Periodic Reporting for period 2 - ALVAR (Advance Laminar Flow Control with Variable Porosity)
Berichtszeitraum: 2021-08-01 bis 2022-05-31
Laminar Flow Control by means of a suction system appears to be one of the most promising technologies to significantly reduce the fuel consumption and hence, pollutant emission of modern large transport aircraft through drag reduction. The consortium, composed of a renowned University (TUBS) and world leading wind tunnel provider DNW, performs high-quality large-scale wind tunnel tests with a full-scale vertical tail plane equipped with a hybrid laminar flow control (HLFC) system, which is based on the tailored skin single duct (TSSD) concept.
The DNW-LLF is one of the world’s largest low speed facilities and offers unique capabilities for wind tunnel tests with large scale models at representative flow conditions. The framework provided for by ALVAR includes state-of-the-art data acquisition, aerodynamic data, lift, drag, static pressure distributions, transition location with infrared and hot films, and the extremely high flow quality of the LLF 8x6 m test section configuration.
One of the root problems in wind tunnel tests with hybrid laminar flow control is that it is technically unfeasible to actually measure the local suction flow rate over individual surface areas, but only the overall suction flow rate including a number of additional values, such as pressures at certain locations, is feasible. However, the local flow rate can be post-processed based on a complex measurement chain, employing data mappings (e.g. calibration of the suction skin), interpolation/extrapolation schemes of the pressure distribution, and geometry measurements. This makes the local flow rate prone to the propagation of uncertainties and errors. ALVAR therefore provides a scientific analysis of the whole measurement chain to establish a precise uncertainty of the local flow rate using Bayesian inference. Based on this, and through analysis, design, optimization and implementation of a specific main mass flow meter including a proven pumping system to provide stable suction rates a comprehensive quantification of boundary layer suction is performed by ALVAR.
The DNW-LLF is one of the world’s largest low speed facilities and offers unique capabilities for wind tunnel tests with large scale models at representative flow conditions. The framework provided for by ALVAR includes state-of-the-art data acquisition, aerodynamic data, lift, drag, static pressure distributions, transition location with infrared and hot films, and the extremely high flow quality of the LLF 8x6 m test section configuration.
One of the root problems in wind tunnel tests with hybrid laminar flow control is that it is technically unfeasible to actually measure the local suction flow rate over individual surface areas, but only the overall suction flow rate including a number of additional values, such as pressures at certain locations, is feasible. However, the local flow rate can be post-processed based on a complex measurement chain, employing data mappings (e.g. calibration of the suction skin), interpolation/extrapolation schemes of the pressure distribution, and geometry measurements. This makes the local flow rate prone to the propagation of uncertainties and errors. ALVAR therefore provides a scientific analysis of the whole measurement chain to establish a precise uncertainty of the local flow rate using Bayesian inference. Based on this, and through analysis, design, optimization and implementation of a specific main mass flow meter including a proven pumping system to provide stable suction rates a comprehensive quantification of boundary layer suction is performed by ALVAR.
The consortium has identified the most promising concept of a suited mass flow meter to be used in conjunction with the planned wind tunnel experiment. The concept comprises two rotary flow gas flow meters, arranged in a parallel configuration. By thorough analysis of measuring errors, it was demonstrated that the concept fulfills the high accuracy requirements of DNW-LLF suction flow experiments. After detailed design, the complete instrument was assembled. While preliminary measurement data indicate the correct function of the instrument, its the final calibration with a norm-meter at the PTB, Braunschweig, during April 2022 revealed that by applying a suited calibration function, systematic errors can be compensated and the error in the relevant measuring range can be reduced to ±0.1 %.
TUBS has established a comprehensive computation chain for inferring the spatial distribution of surface suction velocities and its uncertainties from measurable data. The method comprises a model of the porous suction skin, a re-building of the surface pressure distribution, the readings of internal model sensors, and the wind tunnel onset flow data, and measured wind tunnel model geometry. The input data are accompanied by estimates of uncertainty. The new computation method allows to predict the mean suction velocities and its uncertainty bounds by Bayesian inference.
The method has been successfully employed to evaluate the sensitivities of suction flow with respect to uncertain parameters, and to compute final uncertainty data based on the measurements in the wind tunnel.
DNW and TU Braunschweig established a joint test plan of the wind tunnel experiments and its constraints. The plan comprises detailed suction flow experiments with variations of the relevant flow parameters, and transpiration experiments at off-design conditions at high yawing angle. Based on carefully performed pre-testing of the wind tunnel model and its instrumentation, the windtunnel test campaign took place in February 2022. The test data were carefully evaluated and analysed for pressure distributions, transition location and flow field characteristics. The results show that the TSSD concept of hybrid laminar flow control is viable for large suction inserts and for Reynolds numbers representative for flight application. Uncertainties of the local suction velocities, which are most important for characterising the flow control effect were comprehensively quantified. The project results have resulted in a unique aerodynamic data set, that can be used for future vaildation of development methods in hybrid laminar flow control, and for further developing compliant suction structures.
TUBS has established a comprehensive computation chain for inferring the spatial distribution of surface suction velocities and its uncertainties from measurable data. The method comprises a model of the porous suction skin, a re-building of the surface pressure distribution, the readings of internal model sensors, and the wind tunnel onset flow data, and measured wind tunnel model geometry. The input data are accompanied by estimates of uncertainty. The new computation method allows to predict the mean suction velocities and its uncertainty bounds by Bayesian inference.
The method has been successfully employed to evaluate the sensitivities of suction flow with respect to uncertain parameters, and to compute final uncertainty data based on the measurements in the wind tunnel.
DNW and TU Braunschweig established a joint test plan of the wind tunnel experiments and its constraints. The plan comprises detailed suction flow experiments with variations of the relevant flow parameters, and transpiration experiments at off-design conditions at high yawing angle. Based on carefully performed pre-testing of the wind tunnel model and its instrumentation, the windtunnel test campaign took place in February 2022. The test data were carefully evaluated and analysed for pressure distributions, transition location and flow field characteristics. The results show that the TSSD concept of hybrid laminar flow control is viable for large suction inserts and for Reynolds numbers representative for flight application. Uncertainties of the local suction velocities, which are most important for characterising the flow control effect were comprehensively quantified. The project results have resulted in a unique aerodynamic data set, that can be used for future vaildation of development methods in hybrid laminar flow control, and for further developing compliant suction structures.
The public demand for air travel will create substantial markets for new aircraft will exist. The European aircraft industry can capture a substantial market share if the products will comply with new demands. One of the demands calls for substantial fuel savings of environmentally friendly aircraft for which new technology is needed. The HLFC technology will result in fuel reduction and emission reductions of 10-15%. The ALVAR project showed that the development of the TSSD technology can be done with validated design tools in conjunction with measurements in an industry-proven wind tunnel. At project end the highly accurate measuring equipment developed in this project will be available. Furthermore, comprehensive uncertainties of such wind tunnel measurements that take into account manufacturing variations of geometry and porosity as well as uncertainties of aerodynamic parameters have been determined for the first time. This will create strong confidence in the European aircraft industry and the world-wide air transport community that the concept of HLFC is feasible. The newly developed tools for uncertainty quantification beyond the state of the art are now available and verified to enable future aircraft development.