Characterizing Turbulent Wind Flow Near a Wind Barrier Using Sonic Anemometers

The resulting data analyses can be used to design and develop future computer models of microscale wind flows.

October 1, 2009

Army Research Laboratory

The Holloman High Speed Test Track (HHSTT) facility at Holloman Air Force Base (HAFB), NM, has a test section of 1.8 km that contains an artificial rain field generation capability over the track. The raindrops produced can be excessively deflected when the cross-track wind speed is greater than 1 m/s. To extend testing times to days when moderate wind conditions (1 to 5 m/s) are present, the HHSTT plans to construct a wind barrier for cross-track wind reduction. Before the construction of a complete wind barrier, an observational study was carried out to characterize the mean and turbulent wind fields using a small prototype section of the wind barrier.

The Wind Barrier Setup and Sonic Anemometers arrangement. (a) A wind barrier photograph taken near the southwest corner of the barrier, and (b) a photograph showing the sonic anemometer arrangement across the wind barrier, taken on the east side of the wind barrier looking west.

Wind barriers are widely applied to modify the flow field and other meteorological factors in the nearby area. The shelter effect of a wind barrier has been recognized for many years. Wind breaks or shelterbelts using either tall vegetative stands or artificial materials have been widely applied in agriculture practices and wind erosion control. A prototype wind barrier with a length of 100 m and a height of 5 m was erected parallel to the track at a distance of 20 m from the west side of the track. An observational tower and tripod array were set up perpendicular to and across the barrier fence line and test track at the midpoint of the barrier. A 6-m reference tower was located 30 m to the west of the wind barrier, with two sonic anemometers located at 2-m and 6-m heights. Mounting booms were used to separate the anemometers to a distance of approximately 1.5 m from the center of the tower. Booms were mounted to point south, into the direction of the expected flow.

Five additional tripods, with eight additional sonic anemometers, were located to the east of the wind barrier using alternating anemometer arrangements; three tripods with two sonic anemometers were set up at distances of 5 m, 17 m, and 36 m from the barrier; and two tripods with one sonic anemometer were set up at distances of 10 m and 24 m from the barrier. Each of these tripods had a 2-m anemometer attached at the end of a 1-m boom extension, while the three tripods had an additional sensor at 4-m height. The sonic anemometers were oriented such that their V axis was parallel to the track and wind barrier for the convenience of instrumentation setup and data analysis. The sampling time for all instruments was synchronized with a sampling frequency of 20 Hz.

The data were collected on two laptop computers running the Linux operating system. The data were collected continuously over the observational period and were stored hourly for each instrument. Data files for each instrument were coded according to their instrument position relative to the tower plus the hour and date of collection for ease of use in post-test data processing.

The screen material for the wind break consists of a vinyl-coated polyester, which has 30% porosity. The material was tightly attached, using a series of metal clips, to an array of metal poles with additional metal strut reinforcements.

The average behavior of turbulent winds in the vicinity of a wind barrier was tested. In order to achieve the objective, the continuous data sections used for analysis must be quasi-stationary in terms of wind speed and direction. Unlike controlled laboratory data collections such as in wind tunnel experiments, non-stationary winds are prevalent in atmospheric boundary layer flows and are functions of the wind speed, ground surface morphology, and atmospheric stability conditions. One technique to treat such non-stationary conditions is to sample the data, section by section, according to certain stationarity criteria.

First, the mean wind speed has to be greater than 1 m/s, for the reason that the wind barrier has very little effect at very low wind speeds. Second, the variance of the horizontal wind angle must be less than 10°. Third, the time span of any given stationary section must be at least 5 minutes long in order for the incoming wind flow to establish a roughly uniform pattern throughout the entire 80-m-long horizontal domain where the anemometers are located.

Based on these sampling criteria, data sections were categorized according to the wind speed and direction relative to the wind barrier and the atmospheric stability condition. Due to the placement of the leeward instruments on the east side of the wind barrier, the ambient wind must be upwind from the west side of the barrier where the reference tower was located. During the period of the data collections, most of the winds directed from the south or southwest occurred during the daytime hours, meaning the atmosphere was primarily unstable for most of the applicable cases for analyzing the barrier lee-side winds.

Field observations indicated that the wind barrier with 5-m height and oriented parallel to the test track is very effective in reducing the cross-track wind component, even with large oblique angles of wind. The 2-m cross-track wind speed component (where the raindrops start) was greatly reduced to 10% of the total wind speed of the undisturbed reference. The total wind speed at the lee-ward side of the barrier, however, is very much dependent on the wind oblique angle. The wind barrier lost its efficacy for total wind speed reductions when the wind angle became greater than 35 to 40° relative to the barrier normal vector.

This work was done by Yansen Wang, David Tofsted, Jimmy Yarbrough, David Quintis, Robert Brice, Sean D’Arcy, and Scott Elliott of the Army Research Laboratory; and Thomas Truong and Michael Davalos of Holloman Air Force Base. ARL-0078