Cross-correlation between speed and direction of urban wind fluctuation

Abstract

Active use of natural wind turbulence could be made in the design of effective ventilation methods during the off-seasons of air conditioning systems, especially in an office room equipped with only one window. Ventilation rates through openings by wind turbulence are affected by the stochastic properties not only of the wind speed but also of wind direction. There is little information about the cross-correlations between the fluctuations of wind speed and direction needed to get a better estimation of ventilation rate caused by wind turbulence. In this article, results of turbulence measurements of wind speed and direction in autumn at the northern open district of Kyoto city are indicated. Subsequently, the stochastic properties of fluctuations of wind speed and direction, and the cross-correlations between these two properties, are presented. A distinguishing mark of the measured wind properties is that both the mean value (of 10 min average) and the turbulent intensity were high from 12:00 to 18:00, the mean wind direction was north and its fluctuation was high at noon. Histograms of speed and direction could be approximated by normal distributions. From the 24 cross-correlations between the fluctuations of wind speed and direction every hour, being obtained from the ten-minute periods with measurements at intervals of five seconds, correlations were scarcely found, neither in the daytime nor at any other time. Finally, wind fluctuations of speed and direction should be considered to possess normal distributions and to be independent of each other, though the mean values of speed and direction should be treated as time-dependent ones.