Scott Reynolds, PE M/E Engineering Syracuse NY

Transcription

1 Scott Reynolds, PE M/E Engineering Syracuse NY

2 What is Wind Modeling? Wind modeling studies are used to determine: The flow of pollutants from exhaust stacks on buildings. The determination of pollutant concentrations downstream (and upstream) of a single source or a number of sources. The continual dilution of gases, droplets and particulates as they float in the surrogate wind. The flow of air (wind currents) when buildings, obstacles and street canyons are encountered. Additional information related to wind shielding, wall pressures, and pedestrian comfort. Particulate erosion, deposition and dispersion such as snow drifting. 2

3 Why do Wind Modeling? Wind modeling studies are useful and often required for: Environmental impact statements. Due-diligence to prove that neighboring building are not contaminated by new pollutant sources. The improvement of indoor health and safety by avoiding reentrainment of emissions into a building or group of buildings. Reduction of odors from buildings or surrounding areas. Pedestrian comfort. Snow drift abatement. 3

4 What Techniques are Available? There are 4 basic modeling techniques that are commonly used today for wind studies: 1. Computational Fluid Dynamics (CFD) - Relatively new to the scene (~20 years) 2. Boundary Layer Wind Tunnel - Primary method for decades 3. ASHRAE closed-form, or semi-empirical calculations. - Intended for general population of engineers, but very inaccurate in many cases. 4. AERMOD, CALPUF, AUSTL, and others - Gaussian approximations of plume dispersion; not based on actual physics of airflow. 4

5 Advantages: CFD provides extremely visual capabilities, good accuracy & clientcomputer portable. No physical models to be built or stored. Completely analytical, and does not require empirical data. There are a number of highly qualified and competent firms that perform these studies. CFD can model projects that are difficult or impossible altogether with wind tunnels (e.g. Stack design, helicopter movement). Unlike wind tunnels, CFD will resolve buoyancy issues, stratified temperatures, and temperature differentials between discharge points and the environment (i.e. heated or cooled exhaust, hot roofs and parking lots). Usually significantly less expensive than wind tunnels. 5

6 Disadvantages: CFD is best for problems that are less than 3 miles in size for most consultants. Much larger models are only possible with sufficient computing resources. Because the technique is digital and not analog, quantifying large numbers of wind speeds and directions may not be practical. Requires significant professional judgment to identify the appropriate number of directions and speeds to be executed. CFD validations to field studies do exist, BUT are relatively sparse for atmospheric wind modeling. Some literature attempts to correlate wind tunnel and CFD, but apples-to-apples comparisons are very rare AND, sometimes the wind tunnel comparisons are not as reliable as we would like. (see next slide). 6

7 Field Comparisons w/wind Tunnels ~ 2 orders magnitude At times, wind tunnel comparisons may not be as reliable to benchmark against. ~ 1 order magnitude From internet, US EPA presentation on Wind Tunnel Modeling,

8 CFD wind models fall into two basic categories: Steady or Unsteady Steady Typically k-e or k-w RANS solver. Averaged results over time. Usually more conservative results than unsteady techniques plumes tend to be more potent and bent closer to the ground. Unsteady Typically Large Eddy Simulation (LES) or Detached Eddy. Simulation (DES) DES avoids some mesh related issues of LES. SST turbulence. Usually provides very realistic output compared to wind tunnel or field measurements, but can take a long time to run. Best run in parallel/cluster. 8

9 Steady Solutions 9

10 Urban settings with stacks and other sources of effluence such as traffic 10

11 Hospital settings with stacks and other sources of effluence such as emergency diesel generators 11

12 Lab exhaust effects on building complexes 12

13 CFD for Wind Wake & Chemical Dispersion - Calculate the dispersion of effluent from lab stacks as contamination flows away from building. - Unlike wind-tunnels, CFD accounts for ambient temperature, roof temperature and exhaust temperature. EPA-recognized analysis method currently being used for NYC by DHLS. 13

14 CFD for Wind Wake & Chemical Dispersion - Exceedingly complex structures may be modeled unlike wind tunnels. 14

15 CFD for Wind Wake & Chemical Dispersion - Exceedingly complex structures may be modeled unlike wind tunnels. 15

16 CFD for Wind Wake & Chemical Dispersion - Exceedingly complex structures may be modeled unlike wind tunnels. 16

17 CFD for Wind Wake & Snow Drifting 17

18 Unsteady Solutions 18

19 CFD for Wind Wake & Chemical Dispersion - Hovering helicopters with a time varying wind speed can be modeled with CFD. 19

20 CFD for Wind Wake & Chemical Dispersion - Hovering helicopters with a time varying wind speed and direction can be modeled with CFD. 20

21 CFD for Wind Wake & Chemical Dispersion - Helicopter approach to a landing pad on an urban hospital. 21

22 CFD for Wind Wake & Chemical Dispersion Wind from 280 degrees W at 2 mph, 80 F Dry Bulb Normalized Concentration of Diesel Exhaust 22

23 Normalized Concentration (µg/m 3 )/(g/s) CFD Wind Simulations CFD for Wind Wake & Chemical Dispersion 100 Wind from 280 degrees W at 8 mph, 25 F Dry Bulb Normalized Concentration of Diesel Exhausts at Receptor Points 1000:1 2000:1 4000: NC Odor Threshold for Diesel Fumes is Time (s) R01 R02 R03 R04 R05 R06 23

24 CFD for Wind Wake & Chemical Dispersion - Detached Eddy Simulation for an urban university multi-purpose lab. 24

25 Conclusions Wind studies are commonly used for new or renovated facilities that will be discharging chemical vapors into the atmosphere. Sometimes, auxiliary factors such as pedestrian comfort and wind loads are also required. Several techniques exist to evaluate wind in varying accuracy, cost, complexity and comprehensiveness of solved variables. CFD stands out from the pack because of its capabilities and its accuracy. It is also able to do certain analyses that are impossible for other techniques such as motion, heated surface interactions, etc. 25

26 Questions: Scott Reynolds, MS, PE M/E Engineering, PC Syracuse NY x519 ww.cfd-caes.com 26