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NASA NTRS1992

Instability and transition in three-dimensional supersonic boundary layers

Linear stability of three-dimensional flow past a 5 deg semi-vertex sharp cone at 2 deg incidence in Mach 3.5 free-stream is studied. Computed laminar mean flow profiles along the leeward line of symmetry are highly inflectional and, hence, much more unstable than the profiles along the windward line of symmetry. Accordingly, transition occurs much earlier along the leeward line both in the present calculations (based upon eN approach) and in the experiments of King (1991) performed in NASA Langley 'quiet' tunnel. Boundary-layer transition in the region between the two symmetry lines is influenced by the crossflow generated due to the angle of attack and, as a consequence, it progressively moves upstream with increasing azimuthal angle measured from the windward line. The computed transition front is qualitatively similar to the experimental results, although some differences remain.

Malik, M. R.↗

NASA NTRS2019

Laminar-Turbulent Transition Upstream of the Entropy-Layer Swallowing Location in Hypersonic Boundary Layers

Numerical and experimental studies have demonstrated that modal growth of planar Mack modes is responsible for laminar-turbulent transition on sharp cones at hypersonic speeds. However, the physical mechanisms that lead to transition onset upstream of the entropy-layer swallowing location over sufficiently blunt geometries are not well understood as yet. Modal amplification is too weak or nonexistent to initiate transition at moderate-to-large bluntness values. Nonmodal analysis shows that, with increasing nose bluntness, both planar and oblique traveling disturbances that peak within the entropy layer experience appreciable energy amplification. However, because of the relatively weak signature of the nonmodal traveling disturbances within the boundary-layer region, the route to transition onset subsequent to the nonmodal growth remains unclear. Thus, nonlinear parabolized stability equations (NPSE) and direct numerical simulations (DNS) have been used to investigate the potential transition mechanisms over a 7-degree blunt cone that was tested in the AFRL Mach-6 high-Reynoldsnumber facility. Computations are performed to separately follow the nonlinear development of two classes of inflow disturbances, namely, a pair of oblique traveling waves with equal but opposite angles with respect to the mean flow direction and a planar traveling wave. Results in both cases show an excellent agreement between the NPSE and DNS predictions, establishing that the NPSE is an accurate and efficient technique for predicting the nonlinear development for these particular nonmodal traveling disturbances. Computations reveal that the oblique mode interactions lead to the generation of stationary streaks inside the boundary layer that, in turn, facilitate the growth of a subharmonic sinuous disturbance. For relatively modest amplitudes of the inflow disturbance, the oblique-mode breakdown can lead to transition at the measured location of transition onset during the experiment. On the other hand, the nonlinear development of a planar traveling wave leads to the formation of inclined structures just above the boundary-layer edge and these structures are strongly reminiscent of the transitional events observed during blunt cone experiments by using schlieren flow visualizations.

Entropy↗

NASA NTRS1997

A Review of Hypersonic Boundary Layer Stability Experiments in a Quiet Mach 6 Wind Tunnel

Three recent experimental studies of transition on cones with adverse pressure gradient produced by a flared afterbody and with the additive stability modifiers of wall cooling, angle of attack and bluntness are reviewed. All tests were conducted in a quiet Mach 6 wind tunnel. The dominant instability was found to be the second mode. For the cases examined with linear stability theory, the N factors at mode saturation were in the range of 8.5 to 11. Evidence of a combined second-mode/Gortler transition process was found. Mean, rms and spectral freestream data for the quiet facility is presented and the role of low frequency freestream noise is discussed.

Wilkinson, Stephen P.↗

NASA NTRS2006

Multiphase Flow Technology Impacts on Thermal Control Systems for Exploration

The Two-Phase Flow Facility (TPHIFFy) Project focused on bridging the critical knowledge gap by developing and demonstrating critical multiphase fluid products for advanced life support, thermal management and power conversion systems that are required to enable the Vision for Space Exploration. Safety and reliability of future systems will be enhanced by addressing critical microgravity fluid physics issues associated with flow boiling, condensation, phase separation, and system stability. The project included concept development, normal gravity testing, and reduced gravity aircraft flight campaigns, in preparation for the development of a space flight experiment implementation. Data will be utilized to develop predictive models that could be used for system design and operation. A single fluid, two-phase closed thermodynamic loop test bed was designed, assembled and tested. The major components in this test bed include: a boiler, a condenser, a phase separator and a circulating pump. The test loop was instrumented with flow meters, thermocouples, pressure transducers and both high speed and normal speed video cameras. A low boiling point surrogate fluid, FC-72, was selected based on scaling analyses using preliminary designs for operational systems. Preliminary results are presented which include flow regime transitions and some observations regarding system stability.

McQuillen, John↗

NASA NTRS1991

Compressible stability of growing boundary layers using parabolized stability equations

The parabolized stability equation (PSE) approach is employed to study linear and nonlinear compressible stability with an eye to providing a capability for boundary-layer transition prediction in both 'quiet' and 'disturbed' environments. The governing compressible stability equations are solved by a rational parabolizing approximation in the streamwise direction. Nonparallel flow effects are studied for both the first- and second-mode disturbances. For oblique waves of the first-mode type, the departure from the parallel results is more pronounced as compared to that for the two-dimensional waves. Results for the Mach 4.5 case show that flow nonparallelism has more influence on the first mode than on the second. The disturbance growth rate is shown to be a strong function of the wall-normal distance due to either flow nonparallelism or nonlinear interactions. The subharmonic and fundamental types of breakdown are found to be similar to the ones in incompressible boundary layers.

Chang, Chau-Lyan↗

NASA NTRS1993

Convective Effects During the Physical Vapor Transport Process. II - Thermosolutal Convection

The effect of an inert gas on the diffusive-convective physical vapor transport process is investigated for the case when the temperature gradient is stabilizing, and the concentration gradient destabilizing, for a wide parametric range. When an inert gas is present, the thermal and solutal convection oppose each other. The solutal field is destabilizing while the thermal field and the advective-diffusive flux stabilize the flow field. When the pressure of the inert component is increased, the stabilizing effect of the advective-diffusive flux is decreased; thus, convection becomes more vigorous. The nonlinear dynamics of the flow field here show a transition from quasi-periodic to chaotic state. When both stabilizing mechanisms are present, the flow field shows a transition to a steady state. Toward steady state, growth and amalgamation of rolls occur, which result in an overturning motion. This leads to a superposed flow consisting of one roll and a unidirectional flow. However, when the pressure is increased, the advective-diffusive stability mechanism is decreased. and oscillations of the flow field occur. The low gravity environment is effective at eliminating oscillatory behavior of the flow field and results in uniform temperature and concentration gradients.

Duval, Walter M. B.↗

NASA NTRS1993

Transition prediction and control in subsonic flow over a hump

The influence of a surface roughness element in the form of a two-dimensional hump on the transition location in a two-dimensional subsonic flow with a free-stream Mach number up to 0.8 is evaluated. Linear stability theory, coupled with the N-factor transition criterion, is used in the evaluation. The mean flow over the hump is calculated by solving the interacting boundary-layer equations; the viscous-inviscid coupling is taken into consideration, and the flow is solved within the separation bubble. The effects of hump height, length, location, and shape; unit Reynolds number; free-stream Mach number, continuous suction level; location of a suction strip; continuous cooling level; and location of a heating strip on the transition location are evaluated. The N-factor criterion predictions agree well with the experimental correlation of Fage; in addition, the N-factor criterion is more general and powerful than experimental correlations. The theoretically predicted effects of the hump's parameters and flow conditions on transition location are consistent and in agreement with both wind-tunnel and flight observations.

Masad, Jamal A.↗

NASA NTRS1993

High speed transition prediction

The main objective of this work period was to develop, maintain and exercise state-of-the-art methods for transition prediction in supersonic flow fields. Basic state and stability codes, acquired during the last work period, were exercised and applied to calculate the properties of various flowfields. The development of a code for the prediction of transition location using a currently novel method (the PSE or Parabolized Stability Equation method), initiated during the last work period and continued during the present work period, was cancelled at mid-year for budgetary reasons. Other activities during this period included the presentation of a paper at the APS meeting in Tallahassee, Florida entitled 'Stability of Two-Dimensional Compressible Boundary Layers', as well as the initiation of a paper co-authored with H. Reed of the Arizona State University entitled 'Stability of Boundary Layers'.

Gasperas, Gediminis↗

NASA NTRS1962

Experimental Research on 4-Duct Tandem VTOL Aircraft Configurations

This paper presents a brief summary of several wind-tunnel investigations conducted at the Langley Research Center of the NASA to study the aerodynamic and stability and control characteristics of several VTOL aircraft configurations powered by four tilting ducted propellers arranged in tandem pairs. Specifically the two rear ducts could be mounted close alongside the upper rear portion of the fuselage with small wing panels attached to the outboard side of the ducts or could be mounted outboard on the tips of a small wing located high on the rear portion of the fuselage. The two front ducts were always mounted close inboard on the forward part of the fuselage and could be mounted either in a high or low position on the fuselage. The results of the investigation indicated that aircraft of this type could have acceptable aerodynamic and static longitudinal and lateral stability and control characteristics in both transition and normal cruise flight except for the possible qualification that the lateral force due to sideslip is abnormally high and might cause the aircraft to be too sensitive to side gusts.

VTOL AIRCRAFT↗

NASA NTRS1972

Evaluation of tantalum 316 stainless steel transition joints

Tubular transition joints providing a metallurgically bonded connection between tantalum and 316 stainless steel pipe sections were comparatively evaluated for durability under thermal cycling conditions approximating the operation of a SNAP-8 mercury boiler. Both coextruded and vacuum brazed transition joints of 50mm (2 inch) diameter were tested by thermal cycling 100 times between 730 C and 120 C(1350 F and 250 F) in a high vacuum environment. The twelve evaluated transition joints survived the full test sequence without developing leaks, although liquid penetrant bond line indications eventually developed in all specimens. The brazed transition joints exhibited the best dimensional stability and bond line durability.

Stoner, D. R.↗

NASA NTRS1979

Synthesis of aromatic secondary diamines

A series of N-methyl substituted aromatic polyamides derived from the secondary aromatic diamines, 4,4'-bis(methylamino)diphenylmethane, 3,3'-bis(methylamino) diphenylmethane, 4,4'-bis(methylamino)benzophenone or 3,3'-bis(methylamino)benzophenone and isophthaloyl dichloride, terphthaloyl dichloride or 3,3'diphenylmethane dicarboxylic acid dichloride was prepared by high temperature solution polymerization in s-tetrachloroethane. Compared to analogous unsubstituted and partially N-methylated aromatic polyamides, the full N-methylated polyamides exhibited significantly lower glass transition temperatures, reduced crystallinity, improved thermal stability and good solubility in chlorinated solvents.

Wolfe, J. F.↗

NASA NTRS1980

Wavelength selection and total growth of Gortler vortices

This paper investigates a wavelength selection mechanism under ideal and nonideal flow conditions. The conditions of the total streamwise growth of Gortler vortices generated under ideal and nonideal flow conditions and its effect on the laminar-turbulent transition process is also examined. The stabilizing effects of suction on the total growth of these vortices are analyzed. It is demonstrated that the wavelength selection mechanism and the total growth of the vortices are affected by the departures from ideal flow conditions. It is shown that under ideal flow conditions, a wavelength selection mechanism can be based on the maximum amplification rate of the disturbances. However, this mechanism is easily affected by the departures from ideal flow. The case of streamwise vorticity in the basic flow is analyzed and is shown to affect disturbances through a subharmonic resonance.

Floryan, J. M.↗

NASA NTRS1981

Natural laminar flow data from full-scale flight and wind-tunnel experiments

Experimental results obtained at NASA Langley during studies of natural laminar flow (NLF) over commercially produced aircraft surfaces are reported. The general aviation aircraft examined were light aircraft, yet displayed NLF extents close to the maximum available and equivalent to high performance business aircraft flying envelopes. Sublimating chemicals and acoustic detection techniques were employed to measure the boundary layer transition. Theoretical predictions of boundary layer stability were found to match well with the experimental data, with consideration given to both swept wings and the amplitudes of allowable waves on the airfoil surfaces. The presence of the NLF on the airfoil surfaces confirmed the benefits available from use of composite materials for airfoil surfaces.

Holmes, B. J.↗

NASA NTRS1983

Cobalt ion-containing epoxies

Varying concentrations of an organometallic cobalt complex were added to an epoxy system currently used by the aerospace industry as a composite matrix resin. Methods for combining cobalt (III) acetylacetonate with a tetraglycidyl 4,4 prime - diaminodiphenylmethane-based epoxy were investigated. The effects of increasing cobalt ion concentration on the epoxy cure were demonstrated by epoxy gel times and differential scanning calorimetry cure exotherms. Analysis on cured cobalt-containing epoxy castings included determination of glass transition temperatures by thermomechanical analysis, thermooxidative stabilities by thermogravimetric analysis, and densities in a density gradient column. Flexural strength and stiffness were also measured on the neat resin castings.

Stoakley, D. M.↗

NASA NTRS1989

Condensation polyimides

Polyimides belong to a class of polymers known as polyheterocyclics. Unlike most other high temperature polymers, polyimides can be prepared from a variety of inexpensive monomers by several synthetic routes. The glass transition and crystalline melt temperature, thermooxidative stability, toughness, dielectric constant, coefficient of thermal expansion, chemical stability, mechanical performance, etc. of polyimides can be controlled within certain boundaries. This versatility has permitted the development of various forms of polyimides. These include adhesives, composite matrices, coatings, films, moldings, fibers, foams and membranes. Polyimides are synthesized through both condensation (step-polymerization) and addition (chain growth polymerization) routes. The precursor materials used in addition polyimides or imide oligomers are prepared by condensation method. High molecular weight polyimide made via polycondensation or step-growth polymerization is studied. The various synthetic routes to condensation polyimides, structure/property relationships of condensation polyimides and composite properties of condensation polyimides are all studied. The focus is on the synthesis and chemical structure/property relationships of polyimides with particular emphasis on materials for composite application.

Hergenrother, P. M.↗

NASA NTRS1991

Wettability of Pyrolytic Boron Nitride by Aluminum

The wetting of pyrolytic boron nitride by molten 99.9999 percent pure aluminum was investigated by using the sessile drop method in a vacuum operating at approximately 660 micro-Pa at temperatures ranging from 700 to 1000 C. The equilibrium contact angle decreased with an increase in temperature. For temperatures at 900 C or less, the equilibrium contact angle was greater than 90 deg. At 1000 C a nonwetting-to-wetting transition occurred and the contact angle stabilized at 49 deg.

Chiaramonte, Francis P.↗

NASA NTRS1990

Thermally stable, low dielectric polyquinolines for aerospace and electronics applications

Four new high molecular weight, linear chain polyquinolines have been synthesized and fabricated into high quality free standing films. These polymers are characterized by moderate to high glass transition temperatures, excellent thermal and thermooxidative stability, extremely low dielectric constants and good planarizing characteristics. The polymers absorb very low quantities of moisture. As a consequence, the dielectric constant of one new polyquinoline has been shown to be quite insensitive to exposure to warm/wet conditions. Isothermal aging of one new derivative in air has been carried out at elevated temperatures (250 C to 345 C). The results demonstrate truly outstanding thermooxidative stability. Additional characterizations include molecular weight determinations, solubilities and film-forming characteristics, density measurements, and UV-Vis spectroscopy. The data acquired to date suggest that the polymers may find use as refractive films and coatings and as interlevel planarizers in microelectronics applications.

Hendricks, Neil H.↗

NASA NTRS1991

Multiple-mode reconfigurable electro-optic switching network for optical fiber sensor array

This paper reports the first switching network compatible with multimode fibers. A one-to-many cascaded reconfigurable interconnection was built. A thin glass substrate was used as the guiding medium which provides not only higher coupling efficiency from multimode fiber to waveguide but also better tolerance of phase-matching conditions. Involvement of a total-internal-reflection hologram and multimode waveguide eliminates interface problems between fibers and waveguides. The DCG polymer graft has proven to be reliable from -180 C to +200 C. Survivability of such an electrooptic system in harsh environments is further ensured. LiNbO3 was chosen as the E-O material because of its stability at high temperatures (phase-transition temperature of more than 1000 C) and maturity of E-O device technology. Further theoretical calculation was conducted to provide the optimal interaction length and device capacitance.

Chen, Ray T.↗