These high Reynolds number (ReΓ ∼109) simulation results show flow fields that resemble highly unsteady, turbulent flows with large regions of flow separation, and large eddy size range.

Evidence that superstructures comprise self-similar coherent motions in high Reynolds number boundary layers

We present experimental evidence that the superstructures in turbulent boundary layers comprise smaller, geometrically self-similar coherent motions. The evidence comes from identifying and analysing instantaneous superstructures from large-scale particle image velocimetry datasets acquired at high Reynolds numbers, capable of capturing streamwise elongated motions extending up to 12 times the boundary layer thickness. Given the challenge in identifying the constituent motions of the superstructures based on streamwise velocity signatures, a new approach is adopted that analyses the wall-normal velocity fluctuations within these very long motions, which reveals the constituent motions unambiguously. The conditional streamwise energy spectra of the Reynolds shear stress and the wall-normal fluctuations, corresponding exclusively to the superstructure region, are found to exhibit the well-known distance-from-the-wall scaling in the intermediate-scale range. It suggests that geometrically self-similar motions are the constituent motions of these very-large-scale structures. Investigation of the spatial organization of the wall-normal momentum-carrying eddies, within the superstructures, also lends empirical support to the concatenation hypothesis for the formation of these structures. The association between the superstructures and self-similar motions is reaffirmed on comparing the vertical coherence of the Reynolds-shear-stress-carrying motions, by computing conditionally averaged two-point correlations, which are found to match with the mean correlations. The mean vertical coherence of these motions, investigated for the log region across three decades of Reynolds numbers, exhibits a unique distance-from-the-wall scaling invariant with Reynolds number. The findings support modelling of these dynamically significant motions via data-driven coherent structure-based models.

Particle-scale analysis of thermophysical behaviors of coaxial gas swirling jets with binary-size particle mixture

Understanding the multi-physics coupling of coaxial-jet gas-particle swirling flow is crucial for practical applications. In this study, computational fluid dynamics coupled with the discrete element method is utilized to investigate the flow behavior of a coaxial jet composed of powder and coal within an annular pipe. The influence of particle diameter on particle-scale information is examined. When powder diameter is 450 μm, large particles display high Reynolds and Nusselt numbers, whereas small coal particles demonstrate elevated velocities and temperature. The variation in particle diameter affects momentum and energy exchanges at specific locations, but has a negligible impact on the radial velocity distribution and Nusselt number of powder particles. Specifically, increasing the particle diameter from 50 to 450 μm leads to a reduction in swirling velocity at the outlet of the annular pipe from 15.43 m/s to 9.57 m/s for binary mixtures, and from 18.49 m/s to 7.03 m/s for powders.

Analysis of the immersed boundary method for turbulent fluid-structure interaction with Lattice Boltzmann method

An efficient and new methodology to deal with fluid structure interaction at high Reynolds number flows is presented in this article. It relies on the coupling of the lattice Boltzmann method and the immersed boundary method with a second order predictor corrector model for the structure. The effect of the lagrangian weight of the immersed boundary method is also analyzed in the context of fluid structure interaction. Both curved and moving boundaries are considered, and several methods to calculate the lagrangian weight are compared on relevant test-cases: the laminar flow around a cylinder at Reynolds number Re=100, the Poiseuille flow in a 2D channel and a 3D fluid-structure interaction test case: a deformable flapping flag immersed in a laminar flow. A convergence study in space and time is performed and the three following parameters are investigated: the number of lagrangian markers along the boundary, the value of the lagrangian weight and the shape of the discrete delta function used in the immersed boundary. Finally, the novelty of the paper is two-fold: a new expression of the lagrangian weight which is found to reduce the error by 20% in the case of fluid-structure interaction, and the coupling of a turbulence model with a second order predictor corrector model for improved stability and accuracy. This numerical methodology is found here to be accurate for challenging cases with high added mass effect and high Reynolds number flows.

Numerical study of the wall boundary condition in large-eddy simulation of the flow past a sharp-edged bluff body

Based on the fact that the drag coefficient is relatively insensitive to the Reynolds number in many experiments on the flow past a sharp-edged bluff body (SEBB) at high Reynolds numbers, it is speculated that the effect of the wall viscosity on the aerodynamics of the SEBB can be neglected to some extent. To validate this hypothesis for the large-eddy simulation (LES), the slip-wall boundary condition was applied to the SEBB, and this LES was termed slip-wall LES (SWLES). The flow past a square cylinder was chosen to validate the SWLES. The aerodynamic parameters, such as the drag coefficient and Strouhal number, agreed well with the experimental data and those of the traditional wall-resolved LES (WRLES). The surface pressure distributions and energy spectra were close to those of the WRLES. The Kelvin–Helmholtz instability structures, flow separation, and von Kármán vortex shedding were captured accurately by the SWLES. The computational cost was compared with that of the WRLES. An interpretation of the SWLES was discussed based on flow visualizations. After the validation, the SWLES was applied to the flow past a photovoltaic system. The wind loads of the system were compared with the wind tunnel experimental data at different incident angles. The surface pressure distributions, flow structures, and wake were analyzed. The present study demonstrated that the SWLES has potential applications for the flow around an SEBB with a much lower cost than that of the WRLES.

Self-Adaptive Turbulence Eddy Simulation with a high-order finite differencing method for high Reynolds number complex flows

Accurate prediction of high Reynolds number wall-bounded flows is a challenge for turbulence modeling and numerical discretization. Moreover, complex engineering applications increase the difficulties in implementing robust and low-dissipation numerical schemes. A newly developed unified turbulence approach, Self-Adaptive Turbulence Eddy Simulation (SATES), is assessed in the simulation of a multi-element 30P30N airfoil at a high Reynolds number of 1.71 × 106. To reduce the numerical error, high-order finite differencing-based weighted compact nonlinear schemes (WCNSs) are successfully applied. The effects of grid resolution, numerical schemes and turbulence model are investigated by grouping studies. The SATES shows low sensitivity to grid resolution and predicts satisfactory results even on a coarse grid containing approximately 7.2 million cells. Compared with second-order accurate central differencing, the high-order accurate WCNS has less numerical dissipation and significantly improves the resolution of turbulence structures and the accuracy of numerical results. Z-type nonlinear weighted averaging interpolation, designed for shock capturing property in the WCNS scheme, leads to extra numerical dissipation and affects the accuracy of the results. The numerical results of the SATES are more accurate than those of the Smagorinsky Large Eddy Simulation (LES) and the Improved Delayed Detached Eddy Simulation (IDDES). The SATES provides sufficient modeling of the boundary layer on the coarse grid and a fast ‘transition’ from the RANS to the LES mode for accurate prediction of the separated shear layer and flap boundary layer separation. These results confirm that the SATES turbulence modeling approach is efficient for high Reynolds number flows and that the high-order accurate WCNS schemes show sufficient robustness and accuracy for engineering applications.