Main Flow Direction Research Articles

Influence of incoming turbulence and shear on the flow field and performance of a lab-scale roof-mounted vertical axis wind turbine

Flow conditions in an urban environment are complex, featuring varying levels of turbulence intensity and shear. The influence of these flow characteristics on the performance of a roof-mounted vertical axis wind turbine of the Savonius (drag) type is investigated at lab scale. Five different inflow conditions are generated with an active grid in a wind tunnel, covering turbulence intensities from 0.9% to 11.5% and relative vertical shear from 0% to 17%. The flow field is captured using particle image velocimetry, and the power output of the turbine is assessed through measurements of the converted power. The set-up consists of two-surface mounted cubes aligned with each other in the main flow direction, spaced apart by two cube heights. The turbine is placed on top of these model buildings at six different streamwise positions along the centerline and at two different heights. It was observed that the turbulence intensity in the inflow has a significant impact on the flow field and also on the power output of the turbine. The increasing turbulence intensity leads to smaller regions of recirculating flow. Thus, the turbine experiences higher flow velocities, which is reflected in the measured power. The influence of shear is comparably small on both the flow field and the turbine performance. The higher of the two turbine positions yields higher power output overall. Furthermore, it was shown that the impact of the turbine on the flow field is significant for all inflow conditions and can vary substantially depending on the inflow.

Sustainability assessment of hydrochemical and microbiological disturbances in shallow aquifer by thermal utilization

The use of geothermal systems with shallow groundwater to achieve global carbon neutrality has increased substantially. However, they not only have hydraulic and thermal effects on the environment, but also induce hydrochemical and microbial changes that can worsen the sustainability of water resources. This study focused on evaluating groundwater mixing and pollution in a surrounding shallow aquifer induced by operation of a groundwater heat pump (GWHP) system. To explain these phenomena, hydrogeochemistry, multiple isotopes (O, H, and Sr), Rn, and microbial data were combined with geochemical modeling of the test system. Few studies have been conducted on field measurements using these combined approaches. The hydrogeochemical and microbial data showed specific characteristics accompanying the thermal use of groundwater. The calculated saturation index values and PHREEQC modeling results indicated a high probability of clogging effects in wells located near the boundary of the thermal plume. There was also a decrease in the microbial diversity index. Rn and stable isotopes indicated active mixing between circulating water and ambient groundwater along the main groundwater flow direction during the operation. This study demonstrated that subsurface hydrological and microbial environments, such as those of the GWHP system, responded to impacts of moderate intensity. Because the applied data were able to derive possible environmental and operational problems of shallow geothermal systems, hydrological and microbiological signatures should be considered for sustainable management of groundwater in geothermal sites. Our study can have implications for energy reuse from storage within an aquifer, such as aquifer thermal energy storage (ATES) based on field tests, using combined analysis.

Wetland Management in Recharge Regions of Regional Groundwater Flow Systems with Water Shortage, Nyírség Region, Hungary

Climate change and increasing human impacts are more emphasised in recharge regions, where the main flow direction is downward, resulting in negative water balance. Two wetlands located in the recharge position of regional groundwater flow systems were investigated in the Nyírség region, Hungary, as pilot areas for representing wetlands in similar hydraulic positions. Hydraulic data processes, chemical data evaluations, and numerical simulations revealed that the wetlands are fed via local flow systems, superimposing regional-scale recharge conditions in the area. The wetlands are discharge and flow-through types in connection with local flow systems. Nevertheless, in the case of significant regional water table decline—due to the high vulnerability of recharge areas to climate change—local flows are degraded, so they are not able to sustain the wetlands. To preserve the groundwater-dependent ecosystems in the areas, water retention at the local recharge areas of the wetlands may help in the mitigation of water level decline under present-day conditions. If the regional water table continues to decline, comprehensive water retention solutions are needed in the whole region. The results highlight that understanding the natural wetland–groundwater interactions at different scales is crucial for the preservation of wetlands and for successful water retention planning.

Optimizing Rapid Prototype Development Through Femtosecond Laser Ablation and Finite Element Method Simulation for Enhanced Separation in Microfluidics

In recent years, microfluidic systems have emerged as promising tools for blood separation and analysis. However, conventional methods for prototyping microfluidic systems can be slow and expensive. In this study, we present a novel approach to rapid prototyping that combines femtosecond laser ablation and finite element method (FEM) simulation. The optimization of the prototyping process was achieved through systematic characterization of the laser ablation process and the application of FEM simulation to predict the flow behavior of the microfluidic devices. Using a dean-coupled inertial flow device (DCIFD) that comprises one channel bend and three outlets side-channels. DCIF is a phenomenon that occurs in curved microfluidic channels and is considered by the existence of inconsequential flow patterns perpendicular to the main flow direction. The DCIF can enhance the separation efficiency in microfluidic devices by inducing lateral migration of particles or cells towards specific locations along the channel. This lateral migration can be controlled by adjusting the curvature and dimensions of the channel, as well as the flow rate and properties of the fluid. Overall, DCIF can provide a valuable means of achieving efficient and high-throughput separation of particles or cells in microfluidic devices. Therefore, various microfluidics designs that contain different outlet channels were studied in this research to improve blood plasma separation efficiency. Results from imitated blood flow experiments showed positive results for fluid flow and particle separation. The study also found that incorporating three various channel widths is the key to achieving efficient plasma separation, indicating that this result could serve as a guideline for future microfluidics geometry specifications in the field of blood plasma separation. According to the FEM simulation, the highest separation percentage for both microparticle sizes was obtained by incorporating a variable outlet channel width into the same microfluidic device. The FEM simulation revealed that around 95% of the larger microparticles separated while 98% of the smaller microparticles separated. This is consistent with the imitated blood separation results, which showed that 91% of the larger microparticles separated and around 93% of the smaller microparticles were separated. Overall, our results demonstrate that the combination of femtosecond laser ablation and FEM simulation significantly improved the prototyping speed and efficiency while maintaining high blood separation performance.

Morphology of Anisotropic Banded Structures in an Emulsion under Simple Shear

The formation of flow-induced, oriented structures in two-phase systems, as in this study, is a phenomenon of considerable interest to the scientific and industrial sectors. The main difficulty in understanding the formation of bands of droplets is the simultaneous interplay of physicochemical, hydrodynamic, and mechanical effects. Additionally, banded structure materials frequently show multiple length scales covering several decades as a result of complex time-dependent stress fields. Here, to facilitate understanding a subset of these structures, we studied water in oil emulsions and focused on the effects of three variables specifically: the confinement factor (Co=2R/H), the viscosity ratio (p), and the applied shear rate (γ˙). The confinement (Co) is the ratio between the drop’s diameter (2R) and the separation of (the gap between) the circular rotating disks (H) containing the emulsion. We carried out (a) observations of the induced structure under different simple shear rates, as well as (b) statistical and morphological analysis of these bands. At low shear rates, the system self-assembles into bands along the direction of the flow and stacked normal to the velocity gradient direction. At higher shear rates is possible to observe bands normal to the vorticity direction. Here, we show that a detailed analysis of the dynamics of the band structures is amenable, as well as measurements of flow field anomalies simultaneously observed. The local emulsion viscosity varies in time, increasing in regions of higher droplet concentration and subsequently inducing velocity components perpendicular to the main flow direction. Thus, the emulsion morphology evolves and changes macroscopically. A relatively plausible explanation is attributed to the competitive effects of coalescence and the rupture of drops, where p values less than one predominate coalescence.

Estimating parameters of plate heat exchanger for condensation of steam from mixture with air as a component of heat exchanger network

The condensation of vapours in the presence of non-condensable gas is happening in a number of industries. Its efficient realisation must be considered in optimisation of Heat Exchanger Networks (HENs) to increase efficiency of energy usage. In these processes, Plate Heat Exchanger (PHE) with enhanced heat and mass transfer can be effectively used. The method of preliminary selection of the PHE heat transfer area, plate size and corrugation geometry is proposed for the case of steam condensing from its mixture with air. The method uses the developed earlier one-dimensional mathematical model, modernized for PHE consisting from commercially produced plates. The heat transfer area of PHE is minimised by proper selection of plate sizes and geometrical characteristics of its corrugation. The optimisation variables are the corrugation angle to the main flow direction inside PHE channel, the corrugations depth and the length of plate. The case study illustrates the application of the method in case of utilisation low grade heat from exhaust gases after process of drying by superheated steam. The method is implemented as a computer program which can be included as a Dynamic Link Library module in software for HEN optimisation.

Essential analysis and quantitative evaluation on mechanical-energy loss of fluid momentum wheel

The Fluid Momentum Wheel (FMW), an innovative anti-roll device, requires accurate evaluations of mechanical-energy loss within its flow field due to the direct impact on its anti-roll capability. In this paper, the characteristics of the internal flow field and the essential reasons for the mechanical-energy loss of FMW were analyzed through numerical simulations. The results show that the mechanical-energy loss of FMW can be unified with the form of the traditional empirical formula. However, a substantial relative difference of up to 30.59% was observed between the empirical formula and numerical solutions, indicating limitations of traditional empirical formula in predicting the mechanical-energy loss of the FMW. Secondary flow in the cross-section and the asymmetrical distribution of fluid velocity in the main flow direction were identified as essential factors limiting the traditional formula's accuracy. Next, three dimensionless parameters of Reynolds number, curvature ratio, and center-distance ratio are presented to evaluate the mechanical-energy loss according to the fluid characteristics of FMW. Orthogonal experiments are conducted for parametric sensitivity analysis to study the interactive effects and significance of these parameters on the mechanical-energy loss. Finally, the characteristics of the loss coefficients with the variation of parameters are explained by the results of the flow velocity and turbulent kinetic energy in the cross-section. A linear relationship between the loss coefficients and the logarithm of the Reynolds number at various curvature ratios. Based on the above analysis, a new quantitative evaluation formula for the mechanical-energy loss of FMW is proposed and validated, which can support improved anti-roll control of FMW.

Direct numerical simulations of turbulent flow over the converging and diverging riblets

Different from conventional streamwise-aligned riblets, converging and diverging (C–D) riblets are arranged obliquely along the main-flow direction, intending to induce a large-scale secondary flow that modulates or controls the existing large-scale vortical structures in wall turbulence. In this study, we perform direct numerical simulations of turbulent flow over the C–D riblets at a low Reynolds number and investigate the impacts of the C–D riblets on aerodynamic forces, turbulence statistics, and coherent structures. The results show that the C–D riblets can reduce the skin-friction drag, but at the expense of a significantly increased pressure drag in the diverging region, which leads to an increase in the total drag. We present detailed mean and statistical flow-fields to assess the flow characteristics in the presence of the C–D riblets, including modifications on the distributions of velocities, pressure, Reynolds stresses, and turbulence kinetic energy. Moreover, the re-arrangement of turbulence structures over the C–D riblets is examined in terms of instantaneous flow-fields and two-point correlations. The findings provide more insight into the physics of flow over C–D riblets and may contribute to the further development of this flow control strategy.

Multivariate statistical analysis of the alluvial aquifer of Tadjenanet-Chelghoum Laid (Eastern Algeria)

The region of Chelghoum Laid - Tadjenanet is located in eastern Algeria, in the high plains. This area is characterized by a varied age (Mio-Plio-Quaternary) formations and the human demand is answered by the groundwater hosted into the superficial aquifer of the Mio-Plio-Quaternary formations. However, the overexploitation of this aquifer leads to a drop in the piezometric surface and an increase in salinity. of this aquifer leads to a drop in the piezometric surface and an increase in the salinity of the water. A hydrochemical study using characteristic ratios and statistical tools such as principal component analysis and hierarchical ascending classification, was performed using 28 water samples. Chemical analyzes show that the waters of this aquifer are of the calcium bicarbonate type on the limits of the study area, and calcium sulphate to sodium chloride in the centre. The spatial evolution of chemical water facies is explained by the phenomenon of dissolution and ion exchange between the alkali metals and alkaline earth metals coinciding with the West-East flow direction. The application of the various tools shows that the aquifer is recharged by precipitation at the plains border, where the water acquires its original mineralization from the carbonate formations, and becomes more mineralized in chlorides, sodium and sulphates concentrations in contact with the salt-bearing terrigenous formations of the Mio-Plio-Quaternary. The salinization of the waters seems to be mainly due to the dissolution of gypsum, halite and epsomite. The application of the ascending hierarchical classification and the principal component analysis shows the existence of two groups of water whose salinity increases from the borders towards the center of the plain following the main flow direction.

Review of Developments in Plate Heat Exchanger Heat Transfer Enhancement for Single-Phase Applications in Process Industries

A plate heat exchanger (PHE) is a modern, effective type of heat transfer equipment capable of increasing heat recuperation and energy efficiency. For PHEs, enhanced methods of heat transfer intensification can be further applied using the analysis and knowledge already available in the literature. A review of the main developments in the construction and exploration of PHEs and in the methods of heat transfer intensification is presented in this paper with an analysis of the main construction modifications, such as plate-and-frame, brazed and welded PHEs. The differences between these construction modifications and their influences on the thermal and hydraulic performance of PHEs are discussed. Most modern PHEs have plates with inclined corrugations on their surface that create a strong, rigid construction with multiple contact points between the plates. The methods of PHE exploration are mostly experimental studies and/or CFD modelling. The main corrugation parameters influencing PHE performance are the corrugation inclination angle in relation to the main flow direction and the corrugation aspect ratio. Optimisation of these parameters is one way to enhance PHE performance. Other methods of heat transfer enhancement, including improving the form of the plate corrugations, use of nanofluids and active methods, are considered. Future research directions are proposed, such as improving fundamental understanding, developing new corrugation shapes and optimisation methods and area and cost estimations.

Drag-reduction effect of staggered superhydrophobic surfaces in a turbulent channel flow

In this study, direct numerical simulations of turbulent channel flow with superhydrophobic surfaces (SHS) were performed. The staggered SHS pattern alternating the free-shear and no-slip regions was designed to be robust to variations in the main flow direction. A constant-pressure gradient condition was imposed, and the Reynolds number was set to Reτ=180 and 395. The bulk mean velocity in the SHS case increased compared with the no-slip case, and the effect was enhanced by increasing the size of the free-shear region lb+. The main reason for the increase in bulk mean velocity was the increase in slip velocity (or slip length). If the size of the free-shear region was small, the drag-reduction rate was found to be proportional to the slip velocity. However, the turbulent contribution to the bulk mean velocity remained almost unchanged in the no-slip case owing to the negative coherent component of the Reynolds shear stress.

A Cooling Efficiency Model and Numerical Research of Multiparameter Film Cooling

Abstract Based on the validated simulation method of film cooling and multiphase flow simulation method, a multi-level three-dimensional simulation of forward-leaning fan-shaped film hole, cylindrical film hole with different injection angles, and film hole containing water vapor are established to discuss the effects of film hole structure parameters, hole distance, blowing ratio, injection angle, and water vapor volume on film cooling efficiency. The cooling efficiency of forward-leaning fan-shaped film hole increases as the exit length of film hole increases. After adding water vapor, the cooling efficiency of fan-shaped film hole decreases, and the influence of hole axis length and exit length on cooling efficiency is weak. For the cylindrical film hole, the larger the injection angle of film hole, the larger the film coverage area under the same blowing ratio. After adding water vapor, with the increase of the blowing ratio, the film coverage area increases first and then decreases. However, the film coverage area decreases with the increase of cooling injection angle for film holes containing water vapor. The cooling efficiency of the film hole with and without water vapor is related to the vapor velocity in the rising direction and the velocity in the mainstream direction, respectively. A model of film cooling efficiency with air blowing ratio and injection angle is established and verified with experimental data, based on the law that the average cooling efficiency in the main flow direction grows exponentially with the sine of the injection angle.

An improved method for estimating the dissipation rate of turbulent kinetic energy using structure functions evaluated from the motion of finite-sized neutrally buoyant particles

Statistical relations used for estimating the dissipation rate of turbulent kinetic energy (TKE) in isotropic turbulence from the inertial subrange of Lagrangian temporal and spatial structure functions are extended here to the case of more realistic turbulence spectra that include low-frequency and low-wavenumber ranges. It is shown that using the traditional relations based only on the inertial subrange substantially underestimates the dissipation. The improved relations are better constrained by experimental data from which the dissipation is evaluated, enabling more accurate dissipation estimates. The concept is illustrated using laboratory data from water tank experiments of turbulence generated by an oscillating cylinder, where the dissipation is evaluated in three independent ways: from Lagrangian spectra and from Lagrangian temporal and spatial structure functions calculated from the motion of neutrally buoyant finite-sized particles. An additional correction to the relations for estimating the dissipation from the spatial structure functions is applied to take into account the filtering effect of the particles due to their finite size. It is found that, for these particular experiments, the TKE dissipation rate scales well with dimensionally consistent quantities built using the amplitude of the oscillation of the cylinder and the period of its motion, and the constant of proportionality in this scaling relation is determined using the method proposed here. Although the turbulence under consideration is quite anisotropic, the adopted theoretical framework, which assumes isotropic turbulence, seems to be applicable to the experimental data as long as the turbulence statistics are averaged over the three main flow directions.

Experimental estimation of equivalent loads on upward two-phase crossflow in tube bundles via Kalman filtering

Tube bundles of heat exchangers are permanently exposed to flow-induced forces, which may lead to high amplitude vibrations compromising the lifetime. In this context, Flow-induced vibration (FIV) is the most crucial dynamic issue to deal with in the design and operation of shell-and-tube heat exchangers. The experimental investigation of FIV requires determining the nature of flow-induced forces and their consequences, which are given by the characteristics of tube vibration. However, the measurement of flow-induced forces is challenging due to space restrictions within a tube bundle and their intrinsic spatially-distributed nature, often represented by equivalent point forces. The present work aims to estimate the equivalent flow-induced forces in tube bundles due to two-phase crossflow using Virtual Sensing principles. In order to do that, a device is designed and constructed to replace a tube within a normal triangular (transversal pitch to diameter ratio of 1.26) tube bundle, which is subjected to two-phase flow with void fractions ranging from 30% to 95% and a wide range of mass velocities. The device is instrumented with two couples of accelerometers and strain sensors, aligned with drag and transverse directions. The measured dynamic responses are discussed in terms of void fraction, mass velocities, and flow patterns. The results confirm the severity of FIV under Intermittent-type flow patterns. Further, the equivalent force estimation is performed via the Augmented Kalman Filter (AKF), which requires a model approximation (based on EMA techniques) and the system’s outputs. The values of the input covariance matrix are determined through the L-curve regularization method. Finally, the quality of the estimated equivalent force is tested by simulating system accelerations, whose RMS values are compared with measured data. The results suggest better acceleration predictions in the transverse direction than in the main flow direction for relatively low mass velocities and Bubbles flow pattern.

Influence of streamwise and spanwise wall magnet arrays on near-wall MHD turbulence

ABSTRACT The effect of a permanent, localised and non-uniform magnetic field on the near-wall turbulence in a fully developed channel flow is analysed through direct numerical simulations. The magnetic field distribution is obtained by an arrangement of magnets placed on the upper and lower channel walls, producing preferentially either a streamwise (i.e. in the main flow direction) or a spanwise magnetic field component. The wall shear stress is drastically reduced under the effect of the streamwise arrangement of the wall magnets wherein both streamwise and wall-normal magnetic fields are involved. The magnetic braking effect leads to an important increase of the body force. Paradoxically enough, the small-scale turbulent activity is significantly increased above the low buffer layer in this case. On the opposite, imposing a magnetic field with a predominant spanwise component reduces tremendously the population of the turbulent shear stress producing eddies by directly affecting the regeneration of the buffer layer quasi-streamwise vortices. The flow is quasi-relaminarised between the magnets in the low-buffer and viscous sublayers. The wall shear increases in a predictable and deterministic way over the magnets, wherein the wall normal magnetic field component induces electric current loops.

Transport of a passive scalar in wide channels with surface topography: An asymptotic theory

We generalize classical dispersion theory for a passive scalar to derive an asymptotic long-time convection–diffusion equation for a solute suspended in a wide, structured channel and subject to a steady low-Reynolds-number shear flow. Our asymptotic theory relies on a domain perturbation approach for small roughness amplitudes of the channel and holds for general surface shapes expandable as a Fourier series. We determine an anisotropic dispersion tensor, which depends on the characteristic wavelengths and amplitude of the surface structure. For surfaces whose corrugations are tilted with respect to the applied flow direction, we find that dispersion along the principal direction (i.e. the principal eigenvector of the dispersion tensor) is at an angle to the main flow direction and becomes enhanced relative to classical Taylor dispersion. In contrast, dispersion perpendicular to it can decrease compared to the short-time diffusivity of the particles. Furthermore, for an arbitrary surface shape represented in terms of a Fourier decomposition, we find that each Fourier mode contributes at leading order a linearly-independent correction to the classical Taylor dispersion diffusion tensor.

Experimental investigation of thermal performance of novel type vortex generator in rectangular channel

Vortex generators are objects with geometries specifically designed to increase heat transfer. In this study, the thermal performance of a novel vortex generator that can generate longitudinal vortices in a common flow-down pattern was experimentally investigated with Reynolds numbers in the range of 5000–25,000. The vortex generators were placed on the base surface of the channel in such a way that they were mirror images of each other according to the geometric symmetry plane in the main flow direction. At a fixed inclination angle, the effects of the ratio of the transverse distance between two vortex generators in a row to the channel width (transverse pitch ratio) and the ratio of the longitudinal distance between two vortex rows to the channel height (longitudinal pitch ratio) on heat transfer were analyzed. First, the experiments were performed for a single row by increasing the number of the vortex generators in the row (2, 4, 6, and 8) so that the transverse pitch ratios were 0.50, 0.25, 0.16, and 0.12. According to the results, as the transverse pitch ratio decreased, the Nusselt number and friction factor increased, so the highest increases were realized at the transverse pitch ratio of 0.12, reaching 1.44 and 2.07 times greater values compared with the smooth channel, respectively. However, the highest thermal enhancement factor was obtained at the transverse pitch ratio of 0.16. Then, vortex generators were placed with an inline arrangement keeping the transverse pitch ratio of 0.16 constant, and the experiments were executed for longitudinal pitch ratios of 1, 1.5, 3, and 5. The results revealed that the Nusselt number and friction factor increase with the decrease in longitudinal pitch ratio, but the highest thermal enhancement factor value (1.59) was obtained at the longitudinal pitch ratio of 1.5. In addition, the experiments were also performed using a staggered arrangement, but the inline arrangement proved to be more efficient than the staggered arrangement in terms of thermal performance. The maximal thermal enhancement factor was achieved for the inline vortex arrangement at the transverse pitch ratio of 0.16 and longitudinal pitch ratio of 1.5.

Study on the Oil Displacement Mechanism of Different SP Binary Flooding Schemes for a Conglomerate Reservoir Based on a Microfluidic Model

The objective of this study is to elucidate the oil displacement mechanism of different surfactant- polymer (SP) binary flooding systems in conglomerate reservoirs and to determine the optimal scheme. On the basis of water flooding, the microfluidic model is used to reveal the oil–water flow law of different SP binary flooding systems and then determine the oil displacement mechanism. The experimental results show that the SP binary flooding solution can enhance oil recovery by enhancing the sweep volume; the oil displacement efficiency of the echelon viscosity reducing (EVR) scheme is better than that of the echelon viscosity increasing (EVI) scheme and the single constant viscosity (SCV) scheme. For the EVR scheme, the oil displacement system with a high molecular weight and high concentration preferentially enters the large-scale pore-throat of the reservoir. After a period of time, a high resistance area can be formed in the large-scale pore-throat along the main flow direction of water flooding so as to reduce the mobility of the displacement solution in the dominant seepage channel and decrease the frequency of water channeling and then improve the oil displacement efficiency in the large-scale pore-throat to enhance the oil recovery. When the molecular weight and viscosity of the SP binary solution are both reduced, the oil displacement solution with a low molecular weight and low concentration mainly produces low-permeability layers by adjusting the displacement path to enhance the sweep volume in the small-scale pore-throat. Through the reconstruction of a pressure field and the redistribution of seepage paths of chemical micelles with different sizes, the displacement mechanism of crude oil in pores and throats shows a changing trend from large-scale pore-throats to small-scale pore-throats. After the viscosity reduction of the SP solution, the lower production limit of the pore-throat radius is reduced to 1 μm and the sweep volume of the small-scale pore-throat is enhanced. The stepwise production of crude oil in pores and throats with different radii is realized so as to enhance the oil recovery of the conglomerate reservoir. Finally, a new model of SP binary flooding suitable for “echelon injection and stepwise production” of a conglomerate reservoir is established innovatively.

A partitioned model for predicting the spatial development of flow in ecological vegetated rivers with submerged vegetation

The evolution of the velocity caused by submerged vegetation in river courses with regular cross sections has been poorly investigated using analytical methods. Fully understanding the longitudinal profiles of the velocity is significant and essential to study the hydrodynamic processes taking place in wetland ecosystems, which is the most important indicator of energy transfer and material exchange in fluvial ecosystems. Based on the experimental data in the literature, the streamwise velocity gradually is found to decrease in the submerged vegetation and increase in the free surface layer. A four-panel analytical model derived from exponential decay theory was developed to predict the streamwise velocity occurring both upstream and within vegetation along the main flow direction in a rectangular open channel with a submerged vegetation patch. We aim at the development of velocity, which is averaged across the vegetation and free surface layers separately, and the length scales shown in the velocity evolution. The explicit calculation expression for each parameter in the analytical model was provided, and sensitivity analysis was conducted by changing the parameters to check how the predicted velocity changed. The change in the drag coefficient of vegetation (CD) and the velocity at the leading edge of the vegetation (U0) influenced the development of velocity. The former mainly enlarges the velocity difference between the vegetation and free surface layers after the leading edge as it increases, and the latter mainly diminishes the velocity difference before the leading edge as it increases. The error statistics is conducted and shows that the predicted and measured velocities are consistent within a relative error of 11%, indicating that the partitioned model is applicable in predicting longitudinal velocity in flows with submerged vegetation of various flexibility. The theoretical analysis of velocity development paves the way for the future studies on the flow turbulent structure and sediment transport in partially vegetated rivers.

Hydrodynamic performance of a rim-driven thruster improved with gap geometry adjustment

The hubless rim-driven thruster (RDT) has become increasingly interesting for ship propulsion. Gap flow has been proven as the main feature of RDT that cannot be simply neglected. In this study, based on a classical hubless RDT, the effects of the gap geometry are studied by adjusting its axial passage length, and inlet and outlet oblique angles. The hydrodynamic characteristics of the RDT were simulated with OpenFOAM based on the k – ω shear stress transport turbulence model. Due to the pressure increase after the main flow passes through the rotating blades, the flow inside gap is driven upstream, which is opposite to the main flow direction. It is found that the hydrodynamic efficiency is increased as the gap axial passage length is shortened, which is realized by increasing the oblique angle with the fixed inlet and outlet positions. Moving the inlet and outlet to further downstream and upstream positions has negligible effects on the hydrodynamic efficiency and leads to recirculating flow within the gap near its inlet. These findings shed light on the design of the gap geometry to improve the RDT hydrodynamic performance.