Boundary layer heat transfer coefficients under field conditions

Pearman, G.I.; Weaver, H.L.; Tanner, C. B.

doi:10.1016/0002-1571(72)90009-x

Cited by 100 publications

(43 citation statements)

References 10 publications

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“…(B-22) yields to = 7 s . This agrees with tive and diffusive resistances of flat-plate boundary-layer flows by a factor n, , 1 5 n, I 2 , such that (C-18) (Pearman et al 1972). Another study showed a similar effect on cylinder crossflow resistance (White 1988, p.347).…”

Section: B32 Characteristic Time Of Transient Solutionsupporting

confidence: 75%

Near-ground cooling efficacies of trees and high-albedo surfaces

Levinson

1997

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Portions of thisDaytime summer urban heat islands arise when the prevalence of dark-colored surfaces and lack of vegetation make a city warmer than neighboring countryside. Two frequentlyproposed summer heat island mitigation measures are to plant trees and to increase the albedo (solar reflectivity) of ground surfaces. This dissertation examines the effects of these measures on the surface temperature of an object near the ground, and on solar heating of air near the ground. Near-ground objects include people, vehicles, and buildings.The variation of the surface temperature of a near-ground object with ground albedo indicates that a rise in ground albedo will cool a near-ground object only if the object's albedo exceeds a critical value. This critical value of object albedo depends on wind speed, object geometry, and the height of the atmospheric thermal boundary layer. It ranges from 0.15 to 0.37 for a person. If an object has typical albedo of 0.3, increasing the ground albedo by 0.25 perturbs the object's surface temperature by -1 to +2 K.Comparing a tree's canopy-to-air convection to the reduction in ground-to-air convection induced by tree shading of the ground indicates that the presence of a tree can either increase or decrease solar heating of ground-level air. The tree's net effect depends on the extent to which solar heating of the canopy is dissipated by evaporation, and on the fraction of air heated by the canopy that flows downward and riiixes with the ground-level air.A two-month lysimeter (plant-weighing) experiment was conducted to measure instantaneous rates of water loss from a tree under various conditions of weather and soil-moisture. Calculations of canopy-to-air convection and the reduction of ground-to-air convection based on this data indicate that canopy-induced heating would negate shadowinduced cooling if approximately 45% of the canopy-heated air mixed with ground level air. This critical fraction is comparable to typical downward mixing fractions, so the tree's net heating or cooling effect on near-ground air is small. 2

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Section: B32 Characteristic Time Of Transient Solutionsupporting

confidence: 75%

Near-ground cooling efficacies of trees and high-albedo surfaces

Levinson

1997

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“…Monteith (1965) reviewed boundary-layer resistance data for isolated leaves, and further measurements have been reported by Parkhurst er al. (1968) for leaf models of complex shape, and by Parlange et al (1971Parlange et al ( , 1972 and Pearman et al (1972) who examined the effects of leaf fluttering and behaviour of leaves in a turbulent air stream. Philip (1966) questioned whether wind-tunnel studies on a single leaf are appropriate for leaves in their natural state where turbulence will affect transfer at the leaf surface.…”

Section: Introductionmentioning

confidence: 99%

Heat transfer coefficients for leaves on orchard apple trees

Thorpe

Butler

1977

Boundary-Layer Meteorol

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The coefficient for heat transfer from apple tree leaves was measured from the energy balance of leaves which were prevented from transpiring by applying 'Vaseline' (petroleum jelly). Vaseline had negligible effect on the absorption of short-wave radiation by the leaves. The Nusselt number (Nu) describing heat flux from a leaf in terms of its average temperature was related to Reynolds numbers (Re) in the range lo3 to lo4 bywhere Pr is the Prandtl number. This supports Landsberg and Powell's (1973) wind-tunnel results for transfer from leaves subject to mutual interference.

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“…Note that equation (4) is meant for the canopy scale (with resistances in capitals) and equation (11) applies for the leaf scale (with lower case). The boundary resistance at the leaf scale is given by (Pearman et al, 1972):…”

Section: The Micro-meteorological Modelmentioning

confidence: 99%