Journal Of Applied Fluid Mechanics
Volume:18 Issue: 5, Jan 2025

Stratospheric balloons are an essential part of the scientific research community. In previous stratospheric balloon models used for trajectory prediction and station-keeping, the aerodynamic drag has usually been modeled as similar to that of a sphere. However, with recent proposals to use propulsion systems on the payload of stratospheric balloons to achieve trajectory control in the horizontal plane, it is important to refine our understanding of the drag of stratospheric balloons, especially at low horizontal velocities near transition, where spherical assumptions may deviate significantly. This study conducts a Computational Fluid Dynamics (CFD) investigation into the aerodynamic characteristics of both superpressure balloons (SPBs) and zero pressure balloons (ZPBs) using Large Eddy Simulations (LES). The analysis was conducted over a range of Reynolds numbers that correspond to reasonable forward airspeeds for horizontal stratospheric propulsion-based balloon systems. The results show that both balloons have drag characteristics qualitatively similar to a sphere. This includes an initially high drag coefficient, a drag crisis, and a lower eventual drag coefficient. Quantitatively, however, differences emerge between the balloon aerodynamics and that of a sphere. For example, the drag crisis occurs at a lower Reynolds number for both types of balloons when compared to a sphere. This is critical as proposed propulsion-based balloon systems aim to operate near the Reynolds number where this drag crisis occurs. The drag coefficient for the SPB was found to be less than the ZPB at all Reynolds numbers. A sensitivity analysis revealed that increasing the number of gores decreased the drag coefficient, with the flow separation delayed and the wake narrowing as the gore count increased. For example, a reduction of 32% in drag was observed when the number of gores increased from 30 to 50.

Boiling process in a heated tube is commonly used in different industries such as electronic equipment cooling, power plant, and air conditioning systems. Despite the significance of thoroughly and separately analyzing of heat transfer in different two-phase flow regimes encountered in boiling process, just a few simulations have been conducted. That is because of the lack of proper understanding of the many numerical methods that are now in use and their relative efficacy under various circumstances. This leads to dispersed effort and the application of disparate numerical methods, which incurs significant computational expenses. In this study, Eulerian-Eulerian approach was used to simulate the bubbly flow, which includes vapor bubbles in the rising water flow within a vertical tube. In order to identify the optimal numerical model and the extent of application of available numerical models in the simulation of bubbly flow, volume of fluid (VOF) and Eulerian boiling model of Rensselaer Polytechnic Institute (RPI) models were compared and evaluated. Results demonstrated that while the RPI boiling model results are more appropriate for estimating the heat transfer coefficient and wall temperature in this regime, the VOF model is more effective than the RPI model at simulating the regime, bubble formation and interface between phases. Moreover, RPI model was used to examine how changes in wall heat flux and inlet mass flow rate affected effective parameters. Results revealed that in the bubbly flow regime, a 100% increase in wall heat flux relative to its original value of 5000 W/m2, resulted in a 150% increase in the outlet vapor quality, a 75% rise in temperature difference between the wall and the saturation temperature, and a 20.8% increase in the mean wall heat transfer coefficient. Furthermore, by increasing the inlet mass flow rate, the nucleate boiling zone increases and the outlet vapor quality decreases.

Investigating the aerodynamic characteristics of an ultrahigh-speed elevator between the car and counterweight during the staggering process is crucial for the development of drag reduction and noise abatement technologies. In this study, an actual operating ultrahigh-speed elevator is selected as the research object, and an unsteady flow numerical simulation model for three-dimensional, has been constructed using the method of dynamic mesh. The aerodynamic behaviours of the elevator at various interleaving operating speeds are analysed. The impacts of the counterweight on the flow velocity, pressure, lateral force, aerodynamic drag, and sound pressure level (SPL) of the car are investigated. The results show that a streamlined counterweight can stabilize airflow between the windward areas of the car and counterweight, reducing turbulence, the lateral lift, surface pressure gradients, and SPL, while also lessening the effects of reduced car-counterweight spacing. At a speed of 6 m/s, a bi-arc counterweight with a radius of 250 mm demonstrates superior performance in reducing lateral lift force and aerodynamic drag compared to a traditional rectangular counterweight, with reductions of 12.2% in lateral lift force and 9.3% in aerodynamic drag. Additionally, the simulation and test errors are within 10%, confirming the accuracy of the numerical calculation method.

The objective of this study is to create an innovative blade design that enhances the power efficiency of the Savonius rotors. This is achieved by optimizing the blade shape of the traditional Savonius rotor using the ANSYS Adjoint solver program. The results of the analysis revealed that the total pressure exerted on the optimized shape was 16 times greater than that of the traditional Savonius rotor. To compare performance metrics, the rotor with the optimized blade structure was numerically modeled alongside the traditional and Banesh-type Savonius rotors using the ANSYS Fluent program. The Dynamic Mesh 6DOF method is used in the model domain in order to simulate rotation of the rotor. The rotors were then analyzed in two different configurations: as a single-stage rotor with a phase angle of 0o, and as a three-stage rotor with a phase angle of 60o between each stage while keeping rotor height constant. The optimized blade rotor with 3 stages demonstrated superior performance with a power coefficient of 0.44, outperforming both the Banesh and traditional Savonius rotors, while also displaying power coefficient values 18.9% and 37.5% higher than the Banesh-type Savonius and traditional Savonius rotors, respectively.

To investigate the impact of inlet and outlet diameters on the performance of lobe pumps, this paper analyses six lobe pumps with an inlet and outlet diameter ratio of 1, as well as five lobe pumps with varying inlet and outlet diameter ratios, while keeping other conditions constant. Three-dimensional unsteady numerical simulations of the pumps were conducted using the full cavitation model and the re-normalization group (RNG) k-ε turbulence model. The results show that for the lobe pumps with internal diameters between 40 mm and 100 mm, the shaft power is reduced by 17.6%, the pressure is reduced by 0.4 MPa, the pulsation coefficient is reduced by 18%, the variation of the gas volume fraction at the gap is in the range of 0.2 to 0.8, and the maximum value of the radial force in the X-direction decreases from 58.36 to 17.6 kN, and that the maximum value of the radial force in the Y-direction decreases from 14.56 to 3.25 kN. When the scale is increased from 0.6 to 1, the shaft power decreases by 25.9%, the pressure decreases by 0.1 MPa, the pulsation coefficient decreases by 6%, the volume fraction of gas at the gap varies between 0.1 and 0.8, and the maximum radial force in the X-direction decreases from 17.63 to 12.52 kN, and in the Y-direction decreases from 7.68 to 6.32 kN. This shows that choosing a suitable inner diameter can reduce the cavitation tendency of the lobe pump, enhance its anti-cavitation ability, optimize the fluid flow characteristics, and thus improve its reliability and stability.

The improvement of the fluidic oscillator as an active flow control device is studied in depth. The interior geometry of the fluidic oscillator is modified by adding backward-facing step (BFS). Variations of BFS height (H) are 2, 4, 6, 8, and 10 mm. The study is carried out computationally using OpenFoam. An unstructured mesh is used in this study, with the mesh quality maintained at y+<5. The highest frequency increase occurs at BFS height of 10 mm, which is 36.45%. On the other hand, BFS also increases the average pressure drop by less than 5%, as observed across all height variations. Overall, this study suggests using BFS height of 10 mm. The increase in the momentum of the return flow within the feedback channel leads to a higher oscillation frequency of the fluidic oscillator. The increase in average pressure drop is due to the presence of a recirculation bubble right in the step.

This study simulates the dynamic evolution of demulsification in emulsions under various electric field parameters, using a multicomponent lattice Boltzmann color model that integrates pulsed electric and flow fields. The degree of aggregation of dispersed-phase droplets is quantitatively analyzed using the area-to-circumference ratio. Results of numerical simulation demonstrate the demulsification behavior of dilute emulsions under three types of pulsed electric fields: direct current (DC) pulsed electric field, unidirectional triangular pulsed electric field, and bidirectional triangular pulsed electric field. Findings indicate the occurrence of electrophoretic and oscillatory coalescence in dilute emulsions under pulsed electric fields. The improved bidirectional triangular pulsed electric field shows enhanced efficiency relative to that of either the DC pulsed or the unidirectional triangular pulsed electric field. Moreover, the enhanced bidirectional triangular pulsed electric field effectively demulsifies oil-in-water dilute emulsions and prevents oil droplets disintegration under high-voltage across different component ratios.

This study aims to investigate the simulation of wind impacts on a standard model of a tall building with a novel facades design. The tall building is established by the Commonwealth Advisory Aeronautical Council (CAARC). A substantial volume of data has been generated to improve the living conditions in a building measuring 182.9 m (H) x 45.7 m (B) x 30.5 m (D). A comprehensive investigation is conducted, including a total of 65 cases. These cases involve varying wind velocities, facade angles, and distances between the facades and the building.  To verify the accuracy of the current results, the drag coefficient (CD) values were compared in this study to those from previous experimental and numerical analyses published in the literature. The drag force, velocity, and pressure distribution surrounding the building were computed using computational fluid dynamics (CFD) techniques, considering various wind velocities and geometric characteristics. Research results reveal that both wind velocity and the geometric dimensions of the facades have an important influence on the drag force. The building experiences a significant increase in force as the wind velocity increases from 1 to 5 m/s. The results also indicate that the increasing angle of facades has a noticeable effect on increasing the force produced on the building. This data aims to achieve wind control through the passive flow control method, prevent weathering of the building, decrease wind load, facilitate natural ventilation, save energy, and provide building designers with a wide range of numerical simulations.

The purpose of this study is to enhance the cutting efficiency of high-pressure abrasive water jet (AWJ) by optimizing nozzle structure and jet hydraulic parameters. To achieve nozzle structure optimization, CFD models of various nozzle shapes were established. The results indicate that the conduit length of conical nozzles has minimal impact on the cutting ability of the jet, while the conical nozzle with a taper angle of 40° exhibits excellent guiding characteristics. Furthermore, an infinite SPH AWJ cutting model with different hydraulic parameter settings was developed for the coupled numerical analysis of pump pressure, flow rate, and nozzle diameter. Through extensive numerical simulations, the study plotted curves of cutting depth and volume against pump pressure, flow rate, abrasive concentration, and nozzle diameter. The results show that, under specific hydraulic parameters, there exists an optimal abrasive concentration; and increasing the displacement leads to an increase in this optimal concentration. Furthermore, under constant pump pressure, increasing the nozzle diameter leads to an increase in flow rate. Additionally, both cutting depth and volume initially increase and then decrease, reaching their maximum values when the nozzle diameter ranges from 4mm to 5mm. The research findings provide a solid theoretical basis for abrasive jet cutting technology.

During the chill-down procedure for a space application system, a multiphase flow of extremely cold propellants will undoubtedly occur in the feed lines. Due to flow instability and varying cooling rates, this cooling technique produces dynamic changes that are challenging to control. Over the past few decades, research has extensively examined the multiphase behaviour of various cryogens, such as liquid nitrogen, liquid hydrogen, and liquid oxygen, while few studies focus on liquid methane. This study addresses this gap by investigating the hydrodynamic and thermodynamic characteristics of LCH₄-VCH₄ multiphase flow in a vertical cryogenic pipe using a well-validated, three-dimensional Volume of Fluid (VOF) model. Further, the current study employs an Eulerian flow scheme coupled with an energy equation to accurately capture the multiphase flow dynamics of liquid methane across various inlet velocities and temperatures. The volume fractions are used to investigate the flow pattern, and the interaction between the two phases and the two-phase flow structure is investigated using velocity profiles and phase distribution. Further, the bulk mean and near-wall temperature results were analysed to understand the temperature variation inside the pipe. Increased inlet temperatures at constant velocities enhance vapour volume fractions, while higher velocities reduce vapour generation, leading to decreased bulk mean temperature due to reduced heat transfer. However, a significant rise in vapour flow rates occurs at elevated temperatures with constant velocity. The volume fraction results show the formation of bubble, annular, and slug flow patterns in the flow regime.

Helical axial-flow multiphase (HAFM) pumps experience intermittent gas-blocking events, which negatively impact performance and threaten the stability of the overall pump and pipeline systems. This study applies jet flow field method to HAFM pumps. Active intervention in the gas-liquid separation process, utilizing external energy, results in the reorganization of the flow field within HAFM pumps. The effect of jet location on improving the efficiency of HAFM pumps is assessed, with a focus on the active flow control mechanism through jet influence. The study indicates that the region sensitive to jet site distribution affecting pump performance is 0.5Lc ≤ xr ≤ 0.7Lc, while the weakly sensitive region is 0.15Lc ≤ xr ≤ 0.5Lc. When xr ≤ 0.15Lc, the improvement in head and efficiency under high gas content conditions is reduced. Jet flow field control technology obviously decreases the gas phase accumulation in the downstream flow channel of the moving blade cascade. The optimal position for reducing gas phase agglomeration in the impeller channel is 0.3Lc. The jet site arrangement significantly affects the pressure structure near the cascade trailing edge. Appropriate jet hole positioning significantly improves the pressure structure at the cascade trailing edge, decreases reflux caused by separation vortices at the impeller outlet, and enhances the hydraulic performance in the multiphase pump.

Re-entry capsules' success depends significantly on dynamic and static stability, particularly before deploying the main parachute. Determining the range of dynamic instability and investigating the underlying causes is crucial for designing the entry capsule's control system. Dynamic stability is analyzed in this study based on pitch moment coefficients obtained from forced oscillation experiments conducted in the trisonic wind tunnel for the Orion entry capsule. The results reveal that pressure fluctuations at the aftbody of this model begin at Mach 2. The findings and other research results emphasize the significant role of the aftbody geometry in generating dynamic instability at low supersonic speeds due to its interaction with vortex flow. The results also demonstrate that increasing the Mach number to 2.2 would result in a near zero-pressure coefficient on the capsule's aftbody, which implies that there is no acting force on the aftbody. The results show that as the freestream Mach number increases from M∞ = 1.8 to M∞ = 2.2, the pressure on the aftbody remains unchanged during the pitching motion due to approaching the shear layer towards the body and consequent shrinking of the aftbody vortex. Furthermore, the sensitivity of dynamic stability to the mean angle of attack was investigated. It is shown that a slight increase of approximately 5 degrees in the mean angle of attack can considerably enhance the re-entry capsule's dynamic stability.

The present work attempted to lay a basis for evaluating the compressor aerodynamic performance in service environments. To achieve this goal, a low-speed compressor along with a transonic compressor rotor (NASA Rotor37) were studied. Different damaged blades were established, and three-dimensional viscous flow field simulations were accomplished. The influence of blade damage and the mechanism through which blade damage affects the compressor aerodynamic properties were analyzed. Further, the established numerical method was validated through low-speed compressor experiments. The results showed high resemblance of the simulations to the experimental findings, thus proving the effectiveness of our numerical simulation method. After the blade was damaged, the surge point of the compressor was advanced, the stable working flow range was reduced, and the aerodynamic performance was significantly reduced. The local airflow separation caused by the attack angle enlargement due to the deformation of blade after damage was the primary reason for the whole row of rotors to enter an unstable state. Unlike low-speed compressor, the rotor blade damage of transonic compressor caused the interference of shock wave with the boundary layer separation, and the burst low-speed region blocked the blade channel. This deteriorated the rotor flow state and caused greater flow loss, resulting in a more severe decline in the aerodynamic performance. The proposed numerical simulation approach for aerodynamic performance can effectively predict the steady-state aerodynamic performances of compressors with damaged blades.

In this study, aiming at investigating the formation mechanism of pressure fluctuations of different frequencies in axial flow pumps, the characteristics of pressure fluctuation were determined using fast Fourier transform (FFT) on the basis of a numerical simulation of complex flow fields in the pump. The pressure and velocity modes corresponding to the primary pressure fluctuation frequency in the pump were decoupled and rebuilt using dynamic mode decomposition (DMD). The consequences showed that the primary pressure fluctuation frequencies in impeller were 11fn and 4fn and in diffuser were 4fn and 2fn, respectively, where fn is the shaft natural frequency. Moreover, the pressure fluctuation amplitude in diffuser was significantly larger than that in the impeller. DMD could identify the coherent structures of various frequency pressure fluctuations in the impeller and diffuser. In addition, the used method, which combines both FFT and DMD, revealed that the formation mechanisms of pressure fluctuations at different frequencies are different. In particular, the pressure fluctuation at 4fn in diffuser were caused by rotor–stator interaction (RSI) and flow separation near the suction surface (SS) of diffuser blades. Moreover, the pressure fluctuation at 2fn was caused by flow separation near the SS of diffuser blades and wake vortex shedding. In impeller, the pressure fluctuations at 11fn and 4fn resulted from RSI and flow separation at the leading edge (LE) of impeller blades, respectively.

In order to improve the aerodynamic characteristics and maneuverability of fixed-wing UAV, an improved method to enhance the wing lift by the combination of plasma jet actuator and synthetic jet actuator based on active flow control technique is presented. The aerodynamics of the wing under flow control are calculated by fluent hydrodynamics software. Firstly, the effects of different control modes on the aerodynamic characteristics are compared, including single PJA control, single SJA control, combined PAS control and combined SAP control modes. Lift enhancement mechanism in combined control mode and the advantages are analyzed. Secondly, the effects of flow control parameters, involving the plasma discharge voltage, the maximum exit velocity of the synthetic jet and the deflection angle on the aerodynamic characteristics are investigated in detail. The results show that installing a flow control device at the leading edge of the aileron can substantially increase the lift of both the main wing and aileron when using the PAS flow control mode. The main wing lift is the primary contributor to the total lift increase, accounting for up to 79% of the total lift increase. The hysteresis phenomenon of the pressure recovery on the surface of the main wing is one of the main reasons for the total lift to remain at a high level. Meanwhile, raising the plasma voltage can steadily increase the lift of the wing, while raising the exit velocity of the synthetic jet can cause more lift fluctuations while increasing the lift.