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The target of this investigation is to illustrate the nonlinear fluid flow, heat transfer, and vortex formation phenomena in an orifice pulse tube refrigerator by numerical simulation. Numerical results confirm that jet streaming is generated at the low-temperature end of the pulse tube in between the junction of U-shaped tube and pulse tube. The primary vortex, which is generated due to the minor loss, continuously grows because of velocity change and eventually leads to the formation of secondary eddy. Consequently, flow straighteners have been adopted at the hot side of the regenerator, hot heat exchanger, and both sides of U-shaped tube. It is noticed that, jet streaming has been suppressed at the cold end of the pulse tube. Nevertheless, the streaming effect generated due to the oscillating flow of the gas parcels within the pulse tube, and third type of streaming have not been suppressed with the flow-straightener. Additionally, the numerical simulation shows different gas flow behaviors within the pulse tube that occurs due to the oscillating pressure pulse. An experimental investigation is conducted to compare experimental results with the numerical results and a valid agreement is observed between them. [ABSTRACT FROM AUTHOR]

This study numerically focused investigating the thermal performance of flow oscillations in a rectangular corrugated channel with vertical plates on top wall. The numerical study was performed with the ANSYS Fluent software, and the SIMPLE algorithm was utilized to solve the pressure-velocity coupling. The top wall of the channel was adiabatic and included vertical plates. The bottom wall of the channel was rectangular grooved and kept at Tw=360 K. Suspension of Al2O3 nanoparticles into water was used as the fluid. The particle volume fraction in the suspension was kept constant at φ = 5%. Oscillating amplitude (A) and Strouhal number (St) were maintained constant at A = 1 and St = 2, respectively. In the presented study, the effects of vertical plates, Al2O3-water nanofluid and pulsating flow on flow and heat transfer were analyzed separately at different Reynolds numbers (200 ≤ Re ≤ 800). The Nusselt number (Nu), relative friction factor (frel) and performance evaluation criteria (PEC) were obtained for different Reynolds numbers. The temperature and velocity fields were acquired for varying parameters. The results demonstrated that the flow and temperature structures were significantly influenced by the channel geometry and oscillating flow. Heat transfer considerably enhanced with the oscillating flow at the high Re. At Re = 800, thermal improvement for oscillating flow of the nanofluid in the channel with plates increased by nearly 1.57 times relative to the steady case of the basic fluid in the channel without plates. [ABSTRACT FROM AUTHOR]

In this work, a surface-mounted circular cylinder with a fixed aspect ratio (ratio of height of the cylinder to its diameter) of 5 is subjected to a non-zero mean oscillating flow with a range of frequencies and amplitudes. Three-dimensional direct numerical simulations are then conducted on this finite-height cylinder. The mass and momentum equations are resolved using the finite volume-based Open Source Field Operation and Manipulation (OpenFOAM). A fixed Reynolds number Re = U o D / ν of 250 is used in this study, which is defined based on mean velocity at the inlet ( U o ) and cylinder diameter (D). Here ν is the kinematic viscosity of the working fluid. Non-dimensional velocity oscillation amplitude ( A ∗ = a / U o ) is varied from 0.1 to 0.3, while the non-dimensional oscillation frequency ( f ∗ = f / f o ) takes the values of 0.33, 0.5, 1, 2, and 3. Here a and f are the dimensional oscillation amplitude and frequency, respectively and f o is the vortex shedding frequency corresponding to a uniform flow at Re = 250. The three-dimensional vortex structures, presented with the help of iso- Q surfaces, show that the oscillating flow changes the size and shape of the hairpin-shaped vortices. Wake is found to be synchronized with the oscillation frequency at f * = 2 for each value of the A * and results in the maximum lift force on the cylinder. Hilbert Huang transformation analysis of the transverse velocity signals at a specific point in the wake reveals that the wake is more complex and aperiodic in nature for f * values of 0.33, 0.5, and 1, whereas it is periodic for f * = 2 and 3. In order to further disclose the nonlinearity associated with the oscillating flow, the degree of stationarity is discussed corresponding to each value of A * and f *. Dynamic mode decomposition is exploited to obtain information about the coherent vortical structures and their spatial and temporal behavior in the wake with a change in the value of f *. Effects of A * and f * on the dynamic characteristics are also investigated. [ABSTRACT FROM AUTHOR]

This scientific paper investigates the flow and forced heat transfer characteristics of hybrid nanofluid in a channel with rotating circular cylinders. Two-dimensional simulations are used to analyze the effects of obstacle rotation, number, nanoparticle volume fraction, and Reynolds number on the Nusselt number and friction coefficient. The findings demonstrate that incorporating obstacles enhances heat transfer, while increasing the inlet flow frequency improves the system's performance coefficient. Interestingly, substituting hybrid nanoparticles with inlet frequency reduces the friction coefficient and further enhances the performance coefficient. The study underscores the potential of microscale heat exchangers with oscillating inlet velocity and rotating cylinders in optimizing heat transfer across various industries. Furthermore, it explores pulsating flow as a promising solution for high heat flux density challenges in single-phase cooling systems. Additionally, the investigation investigates the impact of inlet velocity frequency on heat transfer rate and friction coefficient, revealing that adjusting the cycle period and frequency improves the heat transfer rate. By modulating the flow frequency, the performance coefficient improves by mitigating the growth of the friction coefficient compared to a constant inlet velocity. By leveraging the advantages of nanofluids and manipulating the flow mode, we can optimize heat transfer performance without increasing the pump power. The investigation of different configurations of cylinders and oscillating flow conditions can provide valuable insights for engineering applications requiring efficient heat transfer and temperature regulation. The study concludes by highlighting the significance of investigating wake interactions in a three-cylinder system arranged in an intermediate triangular configuration under static and rotating conditions. These complex flow phenomena have substantial relevance to both fundamental fluid flow research and engineering applications. [ABSTRACT FROM AUTHOR]

To characterize the oscillating flow and heat transfer characteristics of the cooling gallery inside a steel piston, a visual experimental platform was designed and constructed. Based on experimental results, simulation models of the cooling gallery under calibration conditions were established, and the oscillating flow and heat transfer characteristics of different oil injection strategies were studied. The results show that adjusting the nozzle diameter has a more significant impact on the fluid charge ratio (FCR) of the cooling gallery compared to the injection pressure. During the reciprocating motion of the steel piston inside the cylinder, the gallery has the largest heat transfer coefficient (HTC) on the Top wall at the top dead center (TDC) and the largest value on the Bottom wall at the bottom dead center (BDC). The average HTC of the Top wall is always the largest, followed by the Bottom wall, the In wall, and the Out wall being the smallest. Compared to the original design, the average HTC of the Top, Bottom, Out, and In walls can be increased by a maximum of 4.39 %, 3.96 %, 8.01 %, and 11.80 %, respectively.

The working gas type had an important effect on the heat transfer performance of the turbulence device in the heater of Stirling engine, but the related researches were scarce, especially under the oscillatory flow. This work investigated the effects of four working gases (i.e., H2, He, N2, and CO2) on the heat transfer characteristics of a heating tube with spiral insert compared with a smooth tube under oscillating flow for a Stirling engine. The transient physical fields under different phase angles in an oscillatory cycle were analyzed, and the Nusselt (Nu) number, pressure loss and outlet temperature of working gas were studied. The results show that the cycle average Nu number of the enhanced tube with H2, He, N2, and CO2 as working gas increased to 1.67, 1.62, 1.61 and 1.72 times when comparing with those of the smooth tube. The cycle average friction coefficient increased to 1.96, 2.37, 2.36 and 2.62 times, respectively. Moreover, the performance evaluation criterion (PEC) values of the enhanced tube using the four types of working gas were all greater than 1 (1.21∼1.33). This implies that the comprehensive heat transfer performance of the heating tube with spiral insert was all improved with the four types of working gas. Moreover, the heat transfer enhancement effect was best when hydrogen was used. While considering the thermodynamic performance and safety reliability, the helium was more recommended. The findings are beneficial to enhance the operating efficiency of a Stirling engine.

The significant importance of present research is to remove the extreme temperature along the magnetic driven horizontal circular cylinder. The induced electromagnetic field is applied around the surface of cylinder. The main novelty of current research is to control thermal and magnetic boundary layer in the presence of viscous dissipation and induced electromagnetic field. The dimensional mathematical form is developed with defined boundary conditions. The dimensional equations are transformed into dimensionless equations to generate physical factors. The primitive form is used to reduce dimensionless equations into convenient form for smooth algorithm. The finite difference method with Gaussian elimination technique is applied for numerical results in FORTRAN language tool. The velocity, temperature and electromagnetic field are sketched graphically with asymptotic sequence. The oscillatory shear stress, oscillating heat rate and periodical current density is plotted graphically and numerically. It is found that fluid velocity improves significantly as buoyancy force increases around each position. It is noticed that the increasing oscillations in heat transfer are sketched for maximum choice of Prandtl number. It is found that the maximum oscillations in current density are obtained for each Eckert parameter. It is noticed that the significant distribution in temperature profile is obtained in the presence of viscous dissipation and magnetic field.

The current work communicates the entropy generation analysis of oscillating flow of magnetohydrodynamic couple stress nanofluid in a vertical channel. The main objective of present study is to examine the entropy analysis of a magnetohydrodynamic couple stress nanofluid. In this study, water and ethylene glycol (50:50) and diamond are used as the base fluid and nanoparticles, respectively. The effects of radiative heat, Ohmic, and viscous dissipation are all considered. By employing the perturbation process, the governing partial differential equations are transformed into the set of ordinary differential equations, which are then deciphered by implementing the Runge-Kutta fourth-order scheme with shooting technique. The obtained outcomes reveal that, amplifying viscous dissipation promising the temperature whereas the reverse is true for the influence of couple stress viscosity and Hartmann number. Heat transfer rate is decelerating with the boost up in Hartmann number at the walls while it is accelerating with the increment in viscous dissipation at the right wall. Entropy is escalating for intensifying viscous dissipation, and thermal radiation whereas the reverse is true for the impression of couple stress viscosity, and volume fraction of nanoparticles. Bejan number is falling for escalating volume fraction of nanoparticles, and viscous dissipation while it is enhancing with escalation in couple stress parameter. [ABSTRACT FROM AUTHOR]

Integral fin with micro-channels have been widely used in small free piston Stirling engines heater due to their high heat transfer rate, low flow loss, and high machining accuracy. A β-type free piston Stirling engine heater with composite cross section mini channels(CCSMH) is designed for heat transfer enhancement applying to solar dish power generation system. Investigation on heat transfer performance of the heater under oscillating flow is carried out by CFD method, which is compared with smooth annular tube heater. The influence of geometry parameters on the performance of heat transfer and pressure loss is investigated. A improved performance evaluation criteria PEC is proposed for the condition of equal flow velocity to evaluate the enhanced heat transfer performance of the heater. The results show that the maximum value of the comprehensive performance evaluation index PEC can reach 3.14, the maximum mean pressure drop increment of CCSMH compared with the smooth tube heater is 38.4 Pa, which is much lower than the minimum pressure of 2.3 MPa and can be neglected. [ABSTRACT FROM AUTHOR]

We quantify the lake dynamics, near‐bottom turbulence, flux of dissolved oxygen (DO) across the sediment‐water interface (SWI) and their interactions during oxygenation in two lakes. Field observations show that the lake dynamics were modified by the bubble plumes, showing enhanced mixing in the near‐field of the plumes. The interaction of the bubble‐induced flow with the internal density structure resulted in downwelling of warm water into the hypolimnion in the far‐field of the plumes. Within the bottom boundary layer (BBL), both lakes show weak oscillating flows primarily induced by seiching. The vertical profile of mean velocity within 0.4 m above the bed follows a logarithmic scaling. One lake shows a larger drag coefficient than those in stationary BBLs, where the classic law‐of‐the‐wall is valid. The injection of oxygen elevated the water column DO and hence, altered the DO flux across the SWI. The gas transfer velocity is driven by turbulence and is correlated with the bottom shear velocity. The thickness of the diffusive boundary layer was found to be consistent with the Batchelor length scale. The dynamics of the surface renewal time follow a log‐normal distribution, and the turbulent integral time scale is comparable to the surface renewal time. The analyses suggest that the effect of bubble plumes on the BBL turbulence is limited and that the canonical scales of turbulence emerge for the time‐average statistics, validating the turbulence scaling of gas transfer velocity in low‐energy lakes. Key Points: Lake seiching induced oscillatory flow is responsible for the turbulent structures in two stratified and low energetic lakesEffect of bubble plume is limited on turbulent structures in the bottom boundary layerShear velocity, turbulent Batchelor length, and turbulent integral time are correlated with oxygen flux at the sediment‐water interface [ABSTRACT FROM AUTHOR]

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Due to excessive heating, various physical mechanisms are less effective in engineering and modern technologies. The aligned electromagnetic field performs as insulation that absorbs the heat from the surroundings, which is an essential feature in contemporary technologies, to decrease high temperatures. The major goal of the present investigation is to use magnetism perpendicular to the surface to address this issue. Numerical simulations have been made of the MHD convective heat and amplitude problem of electrical fluid flow down a horizontally non-magnetized circular heated cylinder with reduced gravity and thermal stratification. The associated non-linear PDEs that control fluid motion can be conveniently represented using the finite-difference algorithm and primitive element substitution. The FORTRAN application was used to compute the quantitative outcomes, which are then displayed in diagrams and table formats. The physical features, including the phase angle, skin friction, transfer of heat, and electrical density for velocity description, the magnetic characteristics, and the temperature distribution, coupled by their gradients, have an impact on each of the variables in the flow simulation. In the domains of MRI resonant patterns, prosthetic heartvalves, interior heart cavities, and nanoburning devices, the existing magneto-hydrodynamics and thermodynamic scenario are significant. The main findings of the current work are that the dimensionless velocity of the fluid increases as the gravity factor R g decreases. The prominent change in the phase angle of current density α m and heat flux α t is examined for each value of the buoyancy parameter at both α = π / 6 and π angles. The transitory skin friction and heat transfer rate shows a prominent magnitude of oscillation at both α = π / 6 and π / 2 positions, but current density increases with a higher magnitude of oscillation. [ABSTRACT FROM AUTHOR]

Due to excessive heating, various physical mechanisms are less interested in engineering and modern technologies. To reduce excessive heating, the aligned magnetic field acts like a coating material to insulate the heat which is a very important mechanism in modern technologies. To solve this problem, the primary goal of the current analysis is to utilize magnetization normal to the surface. Numerical simulations have been made of the magnetohydrodynamic convective heat-transfer phenomena of electrically conductive fluid flow down the horizontally non-magnetized heated cylinder. The associated non-linear PDEs that control fluid motion can be conveniently represented by using the finite-difference algorithm and primitive element substitution. The numerical outcomes are deduced in graphs and tabular form with the help of FORTRAN program. The distinct parameters in the flow model are affected by physical features such skin friction, heat transfer, and current density for velocity profile, magnetic field profile, and temperature profile together with their slopes. The current magnetohydrodynamics and thermal problem is very important in the fields of MRI resonance sequences, artificial heart wolves, internal heart cavities, and nanoburning technologies. Additionally, the increase in Prandtl Pr leads to a reduction in skin friction and heat transmission, which is physically consistent, due to frictional forces between viscous layers with lesser magnitude.

This paper presents a study of the unsteady flow of second grade fluid through a capillary tube, caused by sinusoidally varying pressure gradient, with fractional derivative model. The fractional derivative is taken in Caputo-Fabrizio sense. The analytical solution for the velocity profile has been obtained for non-homogenous boundary conditions by employing the Laplace transform and the finite Hankel transform. The influence of order of Caputo-Fabrizio time-fractional derivative and time parameter on fluid motion is discussed graphically. [ABSTRACT FROM AUTHOR]

Simulative optimization methods often build on an iterative scheme, where a simulation model is solved in each iteration. To reduce the time needed for an optimization, finding the right balance between simulation model quality, and simulation time is essential. This is especially true for transient problems, such as fluid flow within a hydromechanical system. Therefore, we present an approach to building steady-state surrogate models for oscillating flow in a pipe with a local heat source. The main aspect is to model the fluid as a solid with an orthotropic heat transfer coefficient. The values of this coefficient are fitted to reproduce the temperature distribution of the transient case by parametric optimization. It is shown that the presented approach is feasible for different sets of parameters and creates suitable surrogate models for oscillating flow within a pipe with a local heat source. In future works, the presented approach will be transferred from the simplified geometry under investigation to industrial problems. [ABSTRACT FROM AUTHOR]

The driving force of a Stirling engine comes from the heating expansion and cooling compression of working medium. The enhancement of heat transfer capacity of the hot/cold end is the key to improve the Stirling engine performance, while how to objectively evaluate the heat transfer enhancement effect is a precondition. Herein, a polytropic model of a Stirling engine thermal cycle process was firstly established. A method for evaluating the heat transfer enhancement performance under oscillating flow was proposed based on the efficiency and power improvement of a Stirling engine. This method was further used to evaluate the heat transfer enhancement effect of a novel four-leaf helical heating tube under oscillating flow, the efficiency and power improvement factor of which was reached 13.5 % and 6.1 %, respectively. The results show that the efficiency and power improvement evaluation criterion was more comprehensive and intuitive in evaluating the heat transfer enhancement performance under oscillating flow compared with the conventional evaluation indictor like performance evaluation criterion and equivalent tube length and tube number criterion. • Thermal cycle polytropic model of Stirling engine was established. • Heat transfer enhancement evaluation method under oscillating flow was proposed. • Four-leaf helical tube was employed to enhance heat transfer capacity. • Heat transfer enhancement effect was evaluated with different criteria. [ABSTRACT FROM AUTHOR]

In this study, the simulation and experiment of a thermoacoustic Stirling heat engine (TASHE) with a phase-adjuster are demonstrated. The phase-adjuster is used as a tool to tune the acoustic field of the TASHE in order to maintain the high performance of the system. The core components of the TASHE comprising an ambient heat exchanger, regenerator, hot heat exchanger, and feedback pipe, are located in the torus section. Besides, there is a resonator pipe, connecting to the torus, which acts as compliance to maintain the system stability, and the phase-adjuster is installed at the end of the resonator. Firstly, the TASHE is modeled by DeltaEC to search for the optimal configurations of the prototype. Due to the variations of acoustic load or operating conditions from the design criteria, the engine absolutely cannot provide maximum output power. The proposed phase-adjuster could keep its high efficiency by re-matching the frequency. In the experiments, the self-excited and steady-state temperatures of the TASHE are around 755 K and 670 K, respectively. Here, this TASHE can provide an acoustic power of up to 40 W, and an energy conversion efficiency of 12.03%. In comparison, there is a reasonably good agreement between the measured and DeltaEC simulated results. This can reflect on the preciseness of the proposed model. In the case of the TASHE operated under the off-design condition, these scenarios certainly drop its efficiency. Consequently, the role of the phase-adjuster in improving its performance by tuning the acoustic field is presented here.

The heat and mass transfer to solid particles in one-dimensional oscillating flows are investigated in this work. A meta-correlation for the calculation of the Nusselt number (Sherwood number) is derived by comparing 33 correlations and data point sets from experiments and simulations. These models are all unified by their dependencies on the amplitude parameter 10 − 3 ≤ ϵ ≤ 10 3 and the Reynolds number 10 − 1 ≤ R e ≤ 10 6 , while the ϵ - R e plane is applied as a framework in order to graphically display the various models. This is the first study to consider this problem in the entire ϵ - R e plane quantitatively while taking preexisting asymptotic models for various areas of the ϵ - R e plane into account. [ABSTRACT FROM AUTHOR]

The heat and mass transfer characteristics along with Cattaneo-Christov heat flux on oscillating hydromagnetic flow of third grade nanofluid through a permeable channel under entropy generation analysis have been examined in this paper. The impacts of chemical reaction, Brownian motion, thermophoresis, Ohmic heating, and radiative heat have also been taken into consideration. The Buongiorno nanofluid model has been utilized for the present analysis.The investigation related to the present study is helpful in biomedical engineering, manufacturing industries as coolants, energy conservation, cancer treatments (like hyperthermia), dynamics of physiological fluids, biomedicines, and nano-drug suspension in pharmaceuticals. The system of nonlinear ordinary differential equations (ODEs) have been attained and solved by applying the Runge-Kutta 4th order method with the help of shooting process after the execution of the perturbation procedure on nondimensional partial differential equations (PDEs). The results of the present study have been deliberated by plotting graphs for the effects of various non-dimensional parameters on entropy, Bejan number, concentration, velocity, and temperature. The numerical values of heat and mass transfer rates for different physical parameters have been computed. [ABSTRACT FROM AUTHOR]

The forced convection flow and heat transfer aspects of an isothermal sphere in an oscillating stream of power‐law fluids in the range of 5 ≤ Re ≤ 120 and 0.3 ≤ n ≤ 1.5 are numerically investigated. The effects of shear‐dependent viscosity and oscillatory flow features on the temporal variation of streamlines, patterns of streaming flows, drag coefficient, and Nusselt number are examined in depth. The power‐law index modulates the transition of the steady streaming flows between two distinct streaming regimes. Overall, shear‐thinning fluids at low oscillation frequencies are observed to yield significant increase in heat transfer. Due to the phase lag in the momentum boundary layer, the drag coefficient exhibits a phase lag ranging from π/2 to 4π/5 whereas the Nusselt number lags by about 50 % of these values. [ABSTRACT FROM AUTHOR]

A membrane contactor is a device that attains the transfer of gas/liquid or liquid/liquid mass without dispersion of one phase within another. Membrane contactor modules generally provide 30 times more surface area than can be achieved in traditional gas absorption towers and 500 times what can be obtained in liquid/liquid extraction columns. By contrast, membrane contactor design has limitations, as the presence of the membrane adds additional resistance to mass transfer compared with conventional solvent absorption systems. Increasing mass transfer in the gas and solvent phase boundary layers is necessary to reduce additional resistance. This study aims to increase the mass transfer in the gas phase layer without interfering with membrane structure by oscillating the velocity of the feed gas. Therefore, an unsteady state mathematical model was improved to consider feed gas oscillation. The model equation was solved using Comsol Multiphysics version 6.0. The simulation results reveal that the maximum CO2 removal rate was about 30% without oscillation, and at an oscillation frequency of 0.05 Hz, the CO2 percent removal was almost doubled. [ABSTRACT FROM AUTHOR]

Due to the excellent heat transfer performance and low pressure drop, the parallel-plate regenerator (PPR) is very suitable for use in Stirling engines. At present, researches on the application of PPR in Stirling engines are rare. The axial heat conduction loss in PPR is a key factor that affects the heat transfer performance. Therefore, in this paper, the theoretical model of PPR operated under the Stirling engine working conditions is studied numerically, the regenerative effectiveness is taken as the target parameter, and the influence of axial heat conduction and other important parameters are explored. The results show that the axial heat conduction is detrimental to regenerative effectiveness, the regenerative effectiveness does not vary monotonically with the increase in the thermal conductivity, the copper regenerator with higher thermal conductivity has a lower regenerative effectiveness because of its higher axial thermal conductivity. The working frequency and the thermal properties of the working fluids have a comprehensive effect on the regenerative effectiveness, for high thermal diffusivity working fluids, such as helium and hydrogen, the regenerative effectiveness is high, while for low thermal diffusivity working fluid, such as carbon dioxide, the regenerative effectiveness is relatively low, and does not vary monotonously with frequency. The regenerative effectiveness increases with the reduction of the plate spacing, the plate spacing should be determined by considering the regenerative effectiveness and the pressure drop comprehensively. It is recommended that the stainless steel be a suitable material for the plates, and helium be the suitable working fluid. [ABSTRACT FROM AUTHOR]

• Flow sensing of primary cilia numerically explored from the viewpoint of FSI. • The simulation is three-dimensional and two-way fluid-cilia interaction is considered. • Ciliary basal rotations are modelled by attaching a rotational spring to the cilium basal end. • The interaction and synchronization between the primary cilia and the flow are analyzed. • The impact of primary cilia on the wall shear stress field is examined. Primary cilia at mammalian cells play a crucial role in mechanosensing of biological flows. Understanding the fluid-cilia interaction not only help to interpret the role of primary cilia as a flow sensor but also could shed light on the design of bio-inspired flow sensors. This study thus investigates the dynamics of primary cilia in an oscillating viscous flow via a three-dimensional simulation. In our simulations, a two-way fluid-structure interaction is considered using the immersed boundary-lattice Boltzmann method. To reproduce the experimentally observed ciliary basal rotations, the primary cilium is modelled as a slender filament whose basal end is connected to a nonlinear rotational spring. For the scenario considered, the primary cilium is observed to do an in-plane flapping motion which is symmetrical in term of the superimposed cilium profiles. For a cilium undergoes such a flapping motion, the flow-induced curvature (or tensile stress) at its lower part is found to synchronize better with the applied pressure gradient signal. Therefore, the lower part of primary cilia may be more responsible for detecting the real-time variations of the flow information. Our simulation results also suggest that the location of the maximal tensile stress is propagatable rather than staying at a fixed site, e.g., the base point, possibly due to the asynchronous deflection occurs at the cilium's different parts. The presence of primary cilia is also found to reduce the spatial-averaged wall shear stress (WSS) level and alter the oscillation characteristic of the WSS field by making the WSS less oscillatory in some regions. [ABSTRACT FROM AUTHOR]

In the present study, a theoretical model is proposed to calculate the settling velocity of solid particles in turbulence generated by both oscillatory and nearly horizontal flow motions. In contrast to other previous models that typically compartmentalize two flow conditions in their studies, this study takes a more comprehensive approach by considering the combined effects of both conditions. Taking into account the influence of drag nonlinearity, virtual mass and Basset history forces, the new theoretical model is formulated. The proposed model is obtained by solving the particle motion in fluid flow under some reasonable assumptions. Accordingly, we obtain a new dimensionless term to better take into account the effect of turbulence anisotropy on the settling velocity and the role of the sediment damping coefficient. Application of this term for other conditions is discussed in the paper. The present model shows satisfactory agreement with a wide range of experimental and numerical data and with different flow conditions found in the existing literature. These data include homogeneous isotropic turbulence with high resolution direct numerical simulations (DNS), turbulent open channel and vertical oscillation. [Display omitted] • Model calculates particle settling velocity in turbulent and oscillatory flows. • Includes drag nonlinearity, virtual mass, and Basset history forces in new equation. • Validated with experimental and numerical data for various turbulence scenarios. [ABSTRACT FROM AUTHOR]

Efficient engine technologies to convert low-grade heat to electricity are urgently desired. In this work, a conceptual structure of engine for a novel cycle or one-way oscillating flow cycle (OOFC), which consists of two isochoric and two adiabatic processes, is described for low-grade heat utilization. Characteristics of OOFC allows for the working fluid temperature glide to be matched to the decrease in temperature of low-grade heat. Then, thermodynamic model is developed for evaluating the performance. Theoretical simulation results show that maximum specific output works are in the range of 12.2 kJ kg−1 – 79.7 kJ kg−1. Compared to Stirling cycle system, maximum specific output work in OOFC system could be improved by 16.2 %–24.8 %. Compared to ideal Carnot cycle engine system, maximum specific output works in OOFC system is nearly the same and 1.8 %–2.6 % lower. As Carnot cycle engine is ideal while thermodynamic cycle loss and heat transfer loss in cold heat exchangers are considered in OOFC engine, the ratios of maximum specific output work demonstrate that OOFC system could be very promising for low-grade heat utilization as a result of well-matched temperature profile in hot heat exchanger. • Structure of OOFC engine with temperature glide heat addition is described. • Large temperature glide heat rejection is achieved in OOFC engine. • Maximum specific output work of OOFC system is 116 %–125 % of Stirling cycle system. • Maximum specific output work in OOFC system are equivalent to ideal Carnot cycle system. • OOFC engine has well-matched temperature profile for low-grade heat utilization. [ABSTRACT FROM AUTHOR]

Stirling regenerator is one of the emerging heat exchanger systems in the area of cryogenic cooling. Many kinds of research have been conducted to study the efficiency of Stirling regenerators. Therefore, the principles and related knowledge of Stirling refrigerators must be thoroughly understood to design a regenerator with excellent performance for low-temperature and cryogenic engineering applications. In this study, an experimental setup is developed to estimate the pressure drop of the oscillating flow through two different wire-mesh regenerators, namely, 200 mesh and 300 mesh, for various operating frequencies ranging from 3 (200 RPM) to 10 Hz (600 RPM). Transient, axisymmetric, incompressible, and laminar flow governing equations are solved numerically, and source terms are added in the governing equations with the help of the porous media model and the Ergun semiempirical correlation, assuming that the wire meshes are cylindrical particles arranged uniformly. Simulation results show that the numerical predictions of temporal pressure variation are in reasonably good agreement with those of experimental findings. It is also found that the Ergun correlation works more accurately for higher flow rate conditions. [ABSTRACT FROM AUTHOR]

The relaxation of a rigid particle suspended in a one-dimensional oscillating flow is calculated according to different drag models and the results are compared. Conditions are derived under which relaxation can be neglected or drag models can be substituted by simpler ones. This investigation is conducted analytically and graphically via the plane defined by the Reynolds number and amplitude parameter. This work matches various, mostly analytic drag models together to consider simple particle relaxation with a few, broad range input parameters and cover large parts of the plane spanned by Reynolds number and amplitude parameter. [ABSTRACT FROM AUTHOR]

In this article, a comprehensive review of the computational fluid dynamics (CFD)-based modeling approach for thermoacoustic energy conversion devices is proposed. Although thermoacoustic phenomena were discovered two centuries ago, only in recent decades have such thermoacoustic devices been spreading for energy conversion. The limited understanding of thermoacoustic nonlinearities is one of the reasons limiting their diffusion. CFD is a powerful tool that allows taking into consideration all the nonlinear phenomena neglected by linear theory, on which standard designs are based, to develop energy devices that are increasingly efficient. Starting from a description of all possible numerical models to highlight the difference from a full CFD method, the nonlinearities (dynamic, fluid dynamic and acoustic) are discussed from a physical and modeling point of view. The articles found in the literature were analyzed according to their setup, with either a single thermoacoustic core (TAC) or a full device. With regard to the full devices, a further distinction was made between those models solved at the microscopic scale and those involving a macroscopic porous media approach to model the thermoacoustic core. This review shows that there is no nonlinear porous media model that can be applied to the stack, regenerator and heat exchangers of all thermoacoustic devices in oscillating flows for each frequency, and that the eventual choice of turbulence model requires further studies. [ABSTRACT FROM AUTHOR]

Simulative optimization methods often build on an iterative scheme, where a simulation model is solved in each iteration. To reduce the time needed for an optimization, finding the right balance between simulation model quality, and simulation time is essential. This is especially true for transient problems, such as fluid flow within a hydromechanical system. Therefore, we present an approach to building steady-state surrogate models for oscillating flow in a pipe with a local heat source. The main aspect is to model the fluid as a solid with an orthotropic heat transfer coefficient. The values of this coefficient are fitted to reproduce the temperature distribution of the transient case by parametric optimization. It is shown that the presented approach is feasible for different sets of parameters and creates suitable surrogate models for oscillating flow within a pipe with a local heat source. In future works, the presented approach will be transferred from the simplified geometry under investigation to industrial problems.

This work describes the dynamics of an underwater pipeline subjected to an incident flow composed of a uniform flow and a sinusoidal component. The motion equations of the pipeline and the fluid flow around the pipeline are solved simultaneously with a numerical model that considers in-line oscillations (one degree of freedom) and a second numerical model that additionally includes cross flow (two degrees of freedom). The amplitude response and drag forces on the pipe are compared for both models considering parameters such as the difference between the excitation and natural frequencies of pipelines and the relative value between the sinusoidal and uniform components of the incident flow. Important differences in numerical predictions of both models are observed when the excitation frequency is greater than the natural frequency of the system and when the amplitude of the oscillatory component of the incident flow is greater than the amplitude of the uniform flow. [ABSTRACT FROM AUTHOR]

The energy efficiency of systems, equipment, and sensors is nowadays intensively studied. The new generation of microelectronic sensors is very sophisticated and the energy consumption is in the microwatts range. The energy to power the microelectronic devices can be harvested from oscillating flow in small size channels and so replaceable batteries could be eliminated. Piezoelectric elements can convert energy from oscillation to electrical energy. This paper focuses on the simulation of periodic flow in the fluidic oscillator. CFD simulations were performed for several values of the flow rate. Experimental measurement was carried out under the same conditions as the CFD experiment. The main monitored and evaluated parameters were volume flow rate and pressure loss. Fluid oscillations were analysed based on CFD simulations and the theoretical maximum energy available for the deformation of piezoelectric elements and transformable into electrical energy was evaluated. [ABSTRACT FROM AUTHOR]

Liquid film thickness is an important parameter for predicting phase change heat transfer in pulsating heat pipes (PHPs). This study experimentally evaluated the liquid film thickness formed during the oscillation of the liquid column within PHPs. The liquid column was oscillated sinusoidally under various oscillating conditions to simulate the flow phenomena. The average thickness of the liquid film formed during the oscillations was determined by comparing the oscillation amplitudes of the tip of the liquid column with and without the liquid film. It was confirmed that the average thickness of the liquid film varied with the capillary number for each working liquid (water, ethanol, and FC-40). The characteristic velocity of the capillary number was the time-average velocity of the tip of the liquid column, which was derived by assuming that the tip of the liquid column oscillated sinusoidally. Additionally, the average liquid film thicknesses during the liquid column oscillations were evaluated according to the correlations for various flow conditions proposed in previous studies. In this evaluation, the velocity and acceleration values obtained by assuming that the tip of the liquid column oscillated sinusoidally were determined based on the experimental results. This evaluation demonstrated the acceleration effect, thereby indicating that the acceleration made the liquid film thinner when the capillary number was higher than the threshold of the capillary number for each working liquid. Finally, empirical correlations with and without the acceleration effect were proposed for the average liquid film thickness of oscillating flows in terms of capillary and Laplace numbers. It was demonstrated that the proposed correlations could predict the average liquid film thickness within the range of approximately ±15 % accuracy.

The distribution and concentration of dissolved oxygen (DO) play important roles in aerobic heterotroph activities and some slow chemical reactions, and can affect the water quality, biological communities, and ecosystem functions of rivers and lakes. In this work, the transport of high Schmidt number DO at the sediment-water interface of spanwise oscillating flow is investigated. The volume-averaged Navier-Stokes (VANS) equations and Monod equation are used to describe the flow in the sediment layer and the sediment oxygen demand of microorganisms. The phase-averaged velocities and concentrations of different amplitudes and periods are studied. The dependence of DO transfer on the amplitude and period is analyzed by means of phase-average statistical quantities. It is shown that the concentration in the sediment layer is positively correlated with the turbulence intensity, and the DO concentration and penetration depth in the sediment layer increases when the period and amplitude of the oscillating flow increase. Moreover, in the presence of oscillating flow, a specific scaling relationship exists between the Sherwood number/oxygen consumption of aerobic heterotrophs and the Reynolds number. [ABSTRACT FROM AUTHOR]

Annular mini-channel Stirling heater are replacing conventional tubular heater in modern free piston Stirling engines due to reliability and manufacturing issues. Heat transfer characteristics of a new mini channel heater with variable cross section are analyzed under oscillating flow. The effect of oscillating speed and dimensionless fluid displacement (Aₒ) on heat transfer rate is evaluated. The variation of operating speed from 150 to 1200 rpm enhanced heating power requirement from 191 to 581 W for wall temperature in 473–773 K range. However, hot space gas temperature is decreased by 66–162 K under same conditions. The change in Aₒ from 10.98 to 15.09 resulted in 13.2% enhancement in heat transfer at 300 rpm whereas maximum enhancement of 27.6% was noticed at 1200 rpm with maximum hot space temperature drop of 9.5 K. Based on the data, an empirical correlation for heat transfer in annular mini channel Stirling heater is proposed, the correlation covers kinetic Reynolds number (Reω) in 0.48–1.94 range.

The oscillating flow is a problem occurring in the stay vane passage of hydro turbines. The improper shape of the stay vane causes wake formation and sheds large eddies in the trailing edge of the stay vane. The wake at the trailing edge of the stay vane produces pressure fluctuation in the stay vane passage, which leads to noise and vibration during the turbine operation. Therefore, the improper shape of the stay vane may cause flow oscillation in the stay vane passage. Thus, a proper shape design of the stay vane considering the oscillating flow is necessary to mitigate the flow instability. Consequently, experiment and CFD analyses showed that the initial stay vane (ISV) shape causes recirculation and pressure fluctuation. An optimum design methodology is adopted to improve the flow behavior around the stay vane. Optimization of the stay vane is also conducted by using two objective functions (turbine efficiency and flow uniformity) and 12 design variables. The optimal stay vane (OSV) shape is attained by a multi-objective genetic algorithm. Finally, the flow behavior in the stay vane passage with ISV and OSV is compared by experiment and CFD analyses. Thus, the flow instabilities are mitigated with OSV. The installation of OSV improved the flow angle distribution, secondary flow, and pressure fluctuation in the Francis hydro turbine. [ABSTRACT FROM AUTHOR]

• The device with resonance channel generating low frequencies is investigated. • The flapping frequency is generated due to the existence of the resonance channel. • The vortex amplifier creates a time delay in changing the direction of the flow. • The flapping frequency is adjustable by changing the geometrical parameters. In this paper, the performance of the fluidic oscillator with resonance channel and vortex amplifier is numerically investigated. The unsteady flow generated in this oscillator is simulated in the three-dimensional model with the assumption of turbulent flow using the DES model. The effects of different geometric parameters on the output frequency of the oscillator were investigated by changing the height of the resonance channel, placing triangular, square, and circular splitters, changing the splitter distance from the supply nozzle and, changing the state of control nozzles. Results indicate that the highest frequency variation is due to the change of the distance between the supply nozzle and splitter. By changing the splitter distance, the output frequency varies from 11 to 66 Hz. Reducing the height of the resonance channel leads to an increase in the output frequency. It has also been shown in the results that the frequency achieved using a triangular splitter is lower than square and circular splitters. Moreover, the output frequency is shifted with different output angles, achieving the highest frequency (43.48 Hz) using the 40-degree angle. A characteristic curve correlating the oscillation frequency f with the outlet area angle θ was obtained, which is expressed as = - 0.0009 θ 3 + 0.0553 θ 2 + 0.581 θ - 18.4. [ABSTRACT FROM AUTHOR]

The viscoelastic elastomer flowing in a shock absorber has a strong viscoelasticity,and the oscillating flow models for simulating and studying a viscoelastic elastomer in the orifice and gap of shock absorber have been rarely reported. A fractional Maxwell model with quasi-property is proposed to study the oscillating flow of a viscoelastic elastomer shock absorber. According to the actual operating conditions of viscoelastic elastomer shock absorber during periodic reciprocating motion,the oscillating flows in both the orifice and gap of a shock absorber were simplified. The real-world initial and boundary conditions were set,and then the numerical solution of the fractional Maxwell model was obtained using the finite difference method. The velocity distributions at different oscillation frequencies and the influences of the parameters on the stress-strain rate curves were analyzed by comparing the fractional Maxwell model with the Newtonian fluid model. The simulated and experimental results indicate that the non-linearity and frequency dependence of the fractional Maxwell model are stronger than those of the Newtonian fluid model; the stress-strain rate distribution curves of the fractional Maxwell model are all elliptic for different parameters; and the ellipse major axes of stress-strain rate curves of the fractional Maxwell model are counterclockwise rotated as fractional order exponents α and β as well as quasi-state property dimensionless coefficient η increase. The proposed fractional Maxwell model can be used to successfully simulate and predict the shapes and changing trends of hysteretic curves,and the relative average error of energy absorption rate is 3. 60%. [ABSTRACT FROM AUTHOR]

Although single‐hole oscillatory columns have been studied since the 1990s, to this day there is an absence of appropriate dimensionless groups to express the hydrodynamic conditions and power requirement for the moving‐baffle oscillatory baffled column (OBC). This paper uses computational fluid dynamic (CFD) software coupled with moving overset meshing to aid in the derivation of the first dimensionless oscillatory power number for OBCs. In terms of the moving‐baffle OBC, this work marks the first time a power density equation has been derived specifically to account for this column's unique hydrodynamic profile. Equations for period‐averaged Reynolds number and period‐averaged Strouhal numbers were developed to better estimate the fluid intensity within these moving‐baffle columns. This work serves as an example of how complex and challenging flow regimes, such as periodically oscillating flow, can be simplified and analyzed to produce appropriate design equations. [ABSTRACT FROM AUTHOR]

This study investigates self-sustained oscillation of the flow in a double-cavity channel with cavity length–width ratio L/H = 3 using a time-resolved particle image velocimetry (TR-PIV) technique. Three Reynolds numbers based on the cavity length L and the bulk velocity in the narrow section of the channel U 0 , i.e., R e L = 12,500, 24,580, and 49,100, are considered to investigate the influence of Reynolds number on the self-sustained oscillation. As the Reynolds number increases, the oscillation becomes more intense and shifts to the leading edge of the double-cavity channel. However, the power spectra and contour plots of the spatial v–v correlation coefficient reveal that the periodicity of the oscillation becomes less profound as the Reynolds number increases. A further phase-averaged analysis reveals the spatiotemporal evolution process of oscillation and convincingly demonstrates a more intense and complicated process of the oscillation as the Reynolds number increases. [ABSTRACT FROM AUTHOR]

A steady flow with a high flow rate, oscillating flow, and audible sound waves can enhance the heat transfer performance in air-to-air heat exchangers, while the comparison of influence mechanisms and degrees of different methods is ignored. This study investigates flow instability and heat transfer characteristics in an air-to-air heat exchanger under steady flow, oscillating flow and sound waves. The results show that steady flow and oscillating flow with different amplitudes have little effect on the distribution characteristics of velocity and vortices. However, the vortices disappear and generate periodically under 140 dB. Moreover, oscillating flow and sound waves exert distinct influences on flow instability. The cold side experiences the highest increase in turbulence kinetic energy when subjected to high-amplitude oscillating flow, while the greatest increase on the hot side occurs under high-amplitude sound waves, and the influence of steady flow on turbulence kinetic energy is relatively low. Additionally, the steady flow enhances the heat transfer performance by increasing flow rate, while the oscillating flow and sound waves promote the heat transfer between the fluid and surface. Under the effect of steady flow, oscillating flow, and sound waves, the values of heat flux are 1.22, 1.24, and 1.31 times that of the initial condition with the amplitude increasing to 0.683 m/s (140 dB). The results demonstrate that with the increase in amplitude acting on the inlet, the sound waves have the greatest impact on heat transfer performance, followed by oscillating flow, and the effect of steady flow is relatively slight. The research can provide guidelines for the development of heat transfer enhancement in air-to-air heat exchangers. • Effects of steady, oscillating flow, and sound waves on heat transfer are compared. • Oscillating flow and sound waves exert the distinct effects on flow instability. • Sound waves with high amplitude alter the evolution of vortex structure. • Increase in flow instability contributes more to heat transfer than that in velocity. • Sound waves with high amplitude enhance the air heat transfer significantly. [ABSTRACT FROM AUTHOR]

In order to gain a better understanding of the latent heat transfer mechanism in pulsating heat pipes (PHPs), this study experimentally investigated the thickness characteristics of a liquid film that forms during oscillations of the liquid column within PHPs. Accordingly, the liquid column was oscillated sinusoidally under different oscillating conditions to simulate the flow phenomena. A circular tube with an inner diameter of 2 mm was used as the test channel; ethanol and FC-40 were used as working liquids. The average thickness of the liquid film formed due to the liquid column oscillations on the channel wall was determined by comparing the oscillation amplitudes of the tip of the liquid column obtained with and without the liquid film. It was confirmed that the average thickness of the liquid film varied based on the average capillary number for each working liquid. The time-average velocity of the tip of the liquid column, which was derived based on the assumption that the tip of the liquid column oscillates sinusoidally, was used as the characteristic velocity to calculate the average capillary number. The experimental results were compared with various correlations proposed in previous studies. The results show that the acceleration of the oscillating liquid column affected the liquid film thickness for a high average capillary number. For this high average capillary number, the rate of increase in liquid film thickness with the average capillary number decreased, and the liquid film thicknesses during the liquid column oscillation approached those under the steady condition. Furthermore, the liquid film thicknesses for the vertical channel were compared with those for the horizontal channel, and it was made clear that the effect of gravity on liquid film thickness is significant. Thus, the thickness characteristics of a liquid film during the liquid column oscillation within PHPs were clarified.

• Introduced the influence of frequency on the heat transfer performance of AMR. • The frequency correlated characteristic is revealed by synergy principle. • A frequency correlated heat transfer correlation is proposed. • Perforated plate enhances heat transfer performance by more than 20%. This paper investigates the heat transfer characteristics of oscillating flow in active magnetocaloric regenerator (AMR) by numerical simulation. We show that the operating frequency plays an important role to the heat transfer coefficient of the regenerator other than the Reynolds number and the Prandtl number. Moreover, to reveal the mechanism of the frequency correlated heat transfer characteristics, a modified synergy number Fci is proposed based on the field synergy principle. To quantitatively describe the influence of operating frequency, the oscillating Reynolds number is introduced. This paper compares the heat transfer coefficient calculated by three different methods: CFD simulation, correlations with the oscillating Reynolds number and correlations with the conventional Reynolds number. Based on the results of comparison, this paper qualitatively presents a frequency correlated heat transfer correlation for different operating frequency. In addition, a perforated parallel plate matrix regenerator is proposed to improve the heat transfer performance of the AMR. [ABSTRACT FROM AUTHOR]

Frequency change by changing the various geometrical parameters of single feedback loop oscillator. • The single feedback loop fluidic oscillator has been simulated with DES method. • Increase nozzles angle until 18°, f (Hz) increases then decreases by angle increasing. • Feedback loop width has an optimum length to create the highest frequency. • The horizontal elliptic feedback loop oscillator has 25% higher than circle loop. • The highest reduction in frequency is observed by decreasing the splitter distance. In this study, a single feedback loop oscillator with two outlet channels is numerically studied. In order to solve the governing equations, the commercial software of Ansys-Fluent 16.0 is used. A set of CFD simulations is performed for a 2D and turbulent flow using hybrid DES model. The results of a simple parametric study investigating the effects of five geometrical parameters such as splitter distance, angle between outlet channels, feedback loop width and different shape of splitter and feedback loop on the flow-switching frequencies produced in single feedback loop oscillators are reported. It can be observed that the 2D simulation results, which are obtained using DES method, showed more appropriate outcomes compared to various RANS models. Moreover, by changing the oscillator's geometric parameters, there will be an increase in switching jet frequency up to 25% which occurs in certain conditions. The change in the splitter distance from feedback loop showed the most variations after examining the geometric parameters in which the frequency is reduced from 83 to 38 Hz by decreasing the so-called parameter. Results also showed that the increase in outlet channel angle up to 18 ° initially cause an increase in the frequency and then it is followed by a reduction. After investigating the change in the feedback loop width, it is concluded that by increasing the feedback loop width to 4 mm, the outlet frequency of the device increased primarily and followed by the considerable reduction. Also, the obtained frequency by the oscillator with horizontal elliptic loop is 25% higher than the circle feedback loop oscillator. [ABSTRACT FROM AUTHOR]

• Effect of a novel spark-plug plasma synthetic jet on PEM fuel cell is investigated. • Present study provides a new insight into PEM fuel cell. • PEMFC shows 11% increase in the peak power output as compared to SPSJ actuator off. • SPSJ actuator is enhanced the oxygen concentration and diffusion rate. The effect of oscillating flow on the performance of a single cell PEM fuel cell is investigated at the cathode side. The oscillating flow is generated with a Spark-plug plasma synthetic jet (SPSJ) actuator. The PEM fuel cell is directly connected to SPSJ actuator. The significant performance improvement of the PEM fuel cell is by this approach. At two different hydrogen and air flow rates, necessary experiments are performed to obtain the polarization curves and current–time history plots at a constant voltage. The results indicate that the peak power output of the PEMFC increases up to 11% compared to the case without a SPSJ actuator. It can be concluded that the plasma synthetic jet actuator producing oscillating flow enhances the oxygen concentration and diffusion rate at the cathode channel of the PEM fuel cell. [ABSTRACT FROM AUTHOR]