A new capability has been added to study shock-particle interactions in the Sandia High-Temperature Shock Tube (HST). The apparatus to do so featured a high-speed pneumatic actuator with high-pressure engineered seals. Like previous studies in a lower-strength facility, the particle curtain was comprised of 100-micron glass spheres at an initial volume fraction of approximately 20%. A shock-particle interaction was investigated using 210 kHz Schlieren imaging where the incident shock Mach number was 3.3. The initially uniform curtain was distorted by recoil in the HST. Nevertheless, the interaction dynamics were observed to be qualitatively similar to those in previous studies. Future efforts will work to decouple the recoil from the curtain formation and push the interaction towards stronger shocks.
An experimental characterization of the flow environment for the Sandia Axisymmetric Transonic Hump is presented. This is an axisymmetric model with a circular hump tested at a transonic Mach number, similar to the classic Bachalo-Johnson configuration. The flow is turbulent approaching the hump and becomes locally supersonic at the apex. This leads to a shock-wave/boundary-layer interaction, an unsteady separation bubble, and flow reattachment downstream. The characterization focuses on the quantities required to set proper boundary conditions for computational efforts described in the companion paper, including: 1) stagnation and test section pressure and temperature; 2) turbulence intensity; and 3) tunnel wall boundary layer profiles. Model characterization upstream of the hump includes: 1) surface shear stress; and 2) boundary layer profiles. Note: Numerical values characterizing the experiment have been redacted from this version of the paper. Model geometry and boundary conditions will be withheld until the official start of the Validation Challenge, at which time a revised version of this paper will become available. Data surrounding the hump are considered final results and will be withheld until completion of the Validation Challenge.
A new wind tunnel experiment is underway to provide a comprehensive CFD validation dataset of an unsteady, transonic flow. The experiment is based on the work of Bachalo and Johnson; an axisymmetric model with a spherical hump is tested at a transonic Mach number. The flow is turbulent approaching the hump and becomes locally supersonic at the apex. This leads to a shock-wave/boundary-layer interaction, an unsteady separation bubble, and flow reattachment downstream. A suite of diagnostics characterizes the flow: oil-flow surface visualization for shock and reattachment locations, particle image velocimetry for mean flow and turbulence properties, fast pressure-sensitive paint for model pressure distributions and unsteadiness, high-speed Schlieren for shock position and motion, and oil-film interferometry for surface shear stress. This will provide a new level of detail for validation studies; therefore, a blind comparison, or ‘CFD Challenge’ is proposed to the community. Participants are to be provided the geometry, incoming boundary layer, and boundary conditions, and are free to simulate with their method of choice and submit their results. A blind comparison will be made to the new experimental data, with the goal of evaluating the state of various CFD methods for use in unsteady, transonic flows.
Conventional particle image velocimetry (PIV) configurations require a minimum of two optical access ports, inherently restricting the technique to a limited class of flows. Here, the development and application of a novel method of backscattered time-gated PIV requiring a single-optical-access port is described along with preliminary results. The light backscattered from a seeded flow is imaged over a narrow optical depth selected by an optical Kerr effect (OKE) time gate. The picosecond duration of the OKE time gate essentially replicates the width of the laser sheet of conventional PIV by limiting detected photons to a narrow time-of-flight within the flow. Thus, scattering noise from outside the measurement volume is eliminated. This PIV via the optical time-of-flight sectioning technique can be useful in systems with limited optical access and in flows near walls or other scattering surfaces.
Backscatter Particle Image Velocimetry via Optical Time-of-flight Sectioning (PIVOTS) is a novel method of performing PIV in situations where conventional PIV presents difficulties. The PIVOTS technique is introduced along with recent applications and results.
Simultaneous pressure sensitive paint (PSP) and stereo digital image correlation (DIC) measurements on a jointed beam structure are presented. Tests are conducted in a shock tube, providing an impulsive starting condition followed by approximately uniform high-speed flow conditions for 5.0 msec. The unsteady pressure loading generated by shock waves and vortex shedding results in the excitation of various structural modes in the beam. The combined data characterizes the structural loading input (pressure) and the resulting structural behavior output (deformation). Time-series filtering is used to remove external bias errors such as shock tube motion, and proper orthogonal decomposition (POD) is used to extract mode shapes from the deformation data. This demonstrates the utility of using fast-response PSP together with stereo digital image correlation (DIC), which provides a valuable capability for validating structural dynamics simulations.
The spanwise variation of resonance dynamics in the Mach 0.94 flow over a finite-span cavity was explored using stereoscopic time-resolved particle image velocimetry (TR-PIV) and time-resolved pressure sensitive paint (TR-PSP). The TR-PSP data were obtained along the cavity floor, whereas the TR-PIV measurements were made in a planform plane just above the cavity lip line. The pressure data showed relatively coherent distributions across the span. In contrast, the PIV showed a significant variation in resonance dynamics to occur across the span in the plane above the cavity. A substantial influence of the sidewalls appears to stem from spillage vortices. At the first cavity mode frequency, streamwise velocity fluctuations were several times higher near the sidewalls in comparison to the centerline values. Importantly, PSDs of streamwise velocity in the region of the spillage vortices showed a large peak to occur at mode one, indicating velocity fluctuations in these regions can have a preferred frequency. The resonance fluctuations in the velocity fields at modes two and three demonstrated a complex spatial dependence that varied with spanwise location.