Investigations on the Turbulent Wake of a Generic Space Launcher Geometry in the Hypersonic Flow Regime

The turbulent wake flow of generic rocket configurations is investigated experimentally and numerically at a freestream Mach number of 6.0 and a unit Reynolds number of 10 x 10^6. The flow condition is based on the trajectory of Ariane V at an altitude of 50 km, which is used as baseline to address the overarching tasks of wake flows in the hypersonic regime like fluid-structural coupling, reverse hot jets and base heating. Experiments using pressure transducers and high-speed schlieren measurement technique were conducted to gain insight into the local pressure fluctuations on the base and the oscillations of the recompression shock. This experimental configuration features a wedge-profiled strut orthogonally mounted to the main body. Additionally, the influence of cylindrical nozzle extensions attached to the base of the rocket is investigated, which is the link to the numerical investigations. Here, the axisymmetric model possesses a cylindrical sting support of the same diameter as the nozzle extensions. The sting support allows investigations of a undisturbed wake flow. A time-accurate zonal RANS/LES approach was applied to identify shocks, expansion waves, and the highly unsteady recompression region numerically. Subsequently, experimental and numerical results in the strut-averted region are opposed with regard to the wall pressure and recompression shock frequency spectra. For the compared configurations, experimental pressure spectra exhibit dominant Strouhal numbers at about S rD = 0.03 and 0.27 and the recompression shock oscillates at 0.2. In general, the numerical pressure and recompression shock fluctuations agree satisfactorily to the experimental results. The experiments with a blunt base reveal base-pressure spectra with dominant Strouhal numbers at 0.08 at the center position and 0.145, 0.21 − 0.22 and 0.31 − 0.33 at the outskirts of the base

On subsonic near-wake flows of a space launcher configuration with various base geometries

Buffet/buffeting as load imposing mechanism on the base structures of space launcher has been of strong interest ever since it was found as partially responsible for the failed flight 157 of Ariane 5. Several studies suggested that the base region is most excited at Mach 0.8. A preceding study of the current series on base flow effects revealed a differing excitation in comparison to the other subsonic Mach number cases. It featured an especially pronounced excitation in the recirculation region. Thus, the current work attempts to answer the question why this case appears to be distinct. This is done by decreasing the relative nozzle length and focusing on the Reynolds stress distribution. The research question is approached by experiments in the Vertical Test Section Cologne (VMK) on a base model with supersonic, over-expanded exhaust jet exposed to an ambient flow at Mach 0.8 and a Reynolds number of 1.4x10^6 . Data are acquired by means of particle image velocimetry (PIV) and high-speed schlieren imaging. The results reveal that a most unfavorable configuration appears to exist, which is if the mean shear layer reattachment takes place just on the tip of the nozzle

Expression of ECM proteins fibulin-1 and -2 in acute and chronic liver disease and in cultured rat liver cells

Fibulin-2 has previously been considered as a marker to distinguish rat liver myofibroblasts from hepatic stellate cells. The function of other fibulins in acute or chronic liver damage has not yet been investigated. The aim of this study has been to evaluate the expression of fibulin-1 and -2 in models of rat liver injury and in human liver cirrhosis. Their cellular sources have also been investigated. In normal rat liver, fibulin-1 and -2 were both mainly present in the portal field. Fibulin-1-coding transcripts were detected in total RNA of normal rat liver, whereas fibulin-2 mRNA was only detected by sensitive, real-time quantitative polymerase chain reaction. In acute liver injury, the expression of fibulin-1 was significantly increased (17.23-fold after 48 h), whereas that of fibulin-2 was not modified. The expression of both fibulin-1 and -2 was increased in experimental rat liver cirrhosis (19.16- and 26.47-fold, respectively). At the cellular level, fibulin-1 was detectable in hepatocytes, “activated” hepatic stellate cells, and liver myofibroblasts (2.71-, 122.65-, and 469.48-fold over the expression in normal rat liver), whereas fibulin-2 was restricted to liver myofibroblasts and was regulated by transforming growth factor beta-1 (TGF-β1) in 2-day-old hepatocyte cultures and in liver myofibroblasts. Thus, fibulin-1 and -2 respond differentially to single and repeated damaging noxae, and their expression is differently present in liver cells. Expression of the fibulin-2 gene is regulated by TGF-β1 in liver myofibroblasts

Experimental Analysis on Near-Wake Flows of Space Transportation Systems

This work addresses aerodynamic issues in the near-wake of space transportation systems in the subsonic flow regime ranging from Mach 0.5 to 0.9. The activities are driven by the finding of undesired and strong buffeting loads during the ascend of Ariane 5. Thus, the flight of Ariane 5 is simulated in the Vertical Test Section Cologne at the Supersonic and Hypersonic Technologies Department, Cologne, as part of the Institute of Aerodynamics and Flow Technology at of the German Aerospace Center. Particle image velocimetry, high-speed schlieren and unsteady pressure measurements have been performed on a generic base model. The base model investigations varied with respect to the exhaust jet, the ambient subsonic Mach number and the base geometry. The analysis encompasses the mean and turbulent features of the velocity distribution and base pressure, respectively base pressure fluctuations. Further, a proper orthogonal decomposition was applied on the velocity distribution for a modal analysis of the dominant motions. The results suggest an aeroacoustic feed-back mechanism between the screeching jet noise mechanism and the near-wake flow dynamics at Mach 0.8 with running jet. This feed-back mechanism appears to introduce the highest level of turbulence if the detached shear layer from the main body reattaches on average exactly at the tip of the nozzle. If no coupling occurs, the velocity distribution appears to scale with the reattachment length over a wide subsonic Mach number range

EXPERIMENTAL FLOW CHARACTERIZATION AND HEAT FLUX AUGMENTATION ANALYSIS OF A HYPERSONIC TURBULENT BOUNDARY LAYER ALONG A ROUGH SURFACE

Surface roughness increases skin friction drag and convective heat transfer along high-speed flight vehicles. Although the corresponding heat flux augmentation is usually lower compared to increased friction, careful consideration in the prediction of the resulting heat loads is required to define suitable margins in the design of thermal protection systems. In the present study, the response of a hypersonic turbulent boundary layer to a smooth and rough surface along a sharp right-circular cone is examined. Tests were conducted at an inflow of Ma = 6 and Re = 16 Million per meter in the hypersonic wind tunnel H2K at DLR Cologne. The testing time was in the order of 20 seconds. The model consisted of three segments with exchangeable parts to consider smooth or rough surfaces. The roughness topology consisted of square bar elements to enable comparisons to previous experimental campaigns. The roughness-element wavelength was four times the depth of the elements. The model was made of a specific material with low thermal conductivity, in order to measure the surface temperature distribution by means of global quantitative infrared thermography and to avoid lateral heat dissipation. The flow field along the smooth and rough cone was measured in selected regions of interest by Particle Image Velocimetry (PIV). This technique was successfully applied for the first time in the high-speed environment of the H2K. The data is compared and discussed based on comparison to analytical and numerical predictions. The analytical calculations include classical turbulent smooth cone relations as well as correlations for rough surfaces. The data for numerical comparisons was derived by a boundary layer code and full CFD. In case of the boundary layer code a modified Krogstad model was applied to account for the rough wall

Design of a Hot Plume Interaction Facility at DLR Cologne

Space transportation systems are exposed to high thermal and mechanical loads during the ascend in the transonic flow regime. By now, there are still many uncertainties, which can not be solved with state of the art computational fluid dynamic models or experiments with cold jet flows. A test facility with a high degree of similarity to flight with respect to the influence of the hot nozzle flow can contribute to improve the understanding of interaction effects between the hot nozzle flow and the ambient flow by providing reliable data for validation. The objective of the paper at hand is to present the work progress on such a facility. Issues and challenges concerning the base flows are discussed and potential research areas for investigations are considered. Relevant conditions during the ascend of Ariane 5 are used as baseline and appropriate scaling laws are discussed to conclude requirements for the operational conditions for the existing wind tunnel Vertical Test Section Cologne (VMK). These operational conditions are used to develop a concept. After a proof of concept is given by CFD calculations, details to the supply system including the operational range are described and opposed to existing test benches without interaction capabilities

Experiments on a smooth wall hypersonic boundary layer at Mach 6

The turbulent boundary layer along the surface of high-speed vehicles drives shear stress and heat flux. Although essential to the vehicle design, the understanding of compressible turbulent boundary layers at high Mach numbers is limited due to the lack of available data. This is particularly true if the surface is rough, which is typically the case for all technical surfaces. To validate a methodological approach, as initial step, smooth wall experiments were performed. A hypersonic turbulent boundary layer at Ma = 6 (Ma_e = 5.4) along a 7° sharp cone model at low Reynolds numbers Re_theta = 3000 was characterized. The mean velocities in the boundary layer were acquired by means of Pitot pressure and particle image velocimetry (PIV) measurements. Furthermore, the PIV data were used to extract turbulent intensities along the profile. The mean velocities in the boundary layer agree with numerical data, independent of the measurement technique. Based on the profile data, three different approaches to extract the skin friction velocity were applied and show favorable comparison to literature and numerical data. The extracted values were used for inner and outer scaling of the van Driest transformed velocity profiles which are in good agreement to incompressible theoretical data. Morkovin scaled turbulent intensities show ambiguous results compared to literature data which may be influenced by inflow turbulence level, particle lag and other measurement uncertainties

Aeroacoustic Coupling Effect During the Ascent of Space Transportation Systems

After the failure of maiden flight 157 of Ariane 5 Evolution Cryotechnique type A (ECA), the inquiry board responsible for the investigation reported as one of the probable causes the non-exhaustive definition of the loads to which the Vulcain 2 engine is subjected during flight. As a result, many research activities were set up with the objective to isolate the driving mechanisms. Driving mechanisms such as pumping, flapping, or swinging were found to excite oscillations in the base region. Despite this finding, the question of why the pressure fluctuations and base flow excitations are especially pronounced in the subsonic to transonic flow regime remained unclear. This question is addressed in the study at hand. Results from the literature are combined with recent wind-tunnel measurements on the base model of a space transportation system. The measurement data were acquired by means of particle image velocimetry, pressure transducers, and high-speed schlieren imaging. The results suggest that an aeroacoustic coupling takes place between a jet noise generation mechanism called screeching and the near-wake dynamics. Due to the nature of both effects, they are likely to reach a resonating frequency during the ascent. The resonant frequency appears to increase unfavorable fluctuations in the base region, which are held responsible for the amplified unsteady loads. A schematic model concept is proposed, which describes the underlying governing mechanism. It provides an explanation for question why buffeting effects are especially amplified in the high subsonic ambient flow regime