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• Large-eddy simulations (LESs) for an airflow over a simplified urban-like array were conducted. • Three driving methods were used in LESs with periodic boundary conditions. • Effects of driving methods on the pedestrian-level wind were presented. • Agreement between the LESs and the estimations based on Weibull distribution. The effect of urban geometries on the peak wind speed at the pedestrian level was investigated using simplified urban-like arrays in wind tunnel experiments. To scrutinize the turbulent flow's spatial and temporal characteristics, large-eddy simulations (LESs) were adopted to simulate suitable experimental conditions using an external force accelerating the flow with the periodic boundary condition. Although previous studies have revealed that conventional LESs use a constant pressure gradient as a momentum source, driving the flow differs from those in developing boundary layers in experiments, and the effect of the momentum source on peak wind speeds at the pedestrian level remains unknown. Therefore, this study used a series of LESs based on the three driving methods to investigate the impact of the momentum provision on the relevant statistics and peak values. The turbulent statistics showed good agreement among the cases driven by the momentum sources regardless of the shape of the profiles of the momentum source. Peak wind speeds, quantified by percentiles, were estimated using statistical models based on the Weibull distribution. Overall, the results showed a good agreement between the LESs and statistical model estimations when higher-order moments were adopted as the estimation parameters. [ABSTRACT FROM AUTHOR]

Various numerical simulations have been developed to evaluate the mean ventilation rates of a target building; however, the manner in which turbulent flow generated by buildings and surrounding conditions affects the mean and fluctuating ventilation rates is not well understood. Therefore, we have performed large-eddy simulation of flow and pressure fields above two types of block arrays (lattice-type square and staggered pattern) to clarify the turbulent characteristics of estimated ventilation rates based on pressure coefficient distribution on the block faces. The concept of short-term ventilation rates, which are estimated from filtered pressure coefficients, is introduced to investigate the temporal variation in the estimated ventilation rates for different locations of the block faces and arrangements. First, mean and second-order turbulent statistics agree well with previous results obtained from both wind-tunnel experiments and numerical simulations, indicating that the turbulent characteristics of the flow above urban-like arrays are well reproduced. Second, short-term ventilation rates are found to vary temporally and spatially. Therefore, these values instantaneously become larger or smaller than the mean ventilation rates. In addition, complex fluctuation patterns of pressure coefficient distribution are found to be caused by air flow introduction from the block arrays into gaps between the blocks as well as by small-scale turbulence generated by surrounding buildings themselves. Lastly, the temporal statistics of filtered pressure coefficients show that the short-term ventilation rates can possibly become stronger or weaker than the mean ventilation rates, whereas the accumulated fluctuating ventilation rates are almost comparable to those estimated by mean pressure coefficients. [ABSTRACT FROM AUTHOR]

Particle image velocimetry (PIV) is currently the only method which can experimentally quantify the spatial distributions of velocity with high sampling frequencies without causing intrusions into the airflow of the measuring devices. To extend the use of PIV systems for determining the distributions of velocity for wind environmental and engineering purposes, this study showed various applications of wide-ranging PIV systems for an artificially created turbulent flow at the wind-tunnel scale. Various turbulent statistics, which are commonly employed for evaluating the wind structures and environments, were compared among three conditions of the measurement areas covered by PIV systems and a hot-wire anemometry (HWA). The largest measurement covering 500 × 500 mm2 can capture the turbulent motions extending to 40% of the entire boundary layer. In addition, the discrepancies on two-length scales could be discussed using spatial and temporal correlation functions owing to the wide-ranging PIV. Moreover, the limitations of the wide-ranging PIV systems were clarified. While the turbulent statistics compared among three PIV conditions and HWA demonstrated adequate agreement, the PIV with the largest measurement area failed to capture the small-scale turbulent structures due to the coarse grid resolutions. Moreover, the measurement resolutions of PIV systems demonstrated sensitivity in determining the spectrum and probability density functions, whereas a wide-ranging PIV system can cover wide-ranging measurement areas. • Applications of wide-ranging PIVs for turbulent flow at the wind-tunnel scale. • Turbulent motions covering 40% of entire boundary layer captured over 500 × 500 mm2. • Elucidated the limitations of the wide-ranging PIVs. • Sensitivity in determining the spectrum and probability density functions by PIVs. • Wide-ranging PIVs reduce the spatial resolutions of each measurement point. [ABSTRACT FROM AUTHOR]

The interaction between the turbulent boundary layer and wake behind various types of two-dimensional cylinders is examined based on the velocity measurements by hot-wire anemometry and particle image velocimetry. Three types of cylinders, such as circular, square, and 1:5 rectangular cylinders, of two different diameters of 5 ​mm and 10 ​mm were employed in this study. The streamwise velocity distributions of the wake flow showed that its width is sustained in the streamwise direction within the boundary layer because of its interaction with the turbulent boundary layer flow. Interestingly, this phenomenon was observed regardless of the cylinder shapes. The momentum exchange in both spanwise and vertical directions showed that the advective momentum flux considerably contributes to momentum budget in the wake flow owing to the weak vertical flow formed within the boundary layer. Consequently, the spanwise width of the wake does not expand in the streamwise direction due to the alternation of the momentum budget. The results of this study can contribute to clarifying the effect of interaction between the wake and the turbulent boundary layer, as well as explain the sustaining mechanics of the low speed streaks formed within the turbulent boundary layer, even for different origins of the low-speed regions. • The interaction between boundary layer and wake behind two-dimensional cylinders is examined. • The wake width is sustained in the streamwise direction within the boundary layer. • The advective momentum flux contributes to momentum budget in the wake flow. [ABSTRACT FROM AUTHOR]

Most studies on indoor ventilation have utilized wind-tunnel experiments (WTEs) or computational fluid dynamics based approaches under well-controlled flow conditions. However, the effects of urban boundary layer flow with variable wind directions and various turbulence scales on the ventilation driven by the pressure differences between the upwind and downwind sides of a building within a block array is still under discussion. Therefore, we conducted outdoor experiments at comprehensive outdoor scale model (COSMO) experiment sites in an urban climate to clarify the relationships between the building wall pressure differences and the approaching flow. The pressure coefficients for the outdoor site were comparable with those obtained during previous WTEs. Accordingly, temporal variations in the wind speed and pressure coefficient on the target block were investigated in detail using low-pass filtering operations. The relationships between the filtered wind speed and the pressure differences indicate that the slower temporal variations in the wall pressure showed good agreement with the filtered approaching flow. In addition, the correlation coefficient between the filtered wind speed and the wall pressure differences quantified the apparent coherence between the turbulent flow and the ventilation rate. Furthermore, the statistics of the ventilation rate were determined based on the conventional model to clarify the effects of the turbulent flow on the natural ventilation rate. The ratios between the mean and short-term ventilation rates imply that the short-term ventilation rate presented dramatic temporal fluctuations owing to the various scale turbulence generated by the atmospheric flow. • Simultaneous outdoor measurements of atmospheric flow and wall pressure on a cube. • Temporal variations of filtered wind speed and wall pressure are obviously correlated. • Correlation between wind speed and wall pressure increases with filtering period. • Short-term ventilation rate indicates dramatic variations in natural ventilation. [ABSTRACT FROM AUTHOR]