Turbulent Fluid Flow Research Articles

Development of an Interpolated RANS-LES solver for wall-bounded turbulent fluid flows

Scale-resolving hybrid RANS-LES models are being increasingly used to numerically simulate wall-bounded turbulent flows since they are computationally efficient compared to LES (Large Eddy Simulation) while also being more accurate compared to RANS (Reynolds-Averaged Navier–Stokes) models. Existing hybrid RANS-LES models typically either suffer from modeled stress depletion or they can introduce a sharp jump in the mean Reynolds stress across the RANS-LES zone. We present a novel interpolated RANS-LES (IRL) solver implemented in OpenFOAM, in which the RANS equations (for eddy-viscosity) and LES equations (for filtered velocity) are evolved simultaneously on the same computational grid. Based on the distance from the no-slip wall, the flow domain is demarcated into a near-wall, free-stream, and a “hybrid” region, sandwiched between the near-wall and free-stream regions. Spalart Allmaras (S-A) model has been used to evolve the RANS eddy viscosity, while the Wall Adapting Local Eddy Viscosity (WALE) model is used for evolving the filtered LES velocity. Using a novel scheme based on solving a partial differential equation, the turbulent eddy-viscosity is interpolated between the RANS eddy viscosity in the near wall region and the effective eddy viscosity from the resolved LES fluctuations in the free-stream region. The mean subgrid stress in the LES momentum equation is then corrected in the near-wall and hybrid regions using the interpolated turbulent eddy-viscosity. IRL’s grid resolution requirements are the same as DES’s (Detached Eddy Simulation). Grid convergence and sensitivity studies have been carried out for two canonical flow geometries. For turbulent channel flow simulations, the IRL solver does not display the typical mean stress depletion observed in DES while also predicting resolved Reynolds stresses accurately in the free-stream region. For simulations of flow over backward facing step, the IRL solver can predict the friction factor and coefficient of pressure better than that DES, especially after the reattachment point.

Wall-modeled large-eddy simulation of turbulent non-Newtonian power-law fluid flows

For high-fidelity predictions of turbulent flows in complex practical engineering problems, the Wall-Modeled (WM) Large-Eddy Simulation (LES) has aroused great interest. In the present study, we prove that the conventional Wall-Stress Models (WSMs) developed for WMLES of Newtonian fluids fail to predict the shear-thinning-induced drag reduction in power-law fluids. Therefore, we propose novel algebraic, integrated, and Ordinary-Differential-Equation (ODE) WSMs, for the first time, for WMLES of power-law Non-Newtonian (NN) fluids and assess their performance against reference Wall-Resolved (WR) LES solutions. In addition, the effects of the key model parameters, including the WSM type, sampling height, sampling cell, and axial grid resolution are explored, and it is revealed that turbulent NN flow predictions have a much higher sensitivity to the choice of WSM, compared to their Newtonian counterparts. It is manifested that, in contrast to WRLES, accurate modeling of the mean apparent and subgrid-scale NN viscosities in the Non-Newtonian ODE (NNODE) model can improve the predictions considerably. Therefore, closures with lower uncertainties on coarse WMLES grids are sought for these terms. Finally, the best performance for the present test cases is obtained via the integrated NN WSM, sampling at the lower edge of the log layer within the third off-the-wall grid cell. Nevertheless, the new NNODE WSM can have an advantage in the presence of non-equilibrium effects in more complex problems.

Numerical Simulation of Turbulent Fluid Flow in Rough Rock Fracture: 2D Case

We investigate both laminar and turbulent flow regimes in a rough-walled rock fracture via numerical CFD simulations. While previous studies were limited to either fully viscous Darcy or inertial Forchheimer laminar flow regimes, we chose to cover the widest possible Reynolds number range of 0.1–10^6. We introduce CFD simulation of a turbulent flow for rough-walled fractures, implementing RANS approach to turbulence modeling. We focus on 2D fracture geometries and implement changes in both shear displacement and wall roughness, systematically examining their effect on fracture permeability and friction factor in a manner similar to the fundamental studies of the flow in rough-walled pipes. For a curvilinear fracture, laminar flow becomes non-stationary between Re , {sim } , 10^2–10^3, earlier for larger shear displacement and wall roughness. Laminar–turbulent transition starting at Re_{textrm{cr}} , {sim } , 2300 may lead to a sharp drop in permeability depending on the fracture geometry; this gap vanishes for larger shear displacement and wall roughness. Depending on the fracture geometry, bottlenecks closing with shear become a major negative factor for the overall permeability.

Entropy generation studies of turbulent fluid flow through novel hybrid corrugated channels with variable thermophysical properties

The investigation of the effects associated with the temperature-dependent property (viscosity, density and thermal conductivity) variations on fluid flow, entropy generation, and heat transfer in the various hybrid corrugated channels is performed numerically. Water enters the flow domain with a constant inlet temperature of 300[Formula: see text]K and constant inlet velocity corresponding to the different Re values. An in-depth numerical investigation is performed for the two cases that have the least entropy generation among all the cases and these cases are inward arc-outward triangular and inward trapezoidal-outward triangular corrugated channels. The use of corrugated channels with variable properties substantially affects the Nu. The effect of temperature-dependent property variation corresponding to the Re on frictional and heat transfer entropy generation is also presented in this work. The rise in the Re and consideration of variable fluid properties resulted in a decrement in the total entropy generation. Under the considered conditions, the frictional entropy generation contribution in the total entropy generation is very small as compared to the heat transfer entropy generation with constant and variable fluid properties. Furthermore, the consideration of temperature-dependent property variation results in a lower augmentation entropy generation number as compared to the constant property cases at higher Re.

Investigation of propagation of thermal runaway during large-scale storage and transportation of Li-ion batteries

Thermal runaway in Li-ion cells and battery packs impacts the safety and performance of electrochemical energy storage systems. In particular, preventing the propagation of thermal runaway in large-scale battery storage and transportation of batteries is of much importance. While most of the past work in this direction addresses single cells or a small battery pack, only limited literature is available on systems of larger size. This work presents multiphysics simulations of propagation of thermal runaway during large-scale storage and transportation of Li-ion cells. Through simulations that account for multimode heat transfer, Arrhenius heat generation, turbulent fluid flow and combustion, the propagation of thermal runaway from one pallet of cells to an adjacent pallet is studied. The model quantitatively predicts the temperature field in/around the pallets, and, in particular, predicts whether the adjacent pallet will also catch fire or not. The impact of state of charge of the cells on thermal runaway propagation is examined. Results indicate that the gap between pallets plays a key role in determining propagation. A sharp threshold value of the gap is found, beyond which, propagation does not occur. Results from this work may be helpful in ensuring thermal safety during large-scale storage and transportation of Li-ion cells, ultimately contributing towards improved electrochemical energy storage and conversion.

Forecasting small-scale dynamics of fluid turbulence using deep neural networks

Turbulence in fluid flows is characterized by a wide range of interacting scales. Since the scale range increases as some power of the flow Reynolds number, a faithful simulation of the entire scale range is prohibitively expensive at high Reynolds numbers. The most expensive aspect concerns the small-scale motions; thus, major emphasis is placed on understanding and modeling them, taking advantage of their putative universality. In this work, using physics-informed deep learning methods, we present a modeling framework to capture and predict the small-scale dynamics of turbulence, via the velocity gradient tensor. The model is based on obtaining functional closures for the pressure Hessian and viscous Laplacian contributions as functions of velocity gradient tensor. This task is accomplished using deep neural networks that are consistent with physical constraints and explicitly incorporate Reynolds number dependence to account for small-scale intermittency. We then utilize a massive direct numerical simulation database, spanning two orders of magnitude in the large-scale Reynolds number, for training and validation. The model learns from low to moderate Reynolds numbers and successfully predicts velocity gradient statistics at both seen and higher (unseen) Reynolds numbers. The success of our present approach demonstrates the viability of deep learning over traditional modeling approaches in capturing and predicting small-scale features of turbulence.

Investigation and simulation of parabolic trough collector with the presence of hybrid nanofluid in the finned receiver tube

The present study discusses the thermal performance of the receiver tube, which contains a wall with various fin shapes in the parabolic trough collector. Inserted fins and bulge surfaces of the inner wall of the receiver tube increase the turbulent fluid flow. In pursuance of uniform distribution of heat transfer, various fin shapes such as square-shape, circle-shape, triangle-shape, and combined square-circle shapes were inserted, examined, and compared. A study of the temperature differences and fluid flow is meaningful for this project therefore finite volume method was used to investigate heat transfer. Also, hybrid Nano-Fluid AL2O3CuO, TiO2Cu, and Ag-MgO were applied to increase thermal diffusivity. When the combined square-circle-shaped fin was inserted, the thermal peak of fluid flow in the receiver tube was lower than the other studied fin shapes by almost 1%. Besides, the hybrid nano-fluid Ag-MgO Syltherm-oil-800 has lower thermal waste in comparison to others by more than 3%.

Discrete rough surface intensifiers in the thermal decomposition plants: current status and future potential

It is known that roughness affects the drop in hydraulic pressure, increasing the resistance force. The formation of a boundary layer on rough surfaces significantly affects fluid dynamics and the process of heat exchange in convective flows, causing disturbances in the velocity profile and affecting surface resistance, turbulent mixing and heat exchange. Despite the fact that there are a large number of experimental and CFD studies in the field of studying turbulent flow, there is no full-fledged review of this issue. In view of this, it is necessary to systematize studies related to the study of the influence of a rough surface during turbulent fluid flow. In most cases, roughness is quantified only using a single scaling parameter – the equivalent roughness height of a grain of sand, which can be expressed based on statistical parameters. This article presents an overview and generalization of data on the parameters and characteristics of rough surfaces. The correlation method is considered using the standard deviation in the slope of the roughness profile, as well as the effect of the secondary flow on the flow of the coolant in a closed space. The results of this study can be used in the intensification of heat transfer in thermal destruction reactors using intensifiers in the form of discretely rough surfaces. Keywords: discrete-rough surfaces; thermal contact surface;и rough surface; turbulent flow; pyrolysis reactor; thermal destruction; heat exchanger.

A Lattice-Boltzmann-based modelling chain for traffic-related atmospheric pollutant dispersion at the local urban scale

Urban traffic-related air pollution is a major source of environmental and health damage and is difficult to quantify due to its inherent physical complexity. We construct a CFD-based simulation framework coupling an efficient numerical method for turbulent fluid flows with a microscopic traffic model and an emissions model to simulate road transport pollutant dispersion at the urban microscale. We improve the open-source Lattice-Boltzmann based CFD software OpenLB to overcome its original stability deficiencies for high Reynolds number flows. A stable recursive regularization procedure with a double distribution function approach is proposed to solve an advection diffusion equation for passive scalar transport at high Reynolds number. The code is successfully validated on three reference cases of increasing complexity and the traffic model SUMO along with a physical engine emissions model are coupled with OpenLB to simulate traffic-induced pollution from a road network in a realistic complex geometry. Transient flow features are analysed and the time-averaged concentration levels in different neighbourhoods of the considered geometry are evaluated: high concentration levels are observed close to the streets but also inside specific building infrastructures due to complex wind dynamics. Analyses of altitudinal concentration variations show that flow recirculations located close to traffic lights can drive pollutant over the buildings and increase concentration levels inside inner courtyards. Time-averaged concentration maps are constructed using both spatially uniform and non-uniform line sources and it is shown that using uniform sources leads to up to 20% local concentration overestimations inside the urban canopy.

CFD simulation of an industrial steam methane reformer: Effect of burner fuel distribution on hydrogen production

Simulation of the steam methane reformer serves as a launching pad to investigate the optimal amount of hydrogen production. In this study, a 3D-CFD simulation is implemented to model transport phenomena for a turbulent fluid flow with chemical reactions within the reformer. Accordingly, the realizable k-ɛ turbulence model and discrete ordinates (DO) radiation model have been employed to simulate the kinetic mechanism of the SMR process and hydrogen/methane combustion. In addition, the REV-scale approach is considered to model the flow in the porous catalyst bed. Numerical results show that 3D-CFD simulation can accurately model such complex flows in comparison to 2D simulation. In the following, 3D numerical simulations have been performed to investigate the effect of fuel distribution between burners on the amount of heat provided, which affects the amount of hydrogen production and the tube wall temperature. Results indicate that the burner feed flow without purge gas and vent gas streams has the highest efficiency in hydrogen production and leads to a ca. 10% increase in hydrogen production compared to industrial case. Although feeding the burner with purge gas and without vent gas increases hydrogen production by about 7% compared to industrial data, it may be a more suitable option due to lower natural gas consumption and economic savings.

Reconceptualizing the organizational environment: a fluid dynamics perspective for turbulence

PurposeTo reconceptualize the organizational environment in a comprehensive manner, it is important to specify not just the velocity but also other aspects of turbulent environments. Concurrently, the purpose of this paper is to also propose that organizational adaptability and, particularly, the speed of adaptations are critical to moderate the impact of turbulence in the environment on organizational performance.Design/methodology/approachThis paper uses a conceptual methodology to fully specify turbulent environments and commensurate managerial response appropriate for such environments. Based on a perspective borrowed from the field of fluid dynamics used to specify the phenomenon of turbulence, the authors develop a conceptual model with research propositions. Four dimensions that describe turbulence in fluid flow when applied metaphorically offer a comprehensive view of the organizational environment.FindingsAn extreme, unanticipated, sudden onslaught resulting in a prolonged disrupted environment such as during the recent coronavirus crisis is best characterized as having caused turbulent environmental conditions. Management theories have addressed disruptions as high-velocity environments in the context of rapid changes in information technology. With a broadened conceptualization of the organizational environment to better capture extreme disruptions, the authors provide a comprehensive model appropriate for turbulent environmental conditions and offer research directions for scholarly pursuit.Originality/valueThis paper provides a new perspective from the physical sciences to better conceptualize organizational environments during extreme disruptions such as in turbulent environmental conditions.

Modeling of EDM Process Flushing Mechanism.

The study was performed to determine the optimum flushing condition for electrical discharge machining (EDM) of functional material titanium VT6 obtained by plasma cladding with a thermal cycle. Copper is used as an electrode tool (ET) to machine functional materials. The optimum flushing flows are analyzed theoretically by using ANSYS CFX 20.1 software which is also validated by an experimental study. It was observed that while machining the functional materials to adepth of 10 mm or more, the turbulence fluid flow dominates when nozzle angles are 45° and 75°, consequently drastically affecting the quality of flushing and the performance of the EDM. For the highest machining performance, the nozzles should be at an angle of 15° relative to the tool axis. The optimum flushing at deep hole EDM process minimizes the occurrence of debris deposition on tool electrodes, thus facilitating stable machining of functional materials. The adequacy of the obtained models was confirmed experimentally. It has been established that EDM of a hole with a depth of 15 mm, an intense accumulation of sludge, is observed in the processing zone. There arebuild-ups exceeding 3 mm in cross-section after EDM. This build-up leads to a short circuit and a reduction in surface quality and productivity. It has been proven that not correct flushing leads to intensive wear of the tool and a change in its geometric shape and, accordingly, to a decrease in the quality of EDM.

A Comparison of Multiple Odor Source Localization Algorithms

There are two primary algorithms for autonomous multiple odor source localization (MOSL) in an environment with turbulent fluid flow: Independent Posteriors (IP) and Dempster–Shafer (DS) theory algorithms. Both of these algorithms use a form of occupancy grid mapping to map the probability that a given location is a source. They have potential applications to assist in locating emitting sources using mobile point sensors. However, the performance and limitations of these two algorithms is currently unknown, and a better understanding of their effectiveness under various conditions is required prior to application. To address this knowledge gap, we tested the response of both algorithms to different environmental and odor search parameters. The localization performance of the algorithms was measured using the earth mover’s distance. Results indicate that the IP algorithm outperformed the DS theory algorithm by minimizing source attribution in locations where there were no sources, while correctly identifying source locations. The DS theory algorithm also identified actual sources correctly but incorrectly attributed emissions to many locations where there were no sources. These results suggest that the IP algorithm offers a more appropriate approach for solving the MOSL problem in environments with turbulent fluid flow.

Heat transfer and pressure drop performance improvement using curved circular spines for flow through circular pipe

The heat transfer augmentation method of inserting curved circular spines increases the dwelling time of the fluid within the system by inducing swirling motion within the fluid. It is a promising passive heat transfer augmentation technique for improving average and local heat transfer coefficients in the flow across uniform & non-uniform cross-sectioned channels at a high Reynolds number. The spines of circular cross-sections can considerably enhance flow passages employed for various purposes. The heat transfer performance study of tubes mounted with curved circular spines is presented for turbulent fluid flow conditions for Reynolds numbers ranging from 10,000 to 50,000. The curved spines are placed in the inline arrangement at the inner wall of the circular tube, disturb the flow, and break the dominant thermal resistance close to the wall, improving the heat transfer to the flowing fluid. The heat transfer enhancement at the cost of a rise in the pressure drop is also studied. The overall performance factor, R3, at the same pumping power, is presented to account for cost-effectiveness. The numerical results show that the enhancement ratio, Nue/Nuo, and friction factor ratio, fe/fo, using curved spines are 1.5 to 3.6, and 2.0 to 20.0, respectively, compared with the smooth tube at similar flow conditions. The Nusselt number ratio, Nue/Nup, at the same pumping power of the equivalent smooth tube, is 0.8 to 1.6.

ATHEROMA PROGRESSION: PHYSICAL MODELING IN EXPERIMENTAL CARDIOLOGY

Prototype models of intra-arterial circulation is one of the priority aims of experimental cardiology, as well as for the study of atherosclerosis. Purpose: To study the features of intra-arterial hemodynamics in the area of​​atheroma of the artery in physical modeling. Materials and methods: We used an original "Device for modeling intra-arterial circulation". The main parts of the model: glass rotameter tube in the form of a truncated cylinder, inlet and outlet ends of which are fixed with elastic plastic tubes connected to an electric water pump immersed in a container with glycerol solution. Inside the rotameter, using a fitting from the inlet, it is possible to install a pressure sensor that transfers data to the oscilloscope; indicators - a silk thread or dye - ink. The variable pump mode allowed us to simulate a regular heart rhythm, extrasystole (ES) and atrial fibrillation (AF). Results: In the first post-extrasystolic wave, a turbulent fluid flow formed after the plaque, standing waves and waves reflected from the walls of the rotameter were observed; the sensor registered an increase in pressure 1,6 times more compared with a regular heart rate wave. The marginal plaque zones along and against the fluid flow, especially the areas bordering the intact part of the arterial vessel, underwent the main mechanical impact. The same patterns were observed in AF with a maximum duration of a pause between pulse waves of ≥1,5 s. Conclusions: Heart arrhythmias play an important role in the intra-arterial hemodynamics changes and are the part of the pathophysiological changes in the arteries in atherosclerosis. The main danger is not the ES itself, but by the first post-extrasystolic contraction or the first pulse wave after a long pause between ventricular contractions in AF.

Investigation of Thermal Engineering Processes in a Plate Heat Exchanger

Purpose of research. The aim of the study is to identify vortex formation zones in the modified corrugated channel of a plate heat exchanger using computer modeling to increase the intensification of the heat exchange process.Methods. Plate heat exchangers have a compact design, which significantly saves space on production areas and facilitates the installation and dismantling of equipment. In accordance with the purpose of this work, we solved the problem of creating artificial turbulence of the flow to increase the thermal characteristics.To implement this task, plates have been developed that have a special geometry that creates a turbulent fluid flow in the channels between the plates and increases the heat transfer coefficient. This makes it possible to achieve higher heat exchange efficiency with the same exchange area, which reduces equipment and operating costs.Results. The result of this work is the implementation of a computer model of a modified corrugated channel for visual assessment of the impact of artificially created turbulators on the degree of vortex formation.Conclusion. The introduction of plates, the main heat exchange part of which contains spherical technological wells, makes it possible to increase the efficiency of the heat exchange process by increasing the heat transfer coefficient. In addition, an increase in the heat transfer coefficient will help reduce the metal consumption and the cost of heat exchange equipment.

Scaling and intermittency in turbulent flows of elastoviscoplastic fluids

Non-Newtonian fluids have a viscosity that varies with applied stress. Elastoviscoplastic fluids, the elastic, viscous and plastic properties of which are interconnected in a non-trivial way, belong to this category. We have performed numerical simulations to investigate turbulence in elastoviscoplastic fluids at very high Reynolds-number values, as found in landslides and lava flows, focusing on the effect of plasticity. We find that the range of active scales in the energy spectrum reduces when increasing the fluid plasticity; when plastic effects dominate, a new scaling range emerges between the inertial range and the dissipative scales. An extended self-similarity analysis of the structure functions reveals that intermittency is present and grows with the fluid plasticity. The enhanced intermittency is caused by the non-Newtonian dissipation rate, which also exhibits an intermittent behaviour. These findings have relevance to catastrophic events in natural flows, such as landslides and lava flows, where the enhanced intermittency results in stronger extreme events, which are thus more destructive and difficult to predict.

Dynamics of a Newtonian droplet in the turbulent flow of a shear thinning fluid in a microchannel

The dynamics of a single Newtonian drop in a turbulent microscopic flow are strongly influenced by the shear thinning behavior of the continuous phase. The central aim of this study is to map and describe the various dynamic modes of a single Newtonian drop in terms of both the rheology of the continuous phase and the Reynolds number. In the presence of shear thinning the drops are elongated in the form of long filaments, and the stronger the shear thinning behavior of the matrix, the longer their characteristic break time is. Single drop dynamics is found to be dominated by local extension. Our findings could be useful for addressing the turbulent emulsification of non-Newtonian fluids.

Deep reinforcement learning for turbulent drag reduction in channel flows

We introduce a reinforcement learning (RL) environment to design and benchmark control strategies aimed at reducing drag in turbulent fluid flows enclosed in a channel. The environment provides a framework for computationally efficient, parallelized, high-fidelity fluid simulations, ready to interface with established RL agent programming interfaces. This allows for both testing existing deep reinforcement learning (DRL) algorithms against a challenging task, and advancing our knowledge of a complex, turbulent physical system that has been a major topic of research for over two centuries, and remains, even today, the subject of many unanswered questions. The control is applied in the form of blowing and suction at the wall, while the observable state is configurable, allowing to choose different variables such as velocity and pressure, in different locations of the domain. Given the complex nonlinear nature of turbulent flows, the control strategies proposed so far in the literature are physically grounded, but too simple. DRL, by contrast, enables leveraging the high-dimensional data that can be sampled from flow simulations to design advanced control strategies. In an effort to establish a benchmark for testing data-driven control strategies, we compare opposition control, a state-of-the-art turbulence-control strategy from the literature, and a commonly used DRL algorithm, deep deterministic policy gradient. Our results show that DRL leads to 43% and 30% drag reduction in a minimal and a larger channel (at a friction Reynolds number of 180), respectively, outperforming the classical opposition control by around 20 and 10 percentage points, respectively.

Collective dynamics of dense hairy surfaces in turbulent flow

Flexible filamentous beds interacting with a turbulent flow represent a fundamental setting for many environmental phenomena, e.g., aquatic canopies in marine current. Exploiting direct numerical simulations at high Reynolds number where the canopy stems are modelled individually, we provide evidence on the essential features of the honami/monami collective motion experienced by hairy surfaces over a range of different flexibilities, i.e., Cauchy number. Our findings clearly confirm that the collective motion is essentially driven by fluid flow turbulence, with the canopy having in this respect a fully-passive behavior. Instead, some features pertaining to the structural response turn out to manifest in the motion of the individual canopy elements when focusing, in particular, on the spanwise oscillation and/or on sufficiently small Cauchy numbers.