116 research outputs found
Waltzing peakons and compacton pairs in a cross-coupled Camassa-Holm equation
We consider singular solutions of a system of two cross-coupled Camassa-Holm
(CCCH) equations. This CCCH system admits peakon solutions, but it is not in
the two-component CH integrable hierarchy. The system is a pair of coupled
Hamiltonian partial differential equations for two types of solutions on the
real line, each of which separately possesses exp(-|x|) peakon solutions with a
discontinuity in the first derivative at the peak. However, there are no
self-interactions, so each of the two types of peakon solutions moves only
under the induced velocity of the other type. We analyse the `waltzing'
solution behaviour of the cases with a single bound peakon pair (a peakon
couple), as well as the over-taking collisions of peakon couples and the
antisymmetric case of the head-on collision of a peakon couple and a peakon
anti-couple. We then present numerical solutions of these collisions, which are
inelastic because the waltzing peakon couples each possess an internal degree
of freedom corresponding to their `tempo' -- that is, the period at which the
two peakons of opposite type in the couple cycle around each other in phase
space. Finally, we discuss compacton couple solutions of the cross-coupled
Euler-Poincar\'e (CCEP) equations and illustrate the same types of collisions
as for peakon couples, with triangular and parabolic compacton couples. We
finish with a number of outstanding questions and challenges remaining for
understanding couple dynamics of the CCCH and CCEP equations
The Square Root Depth Wave Equations
We introduce a set of coupled equations for multilayer water waves that
removes the ill-posedness of the multilayer Green-Naghdi (MGN) equations in the
presence of shear. The new well-posed equations are Hamiltonian and in the
absence of imposed background shear they retain the same travelling wave
solutions as MGN. We call the new model the Square Root Depth equations, from
the modified form of their kinetic energy of vertical motion. Our numerical
results show how the Square Root Depth equations model the effects of
multilayer wave propagation and interaction, with and without shear.Comment: 10 pages, 5 figure
Error bounds on complex floating-point multiplication
Given floating-point arithmetic with t-digit base-β significands in which all arithmetic operations are performed as if calculated to infinite precision and rounded to a nearest representable value, we prove that the product of complex values z0 and z1 can be computed with maximum absolute error |z0||z1|1/2β 1-t√5. In particular, this provides relative error bounds of 2-24√5 and 2-53√5. for IEEE 754 single and double precision arithmetic respectively, provided that overflow, underflow, and denormals do not occur. We also provide the numerical worst cases for IEEE 754 single and double precision arithmetic
An Euler Poincar\'e framework for the multilayer Green Nagdhi equations
The Green Nagdhi equations are frequently used as a model of the wave-like
behaviour of the free surface of a fluid, or the interface between two
homogeneous fluids of differing densities. Here we show that their multilayer
extension arises naturally from a framework based on the Euler Poincare theory
under an ansatz of columnar motion. The framework also extends to the
travelling wave solutions of the equations. We present numerical solutions of
the travelling wave problem in a number of flow regimes. We find that the free
surface and multilayer waves can exhibit intriguing differences compared to the
results of single layer or rigid lid models
Error Bounds on Complex Floating-Point Multiplication
International audienceGiven floating-point arithmetic with -digit base- significands in which all arithmetic operations are performed as if calculated to infinite precision and rounded to a nearest representable value, we prove that the product of complex values and can be computed with maximum absolute error \abs{z_0} \abs{z_1} \frac{1}{2} \beta^{1 - t} \sqrt{5}. In particular, this provides relative error bounds of and for {IEEE 754} single and double precision arithmetic respectively, provided that overflow, underflow, and denormals do not occur. We also provide the numerical worst cases for {IEEE 754} single and double precision arithmetic
Hydro-morphodynamics 2D modelling using a discontinuous Galerkin discretisation
The development of morphodynamic models to simulate sediment transport accurately is a challenging process that is becoming ever more important because of our increasing exploitation of the coastal zone, as well as sea-level rise and the potential increase in strength and frequency of storms due to a changing climate. Morphodynamic models are highly complex given the non-linear and coupled nature of the sediment transport problem. Here we implement a new depth-averaged coupled hydrodynamic and sediment transport model within the coastal ocean model Thetis, built using the code generating framework Firedrake which facilitates code flexibility and optimisation benefits. To the best of our knowledge, this represents the first full morphodynamic model including both bedload and suspended sediment transport which uses a discontinuous Galerkin based finite element discretisation. We implement new functionalities within Thetis extending its existing capacity to model scalar transport to modelling suspended sediment transport, incorporating within Thetis options to model bedload transport and bedlevel changes. We apply our model to problems with non-cohesive sediment and account for effects of gravity and helical flow by adding slope gradient terms and parametrising secondary currents. For validation purposes and in demonstrating model capability, we present results from test cases of a migrating trench and a meandering channel comparing against experimental data and the widely-used model Telemac-Mascaret
The velocity field of 2MRS Ks=11.75 galaxies: constraints on beta and bulk flow from the luminosity function
Using the nearly full sky Ks=11.75 2MASS Redshift Survey [2MRS]of ~45,000
galaxies we reconstruct the underlying peculiar velocity field and constrain
the cosmological bulk flow within ~100. These results are obtained by
maximizing the probability to estimate the absolute magnitude of a galaxy given
its observed apparent magnitude and redshift. At a depth of ~60 Mpc/h we find a
bulk flow Vb=(90\pm65,-230\pm65,50\pm65) km/s in agreement with the theoretical
predictions of the LCDM model. The reconstructed peculiar velocity field that
maximizes the likelihood is characterized by the parameter beta=0.323 +/- 0.08.
Both results are in agreement with those obtained previously using the ~23,000
galaxies of the shallower Ks=11.25 2MRS survey. In our analysis we find that
the luminosity function of 2MRS galaxies is poorly fitted by the Schechter form
and that luminosity evolves such that objects become fainter with increasing
redshift according to L(z)=L(z=0)(1+z)^(+2.7 +/-0.15).Comment: 10 pages, 6 figure
Non-Standard Structure Formation Scenarios
Observations on galactic scales seem to be in contradiction with recent high
resolution N-body simulations. This so-called cold dark matter (CDM) crisis has
been addressed in several ways, ranging from a change in fundamental physics by
introducing self-interacting cold dark matter particles to a tuning of complex
astrophysical processes such as global and/or local feedback. All these efforts
attempt to soften density profiles and reduce the abundance of satellites in
simulated galaxy halos. In this contribution we are exploring the differences
between a Warm Dark Matter model and a CDM model where the power on a certain
scale is reduced by introducing a narrow negative feature (''dip''). This dip
is placed in a way so as to mimic the loss of power in the WDM model: both
models have the same integrated power out to the scale where the power of the
Dip model rises to the level of the unperturbed CDM spectrum again. Using
N-body simulations we show that that the new Dip model appears to be a viable
alternative to WDM while being based on different physics: where WDM requires
the introduction of a new particle species the Dip stems from a non-standard
inflationary period. If we are looking for an alternative to the currently
challenged standard LCDM structure formation scenario, neither the LWDM nor the
new Dip model can be ruled out with respect to the analysis presented in this
contribution. They both make very similar predictions and the degeneracy
between them can only be broken with observations yet to come.Comment: 4 pages, 3 figures; to appear in "The Evolution of Galaxies III. From
Simple Approaches to Self-Consistent Models", proceedings of the 3rd
EuroConference on the evolution of galaxies, held in Kiel, Germany, July
16-20, 200
Time Protection: the Missing OS Abstraction
Timing channels enable data leakage that threatens the security of computer
systems, from cloud platforms to smartphones and browsers executing untrusted
third-party code. Preventing unauthorised information flow is a core duty of
the operating system, however, present OSes are unable to prevent timing
channels. We argue that OSes must provide time protection in addition to the
established memory protection. We examine the requirements of time protection,
present a design and its implementation in the seL4 microkernel, and evaluate
its efficacy as well as performance overhead on Arm and x86 processors
Amplitude distribution of eigenfunctions in mixed systems
We study the amplitude distribution of irregular eigenfunctions in systems
with mixed classical phase space. For an appropriately restricted random wave
model a theoretical prediction for the amplitude distribution is derived and
good agreement with numerical computations for the family of limacon billiards
is found. The natural extension of our result to more general systems, e.g.
with a potential, is also discussed.Comment: 13 pages, 3 figures. Some of the pictures are included in low
resolution only. For a version with pictures in high resolution see
http://www.physik.uni-ulm.de/theo/qc/ or http://www.maths.bris.ac.uk/~maab
- …