scholarly journals Initial condition effects on large scale structure in numerical simulations of plane mixing layers

2016 ◽  
Vol 28 (1) ◽  
pp. 015111 ◽  
Author(s):  
W. A. McMullan ◽  
S. J. Garrett
Keyword(s):  
Numerical Simulations ◽  
Large Scale ◽  
Large Scale Structure ◽  
Scale Structure ◽  
Mixing Layers ◽  
Initial Condition

scholarly journals Extragalactic filament detection with a layer smoothing method

2014 ◽  
Vol 4 (1-2) ◽  
pp. 42-45 ◽  
Author(s):  
A. Tugay
Keyword(s):  
Numerical Simulations ◽  
Large Scale ◽  
Large Scale Structure ◽  
Density Field ◽  
Scale Structure ◽  
Smoothing Method ◽  
Computer Algorithms ◽  
The Real

Filaments are clearly visible in galaxy distributions, but they are difficult to detect by computer algorithms. Most methods of filament detection can be used only with numerical simulations of a large-scale structure. New simple and effective methods for the real filament detection should be developed. The method of a smoothed galaxy density field was applied in this work to SDSS data of galaxy positions. Five concentric radial layers of 100 Mpc are appropriate for filaments detection. Two methods were tested for the first layer and one more method is proposed.

Far-field noise of a subsonic jet under controlled excitation

1985 ◽  
Vol 152 ◽  
pp. 83-111 ◽  
Author(s):  
K. B. M. Q. Zaman
Keyword(s):  
Flow Field ◽  
Shear Layer ◽  
Strouhal Number ◽  
Large Scale ◽  
Jet Noise ◽  
Large Scale Structure ◽  
Broadband Noise ◽  
Scale Structure ◽  
Initial Condition ◽  
High Speeds

The phenomena of excitation-induced suppression and amplification of broadband jet noise have been experimentally investigated in an effort to understand the mechanisms, especially in relation to the near flow-field large-scale structure dynamics. Suppression is found to occur only in jets at low speeds with laminar exit boundary layers, the optimum occurring for excitation at Stθ ≈ 0.017, where Stθ is the Strouhal number based on the initial shear-layer momentum thickness. The suppression mechanism is linked to an initial-condition effect on the large-scale structure dynamics. The interaction and evolution of laminar-like structures at low jet speeds produce more (normalized) noise and turbulence, compared to asymptotically lower levels at high speeds when the initial shear layer is no longer laminar. The effect of initial condition has been demonstrated by tripped versus untripped jet data. The excitation at Stθ ≈ 0.017 results in a quick roll-up and transition of the laminar shear-layer vortices, yielding coherent structures which are similar to those at high speeds. Thus, the broadband noise and turbulence are suppressed, but at the most to the asymptotically lower levels. When at the asymptotic level, the broadband jet noise can only be amplified by the excitation; the amplification is found to be maximum for excitation in the StD range of 0.65–0.85, StD being the Strouhal number based on the jet diameter. Excitation in this StD range also produces strongest vortexpairing activity. From spectral analysis of the flow-field and the near sound-pressure field, it is inferred that the pairing process induced by the excitation is at the origin of the broadband noise amplification.

Global texture as the origin of large-scale structure: Numerical simulations of evolution

1991 ◽  
Vol 43 (4) ◽  
pp. 1038-1046 ◽  
Author(s):  
David N. Spergel ◽  
Neil Turok ◽  
William H. Press ◽  
Barbara S. Ryden
Keyword(s):  
Numerical Simulations ◽  
Large Scale ◽  
Large Scale Structure ◽  
Scale Structure

Interplanetary Scintillation and Flare — Produced Disturbances

1975 ◽  
Vol 2 (6) ◽  
pp. 378-379 ◽  
Author(s):  
B.D. Ward
Keyword(s):  
Numerical Simulations ◽  
Large Scale ◽  
Large Scale Structure ◽  
Scale Structure ◽  
Interplanetary Scintillation ◽  
Ensemble Averaging ◽  
Multiple Spacecraft ◽  
Spacecraft Data

Determination of the large-scale structure of flare-produced disturbances requires either multiple spacecraft observations or an ensemble averaging of single spacecraft observations of a number of events. There is currently some confusion in the results of studies of shock normals derived from spacecraft data. Chao and Lepping (1974) suggest that the average shock shape near 1 AU is essentially spherical while Bavassano et al. (1973) suggest that the disturbance corresponds closely to the shapes predicted in the numerical simulations of de Young and Hundhausen (1971).

The large-scale structure of homogenous turbulence

1956 ◽  
Vol 248 (949) ◽  
pp. 369-405 ◽  
Keyword(s):  
Large Scale ◽  
Asymptotic Form ◽  
Large Scale Structure ◽  
Scale Structure ◽  
Homogeneous Turbulence ◽  
Wave Number ◽  
Initial Condition ◽  
Triple Correlation ◽  
Leading Term ◽  
Asymptotic Forms

The starting-point for this paper lies in some results obtained by Proudman & Reid (1954) for isotropic turbulence with zero fourth-order cumulants. They showed in that case that the quantity _ /*00 u2 r4f(r) dr is not a dynamical invariant and that the first time derivative of the triple correlation J0 k( r ) is of order r -4 when r is large. The customary assumption that all velocity cumulants in homogeneous turbulence are exponentially small for large separations, and the consequent results, about the large-scale structure of the motion and about the final period of decay of the turbulence, are thus suspect, and we have redeveloped the whole subject ab initio. The fallacy in the old assumption of exponentially small cumulants can be ascribed to the action of pressure forces, which are local in their effect but which have values determined instantaneously by the whole velocity field. If at some initial instant a finite region only of an infinite fluid is in motion, at subsequent instants pressure forces generate a surrounding irrotational velocity distribution that falls off as some integral power of the distance from the central region. Likewise, the action of pressure forces in homogeneous turbulence is to ensure the development of algebraic asymptotic forms of velocity cumulants, the analogue of the finite region of initial motion being a volume of the fluid over which the vorticity is effectively correlated. However, an essential difference between the two cases is that in homogeneous turbulence pressure forces also build up long-range statistical connexions in the vorticity distribution. Having recognized why the old assumption is wrong, it is necessary to consider what kinds of asymptotic forms of velocity cumulants (for large separation) are dynamically persistent, and to consider in particular what asymptotic forms are likely to occur when homogeneous turbulence is generated in the usual way by setting a regular array of rods across a uniform stream. We have been able to find only one kind of large-scale structure that is unchanged by dynamical action, and this is also the kind of large-scale structure that develops from a plausibly idealized initial condition representing the effect of the grid on the stream. This initial condition, according to the hypothesis on which the positive results of this paper are based, is that there is a virtual origin in time at which all integral moments of cumulants of the velocity field converge. The important consequence of this hypothesis is that the effect of pressure forces is subsequently to develop asymptotic forms that are integral power-laws. It is shown, from a consideration of all the time derivatives at the initial instant, that the velocity covariance u{u in general becomes of order r -5 when the separation r is large, the leading term having the property that it makes no contribution to the vorticity covariance 0^X0$> which becomes of order r -8 . This semi-irrotational property of the asymptotic form, which arises from the fact that pressure forces act only indirectly on the vorticity, allows the asymptotic form of to be determined explicitly. By methods that are new in turbulence theory and that involve a good deal of tensor manipulation, it is found that U'} = mnV2 “ $ $ $ V2 ~ drjdr) when r is large, where the coefficient Cpqmn is related to the fourth integral moment of in a known way. There is a corresponding expression for the leading term in the spectrum tensor at small wave-numbers, which is now not analytic. The spectrum function giving the distribution of energy with respect to wave-number magnitude k is in general of the form E(k) = a 4 + 0(^5 In k)9 when k is small. Corresponding expressions are found for the asymptotic forms of the various terms occurring in the dynamical equation giving the rate of change of Both the inertia and pressure terms in this equation are found to be of order r~5, and as a consequence the coefficient Cpqmn (and likewise C) is not a dynamical invariant. It is shown that the integral J*2qzqrmrwdr (which exists, despite the apparent logarithmic divergence at large r) is uniquely related to CpqmnJ and it too varies during the decay, contrary to past belief. The final period of decay is examined afresh, and it is found that the energy then varies as which is also the result found experimentally; the power ( — f) arises from the fact that the spectrum tensor is of order k2 when the wave-number k is small, and is unaffected by the non-analytic character of that leading term. The covariance tqw] does not have a simple form in the final period; it is determined by the parameter on the previous history of the decay in a complicated way. It is rather puzzling that measurements indicate that the longitudinal correlation coefficient has a simple Gaussian form in the final period of decay, as would be the case for an analytic spectrum. We suggest this observation may be true only for turbulence of very low initial Reynolds number, for which the non-analytic part of the spectrum tensor has little time to develop. Finally, the results are specialized to correspond to turbulence which is completely isotropic. For reasons related to the symmetry, is now no larger than 0 ( r -6 ) when r is large (we have been unable to determine the exact order), and the leading term in the spectrum tensor, of order k 2 , is analytic. As suggested by Proudman & Reid’s work, the triple correlation k { r ) is of order r -4 when r is large and d / r«> _ -j-u2 r*f(r) dr> = (w2)1 lim rAk{r)

Effects of heat release on the large-scale structure in turbulent mixing layers

1989 ◽  
Vol 199 ◽  
pp. 297-332 ◽  
Author(s):  
P. A. Mcmurtry ◽  
J. J. Riley ◽  
R. W. Metcalfe
Keyword(s):  
Heat Release ◽  
Turbulent Mixing ◽  
Large Scale ◽  
Mixing Layer ◽  
Large Scale Structure ◽  
Scale Structure ◽  
Mixing Layers ◽  
Turbulent Mixing Layer ◽  
Large Scale Structures ◽  
Scale Structures

The effects of chemical heat release on the large-scale structure in a chemically reacting, turbulent mixing layer are investigated using direct numerical simulations. Three-dimensional, time-dependent simulations are performed for a binary, single-step chemical reaction occurring across a temporally developing turbulent mixing layer. It is found that moderate heat release slows the development of the large-scale structures and shifts their wavelengths to larger scales. The resulting entrainment of reactants is reduced, decreasing the overall chemical product formation rate. The simulation results are interpreted in terms of turbulence energetics, vorticity dynamics, and stability theory. The baroclinic torque and thermal expansion in the mixing layer produce changes in the flame vortex structure that result in more diffuse vortices than in the constant-density case, resulting in lower rotation rates of the large-scale structures. Previously unexplained anomalies observed in the mean velocity profiles of reacting jets and mixing layers are shown to result from vorticity generation by baroclinic torques.

scholarly journals Simulations of Large-Scale Structure in the New Millennium

2005 ◽  
Vol 216 ◽  
pp. 105-119 ◽  
Author(s):  
Yasushi Suto
Keyword(s):  
Large Scale ◽  
Large Scale Structure ◽  
Vital Role ◽  
Scale Structure ◽  
Observational Cosmology ◽  
Initial Condition ◽  
The Galaxy ◽  
The Universe ◽  
Quasar Distribution ◽  
New Millennium

Simulations of large-scale structure in the universe have played a vital role in observational cosmology since the 1980's in particular. Their important role will definitely continue to be true in the 21st century; indeed the requirements for simulations in the precision cosmology era will become more progressively demanding as they are supposed to fill the missing link in an accurate and reliable manner between the “initial” condition at z=1000 revealed by WMAP and the galaxy/quasar distribution at z=0 − 6 surveyed by 2dF and SDSS. In this review, I will summarize what we have learned so far from the previous cosmological simulations, and discuss several remaining problems for the new millennium.

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