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The objective is to study the vortical structural behaviors of a transient pitching hydrofoil and their effects on the hydrodynamic performance. The pitching motion of the hydrofoil is set to pitch up with an almost constant rate from 5° to 15° and then back to 5°, with the Reynolds number 4.4×105 and the frequency 2 Hz. The results show that the main coherent structures around the pitching hydrofoil include small-scale laminar separation bubble (LSB), large-scale second vortex (SV) and trailing edge vortex (TEV) which are all vortical. The relationship between the vortical structure and the lift is investigated with the finite-domain impulse theory. It indicates that the major part of the lift is contributed by the LSB, whereas the shedding and the formation of the SV and TEV cause the fluctuation of the lift. The proper orthogonal decomposition (POD) method is applied to capture the most energetic modes, revealing that the LSB mode occupies a large amount of energy in the flow field. The dynamic mode decomposition (DMD) method accurately extracts the dominant frequency and modal characteristics, with the first mode corresponding to the mean flow, the second mode corresponding to the LSB structure and the third and fourth modes corresponding to the vortex shedding. [ABSTRACT FROM AUTHOR]

Carotid artery diseases, such as atherosclerosis, are a major cause of death in the United States. Wall shear stresses are known to prompt plaque formation, but there is limited understanding of the complex flow structures underlying these stresses and how they differ in a pre-disposed high-risk patient cohort. A 'healthy' and a novel 'pre-disposed' carotid artery bifurcation model was determined based on patient-averaged clinical data, where the 'pre-disposed' model represents a pathological anatomy. Computational fluid dynamic simulations were performed using a physiological flow based on healthy human subjects. A main hairpin vortical structure in the internal carotid artery sinus was observed, which locally increased instantaneous wall shear stress. In the pre-disposed geometry, this vortical structure starts at an earlier instance in the cardiac flow cycle and persists over a much shorter period, where the second half of the cardiac cycle is dominated by perturbed secondary flow structures and vortices. This coincides with weaker favorable axial pressure gradient peaks over the sinus for the 'pre-disposed' geometry. The findings reveal a strong correlation between vortical structures and wall shear stress and imply that an intact internal carotid artery sinus hairpin vortical structure has a physiologically beneficial role by increasing local wall shear stresses. The deterioration of this beneficial vortical structure is expected to play a significant role in atherosclerotic plaque formation. [ABSTRACT FROM AUTHOR]

The performance of a transcatheter aortic valve (TAV) can be evaluated by analyzing the flow field downstream of the valve. However, three dimensional flow and pressure fields, and particle residence time, a quantity closely related to thrombosis risk, are challenging to obtain. This experimental study aims to provide a comprehensive 3D measurement of the flow field downstream of an Edwards SAPIEN 3 using time-resolved 3D particle tracking velocimetry (3D PTV) with Shake-the-Box (STB) algorithm. The valve was deployed in an idealized aorta model and tested in a left heart simulator under physiological conditions. Detailed 3D vortical structures, pressure distributions, and particle residence time were obtained by analyzing the 3D particle tracks. Results have shown large-scale retrograde flow entering the sinuses of the TAV at systole, reducing flow stasis there. However, the 3D particle tracks reveal that the retrograde flow has a high residence time and might have already experienced high shear stress near the main jet. Thus by only focusing on the flow in the sinus region is not sufficient to evaluate the leaflet thrombosis risk, and the flow downstream of the valve should be taken into consideration. The unique perspectives offered by 3D PTV are important when evaluating the performance of the TAVs. [ABSTRACT FROM AUTHOR]

Droplet nuclei dispersion patterns in indoor environments are reviewed from a physics view to explore the possibility of airborne transmission of SARS-CoV-2. This review analyzes works on particle dispersion patterns and their concentration in vortical structures in different indoor environments. Numerical simulations and experiments reveal the formation of the buildings’ recirculation zones and vortex flow regions by flow separation, airflow interaction around objects, internal dispersion of airflow, or thermal plume. These vortical structures showed high particle concentration because particles are trapped for long periods. Then a hypothesis is proposed to explain why some medical studies detect the presence of SARS-CoV-2 and others do not detect the virus. The hypothesis proposes that airborne transmission is possible if virus-laden droplet nuclei are trapped in vortical structures associated with recirculation zones. This hypothesis is reinforced by a numerical study in a restaurant that presented possible evidence of airborne transmission by a large recirculating air zone. Furthermore, a medical study in a hospital is discussed from a physical view for identifying the formation of recirculation zones and their relation with positive tests for viruses. The observations show air sampling site located in this vortical structure is positive for the SARS-CoV-2 RNA. Therefore, the formation of vortical structures associated with recirculation zones should be avoided to minimize the possibility of airborne transmission. This work tries to understand the complex phenomenon of airborne transmission as a way in the prevention of transmission of infectious diseases.

Abstract The high‐temperature molten salt pump (MSP) is the core equipment of the thermal storage system for concentrating solar power plants, which circulates the molten salt of the thermal storage medium. This study employs numerical simulation of a two‐stage MSP with a storage tank to find the correlation between pressure pulsations and the evolutions of vortical structures based on renormalization group k‐ε turbulence model. It is shown that rotor/stator interaction is the most significant excitation source of pressure pulsations with the prominent excitations being the diffuser passing frequency (fDPF) in the impeller and the blade passing frequency (fBPF). Furthermore, Q‐criterion is employed to capture the periodic shedding vortexes with a frequency of about 3fR at the suction surface and the trailing edge in diffusers. Impinging vortex and shedding vortex with the frequency 6fR on the leading edge of the diffuser is captured. The vortex shedding characteristics are significantly different under various conditions, and the separation vortexes appear at the leading edge of pressure surface in the diffusers at higher flow rates. Finally, it is obvious that the excited components and their magnitudes in the pressure spectrum are closely associated with the unsteady vortical structures within the model pump.

A numerical study of two dimensional lid-driven triangular and trapezoidal cavity flow is performed via using the lattice Boltzmann method (LBM) for steady solutions. The equilateral and right-angled isosceles triangular cavity flow at Reynolds numbers, respectively, 500 and 100 is employed as the benchmark case for code validation. The isosceles right-angled triangular cavity flow is studied for Reynolds numbers sweeping from 100 to 8100. Flow topologies are captured and analyzed. The critical Reynolds number of Hopf bifurcation is predicted by calculating the perturbation decay rate. Two different geometries of right-angled isosceles trapezoidal cavities, bowl-shaped and pyramid-shaped trapezoids, are studied at Reynolds numbers 1000 and 7000. For each type of the trapezoidal cavity, a geometric parameter λ (top-line/base-line ratio) is presented to distinguish different geometries of trapezoidal cavities. The flow patterns regarding the streamlines, vortical structures, and velocity profiles are discussed. The impact of parameter λ on the fluid characteristics are investigated. [ABSTRACT FROM AUTHOR]

Tip loading is a common strategy to increase the propulsive efficiency of propellers. Solutions such as contracted and tip-loaded (CLT) and "New generation" CLT propellers exploit the presence of an endplate ("true" or as the result of a dedicated modification of the rake distribution) to sustain the increased load at the tip of the blade, at the cost of more complex vortical structures. Their evolution, and the mutual interaction of secondary vortices originated by the endplate itself, however, has not been completely and deeply investigated. The current paper addresses this topic by improved delayed detached eddy simulations (IDDES) of the flow field around two propellers of this type at different loading conditions. The presence of secondary vortices from the endplate root (or from the bended blade at tip), partially observed in recent experiments, is evidenced by high-fidelity CFD calculations. The interaction mechanism with the primary vortices (those from the endplate tip), and the resulting strengthening of the vortical structures, also through the interaction with the blade trailing vortical wake that promote the leapfrogging phenomenon, is discussed as well, comparing the phenomena in the case of two optimally designed geometries (a CLT and a New Generation CLT propeller) exploiting the same pressure side tip-loading concept in a slightly different way. Results show a rather different instability mechanism depending on the endplate configuration and open the discussion on the effectiveness of splitting a single tip vortex into pairs of vortical structures that may induce similar (or even worse) side effects in terms of pressure minima in the wake and earlier wake destabilization. [ABSTRACT FROM AUTHOR]

Carotid artery diseases, such as atherosclerosis, are a major cause of death in the United States. Wall shear stresses are known to prompt plaque formation, but there is limited understanding of the complex flow structures underlying these stresses and how they differ in a pre-disposed high-risk patient cohort. A ‘healthy’ and a novel ‘pre-disposed’ carotid artery bifurcation model was determined based on patient-averaged clinical data, where the ‘pre-disposed’ model represents a pathological anatomy. Computational fluid dynamic simulations were performed using a physiological flow based on healthy human subjects. A main hairpin vortical structure in the internal carotid artery sinus was observed, which locally increased instantaneous wall shear stress. In the pre-disposed geometry, this vortical structure starts at an earlier instance in the cardiac flow cycle and persists over a much shorter period, where the second half of the cardiac cycle is dominated by perturbed secondary flow structures and vortices. This coincides with weaker favorable axial pressure gradient peaks over the sinus for the ‘pre-disposed’ geometry. The findings reveal a strong correlation between vortical structures and wall shear stress and imply that an intact internal carotid artery sinus hairpin vortical structure has a physiologically beneficial role by increasing local wall shear stresses. The deterioration of this beneficial vortical structure is expected to play a significant role in atherosclerotic plaque formation.

Due to global environmental conditions, the focus of household heating has shifted from fossil fuels towards environmentally friendly and renewable energy sources. Desuperheaters have attracted attention as a domestic provision involving steam-induced direct contact condensation (DCC)to warm the water. The present study is an attempt to investigate the hydrodynamics in the desuperheater vessel experimentally, namely, when the pressurized pulsating steam is injected into the vessel, where the steam jet interacts co-currently with the slow-moving water. Flow visualization showed a circulation region when the pulsating steam was injected into the slow-moving water, and the peaked vorticity corresponded to the steam injection duration of 10–60 s. Sevenhot film anemometers (HFAs) were traversed axially and radially to determine the velocity fluctuations at 0–20 cm from the steam's nozzle exit. Vortical structures indicated the entrainment of the steam with the surrounding moving water. The circulation regions were thus exhibited in relation to the steam's injection durations as well as the downstream axial distances of 2 and 15 cm from the nozzle exit, which showed that the core local circulation at 2 cm downstream of the nozzle exit lost 75–79% of its circulation at 15 cm downstream of the nozzle exit. [ABSTRACT FROM AUTHOR]

This paper presents a direct numerical simulation of turbulent flows over a bump in an open channel to examine the turbulence characteristics near divergent waves emanating from the bump and to investigate the interaction of the turbulences with the divergent waves. To verify the reliability of the simulations, the mean velocity profile and root-mean-square of velocity fluctuations are compared with previous data. The anisotropic invariant maps show that the ratio of the streamwise to spanwise velocity fluctuations plays an important role in characterizing the anisotropic nature of the separated shear layer behind the bump in the vicinity of the free surface. The vortex identification discloses a large-scale streamwise vortical structure from the mean velocity field and a cluster of small coherent structures from the instantaneous velocity field, which are responsible for the anisotropic characteristics of the turbulence beneath the free surface.

The high‐temperature molten salt pump (MSP) is the core equipment of the thermal storage system for concentrating solar power plants, which circulates the molten salt of the thermal storage medium. This study employs numerical simulation of a two‐stage MSP with a storage tank to find the correlation between pressure pulsations and the evolutions of vortical structures based on renormalization group k‐ε turbulence model. It is shown that rotor/stator interaction is the most significant excitation source of pressure pulsations with the prominent excitations being the diffuser passing frequency (fDPF) in the impeller and the blade passing frequency (fBPF). Furthermore, Q‐criterion is employed to capture the periodic shedding vortexes with a frequency of about 3fR at the suction surface and the trailing edge in diffusers. Impinging vortex and shedding vortex with the frequency 6fR on the leading edge of the diffuser is captured. The vortex shedding characteristics are significantly different under various conditions, and the separation vortexes appear at the leading edge of pressure surface in the diffusers at higher flow rates. Finally, it is obvious that the excited components and their magnitudes in the pressure spectrum are closely associated with the unsteady vortical structures within the model pump. [ABSTRACT FROM AUTHOR]

Transfer and transport of a passive scalar as well as flow structures of unbounded flow over a row of 2, 3, 4 and 5 equispaced identical cylinders at Re = 90 and pitch ratios 1.05D ≤ S ≤ 7D are investigated. Studies are performed through high fidelity simulations using a novel sharp-interface immersed boundary method. Five flow regimes are identified based on the nature of flow in the gap between the first two cylinders: dormant in the gap where fluid in the gap is motionless; symmetric in the gap where fluid in the gap behaves like a cavity flow; alternating in the gap where fluid in the gap is trapped but flaps up and down; wake in the gap where periodic vortices are shed in the gap; and bi-stable flow regime where vortices are shed in the gap for some time and then quasi-periodic twin vortices prevail for other times. A jump in values of hydrodynamic forces, pressure coefficient, Strouhal number, and Sherwood number is observed as pitch ratio increases due to change in flow structure. At high pitch ratio, a convectively unstable two-row vortical train is formed by the second cylinder. Member cylinders immersed in the two-row vortical structures have no effect on flow structure downstream or upstream and portray poor scalar transfer characteristics. The effect of configuration size on flow structures is found not to be as prominent as that of pitch ratio. [ABSTRACT FROM AUTHOR]

The lateral thermal plume discharge in the deep cross-flow has been investigated by numerical simulation using the open-source Open FOAM code. Adaptive mesh refinement method has been applied to reduce the computational cost. The numerical simulation results show a good agreement with the previous experimental data. Three-dimensional structures illustrated by instantaneous velocity fields indicate shear layer roll-up vortices around the discharged plume. As the densimetric Froude number (Fr 0) is increased, the buoyant plume penetrates more in the depth of the channel and fewer spreads in the free surface. As the Fr 0 decreases, the coherent structures become increasingly weak, with the faster breakdown of the shear layer roll-up. The instantaneous temperature contours near the free surface exhibit a vortex shedding phenomenon. Three-dimensional streamlines based on the instantaneous velocity vectors illustrate a swirl flow pattern downstream of the main channel. Increasing the Fr 0 results in weakening the swirl flow around the discharged jet core. The mixing ability of discharged plume is investigated by temporal mixing deficiency (TMD) and spatial mixing deficiency (SMD) indices. The statistical analysis of the TMD and SMD reveals the direct relationship between the mixing efficiency and reduced gravity. [ABSTRACT FROM AUTHOR]

This paper studies the vortical structures found in the wake of a ducted propeller (DP) and a rim-driven thruster (RDT) in bollard conditions. Despite a good understanding of the vortical structures in the wake of DPs, an understanding of these structures and their dynamics in the wake of RDTs is largely lacking. At bollard conditions, noise levels are higher compared to other loading conditions due to the higher pressure fluctuations in the wake. Hence, understanding the flow structures in the wake is a necessity to understand the noise production and propagation. Both wake flows were analysed using visualization of the instantaneous vortical structures, frequency spectra at several locations in the flow field and proper orthogonal decomposition (POD). It was shown that the centre of the wake in both the DP and RDT contained several spiralling structures behind the central hub. These structures contained a wide range of frequencies, making the wake very dynamic with a broad range of different coherent fluctuations. In case of the RDT, these structures persisted longer in the flow field before breaking up into smaller turbulent structures. In the DP wake, very strong tip vortices were formed in the gap flow between the duct and the propeller blades, which were not present in the RDT. These tip vortices interacted with the flow around the duct to form a shear layer with large pressure fluctuations. As these tip vortices induce higher sound levels, the RDT shows a significantly better noise performance compared to the DP. • The wake structures of a rim-driven thruster (RDT) and a ducted propeller (DP) are analysed. • The instantaneous vortical structures and their dynamics are presented. • A POD (proper orthogonal decomposition) analysis reveals the large scale vortical structures. • The RDT exhibits less noise than the DP due to the absence of tip vortices. [ABSTRACT FROM AUTHOR]

Tip cavitation remains a complex and critical concern for waterjet pumps, affecting the safety and efficiency of pump operation. This paper aims to explore the cavitation scale's evolution law, the cavitation structure's evolution characteristics and mechanisms, and how these factors affect pump performance. A synchronous experimental system consisting of a test pump, a high-speed camera, and pressure transducers has been established. Results show that the tip cavitation mainly consists of primary leakage vortex cavitation (PLTVC), secondary leakage vortex cavitation (STLVC), and suction surface attached cavitation near the blade tip (SSAC). The gray level-based image processing method is used to reveal the law of cavitation scale growth rate and vapor fraction distribution. The tip cavitation scale's growth rate increased as the cavitation number decreased. The rapid expansion of the attached cavitation promotes the formation of a re-entrant jet, cavity shedding, and the formation of perpendicular cavitating vortices (PCVs). It was then found that for every 0.1 decrease in cavitation number, the PCV scale in the blade overlap region increased by about 14.1 % during σ =0.256–0.206, which severely blocked the tip passage flow and accelerated the degradation of pump performance. The spatial-temporal evolution of the PCVs is classified into three stages: the shedding stage of the attached cavity, the stage that the shedding cavities change the migrating direction and form the PCVs, and the collage stage of the PCVs. With cavitation numbers of 0.256 and 0.223, the PCV evolves at roughly 194 Hz and 144 Hz, which results in pressure fluctuations of approximate frequency. [ABSTRACT FROM AUTHOR]

Vortex is the dominant flow structure in hydro-energy machinery, and its swirling features are mainly determined by the rigid rotation part of vorticity. In this study, the rigid vorticity transport equation is proposed based on the vorticity decomposition. The vortex spatial evolution is described by the stretching term RST, the dilatation term RDT, the Lamb term RCT and the viscous term RVT. The shape change of a vortex tube is mainly affected by RST and RDT, while vortex production and dissipation are mainly reflected by RCT and RVT. Compared with the traditional vorticity transport equation, this new theoretical tool can distinguish between rigid rotation and shearing motion of local fluids, and it can better reveal the intuitive evolution characteristics of vortical structures. Two cases with clear engineering demands are analysed by using the transport equation, and the results show that it can provide a practical method for the analysis and control of vortical flows in hydro-energy machinery.

Vortex is the dominant flow structure in hydro-energy machinery, and its swirling features are mainly determined by the rigid rotation part of vorticity. In this study, the rigid vorticity transport equation is proposed based on the vorticity decomposition. The vortex spatial evolution is described by the stretching term RST, the dilatation term RDT, the Lamb term RCT and the viscous term RVT. The shape change of a vortex tube is mainly affected by RST and RDT, while vortex production and dissipation are mainly reflected by RCT and RVT. Compared with the traditional vorticity transport equation, this new theoretical tool can distinguish between rigid rotation and shearing motion of local fluids, and it can better reveal the intuitive evolution characteristics of vortical structures. Two cases with clear engineering demands are analysed by using the transport equation, and the results show that it can provide a practical method for the analysis and control of vortical flows in hydro-energy machinery. [ABSTRACT FROM AUTHOR]

A numerical study of two dimensional lid-driven triangular and trapezoidal cavity flow is performed via using the lattice Boltzmann method (LBM) for steady solutions. The equilateral and right-angled isosceles triangular cavity flow at Reynolds numbers, respectively, 500 and 100 is employed as the benchmark case for code validation. The isosceles right-angled triangular cavity flow is studied for Reynolds numbers sweeping from 100 to 8100. Flow topologies are captured and analyzed. The critical Reynolds number of Hopf bifurcation is predicted by calculating the perturbation decay rate. Two different geometries of right-angled isosceles trapezoidal cavities, bowl-shaped and pyramid-shaped trapezoids, are studied at Reynolds numbers 1000 and 7000. For each type of the trapezoidal cavity, a geometric parameter λ (top-line/base-line ratio) is presented to distinguish different geometries of trapezoidal cavities. The flow patterns regarding the streamlines, vortical structures, and velocity profiles are discussed. The impact of parameter λ on the fluid characteristics are investigated.

Tip loading is a common strategy to increase the propulsive efficiency of propellers. Solutions such as contracted and tip-loaded (CLT) and “New generation” CLT propellers exploit the presence of an endplate (“true” or as the result of a dedicated modification of the rake distribution) to sustain the increased load at the tip of the blade, at the cost of more complex vortical structures. Their evolution, and the mutual interaction of secondary vortices originated by the endplate itself, however, has not been completely and deeply investigated. The current paper addresses this topic by improved delayed detached eddy simulations (IDDES) of the flow field around two propellers of this type at different loading conditions. The presence of secondary vortices from the endplate root (or from the bended blade at tip), partially observed in recent experiments, is evidenced by high-fidelity CFD calculations. The interaction mechanism with the primary vortices (those from the endplate tip), and the resulting strengthening of the vortical structures, also through the interaction with the blade trailing vortical wake that promote the leapfrogging phenomenon, is discussed as well, comparing the phenomena in the case of two optimally designed geometries (a CLT and a New Generation CLT propeller) exploiting the same pressure side tip-loading concept in a slightly different way. Results show a rather different instability mechanism depending on the endplate configuration and open the discussion on the effectiveness of splitting a single tip vortex into pairs of vortical structures that may induce similar (or even worse) side effects in terms of pressure minima in the wake and earlier wake destabilization.

Aortic valve replacement has become an increasing concern due to the rising prevalence of aortic stenosis in an ageing population. Existing replacement options have limitations, necessitating the development of improved prosthetic aortic valves. In this study, flow characteristics during systole in a stenotic aortic valve case are compared with those downstream of two newly designed surgical bioprosthetic aortic valves (BioAVs). To do so, advanced three-dimensional fluid-structure interaction simulations are conducted and dedicated analysis methods to investigate jet flow configuration and vortex dynamics are developed. Our findings reveal that the stenotic case maintains a high jet flow eccentricity due to a fixed orifice geometry, resulting in flow separation and increased vortex stretching and tilting in the commissural low-flow regions. One BioAV design introduces non-axisymmetric leaflet motion, which reduces the maximum jet velocity and forms more vortical structures. The other BioAV design produces a fixed symmetric triangular jet shape due to non-moving leaflets and exhibits favourable vorticity attenuation, revealed by negative temporally and spatially averaged projected vortex stretching values, and significantly reduced drag. Therefore, this study highlights the benefits of custom-designed aortic valves in the context of their replacement through comprehensive and novel flow analyses. The results emphasise the importance of analysing jet flow, vortical structures, momentum balance and vorticity transport for thoroughly evaluating aortic valve performance., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

This study was aimed at examining the influence of the system rotation as an external action on the development of vortical structures and combustion. Specifically, three-dimensional direct numerical simulations of compressible mixing layers with non-premixed H 2 /air combustion were performed using a detailed chemical reaction scheme. The relationship between the developing vortical structures and chemical reactions in the flow field with the rotation was investigated. The development of combustion changed depending on the vortical structures, and the presence of roller vortices promoted the combustion phenomena. The influence of the vortical structures on the elementary reactions, which contribute to the heat release rate, was small. During the anticyclonic rotation, the roller vortices collapsed and suppressed the combustion. In contrast, the cyclonic rotation resulted in the generation of quasi-2D roller vortices, which enlarged the high-heat-release-rate regions and promoted the combustion. Overall, the vortical structures induced by the rotation can change the development of combustion even though the elementary reactions that contribute to the heat release rate remain unchanged. The presented findings can guide the prediction and control of turbulent combustion in practical situations involving fluid machinery. [ABSTRACT FROM AUTHOR]

Lateral gravity currents can play a critical role in the exchange of materials between terrestrial and marine ecosystems. The three-dimensional flow structure and mixing process of the gravity current, discharged from a lateral rectangular lock into an ambient fluid, are investigated by solving Unsteady Reynolds-averaged Navier-Stokes equations with the RNG k-ε turbulence model. The accuracy and consistency of the developed model are checked using the experimental data of the lock-exchange in the straight channel, lateral flow in the open channel and mass exchange in the cavity. The agreement between measured and simulated flow and concentration fields is reasonable. The lateral gravity current without the main channel discharge spreads radially out and is arrested by the other bank of the main channel. Before reaching the other bank, the lateral gravity current evolves into the acceleration, slumping, and inertial phases. The gravity current remains in the slumping phase at a straight channel without the tributary. The mixing layer is more diluted at the lateral gravity current. As the main channel discharge increases, the symmetry plume is broken, and the dense fluid propagation is limited toward the confluence upstream. The time-evolution of discharged spatially averaged density decreases due to increasing the main channel discharge. The mixing process assessment indicated that the entrainmant of the lateral gravity current without the main channel discharge is more intense compared with the cases having discharge. The decreasing lateral Froude number associated with increasing main channel discharge leads to a decrease in the entrainment. [ABSTRACT FROM AUTHOR]

Due to global environmental conditions, the focus of household heating has shifted from fossil fuels towards environmentally friendly and renewable energy sources. Desuperheaters have attracted attention as a domestic provision involving steam-induced direct contact condensation (DCC)to warm the water. The present study is an attempt to investigate the hydrodynamics in the desuperheater vessel experimentally, namely, when the pressurized pulsating steam is injected into the vessel, where the steam jet interacts co-currently with the slow-moving water. Flow visualization showed a circulation region when the pulsating steam was injected into the slow-moving water, and the peaked vorticity corresponded to the steam injection duration of 10–60 s. Sevenhot film anemometers (HFAs) were traversed axially and radially to determine the velocity fluctuations at 0–20 cm from the steam’s nozzle exit. Vortical structures indicated the entrainment of the steam with the surrounding moving water. The circulation regions were thus exhibited in relation to the steam’s injection durations as well as the downstream axial distances of 2 and 15 cm from the nozzle exit, which showed that the core local circulation at 2 cm downstream of the nozzle exit lost 75–79% of its circulation at 15 cm downstream of the nozzle exit.

Vortical structures of a submarine with appendages are fully turbulent and complex. Thus, flow control and vortex manipulation are of great importance for the hydrodynamic performance and acoustic characteristics. Take the generic submarine model DARPA Suboff as the test case, a vortex tuning method based on the Liutex force field is proposed to manipulate the vorticity field. Viscous flow past the submarine model in straight-line motion at a Reynolds number of 1.2×107 is achieved by solving the Reynolds averaged Navier-Stokes (RANS) equations. Multi-block structured mesh topology is used to discretize the computational domain, and the shear stress transport (SST) k - ω turbulence model is implemented to close the equations. The control of vortex is achieved by introducing additional source terms based on Liutex vortex definition and identification system to the RANS equations. The resistance acting on the submarine, flow field as well as the vortical structures are compared and analyzed. Results show that Liutex force model can effectively reduce the resistance by 9.31% and change the vortical structures apparently. [ABSTRACT FROM AUTHOR]

Film cooling is widely applied on endwalls in high-performance gas turbines to guarantee the safety and endurance of the engines. Micro ramps are employed to optimize the flow and thermal performance in internal and external cooling designs. In the current investigation, flat-surface film cooling at the blowing ratio of 1.0 and the density ratio of 2.0 is investigated with and without the micro ramp by LES. The result shows that the micro ramp effectively enhances the cooling performance by creating the ACRVP, drawing the coolant toward the wall and enlarging the film coverage. Mean and instantaneous vortical structures are analyzed. Endwall film cooling with micro ramps is studied with high and low mass flow ratios (MFR). The cooling effectiveness of the micro ramp case is 89.9% higher than the baseline case when MFR is high, while 16.3% higher than the baseline when MFR is low. Micro ramps are more effective for endwall film cooling with high MFR. The cooling effectiveness of the region in the middle of the passage improves when applying the micro ramps. In the region upstream of the leading edge, cooling performance deteriorates due to the interaction of the horseshoe vortex and the micro ramps. [ABSTRACT FROM AUTHOR]

Embayments, as major storage zones in riverine environments, could be surrounded by gravity currents associated with industrial pollution, heavy impurities, and sewage. The accumulated contaminated matter in embayments significantly impairs aquatic habitats and influences the embayment performance. In this study, the three-dimensional vortical structures of continuous and discontinuous gravity currents in channels connected to an embayment are investigated by solving unsteady Reynolds-averaged Navier-Stokes with algebraic Reynolds stress model (ASM) which has been improved by the buoyancy effects. The lateral embayment with different configurations is simulated to investigate the exchange processes of the dense fluid between the lateral embayment and main stream. The accuracy and consistency of the developed model are checked using the experimental data of continuous and discontinuous gravity currents in the straight channel and mass exchange in the lateral embayment. The findings have revealed that the agreement between measured and simulated flow and concentration fields is reasonable. The current head propagation of the continuous gravity current inside the embayment is similar to the discontinuous gravity current head propagation. At early time, the continuous gravity current leads to rapid diffusion of the concentration in the embayment and main channel downstream. In the discontinuous gravity current with the embayment aspect ratio equal to 1.0, the dense fluid firstly fills the lateral embayment as the most volume of the dense fluid is trapped and then it flushes out into the main channel. The geometric aspect ratio of the lateral cavity slightly affects the exchange coefficient between the main channel and lateral cavity. The time-averaged exchange coefficient is 0.33 in the continuous gravity current. [ABSTRACT FROM AUTHOR]

Vortical structures and wakes of bluff bodies in homogenous and stratified environment are common and important in ocean engineering. Based on the Boussinesq approximation, a thermocline model is proposed to deal with the variable density stratified fluid, and implemented in the commercial software Simcenter STAR-CCM+ framework. The improved delayed detached eddy simulation (IDDES) modeling method is adopted to resolve the coherent vortical structures and turbulent wakes precisely and efficiently. Four conditions consisting of one homogenous and three stratified fluid cases with different density gradient past a sphere at Reynolds number 3 700 are investigated. Results show that density stratification has a great impact on the vortical structures, the vertical motion is suppressed and internal waves will be induced and propagated, which is very different with that of homogenous situation. With the stratification strength increases, the vortical structures are gradually flattened, the asymmetry and anisotropy between vertical and horizontal motions are enhanced. [ABSTRACT FROM AUTHOR]

The culture medium in many fermentations is a non-Newtonian fluid. In bacterial alginate batch production, the broth becomes more pseudoplastic as the alginate concentration increases, which impairs the mixing process. This work characterizes the effect of the interaction between changing broth rheology and impeller mixing on a bioreactor fluid dynamics. Experimentally, a fermentation with evolving broth pseudoplastic rheology is reproduced. Three fermentation stages are mimicked using appropriate solutions of water and xanthan gum. Impeller torque measurements are reported. The weakening of the impellers’ interaction over the fermentation process is identified. To overcome the experimental limitations, CFD is applied to study the evolution of the fermentation fluid flow patterns, velocity field, dead zones, and vortical structures. Precessional vortex macro-instabilities are identified as being responsible for the unstable flow patterns identified at the earlier stages of the fermentation. A stable parallel flow pattern accounts for the weakest impellers’ interaction at the final stage. Overall, this work contributes with a complete workflow to adapt CFD models for characterization and aided design of stirred tanks with changing broth pseudoplastic rheology as well as an evolving flow regime.

Crude oil transport is an essential task in oil and gas industries, where centrifugal pumps are extensively used. The design of a centrifugal pump involves a number of independent parameters which affect the pump performance. Altering some of the parameters within a realistic range improves pump performance and saves a significant amount of energy. The present research investigated the pump characteristics by modifying the number of blades and the exit blade-angles. Reynolds-Averaged Navier-Stokes equations with standard k-ε two-equation turbulence closure were used for steady and incompressible flow of crude oil through the pump. The experimental set-up was installed and the pump performance calculated numerically was compared with the experiments. The investigations showed that the number of blades and the exit blade-angles have a significant influence on the head, shaft power, and efficiency. The vortical flow structures, recirculation and reverse flow characteristics around the impeller were investigated to explain the flow dynamics of impeller and casing. A larger number of blades on the rotor showed dominant streamlined flow without any wake phenomena. The combined effect of the number of blades and exit blade angle has led to an increase in head and efficiency through the parametric optimization.

The impact of aerodynamics on vehicle safety during crossing of passenger cars is investigated, in the absence and presence of 30 ° crosswind. Three-dimensional, unsteady computational fluid dynamics (CFD) simulations were used to simulate these maneuvers. The vortical structures surrounding one car in the case without crosswind were analyzed, establishing the connection between force and moment fluctuations pre-interaction and the shedding frequency of these vortices. The forces and moments acting on a passenger car during a crossing maneuver may change by up to 43 %, with the maximum change associated with the windward car in the presence of crosswind. However, the duration of this increase in forces is at most 0.01 s, which will not affect the stability of vehicles under normal conditions. The presence of crosswind increased the rate of fluctuation of forces and moments. Wind tunnel experimental results are in good agreement with the simulations, and the data available in literature. The analysis results do not show the necessity of enacting new safety policies on highways, but future parametric studies are needed to fully investigate the impact of different crosswind speeds and directions, the impact of discrepancy in vehicles sizes, and different vehicle lateral separating distances during crossing and overtaking. [ABSTRACT FROM AUTHOR]

Abstract Turbulent flow over the DrivAer fastback vehicle model is investigated using large-eddy simulation with particular emphasis on flow separation, vortical structures and unsteady quantities. A systematic and detailed analysis of the flow field is made considering rotating wheels and moving ground floor. Overall features of vortical structures at the cowl top, behind side mirrors, near front and back wheels, at A-, B- and C-pillars and behind the rear end of the vehicle are revealed by investigating velocity and vorticity fields. The rear end is identified to be the main contributor to the pressure force acting on the vehicle, followed by back and front wheels and side mirrors. The resulting pressure force on the upper part of the vehicle, including A-, B-, and C-pillars but excluding the cowl top and side mirrors, is found to be only slightly higher than the contribution of the gap in the cowl top. Flow separation and resulting vortices do not only have an impact on automotive drag, it is also pointed out how unsteadiness in the flow field affects pressure fluctuations. High levels of surface pressure fluctuations are found near side mirrors and front wheels. Similarities in distributions of pressure fluctuations and turbulent kinetic energy are found. Highlights • Turbulent flow over the DrivAer fastback vehicle model is investigated in detail using large-eddy simulation. • Flow separation and resulting vortices at various regions and their impact on automotive drag have been identified. • High levels of surface pressure fluctuations are found near side mirrors and front wheels. [ABSTRACT FROM AUTHOR]

Highlights • PIV measurements of the gas flow above waves are analyzed. • Conditional averaging enables the separation of wave-coherent flow from the full flow field. • At high gas flow rates, vortical structures are shed from the crest and populate the detached shear layer. • Below the detached shear layer is a region populated by opposite signed vortices. The separation between these two regions coincides with the critical height. Abstract We investigate the effect of waves on the airflow in horizontal two-phase pipe flow. Velocity fields in the gaseous phase were acquired by particle image velocimetry (PIV), while interfacial elevation was measured with conductance wave probes. The velocity fields were sampled on a wave-following coordinate system which allows for a decomposition of the velocity field into a mean, wave-coherent and fluctuating component by means of a three-component Reynolds decomposition. Additionally, coherent vortical structures were identified by the swirling strength criterion, and their distribution along the waves is investigated. Results suggest that the interaction between turbulent airflow and propagating waves in a pipe has a number of features reminiscent of wind-wave interaction in open systems. Above waves generated by sufficiently high gas flow rates, there is a distinct region of sheltered airflow, and a lifting of the critical layer on the leeward side of the crest. Streamlines of the phase-averaged flow field show a cat's eye structure located close to the crest in this region. Above waves of moderate steepness, we observe a shear layer that remains adjacent to the wave surface. Above steeper waves and higher gas flow rate, this layer detaches from the surface just downstream of the crest. Shear layer separation above waves is traditionally linked to the onset of wave breaking, and it is interesting to note that the case where we observe a separated shear layer in the phase-averaged vorticity field is in a regime of amplitude saturation. The swirling strength criterion reveals that vortical structures are shed from the interface and populate the detached shear layer above the trough. Below the detached shear layer, there is a region populated by counter-rotating vortices. The critical height coincides with the border between these two regions. [ABSTRACT FROM AUTHOR]

Significant environmental effects from the use of marine outfall discharges have led to increased efforts by both regulatory bodies and research groups to minimize the negative impacts of discharges on the receiving water bodies. Understanding the characteristics of discharges under conditions representative of marine environments can enhance the management of discharges and mitigate the adverse impacts to marine biota. Thus, special attention should be given to ambient cross-flow effects on the mixing behaviors of jet discharges. A buoyant jet in cross-flow has different practical applications such as film cooling and dilution, and provide a higher mixing capability in comparison with free jets or discharges into stationary environments. The main reason for this is believed to be the existence of various complicated vortical structures including a counter-rotating vortex pair as the jet expands downstream. Although tremendous research efforts have been devoted to buoyant jets issuing into cross-flows over the past five decades, the mixing process of an effluent at the discharge point is not yet well understood because of the highly complex fluid interactions and dispersion patterns involved. Therefore, there is a need for a deeper understanding of buoyant jets in cross-flows in order to obtain better predictive methods and more accurate design guidelines. The main aims of this study were (i) to establish the background behind the subject of buoyant jets in cross-flows including the flow structures resulting from the interaction of jets and cross-flows and the impacts of current on mixing and transport behavior; (ii) to present a summary of relevant experimental and numerical research efforts; and finally, (iii) to identify and discuss research gaps and future research directions.

An experimental investigation of detailed flow field measurement in a rectangular duct with truncated prismatic ribs mounted on the bottom wall has been carried out using 2-dimensional particle image velocimetry (2-D PIV) technique. The truncated prismatic ribs have been manufactured by tapering the square rib from both the sides up to the center to provide rib height at the ends ( h ) of 0–8 mm with an increment of 2 mm. The PIV measurements have been performed in vertical as well as in horizontal streamwise planes for different Reynolds numbers and a fixed rib pitch to height ratio of 12. In addition, a number of PIV measurements in vertical symmetry plane have also been performed while covering the streamwise region of 46 ≤ x / e ≤ 120 to observe the nature of the flow in the test section and the flow periodicity. Overall, the emphasis is to investigate the flow structures and their dynamics in the inter-rib region in terms of time-averaged streamlines, mean velocities, vorticity fields, instantaneous velocity vector and corresponding vorticity fields, and fluctuation statistics in both the measurement planes. The close-up inspection based on critical point theory shows the significant evidence of salient critical points and limit cycles such as saddle point, stable and unstable node, bifurcation line and stable and unstable focus in both just upstream and downstream rib regions. The critical flow patterns around the rib confirm three-dimensionality and unsteady nature of the flow. The instantaneous flow characteristics reveal the presence of large-scale coherent vortices in the separated shear layer region, the existence of periodical processes such as intermittent ejection of the recirculation zone into the main flow and motion of the reattachment point. Significant reduction in the size of primary recirculation bubble and thereby reduction in reattachment length is observed for truncated prismatic ribs as compared to square rib in spanwise direction. [ABSTRACT FROM AUTHOR]

Crude oil transport is an essential task in oil and gas industries, where centrifugal pumps are extensively used. The design of a centrifugal pump involves a number of independent parameters which affect the pump performance. Altering some of the parameters within a realistic range improves pump performance and saves a significant amount of energy. The present research investigated the pump characteristics by modifying the number of blades and the exit blade-angles. Reynolds-Averaged Navier-Stokes equations with standard k-ε two-equation turbulence closure were used for steady and incompressible flow of crude oil through the pump. The experimental set-up was installed and the pump performance calculated numerically was compared with the experiments. The investigations showed that the number of blades and the exit blade-angles have a significant influence on the head, shaft power, and efficiency. The vortical flow structures, recirculation and reverse flow characteristics around the impeller were investigated to explain the flow dynamics of impeller and casing. A larger number of blades on the rotor showed dominant streamlined flow without any wake phenomena. The combined effect of the number of blades and exit blade angle has led to an increase in head and efficiency through the parametric optimization. [ABSTRACT FROM AUTHOR]

The two-phase turbulent flow past an interface-piercing circular cylinder is studied using a high-fidelity orthogonal curvilinear grid solver with a Lagrangian dynamic subgrid-scale model for large-eddy simulation and a coupled level set and volume of fluid method for air-water interface tracking. The simulations cover the sub-critical and critical and post critical regimes of the Reynolds and sub and super-critical Froude numbers in order to investigate the effect of both dimensionless parameters on the flow. Significant changes in flow features near the air-water interface were observed as the Reynolds number was increased from the sub-critical to the critical regime. The interface makes the separation point near the interface much delayed for all Reynolds numbers. The separation region at intermediate depths is remarkably reduced for the critical Reynolds number regime. The deep flow resembles the single-phase turbulent flow past a circular cylinder, but includes the effect of the free-surface and the limited span length for sub-critical Reynolds numbers. At different Froude numbers, the air-water interface exhibits significantly changed structures, including breaking bow waves with splashes and bubbles at high Froude numbers. Instantaneous and mean flow features such as interface structures, vortex shedding, Reynolds stresses, and vorticity transport are also analyzed. The results are compared with reference experimental data available in the literature. The deep flow is also compared with the single-phase turbulent flow past a circular cylinder in the similar ranges of Reynolds numbers. Discussion is provided concerning the limitations of the current simulations and available experimental data along with future research.

The two-phase turbulent flow past an interface-piercing circular cylinder is studied using a high-fidelity orthogonal curvilinear grid solver with a Lagrangian dynamic subgrid-scale model for large-eddy simulation and a coupled level set and volume of fluid method for air-water interface tracking. The simulations cover the sub-critical and critical and post critical regimes of the Reynolds and sub and super-critical Froude numbers in order to investigate the effect of both dimensionless parameters on the flow. Significant changes in flow features near the air-water interface were observed as the Reynolds number was increased from the sub-critical to the critical regime. The interface makes the separation point near the interface much delayed for all Reynolds numbers. The separation region at intermediate depths is remarkably reduced for the critical Reynolds number regime. The deep flow resembles the single-phase turbulent flow past a circular cylinder, but includes the effect of the free-surface and the limited span length for sub-critical Reynolds numbers. At different Froude numbers, the air-water interface exhibits significantly changed structures, including breaking bow waves with splashes and bubbles at high Froude numbers. Instantaneous and mean flow features such as interface structures, vortex shedding, Reynolds stresses, and vorticity transport are also analyzed. The results are compared with reference experimental data available in the literature. The deep flow is also compared with the single-phase turbulent flow past a circular cylinder in the similar ranges of Reynolds numbers. Discussion is provided concerning the limitations of the current simulations and available experimental data along with future research