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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

CFD Mixing Analysis of Jets Injected from Straight and Slanted Slots into Confined Crossflow in Rectangular Ducts

A CFD study was performed to analyze the mixing potential of opposed rows of staggered jets injected into confined crossflow in a rectangular duct. Three jet configurations were numerically tested: (1) straight (0 deg) slots; (2) perpendicular slanted (45 deg) slots angled in opposite directions on top and bottom walls; and (3) parallel slanted (45 deg) slots angled in the same direction on top and bottom walls. All three configurations were tested at slot spacing-to-duct height ratios (S/H) of 0.5, 0.75, and 1.0; a jet-to-mainstream momentum flux ratio (J) of 100; and a jet-to-mainstream mass flow ratio of 0.383. Each configuration had its best mixing performance at S/H of 0.75. Asymmetric flow patterns were expected and predicted for all slanted slot configurations. The parallel slanted slot configuration was the best overall configuration at x/H of 1.0 for S/H of 0.75.

Bain, D. B.↗
NASA NTRS1992

Experimental Study of Cross-Stream Mixing in a Rectangular Duct

An experimental investigation of non-reacting cross-stream jet injection and mixing in a rectangular duct was conducted for application in a low emissions combustor. Planar digital imaging was used to measure concentration distributions in planes perpendicular to the duct axis. Mixing rate was measured for 45 deg slanted slot and round orifice injectors. Five areas of inquiry are discussed: (1) mixing improves continuously with increasing momentum-flux ratio; (2) given a momentum-flux ratio, there is an optimum, orifice spacing; (3) mixing is more dependent on injector geometry than mass flow ratio; (4) mixing is influenced by relative slot orientation; and (5) jet structure is different for round holes and slanted slots injectors. The utility of acquiring multipoint fluctuating properties of the flow field is also demonstrated.

Liscinsky, D. S.↗
NASA NTRS1992

A numerical study of two-dimensional vortex shedding from rectangular cylinders

An efficient time-marching, non-iterative calculation method is used to analyze time-dependent flows around rectangular cylinders. The turbulent flow in the wake region of a square section cylinder is analyzed using an anisotropic k-epsilon model. Initiation and subsequent development of the vortex shedding phenomenon is naturally captured once a perturbation is introduced in the flow. Transient calculations using standard eddy-viscosity and an anisotropic k-epsilon model averaged over an integral number of cycles to get the fluctuating energy (organized and turbulent) are compared with experimental data. It is shown that the anisotropic k-epsilon model resolves the anisotropy of the Reynolds stresses and gives mean energy distribution closer to the experiment than the standard k-epsilon model.

Hadid, A. H.↗
NASA NTRS1990

Initial postbuckling response of an unsymmetrically laminated rectangular plate

It was shown that anisotropic plates can have unstable postbuckling behavior resulting in potential imperfection sensitivity. The degree of instability for rectangular, simply-supported, cross-ply laminated plates is quantified. The analysis is based on asymptotic Koiter-type expansion of postbuckling response. The degree of postbuckling instability is quantified in terms of the reduction in load carrying capacity in the immediate postbuckling range. For graphite-epoxy plates it is found that this measure of instability is very small. Only a low aspect ratio plate with a high degree of anisotropy can have any significant reduction in its buckling load.

Haftka, Raphael T.↗
NASA NTRS1992

Pressure measurements on a rectangular wing with a NACA0012 airfoil during conventional flutter

The Structural Dynamics Division at NASA LaRC has started a wind tunnel activity referred to as the Benchmark Models Program. The primary objective of the program is to acquire measured dynamic instability and corresponding pressure data that will be useful for developing and evaluating aeroelastic type CFD codes currently in use or under development. The program is a multi-year activity that will involve testing of several different models to investigate various aeroelastic phenomena. The first model consisted of a rigid semispan wing having a rectangular planform and a NACA 0012 airfoil shape which was mounted on a flexible two degree-of-freedom mount system. Two wind-tunnel tests were conducted with the first model. Several dynamic instability boundaries were investigated such as a conventional flutter boundary, a transonic plunge instability region near Mach = 0.90, and stall flutter. In addition, wing surface unsteady pressure data were acquired along two model chords located at the 60 to 95-percent span stations during these instabilities. At this time, only the pressure data for the conventional flutter boundary is presented. The conventional flutter boundary and the wing surface unsteady pressure measurements obtained at the conventional flutter boundary test conditions in pressure coefficient form are presented. Wing surface steady pressure measurements obtained with the model mount system rigidized are also presented. These steady pressure data were acquired at essentially the same dynamic pressure at which conventional flutter had been encountered with the mount system flexible.

Rivera, Jose A., Jr.↗
NASA NTRS1991

Theoretical and experimental study of the input impedance of the cylindrical cavity-backed rectangular slot antennas

The authors study the input impedance of a cylindrical cavity-backed slot antenna based on mode matching and the complex Poynting theorem. Two cavity-backed slot antennas were fabricated to verify the theory. The numerical results agree very well with measurements. Two resonant frequencies were found from the input impedance. One resonant frequency is attributed to the rectangular slot and the other is due to the cavity. The slot length controls the first resonant frequency and has a much stronger effect on the input impedance at the antenna operating frequency as compared with the cavity length.

Li, Ming-Yi↗
NASA NTRS1991

Microwave absorption by a lossy dielectric sphere in a rectangular cavity

A new theory of absorption of microwave power by a lossy dielectric sphere in a resonant rectangular chamber is developed here. In this treatment, resonant modes of the cavity are resolved into traveling plane-wave fields, and Mie's theory of scattering and absorption of a plane electromagnetic wave by a sphere is utilized. The fields in the cavity containing the sphere are found by superposition of results for the individual scattering problem for each traveling plane wave. Absorbed power is then calculated by evaluating the time-averaged Poynting vector and integrating over the surface of the sphere. Applications of microwave power absorption are discussed, with particular emphasis on processing of materials. Numerical studies based on the newly derived formulas are presented. The results exhibit the manner in which microwave absorption is influenced by a number of different experimentally controllable parameters. The examples considered were chosen with a view toward designing experiments and apparatus for containerless processing of materials in the microgravity environment of space.

Jackson, H. W.↗
NASA NTRS1992

Characteristics of two adjacent rectangular jets

Rectangular supersonic (Design Mach number 1.8), single and twin free jets were tested in a Mach number range of 1.2 to 2.4. The objective was to study the interaction between the jets and the effect of the interaction on the spreading rate of the jets. Nozzle spacing in the range of one to four equivalent exit diameters, and jet Mach number were the parameters investigated. The experimental techniques used were Schlieren visualization and direct mixing (gas sampling) measurements from which jet spreading can be readily obtained. At the design Mach number of the nozzles and above the twin jets interact but the interaction does not affect the spreading rate of the jets. At lower Mach numbers, a moderate increase in the twin jets' spread compared to the single jet's spread was observed. The greater increase was observed for nozzle spacing of two equivalent exit diameters. Similar conclusions are drawn from both visualization and direct mixing results.

Zeierman, S.↗
NASA NTRS1992

Screech noise source structure of a supersonic rectangular jet

The near-field of the screech noise source structure of an under-expanded supersonic rectangular jet was studied in detail. A miniature probe microphone was used along with a reference microphone to determine the amplitude and phase of the sound pressure near and in the high speed flow field. The transverse structure of the unsteady pressure field was investigated by moving the probe microphone sufficiently far into the jet so that pressure fall-off was observed. Five islands of high sound pressure level have been distinguished which may be associated with the actual local sources of sound production. These sources of screech noise are closely associated with the jet shock structure as would be expected, with the peak region of noise level being found slightly downstream of each of the five observed shocks. The third and fourth noise sources have the highest levels and are about equal in strength. All of the apparent noise sources have their peak levels in the subsonic flow region. Strong cancellations in the acoustic field are observed in the downstream and sideline directions which may account for the predominant upstream propagation of the fundamental tone noise.

Rice, E. J.↗
NASA NTRS1992

Initial postbuckling response of anisotropic laminated rectangular plates

It has been shown that anisotropic plates can have unstable postbuckling behavior resulting in potential imperfection sensitivity. The present paper quantifies the degree of instability for rectangular, simply supported asymmetrically laminated plates. The analysis is based on asymptotic Koiter-type expansion of postbuckling response. The degree of postbuckling instability is quantified in terms of the reduction in load carrying capacity in the immediate postbuckling range. For graphite-epoxy plates it is found that this measure of instability is very small unless the lamination asymmetry is very pronounced.

Johnson, Eric R.↗
NASA NTRS1992

Stress analysis of a rectangular implant in laminated composites using 2-D and 3-D finite elements

An analysis method using the FEM based on the Hellinger-Reissner variation principle has been developed to determine the 3-D stresses and displacements near a rectangular implant inside a laminated composite material. Three-dimensional elements are employed in regions where the interlaminar stress is considered to be significant; 2-D elements are used in other areas. Uniaxially loaded graphite-epoxy laminates have been analyzed; the implant was modeled as four plies of 3501/6 epoxy located in the middle of the laminate. It is shown that the interlaminar stresses are an order of magnitude lower than the stress representing the applied far-field load. The stress concentration factors of both the interlaminar and in-plane stresses depend on the stacking sequence of the laminate.

Chow, Wai T.↗
NASA NTRS1992

Asymptotic modal analysis of a rectangular acoustic cavity excited by wall vibration

Asymptotic modal analysis, a method that has recently been developed for structural dynamical systems, has been applied to a rectangular acoustic cavity. The cavity had a flexible vibrating portion on one wall, and the other five walls were rigid. Banded white noise was transmitted through the flexible portion (plate) only. Both the location along the wall and the size of the plate were varied. The mean square pressure levels of the cavity interior were computed as a ratio of the result obtained from classical modal analysis to that obtained from asymptotic modal analysis for the various plate configurations. In general, this ratio converged to 1.0 as the number of responding modes increased. Intensification effects were found due to both the excitation location and the response location. The asymptotic modal analysis method was both efficient and accurate in solving the given problem. The method has advantages over the traditional methods that are used for solving dynamics problems with a large number of responding modes.

Peretti, Linda F.↗
NASA NTRS1992

Study of intensification zones in a rectangular acoustic cavity

The interior acoustic field of a rectangular acoustic cavity, which is excited by the structural vibration of one of its walls, or a portion of the wall, has been studied. Particularly, the spatial variations of sound pressure levels from the peak levels at the boundaries (intensification zones) to the uniform interior are considered. Analytical expressions, which describe the intensification zones, are obtained using the methodology of asymptotic modal analysis. These results agree well with results computed by a discrete summation over all of the modes. The intensification zones were also modeled as a set of oblique waves incident upon a surface. The result for a rigid surface agrees with the asymptotic modal analysis result. In the presence of an absorptive surface, the character of the intensification zone is dramatically changed. The behavior of the acoustic field near an absorptive wall is described by an expression containing the rigid wall result plus additional terms containing impedance information. The important parameter in the intensification zone analysis is the bandwidth to center frequency ratio. The effect of bandwidth is separated from that of center frequency by expanding the expression about the center frequency wave number. The contribution from the bandwidth is second order in bandwidth to center frequency ratio.

Peretti, Linda F.↗
NASA NTRS1992

Heat transfer in thin, compact heat exchangers with circular, rectangular, or pin-fin flow passages

Heat transfer and pressure drop have been measured of three thin, compact heat exchangers in helium gas at 3.5 MPa and higher, with Reynolds numbers of 450 to 36,000. The flow geometries for the three heat exchanger specimens were: circular tube, rectangular channel, and staggered pin fin with tapered pins. The specimens were heated radiatively at heat fluxes up to 77 W/sq cm. Correlations were developed for the isothermal friction factor as a function of Reynolds number, and for the Nusselt number as a function of Reynolds number and the ratio of wall temperature to fluid temperature. The specimen with the pin fin internal geometry had significantly better heat transfer than the other specimens, but it also had higher pressure drop. For certain conditions of helium flow and heating, the temperature more than doubled from the inlet to the outlet of the specimens, producing large changes in gas velocity, density, viscosity, and thermal conductivity. These changes in properties did not affect the correlations for friction factor and Nusselt number in turbulent flow.

Olson, D. A.↗
NASA NTRS1992

New coplanar waveguide to rectangular waveguide end launcher

A new coplanar waveguide to rectangular waveguide end launcher is experimentally demonstrated. The end launcher operates over the Ka-band frequencies that are designated for the NASA Advanced Communication Technology Satellite uplink. The measured insertion loss and return loss are better than 0.5 and -10 dB, respectively.

Simons, R. N.↗
NASA NTRS1992

Boundary Heat Fluxes for Spectral Radiation from a Uniform Temperature Rectangular Medium

The effect of spectral behavior is analytically shown for radiation in a 2D rectangular geometry. The solution provides exact boundary heat flux values that can be used for comparison with values obtained from general computer programs. The spectral solution presented can be easily evaluated by numerical integration for complex variations of the spectral absorption coefficient with wavelength.

Siegel, Robert↗
NASA NTRS1992

LDV measurements on a rectangular wing with a simulated glaze ice accretion

LDV measurement results are presented for the upper surface of a rectangular semispan wing with and without simulated glaze ice accretion. Inspection of the model centerline flow field indicates that a large region of reverse flow exists aft of the ice horn on the iced model. At alpha = 0 deg, this region extends to 7 percent chord, while at alpha = 4 deg the bubble grows to more than 12 percent chord. At alpha = 8 deg, the time-averaged separation bubble is measured well beyond 50 percent chord. Experimental and computational flow visualization support these findings. The flow in the vicinity of the ice shape contains many of the features of flow over a backward-facing step.

Khodadoust, A.↗
NASA NTRS1992

Mixed convection in a horizontal rectangular channel - Experimental and numerical velocity distributions

Mixed convection in a rectangular channel (width/height = 2) with bottom-heated and top-cooled sections is studied by laser Doppler anemometry in nitrogen at Ra = 22,200 and Re = 18.75, 36, and 54. At the lower Re values, symmetry breaking is observed in steady but spatially oscillating flows that prevail over a certain distance from the leading edge of the differentially heated section. Further downstream, unsteady flows are found even for Re = 18.75. Numerical models are used to investigate the effects of adiabatic, conducting (with a conductive-convective heat transfer coefficient), and perfectly conducting side walls; channel tilts and Prandtl number dependence. Good agreement between calculations and experiment is obtained for longitudinal convective roll velocities. The transverse velocities are found to be independent of Re.

Nyce, Thomas A.↗