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Thunderstorm Characteristics of Importance to Wind Engineering
Franklin T. Lombardo, Ph.D.Texas Tech University
Lubbock Severe Weather ConferenceLubbock, Texas
February 18, 2010
PROBLEM STATEMENT“Wind is Wind” Statistics for wind/pressure used in wind load standard (ASCE 7)
• Wind Tunnel Data steady mean and variance stationary (log-law)• 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
Assume that physical and statistical characteristics are the same
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T = 900 sT = 900 st1 = 120s t2 = 120s t1 = 120s t2 = 120s
UAn example of a stationary wind record (left) and a thunderstorm record (right)
INTRODUCTION
Non-Stationary Wind/Pressure Data Wind/Pressure Statistics (e.g. turbulence intensity, pressure coefficient)
• 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 non-stationary data; statistics not representative
Attempt to collect additional thunderstorm data and facilitate comparisons of the two events
UTI u
25.0 U
pCp
Facilities/Instrumentation/Data Collection Wind Engineering Research Field Laboratory (WERFL)
200 Meter Tower• Meteorological instrumentation on 10 different levels 3’ to 656’
INTRODUCTION
• 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
THUNDERSTORM EVENTS “Ramp-Up” Types/Characteristics
• 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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THUNDERSTORM TIME SCALES 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
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RESIDUAL TURBULENCEUsing 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
THUNDERSTORM VARIABILITY 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
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Turbulence Intensity • Compared with SBL data (100 s segments)
• All “ramp-up” events fall within range of SBL (33’) for 15-60 s averaging times
• VORTEX2 case outside of range > 10 second averaging time (7 ‘)
• Inherently additional turbulence, but likely not attributed to surface roughness
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WIND ENGINEERING PARAMETERS
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VORTEX2 CASE May 15, 2009 North Central Oklahoma
• 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)
• 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
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Gust Factor• VORTEX2 case, others, outside of range > 100 seconds, smaller time scales
• 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
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WIND ENGINEERING PARAMETERS
GF (Local) = 0.0031ln(t)3 + 0.017ln(t)2 + 0.0143ln(t) + 1.0171
R2 = 0.9976
GF (Traditional) = 0.0056ln(t)3 - 0.0172ln(t)2 + 0.087ln(t) + 0.978
R2 = 0.9924
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Choi (2002)
Holmes et al. (2008)Akyuz (1994)
ASCE (2006)LocalTraditional
Power Spectral Density, Turbulence Scales• 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
WIND ENGINEERING PARAMETERS
Important for Structural Loading• 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’)
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VERTICAL WIND PROFILES
Other Examples• 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)
• 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
VERTICAL WIND PROFILES
June 19, 2003 June 19, 2008
T= 0
s
T= 50 s
T= 130 s
June 4, 2009
“Impinging Jet” “Uniform” “Log”
Noted in studies (Wu, 2001; Richards and Hoxey, 2004) to induce high negative pressures on roof with positive (upward) angles…NOT vertical wind speed
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
VERTICAL ANGLE OF ATTACK (33’)
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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
BUILDING EFFECTS
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Case 1: June 19, 2003
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BUILDING EFFECTS
• ~ 95 % of ramp-up Cp’s (red) fell within range of WERFL SBL at similar AOA using peak 3-s gust
• All fell within range in conical vortex regions
• Flow features over building are similar
BUILDING EFFECTS 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”
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Distance From Roof Edge (ft)
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a) b)
Determination of:
1) Distance of Strongest Negative Pressure From Roof Edge
2) Aerodynamics Changes
26.3s
m
dt
dv
BUILDING EFFECTS Results
• 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
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Run 1609 Gust Analysis
Mean PressureGust PressureCorrected Gust Pressure
• May actually be gain additional information in wind tunnel where wind/pressure effects can be more easily measured/visualized
EXTREME WIND SPEED ANALYSIS 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
EF-SCALE ISSUES/QUESTIONS 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?
CONCLUSIONS/FUTURE WORK 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
CONCLUSIONS/FUTURE WORK 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
QUESTIONS/COMMENTS