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At least 721 records · Page 40
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

Instabilities orginating from suction holes used for Laminar Flow Control (LFC)

A small-scale wind tunnel previously used for turbulent boundary layer studies has been modified for experiments in laminar flow control. The facility incorporates suction through interchangeable porous test surfaces which are used to stabilize the boundary layer and delay transition to turbulent flow. The thin porous test surfaces are supported by a baffled plenum chamber box which also acts to gather the flow through the surface into tubes which are routed to a high pressure fan. An elliptic leading edge is attached to the assembly to establish a new layer on the test plate. A slot is used to remove the test section flow below the leading edge. The test section was lengthened and fitted with a new ceiling. Substantial modifications were also made to the 3D probe traverse. Detailed studies have been made using isolated holes to explore the underlying instability mechanisms. The suction is perturbed harmonically and data are averaged on the basis of the phase of the disturbance. Conditions corresponding to strong suction and without suction have been studied. In both cases, 3D contour surfaces in the vicinity of the hole show highly three-dimensional T-S waves that fan out away from the hole with streamwise distance. With suction, the perturbations on the centerline are much stronger and decay less rapidly, while the far field is similar to the case without suction. Downstream the contour surfaces of the bow-shaped TS waves develop spanwise irregularities which eventually form into clumps. The contours remain smooth when suction is not applied. Even without suction, the harmonic point source is challenging for CFD; e.g. DNS has been used for streamwise growth. With suction, grid resources are consumed by the hole and this makes DNS even more expensive. Limited DNS results so far indicate that the vortices which emanate from suction holes appear to be stable. The spanwise clumping observed in the experiment is evidence of a secondary instability that could be associated with suction vortices. A typical porous surface for LFC consists of 0.002 inch diameter holes with 0.020 inch grid spacing L, which is too small to resolve disturbances. A 20:1 scale porous test surface has been machined for improved spatial resolution while the L/d is still representative of flight conditions. Designers of porous surfaces use Goldsmith's criterion to minimize crossstream interaction. However nothing is known about the streamwise interactions. Results using two holes, aligned but displaced in the streamwise direction, indicate that partial TS wave cancellation is possible, depending on the hole spacing and disturbance frequency. Using DNS for streamwise interaction studies will be prohibitively expensive if linear superposition cannot be used for the multiple holes.

Watmuff, Jonathan H.↗
NASA NTRS1996

Effects of Microgravity on Quail Eye Development

During embryonic development, the most exposed tissue of the eye, the cornea, becomes differentially bulged outward because of constant intraocular pressure (IOP). The component cells of the cornea secrete a unique, paracrystalline extracellular matrix (the stroma) composed of orthogonal plies of collagen fibrils and proteoglycans. The cornea remains avascular, becomes transparent, and becomes more densely innervated than any other region on the surface of the body. Corneas from chicken embryos that flew on STS-47 contain many more cellular processes in the outermost region of the stroma (Bowman's Layer) than any corresponding region of control corneas. These processes appear to be cross-sections of cytoplasmic extensions of cells and are found in that region of Bowman's Layer immediately beneath the basal lamina of the corneal epithelium. Here, we propose to compare corneas of quail that flew in space on Mir-1 with those of ground controls to determine if the same unusual cellular processes are seen as in the space-flown chicken corneas. In the central regions of such space-flown corneas, the processes appear to be either portions of basal epithelial cells whose pseudopodial extensions have migrated down through their own basal lamina into the stroma, or corneal nerves that have innervated the corneal stroma in an unusual manner. Eyeballs of embryos fixed on Mir-1, control embryos fixed at KSC and clinostated embryos fixed at KSU, will provide corneas for this study. Electron microscopy will be used to assess the distribution of the cellular processes in Bowman's Layer in the central region of each cornea. Attempts also will be made to determine the relative glycosaminoglycan distributions in the corneal stromas by indirect immunofluorscence and to record whole-mount staining patterns of the corneal nerves.

Conrad, Gary W.↗
NASA NTRSDate not supplied

Structure and Dynamics of Bleed-Controlled Impinging Shock/Turbulent-Boundary-Layer Interactions

Porous boundary-layer bleed is often employed to alleviate the detrimental effects of shock/turbulent-boundary-layer interactions (STBLI) in high-speed vehicles. However, outstanding questions remain on the underlying mechanisms through which bleed influences STBLI dynamics. This work addresses this knowledge gap by performing a scale-resolved Large-Eddy Simulation of an impinging STBLI controlled by discrete bleed holes. The simulation has a freestream Mach number of 2.5, an oblique shock generator angle of 8 degrees, and a Reynolds number based on the incoming momentum thickness of 2800. Bleed substantially reduces the separation size and upstream influence of the STBLI compared to the canonical, uncontrolled case. Consequently, the characteristic, low-frequency motion of STBLI-induced separation is mitigated, as revealed by wall-pressure spectra. Energy is shifted from the low frequency separation behavior to a higher-frequency signature caused by the bleed shocks and expansions. The scale-resolved simulations also reveal that these small, localized structures severely limit the turbulence production traditionally seen in the uncontrolled interaction.

computational fluid dynamics↗
NASA NTRS1993

Drag reduction in channel flow using nonlinear control

Two nonlinear control schemes have been applied to the problem of drag reduction in channel flow. Both schemes have been tested using numerical simulations at a mass flux Reynolds numbers of 4408, utilizing 2D nonlinear neutral modes for goal dynamics. The OGY-method, which requires feedback, reduces drag to 60-80 percent of the turbulent value at the same Reynolds number, and employs forcing only within a thin region near the wall. The H-method, or model-based control, fails to achieve any drag reduction when starting from a fully turbulent initial condition, but shows potential for suppressing or retarding laminar-to-turbulent transition by imposing instead a transition to a low drag, nonlinear traveling wave solution to the Navier-Stokes equation. The drag in this state corresponds to that achieved by the OGY-method. Model-based control requires no feedback, but in experiments to date has required the forcing be imposed within a thicker layer than the OGY-method. Control energy expenditures in both methods are small, representing less than 0.1 percent of the uncontrolled flow's energy.

Keefe, Laurence R.↗
NASA NTRS2006

Non-Uniform Thickness Electroactive Device

An electroactive device comprises at least two layers of material, wherein at least one layer is an electroactive material and wherein at least one layer is of non-uniform thickness. The device can be produced in various sizes, ranging from large structural actuators to microscale or nanoscale devices. The applied voltage to the device in combination with the non-uniform thickness of at least one of the layers (electroactive and/or non-electroactive) controls the contour of the actuated device. The effective electric field is a mathematical function of the local layer thickness. Therefore, the local strain and the local bending/ torsion curvature are also a mathematical function of the local thickness. Hence the thinnest portion of the actuator offers the largest bending and/or torsion response. Tailoring of the layer thicknesses can enable complex motions to be achieved.

Su, Ji↗
NASA NTRS1987

Wind-tunnel investigation of a full-scale general aviation airplane equipped with an advanced natural laminar flow wing

An investigation was conducted in the Langley 30- by 60-Foot Wind Tunnel to evaluate the performance, stability, and control characteristics of a full-scale general aviation airplane equipped with an advanced laminar flow wing. The study focused on the effects of natural laminar flow and advanced boundary layer transition on performance, stability, and control, and also on the effects of several wing leading edge modifications on the stall/departure resistance of the configuration. Data were measured over an angle-of-attack range from -6 to 40 deg and an angle-of-sideslip range from -6 to 20 deg. The Reynolds number was varied from 1.4 to 2.4 x 10 to the 6th power based on the mean aerodynamic chord. Additional measurements were made using hot-film and sublimating chemical techniques to determine the condition of the wing boundary layer, and wool tufts were used to study the wing stall characteristics. The investigation showed that large regions of natural laminar flow existed on the wing which would significantly enhance cruise performance. Also, because of the characteristics of the airfoil section, artificially tripping the wing boundary layer to a turbulent condition did not significantly effect the lift, stability, and control characteristics. The addition of a leading-edge droop arrangement was found to increase the stall angle of attack at the wingtips and, therefore, was considered to be effective in improving the stall/departure resistance of the configuration. Also the addition of the droop arrangement resulted in only minor increases in drag.

Murri, Daniel G.↗
NASA NTRS1987

Execution environment for intelligent real-time control systems

Modern telerobot control technology requires the integration of symbolic and non-symbolic programming techniques, different models of parallel computations, and various programming paradigms. The Multigraph Architecture, which has been developed for the implementation of intelligent real-time control systems is described. The layered architecture includes specific computational models, integrated execution environment and various high-level tools. A special feature of the architecture is the tight coupling between the symbolic and non-symbolic computations. It supports not only a data interface, but also the integration of the control structures in a parallel computing environment.

Sztipanovits, Janos↗
NASA NTRSDate not supplied

Embedding Differential Dynamic Logic in PVS

Runtime assurance is a control framework where a complex controller operates under the observation of a monitor. If the monitor detects the controller exhibiting undesirable behavior, control is passed off to a trusted controller until a desirable state is regained. The runtime assurance architecture provides a layer of assurance to the system being controlled, but special care must be taken that the resulting overall system, consisting of the monitors and controllers, is behaving as intended. This talk aims to formally model and reason about runtime assurance-equipped systems as hybrid programs- which are models that consist of both discrete and continuous components. Using the verification tool Plaidypvs, safety properties of some examples involving RTA architectures is shown.

Formal Verification↗
NASA NTRS1998

Correlation of Predicted and Observed Optical Properties of Multilayer Thermal Control Coatings

Thermal control coatings on spacecraft will be increasingly important, as spacecraft grow smaller and more compact. New thermal control coatings will be needed to meet the demanding requirements of next generation spacecraft. Computer programs are now available to design optical coatings and one such program was used to design several thermal control coatings consisting of alternating layers of WO3 and SiO2. The coatings were subsequently manufactured with electron beam evaporation and characterized with both optical and thermal techniques. Optical data were collected in both the visible region of the spectrum and the infrared. Predictions of solar absorptance and infrared emittance were successfully correlated to the observed thermal control properties. Functional performance of the coatings was verified in a bench top thermal vacuum chamber.

Jaworske, Donald A.↗
NASA NTRS1980

Continuous coating of silicon-on-ceramic

Growth of sheet silicon on low-cost substrates has been demonstrated by the silicon coating with inverted meniscus (SCIM) technique. A mullite-based ceramic substrate is coated with carbon and then passed over a trough of molten silicon with a raised meniscus. Solidification occurs at the trailing edge of the downstream meniscus, producing a silicon-on-ceramic (SOC) layer. Meniscus shape and stability are controlled by varying the level of molten silicon in a reservoir connected to the trough. The thermal conditions for growth and the crystallographic texture of the SOC layers are similar to those produced by dip-coating, the original technique of meniscus-controlled growth. The thermal conditions for growth have been analyzed in some detail. The analysis correctly predicts the velocity-thickness relationship and the liquid-solid interface shape for dip-coating, and appears to be equally applicable to SCIM-coating. Solar cells made from dip-coated SOC material have demonstrated efficiencies of 10% on 4-sq cm cells and 9.9% on 10-sq cm cells.

Heaps, J. D.↗
NASA NTRS1987

Structure-borne noise control for propeller aircraft

A laboratory test apparatus was developed which would allow the study and development of propeller wake/vortex-induced structure-borne interior noise control measures. Various methods of wing structural modification, including blocking masses, surface damping treatments, and tuned mechanical absorbers, were evaluated relative to reduced interior noise levels. Inboard wing fuel was found to act as an effective blocking mass. Wing panel add-on damping treatment in the form of a single, constrained layer was not an effective control measure, except in the area of the propeller wake. However, highly damped, tuned mechanical absorbers were found to be the most efficient structure-borne noise (SBN) control measure.

Unruh, James F.↗
NASA NTRS1995

Lagrangian mixed layer modeling of the western equatorial Pacific

Processes that control the upper ocean thermohaline structure in the western equatorial Pacific are examined using a Lagrangian mixed layer model. The one-dimensional bulk mixed layer model of Garwood (1977) is integrated along the trajectories derived from a nonlinear 1 1/2 layer reduced gravity model forced with actual wind fields. The Global Precipitation Climatology Project (GPCP) data are used to estimate surface freshwater fluxes for the mixed layer model. The wind stress data which forced the 1 1/2 layer model are used for the mixed layer model. The model was run for the period 1987-1988. This simple model is able to simulate the isothermal layer below the mixed layer in the western Pacific warm pool and its variation. The subduction mechanism hypothesized by Lukas and Lindstrom (1991) is evident in the model results. During periods of strong South Equatorial Current, the warm and salty mixed layer waters in the central Pacific are subducted below the fresh shallow mixed layer in the western Pacific. However, this subduction mechanism is not evident when upwelling Rossby waves reach the western equatorial Pacific or when a prominent deepening of the mixed layer occurs in the western equatorial Pacific or when a prominent deepening of the mixed layer occurs in the western equatorial Pacific due to episodes of strong wind and light precipitation associated with the El Nino-Southern Oscillation. Comparison of the results between the Lagrangian mixed layer model and a locally forced Eulerian mixed layer model indicated that horizontal advection of salty waters from the central Pacific strongly affects the upper ocean salinity variation in the western Pacific, and that this advection is necessary to maintain the upper ocean thermohaline structure in this region.

Shinoda, Toshiaki↗
NASA NTRS1980

Disturbance amplification rates

Program computes incompressible linear stability characteristics for swept and tapered wings. Amplification rates of boundary-layer disturbances also are calculated. Program is useful in designing tapered, laminar-flow control wings incorporating suction to prevent boundary layer separation. Program is written in FORTRAN IV and Assembler for batch execution on CYBER 70-series.

Srokowski, A. J.↗
NASA NTRS2000

Experimental Verification of Electric Drive Technologies Based on Artificial Intelligence Tools

In this report, a fully integrated prototype of a flight servo control system is successfully developed and implemented using brushless dc motors. The control system is developed by the fuzzy logic theory, and implemented with a multilayer neural network. First, a neural network-based architecture is introduced for fuzzy logic control. The characteristic rules and their membership functions of fuzzy systems are represented as the processing nodes in the neural network structure. The network structure and the parameter learning are performed simultaneously and online in the fuzzy-neural network system. The structure learning is based on the partition of input space. The parameter learning is based on the supervised gradient decent method, using a delta adaptation law. Using experimental setup, the performance of the proposed control system is evaluated under various operating conditions. Test results are presented and discussed in the report. The proposed learning control system has several advantages, namely, simple structure and learning capability, robustness and high tracking performance and few nodes at hidden layers. In comparison with the PI controller, the proposed fuzzy-neural network system can yield a better dynamic performance with shorter settling time, and without overshoot. Experimental results have shown that the proposed control system is adaptive and robust in responding to a wide range of operating conditions. In summary, the goal of this study is to design and implement-advanced servosystems to actuate control surfaces for flight vehicles, namely, aircraft and helicopters, missiles and interceptors, and mini- and micro-air vehicles.

Rubaai, Ahmed↗
NASA NTRS2001

Further Investigations of the Passive Optical Sample Assembly (POSA) - I Flight Experiment

The Passive Optical Sample Assembly-I (POSA-I), part of the Mir Environmental Effects Payload (MEEP), was designed to study the combined effects of contamination, atomic oxygen, ultraviolet radiation, vacuum, then-nal cycling, and other constituents of the space environment on spacecraft materials. The MEEP program is a Phase I International Space Station Risk Mitigation Experiment. Candidate materials for the International Space Station (ISS) were exposed in a specially designed "suitcase" carrier, with identical specimens facing either Mir or space. The payload was attached by EVA to the exterior of the Mir docking module during the STS-76 mission (f'ig. 1). It was removed during the STS-86 mission after an 18-month exposure. During the mission, it received approximately 7 x 1019 atoMS/CM2 atomic oxygen, as calculated by polymer mass loss, and 413 ESH of solar ultraviolet radiation on the Mir-facing side. The side facing away from Mir received significant contaminant deposition, so atomic oxygen fluence has not been reliably determined. The side facing away from Mir received 571 ESH of solar UV. Contamination was observed on both the Mir-facing and space-facing sides of the POSA-I experiment , with a greater amount of deposition on the space facing side than the Mir side. The contamination has been determined to be outgassed silicone photofixed by ultraviolet radiation and converted to silicate by atomic oxygen interaction. Electron spectroscopy for chemical analysis (ESCA) with depth profiling indicated the presence of 26 - 31 nm silicate on the Mir-facing side and 500 - 1000 nm silicate on the space-facing side. The depth profiling also showed that the contaminant layer was uniform, with a small amount of carbon present on the surface and trace amounts of nitrogen, phosphorus, sulfur, and tin. The surface carbon layer is likely due to post-flight exposure in the laboratory and is similar to carbonaceous deposits on control samples. EDAX and FTIR analysis concurred with ESCA for the presence of silicon, oxygen, and carbon. Nearly 400 samples were exposed on POSA-I, which included materials such as thermal control coatings polymeric films, optical materials, and multi-layer insulation blankets. A previous paper discussed the effects of the space environment exposure and contaminant deposition on candidate materials for ISS, including Z93P inorganic thermal control coating, various anodizes, and multi-layer insulation blankets. This paper details the investigation of environmental effects on the remainder of POSA-I samples, particularly the innovative conductive thermal control coatings developed by AZ Technology of Huntsville, AL and HT Research Institute of Chicago, IL. The silicone/silicate contamination had a significant impact on the solar absorptance of white inorganic thermal control coatings on the space-facing side of POSA-I. The effect of contamination on electrical conductivity is discussed. Samples of conductive anodized aluminum developed by Boundary Technologies of Buffalo Grove, IL were also exposed on POSA-I. The effects of the space environment and contaminant deposition on the optical and electrical properties of the conductive anodized aluminum are discussed.

Finckenor, Miria M.↗
NASA NTRS2011

Control of Stationary Cross-Flow Modes in a Mach 3.5 Boundary Layer Using Patterned Passive and Active Roughness

Spanwise-periodic roughness designed to excite selected wavelengths of stationary cross- ow modes was investigated in a 3-D boundary layer at Mach 3.5. The test model was a sharp-tipped 14deg right-circular cone. The model and integrated sensor traversing system were placed in the Mach 3.5 Supersonic Low Disturbance Tunnel (SLDT) equipped with a "quiet design" nozzle at the NASA Langley Research Center. The model was oriented at a 4:2deg angle of attack to produce a mean cross-fl ow velocity component in the boundary layer over the cone. Five removable cone tips have been investigated. One has a smooth surface that is used to document the baseline ("natural") conditions. Two had minute (20 - 40 micron) "dimples" that are equally spaced around the circumference, at a streamwise location that is just upstream of the linear stability neutral growth branch for cross- ow modes. The azimuthal mode numbers of the dimpled tips were selected to either enhance the most amplified wave numbers, or to suppress the growth of the most amplified wave numbers. Two of the cone tips had an array of plasma streamwise vortex generators that were designed to simulate the disturbances produced by the passive patterned roughness. The results indicate that the stationary cross-fl ow modes were highly receptive to the patterned roughness of both passive and active types. The patterned passive roughness that was designed to suppress the growth of the most amplified modes had an azimuthal wavelength that was 66% smaller that that of the most amplified stationary cross- ow mode. This had the effect to increase the transition Reynolds number from 25% to 50% depending on the measurement technique. The application of the research is on turbulent transition control on swept wings of supersonic aircraft. The plasma-based roughness has the advantage over the passive roughness of being able to be adaptable to different conditions that would occur during a flight mission.

Schuele, Chan Yong↗
NASA NTRS1997

Neural Generalized Predictive Control: A Newton-Raphson Implementation

An efficient implementation of Generalized Predictive Control using a multi-layer feedforward neural network as the plant's nonlinear model is presented. In using Newton-Raphson as the optimization algorithm, the number of iterations needed for convergence is significantly reduced from other techniques. The main cost of the Newton-Raphson algorithm is in the calculation of the Hessian, but even with this overhead the low iteration numbers make Newton-Raphson faster than other techniques and a viable algorithm for real-time control. This paper presents a detailed derivation of the Neural Generalized Predictive Control algorithm with Newton-Raphson as the minimization algorithm. Simulation results show convergence to a good solution within two iterations and timing data show that real-time control is possible. Comments about the algorithm's implementation are also included.

Soloway, Donald↗
NASA NTRS1986

Natural laminar flow and airplane stability and control

Location and mode of transition from laminar to turbulent boundary layer flow have a dominant effect on the aerodynamic characteristics of an airfoil section. The influences of these parameters on the sectional lift and drag characteristics of three airfoils are examined. Both analytical and experimental results demonstrate that when the boundary layer transitions near the leading edge as a result of surface roughness, extensive trailing-edge separation of the turbulent boundary layer may occur. If the airfoil has a relatively sharp leading-edge, leading-edge stall due to laminar separation can occur after the leading-edge suction peak is formed. These two-dimensional results are used to examine the effects of boundary layer transition behavior on airplane longitudinal and lateral-directional stability and control.

Vandam, Cornelis P.↗