>Workpackages>Flow Field Measurements
Monday, 10. August 2020

WP 5 Flow Field Measurements

During the initial AIM research program the Particle Image Velocimetry (PIV) was applied in-flight for the first time to show the viability of the flight test technique in principle. The application of PIV to an in-flight environment to ensure reliable measurements presented major challenges and thus only a proof-of-principle was possible. Therefore a reliable in-flight PIV setup and test procedures are developed. Also further research in the field of particle detection and optimisation during flight test could enhance the performance of seeding based flow measurements. In addition, recent developments on the field of PIV hardware for in-flight application such as safer illumination sources and new CCD cameras would reduce operational restrictions and increase the user friendliness of PIV.

Light Detection and Ranging (LIDAR) is a well established measurement method for the prediction of atmospherical motions but also for the determination of distinct flow phenomenon through velocity measurement. Thus within this work package this optical measurement technique is applied to airspeed measurement and system calibration. In particular, the identification of the static error, the angle of sideslip as well as the angle of attack with a LIDAR type device will enhance the quality of the data acquisition process during flight test certification.

The Background Oriented Schlieren Method (BOS) has proven to be a valuable tool for the detection of vortex and shock position in wind tunnel applications. The application of these capabilities to flight test will enable the visualisation of arbitrative aerodynamic entities. Several studies aim to enhance the performance of the BOS system.

Task 5.1. Development & Research on Flow Field Measurements

Task 5.1. deals with the basic improvement of the PIV, LIDAR and BOS method for in-flight applications. Problems relating to integration of these measurement techniques to their particular field of application are addressed.

Subtask 5.1.1. Assessment and Detection of Atmospheric Particles for In-flight Flow Field Measurements

The main objective of this subtask is evaluation and analysis of natural aerosols and cloud droplets for application to airborne PIV and LIDAR systems. The development of viable detection systems such as the forward scattering spectrometer probe (FSSP) will allow in-situ measurement of the size, shape, scattering behaviour and distribution of the atmospherical tracer particles. This in-situ knowledge of the airborne particle properties during the flight test will then allow optimisation of the visualisation techniques PIV and LIDAR. Furthermore, an extension of such a particle measurement system towards ice detection (particle sizes up to 500 µm) will establish a potential tool for certification processes. In the first step of this subtask CU, DLR and ONERA  define the seeding requirements for an airborne PIV system and for LIDAR. DLR and CU do furthermore perform a study of particle detection systems to determine the size, shape, scattering behaviour and number of cloud droplets (including FSSP probe). In addition DLR does check the feasibility for an artificial seeding system.

Subtask 5.1.2. Advancement of Particle Image Velocimetry for In-flight Application

The initial application of in-flight PIV measurements has highlighted a number of areas where substantial improvements could be made for a simpler application of PIV to industrial flight testing. Part of the improvements can immediately come from the availability of new PIV hardware and software systems and further optimisation of the setup of the measurement system to ensure ease of installation for industrial flight test. Furthermore, the latest software developments foresee an online PIV post-processing ability. This option would enable online flow visualisation during the flight test for certification thus providing immediate flow data during changing flight test conditions.

In a first step DLR and CU assess installation and access issues, error sources and operation limitations of current in-flight PIV systems. Following DLR does deeper research on LED illumination for flexible PIV light sheet implementation on different aircraft configurations as well as fields of interest. DLR and CU do also assess PIV imaging systems using low energy CW laser systems with image intensified CCDs and CMOS cameras. DLR is to develop PIV data processing tools with the focus on online monitoring. At the end of this subtask an assessment of mobility, safety and simplicity of PIV setup for measurement around aircraft fuselage, wings and empennage is performed by DLR and CU. DLR does furthermore describe a general procedure and test matrix for in-flight PIV certification process.

Subtask 5.1.3. Improvement of BOS for Industrial In-flight Application

In this subtask DLR does design a BOS imaging and acquisition system suitable for in-flight applications. An assessment of the feasibility of BOS for vortex or shock detection on the wing is performed. The installation and the positioning of different background pattern is investigated by DLR. Also numerical calculations and a digital mock-up (DMU) is used for this. Suitable calibration procedures for the technique will also be investigated.

Subtask 5.1.4. Improvement of Airborne LIDAR towards Air Data Calibration

In the proposed research activity, the LIDAR anemometer shall be evaluated as an alternative means to produce relevant information for air data calibration, where figure of merits used shall be accuracy level, procedure complexity, flight time and overall costs. The feasibility of an aircraft probe has been already proved in past testing using LWIR lidar technology. In this case, SWIR fiber lidar is to be developed using an optical architecture compatible with a compact packaging needed for a future commercial product. ONERA will design and build a suitable LIDAR and will (together with PAI) assess its viability for dynamic testing of AOA and AOS as well as for dynamic testing of static error correction at lower altitude. ONERA and PAI develop suitable tests procedures for certification and define reference data acquisition procedures. To prepare a flight test demonstration PAI and ONERA elaborate a flight test plan and develop suitable equipment installations.

Task 5.2. Research Testing of Flow Field Measurement Techniques

In this task the ‘laboratory’ knowledge about BOS and PIV gained within Task 5.1 is tested to flight tests on research aircraft with the respect to later industrial application. In addition particle detection systems will be applied.

Subtask 5.2.1. Application of Particle Detection Systems

The modified particle detection system developed for LIDAR and PIV measurements are integrated and certified for an aircraft to fly through different cloud layers and meteorological conditions. This prototype generic system can run simultaneously with other AIM² flight tests including in-flight PIV, BOS or LIDAR on the same aircraft. Therefore the developed particle detector has to be certified for in-flight application. CU and DLR work together to get the permission to flight and conduct ground and airborne tests. Finally they perform an assessment of prototype generic systems.

Subtask 5.2.2. PIV Flight Test Campaign on Research Aircraft

The main aim of this subtask is the application and verification of the modified PIV setup (see WP5, Subtask 5.1.2) to flight test. In particular, the application concentrates on the reliable acquisition of PIV images and an easy and safe installation. DLR does define an optimised in-flight PIV setup to measure the instantaneous flow field (e.g. propeller slipstream, flow around selected area of a wing cross section or the interaction between the fuselage and wing flow). The testing is performed with one of DLR’s research aircraft (e.g. DO228) to adopt the system and the measurement parameters in an optimal way. A combination of the in-flight PIV test campaign with the application of the particle detection system is recommended. Therefore CU will integrate a particle detection system with PIV optimisation to the designed flight test installation. DLR will certify the flight test installation and perform the flight tests. After the test the DLR will process the PIV data. An assessment as well as a structuring of the PIV flight test procedure will be done by DLR.

Task 5.3. Demonstration of Air Data Calibration by Means of LIDAR

The established AOA, AOS and static correction system shall be certified and applied to flight tests, including a well proved calibration procedure. The LIDAR system designed by ONERA in subtask 5.1.4 is to be installed on board a Piaggio P180 by PAI. PAI and ONERA will perform flight tests with this setup and analyse the data to be able to evaluate LIDAR for flight test certification.

AIM² Advanced In-Flight Measurement Techniques, c/o German Aerospace Center (DLR), Bunsenstrasse 10, 37075 Goettingen, Germany, Tel: +49 551 709 2252, Fax: +49 551 709 2830, Email: