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Hellmann exponents

Posted: Thu Nov 15, 2018 11:29 pm
by Gerald.Steinfeld
Dear managers of the SWiFT benchmarks, I've got a request concerning the Hellmann exponents that are provided in the description of the different benchmark cases. Could you please add a description how exactly the Hellmann exponents have been derived? I think that this could help the participants to compare the results of their models with the measured atmospheric conditions.

Re: Hellmann exponents

Posted: Mon Nov 19, 2018 4:50 pm
by Paula.Doubrawa
Hi, Gerald --

All of the inflow parameters were obtained from the inflow measurements collected at SWiFT, which were used to define the benchmarks. Specifically regarding your question:

1) For a given benchmark, a set of 10-minute periods is selected (at least five 10-minute periods are used to define each benchmark)
2) For each 10-minute period selected, the temporally averaged vertical wind speed profile is obtained from the measurements
3) The power law exponent is computed based on the mean wind speed values at the top and bottom of the rotor (this is stated in the Glossary in ... ssary.html)
4) The power law exponents of all 10-minute periods (which are similar, since vertical shear across the rotor was one of the criteria used to select the data for the benchmarks) are averaged together and the value provided constitutes an ensemble average, where the ensemble is the set of 10-minute periods used to define the benchmark.

Hope this helps,
Paula D.