Lubbock Severe Weather Conference
Lubbock, Texas
February 18, 2010

 Statistics for wind/pressure used in wind load standard (ASCE 7)
• Wind Tunnel Data
 steady mean and variance
 stationary (log-law)

20
40
• Validated with full-scale data that is stationary in boundary layer (SBL) over periods ranging from 10 minutes to 1 hour (spectral gap)
Extreme events (e.g. thunderstorms, hurricanes) --> non-stationary  control design in most of the US
70 t
1
= 120s t
2
= 120s T = 900 s t
1
= 120s t
2
= 120s T = 900 s
60
35
30
25
U
50
40
30
An example of a stationary wind record (left) and a thunderstorm record (right)
20
15
10

10
0 200 400
Time (s)
600 800
0
0 200 400
Time (s)
600 800
Assume that physical and statistical characteristics are the same

 Wind/Pressure Statistics (e.g. turbulence intensity, pressure coefficient)
TI


U u Cp

0 .
5
 p
U
2
• Use mean wind speeds within the spectral gap
• Thunderstorm usually occur over durations shorter than the spectral gap (~
1-10 min) and display non-stationary characteristics, especially short duration “ramp-up” events


Difficult to make comparisons between stationary and nonstationary data; statistics not representative
Attempt to collect additional thunderstorm data and facilitate comparisons of the two events

 Wind Engineering Research Field Laboratory (WERFL)
• 204 differential pressure taps
(building) (104 walls, 90 roof)
• 30’ sonic  geometric center
• 160’ tower  5 levels
• ~ 150 feet away
• Now at Reese  building remains
• 13’ Tower, 30’ Sonic
 200 Meter Tower
• Meteorological instrumentation on 10 different levels  3’ to 656’


• Exhibit rapid increase/decrease in wind speed over a short period
• Time histories show some similarities but no universal form (wide variability)
• Some occur over “longer” scales (~ 2 min), others “shorter” (~ 10 sec)  9 events
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60 60
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40 40
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20
0
0
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200 400
Time (s)
600 800
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Time (s)
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1500
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0
0
0
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Time (s)
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Time (s)
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1500
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0
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58
500
Time (s)
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1500
0
0 500
Time (s)
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1500
1500




Andrews AFB Microburst (1983)  90-100 seconds
• Standard for wind engineering use ~ 150 mph gust; poor data quality
Lubbock RFD (2002)  100 seconds (Holmes, 2008), 2 – 3 minutes
(Kwon and Kareem, 2009)
• ~ 90 mph gust
 design wind speed for most of the country; high resolution data
Want to determine information of importance to wind engineering
• Previous studies used “time-varying mean” for non-stationary events to quantify information
• Created algorithm to measure durations of “stationary turbulence”
• Stationary turbulence that contained peak wind speed was used
60 50
10
40
40
30
0
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20
-10
10
0
1000 1500 2000 2500 3000 3500
Time (s)
0
1000 1500 2000 2500 3000 3500
Time (s)
-20
1000 1500 2000 2500 3000 3500
Time (s)

Using 17 second averaging time
Mean Residual Turbulence
Duration ~ 150 s
Appropriate time periods for analysis in thunderstorm prone areas should be 60 – 200 seconds
These representations
(using 15 – 60 s averaging time) can be used for further wind engineering statistics (TI, GF, PSD)
Likely areas a higher turbulence on small scales shown in previous figure
(~10s) but would be near impossible to quantify

 So what does the reduced time scale and consideration for thunderstorms in structural design mean?
• Increased Variability
– Other studies (Ponte and Riera, 2007) have shown highly varying time scales for thunderstorms
– Other variability has been shown in vertical wind speed profiles, turbulence, etc…  will show later
• Assuming statistical and physical properties are the same for a moment
0.4
Schroeder (1999)
0.3
0.2
0.1
0
10
0 1
10 10
Averaging Time (s)
2
10
3


• Compared with SBL data (100 s segments)
1.5
• All “ramp-up” events fall within range of SBL (33’) for 15-60 s averaging times
1.5
1.25
1.25
1
0.75
1
0.75
0.5
0.25
0.5
0.25
0
10
0
10
1
10
2
Averaging Time (s)
10
3
0
10
0
10
1
10
Averaging Time (s)
2
• VORTEX2 case outside of range > 10 second averaging time (7 ‘)
10
3
• Inherently additional turbulence, but likely not attributed to surface roughness

VORTEX2 CASE


• Although wind speeds barely exceeded severe levels and are well below
“design” values for a short period, it raises a number of interesting questions for wind engineering as it is a unique time history (TI values different)
70 400
60 350
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40
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10
0
500 550 600 650 700 750 800
Time (s)
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Time (s)
• Multiple rapid changes in wind speed and direction ~ 2 minute period
• Periodic fluctuations on relatively smaller scales (0.03 – 0.05 Hz)
• Also small spatial scale  “probe” ~ 1 mile away did not record event


4.5
4
3.5
• VORTEX2 case, others, outside of range > 100 seconds, smaller time scales
3.5
3
GF (Local) = 0.0031ln(t)
3
+ 0.017ln(t)
2
+ 0.0143ln(t) + 1.0171
R
2
= 0.9976
GF (Traditional) = 0.0056ln(t)
3
- 0.0172ln(t)
2
+ 0.087ln(t) + 0.978
R
2
= 0.9924
Choi (2002)
Holmes et al. (2008)
Akyuz (1994)
ASCE (2006)
Local
Traditional
2.5
3
2.5
2
2
1.5
1.5
1
10
0
1
10
1
Averaging Time (s)
10
2
0 1
2.7
2
7.4
3
20.1
4 5
54.6
148.4
6 7
403.4
1096
8 ln (t) t(s)
• Higher variability noted, few straddle bounds of SBL although most within
– Suggests similar “gustiness” at short time scales
• Ramp-Up GF different than one used in ASCE ~ 60-100 seconds
• V2 GF for a 1500 second record was ~ 9


• Look at “turbulence” in frequency domain; high frequency scales (along-wind component)
• At frequencies > 0.05 Hz, thunderstorm energy is similar to SBL models
• However V2 case shows strong energy at ~0.03-0.05 Hz (not shown)
• Other cases show strong energy at ~ 0.01 Hz


• ASCE 7 assumes modified “log” profile for 3 second gust wind speed
• Evolutionary factors not considered in wind engineering
– Design exceedance at only one or multiple levels
• Taken from 200 meter tower  Reese Field Site
• Transition from SBL to impinging jet  30s
• Momentum works downward with time
• Below maximum wind speed  resembles
SBL profiles (low as 13’)


• Some cases show close to uniform profile; noted in other extreme wind studies
• Compared with SBL 3-second maximum gust profiles
– 0.30 z/zmax compared to 0.88 z/zmax for SBL (highly variable)
June 19, 2003 June 19, 2008 June 4, 2009
“Impinging Jet”
“Uniform” “Log”
• Environmental conditions, storm type (i.e. isolated microburst, bow echo, supercell) need to be further studied
• Highest wind speed at surface similar whereas highest overall wind speed from HP supercell/bow echo

 Noted in studies (Wu, 2001; Richards and Hoxey, 2004) to induce high negative pressures on roof with positive (upward) angles…NOT vertical wind speed
5
0 w 
V
-5
2500 3000 3500
Time (s)
4000 4500
 No significant differences detected versus SBL

• Even in “ramp-up” events due to strong horizontal wind speeds
May be different as surface roughness becomes less dominant
• Strong upward motion in tornadic vortices, for “high-rise” buildings > 60 feet

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5
0
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 Pressure Coefficient vs. Angle of Attack (3 second)
• Use sonic (30’) on top of WERFL assuming (2 events):
– Uniform profile, no angle of attack changes from MRH to 30’
• Use (13’) ~150-200’ from WERFL (1 event)
– Determine any flow field differences over that distance
θ
Case 1: June 19, 2003
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Time (s)
100
Time (s)
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100
Time (s)
150

a) 1
0
-1
• ~ 95 % of ramp-up Cp’s
(red) fell within range of
WERFL SBL at similar
AOA using peak 3-s gust
-2
-3
0 b)
0
-1 • All fell within range in conical vortex regions
• Flow features over building are similar
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-3
0 c) 0.5
0
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-1
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-2.5
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0
100
Angle of Attack (deg)
100 200 300
Angle of Attack (deg)
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300
Angle of Attack (deg)
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100 200 300
Angle of Attack (deg)
100 200 300
Angle of Attack (deg)
100 200 300
Angle of Attack (deg)
400
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0
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1
0
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400
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0
0
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100 200 300
Angle of Attack (deg)
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100 200 300
Angle of Attack (deg)
400
100 200 300
Angle of Attack (deg)
400

 Interest of what happens in separation region during gusting conditions (Murgai et al., 2006;Hwang et al., 2001)
• Temporal acceleration of wind has become area of interest (Doswell et al., 2009)
• Criteria: 20 mph increase in 3s, flow normal to walls (gust, mean), AOA “constant” p
1 p
2 p
3 p
4 p
5
Determination of:
1) Distance of Strongest Negative
Pressure From Roof Edge
2) Aerodynamics Changes a) dv dt

3 .
6 m s
2 b)


• Mean cases  3.9 – 4.1 feet
• Gust cases  2.0 – 5.3 feet  high variability
• Pressure distributions similar when using mean gust speed
• Anemometer ~ 30 feet away  still difficult to determine the effects at smaller time/length scales  correlation of wind and pressure
Run 1609 Gust Analysis
-0.5
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-5
0
Mean Pressure
Gust Pressure
Corrected Gust Pressure
5 10
Distance from Edge (ft)
15
• May actually be gain additional information in wind tunnel where wind/pressure effects can be more easily measured/visualized

 Current ASCE wind map uses “basic” wind speeds (3s gust) without regard for storm type and assumed uniform exposure
• Computation of design pressure on a building for all US (most 90 mph)
• Thunderstorm winds shown to have different probability distributions and dominate most US extreme wind climates including West Texas
• ~ 200 ASOS stations in current analysis; high resolution data (WTM, StickNet), additional ASOS available to enhance current wind estimates (6 exceedances in
8 years); GIS programs to aid with address roughness issues
• Due to small spatial scales (V2, others), wind speeds not in current analysis

 Main application is tornadoes but these research topics would apply to thunderstorm research as well
• Temporal/Spatial character of high winds
– Temporal Acceleration
– Duration vs. Damage; Flow Modification
– Coherence/Correlation
• Wind Speed vs. Damage Relation
• Rapid Wind Direction Changes  affect building pressures
• Additional high resolution measurements
– StickNet, KA Band Radar  near surface wind characteristics
– Pressure measurements on structures similar to hurricanes
• Vertical wind speeds in tornadoes
– Does it offset the strong horizontal wind speeds?

 Extreme thunderstorm events (9) studied for wind engineering purposes
• High Variability (time series, time scales, WE parameters, vertical profiles)
• Time Scales (~ 60 -200 seconds)
– Current method not appropriate for analysis in thunderstorm areas
– Likely “small scale” turbulence regimes not accounted for
• Wind Engineering Parameters
– Turbulence Intensity  SBL, TS similar for prescribed averaging times with exception of V2 case
– Gust Factor  high variability, > 60 seconds no Durst Curve
– Power Spectral Density  periodic fluctuations evident, higher scale turbulence important to most structures similar
– Events like V2 case need additional documentation and study
• Vertical Profiles
– Evolve over short time scales; maximum profiles highly variable
– Peak on average lower the max measuring height

 Extreme thunderstorm events (9) studied for wind engineering purposes
• Vertical Angle of Attack
– No significant differences compared to SBL
– May be different at higher above surface, tornadic cases
• Building Effects
– 3-s Cp mostly within range of SBL  all in “critical areas”
– Rapid increases in wind speed do not seem to alter aerodynamics
– Rapid wind direction changes need study
• Extreme Wind Speeds
– Can be further enhanced with field programs to capture events of small temporal, spatial scales