Taylor Asher URS April 30, 2013

Transcription

1 Taylor Asher URS April 30, 013

2 Goal of Talk Purpose of Wind Scaling Common Values Statistical Analysis Discrepancies Conclusions Future Work On Model Wind Scaling Parameters

3 Determine the purpose and origin of the wind scaling term Reach a consensus on its application and selection On Model Wind Scaling Parameters 3

4 wind speed Wind scaling done to ensure proper mean stress is applied, independent of sampling interval Wind stress proportional to wind speed squared High-frequency variations in wind speeds not captured in winds input to models Variable DWM is used for OWI format (NWS = 1), typically 30-minute mean winds w Aw 10min T min A = scaling parameter w = wind speed Tmin = averaging period in minutes A wˆ 10min wˆ T min time series sin(t)+1 mean rms ADCIRC based on Garratt wind drag law, which may come from 10-minute mean winds time On Model Wind Scaling Parameters 4

5 1.09 for conversion of 30 minute mean to 10 minute mean winds commonly done for OWI format (NWS=1) 1.04 for conversion of 30 minute mean to 10 minute mean winds used to be hard-coded into ADCIRC for the PBL/JAG format (NWS=4) Review of FEMA studies shows 1.00, 1.04, and 1.09 all have been used for NWS=1 w 1 w 10min. 09 w 1 w 10min. 04 w 1 w 10 min.0? 30min 30min 30min OWI (Vince Cardone & Andrew Cox) do not think any adjustment should be made On Model Wind Scaling Parameters 5

6 Generate Gaussian or boxcar random wind speed time series, 1-minute mean values wˆ Calculate 10-minute and 30-minute mean wind speeds A wˆ Determine scale factor necessary for mean stresses to be equal 10 min 30 min Mean 80, variation 80 Mean 80, variation minute average 10 minute average 30 minute average minute 10 minute 30 minute count speed (m/s) time (minutes) x 10 On Model Wind Scaling Parameters speed (m/s)

7 Generate Gaussian or boxcar random wind speed time series, 1-minute mean values wˆ Calculate 10-minute and 30-minute mean wind speeds A wˆ Determine scale factor necessary for mean stresses to be equal 10 min 30 min Mean 80, variation 80 Mean 80, variation minute average 10 minute average 30 minute average minute 10 minute 30 minute count speed (m/s) time (minutes) x 10 On Model Wind Scaling Parameters speed (m/s)

8 Variation denotes the standard deviation for Gaussian and the ±range for boxcar Value of A is quite close to 1, less than 1.01 for reasonable variation Value of A appears to be dependent on ratio of mean to variation A wˆ 10 min wˆ 30 min mean Gaussian variation mean boxcar variation Results disagree with physical justification for parameter On Model Wind Scaling Parameters 8

9 Scale factors may have arisen from a misinterpretation of the usage of gust factors, used to relate peak to mean winds Peak wind increases relative to sample duration due to statistical variability On Model Wind Scaling Parameters 9

10 Although peak wind varies by sampling interval, mean wind does not Gust factors cannot be used to relate average winds of different averaging times On Model Wind Scaling Parameters 10

11 J.R. Garratt (1977) Review of Drag Coefficients over Oceans and Continents. Monthly Weather Review Volume 105. Review of Garratt (1977) does not appear to give any evidence of necessitating 10-minute mean winds The author states the effect of mean-squared winds is quite small ( typically <1.0 over the sea for time scales less than about several hours p. 91) and that the data he used was mostly from averaging periods of minutes (p. 91) On Model Wind Scaling Parameters 11

12 If a physical justification cannot be found, then either Wind scaling is a calibration term No scaling can be used Potential explanations Shortcomings in Garratt wind drag law Changes in sea roughness related to wave conditions Other? On Model Wind Scaling Parameters 1

13 Evaluate other wind drag formulations? Improve relationship between sea state and wind drag? On Model Wind Scaling Parameters 13

14 URS & RAMPP FEMA On Model Wind Scaling Parameters 14

15 On Model Wind Scaling Parameters 15

16 On Model Wind Scaling Parameters 16

17 Text Sub-text equation On Model Wind Scaling Parameters 17

18 First item Text Sub-text On Model Wind Scaling Parameters 18