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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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

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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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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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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

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“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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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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Determination of:

1) Distance of Strongest Negative Pressure From Roof Edge

2) Aerodynamics Changes

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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