Imaging diagnostics that utilize coherent light, such as digital in-line holography, are important for object sizing and tracking applications. However, in explosive, supersonic, or hypersonic environments, gas-phase shocks impart imaging distortions that obscure internal objects. To circumvent this problem, some research groups have conducted experiments in vacuum, which inherently alters the physical behavior. Other groups have utilized single-shot flash x-ray or high-speed synchrotron x-ray sources to image through shock-waves. In this work, we combine digital in-line holography with a phase conjugate mirror to reduce the phase distortions caused by shock-waves. The technique operates by first passing coherent light through the shock-wave phase-distortion and then a phase-conjugate mirror. The phase-conjugate mirror is generated by a four-wave mixing process to produce a return beam that has the exact opposite phase-delay as the forward beam. Therefore, by passing the return beam back through the phase-distortion, the phase delays picked up during the initial pass are canceled, thereby producing improved coherent imaging. In this work, we implement phase conjugate digital in-line holography (PCDIH) for the first time with a nanosecond pulse-burst laser and ultra-high-speed cameras. This technique enables accurate measurement of the three-dimensional position and velocity of objects through shock-wave distortions at video rates up to 5 MHz. This technology is applied to improve three-dimensional imaging in a variety of environments from imaging supersonic shock-waves through turbulence, sizing objects through laser-spark plasma-generated shock-waves, and tracking explosively generated hypersonic fragments. Theoretical foundations and additional capabilities of this technique are also discussed.
Epoxies and resins can require careful temperature sensing and control in order to monitor and prevent degradation. To sense the temperature inside a mold, it is desirable to utilize a small, wireless sensing element. In this paper, we describe a new architecture for wireless temperature sensing and closed-loop temperature control of exothermic polymers. This architecture is the first to utilize magnetic field estimates of the temperature of permanent magnets within a temperature feedback control loop. We further improve performance and applicability by demonstrating sensing performance at relevant temperatures, incorporating a cure estimator, and implementing a nonlinear temperature controller. This novel architecture enables unique experimental results featuring closed-loop control of an exothermic resin without any physical connection to the inside of the mold. In this paper we describe each of the unique features of this approach including magnetic field-based temperature sensing, Extended Kalman Filtering (EKF) for cure state estimation, and nonlinear feedback control over time-varying temperature trajectories. We use experimental results to demonstrate how low-cost permanent magnets can provide wireless temperature sensing up to ~90°C. In addition, we use a polymer curecontrol test-bed to illustrate how internal temperature sensing can provide improved temperature control over both short and long time-scales. In conclusion, this wireless temperature sensing and control architecture holds value for a range of manufacturing applications.
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.
Combustion of aluminum droplets in solid rocket propellants is studied using laser diagnostic techniques. The time-resolved droplet velocity, temperature, and size are measured using high speed digital in-line holography and imaging pyrometry at 20 kHz.
Aluminized ammonium perchlorate composite propellants can form large molten agglomerated particles that may result in poor combustion performance, slag accumulation, and increased two-phase flow losses. Quantifying agglomerate size distributions are needed to gain an understanding of agglomeration dynamics and ultimately design new propellants for improved performance. Due to complexities of the reacting multiphase environment, agglomerate size diagnostics are difficult and measurement accuracies are poorly understood. To address this, the current work compares three agglomerate sizing techniques applied to two propellant formulations. Particle collection on a quench plate and backlit videography are two relatively common techniques, whereas digital inline holography is an emerging alternative for three-dimensional measurements. Atmospheric pressure combustion results show that all three techniques are able to capture the qualitative trends; however, significant differences exist in the quantitative size distributions and mean diameters. For digital inline holography, methods are proposed that combine temporally resolved high-speed recording with lower-speed but higher spatial resolution measurements to correct for size- velocity correlation biases while extending the measurable size dynamic range. The results from this work provide new guidance for improved agglomerate size measurements along with statistically resolved datasets for validation of agglomerate models.
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.
A three year LDRD was undertaken to look at the feasibility of using magnetic sensing to determine flows within sealed vessels at high temperatures and pressures. Uniqueness proofs were developed for tracking of single magnetic particles with multiple sensors. Experiments were shown to be able to track up to 3 dipole particles undergoing rigid-body rotational motion. Temperature was wirelessly monitored using magnetic particles in static and predictable motions. Finally high-speed vibrational motion was tracked using magnetic particles. Ideas for future work include using small particles for measuring vorticity and better calibration methods for tracking multiple particles.
Progress toward quantitative measurements and simulations of 3D, temporally resolved aerodynamic induced liquid atomization is reported. Columns of water and galinstan (liquid metal at room temperature) are subjected to a step change in relative gas velocity within a shock tube. Breakup morphologies are shown to closely resemble previous observations of spherical drops. The 3D position, size, and velocity of secondary fragments are quantified by a high-speed digital inline holography (DIH) system developed for this measurement campaign. For the first time, breakup dynamics are temporally resolved at 100 kHz close to the atomization zone where secondary drops are highly non-spherical. Experimental results are compared to interface capturing simulations using a combined level set moment of fluid approach (CLSMOF). Initial simulation results show good agreement with observed breakup morphologies and rates of deformation.
Remote temperature sensing is essential for applications in enclosed vessels, where feedthroughs or optical access points are not possible. A unique sensing method for measuring the temperature of multiple closely spaced points is proposed using permanent magnets and several three-axis magnetic field sensors. The magnetic field theory for multiple magnets is discussed and a solution technique is presented. Experimental calibration procedures, solution inversion considerations, and methods for optimizing the magnet orientations are described in order to obtain low-noise temperature estimates. The experimental setup and the properties of permanent magnets are shown. Finally, experiments were conducted to determine the temperature of nine magnets in different configurations over a temperature range of 5 °C to 60 °C and for a sensor-to-magnet distance of up to 35 mm. To show the possible applications of this sensing system for measuring temperatures through metal walls, additional experiments were conducted inside an opaque 304 stainless steel cylinder.