Changes In Outflow Rates Research Articles

Numerical modeling of observed effective flow behavior in unsaturated heterogeneous sands

The concept of effective parameters has been introduced in recent years to represent the spatial variability of natural soils in numerical simulation models. In the present study, effective hydraulic properties of unsaturated flow were investigated for the case of a two‐dimensional heterogeneous laboratory tank. Hydraulic parameter estimates obtained from simple statistical averages, inverse procedures, and a stochastic theory were compared to effective retention and hydraulic conductivity characteristics measured for the whole tank at steady state. The applicability of the effective parameter estimates was investigated by comparing transient flow events monitored in the laboratory tank with simulated results based on those estimates. Capillary suction measurements were simulated reasonably well using several straightforward arithmetic and geometric statistical averaging approaches, whereas most averaging approaches simulated too slow a response in the outflow rate. An alternative approach involving a combination of arithmetic and geometric averaging of the measured values more closely simulated the observed relatively fast changes in outflow rate. Generally, the simulations based on the measurements of effective properties performed quite well, indicating that the fundamental concept of effective parameters may be valid for this type of heterogeneous soil system.

Sympathetic nervous control of blood flow in the gills of the Atlantic cod,Gadus morhua

The effects of sympathetic nerve stimulation, adrenaline and isoprenaline on the inflow pressure and efferent arterial and venous flow rates were studied in a cod gill preparation perfused at constant flow rate. The dominant effect of adrenaline was a reduced inflow pressure, accompanied by an increase in arterial flow and a decrease in venous flow. Isoprenaline also decreased the inflow pressure, but the changes in both outflow rates were small or absent. Sympathetic nerve stimulation gave arterial and venous flow changes comparable to the adrenaline effects, but the inflow pressure increased during nerve stimulation. Propranolol has little effect on the nerve responses, but phentolamine abolished or reversed the increase in inflow pressure, and also decreased or abolished the changes in outflow rates. The possible sites of action of the sympathetic fibres, and the distribution of adrenoceptors in the effector tissue is discussed. It is concluded that the main effect of sympathetic nerve stimulation is α-adrenoceptor mediated, involving constriction of the arterio-venous pathway. Theβ-adrenoceptor mediated control of total branchial vascular resistance may largely depend on circulating catecholamines.