Apparatus for reducing aerodynamic noise in a wind tunnel

An apparatus is described for reducing the background noise produced by the porous walls of the test section of a wind tunnel. A finely meshed screen member is placed over the perforations in the test section walls. The mesh wire screen attached to the interior wall provides a smoother surface for the air stream to flow against reducing the vorticies produced by the edges of the perforations in the test section walls

HD 4915: A Maunder Minimum Candidate

We study the magnetic activity cycle of HD 4915 using the \ion{Ca}{2} H \& K emission line strengths measured by Keck I/HIRES spectrograph. The star has been observed as a part of California Planet Search Program from 2006 to present. We note decreasing amplitude in the magnetic activity cycle, a pattern suggesting the star's entry into a Magnetic Grand Minimum (MGM) state, reminiscent of the Sun's Maunder and Dalton Minima. We recommend further monitoring of the star to confirm the grand minimum nature of the dynamo, which would provide insight into the state of the Sun's chromosphere and the global magnetic field during its grand minima. We also recommend continued observations of H \& K emission lines, and ground or space based photometric observations to estimate the sunspot coverage.Comment: To be submitted to AAS Journals; comments welcom

Persistence in the Voter model: continuum reaction-diffusion approach

We investigate the persistence probability in the Voter model for dimensions d\geq 2. This is achieved by mapping the Voter model onto a continuum reaction-diffusion system. Using path integral methods, we compute the persistence probability r(q,t), where q is the number of ``opinions'' in the original Voter model. We find r(q,t)\sim exp[-f_2(q)(ln t)^2] in d=2; r(q,t)\sim exp[-f_d(q)t^{(d-2)/2}] for 2<d<4; r(q,t)\sim exp[-f_4(q)t/ln t] in d=4; and r(q,t)\sim exp[-f_d(q)t] for d>4. The results of our analysis are checked by Monte Carlo simulations.Comment: 10 pages, 3 figures, Latex, submitted to J. Phys. A (letters

The sun's magnetic sector structure

The synoptic appearance of solar magnetic sectors is studied using 454 sector boundaries observed at earth during 1959-1973. The sectors are clearly visible in the photospheric magnetic field. Sector boundaries can be clearly identified as north-south running demarcation lines between regions of persistent magnetic polarity imbalances. These regions extend up to about 35 deg of latitude on both sides of the equator. They generally do not extend into the polar caps. The polar cap boundary can be identified as an east-west demarcation line marking the poleward limit of the sectors. The typical flux imbalance for a magnetic sector is about 4 x 10 to the 21st power Maxwells

Linear response of a grafted semiflexible polymer to a uniform force field

We use the worm-like chain model to analytically calculate the linear response of a grafted semiflexible polymer to a uniform force field. The result is a function of the bending stiffness, the temperature, the total contour length, and the orientation of the field with respect to that of the grafted end. We also study the linear response of a worm-like chain with a periodic alternating sequence of positive and negative charges. This can be considered as a model for a polyampholyte with intrinsic bending siffness and negligible intramolecular interactions. We show how the finite intrinsic persistence length affects the linear response to the external field.Comment: 6 pages, 3 figure

Pure-state quantum trajectories for general non-Markovian systems do not exist

Since the first derivation of non-Markovian stochastic Schr\"odinger equations, their interpretation has been contentious. In a recent Letter [Phys. Rev. Lett. 100, 080401 (2008)], Di\'osi claimed to prove that they generate "true single system trajectories [conditioned on] continuous measurement". In this Letter we show that his proof is fundamentally flawed: the solution to his non-Markovian stochastic Schr\"odinger equation at any particular time can be interpreted as a conditioned state, but joining up these solutions as a trajectory creates a fiction.Comment: 4 page

Persistence of a pinch in a pipe

The response of low-dimensional solid objects combines geometry and physics in unusual ways, exemplified in structures of great utility such as a thin-walled tube that is ubiquitous in nature and technology. Here we provide a particularly surprising consequence of this confluence of geometry and physics in tubular structures: the anomalously large persistence of a localized pinch in an elastic pipe whose effect decays very slowly as an oscillatory exponential with a persistence length that diverges as the thickness of the tube vanishes, which we confirm experimentally. The result is more a consequence of geometry than material properties, and is thus equally applicable to carbon nanotubes as it is to oil pipelines.Comment: 6 pages, 3 figure

Perturbative Wilson loops from unquenched Monte Carlo simulations at weak couplings

Perturbative expansions of several small Wilson loops are computed through next-to-next-to-leading order in unquenched lattice QCD, from Monte Carlo simulations at weak couplings. This approach provides a much simpler alternative to conventional diagrammatic perturbation theory, and is applied here for the first time to full QCD. Two different sets of lattice actions are considered: one set uses the unimproved plaquette gluon action together with the unimproved staggered-quark action; the other set uses the one-loop-improved Symanzik gauge-field action together with the so-called ``asqtad'' improved-staggered quark action. Simulations are also done with different numbers of dynamical fermions. An extensive study of the systematic uncertainties is presented, which demonstrates that the small third-order perturbative component of the observables can be reliably extracted from simulation data. We also investigate the use of the rational hybrid Monte Carlo algorithm for unquenched simulations with unimproved-staggered fermions. Our results are in excellent agreement with diagrammatic perturbation theory, and provide an important cross-check of the perturbation theory input to a recent determination of the strong coupling $\alpha_{\bar{\rm MS}}(M_Z)$ by the HPQCD collaboration.Comment: 14 pages, 8 figure

First Stereoscopic Coronal Loop Reconstructions from Stereo Secchi Images

We present the first reconstruction of the three-dimensional shape of magnetic loops in an active region from two different vantage points based on simultaneously recorded images. The images were taken by the two EUVI telescopes of the SECCHI instrument onboard the recently launched STEREO spacecraft when the heliocentric separation of the two space probes was 12 degrees. We demostrate that these data allow to obtain a reliable three-dimensional reconstruction of sufficiently bright loops. The result is compared with field lines derived from a coronal magnetic field model extrapolated from a photospheric magnetogram recorded nearly simultaneously by SOHO/MDI. We attribute discrepancies between reconstructed loops and extrapolated field lines to the inadequacy of the linear force-free field model used for the extrapolation.Comment: 6 pages, 5 figure

Time-stepping approach for solving upper-bound problems: Application to two-dimensional Rayleigh-Benard convection

An alternative computational procedure for numerically solving a class of variational problems arising from rigorous upper-bound analysis of forced-dissipative infinite-dimensional nonlinear dynamical systems, including the Navier-Stokes and Oberbeck-Boussinesq equations, is analyzed and applied to Rayleigh-Benard convection. A proof that the only steady state to which this numerical algorithm can converge is the required global optimal of the relevant variational problem is given for three canonical flow configurations. In contrast with most other numerical schemes for computing the optimal bounds on transported quantities (e.g., heat or momentum) within the "background field" variational framework, which employ variants of Newton's method and hence require very accurate initial iterates, the new computational method is easy to implement and, crucially, does not require numerical continuation. The algorithm is used to determine the optimal background-method bound on the heat transport enhancement factor, i.e., the Nusselt number (Nu), as a function of the Rayleigh number (Ra), Prandtl number (Pr), and domain aspect ratio L in two-dimensional Rayleigh-Benard convection between stress-free isothermal boundaries (Rayleigh's original 1916 model of convection). The result of the computation is significant because analyses, laboratory experiments, and numerical simulations have suggested a range of exponents alpha and beta in the presumed Nu similar to (PrRa beta)-Ra-alpha scaling relation. The computations clearly show that for Ra <= 10(10) at fixed L = 2 root 2, Nu <= 0.106Pr(0)Ra(5/12), which indicates that molecular transport cannot generally be neglected in the "ultimate" high-Ra regime.NSF DMS-0928098 DMS-1515161 DMS-0927587 PHY-1205219Simons FoundationNSFONRInstitute for Computational Engineering and Sciences (ICES