Narrow Results

Flow control can effectively improve lift and delay flow separation. Sweeping jet (SWJ) actuators have attracted increasing interest for as active flow control actuators. In this paper, the characteristics of the SWJ actuator were studied through large-eddy simulation and experiments. It was discovered that the oscillation frequency of the SWJ actuator is directly proportional to the mass flow rate, and its working principle was determined. Subsequently, SWJ actuators with $C\mu =1.1\%$, 1.9%, and 6.6% were applied to flow control on the hump airfoil. It was observed that with an increase in momentum coefficient, the SWJ actuator was able to push the shear layer toward the hump airfoil surface, leading to a reduction in the size of the separation vortex.

At $C_{\mu }=6.6\%$, it was achieved that the reattachment point of the separation vortex shifted upstream from $x/c=1.2$ to $x/c=0.72$, and the separation vortex size essentially disappeared. During the investigation of the flow control mechanism of the SWJ actuator, it was found that the SWJ actuator periodically generates a separation vortex V1, which has entrainment effects, resulting in the formation of an external jet on the airfoil surface, effectively inhibiting flow separation.

In this paper, we study wormhole routed networks and envision their suitability for real-time traffic in a priority-driven paradigm. A traditional blocking flow control in wormhole routing may lead to a priority inversion in the sense that high priority packets are blocked by low priority packets for unlimited time. The priority inversion causes the frequent deadline missing even at a low network load. This paper therefore proposes two preemptive flow control policies where high priority packets can preempt network resources held by low priority packets. As a result, the proposed flow controls can resolve the priority inversion. Our simulations show that preemptive flow controls significantly reduce deadline miss ratios for various real-time traffic configurations.

The past few years have seen a rise in popularity of massively parallel architectures that use fat-trees as their interconnection networks. In this paper we formalize a parametric family of fat-trees, the k-ary n-trees, built with constant arity switches interconnected in a regular topology. A simple adaptive routing algorithm for k-ary n-trees sends each message to one of the nearest common ancestors of both source and destination, choosing the less loaded physical channels, and then reaches the destination following the unique available path. Through simulation on a 4-ary 4-tree with 256 nodes, we analyze some variants of the adaptive algorithm that utilize wormhole routing with 1, 2 and 4 virtual channels. The experimental results show that the uniform, bit reversal and transpose traffic patterns are very sensitive to the flow control strategy. In all these cases, the saturation points are between 35–40% of the network capacity with 1 virtual channel, 55–60% with 2 virtual channels and around 75% with 4 virtual channels. The complement traffic, a representative of the class of the congestion-free communication patterns, reaches an optimal performance, with a saturation point at 97% of the capacity for all flow control strategies. In this case virtual channels are of little help and the average network latency experiences an increase proportional to the number of virtual channels, due to the multiplexing of several packets onto the same physical links.

Complex-valued Hopfield networks which possess the energy function are analyzed. The dynamics of the network with certain forms of an activation function is decomposable into the dynamics of the amplitude and phase of each neuron. Then the phase dynamics is described as a coupled system of phase oscillators with a pair-wise sinusoidal interaction. Therefore its phase synchronization mechanism is useful for the area-wide offset control of the traffic signals. The computer simulations show the effectiveness under the various traffic conditions.

This study investigates the impact of a rotating tube on the flow characteristics within a tube bank consisting of nine cylinders arranged in an in-line configuration, with a pitch-to-diameter ratio of 1.44, using both experimental and numerical approaches. Experiments are performed in a subsonic wind tunnel to measure pressure distributions at various azimuthal angles along the tubes, using a multi-channel differential pressure system. Drag forces are determined via a wire-strain gauge balance. Numerical simulations are conducted using ANSYS Fluent, employing the URANS-based Shear-Stress Transport (SST) k–$\omega$ model, to replicate turbulent cross-flow at two values of Reynolds numbers $\mathrm{\text{Re}}=0.712\times 10^5$ and $\mathrm{\text{Re}}=1.42\times 10^5$. This study explores the effect of a tube rotation within the array on flow separation angles, pressure distributions and vortex shedding in the wake region, focusing on the effects of different positions of the rotating tube on the flow characteristics of the surrounding tubes. Numerical results closely match the experimental data, demonstrating that the rotational motion significantly mitigates flow separation and reduces drag forces on adjacent tubes. Additionally, the position of the rotating tube within the array plays a critical role in optimizing the fluid forces and pressure distribution, offering enhanced control over the flow within tube banks.

The performance of the communication network of a massively parallel processor depends, among other parameters, on the network topology, the message flow control and the routing mechanisms. This paper analyses the gains in average message latency and maximum sustained throughput that can be achieved using an adaptive routing strategy instead of an oblivious one. Two different bidimensional topologies have been studied, mesh and torus, using cut-through message flow control. First, we have simulated an ideal case in which there is no limit to the temporary storage capacity of the routing node. Then, a more realistic design, that implies the implementation of a deadlock avoidance technique, is analysed. To assure deadlock-free routing, the network is split into several virtual networks. Results show that adaptive routing is not a good election with this kind of networks. The torus topology shows potentially better results than the mesh. In any case, a different deadlock avoidance technique should be implemented if these potential gains are to be exploited.

Nanoparticles-based infusion strategies are presently being employed for a range of clinical interventions either for in vivo or in vitro applications while imposition of magnetic field is also identified as an important technique for fluid manipulation during nanoparticles-based propulsion. The impact of magnetic field to control of the transport of nanoparticles-based blood flow is demonstrated numerically over an elaborate variant of transport mechanisms. Mathematical formulations were undertaken and stability analysis of the mathematical problem was a scrutinized by generation of eigen values using the Lyapunov scheme. The numerical solution based on Chebysehev pseudo-spectra and spectra homotopy analysis method (SHAM) was implemented to handle the combination on nonlinear ordinary differential equations derived from the transport models. We observed that far-field of the stagnation point, nanoparticles specie dispersion increased with higher thermal diffusivity, while the decrease in concentration profile around the vicinity of stagnation point depicts clustering of nanoparticles-embedded blood flow. The observations revealed that higher magnitude of thermophoretic parameters constitute significantly to increase in momentum as well as energy fields during transport of nanoparticles-containing blood flow under magnetic field influence. These findings showed the potentials of magnetic-field for control of suspended particles in transport medium which could be harnessed to manipulate transport of nanoparticles-containing fluids in microfluidic platforms with intricate configurations.

The wind tunnel experiments for high angle of attack aerodynamics were designed from the inspiration of understanding the mechanism and development of an innovative flow control technique. The side force, varying with the different rolling angle, is featured by bi-stable situation, and can be easily switched by a tiny disturbance. A miniature strake is attached to the nose tip of the model. When the strake is stationary, the direction of the side force can be controlled. When the nose tip strake, as an unsteady control means, is swung the flow pattern could be controlled. The results obtained from dynamic measurements of section side force indicate that when the strake swing at lower frequency the side force can follow the cadence of the swinging strake. With increasing frequency, the magnitude of the side force decreases. At still high frequency, the side force diminishes to zero. The side forces could be also changed proportionally. Based on the experimental factors, the mechanism of the asymmetry is discussed.

Both open and closed loop control algorithms have been developed for manipulating wake flows past a solid cylinder in an electrically low-conducting fluid. The intent is to avoid vortex shedding and flow separation from the body, which is achieved through the introduction of localized electromagnetic forces (Lorentz forces) in the azimuthal direction generated by an array of permanent magnets and electrodes on the surface of the circular cylinder. The array of actuators offers the advantage of making the Lorentz force time and space dependent. More specifically, one closed loop control method has been derived from the equations of motion capable of determining at all times the intensity of the Lorentz force in order to control the flow. This is accomplished first, independently of the flow (open loop algorithm) and second, based on some partial flow information measured on the surface of the solid body (closed loop algorithm).

A dual synthetic jets actuator driven by different electrical factors was investigated using particle image velocimetry (PIV). A transfer-phase and sub-frequency technique was provided to capture the arbitrary phase of the dual synthetic jets, and a transfer-phase to equal technique was provided to determine the phase of the dual synthetic jets. The results show that both the amplitude and frequency of the electrical forcing voltages vastly affect the flow-field of the dual synthetic jets actuator. Both the forcing frequency and the driving voltage amplitudes contribute to the pressure difference and the area of the lower pressure, which determines the interactions of dual synthetic jets. The dual synthetic jet actuator exits a circumscription of electrical factors in which the actuator works efficiently.

Forward blowing from a pair of plasma actuators on the leeward surface and near the apex is used to switch the asymmetric vortex pair over a cone of semi-apex angle 10° at high angles of attack. Wind tunnel pressure measurements show that by appropriate design of the actuators and appropriate choice of the AC voltage and frequency, side forces and yawing moments of opposite signs can be obtained at a given angle of attack by activating one of the plasma actuators. Further work is suggested.

Our previous studies in quiescent air environment [Z. J. Zhao et al., AIAA J.53(5) (2015) 1336; J. G. Zheng et al., Phys. Fluids26(3) (2014) 036102] reveal experimentally and numerically that the shock wave generated by the nanosecond pulsed plasma is fundamentally a microblast wave. The shock-induced burst perturbations (overpressure and induced velocity) are found to be restricted to a very narrow region (about 1 mm) behind the shock front and last only for a few microseconds. These results indicate that the pulsed nanosecond dielectric barrier discharge (DBD) plasma actuator has stronger local effects in time and spatial domain. In this paper, we further investigate the effects of pulsed plasma on the boundary layer flow over a flat plate. The present investigation reveals that the nanosecond pulsed plasma actuator generates intense perturbations and tends to promote the laminar boundary over a flat plate to turbulent flow. The heat effect after the pulsed plasma discharge was observed in the external flow, lasting a few milliseconds for a single pulse and reaching a quasi-stable state for multi-pulses.

The structures of a flow field induced by a plasma actuator were investigated experimentally in quiescent air using high-speed Particle Image Velocimetry (PIV) technology. The motivation behind was to figure out the flow control mechanism of the plasma technique. A symmetrical Dielectric Barrier Discharge (DBD) plasma actuator was mounted on the suction side of the SC (2)-0714 supercritical airfoil. The results demonstrated that the plasma jet had some coherent structures in the separated shear layer and these structures were linked to a dominant frequency of $f_0$ = 39 Hz when the peak-to-peak voltage of plasma actuator was 9.8 kV. The high speed PIV measurement of the induced airflow suggested that the plasma actuator could excite the flow instabilities which lead to production of the roll-up vortex. Analysis of transient results indicated that the roll-up vortices had the process of formation, movement, merging and breakdown. This could promote the entrainment effect of plasma actuator between the outside airflow and boundary layer flow, which is very important for flow control applications.

An innovative structure of longitudinal grooves was designed on the volute casing of a centrifugal pump to improve the performance and internal flow characteristics. Comparisons were made between the model with grooved volute casing (GVC) and the model with original volute casing (OVC) operating in different working conditions. To get a better understanding of the influence of grooves on the centrifugal pump, some preliminary studies were performed with a simplified model. The results indicated that the application of GVC offers better pump performance under the large flow rate, also can stabilize the operating range of the centrifugal pump. In addition, the drag reduction effect of GVC is quite considerable compared with the pump with OVC. The pump with modified volute casing can suppress and eliminate the instability flow inside the impeller channels and volute casing. By using the GVC, the pressure and velocity distribution inside the impeller channel ameliorated especially for the 0.85 $R_i$ near the region of rotor–stator interaction between impeller and volute casing. Furthermore, the effects of GVC on flow passing ability may be recognized clearly for the flow area of the model with GVC increased by 5% at one impeller channel compared to the pump with OVC.

Flow around a near-wall circular cylinder with the splitter plate is numerically performed at Reynolds number of 500, with the objective of investigating the wake characteristics and hydrodynamic forces. Five gap ratios $G∕D=0.1,0.3,0.5,0.7$ and $0.9$ (G is the gap between the lower surface of the cylinder and the wall, D is the diameter of the cylinder) are selected, and the splitter plate length $L∕D$ ranges from 0 to $4.5$. The flow characteristics of an isolated cylinder with the splitter plate are investigated first for comparison, and four wake flow modes are observed, which include 2S mode ($L∕D\le 0.15$), P+S mode ($0.3\le L∕D\le 1.0$), 2S+S mode ($1.25\le L∕D\le 2.0$) and 2P mode ($L∕D\ge 3.0$). As $L∕D$ increases from 0 to $0.75$, the mean drag coefficient ($\overline{C_D}$) is decreased, and there is a slight increase of $\overline{C_D}$ for $0.75<L∕D\le 1.25$. In addition, the cases of $L∕D=0.75$ and $L∕D=1.0$ produce a very significant reduction of drag, and the $\overline{C_D}$ is reduced by as much as $39\%$ and $38\%$, respectively. The wake characteristics and hydrodynamic forces of a near-wall cylinder with the splitter plate are investigated in detail, and five wake regimes are observed, which include the wake vortex merging regime I, merged vortex attaching regime II, steady flow regime III, wall shear layer elongation regime IV and upper shear layer attaching regime V. For $G∕D=0.1$, the wake vortex shedding is suppressed. For $G∕D\ge 0.3$, the Strouhal number (St) is decreased as $L∕D$ increases from 0 to 1.0, and there is an increase of St at $L∕D=1.25$. At $L∕D\ge 1.25$, the St of the near-wall cylinder is larger than that of the isolated cylinder, and the increase in St is affected by the deflected gap flow. What is more, the hydrodynamic characteristics are affected by the wall. For $G∕D>0.1$, the variations of $\overline{C_D}$ with $L∕D$ are similar to that of the isolated cylinder. It is found that the cases of $L∕D=0.75$ and $L∕D=1.0$ still produce a significant reduction of drag for $G∕D>0.3$, and the $\overline{C_D}$ is increased for all cases of $L∕D$ as $G∕D$ increases.

As the port count of routers in an interconnection network increases rapidly, the amount of buffers within the router chip also increases greatly. To improve the buffer utilization and to reduce the buffer size, the dynamically allocated multi-queue (DAMQ) algorithm is commonly used. However, traditional DAMQ buffer management suffers from high write latency and read latency, and one virtual channel (VC) monopolizes the entire buffer. To address these issues, we propose a fast and area-efficient DAMQ buffer-management algorithm and a novel flow-control mechanism based on credit with congestion-control support. The simulation results show that the new DAMQ algorithm can achieve low latency and prevent one VC from occupying the entire buffer during periods of congestion. Additionally, it can achieve high throughput with a shallow buffer, which leads to a reduced chip area.

A method for finding reduced-order approximations of turbulent flow models is presented. The method preserves bounds on the production of turbulent energy in the sense of the norm of perturbations from a notional laminar profile. This is achieved by decomposing the Navier–Stokes system into a feedback arrangement between the linearized system and the remaining, normally neglected, nonlinear part. The linear system is reduced using a method similar to balanced truncation, but preserving bounds on the supply rate. The method involves balancing two algebraic Riccati equations. The bounds are then used to derive bounds on the turbulent energy production. An example of the application of the procedure to flow through a long straight pipe is presented. Comparison shows that the new method approximates the supply rate at least as well as, or better than, canonical balanced truncation.

This paper shows the effects of optimal control techniques on pattern formation in a three-dimensional Rayleigh–Bénard problem with nonhomogeneous heating from below. In particular, we consider a cylindrical domain with a heating profile at the bottom localized around the origin. An axisymmetric basic state appears as soon a nonzero horizontal temperature gradient is imposed. The basic state may bifurcate to different solutions as spiral waves, stationary patterns, etc., depending on the vertical and lateral temperature gradients and on the shape of the heating function. An optimal control problem that determines the thermal boundary condition minimizing the enstrophy is proposed. The control boundary condition obtained changes the leading mode in the bifurcation and allows to eliminate the instabilities for some values of the parameters.

In this paper we present a new approach in the study of Aorto–Coronaric bypass anastomoses configurations. The theory of optimal control based on adjoint formulation is applied in order to optimize the shape of the zone of the incoming branch of the bypass (the toe) into the coronary. The aim is to provide design indications in the perspective of future development for prosthetic bypasses. With a reduced model based on Stokes equations and a vorticity functional in the down field zone of bypass, a Taylor-like patch is found. A feedback procedure with Navier–Stokes fluid model is proposed based on the analysis of wall shear stress and its related indexes such as OSI.

Flow control is the dominant technique currently used in communication networks for preventing excess traffic from flooding the network, and for handling congestion. In rate-based flow control, transmission rates of sessions are adjusted in an end-to-end manner through a sequence of operations. In this work, we present a theory of max-min fair, rate-based flow control sensitive to priorities of different sessions, as a significant extension of the classical theory of max-min fair, rate-based flow control to networks supporting applications with diverse requirements on network resources.

Each individual session bears a priority function, which maps the session's priority to a transmission rate; the priority is a working abstraction of the session's priority to bandwidth access. Priority functions enable the specification of requirements on bandwidth access by distributed applications, and the formal handling of such requirements. We present priority max-min fairness, as a novel and well motivated fairness condition which requires that assigned rates correspond, through the priority functions, to priorities comprising a max-min vector. We also introduce priority bottleneck algorithms gradually update a session's rate until when its priority is restricted on a priority bottleneck edge of the network. We establish a collection of interesting combinatorial properties of priority bottleneck algorithms. Most significantly, we show that they can only converge to priority max-min fairness.

As an application of our general theory, we embed priority bottleneck algorithms in the more realistic optimistic framework for rate-based flow control. The optimistic framework allows for both decreases and increases of session rates. We exploit these additionally provided semantics to prove further combinatorial properties for the termination of priority bottleneck algorithms in the optimistic framework. We use these properties to conclude the first optimistic algorithms for efficient, max-min fair, rate-based flow control sensitive to priorities.