An experimental investigation of the near surface flow over air-water and air-solid interfaces

Abstract

Results from an experimental study, investigating the similarities and dissimilarities of the airflow structure over water and solid surfaces for smooth and wavy conditions, are reported. The experiments were conducted at the same location, under identical flow conditions. The two-dimensional velocity fields were measured using particle image velocimetry technique over a wind speed range from 1.5 to 4.4 m s−1. The mean velocity profiles for all surface configurations showed the logarithmic behavior. The profiles of different turbulent properties followed similar trend over water and solid surfaces, however, their magnitudes varied over different surface types. The results show that the level of enhancement of normalized Reynolds stress, rates of energy production, and dissipation from the free-stream region toward the surface is higher over the water surface especially in the presence of waves as compared to that over the solid surface. It is also observed that the normalized magnitudes of turbulent properties are largest above the water surface and smallest above the wavy solid wall. It is concluded that, at a given free stream condition, the flow behavior over the solid surface is not necessarily similar to that over the water surface.

An experimental investigation of the flow structure over a corrugated waveform in a transpired air collector

“…The reattachment point varied non-monotonously as a function of Reynolds number, which was attributed to the recirculation on the windward side. Several studies investigated the flow over sinusoidal waves in turbulent channel flows (Kuzan et al, 1989;Hudson et al, 1996;Calhoun and Street, 2001;Kruse et al, 2006;Breuer et al, 2009;Shaikh and Siddiqui, 2010). A common observation in all these studies was the enhancement of turbulent properties in the separation region above troughs.…”

“…Not only is the corrugation geometry unconventional, but the presence of slit perforations creates a unique form of surface roughness, resulting in a complex flow structure. When the air flows over a corrugated surface in a two dimensional channel it experiences flow separation over the crest of the corrugation and reattachment in the trough region (Kuzan et al, 1989;Breuer et al, 2009;Shaikh and Siddiqui, 2010). There are two primary flow regimes; the bulk flow above the corrugation height, and the local flow within the corrugation trough.…”

“…A common observation in these studies was deviating trends of flow behaviour from that of the conventional flat channel flows. The common types of corrugated geometries previously investigated were continuous sinusoidal waves (Buckles et al, 1984;Hudson et al, 1996;Calhoun and Street, 2001;Nakagawa and Hanratty, 2001;Kruse et al, 2006;Shaikh and Siddiqui, 2010), saw-tooth (V corrugations by Yang and Chen, 2010), and square-wave (Stel et al, 2010). Breuer et al (2009) conducted a numerical and experimental study to understand the flow behaviour over a series of hills for a range of Reynolds numbers.…”

“…They also observed asymmetry in the mean velocity profiles where the location of the maximum mean velocity shifted towards the plane wall. Shaikh and Siddiqui (2010) investigated the impact of surface configuration on the turbulent flow characteristics using PIV. They considered smooth and wavy solid and water surfaces and found that the turbulent behaviour over water and solid surfaces is similar, however, the magnitude was dependent on the surface type.…”

Transfer Across the Air-Sea Interface

“…This mode is often referred to as 'standard PIV'. These PIV techniques have been widely used in laboratory measurements at the airwater interface, both on the water-side (Siddiqui and Loewen 2010;Siddiqui et al 2004;Turney et al 2005;Banner and Peirson 1998) or, more challengingly, on the air-side (Reul et al 2008;Shaikh and Siddiqui 2010;Troitskaya et al 2011). Particularly for anisotropic turbulent flow, 2D measurement of velocities is problematic as out of plane velocities are not captured (Adrian 2005).…”