Turbulence characteristics, short time wind gusts and wind pulsations in urban location.

Transcription

1 Turbulence characteristics, short time wind gusts and wind pulsations in urban location. Krzysztof Fortuniak, Joanna Wibig and Włodzimierz Pawlak Department of Meteorology and Climatology, Uniersity of Łódź, Poland Abstract The work focuses on relation between mean wind speed and wind gust at urban location in Łódź, central Poland. Sonic anemometer data (Hz) was compared with classical cup anemometer records from the same point. Results show that maximum wind speed recorded by sonic anemometer is higher than wind gust measured with the classical method. Relation between maximum wind speeds for different aeraging periods exhibit linear scaling with logarithm of aeraging period. This relation can be applied to predict gusts for different wind speeds. For practical purposes the knowledge on maximum fluctuations is useful to estimate forces acting on surface structures. Results of the present study show ery strong changes in wind speed in relation to its mean alue. For example, for mean wind around m/s, wind speed can change about m/s in second only. A linear-logarithmic scaling between the ratio of maximal wind change and mean wind speed is obsered for a short time step (< s). Due to stress of surface structures such changes can be more destructie and uncomfortable for indiiduals that high wind speed itself.. Introduction High wind speed, a sudden wind increase and wind gusts are fundamental in terms of wind comfort and wind safety. A damage to forests and housing are the most eident of the public consequences of wind extremes. Significant factor of an indiidual comfort is the response of pedestrians to a sudden change in wind speed (Duro, 967; Jordana et al., 8) like the effect of turbulence of indiiduals ability to undertake simple tasks such as walking or putting on a rain coat (Hunt et al., 976). The problem can be ery important under changing climate conditions including changes in wind speed and frequency of stormy weather. Een if many studies focus on problem of respond of anemometric conditions to the climate changes (Lambert, 99; Jönsson and Fortuniak, 99; Carnell and Senior, 998; Knippertz at al., ; Geng and Sugi, ; Lambert, 4; Lambert and Fyfe, 6; Leckebusch et al., 6; Bärring and Fortuniak, 9), majority of the them focus on changes in the maximum wind speed or frequency of high speed eents. Much less information is gien about sudden changes in wind speed related to turbulent nature of wind flow witch result in increased wind-included loads on the structures and indiiduals. The main goal of this work is to present a selected turbulence characteristics of wind speed oer urbanised area and relate the highest alues of wind speed and rapid wind increases to its mean in longer time period.. Side location and data collection The work base on the measurements in Łódź (population ca 7 ), central Poland. The data were collected on the thin mast, m in height located on roof of the 7 m in height building. A building is located at Lipowa 8 str., in the west edge of the old urban core and it is surrounded by mostly - stories buildings (- m) which form a

2 large homogenous settlement of great density. The streets in this region are narrow, oriented in a regular grid pattern, resulting in "canyons" cut into "plateau" of roofs. Meteorological measurements at this site are continuously proided since 996 and wind conditions was first measured by a standard wind wane and cup anemometer and since by a sonic anemometer installed on the top of the mast (SWS-/K Applied Technologies, in the years - and 8 Young since 6) which allows for a fast respond measurements (Hz in this study). Both types of data are used in the present study. Data were collected in one hour files and for each one hour period wind statistics were calculated: mean alue, the highest alue for different aeraging period, standard deiations, drag coefficient etc. (see next section for details). More detailed description on the city structure, site location, instrumentation and data processing can be found in Fortuniak et al. (), Offerle at al. (, 6a, 6b) and Pawlak at al. ().. Results In general, winds from the western sector dominate at the study area, but the wind rose for the measurement point is characterised by a gap in winds from directions 4 o (Fig. ). The explanation for this gap in unclear, but there are some arguments that it can be a result of tunnelling of the flow by an adjoining street (see Fortuniak et al. ). The mean wind speed at the measurement point is about. m s with slightly higher alues in the winter, but in general it is ery similar for all seasons. The highest wind gusts (measured by cup anemometer) reach m s ; the 9% and 99% quantiles are ~9. m s and.8 m s, respectiely % % 4% [m s - ] > - > - > - > number of cases wind speed [m s - ] Fig. : Wind rose and frequency distribution of wind speed at Lipowa point in Łódź data from a cup anemometer from the period It is obious that, a real highest wind speed can be higher than measured by a cup anemometer due to inertia of the instrument. The wind gust measured by this instrument is in fact an aerage alue for the period of a few seconds (depend on the type of anemometer). So, the question can be raised, how a length of aeraging period (instrument inertia) influence on the estimation of the highest wind speed? For a fixed probability of occurrence the alue of the wind gust recorded by a cup anemometer is roughly twice of the mean for hour period (Fig. ). More detailed information of the dependence of maximum recorded wind speed on the sensor aeraging period can be gain from the analysis of sonic high frequency (Hz) data. Here for each one hour data

3 set we found maximum wind speed for different aeraging periods: h, min, min, min, min, min, 4 s, s, s,.4 s,. s,. s,. s, and. s. The clear dependence of the maximum wind gust on the aeraging period is isible on the Figure. The probability distribution for the wind gust measured by a cup anemometer is characteristic for s aeraging period. Moreoer, tails of probability distributions show a linear scaling with logarithm of aeraging period: Prob(> th ) = exp(a th +B). The parameter A of these relations decrease proportionally to the logarithm of the aeraging period, but the strength of these relation changes for time scale around s (Fig. left). The measurement points of a ratio of maximal wind speed to the mean alue or to the standard wind gust are scattered for low wind speed but for the higher winds they become stable (Fig. 4). It allows for estimation of the ratio as a function of aeraging period for igorous winds (Fig. right). The maximal wind speed for the short aeraging period can be significantly higher than wind gust measured by a cup anemometer (for example for a period. s the ratio is about.) and much more higher than hour aerage. - Prob(> th ) th [m s - ] Fig. : The probability distribution of wind speed for different aeraging periods: data from sonic anemometer for aeraging periods: h, min, min, min, min, min, 4 s, s, s,.4 s,. s,. s,. s, and. s blue solid lines from left to right; data from cup anemometer for h aeraging period and (left dashed line) and maximum wind gust (right dashed line). For practical purposes the knowledge on maximum wind fluctuations is useful to estimate forces acting on surface structures. In the present study the maximal wind change, max, in the fixed time steps, t, (. s,. s,. s, s, s, s, s, s, s, s, min, min, min, min, min) was determined for each hour dataset as: { ( u ) } ( ) ( t+ t ut + t+ t t + wt + t wt max = max ) The results show that max is proportional to the mean wind speed, h CUP, and the slope of this relation increase with the time step length (Fig. left). The wind speed change can be ery rapid in the short time. For example, for mean wind around m/s, wind speed can change about m/s in second only. The ratio of max / h CUP (slope of proportionality) as a function of t shows relatiely fast increase for t < s

4 with close to linear relationship to log( t). Next the line is broken and the ratio increase is more moderate or een stops for sufficiently long time step (Fig. right). Een if absolute alue of wind changes is smaller for shorter time steps their acceleration is much higher and in consequence such pulsations can be more destructie and more uncomfortable for indiiduals than a high wind speed itself. -.. parameter A in regression: Prob( > th) = exp(a th + B) max AP / maxcup max AP / h aecup aeraging period [s] aeraging period [s] 4 4 max.s max.s / / Fig. : The dependence of the parameter A on aeraging period in scaling relations, Prob(>th) = exp(a th+b), for tails of probability distributions (left); and a ratio of maximum wind speed calculated for different aeraging periods to the wind gust or to the hour aerage wind speed measured by a cup anemometer for igorous winds (right). [m s-] [m s-] Fig. 4: The ratio of maximal wind speed in aeraging period. s (left) and s (right) to the wind gust measured by a cup anemometer in relation to the wind gust.

5 .8 t=.s t= s t= s t=min h CUP - [m s ] max / h CUP in the time step ( t) - maximum wind change [m s-] max t [s] Fig. : The dependence of maximal wind change, max, in the time step, t, on the mean wind speed, h CUP (left); and the ratio max / h CUP as a function of t. 4. Conclusions The results of the present study shows that the real wind gust can much higher than measured by the standard cup anemometer. The tails of the probability of excedance for different aeraging periods show an exponential decay with parameter proportional to logarithm of an aeraging period. For unknown reasons, this proportionality is broken for an aeraging period about s, but clear linear relation for shorter and longer time allows for effectie estimation of maximal wind speed. The maximal wind change obsered for the fixed time step is proportional to the wind speed and a slope increase roughly proportional to the logarithm of time step. Again, this proportionality is broken for time steps about s. Acknowledgements: Funding for this research was proided by the Polish Ministry of Science and Higher Education (State Committee for Scientific Research) under grants no. N N6 769 and N N6 968 for the years 8 4. References Bärring, L., Fortuniak, K., 9: Multi-indices analysis of southern Scandinaian storminess 78- and links to interdecadal ariations in the NW Europe-North Sea region. Int. J. Climatol. 9 (), Carnell, R., Senior, C., 998: Changes in mid-latitude ariability due to increasing greenhouse gases and sulphate aerosols. Climate Dyn. 4, Duro, A.C., 967: Questions relating to the aerodynamics of high-speed trains. Zheleznodorozhni Transport 49, 4 6. Fortuniak, K., Offerle, B., Grimmond, C.S.B., Oke, T.R., Kłysik, K., Wibig, J., : A system to obsere the urban energy balance: Initial results from winter-time measurements in Łódź. Ann. UMCS, B, LV/LVI 9, Fortuniak. K Pawlak, W., Siedlecki, M.,, Integral turbulence statistics oer a central European city centre, Bound. Layer Meteorol. (submitted) Geng, Q., Sugi, M., : Possible change of extratropical cyclone actiity due to enhanced greenhouse gases and sulfate aerosols - study with a high-resolution AGCM. J. Climate 6, 6-74.

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