Open-loop Flow Control Research Articles

Open-loop flow control design guided by the amplitude-frequency characteristics of the reduced-order model

Unsteady separated flow is a common flow condition causing many detrimental effects in aerospace and other fields. Open-loop control is a potential means to eliminate these drawbacks. At present, the unsatisfactory performance of open-loop control mainly attributes to the high-dimensional parameter optimization procedure and the lack of efficient knowledge-guided control law design methods. This paper proposes a method managing to directly obtain the anti-resonant frequency from the input and output amplitude-frequency characteristic curves of the reduced-order model of the flow-control coupled system. Simultaneously, a negative gain criterion is proposed to judge whether the target control effect can be achieved under the current parameter combination. For the control of low Reynolds number flow around a cylinder, the simulation results show that the optimal open-loop control frequency is 1.268 times the natural frequency of the flow, which is consistent with the anti-resonant frequency, and 26.8% of the lift fluctuation is suppressed. This paper also studies the influence of parameters such as flow frequency change, control start time, jet mass flow rate, and so on. Furthermore, control position is the key parameter affecting the amplitude-frequency characteristics. The anti-resonance points on the curves corresponding to different control positions can also guide the design of the optimal control frequency, and the negative gain criterion is still applicable. This method greatly reduces the time consumption in parameter optimization and improves the engineering application prospect of the open-loop control in unsteady separated flow control.

Subsonic cavity flow control with Micro-Magneto-Mechanical Systems (MMMS) microvalves

Flow control consists in modifying a flow natural state in order to converge towards another state which is considered as favorable, as drag or noise radiation might be reduced. In this paper, open-loop flow control experiments are carried out on a subsonic open-cavity flow. In the case of unstable flow control, the control focus is brought onto the flow fluctuations modifications rather than modification of the mean flow properties. Therefore, the forcing flexibility using arbitrary signals and the forcing linearity are essential for such flow control cases. In that sense, a linear array of Micro Magneto-Electro-Mechanical Systems actuators has been implemented to perform open-loop flow control experiments on an open-cavity. The actuators are able to generate both quasi-steady and pulsed jets with linear behavior. We proved the microvalves efficiency to damp the cavity oscillations. The quasi-steady jets reached a reduction of 20 dB in the cavity fundamental amplitude sound pressure level. Pulsed jets enabled an additional cavity tone amplitude reduction, which depends on the pulsating frequency and on the forcing amplitude. These results are a first step towards the implementation of the closed-loop control of the open-cavity flow.

Impact of the Precessing Vortex Core on NOx Emissions in Premixed Swirl-Stabilized Flames—An Experimental Study

Abstract The reduction of NOx emissions remains a driving factor in the design process of swirl-stabilized combustion systems, to meet legislative restrictions. In reacting swirl flows, hydrodynamic coherent structures, such as periodic large-scale vortices in the shear layer, induce zones with increased heat release rate fluctuations in connection with temperature peaks, which lead to an increase of NOx emissions. Such large-scale vortices can be induced by the helical coherent structure known as precessing vortex core (PVC), which influences the flow and flame dynamics under certain operating conditions. We developed an active flow control system, allowing for a targeted actuation of the PVC, to investigate its impact on combustion properties such as NOx emissions. In this work, a perfectly premixed flame, which slightly damps the PVC, is studied in detail. Since the PVC is slightly damped, it can be precisely excited by means of open-loop flow control. In connection with time-resolved OH*-chemiluminescence and stereoscopic particle image velocimetry (PIV) measurements, the impact of the actuated PVC on flow and flame dynamics is characterized. It turns out that the PVC rolls up the inner shear layer, which results in an interaction of PVC-induced vortices and flame. This interaction considerably influences the measured level of NOx emissions, which grows with increasing PVC amplitude in a perfectly premixed flame. Nearly, the same increase is measured for partially premixed conditions. This is in contrast to previous studies, where the PVC is assumed to reduce the NOx emissions due to vortex-enhanced mixing.

Open-loop and closed-loop flow control based on Van der Pol modeling

A real-time modeling and control architecture for coupled shear layer von Karman instabilities on a wing section wake is developed. Previous wind tunnel experiments highlighted that these wake instabilities give rise to limit cycles on both local and resulting airloads. The two-state Van der Pol system is employed to model the self-sustained oscillations exhibited by the actual flow. Schemes featuring either one or two coupled Van der Pol oscillators in parallel are taken under consideration. Open-loop and closed-loop control laws to manipulate and reduce the self-sustained oscillations are extensively studied. An adaptive control scheme is developed to optimize the performance of the controller, according to the flow conditions. For the open-loop control scheme, the consistency with experimental tests performed on a wind tunnel model is shown.

Extremum seeking to control the amplitude and frequency of a pulsed jet for bluff body drag reduction

Feedback control of fluid flows presents a challenging problem due to nonlinear dynamics and unknown optimal operating conditions. Extremum seeking control presents a suitable method for many flow control situations but involves its own challenges. In this paper, we provide a brief analysis of the extremum seeking method, with attention to modifications that we find to be advantageous. In particular, we present an adaptation for optimisation of the frequency of a harmonic input signal, a common scenario for open-loop flow control systems. We then present results from the experimental implementation of our modified method to the open-loop control system of Oxlade et al. (J Fluid Mech 770:305–318, 2015), an axisymmetric bluff-body wake, forced by a pulsed jet. We find that the system is able to achieve optimal operating conditions in both the amplitude and frequency of the harmonic input signal, and is able to largely reject the disturbances arising from measurements of a highly turbulent flow. We finally show the ability of the extremum seeking system to adapt to changing conditions.

Dynamic characterization of piezoelectric micro-blowers for separation flow control

A micro-blower device, made by Murata™ Manufacturing Co. and originally designed for cooling electronic microprocessors, is used in the present study as an actuator for separation flow control. The dynamical performance of the device, working in pulsed mode, is experimentally investigated to deduce its abilities and limitations regarding active flow control. Both the device design and its dynamical characterization are addressed in the paper, paying particular attention to its ability to control flow. Both hot-wire anemometry and particle image velocimetry have been employed to investigate the dynamical evolution of the pulsed jet and allow the identification of the jet organized flow structures. The phase-averaging technique is applied to distinguish the large scale structure dynamics induced during the ejection phase. An array of active devices is also used to evaluate the flow control efficiency by forcing frequency strategy in order to reduce the separation area of a back facing step flow. Results show good performance in open loop flow control and applicability of such actuator for further closed loop flow control research.

Numerical analysis of periodic open-loop flow control on bluff bodies in ground proximity

The effectiveness of open-loop actuation at the rear of near-ground bluff bodies in achieving drag reduction is explored numerically. The investigations are conducted on an infinite-width two-dimensional flat-back Ahmed body of height H, placed at a height of 0.2H above ground, using large eddy simulations at a Reynolds number of 23,000. This case better mimics the upper and lower shear layers that might be seen behind a heavy vehicle than previous away-from-ground tests, which have shown significant drag reduction is achievable through the attenuation of von Kármán shedding in the near wake. In this study, this effect is shown to essentially vanish once the body is moved close to a moving ground-plane, because the regular formation of strong compact wake vortices in the near wake is prevented. This is also shown to be the case under natural conditions, where a lower drag coefficient than the away-from-ground case is observed. This is interpreted as the result of a mismatch in wake vorticity shed in the upper and lower shear layers, and the disrupting presence of an extra layer of vorticity at the ground, which act together to limit or even suppress vortex shedding into the wake. The periodic actuation strategy, which attempts to control the separating shear layers and, in turn, the formation and shedding of wake vortices, therefore loses applicability. It is postulated that the open-loop strategy may still be effective if employed on the side rear-edges of a heavy vehicle, even though the upper and lower rear-edges are unlikely to yield much success.

A practical interface for coordinated position control of an excavator arm

Abstract This paper presents a new hand controller interface that addresses practical challenges to implementing coordinated position control (CPC) for an excavator arm. The new interface uses a hybrid control scheme with closed-loop CPC of the excavator arm and open-loop flow control of the swing. CPC is achieved using a joystick that is kinematically similar to the excavator arm. The kinematically similar joystick motion is planar. Thus, it can be mounted vertically, matching the excavator arm, or horizontally, which has many advantages, such as reduced operator fatigue. The new interface is compared to a conventional interface in a human subject experiment using a dynamic excavator simulator. The results demonstrate similar improvements in spoil removed and fuel efficiency as previous CPC approaches while being more practically designed. Although the design is for an excavator, the concepts presented can apply to a range of hydraulic manipulators.

Open-Loop Flow-Control Investigation for Airfoils at Low Reynolds Numbers

Within the scope of a flight research project involving dynamically scaled models, active flow control was investigated for a modified NACA airfoil. At low-Reynolds-number conditions, the aerodynamic performance of the modified NACA airfoil is considerably reduced by flow separation. Computational-fluid-dynamics simulations of the natural (uncontrolled) and controlled flow were carried out and provide the basis for a detailed analysis of the underlying physical mechanisms. The simulation results for the natural uncontrolled flow compare well with wind-tunnel measurements. Simulations and experiments show a large trailing-edge separation at low angles of attack and a leading-edge separation bubble at high angles of attack. To control the laminar separation and improve the overall performance of the airfoil at the low-Reynolds-number conditions, time-periodic blowing and suction through a spanwise slot is employed. A strong dependence of the effectiveness of the active flow control on the forcing parameters, such as forcing frequency and forcing slot location, is found. In addition, linear stability analysis is employed to identify the instability mechanisms that are responsible for the effectiveness of the flow-control approach.

Investigation of the benefits of unsteady blowing actuation on a 2D wind turbine blade

This paper investigates the benefit of unsteady blowing actuation over a two-dimensional (2D) airfoil specially designed for wind turbine applications. The experiments were carried out in Syracuse University’s anechoic wind tunnel, both with and without large-scale unsteadiness in the free stream generated by a 2D cylinder upstream of the airfoil. By analyzing both surface pressure through wavelet analysis and Particle Image Velocimetry (PIV) velocity field measurements, we found a drastic change in the flow physics and the aerodynamic loading on the airfoil between steady and unsteady free-stream conditions. When there was no large-scale unsteadiness introduced in the flow, under open-loop flow control conditions with unsteady blowing, the leading-edge separation was delayed and the maximum lift coefficient was increased. For the cases where large-scale unsteadiness was introduced into the flow, the experiments showed that both open-loop and closed-loop control cases were capable of reducing load fluctuations by a measurable amount. However, only the closed-loop control case that utilized dynamic surface pressure information from the airfoil suction side near the leading edge was capable of consistently mitigating the fluctuating load.

Analysis and control of the near-wake flow over a square-back geometry

A 3D numerical simulation, based on the Lattice Boltzmann method is carried out on the near-wake flow behind a generic square-back blunt body to analyze and establish a method to control the near-wake flow. The flow topology is described by the velocity and the pressure fields. The influence of the wake vortices on the aerodynamic drag is clarified and quantified. In order to reduce this drag, an active open-loop flow control is applied by continuous blowing devices distributed around the base periphery. The blowing effect on the behind body flow is a reduction of the wake section and of the total pressure loss in the wake and an increase of the static pressure on the base of the square body. This control leads to a significant drag reduction of Δ C x = −29% with a blowing velocity of 1.5 V 0. The efficiency is then studied, and we found that the most efficient control is obtained for a blowing velocity of 0.5 V 0 and a jet angle of 45°. In this case, a 20% drag reduction is obtained, and the energy needed to control the system is seven times lower than the energy saved by the control.

Logic-Based Active Control of Subsonic Cavity Flow Resonance

We present the results of an experimental investigation for controlling a shallow cavity flow in the Mach number range 0.25‐0.5. The flow exhibits the characteristic staging behavior predicted by the semi-empirical Rossiter formula with multiple modes in the Mach number range 0.32‐0.38 and a single strong mode in other Mach numbers. A survey of the velocity at the exit of the zero net mass flow compression-driver actuator used for control reveals that its amplitude is frequency dependent and its behavior is little influenced by the main flow. Forcing the Mach 0.3 flow indicated that the actuator has good authority over a large range of frequencies, with reduction of spectral peaks observed at some forcing frequencies. We took advantage of this phenomenon to develop a logicbased controller that searches the frequency space and maintains the optimal forcing frequency for peak reduction at each Mach number. Optimal frequencies and the corresponding reduced resonance have been obtained for all of the flow conditions explored. The physics of optimal frequency forcing as well as some characteristics of the logic-based controller are discussed. I. Introduction A PPLICATION of closed-loop control in fluid dynamics is by its nature a challenging and fascinating problem. Although many significant results have been obtained with open-loop flow control, this technique lacks the responsiveness or the flexibility needed for application in dynamic flight environments. In contrast, closed-loop flow control, although in its infancy, appears to be the ideal technique for the successful management of flow in many applications due to its adaptability to variable conditions and to its potential for significantly reducing the power required for controlling the flow. Fo re xample, Cattafesta et al. 1 found that closed-loop control of cavity tones requires an order-of-magnitude less power than open-loop control. The results presented here are part of a larger multidisciplinary effort to develop tools and methodologies that can apply closed-loop aerodynamic flow control for manipulating the flow over maneuvering air vehicles. The first step was to select a particular flowfield relevant to Air Force applications and to utilize it in the development of various components of closed-loop flow control techniques. The case study chosen is the control of shallow cavity flow pressure fluctuations that are characterized by a strong resonance produced by a natural feedback mechanism similar to that occurring in other flows with self-sustained oscillations (e.g., impinging jet, screeching jet). In all of these cases, shear layer structures impacting a discontinuity or obstacle in the flow (e.g., the cavity trailing edge) scatter acoustic waves that propagate upstream and reach the shear layer receptivity region where they tune and enhance the development and growth of shear layer structures. In the case of flow over a weapon bay cavity, these fluctuations can lead to structural damage to the air vehicle or to the stores carried within the bay. Rossiter 2 first developed an empirical formula for predicting the cavity flow resonance frequencies, today referred to as Rossiter frequencies or modes. He also investigated the concept of a dominant mode of oscillation. Later Rockwell and Naudascher 3 ob

FAST, MINIMAL, AND OBLIVIOUS ROUTING ALGORITHMS ON THE MESH WITH BOUNDED QUEUES

This paper studies fast, deterministic permutation routing algorithms with bounded queues on the n×n mesh. Our main result is an O(n)-step, strongly-dimensional (and thus also source-oblivious and minimal) permutation routing algorithm. This algorithm works under a relaxed model in which nodes can freely send data to their neighbors. In a more prevalent model, the standard model, data may be sent only when accompanied by a packet. Under this model we present the following two algorithms: an O(n log n)-step strongly-dimensional algorithm and an O(n)-step oblivious and weakly-dimensional (and thus also minimal) algorithm. As said, all these algorithms store only O(1) packets in a node. Moreover, they use only O( log n) state bits in a node and transfer only O( log n) data bits on an edge in a step. All our routing algorithms are based on the following new technique of open-loop flow control. An algorithm is composed of two stages: setup and transportation. The setup stage computes certain values and stores them in the network. In particular, it computes a rational number α(e) for certain critical edges e. The transportation stage moves the packets to their destinations. It uses the computed values to slow the packets so that the traffic on each critical edge e is bounded byα(e); that is, at most ⌈α(e) · l⌉ packets traverse e during any l consecutive steps. This bounded on the burstiness of the traffic enables the algorithm to avoid hot spots and maintain bounded queues. The algorithm achieves this by an open-loop control; that is, during this stage no information is transferred in a direction opposite to that of the packets. An additional novelty of our algorithms is the application of a dynamic routing problem to solve a static one. The dynamic problem in question seems easy, as its networks is just a linear array. We show, however, that this problem is beyond the scope of the Adversarial Queuing Theory.

Joint encoder and channel rate control of VBR video over ATM networks

Rate control is considered an important issue in video coding, since it significantly affects video quality. We discuss joint encoder and channel rate control for variable bit-rate (VBR) video over packet-switched ATM networks. Since variable bit-rate traffic is allowed in such networks, an open-loop encoder without rate control can generate consistent quality video. However, in order to improve the statistical multiplexing gain (SMG), an encoder buffer is essential to meet traffic constraints imposed by networks and to smooth the highly variable video bitstream. Due to the finite buffer size, some forms of encoder rate control have to be enforced, and consequently, video quality varies. We argue that a rate control scheme has to balance both issues of consistent video quality on the encoder side and bitstream smoothness for the SMG on the network side. We present a joint encoder and channel rate control algorithm for ATM networks with leaky buckets as open-loop source flow control models. The algorithm considers constraints imposed by the encoder and decoder buffers, the leaky bucket control, traffic smoothing, and rate control. The encoder rate control is separated into a sustainable-rate control and a unidirectional instantaneous-rate control. It can improve the problem of leaky bucket saturation exhibited in previous works. Experimental results with MPEG video are presented. The results verify our analysis and show the effectiveness of the proposed algorithm.

Input buffer requirements for round robin polling systems

A packet switched network that can be modeled as a round robin polling system with nonzero switchover times is considered. The input traffic is assumed to obey the (σ, ϱ) regularity conditions recently proposed by Cruz [ IEEE Trans. Inform. Theory 37(1) (1991) 114–141]. These conditions are particularly applicable to traffic from open loop flow control mechanisms, such as the leaky bucket. For partially gated service, upper and lower bounds are provided for buffer requirements to insure no data loss. The bounds are extended to gated and exhaustive service and are modified to provide bounds on guaranteed network delays. Using the bounds, it is demonstrated that partially gated service requires nearly minimal buffering when incoming traffic approaches the capacity of the network. It is also demonstrated that open loop flow control can improve the buffer requirements and guaranteed delays of the network significantly.