Turbulent flow in a porous tube with wall suction.
This study examines the effects of suction (i.e. lateral flow through the walls) on the structure of a fully developed turbulent pipe flow. Also the effect of suction on pressure gradient and pressure change is examined experimentally. The frictional characteristics of Irrigro$\sp{\circler,}$ the porous tubing used in this investigation, is studied by means of a comprehensive experimental program that considered different lengths of porous tubing. Three aspects of flow in porous pipes are investigated in this study, (i) a computational study of the effects of suction on the flow characteristics, (ii) an experimental study of the frictional characteristics of the porous tubing with no suction condition and (iii) an experimental study of pressure change along a porous tubing with lateral flow. The numerical study of turbulent pipe flow with wall suction rates ranging from A = 0 to 13 percent showed that in fully developed pipe flow, wall suction results in a more uniform velocity distribution with increased near-wall velocity values and reduced velocities near the centerline. The near-wall component of radial velocity, $\nu,$ increases with increasing distance from the wall in the zone near the pipe wall. The absolute levels of turbulent kinetic energy decrease with increasing suction rate. Wall suction increases the wall shear stress, $\tau\sb{\rm w},$ along the wall of the tube. The increase in $\tau\sb{\rm w}$ is significant even for the smallest suction rate (up to 30 percent) while such an increase is much higher for A = 13 percent (up to 360 percent). Analysis of the experimental friction loss data obtained for small diameter porous tubing in this study confirmed that the Colebrook and White (C-W) equation is a very accurate predictor of the friction factor for porous tubing with small diameter size and Reynolds numbers less than 100,000. These results are in agreement with the results of Aggarwal et al (1972). The value of the relative roughness obtained in this study showed that the porous tubing under study is smoother than most of the tubing used as laterals in the traditional trickle irrigation. Also, the fact that the friction factors agreed with the Colebrook-White law indicates that the physical roughness in the porous tubing under study corresponds very nearly to the equivalent sand roughness with a relative roughness of about e/D = 0.002. A relationship was established as a convenient and accurate head loss prediction equation (within 5% error) by combining a power function with the Darcy-Weisbach equation. The combination equation is correctable for viscosity changes and accurate for the porous pipe tubing under study. A pressure change and a pressure gradient prediction relationship were established in the transition zone of the Moody diagram for high suction rates, assuming a uniform radial flow rate along the suction region. The relationships presented herein are based on a control volume approach analysis and incorporated the data obtained from laboratory studies on the porous tubing under study.
Source: Dissertation Abstracts International, Volume: 59-07, Section: B, page: 3595.
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