roughness. Based on the data obtain from 52m observation tower, Shanmugasundaram et al. [3] found the power-law exponent of tropical cyclone was large

Transcription

1 The Seventh International Colloquium on Bluff Body Aerodynamics and Applications (BBAA7) Shanghai, China; September 2-6, 2012 Amplification effect of rough underlying surface on the strong wind parameters at typhoon eye wall wenchao Chen a, lili Song b,c, Liu Aijun a, shiqun Zhi a a Guangdong Climate Center, No.6 Fujin Road, Guangzhou, China b Public Meteorological Service Center, CMA, No46 Southstreet, Zhongguancun, Beijing, China c Guangzhou Institute of Tropical and Marine Meteorology, CMA, No.6 Fujin Road, Guangzhou, China ABSTRACT: Based on nearly one year normal wind data and the wind data of typhoon Hagupit and Nesat observed by the meteorological towers located at South China Sea and, the underlying surfaces of the meteorological towers were classified to two exposures, the characteristics of the power-law exponents of the wind profiles, turbulence intensities over different underlying surfaces were analyzed. It were found that when the roughness length of underlying surface was larger than 6cm, the power-law exponent at typhoon condition was times compared with the power-law exponents of exposure D and C (0.3 and 0.22) given in Chinese code. The near surface wind speed decayed more significantly, the power-law exponent and the turbulence intensities were larger in typhoon condition than in normal wind condition, especially over the rough underlying surface. Rough underlying surface has significant amplification effect on the wind parameters in typhoon condition. The amplification factors were calculated and the mathematical relationship between the amplification factors and the underlying surface roughness lengths were established. Based on these mathematical relationships, the wind parameters at the place attacked by typhoon eye wall can be estimated by the roughness length of the underlying surface and the wind parameters in normal wind condition which were more easily to measure. KEYWORDS: landing typhoon, power-law exponent of wind profile, turbulence intensity, roughness length, amplification factor. 1 INTRODUCTION Typhoon has control impact on the urban planning and construction, design of major engineering structure. The impact of strong wind on building and structure can be divided into static load induced by mean wind and dynamic load induced by turbulent wind. The vertical distribution of the surface wind (wind profile for short) is often used to reflect the impact of the mean wind speed and is mainly described by the power-law in the code of wind resistance engineering. The power-law exponent can present the changing rate of the wind speed with height. The power exponents of the wind profiles corresponding to four kinds of exposures which are type A, B, C and D with different roughness in the code [1] are 0.12, 0.16, 0.22 and 0.3. The turbulence intensity is an index to assess the fluctuation of the wind field which is often used in the study of turbulent wind. Some observation facts showed that the characteristics of the wind profile and turbulent wind in typhoon process has significant difference with that in normal wind system. At present, most of the investigations of typhoon wind profiles were based on the wind data at typhoon periphery at which the wind speeds are not so strong. Tamura et al. [2] analyzed the power-law exponent of the wind profiles based on the typhoon periphery wind data observed at Seashore and two inland residential areas by Doppler sodar. The study showed that the power-law exponents are small at Seashore and apt to grow larger as the wind blows over a longer fetch with inland 1832

2 roughness. Based on the data obtain from 52m observation tower, Shanmugasundaram et al. [3] found the power-law exponent of tropical cyclone was larger than that in normal wind condition and the power-law exponent would decrease with the wind speed. Song et al. [4] analyzed the gradient wind data observed at a complex terrain and found the wind profile over complex terrain didn t satisfied with the power-law even in neutral atmospheric condition. Chen [5] found that the gust factor has positive relationship with the roughness length of underlying surface and established an equation to describe the relationship according to the gust factors over different underlying surface. It seems that the typhoon wind parameters would increase to a certain extent by the influence of the special vortex structure and the rough underlying surface. In order to further investigate this amplification effect of power-law exponent of typhoon wind profile over rough underlying surface, the observation wind data of typhoon Hagupit and Nesat and the wind data obtained at the same towers in normal wind condition are compared and analyzed. It is hoped that some results which is meaningful in the engineering apply can be achieved. 2 BASIC DATA SOURCE AND PROCESSING In order to analyze the variation characteristics of the typhoon wind profiles over different underlying surfaces, the observation data of two typhoon cases which can represent the whole typhoon processes and the observation site had obvious different the underlying surfaces were chosen and analyzed. The data in this paper included the measurement at three gradient observation towers near the passage of strong typhoon Hagupit made landfall in Guangdong, China in 2008 and one gradient observation tower near the passage of typhoon Nesat made landfall in Hainan, China in Description of landing typhoon Hagupit and the instrument setting Strong typhoon Hagupit landed to al region of Chencun Town, Dianbai County, Maoming, Guangdong Province at 06:45 am (Beijing time, the same below) on 24th Sep (Fig. 1). The maximum 3sec average gust wind speed recorded at al meteorological station at the center of typhoon landfall was 58m/s while the maximum min mean wind speed was 48m/s. There are three meteorological mast towers located near the passage of strong typhoon Hagupit which were Zhizai Island tower, Qinba tower, Wuyang tower and the shortest distances between these three towers and the center of typhoon Hagupit were 8.5km, 12km, 18.3km, respectively (Fig.1). The shortest distance between Zhizai Island and was 4.5km. The 0m high gradient tower was located at the top of the island m above sea level.the anemometers were equipped at the heights of, 20, 40, 60, 80, 0m and wind vanes were set at the heights of, 60, 0m. But the wind vane at the heights of m and 0m were destroyed during the typhoon Hagupit center passed and the wind direction data were missing. Both of the 80m high Qinba tower and m high Wuyang tower were located at with altitude of 19m and 7m high. The anemometers equipped at Qinba tower were at the heights of,,,, 80m while the anemometers equipped at Wuyang tower were at the heights of,,, m. The wind vanes were installed at m and 75m at Qinba tower and the wind vanes are installed at m and m at Wuyang tower. During the passage of typhoon Hagupit, the maximum min mean wind speed at 0m high of Zhizai Island tower, at m high of Qinba tower and Wuyang tower were 48.5m/s, 33.9m/s and 31.3m/s when the gust (3sec) wind speed were 59.8m/s, 43.6m/s and 44m/s respectively. 1833

3 The Seventh International Colloquium on Bluff Body Aerodynamics and Applications (BBAA7) Shanghai, China; September 2-6, Description of landing typhoon Nesat and the instrument setting Strong typhoon Nesat landed to al area of Wengtian County, Wencchang, Hainan Province at 14: on 14th Oct (Fig. 1) with the minimum air pressure of 960 hpa and the central wind force of 14 grads which had reach 42m/s. Xiuying tower located at the northeast of typhoon track. The shortest distances between these three towers and the center of typhoon Hagupit was 8km. Xiuying tower was located at with line runs northeast-southwest. There were sparse houses at the southeast of the tower. The underlying surface of the tower is near surface sward. The anemometers and wind vanes were equipped at the heights of, 20, 40, 60, 80, 0m.The observed maximum min mean wind speed and gust wind speed were 34.2m/s and 41.3m/s at the height of 0m at Xiuying tower which appeared before the typhoon center passed. The NRG-Symphonie type #40 anemometer and type #200p wind vane were used at four observation towers. Figure 1. Path of typhoon Hagupit, Nesat and the location of the observation tower relative to the typhoon center 2.3 Typhoon strong wind representative assessment Due to the eddy structure of typhoon system, the wind field structure and wind turbulence characteristics are obviously different in different locations relative to typhoon such as typhoon center, typhoon strong wind region and the periphery [6]. Since wind resistance engineering is mainly concerned with the characteristics of strong wind, the selected basic data must represent the whole observation process which included typhoon eye, typhoon strong wind and the periphery wind. According to the eddy structure of typhoon and the Beaufort scale of strong typhoon wind, two criteria should be fulfilled at the same time to prove whether the typhoon center has passed over the observation site [7] : (1) The wind direction with wind speed exceeding 8th grade Beaufort scale ( min mean wind speed of 8th grade is 17.2 m/s) successively alters over 120. (2) The variation of wind speed with time shows an M shape during the passage of typhoon and if a low wind speed less than 11 m/s (5th grade) occurs between two peak values, it can be judged as the typhoon eye area [8]. According to two criteria mention above, it can be claimed that the wind data obtained from these four meteorological towers were able to represent the specific wind characteristics of strong typhoon Hagupit and Nesat. 1834

4 3 CLASSIFICATION OF THE UNDERLYING SURFACE 3.1 Wind type classification base on the characteristics of the underlying surface Considering that the roughness of the underlying surface has direct impact on the characteristics of near surface wind, it is necessary to classify the observed wind according to the characteristics of the underlying surfaces so as to quantitatively investigate the differences between the winds over different underlying surfaces. The wind types were classified by the method recommended in the WMO technical document [9].According to the characteristics of the surrounding surfaces of four observation towers, the wind can be divided into two types: The wind obtained by Zhizai island tower can be seen as inshore sea surface wind: wind came from (clockwise, similarly hereinafter) underlying surface can be seen as onshore wind at inshore sea surface. The wind from other orientation was defined as offshore wind at inshore sea surface (Fig. 2a). Qinba, Wuyang and Xiuying tower can be seen as coastal tower: wind respetively came from 247.5, and underlying surface were onshore wind at and the wind from other orientation was defined as offshore wind at the coast (Fig. 2b-3d). Figure 2. Wind type classification over different underlying surface. (a: Zhizai Island tower; b: Qinba tower; c: Wuyang tower; d: Xiuying tower; shaded arrow: onshore wind orientation; hollow arrow: offshore wind orientation) 3.2 Calculation of the underlying surface roughness length Aerodynamic roughness length is defined as the height at which the wind speed decreases to zero and this is an important parameter to indicate the roughness of the surface. For the purpose of examining that whether the classification of the near surface wind was reasonable. The roughness lengths of different underlying surfaces were calculated based on nearly one year of normal wind data (no influence by typhoon) in neutral atmospheric condition at each gradient tower and the logarithmic wind profile fitting method. According to the wind profile equation given by Monin and Obukhov [] : ( ) ( ) U( z) = u* ln z/ z0 ψ z/ L / κ (1) 1835

5 The Seventh International Colloquium on Bluff Body Aerodynamics and Applications (BBAA7) Shanghai, China; September 2-6, 2012 where κ=karman constant, u*=friction velocity, z0=roughness length, ψ=monin-obukhov function, L=Monin-Obukhov length. Most strong wind happens in the neutral stability atmosphere and in the neutral stability condition, L= and ψ=0. Eq.(1) can be simplified as below: ( ) U(z) = u ln z/ z / κ (2) * 0 where u* and z 0 are considered as the fitting parameters. By the method of the least square fitting, the height at which the wind speed becomes zero can be seen as roughness length z 0. The calculation result of the roughness lengths is given in Table 1. Table 1 also gave the detailed description of the underlying surface at the orientation of land and sea. Compared with the roughness length and the description of the underlying surfaces, the roughness length calculated by logarithmic-law fitting can even describe the slight variations of different underlying surfaces. Table 1. Roughness length of different underlying surface in normal wind condition Observation Orientation Wind type tower (clockwise) Zhizai Island tower Qinba tower Wuyang tower Xiuying tower Offshore wind at inshore sea surface Onshore wind at inshore sea surface Offshore wind at Onshore wind at Offshore wind at Onshore wind at Offshore wind at Onshore wind at Description of the underlying surface Sea surface nearby and there is 4.5km wide sea area between the tower and the land northwest. Open sea area, an island of 3.5 km2 lies on to the tower and about 1 km from the tower Dense forest at with residential area nearby Open sea area and the underlying surface of to the tower is bay coast with cultivated land and sparse forest and residential area Roughness length (cm) Complete open sea surface There are sparse houses at the southeast of the tower. The underlying surface of the tower is near surface sward. Sea surface, there is 21km wide sea area between the tower and the land at the north CHARACTERISTICS OF THE TURBULENCE PARAMETERS OVER DIFFERENT UNDERLYING SURFACES 4.1 Selection of the wind data samples In consideration of the wind resistance engineering mainly focused on the strong wind characteristics, here we only chose the wind data with the wind speed exceeding 8th grade Beaufort scale to analyze. Due to the typhoon wind speed varied significantly with height so in this paper, it was specified that the selection of the 8th grade strong wind at Zhizai Island tower, Qinba tower, Wuyang tower and Xiuying tower were respectively based on the wind speed at the height of 60m, m, m and 60m. When the strong typhoon occurred, the atmosphere was in neutral stability condition. In order to make a comparison with the typhoon wind, the normal wind data were also selected in neutral atmosphere. 1836

6 4.2 The parameter used to describe the characteristics of the wind profile Due to the lack of a universal mathematical model which can describe the complex wind profiles accurately. The power-law fitting (eq.3) is widely recommended to estimate the wind speed at different height in engineering design. The wind profile obtained by the least square fitting which is closest to the real wind is called best power-law fitting (best fitting for short hereafter). But under the consideration of the safety for engineering wind resistance, except for the best fitting, the envelope curve power-law fitting (envelope fitting for short hereafter, the fitting wind speeds obtained by the envelope fitting are larger or equal to the observed wind speed at all the observation heights) are also applied in order to assess and choose the most suitable power-law exponent. u = u1( z/ z1) α (3) Where the u = the wind speed at the height of z, u 1 = the wind speed at the height of z 1, α = power-law exponent which can indicate the variation of the wind speed with height. 4.3 The characteristics of the wind profiles over different underlying surface Figure 3 showed the mean wind profiles observed at four meteorological tower in typhoon and normal condition. All of the normal wind profiles basically accorded with the power-law. But the wind profiles observed at Zhizai Island tower which was the nearest observation site to the center of typhoon Hagupit appeared significant variation. The onshore wind profile showed an S shape and negative wind shear appeared at the height of 40m to 60m. And the wind profiles observed at other three towers were basically satisfied with the power-law. In view of the normal wind profiles, the differences between the offshore wind profile and onshore wind profile were very small at Zhizai Island tower, Wuyang tower and Xiuying tower while that difference was very large for the case of Qinba tower. The offshore wind lower than m decayed more significantly than onshore wind at Qinba tower which maybe induced by the large roughness length difference between two exposures which was 12.38cm. And the roughness length differences between two exposures were 0.05cm, 6.07cm and 7.99cm at Zhizai Island tower, Wuyang tower and Xiuying tower respectively. Whether for normal wind or typhoon wind, the near surface wind speed came from rough underlying surface decayed more significantly than that from smooth underlying surface, especially for typhoon wind. Comparison of the wind profiles from the same underlying surface showed that the typhoon wind decayed more significantly than that in normal wind condition, especially for offshore wind. The differences value between the wind speeds at the lowest two level at Qinba tower, Wuyang tower and Xiuying tower in typhoon wind condition were 2.4times, 5.3 times and 4.8 times compared with that in normal wind condition. onshore wind & nontyphoon offshore wind & nontyphoon onshore wind & typhoon offshore wind & typhoon a Wind Speed(m/s) b Wind Speed(m/s) 1837

7 The Seventh International Colloquium on Bluff Body Aerodynamics and Applications (BBAA7) Shanghai, China; September 2-6, c. Wind Speed(m/s) d. Wind Speed(m/s) Figure 3. Wind profiles over different underlying surface. (a: Zhizai Island tower; b: Qinba tower; c: Wuyang tower; d: Xiuying tower) The amplification effect of different underlying surfaces on power-law exponents of typhoon wind profiles Table 2 listed the power exponents of the best fitting and envelope fitting. The roughness lengths of offshore wind exposure at Qinba tower, Wuyang tower and Xiuying tower were cm, cm and 8.69cm which were close to the class D or C exposures given in Chinese code. And the power exponents of the best fitting observed at the eyewall were 0.453, and while the power exponents of the envelope fitting were 0.661, and which were times compared with the power exponent of class D, C exposures (0.3, 0.22) recommended by the code. The power exponents of the best fitting in normal wind condition were 0.342, 0.1 and while that of the envelope fitting were 0.478, and The power-law exponent at Qinba tower was slightly larger than the value given in code while it were opposite for Wuyang and Xiuying tower. The wind profile exponent of the onshore strong wind which came from smooth sea surface (as the class A exposure in Chinese code) was less than the recommended value (0.12). Table 2. Power-law exponents of typhoon wind profiles and normal wind profiles Observation tower Wind type Amplification factor Roughness Normal wind Typhoon wind length (cm) α1 α2 α3 α4 α3/α1 α4/α2 Zhizai Island Offshore wind tower Onshore wind Qinba tower Offshore wind Onshore wind Wuyang Offshore wind tower Onshore wind Xiuying Offshore wind tower Onshore wind To calculate the ratio between the power-law exponents of typhoon wind profile and normal wind profile which is called amplification factors and figure 4 showed the variation of the amplification factors with roughness length. When the roughness length was small, the typhoon powerlaw exponents were generally less then that of normal wind. When the roughness lengths were larger than 0.6cm, the amplification factor became larger than 1. The amplification factor for best fitting and envelope fitting were and when the roughness length increased to nearly cm. It can be suggested that the rough underlying surface have significant amplification effect on the power-law exponent of strong typhoon wind profile. 1838

8 Aα best fitting envelope fitting roughness length (cm) Figure 4. Relationship between the amplification factors of power-law exponent and the roughness length. In order to quantitatively assess the amplification effect, the linear fittings were done according to equation 4. A = A z B α 0 + (4) where z0 = roughness length, Aα = amplification factor of power-law exponent, A and B were fitting parameters. The result shows that for best fitting: Aα =0.115z and for envelope fitting: Aα =0.128z Characteristics of turbulence intensity profile Turbulence intensity can reflect the wind fluctuant characteristics and the definition showed in equation 5. I i = σ / U (5) i where σ i = standard deviation of turbulent wind u(t), v(t) and w(t) with 0.1s time period for i=1,2,3. U = min mean wind speed. Figure 5 showed the turbulence intensity profiles. The turbulence intensity decreased with height and the turbulence intensity observed at inshore sea tower and the onshore wind observed at coastal tower were obviously less than that of the offshore wind at coastal tower. The typhoon turbulence intensity were generally larger than that in normal wind condition, especially under the influence of the rough exposure. The turbulence intensities at the lowest level at Zhizai Island tower, Qinba tower, Wuyang tower and Xiuying tower were and 0.7 in typhoon condition which were 1.691, 1.123, and times compared with the turbulence intensities in normal wind. onshore wind & nontyphoon offshore wind & nontyphoon onshore wind & typhoon offshore wind & typhoon a Turbulence Intensity b Turbulence Intensity 1839

9 The Seventh International Colloquium on Bluff Body Aerodynamics and Applications (BBAA7) Shanghai, China; September 2-6, 2012 c Turbulence Intensity d Turbulence Intensity Figure 5. Turbulence intensity profiles over different underlying surface. (a: Zhizai Island tower; b: Qinba tower; c: Wuyang tower; d: Xiuying tower) 4.6 The amplification effect of different underlying surfaces on turbulence intensity Ratio between the turbulence intensity of typhoon wind and normal wind were calculated and the results showed the amplification factor were mostly larger than 1 except for some onshore wind samples and this suggested that the typhoon turbulence intensities were basically larger than that of normal wind. Figure 6 showed the relationship between the amplification factor and the roughness length. The amplification factors increased with the roughness length of the underlying surface. The amplification factor can reach when the roughness length increased to about cm. Linear fitting was applied according to equation 6 in order to quantitatively analyzing the amplification effect of the underlying surface on typhoon turbulence intensity. A C z + D = ti 0 where z 0 = roughness length, A ti = amplification factor of turbulence intensity, C and D were fitting parameters. The result showed Ati =0.029z Ati Roughness Length (cm) Figure 6. Variation of amplification factors of turbulence intensity and the roughness length. These equations can applied to estimate the wind parameters at strong typhoon eye wall in the area influenced by typhoon frequently which means only the precise roughness length and the normal wind parameters were needed but not the wind data at typhoon eye wall. In fact, the opportunity to measure the strong typhoon passed over a given gradient tower face to face is rare. (6) 5 SUMMARY AND DISCUSSION Based on the normal wind data and the wind data of typhoon Hagupit and Nesat observed by the meteorological towers located at South China Sea and, the power-law exponent and turbulence intensity over different exposure were analyzed. The results were found as follows. (1) The wind profile in the near surface decayed more significantly than that in normal wind condition, especially for the wind came form rough underlying surface. When the roughness lengths 1840

10 were larger than 6cm, the power-law exponent of typhoon wind profiles were times compared with the power exponent of class D, C exposures (0.3, 0.22) recommended by the code. (2) The amplification factor of best fitting and envelope fitting were and and the amplification factor of turbulence intensity can reach when the roughness length increased to nearly cm. It can be seen that the rough underlying surface have significant amplification effect on the wind parameters of strong typhoon wind. (3)The amplification factors of power-law exponent and turbulence intensity had positive relationship with roughness length. And these relationships can be precise described by linear equations. In the design of wind resistance engineering, the wind parameters at strong typhoon eye wall in the area influenced by typhoon frequently can be estimated based on the roughness length of the underlying surface and the wind parameters observed in normal wind condition. The measured facts and change regulations achieved based on the data analysis of typical wind typhoon Hagupit and Nesat three observation towers were representative to some extent. But due to the complex structure of typhoon, the special area, size of typhoon eye, the thickness of typhoon eye wall and so on varied significantly in different typhoon case and the wind parameters were also influenced by the terrain, horizontal pressure difference, the distance between typhoon center and measuring site and so on. more observation cases are needed to examine the relationship between other influence factors and the wind parameter and find out more universal and regulations and conclusion. 6 ACKNOWLEDGEMENTS The work described in this paper is fully supported by Major Program of National Natural Science Foundation of China (7131), General Program of National Natural Science Foundation of China (407771) and National Ministry of Science and Technology Public Benefit Specific Research Foundation (GYHY ) 7 REFERENCES 1 Ministry of Construction People s Republic of China, Load code for the design of building structures, China Architecture & Building Press, (2002) Y. Tamuraa, Y. Iwatani, K. Hibi, K. Suda, O. Nakamura, T. Maruyama and R. Ishibashi, Profiles of mean wind speeds and vertical turbulence intensities measured at seashore and two inland sites using Doppler sodars, J. Ind. Aerodyn., 95 (2007) J.Shanmugasundaram, P. Harikrishna, S. Gomathinayagam, N. Lakshmanan, Wind, terrain and structural damping characteristics under tropical cyclone conditions. Engineering Structures, 21 (1999) Song L.L., Wu Z.P., Qin P., Huang H.H, Liu A.J., Zhi S.Q., An analysis of the characteristics of strong winds in the surface layer over a complex terrain. Acta Meteorologica Sinica, 67( 3) (2009) Chen W.C., Song L.L., Zhi S.Q., et al, Analysis on gust factor of tropical cyclone strong wind over different underlying surfaces, Science China Technological Sciences, 54() (2011) Song L.L, Pang J.B., Jiang C.L., Huang H.H. and Qin P., Field measurement and analysis of turbulence coherence for Typhoon Nuri at Macao Friendship Bridge. Science China Technological Sciences, 53() (20) Song L.L., Li Q.S., Chen W.C, et al., Wind characteristics of a strong typhoon in marine surface boundary layer. Wind and Structures, 15(1), pp Chen R.S., Typhoon. Fujian Science & Technology Press, (2002) B.A.Harper, J.D.Kepert, J.D.Ginger. Guidelines for converting between various wind averaging periods in tropical cyclone conditions. sixth tropical cyclone RSMCs/TCWCs technical coordination meeting technical document, Brisbane, (2009) 4-4. Simiu, E., Scanlan, R.H, Wind effects on structures: An introduction to wind engineering, version 2. Shanghai. Tongji University Press, (1992)