Interactions among different scales of motions in turbulent flows are a fundamentally important problem in fluid mechanics. This study examines the modulation of small-scale turbulences by large-scale turbulent coherent structures (TCSs) over urban surfaces of differing morphologies. For this, large-eddy simulations are performed for four idealized building arrays with unstaggered/staggered arrangements and even/uneven heights and the amplitude modulation (AM) analysis is made. Overall, the building height heterogeneity exerts greater influence on the vertical profiles of turbulent quantities, TCSs, and modulation of small-scale turbulence by large-scale coherent motions than the building arrangement. The building height heterogeneity notably increases the vertical turbulent momentum flux and turbulent kinetic energy above the canopy layer and the spanwise extension of TCSs regardless of the building arrangement. The modulation of small-scale turbulence by large-scale coherent motions is clearly enhanced near the top of the canopy layer and within the canopy layer when the building height heterogeneity is considered. The two-point time-lagged AM analysis reveals that the large-scale coherent motions more rapidly influence the amplitude of small-scale turbulence with building height heterogeneity than without it. While the staggering of buildings enhances the modulation without building height heterogeneity, it weakens the modulation with building height heterogeneity. The findings of this study can improve the understanding of the impacts of urban morphology on scale interactions in turbulent flows.
Large amounts of synchronous multipoint wind velocity and particle concentration data are acquired from long-term observations in high-Reynolds-number ( Re_τ∼𝒪(10^6) ) particle-laden atmospheric surface layer (ASL) at the Qingtu lake observation array (QLOA) site. Leveraging these observational data, the effects of particles on three-dimensional (3-D) spatial length scale of large-scale coherent structures (streamwise lengths, spanwise widths, and wall-normal heights) and flux transport characteristics are investigated in this study. In addition to the significant structural characteristics whereby the 3-D spatial length scales of the large-scale coherent structures contained in the flow field essentially follow the same linear decay form with increasing particle mass loading, the momentum and dust flux also clearly depend on the particle mass loading. Particles considerably increase the proportion of ejection and sweep events, thus significantly increasing the flux transport efficiency observed at high concentrations, which is visualized by the fact that dust particles tend to cluster in the low-speed region of the flow field, and this tendency becomes more pronounced with increasing particle mass loading and height. In addition, the scales of most flux-carrying structures, for both momentum flux and dust flux (in the streamwise and wall-normal directions), increase significantly under the influence of particles in the upper logarithmic layer. The average transport similarity between the momentum flux and dust flux increases with increasing particle mass loading and height. Furthermore, considering the multiscale nature of turbulence, the scale-dependent transport similarity between momentum flux and dust flux progressively increases with height at large scales because of the presence of large-scale structures. The 3-D spatial length scales of large-scale structures exhibit the same linear decay form with increasing particle mass loading. Particle-enhanced ejection/sweep events improve the flux transport efficency An increase in transport similarity is associated with larger-scale structures at higher positions.
Predicting the transport and fate of microplastics (MPs) requires a reliable understanding of the incipient motion of deposited particles. Recent studies have extended the Shields diagram to MPs, proposing correction frameworks for foreign particles exposed in isolation on a bed of different material. These frameworks quantify how such a configuration departs from the diagram’s original premises—through hiding–exposure and relative static friction—and correct for that departure. Since these corrections vanish under the original premises themselves, such frameworks may implicitly assume a reference condition in which plastic conforms to the classical Shields curve. Whether this reference condition has been verified directly, without correction, remains unclear. Here, the original premises—a flat granular bed of uniformly sized, spherical particles of identical material—were reproduced in an annular flume using 3D-printed plastic spheres (1.05 to 1.31 g/cm³; 1 to 10 mm). All plastic data fall close to the Shields curve, experimentally supporting the reference condition on which those frameworks rest. The thresholds measured within a granular bed—the configuration in which MPs accumulate at depositional sites—lie consistently above an existing single-particle relationship, indicating that packed particles require a greater shear stress, possibly reflecting reduced exposure and mutual support. The Shields diagram may therefore offer a suitable basis for describing MP incipient motion in a granular bed. Shields’ original granular-bed premises were reproduced in an annular flume using 3D-printed plastic spheres. Low-density plastic data fall close to the Shields curve, supporting the reference condition MP models assume. Granular-bed thresholds exceed single-particle ones, suggesting the Shields diagram may suit MPs in a granular bed.
Evaluation of ventilation is essential for maintaining healthy indoor environments. While previous experimental and numerical studies have examined mean and fluctuating flow characteristics around openings, the detailed mechanisms governing ventilation remain insufficiently understood. In this study, shear-induced natural ventilation in a cubic building with side-wall openings is investigated using large-eddy simulation (LES), enabling direct analysis of unsteady flow structures and their influence on ventilation performance. The LES results are validated against wind tunnel experiments, showing good agreement in both mean flow and turbulence statistics. Analysis of instantaneous velocity fields reveals that ventilation is highly unsteady and intermittent, driven by transient turbulent structures such as flow separation and reattachment. Two-point correlation analysis indicates strong synchronization of airflow near the openings. Conditionally averaged results show that weak ventilation produces symmetric inflow–outflow patterns, whereas strong ventilation leads to asymmetric flow, with dominant inflow at one opening and outflow at the other. This behavior is associated with streamwise acceleration near openings and enhanced vertical motion induced by separation. Different ventilation metrics are evaluated, demonstrating that the instantaneous ventilation rate Q_inst provides a more accurate representation than mean-based measures. A Gaussian-based estimation method Q_est reproduces Q_inst within about 10
