Michael Hargather

Retroreflective shadowgraph technique for large-scale flow visualization

A simple and robust retroreflective shadowgraph technique is presented for the visualization of refractive phenomena across a broad range of scales in space and time. Originally developed by Edgerton, it is improved here with techniques for producing coincident shadowgram illumination. The optical components required to construct a simple system are discussed, including the retroreflective screen material. The optical sensitivity of the system is explored for visualization of shock waves and turbulent eddies. The shadowgraph system is used here to visualize experiments performed in the laboratory, on a military test range, and in an open field.

Experimental Characterization and Material-Model Development for Microphase-Segregated Polyurea: An Overview

Numerous experimental investigations reported in the open literature over the past decade have clearly demonstrated that the use of polyurea external coatings and/or inner layers can substantially enhance both the blast resistance (the ability to withstand shock loading) and the ballistic performance (the ability to defeat various high-velocity projectiles such as bullets, fragments, shrapnel, etc. without penetration, excessive deflection or spalling) of buildings, vehicles, combat-helmets, etc. It is also well established that the observed high-performance of polyurea is closely related to its highly complex submicron scale phase-segregated microstructure and the associated microscale phenomena and processes (e.g., viscous energy dissipation at the internal phase boundaries). As higher and higher demands are placed on blast/ballistic survivability of the foregoing structures, a need for the use of the appropriate transient nonlinear dynamics computational analyses and the corresponding design-optimization methods has become ever apparent. A critical aspect of the tools used in these analyses and methods is the availability of an appropriate physically based, high-fidelity material model for polyurea. There are presently several public domain and highly diverse material models for polyurea. In the present work, an attempt is made to critically assess these models as well as the experimental methods and results used in the process of their formulation. Since these models are developed for use in the high-rate loading regime, they are employed in the present work, to generate the appropriate shock-Hugoniot relations. These relations are subsequently compared with their experimental counterparts in order to assess the fidelity of these models.

Recent Developments in Schlieren and Shadowgraphy

Schlieren and shadowgraph techniques are hundreds of years old, yet several important developments have occurred in the last decade, as summarized in this paper. Progress has been made in using the turbulent eddies of a refractive flow as “particle” tracers for seedless velocimetry. This approach will never supplant standard PIV, but “schlieren PIV” can be useful, for example, in cases where particles cannot be seeded in a turbulent flow under study. Background-oriented schlieren (BOS) has become very popular in just a few years. Given modern image-processing software for PIV or digital image correlation, a digital camera and a proper background, schlieren-like images of all sorts are easy to make without parabolic mirrors or even a knife-edge. “Rainbow schlieren” is the name for a quantitative schlieren instrument that uses color to make density, temperature, or species measurements in steady and unsteady planar or axisymmetric flows. Data acquisition and reduction are highly automated and the range of applications is very broad. Finally, shadowgraphy has not been forgotten: it is the simplest of all the optical flow visualization methods, but is often the best choice for imaging shock waves and turbulence. The addition of a retroreflective screen and a high-speed camera makes direct shadowgraphy a robust tool for the study of large-scale events in harsh environments.

Optical Diagnostics for Characterizing a Transitional Shear Layer over a Supersonic Cavity

C AVITY flowfields have been the subject of research since the 1950s, and still remain the subject of active research. The basic mechanisms governing cavity flows were first described by Rossiter [1], but a detailed understanding of the complete flow physics still does not exist. A wide variety of experimental investigations into cavity flowfields have been presented, including subsonic [2,3] and supersonic [4,5] freestream velocities. A detailed review of cavity research efforts was recently given by Cattafesta et al. [6]. Refractive imaging optical diagnostic techniques, such as schlieren and shadowgraphy [7], have been an important part of cavity experimental investigations. Rossiter used shadowgraph imaging in his original work to identify shock-wave structures and general motions [1]. Schlieren optics and photodiodes (schlieren deflectometry [7]) were used by Cattafesta et al. [2] and Garg and Cattafesta [8] to measure the density fluctuations and the frequency content at locations in a cavity shear layer. Zhuang et al. [4,9] have used schlieren images to supplement pressure and particle-imagevelocimetry (PIV) measurements over a supersonic cavity. A recent study by Alvi and Cattafesta has reviewed the range of flowvisualization techniques that have been used for cavity-flow-control experiments [10]. The present work applies high-speed digital shadowgraph, and focusing schlieren imaging to the study of a supersonic cavity flowfield. A range of analyses are presented, including new imageprocessing techniques for examining a transient flow, a frequency analysis similar to that described in other investigations [2,11], and schlieren image velocimetry (SIV) [12]. II. Experimental Methods

Background-oriented schlieren visualization of heating and ventilation flows: HVAC-BOS

There is an important need for simple methods to visualize and measure whole-field airflow patterns in the HVAC field. The background-oriented schlieren (BOS) method is presented here as an answer to this need—a simple and effective method for visualizing refractive HVAC flowfields in situ. The equipment required is simple, portable, inexpensive, and readily available, thus superseding some previous approaches, including lens-and-grid schlieren techniques that require large fixed installations. For BOS a custom random-dot background pattern is used, and imaging is done by a consumer-grade digital single-lens-reflex (SLR) camera. After covering the principles of BOS, examples are shown of visualized airjets and plumes from heating vents and registers, a space-heater, a teakettle, and a human thermal plume and cough. BOS images of candle plumes are also used to explore several different visualization approaches, equipment arrangements, and image color scales. While current results are purely-qualitative visualizations, quantitative temperature measurements can also be made in certain cases where the flow is approximately two dimensional. Finally, BOS lends itself well to certain HVAC chores, such as the diagnosis of commercial kitchen ventilation airflows.

Focusing-Schlieren PIV Measurements of a Supersonic Turbulent Boundary Layer

A focusing-schlieren optical system has been developed for performing velocity measurements in refractive turbulent flows using commercial particle image velocimetry (PIV) algorithms. Focusing-schlieren optics allows the visualization of refractive disturbances within a limited depth-of-focus, resulting in quasi-planar schlieren images. The schlieren “PIV” technique makes use of naturally-occurring refractive-turbulent eddies in a flow as PIV “particles” upon which velocimetry is performed. Current experiments are performed in a small supersonic wind tunnel to measure the Mach 3 turbulent boundary layer mean-velocity profile. Results from both focusing-schlieren PIV and shadowgraph PIV are compared to the velocity profile from a standard pitot-pressure survey. The natural intermittency of the outer part of the turbulent boundary layer plays a role in the schlieren PIV results, but useful measurements of the velocity profile can still be made. We also introduce an important improvement in schlieren “PIV”, the use of a pulsed LED light source in place of the twin pulsed lasers typically required for traditional PIV measurements. This comparatively-inexpensive white-light source eliminates the traditional problems of laser illumination in schlieren optical systems and improves the overall results.

Design of a High-Throughput Chemical Trace Detection Portal That Samples the Aerodynamic Wake of a Walking Person

The aerodynamic wake behind a walking person is first investigated using flow visualization and spectrophotometry in order to characterize the plume-to-wake transition and the decay of a trace body contaminant downstream of a walking human. The buoyant human thermal plume is shown to transition to an aerodynamic wake at a walking speed of 0.2 m/s. At higher walking speeds, the human wake is highly turbulent, resulting in an exponential decay of a passive scalar downstream of the walking human subject. From these results, a high-throughput chemical trace detection portal that uses the natural momentum of the wake to assist its collection is designed. This wake collection portal is shown to be capable of substantial improvement in security-checkpoint throughput over previous trace-detection-screening technology. To collect and sample the trace-bearing wake for a chemical signal, air flow rates nearly an order of magnitude larger than those of the previous collection technology must be accommodated. A high-flow-rate particle impactor was designed and tested to effectively collect the entire human aerodynamic wake. This impactor accommodates a flow rate of 750 L/s with a particle cutoff diameter of 5 μm. An iterative CFD process is described which streamlined the impactor geometry to reduce the total pressure drop across it. Experimental results are presented on the impactor collection efficiency. Preliminary experimental results show that trace chemicals can be detected at a useful level from a range of locations across the body when a human subject passes through the system wearing a chemical particle patch source.

Gram-range explosive blast scaling and associated materials response

Laboratory-scale gram-range explosive blast testing of materials is shown to be feasible. Blast loading from different explosive compounds is coupled to a witness plate through the air by way of a shock wave of known strength, measured optically. The resulting witness-plate deflection is also measured by a high-speed optical method. An attempt is made to relate the material response to blast loading parameters, especially impulse, by scaling arguments. More work is needed on this topic, and a discussion of future research directions is included. The promise of gram-range testing is to take on at least some of the burden now carried by expensive, dangerous, time-consuming full-scale explosive testing.

The Internal Aerodynamics of Cargo Containers for Trace Chemical Sampling and Detection

Millions of air- and sea-cargo containers enter into and are transported throughout the United States each year. The possibility that some might contain terrorist devices and the impracticability of opening and inspecting them all have become highly charged political issues. However, if the interiors of cargo containers could be quickly sampled for trace explosives without opening them, broad and rapid inspections could be conducted. This would enhance security and allow legitimate cargo to flow almost unimpeded through ports and terminals. Here, we present techniques for nonintrusively sampling cargo containers for trace explosive particles and vapors using external suction devices. The experimental results show the ability to successfully detect explosive contamination and the importance of the internal aerodynamics of the cargo containers. This is studied through flow visualization techniques to reveal the effects of “natural air vents,” container geometry, and packing configurations upon the sampling techniques investigated here. A discussion of optimal trace sampling strategies is given based on these results.

Integrated Impactor/Detector for a High-Throughput Explosive-Trace Detection Portal

The design of a very-high-flow-rate inertial particle impactor with integrated sampling for use in a walk-through explosive-trace detection portal is presented. This impactor is designed to collect particles of explosives from the aerodynamic wake of a walking subject, which requires a sampling capability of ~ 1m3/s and must reliably collect particles of ≥ 5-μm diameter. An impactor cross-section of 0.3 × 0.3 m was used, with three linear slot nozzles and corresponding impactor blades. The central impactor blade has a rotating impaction surface with an integrated heater for automated thermal desorption of the impacted material. An ion mobility spectrometer is used to interrogate the desorbed vapor for trace explosives detection. The process of sampling a subject and interrogating its aerodynamic wake for explosives requires about 8 s totally, allowing very-high throughput sampling for security-screening purposes.