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  • 1
    Publication Date: 2000-11-16
    Description: Simultaneous measurements of the velocity and scalar concentration fields have been made in the plume emitting from a two-dimensional line source at the wall. The source is one obstacle height, h, downstream of a two-dimensional square obstacle located on the wall of a turbulent boundary layer. These measurements were made in two fluid media: water and air. In both media particle image velocimetry (PIV) was used for the velocity field measurements. For the scalar concentration measurements laser-induced fluorescence (LIF) was used for the water flow and Mie scattering diffusion (MSD) for the air flow. Profiles of the mean and root-mean-square streamwise and wall-normal velocity components, Reynolds shear stress and mean and root-mean-square scalar concentration were determined at x = 4h and 6h downstream of the obstacle in the recirculation region and above it in the mixing region. At these streamwise stations the scalar fluxes, uc and vc, were also determined from the simultaneous velocity and scalar concentration field data. Both of these fluxes change sign from negative to positive with increasing distance from the wall in the recirculating region at 4h. A conditional analysis of the data was carried out by sorting them into the eight categories (octants) given by the sign combinations of the three variables: ±u, ±v and ±c. The octants with combinations of these three variables that correspond to types of scalar concentration flux motions that can be approximated by mean gradient scalar transport models are the octants that make the dominant contributions to uc and vc. However, in the recirculating zone, counter-gradient transport type motions also make significant contributions. Based on this conditional analysis, second-order mean gradient models of the scalar and the momentum fluxes were constructed; they compare well to the measured values at 4h and 6h, particularly for the streamwise scalar flux, uc. Additional measurements of the velocity and concentration fields were made further downstream of the reattachment location in the wake region of the air flow. The mean velocity deficit profile determined from these measurements at x = 20h compares quite well to a similarity solution profile obtained by Counihan, Hunt and Jackson (1974). Their analysis was extended in the present investigation to the concentration field. The similarity solution obtained for the mean concentration compares well to profiles measured at x = 12h, 15h, and 20h, up to about three obstacle heights above the wall.
    Print ISSN: 0022-1120
    Electronic ISSN: 1469-7645
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
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  • 2
    Publication Date: 1983-06-01
    Description: SUMMARYDiurnal and seasonal changes in the total, osmotic and turgor potentials of winter wheat leaves are compared in two seasons of mild and severe soil water stress. Gradients of total water potential in the soil-plant system are also presented. In both seasons the total water potential of the leaves decreased in parallel with the soil water potential, concurrently leaf osmotic potential also decreased sufficiently to maintain positive leaf turgor potential. Eventually, under severe water stress, soil water potential approached –1·5 MPa and leaf turgor potential tended to zero during the middle of the day.The potential drop across the soil-root system was twice that along the stem. Estimates of the water potential at the root surface varied diurnally and were often lower than the bulk soil water potential. In dry soil plants were unable to equilibrate with the soil water potential overnight. These results are consistent with the existence of significant resistance to water flow across the rhizosphere.
    Print ISSN: 0021-8596
    Electronic ISSN: 1469-5146
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
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  • 3
    Publication Date: 1983-06-01
    Description: SUMMARYA simple model is presented which can account for the hysteresis observed in the relationship between leaf water potential (χ1) and water flow per stem (Q) in winter wheat. Using this model the total hydraulic resistance (R) and capacitance (C) of the soil-plant system were estimated from measured diurnal changes in χ1 and Q. Daily mean values of R were constant when the soil water potential (χ1) was above – 0·6 MPa, but R increased by about 40% when the soil dried to – 1·5 MPa. R also appeared to vary during the day, increasing in the afternoon and evening.The results of experiments on the recovery of χ1 in plants from which transpiration was prevented suggested that the major resistance to flow was in the soil-root system and that it was an increase in this resistance which gave rise to the overall increase in R as the soil dried.There was substantial variation in the estimates of hydraulic capacitance (C) and no systematic diurnal or seasonal trend was evident. Despite the variation in C the mean value for the season, 8 x 10-7 m3/MPa, predicted realistic changes in plant water storage. Estimates of the components of C indicated that the major capacitance was located in the soil-root complex. More information is needed about the components of R and C, particularly those in the soil-root system.
    Print ISSN: 0021-8596
    Electronic ISSN: 1469-5146
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
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  • 4
    Publication Date: 2013-09-24
    Description: The inception of leading-edge stall on stationary, two-dimensional, smooth, thin aerofoils at low to moderately high chord Reynolds number flows is investigated by a reduced-order, multiscale model problem via numerical simulations. The asymptotic theory demonstrates that a subsonic flow about a thin aerofoil can be described in terms of an outer region, around most of the aerofoil’s chord, and an inner region, around the nose, that asymptotically match each other. The flow in the outer region is dominated by the classical thin aerofoil theory. Scaled (magnified) coordinates and a modified (smaller) Reynolds number $(R{e}_{M} )$ are used to correctly account for the nonlinear behaviour and extreme velocity changes in the inner region, where both the near-stagnation and high suction areas occur. It results in a model problem of a uniform, incompressible and viscous flow past a semi-infinite parabola with a far-field circulation governed by a parameter $ilde {A} $ that is related to the aerofoil’s angle of attack, nose radius of curvature, thickness ratio, and camber. The model flow problem is solved for various values of $ilde {A} $ through numerical simulations based on the unsteady Navier–Stokes equations. The value ${ilde {A} }_{s} $ where a global separation zone first erupts in the nose flow, accompanied by loss of peak streamwise velocity ahead of it and change in shedding frequency behind it, is determined as a function of $R{e}_{M} $. These values indicate the stall onset on the aerofoil at various flow conditions. It is found that ${ilde {A} }_{s} $ decreases with $R{e}_{M} $ until some limit $R{e}_{M} $ (${sim }300$) and then increases with further increase of Reynolds number. At low values of $R{e}_{M} $ the flow is laminar and steady, even when stall occurs. The flow in this regime is dominated by the increasing effect of the adverse pressure gradient, which eventually overcomes the ability of the viscous stress to keep the boundary layer attached to the aerofoil. The change in the nature of stall at the limit $R{e}_{M} $ is attributed to the appearance of downstream travelling waves in the boundary layer that shed from the marginal separation zone and grow in size with either $ilde {A} $ or $R{e}_{M} $. These unsteady, convective vortical structures relax the effect of the adverse pressure gradient on the viscous boundary layer to delay the onset of stall in the mean flow to higher values of ${ilde {A} }_{s} $. Computed results show agreement with marginal separation theory at low $R{e}_{M} $ and with available experimental data at higher $R{e}_{M} $. This simplified approach provides a universal criterion to determine the stall angle of stationary thin aerofoils with a parabolic nose.
    Print ISSN: 0022-1120
    Electronic ISSN: 1469-7645
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
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