Time-averaged Flow Research Articles

Velocities of transmission eigenchannels and diffusion

The diffusion model is used to calculate both the time-averaged flow of particles in stochastic media and the propagation of waves averaged over ensembles of disordered static configurations. For classical waves exciting static disordered samples, such as a layer of paint or a tissue sample, the flux transmitted through the sample may be dramatically enhanced or suppressed relative to predictions of diffusion theory when the sample is excited by a waveform corresponding to a transmission eigenchannel. Even so, it is widely assumed that the velocity of waves is irretrievably randomized in scattering media. Here we demonstrate in microwave measurements and numerical simulations that the statistics of velocity of different transmission eigenchannels are distinct and remains so on all length scales and are identical on the incident and output surfaces. The interplay between eigenchannel velocities and transmission eigenvalues determines the energy density within the medium, the diffusion coefficient, and the dynamics of propagation. The diffusion coefficient and all scattering parameters, including the scattering mean free path, oscillate with the width of the sample as the number and shape of the propagating channels in the medium change.

Features of portal hemodynamics in patients with fatty liver disease: mechanisms of formation, clinical significance

There are few data in the literature on the state of portal and hepatic blood flow in patients with fatty liver disease, and portal hemodynamics at the pre-cirrhotic stage is practically not covered. The aim of the study was to study the features of portal hemodynamics in patients with fatty liver disease at the pre-cirrhotic stage. Materials and methods. The study included 43 patients (25 - patients with fatty liver disease and 18- a comparison group who applied to outpatient clinics of therapeutic profile aged 18-89 years (41.9% of men, 58.1% of women). The average age of the patients was 45.1± 19.4 years. Criteria for inclusion in the study: age 18 years and older; presence of fatty liver disease associated with metabolic conditions. Criteria for not being included in the study: the presence of viral hepatitis, alcoholic liver disease, autoimmune liver diseases, pregnancy, lactation, taking any medications, narcotic drugs, the presence of acute or exacerbation of chronic diseases. The anthropometric indicators of patients were evaluated: waist circumference (OT), body mass index (BMI), steatotic indices (St-index, Fatty Liver Index), liver fibrosis index (Fib 4). Portal hemodynamics was assessed using ultrasound Dopplerography on a Philips Affiniti 50 device. Sensor C5-1. The following criteria were evaluated: vessel diameter (mm), its cross-sectional area (PPP, cm2), time-averaged maximum velocity (TAMX, cm/s), time-averaged average blood flow velocity (TAV, cm/s), Doppler perfusion index was calculated using the formula: V vol SPA / V vol SPA + V vol BB), the stagnation index was calculated using the formula: PPP/TAV. The results obtained. In patients with fatty liver disease, structural and hemodynamic shifts in the portal blood flow system are recorded: an increase in the diameter of the portal and inferior vena cava, a larger cross-sectional area, a low time-averaged maximum blood flow velocity and time-averaged average blood flow velocity, an increase in the stagnation index for these vessels, which reflects the process of remodeling the liver as a target organ in fatty liver disease and its vascular bed, forming a “stagnant liver”. Patients have lower values of the time-averaged average blood flow rate through the splenic vein, which indicates the initiation of collateral blood flow. Portal hemodynamics indicators correlate with metabolic parameters (anthropometric characteristics of patients (OT, BMI), steatotic indices (St index, FLI), but not with the liver fibrosis index (Fib 4). These hemodynamic features at the pre-cirrhotic stage of the disease, correlating with the severity of metabolic disorders, exacerbate structural changes in the liver and necessitate their correction in patients with fatty liver disease.

Remodeling of extracranial veins and venous-arterial imbalance in extrinsic stenosis and hypoplasia of the internal jugular veins

The aim of the study was to compare hemodynamic parameters, the sequence of remodeling of the jugular and extrajugular outflow tracts of cerebral venous circulation in extrinsic stenosis and hypoplasia of the internal jugular vein (IJV). Material and methods. Groups with external compression and IJV stenosis (n = 50), with IJV hypoplasia (n = 27), and after IJV ligation/resection (n = 6) as a model of IJV occlusion with a minimal cerebral venous insufficiency and venous encephalopathy and control group (n = 31) were compared. All patients underwent a neurological examination, ultrasound duplex scanning of the IJV (J2 and at the level of stenosis), common carotid and vertebral artery (V2), 2DToF or 3DToF magnetic resonance venography of the neck veins. The degree of stenosis of the IJV, the crosssectional area and the time average blood flow velocity, the calculated indicator of the venous-arterial balance (VAB) were determined according to the modified formula. Results and discussion. In the group of external compression of the IJV, stenosis averaged 64.52 %. The area of the IJV at the site of stenosis was 6 times, and at the standard place (J2) – 3 times less than the area of the opposite IJV. The hypoplastic IJV throughout its entire length (from J1 to J3) had the same area (0.21 ± 0.12 cm2 ), smooth contours, without local expansions or narrowings. A very low blood flow velocity (10.2 ± 11.67 cm/s) is characteristic for stenosis, in contrast to the normal velocity in the hypoplastic vein and in the control group. The blood flow velocity in the contralateral IJV with external stenosis does not differ from normal, but it is increased with hypoplasia (34.62 ± 12.23 cm/s). With MR venography, it is possible to detect a stenosing factor, symptoms of a filling defect, a decrease or loss of a blood flow signal, and an assessment of the overall picture of remodeling of the venous network of the neck. A decrease in the IJV area on the side of the lesion/anomaly corresponds to negative remodeling, while an expansion of the opposite IJV corresponds to positive remodeling. If the obstruction persists, the homolateral, then the contralateral external jugular vein expands, then the vertebral veins and plexuses, spinal epidural veins, anterior jugular and posterior cervical veins expand too. The magnitude of the IJV on the side of the anomaly/pathology with hypoplasia did not differ from the value with stenosis (9 %), but was slightly less than on the opposite IJV (53.39 ± 13.40 and 67.24 ± 18.02 %, respectively, p < 0.06) and significantly less compared to the control (24.16 ± 8.06 % on the left and 33.15 ± 8.27 % on the right, p < 0.0001), which makes this indicator a good additional criterion for outflow abnormality according to the IJV of one side.

Effects of axial distance on flow behaviours of continuous jet impinging on a flat surface using a finite confined nozzle

The velocity fields and impinging pressure characteristics of confined jet impinges on a flat surface subject to the influence of various axial stages of a flat plate were experimentally examined. The instantaneous flow patterns, time-averaged flow patterns and velocity characteristics, velocity histories, and impingement pressure distributions were obtained using flow visualisation, particle image velocimeter (PIV), hot-wire anemometer measurement, and pressure detection techniques, respectively. From the analysis of the flow patterns, three characteristics of flow regimes are identified – laminar vortical wake, transition, and turbulent vortical wake – in the domain of Reynolds number and jet-to-wall distance. In laminar vortical wake mode, a ‘fat' pair of counter-rotating vortices appears between the confining surface of the nozzle and the flat plate. This results in a large region of turbulent intensities due to the entrainment of the jet flow. In transition mode, shear-layer vortices appear on the shear layer of the bifurcated jet. In turbulent vortical wake mode, the large momentum of jet fluids impinges the flat plate and deflects radially. The counter-rotating vortices and shear-layer vortices no longer exist in the wake of the bifurcated jet. The large eddies quickly break up into small eddies as the momentum of the jet-dominated flow. No obvious peak value of oscillating frequency was detected in this flow mode along the shear layers of the bifurcated impinging jet. The momentum of the fluid structures is converted into stagnation pressure on the flat plate when a strong jet impinges on it; therefore, the impingement pressure is large, consequently. The impingement pressure in the mode of the turbulent vortical wake was significantly greater than that in the modes of laminar vortical wake and transition.

A deep-learning model for predicting spatiotemporal evolution in reactive fluidized bed reactor

Detailed information of flow fields is of great significance for designing and optimizing multiphase flow systems. However, predicting spatiotemporal evolution of gas-solid flows using numerical simulation often requires a significant amount of computation and time. In this study, we proposed a 3D convolutional neural network for predicting reactive dense gas-solid flows. We first explored the design of model architecture and extensively evaluated the performance in terms of efficiency, accuracy, long-term prediction stability and generalizability for a non-reactive fluidized bed. Then we extended the method to a biomass fast pyrolysis process. The proposed model achieves real-time prediction, 3–4 orders of magnitude faster than CFD-DEM simulations. The surrogate model reasonably captures bubble-driven flow behaviors and effects of bubble on fast pyrolysis reactions. The predicted bubble characteristics, and time-averaged and RMS flow fields match well with the simulation results. Our approach exhibits excellent long-term stability and has good generalization capability to unseen fluidization velocities. To the best of our knowledge, this is the first time a neural network has been successfully applied to learn spatiotemporal evolution of reactive dense gas-solid flows.

Experimental study of turbulent flow in lower plenum with flow skirt of reactor pressure vessel of pressurized water reactor

The turbulent flow in lower plenum of reactor pressure vessel (RPV) of pressurized water reactor (PWR) is an important phenomenon for design of flow mixing device. A high-precision matched index of refraction (MIR) technique is requisite for time-resolved particle image velocimetry (TR-PIV) measurements in RPV with complicated internals. In this study, turbulent flow in a scaled-down RPV model was measured by TR-PIV under different Reynolds numbers (Re) and asymmetrical flow rate with aid of the high-precision MIR technique of sodium iodide (NaI) solution and acrylic. The time-averaged flow structures include flow separation at the corner of lower support plate, fluid bifurcation around the lower edge of flow skirt, flow turning upward below the vortex suppression plate, horizontal jets at holes of flow skirt, vertical jets at holes of vortex suppression plate, improving turbulent mixing and Reynolds stresses in the lower plenum. The coherent structures were studied by proper orthogonal decomposition (POD) analysis. The flow skirt and vortex suppression plate break down large vortices through them. With Re increasing, Reynolds stresses decrease fast, and the energy fractions of low-order modes transfer into high-order modes, because turbulent vortices break up. In the case of asymmetrical flow rate, the time-averaged velocity and Reynolds stress decrease slightly, and the cumulative energy fraction decreases at flow skirt and vortex suppression plate.

Dynamic Analysis of Tip Leakage Phenomena in Axial Flow Pumps Using a Square-Cavity Jet Model

In the field of pump impeller studies, tip leakage flow (TLF) and the resultant tip leakage vortex (TLV) significantly influence hydraulic efficiency, cavitation, and noise generation. This paper builds a novel square-cavity jet model combined with Large Eddy Simulation (LES) technology to obtain precise the dynamic properties of the TLV, significantly simplifying the computational resources required for numerical simulations. The novel square-cavity jet model simplifies a single blade channel to a square-cavity, and then adds a longitudinal slit on the top wall of the square-cavity. The analysis of both instantaneous and time-averaged flow fields indicates that the interaction between the main flow and the jet is the primary source of TLV generation. This study successfully captures the formation process of the TLV and accurately reveals its turbulent coherent structures. The evolution of the TLV is divided into three main parts: the first part is the jet slot, predominantly characterized by negative vorticity flow. The second part is the TLV formation, which is mainly composed of significant negative streamwise vortices. The third part is the development of the TLV, where positive and negative vorticities begin to interact, resulting in a more complex overall structure. The entire evolution of the TLV phenomenon starts with a concentrated negative vortex, which, after breakdown, develops at a certain angle to the slot and continuously advances towards the sidewall, ultimately resulting in the formation of a large-scale intermingled group of small-scale positive and negative vortices. This research not only provides a new physical model for investigating the tip leakage phenomenon in axial flow pumps but also offers a powerful tool and methodology for future studies in similar complex flow domains.

Time-averaged flow characterisations in a bimodal gravel-bed stream and relative role of sediment depositions on near-bed coherent flow structures

Abstract The present study aims to quantify experimentally the relative role of sediment depositions on near-bed flows and turbulence in a gravel-bed stream. Time-averaged velocity was measured over a sand-filled gravel-bed stream with four cases of sediment depositions and compared with those over a gravel-bed stream without sediment depositions. An acoustic Doppler velocimeter was used to measure the instantaneous velocity of flows. The progressive infilling of void spaces in the gravel-bed stream forms distinct bimodal depositions that alter the mean flows characterised by increasing zero-velocity levels and massive damping in the bed shear stresses. The data plots of turbulent intensity depict enhancement of streamwise turbulent intensity in the near-bed flow region with increasing sand depositions. Moreover, the opposite nature of streamwise and vertical turbulent kinetic energy fluxes in the interfacial sublayer leads to slowing down the time-average Reynolds shear stresses at the vicinity of the gravel-bed surface. At the vicinity of the crest, the ejection and sweep events contributed approximately 86 and 56%, respectively, to the total Reynolds shear stress production in the case of gravel bed under clear-water flow conditions. By contrast, the contributions of turbulent sweep events increased over the sand-filled gravel bed at the same location.

PIV measurements of coherent vortices and turbulence production inside acoustic liner cavity with offset slit

This paper reports an experimental investigation into the vortex dynamics purely excited by periodic acoustic waves within a slit–cavity structure, which is a typical unit element of an acoustic liner for aeroengine applications. Particle image velocimetry (PIV) was used to measure the instantaneous, time-averaged and phase-dependent flow dynamics inside the slit–cavity, which is exposed to high-intensity incident acoustic waves. Different with traditional literature mainly focusing on the acoustic characteristics, we primarily analyze the coherent vortex structures and turbulence production mechanism, accompanying with the variable of slit offset ratio being taken into consideration. The results demonstrate that the acoustic waves can excite the hydrodynamic velocities exceeding almost tens of times than the acoustic particle velocity, yielding multi-scale vortex structures appearing within the enclosed volume. Comparatively, in configuration with offset slit, the maximum hydrodynamic velocities were gradually intensified by wall confinement effect. Subsequent proper orthogonal decomposition analysis revealed the coherent vortex structures and demonstrated the offset configuration will increase their energy proportions and enhance their temporal correlations. However, the phase-dependent flow results revealed that the offset slit can oppositely attenuate the kinematics of the coherent vortices, which were represented by the convection trajectories and convection speeds, especially during the initial acceleration stage. Finally, analysis on turbulence production mechanism confirmed the attenuated turbulent kinetic energy by the offset slit, which was attributed to the less generation of shear stresses.

An in-depth examination of flow structures organization downstream of a delta-winglet pair vortex generator under laminar, transitional, and turbulent flow regimes

In the present study, we experimentally investigate the time-averaged and time-evolving flow structures generated downstream of a delta-winglet pair vortex generator in an airflow channel. The Stereoscopic Particle Image Velocimetry technique is used to measure the velocity fields in the cross-sectional planes at various axial positions along the channel. The analysis of time-averaged flow topology shows that the flow is predominantly composed of two main counter-rotating longitudinal vortices under laminar, transitional, and turbulent regimes. While the time-averaged flow behavior might appear simple, the real-time organization of the instantaneous flow structures presents more complex features. The study of the flow's temporal evolution indicates that the transition from laminar to turbulent regime is controlled by a meandering mechanism referred to the motion of the main vortex centers around their axis and the emergence of secondary structures that evolve periodically over time.

Detached-eddy simulation of the vortex system on the high-lift common research model

A complex vortex system exists on the high-lift transport aircraft with components of the nacelle, pylon, high-lift devices, strakes, and brackets, and the investigation of the vortex system is helpful in profoundly understanding the aerodynamic characteristics of actual high-lift aircraft. To investigate the vortex system on the high-lift transport aircraft and evaluate the simulation capacity of detached-eddy simulation (DES) for the vortex system on the high-lift aircraft configuration, a high-resolution DES numerical investigation is carried out on a standard high-lift transport aircraft configuration, the National Aeronautics and Space Administration (NASA) high-lift common research model (CRM-HL). Reynolds-averaged Navier–Stokes simulation (RANS) is also performed to investigate the differences between DES and RANS in predicting time-averaged aerodynamic data on the CRM-HL. After experimental validation of time-averaged surface load and flow patterns, three vortex identification methods are adopted to extract the vortex structures of the flow field. It is found that the Rortex-based vortex identification method can more distinctly extract the fine vortex structures. The DES method can reliably predict the time-averaged surface load while capturing the complex vortex system on the CRM-HL under the concerned case of this work with high resolution.

Revisiting the surface-mounted cube: An updated perspective of the near wake and near-wall flow field

The flow around a surface-mounted cube has been investigated for decades, yet the fundamentals of the mean flow have not been updated to reflect the latest advances regarding the flow around surface-mounted finite-height square prisms in general. One of the main gaps is the flow field very near the cube walls, especially the sides, and its relationship to the near wake. To investigate these features, large-eddy simulations of the flow around a surface-mounted cube were carried out at a Reynolds number Re =1×104. Two boundary layers were considered: a thin and laminar boundary layer and a thick and turbulent one, to provide an overview of the flow around the cube for contrasting boundary layers. The major flow structures in the mean wake were the horseshoe vortex, the arch vortex and the dipole structures, with other regions of vorticity also present. The time-averaged flow fields presented similar flow features for both boundary layers, but the thicker and turbulent one caused the horseshoe vortex to be located closer to the cube, the wake to become narrower and shorter, the coherent structures and downwash to weaken, the pressure to decrease around the sides and top of the cube, the drag force coefficient to decrease, the normal force coefficient to increase, and the trailing edge vortices to weaken. Intermittent flow reattachment on the top and side faces of the cube, as well as corner vortices, were found for both boundary layers, while the side and free end vortices were found exclusively with the thicker boundary layer, yielding a headband vortex. The three-dimensional flow structures and features were related to the near-wall flow field on the cube and ground plane surfaces, giving a complete and updated description of the mean flow characteristics.

FLOW IN A CATHETERIZED EXPONENTIALLY DIVERGING TUBE WITH A PERIPHERAL LAYER OF DIFFERENT VISCOSITY BY MEANS OF PERISTALTIC WAVES OF DILATING AMPLITUDE

In this paper, we investigate the impact of catheterization on swallowing if esophagus suffers from a hiatus hernia. It is modeled as a partially diverging tube having a peripheral layer of different viscosity besides the core layer. The model duly considers mass conservation in the two layers separately. It is inferred that even in the presence of a catheter inside esophagus, any sliding herniation requires less pressure for flow, i.e., swallowing will be easier if the esophagus diverges more near the cardiac sphincter. However, the presence of a catheter requires esophagus to work more in order to swallow the same quantity. Pressure enhances by many folds, but its distribution remains similar. The time-averaged flow rate and the maximum pressure, restraining fluid from swallowing, give some clues about a catheterized herniated esophagus. The flow enhances for a thinner peripheral layer. A more viscous peripheral layer as well favors flow. However, the thickness of the catheter hinders swallowing. This is to be taken care of while a patient undergoes any catheterized treatment. Increasing wave amplitude and dilating parameters can enhance flow in the presence of a catheter. It is also concluded that the frictional forces exerted by the tube wall on the peripheral layer increase in the catheter’s presence.

Exploring Cerebral Perfusion Transcranial Doppler Parameters in Patients Admitted to Combined Medical Surgical Intensive Care Unit

Introduction: Encephalopathy, a common complication in Intensive Care Unit (ICU) patients, is often linked to poor outcomes. Transcranial Doppler (TCD), a non-invasive tool assesses cerebral perfusion via the Pulsatility Index (PI), Resistivity Index (RI), and Time-Averaged Peak (TAP) or Mean Flow Velocity (MFV). These parameters may offer insights into cerebral perfusion and outcomes in encephalopathic patients. Aim: To describe the PI, RI of the Middle Cerebral Artery (MCA), and MFV or TAP measured by TCD in patients admitted to the ICU, comparing those with and without encephalopathy at the time of admission. Materials And Methods: This cross-sectional observational study was conducted from January 2019 to November 2020, in a combined medical-surgical ICU of a tertiary care hospital, involving 45 enrolled patients. Patients were evaluated within 24 hours of admission and subsequently every 24 hours until ICU discharge, death, or discharge against medical advice. Bilateral Middle cerebral artery TCD studies were conducted using a 1-5 MHz phased array probe or TCD mode through the transtemporal window. The PI, RI, and MFV were measured on both sides, with the higher value used for analysis, and all statistical analyses were performed using Statistics and Data 13 software. Results: In the present study, 88% (n=40) of patients had encephalopathy (GCS<15 and RAAS less than or more than 0). The mean APACHE II score was 19 (14-25), indicating severe illness with a predicted mortality of 30-40%. Patients with encephalopathy had significantly higher APACHE II scores compared to those without (19.5 (16-25) vs 10 (4-19)). Although there was a trend towards a higher Pulsatility Index in encephalopathic patients at admission (1.11±0.378 vs 1.07±0.12, p=0.81), PI, RI, and TAP values did not significantly differ in non-survivors (1.12±0.49 vs 1.11±0.33, p=0.750). Persistent encephalopathy was associated with a trend towards higher PI at admission (1.05±0.24 vs 1.16±0.46; p=0.756), and a moderate correlation was found between decreasing PI and improvement in GCS (rho=-0.489, p=0.001). Conclusions: PI, RI, and TAP at the time of admission were not found to be associated with occurrence and recovery of encephalopathy as well as mortality. The trend of change in PI moderately correlated with improvement in GCS suggesting the importance of trends rather than absolute values.

NUMERICAL SIMULATION OF PSEUDOPLASTIC FLUID FLOW OVER A SQUARE CYLINDER USING ANSYS FLUENT

Flow of any non-Newtonian fluid past a bluff body requires considerable attention due to its involvement in chemical, polymer, and process engineering. In the current study, numerical simulation of the flow past a square cylinder was conducted at Reynolds number Re = 100 in a pseudoplastic fluid flow environment (0.1 ≤ <i>n</i> < 1.0, where <i>n</i> represents the power index of non-Newtonian fluid) using ANSYS Fluent. Instantaneous and time-averaged flow fields along with the flow parameters were calculated to understand the behavior of pseudoplastic flow in the presence of an obstacle (i.e., the square cylinder). It was observed that the drag force value increased in the pseudoplastic fluid flow compared to that in Newtonian fluidflow (<i>n</i> = 1), whereas the vortex shedding frequency decreased as the n value decreased. Close to <i>n</i> = 0.4, clear evidence of flow instability was seen, which drastically changed the vortex shedding mechanism and affected the flow downstream from the cylinder. In addition, at <i>n</i> = 0.1 the fluid flow stood still downstream due to very high flow resistance-a common feature of flow fields with a very high pseudoplastic nature.

Shock-wave/turbulent boundary-layer interaction with a flexible panel

The dynamic coupling between a Mach 2.0 shock-wave/turbulent boundary-layer interaction (STBLI) and a flexible panel is investigated. Wall-resolved large-eddy simulations are performed for a baseline interaction over a flat-rigid wall, a coupled interaction with a flexible panel, and a third interaction over a rigid surface that is shaped according to the mean panel deflection of the coupled case. Results show that the flexible panel exhibits self-sustained oscillatory behavior over a broad frequency range, confirming the strong and complex fluid–structure interaction (FSI). The first three bending modes of the panel oscillation are found to contribute most to the unsteady panel response, at frequencies in close agreement with natural frequencies of the mean deformed panel rather than those for the unloaded flat panel. This highlights the importance of the mean panel deformation and the corresponding stiffening in the FSI dynamics. The time-averaged flow shows an enlarged reverse-flow region in the presence of mean surface deformations. The separation-shock unsteadiness is enhanced due to the panel motion, leading to higher wall-pressure fluctuations in the coupled interaction. Spectral analysis of the separation-shock location and bubble-volume signals shows that the STBLI flow strongly couples with the first bending mode of the panel oscillation. This is further confirmed by dynamic mode decomposition of the flow and displacement data, which reveals variations in the reverse-flow region that follow the panel bending motion and appear to drive the separation-shock unsteadiness. Low-frequency modes that are not associated with the fluid–structure coupling, in turn, are qualitatively similar to those obtained for the rigid-wall interactions, indicating that the characteristic low-frequency unsteadiness of STBLI coexists with the dynamics emerging from the fluid–structure coupling. Based on the present results, unsteady FSIs involving STBLIs and flexible panels are likely to accentuate rather than mitigate the undesirable features of STBLIs.

Experimental study of acoustic streaming induced by a sharp edge at different frequencies and vibrating amplitudes

Acoustic streaming is the time-averaged flow induced by acoustic waves inside the fluid medium. Much attention has been paid to the streaming flow at the microscale, with the rapid development of micro-fluidics and significant demand for the microscale manipulation of fluid or particles. Recently, the streaming flow at the audible or lower frequency (10 Hz~10 kHz) has been found to be closely associated with local structures, like a sharp edge in the micro-channel. By its strong magnitude and low cost, this kind of streaming flow has been applied in various fields. However, the mechanisms behind this non-classical Rayleigh streaming are still not very clear, though its high sensitivity to the thickness of the acoustic boundary-layer and unstable streaming pattern under high forcing amplitude have been demonstrated. In this study, experimental work has been conducted, with the help of the particle imaginary velocimetry platform, to reveal the influence of frequency and vibrating amplitude on the streaming flow field around a sharp edge with 90?, and its characterized spatial dimension. The scaling law concerning the vibration amplitude and streaming velocity has been come up with, and the parameter frequency is also included. The expression f ?1/6va2~vsy,max demonstrates a good prediction in terms of the streaming magnitude, in comparison with experimental results.

Large Eddy Simulation of Pulsed Film Cooling with a Dielectric Barrier Discharge Plasma Actuator

The effects of the coolant pulsation and the plasma aerodynamic actuation (PAA) on the film cooling are herein explored via large eddy simulations. The electrohydrodynamic force derived from the PAA was solved through the phenomenological plasma model. The Strouhal number of the sinusoidal coolant pulsation and the averaged pulsation blowing ratio were 0.25 and 1.0, respectively. Comprehensive analyses were carried out on the time-averaged flow fields, and the results reveal that the pulsed cooling jet might cause a deeper penetration into the crossflow, and this phenomenon could be remarkably mitigated by the downward force of the PAA. Comparing steady film cooling to pulsed film cooling revealed a modest 15.1% reduction in efficiency, while the application of the dielectric barrier discharge plasma actuator (DBDPA) substantially enhanced the pulsed film cooling efficiency by 42.1%. Moreover, the counter-rotating vortex pair (CRVP) was enlarged and lifted off from the wall more poorly due to the coolant pulsation, and the PAA weakened the detrimental lift-off effect and entrainment of the CRVP. Then, the spatial–temporal development of the coherent structures was figured out by the alterations in the centerline temperature, reflecting the formation of the intermittent coherent structures rather than hairpin vortices due to the coolant pulsation, and their size and upcast behaviors were reduced by the PAA; thus, the turbulent integration of the coolant with the crossflow was suppressed fundamentally. Finally, the three-dimensional streamlines confirmed that the coherent structure dynamic behaviors were significantly regulated by the PAA for alleviating the adverse influences of the coolant pulsation. In summary, the PAA can effectively improve the pulsed film cooling efficiency by controlling the spatial–temporal development of the dominant coherent structures.

Numerical simulation and coupling mechanism study of acoustic-inertial micromixer

Acoustic-inertial micromixers exhibit a wider operating range and higher mixing efficiency than conventional mixers. We have carried out experimental studies and numerical simulations of acoustic-inertial micromixers. It is found that the acoustic streaming will be changed in morphology and intensity by the background flow, and the coupling mechanism between them is obtained. Specifically, we developed the numerical model of acoustic-inertial micromixer by using perturbation theory and the Generalized Lagrangian Mean (GLM) theory. By dividing the flow variables into zero-order, first-order and second-order variables, we obtained the control equations representing background flow, acoustic response and time-averaged streaming flow. Then realized the decoupling analysis of the acoustic-inertial coupling phenomenon. It is found that in terms of acoustic streaming flow, the intensity extreme of it increases as the background flow rate increases. The flow velocity at 5Vpp can reach 0.46 m/s under 500 μL/min, which is 30 times for the same condition under 10 μL/min. However, the streaming morphology no longer behaves as a symmetric vortex distributed on both sides of the sharp edge, which is stretched and twisted by the background flow and structure at a high flow rate. The percentage of acoustic contribution at different working conditions is also investigated quantitatively, and the acoustic contribution index (ACI) will generally decrease to less than 0.5 after the background flow dominates the flow field. The results of the study can provide a reference for the design of acoustic-inertial micromixers with larger flow ranges and higher efficiencies.

Numerical simulation of pulsatile blood flow through eccentric double stenosed carotid artery

Purpose This study aims to perform three-dimensional numerical computations for blood flow through a double stenosed carotid artery. Pulsatile flow with Womersley number (Wo) of 4.65 and Reynolds number (Re) of 425, based on the diameter of normal artery and average velocity of inlet pulse, was considered. Design/methodology/approach Finite volume method based ANSYS Fluent 20.1 was used for solving the governing equations of three-dimensional, laminar, incompressible and non-Newtonian blood flow. A high-quality grid with sufficient refinement was generated using ICEM CFD 20.1. The time-averaged flow field was captured to investigate the effect of severity and eccentricity on the lumen flow characteristics. Findings The results show that an increase in interspacing between blockages brings shear layer instability within the region between two blockages. The velocity profile and wall shear stress distribution are found to be majorly influenced by eccentricity. On the other hand, their peak magnitude is found to be primarily influenced by severity. Results have also demonstrated that the presence of eccentricity in stenosis would assist in flow development. Originality/value Variation in severity and interspacing was considered with a provision of eccentricity equal to 10% of diameter. Eccentricity refers to the offset between the centreline of stenosis and the centreline of normal artery. For the two blockages, severity values of 40% and 60% based on diameter reduction were permuted, giving rise to four combinations. For each combination, three values of interspacing in the multiples of normal artery diameter (D), viz. 4D, 6D and 8D were considered.