Turbulent transport mechanism in the roughness sublayers over idealized urban areas and its implication to street-level ventilation

Abstract

Turbulence in the roughness sublayer (RSL) is inhomogeneous compared with that in the inertial sublayer (ISL) of the atmospheric surface layer (ASL) over urban areas. Drag coefficient Cd, which measures aerodynamic roughness, is employed in this paper to examine how (idealized) urban morphology influences ASL dynamics and transport. Wind tunnel experiments are conducted to study the flows and turbulence in response to different configurations of (identical) roughness elements. Statistics, quadrant analysis, and tilt angle evidence the more efficient RSL transport over rougher surfaces even the winds are slower. Although the power spectra of streamwise u’’ and vertical w’’ fluctuating velocities are rather insensitive to Cd, their cospectrum shows a secondary peak at small motion scales λx (≤ 0.1δ where δ is the thickness of turbulent boundary layer) along with the primary peak at integral length scale Λx (≈ δ). It is thus suggested that, regardless of the turbulence intensity, RSL streamwise and vertical winds are more correlated, enhancing the transport. Amplitude (AM) and frequency (FM) modulations signify the positive correlation between RSL large and small motion scales which is amplified over rougher surfaces. Furthermore, RSL turbulence kinetic energy (TKE) production (entrainment) increases with decreasing (increasing) Cd, fostering the basic mechanism of street-level ventilation.