Flow Control Performance Research Articles

Effect of synthetic jets actuator parameters on deep reinforcement learning-based flow control performance in a square cylinder

We conduct an active flow control study on the mass flow rate of synthetic jets on the upper and lower surfaces of a square cylinder using a deep reinforcement learning algorithm, with a focus on investigating the influence of the position and width of the synthetic jets on the flow control performance. At Reynolds numbers (Re) of 100 and 500, it is found that our proposed method significantly reduced the lift and drag coefficients of the square cylinder and completely suppressed vortex shedding in the wake. In particular, at Re = 100, placing the synthetic jets near the tail corner was beneficial for reducing drag, with a maximum drag reduction rate of 14.4%. When Re = 500, positioning the synthetic jets near the leading edge corner resulted in a maximum optimal drag reduction effect of 65.5%. This indicates that locating the synthetic jet at the main flow separation point can achieve optimal control. Furthermore, we notice that when the synthetic jets are positioned near the tail corner, vortex shedding can be completely suppressed. Additionally, a narrower width of the synthetic jets can enhance flow instability and increase the cost of flow control.

Transient cold-front-water through y-shaped aluminium ducts: nature of turbulence, non-equilibrium thermodynamics, and velocity at the converged and diverged outlets

Abstract The interaction between water motion efficiency, outlet control mechanisms, and energy dynamics management hinges significantly on turbulence characteristics. However, understanding the influence of input velocities and duct features on outlets remains elusive. This study employs the realizable k − ɛ viscous model and Reynolds-averaged Navier–Stokes equations (RANS equations) to explore transient water dynamics encountering a cold front through ducts leading to convergence or divergence. Using Ansys Fluent 2023R2 and the waterlight workflow, meticulous meshing of the ducts is executed to capture flow intricacies accurately. Grid independence, suitable boundary conditions, and solver settings are carefully considered to ensure reliable results for investigating four key research questions. Duct bending introduces non-uniformities in velocity distribution, impacting exit velocity and altering flow characteristics and turbulence. In Case III, centrifugal forces from a 90° bend result in higher outlet velocities at the convergent exit and secondary flow patterns like swirls and vortexes. Additionally, entrance velocities influence Reynolds numbers, affecting mixing, heat transfer coefficients, and flow regimes, thereby optimizing thermal conductivity. This comprehensive investigation sheds light on optimizing water dynamics and energy management across various duct configurations, offering valuable insights into efficient flow control and thermal performance enhancement.

Scaling the Momentum Coefficient for Active Flow Control at Various Operating Conditions

The momentum coefficient is one of the most commonly used parameters to quantify the aerodynamic performance and system output of active flow control (AFC). Estimating the momentum coefficient accurately through experiments is challenging. Several approaches exist to estimate the momentum coefficient in laboratory conditions. However, the question arises as to whether these models hold as ambient and supply conditions change during operation in flight. Therefore, computational fluidic dynamic (CFD) simulations were carried out on two exemplary AFC actuators (i.e., a fluidic oscillator and a steady jet nozzle) in a quiescent environment with supply pressure ratios ranging from 1.5 to 4; supply temperatures at 293, 550, and 800 K; and ambient altitude conditions at sea level and 10 km. When scaled with Tsup/P∞, the mass flow for different conditions collapses as a function of the pressure ratio. Furthermore, the momentum coefficient collapses when scaled with 1/P∞. The existing models to calculate the momentum coefficient hold for changing ambient and supply conditions. Therefore, these models, in conjunction with scaling parameters, can be employed during the operation of an AFC system in flight with changing conditions in order to provide the desired momentum output. The paper discusses the details of the CFD results and the derivation of the scaling parameters.

Numerical simulation of streamer, pressure wave, and vortex induced by nanosecond pulsed surface dielectric barrier discharges

In this study, a two-dimensional fluid model is employed to simulate the streamer, pressure wave, and vortex in surface dielectric barrier discharge driven by nanosecond pulse voltage (ns-SDBD). It comprises a numerical model with two interconnected modules: discharge dynamics and gas flow dynamics. These modules are coupled through the physical variables including ‘EHD force’, ‘thermal source’, ‘velocity field’, ‘gas temperature’, and ‘gas pressure’. Our research primarily focuses on the underlying physical mechanisms of pressure waves and vortices for plasma-based flow control. The generation of pressure waves is attributed to the rapid gas heating by pulsed discharge, whereas the formation and development of the vortex are related to the ionic wind (EHD effect) provided by the plasma. To thoroughly understand and optimize flow control performance, an investigation into the effects of various discharge parameters, such as voltage amplitude and polarity, is conducted. Additionally, several SDBD modules are arranged in series, each featuring a dual three-electrode configuration. Subsequently, the dynamic behaviors of multiple streamers, pressure waves, and vortices, along with their interactions, are explored.

Numerical Study on Combustion-Driven Jet Actuation for Aerodynamic Control of Airfoil Flows

In this study, a numerical investigation is conducted on combustion-driven pulsed-jet actuation to control the flow around a lifting surface. Based on relevant experimental measurements and computations, high-speed jets are generated from the impulsive variation in pressure at the actuator boundary. A supersonic jet flow is momentarily generated by combustion in a reaction chamber of the actuator, and the flow interacts with the external flow around the lifting surface and alters the aerodynamic characteristics. The computational results indicate that the flow control performance of the jet actuation is significant at a high-incidence angle of attack, such as beyond the stall angle, whereas the impact is minimal at low angles of attack, such as in the linear lift region. Repetitive jet actuation can produce additional momentum to the external flow and alters the pressure distribution on the suction surface, particularly downstream of the actuator location. This pressure variation from the actuation yields an additional lift force on the lifting surface and reduces the amplitude of the aerodynamic moment at a given angle of attack, thus enhancing the aerodynamic performance of the airfoil.

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.

Multi-objective optimisation of K-shape notch multi-way spool valve using CFD analysis, discharge area parameter model, and NSGA-II algorithm

Multi-way spool valves (MWSVs) with K-shape notches (KSNs) provide advantages such as improvement of the actuator speed control, micro-action, and response performance. However, the pressure drop (PD) associated with the energy consumption is larger of MWSVs under the high-pressure and large-flow condition. Meanwhile, extremely complex flow coefficient and multi-parameter, highly coupled KSNs are the chief factors restricting the flow-pressure characteristics exploration and optimisation design of MWSVs. To address these problems, complete numerical research and experiment are performed in this study, especially concerning the method of MWSV flow-pressure characteristics modelling, and surrogated model-based optimal design of KSN structures. First, the relationships between KSN structure parameters and flow-pressure properties of MWSV are modelled on the innovative discharge area parameter model (DAPM) and response surface methodology (RSM) (RSM-DPAM) using the CFD dataset. Second, to reduce the PD combining the flow control performance, surrogate model-based optimisation design is addressed. During optimisation, six KSN structure parameters are chosen as design variables, PD and flow area relative deviation (FARD) are selected as objective functions, and the RSM is employed as the projected model. Depended on the created surrogate model, the non-dominated sorting genetic algorithm (NSGA-II) is established to search for the optimal KSN structure. To certify the performance of the optimisation, flow field characteristics are analysed. The results demonstrate that the proposed RSM-DAPM-RSM model achieves reliable prediction for PD and FARD with a great correlation coefficient (0.9757 and 0.9946). The average PD reduces as much as 7.23% while the FARD is only 1.28%. Moreover, the region of low-pressure, high-velocity, and high-turbulent kinetic energy in the flow field are reduced. The proposed framework enhances the performance in KSN spool optimisation and could be applied to other kinds of notches.Abbreviations: BOI: Body of influence; BSV: Bucket spool valve; CFD: Computational fluid dynamics; CM: Construction machinery; DAPM: Discharge area parameter model; FARD: Flow area relative deviation; FCC: Flow control characteristic; GSA: Global sensitivity analysis; KSN: K-shape notch; MHAs: Meta-heuristic algorithms; MODE: Multi-objective differential evolution; MOGA: Multi-objective genetic algorithm; MOO: Multi-objective optimisation; MOPSO: Multi-objective particle swarm optimisation; MWSV: Multi-way spool valve; NRMS: Non-road mobile source; NSGA-II: Non-dominated sorting genetic algorithm; OLHD: Optimal Latin hypercube design; PD: Pressure drop; PDCs: Pressure drop characteristics; RSM: Response surface methodology; SA: Sensitivity analysis; TKE: Turbulent kinetic energy; TOPSIS: Technique for order preference by similarity to ideal solution; VFR: Volume flow rate

A numerical study on the benefits of passive-arc plates on drag and noise reductions of a cylinder in turbulent flow

In this study, we introduce a novel arrangement consisting of two arc plates around a cylinder with the privilege of improved fluid flow and noise control. The arc plates are placed symmetrically and concentrically at the rear portion of a circular cylinder. The coverage angle (30 °≤β≤75°) of the plates and the normalized radius of arc plates (1.125≤Rd≤1.625) are varied to find the optimum case in terms of drag and noise reductions. The simulations are performed for a turbulent flow with a Reynolds number of 22 000. The numerical analysis is based on an unsteady Reynolds-averaged Navier–Stokes (URANS) solver and Ffowcs Williams and Hawkings (FW–H) acoustic analogy. It is found that by implementing the arc plates, the noise level and drag coefficient decrease dramatically. The results also reveal a strong correlation between the vortex shedding suppression and the noise reduction. It is shown that as the fluctuation of lift force decreases, the performance of flow and noise control enhances simultaneously. Furthermore, the noise assessment indicates that in a specific configuration of the arc plates, the overall sound pressure level decreases by around 51 dB compared to the uncontrolled case with no arc plates. Also, a maximum noise reduction of 27 dB is achieved, in which the drag coefficient reduces by 39% compared to the case with no arc plates. In conclusion, the results provide strong support for the proposed passive method as a beneficial strategy for noise reduction and wake control of cylindrical structures, which have wide applications in industry.

Research on Precise Flow Control and Anti-interference Performance of Load Sensitive Multi-way Valve with After-valve Compensation

Regarding a load-sensitive system with after-valve compensation, in the case of a single action or composite action, theoretically, the flow of each channel neither varies with the load pressure of this channel, nor is it affected by the flow of other channels, and the anti-interference performance is good. In fact, due to the influence of factors such as the flow passage of the valve body, the matching between the pressure compensation valve and the main spool, its flow control and anti-interference performance are difficult to achieve good results, which has a great impact on the synchronization, rapid action impact and micro motion characteristics of the main engine of construction machinery. This paper studies the simplification and analysis of the system pressure drop diagram, the matching characteristics between the pressure compensation valve and the main valve, accurate modeling and simulation technology, solves the problems of flow fine control and anti-interference performance, improves the flow control accuracy of a single action and compound action, and enhances the anti-interference performance. The real vehicle test shows that the self-developed multi-way valve for compensating for load sensitivity behind the valve is installed on the 6T small excavation. The initial position of the excavator’s track is aligned along a straight line. After the test runs for 20 meters, the track offset is measured. The fast forward offset is only 20mm, and the slow forward offset is only 7mm.

Flow control performance evaluation of a tri-electrode sliding discharge plasma actuator

Tri-electrode sliding discharge (TED) plasma actuators are formed by adding a direct current (DC) exposed electrode to conventional dielectric barrier discharge (DBD) plasma actuators. There are three TED modes depending on the polarity and amplitude of the DC supply: DBD discharge, extended discharge and sliding discharge. This paper evaluates the electrical, aerodynamic and mechanical characteristics of a TED plasma actuator based on energy analysis, particle image velocimetry experiments and calculations using the Navier–Stokes equation. The flow control performances of different discharge modes are quantitatively analyzed based on characteristic parameters. The results show that flow control performance in both extended discharge and sliding discharge is more significant than that of DBD, mainly because of the significantly higher (up to 141%) body force of TED compared with DBD. However, conductivity loss is the primary power loss caused by the DC polarity for TED discharge. Therefore, power consumption can be reduced by optimizing the dielectric material and thickness, thus improving the flow control performance of plasma actuators.

Optimal trajectory exploration large-scale deep reinforcement learning tuned optimal controller for proton exchange membrane fuel cell

To accurately regulate hydrogen flow and guarantee satisfactory output voltage control performance, taking advantage of the high adaptability and robustness of large-scale deep reinforcement learning, an optimal fractional-order proportion integral differential (FOPID) controller for controlling proton exchange membrane fuel cell (PEMFC) output voltage is proposed in this paper. In addition, an optimal trajectory exploration large-scale multi-delay deep deterministic policy gradient (OTEL-MD3PG) algorithm, which naturally considers the baseline FOPID coefficients in the design objective and provides the online coefficient adjusting ability through learning, is designed as the tuner of the controller to improve adaptability and robustness. This algorithm adopts the optimal trajectory exploration policy, whereby a new agent (demonstrator) generates demonstration samples that instruct the agent to learn, and another agent (tracker) adds noise to the action of the demonstrator to explore the limits of its control trajectory, thereby obtaining a more robust control strategy. The simulation results show that this proposed algorithm offers a rapid response, strong anti-interference, and excellent control performance.

Feedback control of transitional shear flows: sensor selection for performance recovery

The choice and placement of sensors and actuators is an essential factor determining the performance that can be realized using feedback control. This determination is especially important, but difficult, in the context of controlling transitional flows. The highly non-normal nature of the linearized Navier-Stokes equations makes the flow sensitive to small perturbations, with potentially drastic performance consequences on closed-loop flow control performance. Full-information controllers, such as the linear quadratic regulator (LQR), have demonstrated some success in reducing transient energy growth and suppressing transition; however, sensor-based output feedback controllers with comparable performance have been difficult to realize. In this study, we propose two methods for sensor selection that enable sensor-based output feedback controllers to recover full-information control performance: one based on a sparse controller synthesis approach, and one based on a balanced truncation procedure for model reduction. Both approaches are investigated within linear and nonlinear simulations of a sub-critical channel flow with blowing and suction actuation at the walls. We find that sensor configurations identified by both approaches allow sensor-based static output feedback LQR controllers to recover full-information LQR control performance, both in reducing transient energy growth and suppressing transition. Further, our results indicate that both the sensor selection methods and the resulting controllers exhibit robustness to Reynolds number variations.

Effects of varied bend turning vanes on flow control and heat transfer performance in a smooth two-pass channel

Turbine blades are always withstanding high temperature and thermal stress from hot gas. Serpentine passages with 180-deg turns are widely used within the turbine blade so as to guide cooling flow and enhance heat transfer. However, there is always a large pressure drop when cooling airflow passes the 180-deg bend. Turning vanes are usually arranged in the bend of two-pass channel to reduce the flow separation and pressure loss, and improve mainstream uniformity in the second passage. In this study, the heat transfer performance and flow field features of six two-pass channels with / without varied turning vanes are investigated by using Computational Fluid Dynamics numerical methods. The studies on effects of shape lines and curvature of the turning vane are conducted by commercial software Fluent based on realizable k - ε turbulent model. The influences of Reynolds number are also included. Numerical results indicate that the friction loss factors of channels with turning vane are decreased by 10.5%–31.4% compared to smooth channel without turning vane. However, the Nusselt number in the second passage is reduced greatly by 12.6%–22.1%. Considering the heat transfer and pressure loss, semicircular turning vane yields the highest thermal performance factor with an improvement by 5.2%. Moreover, turning vane with big curvature shows the best ability to suppress the flow separation and recirculation around the divider in the second passage. Among all the turning vanes studied, the novel airfoil-shaped turning vane produces the highest transfer augmentation at the region downstream the bend.

Duffing–van der Pol nonlinear reduced-order model for explaining the phenomena and mechanism in pulsed jet flow separation control

A two-equation nonlinear reduced-order model is established to describe the interaction between external periodic excitation from a pulsed jet and unsteady flow separation. This model, which is based on a 2D Navier–Stokes equation, the Stuart vortex row model, and the Stuart–Landau model, can approximately describe the typical phenomena occurring in pulsed jet flow separation control. These phenomena include the frequency-dependent, threshold, and lock-on effects, which are related to the frequency, intensity, and phase of excitation. Two indices, namely, the maximum Lyapunov exponent and the entrainment degree, are introduced to identify the mechanism of flow control via external periodic excitation. These indices are used to reflect the order degree and momentum transfer of the flow field. A comparison of the results of the model and those of a numerical simulation in a curved diffuser shows that the nonlinear reduced-order model is effective for qualitatively featuring the behavior of pulsed jet flow separation control. Moreover, the mechanism of pulsed jet flow separation control is explained via the model. Three aspects (i.e., flow instability, flow field ordering, and momentum transfer) are assumed to function together, resulting in an efficient flow control performance.

Analysis of pulsed suction flow control behavior based on a nonlinear reduced-order model

The use of pulsed suction is an effective unsteady flow control method. However, compared with steady suction, which simply eliminates low-energy boundary layer, pulsed suction flow control presents unique phenomena and complex control mechanisms that are not explained fully. Thus, in this paper, pulsed suction flow control in a compressor cascade is investigated by numerical simulation. Numerical results show that the control performance depends on the suction frequency, momentum, and location, as any unsteady active flow control method. To further reveal the mechanism and explain the phenomena of pulsed suction, a nonlinear reduced-order model for pulsed suction flow control is established on the basis of the Navier–Stokes equation, Stuart vortex row model, Oseen vortex model, and Stuart–Landau equation. The entrainment degree and maximal Lyapunov exponent are used as indices to evaluate the flow losses and control performance in this model. By solving the equations of this model under different parameters, the model presents similar phenomena as found in the numerical simulations with various suction frequencies, momentum, and locations. Thus, it is innovative that the model with calibrated parameters has the ability to roughly predict control performance of unsteady pulsed suction in an efficient way. Also, based on this reduced-order model, one can explain that, unlike steady suction, chaos control, momentum transfer and lock-in mechanisms are especially relevant with the control physics of pulsed suction.

Mechanism analysis of magnetohydrodynamic control in hypersonic turbulent flow

Under the assumption of the low magnetic Reynolds number, the coupled model is established for the turbulent flow field and the externally applied magnetic field. The AUSMPW+ scheme and LUSGS method are used to solve turbulent magnetohydrodynamics (MHD) flow equations, in which the Spalart-Allmaras one-equation turbulence model is used. A series of numerical simulations over various geometry configurations, namely, a flat plate and a compression corner, is conducted by using an external electromagnetic field. Results show that the performance of MHD boundary layer flow control is determined mainly by the Lorentz force in the streamwise direction. With an external magnetic field used, the low velocity fluid in the boundary layer can decelerate and increase the static temperature locally. Moreover, the counter-flow Lorentz force always brings a negative effect on the turbulent skin friction coefficient, and the location for the MHD zone has a great influence on the control efficiency of the ramp-induced separation. A reasonable magnetic field layout scheme should be configured in practical engineering application.

Impact of micro-grooves in scroll wrap tips on the performance of a trans-critical CO2 scroll compressor

Radial leakage was commonly regarded as the primary factor restricting the aero-thermodynamic performance of scroll compressors. In this paper, a passive flow control approach of micro-grooves in scroll wrap tip for the radial leakage in the scroll compressor was proposed, and its flow characteristics were numerically investigated. The impact of micro-grooves geometric parameters, such as groove number, depth, and partitions, on the flow control effect of the radial leakage was discussed, which was obtained by analyzing the evolution of the flow field and its influence on the performance of the scroll compressor. Results showed that the adoption of micro-grooves in scroll wrap tip yielded a significant limitation on the radial leakage, and geometrical parameters of the micro-grooves directly affected the flow control performance. Increasing groove numbers and partitions decreased the radial leakage effectively, while the leakage decreased first and then increased with the increase of the groove depth. The energy dissipation induced by the continuous throttling effect, jet expansion, and vortex in the groove cavity was the main flow control mechanism. The quadruple-grooves with the groove width of 0.5 mm and the groove depth of 100 μm were adopted in this study eventually. The volumetric and isentropic efficiency of the scroll compressor raised up 2.1 and 1.0 percentage point compared to the original case, respectively. Through reasonable working conditions and micro-grooves geometry, the aero-thermodynamic performance of the oil-free scroll compressor could be improved effectively by the scroll wrap tip with micro-grooves.

A network sensor location problem for link flow observability and estimation

Effective traffic sensor systems can collect precise and reliable real-time traffic information to improve the performance of traffic guidance and traffic flow control. In particular, link flow data are among the most intuitive sources of information for reflecting road congestion and can be further used to estimate origin-destination (OD) and path demands. To efficiently obtain link flow information, much effort has been devoted to the network sensor location problem for link flow observability and estimation. The objective of the observability problem is to minimize the number of installed sensors while ensuring full link observability, whereas the objective of the link flow estimation problem is to maximize the information gain or minimize the estimation deviations given a limited number of sensors. To the best of our knowledge, our study is the first to integrate link flow observability, flow estimation and the impact of prior link flow uncertainty into the sensor location problem. Accordingly, we propose two new models. In detail, we first present a new mixed-integer optimization model to achieve full link flow observability. Since only topological information is used, this is a generalized version of the existing models as well as a basic representation of other models for the flow estimation problem in our study. The second model integrates linear regression into the traffic sensor location problem. The results of this model provide both location selection and estimation methods for the decision maker. Furthermore, to address the problems of data loss and low computational efficiency in large-scale networks, we exploit the concepts of ambiguity sets and linear decision rules in robust optimization to generate a tractable approximate counterpart to the above sample-based link estimation model. Sample information is further integrated into this tractable approximate model to enhance the simulation performance without incurring a loss in computational efficiency. Numerical experiments are conducted for networks of different sizes. The results show significant improvement in surveillance performance, with a significant reduction in the number of required sensors and good estimation performance. Our work also verifies that when the number of samples is small, compared with the sample-based model, our enhanced tractable approximate model has several advantages, including (i) better out-of-sample performance, (ii) a similar noninferred link distribution, (iii) less dependence on sample size, and (iv) lower computation time requirements for large-scale problems.

Effects of Bending of Fluidic Oscillators on Aerodynamic Performance of an Airfoil with a Flap

The present work investigated the effects of bending the outlet nozzles of fluidic oscillators installed on the NACA0015 airfoil with a flap on the flow control performance and, thus, the aerodynamic performance of the airfoil. The effects of bending on fluidic oscillators have not been reported so far in previous works. The aerodynamic analysis was performed numerically using unsteady Reynolds-averaged Navier-Stokes equations. Three different cases were considered: Case 1 changes only the bending angle with a fixed pitch angle, Case 2 changes only the pitch angle without bending, and Case 3 changes both the bending and pitch angles. Although the bending of the oscillators was introduced inevitably due to a geometrical limitation in the installation, the results indicated that the bending rather improved the lift coefficient and lift-to-drag ratio of the airfoil by improving the characteristics of the fluidic oscillators, such as the jetting angle and peak velocity ratio.

Is flow control in a space-constrained drainage network effective? A performance assessment for combined sewer overflow reduction

Recurring combined sewer overflows (CSOs) can have a significant impact on the ecological condition of receiving water bodies. There are several structural measures, like adding retention basins and switching to low impact development solutions, that have been proposed to reduce the number of sewage overflows. Besides, several flow control strategies have been discussed in scientific literature that take advantage of the space within urban drainage networks, which is assumed to be adequate, for temporary storage. The adequacy of such storage space, however, is not a universally valid assumption as a large fraction of drainage networks frequently operate close to their design discharge. In this paper, we investigate the efficacy of flow control for a space-constrained drainage network. We employ a low-cost, heuristic real time control strategy with the use of flow control devices (FCDs) and available in-sewer space to reduce the magnitude of CSOs. We consider the performance of the proposed control strategy and discuss the effect of FCD location on CSO reduction. Our results, based on over 300 rainfall-event simulations, show that the flow control strategy using limited sewer capacity is more efficient during relatively small rainfall events, where the CSO is large enough to enable reduction using the chosen control rules. The CSO is reduced, to varying degrees, for around 80% of rainfall events with peak intensity between 10 and 20 mm h−1. For larger rainfall events, the flow control is more unstable in response to abrupt water release during control operation, which seems to be unavoidable because of the water accumulation effect and the transition to pressurized pipe flow in space-constrained networks. We also found that the flow control performance is highly sensitive to the FCD location – as it depends on the interplay of the peak rainfall intensity and the water level condition immediately upstream of the FCD. The efficacy of a location for flow control is determined by the unfilled capacity (i.e., effective in-sewer storage potential) in the pipe upstream of the FCD during the rainfall peak; furthermore, the location also has to be resistant to the water accumulation effect. Using our analysis, we substantiate two anticipated caveats to flow control strategies when the storage space is limited in a drainage network: diminished performance in CSO reduction and the appearance of additional control-related challenges, which are otherwise mitigated in more spacious networks.