Session E30: Compressible Flow: Internal Shock-Boundary Layer Interaction

Shock train unsteadiness induced by separation bubble instabilities

A shock train is a highly three-dimensional system of shock and compression waves that gradually decelerates a supersonic flow in a duct and is typically found in the isolator section of high-speed air breathing engines. These fluid systems exhibit what we term inherent unsteadiness, which are self-excited fluctuations of the shock train system about its time-average position even with nominally constant inflow and outflow conditions. We have found that the instabilities of the separation bubbles within the shock train system contribute to the shock unsteadiness. The existence of boundary layer separation along the shock train is generally an accepted or assumed feature of shock trains. However, its properties, such as the point of separation, its length and thickness, are not well defined from works in the literature. Here, we present two-component particle image velocimetry measurements to examine the separation bubble characteristics and determine the physical structure of the perturbations that the separation bubble creates.   

Self-Excited Shock Train Dynamics in a Mach 2 Isolator

A shock train is the complex system of shock waves that forms in a supersonic ducted flow when the back pressure is raised, and it is typically found in the isolator of air-breathing, high-speed systems. Its formation is due to a balance of the inviscid action of a system of shocks in the core of the flow and the viscous effects at walls. Although the typical description and understanding of shock trains is limited to its steady state behavior, a shock train exhibits significant dynamics, most of which are self-excited, even under nominally constant inflow and outflow conditions. Here we evaluate some of the dynamical properties of a shock train generated in a Mach 2.0 ducted flow. Cross-spectral analysis of pressure and shock position fluctuations are used to identify a complex, frequency dependent system of perturbations that affects the unsteady motion of the shock train. Specifically, we have identified two paths of propagation of perturbations that are associated with two different sources, one associated with the regions of separated flow and one external to the shock train, that affect the steadiness of the shock train, thus partially explaining the observed shock train inherent unsteadiness.   

Experimental study of explosively-driven shock wave propagation in scaled two-dimensional geometries

An experimental fixture was developed to study explosively-driven shock wave propagation in two-dimensional geometries. Shock waves are produced using an electric spark gap on a detonator header which is driven by a FS-17 fire set. The spark produces a shock wave, which propagates through the fixture. The fixture itself is made from laser-cut acrylic sheets which are cut to represent varied geometries for shock reflection, diffraction, and complex interactions. This fixture is placed within a schlieren imaging system and high-speed images of explosive events are recorded at various frame rates. The shock wave propagation is quantified in terms of shock speed throughout the models. Piezoelectric pressure gages are used to measure static and reflected pressures at various locations in the geometries to supplement shock wave measurements. Additional measurements of product gas motion and turbulent mixing are presented.   

Shock Wave Propagation in Small Scale Circular Channels

Shock wave propagation through circular channel with inner diameter in the order of 1 and 2 mm at different wall temperature is studied using two fast-response pressure transducers. The shock wave is generated by a modified Split-Hopkinson Pressure Bar shock tube and introduced into the channel. The shock wave propagation speed near the inlet of the channel is about $M \sim 1.2$, and quickly decelerates inside the channel. Pressure profile measured in the channel indicated the sharp increase followed by gradual decrease with fluctuations and several small peaks. Values of peak pressure increase with channel wall temperature, but the inlet shock wave propagation speed has no significant impact. On the other hand, the intervals between peak pressures decrease with increased shock tube pressure while the wall temperature has marginal impact. The inner diameter of the channel also affects the wave propagation speed due to the difference in dissipation.   

Investigation of Dalton's and Amagat's laws for gas mixtures with shock propagation

Dalton’s and Amagat’s laws are two well-known thermodynamic models describing gas mixtures. Our current research is focused on determining the suitability of these models in predicting effects of shock propagation through gas mixtures. Experiments are conducted at the Shock Tube Facility at the University of New Mexico (UNM). The gas mixture used in these experiments consists of approximately 50\% sulfur hexafluoride (SF6) and 50\% helium (He) by mole. Fast response pressure transducers are used to obtain pressure readings both before and after the shock wave; these data are then used to determine the velocity of the shock wave. Temperature readings are obtained using an ultra-fast mercury cadmium telluride (MCT) infrared (IR) detector, with a response time on the order of nanoseconds. Coupled with a stabilized broadband infrared light source (operating at 1500 K), the detector provides pre- and post-shock line-of-sight readings of average temperature within the shock tube, which are used to determine the speed of sound in the gas mixture. Paired with the velocity of the shock wave, this information allows us to determine the Mach number. These experimental results are compared with theoretical predictions of Dalton’s and Amagat’s laws to determine which one is more suitable.   

Experimental Investigation of Reynolds Number Effects on Test Quality in a Hypersonic Expansion Tube

Reynolds number effects on test time and the temporal and spatial flow quality in a hypersonic expansion tube are explored using high-speed pressure, infrared optical, and Schlieren imaging measurements. Boundary layer models for shock tube flows are fairly well established to assist in the determination of test time and flow dimensions at typical high enthalpy test conditions. However, the application of these models needs to be more fully explored due to the unsteady expansion of turbulent boundary layers and contact regions separating dissimilar gasses present in expansion tube flows. Additionally, expansion tubes rely on the development of a steady jet with a large enough core-flow region at the exit of the acceleration tube to create a constant velocity region inside of the test section. High-speed measurements of pressure and Mach number at several locations within the expansion tube allow for the determination of an experimental \textit{x-t} diagram. The comparison of the experimentally determined \textit{x-t} diagram to theoretical highlights the Reynolds number dependent effects on expansion tube. Additionally, spatially resolved measurements of the Reynolds number dependent, steady core-flow in the expansion tube viewing section are shown.