arXiv subjects

D. Wells

Publications and source records attributed to D. Wells.

4 recordsLinked to original sources

arXiv2017

Spatial Filtering for Reduced Order Modeling

Spatial filtering has been central in the development of large eddy simulation reduced order models (LES-ROMs) and regularized reduced order models (Reg-ROMs), In this paper, we perform a numerical investigation of spatial filtering. To this end, we consider one of the simplest Reg-ROMs, the Leray ROM (L-ROM), which uses ROM spatial filtering to smooth the flow variables and decreases the amount of energy aliased to the lower index ROM basis functions. We also propose a new form of ROM differential filter and use it as a spatial filter for the L-ROM. We investigate the performance of this new form of ROM differential filter in the numerical simulation of a flow past a circular cylinder at a Reynolds number $Re=760$.

math.NA↗

arXiv2016

Precise and accurate measurements of strong-field photoionisation and a transferrable laser intensity calibration standard

Ionization of atoms and molecules in strong laser fields is a fundamental process in many fields of research, especially in the emerging field of attosecond science. So far, demonstrably accurate data have only been acquired for atomic hydrogen (H), a species that is accessible to few investigators. Here we present measurements of the ionization yield for argon, krypton, and xenon with percentlevel accuracy, calibrated using H, in a laser regime widely used in attosecond science. We derive a transferrable calibration standard for laser peak intensity, accurate to 1.3%, that is based on a simple reference curve. In addition, our measurements provide a much-needed benchmark for testing models of ionisation in noble-gas atoms, such as the widely employed single-active electron approximation.

physics.atom-ph↗

arXiv2009

Polarimetry and the Long Awaited Superoutburst of BZ UMa

BZ UMa is a cataclysmic variable star whose specific classification has eluded researchers since its discovery in 1968. It has outburst and spectral properties consistent with both U Gem class dwarf novae and intermediate polars. We present new photometric and polarimetric measurements of recent outbursts, including the first detected superoutburst of the system. Statistical analysis of these and archival data from outbursts over the past 40 years present a case for BZ UMa as a non-magnetic, U Gem class, SU-UMa subclass dwarf novae.

astro-ph.SR↗

arXiv2005

Studies of Air Showers above 10^18 eV with the CHICOS Array

CHICOS (California HIgh school Cosmic ray ObServatory) is presently an array of more than 140 detectors distributed over a large area (~400 km^2) of southern California, and will consist of 180 detectors at 90 locations in the near future. These sites, located at area schools, are equipped with computerized data acquisition and automatic nightly data transfer (via internet) to our Caltech lab. The installed sites make up the largest currently operating ground array for ultra-high energy cosmic ray research in the northern hemisphere. The goal of CHICOS is to provide data related to the flux and distribution of arrival directions for ultra-high energy cosmic rays. We have performed detailed Monte-Carlo calculations to determine the density and arrival-time distribution of charged particles in extensive air showers for the CHICOS array. Calculations were performed for proton primaries with energies 10^18 to 10^21 eV and zenith angles out to 50 degrees. We have developed novel parameterizations for both distributions as functions of distance from the shower axis, primary energy, and incident zenith angle. These parameterizations are used in aperture calculations and reconstruction of shower data, enabling preliminary analysis of ultra-high energy shower data from CHICOS.

astro-ph↗