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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 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Fluid-Dynamic Mechanisms Underlying Wind Turbine Wake Control with Strouhal-Timed Actuation

A reduction in wake effects in large wind farms through wake-aware control has considerable potential to improve farm efficiency. This work examines the success of several emerging, empirically derived control methods that modify wind turbine wakes (i.e., the pulse method, helix method, and related methods) based on Strouhal numbers on the O(0.3). Drawing on previous work in the literature for jet and bluff-body flows, the analyses leverage the normal-mode representation of wake instabilities to characterize the large-scale wake meandering observed in actuated wakes. Idealized large-eddy simulations (LES) using an actuator-line representation of the turbine blades indicate that the n=0 and ±1 modes, which correspond to the pulse and helix forcing strategies, respectively, have faster initial growth rates than higher-order modes, suggesting these lower-order modes are more appropriate for wake control. Exciting these lower-order modes with periodic pitching of the blades produces increased modal growth, higher entrainment into the wake, and faster wake recovery. Modal energy gain and the entrainment rate both increase with streamwise distance from the rotor until the intermediate wake. This suggests that the wake meandering dynamics, which share close ties with the relatively well-characterized meandering dynamics in jet and bluff-body flows, are an essential component of the success of wind turbine wake control methods. A spatial linear stability analysis is also performed on the wake flows and yields insights on the modal evolution. In the context of the normal-mode representation of wake instabilities, these findings represent the first literature examining the characteristics of the wake meandering stemming from intentional Strouhal-timed wake actuation, and they help guide the ongoing work to understand the fluid-dynamic origins of the success of the pulse, helix, and related methods.

17 WIND ENERGY↗
NASA NTRS1973

Emission calculations for a scramjet powered hypersonic transport

Calculations of exhaust emissions from a scramjet powered hypersonic transport burning hydrogen fuel were performed over a range of Mach numbers of 5 to 12 to provide input data for wake mixing calculations and forecasts of future levels of pollutants in the stratosphere. The calculations were performed utilizing a one-dimensional chemical kinetics computer program for the combustor and exhaust nozzle of a fixed geometry dual-mode scramjet engine. Inlet conditions to the combustor and engine size was based on a vehicle of 227,000 kg (500,000 lb) gross take of weight with engines sized for Mach 8 cruise. Nitric oxide emissions were very high for stoichiometric engine operation but for Mach 6 cruise at reduced equivalence ratio are in the range predicted for an advanced supersonic transport. Combustor designs which utilize fuel staging and rapid expansion to minimize residence time at high combustion temperatures were found to be effective in preventing nitric oxide formation from reaching equilibrium concentrations.

Lezberg, E. A.↗
NASA NTRS1980

Experimental performance and analysis of 15.04-centimeter-tip-diameter, radial-inflow turbine with work factor of 1.126 and thick blading

The aerodynamic design, the performance, and an internal loss breakdown were examined for a 15.04 cm tip diameter, radial-inflow turbine. The design application was to drive a two stage, 10 to 1 pressure ratio compressor with a mass flow of 0.952 kg/sec and a rotative speed of 70,000 rmp. The turbine inlet temperature was 1478 K, and the turbine was designed with blades thick enough for internal cooling passages. The rotor tip diameter was limited to 86 percent of optimum in order to obtain a reduced tip speed design. The turbine was fabricated with solid, uncooled blading and tested in air at nominal inlet pressure and temperature of 1.379 x 10000 N/sq m and 322.2 K, respectively. Results indicated the turbine total efficiency to be 5.3 points less than design. Analysis of these results has indicated the deficit in performance to be due to stator secondary flow losses, vaneless space surface friction losses, and trailing edge wake mixing losses.

Mclallin, K. L.↗
NASA NTRS2018

Simulated Encounters of the Parker Solar Probe with a Coronal-Hole Jet

Solar coronal jets are small, transient, collimated ejections most easily observed in coronal holes (CHs). The upcoming Parker Solar Probe (PSP) mission provides the first opportunity to encounter CH jets in situ near the Sun and examine their internal structure and dynamics. Using projected mission orbital parameters, we have simulated PSP encounters with a fully three-dimensional magnetohydrodynamic (MHD) model of a CH jet. We find that three internal jet regions, featuring different wave modes and levels of compressibility, have distinct identifying signatures detectable by PSP. The leading Alfvén wave front and its immediate wake are characterized by transAlfvénic plasma flows with mild density enhancements. This front exhibits characteristics of a fast switch-on MHD shock, whose arrival is signaled by the sudden onset of large-amplitude transverse velocity and magnetic-field oscillations highly correlated in space and time. The trailing portion is characterized by supersonic but subAlfvénicout flows of dense plasma with uncorrelated velocity and magnetic-field oscillations. This compressible region contains most of the jet's mass. The volume between the immediate wake and dense jet, the remote wake,mixes and transitions the characteristics of the two other regions. In addition to probing each region separately, we also simulate a corotational PSP-jet encounter. In this scenario, the simulated spacecraft hovers over the jet producing CH, as may occur during the mission's corotational phases, sampling each jet region in turn. We estimate that PSP will encounter numerous CH jets over the lifetime of the mission.

Roberts, Merrill A.↗
NASA NTRS1984

Study of large-scale mixing in developing wakes behind streamlined bodies

Heat-tagging and conditional sampling techniques were used to study the large scale mixing process in the developing wake behind a streamlined body. The results were used to understand the manner in which the large eddy length scale evolves from a value appropriate to a boundary layer to that appropriate to the asymptotic far-wake.

Ramaprian, B. R.↗
NASA NTRS1991

Numerical prediction of wakes in cascades and compressor rotors including the effects of mixing. II - Rotor passage flow and wakes including the effects of spanwise mixing

The results of a numerical investigation to predict the flow field including wakes and mixing in axial-flow compressor rotors are presented. The wake behavior in a moderately loaded compressor rotor is studied numerically using a 3D incompressible Navier-Stokes solver with a high Reynolds number form of a turbulence model. The equations are solved using a time dependent implicit technique. The agreement between the measured data and the predictions is good; including the blade boundary-layer profiles, wake mean-velocity profiles, and decay. The ability of the pseudocompressibility scheme to predict the entire flow field including the near and far wake profiles and its decay characteristics, effect of loading, and the viscous losses of a 3D rotor flow field are demonstrated. The mixing in the downstream regions away from the hub and annulus walls is dominated by wake diffusion. In regions away from the walls the radial mixing is predominantly caused by the transport of mass, momentum, and energy by the radial component of velocity in the wake.

Lakshminarayana, B.↗
NASA NTRS1985

Rotor wake characteristics of a transonic axial flow fan

State of the art turbomachinery flow analysis codes are not capable of predicting the viscous flow features within turbomachinery blade wakes. Until efficient 3D viscous flow analysis codes become a reality there is therefore a need for models which can describe the generation and transport of blade wakes and the mixing process within the wake. To address the need for experimental data to support the development of such models, high response pressure measurements and laser anemometer velocity measurements have been obtained in the wake of a transonic axial flow fan rotor.

Hathaway, M. D.↗
NASA NTRS1985

Rotor wake characteristics of a transonic axial flow fan

State of the art turbomachinery flow analysis codes are not capable of predicting the viscous flow features within turbomachinery blade wakes. Until efficient 3D viscous flow analysis codes become a reality there is therefore a need for models which can describe the generation and transport of blade wakes and the mixing process within the wake. To address the need for experimental data to support the development of such models, high response pressure measurements and laser anemometer velocity measurements were obtained in the wake of a transonic axial flow fan rotor.

Hathaway, M. D.↗
NASA NTRS1991

A viscous flow study of shock-boundary layer interaction, radial transport, and wake development in a transonic compressor

A numerical study based on the 3D Reynolds-averaged Navier-Stokes equation has been conducted to investigate the detailed flow physics inside a transonic compressor. 3D shock structure, shock-boundary layer interaction, flow separation, radial mixing, and wake development are all investigated at design and off-design conditions. Experimental data based on laser anemometer measurements are used to assess the overall quality of the numerical solution. An additional experimental study to investigate end-wall flow with a hot-film was conducted, and these results are compared with the numerical results. Detailed comparison with experimental data indicates that the overall features of the 3D shock structure, the shock-boundary layer interaction, and the wake development are all calculated very well in the numerical solution. The numerical results are further analyzed to examine the radial mixing phenomena in the transonic compressor. A thin sheet of particles is injected in the numerical solution upstream of the compressor. The movement of particles is traced with a 3D plotting package. This numerical survey of tracer concentration reveals the fundamental mechanisms of radial transport in this transonic compressor.

Hah, Chunill↗
NASA NTRS2002

Contrail Formation in Aircraft Wakes Using Large-Eddy Simulations

In this work we analyze the issue of the formation of condensation trails ("contrails") in the near-field of an aircraft wake. The basic configuration consists in an exhaust engine jet interacting with a wing-tip training vortex. The procedure adopted relies on a mixed Eulerian/Lagrangian two-phase flow approach; a simple micro-physics model for ice growth has been used to couple ice and vapor phases. Large eddy simulations have carried out at a realistic flight Reynolds number to evaluate the effects of turbulent mixing and wake vortex dynamics on ice-growth characteristics and vapor thermodynamic properties.

Paoli, R.↗
DOE OSTI2026

Idealized simulations of wind farm interactions with intermittent turbulence in stable boundary layer conditions

Stable atmospheric boundary layer conditions typically correspond to weak turbulence levels, but intermittent periods of elevated turbulence can occur during otherwise quiescent conditions. The interaction between intermittent turbulence and wind turbines is not well understood because of sparse observations, as well as the difficulty in realistically resolving small-scale turbulence during strongly stable conditions with numerical simulations. In this study, an explicit filtering and reconstruction approach for large-eddy simulation (LES) is used to simulate weakly and strongly stable conditions, with surface cooling rates of −0.2 and −2.0 K h −1 , respectively. This approach can sustain resolved background turbulence at relatively coarse grid spacing and stronger stratification compared to conventional closures, permitting more realistic intermittent stable boundary layer (SBL) turbulence. The idealized LES capability of the Weather Research and Forecasting model is employed with turbine rotors parameterized using generalized actuator disks to examine (1) how the presence of turbine wakes affects SBL evolution and (2) the effect of intermittent turbulence on power production and wake recovery. Wakes increase mixing and deepen the SBL, with a stronger effect under strongly stable conditions, primarily because the SBL is shallower and closer to the top of the wind turbine rotor layer. Intermittent turbulence does not have a significant impact on mean power generation and wake recovery because the relevant intermittent turbulent structures in this study only affect the bottom half of the rotor disk. Power production is, however, more variable during periods of elevated turbulence, demonstrating the impact of SBL intermittency. This study uses an idealized configuration, focusing on LES model performance and physical understanding, with the goal of informing future simulations of the conditions observed during the American Wake Experiment.

Energy - Wind↗
NASA NTRS1984

Fully-coupled analysis of jet mixing problems. Part 1. Shock-capturing model, SCIPVIS

A computational model, SCIPVIS, is described which predicts the multiple cell shock structure in imperfectly expanded, turbulent, axisymmetric jets. The model spatially integrates the parabolized Navier-Stokes jet mixing equations using a shock-capturing approach in supersonic flow regions and a pressure-split approximation in subsonic flow regions. The regions are coupled using a viscous-characteristic procedure. Turbulence processes are represented via the solution of compressibility-corrected two-equation turbulence models. The formation of Mach discs in the jet and the interactive analysis of the wake-like mixing process occurring behind Mach discs is handled in a rigorous manner. Calculations are presented exhibiting the fundamental interactive processes occurring in supersonic jets and the model is assessed via comparisons with detailed laboratory data for a variety of under- and overexpanded jets.

Dash, S. M.↗
NASA NTRS1973

A local eddy viscosity model for turbulent shear flow

In the model described, the eddy viscosity is assumed to be a fluid property dependent on the state of the fluid locally, namely the local density, turbulent kinetic energy, turbulence scale, and Mach number. An empirical law was found which related eddy viscosity to these properties satisfactorily for free jets. This law is used without modification for a set of test cases in free shear layers, free-jet decay, coaxial mixing, and wakes. The scale of turbulence is taken as a constant at any axial location equal to the width of the shear layer. By utilizing the boundary-layer order-of-magnitude analysis, a coupled set of fluid dynamic equations is formulated, which of necessity includes the equation for the production of turbulent kinetic energy.

Ortwerth, P. J.↗
NASA NTRS1979

Experimental evaluation of a pilot multinozzle-duct apparatus

A pilot multinozzle and duct were tested at ambient enthalpy to evaluate the suitability of such apparatus for testing thermal protection system panels mounted in the sidewalls of the duct downstream of the nozzle array. The flow field in the duct was complex: effects of wakes and shock waves from the nozzle dominated the flow field; the wakes continually mixed with the surrounding fluid; the boundary layer on the sidewalls of the duct was nonuniform; and near the exit of the duct the sidewall pressure variation was as much as 8.5 percent about the mean wall pressure. Starting loads on the duct walls were higher than those of a similar conventional nozzle and duct. It was concluded that the multinozzle-duct apparatus was not suitable for testing TPS panels, although the design and flow-field information should be of interest to designers of high-energy gasdynamic lasers.

Puster, R. L.↗
NASA NTRS1981

Higher level simulations of turbulent flows

The fundamentals of large eddy simulation are considered and various approaches to this simulation are compared. The subgrid model required by large eddy simulation is discussed as well as the use of this type of simulation in the development of models for the Reynolds-averaged equations and the application of direct simulation to the testing of both subgrid scale and Reynolds-averaged models. Numerical methods used in large eddy and direct simulation are described with emphasis on special purpose methods. Topics covered include the simulation of homogeneous flow, free shear flows, the mixing layer, wakes, and wall-bounded flows including channel flow and the boundary layer. Applications of large eddy simulation in the laboratory as in meteorological and other environmental flows are examined. Directions in which the work is proceeding and what can be expected from higher levels simulated on are examined.

Ferziger, J. H.↗
NASA NTRS1986

Free shear flows - Organized structures and effects of excitation

Recent studies of free shear flows are reviewed. Included are experimental studies of: excited and unexcited circular and elliptic jets, plane wake and mixing layers, and effects of excitation on augmentation and reduction of jet turbulence and jet noise. It is shown that proper excitation can produce both large increases and large decreases in turbulence and Reynolds stress level, mixing and noise generation, suggesting promising technological applications of self-excited jets such as whistler jets. It is also argued that 'ribs' or longitudinal vortices play an important role in production and mixing in free, and perhaps all, turbulent shear flows. The 'cut-and-connect' interaction of vortical structures is proposed as a key mechanism for coherent structure breakdown, mixing, turbulence production, and aerodynamic noise generation. A simple analysis of the viscous vorticity equation using symmetry of configurations of vortical structures before and after the interaction gives realistic predictions.

Hussain, A. K. F.↗
NASA NTRS1993

Local isotropy in distorted turbulent boundary layers at high Reynolds number

This is a report on the continuation of our experimental investigations of the hypothesis of local isotropy in shear flows. This hypothesis, which states that at sufficiently high Reynolds numbers the small-scale structures of turbulent motions are independent of large-scale structures and mean deformations, has been used in theoretical studies of turbulence and computational methods such as large-eddy simulation. Since Kolmogorov proposed his theory, there have been many experiments, conducted in wakes, jets, mixing layers, a tidal channel, and atmospheric and laboratory boundary layers, in which attempts have been made to verify - or refute - the local-isotropy hypothesis. However, a review of the literature over the last five decades indicated that, despite all these experiments in shear flows, there was no consensus in the scientific community regarding this hypothesis, and, therefore, it seemed worthwhile to undertake a fresh experimental investigation into this question.

Saddoughi, Seyed G.↗
NASA NTRS1994

Large-eddy simulation of a plane wake

Previously the theoretical development leading to the dynamic localization model (DLM) for large-eddy simulation (LES) was presented. The method has been successfully applied to isotropic turbulence, channel flow, and the flow over a backward-facing step. Here we apply the model to the computation of the temporally developing place wake. The two main objectives of this project are: (1) Use the model to perform an LES of a time developing plane wake and compare the results with direction numerical simulation (DNS) data to see if important statistical measures can be readily predicted, and to provide a relative evaluation of the several versions of the model in terms of predictive capability and cost; and (2) If the tests in (1) show that the model generates reliable predictions, then use the LES to study various aspects of the physics of turbulent wakes and mixing layers.

Ghosal, Sandip↗