Journal Of Applied Fluid Mechanics
Volume:16 Issue: 4, Apr 2023

In this study the mass transport through free turbulent liquid surfaces, or gas/liquid interfaces, is considered. The main direction of mass transfer is perpendicular to the interface, so that a one-dimensional point of view is followed. The equations for the interfacial gas/liquid transport are presented using the random square waves method (RSW), a statistical tool that models the fluctuations of physical variables as ideal signals. The method defines three statistical functions (partition, reduction, and superposition), related to fluctuations of concentration and velocity, which were introduced into the mass advection-diffusion equation generating a set of differential equations adequate for boundary layer problems. Solution profiles of the partition and reduction functions, and of turbulent fluxes across the boundary layer were obtained for transient situations. The solutions use Taylor series centered at the immersed border of the concentration boundary layer. For practical applications, the series were truncated and the coefficients were calculated in order to satisfy adequate physical conditions. The proposed procedure substitutes coefficients of the higher order parcels of the truncated series, enabling them to satisfy boundary conditions in the two borders of the domain of interest, which is the region of variation of the mass concentration. The theoretical profiles for concentration and turbulent fluxes close to the interface agree with measurements and predictions found in the literature.

Enhancement in the aerodynamic performance of wings and airfoils is very notable when Active Flow Control (AFC) is applied to Short Take-off and Landing aircraft (STOL). The present numerical study shows the application of steady, pulsed and synthetic tangential jets applied to the plain flap shoulder of a modified NASA Trapezoidal Wing. Pulsed jets are modeled by sinusoidal and square waveforms while synthetic jets are modeled only by pure sine waveform. The freestream airflow conditions are Mach number equal to 0.2 and Reynolds number equal to 4.3 million based on the mean aerodynamic chord. The presented simulations are two-dimensional and based on RANS for steady jet cases and URANS for pulsed and synthetic cases, compiled with the open-source suite SU2 and adapted for time varying boundary conditions. Numerical results for modified configurations based on the same baseline wing profile considering different leading edges, jet slot height, flap position, blowing mass flow, type and frequency of the jets are presented. Curves of pressure coefficient distribution revealed a substantial influence upstream of the AFC, around the slat and main element. The jet slot height analysis showed that the lift gain is also influenced by the slot size due to the change of the local flow velocity considering the same blowing momentum coefficient. Regarding the jet frequency, no significant differences on the lift coefficients were found between the reduced frequencies F+ equal to 1 and 2. Results of aerodynamic loads showed an improved lift coefficient in relation to the baseline airfoil when pulsed and steady jets are employed. Pulsed jets under square waveform were effective even at high deflected flap condition at 50°, with a significant gain in the lift coefficient of 36%, in relation to the uncontrolled case, combined with a drag reduction of 20%, and a decrease in mass flow up to 49% in relation to the steady jet for the same lift gain. Although sine and square waveform results presented similar improvements for lift, the drag is around 15% higher for the former. When compared with the steady jet case, the mass flow reduction is 36% for the sinewave. Synthetic jets with zero-net-mass-flux proved superior to the baseline conventional multi-element airfoil only with deployed flap at 50°, where a modest lift improvement of 5% was observed.

This paper is presented to compare various wavy leading-edge protuberances on oscillating hydrofoil performance and power efficiency. The unsteady turbulent 3D flow simulations were carried out by using the StarCCM+ software. The 3D hydrofoil with a straight at the leading-edge is a NACA0015 section with a chord of 0.24 m and an aspect ratio of 7. Four new types of hydrofoils are proposed with wavy leading-edge protuberance. The RANS equations with the realizable k–ε turbulence model are used to predict the turbulent flow around the hydrofoil under different conditions. In order to validate, a comparison of the numerical results of the forces coefficients and power efficiency of non-protuberance oscillating hydrofoil are shown in good agreement with experimental data. Then, four new profiles on the leading-edge of the hydrofoils are simulated and many results of the pressure distribution, vorticity contour, streamline velocity, and power coefficient for five hydrofoil types are presented and discussed. It is concluded that the hydrofoil of Type-M can achieve constantly higher efficiency of over 46% by employing appropriate heave and pitch amplitudes.

Surface structure is used to interfere with the turbulent boundary layer in the groove drag reduction, which is important to the endurance and stability of high-speed and ultrahigh-speed aircraft. The size of the groove structure directly affects the flow in the turbulent boundary layer and changes the drag reduction effect. The drag reduction characteristics of bionic triangular (V-groove) riblets were studied through Particle Image Velocimetry (PIV) experiment and Finite Volume Method (FVM) simulation. Triangular riblets with adjacent height ratios (AHR) of 1.00, 1.74, and 3.02 were considered in this research, and the influence of these groove structures on the flow characteristics of turbulence near the wall is compared with those of the smooth plates. The distribution of time-averaged velocity, turbulence intensity, and coherent structures of turbulent boundary layer on the riblet surface is analyzed to document the effects of the geometric parameters of various groove structures on drag reduction rates. Results showed that the best drag reduction is obtained using the V-groove riblets with adjacent height ratio of 1:1 under the low free-stream velocity. The results can be used as a reference for further optimization of drag reduction structures with surface grooves.

Centrifugal pumps are turbomachines that have wide industrial applications and could perform in different ways such as pump and turbine mode. The maintenance of this equipment is mostly carried out using invasive methods that are expensive, time-consuming, and even complicated. The application of non-invasive methods is sought since they offer the advantage of real-time monitoring without stopping the process, reducing component assembly and disassembly times and providing a faster response. The aim of this work is done an experimental investigation that shows evidence about how the information on the hydraulic variables can be obtained if the electrical variables are monitored for the modes of operation such as pump and turbine. This work is divided into two parts, the first part is based on a statistical analysis to perform a multivariate adjustment through copulas and probability distributions. The second part focuses on the graphical analysis of the power density spectra for the hydraulic variables, the torque, and the defined electrical variables. The amplitude peaks of each variable and which peaks are common between them are determined. A statistically significant fit for Tawn type 2 copula is obtained with the indicator variable of pressure fluctuation and a multivariate transformation of the three-phase network currents. In the spectra analysis, common amplitude peaks are observed between the spectra that indicate the information flow on the phenomena between the hydraulic variables and the electrical variables.

Clean energy sources like wind energy have been receiving much attention, and great emphasis has been given to the design and optimization of horizontal axis wind turbines, but just as important are the vertical axis wind turbines that can be used for generating energy for small businesses, houses, and buildings. This article sought to study the optimal geometrical parameters of a H-Darrieus vertical axis wind turbine using surrogate-based optimization with three different types of surrogate models and compared them. Airfoil chord and thickness were chosen as the design variables and respective ranges set at 0.32-0.6 m and 0.04-0.16 m. All evaluations are carried out for a tip-speed ratio of 1.5. Three different surrogate models were used and compared, namely a quadratic polynomial response surface, an artificial neural network based on radial basis functions called Extreme Learning Machine and a Kriging interpolator. Surrogates were constructed based on an initial sample data distributed according to a full factorial design. A test set was designed to evaluate the accuracy of the surrogates. Both training and testing data sets were generated using 2D CFD modeling to reduce computational cost. From the test set, Extreme Learning Machine surrogate showed the smallest RMSE of 11.24%, followed by Kriging, at 17.64%, and Response Surface of 22.17%. For the optimal designs the same pattern ensued, with optimal power coefficient overestimated by 8.7% for the response surface surrogate, followed by 3.12% and 2.17% for the Kriging interpolator and the Extreme Learning Machine, respectively. Power coefficient curves comparing the three optimal geometries from each surrogate were calculated and plotted. Optimal turbine obtained from Kriging surrogate optimization process resulted in a 7.92% increase in the Cp, whilst Extreme Learning Machine and Response Surface resulted in a 7.86% and 4.29% increase, respectively, all when compared to baseline CFD model. Concluding guidelines are that the quadratic polynomial response surface may not be the best alternative when dealing with complex non-linear relationships as typically present in VAWT simulations. Superior techniques such as Extreme Learning Machine and Kriging could be more suitable for this application.

The Coanda effect is a tendency of flowing fluid to follow the solid surface rather than to separate under the inertial effect. This paper presents the effects of surface curvature on the Coanda angle and jet deflection for a vertical jet impacting a horizontal cylinder (of diameter D) with Froude numbers Fr = 4.74 and 6.33. The penetration depth h of the cylinder in the jet is varied from h/D = 0.01-0.4 while the jet-to-cylinder diameter ratio d/D is varied from 0.11 to 2.0. For Fr = 4.74, the round jet impacting the cylinder evolves into a planar jet for d/D £ 0.40 where the jet deflection is positive, dictated by the Coanda effect. The deflection of the jet is negative for d/D = 0.60 and 0.83 where the bouncing (inertial) effect overwhelms the Coanda effect. Fr = 4.74, the jet deflects in the positive direction, regardless of d/D. With increasing h/D or decreasing d/D, both deflection and Coanda angles monotonically increase. An intermittent switch of the jet from one side of the cylinder to the other is observed for certain curvature of the cylinder, ascribed to the competition between bouncing and Coanda effects.

In this study, airfoil shape optimization of a wind turbine blade is performed using the ANSYS Fluent Adjoint Solver. The aim of this optimization process is to increase the wind turbine output power, and the objective function is to maximize the airfoil lift to drag ratio (Cl/CD ). This study is applied to the NREL phase VI wind turbine, therefore, the S809 airfoil is used as a reference profile. First, for the validation of the applied numerical model, steady-state simulations are carried out for the S809 airfoil at various angles of attack. Then, the optimization is performed with the airfoil set at a fixed angle of attack, , considering three Reynolds numbers, Re =3 105,4.8 105  and 106. Next, computations are performed for the fluid flow around the optimized airfoils at angles of attack AOA= 6.1° ranging from 0° to 20°. The results show that (i) the lift to drag ratios of the optimized airfoils are significantly improved compared to the baseline S809 airfoil, (ii) this improvement is sensitive to the Reynolds number, and (iii) the Cl/CD ratios are also improved for another angle of attack values. Thereafter, the optimized airfoils are used for the design of the NREL Phase VI blade and the aerodynamic performances of this new wind turbine are assessed using the open-source code QBlade. These latter results indicate that when the blades are designed with the optimized airfoils, the wind turbine aerodynamic performances increase significantly. Indeed, at a wind speed of 10 m/s, the power output of the wind turbine is improved by about 38% compared to that of the original turbine.

In recent years, the microfluidics technique has screwed up rising attention and be in progress a fascinating topic. Species mixing is a compelling part of any microfluidic system that abides by the major challenge. In this research, a relative explication of mixing quality of microchannels of two cross-sections square and circular in spiral form is presented by numerical simulation. To perform the visitation, geometric parameters like axial length of microchannels and hydraulic diameter are taken equal for both cases. Computational Fluid Dynamics codes unriddle the Continuity equation, Navier-Stokes equation, and Convection-Diffusion equation. Explication of fluid flow and mixing have been gone through with an extensive limit of Reynolds numbers 1 to 125. The results explicate that the circular section spiral microchannel affords a higher mixing quality admixed to the square section spiral microchannel. Furthermore, in the circumstance of circular section spiral micromixer, mixing index esteem has attained 92% at Re =125.  For both section micromixer, the esteem of mixing index enhancement be contingent on Reynolds numbers. For both cases, pressure downfall has been computed for microchannels of similar lengths. The esteem of pressure downfall in square section spiral mixer is excess than circular section spiral mixer. The simulation outcomes exhibited that the circular section spiral micromixer is an effective design for microfluidic devices like Lab on a chip (LOC).

A numerical simulation is used to investigate the flow field characteristics of a three-dimensional over-under turbine-based combined cycle circular-to-rectangular transition exhaust nozzle when only the turbojet flowpath is in operation. The effect on the exhaust nozzle performance of there being a secondary injection on the ramp of the expansion section of the turbojet flow path is then examined. Finally, the impact of variations in the secondary injection design parameters is further investigated. The results show that a secondary injection can improve the exhaust nozzle's performance by reducing the axial force on the inner wall of the flow path. However, changes in flight status can undermine this improvement. Under the baseline operating condition, a secondary injection with a larger angle and close to the ramp outlet can produce a more significant improvement in nozzle performance. In this study, the axial thrust coefficient, lift and pitch moment of the nozzle can be improved by a maximum of 9.312%, 66.007% and 10.975%, respectively.

This article studies the aerodynamic performance of a novel bypass shock-induced thrust vector nozzle. An arc-shaped bypass is innovatively designed to optimize nozzle performance and equips a variable shrinkage part. The nozzle performance is investigated numerically under diverse shrinkage area ratios. Computational results indicate that both geometry and friction choking have important effects on the nozzle performance. Normally, in the case of without any bypass shrinkage, the flow choking occurs at the bypass outlet. Very small bypass shrinkage is unable to change the flow choking location. The bypass geometry choking comes up at its throat as the shrinkage area ratio of the bypass reaches 0.06. According to computational results, the vectoring angle diminishes with the increasing shrinkage area ratio of the bypass, thrust force ratio, thrust efficiency, specific impulse ratio, and coefficient of discharge increase. As the NPR enlarges, the deflection angle and thrust efficiency decrease, and the thrust force ratio increases.

Venturi bubble generators have been extensively studied because of having a simple structure and high foaming efficiency, while producing a uniform bubble size. The effect of a noncondensable gas on hydraulic cavitation was considered to improve the Zwart-Gerber-Belamri cavitation model. This improved model and a population balance model were used to study the effect of cavitation on bubble fragmentation. The CFD-PBM results were compared with experimental results, and the accuracy of the improved calculation method was verified in terms of the distributions for the cavitation cavity, gas phase, and bubble size. The calculation results showed that increasing the noncondensable gas content over a certain range promoted the development of hydraulic cavitation, and the cavitation intensity could be indirectly controlled by adjusting the noncondensable gas content. With increasing cavitation intensity, the average bubble size decreased, and the bubble size distribution became narrower. Therefore, a high-pressure pulse generated by cavitation could effectively break bubbles. The development process of microbubbles was studied. The main controlling factors for bubble formation were determined to be the turbulent shear force of the fluid and the collapse impact force of the cavitation group, which provides a theoretical basis for optimizing the design of bubble generators.

With the advancement in the modern transport system, it has become imperative to achieve larger aircraft volume at minimum cost wherein aerodynamics plays a major role. The present work aims to investigate the flow over a typical blended wing body (BWB) configuration at different angles of attack adopting experimental and computational techniques. Experiments consisted of the force measurements and oil flow visualizations at a free stream velocity of around 19 m/s. Computations were made using the commercially available software ANSYS 18.1. Results indicated that the use of a Blended Wing Body Configuration resulted in improved aerodynamic characteristics in comparison to other wing configurations such as Delta/ Double delta wings. Surface flow visualization carried out over the BWB configuration indicated the reason behind increased lift coefficients. Reasonable agreement of experiments and computations was observed.

This study addresses the measurement of three-dimensional (3D) bubble rising behaviour in still water with bubble equivalent diameters ranging from 2.61 mm to 5.11 mm using high-speed imaging and virtual stereo vision technology. The bubble shape, 3D trajectory/velocity, displacement angular frequency and terminal velocity of bubbles are analysed. The bubble equivalent diameter is obtained by the elliptic volume method. The bubbles are divided into small and large bubbles with a critical equivalent diameter of 4.49 mm, according to whether they are accompanied by deformation. The small bubbles (deq<4.49 mm) are spherical or ellipsoid, while the large bubbles (deq≥4.49 mm) exhibit ellipsoid, mushroom and hat shapes. The 3D trajectory is obtained by 3D reconstruction of bubble centroid coordinates. The rising trajectory of small bubbles shows 3D spiral motion, while the pitch increases gradually with the increase in the equivalent diameter. The trajectory of large bubbles is a two-dimensional (2D) zigzag. The bubble displacement curves in x- and z-directions are evaluated with third-order Fourier fitting. The results show that the bubble displacement frequency in the x and z-directions decreases with the increasing bubble diameter, and the displacement frequency in the xdirection is larger than that in the z-direction. The relative proportions of the viscous force, buoyancy, surface tension and inertial force on bubbles with different equivalent diameters are different, which leads to three trends in the vertical velocity of bubbles within the diameter range of this study. Finally, the bubble terminal velocity in still water is investigated. The terminal velocity first decreases and then increases with the increase in the equivalent diameter. The minimum value is 16.17 cm/s when the diameter of the bubble equivalent diameter is 4.49 mm. Moreover, the applicability of some classical prediction models is discussed.

The Dual Bell Nozzle is the most ambitious of several supersonic, altitude-compensating nozzle concepts for rocket engines. This design's objective is to enhance performance in two different evolving regimes (Sea-Level and High Altitude Mode) by self-adaptation with no mechanical control. The concept is simple in theory, but the structural efforts involved are significant. The study carried out in this paper is a simulation of the flows in this type of nozzle. Computational fluid dynamics (CFD) is increasingly used as an analytical tool in research and industry. Simulation is not a substitute for experimentation but a complement to it; it allows the analysis of the problem in real conditions (reproduce tests that are done in experimentation to better understand them and at lower cost) or, on the contrary, in extreme or marginal test conditions (extreme climates, installation defects, etc.). Through simulation, the studied system becomes more flexible. We can easily make parametric studies. Simulation almost always takes the form of a program or computer tool. These are commonly called simulation environments. Developments and progress over the past two decades have led to the emergence of a methodology that has become standard. As for any complex system, the control of a phenomenon is based on the identification and modularization of the tasks. Currently, the standard methodology divides the simulation process into four distinct tasks: geometric modeling, meshing, solving, and finally analysis and visualization. In this study, we will present a test case used to validate our computational models that will be used to optimize the profile of a dual bell nozzle. We will use the Ansys-ICEM environment to generate the meshes and the Ansys-Fluent environment to solve the equations of our models. Our results will then be compared with experimental and numerical data from our literature review.

Radial force in low-head axial flow turbines (AFTs) is an influential factor in their operational stability. To explore the transient operating behavior of the radial force in low-head AFTs under different blade numbers, transient numeric computations were executed with the shear stress transport (SST) k-w turbulent model. Turbine performance was numerically computed and compared with results from experiments. Furthermore, the unsteady flow field pulsations were experimentally verified by means of pressure sensors. The radial forces on the runners (z = 2, 3, and 4) were each numerically studied in time, frequency, and joint time–frequency fields. The result reveals that the radial force acting on the runner varies with time, since periodic radial forces reflect the vane number on the stay vanes with minimal runner effect. Moreover, the amplitude of the radial forces is directly proportional to the flow rate. Furthermore, the spectral analysis shows that the radial force frequency is close to the blade passing frequency and also increases radially outward since peak values were recorded in this region. Minimal radial force amplitudes were recorded when z = 3, across all flow conditions, making this configuration suitable for smooth and reliable operation. The unstable pressure and force pulses that affect the noise and vibration produced in the turbine are instigated by the flow exchange that occurs between the guide vane and the runner. In order to optimize turbines for increased operational dependability, the acquired data would be crucial references for noise and vibration analytical investigations.

As a typical microdroplet, double emulsion droplet, has received much attention and been widely used in recent years. For a simplified double-cross-shaped microchannel, the process of preparing double emulsion droplets is numerically simulated in this paper. The mechanism of droplet forming was analyzed, and the effects of the angles of the inner-, middle-, and outer-phase channels of the microchip, length of the focusing hole, and expansion angle on the process and quality of the double emulsion droplet formation were investigated. The variation in angles between each inlet channel affects the droplet area; the change in expansion angle affects the flow pattern of droplets; the change in each geometric parameter affects the monodispersity of droplets. The droplet area is fitted to the microchannel geometric parameters and the functional expressions that represent their relationship are derived. The work in this paper provides a reference for the practical production and research of double emulsion droplets.

Water streams with low heads can be found in India, both naturally as well as through irrigation canals. These natural resources can be used to generate electricity by utilising Hydrokinetic water turbines. The current investigation includes an experimental investigation of the axial flow turbine in order to make use of these naturally available resources. For Axial Flow Turbines (AFT), the influence of the fillet radius at the leading edge corner of the runner blade is studied. The experiments are carried out under various turbine loads and corresponding head conditions. The 3D printed runners with six different fillet radius (Rtu), i.e. 0 mm, 2 mm, 4 mm, 6 mm, 8 mm, and 10 mm are examined experimentally. The results are presented in the form of obtained efficiency (η ) with a diverse Fillet Radius Ratio (FRR) for different Tip Speed Ratio (TSR) and equivalent head (H). The results indicate that, at the head, 0.012 m, with a sharp edge, i.e., FRR = 0 (Rtu = 0 mm), the minimum efficiency of 34.90% is recorded. However, at the same water head, the maximum efficiency of 84.82% is achieved with FRR = 0.046 (Rtu = 2 mm).

The sleeve regulating valve is an important part of a pipeline system and is widely used in the fields of nuclear power and thermal power. In this study, a series of numerical and experimental studies are performed to understand the depressurized flow characteristics inside a new type of multi-layer sleeve regulating valve. In the calculations, the standard k-ԑ turbulence model and the mixture model combined with the Zwart–Gerber–Belamri cavitation model are used to clarify the internal flow and cavitation characteristics in the regulating valve. With the new valve, the results show that when the valve is fully opened, the pressure drop at all levels of the valve is comparatively average (approximately 2–3 MPa for each level) and the fluid velocity in the sleeves at all levels is comparatively uniform at 90 m/s—which can prevent the valve from being eroded by highly changing fluid flow rates, and also offers ideal pressure reduction performance. To reduce the degree of cavitation, it is recommended to adjust the outlet pressure of the valve to 0.7 MPa.

During the operation of the floating ring gas film seal, a certain amount of heat is generated inside the seal gap, giving rise to thermal deformation of the seal rings, and further leading to operation unstable and increased leakage rate. Based on the gas lubrication theory, the control equations of gas pressure and gas film thickness of the floating ring gas film seal are obtained. And the energy and temperature-viscosity equation are also introduced. The above equations were solved by the finite difference method and their correctness was verified by experiments. The variation of opening force, leakage rate, friction force, and gas film temperature rise with rotating speed, inlet pressure, and eccentricity were analyzed. The results reveal that, for leakage rate, the difference between the modeled and tested values is only 2.94% at high speeds, taking into account the influence of the temperature-viscosity effect. The experiment substantiates that the temperature-viscosity effect model is scientifically valid. Operation parameters also have different effects on sealing performance. Compared with isothermal flow, the pressure distribution in the gas film flow field will change significantly with increasing gas temperature, which means that the temperature-viscosity effect cannot be neglected in the flow field calculation. These results provide grounds for further study of the thermoelastic effect of air film seal of floating ring and have important engineering significance.