Bibliothek

Notizen: The coupled interaction of vorticity ω and rate-of-strain S in homogeneous sheared turbulence is investigated using direct numerical simulation. Conditional sampling and comparison with linear simulations reveal various aspects of the structure and dynamics. Due to the influence of the imposed ω and S, distinct directional features develop. Initial stretching of fluctuating ω by mean extensional strain and the presence of mean vorticity establish a predominant misalignment of ω with respect to the principal axes of S. The associated locally induced rotation of the S axes results in preferred orientations in ω and S. In high amplitude rotation-dominated regions of the flow, distinct characteristics are exhibited by the pressure Hessian Π due to the presence of small-scale spatial structure. Nonlocally induced S axes rotation through Π tends to counteract locally induced rotation in these regions. These features are absent in the linear flow which suggests a lack of spatial coherence in the corresponding intense ω2 regions. High amplitude strain-dominated and comparable rotation-strain regions are also considered. In general, the high amplitude conditional samples capture the main features of the flow. The underlying behavior of ω and S is essentially the same as in isotropic turbulence; the directional preferences observed in shear flow demonstrate the physical implications of the associated mechanisms. Although there is greater directional variation in flows with high Reλ/Sh, results indicate the significance of the persistence of mean shear. © 2000 American Institute of Physics.

Notizen: The structure and dynamics of stably stratified homogeneous sheared turbulence is investigated in terms of the triadic interaction of vorticity ω, rate-of-strain S, and scalar (density fluctuation) gradient G≡∇ρ′. Results of direct numerical simulations are presented. Due to the presence of the mean velocity and scalar gradients, distinct directional preferences develop which affect the dynamics of the flow. The triadic interaction is described in terms of the direct coupling of primary mechanism pairs and influential secondary effects. Interaction of ω and S is characterized by the coupling of vortex stretching and locally-induced rotation of the S axes. Due to the intrinsic directionality of baroclinic torque, the generated ω acts to impede S axes rotation. Interaction of ω and G involves an inherent negative feedback between baroclinic torque and reorientation of G by ω. This causes baroclinic torque to act as a sink which promotes decay of ω2. Interaction of S and G is characterized by a positive feedback between differential acceleration and gradient amplification by compressive straining which promotes persistence in vertical G. In high-amplitude, rotation-dominated regions of the flow, differential acceleration effects enhance the attenuation of vertical ω while shear and baroclinic torque tend to maintain horizontal ω. This leads to a predominance of horizontal ω in these regions which manifests itself as collapsed vortex structures. As the flow develops, the third invariant of the velocity gradient tensor tends towards zero indicating locally two-dimensional flow. © 2000 American Institute of Physics.

Notizen: Summary Background  Perforating skin dermatoses include elastosis perforans serpiginosa (EPS), reactive perforating collagenosis, Kyrle's disease and perforating folliculitis. In addition to these four diseases, an acquired form of perforating dermatosis associated with diabetes mellitus and/or chronic renal failure has been reported for which the term acquired perforating dermatosis (APD) was proposed. The molecular mechanism of transepidermal elimination of dermal components in perforating skin dermatoses remains unclear. We recently demonstrated that the 67-kDa elastin receptor can be detected in the epidermis eliminating altered elastic fibres in EPS, suggesting that the elastin–keratinocyte interaction may play a role in transepidermal elimination in EPS.Objectives  To determine whether the 67-kDa elastin receptor is involved in other perforating diseases.Methods  Paraffin-embedded skin specimens from new cases of EPS (n = 2), APD (n = 15) and perforating granuloma annulare (PGA; n = 2) were studied immunohistochemically using a specific antibody to the 67-kDa elastin receptor. In one case of EPS, two different sites from a single lesion, a central atrophic area and a peripheral keratotic area, were studied.Results  Expression of the elastin receptor was detected in the epidermis surrounding the elastic materials in both cases of EPS. The elastin receptor was not detected in the central inactive area, whereas it was expressed strongly in the peripheral keratotic active area. The elastin receptor was also detected in three of 15 cases of APD in which a few elastic fibres were found in the eliminated dermal materials. In one case of APD, the elastin receptor was not detected in spite of the presence of a few elastic fibres in the eliminated materials. The elastin receptor was not detected in either case of PGA.Conclusions  Expression of the elastin receptor in EPS was seen in both cases studied and was dependent on the stage of the lesion. Expression of the elastin receptor in APD appeared to be related to the amount of elastic fibres in the eliminated materials. Thus, expression of the elastin receptor in perforating skin disorders may depend on the stage of the lesion and/or the content of elastic fibres in the dermal materials being eliminated.

Notizen: A case of solitary malignant schwannoma, appearing as a painless mass near the right ankle of a 46-year-old female, is presented. Solitary malignant schwannoma without neurofibromatosis is considered to be quite rare. However, this disorder has a better prognosis than neurofibromatosis. Previous reports indicate that the treatment usually chosen is wide local excision without lymph node dissection. unless clinically involved nodes are present.

Notizen: The influence of Ag-Au and Ag-Cu alloy substrates on the microstructure and superconducting properties of Bi2Sr2CaCu2Ox (Bi-2212) was studied. The critical current density (Jc) of Bi-2212/(Ag-Au) was not degraded by increasing the Au content of the Ag-Au alloy, while the Jc of Bi-2212/(Ag-Cu) was drastically degraded by increasing the Cu content of the Ag-Cu alloy. The microstructure of Bi-2212 on the Ag-Au alloy was uniform and no precipitates were observed. The Jc degradation of Bi-2212 in contact with the Ag-Cu alloy was understood by the fact that Cu absorption of Bi-2212 from the Ag-Cu alloys changed the composition of the oxide layer. Ag-Au alloys are superior substrate materials for Bi-2212 for power current leads for superconducting magnets, taking into account their low thermal conductivity.

Notizen: Oxide superconductor tape with Ag-Au sheath was studied for current leads. A 100-cm-long current lead of 1000 A with 0.2 W/kA heat leakage was designed using Ag-11 at. % Au alloy sheath with low thermal conductivity. The overall current density of the superconducting tape is required to be 350–770 A/cm2 at 77 K when the superconductor cross-section ratio in the tape is 0.35–0.7. Bi-Pb-Sr-Ca-Cu-O superconducting tape with Ag-11 at. % Au alloy sheath was fabricated and the critical current density Jc was measured. The overall-Jc was 1700 A/cm2 at 77 K under 0 T. The possibility of the current lead of 1000 A under 0.28 T with 0.2 W/kA was made clear.

Notizen: The characteristics of Ag-(Mg,Ni) and Ag-Mn alloys as substrate materials for Bi2Sr2CaCu2Ox (Bi-2212) tapes and their influence on the superconducting properties were studied. Alloying with Mg+Ni or Mn drastically decreased the grain size of Ag after heat treatment. The additives in Ag-(Mg,Ni) and Ag-Mn alloys were oxidized during heat treatment in air. Both the fine grain structure and the internal oxidation of the additives increased the mechanical strength of the Ag alloys. Both alloys were 3–6 times stronger than pure Ag. The superconducting properties of Bi-2212 prepared on these Ag alloys were slightly degraded; however the critical current densities at 4.2 K in 8 T were still well over 104 A/cm2.

Notizen: Ag-sheathed Bi2Sr2Ca1Cu2Ox(Bi-2212) tapes with various oxide superconductor core (SC-core) thicknesses, SC-core ratios (SC-core area to overall cross-sectional area), and tape configurations were prepared by a partial melting and slow cooling process. For single-core tapes the critical current density, Jc(core), for SC-core, increases with decreasing SC-core thickness, due to the highly aligned Bi-2212 grains, and the highest Jc(core) of 1.4×105 A/cm2 at 4.2 K in 10 T is achieved at a core thickness of 15 μm. Furthermore, the Jc(core) increases with increasing SC-core ratio (i.e., with decreasing Ag-sheath thickness). From the practical point of view, high Jc(overall) as well as high critical current (Ic) is more important than Jc(core). This requirement is attained by the double-tube method, which shows the highest Jc(overall) value of 3.0×104 A/cm2 at 4.2 K in 10 T. In the tape fabrication process, the slow heating rate just below the partial melting becomes important, because oxygen released from the SC-core during heating makes void swelling in the Ag sheath and leads to the degradation of Jc.