Journal Of Applied Fluid Mechanics
Volume:15 Issue: 5, Sep-Oct 2022

Two immiscible fluids flowing in microchannels are essential for microdevices to achieve efficient transfer of fluid reactions and heat, droplet mixing, extraction, and emulsification. In this study, a numerical investigation of the flow regime of droplet generation and the droplet breakup behavior of immiscible fluids (water and oil) in various microchannel structures was undertaken. To predict the influence of the microchannel structure on droplet generation and the breakup process, a two-phase level set method was implemented. The generated droplets were validated with experimental results of the T-shape microchannel structure. The obtained numerical results were in good agreement with the experimental results. Furthermore, the validated model was used to investigate the effect of various types of microchannel structures on droplet generation and breakup behavior. Also, the effects of different viscosities, wetted wall contact angles, surface tension, the size of continuous and dispersed channel widths, and the continuous flow rate for droplet generation and breakup in the microchannel were studied. This work contributes to better understanding of effective microchannel design.

Radiation heat transfer is often ignored in several studies as it has few significant effects in some cases. However, when using a participating fluid, where the molecules interact with the radiative spectrum, these effects cannot be disregarded. A numerical study of the heat transfer by natural convection and radiation in two square enclosures (with and without protrusions) using a transparent (non-participating) and semi-transparent (participating) fluid medium was carried out in this study. The governing equations were discretized using the finite volume technique and solved using a CFD code ANSYS CFX. The heat transfer by radiation was modeled using the differential approach. The model proposed in this study was validated with the data available in the literature with errors of less than 3%. The results showed that the addition of the participant fluid (CO2) promotes a better condition for heat transfer. It was proven that the use of the participating medium caused an increase in the Nusselt number, indicating an increase in heat transfer by convection. The presence of protrusions reduces the thermal stratification zone for the pure convection case (CP) and provides a better temperature distribution for the cases conjugated with air (CRAIR) and CO2 (CRCO2) when compared to the cases without protrusions. It is observed that for all cases, the geometry with protrusion presented the highest values for the Nusselt number, indicating that the insertion of the protrusion increases the heat transfer in the enclosure by up to 11%. The airflow values for the conjugated cases are more than 300% higher than those presented for the pure convection case for any Rayleigh number value. The heat flow increased by up to 4 times when the radiation effect was considered. The average Nusselt number increased with the increase in the Rayleigh number and with the coupling of radiation in the energy equation. This indicates that the effect of radiation cannot be disregarded in the study of heat transfer in enclosures.

Aggregation of fluidization media may appear at the dense phase region of the pant-leg fluidized bed near the incline walls. When the particles flow along the inclined wall, the friction and drag force will cause the particles to accumulate on the inclined wall, resulting in an uneven distribution of particles. The stagnant zones can be minimized by correctly arranging secondary air. Computational particle fluid dynamics (CPFD) method was used to simulate the gas-solid two-phase flow pattern in the dense phase region of pant-leg fluidized bed. Cold tests were performed on a benchtop pant-leg fluidized bed. A high speed imaging technology was used to monitor the flow pattern in the dense phase area, whereas the bubble size and residence time were compared to verify the accuracy of the simulation. The gas-solid flow patterns under various models were simulated. The influence of different secondary air velocities on the reduction of stagnant zone in the dense phase zone of the fluidized bed were predicted. The results indicated that the introduction of secondary air could effectively promote the mixing of particles, and weaken the accumulation of particles on the inclined wall surface. Moreover, secondary air can effectively promote the flow between the gas-solid two-phases and improve the combustion characteristics in the furnace.

The passive micromixer is one of the essential devices that can be integrated into the Lab on Chip (LoC) system. Micromixer is needed to increase mixing efficiency. In this paper, two Koch fractal obstacle-based micromixer models of Secondary Snowflakes Fractal Micromixer (SSFM) and Tertiary Snowflakes Fractal Micromixer (TSFM) were designed. The effect of the Koch fractal resistance angles (15o, 30o, 45o, 315o, 330o, 345o) and the influence of the inlet (T and T-vortex) were studied in this paper using COMSOL Multiphysics numerical simulations. The results showed that the TSFM structure with a 30o angle on the T-vortex inlet is optimal. The deflection phenomena generated by the TSFM obstacle enhance the contact area between the two fluids and chaotic convection can be increased at Reynolds Number (Re) 0.05 and Re 100. This paper examines concentration curves along the channel ranging from 1 mol/L to 5 mol/L. This clearly shows that the fluid flow direction changes within the microchannel. This work provides a new design for the micromixer.

A multi-objective optimization study and sensitivity analysis of a SI engine piston-rings pack using dynamics analysis software (AVLExcite Piston&Rings) and optimizer software (modeFRONTIER) are presented. The effects of changing the piston rings' tangential force and face profile on the oil and gas flow behavior inside the piston-rings pack are investigated by calculating the lubrication oil consumption, blow-by, and power losses. The feasibility of the simulation model was determined by comparing it to empirical data obtained from experimental testing of the engine to estimate the amount of oil consumption and blow-by gas flow. Using the statistical modeling algorithm SS-ANOVA, multi-objective optimization investigates the individual and interaction effects of the three rings' tangential forces. This method significantly reduces the time and cost required to find the optimal design, an approach not reported in previous studies. The results showed a strong correlation between simulation and experimental test results, indicating an acceptable match during model validation. Furthermore, the predictions show that tangential forces affect sealing performance; thus, modifying the tangential force resulted in a 30% reduction in oil consumption and less than a 0.8 percent increase in friction. Furthermore, the LKZ oil control ring model efficiently reduces oil consumption by 25% while slightly increasing friction (about 10 percent without face coating).

Contact angle hysteresis (CAH) is a significant factor affecting the drop motion on solid substrates. A model of CAH is introduced to explore the influence of CAH on the dynamics of a sessile drop on a uniformly heated surface, and a two-dimensional evolution equation of the drop thickness is established using the lubrication approximation and Navier slip boundary conditions. A numerical simulation is performed to examine the dynamic behaviors of an evaporating drop, and the drop profile, contact angle, contact line, and moving speed are investigated. Simulated results indicate that the drop evolution process involves drop spreading, pinning, and depinning of the contact line. In the drop spreading stage, when the hysteresis angle increases, the spreading period is shortened, and the spreading radius and spreading speed are reduced; in contrast, the pinning period is raised, and the mass of the drop is apparently reduced with increasing hysteresis angle. In the depinning stage, the CAH declines the contact angle, and a flatter pattern is evolved, thereby improving the heat transfer performance, promoting drop evaporation, and shortening the depinning time. The presence of CAH can speed up the drying of the drop, and the large hysteresis angle leads to faster evaporation. Regulating the CAH is an effective way to manipulate the motion of the contact line for an evaporating drop.

To investigate the movement characteristics of cubic particles in a pump, a deep-sea mining lift model pump with a specific speed of 94 is used as the research object in this study. The discrete element method is coupled with the computational fluid dynamics method to simulate the solid–liquid two-phase flow of cubic particles with different densities in the pump while the effect of particle shape on the solid–liquid two-phase flow in the pump is considered. Results show that the cubic particle movement rules for the same flow component are the same. The cubic particle density imposes a more significant effect on the number of particles in the low-velocity zone than in other zones. The number of particles in the low-velocity zone increases with the increase of density. The cubic particle velocity gradient in the impeller decreases as the particle density increases, and the particles exhibit unsatisfactory following performance in the fluid. As the density increases, the collision exhibited by the cubic particles is primarily particle-to-particle collisions, (i.e., more than 37%), and the collision rate between the cubic particles and first-stage guide vane decreases significantly. Compared with cubic particles, spherical particles are likely to obstruct the flow channel in the guide vane. The collision exhibited by the spherical particles in the pump is primarily particle-to-guide vane collision, and the collision rate between the spherical particles decreases by 15.92%.

Wind flow on and around buildings attains more importance among architectures, builders, urban planners, structural engineers, and wind engineers. Wind tunnel experiments and wind flow assessments of full-scale buildings are expensive and complex in varied terrain conditions. Hence, wind flows are extensively assessed using Computational Fluid Dynamics (CFD). By following the turbulence parameters, CFD turbulence models create the wind tunnel and atmospheric environments. No literature has till elucidated which CFD turbulence model is more suitable for predicting the terrain wind flow on and around high-rise buildings. The efficiency of the CFD models, their performance, and their accuracy must be validated with experimental results, which is indispensable before using the turbulence model in practice. Therefore, this investigation aims to validate the Unsteady Reynolds–Averaged Navier-Stokes (URANS) simulations for a setback tall building under open terrain wind conditions enclosed within the wind tunnel dimensions. The URANS simulation is accompanied with Standard k– ε, Realizable k-ε, RNG k-ε, Standard k–ω, k–ω SST and RSM. The k –ω SST and RSM turbulence models have reproduced the wind pressure coefficients observed from the wind tunnel. However, all turbulence models failed to produce the same velocity profiles at downstream recirculation, as they vary with sampling time. The transient feature, RMS (Root Mean Square), is better reproduced by RSM and k–ω SST models, while the most unsteady features like across wind spectra and eddies were captured by Realizable, RSM and SST using iso-surface. k–ω SST and RSM models predict similar results with the experiment. Where less computational time was required for the  SST, it is promising that this model provides both mean pressure and unsteady feature, encouraging more accurate simulation around the buildings.

In previous studies, researchers established mathematical models for predicting the pressure coefficient in simple cavities and tubed vortex reducers based on the assumptions of incompressibility and adiabatic reversibility. However, these mathematical models are not suitable for engineering design and cannot predict the internal pressure and temperature. In this study, we first derived mathematical equations for predicting the pressure drop and temperature change in a tubed vortex reducer, by considering the irreversible loss at the tube inlet. To compensate for the shortcomings of the incompressibility assumption, we developed an iterative alternating calculation method that revises the density. Subsequently, we established a coupled prediction model based on the aforementioned equations and methods. The verified Reynolds stress model results proved that the coupled prediction model and the single prediction model, which represents the incompressible case, yield similar results in predicting pressure under low-rotating-speed conditions. However, as the rotating speed was increased, the error of the single prediction model gradually increased, whereas the coupled prediction model still had good prediction accuracy. With an increase in the length and number of tubes, the pressure drop showed a decreasing trend, whereas the temperature change did not fluctuate significantly.

The flow boiling heat transfer characteristics of R245fa in segmented internally-threaded tubes are studied experimentally. The heat transfer performances of smooth tube, front-threaded tube, rear-threaded tube, and full-threaded tube are compared. The experiments are carried out with heat flux ranging from 14.01 to 48.79 kW·m-2 and mass flux ranging from 125 to 375 kg·m-2·s-1. The experimental results show that the internal thread structure facilitates the heat transfer. At low mass fluxes and low heat fluxes, the heat transfer performance of the full-threaded tube is the best, while at high heat fluxes and high mass fluxes, that of the front-threaded tube is the best. Convective boiling is dominant in the smooth tube and the front-threaded tube, and corresponding heat transfer coefficients increase significantly with increasing mass flux. In contrast, nucleate boiling dominates in the rear- and full-threaded tube, and the heat transfer coefficients almost keep unchanged with increasing mass flux. The front- and rear-threaded tubes exhibit significantly different heat transfer characteristics, although they have the same heat transfer area. The effect of convective perturbation and bubble nucleation caused by the internal thread structure varies with the zone and intensity, leading to the change of the dominant mechanism of boiling heat transfer.

This study experimentally investigated various spraying modes in electrospraying, an atomization method in which a high voltage is applied to the auxiliary device at the tip of the nozzle. The spraying modes were generated depending on the experimental parameters (voltage, current, and flow rate) and characteristics of two test solutions (S and C), which were a mixture of ethanol, glycerol, citric acid, and water. Solution C had a higher electrical conductivity than solution S. Eleven spray modes were identified in the study. From a comparison of the spray modes, a maximum Sauter mean diameter (SMD) of the cone jet of solution S was 1.7 times that of solution S. The standard deviation of SMD for the unstable, rotating-jet, and pulsed-jet modes were more than two times that for the cone-jet mode. With an increase in flow rate in the cone jet, the SMD and SMD standard deviation of solution C increased linearly, and the SMD value of solution C was ~5% lower than that of solution B. The SMD standard deviations for both S and C solutions were small at low flow rates, and the standard deviation for solution C (with high conductivity) was smaller than that of solution S. For a given SMD, the current associated with solution C was higher than that associated with solution S. The study presented the comprehensive data for SMD, SMD standard deviation, and current in an electrospray system for the two fluids of different electrical conductivities under various experimental conditions.

Experiments on a two-phase rotating detonation combustor operating with gasoline and high total temperature air were conducted to investigate the initiation characteristics, operation mode, and propagation characteristics of a two-phase rotating detonation wave (RDW). The outer diameter, inner diameter, and length of the annular combustor were 204 mm, 166 mm, and 155 mm, respectively. The initiation characteristics, operation mode, and propagation characteristics of the two-phase RDW were studied by varying the total air temperature. The experimental results show that the initiation time of the RDW first decreases and then increases with an increase in the total air temperature and reaches an extreme value at a total air temperature of 713 K. Four operation modes (failure, intermittent detonation, single wave, coexistence of double wave collision, and single wave) of the detonation combustor were found for different total air temperatures. The effect of the total air temperature on the peak pressure stability and propagation frequency of the RDW was studied in detail. From the results, the effect of the equivalence ratio on the working characteristics of a rotating detonation engine (RDE) was investigated at a total air temperature of 713 K. Four detonation propagation modes (sporadic detonation, intermittent detonation, single-wave mode, coexistence of double-wave collision and single wave) were obtained in the combustor. When the equivalence ratio was 0.52, the detonation initiation failed. The pressure characteristics in the combustor and propagation frequency of the RDW were studied with different equivalence ratios. In addition, a long-duration test was performed for 3 s to verify the continuous working feasibility of the two-phase RDE.

In order to improve the performance of single-stage turbines, blade profiling optimization was conducted for the guide vane and rotor under design condition. Support vector regression (SVR) and non-dominated sorting genetic algorithm-II (NSGA-II) were used to execute the optimization, with the objective of maximizing the efficiency and total pressure ratio of single-stage turbines. The gas turbine chosen for the initial study was the KJ66, which is one of the most robust and primitive small gas turbine designs available. The influence mechanism of the stator and rotor profiling on flow field and performance was discussed. The results revealed that compared with the prototype, the adiabatic efficiency increased by 5.95% and the total pressure ratio increased by 0.9%. Furthermore, the matching of flows between the stator blade and rotor blade obviously improved. The optimized guide vane suppressed the flow separation by increasing the leading edge and improving the distribution of the inlet angle of attack. The load distribution of rotors with a 50% spanwise position changed from the original "C" loaded to post-loaded. The leading load obviously decreased, and the angle of attack was smaller than that of the prototype, which effectively weakened the flow separation at the leading edge of the rotor. Compared with the original rotor, the higher lean angle and pressure ratio of the turbine stage also improved. However, the leakage loss near the shroud of the rotor increased, which led to decreased efficiency.

The study confirms the feasibility of using information from flow-induced vibration generated by bluff bodies for measurement of flow. The proposed methodology is based on experiments conducted for recording acceleration signals from vibration sensors embedded along with bluff bodies in cross flow. The studies were undertaken in a wide Reynolds number range of 4700 to 5.655x105, with five different geometries of bluff bodies for shape optimization. Resonance and vortex shedding frequencies of samples are determined experimentally. The recorded acceleration signals are analysed using three different techniques such as frequency shift method, resonance amplitude method and overall acceleration estimation method. In this, the frequency shift method shows correlation at low flow velocities. In the resonance amplitude method and overall acceleration method, acceleration levels show a cubic relation with the flow rate. In the resonance amplitude method, obtained R2 values are from 0.93 to 0.99. Overall acceleration estimation method has shown better correlation with flow rate and its R2 values were above 0.99. The study confirms that application of all three methods together yields reasonably good estimation for a wide range of flow.

As one of the core components of the fire-fighting water-supply system, the performance of a fire pump directly determines the extent of damage caused by the fire. Compared with conventional pumps, the design requirements of fire pumps not only need to ensure that the head of the pump is at 0Qd, 1.0Qd, and 1.5Qd and its efficiency is at 1.0Qd but also consider the cavitation performance at each flow rate, which presents a greater challenge for the design of high-performance fire pumps. By optimizing the design of a centrifugal fire pump with a specific speed of 24.7, numerical calculations were performed to obtain the best optimized scheme Y4. The results show that at the design flow rate the best optimized scheme improves the efficiency by 9.17% compared with the original scheme, and the head meets the design requirements of the fire pump while avoiding the hump phenomenon. Through a comparative analysis, it was found that the optimized scheme Y4 can reduce the pressure pulsations at the outlet of the pump and improve the cavitation performance at each flow rate. The experiment verifies that the head of the best optimized scheme at the design flow rate is 74.43m, the pump efficiency is 40.22%, and there is no hump in the head curve, which can meet the design and use requirements of the fire pump. The maximum reduction in the outlet pressure pulsations coefficient in the best optimized scheme was 47.12% on average. Compared with the original scheme, the critical net positive suction head (NPSHr) of the best optimized scheme was reduced by 21.5%, 17.6%, 15.7%, and 16.8%, respectively.

This work is devoted to the development of a new model for the non-atomizing sprinkler irrigation jet, for calculating the trajectories and landing positions of water droplets. The novelty of the proposed model is that the secondary breakup of the droplets can be calculated during the spraying process. For irrigation jet with a second wind-induced breakup regime, the model is optimized based on the ballistic theory by considering the secondary breakup of droplets and the jet breakup length. The wave-breaking model is used to determine the secondary breakup of the droplets. The output of this model is the water application rate that is calculated by using the cumulative volume of droplets along the radial spraying direction. A comparison of the results obtained using the proposed model with experimental data is conducted to verify the accuracy and reliability of the proposed model. The results show a good agreement of the peak water application rate between the optimized model and the experimental data, with an average error ranging within 6%. The droplets in the front spraying area usually have a diameter of 0-2 mm. This is computed by using the droplet secondary breakup sub-model, resulting in a considerably improved accuracy of the optimized model in the prediction of the water application rate of a sprinkler.

Vortex tube is a device without moving parts with ability to separate pressurized gas into two streams: cold and hot. This is a consequence of the Eckert-Wiese effect, which is responsible for spontaneous redistribution of total energy within the flow domain. In order for vortex tubes to work properly, there are some constraints which have to be fulfilled. The most important constraint in that sense is the L/D ratio. One part of this paper is dedicated to the research of the influence of L/D ratio on the energy separation in a vortex tube, i.e. to the values of total temperatures on cold and hot outlets of the device. On the other hand, experimental research of the inner flow is quite challenging since vortex tube is a device of small dimensions. Hence, we are relaying on numerical computations. One of important quantities that has to be prescribed in these computations is the turbulent Prandtl number PrT. Because of that, the other part of this paper is dedicated to research of the influence of PrT on the results of numerical computations. The research is conducted using open-source software OpenFOAM. Turbulence is modelled using two-equation and RST models. For small L/D ratios there is a secondary circulation that acts as a refrigeration cycle, and for greater L/D ratios distribution of velocity and temperature inside the vortex tube remains the same, regardless of the stagnation point presence. It is not justified to increase the length of the vortex tube beyond 20D since the change in cold total temperature inside the vortex tube as well at the cold outlet is practically null. For L/D variation from 1.8 to 10, the cold outlet temperature changes from 270.9 K to 266.8 K, and then rises to its final value of 270.5 K. For L/D ratio from 20 to 60, the total temperature at cold end remains unchanged at 271.3 K. We obtained good results with the unit value of turbulent Prandtl number, and demonstrated that increasing the PrT beyond unit value is not necessary in order to numerically obtain the energy separation inside the vortex tube.

This paper investigates the flow structures on the near- and far-wake of a 1/8th scaled simplified heavy vehicle model called Ground Transportation System (GTS) model using a steady-state Reynolds-averaged Navier-Stokes (RANS) with k-ω Shear Stress Transport (SST) turbulence model at Reynolds number (Re) of 1.6 × 106 and yaw angles (Ψ) = 0o-14o. The current CFD results have been validated using experimental data from the literature. Two crosswind conditions based on the crosswind incidence angle (β) are adopted; β < 90o is called crosswind, and β = 90o (perpendicular to the GTS side surface) is called pure crosswind. Vortex detection scheme based on Ω method and total pressure coefficient (Cpt) contours is used to visualize the wake structure. With Ψ, vortices on the GTS roof take birth as a result of pressure differences between the windward and leeward sides. These vortices grow in size as they travel downstream. The growth in size is related to the Helmholtz theorem of vorticity and Kelvin’s Circulation Theorem. The vortices merged at Z/W > -4 (Z/W = 0 is the GTS rear surface) downstream of the GTS for Ψ = 7.5o and 14o. The merged vortex dissipates at Z/W > -6 and Z/W > -8 for Ψ = 7.5o and 14o, respectively. In the pure crosswind condition, the merged vortex attaches to the ground due to the velocity difference between the freestream and the moving computational ground used in the present simulation. At Ψ = 14o, surface streamlines on the GTS surface show the creation of two co-rotating vortices on the windward roof. For the present Ψ, similar flow structures between the two crosswind conditions are shown. Initial results show that the aerodynamic crosswind stability of a truck is related to the spanwise pressure difference between the windward and leeward surfaces of the truck.

At stations, high-speed trains frequently pass through the platform without stopping, where a combination of two island platforms represents the most common layout. The interaction between the train and the platform leads to certain problems, such as reductions in the comfort of the waiting environment and the safety of people around the platform. However, in the literature, there are few studies on the aerodynamic response between the train and the platform and on the airflow field characteristics above the platform when the train passes through the platform under different crosswind speeds. Therefore, we attempted to fill this gap using numerical methods to study the aerodynamic characteristics of the train passing through island platforms at 350 km/h under different crosswind speeds (10, 15, 20, 25, and 30 m/s). The aerodynamic response of high-speed trains combined with the flow field distribution is discussed in depth. We studied the wind speed distribution at different longitudinal distances above the platform, and obtained the position of the maximum wind speed when the head and tail car passed through the platform. Based on this, the wind speed distribution at different lateral distances above the platform was studied, and the reasons for the airflow changes above the platform were analyzed. The research results show that when a train enters a platform at 350km/h under a crosswind speed of 30 m/s, the reductions in the drag and lateral force of the whole vehicle reach their maximum, which are 50.44% and 66.51%, respectively. However, the change trend in the whole car lift force is opposite to that of the drag and lateral force, which increase when the train enters the platform and decrease when it leaves the platform. The largest growth in lift force is 102.39%, which occurred at a wind speed of 30m/s. The airflow velocity above the platform will increase rapidly as the head and tail car pass through the platform. A higher crosswind speed will result in the monitoring point of platform reaching its maximum airflow speed to an earlier time as the tail car passes through the platform. Meanwhile, we found that the lateral distance 1 – 2m above the platform is the area with the largest wind speed attenuation.

Surface-Piercing Propellers (SPPs) are essential categories of high-performance propulsion systems usually used for high-speed boats, which are designed to operate in semi-submerged conditions. In such conditions, a propeller performs in a two-phase mixed environment, consisting of water and air concurrently. Due to the intrinsic complexity of the working environment, describing the performance of an SPP is complex and cannot be recognized with the traditional submerged propellers. The present study aims to assess the effect of immersion depth on semi-submersible propellers. Accordingly, experimental tests in a towing tank were used along with a numerical method to achieve reliable results. In the numerical method, a sliding mesh was used to simulate the propeller's motion, and the volume of fluid was used to model the free surface. The hydrodynamic coefficients of the SPP, measured in the towing tank, were used to validate the numerical method. The outcomes of the numerical method were revealed to be in good agreement with the experimental data. The results showed that the critical advance coefficient decreased with the rise in the immersion depth. In detail, altering the immersion depth from 0.4 to 0.75 reduced the critical value of the advance coefficient from 0.8 to 0.7. The ventilation pattern also changed with increasing the immersion ratio. For a constant advance coefficient, the amount of ventilation increased at shallower depths of immersion.

Serpentine nozzle can effectively suppress the infrared radiation signatures of the aero-engine exhaust system. However, it experiences the remarkable fluid-structure interaction (FSI) process at the work condition. In this paper, the deformation behavior of the serpentine nozzle and its flow characteristic were investigated numerically. Then, the influences of the wall thickness and the geometric configuration on the FSI effect were also explored. The results show that, the mechanism of the fluid-structure interaction is formed through the data transfer of the force and the displacement at the FSI interface. Under the FSI effect, there occur the ballon-like swellings at the second S passage, and the linear section bends upward along the Y direction. They induce the special flow features including the flow separation, the shock wave and the plume vector angle. As the value of the wall thickness increases from 3mm to 6mm, the maximum of the deformation displacement of the serpentine nozzle decreases 68.5mm. As compared to the uncoupled state, the variation of the axial thrust decreases from 2.70% to 0.70% at the coupled state. The circular-to-rectangular profile and the S-shaped passage enlarge the deformation behavior of the nozzle structure. The value of the axial thrust of the serpentine nozzle with 5mm wall thickness for the coupled state is lower 1.92% than these for the uncoupled state.

In this paper, numerical investigations of the harnessed power from Flow-Induced Vibrations of a new modified circular cylinder are performed. The proposed cylinder modification consists in adding two slots located on the front surface of the cylinder, instead of the baseline configuration, usually applied, which consists of a Passive Turbulence Control in form two straight strips. The computations are based on the solution of the Unsteady Reynolds- Averaged Navier-Stokes equations (URANS) coupled with the dynamic equations system describing the cylinder motion, where turbulence is modeled using the two-equation SST k – ω model. The harvested and the harnessed powers are thereafter calculated according to the amplitude and the frequency of the cylinder oscillatory motion. The numerical results show that the slots lead to shift the flow separation point toward the leading edge, which involves higher hydrodynamic instabilities resulting in higher oscillations amplitudes, and thereby a significant enhancement of the harnessed power is noticed.

Surface characteristics have an important role in defining hydrodynamics of the flow through hydraulic machines. Surface roughness is a critical parameter that contributes to altering near-wall flow features and promotes frictional losses in various components of a water turbine. The nature of flow through the Francis turbine runner is highly complex, especially at cavitating regimes, and the surface roughness effects add to the flow complexity. The present work is aimed at evaluating surface roughness effects on the cavitation performance of a low head prototype Francis turbine computationally. Complete cavitation characteristic of the turbine is derived with the consideration of smooth and rough boundaries by implementing SST k-ω turbulence model and cavitation model based on the Rayleigh-Plesset equation, and a comparative study is carried out comprehensively. For the analysis, the complete flow domain of the turbine is considered, and the simulations are conducted for four different operating conditions from the part load of 60% to overload of 120%. Different values of equivalent sand grain roughness, ks, are assigned to different components of the turbine by following the International Electro-Technical Commission standard IEC 62097 Edition-2. It is concluded that the surface roughness effects on the performance of the turbine in the absence of cavitation are not significant for operation at BEP but for the part load of 60% and overload operations, it has considerable hydrodynamic effects. However, these effects become more detrimental at developed cavitation regimes. The obtained computational results are found in a fair agreement with the available experimental results and are quite consistent with the previous research.

The erosion wear of a gas–solid flow is a major challenge for an S Zorb reactor; it affects the production safety and stability. This study aims to investigate the erosion wear problem of a gas–solid flow on an assignment plate in an S Zorb reactor using CFD–DEM. The erosion wear behavior of the assignment plate is studied through flow field and particle trajectory analyses. Moreover, five polyhedral particle models were established by DEM to study the effect of particle shape on erosion behavior. The numerical calculation results show that the special structure of the bubble cap affords a special serrated erosion wear area. According to the movement process of particles, the erosion wear of the assignment plate is divided into two stages: first a large range of slight erosion wear occurs and then seriously concentrated erosion wear occurs. With the decrease of particle sphericity φ, the erosion wear rate first decreases rapidly and then increases slowly. φ=0.85 is the critical value of change, and the assignment plate has the lowest erosion wear rate. The simulation studies are helpful for reducing the erosion wear of the assignment plate and improving its service life.

Double- to single-loop pattern transition and a significant reduction in the power number with a decrease in the clearance of the Rushton turbine impeller in a baffled reactor was elucidated in earlier research works. The present work investigates the physical reasons behind these phenomena using the computational fluid dynamics approach. The Reynolds Averaged Navier–Stokes equations with standard  turbulence model closure were used to model the turbulent flow conditions in the reactor vessels. The Multiple Reference Frame (MRF) approach was adopted to model the impeller baffle interactions in the reactor vessels. The implicit Volume of Fluid (VOF) method was employed to simulate the aeration process in the reactor configurations considered. The development of a low pressure region below the impeller of a low clearance vessel deflects the discharge streams downward, leading to the formation of a single-loop pattern. The downward movement of the discharge streams reduces the vortex activity behind the impeller blades, leading to weaker form drag and a decrease in the power number of the impeller. Similarly, a high clearance vessel provides a low pressure region above the impeller which deflects the discharge streams above the impeller, resulting in a single-loop pattern and a considerable increase in the air entrainment due to superior vortex and turbulence activity present near the free liquid surface. The standard reactor vessel was found to provide superior bulk mixing of fluid as the overall turbulent dissipation rate is 35% more than that associated with low and high clearance vessels.