The interaction of helical tip and root vortices in a wind turbine wake

Analysis of the helical vortices measured behind a model wind turbine in a water channel are reported. Phase-locked measurements using planar particle image ve- locimetry are taken behind a Glauert rotor to investigate the evolution and breakdown of the helical vortex structures. Existing linear stability theory predicts helical vortex filaments to be susceptible to three unstable modes. The current work presents tip and root vortex evolution in the wake for varying tip speed ratio and shows a breaking of the helical symmetry and merging of the vortices due to mutual inductance between the vortical filaments. The merging of the vortices is shown to be steady with rotor phase, however, small-scale non-periodic meander of the vortex positions is also ob- served. The generation of the helical wake is demonstrated to be closely coupled with the blade aerodynamics, strongly influencing the vortex properties which are shown to agree with theoretical predictions of the circulation shed into the wake by the blades. The mutual inductance of the helices is shown to occur at the same non-dimensional wake distance

Reply to the comment A.J. Kassman and D.B. Losee on the "Effect of gallium ions and of preparation methods on the structural properties of cobalt-molybdenum-alumina catalysts"

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Mutual inductance of two helical vortices

The pairing of helical tip vortices in the wake of a two-bladed rotor is investigated experimentally. Time-resolved particle image velocimetry measurements provide a clear temporal and spatial evolution of the vortical structures, highlighting the transition to instability and the effect of tip speed ratio and helical spacing. The temporal growth rate of the vortex system instabilities were measured and are shown to be dependent on helical spacing. The evolution of filaments and their growth rates support the argument that the mutual inductance mode is the driving mechanism behind the transition to an unstable wake. The measurements are in agreement with maximum growth rates predicted by linear stability analyses of single- and double-helix arrangements. In addition, the wake topology due to varying rotor load through tip speed ratio variation is shown to play an important role in the initial symmetry breaking that drives the wake transition

Mitral regurgitation due to caseous calcification of the mitral annulus: two case reports

Caseous calcification is a rare variant of mitral annular calcification, occurring in about 0.06% of echocardiographic studies performed. It is usually a benign lesion, but it should be differentiated by abscess and tumors. Echocardiography is the most sensitive method to identify caseous calcification which appears typically as a round, calcified mass with an echo-lucent, liquid-like inner part

Mutagenic activation of benzo[a]pyrene by human red blood cells.

The induction of sister-chromatid exchanges (SCEs) and micronuclei (MN) was used as an endpoint to evaluate the cytogenetic effects of benzo[a]pyrene (B(a)P) activated by human red blood cells and S9 mix. Human erythrocytes can metabolically activate B(a)P. It was shown that both human erythrocytes and S9 mix activate B(a)P and that the resulting excess SCE and MN depend in a linear manner on the B(a)P dose. HPLC analysis suggested that quinone derivatives formed by the red blood cells are responsible for the cytogenetic abnormalities observed

Magnetic and spectroscopic investigations on cobalt-alumina and cobalt-molybdenum-alumina. Electron spin resonance of the oxidized, sulfided, and reduced catalysts

The ESR spectra of CoO-Al2O3, MoO3-Al2O3 and CoO-MoO3-Al2O3 were investigated in the oxidized state and after redn. with H2 + H2S (sometimes followed by a further treatment with H2) or by H2. All Co-contg. samples produce signals at 1200 Oe (for g-Al2O3) which shift to 1750 and 4300 Oe after treatment at 1300 Deg (formation of a-Al2O3). The signals are assigned to Co2+ in tetrahedral sites in the support; they are not affected by sulfiding or redn. If no Mo is present, redn. leads to the formation of an extremely broad and strong signal ascribed to Co metal; it is eliminated by the presence of Mo. In the absence of Co and in the oxidized state Mo-contg. samples develop a weak signal at g = 1.93, assigned to traces of Mo5+ in free MoO3; in the presence of Co this signal disappears. Pretreatment of Mo-contg. samples with H2 + H2S leads first to a hundredfold enhancement of the signal which, however, disappears after application of longer redn. times or higher temps. No signal connected with Mo3+ could be found. Finally, H2 after H2 + H2S treatments produced a signal at g = 2.16 if Co was present; it is assigned to either Co+ or Co2+ (low spin). A catalyst model is proposed containing 2 solid states. State I is the support with Co2+ and Mo6+ in its subsurface layers. Its fate after redn. is uncertain but Co2+ is not changed. State II is a separate system of compds. Co3O4, MoO3, or CoMoO4, depending on the compn. Treatments with H2 + H2S lead, resp., to Co + CoS, MoS2, or CoS + MoS2 (CoMo2S4)