Estimation of Angle of Deflection of Rain at Driving Rain Prone Stations in India * 1 and P. K. Bhargava 2 1 Scientist Co-ordinator, Central Building Research Institute, Roorkee, India 2 Scientist, Central Building Research Institute, Roorkee, India Abstract A simple method for estimation of mean angle of deflection i.e. slant angle of wind driven rain from vertical has been recapitulated. The method has been used for computation of average slant angles of rain for twelve driving rain prone stations, viz., Bhopal, Mumbai, Calcutta, Cochin, Indore, Kodaikanal, Mangalore, Port Blair, Sagar Island, Tiruchchirapalli, Tiruvananthapuram, and Veraval. Based on ten years data on mean hourly wind speed and hourly rainfall, the cumulative probability distribution of angle of deflection of rain at these stations were determined and optimum values of these angles were worked out. The values vary broadly from 50 o to 65 o for the stations covered in the present study. Keywords: driving rain; angle of deflection; wind speed; driving rain index Introduction Rain carried along at an angle to the vertical by the wind is usually known as Driving Rain. Knowledge of angle of deviation of rain from the vertical is needed for design and development of rain protecting systems for walls and windows. Earlier studies related to driving rain carried out at Central Building Research Institute, Roorkee [1] provide comprehensive information about the angle of deflection of raindrops of various sizes for different values of wind speed. In addition to this, the percentage bulk of water content associated with drops of different sizes for rains of different intensities are also available in these studies. This information is quite useful for estimating the quantitative distribution of rainwater impinging at various angles on a wall for a given wind speed and nature of the rainfall. Since data archived by India Meteorological Department are available in the form of mean hourly rainfall and wind speed, the aforesaid information available in the literature is difficult to be used for estimation of mean angle of deflection i.e. slant angle of rain at a given station. Further, to design and develop rain protecting devices for walls and windows at a given station, it is quite pertinent to know the most probable and optimum values of mean angle of deflection of rain. This necessitates knowledge of the frequency of occurrence corresponding to different values of mean angle of deflection of rain. To generate this information, an alternative approach reported by World Meteorological Organisation [2] has been followed. The frequency distribution of angle of deflection of rain was studied for twelve stations which belong to the category of driving rain prone stations in India. Finding of these studies are reported in this paper. Driving rain prone stations Driving Rain Index proposed by Lacy [3] is a well known basis [4] for assessing the level of exposure of a wall to wind driven rain. Stations that have average annual driving rain index more than three are considered to be prone to driving rain. These stations as identified in an earlier study [5] are depicted in Fig.1. Though it was desirable to work out optimum slant angles of rain for all these stations, the exercise had been possible only for the twelve stations for which hourly records of co-occurrence of rainfall and wind could be procured from India Meteorological Department. The details of these stations and the period of data used for the present study are given in Table1. Table 1. Stations Covered and Duration of Data Used *Contact Author:, Scientist Co-ordinator, Central Building Research Institute, Roorkee-247667, India Tel: +91-1332-283228 ; +91-1332-283235 Fax: +91-1332-272272,272543 e-mail:sharmaishwarchand@rediffmail.com; bhargavapk@ rediffmail.com ( Received July 1, 2004 ; accepted March 22, 2005 ) Journal of Asian Architecture and Building Engineering/May 2005/229 223
Fig.1. Driving Rain Prone Stations Computation of angle of deflection of rain from the vertical Based on the experimental studies carried out at the Norwegian Building Research Institute, Sagadashvili [6] proposed following relationship between the intensity of rainfall and mean speed of falling rain drops, V = 4.5 I 0.107 (1) Where, V = Mean speed of rain drops (m/sec) I = Intensity of rainfall (mm/hr) If the speed of wind accompanying rain fall is u m/ sec, then the mean angle of deflection (θ) of the rain from the vertical is given by 224
tan θ = u / (4.5 I 0.107 ) (2) With hourly data of mean wind speed and rainfall in hand, the above expression was used for computation of mean angle of deflection of rain for each hour for the different stations. Then frequencies corresponding to the different values of q varying in steps of 5 o from 0 o to 80 o were determined. The results are shown in Fig. 2. Using these data, values of rain deflection angles with maximum frequency of occurrence can be easily determined. Further processing of data was also carried out to estimate the values of cumulative frequencies, and cumulative probability of occurrence of various angles of deflection of rain. The results are depicted in Fig3. The data generated are useful for determination of the probable angle of deflection of rain corresponding to a desired value of cumulative probability. Fig.2. Frequency Distribution of Angle of Deflection of Rain 225
Fig.2. (Contd.) Frequency Distribution of Angle of Deflection of Rain 226
Fig.3. Cumulative Probability Distribution of Angle of Deflection of Rain 227
Fig.3. (Contd.) Cumulative Probability Distribution of Angle of Deflection of Rain 228
Optimisation of angle of deflection of rain It is evident from equation 2 that larger angles of deflection of rain correspond to rain of low intensities and high wind speeds. Since intensity of rainfall on vertical surfaces is proportional to the intensity of rainfall on horizontal surface, hence the amount of water carried on to walls by the rain which has been too much deviated from the vertical might not cause too serious problem. Further, situations with simultaneous occurrence of heavy rain and high wind speeds do not occur frequently. Hence angle of deflection of rain from vertical with cumulative probability equal to 0.9 provides an appropriate basis for development of rain protecting devices. Such angles along with the most probable angles of deflection of rain for all the stations covered in the study are given in Table 2. It is observed that at about 90% occasions the angle of deflection of rain normally lies between 50 o to 65 o. Hence rain protecting devices with their design based on these values should be good enough to provide protection against the driving rain for major period of the rainy season in the driving rain prone areas of the country. Table 2. Angle of Deflection of Rain Discussion Necessity of the knowledge of the angle of deflection of wind driven rain from vertical for design and development of devices for protection of walls and windows against the rain has been emphasized. A simple method for the estimation of these angles has been recapitulated. Based on this method, the mean slant angles of rain were determined for twelve driving rain prone stations in India. The stations covered are Bhopal, Mumbai, Calcutta, Cochin, Indore, Kodaikanal, Mangalore, Port Blair, Sagar Island, Tiruchchirapalli, Tiruvananthapuram, and Veraval. Using ten years data of mean hourly wind speed and rainfall, cumulative probability distribution of the slant angles of rain at these stations were worked out and optimum values of these angles were determined. The values were found to vary broadly from 50 o to 65 o for the stations covered in the present study. Acknowledgements The study forms a part of the research programme of the institute and the paper is published with the permission of the Director, Central Building Research Institute, Roorkee. References 1) Sharma M.R.(1964) Driving rain and protecting devices in windows in buildings. The Indian Architect, 6(3), 24-30. 2) Kobysheva N.V.(1992)Guidance material on the calculation of climatic parameters used for building purposes. WMO Technical Note No. 187, 97-101. 3) Lacy R.E.(1971) An index of exposure to driving rain. Building Research Station Digest No. 127, 1. 4), Bhargava P.K(1999) Driving rain studies - state of the art. Proceedings of the South - Asian Countries Conference on Challenges to Architects and Civil Engineers During Twenty First Century, Nepal Engineering College, Kathmandu, 568-573. 5), Bhargava P.K.(2002) Estimation of driving rain index for India. Building & Environment, Vol. 37, 549-554 6) Sagadashvili G.P., Kartvelishvili L.G.(1982) Methods of processing meteorological observational data for the assessment of driving rain parameters. Proceedings of the Symposium on Building Climatology, Moscow, 619-629. 229