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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS2006

Planar laser-induced fluorescence (PLIF) investigation of hypersonic flowfields in a Mach 10 wind tunnel

Planar laser-induced fluorescence (PLIF) of nitric oxide (NO) was used to visualize four different hypersonic flowfields in the NASA Langley Research Center 31-Inch Mach 10 Air wind tunnel. The four configurations were: (1) the wake flowfield of a fuselage-only X-33 lifting body, (2) flow over a flat plate containing a rectangular cavity, (3) flow over a 70deg blunted cone with a cylindrical afterbody, formerly studied by an AGARD working group, and (4) an Apollo-geometry entry capsule - relevant to the Crew Exploration Vehicle currently being developed by NASA. In all cases, NO was seeded into the flowfield through tubes inside or attached to the model sting and strut. PLIF was used to visualize the NO in the flowfield. In some cases pure NO was seeded into the flow while in other cases a 5% NO, 95% N2 mix was injected. Several parameters were varied including seeding method and location, seeding mass flow rate, model angle of attack and tunnel stagnation pressure, which varies the unit Reynolds number. The location of the laser sheet was as also varied to provide three dimensional flow information. Virtual Diagnostics Interface (ViDI) technology developed at NASA Langley was used to visualize the data sets in post processing. The measurements demonstrate some of the capabilities of the PLIF method for studying hypersonic flows.

Danehy, Paul M.↗
NASA NTRS1991

Passive control of supersonic cavity flowfields

A computational investigation has been conducted to study the effect and mechanisms of the passive control of a supersonic flow over a rectangular two-dimensional cavity. The passive control was included through the use of a porous surface over a vent chamber in the floor of the cavity. The passive control effectively suppressed the low-frequency pressure oscillations for the open type cavity, (length-to-depth ratio = 6.0). The mechanism for the suppression was observed to be the stabilization of the motion of the free shear layer. For the closed type cavity flow, (length-to-depth ratio = 17.5), the passive control modified the flowfield to nearly that of an open type cavity flow; further the cavity drag was reduced by a factor of four. The computational results of both cases showed good agreement with the available experimental data and the predictions of a semiempirical formula. This study demonstrates that the passive control concept can be used to improve the aerodynamic characteristics of open and closed cavity flowfields.

Chokani, N.↗
NASA NTRS1995

The acoustic characteristics of turbomachinery cavities

Internal fluid flows are subject not only to self-sustained oscillations of the purely hydrodynamic type but also to the coupling of the instability with the acoustic mode of the surrounding cavity. This situation is common to turbomachinery, since flow instabilities are confined within a flow path where the acoustic wavelength is typically smaller than the dimensions of the cavity and flow speeds are low enough to allow resonances. When acoustic coupling occurs, the fluctuations can become so severe in amplitude that it may induce structural failure of engine components. The potential for catastrophic failure makes identifying flow-induced noise and vibration sources a priority. In view of the complexity of these types of flows, this report was written with the purpose of presenting many of the methods used to compute frequencies for self-sustained oscillations. The report also presents the engineering formulae needed to calculate the acoustic resonant modes for ducts and cavities. Although the report is not a replacement for more complex numerical or experimental modeling techniques, it is intended to be used on general types of flow configurations that are known to produce self-sustained oscillations. This report provides a complete collection of these models under one cover.

Lucas, M. J.↗
NASA NTRS1981

Flow separation in shear-layer-driven cavities

A study of the shear-layer flow over a range of open-top cavity configurations is reported. Emphasis is placed on the effect that altering the cavity's span length and aspect ratio has on the development of the shear layer. Computational results are obtained using an interactive method which adapts the compressible boundary-layer model for the flow above the cavity and incompressible Navier-Stokes equations within the enclosure. Interaction of this composite model with the outer, inviscid supersonic flow is also considered in one case. The results show that the location of the stagnation points is sensitive primarily to the variation of the span. When the span was fixed and the aspect ratio varied, the shear layer was nearly unaffected except at aspect ratios less than 0.5. Interaction with the outer flow had a smoothing effect on the shape of the dividing streamline but did not significantly affect the location of the stagnation points.

Brandeis, J.↗
NASA NTRS1995

Effects of cavity dimensions, boundary layer, and temperature on cavity noise with emphasis on benchmark data to validate computational aeroacoustic codes

This report documents the results of an experimental investigation on the response of a cavity to external flowfields. The primary objective of this research was to acquire benchmark of data on the effects of cavity length, width, depth, upstream boundary layer, and flow temperature on cavity noise. These data were to be used for validation of computational aeroacoustic (CAA) codes on cavity noise. To achieve this objective, a systematic set of acoustic and flow measurements were made for subsonic turbulent flows approaching a cavity. These measurements were conducted in the research facilities of the Georgia Tech research institute. Two cavity models were designed, one for heated flow and another for unheated flow studies. Both models were designed such that the cavity length (L) could easily be varied while holding fixed the depth (D) and width (W) dimensions of the cavity. Depth and width blocks were manufactured so that these dimensions could be varied as well. A wall jet issuing from a rectangular nozzle was used to simulate flows over the cavity.

Ahuja, K. K.↗
NASA NTRS2016

Hybrid Reynolds-Averaged/Large Eddy Simulation of the Flow in a Model SCRamjet Cavity Flameholder

Steady-state and scale-resolving simulations have been performed for flow in and around a model scramjet combustor flameholder. Experimental data available for this configuration include velocity statistics obtained from particle image velocimetry. Several turbulence models were used for the steady-state Reynolds-averaged simulations which included both linear and non-linear eddy viscosity models. The scale-resolving simulations used a hybrid Reynolds-averaged/large eddy simulation strategy that is designed to be a large eddy simulation everywhere except in the inner portion (log layer and below) of the boundary layer. Hence, this formulation can be regarded as a wall-modeled large eddy simulation. This e ort was undertaken to not only assess the performance of the hybrid Reynolds-averaged / large eddy simulation modeling approach in a flowfield of interest to the scramjet research community, but to also begin to understand how this capability can best be used to augment standard Reynolds-averaged simulations. The numerical errors were quantified for the steady-state simulations, and at least qualitatively assessed for the scale-resolving simulations prior to making any claims of predictive accuracy relative to the measurements. The steady-state Reynolds-averaged results displayed a high degree of variability when comparing the flameholder fuel distributions obtained from each turbulence model. This prompted the consideration of applying the higher-fidelity scale-resolving simulations as a surrogate "truth" model to calibrate the Reynolds-averaged closures in a non-reacting setting prior to their use for the combusting simulations. In general, the Reynolds-averaged velocity profile predictions at the lowest fueling level matched the particle imaging measurements almost as well as was observed for the non-reacting condition. However, the velocity field predictions proved to be more sensitive to the flameholder fueling rate than was indicated in the measurements.

Baurle, R. A.↗
NASA NTRS1976

Measurements of sound radiation from cavities at subsonic speeds

To better understand the physical mechanisms responsible for noise generated by flow over cavities with a range of dimensions common to commercial aircraft, a series of measurements of farfield sound pressure level, fluctuating surface pressure, and drag coefficient was made. An examination of the resultant data shows that two theoretical models are needed for this range of dimensions. The shallow cavity (width-to-depth ratio larger than 1.0) can be approximated by a dipole located near the downstream edge of the cavity and the deep cavity (ratio less than 1.0) can be approximated by a dipole located near the downstream edge with the superimposed effects of the upstream edge of the cavity.

Yu, Y. H.↗
NASA NTRS1985

Sound generation by flow over relatively deep cylindrical cavities

To develop a system for acoustic coding of moving objects containing drilled cylindrical cavities, the production of high-intensity tones by deep cylindrical cavities in a flat surface at low (0.12-0.24) Mach numbers was investigated. The sound intensity and frequency have been determined as functions of flow velocity, diameter, and depth of the cavities. It is shown that whistles can be designed for a given frequency (in the range of 5-17 kHz) and flow, and the sound pressure levels can be calculated by the equations given. Using these equations a whistle producing 106 dB at a 30.5-cm distance from a cylindrical cavity of 0.508 cm in diameter and 1.32 cm in depth with an airflow of 57.7 m/s past the cavity was designed.

Parthasarathy, S. P.↗
NASA NTRS2001

Identification of complex flows in Taylor-Couette counter-rotating cavities

The transition in confined rotating flows is a topical problem with many industrial and fundamental applications. The purpose of this study is to investigate the Taylor-Couette flow in a finite-length cavity with counter-rotating walls, for two aspect ratios L=5 or L=6. Two complex regimes of wavy vortex and spirals are emphasized for the first time via direct numerical simulation, by using a three-dimensional spectral method. The spatio-temporal behavior of the solutions is analyzed and compared to the few data actually available. c2001 Academie des sciences/Editions scientifiques et medicales Elsevier SAS.

Non-NASA Center↗
NASA NTRS1990

Tone generation by aeroacoustic sources in pipes with flow

Sound production by axisymmetric ring cavities in internal flow has been investigated experimentally. These ring sources generate sound when the acoustic motion in the cavity is in resonance in one of many possible modes. Experimental results agree with predictions of a model eddy-driven oscillator. It is observed that the standing waves produced by reflections from regions upstream and downstream of the source are adjusted by the source such that the source is at a pressure antinode at the condition of maximum sound emission.

Shakkottai, P.↗
NASA NTRS1984

Multi-grid technique for the solution of incompressible Navier-Stokes equations

A computationally efficient method is proposed for obtaining fine-mesh solutions of the Navier-Stokes equations for high-Re flow, including separation. The method involves implementing a multi-grid solution procedure with suitably chosen elements, such that the resulting overall solution technique is efficient as well as robust. The robustness and efficiency of the solution technique are demonstrated by applying it to three model problems: flow in a driven cavity, downstream asymptotic flow in curved ducts of square and polar sections, and Newmann boundary-value problem in clustered curvilinear orthogonal coordinates.

Ghia, K. N.↗
NASA NTRS1995

Study of the Effect of Two-Dimensional Cavities on the Boundary Layer in an Adverse Pressure Gradient

One concept for high lift aerodynamics is the trapped-vortex. Recently it has been suggested that several spoilers located at different chordwise locations on an airfoil could be deployed to form several cavities in the chordwise direction. This may provide a means of increasing upper surface camber and thereby increase wing lift. It is envisioned that the cavities would be located on an airfoil in its pressure recovery region. Investigations of the effect of one or more cavities on the adjacent boundary layer in an adverse boundary layer were not found in a literature survey. Since flow separation can be caused by flow over cavities, boundary layer measurements were made in the vicinity of cavities which were located in an adverse pressure gradient to study the associated flow in an effort to identify any benefits. The experimental data from the present investigation indicate a minimal impact on the boundary layer profiles is caused by the presence of cavities in either a constant pressure freestream or an adverse pressure gradient. The data from the experimental investigation has been obtained and analyzed. The computational investigation using an incompressible Navier-Stokes code show that reasonable results are being obtained when they are compared with the experimental data.

Margason, Richard J.↗
NASA NTRS1988

3-D laser anemometer measurements in a labyrinth seal

The flow field inside a seven cavity labyrinth seal with a 0.00127 m clearance was measured using a 3-D laser Doppler anemometer system. Through the use of this system, the mean velocity vector and the entire Reynolds stress tensor distributions were measured for the first, third, fifth, and seventh cavities of the seal. There was one large recirculation region present in the cavity for the flow condition tested, Re = 28,000 and Ta = 7,000. The axial and radial mean velocities as well as all of the Reynolds stress term became cavity independent by the third cavity. The azimuthal mean velocity varied from cavity to cavity with its magnitude increasing as the flow progressed downstream.

Morrison, G. L.↗
NASA NTRS1995

Measurements of store forces and moments and cavity pressures for a generic store in and near a box cavity at subsonic and transonic speeds

An experimental force and moment study was conducted in the Langley 8-Foot Transonic Pressure Tunnel for a generic store in and near rectangular box cavities contained in a flat-plate configuration at subsonic and transonic speeds. Surface pressures were measured inside the cavities and on the flat plate. The length-to-height ratios were 5.42, 6.25, 10.83, and 12.50. The corresponding width-to-height ratios were 2.00, 2.00, 4.00, and 4.00. The free-stream Mach number range was from 0.20 to 0.95. Surface pressure measurements inside the cavities indicated that the flow fields for the shallow cavities were either closed or transitional near the transitional/closed boundary. For the deep cavities, the flow fields were either open or near the open/transitional boundary. The presence of the store did not change the type of flow field and had only small effects on the pressure distributions. For transitional or open transitional flow fields, increasing the free-stream Mach number resulted in large reductions in pitching-moment coefficient. Values of pitching-moment coefficient were always much greater for closed flow fields than for open flow fields.

Stallings, Robert L., Jr.↗
NASA NTRS1989

Analysis of the SSME HPOTP bearing inlet cavity

Analysis of the flow in the Space Shuttle Main Engine (SSME) high pressure oxygen turbopump (HPOTP) bearing no. 1 inlet cavity was completed in support of return-to-flight. With the incorporation of several design changes in the Phase 2 turbopump, rotordynamic stability of the pumps was enhanced, but the durability and life of the LOX-cooled bearings has decreased. During the post-Challenger SSME recertification, the causes of limited bearing durability were investigated. One topic addressed was the flow environment upstream of the pump-end bearing and the effect of seal exit swirl and a cavity anti-vortex rib on the bearing environment and life. The objective is to define the hydrodynamic environment upstream of the pump-end bearing and determine the effect of seal exit swirl and the anti-vortex rib on bearing inlet swirl. The problem was posed as an axisymmetric cavity flow with the computational domain extending from the seal exit to the bearing inlet. This domain was discretized with 22800 grid points. Boundary conditions were obtained from a 1-D model of the SSME coolant path. The inlet Mach number was 0.19 and the problem was solved with the CMINT code utilizing the Briley-McDonald/Beam-Warming algorithm with preconditioning to speed convergence at low Mach numbers. Three parametric cases with inlet swirl of 50 percent shaft speed (labyrinth seal), 20 percent shaft speed (damping seal), and no inlet swirl were considered. Computational results indicate large vortical flow structures in the cavity, with the labyrinth, damping, and no-swirl cases yielding bearing inlet swirl rates of 14, 10, and 9 percent of shaft speed, respectively. When these results were used as input to the SHABRETH bearing model, limited durability could not be explained by these small differences in swirl. Also, based on these results, a proposed design change for the cavity anti-vortex rib was not implemented by the SSME chief engineer.

Mcconnaughey, P. K.↗
NASA NTRS1987

Computational and experimental investigation of cavity flowfields

This paper presents a computational and experimental investigation of supersonic flow past a cavity in a flat plate. The source of the particular interest in this problem is the ongoing study of the aerodynamic interference effects between a separating store and its bay in the parent body. An upwind relaxation scheme, utilizing flux vector splitting and line-Gauss-Seidel iterations, is used to solve Reynolds-averaged Navier-Stokes equations. Spatial discretizations of this two-dimensional analysis are based on implicit and finite-volume methods. Turbulence is modeled and shocks are captured. The flowfield of the symmetry plane at the half-width is computationally visualized and all flow properties are computed. Experimental tests are conducted in the Langley Unitary Plan Wind Tunnel to measure wall pressures and to capture schlieren photographs. Qualitative as well as quantitative data of computations and experiments agree very well. These two vehicles of investigation are merged to show open, closed and transitional cavity flow behaviors.

Baysal, O.↗