Smart Flow Control Processes in Micro Scale Printed Edition of the Special Issue Published in Processes www.mdpi.com/journal/processes Bengt Sunden, Jin-yuan Qian, Junhui Zhang and Zan Wu Edited by Volume 1 Smart Flow Control Processes in Micro Scale Smart Flow Control Processes in Micro Scale Volume 1 Special Issue Editors Bengt Sunden Jin-yuan Qian Junhui Zhang Zan Wu MDPI • Basel • Beijing • Wuhan • Barcelona • Belgrade • Manchester • Tokyo • Cluj • Tianjin Special Issue Editors Bengt Sunden Lund University Sweden Jin-yuan Qian Zhejiang University China Junhui Zhang Zhejiang University China Zan Wu Lund University Sweden Editorial Office MDPI St. Alban-Anlage 66 4052 Basel, Switzerland This is a reprint of articles from the Special Issue published online in the open access journal Processes (ISSN 2227-9717) (available at: https://www.mdpi.com/journal/processes/special issues/ Flow Micro Scale). 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Contents About the Special Issue Editors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii Preface to ”Smart Flow Control Processes in Micro Scale” . . . . . . . . . . . . . . . . . . . . . . ix Jin-yuan Qian, Junhui Zhang, Zan Wu and Bengt Sunden Special Issue: Smart Flow Control in Micro Scale Reprinted from: Processes 2020 , 8 , 550, doi:10.3390/pr8050550 . . . . . . . . . . . . . . . . . . . . . 1 Chenggang Yuan, Vinrea Lim Mao Lung, Andrew Plummer and Min Pan Theoretical and Experimental Studies of a Digital Flow Booster Operating at High Pressures and Flow Rates Reprinted from: Processes 2020 , 8 , 211, doi:10.3390/pr8020211 . . . . . . . . . . . . . . . . . . . . . 7 Jianyong Qian, Qi Su, Fu Zhang, Yun Ma, Zifan Fang and Bing Xu Static Deformation-Compensation Method Based on Inclination-Sensor Feedback for Large-Scale Manipulators with Hydraulic Actuation Reprinted from: Processes 2020 , 8 , 81, doi:10.3390/pr8010081 . . . . . . . . . . . . . . . . . . . . . 17 Jin Zhang, Baolei Liu, Ruiqi L ̈ U, Qifan Yang and Qimei Dai Study on Oil Film Characteristics of Piston-Cylinder Pair of Ultra-High Pressure Axial Piston Pump Reprinted from: Processes 2020 , 8 , 68, doi:10.3390/pr8010068 . . . . . . . . . . . . . . . . . . . . . 35 Hao Li, Hong Li, Xiuqiao Huang, Qibiao Han, Ye Yuan and Bin Qi Numerical and Experimental Study on the Internal Flow of the Venturi Injector Reprinted from: Processes 2020 , 8 , 64, doi:10.3390/pr8010064 . . . . . . . . . . . . . . . . . . . . . 51 Xiaoming Yuan, Xuan Zhu, Chu Wang, Lijie Zhang and Yong Zhu Research on the Dynamic Behaviors of the Jet System of Adaptive Fire-Fighting Monitors Reprinted from: Processes 2019 , 7 , 952, doi:10.3390/pr7120952 . . . . . . . . . . . . . . . . . . . . . 65 Yan Jin, Xiaoke He, Ye Zhang, Shanshan Zhou, Hongcheng Chen and Chao Liu Numerical and Experimental Investigation of External Characteristics and Pressure Fluctuation of a Submersible Tubular Pumping System Reprinted from: Processes 2019 , 7 , 949, doi:10.3390/pr7120949 . . . . . . . . . . . . . . . . . . . . . 83 Qiaorui Si, Chunhao Shen, Asad Ali, Rui Cao, Jianping Yuan and Chuan Wang Experimental and Numerical Study on Gas-Liquid Two-Phase Flow Behavior and Flow Induced Noise Characteristics of Radial Blade Pumps Reprinted from: Processes 2019 , 7 , 920, doi:10.3390/pr7120920 . . . . . . . . . . . . . . . . . . . . . 95 Can Luo, Hao Liu, Li Cheng, Chuan Wang, Weixuan Jiao and Di Zhang Unsteady Flow Process in Mixed Waterjet Propulsion Pumps with Nozzle Based on Computational Fluid Dynamics Reprinted from: Processes 2019 , 7 , 910, doi:10.3390/pr7120910 . . . . . . . . . . . . . . . . . . . . 115 Jian Zhang, Xinhai Yu and Shan-Tung Tu Lattice Boltzmann Simulation on Droplet Flow through 3D Metal Foam Reprinted from: Processes 2019 , 7 , 877, doi:10.3390/pr7120877 . . . . . . . . . . . . . . . . . . . . . 137 v Chang-Bin Guan, Yan Shen, Zhao-Pu Yao, Zhao-Li Wang, Mei-Jie Zhang, Ke Nan and Huan-Huan Hui Design, Simulation, and Experiment of an LTCC-Based Xenon Micro Flow Control Device for an Electric Propulsion System Reprinted from: Processes 2019 , 7 , 862, doi:10.3390/pr7110862 . . . . . . . . . . . . . . . . . . . . . 153 Liang Lu, Qilong Xue, Manyi Zhang, Liangliang Liu and Zhongyu Wu Non-Structural Damage Verification of the High Pressure Pump Assembly Ball Valve in the Gasoline Direct Injection Vehicle System Reprinted from: Processes 2019 , 7 , 857, doi:10.3390/pr7110857 . . . . . . . . . . . . . . . . . . . . 165 Chang Qiu, Cheng-Hang Jiang, Han Zhang, Jia-Yi Wu and Zhi-Jiang Jin Pressure Drop and Cavitation Analysis on Sleeve Regulating Valve Reprinted from: Processes 2019 , 7 , 829, doi:10.3390/pr7110829 . . . . . . . . . . . . . . . . . . . . . 177 Zhi Zheng, Zhijun Wang, Yong Zhu, Shengnan Tang and Baozhong Wang Feature Extraction Method for Hydraulic Pump Fault Signal Based on Improved Empirical Wavelet Transform Reprinted from: Processes 2019 , 7 , 824, doi:10.3390/pr7110824 . . . . . . . . . . . . . . . . . . . . . 193 Xiaoming Yuan, Xuan Zhu, Chu Wang, Lijie Zhang and Yong Zhu Natural Frequency Sensitivity Analysis of Fire-Fighting Jet System with Adaptive Gun Head Reprinted from: Processes 2019 , 7 , 808, doi:10.3390/pr7110808 . . . . . . . . . . . . . . . . . . . . 215 Hui Wu, Jun-ye Li and Zhi-xin Gao Flow Characteristics and Stress Analysis of a Parallel Gate Valve Reprinted from: Processes 2019 , 7 , 803, doi:10.3390/pr7110803 . . . . . . . . . . . . . . . . . . . . . 239 Qiaorui Si, Biaobiao Wang, Jianping Yuan, Kaile Huang, Gang Lin and Chuan Wang Numerical and Experimental Investigation on Radiated Noise Characteristics of the Multistage Centrifugal Pump Reprinted from: Processes 2019 , 7 , 793, doi:10.3390/pr7110793 . . . . . . . . . . . . . . . . . . . . . 253 Yongshun Zhang, Wanlu Jiang, Yong Zhu and Zhenbao Li Research on the Vertical Vibration Characteristics of Hydraulic Screw Down System of Rolling Mill under Nonlinear Friction Reprinted from: Processes 2019 , 7 , 792, doi:10.3390/pr7110792 . . . . . . . . . . . . . . . . . . . . . 277 Yong Zhu, Shengnan Tang, Chuan Wang, Wanlu Jiang, Jianhua Zhao and Guangpeng Li Absolute Stability Condition Derivation for Position Closed-Loop System in Hydraulic Automatic Gauge Control Reprinted from: Processes 2019 , 7 , 766, doi:10.3390/pr7100766 . . . . . . . . . . . . . . . . . . . . . 291 Jin-yuan Qian, Min-rui Chen, Zan Wu, Zhi-jiang Jin and Bengt Sunden Effects of a Dynamic Injection Flow Rate on Slug Generation in a Cross-Junction Square Microchannel Reprinted from: Processes 2019 , 7 , 765, doi:10.3390/pr7100765 . . . . . . . . . . . . . . . . . . . . . 307 vi About the Special Issue Editors Bengt Sunden received his M.Sc. in 1973, Ph.D. in 1979, and was appointed Docent in 1980, all at Chalmers University of Technology, Gothenburg, Sweden. He was appointed Professor of Heat Transfer at Lund University, Lund, Sweden, in 1992. He has served as Professor Emeritus and Senior Professor since 2016. His main research interests include heat transfer enhancement techniques, gas turbine heat transfer, and computational modeling and analysis of multiphysics and multiscale transport phenomena for fuel cells. He serves as Guest Professor of numerous prestigious universities. He is a Fellow of ASME, regional editor for Journal of Enhanced Heat Transfer since 2007, and associate editor of Heat Transfer Research since 2011, the ASME J. Thermal Science, Engineering and Applications (2010–2016), and ASME Journal of Electrochemical Energy Conversion and Storage since 2017. He is a recipient of the ASME Heat Transfer Memorial Award 2011 and Donald Q. Kern Award 2016. He received the ASME HTD 75th Anniversary Medal 2013. He has edited 30 books and authored three textbooks. He has published over 400 papers in numerous journals, with a h-index of 39 and over 6400 citations. Jin-yuan Qian is presently Lecturer at the Institute of Process Equipment, College of Energy Engineering, Zhejiang University, China, a position he has held since his appointment in 2018. He received his B.Sc. and Ph.D. degrees, both in Chemical Process Equipment, from Zhejiang University, China, in 2011 and 2016, respectively. He was a joint Ph.D. student at TU Bergakademie Freiberg, Germany, from 2013 to 2014 and a Postdoc Researcher in the Department of Energy Sciences, Lund University, Sweden, from 2016 to 2017. His research interests include heat transfer, multiphase flow, flow control, and computational fluid dynamics. He has co-authored around 50 papers in international journals and conference proceedings. Junhui Zhang is presently Research Professor at College of Mechanical Engineering, Zhejiang University, China, a position he has held since 2013, and has been Deputy Director of the State Key Laboratory of Fluid Power and Mechatronic Systems since 2019. He received his B.Sc. and Ph.D. degrees, both in Mechatronic Engineering, from Zhejiang University, China, in 2007 and 2012, respectively, and won the Outstanding Youth Science Foundation in 2019. His research interests are focused on the design and measurement of hydraulic components, especially the high-power-density axial piston pump. He has co-authored about 60 papers in international journals and conference proceedings. Zan Wu is presently Senior Lecturer in the Department of Energy Sciences, Lund University, Lund, Sweden. He received his B.Sc. in Energy and Environmental System Engineering in 2008, and Ph.D. in Energy Engineering, both from Zhejiang University, Hangzhou, China. His research interests include multiphase flow, phase-change heat transfer enhancement techniques, microfluidics, surface modification, nanofluids, thermophysical properties, compact heat exchangers, and proton exchange membrane fuel cells. He has co-authored around 70 papers in international journals and conference proceedings as well as four book chapters. vii Preface to ”Smart Flow Control Processes in Micro Scale” In recent years, microfluidic devices with a large surface-to-volume ratio have witnessed rapid development, allowing them to be successfully utilized in many engineering applications. Within microfluidic devices, the fluid flow at microscale shows obvious differences and unique flow characteristics compared to that at the common macroscale. Thus, the flow behaviors at microscale have attracted many researchers for the purpose of innovative heat and mass transfer enhancement. A smart control process has been proposed for many years, while many new innovations and enabling technologies have been developed for smart flow control, especially concerning “smart flow control” at the microscale. This Special Issue aims to highlight the current research trends related to this topic, presenting a collection of 33 papers from leading scholars in this field. Among these include studies and demonstrations of flow characteristics in pumps or valves as well as dynamic performance in roiling mill systems or jet systems to the optimal design of special components in smart control systems. We do think smart flow control at the microscale will continue to become more and more useful in the near future. To end, we would like to express our heartful gratitude to all the scientific contributors of the papers submitted to this Special Issue. Bengt Sunden, Jin-yuan Qian, Junhui Zhang, Zan Wu Special Issue Editors ix processes Editorial Special Issue: Smart Flow Control in Micro Scale Jin-yuan Qian 1,2,3 , Junhui Zhang 2 , Zan Wu 3 and Bengt Sunden 3, * 1 Institute of Process Equipment, College of Energy Engineering, Zhejiang University, Hangzhou 310027, China; qianjy@zju.edu.cn 2 State Key Laboratory of Fluid Power and Mechatronic Systems, Zhejiang University, Hangzhou 310027, China; benzjh@zju.edu.cn 3 Department of Energy Sciences, Lund University, P.O. Box 118, SE-22100 Lund, Sweden; Zan.Wu@energy.lth.se * Correspondence: bengt.sunden@energy.lth.se; Tel.: + 46-462228605 Received: 28 April 2020; Accepted: 28 April 2020; Published: 8 May 2020 1. Introduction Smart control processes have been proposed for many years, while for smart flow control—especially when “smart flow control” comes at the microscale—it turns out that many new innovations and enabling technologies are possible. For instance, precise flow rate in a microreactor means high reaction e ffi ciency. Similarly, for micromixers, smart fluid control can improve the precise distribution of every constituent. Such systems are made up of micropumps, microchannels, and microvalves, etc. In this Special Issue on “Smart Flow Control Processes at the Microscale”, 33 papers have been published, ranging from studies of flow characteristics in pumps or valves, dynamic performances in roiling mill systems or jet systems, to optimal design of special components in smart control systems. The Special Issue is available online at the following link: https: // www.mdpi.com / journal / processes / special_issues / Flow_Micro_Scale. The contributions are summarized in four parts as follows: 2. Smart Flow Control in Pumps A pump is one of the most important devices in fluid transportation systems. Research aiming at the flow field and pressure characteristics is of great importance for improvement of the operating performance. Bai et al. [ 1 ] numerically studied the influence of pressure fluctuations and unsteady flow patterns in a pump flow channel with di ff erent di ff user vane numbers. The results indicate that the lower number of di ff user vanes was beneficial to obtain weaker pressure fluctuation intensity. Cao et al. [ 2 ] investigated the whole flow field of a low specific speed centrifugal pump with five blades at di ff erent flow rates in order to study the near-wall region flow characteristics in a low-specific-speed centrifugal impeller. The main contribution of this work is the illustrations of pressure distribution and relative velocity distribution profiles on the pressure side of di ff erent blades. Si et al. [ 3 ] studied the mechanism of radiated noise and its relationship with hydraulics in centrifugal pumps via a numerical method combined with an experimental approach. The results reveal that the radiated noise exhibits a typical dipole characteristic behavior and its directivity varies with the flow rate. In addition, Si et al. [ 4 ] carried out an experimental and numerical study aiming at the internal flow characteristics under gas-liquid two-phase flow in a miniature drainage pump. The pump performance and emitted noise measurements were monitored at various conditions. The study is a good reference for low noise design of drainage pumps. Wang et al. [ 5 ] also analyzed the gas-water two-phase flow in a self-priming centrifugal pump. The results illustrate the three stages in a self-priming process. The e ff ect of the middle stage is highlighted, which further determines the length of the self-priming time. Processes 2020 , 8 , 550; doi:10.3390 / pr8050550 www.mdpi.com / journal / processes Processes 2020 , 8 , 550 Jiao et al. [ 6 ] studied the three-dimensional cavitation flow in a waterjet propulsion pump based on Zwart–Gerber–Belamri cavitation model and the RNG (Renormalization Group) k- ε model. The study demonstrates that the potential dangerous regions of cavitation are the lip of the inlet passage and the upper and lower connecting curved section of the inlet passage. Luo et al. [ 7 ] analyzed the unsteady flow process in waterjet pumps to improve the overall performance and optimization of the structure design. The surface vortex of the blade and the unsteady flow process of the propulsion pump at di ff erent times of the same period were demonstrated. Cao et al. [ 8 ] studied the evolution of vortex structures in a laminar boundary layer over a flat plate by the Fourier spectral hybrid method. Results show that the maximum amplitudes of the vortex structures experience a process of linear growth and nonlinear rapid growth. The change in the mean flow profile further induces or promotes the growth or formation of vortex structures. Jin et al. [ 9 ] investigated the external flow characteristics and pressure fluctuation in a submersible tubular pumping system. Results indicate that the pressure pulsation is less a ff ected by the blade frequency with an increase of the measuring point from the impeller. Xue et al. [ 10 ] proposed a design method based on Amesim and a Python script for the purpose of multi-objective optimization in static and dynamic performances of a pump-driven actuator. The mapping between the design parameters and the relations between the objectives are plotted. The results highlight the feasibility of the proposed method in achieving the multi-objective optimization. Zhang et al. [ 11 ] investigated the structural characteristics of an ultra-high pressure axial piston pump. Via an analysis of the oil film pressure and thickness in di ff erent rotating angles of the piston–cylinder pair, it was found that the oil film pressure achieves the maximum value when the rotating angle increases to 90 ◦ , while the film thickness reaches the minimum at the same time. Zheng et al. [ 12 ] provided a fluid pressure signal method for hydraulic pumps based on Autogram for solving the fluid pressure fluctuations caused by the center spring wear faults. The results highlight the superiority of standard Autogram on the extraction of fault feature information on center spring wear when comparing with upper Autogram and lower Autogram. Moreover, a novel method named as improved wavelet transform (IEWT) was proposed by Zheng et al. [ 13 ] in order to solve the segment over-decomposition obtained by the empirical wavelet transform (EWT). The proposed method was shown to be superior for eliminating the over-decomposition of the fault feature information. 3. Smart Flow Control in Valves Valves play a significant role to change the flow rate, pressure and directions of fluids. Smart valves can turn out smart control of fluids. Lei et al. [ 14 ] suggested a novel method depending on the Machine Learning Service (MLS) HUAWEI CLOUD to achieve accurate diagnosis of hydraulic valve faults. The method combines advantages of Principal Component Analysis (PCA) in dimensionality reduction and the eXtreme Gradient Boosting (XGBoost) algorithm and proves to be highly e ff ective for identifying valve faults in the hydraulic directional valve. Liu et al. [ 15 ] studied the throttling characteristics of the diaphragm valve. In order to identify the optimal design of the flow path profile, two-dimensional simulation of the Weir diaphragm valve flow field was conducted. The study shows that the flatting of the ridge side wall, widening of the ridge top and the gentle flatting of the internal protruding of the flow path prove to be three positive approaches for the improvement of the throttling characteristics. Lu et al. [ 16 ] presented an investigation aiming at the oscillating flow field of the double-nozzle flapper servo valve pre-stage through Large Eddy Simulation (LES) turbulent modeling. Meanwhile, the User-Defined Function (UDF) was introduced to control the main stage movement. The results highlight the structure and flow parameter e ff ect on the oscillating flow. In order to illustrate the damage caused by the increase of the injection pressure in the high pressure pump unloading valve ball, a theoretical calculation of the pressure relieve valve and the fatigue numerical simulation was carried out by Lu et al. [ 17 ]. Results indicate that the high pressure relief valve ball in the direct injection high Processes 2020 , 8 , 550 pressure pump should not be a traditional structural damage under high pressure conditions and the surface damage of the valve ball is microscopic damage, such as fretting wear. Qiu et al. [ 18 ] investigated the pressure drop and cavitation characteristics in the sleeve-regulating valve in di ff erent pressure di ff erences and valve core displacements using the multiphase cavitation model. The results show that the decrease of the valve core displacement induces the enlargement of the vapor distribution region and the increase of the vapor density. The e ff ects of the pressure di ff erence on the cavitation intensity are more prominent with the decrease of the valve core displacement. The work provides valuable instructions for the cavitation control of the sleeve regulating valves. Wu et al. [ 19 ] studied the flow and loss coe ffi cients in a wedge-type double disk parallel gate valve. E ff ects of the Reynolds number, valve opening degree and groove depth were analyzed. The results suggest that a large groove depth should be selected to provide a large flow coe ffi cient during the design process. However, during the machining process, the machining accuracy should be satisfied in order to avoid stress concentration of the bolt. Besides, bileaflet mechanical heart valves (BMHVs) are widely used as the alternatives of diseased heart valves. Xu et al. [ 20 ] performed simulations of unsteady flow in a BMHV and pressure pulsation characteristics under di ff erent flow rates and leaflet fully opening angle conditions were investigated. The work provides a good reference for the alleviation of leaflet vibration phenomenon in BMHVs. 4. Smart Flow Control in Microfluidics Droplet flow and microflow control in microfluidics are extensively studied. Qian et al. [ 21 ] investigated the characteristics of droplets in a dynamic injection flow rate by the Volume of Fluid (VOF) method combined with UDF. The study presents a novel aspect of the droplet flow since the droplet generation is always at a constant flow rate of two phases in most researches. Zhang et al. [ 22 ] studied the hydrodynamics of droplets passing through metal foam by the lattice Boltzmann method (LBM). The critical capillary number was identified. Results show that the droplet continues to be deformed until it breaks up when the capillary number is larger than 0.61. In order to avoid the calescence of the adjacent droplet, the distance between the droplets should be larger than three times the diameter of the droplet. Li et al. [ 23 ] investigated the two-phase flow inside a grooved rotating-disk system both in experimental and numerical methods. Visualization tests indicated that the flow field of the system was an air–oil flow. The stable interface between the continuous oil phase and the two-phase area could be formed and observed. Guan et al. [ 24 ] proposed a miniaturized, easily processed, and inexpensive xenon micro flow control device (XMFCD) in order to reduce the volume and weight of the traditional XMFCDs. The design of the proposed XMFCD is based on complex three-dimensional (3D) microfluidic channels while the fabrication process is based on low-temperature co-fired ceramic (LTCC) technology and it was illustrated in detail. 5. Smart Flow Control in Mechatronic Systems In mechatronic systems, there are many flow control issues, and smart flow control can improve the e ffi ciency of mechatronic systems significantly. A rolling mill with a hydraulic system is widely used in the strip steel industry. The vertical vibration seriously a ff ects the stability of the rolling mill system. Zhang et al. [ 25 ] analyzed the e ff ects of the equivalent damping coe ffi cient, leakage coe ffi cient, and proportional coe ffi cient of the controller on the hydraulic screw-down system of the rolling mill. Results suggest that in the closed-loop state, when Proportional–Integral–Derivative (PID) controller parameters are fixed, the system will have parameter uncertainty due to the change of the equivalent damping coe ffi cient and internal leakage coe ffi cient. Yuan et al. [ 26 ] investigated the dynamics, flow responses and power consumption theoretically and experimentally in hydraulic systems using the switched inertance hydraulic converter (SIHC). Results highlight the superiority of the SIHC in operation involving high pressures and delivery-flow rates. Processes 2020 , 8 , 550 Qian et al. [ 27 ] proposed a static deformation-compensation method based on inclination sensor feedback for large-scale manipulators to reduce the deviation of the endpoint in manipulators with hydraulic actuation. Compared to the finite element method, the proposed method considers less boundary conditions, which are uncertain for flexible manipulators in most situations. Zhu et al. [ 28 ] revealed the bifurcation characteristic of the load vertical vibration of the hydraulic automatic gauge control (HAGC) system through the investigation of the nonlinear factors such as excitation force, elastic force and damping force. Results point out that the resonance region can be e ff ectively avoided by adjusting the nonlinear sti ff ness coe ffi cient and the stability of the system will be promoted as well. In addition, Zhu et al. [ 29 ] described the function of the key position closed-loop system in HAGC. Results indicate that the absolute stability conditions of the position closed-loop system are derived whether the spool displacement is positive or negative. Li et al. [ 30 ] introduced a new method for the evaluation of the blood cell damage and the observation of the real-time characteristics of blood flow patterns in vitro using rheometer and bionic microfluidic devices. The damaged erythrocytes were collected and injected into a bionic microfluidic device. Analysis of the captured images indicate that with the increase of shear stress su ff ered by the erythrocyte, the migration rate of damaged erythrocyte in bionic microchannel is significantly decreased. Yuan et al. [ 31 , 32 ] investigated the natural frequency sensitivity and dynamic behaviors of the fire-fighting jet system. An adaptive gun-head design was proposed to achieve the fluid–structure interaction and discrete–continuous coupling characteristics and the sensitivity calculation formulas of the natural frequency was derived of the jet system to typical design parameters [ 31 ]. Focusing on the adaptive fire-fighting monitor, influence of the nonlinear fluid spring force on the dynamic characteristics was investigated. Results indicated that in the design of a fire-fighting system, the interval of the input shaft speed of the pump, and the pulsation frequency of the output fluid should be avoided [32]. Finally, in order to analyze the appropriate numerical simulation method for the investigation of the hydraulic performance, the mixing process and the flow law in the venturi injectors were considered by Li et al. [ 33 ]. Flow characteristics of the internal flow field obtained with and without the cavitation model were both compared with the experiments. Results indicate that the cavitation model has better agreement with experiments. 6. Conclusions In this special issue, 33 papers are presented and they relate to smart flow control in pumps, valves, microfluidics and mechatronic systems. We believe that smart flow control, especially at microscales, will become more important and useful in the near future. We would like to express our heartfelt gratitude to all the scientific contributors of the papers submitted to this Special Issue. Author Contributions: Conceptualization, J.-y.Q. and J.Z.; Methodology, J.-y.Q. and Z.W.; Data Curation, J.-y.Q., J.Z. and Z.W.; Writing—Original Draft Preparation, J.-y.Q.; Writing—Review & Editing, J.-y.Q. and B.S. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China, through grant number 51922093 and 51805470; the National Key R&D Program of China, through grant number 2019YFB2005101; and the Yucai Project of Zhejiang Association for Science and Technology. Conflicts of Interest: The authors declare no conflict of interest. References 1. Bai, L.; Zhou, L.; Han, C.; Zhu, Y.; Shi, W. Numerical Study of Pressure Fluctuation and Unsteady Flow in a Centrifugal Pump. Processes 2019 , 7 , 354. [CrossRef] 2. Cao, W.; Jia, Z.; Zhang, Q. Near-Wall Flow Characteristics of a Centrifugal Impeller with Low Specific Speed. Processes 2019 , 7 , 514. [CrossRef] Processes 2020 , 8 , 550 3. Si, Q.; Wang, B.; Yuan, J.; Huang, K.; Lin, G.; Wang, C. Numerical and Experimental Investigation on Radiated Noise Characteristics of the Multistage Centrifugal Pump. Processes 2019 , 7 , 793. [CrossRef] 4. Si, Q.; Shen, C.; Ali, A.; Cao, R.; Yuan, J.; Wang, C. 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This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http: // creativecommons.org / licenses / by / 4.0 / ). processes Article Theoretical and Experimental Studies of a Digital Flow Booster Operating at High Pressures and Flow Rates Chenggang Yuan, Vinrea Lim Mao Lung, Andrew Plummer and Min Pan * Centre for Power Transmission and Motion Control, Department of Mechanical Engineering, University of Bath, Claverton Down BA2 7AY, UK; cy466@bath.ac.uk (C.Y.); vlml20@bath.ac.uk (V.L.M.L.); arp23@bath.ac.uk (A.P.) * Correspondence: m.pan@bath.ac.uk Received: 18 December 2019; Accepted: 4 February 2020; Published: 10 February 2020 Abstract: The switched inertance hydraulic converter (SIHC) is a new technology providing an alternative to conventional proportional or servo-valve-controlled systems in the area of fluid power. SIHCs can adjust or control flow and pressure by means of using digital control signals that do not rely on throttling the flow and dissipation of power, and provide hydraulic systems with high-energy e ffi ciency, flexible control, and insensitivity to contamination. In this article, the analytical models of an SIHC in a three-port flow-booster configuration were used and validated at high operating pressure, with the low- and high-pressure supplies of 30 and 90 bar and a high delivery flow rate of 21 L / min. The system dynamics, flow responses, and power consumption were investigated and theoretically and experimentally validated. Results were compared to previous results achieved using low operating pressures, where low- and high-pressure supplies were 20 and 30 bar, and the delivery flow rate was 7 L / min. We concluded that the analytical models could e ff ectively predict SIHC performance, and higher operating pressures and flow rates could result in system uncertainties that need to be understood well. As high operating pressure or flow rate is a common requirement in hydraulic systems, this constitutes an important contribution to the development of newly switched inertance hydraulic converters and the improvement of fluid-power energy e ffi ciency. Keywords: digital hydraulics; switched inertance hydraulic systems; high-speed switching valves; pressure booster; flow booster; e ffi cient fluid power 1. Introduction Digital hydraulics is a new technology providing an alternative to conventional proportional or servo-valve-controlled systems in the area of fluid power. It promises hydraulic systems with high-energy e ffi ciency, flexible control, and insensitivity to contamination [ 1 – 5 ]. The switched inertance hydraulic converter (SIHC) concept is a subdomain of digital hydraulics [ 5 – 7 ], which is analogous to the electrical buck converter. It makes use of the inherent reactive behaviour of hydraulic components, including high-speed switching valves (switch function), small diameter tubes (inductive e ff ect), and accumulators (capacitive e ff ect) acting as a switch, an inductor, and a capacitor in the electrical circuit. Figure 1 shows a schematic of a three-port flow booster, which is a typical configuration of SIHCs [ 8 – 11 ]. The three- / two-way high-speed switching valve alternatively switches between the high- and low-pressure supply port. When the high-speed valve connects to the supply pump, the high-velocity fluid passes from pump to load; when the valve switches from the pump to the low-pressure supply port, the momentum of the fluid in the inertance tube draws the continuous flow from the low-pressure supply port to the load despite the adverse pressure gradient. As long as the switching time of the valve is short, the reduction in delivery flow is very small, and the average delivery flow is boosted and could be significantly higher than the supply flow. Processes 2020 , 8 , 211; doi:10.3390 / pr8020211 www.mdpi.com / journal / processes Processes 2020 , 8 , 211 /RDG ,QHUWDQFHWXEH $FFXPXODWRU 6ZLWFKLQJYDOYH 3XPS 5HVHUYRLU Figure 1. Schematic of three-port flow booster. The concept of SIHCs was initially proposed by Brown et al. in 1987 [ 12 ]. The team proposed and investigated a series of SIHC configurations, including a step-down transformer (flow booster), step-up transformer (pressure booster), switching gyrator, and four-port SIHC analogously to electrically switch inductance transformers. They concluded that hydraulic transformers have clear potential to improve hydraulic-system bandwidth and energy e ffi ciency on the basis of comprehensive theoretical and experimental studies [ 12 – 14 ]. However, due to the limitations of manufacturing high-speed switching valve in the 1980s, continuous work was limited. In the past decade, SIHC research came from Linz (Austria), the United States, Canada, the Nordic countries, Brazil, and Bath (UK). This research can be categorised as SIHC characteristics, SIHC optimisation, and high-speed switching-valve design. Scheidl et al. design