Analysis of Viscoelastic Fluid Flow over a Suddenly Moved Flat Plat

The velocity field and the associated tangential tension corresponding to the flow of the viscoelastic fluid over an infinite rigid plate are determined in this paper. The characteristic of the non-Newtonian viscoelastic fluid flow is analyzed using the Stehfest method of the numerical inversion of Laplace transform. It indicates that the flow of the viscoelastic fluid is very sensitive to the material constants. Finally, some comparative diagrams concerning the velocity and tangential tension profiles are presented.

CHARGED MEMBRANE AS A SOURCE FOR REPULSIVE GRAVITY

We demonstrate an alternative (with respect to the ones existing in the literature) and more-typical-for-physicists derivation of an exact solution to the Einstein–Maxwell equations for the motion of a charged spherical membrane with tangential tension. We stress that there exists physically acceptable range of parameters for which the static and stable state of the membrane producing the Reissner–Nordstrom (RN) repulsive gravity effect exists. The concrete realization of such a state for the Nambu–Goto membrane is described. The point is that the membrane is able to cut out the central naked singularity region and at the same time join in an appropriate way the RN repulsive region. As a result we have a model of an everywhere-regular material source exhibiting a repulsive gravitational force in the vicinity of its surface: this construction gives a more sensible physical status to the RN solution in the naked singularity case.

Carotid arterial hemodynamics in response to LBNP in normal subjects: methodological aspects

Pulsatile changes in blood pressure and arterial diameter were studied noninvasively with applanation tonometry and echo-tracking techniques at the sites of the common carotid artery (CCA) and the carotid arterial bulb (CAB) in 12 healthy volunteers. Determinations were performed before and during application of -10 and -40 mmHg lower body negative pressure (LBNP) to investigate noninvasively the tensile forces acting on the CAB. Together with significantly decreased mean arterial pressure, increased heart rate, forearm vascular resistance, and plasma norepinephrine, the -40 mmHg LBNP stimulus produced the following significant changes in CCA and CAB hemodynamics: 1) for the same decrease in mean arterial pressure, a greater decrease in carotid than in brachial pulse pressure was observed (P < 0.01) due to a significant change in pressure wave transmission and in the timing of the carotid backward pressure wave; and 2) a highly significant decrease in pulsatile changes in diameter and tangential tension occurred, with a greater decrease in systolic than in diastolic tangential tension. Subsequently, cyclic tangential tension decreased more substantially than mean tangential tension. The cyclic changes in tension were quite significant after -40 mmHg LBNP but were already observed for mild -10 mmHg LBNP in which mean systemic blood pressure and heart rate were not modified. During -10 and -40 mmHg LBNP, CCA and CAB compliance and distensibility were unchanged. This study provides evidence that the autonomic nervous system activation produced by the LBNP procedure is associated with significant changes in pressure-wave amplification and in cyclic tensile forces acting on the CAB. These changes, which may occur even for mild LBNP, should be taken into account when interpreting results of the LBNP procedure in humans.

Microgravity and orthostatic intolerance: carotid hemodynamics and peripheral responses

A ground-based model [24 h of bed rest (BR) with head-down tilt (HDT)] was used to investigate the cardiovascular deconditioning responsible for orthostatic intolerance, frequently observed after weightlessness flights. This experimental deconditioning is shown to be distinguished by an increase of mean blood pressure (P < 0.05), with increased total peripheral resistances (TPRs). Systolic tangential tension of the carotid arterial wall, cardiac output and frequency (spectral analysis), and plasma norepinephrine and epinephrine were not significantly altered, while plasma dopamine was increased (P < 0.05). Cardiovascular homeostasis was challenged before and after 24 h of BR with HDT through -40 mmHg lower body negative pressure (LBNP). Systolic tangential tension of the carotid wall was decreased, with a decrease of systolic pressure and cardiac output; increased heart rate was likely due to an increase of sympathetic drive with a decrease of vagal braking. The overall picture was not changed after 24 h of BR with HDT, except for a lack of increase of TPRs: their increase (+13.7%, P < 0.05) before was no longer observed after (-2.6%) 24 h of BR with HDT. This apparent deficiency cannot be explained. However, a heterogeneity in the response of TPR should be considered because the magnitude of the increase of blood pressure to cold pressor test was the same after 24 h of BR with HDT as it was before.(ABSTRACT TRUNCATED AT 250 WORDS)