The Influence of Slopes of Isolated Three-Dimensional Valleys on Near-Surface Turbulence

Abstract

Motivated by a limited understanding of how valleys affect near-surface turbulence, characterizations of neutrally stable atmospheric-boundary-layer flows over isolated valleys are presented. In particular, the influence of the slopes of the three-dimensional ridges that form the idealized valleys are investigated. Flows over three distinct symmetric valley geometries were modelled in a large boundary-layer wind tunnel. For each valley geometry, the high-resolution measurements from the crests of each of the ridges and the midpoint between them are compared with an undisturbed moderately rough classed boundary-layer flow over flat terrain with homogeneous surface roughness. Flow separation originates above the crests of the first ridges of all geometries and generates recirculation zones. These are characterized by slope-dependent increases in three-dimensional near-surface turbulence when compared with the attached flows further upstream. The recirculation zones longitudinally extend to roughly half the valley width. Above the crests of the second ridges, the longitudinal velocity component decreases and turbulence intensity increases when compared with the flows above the crests of the first ridges. Results also exhibit significant increases of turbulence above the inner-valley regions of all geometries.

The Effects of the Width of an Isolated Valley on Near-Surface Turbulence

“…The functional relationship between the normalized longitudinal integral length scales and roughness length yielded a moderately rough ABL flow at z/H ≤ 0.25 [14,22]. At higher altitudes, the length scales shifted to slightly rough class reference values [14]. Taking into account that the integral length scales are the most challenging turbulence parameter to match with the full scale, we consider the data physically consistent with a moderately rough ABL flow.…”

“…The functional relationship between the normalized longitudinal integral length scales and roughness length yielded a moderately rough ABL flow at z/H ≤ 0.25 [14,22]. At higher altitudes, the length scales shifted to slightly rough class reference values [14]. Taking into account that The full-scale aerodynamic surface roughness length of the modeled ABL flow is approximately 0.025 ± 0.015 m for a model scale of 1:1000.…”

“…A moderately rough classed flow also resulted from the semilogarithmic relationship between roughness length and profile exponent. The functional relationship between the normalized longitudinal integral length scales and roughness length yielded a moderately rough ABL flow at z/H ≤ 0.25 [14,22]. At higher altitudes, the length scales shifted to slightly rough class reference values [14].…”

“…However, idealized terrain features are useful in providing maximal data transferability between different landforms of equivalent types (e.g., ridgetype landforms) and in ascertaining the effects of individual geometric parameters (e.g., slopes) of the terrain on the flows. Moreover, they have no requirements for modeling site-specific ABL flow characteristics [14]. The simple geometries of idealized landforms and their smaller study domains are also beneficial in providing validation data for numerical models [15].…”

“…Ref. [14] investigated near-surface flows over three-dimensional valleys of constant depth (H) and width (A = 12H) and varying slopes (10 • , 30 • , and 75 • ). Inner-valley recirculation zones were observed for all geometries and their lengths were found to be independent of the slope inclinations.…”