Boundary-layer resistances of artificial leaves in turbulent air II: Leaves inclined to the mean flow

Abstract

The effect of turbulence on boundary-layer resistances to heat and water vapour transfer from leaves inclined to the mean airflow has.been studied using heated square plates in a wind tunnel. Heat and water vapour transfer coefficients increased with streamwise turbulence intensity for all angles of inclination of the plates to the mean flow, and the increase was dependent on the ratio of the longitudinal integral length scale to the plate dimension. This dependence on the turbulence length scale probably results from a resonant interaction between the boundary layer on the plate and the turbulence in the approaching mean flow.The paper also presents results of experiments with heated plates having serrated leading edges and/or a transverse ridge on the surface, conducted in an attempt to understand the aerodynamic importance of morphological irregularities on the leaf surface. The irregularities studied here disturbed the boundary layer on the plate, and greatly increased heat transfer when the angle of inclination of the plates to the mean wind was small, but had little effect when the angle of inclination exceeded 40".

Tansley Review No. 59 Leaf boundary layers

“…in such cases. On the downstream (wake) side of an inclined plate, maximum enhancement was P K 2-\S at L^JL of 0'5 and 0'9 for inclination angles of 20° and 90°, respectively (Chen, Ibbetson & Milford, 19886). This clearly suggests a resonance effect between wake flow and free stream turbulence, reminiscent of the observations of van der Hegge Zijnen (1958).…”

“…Effects of serration near the leading edge increased transfer on square plates by up to factors of 1-4 at low angles of attack, broadening the range of angles 0 over which significant transfer enhancement is observed (Chen et al, 19886;section V.2a).…”

“…4a) as the angle of attack 0 increases from 0° to 90° (e.g. Powell, 1940;Haseba, 19736;Parkhurst & Pearman, 1974;Chen, Ibbetson & Milford, 19886). Transfer enhancement with inclination on the forward surface, due to compression of BL thickness, is always observed unless flow separation at a blunt front edge causes a recirculating bubble (Motwani, Gaitoiide & Sukhatme, 1985).…”

Modelled Influences of Non-exchanging Trichomes on Leaf Boundary Layers and Gas Exchange

“…Moreover, increased surface roughness can lead to increased heat exchange across surfaces of leaves (Chen et al, 1988) due to increased turbulence. Others have reported that changes in boundary layer characteristics can alter the response of plants to ambient CO concentrations (Aphalo & Jarvis, 1993).…”

Boundary-layer resistances of artificial leaves in turbulent air: I. Leaves parallel to the mean flow

“…This report presents results from experiments with the plate held parallel to the mean flow: a second paper (Chen et al, 1988, hereafter referred to as II) presents the results from inclined plates.…”