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At least 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Control of an axisymmetric turbulent jet by multi-modal excitation

Experimental measurements of naturally occurring instability modes in the axisymmetric shear layer of high Reynolds number turbulent jet are presented. The region up to the end of the potential core was dominated by the axisymmetric mode. The azimuthal modes dominated only downstream of the potential core region. The energy content of the higher order modes (m is greater than 1) was significantly lower than that of the axisymmeteric and m = + or - 1 modes. Under optimum conditions, two-frequency excitation (both at m = 0) was more effective than single frequency excitation (at m = 0) for jet spreading enhancement. An extended region of the jet was controlled by forcing combinations of both axisymmetric (m = 0) and helical modes (m = + or - 1). Higher spreading rates were obtained when multi-modal forcing was applied.

Raman, Ganesh↗
NASA NTRS1991

Control of an axisymmetric turbulent jet by multi-modal excitation

Experimental measurements of naturally occurring instability modes in the axisymmetric shear layer of high Reynolds number turbulent jet are presented. The region up to the end of the potential core was dominated by the axisymmetric mode. The azimuthal modes dominated only downstream of the potential core region. The energy content of the higher order modes (m is greater than 1) was significantly lower than that of the axisymmetric and m = + or - 1 modes. Under optimum conditions, two-frequency excitation (both at m = 0) was more effective than single frequency excitation (at m = 0) for jet spreading enhancement. An extended region of the jet was controlled by forcing combinations of both axisymmetric (m = 0) and helical modes (m = + or - 1). Higher spreading rates were obtained when multi-modal forcing was applied.

Raman, Ganesh↗
NASA NTRS1983

Effects of control saturation on the command response of statically unstable aircraft

Hanson and Stengel (1981) have described the effects of saturating controls on the initial-condition response of statically unstable aircraft. In an analysis of the stability augmentation system, consideration was given to maximizing the region of stable response. The present investigation is concerned with an extension of the control saturation analysis to the problem of command response, taking into account the main problem of control design for satisfactory flying qualities (as perceived by the pilot). A model for examining the effects of control saturation on longitudinal stability and command response is developed and applied to an aircraft with a static instability. Three parameters prescribing longitudinal flying qualities are examined with respect to their ability to identify proper flying qualities in the presence of control saturation. The presented analysis provides also satisfactory guidelines for establishing command saturation/stability boundaries.

Hanson, G. D.↗
NASA NTRS1987

Plasma instability in the presence of negative ions

Ion density fluctuations are studied in a diffusion-controlled argon plasma containing, as a dilutant, two electron-attaching species, carbon dioxide and sulfur hexafluoride. It is found that the system becomes increasingly unstable as the concentration of the electron-attaching species is increased. Nonlinear mode-mode couplings have been identified and the coupling coefficients for these interactions have been computed. Turbulent fluctuations are observed to have pronounced three-dimensionality with distinct axial and azimuthal behaviors. The power spectra of these fluctuations are composed of many discrete modes. The importance of changes in the total discharge pressure is dependent on the electron-attachment cross section of the dilutant species. From these results it is shown that the ionization instability is probably responsible for the observed phenomenon.

Johnson, J. A., III↗
NASA NTRS1992

Onset conditions for equatorial spread F

The problem of day-to-day variability in the occurrence of equatorial spread F (ESF) is addressed using multidiagnostic observations and semiempirical modeling. The observational results are derived from a two-night case study of ESF onset conditions observed at Kwajalein Atoll (Marshall Islands) using the ALTAIR incoherent scatter radar and all-sky optical imaging techniques. The major difference between nights when ESF instabilities did not occur (August 14, 1988) and did occur (August 15, 1988) in the Kwajalein sector was that the northern meridional gradient of 6300-A airglow was reduced on the night of limited ESF activity. Modeling results suggest that this unusual airglow pattern is due to equatorward neutral winds. Previous researchers have shown that transequatorial thermospheric winds can exert a control over ESF seasonal and longitudinal occurrence patterns by inhibiting Rayleigh-Taylor instability growth rates. Evidence is presented to suggest that this picture can be extended to far shorter time scales, namely, that 'surges' in transequatorial winds acting over characteristic times of a few hours to a day can result in a stabilizing influence upon irregularity growth rates. The seemingly capricious nature of ESF onset may thus be controlled, in part, by the inherent variability of low-latitude thermospheric winds.

Mendillo, Michael↗
NASA NTRS1991

Dynamics and control of hypersonic vehicles - The integration challenge for the 1990's

The dynamic characteristics of hypersonic vehicles are reviewed, with special attention to the interactions between the airframe, engine, and structural dynamics. Based on a preliminary investigation of the dynamics of a generic vehicle configuration similar to the X-30 with Scramjet propulsion, an assessment of these interactions is presented. The control effectors include aerodynamic pitch-control surfaces, as well as engine fuel flow and diffuser area ratio. The study configuration is statically instable in pitch, and exhibits strong airframe/engine/elastic coupling in the attitude dynamics and engine responses. This strong coupling will require a highly integrated airframe-engine control system, and the performance of the attitude control system will be contingent upon the ability to adequately deal with the structural aeroelastic response and engine dynamics.

Schmidt, David K.↗
NASA NTRS1989

Numerical simulation of controlled large space structures

Large Space Structures do not have much damping, which necessitates the installation of a controller onto the structure. If the controller is improperly designed, the structure may become unstable and be destroyed. Since Large Space Structures are extremely expensive pieces of hardware, new controllers must not be tested first on the structure. They must first be tested in computer simulations. Until now, the usual procedure for simulating controlled Large Space Structures is to compute a reduced order modal representation of the structure and then apply the controller. However, this procedure entails modal truncation error. A new software package which is free from this error is currently under development within the Center for Space Construction. The more accurate finite element representation of the structure is used in the simulation, instead of the less accurate reduced order modal representation. This software also features an efficient matrix storage scheme, which effectively deals with the asymmetric system matrices which occur when control is added to the structure. Also, an integration algorithm was chosen so that the simulation is a reliable indicator of system stability or instability. The software package is fairly general in nature. Linearity of the finite element model and of the controller is the only assumption made. Actuator dynamics, sensor dynamics, noise, and disturbances can be handled by the package. In addition, output feedback of displacement, velocity, and/or acceleration signals can be simulated. Kalman state estimation was also implemented. This software was tested on a finite element model of a real Large Space Structure: The Mini-Mast Truss. Mini-Mast is a testbed at NASA-Langley which is currently under development. A 714 degree of freedom finite element model was computed, and a 19 state controller was designed for it. Torque wheel dynamics were added to the model, and the entire closed loop system was simulated with the software package.

Quan, Ralph↗
NASA NTRS2004

Extraterrestrial Hemorrhage Control: Terrestrial Developments in Technique, Technology, and Philosophy with Applicability to Traumatic Hemorrhage in Space

Managing injury and illness during long duration space flight limits efforts to explore beyond low earths orbit. Traumatic injury may be expected to occur in space and is a frequent cause of preventable deaths, often related to uncontrolled or ongoing hemorrhage (H). Such bleeding causes 40% of terrestrial injury mortality. Current guidelines emphasize early control of H compared to intravenous infusions. Recent advances in surgical and critical care may be applicable to trauma care in space, with appropriate considerations of the extreme logistical and personnel limitations. Methods: Recent developments in technique, resuscitation fluids, hemoglobin (Hb) substitutes, hemostatic agents, interventional angiography, damage control principles, and concepts related to suspended animation were reviewed. Results: H associated with instability frequently requires definitive intervention. Direct pressure should be applied to all compressible bleeding, but novel approaches are required for intracavitary noncompressible bleeding. Intravenous hemostatic agents such as recombinant Factor VII may facilitate hemostasis especially when combined with a controlled hypotension approach. Both open and laparoscopic techniques could be used in weightlessness, but require technical expertise not likely to be available. Specific rehearsed invasive techniques such as laparotomy with packing, or arterial catherterization with with robotic intravascular embolization might be considered . Hemodynamic support, thermal manipulation, or pharmacologic induction of a state of metabolic down regulation for whole body preservation may be appropriate. Hypertonic saline, with or without dextran, may temporize vascular support and decrease reperfusion injury, with less mass than other solutions. Hb substitutes have other theoretical advantages. Conclusions: Terrestrial developments suggest potential novel strategies to control H in space, but will required a coordinated program of evaluation and training to evaluate.

Kirkpatrick, Andrew↗
NASA NTRS2004

Multi-Element Unstructured Analyses of Complex Valve Systems

The safe and reliable operation of high pressure test stands for rocket engine and component testing places an increased emphasis on the performance of control valves and flow metering devices. In this paper, we will present a series of high fidelity computational analyses of systems ranging from cryogenic control valves and pressure regulator systems to cavitating venturis that are used to support rocket engine and component testing at NASA Stennis Space Center. A generalized multi-element framework with sub-models for grid adaption, grid movement and multi-phase flow dynamics has been used to carry out the simulations. Such a framework provides the flexibility of resolving the structural and functional complexities that are typically associated with valve-based high pressure feed systems and have been difficult to deal with traditional CFD methods. Our simulations revealed a rich variety of flow phenomena such as secondary flow patterns, hydrodynamic instabilities, fluctuating vapor pockets etc. In the paper, we will discuss performance losses related to cryogenic control valves, and provide insight into the physics of the dominant multi-phase fluid transport phenomena that are responsible for the choking like behavior in cryogenic control elements. Additionally, we will provide detailed analyses of the modal instability that is observed in the operation of the dome pressure regulator valve. Such instabilities are usually not localized and manifest themselves as a system wide phenomena leading to an undesirable chatter at high flow conditions.

Sulyma, Peter↗
NASA NTRS1994

Screech tones from free and ducted supersonic jets

It is well known that screech tones from supersonic jets are generated by a feedback loop. The loop consists of three main components. They are the downstream propagating instability wave, the shock cell structure in the jet plume, and the feedback acoustic waves immediately outside the jet. Evidence will be presented to show that the screech frequency is largely controlled by the characteristics of the feedback acoustic waves. The feedback loop is driven by the instability wave of the jet. Thus the tone intensity and its occurrence are dictated by the characteristics of the instability wave. In this paper the dependence of the instability wave spectrum on the azimuthal mode number (axisymmetric or helical/flapping mode, etc.), the jet-to-ambient gas temperature ratio, and the jet Mach number are studied. The results of this study provide an explanation for the observed screech tone mode switch phenomenon (changing from axisymmetric to helical mode as Mach number increases) and the often-cited experimental observation that tone intensity reduces with increase in jet temperature. For ducted supersonic jets screech tones can also be generated by feedback loops formed by the coupling of normal duct modes to instability waves of the jet. The screech frequencies are dictated by the frequencies of the duct modes. Super resonance, resonance involving very large pressure oscillations, can occur when the feedback loop is powered by the most amplified instability wave. It is proposed that the observed large amplitude pressure fluctuations and tone in the test cells of Arnold Engineering Development Center were generated by super resonance. Estimated super-resonance frequency for a Mach 1.3 axisymmetric jet tested in the facility agrees well with measurement.

Tam, C. K. W.↗
NASA NTRS1992

Marangoni instability induced convection in an evaporating liquid droplet

The processes occurring when a liquid drop undergoes evaporation are described. When a liquid drop undergoes evaporation, its surface temperature decreases. If the droplet is free floating in a microgravity environment, the heat transfer process inside the droplet is initially condition controlled. As the process continues, a radial temperature gradient builds up at the free surface until the critical Marangoni number is exceeded. Then the onset of instability induces thermocapillary convective flows, which in turn speed up the evaporation. The convective flows will subside when the interior of the droplet reaches a certain equilibrium temperature, and the process will return to the diffusion controlled mode. Both preliminary modeling and recent laboratory data have confirmed that Marangoni instability induced convection can and does occur in the droplet evaporation process. Mathematical models representing Marangoni instability in an evaporating liquid drop are presented. An ideal space experiment to study and characterize the onset of Marangoni instability in an evaporating liquid droplet and to establish the effect of Marangoni instability induced convection on the droplet evaporation rate is outlined and the need for conducting such experiments in space is highlighted. However, before an opportunity to conduct experiments in space arises, ground based experiments have to be conducted to study feasibility issues and proof of concept. A ground based experiment of this type is outlined.

Chai, An-Ti↗
NASA NTRS2005

Model-Based, Multiscale Self-Tuning Controller Developed for Active Combustion Control

New challenges concerning system health-monitoring and life-extending robust controls for the Ultra-Efficient Engine Technology Project, as well as other advanced engine and power system concepts at NASA and elsewhere, have renewed the control community s interest in smart, model-based methods. In particular, these challenges have further motivated efforts at the NASA Glenn Research Center to exploit the versatility and superiority of the dynamic features extraction of multiscale analysis for controls--such as with "wavelets" and "wavelet filter-banks.' The accomplishments reported herein pertain to the active suppression of combustion instabilities in liquid-fuel combustors via fuel modulation. The fundamentals and initial success of this innovation were reported for a unique demonstration of active combustion control (a research collaboration of NASA Glenn with Pratt & Whitney and the United Technologies Research Center, UTRC). This demonstration, conducted in 2002 at UTRC on the NASA single nozzle rig (SNR) combustor, was the first known suppression of high-frequency instability with a liquid-fueled combustor. The SNR is based on a high-powered military engine combustor that exhibited well-known instabilities.

Le, Dzu K.↗
NASA NTRS1976

Elementary Applications of a Rotorcraft Dynamic Stability Analysis

A number of applications of a rotorcraft aeroelastic analysis are presented to verify that the analysis encompasses the classical solutions of rotor dynamics, and to examine the influence of certain features of the model. Results are given for the following topics: flapping frequency response to pitch control; forward flight flapping stability; pitch/flap flutter and divergence; ground resonance instability; and the flight dynamics of several representative helicopters.

Johnson, W.↗
NASA NTRS1976

The Tethered Balloon Current Generator - A space shuttle-tethered subsatellite for plasma studies and power generation

The objectives of the Tethered Balloon Current Generator experiment are to: (1) generate relatively large regions of thermalized, field-aligned currents, (2) produce controlled-amplitude Alfven waves, (3) study current-driven electrostatic plasma instabilities, and (4) generate substantial amounts of power or propulsion through the MHD interaction. A large balloon (a diameter of about 30 m) will be deployed with a conducting surface above the space shuttle at a distance of about 10 km. For a generally eastward directed orbit at an altitude near 400 km, the balloon, connected to the shuttle by a conducting wire, will be positive with respect to the shuttle, enabling it to collect electrons. At the same time, the shuttle will collect positive ions and, upon command, emit an electron beam to vary current flow in the system.

Williamson, P. R.↗
NASA NTRS1978

Entry flight control system downmoding evaluation

A method to desensitize the entry flight control system to structural vibration feedback which might induce an oscillatory instability is described. Trends in vehicle response and handling characteristics as a function of gain combinations in the FCS forward and rate feedback loops were described as observed in a man-in-the-loop simulation. Among the flight conditions considered are the effects of downmoding with APU failures, off-nominal trajectory conditions, sensed angle of attack errors, the impact on RCS fuel consumption, performance in the presence of aero variations, recovery from large FCS upsets, and default gains.

Barnes, H. A.↗
NASA NTRS2001

Fluid Physics Under a Stochastic Acceleration Field

The research summarized in this report has involved a combined theoretical and computational study of fluid flow that results from the random acceleration environment present onboard space orbiters, also known as g-jitter. We have focused on a statistical description of the observed g-jitter, on the flows that such an acceleration field can induce in a number of experimental configurations of interest, and on extending previously developed methodology to boundary layer flows. Narrow band noise has been shown to describe many of the features of acceleration data collected during space missions. The scale of baroclinically induced flows when the driving acceleration is random is not given by the Rayleigh number. Spatially uniform g-jitter induces additional hydrodynamic forces among suspended particles in incompressible fluids. Stochastic modulation of the control parameter shifts the location of the onset of an oscillatory instability. Random vibration of solid boundaries leads to separation of boundary layers. Steady streaming ahead of a modulated solid-melt interface enhances solute transport, and modifies the stability boundaries of a planar front.

Vinals, Jorge↗
NASA NTRS2010

Elevator Sizing, Placement, and Control-Relevant Tradeoffs for Hypersonic Vehicles

Within this paper, control-relevant vehicle design concepts are examined using a widely used 3 DOF (plus flexibility) nonlinear model for the longitudinal dynamics of a generic carrot-shaped scramjet powered hypersonic vehicle. The impact of elevator size and placement on control-relevant static properties (e.g. level-flight trimmable region, trim controls, Angle of Attack (AOA), thrust margin) and dynamic properties (e.g. instability and right half plane zero associated with flight path angle) are examined. Elevator usage has been examine for a class of typical hypersonic trajectories.

Dickeson, Jeffrey J.↗
NASA NTRS1996

An Experimental Study of Richtmyer-Meshkov Instability

Richtmyer-Meshkov (RM) instability occurs when a planar interface separating two fluids of different density is impulsively accelerated In the direction of its normal. It is one of the most fundamental of fluid instabilities and is of importance in fields ranging from astrophysics to materials processing. Because RM instability experiments are normally carried out in shock tubes, where the generation of a sharp well controlled interface between gases is difficult, there is a scarcity of good experimental results. The experiments presented here utilize a novel technique which circumvents many of the experimental difficulties that have previously limited the study of RM instability. In this system, the instability is generated by bouncing a thin rectangular tank containing two liquids off of a fixed spring. Results obtained from these experiments yield particularly well visualized images of the nonlinear development of the instability. However, because the run time in these experiments is limited, new experiments capable of achieving longer run times are planned.

Jacobs, Jeffrey W.↗