Journal Of Applied Fluid Mechanics
Volume:16 Issue: 3, Mar 2023

This manuscript bridges the fields of jet propulsion, synthetic jets and thermoacoustic engines. We introduce the case that synthetic jets in air can be considered a consistent approach for propulsion systems. To date, studies on synthetic jet propulsion have been scattered and rather subsidiary. Furthermore, investigating the perspective of propulsion can provide new insights into the field of synthetic jets and the physics of propulsion. In this work, synthetic jet propulsion in air is demonstrated and studied for acoustic and thermoacoustic cases. We developed synthetic jet systems and characterized the resulting propulsion regarding several relevant parameters such as geometric factors and the frequency and power of acoustic jets electromechanically and thermally produced. We demonstrated the 10.4-mN and 2.7-mN thrust in the acoustic and thermoacoustic modes, respectively, using compact tabletop assemblies. The physical mechanism of the jets has been modeled, simulated, verified with Schlieren imaging and laid in the perspective of adherent literature. Similarities and differences regarding traditional jet propulsion systems are discussed. Synthetic jet propulsion in air using a thermal cycle is experimentally demonstrated for the first time.

The aerodynamic noise from the new high-speed Maglev train (NHMT) is significant. One way to reduce it is to use a fully enclosed sound barrier (FESB). This paper studies the aerodynamic noise characteristics of the NHMT operating inside a FESB at 600 km/h, taking into account the compressibility of air and the quadrupole sound sources. The transient flow field around the NHMT is simulated by adopting an improved delayed detached-eddy simulation. According to Lighthill acoustic analogy theory, the aerodynamic noise inside and outside the FESB can be simulated using the acoustic finite element method. The sound insertion loss (IL) of the FESB is analyzed by comparing the noise outside the FESB with the noise generated by the NHMT operating on open tracks. The results indicate that the noise is distributed in the streamlined shoulder area of the head and tail car and the wake. The far-field noise belongs to broadband noise, and the noise outside the FESB is similar to an incoherent line source. The IL of the FESB is 25.2 dB(A) at 25 m from the track centerline and 3.5 m above the track surface. Therefore, the noise reduction effect of a FESB is much better than a traditional upright sound barrier.

Numerical simulation of interactions between acoustic waves and flames is of utmost importance in thermo-acoustic instability research. In this study, interactions between a one-dimensional Methane-Air laminar premixed flame and acoustic waves with a frequency of 50 to 50000 Hz are simulated by simultaneously solving the equations for energy conservation, chemical species transport, state and continuity in one-dimensional space. By assuming that the flame thickness is smaller than the acoustic wavelength, the spatial pressure fluctuations can be neglected and the flame experiences only a time-varying acoustic pressure. The GRI mechanisms, as well as their reduced mechanisms, are considered to obtain results for steady flames without acoustic waves, and the interaction of unsteady flames with acoustic waves. Results show that the total heat-release-rate fluctuations for the flame is affected by increasing the frequency of the acoustic wave. An increase in frequency first increases the total heat released, and then decreases it. The obtained results are in good agreement with those of other researchers. Furthermore, at the presence of acoustic waves, various chemical species can affect the total heat-release-rate fluctuations. With Rayleigh's instability criterion, it can be shown that H2O, CO2 and O2 are the main species to the fluctuations of the total heat release rate and lead to flame instability. Results show that heat-release-rate of H2O specie is the most important on the total heat-release-rate. Therefore, for the flame-acoustic waves interaction problem, the best mechanism is the one that could predict the concentration of H2O more precisely.

Complex features of transonic flow over a turret make it challenging to use passive flow control to reduce aero-optical effects. In this study, the influence of different flow features on wavefront distortions is numerically investigated through improved delayed detached eddy simulation coupled with a modified sub-grid scale. The proper orthogonal decomposition (POD) method is used to study the spatiotemporal characteristics of the flow features. The flow field changes in the wake along with the motion of the shock. Two features, namely, lateral shift (dominant in modes 1 and 2) and wall-normal fluctuation (dominant in modes 3 and 4) of the wake, are the most dominant in the flow field. All beams share the common feature of transmitting the flow field, in which a large component of optical path difference (OPD) appears at St=0.35, indicating the high impact of wall-normal fluctuation on the distortion of the wavefront. After the different POD modes, which contain 30% of the mode energy, are removed, all beams transmitted through different reconstructed fields show very different features for OPD. The flow features that do not exhibit higher-order modes from modes 21 to 92 affect the OPD slightly, as the OPD components in the low-St region are almost unchanged. With the removal of modes from 3 to 32, wavefront distortions are considerably reduced, particularly at St=0.35. The wavefront distortions are most reduced after the lower order modes from 1 to 20 are removed, as the components of OPD in the low-St region are dramatically reduced. The significant relations between OPD and the flow features reveal that controlling the dominant flow features has significant potential for reducing aero-optical effects.

A numerical study was conducted for natural turbulent and laminar convection induced by a circular hot obstacle in two different positions, the heat obstacle is introduced at the inlet of the cylinder and inside the cylinder. To determine the effects of a circular hot obstacle on the regime of the thermosiphon flow generated by a heated cylindrical channel, we suggest a theoretical and numerical model using the Navier–Stokes equations and the finite volume method. An experimental apparatus was used for the validation of the numerical model. A comparison of the numerical and experimental results showed a good agreement in the global flow behavior. The temperature and vertical velocity profiles demonstrated the presence of a boundary-layer regime along the heated channel's wall in the absence of a hot obstacle and the presence of the hot obstacle at different positions. The introduction of the hot obstacle generates a significant modification of the evolution of the flow above the hot obstacle. The numerical simulation was conducted for a specific radius of the hot obstacle that ensures the existence in the laminar flow regime, and different Rayleigh numbers (Ra=105; 107; 108; 109; and1010). The flow rate, velocity, temperature and Nusselt number profiles were presented and discussed. The effects of the hot obstacle are presented in our results by two maxima of the vertical component of the velocity profile. The temperature profile at the entrance of the vertical cylinder is presented by the appearance of a maximum above the hot obstacle. As the cylinder height increases, the air temperature decreases. The vertical displacement of the obstacle upward induces a modification in the behavior of the dynamical flow. A CFD code was used to study the different natural convection regimes presented by three different form factors, including the critical regime with a form factor of A=0.1.

The influence of inlet gas volume fractions on flow-induced noise in centrifugal pumps during the transition process of gas injection and the generation mechanism of noise in this process was studied. To this end, numerical simulations of the flow and sound fields as well as visual experiments of the flow field were conducted. The variation laws of the flow field, frequency-domain sound field, and time-domain sound field during the process were investigated. The relationship between the pressure pulsation and flow-induced noise was analysed in detail. The results indicate that it is more reasonable to consider the outlet gas volume fraction as the judgement criterion for the end of the transition process. The axial and blade passing frequencies are characteristic discrete frequencies in this process. The pressure pulsation and sound pressure level at the blade passing frequency are stable in the early stage of the process. Subsequently, they decrease first and then increase in the middle stage and return to a stable state in the final stage. The variation laws of the two exhibit consistency, indicating that the pressure pulsation directly induces a change in the sound pressure level. Furthermore, the injection of gas can weaken the disturbance effect of the blade.

Employing FVM, we have investigated numerically the rheological behavior of bifurcation phenomena of blood flow at various Reynolds numbers (Re) and at various values of contraction ratio (h), defined as the ratio of the inlet to narrow sections width of a two-dimensional planner contraction-expansion channel. Blood flow bifurcation through a planar contraction-expansion channel is analogous to the case of regurgitation (i.e., abnormal leakage of blood) in the mitral valve. In this work, we have studied the blood flow bifurcation characteristics including the normalized axial velocity profile, velocity gradient, dimensionless pressure, dimensionless longitudinal pressure gradient, pressure and skin friction coefficients on both the channel walls and analyzed the pressure drop, excess pressure drop for different values of Re. Secondly, blockage in the mitral valve is studied for different values of h. Pressure drops for various values of h are also studied to measure blood pressure. Correlation analyses are presented for normalized vortex length in terms of critical values of Re and h. It is revealed that if Re goes on increasing to 14.4 or more, flow breaks the symmetry at h = 15, and for each , recirculation length increases linearly with the increase in Re but decreases valve flaps that reduce blood flow to the heart muscles.

Inspired by the challenging and nimble flight dynamics of flying insects and birds, this research investigates bionic propulsion technology to develop an improved flapping wing micro air vehicle (FWMAV) design. Following the bionic formula, a prototype is preliminarily designed to achieve multi-attitude flight. Then, kinematic modeling is employed for further data analysis. A meshless particle hydrodynamics method is adopted to explore an optimized flapping driving mechanism and understand the influence of the flapping frequency, flapping amplitude, and quick-return characteristics of one side of the symmetrical mechanism on aerodynamic performance. Based on the aerodynamic model, force measurement experiments are developed to verify simulation availability and investigate the importance of wing flexibility. The numerical analysis results demonstrate that the average lift is approximately proportional to the flapping frequency, flapping-wing amplitude, and quick-return characteristics. Further optimization is conducted to find the best design parameters setting because of the complicated coupling relationship between the flapping wing amplitude and quick-return characteristics. Moreover, the optimized wing property supports high aerodynamic performance via experimental analysis in hovering flight.

This comprehensive study focuses on the relationship between the inlet vane and wrap angles of a centrifugal pump impeller and its cavitation behavior. The objective is to propose a new design for cavitation performance enhancement of the pump by reducing the required net positive suction head (NPSHR). To render a detailed study, the variations of the mentioned angles were examined both at hub and shroud. A three-dimensional two-phase parametric computational fluid dynamics (CFD) study was conducted and variations of inlet blade and wrap angles at hub and shroud were investigated separately. After validating the experimental results with the pump model, the pump parameterization process was carried out, and pump geometry was re-defined with its geometrical properties. The characteristic curves of the parameterized pump and pump model were in agreement with each other. Numerical cavitation tests were performed and the NPSHR values of each design were calculated for four different operating conditions. Results showed that cavitation performance increase was achieved with angle variations at the best efficiency point (BEP) but, this situation led the pump to operate exposed to cavitation at high flows (HF) for some designs. Therefore, the design with the best cavitation performance increased at BEP and HF was proposed as a new design. During numerical simulations, shear stress transport (SST) turbulence model and Rayleigh–Plesset equation were used.

Unsteady numerical simulation of single passage was carried out on NASA Rotor 35 to study the influence of the radial inclined angle of self-circulation casing treatment (SCT) on the performance and stability of a transonic compressor at 100% design speed. The radial inclined angles were set to 0°(D0), °(D30) and 60°(D60), respectively. The calculated result indicates an increase in the stall margin improvement (SMI) and the design efficiency improvement (DEI) as the radial inclined angle increases gradually. The SMI of the SCT with a 60° radial inclined angle is 12.5%, the biggest among the three SCTs, and the peak efficiency improvements (PEI) of the SCTs are almost the same. The radial inclined angle is provided with the effect of strengthening the self-circulation casing treatment effect, which further improves the stable working range of the compressor, and the efficiency loss is also lower than that of the 0° angle structure. The flow conditions inside the bleeding part can be improved by radially skewing the self-circulating structure toward the rotor rotating direction.

This paper presents a computational study characterizing the swirl intensity distribution and Internal Recirculation Zone (IRZ) formed in a cylindrical domain with tangential injections and isothermal flow. The range of inlet boundary conditions investigated is 5o to 25o for the injection angle and 7190 to 100711 for the bulk flow Reynolds number. The evolution of swirl intensity is presented with and without incorporating effects of the accompanying pressure variations. The Shear Stress Transport (SST) k-ω is used to model turbulence. Results show that the Swirl strength created by such tangential injections strongly depends on the injection angle but does not vary with bulk flow Reynolds number (Re), except for low Re values. The swirling flow is shown to result in IRZ formation at injection angles 6o and above or when asymptotic value of the maximum Swirl Number in the domain exceeds approximately 0.6, same as the transition value of inlet Swirl Number in swirling flows with axial injections. The IRZ length increases with injection angle and varies with Re for lower values of Re at a given injection angle but asymptotes for higher values above 40000. The conventional Swirl Number rises rapidly downstream of the injection plane followed by a slow decline. On the other hand, an alternative Swirl Number, which incorporates the gauge pressure variation, shows slow and consistent decay all the way downstream of the injection plane. The Swirl Number incorporated with gauge pressure term subsumes interconversions between the axial momentum and pressure in the regions of vortex breakdown and IRZ formation, thereby presenting an alternative picture of swirl intensity evolution in swirling flows.

Hydrophilicity is one of the most vital characteristics of titanium (Ti) implants. Surface structure design is a powerful and efficient strategy for improving the intrinsic hydrophilic ability of Ti implants. Existing research has focused on experimental exploration, and hence, a reliable numerical model is needed for surface structure design and corresponding hydrophilicity prediction. To address this challenge, we proposed a numerical model to analyze the droplet dynamics on Ti surfaces with specific microstructures designed through the lattice Boltzmann method (LBM). In this work, a Shan-Chen (SC) model was applied in the simulations. We simulated droplets spreading on smooth and micropillar surfaces with various wettability and provided a comprehensive discussion of the edge locations, contact line, droplet height, contact area, surface free energy, and forces to reveal more details and mechanisms. To better tune and control the surface hydrophilicity, we investigated the effects of micropillar geometric sizes (pillar width a, height h, and pitch b) on hydrophilicity via single factor analysis and the response surface method (RSM). The results show that the hydrophilicity initially increases and then decreases with an increasing a, increases with an increasing h, and decreases with an increasing d. In addition, the interaction effects of a-d and h-d are significant. The optimization validation of the RSM also demonstrates the accuracy of our lattice Boltzmann (LB) model with an error of 0.687%. Here, we defined a dimensionless parameter ξ to integrate the geometric parameters and denote the surface roughness. The hydrophilicity of Ti surfaces improves with an increasing surface roughness. In addition, the effect of the microstructure geometry shape was investigated under the same value of surface roughness. Surfaces with micropillars show the best hydrophilicity. Moreover, this study is expected to provide an accurate and reliable LB model for predicting and enhancing the intrinsic hydrophilicity of Ti surfaces via specific microstructure and roughness designs.

Wind energy is an alternative future energy to fossil fuels since it is abundant and green energy. As a result, high performance unique design is proposed that has a diffuser augmented wind turbine including intake funnel with guide vanes, natural fan, straight flow section, exit splitter with air openings and end flange. This proposed design is called as Integrated omni-directional Intake funnel, Natural fan, Straight diffuser, Splitter and Flange (I2NS2F) Design. To construct this I2NS2F configuration, four distinct wind turbines were developed: a) bare wind turbine, b) wind turbine diffuser design with single rotor turbine, c) bend diffuser, intake funnel, natural fan, splitter and flange and d) I2NS2F design. The proposed designs are numerically studied using MATLAB Simulink and Ansys Fluent. These designs are optimized by Random Search Optimization with Supervisory Control and Data Acquisition technique to evaluate the wind velocity and the performance is comparatively estimated. From this analysis, I2NS2F design achieves 53m/s of wind velocity at turbine region for 5.5m/s inlet wind and it could be considered as highest wind velocity than other three designs. It is evidently proved and concluded that proposed I2NS2F design augments natural wind, resulting in greater green power generation.

Tuned liquid dampers (TLDs) have been broadly applied to suppress structural vibrations. In the present study, a novel vibration mitigation device consisting of non-Newtonian fluids coupled with an elastic baffle is proposed. The fluid-structure interaction is studied numerically. To optimize the system, different fluids, including the Bingham fluid, the Pseudoplastic fluid, and the Dilatant fluid are used as the damping fluids and the vibration suppression ability of each fluid is studied. Moreover, the energy dissipation mechanisms of different liquids are obtained. The results show that the optimal vibration suppression in the container without a baffle can be achieved by using the Bingham fluid. In this case, the average amplitude decay rate of the container is 12.662% with about 0.199% improvement in the damping ratio when compared to water. In the container with an elastic baffle, however, both the Pseudoplastic fluid and the Dilatant fluid outperform water in the damping capacity. The average amplitude decay rates of these fluids are 50.960% and 43.794%, respectively. Moreover, their damping ratios are 0.035% and 0.019% higher than that of water, respectively.

A high-temperature gas mixing chamber is a type of mechanical device which has been widely used to obtain airflow at a specific high temperature. When a temperature environment test for aviation equipment is performed, the airflow temperature should exceed 1,000 K and change fast. However, the existing mixing devices cannot meet the requirements for mixing speed and uniformity of temperature environment tests. To solve the problem of low mixing speed and uniformity, this paper proposes an innovative gas mixing chamber design. The proposed design allows cold and hot gases to be injected in directions perpendicular to each other, which increases the collision and heat exchange of gases with different temperatures. In this way, a faster mixing process and a more uniform outlet temperature field are obtained. Moreover, a genetic optimization method is used to improve the performance of a mixing chamber. This method considers temperature distribution, the velocity distribution of the outlet airflow, and the mixing speed. Simulation results show that the proposed method can reduce the mixing time from 4.4 s to 2.3 s, the standard deviation of the outlet temperature distribution from 25.2 to 14.7, and the standard deviation of velocity distribution from 1.3 to 0.36. The experimental results show good consistency with the simulation results, indicating that the simulation and optimization results are reliable, and the mixing performance of the cold and hot gas mixing chamber is significantly improved by the proposed method. The proposed method is important for optimizing the design of similar orifice plates and flow mixing structures.