Turbulent Flow Research Articles

Turbulent Flow Through Sluice Gate and Weir Using Smoothed Particle Hydrodynamics: Evaluation of Turbulence Models, Boundary Conditions, and 3D Effects

Understanding flow dynamics around hydraulic structures is essential for optimizing water management systems and predicting flow behavior in real-world applications. In this study, we simulate a 3D flow control system featuring a sluice gate and a weir, commonly used in hydraulic engineering. The focus is on accurately incorporating modified dynamic boundary conditions (mDBCs) and viscosity treatment to improve the simulation of complex, turbulent flows. We assess the performance of the Smoothed Particle Hydrodynamics (SPH) method in handling these challenging conditions. Especially when the boundary conditions and applicability to industry are two of the SPH method’s grand challenges. Simulations were conducted on a Graphics Processing Unit (GPU) using the DualSPHysics code. The results were compared to theoretical predictions and experimental data found in the literature. Key hydraulic characteristics, including 3D flow effects, hydraulic jump formation, and turbulent behavior, are examined. The combination of mDBCs with the Laminar plus sub-particle scale turbulence model achieved the correct simulation results. The findings demonstrate agreement between simulations, theoretical predictions, and experimental results. This work provides a reliable framework for analyzing turbulent flows in hydraulic structures and can be used as reference data or a prototype for larger-scale simulations in both research and engineering design, particularly in contexts requiring robust and precise flow control and/or environmental management.

Mode-to-mode nonlinear energy transfer in turbulent channel flows

Mode-to-mode nonlinear energy transfer in turbulent channel flows

CFD modelling and simulations of atomization-based processes for production of drug particles: A review.

Atomization-based techniques are widely used in pharmaceutical industry for production of fine drug particles due to their versatility and adaptability. Key performance measure of such techniques is their ability to provide control over critical quality attributes (CQAs) of produced drug particles. CQAs of drug particles produced via atomization critically depend on fluid dynamics of sprays; resulting mixing, heat and mass transfer; distribution of supersaturation and subsequent nucleation and growth of particles. It is essential to develop and use computational fluid dynamics (CFD) models for adequate understanding of multi-scale transport processes ranging from molecular scale mixing and particle scale processes, and from atomizer nozzle to overall spray chamber scale establishing relationships between CQAs and design and operating parameters of spray nozzle and chamber. In this work, we critically review past and current research efforts on CFD modelling of pharmaceutical atomization-based processes with an objective to provide clear assessment of the state of the art and to provide recommendations. An overview of the key atomization-based methods for producing drug particles with desired CQAs is presented. Key underlying physical processes and relevant concepts are then outlined. This discussion is related to the demands on CFD models; and state of the art is then discussed with respect to the process needs. Recommendations are provided towards higher fidelity and more efficient models of atomized multiphase flow dynamics and turbulence, drying modelling for the produced particles, and validation approaches. We conclude by highlighting a perceived need for numerical atomization studies with a pharmaceutical context; then, we deliver an outlook on current promising active control and machine learning strategies to augment the shift towards quality-by-design approaches in pharmaceutical manufacturing.

A new forcing approach for wall-modeled simulation of rough-wall turbulent flows

The effect of rough surface on turbulent flows is usually investigated using direct numerical simulation (DNS) with either body-fitted grids or immersed boundary method. Both methods, however, require a significant number of grids to resolve all rough scales, resulting in intolerable computational cost. In this paper, a new volume forcing approach is developed to simulate wall-bounded turbulent flows with surface roughness at the same computational cost as that with smooth walls. Specifically, two extra forcing terms are introduced in the Navier–Stokes equations to mimic roughness effects. One is the feedback force operating within the roughness cell and ensuring zero velocity at the grid points located therein. The other represents the vortex force around the roughness surface. A direct comparison with the model proposed in literature shows a significant improvement in the prediction of mean flow and Reynolds stresses. To further validate the new forcing model, simulation of channel flow with cubical roughnesses is implemented, which shows a good agreement with fully resolved DNS.

EXPERIMENTAL STUDY OF FULLY DEVELOPED TURBULENT FLOW IN INTERNALLY FINNED TUBES

A steady, hydrodynamically and thermally fully developed turbulent water flow in internally finned tubes with constant-heat-flux boundary condition is investigated experimentally. In this experiment, 70°C hot water passes the inner tube while 20°C cold water flows through the annulus passage. In order to eliminate thermal contact resistance, the circular tube having a length of 1400 mm with inner fins is directly milled from a cylinder. Apart from a smooth tube, three internal-finned-tube heat exchangers having 4, 6, and 8 longitudinal trapezoidal fins are tested. For Re = 5000, the tested tubes with 4, 6, and 8 fins can improve thermal performance by approximately 35.4, 44.6, and 67.2%, respectively, but merely increase friction factor by 1.92, 4.8, and 6.73%, respectively, compared to a smooth tube. The increases with respect to to a smooth tube in both Nu and <i>f</i> become smaller at a higher Re for <i>N</i> = 4, 6, and 8. As pressure drop, increases, the <i>Q</i> of the tested finned tubes first grows rapidly, and this growth curve gradually becomes flatter at a higher Δ<i>P</i>. Consequently, the heat-transfer performance will become worse as Δ<i>P</i> > 3.5 kPa for the proposed heat exchangers. Additionally, the performance evaluation criterion (PEC) is proposed to assess the performances of the proposed internally finned tubes. It is shown that the heat exchanger with 8 fins has the best thermo-hydraulic performance. Finally, correlating equations for Nu and f with respect to Re are presented for a range of 3000 ≤ Re ≤ 7250 and <i>N</i> = 0, 4, 6, and 8.

Numerical study of turbulent flow boiling heat transfer in structured cooling channels using lattice Boltzmann method with advanced outlet boundary conditions

This study investigated the application of the lattice Boltzmann (LB) method to simulate turbulent flow boiling in structured cooling channels. The simulation used a central moment-based pseudopotential LB model with advanced characteristic boundary conditions to address numerical instabilities associated with turbulent multiphase flow. This approach ensures prolonged stability, facilitating the accurate modeling of complex flow boiling dynamics. The simulation results revealed that structured cooling channels considerably enhanced thermal performance, achieving a 27.3% increase in critical heat flux compared to flat channels. This improvement is induced by the fin structure of structured cooling channel, which makes heat distribution and promotes lower local wall heat flux compared with incident heat flux. Moreover, the simulations showed that fin structures manage bubble detachment more effectively, thereby enhancing heat transfer during the phase-change process. The study suggests that advanced outlet boundary conditions are crucial in stabilizing simulations for fin-structured channels, and the findings provide significant insights into heat dissipation mechanisms in high-heat-flux applications.

Adaptive detached eddy simulations of incident shock-induced separation

The low-frequency unsteadiness associated with shock wave turbulent boundary layer interactions (STBLI) poses significant numerical challenges for Reynolds–averaged Navier–Stokes models as well as scale-resolving methods such as direct numerical simulations and large-eddy simulations and requires hybrid methods like detached eddy simulations (DES) from an engineering perspective. The recently developed adaptive DES (ADES) method [Yin and Durbin, “An adaptive DES model that allows wall-resolved eddy simulation,” Int. J. Heat Fluid Flow 62, 499 (2016)], which was originally introduced for low-speed flows and extended to transonic flows, is a promising approach. However, its efficacy for high-speed flows needs to be assessed. In the present study, a compressible version of the ADES method has been implemented and set to test for supersonic flows with complex STBLI features. Impinging STBLI at M∞=2.30 is investigated numerically using compressible ADES method. No inflow turbulence fluctuations are provided in the upstream (time-averaged) boundary layer profile so as to verify the ADES method's capability to predict the low-frequency “breathing” motion of the bubble and the shear layer flapping/shedding in the mid-frequency range, thus demonstrating the significance of downstream mechanism in the unsteadiness associated with STBLI. Time series analysis of surface pressure distribution and modal analysis of numerical schlieren using proper orthogonal decomposition are carried out to understand the dominant mechanisms and associated frequencies. In the interaction zone, starting from the intermittent region to the region downstream of reattachment, a transition of the dominant frequency from low-to-mid-to-high ranges is observed. The separation shock motion and the shear layer shedding are observed to have dominant Strouhal numbers [Stδ0, defined based on the boundary layer thickness (δ0) upstream of interaction] in the ranges of 0.01 and 0.1, respectively, consistent with the literature. From correlation and coherence analyses, it is also observed that the separation bubble is exercising breathing motions at lower frequencies [Stδ0∼O(0.01)] despite lacking the inflow turbulence, thus supporting the downstream influence arguments. The present study demonstrates that the ADES model is capable of predicting essential features of STBLI, including low- and mid-frequency events and a part of high-frequency phenomena.

Turbulent cavitating flows under periodic inflow perturbations

Turbulent cavitating flows under periodic inflow perturbations

AutoTurb: Using large language models for automatic algebraic turbulence model discovery

Symbolic regression (SR) methods have been extensively investigated to explore explicit algebraic Reynolds stress models (EARSM) for turbulence closure of Reynolds-averaged Navier-Stokes (RANS) equations. The deduced EARSM can be readily implemented in existing computational fluid dynamic (CFD) codes and promotes the identification of physically interpretable turbulence models. Recently, large language models (LLMs) trained on large amounts of publicly available source code have drawn great attention for their abilities to automatically discover equations with more general free-text inputs and problem descriptions and provide wider possibilities with novel insights. This work proposes a novel framework, named “AutoTurb,” using LLMs to automatically discover algebraic expressions for correcting the linear Reynolds stress model. The direct Reynolds stress and the indirect RANS output (e.g., velocity field) are both involved in the training objective to guarantee data consistency and avoid numerical stiffness. An evolutionary search framework is used for global optimization, where constraints on functional complexity and simulation convergence are integrated into the objective to manage the extensive flexibility of LLMs. The proposed method is performed for separated flow over periodic hills. The generalizability of the discovered model is verified on a set of 2D turbulent separated flows with different Reynolds numbers and geometries. Results show that the corrected RANS enhances predictions of both Reynolds stress and mean velocity fields. Compared to models from other studies, our discovered model shows superior accuracy and generalization capability. The proposed approach provides a promising paradigm for using LLMs to improve turbulence modeling for a given class of flows.

Effects of turbulence spreading and symmetry breaking on edge shear flow during sawtooth cycles in J-TEXT tokamak

The effect of sawteeth on plasma performance and transport in the plasmas of tokamak is an important issue in the fusion field. Sawtooth oscillations can trigger heat and turbulence pulses that propagate into the edge plasmas, and thus enhance the edge shear flow and induce a transition from low confinement mode to high confinement mode. The influences of turbulence spreading and symmetry breaking on edge shear flow with sawtooth crashes are observed in the J-TEXT tokamak. The edge plasma turbulence and shear flow were measured using a fast reciprocating electrostatic probe array. The experimental data were analyzed using methods such as conditional average and probability distribution function. After sawtooth crashes, the heat and turbulence pulses in the core propagate to the edge, with the turbulence pulse being faster than the heat pulse. Figures 1 (a)-(e) show the core electron temperature, and the edge electron temperature, turbulence intensity, turbulence drive and spreading rates, Reynolds stress and its gradient, and shearing rates, respectively. Following sawtooth crashes, the edge electron temperature increases and the edge turbulence is enhanced, with turbulence preceding temperature. The enhanced edge turbulence is mainly composed of two parts: turbulence driven by local gradient and turbulence spreading from core to edge. The development of the estimated turbulence spreading rates is prior to that of the turbulence driving rates. The increase in the turbulence intensity can cause the enhancements of the turbulent Reynold stresses and its gradient, thereby enhancing shear flows and radial electric fields. Turbulence spreading leads to the development of edge Reynolds stresses and shear flow faster than edge electron temperature. The Reynolds stress arises from the symmetry breaking of the turbulence wave number spectrum. After sawtooth collapse, the joint probability density function of radial and poloidal wave numbers of turbulence intensity became highly skewed and anisotropic, exhibiting strong asymmetry, as seen in the figures 1 (f) and (g). The development of turbulence spreading flux at the edge is also prior to the particle flux driven by turbulence, indicating that turbulent energy transport is not simply accompanied by turbulent particle transport. These results show that turbulence spreading and symmetry breaking can enhance turbulent Reynolds stress, thereby driving shear flows, after sawtooth crashes.

Uncertainty quantification for the drag reduction of microbubble-laden fluid flow in a horizontal channel

Over three decades, much research has proven the bubble drag reduction (BDR) technique. Recently, the improvement of computing performance has enabled the simulation of multi-phase flows. The present work simulated the microbubble-laden turbulent horizontal channel flow by Nek5000 code, which is based on the spectral element method. To resolve the microbubble dynamics, the 2-way coupling Euler–Lagrange approach was combined with Nek5000 code. Furthermore, for high accuracy, high-order Lagrange interpolation was adopted to track the microbubble's location and velocity in this code. All microbubbles were assumed as non-deformable, spherical, and immiscible. For the investigation of the drag reduction effect of microbubble size and the number of microbubbles, the uncertainty quantification (UQ) method was adopted with the non-intrusive polynomial chaos method. The Latin hypercube sampling method was used to obtain precision with lesser number of samples than the Monte Carlo method. The distribution of random variables was assumed to be Gaussian and generalized polynomial chaos expansion (gPC) was applied to build the surrogate model. The mean value (μ) of random variables was 110 µm, 6,345 each, while the standard deviation (σ) was ± 0.33 μ. As a result, the uncertainty propagation of velocity, second-order turbulence statistics, and drag reduction were achieved.

Effect of curvature on the hypersonic turbulent boundary over the curved compression ramp

Direct numerical simulation of the shock wave/turbulent boundary layer interaction on a compression ramp and curved compression ramps with different radii of the curvature at the Mach number Ma=5.0 and Reynolds number Re=16 800/mm is performed, and the purpose of the study is to investigate the impact of different radii of the curvature on the development of the flow. The flow structure and turbulence properties are analyzed. As the curved angle radius increases, the range of flow deceleration and the impact of the shock wave interaction on the turbulent boundary layer gradually decrease, and the peak value of turbulent pulsation amplification in the interaction zone becomes smaller. Mean skin friction decomposition is carried in upstream undisturbed region and reattachment region. The skin friction coefficient in the upstream is primarily composed of the viscous dissipation term Cf,V and the turbulent kinetic energy production term Cf,T. While in the reattachment zone, it is mainly balanced by the term Cf,T and the spatial growth term Cf,G. Bidimensional empirical mode decomposition is applied to further study the contribution of the turbulent motion at different scales to Cf,T, and the result shows that for the compression ramp, Cf,T is mainly contributed by the large-scale vortex structure generated in the interaction zone, while for the curved compression ramp, it is mainly contributed by the rapid amplification of turbulent pulsations caused by the shock wave interaction. This study is not only a new parametric study of the shock wave/turbulent boundary interaction but also provides a reference for the aerodynamic design of hypersonic vehicles.

TO EVALUATE THE PERFORMANCE OF MOLTEN SALT COOLANTS IN PARABOLIC TROUGH COLLECTORS (PTCs)

Renewable energy sources such as solar, wind, hydro, and geothermal are potential energy sources with the capacity to mitigate environmental impacts. Parabolic trough collectors (PTCs) are the widely used application of solar energy. Molten salts are promising high-temperature, low-pressure coolants for thermal energy storage of concentrated solar power plants. The present novel work is aimed to develop a computational fluid dynamics (CFD) methodology for candidate molten salt coolants (FLiBe, Nafzirf, FliNaK, and NaNO<sub>3</sub>-NaNO<sub>2</sub>-KNO<sub>3</sub>) to compare their thermal hydraulic characteristics in terms of friction factor and Nusselt number distribution for a simple circular tube under turbulent flow conditions. This study is necessary for the thermal-hydraulic design of PTCs. The axial distributions of friction factor and Nusselt number of FliNaK, FLiBe, Nafzirf, and NaNO<sub>3</sub>-NaNO<sub>2</sub>-KNO<sub>3</sub> salts in a simple circular tube under Reynolds number range of 20,000-100,000 have been compared. The predicted friction factor f<sub>∞</sub>), Nusselt number (Nu<sub>∞</sub>), and hydrodynamic entrance length (L<sub>H</sub>) have been compared with available correlations for fully developed conditions. Blasius and Petukhov correlations adequately guessed the predicted f<sub>∞</sub> of salts, whereas the Gneilinski correlation is found to be in close agreement with the predicted Nu<sub>∞</sub> of salts, hence validating the proposed numerical scheme for investigating thermal-hydraulic characteristics of molten salts. NaNO<sub>3</sub>-NaNO<sub>2</sub>-KNO<sub>3</sub> seems to be the potential candidate with high values of thermal efficiency for solar applications.

Dynamic mode decomposition and reconstruction of the transient pump-jet propulsor wake

Comprehensively grasping the wake dynamics of pump-jet propulsor (PJP) lies at the core of developing and fine-tuning future PJP design, particularly the exciting forces suppression and noise reduction. In this work, a pre-swirl stator PJP is considered to investigate its wake dynamics and evolution mechanics. The stress-blended eddy simulation (SBES) is implemented for obtaining turbulent flow, and dynamic mode decomposition (DMD) method is utilized to analyze the wake flow evolution. The numerical results align with the experimental data within an acceptable error and are employed to establish the dataset for DMD. With introducing the modal selection “DMD with criterion (DMDc),” the featured modes of the PJP wake are discussed in detail. Those dominant modes provide a multi-level perspective to analyze flow phenomena and enable the reconstruction of the original flow field within reasonable bounds, achieving the compression of flow information. Modal analysis reveals diverse flow patterns appearing at specific frequencies including the mean flow, tip leakage flow, rotor trailing vortices, as well as multiscale duct and hub wake flow. The turbulence instability in the PJP wake is primarily determined by the modes at the rotor blade passing frequency. The error between the wake flow reconstructed from the top six modes and obtained through SBES is less than 7%. This work broadens the cognition for the evolution mechanics of PJP wake flow field, showing excellent prospects in simplifying the analysis process and the flow simulation, as well as intelligently predicting the future evolution of the flow field.

Turbulent flow pattern and flow resistance characteristics of cross flow over square-arranged tube bundles with different pitch to diameter ratios

Helical tube bundles are used in steam generators and intermediate heat exchangers of high temperature gas-cooled reactors due to their compactness and good thermal expansion adaptation performance. However, the characteristics of cross flow over inline tube bundles are sensitive to pitch to diameter ratios (S/D) and Reynolds numbers (Re), and the influence of S/D and Re variation on the flow patterns (especially wake patterns) and pressure drop coefficients (ξ) remains unclear. In the current investigation, the flow characteristics of tube bundles with S/D = 1.58 and 1.88 with various Re are investigated with the aid of wind tunnel experiments and large eddy simulations (LES). For the tube bundle with S/D = 1.88, the results of LES with periodical geometry align well with wind tunnel experiments, successfully predicting the decreasing recirculation region size as Re increases. In contrast, for the tube bundle with S/D = 1.58, the recirculation region size remains almost constant with varying Re, filling the space between two adjacent tubes even when Re = 36 000. However, the LES with periodical geometry only successfully predicts flow phenomena up to Re = 25 285. The wakes become oblique when Re = 36 000, which differs from experiments. Based on the force balance on the recirculation region, the pressure drop coefficient (ξ) of the cross flow over tube bundles is related to the size of the recirculation region. For the tube bundle with S/D = 1.88, the recirculation region size decreases as Re increases when Re ≥ 18 000, resulting in a larger flow cross section area in the main flow region. The increasing main flow cross section area leads to an increased pressure coefficient along the separation streamlines. Since the Reynolds normal and shear stresses cancel each other out, the increasing pressure coefficients along the recirculation region boundary lead to the increasing surface pressure coefficients in the rear side of the tube and ultimately lead to the ξ decrease from 0.26 to 0.24 when Re increases from 18 000 to 44 571. Conversely, when Re ≤ 10 285, the recirculation area fills the space between two adjacent tubes, so the ξ barely change with Re. For tube bundle with S/D = 1.58, the recirculation area fills the space between tubes even when Re ≥ 18 000, leading to almost the unchanged ξ (≈0.27) with Re. The higher ξ at Re = 10 285 is related to the bounding wall effects on the near wall tubes.

Spatiotemporal super-resolution forecasting of high-speed turbulent flows

Spatiotemporal super-resolution forecasting of high-speed turbulent flows

Neural refractive index field: Unlocking the potential of background-oriented schlieren tomography in volumetric flow visualization

Background-oriented schlieren tomography is a prevalent method for visualizing intricate turbulent flows, appreciated for its ease of implementation and ability to capture three-dimensional distributions of a multitude of flow parameters. However, the voxel-based meshing scheme leads to significant challenges, such as inadequate spatial resolution, substantial discretization errors, poor noise immunity, and excessive computational costs. This study presents an innovative reconstruction approach termed neural refractive index field (NeRIF), which implicitly represents the flow field using a neural network trained with specialized strategies. Numerical simulations and experimental results on turbulent Bunsen flames demonstrate that this approach can substantially improve the reconstruction accuracy and spatial resolution while concurrently reducing computational expenses. Although showcased in the context of background-oriented schlieren tomography here, the key idea embedded in the NeRIF can be readily adapted to various other tomographic modalities including tomographic absorption spectroscopy and tomographic particle imaging velocimetry, broadening its potential impact across different domains of flow visualization and analysis.

Experimental and numerical hydrodynamic study of millimetric zigzag channels in laminar and turbulent flow regimes

Experimental and numerical hydrodynamic study of millimetric zigzag channels in laminar and turbulent flow regimes

THERMOHYDRAULIC PERFORMANCE ANALYSIS OF ABSORBER TUBE WITH COILED WIRE TURBULATORS IN SMALL APERTURE PARABOLIC TROUGH COLLECTOR

Three-dimensional (3D) simulation using computational fluid dynamics (CFD) has been performed to investigate the impact of a coiled wire turbulator (CWT) on heat transfer and fluid flow in the absorber tube of a small aperture parabolic trough collector (PTC) under a turbulent flow regime. The PTC was modeled with a projected aperture area of 0.64 m<sup>2</sup> and an optimized value of rim angle 90 degrees using the ray tracing tool SolTrace based on an optimum value of average heat flux (8.58 kW/m<sup>2</sup>), and this optimum average heat flux was used as one of the boundary conditions in CFD simulation. The CFD-simulated findings for the smooth absorber tube of the PTC model are compared with the Dittus-Boelter equation for heat transfer and the Filonenko equation for friction factor. The simulated result showed an average deviation of 2.54% and 14.68% in the Nusselt number and friction factor, respectively. The effect of a wall-detached type of CWT (D<sub>m</sub> = 10 mm) on thermohydraulic performance with coil pitch ratios (P/d<sub>h</sub>) of 1, 1.5, and 2 and height ratios (e/d<sub>h</sub>) of 0.053, 0.063, and 0.074 is considered for the analysis in the Reynolds number (Re) range of 4000-12000. The optimal value of thermohydraulic performance (η<sub>thp</sub>) is obtained 1.56 for P/d<sub>h</sub> = 2, e/d<sub>h</sub> = 0.053, and Re = 4000, corresponding to which enhancement ratio of Nusselt number (Nu<sub>r</sub>/Nu<sub>s</sub>) and friction factor (f<sub>r</sub>/f<sub>s</sub>) are 2.28 and 3.14, respectively.

CFD study on the effect of the baffles geometry in sedimentation efficiency in wastewater treatments through Large Eddy Simulations

Sedimentation tanks represent one of the most important components of any water and wastewater treatment plants. The lack of knowledge of hydraulics in sedimentation tank leads to unnecessary capital and operating costs as well as water pollution in the form of excessive sludge. Improper and inadequate design cause overloading of filters, and lead to frequent backwashing, which in turn waste a significant percentage of treated water. Sedimentation tanks require a uniform flow field to ensure the correct operating conditions. Unfortunately, the development of circulation zones causes deep deviation from the uniformity, bringing negative effects on the efficiency of the tank. The focus of this study is to apply Computational Fluid Dynamics (CFD) to study and to improve the sedimentation efficiency through the optimization of the hydrodynamics inside settling tanks. In the present analysis, Large Eddy Simulation (LES) is used to simulate three-dimensional turbulent flow and scalar tracer transport in different settlers using a structured finite-volume discretization. The results show how the baffles’ geometry, as well as the inlet design, influences the retention time distribution of the tank, varying the sedimentation efficiency. In particular, the increase of baffles inside the tank reduces the free space and allows a more uniform distribution of velocity vectors. This reduces both short circuits and recirculation zones bringing the flow closer to the ideal condition. On the other hand, the inlet position influences the velocity on the tank’s bottom, with possible particles re-suspension effects.