Local Flow Field Research Articles

Mechanistic model of critical heat flux in rod bundles based on a high-precision subchannel code

A mechanistic model for predicting critical heat flux (CHF) in rod bundles is developed based on the high-precision subchannel code ATHAS. To account for the non-uniform distribution of quality in subchannels caused by the presence of mixing vane grids (MVGs) and guide tubes (GTs), further subdivision of conventional subchannels and the incorporation of MVG crossflow models are implemented in ATHAS to provide a more detailed local flow field for the CHF model. The CHF mechanism model is developed based on the concepts of bubble crowding and assumption of local phenomenon. An equivalent-tube concept is proposed to convert the turbulent intensity distribution on the tube surface to that on the rod bundle surface. The present model is implemented in ATHAS and evaluated against CHF experimental data of rod bundles under the conditions of interest for pressurized water reactors (PWRs). The predicted results show good agreement with the experimental CHF data, with an average error and root mean square of 0.99% and 4.69%, respectively, for a total of 601 data points. Furthermore, owing to the use of the subchannel subdivision method, the present model does not require any cold wall effect correction and can accurately predict the radial position of CHF.

Laser-induced indirect ignition of non-premixed turbulent shear layers

This study uses direct numerical simulations to investigate the forced ignition of temporally-evolving turbulent subsonic shear layers separating a gaseous stream of methane (CH4) and a stagnant gas environment of molecular oxygen (O2). Ignition is forced by a thermal-energy source that heats up a small volume of gas during periods of time much shorter than the characteristic acoustic time scale. The kernel, including its initial rounded conical shape, resembles experimental observations after optical breakdown is achieved in a gas irradiated by a focused laser. Particular emphasis is placed on ignition phenomena observed when the laser is focused on the O2 environment outside the turbulent shear layer, where the local composition is far beyond the lean flammability limit. This represents an indirect mode of non-premixed ignition that develops after a relatively long period of time has passed since laser-energy deposition, when the eddies near the oxidizer edge of the turbulent shear layer are intercepted by a baroclinically-generated subsonic vortex of hot dissociated O2 ejected from the laser-energy deposition zone. The success of indirect ignition depends on the orientation of the laser beam, the thickness of the shear layer, and the kernel standoff distance from the oxidizer edge of the shear layer. An ignition Damköhler number is defined that accounts for these averaged effects in six different simulation cases. For near-unity ignition Damköhler numbers, the solution is also sensitive to the local instantaneous flow field prevailing near the oxidizer edge of the shear layer, in that a modification of the instantaneous pre-deposition flow field, while preserving its turbulence statistics, can produce different ignition outcomes.Novelty and significance statement This work investigates forced ignition in a canonical flow configuration – a non-premixed turbulent CH4/O2 shear layer – and analyzes mechanisms by which ignition can be unexpectedly achieved in adverse conditions. In particular, an ensuing laser-generated advection of hot dissociated gas is observed to ignite the shear layer even when the mixture at the laser focal point is not flammable, an observation first made in early experiments. This flow phenomenon, whose consequences on ignition of non-premixed turbulent flows have not previously been analyzed by direct numerical simulation, can significantly separate the laser-energy deposition zone from the ignition location and is linked to baroclinically generated vorticity due to the rounded conical shape of the energy kernel. The phenomenology analyzed here may impact the design of laser-based ignition systems for chemical propulsion.

Experimental reconstruction of the local flow field in a Wells turbine using a three-dimensional pressure probe

Among the various solutions suggested for wave energy harvesting, the ones based on the oscillating water column (OWC) principle are considered as the most promising, due to their constructive simplicity and reliability. These systems convert sea wave energy into pneumatic energy in the form of a bi-directional airflow that can conveniently turned into mechanical energy by a Wells turbine. Since their introduction, Wells turbines have been studied extensively in order to characterize their performance. Most of the experimental studies have focused on global machine performance analyses, while the studies focusing on local performance analyses are limited. This work presents a detailed experimental investigation of a small-scale Wells turbine coupled to an OWC simulator. The turbine aerodynamic characteristic has been identified with global measurements, while a miniaturized aerodynamic probe has been used to evaluate local performance, by reconstructing the three-dimensional flow field upstream and downstream of the turbine during a complete regular wave period. Local analyses aid to explain global turbine performance, highlighting the main differences between inflow and outflow phases. Moreover, they allow to describe the variation in loading along the blade radius, and to evaluate the blade design law, which justifies the limited Wells turbine aerodynamic performance.

A unified stochastic particle method with spatiotemporal adaptation for simulating multiscale gas flows

Recently, the Unified Stochastic Particle (USP) method has emerged as a promising approach for multiscale particle simulations, tailored specifically for non-equilibrium gas flow scenarios. However, in typical simulations of multiscale flow fields, the time step and grid size of the original USP method remain constrained by the smallest spatiotemporal scale of local flow field, limiting its true multiscale simulation potential. To overcome this critical issue, this work introduces a spatiotemporal adaptive USP method. Initially, we analyze the sources of errors in the traditional single-scale stochastic particle methods under large time steps, which leads to the derivation of the governing equations of USP. Building upon this foundation, we propose principles that dictate the rational time step and grid size for an arbitrary USP simulation. Subsequently, we introduce a spatiotemporal adaptive approach primarily based on the local gradient length, leading to the development of a USP solver endowed with spatiotemporal adaptability. The universality, reliability, and efficiency of the algorithm have been verified through an examination of three cases: two-dimensional flow over a cylinder, three-dimensional flow over a blunted body, and flow over a scaled X38-like model. This work forms an integral component of the development initiative for the general USP solver—SPARTACUS—with relevant source code being open-source in an online repository under the GNU General Public License (GPL).

Numerical simulation of fiber orientation kinetics and rheology of fiber-filled polymers in uniaxial extension

During processing of fiber composites, the fiber-induced stresses influence the local flow fields, which, in turn, influence the stress distribution and the fiber orientation. Therefore, it is crucial to be able to predict the rheology of fiber-filled polymer composites. In this study, we investigate the fiber orientation kinetics and rheological properties of fiber composites in uniaxial extensional flow by comparing direct numerical finite element simulations to experimental results from our previous study [Egelmeers et al., “In-situ experimental investigation of fiber orientation kinetics during uniaxial extensional flow of polymer composites,” J. Rheol. 68, 171–185 (2023)]. In the simulations, fiber–fiber interactions only occur hydrodynamically and lubrication stresses are fully resolved by using adaptive meshing. We employed a 7-mode and a 5-mode viscoelastic Giesekus material model to describe the behavior of, respectively, a strain hardening low-density polyethylene (LDPE) matrix and a non-strain hardening linear LDPE matrix, and investigated the influence of the Weissenberg number, strain hardening, and fiber volume fraction on the fiber orientation kinetics. We found that none of these parameters influence the fiber orientation kinetics, which agrees with our experimental data. The transient uniaxial extensional viscosity of a fiber-filled polymer suspension is investigated by comparing finite element simulations to a constitutive model proposed by Hinch and Leal [“Time-dependent shear flows of a suspension of particles with weak Brownian rotations,” J. Fluid Mech. 57(4), 753–767 (1973)] and to experimental results obtained in our previous study [Egelmeers et al., “In-situ experimental investigation of fiber orientation kinetics during uniaxial extensional flow of polymer composites,” J. Rheol. 68, 171–185 (2023)]. The simulations describe the experimental data well. Moreover, high agreement is found for the transient viscosity as a function of fiber orientation between the model and the simulations. At high strains for high fiber volume fractions, however, the simulations show additional strain hardening, which we attribute to local changes in microstructure.

Toroidal flow around the Tonga slab moved the Samoan plume during the Pliocene

Abstract Age-progressive seamount tracks generated by lithospheric motion over a stationary mantle plume have long been used to reconstruct absolute plate motion (APM) models. However, the basis of these models requires the plumes to move significantly slower than the overriding lithosphere. When a plume interacts with a convergent or divergent plate boundary, it is often deflected within the strong local mantle flow fields associated with such regimes. Here, we examined the age progression and geometry of the Samoa hotspot track, focusing on lava flow samples dredged from the deep flanks of seamounts in order to best reconstruct when a given seamount was overlying the mantle plume (i.e., during the shield-building stage). The Samoan seamounts display an apparent local plate velocity of 7.8 cm/yr from 0 to 9 Ma, 11.1 cm/yr from 9 to 14 Ma, and 5.6 cm/yr from 14 to 24 Ma. Current fixed and mobile hotspot Pacific APM models cannot reproduce the geometry of the Samoa seamount track if a long-term fixed hotspot location, currently beneath the active Vailulu’u Seamount, is assumed. Rather, reconstruction of the eruptive locations of the Samoan seamounts using APM models indicates that the surface expression of the plume migrated ~2° northward in the Pliocene. Large-scale mantle flow beneath the Pacific Ocean Basin cannot explain this plume migration. Instead, the best explanation is that toroidal flow fields—generated by westward migration of the Tonga Trench and associated slab rollback—have deflected the conduit northward over the past 2–3 m.y. These observations provide novel constraints on the ways in which plume-trench interactions can alter hotspot track geometries.

Motion of microswimmers in cylindrical microchannels.

Biological and artificial microswimmers often have to propel through a variety of environments, ranging from heterogeneous suspending media to strong geometrical confinement. Under confinement, local flow fields generated by microswimmers, and steric and hydrodynamic interactions with their environment determine the locomotion. We propose a squirmer-like model to describe the motion of microswimmers in cylindrical microchannels, where propulsion is generated by a fixed surface slip velocity. The model is studied using an approximate analytical solution for cylindrical swimmer shapes, and by numerical hydrodynamics simulations for spherical and spheroidal shapes. For the numerical simulations, we employ the dissipative particle dynamics method for modelling fluid flow. Both the analytical model and simulations show that the propulsion force increases with increasing confinement. However, the swimming velocity under confinement remains lower than the swimmer speed without confinement for all investigated conditions. In simulations, different swimming modes (i.e. pusher, neutral, puller) are investigated, and found to play a significant role in the generation of propulsion force when a swimmer approaches a dead end of a capillary tube. Propulsion generation in confined systems is local, such that the generated flow field generally vanishes beyond the characteristic size of the swimmer. These results contribute to a better understanding of microswimmer force generation and propulsion under strong confinement, including the motion in porous media and in narrow channels.

Flow-induced fabrication of ZnO nanostructures in pillar-arrayed microchannels.

The emergence of microfluidic devices integrated with nanostructures enables highly efficient, flexible and controllable biosensing, among which zinc oxide (ZnO) nanostructure-based fluorescence detection has been demonstrated to be a promising methodology due to its high electrical point and unique fluorescence enhancement properties. The optimization of microfluidic synthesis of ZnO nanostructures for biosensing on chip has been in demand due to its low cost and high efficiency, but still the flow-induced growth of ZnO nanostructures is not extensively studied. Here, we report a simple and versatile strategy that could manipulate the local flow field by creating periodically arranged micropillars within a straight microchannel. We have explored the effects of perfusion speed and flow direction of seed solution, localized flow variation of growth solution and growth time on the morphology of nanostructures. This provided a comprehensive understanding which governs nanostructure fabrication controlled by flow. The results demonstrated that localized flow in microfluidic devices was essential for the initiation and growth of zinc oxide crystals, enabling precise control over their properties and morphology. Furthermore, a model protein was used to demonstrate the intrinsic fluorescence enhancement of ZnO nanostructures as an example to reveal the morphology-related enhancement properties.

Ultrasound Particle Image Velocimetry to Investigate Potential Hemodynamic Causes of Limb Thrombosis After Endovascular Aneurysm Repair With the Anaconda Device.

To identify potential hemodynamic predictors for limb thrombosis (LT) following endovascular aneurysm repair with the Anaconda endograft in a patient-specific phantom. A thin-walled flow phantom, based on a patient's aortic anatomy and treated with an Anaconda endograft, that presented with a left-sided LT was fabricated. Contrast-enhanced ultrasound particle image velocimetry was performed to quantify time-resolved velocity fields. Measurements were performed in the same phantom with and without the Anaconda endograft, to investigate the impact of the endograft on the local flow fields. Hemodynamic parameters, namely vector complexity (VC) and residence time (RT), were calculated for both iliac arteries. In both limbs, the vector fields were mostly unidirectional during the peak systolic and end-systolic velocity phases before and after endograft placement. Local vortical structures and complex flow fields were observed at the diastolic and transitional flow phases. The average VC was higher (0.11) in the phantom with endograft, compared to the phantom without endograft (0.05). Notably, in both left and right iliac arteries, the anterior wall regions corresponded to a 2- and 4-fold increase in VC in the phantom with endograft, respectively. RT simulations showed values of 1.3 to 6 seconds in the phantom without endograft. A higher RT (up to 25 seconds) was observed in the phantom with endograft, in which the left iliac artery, with LT in follow-up, showed 2 fluid stasis regions. This in vitro study shows that unfavorable hemodynamics were present mostly in the limb that thrombosed during follow-up, with the highest VC and longest RT. These parameters might be valuable in predicting the occurrence of LT in the future. This in-vitro study aimed to identify potential hemodynamic predictors for limb thrombosis following EVAR using ultrasound particle image velocimetry (echoPIV) technique. It was shown that unfavorable hemodynamic norms were present mostly in the thrombosed limb. Owing to the in-vivo feasibility of the echoPIV, future efforts should focus on the evaluation of these hemodynamic norms in clinical trials. Thereafter, using echoPIV as a bedside technique in hospitals becomes more promising. Performing echoPIV in pre-op phase may provide valuable insights for surgeons to enhance treatment planning. EchoPIV is also applicable for follow-up sessions to evaluate treatment progress and avoid/predict complications.

Adaptive control of a VTOL uncrewed aerial vehicle for high-performance aerobatic flight

We present two adaptive control methods conceived to enable a vertical take-off and landing (VTOL) uncrewed aerial vehicle (UAV) to perform a class of aerobatic maneuvers in the presence of aerodynamic-coefficient and torque-latency variations induced by changes of the local flow fields during high-speed aerobatic flight. First, we introduce a linear time-varying (LTV) dynamical model, which is assumed to have unknown time-dependent parameters, to describe the aerodynamic effects acting on the actuation dynamics of the system. Then, we present the design of a Lyapunov-based adaptive control scheme aimed to compensate for undesired behavior of the LTV actuator dynamics, according to which we derive the control and adaptation laws from a single Lyapunov candidate function. Next, we propose a modular adaptive control scheme to address the same problem, but in which the adaptation law is specified separately from that of the controller. We use modern nonlinear theory to deduce and analyze the conditions that guarantee the global asymptotic stability of both adaptive control strategies. In order to exemplify the controller synthesis procedures, we implemented both adaptive control methods on a quadrotor UAV to perform three different types of aerobatic-flight maneuvers—namely, triple flips, Pugachev’s cobras, and mixed flips. The obtained experimental results provide compelling evidence of the effectiveness of the two proposed methods to compensate for the undesired effects induced by aerodynamic-coefficient and torque-latency variations. Furthermore, the experimental data demonstrate that both adaptive schemes significantly improve the flight performance of the quadrotor UAV during the execution of aerobatic maneuvers, compared to those achieved with a controller that disregards the LTV actuator dynamics induced by high-speed aerodynamic effects. The suitability of the time-varying approach used to model the influence of high-speed local flow fields on the dynamics of the controlled UAV was indirectly validated through data obtained during the aerobatic-flight experiments presented here.

Local flow assisted platelet template alignment and corresponding alumina texturing in ceramic stereolithography process

Using local slurry laminar flow to align alumina platelet in the slurry and fabricate textured alumina by ceramic stereolithography has been shown with potential. However, the alignment process of alumina platelet and the corresponding texturing of alumina based on this method have not yet been revealed. This study systematically analyzed the mechanism of aligning alumina platelet in the local flow and the evolution of the microstructure of textured grain. The results showed that with the increase of the alumina platelet content in the slurry, the thickness of the grains gradually decreased. After sintering at 1600 °C for 2 h, textured alumina had mainly the crystal plane of (1010). The textured alumina with 15 vol% alumina platelet addition had the highest Lotgering factor (65 %) of (1010). The alumina platelet located in the middle and bottom of the local flow field aligned slower during the flow process. The distribution of flow velocity gradient was the main factor determining the alignment of alumina platelet.

Numerical study on jet-wake flow and its evolution in a centrifugal pump with alternating stall

Stall and jet-wake flow are two typical forms of unstable flow phenomena in centrifugal pumps, significantly affecting their stability. This paper investigates the interaction mechanisms between stall and jet-wake flow in a centrifugal impeller under different working conditions. The unsteady numerical study utilizes the partially averaged Navier–Stokes (PANS) model with a new dynamic fk expression derived from the rotation-corrected energy spectrum. The results reveal four stages in the flow field evolution of the centrifugal pump under different working conditions. In stage I, no stall vortices are present, and the jet-wake flow occurs. The velocity distribution at the impeller outlet depends on the pressure difference distribution between the pressure and suction sides within the flow passage. As the flow rate decreases, the pressure difference between the two sides increases, intensifying the jet-wake phenomenon. Under part-load conditions (stages II, III, and IV), the presence of stall vortices becomes the main factor affecting the jet-wake flow. These stall vortices influence the local and downstream flow fields, thus altering the distribution of the jet-wake. When the stall vortex is on the pressure side, it reduces the velocity near the pressure side, weakening the intensity of the jet-wake. On the other hand, when the stall vortex is on the suction side, it reduces the velocity near the suction side, enhancing the intensity of the jet-wake.

Effect of pulsation parameters on the spatial and temporal variation of flow and heat transfer characteristics in liquid metal cross flow the in-line tube bundle

Liquid metal has a broad application prospect in tube bundle heat exchangers because of its excellent thermal conductivity. However, conventional in-line tube bundle arrangements often exhibit limited secondary flow, so new methods are urgently needed to improve heat transfer. The topic of this paper is to combine two methods, liquid metal cross flow tube bundle and pulsating flow, to improve the heat transfer performance in-line tube bundles. Firstly, the applicability of the model was validated through experimental data from existing literature, encompassing overall flow dynamics, heat transfer performance, and circumferential pressure distribution around the tubes. Subsequently, numerical simulations are conducted involving liquid metal cross flow in-line tube bundles at varying pulsating frequencies, amplitudes, and Reynolds numbers. Circumferential pressure distributions and temperature profiles for individual tubes were analyzed and compared. The amplitude of reduction in thermal resistance compared to the no-oscillation condition increases gradually from a minimum of 8 % in the first row to a maximum of 40 % in the third row, while the reduction in the rear row of tubes is similar to that of the third row. Remarkably, a reversal in the trend of thermal resistance for the first three rows of tubes under pulsating flow were observed, contrasting conventional flow except for the case with an amplitude of 0.1. By analyzing the local transient flow field distribution near single tube and the corresponding circumferential heat transfer performance, the influence of the vortex evolution characteristics on the heat transfer performance at different phases of the pulsating velocity is revealed, which transient Nu is up to three times that of flow with no pulsating. Finally, a series of coherence analyses reveal that as frequency rises, turbulence exhibits a richer spectrum of small-scale vortex structures, intricately linked to enhanced energy cascade efficiency. Amplitude augmentation intensifies velocity fluctuations, particularly at high amplitudes, resulting in substantial energy peak amplification. The integrated heat transfer performance PEC factor varies in the range of 1.25 to 1.51 for all the conditions simulated in this paper, which indicates that the liquid metal coupled pulsating flow approach can significantly increase the comprehensive performance of the tube bundle heat exchangers. These findings hold significant promise for advancing heat exchanger design and enhancing thermal efficiency, with implications for various engineering applications.

Directional and Reconfigurable Assembly of Active Colloidal Motors Triggered by Chemical Communications

AbstractAnisotropic colloidal particles can directionally self‐assemble, but the resulting assemblies cannot reconfigure unless the surface properties of colloidal particles change. Although external fields with adjustable inputs can guide the reconstruction of active structures after stimulating the dynamic assembly of active colloidal motors, the interaction between colloidal motors is unclear. Here, the study reports a bioinspired communication and directed assembly of peanut‐shaped colloidal motors with the emergence of ordered ribbon structures triggered by self‐generated chemical fields. The colloidal motors photocatalytically decompose the fuel to create the chemical concentration gradient fields and exert long‐range attractions on the colloidal scale, which enable colloidal motors to communicate by sensing each other, synchronizing orientation, and assembling side‐by‐side after the collision between their local flow fields. The inter‐individual communication occurs at a critical distance of ≈4.4 µm, resulting from the interplay between hydrodynamic effects, long‐ranged phoretic attraction, and short‐ranged magnetic interaction. The linear ribbons are still active owing to the symmetry breaking of the local fluid field and can be switched between rigid and flexible states according to light conditions to simulate the polymer conformation. The complexity of colloidal motors can be combined with external manipulations to manufacture active soft materials with precise and adaptive functions.

Intelligent perception of tandem flapping wings' flow field environment with sparse detectors

Bionic flapping wing vehicles have great potential for civil and defense applications due to their flexibility and concealment at low Reynolds numbers. Since traditional flow field pattern recognition methods are difficult to identify effective information from the measured local flow field and deduce the state information of the moving body, this study uses an artificial intelligence method to establish the internal correlation between flow field pattern and state information. Specifically, a fully connected neural network is adopted to recognize the tandem flapping wings' flow field pattern by using different data acquisition methods and detector array distribution methods. Compared with the neural network based on time series data, the neural network based on spatial distribution data can realize the real-time judgment of flow field environment, which is closer to the real-time requirements in practical applications. In the paper, the intelligent perception of multi-flapping wings' flow field environment with sparse detectors is carried out and lays the theoretical foundation for autonomous navigation and obstacle avoidance of flapping wing aircrafts.

A simple model of wake capture aerodynamics.

Flapping wings may encounter or 'capture' the wake from previous half-stroke, leading to local changes in the instantaneous aerodynamic force on the wing at the start of each half-stroke. In this paper, I developed a simple approach to integrating prediction of these wake capture effects into existing analytical quasi-steady models for hovering insect flapping flight. The local wake flow field is modelled as an additional induced velocity component normal to the stroke plane of the flapping motion that is blended/switched in at the start of each half-stroke. Comparison of model results against experimental data in the literature shows satisfactory agreement in predicting the wake capture lift and drag variations for eight different test cases. Sensitivity analysis shows that the form of the translation velocity time history has a significant effect on the magnitude of wake capture forces. Profiles that retain high translational velocity right up to stroke reversal evoke a much larger effect from wake capture compared with sinusoidal. This result is significant because while constant flapping translation velocity profiles can be generated in the laboratory, the very high accelerations required near stroke reversals incur high mechanical cost that prevents practical adoption in nature or engineered flapping flight vehicles.

Pyrolysis of Spherical Wood Particles in a Packed Bed – Comparison between Resolved and Unresolved Discrete Element Method/Computational Fluid Dynamics

AbstractThe combined discrete element method/computational fluid dynamics (DEM/CFD) approach allows for the description of reacting granular assemblies passed by a gas flow. Especially for thermally thick particles, the spatial resolution of the solid object volumes, their surfaces, and of the interstitial flow domain controls the quality of results while at the same time driving computational costs. To evaluate the differences between resolved and unresolved DEM/CFD approaches, pyrolysis of a bulk of spherical wood particles enclosed by a cylindrical heating surface and passed by hot nitrogen is numerically examined. The unresolved simulation is based on the averaged volume method (AVM). For the resolved simulation, the so‐called blocked‐off (BO) method is applied. The results show that the total mass conversion rate of solid particles into gaseous volatiles is faster when employing the resolved BO approach. The better spatial resolution of local flow field and particle surface representation leads to a more detailed prediction of convective and radiative heat transfer to the particles, but is associated with the penalty of a six‐fold computing time.

Is the Actuator line method able to reproduce the interaction between closely-spaced Darrieus rotors? a critical assessment on wind and hydrokinetic turbines

The deployment of multiple closely-spaced Darrieus turbines has recently gained interest in the academic and industrial sectors due to their potential to increase power output and wake recovery. However, the computational cost of accurate three-dimensional, blade-resolved CFD analyses rapidly becomes unfeasible as long as multiple rotors are added, putting emphasis on the need for a more practical, yet accurate, simulation method. Based on recent assessments, the Actuator Line Method (ALM) has been shown to represent the most interesting solution for Darrieus turbines’ simulations. However, since the blade-flow interaction is not solved, no evidence was available to date on whether ALM can capture all the physical phenomena related to turbine mutual interaction.In this study, the reliability and limitations of the ALM in simulating closely-spaced Darrieus turbines were assessed using two pairs of turbines with varying geometrical and operational characteristics, focusing on turbine loads, local flow field, and wake development. The results demonstrate that the ALM is effective for simulating twin-rotor Darrieus turbines, particularly at medium and high tip-speed ratios, providing satisfactory predictions of instantaneous blade loads and capturing the increase in torque due to the mutual interaction between adjacent rotors. The ALM is also able to accurately reproduce the flow field across the twin rotors, as demonstrated through Proper Orthogonal Decomposition (POD) analysis of the wake flow field. With an average simulation time of 7 h/rev/core, compared to 508 h/rev/core for blade-resolved CFD, this study provides evidence that the ALM is a low-cost and reliable tool for simulating closely spaced Darrieus turbines, thus representing the most suitable tool to develop new VAWT applications like floating wind turbines or hydrokinetic installations.

Probing pipe flow impact corrosion monitoring effectiveness under corrosion monitor coupons conditions

To reduce the corrosion failure frequency of oil and gas pipelines, corrosion monitoring coupons are often used to evaluate the internal corrosion status of pipelines. However, the local flow field varies after the installation of the coupons, so the monitoring corrosion rate is different from that of the pipe wall. To evaluate the effectiveness of corrosion coupons monitoring, a lab-scale flow loop system was used to carry out flow corrosion experiments to obtain the local corrosion rates of corrosion monitoring coupons and pipe wall, respectively. Combined with the characterization of corrosion products and multiphase flow, a mechanism model of corrosion monitoring effectiveness was established. It was clear that the deviation of corrosion monitoring rate was related to the flow rate, shear force and turbulent kinetic energy. The accumulation of corrosion media and the detachment of corrosion product films resulted in variation in the morphology and the monitoring effectiveness.

Experimental Analysis of the Three Dimensional Flow in a Wells Turbine Rotor

An experimental investigation of the local flow field in a Wells turbine has been conducted, in order to produce a detailed analysis of the aerodynamic characteristics of the rotor and support the search for optimized solutions. The measurements were conducted with a hot-wire anemometer (HWA) probe, reconstructing the local three-dimensional flow field both upstream and downstream of a small-scale Wells turbine. The multi-rotation technique has been applied to measure the three velocity components of the flow field for a fixed operating condition. The results of the investigation show the local flow structures along a blade pitch, highlighting the location and radial extension of the vortices which interact with the clean flow, thus degrading the turbine’s overall performance. Some peculiarities of this turbine have also been shown, and need to be considered in order to propose modified solutions to improve its performance.