Reynolds Number Region Research Articles

Experimental study on the aerodynamic force and vibration of a scratched stay cable perpendicular to the wind

The wind load and wind-induced vibration of stay cables, as the primary load-bearing component in cable-stayed bridges, are highly important. Inevitably, cables experience scratches during production, transportation, and installation. Scratches may have an important effect on a cable's aerodynamic force and wind-induced vibration. Consequently, three sectional cable models with varying scratches were prepared for wind tunnel tests. The incoming wind was perpendicular to the cable model. The aerodynamic and vibration responses in precritical, critical Reynolds number regions were investigated. The results showed that scratched cables exhibit Reynolds number effects similar to those of smooth cables. Within attack angles ranging from 50° to 85°, scratching significantly reduced the corresponding Reynolds numbers of the TrBL0-1 and TrBL1-2 transitions. This caused the scratched cable to vibrate substantially at a lower Reynolds number (wind speed) than the smooth cable. There is a smaller attack angle range between 50° and 90° in which scratches cause the average lift coefficient to climb slowly with the Reynolds number, without a bistable phenomenon. This implies that there is no significant vibration. The critical Reynolds number effect effectively predicts the vibration of the scratched cable, and the complex flow in the critical Reynolds number region limits the accuracy of the Den Hartog criterion.

Turbulence model adaptability at critical Reynolds numbers and applications in wake control via fairings

The water-drop shaped fairings with varying shape angles are attached to a circular cylinder to achieve wake control and vortex suppression at critical Reynolds numbers. To ensure the capability of Reynolds averaged Navier–Stokes (RANS), detached eddy simulation (DES) and large eddy simulation (LES) models at the critical Reynolds number region, three representative turbulence models are employed: LES with σ subgrid-scale (SGS) model, delayed DES model with improved wall-modeling capability (IDDES) and shear stress transport (SST) k−ω RANS model. These models are utilized to simulate flow around a circular cylinder at Reynolds number Re=2.5×105. The solver used in this paper is further developed based on the high-resolution algorithm platform for incompressible flow (HRAPIF). The comparative analysis of the results from the three turbulence models has been rigorously validated and investigated. An exhaustive examination of the mean flow field, Reynolds stresses, characteristic lengths, and instantaneous flow fields among the models reveals instructive insights. The IDDES and σ-LES models predict the hydrodynamic forces, the so-called ‘drag crisis’, alongside the pressure distribution and skin friction coefficient with high precision. The σ-LES model stands out for its superior accuracy, while the IDDES model is also a viable alternative, offering commendable accuracy with a reduced demand for mesh density. Subsequently, the IDDES model is selected for wake control calculations using fairings with five distinct shape angles (30∘,45∘,60∘,75∘ and 90∘) at Re=2.5×105. In-depth comparisons to the bare cylinder and subcritical results reveal that the wake control effect varies at the critical Reynolds region. The fairing with a 30° shape angle substantially suppresses hydrodynamic forces. The lift coefficient experiences a remarkable decrease of approximately 96%, while the drag coefficient diminishes by about 90%. Concurrently, fairings with angles from 45° to 90° lead to reductions in drag coefficient of 11.6%, 10%, 3% and 4%, respectively. A 75% lowering in the lift force coefficient is found even with a short fairing with shape angle α=90∘, which may be a meaningful instruction to industrial design.

A wall-modeled hybrid RANS/LES model for flow around circular cylinder with coherent structures in subcritical Reynolds number regions

In the present paper, a hybrid RANS/LES model with the wall-modelled LES capability (called WM-HRL model) is developed to perform the high-fidelity CFD simulation investigation for complex flow phenomena around a bluff body with coherent structure in subcritical Reynolds number region. The proposed method can achieve a fast and seamless transition from RANS to LES through a filter parameter <i>r</i><sub>k</sub> which is only related to the space resolution capability of the local grid system for various turbulent scales. Furthermore, the boundary positions of the transition region from RANS to LES, as well as the resolving capabilities for the turbulent kinetic energy in the three regions, i.e. RANS, LES and transition region, can be preset by two guide index parameters <i>nr</i><sub>k1-Q</sub> and <i>nr</i><sub>k2-Q</sub>. Through a series of numerical simulations of the flow around a circular cylinder at Reynolds number <i>Re</i> = 3900, the combination conditions are obtained for such two guide index parameters <i>nr</i><sub>k1-Q</sub> and <i>nr</i><sub>k2-Q</sub> that have the capability of high-fidelity resolving and capturing temporally- and spatially-developing coherent structures for such complex three-dimensional flows around such a circular cylinder. The results demonstrate that the new WM-HRL model is capable of accurately resolving and capturing the fine spectral structures of the small-scale Kelvin-Helmholtz instability in the shear layer for flow around such a circular cylinder. Furthermore, under a consistent grid system, through different combinations of these two guide index parameters <i>r</i><sub>k1</sub> and <i>r</i><sub>k2</sub>, the fine structuresof the recirculation zones with two different lengths and the U-shaped and V-shaped distribution of the average stream-wise velocity in the cylinder near the wake can also be obtained.

Partitioning model study of the traction coefficient in a droplet model in a wellbore

AbstractGas wells need to face the problem of gas well outflow during production, and the main way to deal with gas well fluid accumulation is to calculate the critical liquid carrying velocity and other factors by using the prediction model of excess fluid accumulation. When the prediction results of the hydrops prediction model are biased, hydrops will be generated at the bottom of the wellbore, resulting in a decrease in gas well productivity. Aiming at the problem that the drag coefficient of the wellbore droplet movement model changes greatly in the process of natural gas production, which leads to the error of the wellbore effusion prediction, the commonly used droplet models and the common drag coefficient models are analyzed and evaluated. Compared with the model obtained by the partition and the existing drag model and the experimental value, it is found that the model can effectively reduce the average error rate between the calculated results and the experimental value. The findings of this study can help for better understanding of change of critical liquid carrying velocity of droplet in different Reynolds number region, which is of reference value for the subsequent drainage and mining process.

Flow and heat transfer performance of bionic heat transfer structures with hybrid triply periodic minimal surfaces

Efficient energy conversion technologies are essential for mitigating the environmental impact of industrial activities. The development and application of high-efficiency heat exchangers are key components in this endeavor. Heat exchangers based on triply periodic minimal surfaces (TPMS) are widely acknowledged for their exceptional thermal-hydraulic performance. However, current research mainly focuses on the design and optimization of heat exchangers with standard TPMS structures. In this study, a hybrid process is utilized to construct ten novel heat exchangers, followed by comprehensive investigations of their internal flow fields using three-dimensional numerical simulations. In order to establish a baseline, five standard TPMS structures are also introduced as reference cases. The results reveal that complex secondary flow distributions and velocity variations are critical factors influencing the thermal-hydraulic performance. Moreover, the influence of different standard TPMS cells on the flow and heat transfer characteristics varies during the hybridization process. Specifically, the inclusion of the Schwarz cell generally leads to an improvement in heat transfer, while the presence of the Lindinoid cell may result in a deterioration of heat transfer. The Diamond and Lindinoid cells have an adverse impact on the flow characteristics, while the Gyroid and Neovius cells enhance the flow characteristics. To evaluate the overall thermal-hydraulic performance of different channels, a comprehensive performance assessment is introduced. The Diamond channel and the Gyroid-Neovius channel show superior overall performance in their respective Reynolds number regions, with the former exhibiting better flow characteristics and the latter has outstanding heat transfer performance.

A Review of Flapping Mechanisms for Avian-Inspired Flapping-Wing Air Vehicles

This study focuses on the flapping mechanisms found in recently developed biometric flapping-wing air vehicles (FWAVs). FWAVs mimic the flight characteristics of flying animals, providing advantages such as maneuverability, inconspicuousness, and excellent flight efficiency in the low Reynolds number region. The flapping mechanism is a critical part of determining the aerodynamic performance of an FWAV since it is directly related to the wing motion. In this study, the flight characteristics of birds and bats are introduced, the incorporation of these flight characteristics into the development of FWAVs is elucidated, and the utilization of these flight characteristics in the development of FWAVs is explained. Next, the classification and analysis of flapping mechanisms are conducted based on wing motion and the strategy for improving aerodynamic performance. Lastly, the current research gap is elucidated, and potential future directions for further research are proposed. This review can serve as a guide during the early development stage of FWAVs.

Designing air handling unit in data center and estimating its performance

In this paper, the construction process of the enhanced plate, the heat transfer and fluid flow characteristics, and the domain decomposition method for AHU performance estimation are presented. The AHU heat exchanger 3D model is built from plate modeling to heat exchanger assembling. The heat transfer plate modeling starts from a basic flat plate, elliptic cylindrical dimples (ECDs) are firstly added to the fixed location, and then spherical crown dimples (SCDs) are applied to the flat surface among the ECDs. For the enhanced channels, the effect of channel height is weaker than SCD depth on the Nusselt number and friction factors. For all enhanced channels, the PECs are larger than 1.1 within Re from 5000 to 35,355. The performance of a deeper depth of SCD is superior to that of a shallow one when the channel height is the same. Among the nine channels studied, the highest enhanced effects are obtained for “H12-R10-h4” (PEC=1.45 at Re=5000). And all nine enhanced channels can enhance heat transfer at the same pressure drop among the Reynolds number of 5000 to 35,355. The energy saving performance decreases with Re, and the channels with smaller SCD depth can save more energy when channel height is identical at the higher Reynolds number region. A decomposition thermal design method is proposed for the cross-flow AHU heat exchanger design, and the typical AHU design result is picked based on the heat exchange rate and flow loss power. The recommended heat exchanger is “H7.5-R10-h3” whose heat exchange rate is 40.5 kW when flow loss power is 3.25 kW.

Numerical Study on the Distribution of Rodlike Particles in Laminar Flows of Power Law Fluids Past a Cylinder.

The contraction/expansion laminar flow containing rodlike particles in power-law fluid is studied numerically when the particles are in a dilute phase. The fluid velocity vector and streamline of flow are given at the finite Reynolds number (Re) region. The effects of Re, power index n and particle aspect ratio β on the spatial and orientation distributions of particles are analyzed. The results showed that for the shear-thickening fluid, particles are dispersed in the whole area in the contraction flow, while more particles are gathered near the two walls in the expansion flow. The spatial distribution of particles with small β is more regular. Β has a significant, n has a moderate, but Re has a small impact on the spatial distribution of particles in the contraction and expansion flow. In the case of large Re, most particles are oriented in the flow direction. The particles near the wall show obvious orientation along the flow direction. In shear-thickening fluid, when the flow changes from contraction to expansion, the orientation distribution of particles becomes more dispersed; while in shear-thinning fluid, the opposite is true. More particles orient to the flow direction in expansion flow than that in contraction flow. The particles with a large β tend to align with the flow direction more obviously. Re, n and β have great influence on the orientation distribution of particles in the contraction and expansion flow. Whether the particles initially located at the inlet can bypass the cylinder depends on the transverse position and initial orientation of the particles at the inlet. The number of particles with θ0 = 90° bypassing the cylinder is the largest, followed by θ0 = 45° and θ0 = 0°. The conclusions obtained in this paper have reference value for practical engineering applications.

Experimental study of convective heat transfer characteristics of ventilated passages formed by uniform diameter circular pin fins of disc brake

The heat transfer ability of ventilated passage of the disc brake has important effect on heat dissipation rate of the disc brake. The disc brake using uniform diameter circular pin fins to form ventilated passages has been widely used, but there is less reported heat transfer performance data of this type of ventilated passage. This paper investigated the heat transfer characteristics of the ventilated passage which is 10 times smaller than the real ventilated passage. The effects of the geometric parameters of the passage on its heat transfer ability are studied. The results show that the diameter and the number of the pin fin have stronger effect than the height of the pin fin has on the heat transfer. The results show great difference between the results predicted by the available correlation for ventilated passage formed by straight vane at small or large Reynolds number region. The reasons that cause the difference between the present results and other reported results are discussed. A correlation of Nusselt number vs. Reynolds number, the diameter, the number and the height of the pin fin is provided.

Investigation on Convection Heat Transfer Augment in Spirally Corrugated Pipe

A numerical simulation on the heat transport augmentation and flow drag behavior of spirally corrugated pipes was performed. The simulation was conducted on the basis of the experimental results documented in the published literature. The influence of the thread height and pitch on the hydraulic–thermal performance as well as the mechanism of the convection heat transport development inside the spirally corrugated pipe were explored. It was discovered that the convection heat transport performance elevates in the Reynolds number region of 4000~13,000 as the thread height rises or the Reynolds number enlarges, but it declines when the thread pitch extends. The convection heat transport performance marked by the Nusselt number of the spirally corrugated pipe could reach 2.77 times that of the plain pipe, while the flow resistance coefficients of spirally corrugated pipes are 89~324% above that of the plain pipe. It enlarges with the rise in thread height but declines with the extension of the thread pitch. It also reduces when the Reynolds number enlarges. The factors of overall heat transmission performance for all the spirally corrugated pipes are above 1.00, and they increase in the Reynolds number region of 4000~7000 and then decrease in the Reynolds number region of 7000 to 13,000. The secondary flow at the cross-sections and the vortex between two adjacent corrugated grooves are the basic causes of the promotion of convection heat transport inside the spirally corrugated pipes. The secondary flow near the pipe wall both disrupts the border layer and boosts the radial interfusion of the fluid. In addition, the existence of vortexes makes the secondary flow act on the convection heat transmission continuously and positively in the region close to the pipe wall.

The influence of material on the power performance of Savonius turbines in wind and water applications

Savonius turbines are practical for rivers with low speeds due to their simple design and good self-starting capabilities. Despite these advantages, real implementation is still limited due to their low power efficiency. However, the experimental studies are scarce as they mainly focus on power performance, neglecting the impact of materials exposed to the wind or water medium. Therefore, the current study aims to address this gap by investigating the influence of materials on the power performance of Savonius turbines in wind and water, particularly the effect of their mass moment of inertia (I). Aluminum (ALU) and Polylactic Acid (PLA) were selected as the materials for the turbines as they are the typical materials frequently used to fabricate the turbines. They were tested experimentally in a wind tunnel and a water flume with Reynolds numbers ranging from 52, 000 to 83, 600. The findings discovered that although the turbine materials have the exact mass moments of inertia, they perform differently in water and wind. The turbines exhibit higher power coefficient with increasing mass moment of inertia in the wind at low Reynolds numbers (Re < 70,000), which is most likely due to reduced bearing friction that is more pronounced in the low Reynolds number region. The power coefficient was more likely to be affected in the wind than in water due to the effect of water buoyancy force, which reduces the influence of the increased mass moment of inertia. The results also revealed that the ALU and PLA with the same overall mass moment of inertia had maximum power coefficients of 11% and 18% in wind and water, respectively. There is no significant difference in terms of self-starting speed due to the variation in the moment of inertia. The Reynolds number and the turbine starting angle appeared to have a greater effect on self-starting capabilities than the moment of inertia. The findings of this study help to establish the types of materials that are optimal for enhancing the turbine performance in wind or water applications and could potentially save a lot of costs in generating power.

Onset of instability with collapse observed in relatively high density and medium beta regions of LHD

Edge MHD instabilities with pressure collapse are found in relatively high beta and low magnetic Reynolds number regions with a magnetic axis torus outward-shifted configuration of the large helical device (LHD), and characteristics and onset conditions of the instability are investigated. The instability has a radial structure with an odd parity around the resonant surface, which is different from that of the interchange instability typically observed in the LHD. The onset condition dependence on the magnetic axis location shows that the onset beta increases as the magnetic axis location moves more torus inwardly, and the instability appears only in limited configurations where the magnetic axis is located between 3.65 and 3.775 m. In such configurations, the resonant surface location is close to an index of the plasma boundary. This fact suggests that the distance between the resonant surface location and the plasma boundary plays an important role in the onset, and a possibility that the instability is driven by an external mode.

Study of the Influence of Aspect Ratios on Hydrodynamic Performance of a Symmetrical Elliptic Otter Board

The otter board, which is designed to maintain the horizontal opening of trawl nets, is a vital component of a trawl system. It requires a high lift-to-drag ratio, which is directly related to the trawling efficiency and economic effectiveness of the single trawler. To improve the hydrodynamic efficiency of a symmetrical elliptic otter board, four model otter boards, i.e., aspect ratio (AR) = 0.507, 0.640, 0.766, and 0.895, were designed in the present work and the effects of aspect ratios on the hydrodynamic performance of the otter board were investigated by flume tank experiments. Further, the k-ε EARSM turbulence model was adopted to analyze the hydrodynamic coefficients and the flow distribution around the otter board using the computational fluid dynamics (CFD) method. The optimal aspect ratio was obtained based on the analysis of experimental data, wherein the lift coefficient, the drag coefficient, and the lift-to-drag ratio at different angles of attack (AOA) were measured. The results show that the symmetrical elliptic otter board model is within the critical Reynolds number region when the Reynolds number is larger than 1.682 × 105, and its hydrodynamic coefficient is consistent with the real otter board. When the AR was 0.766, the elliptic otter board had the best hydrodynamic performance, of which the lift coefficient and the lift-to-drag ratio were 1.05 and 1.14 fold that of the initial otter board (AR = 0.640), and the volume of the wing-tip vortex reaches a maximum. The results show the hydrodynamic performance of the symmetrical elliptic otter board, and parameter optimization of the otter board has also been provided for reference.

Three-dimensional reynolds number effects and wind load models for cylindrical storage tanks with low aspect ratios

The wind loads of cylindrical storage tanks may be significantly affected by the Reynolds number (Re), while it is different from the infinite-length cylinder because the aspect ratios (H/D) of storage tanks are usually very low. The flow around tanks changes along the spanwise direction and presents a three-dimensional characteristic. In this study, the wind pressure on cylindrical storage tanks with 4 aspect ratios (H/D = 0.2–1.5) are measured by wind tunnel tests. Two kinds of flow field, uniform and atmospheric boundary layer (ABL) flow are set in the test with Re varying from 105 to 106. The drag and lift coefficients, wind pressure distributions and power spectra are analyzed to reveal the three-dimensional characteristics of the aerodynamic loads. Consequently, as the H/D of cylinder decreases, the Reynolds number effects are attenuated or even disappear gradually. The transition of separation occurs at Re in the range of 2.2 × 105–3.0 × 105 and 1.8 × 105–2.6 × 105 for tanks with H/D = 1.5 and 1.0 respectively, which is lower than that of two-dimensional cylinders. Moreover, the circumferential wind pressure distributions near the free end enter the critical Reynolds number region at lower Re than that at the middle sections. Parameters like the minimum pressure coefficient and separation angle are not exactly the same at different heights. Finally, a three-dimensional mean wind pressure distribution model of the storage tanks in the post-critical Re-independent region is proposed, the wind loads computed from which are accurate enough with only about 5% relative error to be used for design reference.

Analysis of heat transfer characteristics and entransy evaluation of high viscosity fluid in a novel twisted tube

• A novel twisted tube is to improve the secondary flow distribution inside the tube. • Heat transfer performance and entransy evaluation are investigated numerically. • The novel twisted tube reduces the entransy dissipation compared with the plain tube. • Effects of geometry parameters and Reynolds number are analyzed comprehensively. The non-uniformity of the secondary flow distribution in the twisted oval tube has impeded its further improvement of the heat transfer performance. A novel twisted tube (NTT) improves the distribution of the secondary flow and enlarges the effect of the secondary flow on the main flow, intensifying the radial mixing and equalizing the temperature distribution of the fluid, which strengthens the convective heat delivery in the near wall region and the core zone comprehensively. Heat transfer characteristics and entransy evaluation of water and engine oil in the NTT are investigated in the low Reynolds number ( Re ) region. The heat delivery performance of the NTT improves with the reduction of the distance ratio ( DR ) and the twist pitch ratio ( PR ), and also raises with the increase of Re . The friction factor of the NTT increases with the reduction of DR and PR , and decreases with the rise of Re . Compared with the twisted oval tube, the NTT could elevate the Nusselt number and enlarge the friction factor by 1.49–1.56 times and 1.32–1.41 times, respectively. Compared with the plain tube, the NTT could improve the Nusselt number and increase the friction factor by 2.42–2.76 times and 1.48–1.56 times, respectively. The equivalent thermal resistance of the NTT decreases with the reduction of DR and PR and reduces with the increase of Re . The Case 4 with the minimum DR and the smallest PR shows the supreme heat transfer performance and the lowest equivalent thermal resistance. The NTT could reduce the entransy dissipation remarkably, compared with the plain tube, the NTT could decrease the equivalent thermal resistance by up to 58% for water under the condition of constant wall temperature and could reduce the equivalent thermal resistance by as much as 53% for engine oil under the condition of given heat flux on the tube wall.

Weighted Moving Square-Based Solver for Unsteady Incompressible Laminar Flow Simulations

For computational fluid dynamics simulations, grid systems are generally used in the Eulerian frame for both structured and unstructured grids and solvers designed for the chosen grid systems. In contrast to the grid-based method, in which the connection information with neighboring grids must be maintained, gridless methods do not require an underlying connectivity in the form of control volumes or elements. Hence, gridless methods are feasible and robust for the problems with moving boundary and/or complicated boundary shapes. In this study, a Eulerian gridless solver is proposed, and its application for simulating two-dimensional unsteady viscous flows in low Reynolds number regions is investigated. The solver utilizes the weighted moving square method to obtain the spatial derivatives of the governing equations and solve the pressure Poisson equation iteratively. Simple remedies to satisfy the boundary conditions in the finite difference method are applied. The fractional step method with the second-order Adams–Bashforth method is used for time integration. Some benchmark problems were solved using the developed solver, and the results were compared with those of other experimental and computational methods. Good agreement with the results of other methods confirmed the validity of the proposed solver.

Unstable spectra of plane Poiseuille flow with a uniform magnetic field

The unstable spectra of plane Poiseuille flow (PF) in the presence of a longitudinal magnetic field are numerically investigated using an eigenvalue solver of incompressible magnetohydrodynamic equations. It is found that the strength of the magnetic field and the dissipative effect of the magnetic perturbation have played different roles in different parameter regions. The magnetic field has a strong suppression effect on the classical plane PF instability with a large Reynolds number in the region with the magnetic Prandtl number or the magnetic Reynolds number . Here, the Reynolds number and the magnetic Reynolds number are defined as and , where a, V 0, ν and η are the typical length, velocity, viscosity and resistivity, respectively. The magnetic Prandtl number is defined as , which is proportional to the ratio of the viscosity and the resistivity of the fluid medium. As the strength of the magnetic field increases, the PF instability can be completely stabilized in the limit of or/and . It is interestingly found that a new instability branch is excited in the small magnetic Prandtl number () or moderate magnetic Reynolds number () and large Reynolds number () regions. This new type of instability is verified to be driven by the magnetic Reynolds stress and modulated by the dissipative effect of the magnetic perturbation. The wavelength of the original PF instability gradually shifts to the long wavelength region, but the wavelength of the new branch is almost unchanged, as increases with fixed . However, the wavelength of the original instability branch is almost unchanged, but the wavelength of the new instability branch shifts to the long wavelength region, as increases with fixed .

Multi-objective optimization of micro-fin helical coil tubes based on the prediction of artificial neural networks and entropy generation theory

The heat transfer, flow resistance and entropy generation characteristics of micro-fin helical coil tubes (MFHCTs) are investigated numerically. MFHCT with different fin numbers (2≤N≤6), coil pitches (150mm≤P≤450mm), coil diameters (600mm≤D≤1200mm) and Reynolds numbers (10945≤Re≤30845) are examined. The effects of these geometric parameters on the Nusselt number (Nu), friction factor (f) and improved entropy generation number (Ns′) are discussed. The performance of MFHCT is then compared to that of a smooth helical coil tube (SHCT). The results show that MFCHT always performs better than SHCT, especially in the lower Reynolds number region. Moreover, artificial neural networks (ANNs) are established to predict Nu, f and Ns′, which are trained by simulation data. This model fits the simulation results better than the multiple linear regression, and the maximum error is no greater than 8%. With the prediction of the network, the micro-fin helical coil tubes are optimized by the entropy minimization method and NSGA-III algorithm. Through optimization, the distribution of design variables is examined. The results demonstrate that a higher Reynolds number and a larger coil diameter and coil pitch lead to a better performance. Additionally, the optimal Pareto points can be utilized to guide the design and operation conditions of micro-fin helical coil tubes.

Effect of periodic metamaterial structures with different arrangement patterns on the effectiveness of hydroelastic energy harvesters: Computational investigation

This study introduces the concept of periodic metamaterials to determine an optimal arrangement for performance enhancement of flow-induced vibration energy harvesters. Three two-dimensional metamaterial structures including four-interference, six-interference, and eight-interference cylindrical structures are computationally explored and discussed. The displacement of the cylinder is calculated by a user-defined function based on the fourth-order Runge-Kutta algorithm. The computational simulation is set under a low Reynolds number region (64 ≤ Re ≤ 388), and the increase of Reynolds number is controlled by the reduced velocity (2 ≤ U* ≤ 12). It is found that the output harvested power of the three metamaterial structures is more significant than the single-cylinder system. The four-interference cylindrical structure shows both maximum output power of 2.4 μW when U* = 5, and good suppression when 10 ≤ U* ≤ 12. Besides, the six-interference cylindrical structure shows a trend of “advance” and the eight-interference cylindrical structure displays “beating” when U* = 9 and 10. In the comparison scheme of reducing the radius, the dimensionless amplitude of the metamaterial scheme is still larger than that of the single-cylinder scheme, however, the property of expanding “lock-in” has not been found. This study shows the possible advantages and disadvantages of considering metamaterial structures to enhance the effectiveness of flow-induced vibration energy harvesters.

Numerical Study of Thermal-Hydraulic Performance of a New Spiral Z-Type PCHE for Supercritical CO2 Brayton Cycle

Printed circuit heat exchangers (PCHEs) have the characteristics of high temperature and high pressure resistance, as well as compact structure, so they are widely used in the supercritical carbon dioxide (S-CO2) Brayton cycle. In order to fully study the heat transfer process of the Z-type PCHE, a numerical model of traditional Z-type PCHE was established and the accuracy of the model was verified. On this basis, a new type of spiral PCHE (S-ZPCHE) is proposed in this paper. The segmental design method was used to compare the pressure changes under 5 different spiral angles, and it was found that increasing the spiral angle θ of the spiral structure will reduce the pressure drop of the fluid. The effects of different spiral angles on the thermal-hydraulic performance of S-ZPCHE were compared. The results show that the pressure loss of fluid is greatly reduced, while the heat transfer performance is slightly reduced, and it was concluded that the spiral angle of 20° is optimal. The local fluid flow states of the original structure and the optimal structure were compared to analyze the reason for the pressure drop reduction effect of the optimal structure. Finally, the performance of the optimal structure was analyzed under variable working conditions. The results show that the effect of reducing pressure loss of the new S-ZPCHE is more obvious in the low Reynolds number region.