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  • FLUID MECHANICS AND HEAT TRANSFER  (19)
  • SPACE RADIATION  (8)
  • Environment Pollution  (5)
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  • 11
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    Modeling the dissipation rate in rotating turbulent flows (1990)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: A variety of modifications to the modeled dissipation rate transport equation that have been proposed during the past two decades to account for rotational strains are examined. The models are subjected to two crucial test cases: the decay of isotropic turbulence in a rotating frame and homogeneous shear flow in a rotating frame. It is demonstrated that these modifications do not yield substantially improved predictions for these two test cases and in many instances give rise to unphysical behavior. An alternative proposal, based on the use of the tensor dissipation rate, is made for the development of improved models.
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: NASA-CR-187485 , ICASE-90-88 , NAS 1.26:187485 , AD-A232079
    Format: application/pdf
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  • 12
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    An analysis of RNG based turbulence models for homogeneous shear flow (1991)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: In a recent paper, the authors compared the performance of a variety of turbulence models including the k-epsilon model and the second-order closure model based on Renormalization Group (RNG) Methods. The performance of these RNG models in homogeneous turbulent shear flow was found to be quite poor, apparently due to the value of the constant C(sub epsilon1) in the modeled dissipation rate equation which was substantially lower than its traditional value. However, recently a correction has been made in the RNG based calculation of C(sub epsilon1). It is shown that with the new value of C(sub epsilon1), the performance of the RNG k-epsilon model is substantially improved. On the other hand, while the predictions of the revised RNG second-order closure model are better, some lingering problems still remain which can be easily remedied by the addition of higher order terms.
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: NASA-CR-187552 , NAS 1.26:187552 , ICASE-91-37 , AD-A236667
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  • 13
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    Modeling the pressure-strain correlation of turbulence: An invariant dynamical systems approach (1990)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The modeling of the pressure-strain correlation of turbulence is examined from a basic theoretical standpoint with a view toward developing improved second-order closure models. Invariance considerations along with elementary dynamical systems theory are used in the analysis of the standard hierarchy of closure models. In these commonly used models, the pressure-strain correlation is assumed to be a linear function of the mean velocity gradients with coefficients that depend algebraically on the anisotropy tensor. It is proven that for plane homogeneous turbulent flows the equilibrium structure of this hierarchy of models is encapsulated by a relatively simple model which is only quadratically nonlinear in the anisotropy tensor. This new quadratic model - the SSG model - is shown to outperform the Launder, Reece, and Rodi model (as well as more recent models that have a considerably more complex nonlinear structure) in a variety of homogeneous turbulent flows. Some deficiencies still remain for the description of rotating turbulent shear flows that are intrinsic to this general hierarchy of models and, hence, cannot be overcome by the mere introduction of more complex nonlinearities. It is thus argued that the recent trend of adding substantially more complex nonlinear terms containing the anisotropy tensor may be of questionable value in the modeling of the pressure-strain correlation. Possible alternative approaches are discussed briefly.
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: NASA-CR-181979 , NAS 1.26:181979 , ICASE-90-5 , AD-A227187
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  • 14
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    On the prediction of free turbulent jets with swirl using a quadratic pressure-strain model (1994)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Data from free turbulent jets both with and without swirl are used to assess the performance of the pressure-strain model of Speziale, Sarkar and Gatski which is quadratic in the Reynolds stresses. Comparative predictions are also obtained with the two versions of the Launder, Reece and Rodi model which are linear in the same terms. All models are used as part of a complete second-order closure based on the solution of differential transport equations for each non-zero component of the Reynolds stress tensor together with an equation for the scalar energy dissipation rate. For non-swirling jets, the quadratic model underestimates the measured spreading rate of the plane jet but yields a better prediction for the axisymmetric case without resolving the plane jet/round jet anomaly. For the swirling axisymmetric jet, the same model accurately reproduces the effects of swirl on both the mean flow and the turbulence structure in sharp contrast with the linear models which yield results that are in serious error. The reasons for these differences are discussed.
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: NASA-CR-194964 , NAS 1.26:194964 , ICASE-94-70
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  • 15
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    Modeling the dissipation rate in rotating turbulent flows (1992)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: A variety of modifications to the modeled dissipation rate transport equation that have been proposed during the past two decades to account for rotational strains are examined. The models are subjected to two crucial test cases: the decay of isotropic turbulence in a rotating frame and homogeneous shear flow in a rotating frame. It is demonstrated that these modifications do not yield substantially improved predictions for these two test cases and in many instances give rise to unphysical behavior. An alternative proposal, based on the use of the tensor dissipation rate, is made for the development of improved models.
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: In: Studies in turbulence (A94-12376 02-34); p. 129-151.
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  • 16
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    An alternative assessment of second-order closure models in turbulent shear flows (1994)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The performance of three recently proposed second-order closure models is tested in benchmark turbulent shear flows. Both homogeneous shear flow and the log-layer of an equilibrium turbulent boundary layer are considered for this purpose. An objective analysis of the results leads to an assessment of these models that stands in contrast to that recently published by other authors. A variety of pitfalls in the formulation and testing of second-order closure models are uncovered by this analysis.
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: AD-A279072 , NASA-CR-194881 , ICASE-94-10 , NAS 1.26:194881
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  • 17
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    Effects of drift on the transport of cosmic rays. IV - Modulation by a wavy interplanetary current sheet (1981)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The effect of a wavy interplanetary current sheet on the solar modulation of galactic cosmic rays is studied. A simple model of the heliospheric magnetic field is employed in which there is an Archimedean spiral magnetic field directed in one direction above the current sheet and another direction below. The current sheet itself is taken to be the extension by the solar wind of a surface that, at the sun, is a plane inclined at an angle alpha to the solar equator. Two magnetic sectors will therefore be seen by an observer within an angle alpha of the heliographic equator. A simple approximate model is used to investigate modulation. As the tilt angle alpha is increased from zero, it is found that the cosmic-ray intensity decreases. For reasonable variation of alpha from approximately 10 deg to approximately 30 deg, the change in intensity can be compared to that observed as one goes from solar minimum to solar maximum. It is concluded that changes in the waviness of the interplanetary current sheet can have an important, and perhaps dominant, effect in producing solar modulation of galactic cosmic rays.
    Keywords: SPACE RADIATION
    Type: Astrophysical Journal; vol. 243
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  • 18
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    the role of shock waves in modulation of galactic cosmic rays (1985)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The understanding of modulation of the galactic cosmic rays has considerably progressed by the exploration by space probes of major heliospheric structures, such as the Corotating Interaction Regions, the neutral sheet, and the compression regions of intense heliospheric magnetic fields. Also relevant in this context were the detections in the outer heliosphere of long lasting Forbush type decreases of cosmic ray intensity. The results of recent theoretical studies on the changes in intensity and energy, at different location from the Sun, induced by the passage of shocks across the heliosphere are presented. In this version of the research, the simplest cases of modulation of uGV and 2GV particles by single or several shocks during periods of positive and negative solar field polarity are reviewed. The results of the theoretical aspects of the search is reported. The comparison of the theoretical predictions with space probe data allows conclusions to be drawn on the role of shocks on the modulation on both the 11 and 22 year galactic cosmic ray cycles in the outer heliosphere and on the plausibility of the models and parameters used.
    Keywords: SPACE RADIATION
    Type: SH-4.1-7 , 19th Intern. Cosmic Ray Conf - Vol. 4; p 384-387; NASA-CP-2376-VOL-4
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  • 19
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    Solar-flare-induced Forbush decreases - Dependence on shock wave geometry (1984)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: It is argued that the principal mechanism for the association of Forbush decreases with the passage of a solar flare shock wave is prolonged containment of cosmic ray particles behind the flare compression region, which acts as a semipermeable obstacle to particle motion along the field lines, leading to additional adiabatic cooling of the particles. Liouville's theorem is used to calculate the instantaneous distribution function at 1 AU for each particle arriving at the earth. By averaging over a large number of individual estimates, a representative estimate of the omnidirectional phase space density and the corresponding particle intensity is obtained. The energy change of individual particles at the shocks is found to be small in comparison to the energy lost by adiabatic cooling of the cosmic rays between the shock wave and the sun. The effects of particle rigidity, diffusion coefficient, and flare longitude on the magnitude of the Forbush decrease are quantitatively investigated.
    Keywords: SPACE RADIATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 89; 2991-299
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  • 20
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    A theoretical interpretation of Forbush decreases (1981)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: An analysis is presented of the intensity variations of relativistic cosmic rays, at 1 AU, which are associated with the passage of a flare shock wave. The magnitude and time profile obtained is similar to that observed in flare-induced Forbush decreases, and the principal cause of the energy reduction is an increase in adiabatic cooling of the arriving particles due to prolonged containment behind the compressed field of the flare shock wave. The large decreases calculated for the smaller diffusion coefficients are inconsistent with observation, implying larger mean free paths than those traditionally assumed at these rigidities. The method presented may be used to study intensity variations associated with other large-scale structures of the interplanetary field, such as corotating interaction regions.
    Keywords: SPACE RADIATION
    Type: International Cosmic Ray Conference; Jul 13, 1981 - Jul 25, 1981; Paris; France
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