Overview
The main objective of the present proposal was the performance of wind tunnel testing of a half model at transonic speed and near flight Reynolds number. The model wing featuring a natural laminar flow (NLF) design was subject to artificially generated surface steps and imperfections allowing investigating the resulting effect on the flow behaviour on the wing. The proposed activities comprise the design and provision of a controlled high pressure air supply inside the model, the model preparation and the test performance. Basic investigations on NLF wings have already been performed in a recently completed EU research project ensuring the availability of test capabilities, tools and appropriate techniques for an achievement of the defined objectives. Forces and moments will be measured with a classical strain gauge balance inside the tunnel ceiling while the PSI system is going to record the large amount of tap based pressure data. Model deformation in wing twist and bending due to aero loads will be monitored by the highly qualified stereo pattern tracking system. The visualisation of laminar flow areas as well as the transition line will be achieved by applying the temperature sensitive paint (TSP) technique, which has been developed and enhanced by DLR and ETW to a mature state over the recent decade. A team of long experienced and highly skilled staff will take the responsibility for a successful performance of the described activities.
To meet concerns of the given complexity of the work an extended risk assessment with corresponding contingency plans has been prepared. Exploitation and an enhanced dissemination address the uniqueness of the expected results.
The proposed investigations on a Natural Laminar Flow model refer to a key technology stream within the SFWA programme contributing to achieving the technology readiness level 6, thus supporting the realization of the Vision 2020 goals of the European Union.
Funding
Results
Executive summary:
The project HIRELF supports the realisation of the ACARE Vision 2020 for designing significantly greener aircraft. Targeting for a new smart wing design with a robust 'laminar performance' under typical flight conditions will reduce aircraft drag and, hence, fuel consumption resulting in a reduction of greenhouse gases. Such developments are simultaneously increasing the European competitiveness in the field of air transport.
On the way for maturing the natural laminar flow technology to a readiness level of 6 are high Reynolds number experiments on a suitable half model which have been performed in the European Transonic Wind tunnel (ETW).
Due to the provided controllable pressurisation system for cavities inside the wing local shape variations could be generated on the upper surface, while forming artificial steps required manual modifications. Using temperature sensitive paint for transition monitoring the sensitivity of laminar flow development to surface imperfections or manufacturing tolerances could be assessed at realistic Mach and close to flight Reynolds numbers. A worldwide unique dataset was gathered holding about 1000 images representing 300 test conditions supplemented by force, moment and pressure data.
Further, it has been demonstrated that using the cryogenic test capability of ETW allows high Reynolds number simulations of laminar wing flow providing industrial quality standards.
Project context and objectives:
The global objective of the project was seen in the answer in how far modifications of the local wing shape and surface imperfections may affect the development and stability of laminar flow and the corresponding aerodynamic parameters. Responding to the complexity of the considered investigations the defined activities were distributed to five work packages.
All project related management and coordination work as well as dissemination and exploitation issues are managed by WP 0.
The second package dealt with the provision of a reliable internal gas supply system for the model allowing a remotely controlled pressurisation and de-pressurisation of individual leak-tight chambers.
The third work package covered the preparation of model and tunnel for a successful test performance. Beside the classical force and moment measurements by a strain gauge balance the most important applied technique is the transition monitoring by temperature sensitive paint (TSP). Extreme care has to be given to the required surface coating for achieving the specified surface finish for laminar flow investigations. Additionally, the tunnel itself and the model preparation areas are subject of careful cleaning as defined by developed handling procedures.
Test performance and data acquisition are part of the fourth working package. Defined by the agreed test programme, variations of test conditions as well as artificially introduced surface imperfections are to be realised. TSP images combined with the corresponding aerodynamic data have to be acquired while information about number and size of particles in the main tunnel flow will be gathered.
The final work package focused on the correction and subsequent distribution of test data. Classical wind tunnel corrections will be applied before releasing the final data set and the test report including model surface surveys, images and documentation.
Project results:
Work performed and main achieved results
One of the prerequisites for a reliable operation of the chamber pressurisation test was the development of a qualified sealing concept. Investigations on provided samples led to the identification of a suitable grease subsequently used for all model related applications. In the next step an internal pressure system for the model has been developed. Using a gas-bottle based supply system outside the tunnel, three individually controllable pressure lines were routed into the model wing. As the system operation and control is handled by the ETW main acquisition system a new dedicated interface had been designed and established linked to the actual tunnel conditions allowing for an individual control of chamber shapes by the client. The proper functioning has been proven by DLR operating their optical 3d-survey system PROPAC.
The model instrumentation with respect to pressure plotting represented a challenging issue due to the high number of nearly 400 taps to be tubed and checked. Nevertheless, the monitoring of laminar flow and boundary layer transition by the temperature sensitive paint technology posed the most important measurement technique regarding the project objectives. Following a specific cleaning process a primer, basic coat and the final TSP-layer had to be sprayed on the wing. The required surface finish of better than Ra = 0.03 µm has been achieved by careful manual polishing while paint thickness and surface waviness was permanently monitored by the experts from the German Aerospace Centre DLR being subcontracted by the client. They were also in charge of TSP operation, data acquisition as well as the presentation and analysis of the results.
The test programme was compiled of 3 blocks each dedicated to an individual height of an artificial step generated on the upper wing surface. Besides testing at variable Mach numbers up to M = 0.8 for Reynolds numbers ranging from 16 to 20 million, the bulk of test conditions was performed around Re = 17 million. While forces and moment have been acquired for about 50 polars, TSP images were gathered at 300 test conditions leading to nearly 1000 images taken in total across the campaign. The achieved high level of productivity on the TSP side allowed for testing at additional conditions and repeat enhancing the achieved reliability and data quality.
A unique set of data was acquired undergoing checking and the application of classical corrections for wall interference. Two sets of data, an uncorrected and a fully corrected one were delivered to the client together with survey and inspection reports especially regarding the wing shape and surface finish at the end of the campaign.
A newly installed optical particle counting system has been operated over the full duration of the test campaign. Data analysis after post-processing revealed useful information about the number and size of particles in the main gas stream as function of operating conditions, operating sequences and model transport.
Potential impact:
At the end of the project HIRELF two substantial questions with respect to the design of natural laminar flow wings can be answered. At first, it has been demonstrated that the cryogenic wind tunnel technology and especially ETW owns the potential and the capabilities for performing relevant tests at Reynolds numbers up to about 20 million. The resulting data quality fulfils industrial requirements with respect to data accuracy and repeatability. The refined model preparation and handling procedures on one side as well as the developed tunnel and model transport sequences on the other side present a high level of maturity forming the prerequisite for this type of experimental investigations. Using temperature sensitive paint with the demonstrated high productivity on image acquisition and online pre-presentation forms a powerful unique tool for the industry for designing and validating such type of modern wings.
Secondly, from the industrial point of view substantial knowledge has been gathered regarding the robustness of laminar flow and transition. The performed investigations provided scalable data about the effect of forward and backward facing contour steps, which may be generated on a real wing by joining different pieces in a non-perfect way, and deviations from the designed wing shape. Such information will allow a more reliable definition of manufacturing tolerances and surface quality for future NLF wings.
Using an industrial model and performing experimental investigations generally limits any dissemination of results due to confidentiality issues. In the present case an agreement with industry has been achieved regarding a dissemination of project related information and the subsequent use for exploitation activities. On this basis, an oral presentation was given in the High Reynolds number Session of the largest worldwide aeronautical conference, the AIAA Aerospace Science Meeting, in Grapevine, USA in January 2013 (Kühn, W. & Quest, J., 'Testing an NLF wing at flight Reynolds number in the ETW'). Additionally, a short video clip has been produced by a professional company featuring an impression of the model preparation and test performance. This video is also available on the ETW website http://www.etw.de/cms/index.php?mact=News,cntnt01,detail,0&cntnt01articleid=20&cntnt01returnid=81 informing potential clients about the successful performance of the Clean Sky project HIRELF and the achieved quality of the demonstrated test capability.
Project website:
https://www.etw.de/cms/index.php?mact=News,cntnt01,detail,0&cntnt01articleid=17&cntnt01returnid=81