Abstract
With the aim of ascertaining the effects of the widths (A) of valleys on near-surface turbulence, flows over an isolated symmetric three-dimensional valley of constant depth (H) and slopes are characterized in a large-boundary-layer wind tunnel. Starting at A = 4H, valley widths were systematically varied to A = 12H with constant increments of 2H. High-resolution laser-Doppler velocimetry measurements were made at several equivalent locations above each of the resulting valley geometries and compared with data from undisturbed flows over flat terrain. Flow separation caused by the first ridges generated inner-valley recirculation bubbles with lengths dependent on the valley widths. Secondary recirculation zones were also observed downstream from the crests of the second ridges. Results show that the width modifications exert the strongest effects on turbulence within the valleys and the vicinities of the second ridges. Above these locations, maximal magnitudes of turbulence are generally found for the larger width geometries. Furthermore, lateral turbulence overpowers the longitudinal counterparts nearest to the surface, with maximal gains occurring for the smaller widths. Our data indicate that valley widths are impactful on near-surface flows and should be considered together with other more established geometric parameters of influence.
Highlights
Atmospheric boundary layer (ABL) flows are affected by irregular orography, which impacts a wide variety of applications, such as wind engineering, air quality, and agriculture, amongst others
This results in a Reynolds roughness number of about 0.4, considered permissible for ABL flows over complex terrain [13]
We have studied the effects of varying valley widths on microscale near-surface atmospheric boundary layer (ABL) flows through physical modeling in a large-boundarylayer wind tunnel
Summary
Atmospheric boundary layer (ABL) flows are affected by irregular orography, which impacts a wide variety of applications, such as wind engineering, air quality, and agriculture, amongst others. The first major contribution to the field of ABL flows over complex terrain came in 1975 with the Jackson and Hunt linear theory of flows over low two-dimensional hills [2]. Their analytical theory applied to attached flows over gentlesloped hills and motivated a number of extensions and alternative theories proposed throughout the following decade [1,2,3]. The advent of the wind energy industry led to increased efforts to understand near-surface microscale flow characteristics and to improve the accuracy of numerical models [1,3]. Fewer efforts have been made to understand how isolated orography affects near-ground turbulence characteristics, and further investigation is required
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