Biomedical subjects
Publications and source records attributed to A Tilgner.
A coherent structure is revealed experimentally by a dyeing technique in the boundary layer of Rayleigh-Bénard convection in water. Dye accumulates in streaks aligned with the mean flow. Possible mechanisms for the formation of these streaks are discussed.
Direct numerical simulations of Rayleigh-Bénard convection in a plane layer with periodic boundary conditions at Rayleigh numbers up to 10(7) show that flow structures can be objectively classified as large or small scale structures because of a gap in spatial spectra. The typical size of the large scale structures does not always vary monotonically as a function of the Rayleigh number but broadly increases with increasing Rayleigh number. A mean flow (whose average over horizontal planes differs from zero) is also excited but is weak in comparison with the large scale structures. The large scale circulation observed in experiments should therefore be a manifestation of the large scale structures identified here.
Peffley, Cawthorne, and Lathrop [Phys. Rev. E 61, 5287 (2000)] have reported on an experiment using liquid sodium, which studies the approach toward a self-generating dynamo. Their results challenge the traditional views of kinematic dynamo theory because (i) the modes of the magnetic field with the smallest decay rates appear to be nearly axisymmetric and (ii) the observed decay rates vary spatially. This report shows how these observations can be reconciled with kinematic dynamo theory.
We provide examples of alpha(2) dynamos in spheres which generate magnetic fields that are confined to the conductor and are therefore undetectable in the surrounding vacuum.
We investigate kinematic dynamos in cylinders and spheres surrounded by an insulator. The flow volume is divided into an inner region, in which the conducting fluid is in motion, and an outer region enclosing the inner one, in which the conductor is at rest. The dependence of the critical magnetic Reynolds number on the thickness of the outer conducting shell is discussed as well as implications for the design of experimental dynamos.
It is found that for Taylor numbers of the order 10(8) quadrupolar dynamos aligned with the axis of rotation are preferred in comparison with dipolar dynamos. This preference holds for a range of Prandtl numbers P and magnetic Prandtl numbers P(m) in the neighborhood of unity. The main time-dependent feature of the quadrupolar dynamos are polward traveling waves.
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The influence of chlorpromazine hydrochloride on failing, cranially-based dorsal flaps of female rats (Uje:WIST) was studied. The drug was injected subcutaneously into the abdomen pre- and postoperatively. The dosage was 1.5 mg/100 g preoperatively and 0.75 mg/100 g body weight every 12 hours following flap replantation for 10 consecutive days. No improvement in the survival rate was achieved by administration of chlorpromazine hydrochloride in this rat strain.
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The neovascularisation was investigated in 51 rectus abdominis myocutaneous island flaps (1.5 x 3.0 cm) in rats (Uje: WIST) following ligation of the pedicle vessels (superior epigastric artery and vein) without and with incision of the cranial flap border. The ligation was done on the 3rd, 4th, 5th, and 7th postoperative day. The efficiency of the neovascularisation was estimated quantitatively on the 3rd, 5th, and 7th day after vascular ligation by measurement of the necrotic flap tissue. The findings are expressed in percent of the whole flap area. Vascular pedicle ligation on the 3rd or 4th postoperative day caused an average necrosis rate of 98% and 84% respectively. No flap survived in these experimental group. In case of pedicle vessel occlusion on the 5th postoperative day without incision of the cranial flap border all myocutaneous flaps survived, partly with focal loss of the epidermis. Placing the ligation after incision of the flap border on the 5th day an average necrosis rate of 34% was found, only 5 out of 11 flaps survived. All flaps (n = 9) remained viable following vascular pedicle ligation on the 7th postoperative day. It seems necessary to observe myocutaneous flaps carefully until the 14th postoperative day, because tissue damages due to vascular pedicle occlusion on the 4th day may become visible not before the 10th or 11th postoperative day. Opening of the myocutaneous flap borders between the 5th and 7th postoperative day should be avoided.
In rectus abdominis myocutaneous island flaps (3.0 x 1.5 cm) of rats (Uje:WIST) skin surface temperature was measured by an infrared pyrometer for monitoring flap viability following experimentally induced blood flow insufficiency. The insufficiency was caused by ligation of the pedicle vessels (epigastric superior artery and vein) on the 3rd, 4th, 5th and 7th d after flap replantation, and by occlusion of these vessels with microclips for 2 h. Only the temperature differences between the intact and the operated skin surface were used to estimate blood flow efficiency not, the absolute values. These differences were statistically insignificant between the intact right and left abdominal skin areas and between the intact skin and the vital flap surfaces. In these cases the maximal temperature differences were 0.52 degree C. Only during the early postoperative period (3-days ligation group) a correlation between blood flow insufficiency and flap viability is expressed by temperature differences (2.14 degrees C). Later on (7-days ligation group) a temperature difference of 1.87 degrees C was no evidence of disturbed wound healing. Thus in advanced stages in myocutaneous flaps differences of blood flow and neovascularization respectively cannot be estimated reliably by measurements of skin surface temperature. However, temperature differences greater than 2 degrees C signal impaired flap viability.
In rectus abdominis myocutaneous island flaps (3.0 x 1.5 cm) of 86 rats (Uje: WIST) transcutaneous oxygen tension measurements (tcPO2) were made by a Universal-PO2-Meter MO 10.1 (PRACITRONIC, Dresden, GDR) for monitoring flap viability following blood flow insufficiency produced experimentally. This insufficiency was induced by ligation of the pedicle vessels (superior epigastric artery and vein) on the 3rd, 4th, 5th and 7th day after flap reposition and by occlusion of these vessels with microclips for two hours. Only the tcpO2 differences between the intact skin and the contralateral myocutaneous island flap were used to assess effects on blood flow and not the absolute values. These differences were statistically insignificant between the right and left non-operated abdominal skin (0.24 +/- 0.11 kPa), the non-operated skin and the rectus abdominis myocutaneous island flap with intact pedicle vessels (group 2) (0.59 +/- 0.14 kPa) as well as flaps ligated at 5-days (0.71 +/- 0.18 kPa) in which vascular pedicle ligation were induced following incision of the cranial flap border. In the two-hour-ischemia group, the 3-days, 4-days and 7-days ligation groups the average tcpO2 differences were greater (1.17 +/- 0.40; 1.58 +/- 0.31; 1.24 +/- 0.22; 1.22 +/- 0.28 kPa) and statistically significant (p less than 0.05) in comparison to the myocutaneous flaps of group 2. Transcutaneous oxygen tension monitoring was useless in predicting flap survival in later postoperative blood flow insufficiency and in assessing the neovascularization. Only during the intraoperative manipulations or the early postoperative period (24 h) tcpO2 monitoring was valuable in assessing the blood supply of myocutaneous flaps in rats.
Using a method of vascular pedicle ligation without skin incision, some minor differences were found between the progress of neovascularization in microvascular free skin-fat flaps and island flaps, both with and without 1-hr ischemia in rats. Viable flap areas were established following vascular pedicle ligation (both vessels or only artery or vein), on the third day after flap replantation in both island and free flaps. Vascular pedicle occlusion on the fifth day after operation resulted in complete survival of free flaps only. Island flaps survived completely following vascular pedicle ligation on the seventh postoperative day. A clear correlation existed between the timing of vascular pedicle ligation and the necrosis and shrinkage rates. In free flaps, clinical signs of viability disturbances were seen earlier in single or total vascular pedicle occlusions, on the third or fourth day compared with island flaps. Insufficiency of the pedicle vein in island or free flaps was tolerated earlier than arterial or total pedicle occlusion. Venous occlusion three days after flap replantation caused significantly higher necrosis and shrinkage rates (p less than 0.05) in free flaps than in island flaps.