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At least 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
NASA NTRS1999

Variability of Fram Strait Ice Flux and North Atlantic Oscillation

An important term in the mass balance of the Arctic Ocean sea ice is the ice export. We estimated the winter sea ice export through the Fram Strait using ice motion from satellite passive microwave data and ice thickness data from moored upward looking sonars. The average winter area flux over the 18-year record (1978-1996) is 670,000 square km, approximately 7% of the area of the Arctic Ocean. The winter area flux ranges from a minimum of 450,000 sq. km in 1984 to a maximum of 906,000 sq km in 1995. The daily, monthly and interannual variabilities of the ice area flux are high. There is an upward trend in the ice area flux over the 18-year record. The average winter volume flux over the winters of October 1990 through May 1995 is 1745 cubic km ranging from a low of 1375 cubic km in 1990 to a high of 2791 cubic km in 1994. The sea-level pressure gradient across the Fram Strait explains more than 80% of the variance in the ice flux over the 18-year record. We use the coefficients from the regression of the time-series of area flux versus pressure gradient across the Fram Strait and ice thickness data to estimate the summer area and volume flux. The average 12-month area flux and volume flux are 919,000 sq km and 2366 cubic km. We find a significant correlation (R =0.86) between the area flux and positive phases of the North Atlantic Oscillation (NAO) index over the months of December through March. Correlation between our six years of volume flux estimates and the NAO index gives R =0.56. During the high NAO years, a more intense Icelandic low increases the gradient in the sea-level pressure by almost 1 mbar across the Fram Strait thus increasing the atmospheric forcing on ice transport. Correlation is reduced during the negative NAO years because of decreased dominance of this large-scale atmospheric pattern on the sea-level pressure gradient across the Fram Strait. Additional information is contained in the original.

Kwok, Ron↗
NASA NTRS1979

Estimating tropical cyclone outer surface winds from satellite microwave data

Warm temperature anomalies over tropical cyclones are sensed by the 55.45 GHz channel of the Nimbus 6 Scanning Microwave Spectrometer. Radial brightness temperature gradients are related to radial surface pressure gradients through the hydrostatic and radiative transfer equations. Surface wind speeds outside of the radius of maximum wind are calculated from the pressure gradients and compared with observations.

Kidder, S. Q.↗
NASA NTRS1974

A review of turbulence measurements in compressible flow

A survey of turbulence measurements in compressible flows is presented. The majority of turbulence measurements at super- and hypersonic speeds were made for the zero pressure gradient, turbulent boundary layer. It was found that the nondimensional turbulent stress terms for the zero pressure gradient flow appear to agree closely with equivalent incompressible measurements in the outer part of the boundary layer. The stress terms were nondimensionalized by the wall value of shear stress and plotted versus the distance from the wall, nondimensionalized by the boundary-layer thickness. Indirect evaluation of the total shear stress distribution from mean velocity measurements for both super- and hypersonic flows (zero pressure gradient, two-dimensional flows) indicate a near universal distribution. These total shear stress curves also agree very closely with measured incompressible shear stress distributions. Recent laser anemometer measurements of the turbulent Reynolds shear stress (puv), reported by Johnson and Rose for a Mach number 2.9 flow, are in reasonable agreement with the expected total shear stress curve over the outer 60% of the boundary layer.

Sandborn, V. A.↗
DOE OSTI2024

Characterization of the ELM-free negative triangularity edge on DIII-D

Tokamak plasmas with strong negative triangularity (NT) shaping typically exhibit fundamentally different edge behavior than conventional L-mode or H-mode plasmas. On DIII-D, every plasma with sufficiently negative triangularity (δ < δ crit ≃ -0.12) is found to be inherently free of edge localized modes (ELMs), even at injected powers well above the predicted L-H power threshold. It is also possible to access an ELM free state at weaker average triangularities provided that at least one of the two x-points is still sufficiently negative. Access to the ELM-free NT scenario is found to coincide with the closure of the second stability region for infinite-n ballooning modes, suggesting that ballooning stability may play a role in limiting the accessible pressure gradient in NT plasmas. Despite this, NT plasmas are able to support small pedestals and are typically characterized by an enhancement of edge pressure gradients beyond those found in traditional L mode plasmas. Further, the pressure gradient inside of this small pedestal is unusually steep, allowing access to high core performance that is competitive with other ELM-free regimes previously achieved on DIII-D. Since ELM-free operation in NT is linked directly to the magnetic geometry, NT fusion pilot plants are predicted to maintain advantageous edge conditions even in burning plasma regimes, potentially eliminating reactor core-integration issues caused by ELMs.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1990

Fully elliptic incompressible flow calculations on regular grid by a new pressure substitution method

A new method is presented for the solution of incompressible flow in generalized coordinates. This method is based on the substitution of the pressure weighted form of the momentum equations into the continuity equation. The algorithm is rigorously derived and a Fourier analysis is used to assess its suitability to act as an error smoother. Linear stability analysis results indicate that the performance of the new pressure substitution method (PSM) and the pressure correction method (PCM) is about the same at low Reynolds numbers, with no significant pressure gradient. At high Reynolds numbers the PSM shows much faster convergence. Likewise prediction of various flows indicate that the PSM has better accuracy for high Reynolds number flows with significant pressure gradients. Since most practical aerodynamic flows have significant pressure gradients, the PSM seems to be attractive for such flows. Solutions for both laminar and turbulent flow are compared with the experimental data. A two-equation low Reynolds number turbulence model is used to resolve the turbulent flowfield.

Hobson, G. V.↗
NASA NTRS1988

Leakage predictions for Rayleigh-step, helium-purge seals

Rayleigh-step, helium purge, annular shaft seals, studied for use in liquid oxygen turbopumps, generate a hydrodynamic force that enables the seal to follow shaft perturbations. Hence, smaller clearances can be used to reduce seal leakage. FLOWCAL, a computer code developed by Mechanical Technology Incorporated, predicts gas flow rate through an annular seal with an axial pressure gradient. Analysis of a 50-mm Rayleigh-step, helium-purge, annular seal showed the flow rate increased axial pressure gradient, downstream pressure, and eccentricity ratio. Increased inlet temperature reduced leakage. Predictions made at maximum and minimum clearances (due to centrifugal and thermal growths, machining tolerances and + or - 2 percent uncertainty in the clearance measurement) placed wide boundaries on expected flow rates. The widest boundaries were set by thermal growth conditions. Predicted flow rates for a 50-mm Rayleigh-step, helium-purge, annular seal underestimated measured flow rates by three to seven times. However, the analysis did accurately predict flow rates for choked gas flow through annular seals when compared to flow rates measured in two other independent studies.

Proctor, Margaret P.↗
NASA NTRS1991

Similarity analysis of compressor tip clearance flow structure

A new approach is presented for analyzing compressor tip clearance flow. The basic idea is that the clearance velocity field can be (approximately) decomposed into independent throughflow and crossflow, since chordwise pressure gradients are much smaller than normal pressure gradients in the clearance region. As in the slender body approximation in external aerodynamics, this description implies that the three-dimensional steady clearance flow can be viewed as a two-dimensional, unsteady flow. Using this approach, a similarity scaling for the crossflow in the clearance region is developed and a generalized description of the clearance vortex is derived. Calculations based on the similarity scaling agree well with a wide range of experimental data in regard to flow features such as crossflow velocity field, static pressure field, and tip clearance vortex trajectory.

Chen, G. T.↗
DOE OSTI2026

Evaluation and optimization of flow boiling frictional pressure drop correlations using the data from traditional and next-generation refrigerants in a micro-fin tube

This study presents an experimental evaluation and optimization of flow boiling frictional pressure drop correlations for conventional and next-generation refrigerants in a horizontal micro-fin tube, with particular emphasis on the newly emerging refrigerant blends R-454C and R-455A, for which pressure-drop data in enhanced tubes remain limited. Experiments were conducted with R-410A, R-454C, R-455A, R-134a, R-1234yf, and R-1234ze(E) in a copper micro-fin tube with an inner diameter of 8.468 mm, over mass fluxes ranging from 100 to 300 kg/(m²·s) depending on the refrigerant, and evaporation temperatures of 7, 12, and 14 °C. Frictional pressure gradients were determined from measured total pressure drops after subtracting acceleration pressure drop, and the resulting database was used to assess four existing models: Kuo and Wang (1996), Cavallini et al. (1997), Goto et al. (2001), and Diani et al. (2014). The measured frictional pressure gradient increased with vapor quality and mass flux for all refrigerants and increased further at lower evaporation temperatures, with the overall trend strongly related to liquid viscosity. Among the four correlations, the Goto et al. (2001) model provided the best overall agreement with the measured data before optimization. To further improve prediction accuracy, the Kuo and Wang (1996) and Goto et al. (2001) models were optimized using the complete experimental database. After optimization, both models reduced the overall mean absolute deviation to below 15%, while the optimized Goto et al. (2001) model maintained the best and most consistent overall performance. The results provide new pressure-drop data for next-generation refrigerants and demonstrate that parameter optimization can significantly enhance the applicability of existing micro-fin-tube correlations.

Hu, Yifeng [ORNL] (ORCID:0000000242875185)↗
NASA NTRS1989

Remote sensing of the ring current using energetic neutral atoms

Energetic neutral atom images of the storm-time ring current obtained from the ISEE-1 spacecraft provide information for a 'zero-order' global model of the energetic ion distribution. With the assumption of isotropic pressure and magnetostatic, nonconvective pressure balance, the global system of electrical currents driven by the ion pressure can be calculated using Euler potentials for the divergenceless current density. Radial pressure gradients drive azimuthal currents, and azimuthal pressure gradients drive radial currents. The radial currents cause current lines in the inner magnetosphere to close in the ionosphere, forming a partial ring current. The intensities and locations of these field-aligned currents driven into and out of the ionosphere resemble those of the observed Region 2 current system, but not all observed properties of the Region 2 system are reproduced by the zero-order model.

Roelof, Edmond C.↗
NASA NTRS2000

A Real-Time Method for Estimating Viscous Forebody Drag Coefficients

This paper develops a real-time method based on the law of the wake for estimating forebody skin-friction coefficients. The incompressible law-of-the-wake equations are numerically integrated across the boundary layer depth to develop an engineering model that relates longitudinally averaged skin-friction coefficients to local boundary layer thickness. Solutions applicable to smooth surfaces with pressure gradients and rough surfaces with negligible pressure gradients are presented. Model accuracy is evaluated by comparing model predictions with previously measured flight data. This integral law procedure is beneficial in that skin-friction coefficients can be indirectly evaluated in real-time using a single boundary layer height measurement. In this concept a reference pitot probe is inserted into the flow, well above the anticipated maximum thickness of the local boundary layer. Another probe is servomechanism-driven and floats within the boundary layer. A controller regulates the position of the floating probe. The measured servomechanism position of this second probe provides an indirect measurement of both local and longitudinally averaged skin friction. Simulation results showing the performance of the control law for a noisy boundary layer are then presented.

Whitmore, Stephen A.↗
DOE OSTI2023

Large-Eddy Simulation of Flow Over Boeing Gaussian Bump Using Multiagent Reinforcement Learning Wall Model: Preprint

We develop a wall model for large-eddy simulation (LES) that takes into account various pressure-gradient effects using multi-agent reinforcement learning. The model is trained using low-Reynolds-number flow over periodic hills with agents distributed on the wall at various computational grid points. It utilizes a wall eddy-viscosity formulation as the boundary condition to apply the modeled wall shear stress. Each agent receives states based on local instantaneous flow quantities at an off-wall location, computes a reward based on the estimated wall-shear stress, and provides an action to update the wall eddy viscosity at each time step. The trained wall model is validated in wall-modeled LES of flow over periodic hills at higher Reynolds numbers, and the results show the effectiveness of the model on flow with pressure gradients. The analysis of the trained model indicates that the model is capable of distinguishing between the various pressure gradient regimes present in the flow. To further assess the robustness of the developed wall model, simulations of flow over the Boeing Gaussian bump are conducted at a Reynolds number of 2 x 10^6, based on the free-stream velocity and the bump width. The results of mean skin friction and pressure on the bump surface, as well as the velocity statistics of the flow field, are compared to those obtained from equilibrium wall model (EQWM) simulations and published experimental data sets. The developed wall model is found to successfully capture the acceleration and deceleration of the turbulent boundary layer on the bump surface, providing better predictions of skin friction near the bump peak and exhibiting comparable performance to the EQWM with respect to the wall pressure and velocity field. We also conclude that the subgrid-scale model is crucial to the accurate prediction of the flow field, in particular the prediction of separation.

boundary layer↗
NASA NTRS1993

Surface winds over West Antarctica

Five winter months (April-August 1988) of thermal infrared satellite images were examined to investigate the occurrence of dark (warm) signatures across the Ross Ice Shelf in the Antarctic continent. These features are inferred to be generated by katabatic winds that descend from southern Marie Byrd Land and then blow horizontally across the ice shelf. Significant mass is added to this airstream by katabatic winds blowing from the major glaciers that flow through the Transantarctic Mountains from East Antarctica. These negatively buoyant katabatic winds can reach the northwestern edge of the shelf - a horizontal propagation distance of up to 1,000 km - 14 percent of the time. Where the airstream crosses from the ice shelf to the ice-covered Ross Sea, a prominent coastal polynya is formed. Because the downslope buoyancy force is near zero over the Ross Ice Shelf, the northwestward propagation of the katabatic air mass requires pressure gradient support. The study shows that the extended horizontal propagation of this atmospheric density current occurred in conjunction with the passage of synoptic cyclones over the southern Amundsen Sea. These cyclones can strengthen the pressure gradient in the interior of West Antarctica and make the pressure field favorable for northwestward movement of the katabatic winds from West Antarctica across the ice shelf in a geostrophic direction. The glacier winds from East Antarctica are further accelerated by the synoptic pressure gradient, usually undergo abrupt adjustment beyond the exit to the glacier valley, and merge into the mountain-parallel katabatic air mass.

Bromwich, David↗
NASA NTRS1990

A self-consistent two-dimensional resistive fluid theory of field-aligned potential structures including charge separation and magnetic and velocity shear

A self-consistent two-fluid theory that includes the magnetic field and shear patterns is developed to model stationary electrostatic structures with field-aligned potential drops. Shear flow is also included in the theory since this seems to be a prominent feature of the structures of interest. In addition, Ohmic dissipation, a Hall term, and pressure gradients in a generalized Ohm's law, modified for cases without quasi-neutrality, are included. In the analytic theory, the electrostatic force is balanced by field-aligned pressure gradients (i.e., thermal effects in the direction of the magnetic field) and by pressure gradients and magnetic stresses in the perpendicular direction. Within this theory, simple examples of applications are presented to demonstrate the kind of solutions resulting from the model. The results show how the effects of charge separation and shear in the magnetic field and the velocity can be combined to form self-consistent structures such as are found to exist above the aurora, suggested also in association with solar flares.

Hesse, Michael↗
NASA NTRS1992

Navier-Stokes analysis and experimental data comparison of compressible flow within ducts

Many aircraft employ ducts with centerline curvature or changing cross-sectional shape to join the engine with inlet and exhaust components. S-ducts convey air to the engine compressor from the intake and often decelerate the flow to achieve an acceptable Mach number at the engine compressor by increasing the cross-sectional area downstream. Circular-to-rectangular transition ducts are used on aircraft with rectangular exhaust nozzles to connect the engine and nozzle. To achieve maximum engine performance, the ducts should minimize flow total pressure loss and total pressure distortion at the duct exit. Changes in the curvature of the duct centerline or the duct cross-sectional shape give rise to streamline curvature which causes cross stream pressure gradients. Secondary flows can be caused by deflection of the transverse vorticity component of the boundary layer. This vortex tilting results in counter-rotating vortices. Additionally, the adverse streamwise pressure gradient caused by increasing cross-sectional area can lead to flow separation. Vortex pairs have been observed in the exit planes of both duct types. These vortices are due to secondary flows induced by pressure gradients resulting from streamline curvature. Regions of low total pressure are produced when the vortices convect boundary layer fluid into the main flow. The purpose of the present study is to predict the measured flow field in a diffusing S-duct and a circular-to-rectangular transition duct with a full Navier-Stokes computer program, PARC3D, and to compare the numerical predictions with new detailed experimental measurements. The work was undertaken to extend previous studies and to provide additional CFD validation data needed to help model flows with strong secondary flow and boundary layer separation. The S-duct computation extends the study of Smith et al, and Harloff et al, which concluded that the computation might be improved by using a finer grid and more advanced turbulence models. The present study compares results for both the Baldwin-Lomas and k-epsilon turbulence models and is conducted with a refined grid. For the transition duct, two inlet conditions were considered, the first with straight flow and the second with swirling flow. The first case permits examination of the effects of the geometric transition on the flow field, while the second case includes the rotational flow effect characteristic of a gas turbine engine.

Harloff, G. J.↗
NASA NTRS1988

Vascular pressures and passage of gas emboli through the pulmonary circulation

Doppler technique was used to measure the pulmonary vascular pressure gradients in dogs that received venous air emboli (VAE) at the rate of 0.35 ml/kg per min, at which the spillover of bubbles into the systemic arteries occur. It was found that, in 60 percent of dogs, venous bubbles spilled over into the arterial circulation at the pulmonary vascular pressure gradient of about 34.7 mm Hg. When the pulmonary vascular pressure gradient was raised to about 52 mm Hg, the spillover of venous bubbles occurred 100 percent of time. It is concluded that venous bubbles can cross the lungs of anesthetized dogs when the driving pressures are sufficient to overcome the normal filtering function.

Butler, B. D.↗
NASA NTRS2011

Implementation and Validation of an Impedance Eduction Technique

Implementation of a pressure gradient method of impedance eduction in two NASA Langley flow ducts is described. The Grazing Flow Impedance Tube only supports plane-wave sources, while the Curved Duct Test Rig supports sources that contain higher-order modes. Multiple exercises are used to validate this new impedance eduction method. First, synthesized data for a hard wall insert and a conventional liner mounted in the Grazing Flow Impedance Tube are used as input to the two impedance eduction methods, the pressure gradient method and a previously validated wall pressure method. Comparisons between the two results are excellent. Next, data measured in the Grazing Flow Impedance Tube are used as input to both methods. Results from the two methods compare quite favorably for sufficiently low Mach numbers but this comparison degrades at Mach 0.5, especially when the hard wall insert is used. Finally, data measured with a hard wall insert mounted in the Curved Duct Test Rig are used as input to the pressure gradient method. Significant deviation from the known solution is observed, which is believed to be largely due to 3-D effects in this flow duct. Potential solutions to this issue are currently being explored.

Watson, Willie R.↗
NASA NTRSDate not supplied

Status of the Microwave Barometric Radar and Sounder (MBARS)

Atmospheric surface pressure and pressure profiles are essential variables in weather modeling and forecasting. Pressure gradients generate atmospheric motion and are essential to the air-sea heat exchange feedbacks within the planetary boundary layer (PBL) that lead to convective storms and heavy precipitation. Despite the importance of pressure and pressure gradients on meso-, synoptic-, and global-scale weather patterns, current technology relies almost entirely on buoy measurements over the majority of the ocean. These measurements are too sparse to capture pressure gradients of even many synoptic scale events, and leave models starved of information. NASA/Goddard Space Flight Center (GSFC), NASA/Langley Research Center (LaRC), and Tomorrow.io have begun development of the Microwave Barometric Radar and Sounder (MBARS), an airborne sensor to retrieve surface air pressure and vertical pressure profiles, especially over oceans. MBARS is a new combined active/passive microwave instrument in the O2 absorption V-band (64-70 GHz) funded by the NASA Earth Science Technology Office (ESTO) Instrument Incubator Program (IIP). This instrument consists of an innovative scanning multi-channel differential absorption radar (DAR) to provide an estimation of total atmospheric column oxygen content and thus the surface dry air pressure. MBARS also will provide hyperspectral radiometric temperature profiling to enables vertical pressure profiles using the hypsometric relationship between pressure and temperature. A co-located microwave radiometer will provide the integrated water vapor mass contribution to atmospheric pressure. This presentation will summarize the project, retrieval concept, and engineering status of the MBARS project.

Radar, Radiometer↗
NASA NTRS1991

Reynolds shear stress measurements in a separated boundary layer flow

Turbulence measurements were obtained for two cases of boundary layer flow with an adverse pressure gradient, one attached and the other separated. A three-component laser Doppler velocimeter system was used to measure three mean velocity components, all six Reynolds stress components, and all ten velocity triple product correlations. Independent measurements of skin-friction obtained with a laser oil-flow interferometer were used to examine the law of the wall in adverse pressure gradient flows where p(+) is less than 0.05. Strong similiarities were seen between the two adverse pressure gradient flows and free shear layer type flows. Eddy viscosities, dissipation rates, and pressure-strain rates were deduced from the data and compared to various turbulence modeling assumptions.

Driver, David M.↗