The Effect of Rotorwash on Atmospheric Profiling

Ryan Martz Author

03/30/2021 Added

11 Plays

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An analysis of how the rotation of rotors on an unmanned aerial vehicle affects atmospheric measurments

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[00:00:01.630]Hello. My name is Ryan Martz
[00:00:03.330]and my UCARE project this school year
[00:00:04.980]was about Using Unmanned Aerial Vehicles,
[00:00:07.100]which will be referred to as UAVs,
[00:00:09.230]for Atmospheric Profiling, which is basically
[00:00:12.020]finding how different atmospheric variables,
[00:00:14.200]like temperature and humidity change with height.
[00:00:16.600]And my supervisor for this project was Dr. Adam Houston.
[00:00:20.610]So the motivation for this project
[00:00:22.490]was each National Weather Service site
[00:00:24.940]currently launches two weather balloons each day,
[00:00:27.860]12 hours apart, and thousands of miles apart
[00:00:30.620]between these sites.
[00:00:32.420]So there's a large time gap and space gap,
[00:00:35.030]which makes it hard to get a good depiction
[00:00:37.210]of the atmosphere across the country.
[00:00:39.510]And it's also wasteful as the weather balloons
[00:00:41.710]are almost never retrieved,
[00:00:43.180]so they need new sensor equipment
[00:00:45.100]and new balloons every day, twice a day.
[00:00:48.010]And so the idea is that UAVs could be used
[00:00:50.230]to fill in the gaps in space and time,
[00:00:52.170]and can be also be a more reusable alternative.
[00:00:57.920]The problem with UAVs that I'll be investigating
[00:01:00.688]is that the spinning rotors can mix up the air
[00:01:03.810]and pull air from above and push it out below.
[00:01:06.700]And so, as you can see on the diagram from right,
[00:01:10.640]this can create some big impacts
[00:01:13.780]with where you're actually measuring the air from.
[00:01:16.230]It could be pulling air from below
[00:01:17.820]and it could be pushing it out from,
[00:01:19.970]pulling air from above and pushing it out below,
[00:01:21.970]and it could recirculate back to the rotors,
[00:01:24.380]so you don't necessarily know where exactly
[00:01:27.180]the air is coming from that you're measuring.
[00:01:31.800]And so the hypothesis that we found
[00:01:33.690]through background research
[00:01:35.230]is about a two meter downwash for our experiment.
[00:01:38.880]And two things that would impact this would be,
[00:01:41.780]wind, shearing the top off of this and making it smaller,
[00:01:44.940]and also the ascent and descent speed
[00:01:47.250]creating a vertical wind.
[00:01:48.990]An ascent would probably
[00:01:50.695]make the two meter downwash smaller
[00:01:53.340]and a decent would likely make it larger.
[00:01:55.950]And the diagram on the right shows
[00:01:57.370]what happens to the rotor wake,
[00:01:58.750]so this is below the rotors as opposed to above,
[00:02:01.510]but it shows the effect that wind has on that rotor wake.
[00:02:04.410]And the idea is that wind might have a similar effect
[00:02:06.810]on the rotor or wash from above.
[00:02:10.500]So for our methods, the set up for this experiment,
[00:02:13.420]first we used eight Vaisala or six Vaisala XQ2 sensors
[00:02:17.750]and two XQ!s for a total of eight sensors.
[00:02:20.780]And we first did an intercomparison
[00:02:22.450]of these sensors to calibrate them.
[00:02:24.490]We zip tied them to a horizontal bar
[00:02:27.940]and we let them record for about 10 minutes
[00:02:30.080]and then compared their temperatures,
[00:02:32.130]pressures and relative humidities
[00:02:33.690]to make sure that there were no outliers.
[00:02:35.710]And if there were, to figure out how to correct for them
[00:02:38.740]and adjust for their biases.
[00:02:41.320]After we did this, we tied them
[00:02:45.130]to a tether connected to a weather balloon,
[00:02:47.727]and the tether was about 50 meters long,
[00:02:50.580]and we put sensors at 10, 20, 35
[00:02:53.640]and 50 meters along this tether.
[00:02:56.130]The 10 and 35 meter sensors were the two XQ1s
[00:03:00.137]and they were both out of battery
[00:03:01.640]by the time that the flights began.
[00:03:03.400]So we only had sensors from XQ2 sensors
[00:03:06.530]at 20 meters and 50 meters.
[00:03:09.090]And that spacing, the 20 and 50 meters is a longer rope,
[00:03:12.940]but as you can see from the right in that picture,
[00:03:15.670]that wasn't necessarily reflective
[00:03:17.280]of the height above ground level.
[00:03:19.810]Those dots represent where the sensors were
[00:03:22.219]along the tether, and it became very slanted,
[00:03:27.670]so that 20 meters and 50 meters
[00:03:29.440]was not necessarily 20 and 50 meters above the ground
[00:03:32.560]because the buoyancy from the balloon
[00:03:34.350]was too weak to lift it,
[00:03:35.860]and there was also wind that was pushing it off to the side.
[00:03:40.040]In addition, there was a GPS unit at 50 meters
[00:03:43.250]along the tether just beneath the balloon,
[00:03:45.480]in addition to the GPS unit on the drone
[00:03:49.130]and two XQ2 sensors attached to the drone.
[00:03:53.210]So those sensors I was talking about
[00:03:55.120]are located inside those yellow boxes there,
[00:03:57.960]just underneath the trumpets,
[00:03:59.300]and the trumpets that I'll be referring to
[00:04:01.040]are those white cones on either side of the drone.
[00:04:04.370]So for experiment one, we did a stepped hovering,
[00:04:06.990]which basically, we flew to 20 meters,
[00:04:10.370]we hovered for 10 seconds and then stepped
[00:04:12.610]up one meter and repeated this process
[00:04:15.140]of stepping up and hovering until we reached 30 meters,
[00:04:18.180]which is the approximate altitude
[00:04:20.240]of that highest sensor above the ground.
[00:04:22.590]We repeated this process three times
[00:04:24.490]and then we rotated the drone
[00:04:26.710]so that the left trumpet was facing into the wind,
[00:04:28.710]and we repeated it another three times.
[00:04:32.220]For experiment two, we did a mini profile
[00:04:35.000]where we flew up to four meters and then stabilize ourselves
[00:04:38.900]and ascended at one meter per second to 50 meters,
[00:04:42.010]waited for 10 seconds and then descended back
[00:04:44.380]to four meters at one meter per second again,
[00:04:46.990]we repeated that process three times
[00:04:49.300]and then repeated it three more times
[00:04:51.000]after rotating the drone 90 degrees like in experiment one.
[00:04:55.740]To analyze this, I use potential temperature
[00:04:58.650]because it's basically a pressure corrective temperature.
[00:05:02.070]And then this potential temperature became constant
[00:05:04.600]approaching and above 30 meters, which is where that second
[00:05:08.290]or that higher sensor was on the tether.
[00:05:11.960]And so that first experiment
[00:05:13.870]where we were stepping up to that altitude
[00:05:15.720]to find where the air was being taken from,
[00:05:18.000]was pretty difficult to analyze,
[00:05:19.450]and we weren't really able to get results from it,
[00:05:21.240]due to that constant potential temperature.
[00:05:24.210]The altitude information
[00:05:25.300]was also unreliable from the sensors
[00:05:27.410]and the balloon mounted GPS system was also broken.
[00:05:30.520]And so I had to use a recorded pressure
[00:05:32.570]to compute an approximate altitude above sea level,
[00:05:34.950]as opposed to using actual altitude measurements.
[00:05:38.500]To analyze this, I split the data,
[00:05:40.920]to analyze it I split the data into four groups,
[00:05:43.350]ascent and descent for profile one,
[00:05:45.800]the trumpet perpendicular to the wind and profile two,
[00:05:48.240]the trumpet into the wind.
[00:05:50.320]And for each instance where the potential temperature
[00:05:52.670]was within 0.02 degrees Celsius of the potential temperature
[00:05:55.980]at the lower sensor, so at about 20 meters,
[00:05:58.920]I subtracted the altitude of the drone
[00:06:01.130]from the altitude of the tether to try to find a rotor wash.
[00:06:05.290]So this graph at the bottom shows approximately
[00:06:07.700]what we were trying to analyze,
[00:06:08.830]those black dots show where the M600 left and right sensors
[00:06:12.850]intersected with the 20 meter XQ2 sensor,
[00:06:17.160]and so we can find the time
[00:06:18.510]where they had the same potential temperature,
[00:06:20.830]and then having that time
[00:06:22.040]find the altitudes of each of them.
[00:06:24.330]And so I found that the air was taken from below the drone
[00:06:28.000]by about 6-10 meters, but with a large standard deviation.
[00:06:31.940]And this was largely due
[00:06:33.240]to the highly variable data that we had,
[00:06:35.650]and also the limited number of data points
[00:06:38.190]as we were only able to analyze experiment two,
[00:06:40.800]we only had six flights to analyze,
[00:06:43.100]and which each flight, there were only a few times
[00:06:45.316]where the potential temperatures were actually equal
[00:06:49.456]between the drone and the sensor.
[00:06:52.150]So that led to the large standard deviations.
[00:06:56.440]So these were only preliminary flights
[00:06:58.300]and a more robust experiment is taking place in Oklahoma,
[00:07:01.220]the week of March 28th.
[00:07:03.070]Some things that we'll be correcting
[00:07:04.490]is using a second balloon to provide a stronger force uplift
[00:07:08.527]to make the tether hopefully more vertical.
[00:07:12.030]In addition, there'll be an onsite tower
[00:07:13.770]with attached sensors and that'll provide constant altitude
[00:07:16.710]a more reliable data.
[00:07:18.460]And we've also learned what went wrong with the GPS set up
[00:07:20.910]and should have more accurate altitude data
[00:07:22.860]from that with the balloon in the next experiment.
[00:07:26.700]Some acknowledgements and references to Dr. Adam Houston
[00:07:29.730]for guiding me through each step
[00:07:31.210]and knowing what to do in the research.
[00:07:33.110]Ashraful Islam and Daniel Rico for operating the drones
[00:07:36.338]and helping with planning and setup of the experiment.
[00:07:39.510]And Dr. Carrick Detweiler also
[00:07:41.340]with helping to set up the experiment and also
[00:07:43.470]taking the pictures that were used in this presentation.
[00:07:46.640]Thank you for your time.

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The Effect of Rotorwash on Atmospheric Profiling

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