3,733 research outputs found
Progress of simulations for reacting shear layers
An attempt was made to develop a high speed, chemically reactive shear layer test rig. The purpose of the experiment was to study the mixing of oxidizer and fuel streams in reacting shear layers for various density, velocity, and Mach number. The primary goal was to understand the effects of the compressibility upon mixing and combustion in a fundamental way. Therefore, a two-dimensional shear layer is highly desirable for its simplicity to quantify the compressibility effects. The RPLUS 2D code is used to calculate the flow fields of different sections of the test rig. The emphasis was on the supersonic nozzle design, the vitiation process for the hot air stream and the overall thermodynamic conditions of the test matrix. The k-epsilon turbulence model with wall function was successfully implemented in the RPLUS code. The k and epsilon equations are solved simultaneously and the LU scheme is used to make it compatible with the flow solver
The least-squares finite element method for low-mach-number compressible viscous flows
The present paper reports the development of the Least-Squares Finite Element Method (LSFEM) for simulating compressible viscous flows at low Mach numbers in which the incompressible flows pose as an extreme. Conventional approach requires special treatments for low-speed flows calculations: finite difference and finite volume methods are based on the use of the staggered grid or the preconditioning technique; and, finite element methods rely on the mixed method and the operator-splitting method. In this paper, however, we show that such difficulty does not exist for the LSFEM and no special treatment is needed. The LSFEM always leads to a symmetric, positive-definite matrix through which the compressible flow equations can be effectively solved. Two numerical examples are included to demonstrate the method: first, driven cavity flows at various Reynolds numbers; and, buoyancy-driven flows with significant density variation. Both examples are calculated by using full compressible flow equations
Resummation Prediction on Higgs and Vector Boson Associated Production with a Jet Veto at the LHC
We investigate the resummation effects for the SM Higgs and vector boson
associated production at the LHC with a jet veto in soft-collinear effective
theory using "collinear anomalous" formalism. We calculate the jet vetoed
invariant mass distribution and the cross section for this process at
Next-to-Next-to-Leading-Logarithmic level, which are matched to the QCD
Next-to-Leading Order results, and compare the differences of the resummation
effects with different jet veto and jet radius . Our
results show that both resummation enhancement effects and the scale
uncertainties decrease with the increasing of jet veto and
jet radius , respectively. When GeV and ,
the resummation effects reduce the scale uncertainties of the Next-to-Leading
Order jet vetoed cross sections to about , which lead to increased
confidence on the theoretical predictions. Besides, after including resummation
effects, the PDF uncertainties of jet vetoed cross section are about .Comment: 22 pages, 10 figures and 2 tables; final version in JHE
violation induced by the double resonance for pure annihilation decay process in Perturbative QCD
In Perturbative QCD (PQCD) approach we study the direct violation in the
pure annihilation decay process of
induced by the and
double resonance effect. Generally, the violation is small in the
pure annihilation type decay process. However, we find that the violation
can be enhanced by double interference when the invariant masses
of the pairs are in the vicinity of the resonance. For
the decay process of , the
maximum violation can reach 28.64{\%}
Soft gluon resummation in the signal-background interference process of
We present a precise theoretical prediction for the signal-background
interference process of , which is useful to constrain the
Higgs boson decay width and to measure Higgs couplings to the SM particles. The
approximate NNLO -factor is in the range of (),
depending on , at the 8 (13) TeV LHC. And the soft gluon resummation
can increase the approximate NNLO result by about at both the 8 TeV and
13 TeV LHC. The theoretical uncertainties including the scale, uncalculated
multi-loop amplitudes of the background and PDF are roughly
at . We also confirm that the approximate
-factors in the interference and the pure signal processes are the same.Comment: 18 pages, 9 figures; v2 published in JHE
Transverse-Momentum Resummation for Gauge Boson Pair Production at the Hadron Collider
We perform the transverse-momentum resummation for , , and
pair productions at the next-to-next-to-leading logarithmic accuracy
using soft-collinear effective theory for and
at the LHC, respectively. Especially, this is the
first calculation of transverse-momentum resummation. We also
include the non-perturbative effects and discussions on the PDF uncertainties.
Comparing with the next-to-leading logarithmic results, the
next-to-next-to-leading logarithmic resummation can reduce the dependence of
the transverse-momentum distribution on the factorization scales significantly.
Finally, we find that our numerical results are consistent with data measured
by CMS collaboration for the production, which have been only reported by
the LHC experiments for the unfolded transverse-momentum distribution of the
gauge boson pair production so far, within theoretical and experimental
uncertainties.Comment: 22 pages, 6 figures, re-versio
Top quark pair production at small transverse momentum in hadronic collisions
We investigate the transverse momentum resummation for top quark pair
production at hadron colliders using the soft-collinear effective theory and
the heavy-quark effective theory. We derive the factorization formula for
production at small pair transverse momentum, and show in detail the
procedure for calculating the key ingredient of the factorization formula: the
next-to-leading order soft functions. We compare our numerical results with
experimental data and find that they are consistent within theoretical and
experimental uncertainties. To verify the correctness of our resummation
formula, we expand it to the next-to-leading order and the
next-to-next-to-leading order, and compare those expressions with the exact
fixed-order results numerically. Finally, using the results of transverse
momentum resummation, we discuss the transverse-momentum-dependent
forward-backward asymmetry at the Tevatron.Comment: 39 pages, 7 figures, 1 table; final version in PR
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