One of the main objectives of SENS4ICE is to develop and mature different ice detection technologies. Ten different ice detectors have been developed by the consortium partners. Most of these ice detectors target detection of and discrimination between the Appendix O (App. O) and Appendix C (App. C) conditions.

Extensive icing wind tunnel tests have been performed in the project to evaluate the detector’s performance in relevant environment at icing wind tunnel. This icing testing reproduces natural icing conditions in compliance with aeroplane certification specifications (CS-25/ 14 CFR Part 25, formerly known as FAR 25) App. C and the extension to Supercooled Large Droplets (SLD) cloud conditions (App. O). 28 weeks of testing time have been distributed over four facilities participating in SENS4ICE. The results of these icing wind tunnel tests provided data for technology selection and development towards natural icing flight tests.

In the SENS4ICE project, Technische Universität Braunschweig (TU Braunschweig) provides the services of the Icing Wind Tunnel in Braunschweig, Germany. The researchers and engineers involved in these activities tell us how to prepare and perform a successful icing wind tunnel test campaign.

Question 1 (Q1): The icing wind tunnels of the TU Braunschweig is a state-of-the art facility. However, it needed to enhance some technological features to fit the testing requirements set in SENS4ICE. What were the challenges of upgrading the icing wind tunnels to the needs of the project?

Answer 1 (A1): TU Braunschweig , established in 1745, is the oldest technical university in Germany with a core research focus in mobility, metrology, infection and therapeutics and future cities. The Institute of Fluid Mechanics at the TU Braunschweig, renowned for its expertise in wind tunnel testing, designed and commissioned the Braunschweig Icing Wind Tunnel (BIWT), a state-of-the-art icing tunnel with proven App. C capability conforming with SAE ARP-5905. The active engagement of the BIWT in several international icing research projects in collaboration with research organizations like DLR, CIRA and ONERA has contributed significantly to understanding the icing physics and the development of numerical icing codes. The perseverance of the aviation community in making aviation safer has resulted in the new icing certification requirements listed in App. O, which are more hazardous and differ substantially from App. C conditions. To simulate App. O conditions, BIWT is upgraded with a new spraying system with an independent control system. The key challenge in augmentation to App. O is to obtain the characteristic wider spectrum of droplets (1µm to 2000 µm) while satisfying the conflicting lower LWC limits. The calibration of the upgraded tunnel is a further challenge due to the lack of accepted instrumentation and the large uncertainties produced by large droplets with a very low probability. Extensive in-house calibration and collaboration with DLR enabled the realisation of Freezing Drizzle conditions in BIWT.

Q2: Which ice detectors have been tested by the TU Braunschweig? 

A2: TU Braunschweig was pleased to welcome several SENS4ICE sensor developers, including five development sensors and two reference sensors. The testing of CM2D sensor developed by DLR was carried out along with cloud characterisation with the CCP  and Nevzorov probes. The novel conceptual sensors tested in the SENS4ICE project are the Local Ice Layer Detector (LILD) sensor from DLR and the Atmospheric Hydrometeor Detector based on ELectrostatics (AHDEL) from ONERA. IWT tests of the Primary in-Flight Ice Detection Sensor (PFIDS) and the Appendix O Discriminator (AOD) sensors from Safran Aerotechnics were postponed to October 2021 due to the Covid-19 situation.

Q3: A wide testing envelope represents a key asset in the experimental effort. What were the main testing conditions?

A3: The test points were derived to enable the testing of sensors in multiple aspects like their sensitivity to the LWC and their ability to discriminate between App. C and App. O type of clouds. The response of the sensor to the changes in the operating temperature, particle size, LWC and velocity are studied. The App. C distributions in most wind tunnels are identical, however, the droplet distributions and LWC of App. O clouds are found to be different across the tunnels. In order to produce a common test ground, the expected response times are scaled to produce 0.3 mm thick ice on a cylinder with 25 mm diameter, which is an accepted indicator in the aerospace industry and defined within ED-103. With the consensus of the SENS4ICE partners, a robust test process is employed that essentially tests the reliability of the sensor in terms of positivity and false negative rates, annunciation ability, reproducibility and endurance when subject to harsh conditions for longer durations. In addition to the agreed test matrix, several tests were carried out rigorously with the individual sensor developers to enhance the understanding of the ice accretion and the specific sensor’s behaviour.

Q4: The sanitary measures taken in the framework of the Covid-19 pandemic put a stop to international business travel which means that not all sensor developers in the project could witness the icing wind tunnel tests on-site. In this context, how did you manage the test campaign? 

A4: Certainly, Covid-19 hindered the planning and levied additional costs in time and efforts. As most sensors are in a maturation stage, remote testing was not a viable option. To fully harness the benefits of the proposed IWT tests, the presence of sensor developers was deemed necessary. TU Braunschweig has given utmost priority to the safety of employees and guests and ensured it by practicing a detailed hygiene plan drafted in consultations with the university health department and the city health office.

Q5: What are the preliminary observations/results/conclusions of the tests from the point of view of the icing wind tunnel operator?

A5: Unique innovative icing environment detection technologies have been developed in SENS4ICE. The successful IWT tests have demonstrated the soundness of the concepts. The sensors tested have shown exemplary performance in detecting the icing conditions, the response in most cases is as instantaneous as the cloud. Evaluation of these diverse technologies in controlled environment should have enhanced the understanding of the detection principles and shortcomings. The tests will enable the developers to improve the architecture to be accepted as primary ice detection systems. Besides, the concepts have enormous potential to serve applications beyond icing.

Q6: What is the way forward for the TU Braunschweig after the SENS4ICE icing wind tunnel test campaign?

A6: The Multiphase Flows and Icing Group of the TU Braunschweig has experience with several measurement technologies, and participation in the SENS4ICE testing has further strengthened the group’s interest in novel measurement principles. SENS4ICE and other EU projects enabled the upgrade of BIWT to SLD conditions on par with leading world class facilities. The calibration of the SLD conditions is challenging, the TU Braunschweig will continue its collaboration with DLR in this regard to ascertain the uncertainties and contribute to establishing acceptance criteria. Further, the upgraded facility will be utilized in studying the critical aspects of SLD icing like droplet splashing and its influence on the accretion rate, extended icing limits due to large droplets and heat transfer investigations. All experiments will help to get a deeper understanding of the physics of ice accretion and will lead to improved numerical models.

Q7: Do you have other comments that you would like to share with the members of the SENS4ICE community?

A7: TU Braunschweig is delighted to be a part of the SENS4ICE project and thanks the European Union for supporting the activity. The consortium brought together niche expertise across the world; the seamless collaboration of the partners has enabled a  definition of new standards for future testing. We look forward to future collaboration.

DISCLAIMER

The information, statements and opinions in the above interview are personal views of the individuals involved in the SENS4ICE project and do not necessarily reflect the views of the SENS4ICE consortium as a whole, nor of the European Commission. None of them shall be liable for any use that may be made of the information contained herein.