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At least 451 records · Page 25

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NASA NTRS2000

Closed-loop Separation Control Using Oscillatory Flow Excitation

Design and implementation of a digital feedback controller for a flow control experiment was performed. The experiment was conducted in a cryogenic pressurized wind tunnel on a generic separated configuration at a chord Reynolds number of 16 million and a Mach number of 0.25. The model simulates the upper surface of a 20% thick airfoil at zero angle-of-attack. A moderate favorable pressure gradient, up to 55% of the chord, is followed by a severe adverse pressure gradient which is relaxed towards the trailing edge. The turbulent separation bubble, behind the adverse pressure gradient, is then reduced by introducing oscillatory flow excitation just upstream of the point of flow separation. The degree of reduction in the separation region can be controlled by the amplitude of the oscillatory excitation. A feedback controller was designed to track a given trajectory for the desired degree of flow reattachment and to improve the transient behavior of the flow system. Closed-loop experiments demonstrated that the feedback controller was able to track step input commands and improve the transient behavior of the open-loop response.

Allan, Brian G.↗

NASA NTRS1994

Turbulent Heat Transfer Characteristics in the Shear Layer of a Separated Flow

Experiments were performed to study the evolution of the heat transfer structure in a separated free shear layer region of an incompressible separated turbulent boundary layer flow behind a backward-facing step. While there is an abundance of velocity field measurements of separated flows, heat transfer measurements are rather scarce, thus limiting assessment of the heat transfer physics and its accurate modeling. The purpose of the paper is twofold: to improve an understanding of effects of flow separation on heat transfer characteristics, and to provide data for turbulence modeling and computation. The boundary layer upstream of the step was turbulent and fully developed. A constant temperature surface boundary condition was imposed upstream and downstream of the step for the heat transfer study. An internal mixing-layer like flow forms and grows from the step lip within the original boundary layer. The turbulent structure of the flow evolving downstream, however, does not switch immediately to that of a mixing layer over the entire shear layer thickness. Measurements of mean and fluctuating velocity and temperature fields indicate that the internal layer spreads gradually in the transverse direction while the outer part of the original boundary layer is effectively unperturbed. The results in this paper have not been previously reported.

Jovic, S.↗

NASA NTRS2001

Hyper-X Stage Separation: Simulation Development and Results

This paper provides an overview of stage separation simulation development and results for NASA's Hyper-X program; a focused hypersonic technology effort designed to move hypersonic, airbreathing vehicle technology from the laboratory environment to the flight environment. This paper presents an account of the development of the current 14 degree of freedom stage separation simulation tool (SepSim) and results from use of the tool in a Monte Carlo analysis to evaluate the risk of failure for the separation event. Results from use of the tool show that there is only a very small risk of failure in the separation event.

Reubush, David E.↗

NASA NTRS2000

AST Critical Propulsion and Noise Reduction Technologies for Future Commercial Subsonic Engines: Separate-Flow Exhaust System Noise Reduction Concept Evaluation

This report describes the work performed by General Electric Aircraft Engines (GEAE) and Allison Engine Company (AEC) on NASA Contract NAS3-27720 AoI 14.3. The objective of this contract was to generate quality jet noise acoustic data for separate-flow nozzle models and to design and verify new jet-noise-reduction concepts over a range of simulated engine cycles and flight conditions. Five baseline axisymmetric separate-flow nozzle models having bypass ratios of five and eight with internal and external plugs and 11 different mixing-enhancer model nozzles (including chevrons, vortex-generator doublets, and a tongue mixer) were designed and tested in model scale. Using available core and fan nozzle hardware in various combinations, 28 GEAE/AEC separate-flow nozzle/mixing-enhancer configurations were acoustically evaluated in the NASA Glenn Research Center Aeroacoustic and Propulsion Laboratory. This report describes model nozzle features, facility and data acquisition/reduction procedures, the test matrix, and measured acoustic data analyses. A number of tested core and fan mixing enhancer devices and combinations of devices gave significant jet noise reduction relative to separate-flow baseline nozzles. Inward-flip and alternating-flip core chevrons combined with a straight-chevron fan nozzle exceeded the NASA stretch goal of 3 EPNdB jet noise reduction at typical sideline certification conditions.

Janardan, B. A.↗

NASA NTRS1996

Control of Flow Separation Using Adaptive Airfoils

A novel way of controlling flow separation is reported. The approach involves using an adaptive airfoil geometry that changes its leading edge shape to adjust to the instantaneous flow at high angles of attack such that the flow over it remains attached. In particular, a baseline NACA 0012 airfoil, whose leading edge curvature could be changed dynamically by 400% was tested under quasi-steady compressible flow conditions. A mechanical drive system was used to produce a rounded leading edge to reduce the strong local flow acceleration around its nose and thus reduce the strong adverse pressure gradient that follows such a rapid acceleration. Tests in steady flow showed that at M = 0.3, the flow separated at about 14 deg. angle of attack for the NACA 0012 profile but could be kept attached up to an angle of about 18 deg by changing the nose curvature. No significant hysteresis effects were observed; the flow could be made to reattach from its separated state at high angles by changing the leading edge curvature. Interestingly, the flow over a nearly semicircular nosed airfoil was separated even at low angles.

Chandrasekhara, M. S.↗

NASA NTRS2002

Membrane Separation Processes at Low Temperatures

The primary focus of Kennedy Space Center's gas separation activities has been for carbon dioxide, nitrogen, and argon used in oxygen production technologies for Martian in-situ resource utilization (ISRU) projects. Recently, these studies were expanded to include oxygen for regenerative life support systems. Since commercial membrane systems have been developed for separation of carbon dioxide, nitrogen, and oxygen, initially the studies focused on these membrane systems, but at lower operating temperatures and pressures. Current investigations art examining immobilized liquids and solid sorbents that have the potential for higher selectivity and lower operating temperatures. The gas separation studies reported here use hollow fiber membranes to separate carbon dioxide, nitrogen, and argon in the temperature range from 230 to 300 K. Four commercial membrane materials were used to obtain data at low feed and permeate pressures. These data were used with a commercial solution-diffusion modeling tool to design a system to prepare a buffer gas from the byproduct of a process to capture Martian carbon dioxide. The system was designed to operate, at 230 K with a production rate 0.1 sLpm; Feed composition 30% CO2, 44% N2, and 26% Ar; Feed pressure 104 kPa (780); and Permeate pressure 1 kPa (6 torr); Product concentration 600 ppm CO2. This new system was compared with a similar system designed to operate at ambient temperatures (298 K). The systems described above, along with data, test apparatus, and models are presented.

Parrish, Clyde↗

NASA NTRS1997

Free Flight and Self-Separation from the Flight Deck Perspective

The concept of "free flight", while still being developed, is intended to emphasize more, flexibility for operators in the National Airspace System (NAS) by providing more separation responsibility to pilots, New technologies, procedures, and concepts have been suggested by the aviation community to enable this task; however, much work needs to be accomplished to help define and evaluate the concept feasibility. The purpose of this simulation was to begin examining some of the communication and procedural issues associated with self-separation in the enroute environment. A simulation demonstration was conducted in the Boeing 747-400 simulator at NASA Ames Research Center. Commercial pilots (from a U.S. domestic carrier) current on the B747-400 aircraft were the participants. Ten flight crews (10 captains, 10 first officers) flew in the Denver enroute airspace environment. A new alerting logic designed to allow for airborne self-separation was created for this demonstration. This logic assumes automatic dependent surveillance broadcast (ADS-B) capability and represented aircraft up to 120 nautical miles on the display. The new flight deck display features were designed and incorporated on the existing navigational display in the simulator to allow for increased traffic and maneuvering information to the flight crew. New tools were also provided to allow the crews to assess conflicts and potential maneuvers before implementing them. Each of the flight crews flew eight different scenarios in the Denver enroute airspace. The scenarios included eight to ten other aircraft, and each scenario was created with the intent of having one of the other aircraft become an operational conflict for our simulator aircraft. Different types of conflict geometries were represented across the eight scenarios. Also, some scenarios allowed for more time to detect a potential clearance, while others allowed for less time for'detection. Additionally, the crews were asked to a ply the Visual Flight Rules (VFR) right of way rules when determining who should maneuver in a conflict situation; therefore, the scenarios were designed to test different applications of those recommendations, Data analyses include an evaluation of crew procedures and communication. The application of the VFR right-of-way rules are being explored. Timing variables are being examined to determine potential efficiency differences between scenarios and conflict types. Proximity of aircraft will be assessed as one indication of the operational safety. The intent of these evaluations is to help provide definitions and guidelines of negotiation procedures in a self-separation environment assuming automated data link technology (ADS-B). Also, definitions of likely flight crew maneuvers and application to current VFR right-of-way rules may be obtained, along with guidelines for negotiation procedures between flight crews.

Lozito, Sandra↗

NASA NTRS2002

Low-Pressure Turbine Separation Control: Comparison With Experimental Data

The present work details a computational study, using the Glenn HT code, that analyzes the use of vortex generator jets (VGJs) to control separation on a low-pressure turbine (LPT) blade at low Reynolds numbers. The computational results are also compared with the experimental data for steady VGJs. It is found that the code determines the proper location of the separation point on the suction surface of the baseline blade (without any VGJ) for Reynolds numbers of 50,000 or less. Also, the code finds that the separated region on the suction surface of the blade vanishes with the use of VGJs. However, the separated region and the wake characteristics are not well predicted. The wake width is generally over-predicted while the wake depth is under-predicted.

Garg, Vijay K.↗

NASA NTRS2003

Demonstration of Separation Control Using Glow-Discharge Plasma Actuators

Active flow control of boundary-layer separation using glow-discharge plasma actuators is studied experimentally. Separation is induced on a flat plate installed in a closed-circuit wind tunnel by a shaped insert on the opposite wall. The flow conditions represent flow over the suction surface of a modem low-pressure-turbine airfoil. The Reynolds number, based on wetted plate length and nominal exit velocity, is varied from 50,000 to 300,000, covering cruise to takeoff conditions. Low (0.2%) and high (2.5%) free-stream turbulence intensities are set using passive grids. A spanwise-oriented phased-plasma-array actuator, fabricated on a printed circuit board, is surface-flush-mounted upstream of the separation point and can provide forcing in a wide frequency range. Static surface pressure measurements and hot-wire anemometry of the base and controlled flows are performed and indicate that the glow-discharge plasma actuator is an effective device for separation control.

Hultgren, Lennart S.↗

NASA NTRS2003

Enhanced Design of Turbo-jet LPT by Separation Control Using Phased Plasma Actuators

This work deals with the documentation and control of flow separation that occurs over turbine blades in the low-pressure turbine stage at low Reynolds numbers that exist at high altitude cruise. We utilize a specially constructed linear cascade that is designed to study the flow field over a generic LPT cascade consisting of Pratt & Whitney 'Pak B' shaped blades. This facility was constructed under a previous one-year NASA Glenn RC initiative. The center blade in the cascade is instrumented to measure the surface pressure coefficient distribution. Optical access allows two-component LDV measurement for boundary layer profiles. Experimental conditions have been chosen to give a range of chord Reynolds numbers from 10 to 100K, and a range of free-stream turbulence levels from u'/U(sub infinity)= 0.08 to 3 percent. The surface pressure measurements were used to define a region of separation and reattachment that depend on the free-stream conditions. The location of separation was found to be relatively insensitive to the experimental conditions. However, reattachment location was very sensitive to the turbulence level and Reynolds number. Excellent agreement was found between the measured pressure distributions and predictions from Euler and RANS simulations. Two-component LDV measurements are presently underway to document the mean and fluctuating velocity components in the boundary layer over the center blade for the range of experimental conditions. The fabrication of the plasma actuator is underway. These are designed to produce either streamwise vortices, or a downstream-directed wall jet. A precursor experiment for the former approach was performed with an array of vortex generators placed just upstream of the separation line. These led to reattachment except for the lowest Reynolds number. Progress has also been made on the proposed concept for a laterally moving wake. This involved constructing a smaller wind tunnel and molding an array of symmetric airfoils to form an array. Following its development, it will be scaled up and used to introduce lateral moving wakes upstream up the Pak-B cascade.

Ashpis, David↗

NASA NTRS2003

On the Physics of Flow Separation Along a Low Pressure Turbine Blade Under Unsteady Flow Conditions

The present study, which is the first of a series of investigations dealing with specific issues of low pressure turbine (LPT) boundary layer aerodynamics, is aimed at providing detailed unsteady boundary flow information to understand the underlying physics of the inception, onset, and extent of the separation zone. A detailed experimental study on the behavior of the separation zone on the suction surface of a highly loaded LPT-blade under periodic unsteady wake flow is presented. Experimental investigations were performed at Texas A&M Turbomachinery Performance and Flow Research Laboratory using a large-scale unsteady turbine cascade research facility with an integrated wake generator and test section unit. To account for a high flow deflection of LPT-cascades at design and off-design operating points, the entire wake generator and test section unit including the traversing system is designed to allow a precise angle adjustment of the cascade relative to the incoming flow. This is done by a hydraulic platform, which simultaneously lifts and rotates the wake generator and test section unit. The unit is then attached to the tunnel exit nozzle with an angular accuracy of better than 0.05 , which is measured electronically. Utilizing a Reynolds number of 110,000 based on the blade suction surface length and the exit velocity, one steady and two different unsteady inlet flowconditions with the corresponding passing frequencies, wake velocities and turbulence intensities are investigated using hot-wire anemometry. In addition to the unsteady boundary layer measurements, blade surface pressure measurements were performed at Re=50,000, 75,000, 100,000, and 125,000 at one steady and two periodic unsteady inlet flow conditions. Detailed unsteady boundary layer measurement identifies the onset and extent of the separation zone as well as its behavior under unsteady wake flow. The results presented in ensemble-averaged and contour plot forms contribute to understanding the physics of the separation phenomenon under periodic unsteady wake flow. Several physical mechanisms are discussed.

Schobeiri, Meinhard T.↗

NASA NTRS2003

Stage Separation Wind Tunnel Tests of a Generic Two-Stage-to-Orbit Launch Vehicle

In support of NASA s Space Launch Initiative Program, stage separation wind tunnel tests of a generic two-stage-to-orbit (TSTO) launch vehicle were conducted to determine the interference aerodynamic forces and moments and to determine the proximity flow environment. The tests were conducted in the NASA Marshall Space Flight Center s Aerodynamic Research Facility using a manual separation fixture for a Mach number range of 2.74 to 4.96 and separation distances up to 80 percent and 35 percent of the body length in the vehicle X and Z coordinates, respectively. For the TSTO bimese, winged-body vehicle configuration, both wing-to-wing and wing-to-fuselage configurations were tested. Individual-body force and moment, schlieren, and surface pressure data were acquired. The results showed that the proximity aerodynamics were dominated by complex bow shock interactions, and that he booster was statically unstable at several separation positions. As compared to the isolated body, the proximity normal force change with pitch angle was found to be nearly the same, and the proximity axial force increased, in general, by 3% for both bodies.

Bordelon, Wayne J., Jr.↗

NASA NTRS2003

Design and Demonstration of Bolt Retractor Separation System for X-38 Deorbit Propulsion Stage

A separation system was designed for the X-38 experimental crew return vehicle program to allow the Deorbit Propulsion Stage (DPS) to separate from the X-38 lifting body during reentry operations. The configuration chosen was a spring-loaded plunger, known as the Bolt Retractor Subsystem (BRS), that retracts each of the six DPS-to-lifting body attachment bolts across the interface plane after being triggered by a separation nut mechanism. The system was designed to function on the ground in an atmospheric environment as well as in space. The BRS provides the same functionality as that of a completely pyrotechnic shear separation system that would normally be considered ideal for this application, but at a much lower cost. This system also could potentially be applied to future space station crew return vehicles. The design goal of 40 ms retraction time was successfully met in a series of demonstrations performed at the NASA Marshall Space Flight Center s Pyrotechnic Shock Facility (PSF) and Flight Robotics Laboratory (FRL). It must be emphasized that a full-scale test series was not performed on the BRS due to program schedule and cost constraints.

Ahmed, Raf↗

NASA NTRS2003

Analysis of Extensive Cross-Flow Separation using Higher-Order RANS Closure Models

The turbulent flow fields associated with the incompressible flow over a 6:1 prolate spheroid at high angle of attack, and the supersonic flow over an ogive cylinder are studied. Both these flows are characterized by large separation and vortical flow regions and therefore provide a challenging database for comparison of turbulent closure models. Of interest is the ability to predict the effects of separation and associated vortical motion common to both flows. Two turbulent models are investigated that each represent the class of linear eddy-viscosity models (LEVMs) and explicit algebraic stress models (EASMs). Since the EASM accounts for anisotropic effects, the in- fluence of these effects on flow field predictions can be assessed. The EASM model is shown to both improve the separation location prediction and pressure trough under the secondary vortex on the 6:1 prolate spheroid at high angle of attack and high Reynolds number, and improve the prediction of the separation location on a supersonic ogive cylinder.

Morison, J. H.↗

NASA NTRS2003

Analysis of Extensive Cross-Flow Separation using Higher-Order RANS Closure Models

The turbulent flow fields associated with the incompressible flow over a 6:1 prolate spheroid at high angle of attack, and the supersonic flow over an ogive cylinder are studied. Both these flows are characterized by large separation and vortical flow regions and therefore provide a challenging database for comparison of turbulent closure models. Of interest is the ability to predict the effects of separation and associated vortical motion common to both flows. Two turbulent models are investigated that each represent the class of linear eddy-viscosity models (LEVMs) and explicit algebraic stress models (EASMs). Since the EASM accounts for anisotropic effects, the influence of these effects on flow field predictions can be assessed. The EASM model is shown to both improve the separation location prediction and pressure trough under the secondary vortex on the 6:l prolate spheroid at high angle of attack and high Reynolds number, and improve the prediction of the separation location on a supersonic ogive cylinder.

Morrison, J. H.↗

NASA NTRS2004

Effects of Elevated Free-Stream Turbulence on Active Control of a Separation Bubble

The Effects of elevated free-stream turbulence (FST) on the natural and periodically excited separation bubbles were studied experimentally, due to the relevance of this flow to low-pressure turbine blades at low Reynolds numbers. A bubble was formed at the leading edge of a flat plate and the FST level was altered by placing a grid across the flow at different locations upstream of the plate. The mixing across the separated shear-layer, forming the free boundary of the bubble, increased due to the elevated FST and due to nominally two-dimensional periodic excitation, both flattening and shortening the bubble. Periodic excitation at frequencies that were at least an order of magnitude lower than those associated with the initial shear-layer instability, were very effective at low FST, because the amplitudes of the excitation frequency and its harmonic were amplified over the bubble. High frequency excitation (F+ 3, based on the length of the baseline low FST bubble) had a major effect close to the separation location, while farther downstream the excited fluctuations rapidly decayed in the reattachment region. Low frequency excitation, that generated waves comparable to the length of the unperturbed bubble (F+ 1) were less effective and their magnitude decayed at a slower rate downstream of reattachment. An increase in the level of the FST reduced the net effect of the periodic excitation on the mixing enhancement and subsequent reattachment process, probably due to a destructive interference between the nominally 2D excitation and the random (in space and time) FST, reducing the spanwise coherence and therefore the effectiveness of the current control strategy. However, even at the reduced effectiveness of 2D periodic excitation at elevated FST, it accelerated the reattachment process and the recovery rate of the reattached boundary layer, enhancing the boundary layer resistance to repeat separation and reducing its momentum loss further downstream.

Ashpis, D.↗

NASA NTRS2004

Control and Identification of Turbulent Boundary Layer Separation

Effective delay of turbulent boundary layer separation could be achieved via closed-loop control. Constructing such a system requires that sensor data be processed, real-time, and fed into the controller to determine the output. Current methods for detection of turbulent boundary layer separation are lacking the capability of localized, fast and reliable identification of the boundary layer state. A method is proposed for short-time FFT processing of time series, measured by hot-film sensors, with the purpose of identifying the alternation of the balance between small and large scales as the boundary layer separates, favoring the large scales. The method has been validated by comparison to other criteria of separation detection and over a range of baseline and controlled flow conditions on a simplified high-lift system, incorporating active flow control.

Seifert, Avi↗

NASA NTRS1994

Responses of soybean to ammonium and nitrate supplied in combination to the whole root system or separately in a split-root system

To address the questions of whether allocation of carbohydrates to roots is influenced by ionic form of nitrogen absorbed and whether allocation of carbohydrates to roots in turn influences proportionality between NH4+ and NO3- uptake from mixed sources, NH4+ and NO3- were supplied separately to halves of a split-root hydroponic system and were supplied in combination to a whole-root system. Dry matter accumulation in the split-root system was 18% less in the NH4(+)-fed axis than in the NO3(-)-fed axis. This, however, does not indicate that partitioning of carbohydrate between the two axes was different. Most of the reduction in dry matter accumulation in the NH4(+)-fed axis can be accounted for by the retransport of CH2O equivalents from the root back to the shoot with amino acids produced by NH4+ assimilation. Uptake of NH4+ or NO3- by the respective halves of the split-root system was proportional to the estimated allocation of carbohydrate to that half. When NH4+ and NO3- were supplied to separate halves of the split-root system, the cumulative NH4+ to NO3- uptake ratio was 0.81. When supplied in combination to the whole-root system, the cumulative NH4+ to NO3- uptake ratio was 1.67. Thus, while the shoot may affect total nitrogen uptake through the export of carbohydrates to roots, the shoot (common for halves of the split-root system) apparently does not exert a direct effect on proportionality of NH4+ and NO3- uptake by roots. For whole roots supplied with both NH4+ and NO3-, the restriction in uptake of NO3- may involve a stimulation of NO3- efflux rather than an inhibition of NO3- influx. While only the net uptake of NH4+ and NO3- was measured by ion chromatography, monitoring at approximately hourly intervals during the first 3 days of treatment revealed irregularly occurring intervals of both depletion (net influx) and enrichment (net efflux) in solutions. In the case of NH4+, numbers of net efflux events were similar (21 to 24 out of 65 sequential sampling intervals) whether NH4+ was supplied with NO3- to whole-root systems or separately to an axis of the split-root system. In the case of NO3-, however, the number of net efflux events increased from 8 when NO3- was supplied to a separate axis of the split-root system to between 19 and 24 when NO3- was supplied with NH4+ to whole-root systems.

Non-NASA Center↗