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The correlation of flow structure, displacement efficiency and relative permeability for two-phase flow in variable-aperture fractures is critical for many fractured rock applications. A systematic visualization method for two-phase flow experiments was developed and four transparent variable-aperture fracture samples were duplicated by epoxy casting method. A total of 112 times two-phase flow experiments of nitrogen displacing water under various gas flow rates were performed and the corresponding results of images and pressure data during flow process were recorded and analyzed based on optical principles. On the basis of the competition between capillary resistance and inertia force of gas, the flow structure can be generally divided into four states including capillary resistance dominant (CR) state with few flow paths in two lateral sides without fingering flow, competitive (CP) state with the occurrence of clear fingering flow phenomenon, inertia force of gas dominant (IF) state showing more chaotic gas distributions, and steady (SD) state that has few changes in flow paths. The growth of displacement efficiency is greatly affected by the four transition states, in which the increment of displacement efficiency in Frac-1 that has gone through the three states of flow structure approaches 30%, and that of Frac-2 and Frac-3 are about 15%, while Frac-4 with little change of flow structure is less than 10%. As a consequence, the relationship between saturation and relative permeability of gas becomes steeper from Frac-1 to Frac-4 with greater deviation from X, V–C and Corey models due to the smaller variation range of gas saturation, and an improved v -type model is obtained where the exponent n shows a positive correlation to fracture mean aperture. [ABSTRACT FROM AUTHOR]

Understanding the mechanisms of leakage flow control and heat load distribution is important in optimizing the structure of cavity tips in gas turbines. This paper investigates the aerothermal performance of cavity tip in a transonic high-pressure turbine stage. By analyzing flow structure, the effects of squealer height, squealer width and gap height on aerothermal performance are explained. It has been found that both the leakage flow and heat load distribution are influenced by the structure of vortices in the cavity. The leakage flow is controlled by the suction-side cavity vortex and the scraping vortex. The heat load distribution relates to the "M-shaped" leakage flow induced by the scraping vortex. The geometric parameters can affect the structure of vortices, which then impacts the aerothermal performance of cavity tip. As the squealer height is 2.5 τ 0 , both the leakage flow rate and average heat transfer coefficient decrease by 0.64 % and 6.6 %, respectively. A multi-objective optimization with a feedforward neural network prediction is conducted to determine the distribution characteristics of optimal design parameters. The optimal values are approximately 2.34 for the expansion ratio, 0.5 τ 0 for the squealer width and 0.5 τ 0 for the gap height. The optimal squealer height ranges from 1.5 τ 0 to 2.5 τ 0. • Effects of flow structure on the aerothermal performance of cavity tip are studied. • A multi-objective optimization determines the characteristics of optimal parameters. • The leakage flow is controlled by suction-side cavity vortex and scraping vortex. • The heat load distribution relates to the "M-shaped" leakage flow. [ABSTRACT FROM AUTHOR]

• The residual circulation of the front bend would affect the flow characteristics in the back bend. • The critical length of the TS to fully decay the residual circulation of the front bend is 4.8 B. • The central bar in the transition tends to accelerate the decay of residual circulation of the front bend. The transition section (TS), linking the front bend and the back bend in consecutive river bends, plays a key role in the flow hydraulic characteristics, sediment transport, and riverbed evolution in meandering rivers, which is crucial for flood control, navigation, and river stability. Most existing studies on the hydraulic effect of the TS focus on flat-bed scenarios, rendering an inadequate understanding of the TS in consecutive bends with point bars, often found in natural rivers. Based on three-dimensional numerical simulations, the effects of the length and the topography of the TS are investigated in detail on the flow structure in consecutive bends with point bars, respectively. The results reveal that without an adequate TS length, the residual circulation of the front bend (RC) inhibits the development of the main circulation in the back bend (MC), causing the positions of the maximum strength of the MC and the maximum value of cross-section-averaged turbulent kinetic energy (< t k e ˜ >) in the back bend moving upstream. As the length of the TS increases, the above effect of the front bend gradually decreases. The critical length of the TS to fully decay the RC is 4.8 times the bend width (B) in consecutive bends with point bars, longer than that (4 B) in the flat-bed bend. Furthermore, the central bar in the TS is found to facilitate the generation of the outer bank cell and accelerate the decay of the RC. These effects of the TS can profoundly change the flow hydraulics, sedimentation, and stream stability of consecutive bends. The results of the study will help to further understand the linkage role of the TS in consecutive bends and may serve as a rational reference for managing natural meandering rivers with multiple hydrological, geomorphological, and/or ecological goals. [ABSTRACT FROM AUTHOR]

To reveal how buoyancy affects heat transfer of supercritical carbon dioxide (sCO 2), the flow and heat transfer characteristics of sCO 2 in circular tubes with an inner diameter of 19 mm at different inclination angles were numerically investigated. The calculation results of RNG k − ϵ , RKE k − ϵ , SST k − ω and SST k − ω using a variable turbulent Prandtl number model (TWL model) have been compared with experimental data. The TWL model is chosen for numerical calculations because of its significant advantage in predicting wall temperature. Both forced convection (Richardson number Ri < 0.1, weak buoyancy) and mixed convection (Ri > 0.1, strong buoyancy) of sCO 2 are calculated. Result shows that the position of the velocity peak moves upward, then downward as the flow changes from vertical upward (inclination angle α = 0°) to horizontal flow (α = 90°) due to the secondary flow caused by radial buoyancy and the increase in velocity magnitude in the near top wall region caused by axial buoyancy. Under strong buoyancy effects, as α increases from 0° to 90°, the heat transfer deterioration (HTD) becomes severe when α < 15°, then alleviated and finally severe again when α approaches 90° due to the change of the heat transfer mechanism. This work deepens our understanding of sCO 2 heat transfer and may provide reference for the design of heat exchangers in nuclear reactors. • Flow structure and heat transfer of sCO 2 in inclined tubes were investigated. • The variable Pr t model used could accurately predict heat transfer deterioration. • The mechanisms of buoyancy affecting flow structure and heat transfer were discussed. • A slight inclination weakened heat transfer for upward flow with strong buoyancy. • A slight inclination enhanced heat transfer for horizontal flow with strong buoyancy. [ABSTRACT FROM AUTHOR]

Most of the numerical studies adopted dry gas model to predict transonic and supersonic flow in steam ejectors, but it cannot accurately reflect flow characteristics in ejectors, since the condensation and evaporation of wet steam is ignored. The current work develops and validates a CFD model describing spontaneous condensation to simulate complicated flow behaviors and phase transition inside an ejector, and illustrates the impact of initial state of primary fluid including wet, saturated and superheated state on flow characteristics, mass and heat exchange and ejector performance. It is found that the initial steam wetness obviously impacts the average liquid fraction in the ejector. Both the superheated and wet initial primary steam suppress the process of mass and heat exchange during nucleation, and the position and intensity of nucleation are influenced by the inlet steam state. Compared with the case of dry saturated inlet steam, the superheated inlet steam improves entrainment ratio by 3.7%, and decreases compression ratio by 2.5%. • A wet steam model with real gas effect considering condensation is established. • Inlet steam state affects the position and intensity of nucleation in the nozzle. • The rise of initial unsaturated degree suppresses heat and mass transfer process. • The existence of initial liquid phase improves the compression ratio. [ABSTRACT FROM AUTHOR]

• The liquid metal cross flow tube bundle under pulsation condition is simulated. • The effect of different pulsation frequency and amplitude are analyzed. • Circumferential flow field and temperature field of the single tube are analyzed. • The vortex characteristics and turbulent structure are analyzed with PSD and CWT. 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. [ABSTRACT FROM AUTHOR]

In this paper, the characteristics of hot spots on an accretor surface are investigated for two types of polars: the eclipsing synchronous polar V808 Aur and the non-eclipsing asynchronous polar CD Ind in configuration of an offset and non-offset magnetic dipole. The drift of hot spots is analyzed based on the results of numerical calculations and maps of the temperature distribution over the accretor surface. It is shown that a noticeable displacement of the spots is determined by the ratio of ballistic and magnetic parts of the jet trajectory. In the synchronous polar, the dominant influence on the drift of hot spots is exerted by variations in the mass transfer rate, which entail a change in the ballistic part of the trajectory. It was found that when the mass transfer rate changes within the range of 10−10M⊙/year to 10−7M⊙/year, the displacement of the hot spot in latitude and longitude can reach 30∘. In the asynchronous polar, a change in the position of hot spots is mainly defined by the properties of the white dwarf magnetosphere, and the displacement of hot spots in latitude and longitude can reach 20∘.

Investigations of combustors and turbines separately have been carried out for years by research institutes and aircraft engine companies, but there are still many questions about the interaction effect. In this paper, a prediction of a turbine stator’s potential effect on flow in a combustor and the clocking effect on temperature distribution in a nozzle guide vane are discussed. Numerical simulation results for the combustor simulator and the nozzle guide vane (NGV) of the first turbine stage are presented. The geometry and flow conditions were defined according to measurements carried out on a test section within the framework of the EU FACTOR (full aerothermal combustor–turbine interactions research) project. The numerical model was validated by a comparison of results against experimental data in the plane at a combustor outlet. Two turbulence models were employed: the Spalart–Allmaras and Explicit Algebraic Reynolds Stress models. It was shown that the NGV potential effect on flow distribution at the combustor–turbine interface located at 42.5% of the axial chord is weak. The clocking effect due to the azimuthal position of guide vanes downstream of the swirlers strongly affects the temperature and flow conditions in a stator cascade.

Flow over shallow cavities is used to model the flow field and heat transfer in a solar collector and a variety of engineering applications. Many studies have been conducted to demonstrate the effect of cavity aspect ratio (AR), but very few studies have been carried out to investigate the effect of cavity height ratio (HR) on shallow cavity flow behavior. In this paper, flow field structure and heat transfer within the 3-D shallow cavity are obtained numerically for two height ratio categories: HR = 0.0, 0.25, 0.5, 0.75, and 1.0 and HR = 1.25, 1.5, 1.75, 2.0, 2.25, and 2.5. The governing equations, continuity, momentum, and energy are solved numerically and using the standard (K-ε) turbulence model. ANSYS FLUENT 14 CFD code is used to perform the numerical simulation based on the finite volume method. In this study, the cavity aspect ratio, AR = 5.0, and Reynolds number, Re = 3 × 105, parameters are fixed. The cavity’s bottom wall is heated with a constant and uniform heat flux (q = 740 W/m2), while the other walls are assumed to be adiabatic. For the current Reynolds number and cavity geometry, a single vortex structure (recirculation region) is formed and occupies most of the cavity volume. The shape and location of the vortex differ according to the height ratio. A reverse velocity profile across the recirculation region near the cavity’s bottom wall is shown at all cavity height ratios. Streamlines and temperature contours on the plane of symmetry and cavity bottom wall are displayed. Local static pressure coefficient and Nusselt number profiles are obtained along the cavity’s bottom wall, and the average Nusselt number for various height ratios is established. The cavity height ratio (HR) is an important geometry parameter in shallow cavities, and it plays a significant role in the cavity flow behavior and heat transfer characteristics. The results indicate interesting flow dynamics based on height ratio (HR), which includes a minimal value in average Nusselt number for HR ≈ 1.75 and spatial transitions in local Nusselt number distribution along the bottom wall for different HRs.

The flow over a backward-facing step (BFS) is a typical separation and reattachment flow. Its flow structures and unsteady mechanisms are still not well explored. In this paper, the global velocity fields of a BFS are obtained by a synchronous particle image velocimetry (PIV) system with Reh = 5 345 (Reynolds number) and Er = 2 (expansion ratio). Flow structures are distinguished and defined by the fraction of the negative velocity (upn). The reattachment zone (Lr) is quantitatively defined as (u0.9, u0.1) on the bottom wall. Spatial distribution of the large-scale vortices couples well with the divided flow structures and their temporal evolution presenting four stages (forming, developing, shedding and redeveloping) when travel downstream. The unsteady motions with various low frequencies are well explained by the coherent vortices and flow structures. Among the unsteady low frequency motions, the Kelvin Helmholtz (KH) vortices and the oscillation of Xr (OX) come likely from the free shear layer. The KH vortices contribute to the unsteadiness of the temporal flow, and the OX is the primary response to the vortical fluctuations. [ABSTRACT FROM AUTHOR]

The paper presents velocity fields with ~3‐km spatial resolution of Saturn's north polar vortex (NPV) retrieved using the optical flow method from a sequence of polar‐projected cloud images captured by the Imaging Science Subsystem camera on board NASA's Cassini spacecraft. The fields of the velocity magnitude, velocity variation, relative vorticity, divergence, and second invariant are determined to characterize the flow structures of the inner core of the NPV. The mean zonal and mean meridional velocity profiles of the NPV are compared with previous measurements. We also describe the relevant details of application of the optical flow method to planetary cloud‐tracking wind measurements. The mean zonal velocity profile is consistent with the previous measurements using correlation image velocimetry methods. The small but significant meridional velocity corresponds to outwardly spiraling streams observed in the region near the north pole (NP). The concentrated vorticity and second invariant within 1° planetographic latitude of the NP indicate strong rotational motion of the fluid. An analysis is presented to explore a possible physical origin of the observed spiraling node at the NP. Plain Language Summary: A swirling flow pattern with wind speeds peaking at about 100 m/s was revealed in Saturn's north polar vortex in high‐resolution images captured by the Imaging Science Subsystem camera on board NASA's Cassini spacecraft in November 2012. Using sequences of images that show clouds in the north polar vortex, the motions of these clouds were analyzed to measure the wind speeds in the north polar region. The high‐precision wind measurements presented in the current report are enabled by the optical flow cloud‐tracking method. The time‐averaged wind field shows a well‐defined counterclockwise (cyclonic) vortex at the pole. In particular, the observed flow structures and wind shear near the pole indicate strong rotational motion of the north polar atmosphere with upwelling at the center. Key Points: The high‐resolution velocity fields of Saturn's north polar vortex (NPV) are extracted from cloud images by using the optical flow methodThe vorticity, divergence, and second invariant are obtained to characterize the flow structures of the NPVThe strong rotational motion of the fluid with upwelling is found near the pole [ABSTRACT FROM AUTHOR]

With the escalating production of automobiles, energy efficiency and environmental friendliness have always been a major concern in the automotive industry. In order to effectively lower the energy consumption of a vehicle, it is essential to develop air-conditioning systems that can make good use of combustion waste heat. Ejector refrigeration systems have become increasingly popular for this purpose due to their energy efficiency and ability to recycle waste heat. In this article, the elements affecting the performance of a typical ejector refrigeration system have been explored using both experimental and numerical approaches. For the first time, the internal flow structure was characterized by means of comprehensive numerical simulations. In essence, three major sections of the steam ejector were investigated. Two energy processes and the shock-mixing layer were defined and analyzed. The results indicated that the length of the choking zone directly affects the entertainment ratio under different primary fluid temperature. The optimum enterainment ratio was achieved with 138 °C primary fluid temperature. The shock-mixing layer was greatly affected by secondary fluid temperature. With increasing of back pressure, the normal shock gradually shifted from the diffuser towards the throat, while the shock train length remains lunchanged. [ABSTRACT FROM AUTHOR]

Shock wave boundary layer interaction prefers to induce boundary layer separation, and it's necessary to analyze the flow structure, local high pressures and heat flux, and the separation bubble size. In this paper, we propose an analytical model for this typical flow structure, and the polar theory is used for auxiliary analysis. Based on the FIT. classical shock relationship, and fining formula of separation length, the analytical model can be solved. With the analytical model, we can quickly gain the flow structure and parameters which compares well with numerical simulation. Also, we use this model to analyze the height of separation bubble. The height increases with increasing flight altitude and deflection angle for incident shock wave, and it first decreases slightly then increases as incoming Mach number increases. [ABSTRACT FROM AUTHOR]

East and west artificial islands are used to realize the transition from bridge to tunnel for Hong Kong-Zhuhai- Macao Cross-sea Tunnel. Immersed tunneling method is applied in sea tunnel and the total length of the sinking section is 5 664 m. The tube sections of El—E33 are arranged from west artificial island. The standard pipe section is 180 m long, 37.95 m wide, 11.4 m high and weighs about 80 000 tons. The connection area of the island and tunnel is affected by the deflecting flow1 and the sudden change of the water depth during the foundation trough excavation, and the flow pattern is complicated. Moreover, the flow pattern will be changed when the sinking joint is subsided. It is necessary to obtain the magnitude of water flow force to guarantee the safe sinking and accurate docking of the immersed tube. By using the undistorted model in a wide flume with the geometric scale of 1: 100, the experimental study on the flow distribution and water flow force during the sinking process of the island tunnel is carried out. The results indicate that, when there is no protective measure at the head of the island, the deflecting flow would affect the middle part of tube El and the flow velocities on the surface and at the bottom layers are larger than that of the middle layer. When sinking tube EI, the maximum longitudinal flow force can reach 4 601 kN, and the averaged longitudinal flow force of tube E2 is 5 149 kN. After sheltering tube El, the flow velocity at tube El decreases significantly. The middle part of tube E2 is mostly affected by the sheltering body, the maximum increment in the flow velocity reaches to 40%, and the flow velocity of the middle layer is larger than those on the surface and at the bottom layers. The average longitudinal flow force of tube E2 is 5 240 kN. When the sinking tube is completely immersed into the water, the flow velocity in the foundation trench is relatively small, and the water flow force during the sinking process decreases gradually. The vertical distribution of water flow in the connection area of island and tunnel is not exponential one, and the velocity distribution along the length of the pipe section is not uniform due to the influence of deflecting flow on the island head wall. The formula of floating water flow force at the pipe section is not suitable for calculating the magnitude of the water flow force during the sinking process of immersed pipe. [ABSTRACT FROM AUTHOR]

The critical state and steady state theories are widely accepted for simulating the soil behavior and providing a basis for liquefaction evaluation and constitutive modeling. However, there was conflicting evidence regarding the critical state and steady state of sands in the last decades. In some cases, the critical state and steady state are taken to be identical and in others they differ significantly. Whether they are the same or not remain a hot academic debate to date. This paper provides a critical review on the origin and history of the critical state and steady state of sands, which identifies conflicting evidences and discusses the influencing factors on the existence and uniqueness of critical state and steady state. Based on a critical review of literature, it is concluded that the steady state concept is a refinement of the critical state concept and the uniqueness of the critical state and steady state was affirmed. Special care should be paid in the laboratory techniques in order to accurately measure the critical state and steady state due to the numerous influencing factors of them. [ABSTRACT FROM AUTHOR]

Purpose: From the correlation between the blood flow dynamics and wall dynamics in the left ventriocle (LV) analyzed using echo-dynamography, the ejection mechanisms and role of the intra-ventricular vortex in the LV were elucidated in detail during the pre-ejection transitional period (pre-ETP), the very short period preceding LV ejection. Methods: The study included 10 healthy volunteers. Flow structure was analyzed using echo-dynamography, and LV wall dynamics were measured using both high-frame-rate two-dimensional echocardiography and a phase difference tracking method we developed. Results: A large accelerated vortex occurred at the central basal area of the LV during this period. The main flow axis velocity line of the LV showed a linearly increasing pattern. The slope of the velocity pattern reflected the deformity of the flow route induced by LV contraction during the pre-ETP. The centrifugal force of the vortex at its junction with the main outflow created a stepwise increase of about 50% of the ejection velocity. Conclusion: Ejection of blood from the LV was accomplished by the extruding action of the ventricular wall and the centrifugal force of the accelerated vortex during this period. During ejection, acceralated outflow was considered to create a spiral flow in the aorta with help from the spherical structure of the Valsalva sinus. [ABSTRACT FROM AUTHOR]

When a composite bank fails,slump blocks deposit at the toe of the bank. This alters the flow structure near the bank and affects the bank erosion processes. To investigate the effects of slump block at different locations on the flow structure near the outer bank,a generalized model of Shishou reach was conducted. Acoustic Doppler Velocimeters (ADV) was used to precisely measure the three-dimensional flow velocities. Experimental results show that the slump block upstream of the head-on scouring point shifts the region of maximum streamwise velocity away from the outer bank and reduces the averaged bed shear stress between the slump block and the bank toe. In contrast,the slump block downstream of the head-on scouring point makes the region of maximum streamwise velocity closer to the outer bank and increases the averaged bed shear stress between the slump block and the bank toe. This study enriches the understanding of composite bank erosion process,and helps to develop and improve the erosion model of composite bank. [ABSTRACT FROM AUTHOR]

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A mathematical model of mass transfer in an atmospheric bubbling deaerator storage tank at thermal power stations (TPS) to calculate the removal efficiency of dissolved gases from water is considered. For mathematical simulation and calculation of the deaerator efficiency, a system of differential equations of mass and heat transfer is written in a local form for the liquid phase of the layer with volume interphase sources of mass and heat. Mass transfer of the dissolved gases from water to the vapor phase is taken into account using volume interphase sources and the balance equations. Parameters of the interphase sources and correlations for calculating the eddy viscosity of the liquid phase depending on bubbling conditions are given. Results from the system of equations' numerical solution are presented and compared with the available experimental data on the final concentration of dissolved oxygen at the deaerator outlet. Curves of the efficiency of dissolved oxygen removal depending on the operating conditions and design characteristics of the bubbling process are presented, and scientific-and-engineering solutions have been developed to upgrade the bubbling storage tank. It has been found that an increase in gas velocity sharply raises the deaeration efficiency to a certain value (depending on the specified initial parameters), and the efficiency growth is then retarded. Similarly, increasing the length of a bubbling device improves the efficiency in removal of dissolved oxygen from water. One of the causes of a decrease in the water deaeration efficiency is likely to be back mixing of the flow. Upgrading of the deaerator tank is in the installation of perforated baffles across the flow to considerably reduce back mixing of the liquid phase. This increases the driving force of mass transfer, and its efficiency improves by five to 20% depending on the steam velocity. The presented mathematical model can be generalized to a wide range of bubbling apparatuses with a high two-phase layer. [ABSTRACT FROM AUTHOR]

Abstract The ultra-small axial flow hydraulic turbine is a portable, palm-sized hydraulic turbine that can be applied to low head applications. By redesigning the original runner of this turbine using a method which uniformized the axial flow velocity, we succeeded in improving the performance of the runner. However, the mechanism of the improved performance of the optimized runner was not sufficiently clear; furthermore, the internal flow of the ultra-small axial hydraulic turbine had hardly been elucidated. This study aims to elucidate the internal flow structure of the ultra-small axial flow hydraulic turbine and to clarify the mechanism behind the improved performance of the optimized runner. To this end, we investigated in detail the internal flow structure of the original and optimized runners using PIV measurements and numerical analysis, focusing on the vortex structures. As a result, it was found that losses due to the tip leakage vortex and vortices near the hub at the blade outlet were smaller for the optimized runner than for the original runner. In particular, it was found that the turbine efficiency was greatly improved because vortices due to the swirling flow downstream of the runner were suppressed, and losses resulting from these vortices were thus reduced. Highlights • Ultra-small axial flow hydraulic turbine is palm-sized. • The internal flow structure of the turbine was revealed. • Vortexes that cause losses were identified. • The mechanism behind the improved performance of the optimized runner was clarified. [ABSTRACT FROM AUTHOR]

Abstract Rotodynamic multiphase pump is vital for energy conversion and oil-gas exploitation. In the present work, the complicated transient characteristics of gas-liquid two phase flow in a multiphase pump under 10% and 20% inlet GVFs are investigated by Dynamic Mode Decomposition (DMD). Results show that the strip-shaped gas volume develops in impeller and then leads to jet impingement at diffuser inlet, which induces the inception of gas pocket in diffuser passage. The major vortex structures in the multiphase pump consist of leading edge vortex in impeller and diffuser, separation vortex in impeller, tip leakage vortex in impeller, trailing edge vortex in impeller and diffuser. Dynamic modes under significant frequency show that, the main structures are the radial inhomogeneity near pressure side in impeller, and the back flow phenomenon near suction side in diffuser. Highlights • 10% and 20% inlet GVFs in a rotodynamic multiphase pump are investigated. • Strip-shaped gas volume is vital for gas-liquid flow pattern. • Dynamic mode decomposition is used to decompose coherent structure of multiphase flow. • Dominant frequency and main structure under various inlet GVFs are revealed. [ABSTRACT FROM AUTHOR]

Oblique ribs are widely applied to the internal cooling of turbine blades to promote the heat transfer between blade wall and coolant. In this study, the effect of several new types of truncated ribs on the heat transfer characteristics in 45° oblique rib channels is investigated experimentally and numerically. The numerical results obtained by the SST k-ω turbulence model agree well with the experimental data for the Reynolds number ranging from 10000 to 60000. The results indicate a significant entrance effect on the heat transfer in truncated rib channels. The numerical results show that ribs continuously truncated at 3.8 mm gives the best heat transfer performance among the newly truncated ribs. Compared with the original structure, the Nusselt number and heat transfer enhancement factor of newly truncated ribs increased by 24.6 % and 17.8 %, respectively. Concurrently, the friction factor is reduced by 5.1 %. [ABSTRACT FROM AUTHOR]

This study designs and optimizes a high-loaded transonic fan stage using a curvature control method. A fan stage with a total pressure ratio of 2.49 and adiabatic efficiency of over 87 % at the design point is obtained after 3D optimizations. The geometric curvature of the blade passage has important driving and adjusting effects on the mainstream and on the flow in the boundary layer. The reasonable optimization of camber line curvature can weaken the radial secondary flow on the blade surface and decrease shock losses effectively. It can also postpone or suppress boundary layer separation by reorganizing aerodynamic load. Hence, secondary flow losses are reduced, and the overall stall margin is expanded. This method features high precision, high operability, and short design cycle, hence providing highloaded fans/compressors with new freedom to improve design ability and offering new possibility to design system development. [ABSTRACT FROM AUTHOR]

• Natural circulation was studied with in simulated core catcher cooling system. • Two-phase flow structure were studied in the coolant channel. • Data on void fraction, re-wetting time, and heat transfer coefficients were obtained. • Effects of subcooling, heat flux and system pressure were studied. • Cooling capability of core catcher system was assessed. Passive cooling based on natural circulation is utilized in ex-vessel core catcher system of an advanced nuclear reactor to handle severe accident scenario. The core catcher coolant channel has a unique geometry which consists of heated downward-facing slightly inclined and vertical surfaces. A full height experimental facility with natural circulation driven flow to model ex-vessel core catcher system was designed using scaling analysis. In this study, the cooling capability and heat transfer characteristics of the ex-vessel core catcher system was carried out. Two-phase flow parameters and wall temperatures were measured under a uniform heat flux condition. Two-phase flow structures were identified by high-speed camera visualization along with measurements of two local parameters, void fraction and re-wetting time. The wall temperature and local heat transfer coefficient distribution along the cooling channel were obtained by direct measurements of the heater surface and liquid temperature. The cooling performance of the core catcher system was analyzed based on the experimental results. The results indicated that the core catcher coolant system provides adequate cooling and maintains the integrity of the core catcher plate for prototype heat flux conditions. [ABSTRACT FROM AUTHOR]

Various methods had been taken for the flow field studies, among them, qualitative visualizations and probe intrusive measurements have a long history and had been adopted since last 50s, while laser non-intrusive measurements and numerical simulations are the emerging methods, especially the former. Nowadays, more and more researchers have realized there are specific and regular flow structures in the strong turbulence of vortex tube, and the flow structures have great significance on understanding the energy separation process and performance. However, there still did not exist a review on the flow structure studies. The aim of this paper is to offer a critical review and discussion on the current studying methods, findings and differences on the flow structures in vortex tube. In addition, future scopes were proposed on the experimentally and theoretically verification of these flow structures and their effects on the energy separation process and performance. [ABSTRACT FROM AUTHOR]

The trailing edge regions of high-temperature gas turbine blades are subjected to extremely high thermal loads and are affected by the external wake flow during operation, thus creating great challenges in internal cooling design. With the development of cooling technology, the dimple and protrusion have attracted wide attention for its excellent performance in heat transfer enhancement and flow resistance reduction. Based on the typical internal cooling structure of the turbine blade trailing edge, trapezoidal cooling channels with lateral extraction slots are modeled in this paper. Five channel outlet configurations, i.e., no second passage (OC1), radially inward flow second passage (OC2), radially outward flow second passage (OC3), top region outflow (OC4), both sides extractions (OC5), and three dimple/protrusion arrangements (all dimple, all protrusion, dimple–protrusion staggered arrangement) are considered. Numerical investigations are carried out, within the Re range of 10,000–100,000, to analyze the flow structures, heat transfer distributions, average heat transfer and friction characteristics and overall thermal performances in detail. The results show that the OC4 and OC5 cases have high heat transfer levels in general, while the heat transfer deterioration occurs in the OC1, OC2, and OC3 cases. For different dimple/protrusion arrangements, the protrusion case produces the best overall thermal performance. In conclusion, for the design of trailing edge cooling structures with lateral slots, the outlet configurations of top region outflow and both sides extractions, and the all protrusion arrangement, are recommended. [ABSTRACT FROM AUTHOR]

The internal flow structure and the energy loss in the first stage impeller side chamber of a molten salt pump for solar thermal power generation were investigated numerically. The flow field in the model pump was simulated based on the RANS equation using the standard k-ε turbulence model. The results indicate that the rotating speed of core flow in the front impeller side chamber is higher than the tangential velocity at the maximal radius of the front shroud. However, the core flow in the rear impeller side chamber gives an opposite trend. Meanwhile, the radial velocity at the boundary-layer separation point on the front impeller side chamber stationary wall decreases initially and then increases with the radius while it only decreases in the rear impeller side chamber. For the energy loss, the percentage of the disk friction loss to total energy consumption reduces as the flow rate increases, while the absolute value of disk friction loss on the front shroud keeps almost constant and the loss on the rear shroud decreases with the increasing flow rate. [ABSTRACT FROM AUTHOR]

• Multiple plasma actuators are arranged to control the TLF and Kriging surrogate model is adopted to obtain the optimal cases. • The physical mechanism of flow structures inside the gap that control leakage flow is presented. • The mixing loss conditions are investigated by considering the leakage flow as a jet. • The arranged position of plasma actuators have a great influence on loss conditions. In this paper, the effects of multiple dielectric barrier discharge (DBD) plasma actuators on the leakage flow structures and loss conditions have been numerically studied in an axial turbine cascade. Kriging surrogate model is adopted to obtain the optimal cases. The physical mechanism of flow structures inside the gap that control leakage flow is presented, which is obtained by analyzing the flow topology, the evolution of the flow structures and its influence on the secondary velocity and loss conditions in the passage as well. The results show that the induced vortex caused by DBD actuators can change the leakage flow direction inside the tip gap and make the separation bubble break earlier, leading to a new type of the flow pattern. When the actuators are applied, the speed of leakage flow is significantly reduced and the angle between leakage flow and main flow has an obviously diminution, causing the reduction of mixing losses in the passage compared with the Baseline case. Furthermore, the comparison of secondary velocity shows that the tip leakage vortex (TLV) approaches the suction surface, resulting in reduced affected area and weakened loss strength. Plasma actuators can diminish the loss coefficient in both TLV and passage vortex near the casing (PVC) zones. The actuators arranged near the trailing edge mainly affect the strength of TLV, while the actuators in the leading edge area contribute to the loss reduction in the zone of PVC. [ABSTRACT FROM AUTHOR]

The flow characteristics of a sweeping jet ejecting from a typical feedback channel fluidic oscillator were investigated using time-resolved particle image velocimetry. The three-dimensional flow structure of the jet was examined through the instantaneous and averaged velocity fields measured at the streamwise center plane and cross-sectional planes of the jet for a range of Reynolds numbers (8000, 12,000, 16,000, 20,000, and 24,000). As the Reynolds number increases, the frequency of the sweeping jet linearly increases from 0.7 to 2 Hz. However, the switching time of the jet flow direction from one side to the other is very short compared to the sweeping period. As a result, the ensemble-averaged flow patterns of the sweeping jet at the streamwise center plane look like twin jets issued from a V-shaped nozzle, regardless of the Reynolds number. The rapidly switching jet contains a counter-rotating vortex at the cross-sectional plane of the jet near the nozzle, similar to a cross-flow jet. The coherent vortex structures are rapidly destroyed and become random when the jet moves downstream. [ABSTRACT FROM AUTHOR]

Abstract Sidewall proximity, characterized by the gap distance (G) between border pin-fin column and sidewall in a pin-finned microchannel with in-line arrangement, plays a significant role on pressure drop and heat transfer characteristics. To better understand the thermal performance and explore the underlying mechanisms, a comprehensive comparison is numerically developed among three representative microchannels with gap-to-diameter ratios (G / D) of 0.6, 1.0 and 1.4, respectively. The Reynolds number investigated in this paper varies from 13 to 202. It is found that the gap distance severely influences flow distribution, streamline structure, velocity field and temperature distributions in a pin-finned microchannel. At a fixed Reynolds number, pressure drop of microchannel is continuously decreased while heat transfer is first enhanced and then reduced with the increase of gap distance. Among the three models, the microchannel with G / D = 1.0 possesses a comparatively superior heat transfer performance. In addition, extremely low local Nusselt numbers on both the base surface and the pin-fin surface near sidewall seriously deteriorate the overall heat transfer performance of the microchannel with G / D = 0.6. Furthermore, unremarkable heat transfer performance is also observed from the microchannel with G / D = 1.4 for its obvious decline of local Nusselt number on inner regions in spite of a rise on border region. Taking heat transfer and pressure drop into account simultaneously, the results show that a very small gap distance (i.e., G / D = 0.6) should be avoided for design of a pin-finned microchannel. Microchannels with middle gap distances (i.e., G / D = 0.9, 1.0, 1.1) have a relatively better overall thermal performance, which separately provide a superiority of 10.6–13.6% (G / D = 0.9), 10.0–13.5% (G / D = 1.0), 8.2–14.4% (G / D = 1.1) compared to the microchannel with G / D = 0.6. Finally, new correlations of friction factor and Nusselt number are developed by considering the effects of sidewall proximity. Highlights • Sidewall proximity influences on a pin-finned microchannel are observed. • Details of flow characteristics and heat transfer are discussed. • Suitable sidewall proximity is of importance to improve thermal performance. • New correlations of friction factor and Nusselt number are developed. [ABSTRACT FROM AUTHOR]

The flow structure and the unsteady mechanism of the unsteady cavitating flow are reviewed in this paper. The flow patterns and structures in different cavitation regime, for the attached cavitation and the vortical cavitation, are shown with both the visualization and the quantitative information. The attached cavitating flow around the Clark-Y hydrofoil and the vortical cavitating flow around the Tulin hydrofoil are considered. In particular, the phenomena such as the large-scale vortex structure and the re-entrant flow associated with the cloud cavitation, and the cavitating vortex street's forming and crumbling are described. The evolution of the cavitation structure in the transient sheet/cloud cavity forming, along with the cavity collapse induced by the re-entrant flow and the shock wave propagation are discussed. The perspective future research of higher fidelity simulations, and the accurate identifications of the cavitating vortex structure is commented. [ABSTRACT FROM AUTHOR]

This work predicts hydrodynamic characteristics for a rolling ship hull section. Flows about the mid-section of a KVLCC2 under harmonic excited roll motions are simulated with solutions of Navier-Stokes (NS) equations filtered by a large eddy simulation (LES) with a finite volume (FV) solver. This solver applies a fourth-order compact scheme to discrete spatial terms, a fourth-order Runge-Kutta scheme to advance equations, a momentum interpolation method (MIM) to avoid pressure and velocity decoupling, a volume of fluid (VOF) method to capture free surface, and a ghost cell immersed boundary method (GCIBM) to simulate rolling ship. Firstly, verification is implemented to confirm the convergency. Validation is followed by comparison with experimental data. Then, systematic assessments are made on the effects of roll amplitude, frequency and bilge keels by analyzing the tendency of roll moment, damping and added mass moment of inertia as well as flow structures. Wave radiation and viscous effects are elucidated through flow structures. As a result, viscous effect is dominant. Roll amplitude, frequency and bilge keels influence roll hydrodynamic characteristics substantially. Besides, causes of vortex generation, shedding and movement, variation of dynamic pressure field and mechanism of bilge keel to mitigate roll motion are disclosed. [ABSTRACT FROM AUTHOR]

In this study, transparent vortex tubes of diameter 30 mm were designed for the main tube visualisation and a 2D laser Doppler velocimetry (LDV) measurement was adopted to investigate the mean flow field on the meridian plane. The distributions of the axial and radial velocities, and particularly the reverse flow boundary, were obtained for understanding the characteristics of large-scale vortex structures. Various operation conditions with different cold mass fractions (0.1–0.9), tube lengths (360 mm, 600 mm, and 900 mm), and inlet pressures (0.1 bar and 0.2 bar) were studied to reveal how the reverse flow boundary affects the energy separation performance. The LDV results show that the mean axial velocity distribution present good axial symmetry, and increasing the cold mass fraction results in an expansion of the reverse flow boundary as reported for previous numerical calculations. A deterioration in the energy separation performance due to an excessively short or long tube was revealed, and the proposed tube optimization criteria were proven. [ABSTRACT FROM AUTHOR]

Visual studies of fluid motion revealed a velocity distribution structure in the cross section of the channels. The analysis of two areas of research: the ideas of I. Prigogine search for external forces to organize the structure or system; as well as the hypothesis of M. Velikanov on the recognition of the form of existence of material media with their inherent self-organization of coherent or discrete formations. Revealing the structure of flows that is stable in space and time explains: why the world around us shows a high degree of organization and order in spite of the dominance of the turbulence chaos model and the assertion of an increase in entropy. Hydraulic experiments have proved the influence of the flow structure on the distribution of parameters during the movement of liquids and gases. The hydraulic friction coefficient for given initial parameters depends not only on the Reynolds number and roughness, but also depends on the transverse dimensions of the channel. The wave nature of the distribution of parameters was obtained both under turbulent and laminar flow regimes. The wave nature of the change in the coefficient of hydraulic friction removes the problem of uncertainty in calculating the pressure loss and other energy indicators of the equipment. The research results show the possibility of forming the flow structure during the movement of liquids and gases.To the traditional concept of the parabolic distribution law of the diagram of averaged velocity values, the wave character of the distribution of the pulsation velocity components has been added. Summing the plot of the averaged values of the flow velocity at each point of the flows with the wave nature of the distribution of the pulsating velocity components gives a plot of the real velocity values at each point of the channel cross section. An experiment with capillaries of different sizes showed that the wavelength of the flow structure revealed in visual studies also forms a deviation of the height of the capillary rise of water from the average by more than 10%. The experiments showed that the deviation of the velocity parameters, the coefficient of hydraulic friction, the heat transfer coefficient, the height of the capillary rise of water from the averaged values for a given value of the initial pressure when changing the transverse dimensions of the flow parts has a wave character with a stable wavelength size. [ABSTRACT FROM AUTHOR]

The performance of external loop airlift reactor (ELALR) strongly depends on local prevailing hydrodynamics. In this work, the hydrodynamics characterization of the gas-liquid flow is performed in a laboratory scale ELALR. The bubbles plume and the velocity field of the riser are captured using Particle Image Velocimetry (PIV) technique and a three-dimensional CFD-PBM simulation is implemented to provide a full description of the flow structure. The effects of superficial gas velocity on the flow structure and the time-averaged liquid velocity are quantitatively investigated and the numerical results are in reasonable agreement with the PIV measurement. The results show that the gas-liquid two-phases flow in the riser is always unsteady, and the bubble plume meandering behavior is observed clearly. Gas holdup increases with the increase of superficial gas velocity, while the liquid axial velocity shows a trend of first increase and then decrease and the maximum is observed at superficial gas velocity of 30 mm/s. With the increase of superficial gas velocity, the probability density function curve becomes flatter, and changes from unimodal to bimodal. This work helps to gain in-depth understanding of the complex fluid hydrodynamics behavior inside an ELALR. [ABSTRACT FROM AUTHOR]

Highlights • Heat transfer, pressure loss and turbulent flow structure of miniature structured ribs are presented. • Experiments and numerical simulations have been done. • W-shaped miniature ribs have the best thermal performance by vortex flow. • Miniature structured ribs can achieve high heat transfer performance while having relatively low pressure losses. Abstract This paper presents an experimental and numerical study of turbulent flow heat transfer and pressure loss in channels with structured miniature ribs on one wall with the aim to improve the internal cooling performance of gas turbine blades and combustor liners. Miniature transverse, angled, V-shaped and W-shaped ribs are investigated respectively in the study, and the aspect ratio of the channels is kept to be 6:1, and the rib pitch to height ratio is 10 and the rib height to mean hydraulic diameter ratio is 0.029. The experiments were carried out at the Reynolds numbers ranging from 10,000 to 60,000, and a low-Reynolds number k -ε turbulence model was also adopted to simulate the turbulent flow and heat transfer in the channels. Both experimental and numerical results have shown that the shapes of miniature ribs have appreciable influences on the heat transfer and pressure loss performance in the channels. The experiments showed that the W ribs have the highest heat transfer and pressure loss performance in the channel, and the angled ribs have the lowest heat transfer and pressure loss performance in the channel. The W-ribbed channel shows respectively the globally averaged Nusselt numbers and overall Nusselt numbers of 2.2–2.6 and 2.9–3.3 times those of a smooth channel, and the friction factors of 2.5–3.7 times those of the smooth channel, which are higher than those of the V-ribbed channel and the transverse-ribbed channel. The miniature structured ribs show appreciable higher thermal performance than the conventional sized ribs. Furthermore, the numerical simulations show different vortex flow structures near the wall induced by the structured miniature ribs on the wall, which are responsible for the high-performance heat transfer enhancement. [ABSTRACT FROM AUTHOR]

Abstract In these years, many works have been done on the combustion performance and flow structure of a trapped vortex combustor (TVC). However, to the best of our knowledge, little works have been done so far to unravel the effect of combination of various factors on cavity flow structure. In this study, a TVC has been studied to understand the effects of the angle and location of primary injection on the cavity flow structure with both experimental and numerical methods. The results indicate that the computational results match well with the experimental results. Two different flow structures have been found out with different combinations of θ and H , which are termed as "broken" and "closed". Besides, when the distance between the primary injection and the bottom wall of the cavity (H) increases and the inlet angle of the primary injection (θ) is kept constant, the flow structure will be "broken". If θ increases at this moment, the flow structure changes from "broken" to "closed". The results are helpful for providing a good deal of insight into the flow structure and flow field organization of TVC. [ABSTRACT FROM AUTHOR]

Abstract Expanding the existing knowledge of the role of roughness on near-bed flow properties is vital to provide deeper insights into the interactions between the flow, sediment transport, morphology and ecology. This study uses four fixed beds, from both the laboratory and the field, to characterise the structure of near-bed flow properties. Dense measurements over a grid of 6 mm spacing were conducted across each cast, using an Acoustic Doppler Velocimeter. These measurements followed the line of the bed topography, with the sampling location being 2 mm away from the maximum bed elevations. Near-bed flow properties were calculated across the surface; including velocity, turbulence intensity, turbulent kinetic energy, Reynolds stress and quadrant analysis. Improved visualisation of flow properties involves contour plots of the spatial distribution of flow over the DEM of the gravel bed to facilitate attributions of flow patterns to roughness and topography. Using the standard deviation of elevations as a proxy for grain roughness of each surface, we observed that a decrease in roughness led to an increase in the average near-bed velocity. In contrast, an increase in roughness resulted in an increase in both turbulence intensity and turbulent production (TKE). Analysis of quadrant events dominating across the surface can provide indications into the potential for erosion and/or deposition across the natural mobile surfaces. Finally, second-order structure function analysis is used to quantify the spatial structure of flow properties, presenting novel isopleth maps of flow properties. For the majority of flow properties across all surfaces, the range in the spatial extent of flow structures is between 1 and 2.25 × D 50A, except for transverse and vertical velocities, which have a wider range up to 3.5 × D 50A. Highlights • Novel flow measurement methodology that tracks the grain-scale topography • Observed increasing near-bed velocity over surfaces with lower roughness • Increased turbulence and Reynolds stress observed with increased roughness. • Uniformity in the spatial extent of flow structures exists across surfaces. • Presenting novel flow visualisation by overlaying flow properties onto DEMs [ABSTRACT FROM AUTHOR]

Abstract A comparative study was performed to investigate flame propagation in a square-cross section channel filled with either flexible- or rigid-obstacles with blockage ratio (BR) of 0.429. Experiments were conducted in premixed hydrogen-air mixtures with different equivalence ratios, at initial conditions of 100 kPa and 298 K. High-speed Schlieren photography was used to obtain the detailed flow structure, flame front evolution and the flame tip velocity. Also, pressure transducers were employed to monitor the pressure around the obstacles. Flame propagation across the obstacles was found to be strongly affected by flow contraction induced by obstacles and separated flow pattern downstream of obstacles. Flame propagation with rigid obstacles is mainly governed by the turbulent burning of the fresh gas in the pockets. For the flexible cases, the flow structure is characterized by the shear layer coming off the obstacles leading-corner and the vortex downstream from the obstacles. These special flow structures together provide a flow contraction and constrict flame propagation in the obstacles-free channel, and therefore the flame maintains acceleration. Most notable, the gas flow ahead of the flame purges the flexible obstacle to tilt, yielding an increase in BR, which is correlated with the stronger acceleration as the flame propagates through the obstacles. However, exposing the obstacles to the overpressure for a long period also induces too much deformation. Therefore, the instantaneous BR (BR real) will also decelerate slightly. Interestingly, BR real is closely related with the overpressure level. Highlights • Premixed H 2 -Air flames across flexible and rigid obstacles were compared. • Unburned gas flow field just ahead of flame front was visualized. • Deformation of flexible obstacles was correlated to the overpressure of flame front. • Flame propagation mechanisms were proposed. [ABSTRACT FROM AUTHOR]

This study revisits the design of tree-shaped flow networks in disc-shaped bodies, focusing on thermal management applications. The key objectives are to achieve equal point distribution along the disk periphery and minimize flow resistance at each bifurcation point. The literature highlights the challenge of balancing fluid and thermal performance simultaneously. To address this, the authors propose a multi-branching approach and hydraulic diameter setting, allowing flexibility in network growth. The optimization process involves minimizing flow resistance as the objective function, employing a constrained minimization technique with an interior point algorithm. By solving this optimization problem, the geometric features of the network are determined. Notably, the research uncovers a memory effect in the geometry and overcomes computational time limitations. The Svelteness, defined using the concept of configuration area, aids in distinguishing between different design possibilities and identifying the evolutionary direction. Increasing the complexity of the network while minimizing the total branch length leads to the discovery of the optimal architecture configuration. These findings provide valuable insights for optimizing symmetric tree designs, improving thermal and hydraulic performance. • Radial tree-shaped flows in disc-shaped bodies are generalized to multi-branching. • A memory effect in the geometry overcomes computation time limitation in design. • Svelteness based on the configuration area allows differentiating configurations. • Performance shows evolutionary direction to few initial channels and less branches. [ABSTRACT FROM AUTHOR]

• Peak bed shear occurs in the wave front and is significantly higher in the floodplain. • Transverse transport weakens the formation of coherent eddies at the interface. • Transverse currents have great contribution to the momentum transport. In recent decades, the study of steady flow in the compound channel has received much attention, while the flow characteristics of flood in the compound channel are rarely reported. In this paper, the characteristics of the flood propagation under different initial main channel depths are studied using the laboratory experiments and numerical simulation. Results show that the overflowing water from the main channel soon forms the reverse flow after hitting the floodplain sidewall. This reverse flow influences the subsequent overflowing process and significantly enhances the bed shear stress on the floodplain. However, this effect gradually decreases with the flood evolution distance and the increase of the initial downstream water depth. Unsteady transverse transport during flooding inhibits the formation of coherent eddies at the interface between the main channel and floodplain, while the high shear region will extend to the internal of the main channel or floodplain. Complex multi-vortex structures are found on the floodplain due to transverse interaction behavior. The main transverse mass transport takes place in the first half of the period after the flood arrival. It is found that the weir flow equations used in the previous studies are unable to accurately describe the transverse mass transfer between the main channel and floodplain. Secondary currents have a little contribution to the unsteady transverse momentum transport, while the transverse currents and transverse Reynolds stresses have a significant effect. The time when the peak transverse current arrives is later than that when the peak transverse Reynolds stresses does in the current study. [ABSTRACT FROM AUTHOR]

The Couette-Taylor flow of compressible gas at high Reynolds numbers is studied. Problem statement is formulated on the basis of the differential equation system (the Navier-Stockes equations) for compressible gas in cylindrical coordinates. On the basis of physical model built in CFD, the numerical results for various temperatures of the cylinder’s surfaces and rotational speed are obtained. The relation between vortice dimensions and surface temperatures and rotational speed is established.

In the present computational investigation, the characteristics of heat transfer from cylinders in cross flow of air has been reported, using the commercial CFD software, FLUENT. Unsteady, constant heat flux condition has been applied at the surface and the turbulent flows were simulated using k-kl-ω turbulence model. Comparing for single cylinder, with k-ω (standard), k-ω (SST) and SA models, the k-kl-ω model has been found best suitable models for simultaneous computation of flow field and heat transfer parameters. The results were also compared with experiments performed as well as from reported literatures for Reynolds number range 1.1x104≤Re≤4.1x104 to prove the capabilities of k-kl-ω model. High aspect ratio (18.75) and very low blockage ratio (0.053), has been used to conduct experiments. An analysis of data revealed that the average deviation from the experimental as well as reported values is less than 1%.

The objective of this paper was to investigate acoustic pressure waves and the transient flow structure emitted from the single bubble near an elastic boundary based on the particle image velocimetry (PIV). A combination of an electric-spark bubble generator and PIV were used to measure the temporal bubble shapes, transient flow structure, as well as the mid-span deflection of an elastic boundary. Results are presented for three different initial positions near an elastic boundary, which were compared with results obtained using a rigid boundary. A formula relating velocity and pressure was proposed to calculate the acoustic pressure contours surrounding a bubble based on the velocity field of the transient flow structure obtained using PIV. The results show the bubbles near the elastic boundary presented a “mushroom” bubble and an inverted cone bubble. Based on the PIV-measured acoustic pressure contours, a significant pressure difference is found between the elastic boundary and the underside of the bubble, which contributed to the formation of the “mushroom” bubble and inverted cone bubble. Furthermore, the bubbles had opposite migration direction near rigid and elastic boundaries, respectively. In detail, the bubble was repelled away from the elastic boundary and the bubble was attracted by the rigid boundary. The resultant force made up of a Bjerknes force and buoyancy force dominated the migration direction of the bubble.

To find the influence of different types of steel poles on measurements by wind speed sensors along highspeed railways, the three-dimensional Reynolds-averaged Navier-Stokes equations, combined with the k-ε turbulence model, were solved on an unstructured grid with a boundary layer using the finite volume method. The grid-independent validation was firstly conducted, and the accuracy of the present numerical simulation method was validated by experiments and simulations carried out by previous researchers. To ascertain angles of influence at different distances between the sensor and the virtual one, the flow field around a sensor was investigated with the method of altering the relative coordinates between the two sensors. After that, the flow fields and velocity distributions around steel poles were studied. It can be stated that behind the sensor, the closer the distance from the sensor center line, the larger the angle of influence. However, as the distance is varied from 0.3 to 1.0 m, the most adverse angles are not in excess of ±20°. In addition, the steel poles have a certain influence on the measurement results of sensors. A “two-sided petal acceleration region” with a “central pistil deceleration zone” comes into being. From the perspective of regions of influence in different wind directions, the influence region of the annulus pole is basically the same. For the square and H types, when the angle is 45°, the region of effect is the largest. For the same distance between the sensor and the pole, the space required between two sensors for the single H type is larger than that required by the annulus type. Thus, it is suggested that the distance between sensors and the pole should be 1.0 m with the anemometer located on the upstream side, and the distance between two anemometers should be 0.8 m.