7,957 research outputs found
Three regularization models of the Navier-Stokes equations
We determine how the differences in the treatment of the subfilter-scale
physics affect the properties of the flow for three closely related
regularizations of Navier-Stokes. The consequences on the applicability of the
regularizations as SGS models are also shown by examining their effects on
superfilter-scale properties. Numerical solutions of the Clark-alpha model are
compared to two previously employed regularizations, LANS-alpha and Leray-alpha
(at Re ~ 3300, Taylor Re ~ 790) and to a DNS. We derive the Karman-Howarth
equation for both the Clark-alpha and Leray-alpha models. We confirm one of two
possible scalings resulting from this equation for Clark as well as its
associated k^(-1) energy spectrum. At sub-filter scales, Clark-alpha possesses
similar total dissipation and characteristic time to reach a statistical
turbulent steady-state as Navier-Stokes, but exhibits greater intermittency. As
a SGS model, Clark reproduces the energy spectrum and intermittency properties
of the DNS. For the Leray model, increasing the filter width decreases the
nonlinearity and the effective Re is substantially decreased. Even for the
smallest value of alpha studied, Leray-alpha was inadequate as a SGS model. The
LANS energy spectrum k^1, consistent with its so-called "rigid bodies,"
precludes a reproduction of the large-scale energy spectrum of the DNS at high
Re while achieving a large reduction in resolution. However, that this same
feature reduces its intermittency compared to Clark-alpha (which shares a
similar Karman-Howarth equation). Clark is found to be the best approximation
for reproducing the total dissipation rate and the energy spectrum at scales
larger than alpha, whereas high-order intermittency properties for larger
values of alpha are best reproduced by LANS-alpha.Comment: 21 pages, 8 figure
Optimal model parameters for multi-objective large-eddy simulations
A methodology is proposed for the assessment of error dynamics in large-eddy simulations. It is demonstrated that the optimization of model parameters with respect to one flow property can be obtained at the expense of the accuracy with which other flow properties are predicted. Therefore, an approach is introduced which allows to assess the total errors based on various flow properties simultaneously. We show that parameter settings exist, for which all monitored errors are "near optimal," and refer to such regions as "multi-objective optimal parameter regions." We focus on multi-objective errors that are obtained from weighted spectra, emphasizing both large- as well small-scale errors. These multi-objective optimal parameter regions depend strongly on the simulation Reynolds number and the resolution. At too coarse resolutions, no multi-objective optimal regions might exist as not all error-components might simultaneously be sufficiently small. The identification of multi-objective optimal parameter regions can be adopted to effectively compare different subgrid models. A comparison between large-eddy simulations using the Lilly-Smagorinsky model, the dynamic Smagorinsky model and a new Re-consistent eddy-viscosity model is made, which illustrates this. Based on the new methodology for error assessment the latter model is found to be the most accurate and robust among the selected subgrid models, in combination with the finite volume discretization used in the present study
Leray and LANS- modeling of turbulent mixing
Mathematical regularisation of the nonlinear terms in the Navier-Stokes
equations provides a systematic approach to deriving subgrid closures for
numerical simulations of turbulent flow. By construction, these subgrid
closures imply existence and uniqueness of strong solutions to the
corresponding modelled system of equations. We will consider the large eddy
interpretation of two such mathematical regularisation principles, i.e., Leray
and LANS regularisation. The Leray principle introduces a {\bfi
smoothed transport velocity} as part of the regularised convective
nonlinearity. The LANS principle extends the Leray formulation in a
natural way in which a {\bfi filtered Kelvin circulation theorem},
incorporating the smoothed transport velocity, is explicitly satisfied. These
regularisation principles give rise to implied subgrid closures which will be
applied in large eddy simulation of turbulent mixing. Comparison with filtered
direct numerical simulation data, and with predictions obtained from popular
dynamic eddy-viscosity modelling, shows that these mathematical regularisation
models are considerably more accurate, at a lower computational cost.Comment: 42 pages, 12 figure
Non-local modulation of the energy cascade in broad-band forced turbulence
Classically, large-scale forced turbulence is characterized by a transfer of
energy from large to small scales via nonlinear interactions. We have
investigated the changes in this energy transfer process in broad-band forced
turbulence where an additional perturbation of flow at smaller scales is
introduced. The modulation of the energy dynamics via the introduction of
forcing at smaller scales occurs not only in the forced region but also in a
broad range of length-scales outside the forced bands due to non-local triad
interactions. Broad-band forcing changes the energy distribution and energy
transfer function in a characteristic manner leading to a significant
modulation of the turbulence. We studied the changes in this transfer of energy
when changing the strength and location of the small-scale forcing support. The
energy content in the larger scales was observed to decrease, while the energy
transport power for scales in between the large and small scale forcing regions
was enhanced. This was investigated further in terms of the detailed transfer
function between the triad contributions and observing the long-time statistics
of the flow. The energy is transferred toward smaller scales not only by
wavenumbers of similar size as in the case of large-scale forced turbulence,
but by a much wider extent of scales that can be externally controlled.Comment: submitted to Phys. Rev. E, 15 pages, 18 figures, uses revtex4.cl
Response maxima in time-modulated turbulence: Direct Numerical Simulations
The response of turbulent flow to time-modulated forcing is studied by direct
numerical simulations of the Navier-Stokes equations. The large-scale forcing
is modulated via periodic energy input variations at frequency . The
response is maximal for frequencies in the range of the inverse of the large
eddy turnover time, confirming the mean-field predictions of von der Heydt,
Grossmann and Lohse (Phys. Rev. E 67, 046308 (2003)). In accordance with the
theory the response maximum shows only a small dependence on the Reynolds
number and is also quite insensitive to the particular flow-quantity that is
monitored, e.g., kinetic energy, dissipation-rate, or Taylor-Reynolds number.
At sufficiently high frequencies the amplitude of the kinetic energy response
decreases as . For frequencies beyond the range of maximal response,
a significant change in phase-shift relative to the time-modulated forcing is
observed.Comment: submitted to Europhysics Letters (EPL), 8 pages, 8 Postscript
figures, uses epl.cl
Flow and bubble statistics of turbulent bubble-laden downflow channel
Direct numerical simulations of fully developed turbulent channel downflow at bulk Re equal to 6300, loaded with monodisperse bubbles at gas volume fractions α=0.5% , α=2.5% and α=10 have been carried out. Bubble deformability, surface tension, as well as discontinuity in the material properties across the bubble interfaces are explicitly accounted for. A full-scale channel of size 4πH × 2H × 4πH/3 in terms of the channel half-width H containing a number of bubbles up to O(103) is considered. The statistical structure of the bubbles, the probability density function describing the bubble velocity and the liquid kinetic energy spectra have been determined. A close range preferential clustering of the bubbles was found with a maximum density independent of the gas volume fraction at a separation distance of about 2.2R, with R the bubble radius. Preferential horizontal alignment and a general tendency to repulsion is shown for separation distances smaller than 3R. At larger separation distances a close to random distribution is observed for α=2.5% and α=10%, while tendency to vertical alignment is observed for α=0.5% . The pdf of the bubble velocity fluctuations was found to be well approximated by a Gaussian distribution. The liquid kinetic energy spectra in the channel core do not show a marked -3 scaling, which was previously reported for homogeneous isotropic turbulence and pseudo-turbulence
Studying patterns of use of transport modes through data mining - Application to U.S. national household travel survey data set
Data collection activities related to travel require large amounts of financial and human resources to be conducted successfully. When available resources are scarce, the information hidden in these data sets needs to be exploited, both to increase their added value and to gain support among decision makers not to discontinue such efforts. This study assessed the use of a data mining technique, association analysis, to understand better the patterns of mode use from the 2009 U.S. National Household Travel Survey. Only variables related to self-reported levels of use of the different transportation means are considered, along with those useful to the socioeconomic characterization of the respondents. Association rules potentially showed a substitution effect between cars and public transportation, in economic terms but such an effect was not observed between public transportation and nonmotorized modes (e.g., bicycling and walking). This effect was a policy-relevant finding, because transit marketing should be targeted to car drivers rather than to bikers or walkers for real improvement in the environmental performance of any transportation system. Given the competitive advantage of private modes extensively discussed in the literature, modal diversion from car to transit is seldom observed in practice. However, after such a factor was controlled, the results suggest that modal diversion should mainly occur from cars to transit rather than from nonmotorized modes to transi
Simple Front End Electronics for Multigap Resistive Plate Chambers
A simple circuit for the presentation of the signals from Multi-gap Resistive
Plate Chambers (MRPCs) to standard existing digitization electronics is
described. The circuit is based on "off-the-shelf" discrete components. An
optimization of the values of specific components is required to match the
aspects of the MRPCs for the given application. This simple circuit is an
attractive option for the initial signal processing for MRPC prototyping and
bench- or beam-testing efforts, as well as for final implementations of
small-area Time-of-Flight systems with existing data acquisition systems.Comment: submitted to Nucl. Inst. and Methods, Section
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