Flow Control Scheme Research Articles

Reinforcement learning-based active flow control of oscillating cylinder for drag reduction

This study explores the use of a reinforcement learning (RL)-based active flow control strategy to reduce the drag of a transversely oscillating cylinder confined between two walls. We incorporate wake kinematic information of the oscillating cylinder from direct numerical solution into the agent to actively adjust the oscillating amplitude. Our findings reveal two strategies that significantly improve drag reduction. First, the oscillating frequency should be chosen within the lock-in region. Second, placing probes in the active region of turbulent kinetic energy enables the full utilization of physical information for achieving more efficient and stable control. The results show that an effective control strategy can reduce the drag by 8.4% compared to the case of stationary cylinder at Re = 200. We identify three characteristics of the controlled flow that contribute to drag reduction: an elongated recirculation zone, an increased energy transport coefficient, and asymmetric cylinder oscillation. We observed that this well-behaved controlled flow can be obtained through real-time RL-based control, as it consistently attenuates the asymmetric dynamic mode decomposition modes of the flow. Furthermore, we find that the asymmetry cylinder oscillation synchronizes with the vortex shedding, indicating that the RL-based control can expand the lock-in region.

Joint Optimal Train Rescheduling and Passenger Flow Control for Speed Limit and High-Demand Scenarios of Urban Rail Transits

In the operation of urban rail transits, train delays occur frequently, and emergency management is one of the key factors to ensure the service quality. For speed limit scenarios with high demand, this paper takes the train running in the restricted manual (RM) mode in the speed limit zone as an example and discusses a coping method by jointing train rescheduling and passenger flow control. With the goal of maximizing the number of passengers served and minimizing total train delay, a nonlinear optimization model is constructed by taking the train operation-related requirements and passenger flow control-related indicators as constraints, and the model is reformulated to a mixed-integer programming (MIP) model with quadratic constraints, which can be solved by the Gurobi solver. In order to obtain effective solutions faster, a two-stage approach is discussed, which first obtains a rescheduling timetable and then dynamically adjusts the requirement of boarding equalization to obtain the passenger flow control scheme. The validity of the model and the solution approach are discussed with the help of numerical experiments. The results suggest that the model and solution approach are feasible. When the number of trains is fixed, the reasonable implementation of passenger flow control will help to increase the number of passengers served, and the pursuit of a higher equalization of boarding is not conducive to the number of passengers served and the full utilization of transport capacity. The two-stage approach has certain advantages over the direct computing method. The methods in this paper have a guiding value for emergency decision-making in similar delay scenarios.

A novel approach to performance improvement of a VAWT using plasma actuators

The numerous advantages of Vertical-Axis Wind Turbines (VAWTs) have made them suitable for urban areas compared to other types of wind turbines; however, the occurrence of phenomena such as vortices and flow separation around the blades which have adverse effects on the turbine performance, indicates the necessity of using control methods to improve the behavior of the flow around the blades. This study numerically examines the effect of different installation positions and activation timing of plasma actuators and presents new strategies for flow control and turbine performance improvement. The model presented by Shyy et al. was used for the numerical modeling of the plasma actuator. The results showed that applying actuators in ten different positions from 10% to 90% of the airfoil chord length on the outer and inner edges of the blades, can lead to an increase in the turbine production power; however, the high-power consumption of the actuator clarifies the necessity of reducing the actuator activity time to smaller azimuthal intervals. The results revealed that applying the actuators under the optimal combined strategy in a 90° interval, increases the turbine power coefficient by 29.2%, while it leads to the lowest power consumption of the actuator.

Optimized fuzzy proportional integral controller for improving output power stability of active hydrogen recovery 10-kW PEM fuel cell system

In conventional PEM fuel cell systems with active hydrogen recovery, a significant drop in the output power occurs during hydrogen purging. In the present study, this problem is addressed through oxygen and hydrogen flow control strategies based on a proportional integral (PI) controller, fuzzy PI controller, and optimized fuzzy PI controller, respectively. When the optimized fuzzy PI controller is applied to the hydrogen supply side, the output power increases to 9112 W, corresponding to an improvement of 16.82%. In addition, when the controller is applied to both the hydrogen supply side and the oxygen supply side, representing an efficiency improvement of 15.62%. Overall, the results show that the proposed fuzzy PI controller improves both the stability and the fuel efficiency of the considered 10-kW PEM fuel cell system.

Digital process control of multi-step assays on centrifugal platforms using high-low-high rotational-pulse triggered valving.

Due to their capability for comprehensive sample-to-answer automation, the interest in centrifugal microfluidic systems has greatly increased in industry and academia over the last quarter century. The main applications of these "Lab-on-a-Disc" (LoaD) platforms are in decentralised bioanalytical point-of-use / point-of-care testing. Due to the unidirectional and omnipresent nature of the centrifugal force, advanced flow control is key to coordinate multi-step / multi-reagent assay formats on the LoaD. Formerly, flow control was often achieved by capillary burst valves which require gradual increments of the spin speed of the system-innate spindle motor. Recent advanced introduced a flow control scheme called 'rotational pulse actuated valves'. In these valves the sequence of valve actuation is determined by the architecture of the disc while actuation is triggered by freely programmable upward spike (i.e. Low-High-Low (LHL)) in the rotational frequency. This paradigm shift from conventional 'analogue' burst valves to 'digital' pulsing significantly increases the number of sequential while also improving the overall robustness of flow control. In this work, we expand on these LHL valves by introducing High-Low-High (HLH) pulse-actuated (PA) valving which are actuated by 'downward' spike in the disc spin-rate. These HLH valves are particularly useful for high spin-rate operations such as centrifugation of blood. We introduce two different HLH architectures and then combine the most promising with LHL valves to implement the time-dependent liquid handling protocol underlying a common liver function test panel.

A genetic algorithm based flow control scheme for LTE-NR dual connectivity networks

A genetic algorithm based flow control scheme for LTE-NR dual connectivity networks

Transient stability enhancement in renewable energy integrated multi-microgrids: A comprehensive and critical analysis

Multi-microgrids offer various benefits including the decreased overloading of a single microgrid, more options during faulty conditions, and more utilization of renewable energy resources. However, the implementation of a multi-microgrid brings the challenges such as power system complexity, interconnection issues, bidirectional power flow management, and power flow balancing. In the presence of these challenges, the power flow stability of the multi-microgrids is a challenging problem. In this context, this study evaluates a transient stability analysis model in multi-microgrids using solar photovoltaics, wind power, and a unified power flow controller (UPFC). UPFC offers a more robust power flow control strategy compared with other flexible alternating current transmission systems (FACTS) devices. First, a multi-microgrid structure consisting of the two microgrids was designed in DIgSILENT PowerFactory software. Second, the load flow calculation was performed in the absence and presence of UPFC, short circuit fault, and grid connection. Third, the electromagnetic transients (EMT) simulation was performed for all these situations. The results exhibited that the UPFC would offer significant power flow stability in the multi-microgrids. It was observed that the UPFC resulted in more transient stability in the microgrid where it was located. However, it improved the power flow quality at all the locations in the multi-microgrids. In addition, UPFC offered significant transient stability during the fault occurrence. The results offer various insights into power flow management in multi-microgrids.

Studies on Flow Characteristics of Gas–Liquid Multiphase Pumps Applied in Petroleum Transportation Engineering—A Review

Flow and phase separation in gas–liquid multiphase pumps is easy to occur, which deteriorates their performance and mixed transportability. Many research achievements have been made in the experiment, CFD simulation and performance improvement of multiphase pumps. However, there are many challenges for the test technology, accurate numerical model development and gas–liquid flow control. This paper is mainly aimed at critically reviewing various technologies for experimental observation, flow calculation and analysis, and the optimization design of gas–liquid multiphase pumps. In this regard, the experimental results including the energy performance, flow pattern and bubble movement in the multiphase pump are presented in detail. Discussions on the turbulence model, multiphase flow model and bubble balance model are carried out for the flow prediction in such pumps. Various numerical results are presented, including energy performance, bubble distribution, vorticity, phase interaction and pressure fluctuation. Moreover, the flow control and optimization strategy are briefly introduced. Having carried out an extensive literature review of flow characteristics in multiphase pumps, the deficiencies of relevant fields and suggestions for future research direction are given.

Collaborative passenger flow control optimization of urban rail transit network based on sliding window mechanism

In order to enhance the operational performance of urban rail transit networks under overloaded conditions, this study proposes a novel network-level collaborative passenger flow strategy based on a sliding window mechanism. The main idea is to categorize arriving passengers at stations into different groups using variable-length windows and allocate them to specific trains. With the aim of minimizing the total passenger extra waiting time under cost budget, we establish a mixed integer programming model with sliding window constraints, flow distribution constraints, and node flow constraints to build the optimal matching relation between passenger flow demands and transportation resources. Specifically, the model incorporates precise quantity constraints related to transfers, facilitating the accurate quantification of both internal and external demands of network passenger flow. Then we use logical constraint transformation and linearization techniques to enable the model to be solved directly by commercial optimization solvers. For large-scale problems, we adopt a rolling horizon approach to improve computing efficiency. To validate the effectiveness of the sliding-window based passenger control method, we conduct case studies including a small designed example and real-world research on the Beijing Subway network. The experimental results demonstrate that our proposed model assists managers in developing flexible and accurate network-level collaborative passenger flow control schemes.

Drag Assessment for Boundary Layer Control Schemes with Mass Injection

The present study considers uniform blowing in turbulent boundary layers as active flow control scheme for drag reduction on airfoils. The focus lies on the important question of how to quantify the drag reduction potential of this control scheme correctly. It is demonstrated that mass injection causes the body drag (the drag resulting from the stresses on the body) to differ from the wake survey drag (the momentum deficit in the wake of an airfoil), which is classically used in experiments as a surrogate for the former. This difference is related to the boundary layer control (BLC) penalty, an unavoidable drag portion which reflects the effort of a mass-injecting boundary layer control scheme. This is independent of how the control is implemented. With an integral momentum budget, we show that for the present control scheme, the wake survey drag contains the BLC penalty and is thus a measure for the inclusive drag of the airfoil, i.e. the one required to determine net drag reduction. The concept of the inclusive drag is extended also to boundary layers using the von Kàrmàn equation. This means that with mass injection the friction drag only is not sufficient to assess drag reduction also in canonical flows. Large Eddy Simulations and Reynolds-averaged Navier–Stokes simulations of the flow around airfoils are utilized to demonstrate the significance of this distinction for the scheme of uniform blowing. When the inclusive drag is properly accounted for, control scenarios previously considered to yield drag reduction actually show drag increase.

Decentralized Power Flow Control Strategy Using Transition Operations of DC-Bus Voltage for Detection of Uncertain DC Microgrid Operations

To enhance the reliability and flexibility of DC microgrids (DCMGs), this paper presents a decentralized power flow control strategy (PFCS) by using the transition operation modes. The transition operation modes are introduced as an effective communication method among power units, eliminating the use of additional digital communication links (DCLs) for the purpose of ensuring the power balance as well as the voltage regulation even under uncertain conditions. During the transition operation modes, the power unit which transmits the information shifts the DC-link voltage level, and the power unit which receives the information continuously monitors the DC-link voltage with predetermined time. When uncertain conditions occur in a particular power unit, this power unit triggers the transition operation modes to send this information to all power units in the DCMG system. The proposed PFCS can maintain the DC-link voltage at the nominal value for steady-state conditions both in the grid-connected mode and islanded mode. Moreover, the proposed PFCS significantly enhances the overall reliability of the decentralized DCMG system by effectively addressing several uncertainties stemmed from electricity price fluctuations, grid availability, battery state-of-charge (SOC) levels, and wind power variations. The scalability of the DCMG system is also demonstrated by incorporating an electric vehicle (EV) unit as an additional energy storage system (ESS). The EV unit seamlessly cooperates with the existing battery unit, functioning as additional ESS to regulate the DC-link voltage when the battery SOC level is low. Simulation and experimentation results under various conditions demonstrate the effectiveness of the proposed PFCS.

Multi-input multi-output phononic subsurfaces for passive boundary layer transition delay

This paper documents progress towards developing a passive boundary layer transition delay strategy using a phononic subsurface (PSub). High-order implicit large-eddy simulation (ILES) is used to explore control requirements for boundary layer transition delay in the context of guiding passive flow control strategies. Positive phasing of surface displacement relative to forcing from a Tollmien-Schlichting (T-S) wave is shown to be effective at attenuating instability for the present flow conditions. Although difficult to achieve with a single input system, positive phasing can be realized with a multi-input, multi-output (MIMO) system. To this end, a MIMO PSub was developed and a reduced-order model was coupled with the flow solver. Positive phasing, and therefore a small degree of passive transition delay, was demonstrated on a flat plate boundary layer. Further optimization of subsurface properties and placement of multiple devices are likely to improve controller efficacy.

Direct numerical simulations of turbulent flow over the converging and diverging riblets

Different from conventional streamwise-aligned riblets, converging and diverging (C–D) riblets are arranged obliquely along the main-flow direction, intending to induce a large-scale secondary flow that modulates or controls the existing large-scale vortical structures in wall turbulence. In this study, we perform direct numerical simulations of turbulent flow over the C–D riblets at a low Reynolds number and investigate the impacts of the C–D riblets on aerodynamic forces, turbulence statistics, and coherent structures. The results show that the C–D riblets can reduce the skin-friction drag, but at the expense of a significantly increased pressure drag in the diverging region, which leads to an increase in the total drag. We present detailed mean and statistical flow-fields to assess the flow characteristics in the presence of the C–D riblets, including modifications on the distributions of velocities, pressure, Reynolds stresses, and turbulence kinetic energy. Moreover, the re-arrangement of turbulence structures over the C–D riblets is examined in terms of instantaneous flow-fields and two-point correlations. The findings provide more insight into the physics of flow over C–D riblets and may contribute to the further development of this flow control strategy.

Influence of flow control schemes of the three-strut injector on the mixing efficiency and total pressure loss for a scramjet combustor

Mixing efficiency and total pressure loss of a scramjet combustor with a three-strut injector under different flow control schemes are numerically investigated using the two-dimensional Reynolds-Averaged Navier-Stokes equations with the SST k-ω model. The flow structure is analyzed in detail to reveal the mixing enhancement mechanism of the three-strut injector configuration. The results demonstrate that the three-strut injector configuration significantly improves the mixing efficiency in the ignition region of the scramjet combustor by about 20% and only about 2% increase in total pressure loss compared to the single-strut injector configuration. The strut and nozzle size distributions of the central and auxiliary struts has negligible impact on the mixing efficiency, but they can alter the flow structure. An increase in the separation distance between the central and auxiliary struts promotes the interaction between shock waves and shear layers, resulting in a reduction of the complete mixing distance by nearly 80 mm. In addition, increasing the inward injection angle of the auxiliary struts improves the mixing efficiency of air and hydrogen in the ignition zone by nearly 15%. The rise in total pressure loss caused by the flow control schemes for the three-strut injector configuration is negligible.

Analytical Approximation-Based Approach for Passenger Flow Control Strategy in Oversaturated Urban Rail Transit Systems

Focusing on a heavily congested urban rail corridor, this study investigates the passenger flow control strategy optimization problem from a mesoscopic perspective to reduce platform congestion and enhance service quality. Based on a quadratic functional approximation for passenger arrival rates, an analytical formula for calculating passenger waiting time is derived based on the classic deterministic queueing theory. We formulate the problem as a continuous nonlinear programming model to minimize the total passenger waiting time within transportation capacity constraints. A Lagrangian relaxation approach effectively transforms the original complex problem into an unconstrained minimization program. The analytical solution relating to optimal flow control strategy is derived by directly solving the unconstrained program. To further provide an integrated optimization framework from both the supply and demand sides, we extend the abovementioned passenger flow control optimization model into an integrated mixed-integer nonlinear programming model to jointly optimize the passenger-flow control strategy and train frequency setting. Numerical examples are presented to demonstrate the applicability and effectiveness of the proposed models. The computational results show that the produced high-quality passenger flow control strategy significantly reduces total passenger delay.

Collaborative passenger flow control for an urban rail transit network

AbstractAmid worsening passenger congestion, passenger flow control has become a major need in urban rail transit. However, existing passenger flow control strategies are fixed and do not consider the impacts of the spatial and temporal variations in passenger flow. To address the problems mentioned, a method for collaborative passenger flow control in urban rail transit is proposed. First, the temporal and spatial distribution of passengers in the network is obtained. Then, a model of collaborative passenger flow control for urban rail transit is built to reduce the number of controlled source points and controlled passengers and minimize the difference between the expected number of passengers on bottleneck links and the number of passengers with control. Next, a multilevel network that considers the time dimension is established based on the model. A forward–backward algorithm is introduced to make full use of the adaptive learning rate to solve the model. A case study based on a small‐scale network shows that the forward–backward algorithm has good convergence. To verify the effectiveness of the method, the passenger flow of Chengdu Metro during COVID‐19 is analyzed. The objective function of the forward–backward algorithm is 44% lower than that of the gradient descent method and 36% lower than that of adaptive moment estimate (ADAM). Its computational speed is also acceptable. The results show that compared with the fixed passenger flow control strategy of metro operators, the obtained strategy is more effective in reducing the number of controlled passengers and source points and alleviating congestion of bottleneck links.

Comparative Study of the Performance Improvement of an Axial Compressor with a Micro-Vortex Generator and a Suction Slot

Flow control methods have been gradually applied to improve the flow field characteristics of axial compressors. However, most of the current research is focused on the cascade, and few studies have taken a compressor as the research object. Therefore, a 3.5-stage transonic axial compressor is adopted to explore the characteristics of different flow control schemes. The effects of the micro-vortex generator (MVG), segmented boundary layer suction (BLS), and combined technology (COM) on the MVG and BLS are compared by the numerical simulation method. The research results are summarized as follows. The excessive accumulated low-energy fluid in the blade passage induces the occurrence of a corner stall in the last stage stator of an axial compressor. At this time, the compressor can still work, but the performance has accelerated deterioration. The flow characteristics are effectively improved when the MVG is introduced, the stall margin improvement ΔSM and the peak efficiency improvement Δηe of the last stage are 2.1% and 1.02%, respectively. Moreover, the BLS shows advantages in removing the three-dimensional reverse flow and decreasing the total pressure loss compared with the MVG, the ΔSM of the last stage is 2.69%, and the Δηe is 1.83%. When the combined technology is applied, it shows a significant advantage in delaying the occurrence of a corner stall, and the stall margin of the last stage is improved by 2.71%. Based on a quantitative analysis method using loss sources, the three flow control schemes show significant advantages in reducing the secondary flow loss sources and the wake loss sources. Above all, the BLS shows significant advantages in reducing the total pressure loss, and the COM shows an advantage in expanding the stable operating range. The research results will provide a reference for studies of flow control methods related to reducing flow losses and widening stability margins.

Quantifying How Turbulence Enhances Boundary Layer Skin Friction and Surface Heat Transfer

Transitional and turbulent boundary layer (BL) flows possess dramatically larger wall-shear stresses and surface heat fluxes than their laminar counterparts. Locally, this can be explained by appealing to the presence of spatiotemporally coherent velocity and temperature regions that enhance the momentum and thermal transport across the BL. However, these coherent structures are seldom present alone, but usually influenced by other physical phenomena, such as the streamwise growth of BLs or freestream pressure gradients. A moment of enthalpy integral equation is introduced to quantify how turbulence and other flow phenomena enhance surface heat flux. Results from a direct numerical simulation of a transitional and turbulent BL are used to demonstrate the proposed analysis method and to form a quantitative assessment of the BL physics. The integral analysis demonstrated in this paper for canonical turbulent flows provides an interpretable analysis tool for unlocking key BL physics, including future extensions to compressible BLs, freestream pressure gradients, and the evaluation of flow control schemes.

Efficient and fast contention resolution for AWGR based optical data center networks

A packet contention resolution scheme featuring efficient bandwidth utilization (CRSebu) is proposed for arrayed waveguide grating router (AWGR)-based optical data centers (DCNs). Optical bandwidth can be efficiently provided in this proposed scheme to adapt the burst network traffic by flexibly arranging the timeslot and wavelength. Moreover, the CRSebu scheme implements a “Dislocation Pipeline” strategy to prevent the conventional “Request-Grant” scheduling caused extra traffic waiting delay. Experimental investigations prove the CRSebu scheme can completely prevent packet contention and also improves the network performance in terms of packet loss and server-to-server latency due to the high bandwidth utilization. Numerical assessments also validate the excellent scalability of this proposed contention resolution scheme in large-scale AWGR-based DCNs. Compared with the typical OPSquare structure deploying an Optical Flow Control scheme to prevent packet contention, the CRSebu-equipped optical DCN eliminates the retransmission of conflicted packets, which significantly improves the network performance. Experimental investigations prove the proposed CRSebu strategy can completely prevent packet contention. Numerical assessments show that the network deployed with the CRSebu scheme decreases 73.6% packet loss, and 11.9% server-to-server latency compared with the static scheme.

Simulating the helix wake within an actuator disk framework: verification against discrete-blade type simulations

Dynamic flow control strategies are raising interest for wake mitigation purposes. Among the different strategies, the so-called helix one relies on individual pitch control (IPC). The numerical simulation of the helix is thus readily performed by means of discrete-blade capturing methods. Yet, if this control strategy is considered at the scale of wind farms, the resolution required by such methods becomes prohibitive and actuator disk (AD) models should be envisioned. It is however not trivial to translate IPC strategies to an AD framework which by definition considers rotor-averaged effects. This work assesses the ability of an AD method to simulate the helix strategy by comparing it to a higher fidelity approach relying on a discrete-blade capturing model. Results show that the disk-type approach supplemented with a disk-adapted IPC scheme is able to capture both the forced motion of the wake at low turbulence and the faster wake recovery at moderate turbulence. From a quantitative perspective, the disk-type approach predicts bigger power gains, compared to those foreseen by the discrete-blade type approach, for a downstream turbine in the wake of a helix-operated one.