A Theory of the Role of Magnetic Activity During Star Formation

Under the assumption that magnetic activity is due to the action of a magnetic field in a rotating star with a convective zone, it is possible to draw the following picture of a sequence of events in the Hertzsprung-Russell diagram. The H-R diagram is divided into two regions by an almost vertical line. Stars of later type have a hydrogen convective zone of great extension (region C); stars of earlier type have no hydrogen convective zone or a convective zone of small extension (region D). Stars in region C show stellar activity of electrodynamic origin like solar activity; stars in region D show little or no stellar activity. When matter is ejected by a star, the magnetic field compels the matter to rotate with the star even at very large distances where it carries away large angular momentum per unit mass. Stars which reach the main sequence in region C may lose a large amount of angular momentum, while stars reaching the main sequence in region D lose only a little

Penalty approximation for non smooth constraints in vibroimpact

We examine the penalty approximation of the free motion of a material point in an angular domain; we choose an over-damped penalty, and we prove that if the first impact point is not at the vertex, then, the limit of the approximation exists and is described by Moreau's rule for anelastic impacts. The proofs rely on validated asymptotics and use some classical tools in the theory of dynamical systems.Comment: two figures, 32 pages, LaTeX2

Acoustic Testing of the Bell 699 Rotor on the Tiltrotor Test Rig (TTR) in the National Full-Scale Aerodynamics Complex 40- by 80- Foot Wind Tunnel in Conversion and Airplane Configuration

The Tiltrotor Test Rig (TTR) provides new proprotor testing capabilities to the National Aeronautics and Space Administration (NASA) and the Department of Defense (DoD). A checkout test for the TTR with the 699 proprotor was performed in the National Full-Scale Aerodynamics Complex (NFAC) 40- by 80 Foot Wind Tunnel from 2017 to 2018. Four microphones were placed around the rotor to capture acoustic data. Revolution based acoustic data processing techniques are evaluated for appropriateness in both helicopter and airplane configurations. This paper presents the acoustic data acquired for the rotor in conversion and airplane configuration

An inviscid model for the vortex-street wake

An inviscid model for the Karman vortex street, containing vortices of uniform vorticity surrounded by irrotational fluid, is related to the wake behind a bluff body by a global analysis requiring the conservation of momentum, energy and vorticity. Some comparison is made with experimental results reported in the literature. A qualitative procedure is proposed whereby the slow evolution of the wake through viscous effects is approximated. Some comments are made regarding the relevance of the stability properties of the inviscid street. Some calculations are made for the ‘secondary vortex street’ that is observed after breakdown and rearrangement, and comparison is made with experiment

Stability of a vortex street of finite vortices

The stability of the finite-area Kármán ‘vortex street’ to two-dimensional disturbances is determined. It is shown that for vortices of finite size there exists a finite range of spacing ratio κ for which the array is stable to infinitesimal disturbances. As the vortex size approaches zero, the range narrows to zero width about the classical von Kármán value of 0·281

The linear two-dimensional stability of inviscid vortex streets of finite-cored vortices

The stability of two-dimensional infinitesimal disturbances of the inviscid Karman vortex street of finite-area vortices is reexamined. Numerical results are obtained for the growth rate and oscillation frequencies of disturbances of arbitrary subharmonic wavenumber and the stability boundaries are calculated. The stabilization of the pairing instability by finite area demonstrated by Saffman & Schatzman (1982) is confirmed, and also Kida’s (1982) result that this is not the most unstable disturbance when the area is finite. But, contrary to Kida’s quantitative predictions, it is now found that finite area does not stabilize the street to infinitesimal two-dimensional disturbances of arbitrary wavelength and that it is always unstable except for one isolated value of the aspect ratio which depends upon the size of the vortices. This result does agree, however, with those of a modified version of Kida’s analysis