Journal Of Applied Fluid Mechanics
Volume:17 Issue: 4, Apr 2024

This article explores how a chemically reactive solute will disperse across mobile to immobile phase when injected into the fluid flowing within a long circular tube. To model this process, we utilized mathematical modeling, including advection-diffusion equations for flow of fluid within the tube and first-order chemical reaction equations to account for reversible and irreversible reactions on the tubes’ wall. We proposed a numerical method based on an explicit finite difference scheme to solve the governing equations for the dispersion of a chemically reactive solute. We used an upwind method with a conservative representation in the diffusion component to discretize the advection-diffusion equation. To ensure the stability of our proposed numerical scheme, we computed the time step constraint condition so that the maximum principle for the discrete governing equation holds. We also verified the performance of our proposed scheme through computational results that were compared with previous studies. One of our key findings was that the depletion coefficient D0 achieved a quasi-steady state for larger absorption rates. We also observed that the advection coefficient  initially increased with an increasing absorption rate, but eventually declined due to phase exchange kinetics. The dispersion coefficient  also decreased with a rising absorption rate due to a low-velocity gradient in the middle region. Our study showed that rapid distributions are possible under certain conditions, such as a high Damköhler number (Da≥10 ) and a high absorption rate (Γ>5). Computational results show that the proposed scheme can be useful in developing an efficient pulmonary drug delivery system for periodic inhalation of drugs to determine the optimal frequency of injection.

A computational fluid dynamic (CFD) and machine learning approach is used to investigate heat transfer on NASA airfoils of type NACA 0012. Several different models have been developed to examine the effect of laminar flow, Spalart flow, and Allmaras flow on the NACA 0012 airfoil under varying aerodynamic conditions. Temperature conditions at high and low temperatures are discussed in this article for different airfoil modes, which are porous mode and non-porous mode. Specific parameters included permeability of 11.36 x 10-10 m2, porosity of 0.64, an inertia coefficient of 0.37, and a temperature range between 200 K and 400 K. The study revealed that a temperature increase can significantly increase lift-to-drag. Additionally, employing both a porous state and temperature differentials further contributes to enhancing the lift-to-drag coefficient. The neural network also successfully predicted outcomes when adjusting the temperature, particularly in scenarios with a greater number of cases. Nevertheless, this study assessed the accuracy of the system using a SMOTER model. It has been shown that the MSE, MAE, and R for the best performance validation of the testing case were 0.000314, 0.0008, and 0.998960, respectively, at K = 3. However, the study shows that epoch values greater than 2000 increase computational time and cost without improving accuracy. This indicates that the SMOTER model can be used to classify the testing case accurately; however, higher epoch values are not necessary for optimal performance.

The results of large-eddy simulations are presented to illustrate the flow structures generated by the interaction of synthetic jets with a crossflow. The coupled calculations involving the internal flow of the actuator cavity and the external flow are performed using the ANSYS-Fluent software. The influence of the orifice shape (round orifice and rectangular orifices with aspect ratio of 6, 12, or 18) on the evolution of coherent structures is analyzed, and the effects of the jet-to-crossflow velocity ratio (0.5, 1.0, or 1.5) on the turbulent flow behavior are examined. The results show that the first vortex ring shed from the rectangular orifice lip behaves as a plate-like vortex. The horseshoe vortex and first vortex ring are followed by a trailing jet in the case of a round orifice, but this configuration is rarely identified when the orifice is rectangular. For the rectangular orifice with an aspect ratio of 18, the plate-like vortex splits into vortex filaments that become interwoven with the center of the synthetic jet. In general, at the same characteristic velocity, the round-orifice synthetic jet has a stronger capacity for normal penetration into the crossflow, whereas the rectangular-orifice synthetic jet with a large aspect ratio develops closer to the wall. For the rectangular orifice with a large aspect ratio, the development of the synthetic jet is restricted to a small region near the wall at a small jet-to-crossflow velocity ratio.

The hydraulic turbines, especially Francis turbines, frequently run at part load (PL) conditions to meet the dynamic energy needs. The flow field at the runner exit changes significantly with a change in the operating point. At PL, flow instabilities such as the Rotating Vortex Rope (RVR) form in the draft tube of the Francis turbine. The present paper compares the features of the velocity and vorticity field of the Francis turbine draft tube at the best efficiency point (BEP) and PL operations using the Proper Orthogonal Decomposition (POD) of the 2D-PIV data. The POD analysis decomposes the flow field into coherent and incoherent structures describing the spatiotemporal behavior of the flow field. A visual representation of the coherent structures and the turbulent length scales in the flow field is extracted and analyzed for BEP and PL, respectively. The study highlights the salient features of the draft tube flow field, which differentiate the BEP and PL operation. The fast Fourier transform of the temporal coefficients confirms the presence of RVR frequency (0.29 times the runner frequency) at PL. The phase portraits of different modes elucidate the relationship between different harmonics of the RVR frequency at PL.

The helium compressor has the inherent characteristics of a lower single-stage pressure ratio and a higher number of stages than an air compressor. The highly loaded design method effectively addressed the compressibility issue of the helium compressor. However, the compressor designed with this technique has narrow passages, short blades, and a large bending angle, making the end-wall secondary flow more intense than a conventional compressor. In this paper, numerical simulation and experimental validation has been conducted to identify the effectiveness of the axial slot close to the blade root in improving end-wall secondary flow in a high-load helium compressor cascade, and to provide data and experimental support for the engineering application of high-load helium compressors. The analytical results show that slotting can utilize the self-pressure difference to generate gap leakage vortices, and the axial momentum generated by the leakage vortices blows away the vortices formed due to the separation of corner area. The airflow flows close to the suction surface of the blade and breaks away at the trailing edge of the blade, merges with the main flow and forms a new vortex. As the height of the channel increases, the blowing away of the vortices in the corner region becomes more pronounced and the cascade improvement performance is better. The test results show that the total pressure loss coefficient at the design operating point is reduced by 6.167% when a slot height of 8.53 mm is positioned at 65% Ca (axial chord length). The improvement effect becomes 16.469% better at a 4° attack angle.

The Pumps as Turbines (PAT) is a well-established technology suitable for standalone micro-hydropower plants and energy recovery systems. But being lower performance than the dedicated turbines, there are continuous efforts to improve it keeping cost benefits intact. One of the recent modifications of the back cavity filling plays a crucial role in the performance of PAT which is not investigated in detail. In the present paper, the PAT back cavity is filled with a solid ring of various sizes and shapes (back cavity filling) to explore its impact. The developed test facility is used to validate the experimental results with the numerical results for the base case. A numerical model after validation has been employed to investigate the impact of back cavity filling on the internal flow dynamics and the PAT performance. Additionally, the study explored the influence of axial clearance on flow physics, associated losses, and the PAT performance, an aspect rarely discussed by researchers in the PAT mode. After back cavity filling, secondary flow-based disk friction losses were reduced, leading to a 3.5 % increase in PAT efficiency. An analysis of the axial clearance showed that increasing it from 0.015 to 0.076 (mean axial gap/impeller radius) led to a 2.6 % reduction in PAT efficiency. This decline can be primarily attributed to elevated losses associated with disk friction, increased volumetric losses, and the formation of a mixing zone at the impeller inlet, which impeded the flow into the impeller's flow zone.

A closed-loop control framework is developed for the co-flow jet (CFJ) airfoil by combining the numerical flow field environment of a CFJ0012 airfoil with a deep reinforcement learning (DRL) module called tensorforce integrated in Python. The DRL agent, which is trained through interacting with the numerical flow field environment, is capable of acquiring a policy that instructs the mass flow rate of the CFJ to make the stalled airfoil at an angle of attack (AoA) of 18 degrees reach a specific high lift coefficient set to 2.0, thereby effectively suppressing flow separation on the upper surface of the airfoil. The subsequent test shows that the policy can be implemented to find a precise jet momentum coefficient of 0.049 to make the lift coefficient of the CFJ0012 airfoil reach 2.01 with a negligible error of 0.5%. Moreover, to evaluate the generalization ability of the policy trained at an AoA of 18 degrees, two additional tests are conducted at AoAs of 16 and 20 degrees. The results show that, although using the policy gained under another AoA cannot help the lift coefficient of the airfoil reach a set target of 2 accurately, the errors are acceptable with less than 5.5%, which means the policy trained under an AoA of 18 degrees can also be applied to other AoAs to some extent. This work is helpful for the practical application of CFJ technology, as the closed-loop control framework ensures good aerodynamic performance of the CFJ airfoil, even in complex and changeable flight conditions.

The Savonius hydrokinetic turbine (SHT) is widely used for generating electricity from running water. However, most optimization work has been carried out on conventional blades with similar concave and convex profiles. This study aims to enhance SHT performance by modifying the rotor blades' outer surface radius (0.079, 0.087 and 0.095 m) to create a semi-elliptical shape, thus reducing opposing forces. The tip speed ratio (TSR) varies from 0.5 to 1.3 with an interval of 0.1. A constant channel velocity of 0.8 m/s at Re = 2.25 × 105 is considered for the analysis. The flow field has been numerically investigated using the SST k - ω model. This study comprises the angular variation in the coefficients of power (Cp) and torque (Cm), performance curves of the rotor, and pressure distribution on the blade surface at different angular positions. It is observed that the rotor with a radius of 0.095 m has a maximum Cp value of 0.142, which is 7.57% and 18.33% higher than the Cp values of rotors with radii of 0.079 m and 0.087 m, respectively. The maximum power output of the rotor with a radius of 0.095 m is 2.32 W, whereas the power outputs of the rotors with radii of 0.087 m and 0.079 m are 2.16 W and 1.96 W, respectively. An increase in the instantaneous values of Cm between rotation angles 0◦ to 115◦ is observed, during which the returning blades mainly interact with the incoming stream. The pressure decreases as the radius of the semi-elliptical outer surface increases at rotor positions ranging from 0◦ to 225◦, but it increases at rotor positions ranging from 270◦ to 315◦.

The ability of water mist to mitigate blast loads has been widely recognized. However, the effects of the mean droplet diameter and volume fraction of water mist on the blast mitigation effect and underlying mechanisms have not been comprehensively examined. In this study, a three-dimensional numerical simulation based on the Euler-Lagrangian approach was carried out to study the dissipation process of blast wave energy and momentum by water mist, as well as the impact of varying mean droplet diameters (255-855 μm) and volume fractions (2.4×10-3-5.4×10-3) on blast mitigation. The numerical model incorporates interphase mass, momentum, and energy exchanges, as well as droplet breakup and size distribution. The results showed that the most efficient transfer of momentum and energy between the blast wave and water mist occurred at the air/water mist interface. Subsequently, the efficiency of momentum and energy transfer decreased as the blast wave propagated within the water mist due to the blast wave mitigation. The reduction in the mean droplet diameter and the increase in the volume fraction result in an increase in both the total the surface area and number of water droplets, thereby enhancing the efficiency of energy and momentum absorption by droplets and improving their ability to mitigate blasts.

Different flow characteristics namely sequent depth ratio, relative height of jump, relative energy loss, efficiency, relative length of jump and relative length of roller in suddenly expanding channel against inflow Froude number varying between 2 to 9 at different expansion ratios B1/B2 (0.4, 0.5, 0.6 and 0.8) as third variable are experimentally studied. Physical explanations of the variation of these characteristics with Froude number are discussed based on the results from experiments. Empirical models are proposed for all the six characteristics for rectangular and suddenly expanding channels using Buckingham π-method which gives quite satisfactory results when compared with other researchers result. Effectiveness of baffle blocks and sills (with different configurations) were also discussed in dissipating maximum energy. Due weightage has been given to Froude and Reynold’s number in present study as reported literature as well. As a result, baffle block and sills caused a significant improvement in sequent depth ratio by an amount of 30%, reduction in relative length of jump ratio and relative length of the roller by an amount of 38% and 37% respectively. Hence, energy dissipation increases due to appurtenances.

Under the influence of crosswind, when high-speed trains (HSTs) meet on a bridge, they produce complex vortexes, strong aerodynamic loads, and other aerodynamic effects. The purpose of this paper is to reveal the influences of crosswind and windbreaks on the vortexes generated by HSTs, the pressure distributions on the surfaces of the trains, and the aerodynamic load coefficients of the trains when they meet on a bridge, as well as the influence of the pressure waves generated by the trains on the windbreaks. The three-dimensional incompressible improved delayed detached eddy simulation (IDDES) method based on the SST k-ω turbulence model is used for numerical calculation purposes, and the overset grid method is used to realize the relative motions of the trains. The results show that the windbreaks can reduce the negative pressure (NP) imposed on the train surface and effectively improve the pressure distribution; crosswinds have a significant impact on the vortexes generated by trains, and the vortexes generated by the upstream train affect the stability of the downstream train; windbreaks can reduce the aerodynamic load applied when trains meet and thus improve the safety of the trains; and the head and tail waves generated by trains impose pressure on the windbreaks, which affects the reliability of the windbreaks installations. The simulation results can provide a preliminary reference for future research.

Imitating Dolphin fish-like movement is productive method for enhancing their hydrodynamic capabilities. This work aims to analyze and understand the oscillations of tail fluke of Dolphin, which can be used as a propulsive mechanism for underwater fish robots and vehicles. The objective of the work is to achieve the desired oscillating amplitude by simulating the NACA 0012 profile using computational models and Set up the swimming movement of the dolphin, imitating a fish like model. Computational techniques were employed to examine the propulsive capabilities of the oscillating hydrofoil, inspired by the dolphin's biological propulsion. The evolutionary of fluid pattern in the field surrounding both Dolphin fish model and the NACA0012 hydrofoil, from initial motion to cruising, was established, and the hydrodynamic impact was subsequently studied. An user-defined function (UDF) was developed to create a dynamic mesh interface with CFD code ANSYS FLUENT for establishing the oscillations of Dolphin tail across the flow field. Influencing hydrodynamic coefficients such as lift and drag coefficients at different frequencies were also obtained. The findings shown that when the acceleration of the Dolphin fish model increases, the time averaged drag force coefficient drops because The wake field's vortex disperses to have some beneficial effects and pressure of water surrounding the fish head intensifies to produce a large resistance force. Simulation results show a 98% agreement at lower frequency and speed levels but a 5% deviation at higher frequency and speed due to turbulence effects in both models. It was established that the vortex superposition enhances the Dolphin fish like model rather than lowering its positive impacts.  The Strouhal number, which is obtained by the fluid field's evolution rule, can be linked to the Kármán vortex street span with reverse.

Isolated centrifugal fans are widely utilized in air conditioning system and their performance is closely related to the energy consumption of the system. The purpose of this work is to enlarge the fan operation range without reducing peak efficiency. An improved class and shape function method coupled the Kriging model is proposed to design fan airfoils and to optimize the efficiency and static pressure of the fan. Numerical simulation and experimental study show that both pressure coefficients in large and small flow rate are improved by this optimized fan, and the peak efficiency remains to be unchanged almost under design condition. Optimized pressure coefficient increased by 2.17% in design condition. Furthermore, operation range of the isolated centrifugal fan is enlarged via optimization design scheme and surrogate model. A better understanding of the physical mechanisms involved isolated centrifugal fan is further investigated.

Compared with the globe valves and other on-off valves, cavitation and noise are more severe in the throttle control valve of the refrigerating system due to the higher saturated vapor pressure of the refrigerant. A short tube throttle valve (STTV) is an important throttle control component and cavitation noise easily occurs because of the pressure drop caused by flow throttling. To suppress the cavitation noise, this paper proposed a numerical investigation verified by experimental data into cavitation and noise characteristics inside the STTV. Three kinds of typical throttle valve structures are proposed in this study. The flow field and cavitation noise of the STTV under different refrigerant flow rates were analyzed numerically. The noise levels of the prototype and three optimized structures are comparatively assessed and analyzed. The findings indicate that the level of cavitation noise rises with an increase in flow rate through the STTV. Specifically, when the flow rate transitions from 0.014 kg/s to 0.024 kg/s, there is a corresponding increase in noise levels, moving from 102.8 dB to 122.5 dB. There is less cavitation noise upstream and the flow is stable, while the noise is mainly concentrated downstream with symmetrical distribution characteristics. The distribution seems to have small noise near the internal wall, while large noise is in the center of the downstream in the STTV, and the maximum noise is observed at the corner of the valve seat of the valve outlet. The optimized valve featuring a slope structure on the valve seat significantly reduces cavitation noise, with a maximum noise reduction of 24.94 dB compared to the prototype valve.

Cavitation in hydraulic installations is still a major concern when it comes to producing electricity. We are aware of the danger of cavitation and the damage it can cause. We have carried out a study of the behavior of the Francis de song lou lou turbines in operation, their areas of application and their various modes of degradation. The first step was to identify the nature of cavitation during operation. We used the three-dimensional Naviers Stokes equations. The pressure and velocity fields obtained are examined to observe the various fluctuations that disrupt the correct operation of the turbine, and consequently reduce production efficiency. The characteristics of the turbine at the Song Loulou hydroelectric power station in Cameroon were used. The results obtained, compared with those in the literature, are satisfactory. This modelling is an effective tool for detecting the presence of cavitation in the Francis turbine