Photoacoustic spectroscopy is a highly sensitive technique, well suited for and used in applications targeting the accurate measurement of water vapor in a wide range of concentrations. This work demonstrates the nonlinear photoacoustic response obtained for water vapor in air at typical atmospheric concentration levels, which is a result of the resonant vibrational coupling of water and oxygen. Relevant processes in the relaxation path of water in a mixture with air, excited with nearinfrared radiation, are identified and a physical model for the acoustic signal measured with a resonant photoacoustic cell is presented. The model is valid for modulation frequencies typical for conventional and quartz-enhanced photoacoustic spectroscopy and provides a simplified means of calibration for photoacoustic water vapor sensors. Estimated values for comprised model coefficients are evaluated from photoacoustic measurements of water vapor in synthetic air. Furthermore, it is shown experimentally that the process of vibrational excitation of nitrogen is of negligible importance in the relaxation path of water vapor and thus insignificant in the photoacoustic heat production in atmospheric measurement environments.

Abstract. This work describes the latest design, calibration and application of a near-infrared laser diode-based photoacoustic (PA) hygrometer developed for total water content measurement in simulated atmospheric freezing precipitation and high ice water content conditions with relevance in fundamental icing research, aviation testing, and certification. The single-wavelength and single-pass PA absorption cell is calibrated for molar water vapor fractions with a two-pressure humidity generator integrated into the instrument. Laboratory calibration showed an estimated measurement accuracy better than 3.3 % in the water vapor mole fraction range of 510–12 360 ppm (5 % from 250–21 200 ppm) with a theoretical limit of detection (3σ) of 3.2 ppm. The hygrometer is examined in combination with a basic isokinetic evaporator probe (IKP) and sampling system designed for icing wind tunnel applications, for which a general description of total condensed water content (CWC) measurements and uncertainties are presented. Despite the current limitation of the IKP to a hydrometeor mass flux below 90 gm-2s-1, a CWC measurement accuracy better than 20 % is achieved by the instrument above a CWC of 0.14 g m−3 in cold air (−30 ∘C) with suitable background humidity measurement. Results of a comparison to the Cranfield University IKP instrument in freezing drizzle and rain show a CWC agreement of the two instruments within 20 %, which demonstrates the potential of PA hygrometers for water content measurement in atmospheric icing conditions.

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<div class="section abstract"><div class="htmlview paragraph">Research institutes and companies are currently working on 3D numerical icing tools for the prediction of ice shapes on an international level. Due to the highly complex flow situation, the prediction of ice shapes on three-dimensional surfaces represents a challenge. An essential component for the development and subsequent validation of 3D ice accretion codes are detailed experimental data from ice shapes accreted on relevant geometries, like wings of a passenger aircraft for example. As part of the Republic of Austria funded research project JOICE, a mockup of a wingtip, based on the National Aeronautics and Space Administration common research model CRM65 was designed and manufactured. For further detailed investigation of electro-thermal de-icing systems, various heaters and thermocouples were included. The mockup was investigated in the Icing Wind Tunnel of Rail Tec Arsenal in Vienna, Austria under various Appendix C and Appendix O icing conditions with and without activated heating systems. The resulting ice structures were documented and analyzed by using 3D and 4D scanning systems. This paper provides information about the design of the mockup, the test setup in the Icing Wind Tunnel well as the applied documentation procedures. Furthermore, an overview of the conducted experimental investigations is given, and selected results and evaluations are presented.</div></div>

<div class="section abstract"><div class="htmlview paragraph">If an Unmanned Aerial Systems (UAS) encounters icing conditions during flight, those conditions might result in degraded aerodynamic performance of the overall UAS. If the UAS is not reacting appropriately, safety critical situations can quickly arise. Thereby, the rotors, respectively the propellers of the UAS are especially susceptible due to the increased airflow through their domain and the corresponding higher impingement rate of supercooled water droplets. In many cases, the UAS cannot be properly operated if the rotors are not fully functional, as they are a vital component. The FFG/BMK funded research and development project</div><div class="htmlview paragraph">“All-weather Drone” is investigating the icing phenomenon on UAS rotors for a 25 kg maximum take-off weight (MTOW) multirotor UAS and evaluating the feasibility of possible technical ice detection and anti-/de-icing solutions. This paper presents results from the investigation carried out at the Rail Tec Arsenal (RTA) icing wind tunnel (IWT) in Vienna, Austria, where UAS rotors were exposed to defined icing conditions based on EASA CS-25 Appendix C. The experimental tests featured various rotors which were exposed to icing conditions without any protective measures to better understand the influence of ice accretion on the aerodynamic performance. In addition, possible technical solutions in form of an electrothermal and chemical anti-/de-icing system, as well as an ice-repellent surface coating were investigated. During the tests, the performance (power, thrust, torque) of the UAS rotors was monitored. The final ice accretion was documented by 3D laser scanning and photographs. The objective of this work is to contribute to a better understanding of icing of UAS rotors, while also investigating solutions that might enable the safe operation of multirotor UAS in icing conditions in the future.</div></div>