German Aerospace Center Research Articles

Thermal analysis and constraints for the MASCOT landing site selection on the Asteroid Ryugu

In June 2018, after 4 years of cruise, the Japanese space probe Hayabusa2 [1-Watanabe S. et al.: Hayabusa2 Mission Overview. (2017)] reached the Near-Earth Asteroid (162173) Ryugu. Hayabusa2 carried a small Lander named MASCOT (Mobile Asteroid Surface Scout) [2-Ho T. M. et al.: MASCOT-The Mobile Asteroid Surface Scout on-board the Hayabusa2 mission. (2017)], jointly developed by the German Aerospace Center (DLR) and the French Space Agency (CNES), to investigate Ryugu's surface structure, composition and physical properties including its thermal behaviour and magnetization in-situ. The Microgravity User Support Centre (DLR-MUSC) in Cologne was in charge of providing all thermal conditions and constraints necessary for the selection of the final landing site and for the final operations of the Lander MASCOT on the surface of the asteroid Ryugu.This article provides a comprehensive assessment of these thermal conditions and constraints, based on predictions performed with the Thermal Mathematical Model (TMM) of MASCOT using different asteroid surface thermal models, ephemeris data for approach as well as descent and hopping trajectories, the related operation sequences and scenarios and the possible environmental conditions driven by the Hayabusa2 spacecraft.A comparison with the real telemetry data confirms the analysis and provides further information about the asteroid characteristics.

Local anomaly detection in hybrid rocket combustion tests

Local anomaly detection was applied to image data of hybrid rocket combustion tests for a better understanding of the complex flow phenomena. Novel techniques such as hybrid rockets that allow for cost reductions of space transport vehicles are of high importance in space flight. However, the combustion process in hybrid rocket engines is still a matter of ongoing research and not fully understood yet. Since 2013, combustion tests with different paraffin-based fuels have been performed at the German Aerospace Center (DLR) and the whole process has been recorded with a high-speed video camera. This has led to a huge amount of images for each test that needs to be automatically analyzed. In order to catch specific flow phenomena appearing during the combustion, potential anomalies have been detected by local outlier factor (LOF), an algorithm for local outlier detection. The choice of this particular algorithm is justified by a comparison with other established anomaly detection algorithms. Furthermore, a detailed investigation of different distance measures and an investigation of the hyperparameter choice in the LOF algorithm have been performed. As a result, valuable insights into the main phenomena appearing during the combustion of liquefying hybrid rocket fuels are obtained. In particular, fuel droplets entrained into the oxidizer flow and burning over the flame are clearly identified as outliers with respect to the main combustion process.Graphic abstract

Estimating Upper Silesian coal mine methane emissions from airborne in situ observations and dispersion modeling

Abstract. Abundant mining and industrial activities located in the Upper Silesian Coal Basin (USCB) lead to large emissions of the potent greenhouse gas (GHG) methane (CH4). The strong localization of CH4 emitters (mostly confined to known coal mine ventilation shafts) and the large emissions of 448 and 720 kt CH4 yr−1 reported in the European Pollutant Release and Transfer Register (E-PRTR 2017) and the Emissions Database for Global Atmospheric Research (EDGAR v4.3.2), respectively, make the USCB a prime research target for validating and improving CH4 flux estimation techniques. High-precision observations of this GHG were made downwind of local (e.g., single facilities) to regional-scale (e.g., agglomerations) sources in the context of the CoMet 1.0 campaign in early summer 2018. A quantum cascade–interband cascade laser (QCL–ICL)-based spectrometer adapted for airborne research was deployed aboard the German Aerospace Center (DLR) Cessna 208B to sample the planetary boundary layer (PBL) in situ. Regional CH4 emission estimates for the USCB are derived using a model approach including assimilated wind soundings from three ground-based Doppler lidars. Although retrieving estimates for individual emitters is difficult using only single flights due to sparse data availability, the combination of two flights allows for exploiting different meteorological conditions (analogous to a sparse tomography algorithm) to establish confidence on facility-level estimates. Emission rates from individual sources not only are needed for unambiguous comparisons between bottom-up and top-down inventories but also become indispensable if (independently verifiable) sanctions are to be imposed on individual companies emitting GHGs. An uncertainty analysis is presented for both the regional-scale and facility-level emission estimates. We find instantaneous coal mine emission estimates of 451/423 ± 77/79 kt CH4 yr−1 for the morning/afternoon flight of 6 June 2018. The derived fuel-exploitation emission rates coincide (±6 %) with annual-average inventorial data from E-PRTR 2017 although they are distinctly lower (−28 %/−32 %) than values reported in EDGAR v4.3.2. Discrepancies in available emission inventories could potentially be narrowed down with sufficient observations using the method described herein to bridge the gap between instantaneous emission estimates and yearly averaged inventories.

IFAR liner benchmark challenge #1 – DLR impedance eduction of uniform and axially segmented liners and comparison with NASA results

This paper presents the contribution from the German Aerospace Center (DLR) to the first liner benchmark challenge under the framework of the International Forum for Aviation Research (IFAR). Therefore, two sets of acoustically damping wall treatments, called ‘liner samples’, have been produced by additive manufacturing based on the design data provided by NASA coordinating this benchmark. These liner samples have been integrated and acoustically characterized in the liner flow test facility DUCT-R at DLR Berlin as well as in the liner flow test facility GFIT at NASA Langley. Besides the dissipation coefficients and the axial pressure profiles, the liner wall impedance was educed by first determining the axial wave numbers and then applying a straightforward method based on the one-dimensional Convected Helmholtz Equation. Finally, the comparison of the liner impedance values to the NASA results show a fairly good agreement.

What Are Good Muscle Endpoints for Translational Studies?

What Are Good Muscle Endpoints for Translational Studies? Jörn Rittweger()1,2 1Institute of Aerospace Medicine, German Aerospace Center, Cologne, Germany 2Department of Pediatrics and Adolescent Medicine, University of Cologne, Cologne, Germany © The Author Abstract Muscles matter to our health because of their size, their involvement in energy metabolism and their relevance for locomotion. Adequate selection of good endpoints is crucial for successfully designing translational studies. At least eight different muscle functions matter to health, namely the mechanical functions of exerting force, velocity, power, elastic storage and braking power, the two metabolic functions of substrate uptake (e.g. carbohydrates, lipids and amino acids) and substrate provision (e.g. lactate and amino acids) and secretory functions. However, specific endpoint tests have been validated for muscle force and power only. Walking speed and grip strength demonstrate good predictive value for hard clinical endpoints, such as disability, loss of autonomy and death. Vertical jump power also has good ecological validity and construct validity, and it depicts excellent test-retest reliability, which is an important advantage with regard to the study of power. Assessment of muscle mass, e.g. by magnetic resonance imaging, dual energy X-ray absorptiometry or bioelectrical impedance, should be considered as an important secondary endpoint to enhance construct validity. Further secondary endpoints should be included wherever they are likely to enhance the plausibility of the study outcome and assessment of test-retest reliability at baseline is always recommended. Well-established methods exist for three relevant muscular endpoints, namely power, strength and muscle mass, and these endpoints lend themselves to utilization in clinical studies. However, such validated methods lack a number of additional muscle functions that are scientifically only emerging. This applies foremost to the metabolic function of muscles but also to its role in storage and dissipation of mechanical energy.

Analysis of Pollutant Emissions on City Arteries—Aspects of Transport Management

The aim of the study is to present a methodology for analyzing pollution emissions in a medium-sized city using modern traffic simulations in the aspect of minimizing exhaust emissions. The scope of the research and the methods of analysis used differ from those applied in big cities projects that can be found in the literature, Therefore the progressive elaboration model has been applied methodically to formulate and carry out the feasibility study. To perform microscopic traffic simulations, the software Simulation of Urban Mobility (SUMO—German Aerospace Center (DLR), Berlin, Germany) was applied. Thanks to the simulations, changes in traffic organization were accurately identified in the context of pollution emissions before they were implemented. The proposed approach allows a smooth flow of vehicles and a reduction of exhaust emissions. The experiments, supported by visual modelling of traffic with respect to pollution emissions, were performed on one of the main arteries of the city of Czestochowa (Poland). The results were used to explain the benefits of planned roadworks and convince the city government of the necessity to modernize the communication network.

Similarity and Key Parameters of Retropropulsion Assisted Deceleration in Hypersonic Wind Tunnels

Retropropulsion, a deceleration technique where the thrust is directed along the flight path, is the driving technology for vertically landing, reusable launchers. Experiments, which are indispensable for the full comprehension of retropropulsion flows, pose serious challenges to the operability of wind tunnels and interpretation of these subscale experiments. The key factors to facilitate meaningful results of supersonic and hypersonic retropropulsion experiments are addressed in this study. Similarity parameters for subscale experiments are evaluated, the interaction of retropropulsion flows with the test facility is investigated, and condensation within these flows is analyzed. For this purpose, computational fluid dynamics simulations were performed; subsequently, cold gas experiments for a generic retropropulsion launcher configuration with a single central nozzle were carried out in the hypersonic wind tunnel Cologne H2K at the Supersonic and Hypersonic Technologies Department of the DLR, German Aerospace Center. Based on these data, an improved similarity parameter related to the exit condition of the engine exhaust flow is proposed, and a universally applicable scaling law for supersonic retropropulsion wind-tunnel models is derived and validated. Various condensation mitigation techniques for the conducted experiments are presented, and their varying effectiveness is highlighted.

The long period of 3He-rich solar energetic particles measured by Solar Orbiter 2020 November 17-23

The Suprathermal Ion Spectrograph (SIS) is a European facility instrument funded by ESA. The SIS instrument was constructed by the JHU/Applied Physics Lab. and CAU Kiel. We thank the many individuals at ESA and within the Energetic Particle Detector team for their support in its development. Post launch operation of SIS at APL is funded by NASA contract NNN06AA01C, and we thank NASA headquarters and the NASA/GSFC Solar Orbiter project office for their continuing support. The UAH team acknowledges the financial support by the Spanish Ministerio de Ciencia, Innovacion y Universidades FEDER/MCIU/AEI Projects ESP2017-88436- R and PID2019-104863RB- I00/AEI/10.13039/501100011033. The CAU Kiel team thanks the German Federal Ministry for Economic Affairs and Energy and the German Space Agency (Deutsches Zentrum fur Luft- und Raumfahrt, e.V., (DLR)) for their unwavering support under grant numbers 50OT0901, 50OT1202, 50OT1702, and 50OT2002; and ESA for supporting the build of SIS under contract number SOL.ASTR.CON.00004, and the University of Kiel and the Land Schleswig-Holstein for their support of SIS. R. Bucik is supported ˇ in part by NASA grant 80NSSC19K0079. F. Carcaboso acknowledges the financial support by the Spanish MINECO-FPI-2016 predoctoral grant with FSE. V. Krupaˇr acknowledges the support by NASA under grants 18-2HSWO2182- 0010 and 19-HSR-192-0143. L. Balmaceda acknowledges the support from the NASA program NNH17ZDA001N-LWS (Awards Nr. 80NSSC19K1261 and 80NSSC19K1235).

Centrifugal Compressor Stall Control by the Application of Engineered Surface Roughness on Diffuser Shroud Using Numerical Simulations.

Downsizing in engine size is pushing the automotive industry to operate compressors at low mass flow rate. However, the operation of turbocharger centrifugal compressor at low mass flow rate leads to fluid flow instabilities such as stall. To reduce flow instability, surface roughness is employed as a passive flow control method. This paper evaluates the effect of surface roughness on a turbocharger centrifugal compressor performance. A realistic validation of SRV2-O compressor stage designed and developed by German Aerospace Center (DLR) is achieved from comparison with the experimental data. In the first part, numerical simulations have been performed from stall to choke to study the overall performance variation at design conditions: 2.55 kg/s mass flow rate and rotational speed of 50,000 rpm. In second part, surface roughness of magnitude range 0–200 μm has been applied on the diffuser shroud to control flow instability. It was found that completely rough regime showed effective quantitative results in controlling stall phenomena, which results in increases of operating range from 16% to 18% and stall margin from 5.62% to 7.98%. Surface roughness as a passive flow control method to reduce flow instability in the diffuser section is the novelty of this research. Keeping in view the effects of surface roughness, it will help the turbocharger manufacturers to reduce the flow instabilities in the compressor with ease and improve the overall performance.

Retrieval of Tropical Forest Height and Above-Ground Biomass Using Airborne P- and L-Band SAR Tomography

Synthetic aperture radar tomography (TomoSAR) at different radar wavelength can be used to measure different structural elements of forests. In this letter, we compared the airborne P- and L-band synthetic aperture radar (SAR) tomograms and TomoSAR-measured canopy height model (CHM) and above-ground biomass (AGB) over a tropical forest in Lope, Gabon. The SAR data sets were acquired by German Aerospace Center (DLR)'s F-SAR system during the AfriSAR2016 campaign. First, the Weighted covariance fitting-based Iterative Spectral Estimator (WISE) was applied to obtain tomograms. CHM was then retrieved based on the canopy phase center derived from the tomograms. Finally, AGB was estimated via an empirical logarithmic model developed from field measurements and the tomographic backscatter power of vegetation layers between 40 and 50 m above ground. Compared with the classical approaches of Capon and Wavelet-based Compressed Sensing, the WISE method can achieve better resolution with higher computational efficiency and reduce the ambiguity level of L-band tomograms successfully. The experimental results also show that there is no substantial difference between P- and L-band TomoCHM, while P-band tomographic intensity is more sensitive than the L-band for the inversion of tropical forest AGB at a resolution of 50 m x 50 m.

The DLR FireBIRD Small Satellite Mission: Evaluation of Infrared Data for Wildfire Assessment

Wildfires significantly influence ecosystem patterns and processes on a global scale. In many cases, they pose a threat to human lives and property. Through greenhouse gas emissions, wildfires also directly contribute to climate change. The monitoring of such events and the analysis of acquired data is crucial for understanding wildfire and ecosystem interactions. The FireBIRD small satellite mission, operated by the German Aerospace Center (DLR), was specifically designed for the detection of wildfires. It features a higher spatial resolution than available with other Earth-observation systems. In addition to the detection of active fire locations, the system also allows the derivation of fire intensity by means of the Fire Radiative Power (FRP). This indicator can be used as a basis to derive the amount of emitted pollutant, which makes it valuable for climate studies. With the FireBIRD mission facing its end of life in 2021, this study retrospectively evaluates the performance of the system through an inter-comparison with data from two satellite missions of the National Aeronautics and Space Administration (NASA) and discusses the potential of such a system. The comparison is performed regarding both geometrical and radiometric aspects, the latter focusing on the FRP. This study uses and compares two different methods to derive the FRP from FireBIRD data. The data are analyzed regarding six major fire incidents in different regions of the world. The FireBIRD results are in accordance with the reference data, showing a geometrical overlapping rate of 83% and 84% regarding MODIS (Moderate-resolution Imaging Spectroradiometer) and VIIRS (Visible Infrared Imaging Radiometer Suite) overpasses in close temporal proximity. Furthermore, the results show a positive bias in FRP of about 11% compared to MODIS.

Lagrangian matches between observations from aircraft, lidar and radar in a warm conveyor belt crossing orography

Abstract. Warm conveyor belts (WCBs) are important airstreams in extratropical cyclones, often leading to the formation of intense precipitation and the amplification of upper-level ridges. This study presents a case study that involves aircraft, lidar and radar observations in a WCB ascending from western Europe towards the Baltic Sea during the Hydrological Cycle in the Mediterranean Experiment (HyMeX) and T-NAWDEX-Falcon in October 2012, a preparatory campaign for the THORPEX North Atlantic Waveguide and Downstream Impact Experiment (T-NAWDEX). Trajectories were used to link different observations along the WCB, that is, to establish so-called Lagrangian matches between observations. To this aim, an ensemble of wind fields from the global analyses produced by the European Centre for Medium-Range Weather Forecasts (ECMWF) Ensemble of Data Assimilations (EDA) system were used, which allowed for a probabilistic quantification of the WCB occurrence and the Lagrangian matches. Despite severe air traffic limitations for performing research flights over Europe, the German Aerospace Center (DLR) Falcon successfully sampled WCB air masses during different phases of the WCB ascent. The WCB trajectories revealed measurements in two distinct WCB branches: one branch ascended from the eastern North Atlantic over southwestern France, while the other had its inflow in the western Mediterranean. Both branches passed across the Alps, and for both branches Lagrangian matches coincidentally occurred between lidar water vapour measurements in the inflow of the WCB south of the Alps, radar measurements during the ascent at the Alps and in situ aircraft measurements by Falcon in the WCB outflow north of the Alps. An airborne release experiment with an inert tracer could confirm the long pathway of the WCB from the inflow in the Mediterranean boundary layer to the outflow in the upper troposphere near the Baltic Sea several hours later. The comparison of observations and ensemble analyses reveals a moist bias in the analyses in parts of the WCB inflow but a good agreement of cloud water species in the WCB during ascent. In between these two observations, a precipitation radar measured strongly precipitating WCB air located directly above the melting layer while ascending at the southern slopes of the Alps. The trajectories illustrate the complexity of a continental and orographically influenced WCB, which leads to (i) WCB moisture sources from both the Atlantic and Mediterranean, (ii) different pathways of WCB ascent affected by orography, and (iii) locally steep WCB ascent with high radar reflectivity values that might result in enhanced precipitation where the WCB flows over the Alps. The linkage of observational data by ensemble-based WCB trajectory calculations, the confirmation of the WCB transport by an inert tracer and the model evaluation using the multi-platform observations are the central elements of this study and reveal important aspects of orographically modified WCBs.

Sequential 90° Rotation of Dual-Polarized Antenna Elements in Linear Phased Arrays with Improved Cross-Polarization Level for Airborne Synthetic Aperture Radar Applications

In this work, a novel rotation approach for the antenna elements of a linear phased array is presented. The proposed method improves by up to 14 dB the cross-polarization level within the main beam by performing a sequential 90° rotation of the identical array elements, and achieving measured cross-polarization suppressions of 40 dB. This configuration is validated by means of simulation and measurements of a manufactured linear array of five dual-polarized cavity-box aperture coupled stacked patch antennas operating in L-Band, and considering both uniform amplitude and phase distribution and beamforming with amplitude tapering. The analysis is further extended by applying and comparing the proposed design with the 180° rotation and non-rotation topologies. This technique is expected to be used for the next generation L-Band Airborne Synthetic Aperture Radar Sensor of the German Aerospace Center (DLR).

Elastic Structure Preserving Control for Compliant Robots Driven by Agonistic-Antagonistic Actuators (ESPaa)

The regulation of the link positions of compliant robots, damping out undesired link oscillations while preserving the system's inherent elasticity is still a challenging task in practical applications. This task becomes even harder to be tackled in the case of compliant robots driven by agonistic-antagonistic variable stiffness actuators in which there are two motors associated with each joint of the system. In this work, leveraging on the physical realization of the elastic mechanism of such actuators, we propose a novel control law able to simultaneously achieve a good regulation performance and a desired damped behavior at the link, while preserving the elastic structure of the system as well as the possibility of adjusting the passive stiffness at the joints. Simulations on the agonistic-antagonistic actuators of the forearm and wrist joints of the Hand Arm System from the German Aerospace Center (DLR) and experiments on a planar platform with an analogous actuation unit, namely qbMove Advanced, validate the proposed method.

Airborne Hyperspectral Data Acquisition and Processing in the Arctic: A Pilot Study Using the Hyspex Imaging Spectrometer for Wetland Mapping

A pilot study for mapping the Arctic wetlands was conducted in the Yukon Flats National Wildlife Refuge (Refuge), Alaska. It included commissioning the HySpex VNIR-1800 and the HySpex SWIR-384 imaging spectrometers in a single-engine Found Bush Hawk aircraft, planning the flight times, direction, and speed to minimize the strong bidirectional reflectance distribution function (BRDF) effects present at high latitudes and establishing improved data processing workflows for the high-latitude environments. Hyperspectral images were acquired on two clear-sky days in early September, 2018, over three pilot study areas that together represented a wide variety of vegetation and wetland environments. Steps to further minimize BRDF effects and achieve a higher geometric accuracy were added to adapt and improve the Hyspex data processing workflow, developed by the German Aerospace Center (DLR), for high-latitude environments. One-meter spatial resolution hyperspectral images, that included a subset of only 120 selected spectral bands, were used for wetland mapping. A six-category legend was established based on previous U.S. Geological Survey (USGS) and U.S. Fish and Wildlife Service (USFWS) information and maps, and three different classification methods—hybrid classification, spectral angle mapper, and maximum likelihood—were used at two selected sites. The best classification performance occurred when using the maximum likelihood classifier with an averaged Kappa index of 0.95; followed by the spectral angle mapper (SAM) classifier with a Kappa index of 0.62; and, lastly, by the hybrid classifier showing lower performance with a Kappa index of 0.51. Recommendations for improvements of future work include the concurrent acquisition of LiDAR or RGB photo-derived digital surface models as well as detailed spectra collection for Alaska wetland cover to improve classification efforts.

Precise orbit and Earth parameter determination supported by LEO satellites, inter-satellite links and synchronized clocks of a future GNSS

The paper presents results of precise orbit and Earth parameter determination based on full-scale simulations of a potential future GNSS, with the name of Kepler, recently proposed by the German Aerospace Center. The space segment of the system consists of 24 Galileo-like Medium Earth Orbit (MEO) satellites and 6 Low Earth Orbit (LEO) satellites equipped with ultra-stable optical clocks. All spacecraft carry terminals for two-way optical inter-satellite links (ISLs) enabling high-rate data communication, constellation-wide clock synchronization and precise absolute ranging. The clock synchronization is assumed to create an extremely stable reference time scale based on ensemble of the optical clocks. A series of simulation scenarios are defined, starting from the Galileo constellation as benchmark, and gradually incorporating additional features including LEOs, ISLs and synchronized clocks to assess the impact on orbits, singnal-in-space range errors (SiSREs) and a geodetic quality Earth parameters like polar coordinates, length of day (LOD) and geocenter coordinates. Precise orbit determination (POD) is done at the observation level using the same standards and setups as in real data processing with ground network containing up to 18 stations. The MEO SiSRE, the most relevant to navigation applications, obtained for the Kepler constellation with perfect models and 18 ground stations was found to be 0.009 cm, 160 times better than 1.4 cm found for Galileo. It was demonstrated, that for operational POD for the Kepler system the ground network can be significantly reduced to only two stations if Earth rotation parameters are to be estimated, or to just one otherwise. A SiSRE value of 0.037 cm was obtained with two stations, and 0.018 cm with one station. For more realistic results a number of modeling errors are being considered. These concern observations – multipath, GNSS code hardware delays and ISL range biases, the dynamics – solar radiation pressure (SRP), gravity field, air drag, MEO antenna thrust, Earth tide potential, and the geometry – ocean loading and phase center offsets (PCOs) of space GNSS antennas. Any other satellite and station equipment contribution to mismodeling has been disregarded. It is shown, that for the Kepler the modeling errors are clearly visible in ISL observation residuals, in contrast to Galileo case, where they are to a large extent absorbed by the large number of estimated clock parameters. This was exploited to efficiently mitigate the dynamic modeling errors by estimation of empirical accelerations. With this setup the MEO SiSRE of 5 cm was obtained for Galileo and 0.3 cm, 0.4 cm and 0.7 cm for Kepler with 18, two and one ground station, respectively. The key to obtaining such low values are ISLs and synchronized satellite clocks. In the Kepler constellation with a single ground station the ISL range biases (RBs) up to 5 mm and 20 mm increase the SiSRE to 0.74 cm and 1.67 cm. Estimation of the ISL RBs is not recommended in the presence of modeling errors since they are significantly biased, which leads to increase of SiSRE to 2.2 cm. The Earth parameter determination, based on global network of 124 observing IGS stations, also benefits significantly from the Kepler constellation. The initial results show that the impact of modeling errors on Earth parameters can, as with POD, be effectively reduced in the Kepler system. The 30 cm error in integrated LOD, which has an impact on UT1-UTC, obtained for Galileo due to combined modeling errors could be reduced to 0.5 cm for the Kepler system. The same is true for the Z-coordinate of the geocenter, whose 14 mm error in the MEO-only solution could be reduced to 0.2 mm in the Kepler solution. The Earth parameters completely free of bias are obtained when the number of the modeling errors is reduced to only two: MEO SRP and PCOs. The ISL range biases up to 5 mm were found to have negligible impact on the estimated EPs. The largest differences (up to 0.2 mm) are found for UT1-UTC, for all other parameters they do not exceed 0.02 mm. The estimation of PCOs, being an integral part of Earth parameter determination, also benefits enormously from the Kepler design. It was demonstrated, that using only ten days of data they could be estimated with millimeter accuracy for MEOs and sub-millimeter accuracy for LEOs in all three spacial directions even in the presence of ISL range biases of up to 5 mm. Such accuracy of PCOs is not achievable with the Galileo constellation.

Isothermal furnace for long-term in situ and real-time X-radiography solidification experiments.

A new x-ray isothermal furnace has been developed, suitable for in situ observations of semi-solid processes including the transition from dendritic to globulitic grain morphology and grain coarsening in metallic samples. A homogeneous, isothermal temperature field is achieved using a novel heater concept. The furnace structure is sandwich-like with heating elements positioned in the beam line and parallel to the sample. Planar heat transfer to the sample enables measurements with low cooling rates and a minimized temperature gradient. Cooling rates from 0.1 to 15K min-1 can be controlled in the temperature range 1170-670K. The furnace setup is integrable in the existing rotatable laboratory x-ray facility (X-RISE) at the German Aerospace Center (DLR). In this setup, an effective pixel size of 3 μm and a field of view of 8mm in diameter can be achieved. Preliminary solidification and semi-solid experiments in the hypo-eutectic alloy systems Al-Ge and Al-Cu, inoculated with Al-5Ti-1B grain refiner, are presented. They indicate a very uniform temperature distribution in the sample.

On the Importance of Transition Control at Transonic Compressor Blades

AbstractThe flow through a transonic compressor cascade is characterized by high unsteadiness and a high loss level. This results from the shock waves in the blade cascade and their interaction with the blade suction side boundary layer. In the case of a laminar shock wave boundary layer interaction, the loss level is higher due to the occurrence of a laminar separation bubble below the shock wave compared to the shock wave interaction with a turbulent boundary layer. In addition, the oscillation of the shock position in both cases influences the working range concerning the point of stall onset as well as leading to an unsteady interaction with the blade called buffeting. The reduction of losses and of unsteadiness in the shock wave oscillation, connected to a decrease of the blade buffeting effect, is the aim of the current investigation. Therefore, experimental investigations using a roughness patch as well as air jet vortex generators in order to control the transition in a transonic compressor cascade have been conducted at the transonic cascade wind tunnel of the German Aerospace Center (DLR) at Cologne. At an inflow Mach number of 1.21, a loss reduction for both transition control cases is achieved. In spite of a nearly uninfluenced fluctuation range of the passage shock wave compared to the reference cascade, the oscillation spectra of the transition control cases show a reduction of the shock movement amplitude at a frequency below 500 Hz and above 1 kHz. In the closing section of the paper, a detailed discussion on the reasons for the resulting flow behavior based on particle image velocimetry and high-speed shadowgraphy data is given. The resulting conclusion of the study is that the consideration of transition control at transonic compressor blades is very important in order to reduce losses and flow unsteadiness that directly influences blade buffeting and the numerical prediction quality of the stall onset.

Effect of added struts and intake velocity on flame stabilization in supersonic combustors

In the present computational study, the impact of strut positions and the effect of inlet Mach numbers on the combustion efficiency are investigated in a strut-based supersonic combustor. An experimentally investigated model combustor developed at the German Aerospace Center (DLR) is simulated and validated. Then, a model combustor with three struts placed at different positions is investigated. Two-dimensional, compressible, reacting-flow governing equations are solved along with single-step chemistry reaction and k-ω SST turbulence model using a commercial CFD code FLUENT. The oblique shock from the struts has a profound influence on the mixing and combustion process. The H2O mole fraction, H2 mole fraction contours, and combustion efficiency of various configurations are compared for finding better mixing and flame stabilization. The combustion efficiency reduces when the two struts are located in farther downstream or placed at the same downstream location. At higher Mach numbers, the combustion is delayed, and the mixing of fuel with the supersonic mainstream is incomplete.

Numerical evaluation of shock wave effects on turbulent mixing layers in a scramjet combustor

The effects of shock waves on turbulent mixing layers in a supersonic combustor are investigated. An asymmetric scramjet combustor with strut designed and tested by German Aerospace Center is taken as the physical model. Then Reynolds-averaged Navier-Stokes numerical simulations of the non-reacting cold flow field are carried out. The results demonstrate that the mixing zone containing two mixing layers exhibits a complex thickness variation under the actions of multiple shock waves. In this variation, the thickness decrease at shock wave action positions can be quantitatively characterized by the relative decrease of thickness of mixing layers, while the recovery of thickness increase in the vicinities downstream of shock wave action positions can be quantitatively characterized by the slope increase of thickness variation curve of mixing layers. Both of these characterization parameters are positively correlated with the shock intensity. The physical mechanism of the thickness decrease at the shock wave action positions is that shock waves cause density increase which then lead the thickness to decrease. The physical mechanism of the recovery of thickness increase in the vicinities downstream of the shock wave action positions is that shock waves induce vorticity enhancement which then results in the thickness growth rate to increase.