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MIT2_017JF09_p21

# MIT2_017JF09_p21 - 21 HURRICANE WINDS 21 65 Hurricane Winds...

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21 HURRICANE WINDS 65 21 Hurricane Winds A 1959 paper by Isaac Van der Hoven gives the spectrum of wind speeds during Hurricane Connie, measured on a tower at Brookhaven National Laboratory. His curve for S + ( ω ) is approximated by the points below: Frequency, S + ( ω ) cycles/hr m 2 /s 0 0.00 10 0.50 14 0.65 20 1.00 32 2.80 50 3.10 72 2.80 100 2.00 141 1.60 200 1.20 316 0.80 500 0.60 717 0.50 1000 0.40 1410 0.20 2000 0.00 This one-sided spectrum is given in units of m 2 /s , i.e., velocity squared divided by ω (rad/s), so that the area under it is equal to the variance. The mean wind speed during most of the hurricane was 13 m/s , but for one hour at the peak it was 20 m/s . 1. Make a plot of this spectrum data - S + ( ω ) vs. ω (rad/s). 2. What is the characteristic frequency of the windspeed ﬂuctuations? What is the ap- proximate standard deviation of wind velocity, and the significant amplitude ¯ a 1 / 3 ? Solution: The peak frequency is apparently at about fifty cycles per hour, or one cycle per 72 seconds. To get σ and a ¯ 1 / 3 = 2 σ , we have to get the area under the spectrum. The attached code shows how to

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MIT2_017JF09_p21 - 21 HURRICANE WINDS 21 65 Hurricane Winds...

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