Flow Control Concept Research Articles

Design of Adjoint-Based Laws for Wing Flutter Control

An airplane, by its nature of being, is constructed so that it is as light as possible. The structural design is guided by static and dynamic factors. The more stringent constraints on the structural design are due to dynamic loads, caused by aero-elastic interactions. One of the most commonly encountered problems in aeroelasticity is flutter, a term that is used to recognize the transfer of energy from unsteady aerodynamics associated with the surrounding fluid to the wing structure, resulting in rapidly divergent behaviour. If flutter can be controlled at cruise speeds, we can design lighter wings and consequently more efficient airplanes. It is therefore, in the aircraft designer’s best interest to design innovative ways in which flutter can be controlled without making the resulting structure too heavy. There are three important choices to make while designing active control strategies for suppressing flutter. The first is the choice of actuator. In this paper, the actuators we use are jets in the walls through which there is a small mass flow, either by way of blowing or suction. The second is to define a clear control objective. Finally, we need to design a control law that will make suitable state measurements and drive the actuators so that the desired control objective is achieved. The concept of Active Flow Control is fast gaining popularity in Fluid Mechanics circles. Indeed, it is important to realize that adding or removing fluid at the wing surface is equivalent to effecting a shape modification. Flow control using surface jets should, in principle, have an effect very similar to that of morphing surfaces. The capability to directly alter the flow field offers a huge realm of possibilities. Seifert, Theofilis and Joslin categorize the problems that are amenable to using Active Flow Control:

Interaction of Synthetic Jet with Boundary Layer Using Microscopic Particle Image Velocimetry

The aerodynamic interaction between an unsteady, inclined synthetic jet and a crossflow boundary layer was studied as a precursor toward applying active flow control concepts for rotor applications, such as dynamic stall control and fuselage drag reduction. Because the flowfield offered numerous challenges from a measurement perspective, several experiments were carried out using a phase-locked, two-dimensional microscopic particle image velocity technique in a building block approach, by adding one complexity after another. The procedure began with boundary layer measurements made on a simple flat plate using the microscopic particle image velocity technique. Velocity measurements were made deep in the viscous sublayer, as close as 20μm from the surface. Following this, the synthetic jet actuator was characterized while operating in quiescent air as well as in crossflow. The results showed that the evolution of the synthetic jet in crossflow was substantially different from its evolution in quiescent air, suggesting that any flow physics or performance prediction (for example, the depth of penetration of the jet into the boundary layer) made based on the quiescent flow conditions may not be applicable in crossflow. All the momentum added to the boundary layer had its source from the synthetic jet actuator, and the penetration of the jet was limited to the viscous sublayer and log layer; the outer layer was unaffected, despite using a jet to freestream velocity ratio of four. Significant effort was also made to validate the microscopic particle image velocity technique and evaluate its capability to accurately resolve such a complex flowfield. To this end, microscopic particle image velocity measurements were compared with hot-wire measurements made on a simple steady jet, as well as an unsteady, periodic synthetic jet. Excellent correlation was found between the two techniques, validating microscopic particle image velocity measurements.

Open source development of a safety critical dual redundant (Ada95/C++) signal control program environment (SCOPE)

Advanced Technologies (ATI) is in the process of developing a dual redundant open source traffic intersection control system for the U.S. Department of Transportation. This system is known as a Signal Control Program Environment (SCOPE). A unique aspect of this effort is that we use all open source non-supported toolsets from requirements though formal qualification testing. This paper will describe our development process, prototype, and current work. The development of the SCOPE is a two part effort. The first part, already completed, was to build a prototype intersection control system that implemented a subsection of newly developed intersection traffic flow control and preemption concepts. In six months, two ATI engineers performed a complete software development lifecycle. The result was a working Ada95/C++ prototype that executes under Linux and can be easily ported to any hardware architecture .

Inverted Jet Spoilers for Aerodynamic Control

This paper reviews concepts for flow control based on jet spoilers and presents a new device based on the jet principle for application to fixed-wing aircraft. The new concept relies on the naturally blown inverse spoiler. Thejet is blown parallel to the surface and points against the freestream. The theoretical basis of the device is presented. Calculations are performed with a Navier-Stokes program to investigate the key design parameters. These parameters include the spoiler deflection (equal or differential), the position of the hinge points, the shape of the bypass channel, the effects of backward- and forward-facing steps in a partial deflection, and the nature and magnitude of the control forces. A comparison is done with conventional inlay spoilers. Finally, a multidimensional optimization has been carried out to study the combined effects of hinge position, deflection, and spoiler length. Optimal configurations with respect to jet momentum and drag coefficient have been analyzed.

Dynamics of Shock Dispersion and Interactions in Supersonic Freestreams with Counterflowing Jets

This study describes an active flow control concept that uses counterflowing jets to significantly modify external flowfields and strongly disperse the shock waves of supersonic and hypersonic vehicles to reduce aerothermal loads and wave drag. The potential aerothermal and aerodynamic benefits of the concepts were investigated by conducting experiments on a 2.6%-scale Apollo capsule model in Mach 3.48 and 4.0 freestreams in a trisonic blowdown wind tunnel, as well as pretest computational fluid dynamics analyses of the flowfields, with and without counterflowing jets. The model employed three sonic and two supersonic (with design Mach numbers of 2.44 and 2.94) jet nozzles with exit diameters ranging from 0.25 to 0.5 in. The schlieren images were consistent with the pretest computational fluid dynamics predictions, showing a long penetration mode jet interaction at low jet flow rates of 0.05 and 0.1 Ib m /s, whereas a short penetration mode jet was revealed at higher flow rates. The long penetration mode jet appeared to be almost fully expanded and was unsteady, with the bow shock becoming so dispersed that it was no longer discernible. High-speed camera schlieren data revealed the bow shock to be dispersed into striations of compression waves, which suddenly coalesced to a weaker bow shock with a larger standoff distance as the flow rate reached a critical value. Heat transfer results showed a significant reduction in heat flux, even giving negative heat flux for some short penetration mode interactions, indicating that the flow wetting the model had a cooling effect, instead of heating, which could significantly impact thermal protection system requirements and design. The findings suggest that high-speed vehicle design and performance can benefit from the application ofcounterflowing jets as an active flow control.

Combustion Powered Actuator with Integrated High Frequency Oscillator

A new actuation concept for flow control has been tested combining the high jet strength of chemically-based, combustion powered actuation with the high frequency of fluidic oscillators. The device exhausts the jet from a combustion powered actuator through a fluidic oscillator and embeds a high frequency [∼ O(1-10) kHz] oscillation into the lower frequency [∼ O(10-100) Hz] of the repetitively ignited combustion bursts. The present work includes characterization of the fluidic oscillator in steady operation for the properties which vary over the combustion burst (pressure, temperature, and gas composition) and results from test firing of the fully integrated device successfully indicating high frequency oscillation in the high speed actuation jet while maintaining a no moving parts design.

The MAESFLO Device: A Complete Microfluidic Control System

AbstractIn spite of considerable efforts, flow control in micro-channels remains a challenge owing to the very small ratio of channel/supply-system volumes, as well as the induction of spurious flows by extremely small pressure or geometry changes. We present here a robust and complete system for flow control in complex microchannel network that both monitors and controls all the flow relevant parameters, that is to say flow rate and pressure. Based on a dynamic control of reservoir pressures at the end of each channel and external thermal flow-sensors, all the parameters are measured with a precision down to 25 μBar and 2nL/min. Thanks to adaptative feed back control loop, the MAESFLO can control either the flow rate or the pressure with high stability over long period whatever the microsystem characteristics. Compared to classic pumps, a significant increase of stability has been reached as no mechanical parts are involved. Indeed the flow rate is pulse free and is stable down to 0.1% of the full scale. Besides, pressure control enables to achieve short response time (less than hundreds of millisec). The MAESFLO is thus a unique system to control flow in complex network architecture and can be considered as an alternative to integrated micro-valves using only external equipments. Indeed, the MAESFLO can stop the flow to nearly zero in one or several branches of a complex microfluidic network while keeping other flows constant. Sequential manipulation of liquids in a definite part of a micro-device is thus possible without expensive and time consuming fabrication processes. It can be particularly useful when dealing with washing steps in the case of biological assay for example. Controlling flow with short response time along with high precision is also a key issue in microfluidic. By combining pressure actuation with flowrate monitoring, short response time are achievable keeping a high precision flow rate. It can be particularly useful for droplet generation and size control, droplet on demand generation, long time living cell perfusion and drug injection… In this work we will present the benefit to control and monitor both pressure and flow rate with the MAESFLO. A lot of information can be extracted from these simple parameters, as hydraulic resistance, monophasic and biphasic apparent viscosity, the volume and the position of a trapped air bubble and many more. The proof of concept of stop flow control will also be shown with experimental results stressing the advantages of the “virtual micro-valve”.

The Influence of Shroud and Cavity Geometry on Turbine Performance: An Experimental and Computational Study— Part II: Exit Cavity Geometry

The geometry of the exit shroud cavity where the rotor shroud leakage flow reenters the main passage flow is very important due to the dominant influence of the leakage flow on the aerodynamics of low aspect ratio turbines. The work presented in this paper investigates, both experimentally and numerically, possibilities for the control of shroud leakage flow by modifications to the exit shroud cavity. The processes through which the leakage flow affects the mainstream aerodynamics identified in the first part of this study were used to develop promising strategies for reducing the influence of shroud leakage flow. The experimental program of this study was conducted on a three-stage model air turbine, which was extensively supported by CFD analysis. Three different concepts for shroud leakage flow control in the exit cavity were analyzed and tested: (a) profiled exit cavity downstream end wall, (b) axial deflector, and (c) radial deflector concepts. Reductions in aerodynamic losses associated with shroud leakage were achieved by controlling the position and direction at which the leakage jet reenters the mainstream when it leaves the exit shroud cavity. Suggestions are made for an optimum shroud and cavity geometry.

Acoustic analysis of low-noise actuator design for active flow control

This paper presents an acoustic analysis of low noise actuation design concepts for active flow control by pulsed jets. The low noise concepts are based on the principle of phase cancellation of coherent unsteady fluctuations. It is shown that by suitably arranging the actuators with prescribed phase variations, active flow control actuators using pulsed jets can achieve very low levels of near-field unsteady fluctuations and far-field noise, while maintaining the same effectiveness in flow control as those conventional designs without the phase cancellation implementation. To demonstrate the concepts, flow separation control on an airfoil is discussed in detail, and numerical simulations are given to show the flow control effectiveness and the low noise features of the actuation design. Simple analytical models are presented to reveal the characteristics of noise radiation from pulsed jet actuators, which relates the near-field pressure and far-field noise to various parameters such as the mean flow Mach number, the frequency of the pulsed jets and the phase variation algorithm. The explicit analytical results can be used as a quick design guide in practical applications.

High-Performance Airfoil Using Coflow Jet Flow Control

T O ACHIEVE revolutionary aircraft performance, advanced technologies should be pursued to drastically reduce the weight of aircraft and fuel consumption and significantly increase aircraft mission payload and maneuverability. Both the military and commercial aircraft will benefit from the same technology. Flow control is a promising technology to break through the limits of the conventional aerodynamic concepts. Recently, a novel active airfoil flow control concept with zero-net mass flux, the coflow jet (CFJ) airfoil, has been developed by Zha et al. [1–5]. The CFJ airfoil achieves three effects simultaneously in a dramatic fashion: lift augmentation, stall margin increase, and drag reduction. The energy expenditure of the CFJ airfoil is low [1], and the CFJ airfoil concept is straightforward to implement. The CFJ airfoil may create a new concept of an “engineless” airplane, which uses the CFJ to generate both lift and thrust without conventional propulsion systems of propellers or jet engines [6]. A CFJ airfoil [1–5] uses an injection slot near the leading edge (LE) and a suction slot near the trailing edge (TE) on the airfoil suction surface. Similar to tangential blowing, the LE jet is in the same direction of the main flow, but the same amount of mass flow that is injected is removed via suction near the TE, resulting in zeronet mass-flux flow control. A proposed fundamental mechanism [2] is that the severe adverse pressure gradient on the suction surface strongly augments turbulent mixing between the main flow and the jet [7]. The mixing then creates the lateral transport of energy from the jet to the main flow and enables the main flow to overcome the large adverse pressure gradient and remain attached at high angle of attack (AOA). The stall margin is hence significantly increased. At the same time, the high-momentum jet drastically increases the circulation, which significantly augments lift, reduces drag, or even generates thrust (negative drag). The objective of this paper is to demonstrate the high performance of the CFJ airfoil with the windtunnel test results.

Fluid flow analysis and vertical gradient freeze crystal growth in a travelling magnetic field

Introduction. In bulk crystal growth of semiconductors the concept of remote flow control by means of alternating magnetic fields has attracted considerable interest (see, e.g., [1-6]). In this way the melt flow can be tailored for growth under optimised conditions to improve the crystal properties and/or the growth yield. A promising option is to apply an axially travelling magnetic wave to the melt (Travelling Magnetic Field – TMF). It introduces a mainly axial Lorentz force which leads to meridional flow patterns. In recent numerical studies [3],[6] the TMF has been recognised to be a versatile and efficient tool to control the heat and mass transport in the melt. For the Vertical Bridgman/Vertical Gradient Freeze (VB/VGF) growth, the beneficial effect of an adequately adjusted, TMF induced flow was clearly demonstrated in [6] in terms of the reduction of thermal shear stress at the solid-liquid interface. In this paper we present experimental and numerical results on the TMF driven convection in an isothermal model fluid as well as first VGF-TMF crystal growth experiments. The model investigations are focused on the transition from laminar to instationary flow conditions that should be avoided in crystal growth applications. The VGF experiments were performed growing Ga doped germanium single crystals under the influence of the travelling field in a newly developed VGF-TMF equipment.

Metabolic coronary flow regulation—Current concepts

The concept of metabolic coronary flow control provides a rationale for the close relationship of coronary flow and myocardial metabolic rate of oxygen. The concept is based on the presence of an oxygen (metabolic) sensor coupled functionally to effector mechanisms, which control vascular tone. Four modes of metabolic control models have been proposed. 1) An oxygen sensor located in the wall of coronary vessels coupling to smooth muscle tension. Endothelial prostaglandin production may support this concept. 2) An oxygen sensing mechanism located in the myocardium and changing metabolism in response to changes of local pO(2). Adenosine is a metabolite produced at an accelerated rate when the supply-to-demand relationship for oxygen falls. 3) Sensing of oxygen turnover may be achieved by carbon dioxide production and, potentially, by mitochondrial production of reactive oxygen species. 4) The red blood cell might serve as an oxygen sensor in response to changes of haemoglobin oxygenation. A potential link to vessel relaxation may be red cell ATP release. A large body of experimental evidence supports the notion that K(ATP) channels play a significant role causing smooth muscle hyper-polarization. However, additional yet unknown effector mechanisms must exist, because block of K(ATP) channels does not lead to deterioration of coronary flow control under conditions of exercise. Thus, although several lines of evidence show that metabolic flow regulation is effective during hypoxic conditions,mechanisms mediating normoxic metabolic flow control still await further clarification.

Vortex Structure and Scale on an Unsteady Air foil(Wing and Airfoil)

A studies on the unsteady flow at the low Reynolds number region have been catalyzed in recent years by an interest in the Micro-Electro-Mechanical-Systems based on the concept of flow control, Micro-Air-Vehicle and micro flight robot. In order to clarify the detailed vortex structure and vortex scale on an unsteady airfoil, such as a pitching airfoil and a heaving airfoil, at low Reynolds number region. We have carried out the flow visualization around unsteady airfoils at Re=4000 using three visualization techniques, such as schlieren visualization, dye flow visualization and PIV measurement. Especially, we report the dynamic behavior of vortices shed from the leading edge, the number of vortex shedding and its scale. The small discrete vortices shed from the leading edge one after another in a pitching airfoil and a heaving airfoil at low Reynolds number region. When the non-dimensional pitching rate in a pitching airfoil and the non-dimensional heaving velocity become high, the clear discrete vortices were visualized. The number of vortex shedding from the leading edge during one pitching cycle on a pitching airfoil was strongly depends on the non-dimensional pitching rate and independent of the airfoil configuration, mean angle of attack and pitching amplitude. At the low non-dimensional pitching rate in a pitching airfoil, a lot of discrete vortices shed from the leading edge and its scale was very small. At the high non-dimensional pitching rate in a pitching airfoil and the high non-dimensional heaving velocity in a heaving airfoil, on the other hand, the number of vortex shedding from the leading edge during one cycle was low. However, its scale was about one fourth of the chord length and it did not depend on the airfoil configuration. Moreover, these vortices consisted of the re-circulation region on the suction surface.

A Three-Dimensional Flow Control Concept for Single-Cell Experiments on a Microchip. 2. Fluorescein Diacetate Metabolism and Calcium Mobilization in a Single Yeast Cell As Stimulated by Glucose and pH Changes

A Three-Dimensional Flow Control Concept for Single-Cell Experiments on a Microchip. 2. Fluorescein Diacetate Metabolism and Calcium Mobilization in a Single Yeast Cell As Stimulated by Glucose and pH Changes

A three-dimensional flow control concept for single-cell experiments on a microchip. 1. Cell selection, cell retention, cell culture, cell balancing, and cell scanning.

An ideal microchip for single-cell experiments should be able to allow us to culture cells, to select any desired single cell from a group, to retain the cell for convenient cellular signal detection, and to deliver any buffer or reagent directly to the cell at any time during continual detection and observation. Most importantly, any negative impact on the live cell should be minimized. To accomplish all these functions, we developed a three-dimensional liquid flow control concept and employed special liquid flow fields to manipulate and retain a single yeast cell freely in the chip. A zero-speed point was controlled to retain the cell for three-dimensional cell balancing and cell scanning. A dispersive flow delivered reagents at a high speed to very near the cell and provided them to the cell at a low speed. No force stronger than its gravitational force was exerted on the cell, which could be balanced on different positions on an arc-sloping wall, thus minimizing any negative impact on the cell due to strong liquid flows. Specifically, we demonstrate on-chip single-cell culture, cell wall removal, and reagent delivery. Subsequently, single-cell fluorescence detection was performed, and noise filtering and background correction were applied for data processing.

A three-dimensional flow control concept for single-cell experiments on a microchip. 2. Fluorescein diacetate metabolism and calcium mobilization in a single yeast cell as stimulated by glucose and pH changes.

Using a three-dimensional flow control concept to manipulate and retain a single yeast cell in a microchip, we were able to study the kinetics of intracellular metabolism and calcium mobilization at the single-cell level, as stimulated by glucose and pH changes. As a model study, the fluorogenic substrate fluorescein diacetate (FDA) was chosen to study how the intracellular carboxylesterase metabolize it. A single yeast cell was first cultured in the microchip. Thereafter, under a constant concentration of FDA, influx of FDA into the yeast cell occurred and FDA was hydrolyzed or metabolized. It was found that changes in both pH and glucose stimulated the FDA metabolism in a yeast cell, and the stimuli can elicit multiple responses from the cell. Since it was carried out within the microchip, the whole experiment on one single yeast cell could last for as long as 10 h. The dormant cell, budding cell, and pretreated budding cell (in low-pH buffer) of yeast resulted in different responses. Experimental data provided details on the FDA metabolism at the single-cell level and revealed strong correlations between FDA metabolism and calcium mobilization. Furthermore, efflux of the FDA metabolite fluorescein could start spontaneously if there was glucose in the medium. The experiments on a single cell were of the "human cell conservation" style because the cell responded to the reagent changes implemented by the human researcher. A mathematical model was also developed to study the influx-hydrolysis-efflux processes of the FDA metabolism using single-cell fluorescent data. These long overdue single-cell experiments are now rendered possible using the three-dimensional flow control in the microchip.

Instrumentation for advanced microseparations in pharmaceutical analysis and proteomics

Small-volume chromatographic columns are only able to generate narrow peaks when flow rates, injection volume and instrument components, such as detector, connecting tubing and fittings, are matched to the peak dispersion from the column. Criteria for the proper design of chromatographic instrumentation are therefore derived from a general model on total dispersion. The performance of such a system is then experimentally evaluated from applications run on narrow-bore, small-volume columns. In order to achieve flow rates that match the dimensions of such columns, a new concept for electronic flow control (EFC) is introduced. A theoretical optimization of column efficiency and throughput is discussed and the results verified with practical examples on short, narrow-bore columns packed with small, porous and superficially porous particles. For complex sample mixtures, the concept of peak capacity is introduced and applied to orthogonal separation principles in multiple chromatographic dimensions through column switching techniques.

Hingeless Flow Control over a Delta-Wing Planform

An experimental investigation of hingeless flow control by vortex breakdown manipulation over a delta wing is described. Employed as a method of longitudinal control for slender-wing planforms, the performance of this active flow control technique was evaluated as a possible replacement to conventional moving or hinged control surfaces. Testing was conducted with a 60-deg delta-wing model incorporating pneumatic vortex control (PVC) actuators. These actuators delivered high-pressure jets of varying parameters on the upper surface of the model. The jets were used to alter the flowfield over the wing caused by leading-edge vortices. The natural location of vortex breakdown was shifted by three sets of PVC actuators strategically positioned on the model. This effect was intended to control the chordwise lift distribution over the wing by inducing both premature and delayed vortex breakdown, resulting in nose-up and nose-down pitch responses, respectively. A comparative analysis of this hingeless flow control concept was conducted in Texas A&M University's 3 × 4 ft low-speed wind tunnel and 2 x 3 ft water tunnel

Forebody Flow Control at Conditions of Naturally Occurring Separation Asymmetry

Extensive efforts have been directed toward the development of the means by which the flow separation asymmetry occurring on an axisymmetric slender forebody at very high angles of attack could be controlled in order to improve the agility of advanced aircraft and missiles. To control the asymmetric flow separation represents a great challenge, and only a few of the investigated flow-control concepts have led to viable solutions. The fluid-mechanical processes associated with steady and alternating-pulsed blowing applications are analyzed in an effort to define flow mechanisms that can cause the observed anomalous control characteristics

Hypersonic Flow Control Using Upstream Focused Energy Deposition

A numerical study of centerline and off-centerline power deposition at a point upstream of a two-dimensional blunt body at Mach 6.5 at 30 km altitude are presented. The full Navier-Stokes equations are used. Wave drag, lift, and pitching moment are presented as a function of amount of power absorbed in the flow and absorption point location. It is shown that wave drag is considerably reduced. Modifications to the pressure distribution in the flow field due to the injected energy create lift and a pitching moment when the injection is off-centerline. This flow control concept may lead to effective ways to improve the performance and to stabilize and control hypersonic vehicles.