Search

This work incorporates in-situ calibration into the analytical inverse heat transfer solution for a finite base in temperature-sensitive-paint (TSP) measurements in high-enthalpy short-duration hypersonic wind tunnels. The thermal penetration parameter of the polymer layer (TSP) in the analytical inverse solution is determined by in-situ calibration based on the data obtained using a reliable heat flux sensor at a reference location. This method is applied to TSP images obtained on a wedge in a high-enthalpy shock tunnel.

In the presence of surface imperfections, the extent of the laminar flow regions on natural laminar flow (NLF) aircraft components can be significantly reduced, as compared to design. The present work focuses on surface bumps, which can occur, e.g., at or between structural joints, ribs and stringers. Bumps can induce a marked amplification of boundary-layer instabilities and thus lead to premature transition to turbulence. The influence of bumps on boundary-layer transition was systematically studied in this work in combination with the effect of variations in the following parameters: streamwise pressure gradient, freestream Mach number and chord Reynolds number. The experiments were conducted in a quasi-two-dimensional flow in the Cryogenic Ludwieg-Tube Göttingen, where chord Reynolds numbers up to 10 million, freestream Mach numbers up to 0.77 and various streamwise pressure gradients were examined. Essentially two-dimensional bumps, with a sinusoidal shape in the streamwise direction (positive half of a sine), fixed length and three different heights, were mounted on a two-dimensional flat-plate configuration. The wind-tunnel model was instrumented with a temperature-sensitive paint for non-intrusive, global transition detection, and also with pressure taps for the measurement of the surface pressure distributions. Boundary-layer transition was found to occur at a more upstream location with increasing bump height-to-length ratio. Transition induced by the small bump was shown to be dependent on streamwise pressure gradient, freestream Mach number and chord Reynolds number. In the case of bumps with larger height-to-length ratios, however, the influence of the aforementioned parameters on bump-induced transition was significantly reduced. The measured surface pressure distributions served also as an input for boundary-layer computations, and the calculated boundary-layer velocity profiles were analyzed according to linear, local stability theory. The results of the stability analysis were then correlated with the transition locations measured in the experiments. Detailed results will be presented in the final version of this contribution, where the importance of considering the effect on transition of pressure gradient, Mach number and Reynolds number in criteria for allowable size of bumps on NLF surfaces will be also discussed.

This work presents a method to automatically detect boundary-layer transition on thermographic images of aerodynamic surfaces, acquired using the temperature-sensitive paint measurement technique. It is applied to surface temperature distributions mapped onto three-dimensional grids representing the surfaces of interest. Motivated and substantiated by earlier work, the transition location is determined at the location corresponding to the maximal value of the streamwise gradient of the surface temperature in the transitional region. The robustness of the transition detection method was proven for a challenging case with low signal-to-noise ratio and moderate streamwise temperature gradients. Moreover, the present method was validated against transition data obtained using an uncorrelated transition detection method based on the measurement of surface pressure fluctuations. The developed transition detection algorithm enabled the accurate, reliable, and consistent detection of the transition location under a variety of test conditions, including experiments on natural laminar flow airfoil and wing models at high Reynolds numbers and at both subsonic and transonic Mach numbers.

The effect on transition of a non-adiabatic surface was systematically studied in the present experimental work in combination with the influence of variations in Mach number and pressure gradient. The investigations were carried out in a (quasi-) two-dimensional flow at four different subsonic Mach numbers and chord Reynolds numbers up to 13 million. Various streamwise pressure gradients and wall temperature ratios were examined. The experiments were conducted in the low-turbulence Cryogenic Ludwieg-Tube Gottingen on a two-dimensional flat-plate configuration designed for an essentially uniform pressure gradient on the model upper surface. The model was instrumented with a temperature-sensitive paint to measure globally and non-intrusively the surface temperature and thus the boundary-layer transition. A marked influence of a variation in the wall temperature ratio on transition was observed for all considered Mach numbers, being this effect more pronounced at lower Mach numbers. The measured transition locations were also correlated with the results of linear local stability analysis. Smaller disturbance amplification factors were found at transition for larger Mach numbers and, in most of the examined cases, for smaller wall temperature ratios and stronger flow acceleration.

The influence of suction on gap- and step-induced boundary-layer transition was investigated experimentally and systematically for large chord Reynolds numbers (up to \(16 \times 10^{6}\)) and various pressure gradients at Mach number 0.6. The experiments were conducted in the low-turbulence Cryogenic Ludwieg-Tube Gottingen with a two-dimensional wind-tunnel model. Transition is detected non-intrusively by means of temperature-sensitive paint. Forward-facing steps and spanwise gaps have caused transition to occur at a more upstream location as compared to a smooth configuration at the same test conditions. Applying suction through the gap, however, was shown to significantly delay transition and even overcompensate the adverse effect of steps and gaps. Further increasing suction rate shifts transition even more downstream until a certain value of suction is reached, above which there is effectively no change in transition location even for larger suction rates.

The influence of suction on step-induced boundary-layer transition has been experimentally investigated in the Cryogenic Ludwieg-Tube Goettingen at large chord Reynolds numbers (up to 16 · 10 6 ), Mach numbers from 0.35 to 0.77 and various streamwise pressure gradients by means of temperature-sensitive paint. Surface imperfections, implemented as combination of gap and forward-facing step, caused transition to occur at a location more upstream than in the case of a smooth surface (i.e. without gap and step). For this combination of imperfections, it was demonstrated for the first time in experiments that suction, achieved passively by exploiting the pressure difference between upper and lower side of the model, induced a movement of transition to a more downstream location than without suction, and in most cases even more downstream than on the smooth configuration at the same test conditions. Thus, the effect of suction was to even overcompensate the adverse effect of the combination of gap and forward-facing step on boundary-layer transition for the investigated test conditions.

The influence of surface bumps on boundary-layer transition was experimentally investigated in the present work. The experiments were conducted in a (quasi-) two-dimensional flow at low to high subsonic Mach numbers and chord Reynolds numbers up to 10 million in the low-turbulence Cryogenic Ludwieg-Tube Gottingen. Various streamwise pressure gradients relevant for natural laminar flow surfaces were examined. Quasi-two-dimensional bumps, with a sinusoidal shape in the streamwise direction, fixed length and three different heights, were installed on a two-dimensional flat-plate model. The model was equipped with temperature-sensitive paint for non-intrusive transition detection and with pressure taps for the measurement of the surface pressure distributions. Boundary-layer transition was shown to occur at a more upstream location with increasing bump height-to-length ratio. This was mainly due to the local adverse pressure gradient on the downstream side of the bump, which was particularly pronounced in the case of the bump with the largest height-to-length ratio, thereby inducing boundary-layer separation (as verified via oil-film visualizations). In the case of the bump with the smallest height-to-length ratio, bump-induced transition was found to be dependent on global pressure gradient, Mach number and Reynolds number; however, the influence of these parameters on transition induced by bumps with larger height-to-length ratios was significantly reduced. The sensitivity of boundary-layer transition to the effect of the bumps was shown to be more pronounced with stronger global flow acceleration and at smaller Mach numbers.

The phenomenon of “cloud voids”, i.e., elongated volumes inside a cloud that are devoid of droplets, was observed with laser sheet photography in clouds at a mountain-top station. Two experimental cases, similar in turbulence conditions yet with diverse droplet size distributions and cloud void prevalence, are reported. A theoretical explanation is proposed based on the study of heavy inertial sedimenting particles inside a Burgers vortex. A general conclusion regarding void appearance is drawn from theoretical analysis. Numerical simulations of polydisperse droplet motion with realistic vortex parameters and Mie scattering visual effects accounted for can explain the presence of voids with sizes similar to that of the observed ones. Clustering and segregation effects in a vortex tube are discussed for reasonable cloud conditions.

The application of Hybrid Laminar Flow Control (HLFC) by suction of the boundary layer has the potential to delay the location of laminar-turbulent transition and, hence, reduce the drag caused by skin friction. Up to now most of the research on HLFC has been performed at low Reynolds and Mach numbers. In order to investigate the technology at flow conditions relevant for today’s commercial flight additional work at higher Reynolds numbers is required. At high Reynolds numbers the major challenge is an appropriate scaling of the perforation used for sucking the boundary layer, leading to very small perforation diameters of only a few microns. In this paper, the applicability of HLFC on a two-dimensional wind tunnel profile is investigated in the cryogenic Pilot European Transonic Windtunnel (PETW). The natural laminar-turbulent boundary-layer transition is visualized with temperature-sensitive paint (TSP). The transition location obtained with an active HLFC system is compared to a reference case without HLFC for different angles-of-attack, Reynolds numbers, free stream Mach numbers and suction rates. Furthermore, the influence of the perforation diameter is also investigated. Results of an alternative approach, using a sintered material instead of a regular perforation for sucking the boundary layer, are also presented.

The phenomenon of cloud voids, i.e., elongated volumes inside a cloud that are devoid of droplets, was observed with laser sheet photography in clouds at a mountain-top station. Two experimental cases, similar in turbulence conditions yet with diverse droplet size distributions and cloud void prevalence, are reported. A theoretical explanation is proposed based on the study of heavy inertial sedimenting particles inside a Burgers vortex. A general conclusion regarding void appearance is drawn from theoretical analysis. Numerical simulations of polydisperse droplet motion with realistic vortex parameters and Mie scattering visual effects accounted for can explain the presence of voids with sizes similar to that of the observed ones. Preferential concentration and sorting effects in a vortex tube are discussed for reasonable cloud conditions.

The effect of geometric forward-facing steps on boundary layer transition was experimentally investigated at a high subsonic Mach number in a blow-down wind tunnel facility in Gottingen, Germany. Boundary layer transition was detected non- intrusively by means of the Temperature-Sensitive Paint (TSP) technique. Forward-facing steps of different height were installed on a spanwise invariant wind tunnel model. Streamwise pressure gradient and high chord Reynolds number were systematically varied and their effect on boundary layer transition was studied in the presence of forward-facing steps on the model surface. At all tested stability situations, the surface imperfections were shown to reduce the extent of the laminar region. Transition was observed to move gradually towards the step location with increasing step Reynolds numbers and increasing relative step height. For a given combination of step height and chord Reynolds number, more pronounced negative pressure gradients led to an increase in transition Reynolds number. The reduction in transition Reynolds number due to the effect of the surface imperfection was more marked at larger flow acceleration. The plots of the relative change in transition location as a function of the step Reynolds number and of the relative step height gave good correlation of the results. The correlations were found to be practically independent of the streamwise pressure gradient in the examined range. Criteria for the allowable tolerances on low- sweep Natural Laminar Flow surfaces can now be derived from the functional relations determined in this work.

The influence of Mach number (\(M = 0.35\)–0.65), chord Reynolds number (\(Re_c=6\times 10^6\) to \(10\times 10^6\)) and pressure gradient (\(dc_p/dx = -0.6\)–0.07 m\(^{-1}\)) on laminar-turbulent boundary layer transition was experimentally investigated in the Cryogenic Ludwieg-Tube Gottingen (DNW-KRG). For this investigation the existing two-dimensional wind tunnel model, PaLASTra, which offers a quasi-uniform streamwise pressure gradient, was modified in order to reduce the size of the flow separation at its trailing edge. The streamwise temperature distribution and the location of laminar-turbulent transition were measured by means of temperature-sensitive paint (TSP). It was found that the transition Reynolds number exhibits a linear dependence on the pressure gradient, characterized by the Hartree parameter, and that an increasing Mach number leads to a linear decrease of the transition Reynolds number. The latter effect is likely due to an increase of the total pressure turbulence level with Mach number in DNW-KRG. The measured pressure and temperature distributions served as input for boundary layer calculations and linear-stability analysis. N-factors were calculated according to compressible and incompressible stability theory. At zero pressure gradient a critical N-factor of approximately 9.5 and 9.0 was found for incompressible and compressible calculations, respectively.

Heat loads on spacecraft traveling at hypersonic speed are of major interest for their designers. Several tests using temperature-sensitive paints (TSP) have been carried out in long duration shock tunnels to determine these heat loads; generally paint layers were thin, so that certain assumptions could be invoked to enable a good estimate of the thermal parameter ρck (a material property) to be obtained—the value of this parameter is needed to determine heat loads from the TSP. Very few measurements have been carried out in impulse facilities [viz. shock tunnels such as the High Enthalpy Shock Tunnel Gottingen (HEG)], where test times are much shorter. Presented here are TSP temperature measurements and subsequently derived heat loads on a ramp model placed in a hypersonic flow in HEG (specific enthalpy h 0 = ~3.3 MJ kg−1, Mach number M = 7.4, temperature T ∞ = 277 K, density ρ ∞ = 11 g m−3). A number of fluorescence intensity images were acquired, from which, with the help of calibration data, temperature field data on the model surface were determined. From these the heat load into the surface was calculated, using an assumption of a 1D, semi-infinite heat transfer model. ρck for the paint was determined using an insitu calibration with a Medtherm coaxial thermocouple mounted on the model; Medtherm ρck is known. Finally presented are sources of various measurement uncertainties, arising from: (1) estimation of ρck; (2) intensity measurement in the chosen interrogation area; (3) paint time response.

Experiments were conducted at a subsonic Mach number in the Cryogenic Ludwieg-Tube Göttingen to study the influence of surface imperfections on boundary layer transition. Forward-facing steps of different height were installed on two spanwise invariant wind tunnel models and their effect on transition was examined at high chord Reynolds numbers and various streamwise pressure gradients. Transition, detected non-intrusively by means of the Temperature-Sensitive Paint technique, was found to gradually move upstream towards the step location with increasing step Reynolds numbers and relative step height. Larger flow acceleration had a favorable influence on the transition Reynolds number even in the presence of forward-facing steps, but also increased the sensitivity of boundary layer transition to the surface excrescences. The plots of the relative change in transition location as a function of step Reynolds number and relative step height gave good correlations of the results obtained with both wind tunnel models. The correlations were practically independent of the streamwise pressure gradient. Criteria for allowable tolerances on low-sweep Natural Laminar Flow surfaces can now be established from the functional relations determined in this work.

Cloud measurements are usually carried out with airborne campaigns, which are expensive and are limited by temporal duration and weather conditions. Ground based measurements at high-altitude research stations therefore play a complementary role in cloud study. Using the meteorological data (wind speed, direction, temperature, humidity, visibility, etc.) collected by the German Weather Service (DWD) from 2000 to 2012 and turbulence measurements recorded by multiple ultrasonic sensors (sampled at 10 Hz) in 2010, we show that the Umweltforschungsstation Schneefernerhaus (UFS) located just below the peak of Zugspitze in the German Alps, at a height of 2650 m, is a well-suited station for cloud-turbulence research. The wind at UFS is dominantly in the east–west direction and nearly horizontal. During the summer time (July and August) the UFS is immersed in warm clouds about 25% of the time. The clouds are either from convection originating in the valley in the east, or associated with synoptic-scale weather systems typically advected from the west. Air turbulence, as measured from the second and third order velocity structure functions that exhibit well-developed inertial ranges, possesses Taylor microscale Reynolds numbers up to 104, with the most probable value at ~ 3000. In spite of the complex topography, the turbulence appears to be nearly as isotropic as many laboratory flows when evaluated on the so called "Lumley-triangle".