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At least 775 records · Page 43

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NASA NTRS1990

Performance evaluation of a Doppler radar system for wind shear detection

Nonlinear stochastic differential equations are used to model wind shear, and extended Kalman filters are used to generate state estimates from measurements received from a Doppler radar onboard an aircraft. Likelihood-ratio tests are then used to detect the presence of wind shear. The performance of the system is evaluated by deriving theoretical expressions for the false alarm and miss error probabiilties. The approach uses a Fokker-Planck equation. The overall methodology is general and should be of interest in other applications.

Khalaf, Camille S.↗

NASA NTRS1990

Transition in high-speed free shear layers

The laminar free-shear layers considered in the study are formed by combinations of the velocities and momentum thicknesses of two adjacent parallel flows. Transition in wakes, pure free-shear layers of the Chapman type, and separate and partition flows are discussed. A stability-transition connection is emphasized, and it is suggested that a recurring deficiency in some stability calculations is the use of overly simplified laminar profiles. It is also noted that physical principles can be used for estimating the transition location or providing the factors affecting it. One such approach, a threshold theory, is discussed by way of example.

Demetriades, A.↗

NASA NTRS1991

Evolution and dynamics of shear-layer structures in near-wall turbulence

Near-wall flow structures in turbulent shear flows are analyzed, with particular emphasis on the study of their space-time evolution and connection to turbulence production. The results are obtained from investigation of a database generated from direct numerical simulation of turbulent channel flow at a Reynolds number of 180 based on half-channel width and friction velocity. New light is shed on problems associated with conditional sampling techniques, together with methods to improve these techniques, for use both in physical and numerical experiments. The results clearly indicate that earlier conceptual models of the processes associated with near-wall turbulence production, based on flow visualization and probe measurements need to be modified. For instance, the development of asymmetry in the spanwise direction seems to be an important element in the evolution of near-wall structures in general, and for shear layers in particular. The inhibition of spanwise motion of the near-wall streaky pattern may be the primary reason for the ability of small longitudinal riblets to reduce turbulent skin friction below the value for a flat surface.

Johansson, Arne V.↗

NASA NTRS1991

Simulations of free shear layers using a compressible k-epsilon model

A two-dimensional, compressible Navier-Stokes equations with a k-epsilon turbulence model are solved numerically to simulate the flows of compressible free shear layers. The appropriate form of k and epsilon equations for compressible flows are discussed. Sarkar's modeling is adopted to simulate the compressibility effects in the k and epsilon equations. The numerical results show that the spreading rate of the shear layers decreases with increasing convective Mach number. In addition, favorable comparison was found between the calculated results and Goebel and Dutton's experimental data.

Yu, S. T.↗

NASA NTRS1991

Shear-induced instability and arch filament eruption - A magnetohydrodynamic (MHD) numerical simulation

A situation wherein a bipolar magnetic field embedded in a stratified solar atmosphere undergoes symmetrical shear motion at the footpoints is investigated via a 2D (nonplanar) MHD simulation. It was found that the vertical plasma flow velocities grow exponentially, leading to a new type of global MHD instability. The growth rate increases almost linearly until it reaches the same order of magnitude as the Alfven speed. Then a nonlinear MHD instability occurs beyond this point. It was found that the central loops are pinched by opposing Lorentz forces, and the outer closed loops stretch upward with the vertically-rising mass flow. The nonlinear dynamical shearing instability is illustrated by a numerical example that is given for three different values of the plasma beta that span several orders of magnitude.

Wu, S. T.↗

NASA NTRS1990

Non-linear coherent mode interactions and the control of shear layers

A nonlinear integral formulation, based on local linear stability considerations, is used to study the collective interactions between discrete wave-modes associated with large-scale structures and the mean flow in a developing shear layer. Aspects of shear layer control are examined in light of the sensitivity of these interactions to the initial frequency parameter, modal energy contents and modal phases. Manipulation of the large-scale structure is argued to be an effective means of controlling the flow, including the small-scale turbulence dominated region far downstream. Cases of fundamental, 1st and 2nd subharmonic forcing are discussed in conjunction with relevant experiments.

Nikitopoulos, D. E.↗

NASA NTRS1992

Compressible homogeneous shear: Simulation and modeling

Compressibility effects were studied on turbulence by direct numerical simulation of homogeneous shear flow. A primary observation is that the growth of the turbulent kinetic energy decreases with increasing turbulent Mach number. The sinks provided by compressible dissipation and the pressure dilatation, along with reduced Reynolds shear stress, are shown to contribute to the reduced growth of kinetic energy. Models are proposed for these dilatational terms and verified by direct comparison with the simulations. The differences between the incompressible and compressible fields are brought out by the examination of spectra, statistical moments, and structure of the rate of strain tensor.

Sarkar, S.↗

NASA NTRS1991

Simulations of free shear layers using a compressible kappa-epsilon model

A two-dimensional, compressible Navier-Stokes equation with a k-epsilon turbulence model is solved numerically to simulate the flow of a compressible free shear layer. The appropriate form of k and epsilon equations for compressible flow is discussed. Sarkar's modeling is adopted to simulate the compressibility effects in the k and epsilon equations. The numerical results show that the spreading rate of the shear layers decreases with increasing convective Mach number. In addition, favorable comparison was found between the calculated results and experimental data.

Yu, S. T.↗

NASA NTRS1992

Modeling of the reactant conversion rate in a turbulent shear flow

Results are presented of direct numerical simulations (DNS) of spatially developing shear flows under the influence of infinitely fast chemical reactions of the type A + B yields Products. The simulation results are used to construct the compositional structure of the scalar field in a statistical manner. The results of this statistical analysis indicate that the use of a Beta density for the probability density function (PDF) of an appropriate Shvab-Zeldovich mixture fraction provides a very good estimate of the limiting bounds of the reactant conversion rate within the shear layer. This provides a strong justification for the implementation of this density in practical modeling of non-homogeneous turbulent reacting flows. However, the validity of the model cannot be generalized for predictions of higher order statistical quantities. A closed form analytical expression is presented for predicting the maximum rate of reactant conversion in non-homogeneous reacting turbulence.

Frankel, S. H.↗

NASA NTRS1990

Organized motions underlying turbulent shear flows

The objective of this project is to determine the nature and significance of the organized motions underlying turbulent shear flow. There is considerable experimental evidence for the existence of such motions. In particular, one consistently observes longitudinal streaks with a spacing of about 100 in wall units in the near-wall region of wall-bounded shear flows. Recently, an analysis based on the direct resonance mechanism has predicted the appearance of streaks with precisely such a spacing. Also, the minimum channel simulations of Jimenez and Moin have given a strong dynamical significance to that spanwise length scale. They have shown that turbulent-like flows can not be maintained when the spanwise wavelength of the motion is constrained to be less than about that critical number. A critical review of the direct resonance ideas and the non-linear theory of Benney and Gustavsson is presented first. It is shown how this leads to the later mean flow-first harmonic theory of Benney. Finally, we note that a different type of analysis has led to the prediction streaks with a similar spacing. This latter approach consists of looking for optimum fields and directly provides deep insights into why a particular structure or a particular scale should be preferred.

Waleffe, F.↗

NASA NTRS1990

Resonant instability of supersonic shear layers

A computer simulation of possible resonant instability of a supersonic shear layer is carried out. The resonance of two acoustic duct modes of the flow induced by periodic Mach waves generated by a wavy wall is sought. Results of the simulations are reported. Simulations are unable to document a resonant instability and the mixing characteristics remain unchanged. Possible weakness of the present simulations are discussed. A second set of simulations involving a mixing layer separating a supersonic and a subsonic stream were performed. A wavy wall placed adjacent to the supersonic stream to produce a set of periodic Mach waves terminating at the shear layer is modelled. The entire flow field is similar to that of an imperfectly expanded supersonic jet discharging into a subsonic coflowing stream for which enhanced mixing due to the onset of screech (feedback instability) is known to occur. The purpose of these simulations is to see if enhanced mixing and feedback instability would, indeed, take place. Some evidence of feedback oscillations is found in the simulated flow.

Tam, C. K. W.↗

NASA NTRS1992

Reynolds stress calculations of homogeneous turbulent shear flow with bounded energy states

Reynolds stress calculations of homogeneous turbulent shear flow are conducted with a second-order closure model modified to account for non-equilibrium vortex stretching in the dissipation rate transport equation, as recently proposed by Bernard and Speziale. As with the earlier reported k-epsilon model calculations incorporating this vortex stretching effect, a production-equals-dissipation equilibrium is obtained with bounded turbulent kinetic energy and dissipation. However, this equilibrium is not achieved until the dimensionless time greater than 60, an elapsed time that is at least twice as large as any of those considered in previous numerical and physical experiments on homogeneous shear flow. Direct quantitative comparisons between the model predictions and the results of experiments are quite favorable. In particular, it is shown that the inclusion of this non-equilibrium vortex stretching effect has the capability of explaining the significant range of production to dissipation ratios observed in experiments.

Speziale, Charles G.↗

NASA NTRS1990

Stochastic prediction techniques for wind shear hazard assessment

The threat of low-altitude wind shear has prompted development of aircraft-based sensors that measure winds directly on the aircraft's intended flight path. Measurements from these devices are subject to turbulence inputs and measurement error, as well as to the underlying wind profile. Stochastic estimators are developed to process on-board Doppler sensor measurements, producing optimal estimates of the winds along the path. A stochastic prediction technique is described to predict the hazard to the aircraft from the estimates as well as the level of uncertainty of the hazard prediction. The stochastic prediction technique is demonstrated in a simulated microburst wind shear environment. Use of the technique in a decision-making process is discussed.

Stratton, D. A.↗

NASA NTRS1991

Coronal current-sheet formation - The effect of asymmetric and symmetric shears

A 2.5D numerical code is used to investigate the results of an asymmetric shear imposed on a potential quadrupolar magnetic field under two sets of atmospheric boundary conditions - a low-beta plasma with line tying at the base, similar to the line-tied analytic model, and a hydrostatic-equilibrium atmosphere with solar gravity, typical of the observed photosphere-chromosphere interface. The low-beta simulation confirms the crucial role of the line-tying assumption in producting current sheets. The effects of a symmetric shear on the same hydrostatic-equilibrium atmosphere is examined, using more grid points to improve the resolution of the current structures which form along the flux surfaces. It is found that true current sheets do not form in the corona when a more realistic model is considered. The amount of Ohmic dissipation in the thick currents is estimated to be two to four orders of magnitude below that required to heat the corona. It is concluded that magnetic topologies of the type examined here do not contribute significantly to coronal heating.

Karpen, Judith T.↗

NASA NTRS1991

Coronal evolution due to shear motion

Numerical solutions of the compressible MHD equations are used here to simulate the evolution of an initially force-free magnetic field in a static corona as a result of slow photospheric motion of the magnetic field footpoints. Simulations have been completed for values of plasma beta from 0.1 to 0.5, maximum shear velocities from 0.5 to 10.3 km/s, and with various amounts of resistive and viscous dissipation. In all cases the evolution proceeds in two qualitatively different stages. In the earlier stage, the field evolves gradually with the field lines, expanding outward at a velocity not unlike the shear velocity. Then, the field begins to expand much more rapidly until it reaches velocities exceeding a characteristic Alfven velocity. Inclusion of the thermodynamics, gravity, and compressibility is shown to have only a quantitative effect on the onset of the eruptive phase, illustrating that the primary interactions are between the dynamics and the magnetic field evolution.

Steinolfson, R. S.↗

NASA NTRS1992

Airborne in situ computation of the wind shear hazard index

An algorithm for airborne in situ computation of the wind shear hazard index (F-factor) was developed and evaluated in simulation and verified in flight. The algorithm was implemented on NASA's B-737-100 airplane, and tested under severe maneuvering, nonhazardous wind conditions, and normal takeoffs and landings. The airplane was flown through actual microburst conditions in Orlando, FL, where the algorithm produced wind shear measurements which were confirmed by an independent, ground-based radar measurement. Flight test results indicated that the in situ F-factor algorithm correctly measured the effect of the wind environment on the airplane's performance, and produced no nuisance alerts.

Oseguera, Rosa M.↗

NASA NTRS1992

Experiments on shear layer mixing at hypervelocity conditions

Utilization of an expansion tube to acquire turbulent, compressible free shear layer data with gases of direct relevance to hypersonic combustion systems (nitrogen, hydrogen, and oxygen) is demonstrated. Visualizations of the free shear layer by laser holographic interferometry and schlieren imaging show a distinctly asymmetric growth rate that is highly preferential toward the lower speed (secondary stream) side. For the nitrogen/nitrogen case, for which the convective velocity was lowest and convective Mach number highest, the growth rate on the primary side was virtually zero until the mixing layer reached the opposite (secondary) wall.

Erdos, John↗

NASA NTRS1990

Filament cooling and condensation in a sheared magnetic field

Thermal instability driven by optically thin radiation in the corona is believed to initiate the formation of solar filaments. The fact that filaments are observed generally to separate regions of opposite, line-of-sight, magnetic polarity in the differentially rotating photosphere suggests that filament formation requires the presence of a highly sheared magnetic field. The coupled energetics and dynamics of the most important condensation modes, those due to perpendicular thermal conduction at short wavelengths are discussed. Linear structure in the sheared field and their growth rates is described, and 2D, nonlinear, MHD simulations of the evolution of these modes in a force-free field are conducted. The simulations achieve the fine thermal structures, minimum temperatures and maximum densities characteristic of observed solar filaments.

Van Hoven, Gerard↗