Atmospheric Measurements and Observations II EAS 535 Anemometry Anemometer Calibration Exercise Prof. J. Haase http://web.ics.purdue.edu/~jhaase/teaching/eas535/index.html
Class Objectives How is wind measured? Understand components of the temperature profile measurement system Estimate a calibration function for anemometers using data collected in the wind tunnel
How is wind measured? Cup and propeller anemometers Wind vane Pitot static tube Hot-wire or hot-film anemometers Sonic anemometers Wind profilers Balloon borne sounding system
Pitot Static Tube
Hot-wire or hot-film anemometers Earth and Atmospheric Sciences Infer wind speed from cooling of heated wire or film Platinum wires Temperature proportional to current, I Calibrated relation between speed,v, and I I 2 = A + B V
Sonic Anemometer Travel time of acoustic pulse = (C + V) d C is the speed of sound (not in a vacuum, depends on density therefore temperature) C 2 =! RT d is the distance between source and receiver Measures 3 component velocity
Balloon borne sounding systems Earth and Atmospheric Sciences Position and velocity calculated by GPS receiver in radiosonde package Sampled every 1 sec Velocity accuracy 0.2 m/sec Horizontal velocity only
Radio Acoustic Sounding System Earth and Atmospheric Sciences Radar transmission and reception of backscattered energy from refractive index fluctuations that move with mean wind. Samples in the vertical direction and in two tilted planes => three component wind 915 MHz Doppler radar profiler 60 to 200 m vertical resolution 1 min sampling 0 to 3-4 km AGL (clear air) 2 khz Doppler sodar 25 m vertical resolution 15 min sampling 50 m to 200-500 m AGL
Cup anemometer Drag coefficent C DV is less than drag coefficient C DC therefore there is a net force acting on the cups that is proportional to wind velocity squared V 2 Measures horizontal components only 1 Fd = Cd! AV 2 2
Standards for assuring representativeness Exposure height: 10 m Exposure radius: 3 km Maximum obstruction height: 3 o above the horizon Minimum distance to obstruction:20 times the obstruction height (~3 o ) Averaging time: 10 minutes Other parameters: Maximum 3 s wind, speed, and direction within 10 minutes Maximum 1 minute wind, average wind speed, and direction Std dev of speed and direction over the averaging period Gust peak speed, duration, magnitude, amplitude, lull speed
Anemometer Instrument model Earth and Atmospheric Sciences
Calibration procedure Construct a calibration curve between known wind speed: U = 2( p t! p o ) " p o is the manometer pressure with the wind tunnel off (Pa) p is the manometer pressure with the wind tunnel on (Pa) " is the air density (kg/m 3 ) And anemometer voltage U = a! V + b
Anemometer calibration setup Earth and Atmospheric Sciences
Lab Procedure Record manometer pressure (in H20) before turning on Target pressure reading Calculated target pressure reading Actual pressure reading (in H20) at start of 30 seconds Actual pressure reading (in H20) at end of 30 seconds Actual calculated pressure p t -p o (Pa) 1in H20 = 249Pa Calculated pitot tube wind speed U = ( p " p )! 2 t o Approximate anemometer voltage reading Time of logged voltages Average of logged anemometer voltage values p o (tunnel off) p 1 (tunnel on, baffles closed) p o + 0.005 p o + 0.02 p o + 0.03 p o + 0.04 p o + 0.08 p o + 0.15 p o + 0.30 p o + 0.50 key: calculated by hand noted in lab calculated later with excel
Coming Up Sept 27 Meet at wind tunnel Drive to airport, cross railroad tracks, turn left 1 story building at end of road, enter last door Wind tunnel anemometer calibration lab