Flow Regime Research Articles

Computational and experimental exploration of a morphable blade concept for wind turbines

Regarding the further development of wind energy solutions, this study explores the idea of morphing wind turbine blades as a means to improve their aero-structural merits. To this end, the aerodynamics of a representative, morphable wind turbine blade section is assessed using numerical simulation, this being done for a significant range of flow conditions (speed, incidence), with various levels of morphing applied. Upon this, diverse morphing scenarios are inferred, either to alleviate the aero-structural loads exerted on the blade, or to rather maximize its generated aerodynamic power. It is shown that these morphing strategies translate into significant benefits, among which substantial gains of power delivered across all flow regimes. The phenomenology behind the benefits brought by the morphing is then explored, which confirms that morphing a blade is superior to simply pitching or twisting it – as classically done or envisioned. Finally, the outcomes obtained from the computational investigation are replicated through a dedicated, small-scale experiment, thereby further confirming the aerodynamic merits brought by the morphing. All this advocates for a further exploration of morphable wind turbine blades.

Erratum for “Identifying Functional Flow Regimes and Fish Response for Multiple Reservoir Operating Solutions”

Erratum for “Identifying Functional Flow Regimes and Fish Response for Multiple Reservoir Operating Solutions”

A new accurate solution method for steady forced convection in an unbounded flow around a circular cylinder in continuum and slip regimes

A new accurate numerical solution method for steady forced convection in an unbounded bidimensional flow around a circular cylinder is presented, addressing both the no-slip and slip regimes for Reynolds numbers and Knudsen numbers within the range 0.01 ≤ Re ≤ 47 and 0 ≤ Kn ≤ 0.1. The challenge of singular behaviour in infinite domains, which degrades numerical precision, is effectively mitigated by our new approach, unlike previous methods that truncate the flow domain. This method provides complete and correct description of the flow near the cylinder and in the far field perfectly reproducing the asymptotic solutions at large distances. We also present a new effective analytical solution for Oseen flow with slip effects, accurately representing flow at low Reynolds numbers. The validity of our numerical solution is confirmed by extensive comparisons with analytical solutions and numerical and experimental data from the literature. Additionally, we derive a new accurate correlation formula for the Nusselt number in slip flow regimes, involving both scenarios with and without temperature jump. This formula is crucial for engineering applications, especially in constant temperature hot-wire anemometry, as it avoids the requirement to correct heat transfer measurements due to slip and temperature jump effects when using very small diameter wires.

Application of oscillating elastic surfaces for heat transfer enhancement from heat dissipating electronic chips

The present study investigates the possibility of heat transfer enhancement from discrete heat sources by an oscillating elastic surface. The three liquid cooled heat sources are located on the bottom surface of a rectangular channel and are influenced by oscillation of the upper surface. Finite element simulations are carried out at several Reynolds numbers in the laminar flow regime for two different cases of oscillation by cosine and step loads with various periods and amplitudes. It is observed that oscillation of the channel surface alters the direction of the pressure gradient and causes backflow and several vortices along the channel. It also leads to the thermal boundary layer disturbance and flow acceleration on the heat sources and consequently intensifies the heat transfer. Results indicate that reducing the period of the oscillations improves the flow mixing and thus increases the heat transfer rate to an optimum value. Further decrease in the period has a reverse effect on the heat transfer enhancement. Maximum heat transfer enhancement of 223.4 %, 162.2 %, 137.8 %, and 150.5 % are obtained for Reynolds numbers of 200, 600, 1000, and 1400, respectively. It is worth mentioning that the heat transfer enhancement is achieved at the expense of a considerable pressure drop increase. The overall performance factor implies that the heat transfer enhancement dominates the pressure drop increase only at low amplitudes and periods of oscillations. Comparison between the cosine and step loads reveals that the cosine and step forces exhibit better heat transfer performance at low and high periods, respectively.

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

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

Numerical Investigation of Unsteady Flow Characteristics of Planar Plug Nozzle in Over-Expanded Conditions

Abstract Plug nozzles are frequently regarded as a strong contender for futuristic single-stage-to-orbit space missions due to their inherent altitude-compensating properties. Optimizing the design of these nozzles to achieve peak performance during the initial ascent phase is of preeminent importance. The current research aims to deepen our comprehension of the unsteady behavior of plug nozzles during this ascent phase, commonly referred to as the overexpanded flow regime. It utilizes a full-spike plug nozzle and employs a finite volume-based solver within the OpenFOAM framework to investigate flow features across Nozzle Pressure Ratios (NPRs) ranging from 3 to 8. Additionally, the Ffowcs Williams Hawkings (FW-H) analogy is employed to assess the influence of acoustics in the nozzle's flow field at different NPRs.Spectral analysis and flow visualizations are employed to study the frequency content and the respective flow features of the system. It is observed that the flow exhibits separation beyond NPR 4 and is highly unsteady at NPR 6, which is inferred from the prominence of the recirculation bubble. However, beyond NPR 6, the flow tends to reattach the plug surface, dampening the disturbance.

Dated radar-stratigraphy between Dome A and South Pole, East Antarctica: old ice potential and ice sheet history

Abstract An array of information about the Antarctic ice sheet can be extracted from ice-sheet internal architecture imaged by airborne ice-penetrating radar surveys. We identify, trace and date three key internal reflection horizons (IRHs) across multiple radar surveys from South Pole to Dome A, East Antarctica. Ages of ~38 ± 2.2, ~90 ± 3.6 and ~162 ± 6.7 ka are assigned to the three IRHs, with verification of the upper IRH age from the South Pole ice core. The resultant englacial stratigraphy is used to identify the locations of the oldest ice, specifically in the upper Byrd Glacier catchment and the Gamburtsev Subglacial Mountains. The distinct glaciological conditions of the Gamburtsev Mountains, including slower ice flow, low geothermal heat flux and frozen base, make it the more likely to host the oldest ice. We also observe a distinct drawdown of IRH geometry around South Pole, indicative of melting from enhanced geothermal heat flux or the removal of deeper, older ice under a previous faster ice flow regime. Our traced IRHs underpin the wider objective to develop a continental-scale database of IRHs which will constrain and validate future ice-sheet modelling and the history of the Antarctic ice sheet.

Cross-border data flow in China: Shifting from restriction to relaxation?

This article examines China's latest development in the governance of cross-border data flow. Under the general framework of Cyber Security Law, Data Security Law, and Personal Data Protection Law, China established its own regime of cross-border data flow. In recent years, contrary to the general international perception that China imposes strict restrictions especially due to national security concerns, China has been de facto relaxing its regulations on cross-border data flow, especially for digital trade. This article suggests three underlying incentives. First, China is in an increasing need to gain economic growth through international trade and investment. Second, China intends to compete in technology development and take the lead in shaping international rules on data governance. Third, China is seeking to adhere to international standards, particularly those prescribed in international free trade agreements. This article further submits that this paradigm shift would have international implications. First, China's practices need to be examined under the domestic regulatory frameworks of international free trade agreements. Second, China's current legislative and judicial practices are multifaceted, taking into account various factors, including international business, national security, and data protection, which may contribute to the further development of international cross-border data flow rules.

Thermal and flow dynamics of an inclined air heat exchanger equipped with spring turbulators in the transition flow regime

AbstractThe research involves an experimental investigation into the performance of a flow assisting air heat exchanger under varying angular orientation and uniform external heat fluxes without and with spring turbulators. The investigation was performed for Reynolds numbers ranging from 511 to 9676 and inclination angle 15° and 30°. Three heat fluxes (2, 3, and 4 kW/m2) were applied to the test section to investigate the effect of external surface heating on the range of transition flow regime and thermohydraulic performance. Transition from laminar to turbulent flow for plain channel at different heat fluxes and inclinations occurs within specific Reynolds number ranges: 2436–4446 for 15° inclination at 4 kW/m2, 2574–4289 at 3 kW/m2, and 2850–4152 at 2 kW/m2; for 30° inclination, the ranges are 2518–4151, 2712–4361, and 2992–4346 at the respective heat fluxes. When it comes to the effect of inclination on Nusselt number, the transition occurs sooner at lower angles, but is delayed as the angle increases. Additionally, the Nusselt number decreases as the angle of inclination increases. When comparing the Nusselt numbers of plain tubes to those with spring turbulators, the latter shows a significantly greater enhancement. In laminar flow, a maximum 100% deviation exists between highest and lowest friction factors, decreasing to 75% with increasing Reynolds number; all insert configurations exhibit highest friction factor at 15° due to stronger buoyancy forces.

Airflow characteristics of the stepped dropshaft in the deep tunnel storage system

ABSTRACT The deep tunnel storage system, mainly consisting of underground tunnels and dropshafts, is effective for dealing with urban waterlogging. A stepped dropshaft is suitable for the system to safely transport water to underground tunnels and efficiently release air carried by water. In this study, the air inflow discharge, air vent discharge, and air discharge into the underground tunnel are investigated experimentally. As the dimensionless water flow discharge increases to 0.56, the water flow successively forms the nappe flow, transition flow, and skimming flow regimes, the relative air inflow discharge decreases from 0.97 to 0.32, the relative air vent discharge decreases from 0.85 to 0.22, and the relative air discharge into the underground tunnel fluctuates below 0.20. The exhaust capacity (the ratio of the air vent discharge to the air inflow discharge) reaches a maximum of 0.87 at the threshold between the nappe flow and the transition flow. Influences of flow discharge, step height, step rotation angle, outlet diameter of the central exhaust pipe, and underground tunnel blockage on airflow characteristics are revealed. The prediction formulas for air inflow discharge, air vent discharge, and exhaust capacity are obtained with accuracies over 80%, providing a guide to the practical design and operation of stepped dropshafts.

Comprehensive assessment of cascading dams-induced hydrological alterations in the lancang-mekong river using machine learning technique

The development of cascading hydropower dams in river basins has significantly altered natural flow regimes in recent decades. This study investigates hydrological alterations caused by cascading hydropower dams in the Lancang-Mekong River Basin (LMRB) by integrating the Indicators of Hydrologic Alteration (IHA) method with non-regulated flow predicted using the Random Forest (RF) machine learning (ML) technique. The analysis focuses on four hydrological stations: Chiang Saen, Mukdahan, Pakse, and Stung Treng across pre-impact (1961–1991), transition (1992–2008), and post-impact (2009–2021) periods. RF models predict streamflow altered primarily by human activities, particularly hydropower development using spatially averaged daily precipitation, cumulative precipitation, and temperature data. This approach isolates human-induced impacts, unlike previous studies that combined climate change and human activities. Results indicate that post-impact alterations were most pronounced at Chiang Saen (77.3%) and decreased downstream. Alterations increased from the transition to post-impact periods by 31.5%, 22.2%, 26.5%, and 17.6% at Chiang Saen, Mukdahan, Pakse, and Stung Treng, respectively. The median monthly flow, annual extreme conditions, and rate and frequency of flow change groups in the IHA were highly altered compared to natural conditions in the post-impact period. Human activities contributed over 50% of streamflow changes (2017–2021). This approach provides a more precise assessment of dam-induced hydrological alterations, aiding hydropower management by isolating human impacts and offering high-resolution predictions.

Computational Fluid Dynamics Modeling of Pressure-Retarded Osmosis: Towards a Virtual Lab for Osmotic-Driven Process Simulations

Pressure-Retarded Osmosis (PRO) is an osmotically driven membrane-based process that has recently garnered significant attention from researchers due to its potential for clean energy harvesting from salinity gradients. The complex interactions between mixed-mode channel flows and osmotic fluxes in real PRO membrane modules necessitate high-fidelity modeling approaches. In this work, an efficient CFD framework is developed for the 3D simulation of osmotically driven membrane processes. This approach is based on a two-way coupling between a CFD solver, which captures external concentration polarization (ECP) effects, and an analytical representation of internal concentration polarization (ICP). Consequently, the osmotic water flux and reverse salt flux (RSF) can be accurately determined, accounting for all CP effects without any limitations on the geometrical complexity of the membrane chamber or its flow mode/regime. The proposed model is validated against experimental data, showing good agreement across various PRO case studies. Additionally, the model’s flexibility to simulate other types of osmotically driven processes such as forward osmosis (FO) is examined. Thus, the contributions of ECP and ICP effects in local osmotic pressure drop along the membrane chamber are comprehensively compared for FO and PRO modes. Finally, the capability of the CFD model to simulate a lab-scale PRO module is demonstrated across a range of Reynolds numbers from low-speed laminar up to turbulent flow regimes.

Generalization Limits of Data-Driven Turbulence Models

AbstractMany industrial applications require turbulent closure models that yield accurate predictions across a wide spectrum of flow regimes. In this study, we investigate how data-driven augmentations of popular eddy viscosity models affect their generalization properties. We perform a systematic generalization study with a particular closure model that was trained for a single flow regime. We systematically increase the complexity of the test cases up to an industrial application governed by a multitude of flow patterns and thereby demonstrate that tailoring a model to a specific flow phenomenon decreases its generalization capability. In fact, the accuracy gain in regions that the model was explicitly calibrated for is smaller than the loss elsewhere. We furthermore show that extrapolation or, generally, a lack of training samples with a similar feature vector is not the main reason for generalization errors. There is actually only a weak correlation. Accordingly, generalization errors are probably due to a data-mismatch, i.e., a systematic difference in the mappings from the model inputs to the required responses. More diverse training sets unlikely provide a remedy due to the strict stability requirements emerging from the ill-conditioned RANS equations. The universality of data-driven eddy viscosity models with variable coefficients is, therefore, inherently limited.

Toroidal torques due to n=1 magnetic perturbations in ITER baseline scenario

Abstract Toroidal torques, generated by the resonant magnetic perturbation (RMP) and acting on the plasma column, are numerically systematically investigated for an ITER baseline scenario. The neoclassical toroidal viscosity (NTV), in particular the resonant portion, is found to provide the dominant contribution to the total toroidal torque under the slow plasma flow regime in ITER. While the electromagnetic torque always opposes the plasma flow, the toroidal torque associated with the Reynolds stress enhances the plasma flow independent of the flow direction. A peculiar double-peak structure for the net NTV torque is robustly computed for ITER, as the toroidal rotation frequency is scanned near the zero value. This structure is found to be ultimately due to a non-monotonic behavior of the wave-particle resonance integral (over the particle pitch angle) in the superbanana plateau NTV regime in ITER. These findings are qualitatively insensitive to variations of a range of factors including the wall resistivity, the plasma pedestal flow and the assumed frequency of the rotating RMP field.

Unveiling the fundamentals of flow boiling heat transfer enhancement on structured surfaces.

Micro- and nanostructured surfaces offer the potential to enhance two-phase heat transfer. However, the mechanisms behind these enhancements are not well-understood due to insufficient diagnostic methods, leading to reliance on trial-and-error surface development. We introduce in situ boroscopy to investigate microscale bubble dynamics during flow boiling nucleation and subsequent flow regime development. This method was applied in saturated flow boiling experiments within chemically etched aluminum and copper tubes. Although the surfaces have self-similar surface structures, our findings revealed varied heat transfer coefficient enhancements, with increases of up to 391% on aluminum and 41% on copper. Using boroscopy, we identified key mechanisms of heat transfer enhancement. We further used mercury porosimetry to determine the impact of pore size distribution on thermal performance. The boroscopy technique introduced here not only elucidates the underlying processes of flow boiling heat transfer enhancement but also has potential applications for the study of other two-phase phenomena.

IMPROVING METHODS FOR PREDICTING THE STRUCTURE OF GAS-LIQUID FLOW IN HORIZONTAL PIPES

Analysis of existing research on the hydrodynamics of two-phase mixtures allows us to trace the main trends in the development of engineering calculation methods, as well as identify the most promising approaches for developing algorithms for predicting the patterns of gas-liquid flows in horizontal pipes. Predicting the patterns of two-phase flows is necessary to solve a number of applied problems related to the design of field pipeline systems, monitoring of operational parameters during the transportation of well products, calculation of hydraulic pressure drop in pipes, etc. Conventionally, in creating methods for predicting flow regimes in horizontal pipes, there are stages based on empirical approaches, mechanistic modeling and unification of hydrodynamic criteria for assessing the processes of patterns transformations of two-phase flows. Approaches based on the unification of hydrodynamic models of gas-liquid flows are noted as promising directions in the development of hydrodynamic forecasting criteria. The principles of unification consist in bringing to uniformity a mathematical apparatus capable of predicting all possible patterns of gas-liquid flow in vertical, inclined and horizontal pipes based on known volumetric flow rates of liquid and gas. The article proposes the improvement of known unification approaches in the development of criteria for predicting the annular and dispersed-bubble patterns of gas-liquid flow in field pipes. The verification of the obtained algorithms for pattern prediction of annular and dispersed-bubble modes of horizontal gas-liquid flow was carried out.

Heat and mass transfer analysis in flow of Walter's B nanofluid: A numerical study of dual solutions

AbstractThis study investigates the behavior of Walter's liquid B fluid over a biaxially stretching/shrinking surface for Homann stagnation point flow, focusing on the rheological properties that are crucial for understanding physical phenomena in engineering and industrial processes. The problem is modeled using Buongiorno's model in the presence of a permeable surface subject to heat generation/absorption, Ohmic heating, and chemical reactions influenced by Arrhenius activation energy. The governing partial differential equations are solved numerically using an appropriate similarity transformation, and the results are graphically analyzed via MATLAB's bvp5c solver. The study highlights the importance of characterizing the intricate properties of viscoelastic fluids, with potential applications in biomedical engineering and materials science. The findings reveal the coexistence of two distinct flow regimes and dual solutions are obtained. It is noted that the activation energy parameter is shown to enhance mass distribution in both solutions, while chemical reactions accelerate reactant conversion but reduce mass distribution. Additionally, an increase in viscoelasticity shifts the fluid's behavior towards elasticity, creating greater resistance to shear deformation and reducing both velocity profiles.

Numerical Assessment of the Thermal Performance of Microchannels with Slip and Viscous Dissipation Effects

Microchannels are widely used across various industries, including pharmaceuticals and biochemistry, automotive and aerospace, energy production, and many others, although they were originally developed for the computing and electronics sectors. The performance of microchannels is strongly affected by factors such as rarefaction and viscous dissipation. In the present paper, a numerical analysis of the performance of microchannels featuring rectangular, trapezoidal and double-trapezoidal cross-sections in the slip flow regime is presented. The fully developed laminar forced convection of a Newtonian fluid with constant properties is considered. The non-dimensional forms of governing equations are solved by setting slip velocity and uniform heat flux as boundary conditions. Model accuracy was established using the available scientific literature. The numerical results indicated that viscous dissipation effects led to a decrease in the average Nusselt number across all the microchannels examined in this study. The degree of reduction is influenced by the cross-section, aspect ratio and Knudsen number. The highest reductions in the average Nusselt number values were observed under continuum flow conditions for all the microchannels investigated.

Hybrid Nanofluid Flow in Microchannel With Fins: Thermal-Hydraulic, Entropy Generation and Energy Management Analysis

This study aims to numerically analyze the thermal-hydraulic performances and total entropy generation of mono-nanofluid and hybrid nanofluid in microchannel heat sinks (MCHS) with fins for laminar flow regime. Besides, the performance evaluation plots (PEPs) are plotted to explore the possibilities of energy saving for mono-nanofluid and hybrid nanofluid flow in MCHS with different fin configurations. The two-phase model is adopted to model the flow of nanofluid in the MCHS. The fin height, fin number, and nanoparticles loading are varied to investigate their effect on the heat transfer, flow characteristic, and entropy generation in the MCHS. It is discovered that 1.0% Al2O3–Cu hybrid nanofluid gives the least total entropy generation if compared to water and Al2O3 mono-nanofluid owing to the significant heat transfer enhancement it offered. The MCHS with 5 fins and a fin height of 0.8 mm configuration is found to be the optimal design for heat removal in this study. Moreover, the plotted PEPs show that the use of Al2O3–Cu hybrid nanofluid and Al2O3 nanofluid (with the concentration range of 0.1%–1.0%) in the MCHS can save energy at either constant pressure drop or flow rate.

Capabilities and limitations of smoothed particle hydrodynamics for the simulation of two‐phase flow instabilities

AbstractSmoothed particle hydrodynamics (SPH) is a mesh‐free, Lagrangian particle‐based method that is able to simulate multiphase flows in an economical manner. However, its ability to capture the flow regimes and regime transitions in two phase (liquid‐gas) internal flows, such as pipe or channel flows is not yet generally established. To address this lack in understanding, we first examine a laminar rising bubble case in order to evaluate the fluid‐fluid interface representation and transient interface evolution by the solver. With a focus towards the transition mechanism from a stratified flow regime to a slug flow regime, we investigate the Kelvin–Helmholtz instability (KHI) both qualitatively and quantitatively, initially focusing on a low density ratio () and then extending it to a high density ratio (). For the low density ratio, we conduct an analysis of the temporal evolution and demonstrate that the SPH solver captures the initial exponential growth in qualitative agreement with inviscid linear stability theory (LST) and reference numerical data for shear‐dominated flow with Richardson number . By conducting eight additional simulations for various for the high density ratio, we demonstrate that the numerically obtained parameter value for instability is around , which is in reasonable agreement with the theoretically expected value of . Based on the SPH results obtained for the range , we suggest a simple parameterization of the reduction of the effective growth rate proportional to .