Alexander Wagner

Time-Resolved Visualization of Instability Waves in a Hypersonic Boundary Layer

LAMINAR-TURBULENT transition in hypersonic boundary layers remains a challenging subject. This is especially true of the hypervelocity regime, in which an intriguing phenomenon is the possible damping of second-mode disturbances by chemical and vibrational nonequilibrium processes. To generate flows with sufficiently high enthalpy to investigate such effects, the use of shock-tunnel facilities is necessary; furthermore, it is now generally accepted that direct measurements of the instability mechanisms active within the boundary layer, together with a characterization of the freestream disturbance environment, are required, as simple measurements of transition locations can lead to ambiguous conclusions. However, as difficult as the accurate measurement of instability waves in conventional hypersonic facilities can be, in shock tunnels it is appreciably more so. For identical unit Reynolds numbers, the higher stagnation temperature in a shock tunnel means that the dominant second-mode disturbances lie at even higher frequencies (typically hundreds of kHz or higher); moreover, because of the destructive testing environment, hot-wire techniques, a staple for instability measurements in conventional tunnels, cannot be used. Fast-response pressure transducers are an obvious alternative, but recent experiments have highlighted the challenging nature of interpreting data from mechanically sensitive sensors in the high-noise environment of a shock tunnel, especially without accompanying stability computations. Measurements with recently developed atomic-layer thermopile (ALTP) heat-flux sensors show promise, though their use has yet to be demonstrated in shocktunnel facilities.

Experimental investigation of hypersonic boundary-layer stabilization on a cone by means of ultrasonically absorptive carbon-carbon material

Hypersonic boundary-layer transition on a 7 half-angle cone using ultrasonically absorptive carbon-carbon material was investigated experimentally in the DLR High Enthalpy Shock Tunnel G�ottingen (HEG) at Mach 7.5. The cone model was tested at zero angle of attack with a nose radius of 2.5 mm and a total length of 1077 mm. One third of the model surface in circumferential direction was equipped with an in-house manufactured porous carbon-carbon material. The remaining model surface consisted of polished steel and served as reference surface. The model was equipped with co-axial thermocouples to determine the transition location by means of the surface heat flux distribution. Flush mounted piezoelectric fast-response pressure transducers were used to measure the pressure fluctuations in the boundary-layer associated with second-mode instabilities. The free-stream unit Reynolds number was varied over a range of Rem = 1.46 10^6 /m to Rem = 6.35 10^6 /m at a stagnation enthalpy of h0 = 3.2 MJ/kg and a temperature ration of Tw/T0= 0.1 . The present study revealed a signicant damping of the second-mode instabilities and an increase of the transition Reynolds number by means of an ultrasonically absorptive C/C material with random microstructure. Furthermore, the eect of small angle of attack variations on the transition location on the model was investigated.

Application of Temperature and Pressure Sensitive Paints to DLR Hypersonic Facilities: “lessons learned”

PSP and TSP measurements have been carried out on the following DLR facilities: small test shock tube, High Enthalpy Shock Tunnel (both in Gottingen) and the Hypersonic Wind Tunnel (in Cologne). Based on the results from these measurements, various challenges and problem areas could be identified: the role played by these in hindering a quantitative application of PSP/TSP to the facilities is discussed, and ways and means are presented for either overcoming them or at least ameliorating their influence.

The Potential of Ultrasonically Absorptive TPS Materials for Hypersonic Vehicles

The potential of ultrasonically absorptive carbon fiber reinforced carbon (C/C) based materials for passive boundary layer transition control on hypersonic flight vehicles is investigated. Based on previous studies on C/C, optimized C/C based materials are developed and studied with respect to their temperature stability and ultrasonic absorption properties at hypersonic flight regimes of interest. Generic vehicle shapes providing an essentially two dimensional flow field allowing the second mode instability to be the dominant boundary layer instability are studied using an engineering approach. Furthermore, the study provides an overview on the test procedures established in DLR to assess the ultrasonic absorption properties of porous materials and to experimentally investigate the impact of potential candidates on the boundary layer transition process.

Shock Tunnel Testing of the Transpiration-Cooled Heat Shield Experiment AKTiV

The present paper presents shock tube testing of the transpiration cooling experiment AKTiV flown on the sub-orbital re-entry body SHEFEXII. The campaign comprises testing of a 1:3 subscale model at original Reynolds number, Mach number and velocity, so that the subscaling has to be compensated for by rising the gas density. This, in turn, results in increased heat flux to the surface compared to the original. The shock tube experiments are supported by fluid dynamics computations by code TAU. Moreover, for lay out and design of the transpiration-cooled thermal protection element, the semi-analytical tool HEATS is introduced, based on a transient heat balance at the surface. Heat flux to the structure is measured just up- and downstream of the cooled sample. The measurement shows that heat flux is reduced upon coolant exhaust. Comparison of these results with HEATS shows that the heat flux predicted with HEATS is in good concurrence with the measured heat flux values at various angles of attack. HEATS results enables prediction of the cooling efficiency, giving an average heat flux reduction to 11.11% upon transpiration cooling for a coolant gas flow of 0.39 g/s and a variety of angles of attack.

Post Flight Analysis of SHEFEX I: Shock Tunnel Testing and Related CFD Analysis

The SHarp Edge Flight EXperiment (SHEFEX) program of the German Aerospace Center (DLR) is primarily focused on the investigation of the potential to utilise improved shapes for space vehicles by considering sharp edges and facetted surfaces. One goal is to set up a sky based test facility to gain knowledge of the physics of hypersonic flow, complemented by numerical analysis and ground based testing. Further, the series of SHEFEX flight experiments is an excellent test bed for new technological concepts and flight instrumentation, and it is a source of motivation for young scientist and engineers providing an excellent school for future space-program engineers and managers. After the successful first SHEFEX flight in October 2005, a second flight is scheduled for September 2011 and additional flights are planned for 2015 ff. With the SHEFEX-I flight and the subsequent numerical and experimental post flight analysis, DLR could for the first time close the loop between the three major disciplines of aerothermodynamic research namely CFD, ground based testing and flight.

High Frequency Free-Stream Disturbance Measurements in Hypersonic Wind Tunnels by Means of a Slender Wedge Probe

Experimental investigations of the free-stream disturbance spectrum by means of a slender wedge probe were conducted in the DLR High Enthalpy Shock Tunnel Gottingen (HEG) at Mach 7.4, the DNW Ludwieg tube (RWG) at Mach 3 and Mach 6 as well as in the TU Braunschweig Ludwieg tube (HLB) at Mach 6. The identical measurement chain, including the probe, the amplifier and the type of transducer were used in all facilities. A wide range of fast response pressure, temperature and heat flux transducers were used to measure the corresponding fluctuations on the probe surface behind the oblique shock. Quantitative results were obtained as function of frequency taking into account the limitations imposed by each transducer. The wedge probe was shown to be a robust and easy to implement probe particularly suited for use in harsh test environments which do not allow the application of hot wire techniques or flush mounted transducers in a Pitot probe configuration.

Experimental and Numerical Analysis of SHEFEX-II

The SH arp E dge F light EX periment (SHEFEX) of the German Aerospace Center (DLR) has been established to demonstrate the feasibility of space vehicles with facetted Thermal Protection System (TPS). In this study, first results to the aerothermodynamic behavior of SHEFEX − II in the High Enthalpy Shock Tunnel Gottingen (HEG) are presented.The main focus is on the experimental and numerical investigation of the active cooling experiment AKTiV. The measured and calculated results confirm that a cooling effect is feasible.

Hypersonic boundary-layer stabilization by means of ultrasonically absorptive carbon-carbon material Part 2: Computational Analysis

The damping of acoustic second mode instabilities by passive porous surfaces is investigated numerically and compared with experiments for different hypersonic boundary-layer flows. The used geometry is a blunt 7° half-angle cone model with exchangeable nose. The cone surface consists partly of an ultrasonically absorptive material, which has a natural, random porosity. For the analyses of the second modes the DLR stability code NOLOT, NOnLocal Transition analysis code, is used. The influence of the nose radius and the unit Reynolds number on the second modes is investigated using a smooth surface. These results of the smooth surface are compared with those of the porous surface to study the damping effect on the second modes and the transition shift. The numerical results are compared with wind tunnel measurements, which were performed in the DLR High Enthalpy Shock Tunnel Gottingen (HEG).

Combined free-stream disturbance measurements and receptivity studies in hypersonic wind tunnels by means of a slender wedge probe and direct numerical simulation

Combined free-stream disturbance measurements and receptivity studies in hypersonic wind tunnels were conducted by means of a slender wedge probe and direct numerical simulation. The study comprises comparative tunnel noise measurements at Mach 3, 6 and 7.4 in two Ludwieg tube facilities and a shock tunnel. Surface pressure fluctuations were measured over a wide range of frequencies and test conditions including harsh test environments not accessible to measurement techniques such as Pitot probes and hot-wire anemometry. A good agreement was found between normalized Pitot pressure fluctuations converted into normalized static pressure fluctuations and the wedge probe readings. Quantitative results of the tunnel noise are provided in frequency ranges relevant for hypersonic boundary-layer transition. Complementary numerical simulations of the leading-edge receptivity to fast and slow acoustic waves were performed for the applied wedge probe at conditions corresponding to the experimental free-stream conditions. The receptivity to fast acoustic waves was found to be characterized by an early amplification of the induced fast mode. For slow acoustic waves an initial decay was found close to the leading edge. At all Mach numbers, and for all considered frequencies, the leading-edge receptivity to fast acoustic waves was found to be higher than the receptivity to slow acoustic waves. Further, the effect of inclination angles of the acoustic wave with respect to the flow direction was investigated. An inclination angle was found to increase the response on the wave-facing surface of the probe and decrease the response on the opposite surface for fast acoustic waves. A frequency-dependent response was found for slow acoustic waves. The combined numerical and experimental approach in the present study confirmed the previous suggestion that the slow acoustic wave is the dominant acoustic mode in noisy hypersonic wind tunnels.