Hybridization between σ- andz-co-ordinates for improving the internal pressure gradient calculation in marine models with steep bottom slopes

Author(s):  
Hans Burchard ◽  
Ole Petersen ◽  
Ole Petersen
2020 ◽  
pp. 26-31
Author(s):  
V.P. Shakshin ◽  
◽  
A.A. Mokrev ◽  
V.S. Vasil’ev ◽  
S.A. Belov ◽  
...  

2012 ◽  
Vol 42 (2) ◽  
pp. 261-271 ◽  
Author(s):  
Hans Burchard ◽  
Henk M. Schuttelaars

Abstract Tidal straining, which can mathematically be described as the covariance between eddy viscosity and vertical shear of the along-channel velocity component, has been acknowledged as one of the major drivers for estuarine circulation in channelized tidally energetic estuaries. In this paper, the authors investigate the role of lateral circulation for generating this covariance. Five numerical experiments are carried out, starting with a reference scenario including the full physics and four scenarios in which specific key physical processes are neglected. These processes are longitudinal internal pressure gradient forcing, lateral internal pressure gradient forcing, lateral advection, and the neglect of temporal variation of eddy viscosity. The results for the viscosity–shear covariance are correlated across different experiments to quantify the change due to neglect of these key processes. It is found that the lateral advection of vertical shear of the along-channel velocity component and its interaction with the tidally asymmetric eddy viscosity (which is also modified by the lateral circulation) is the major driving force for estuarine circulation in well-mixed tidal estuaries.


2016 ◽  
Vol 139 (2) ◽  
Author(s):  
Lei Cao ◽  
Ashish Karn ◽  
Roger E. A. Arndt ◽  
Zhengwei Wang ◽  
Jiarong Hong

A numerical study has been conducted on the internal pressure distribution of a ventilated supercavity generated from a backward facing cavitator under different air entrainment coefficients, Froude numbers, and blockage ratios. An Eulerian multiphase model with a free surface model is employed and validated by the experiments conducted at St. Anthony Falls Laboratory of the University of Minnesota. The results show that the internal pressure in the major portion of the supercavity is primarily governed by the hydrostatic pressure of water, while a steep adverse pressure gradient occurs at the closure region. Increasing the air entrainment coefficient does not largely change the pressure distribution, while the cavity tail extends longer and consequently the pressure gradient near the closure decreases. At smaller Froude number, there is a more pronounced gravitational effect on the supercavity with increasing uplift of the lower surface of the cavity and a decreasing uniformity of the pressure distribution in the supercavity. With the increase of blockage ratio, the overall pressure within the supercavity decreases as well as the pressure gradient in the main portion of the supercavity. The current study shows that the assumption of uniform pressure distribution in ventilated supercavities is not always valid, especially at low Fr. However, an alternative definition of cavitation number in such cases remains to be defined and experimentally ascertained in future investigations.


1979 ◽  
Vol 44 ◽  
pp. 307-313
Author(s):  
D.S. Spicer

A possible relationship between the hot prominence transition sheath, increased internal turbulent and/or helical motion prior to prominence eruption and the prominence eruption (“disparition brusque”) is discussed. The associated darkening of the filament or brightening of the prominence is interpreted as a change in the prominence’s internal pressure gradient which, if of the correct sign, can lead to short wavelength turbulent convection within the prominence. Associated with such a pressure gradient change may be the alteration of the current density gradient within the prominence. Such a change in the current density gradient may also be due to the relative motion of the neighbouring plages thereby increasing the magnetic shear within the prominence, i.e., steepening the current density gradient. Depending on the magnitude of the current density gradient, i.e., magnetic shear, disruption of the prominence can occur by either a long wavelength ideal MHD helical (“kink”) convective instability and/or a long wavelength resistive helical (“kink”) convective instability (tearing mode). The long wavelength ideal MHD helical instability will lead to helical rotation and thus unwinding due to diamagnetic effects and plasma ejections due to convection. The long wavelength resistive helical instability will lead to both unwinding and plasma ejections, but also to accelerated plasma flow, long wavelength magnetic field filamentation, accelerated particles and long wavelength heating internal to the prominence.


Author(s):  
J. J. Kelsch ◽  
A. Holtz

A simple solution to the serious problem of specimen contamination in the electron microscope is presented. This is accomplished by the introduction of clean helium into the vacuum exactly at the specimen position. The local pressure gradient thus established inhibits the migration of hydrocarbon molecules to the specimen surface. The high ionization potential of He permits the use of relatively large volumes of the gas, without interfering with gun stability. The contamination rate is reduced on metal samples by a factor of 10.


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