Dynamic Evolution of a Transient Supersonic Trailing Jet Induced by a Strong Incident Shock Wave

The dynamic evolution of a highly underexpanded transient supersonic jet at the exit of a pulse detonation engine is investigated via high-resolution time-resolved schlieren and numerical simulations. Experimental evidence is provided for the presence of a second triple shock configuration along with a shocklet between the reflected shock and the slipstream, which has no analogue in a steady-state underexpanded jet. A pseudo-steady model is developed, which allows for the determination of the post-shock flow condition for a transient propagating oblique shock. This model is applied to the numerical simulations to reveal the mechanism leading to the formation of the second triple point. Accordingly, the formation of the triple point is initiated by the transient motion of the reflected shock, which is induced by the convection of the vortex ring. While the vortex ring embedded shock move essentially as a translating strong oblique shock, the reflected shock is rotating towards its steady state position. This results in a pressure discontinuity that must be resolved by the formation of a shocklet

Acoustic modes in a plenum downstream of a multitube pulse detonation combustor

The operation of a multitube pulse detonation combustor (PDC) evokes undesirable pressure fluctuations for a turbine downstream of the combustor. Pressure measurements in a downstream annular plenum reveal distinct pressure fluctuations. Although pressure peaks due to the shock waves emanating from the PDC tubes have been investigated in previous studies, pressure oscillations throughout the remaining cycle duration have received far less attention. Based on azimuthal and frequency mode decomposition of pressure signals at six circumferential positions, these oscillations are found to result from acoustic modes that are excited by PDC operation. This conclusion is verified by comparison with numerically calculated eigenmodes of the plenum. The impact of different operating parameters and the plenum outlet geometry on the acoustic oscillations are determined. The sequential firing of the PDC tubes at the lowest firing frequency in combination with a small blockage ratio was identified as the favorable configuration for the smallest acoustic fluctuations.DFG, 200291049, SFB 1029: TurbIn - Signifikante Wirkungsgradsteigerung durch gezielte, interagierende Verbrennungs- und Strömungsinstationaritäten in Gasturbine

Auslegung und Aufbau eines skalierten Modellwindkanals zur Untersuchung von strömungsbeeinflussenden Maßnahmen in der Vorkammer des DLR-Verdichter-Prüfstands M2VP

Im Rahmen akustischer Untersuchungen am Mehrstufen-Zweiwellen-Verdichter-Prüfstand (M2VP) des Deutschen Zentrums für Luft- und Raumfahrt (DLR) in Köln wurde gezeigt, dass Störgeräusche aufgrund großskaliger Turbulenzstrukturen im Einlaufkanal des Prüfstands auftreten. Diese führen z. B. dazu, dass die erste Blattfolgefrequenz akustisch dominant gemessen wird, obwohl diese aufgrund des Cut-off-Designs der Schaufeln bei Approach-Bedingungen nicht ausbreitungsfähig sein dürfte. Weiterhin weisen die zweiten und dritten Harmonischen der Blattfolgefrequenz sogenannte Sockel auf, welche auch auf die Störung der Zuströmung und ein erhöhtes Breitbandlärmniveau hinweisen. Die Interpretation der akustischen Messungen – insbesondere die Identifizierung der dominanten Schallquellen – wird dadurch signifikant erschwert und erlaubt nur sehr eingeschränkt die Validierung von Lärmvorhersage-Tools. Die Aufgabe dieser Arbeit beinhaltet, einen Modellwindkanal auszulegen und zu fertigen, an dem experimentelle Untersuchungen durchgeführt werden können. Ziel dieser Untersuchungen ist eine Verbesserung der Zuströmbedingungen am M2VP. Es gilt zunächst, die Strömungsverhältnisse am Einlauf und an der Vorkammer des Modellwindkanals zu vermessen und zu zeigen, dass die Ergebnisse auf den M2VP übertragbar sind. Hierbei wird der Einfluss des Ansaugturms und der darauffolgende Umlenkung der Strömung am Einlauf vernachlässigt. Aus der Analyse der Strömungsverhältnisse in der Vorkammer gilt es, Maßnahmen abzuleiten, die zu einer Reduzierung der Turbulenz und zu einer Gleichrichtung der Strömung führen. Mögliche Maßnahmen sind der Einsatz von Sieben, Diffusoren und deren Kombinationen. Hierzu sollen Voruntersuchungen und Machbarkeitsstudien durchgeführt werden. Diese werden exemplarisch anhand eines Siebes, eines Diffusors und deren Kombination umgesetzt und bewertet

Diffraction of shock waves through a non-quiescent medium

An investigation of shock diffraction through a non-quiescent background medium is presented using both experimental and numerical techniques. Unlike diffracting shocks in quiescent media, a spatial distortion of the shock front occurs, producing a region of constant shock angle. An example of this process arises in the exhaust from a pulse-detonation combustor. As the background velocity is increased, such as through the inclusion of a converging nozzle at the exhaust, the spatial distortion becomes more apparent. Numerical simulations using a compressible Euler solver demonstrate that the distortion is not due to the geometrical influence of the nozzle, but rather is a function of the magnitude of the background flow velocity. The distortion is studied using a modified geometrical shock dynamics formulation which includes the background flow and is validated against experiments. A simple model is presented to predict the shock distortion angle in the weak-shock limit. Finally, the axial decay behaviour of the shock is investigated and it is shown that the advection of the shock by the background flow delays the arrival of the head and tail of the expansion characteristic at the centreline. This leads to an increase in the rate of decay of the shock Mach number as the background flow velocity is increased

Uncertainty Quantification of Kiel Probes for RDC Applications

Kiel probes have the potential to be a versatile tool for determining stagnation pressure gain in rotating detonation combustors (RDCs). Although average pressure gain values determined with Kiel probes are comparable to those from thrust stand experiments, one can expect several interferences from the probe in unsteady trans- and supersonic flow. This work investigates the response of a Kiel probe to highly unsteady flow, similar to that in an RDC. The probe is subjected to an underexpanded starting jet behind an incident shock with Mach numbers of 1.6 to 2.7, emanating from a shock tube with a reservoir ratio of about 394. The incidence angle of the probe is varied between 0° and 90°, as is the probe’s axial location with respect to the tube’s exit plane. High-speed schlieren images reveal the Mach number of the moving shock wave and the structure of the detached bow shock at the Kiel head, which is similar to that of a bluff body. It is shown that the measured stagnation pressure signal is independent of inflow angle over a range of 45°, and that signal attenuation is caused by gas processing through the bow shock and viscous losses in the probe’s capillary. Moving the probe downstream of the shock tube’s exit plane causes a 7% reduction in the measured stagnation pressure, due to the expansion process. The frequency response of the Kiel probe to sinusoidal, small-amplitude pressure fluctuations is determined up to 5600 Hz, confirming that no unwanted Helmholtz resonance is present in the probe. A Berg-Tijdeman representation delivers amplitude ratio and phase lag of comparable magnitude