The atmospheric dispersion of contaminants in the wake of a large urban structure is a challenging fluid mechanics problem of interest to the scientific and engineering communities. Magnetic Resonance Velocimetry (MRV) is a relatively new technique that leverages diagnostic equipment used primarily by the medical field to make 3D engineering measurements of flow and contaminant dispersal. SIERRA/Fuego, a computational fluid dynamics (CFD) code at Sandia National Labs is employed to make detailed comparisons to the dataset to evaluate the quantitative and qualitative accuracy of the model. The comparison exercise shows good comparison between model and experimental results, with the wake region downstream of the tall building presenting the most significant challenge to the quantitative accuracy of the model. Model uncertainties are assessed through parametric variations. Some observations are made in relation to the future utility of MDV and CFD, and some productive follow-on activities are suggested that can help mature the science of flow modeling and experimental testing.
Intense, dynamic radiant heat loads damage and ignite many common materials, but are outside the scope of typical fire studies. Explosive, directed-energy, and nuclear-weapon environments subject materials to this regime of extreme heating. The Solar Furnace at the National Solar Test Facility simulated this environment for an extensive experimental study on the response of many natural and engineered materials. Solar energy was focused onto a spot (∼10 cm2 area) in the center of the tested materials, generating an intense radiant load (∼100 kW m−2 –1000 kW m−2) for approximately 3 seconds. Using video photography, the response of the material to the extreme heat flux was carefully monitored. The initiation time of various events was monitored, including charring, pyrolysis, ignition, and melting. These ignition and damage thresholds are compared to historical ignition results predominantly for black, α-cellulose papers. Reexamination of the historical data indicates ignition behavior is predicted from simplified empirical models based on thermal diffusion. When normalized by the thickness and the thermal properties, ignition and damage thresholds exhibit comparable trends across a wide range of materials. This technique substantially reduces the complexity of the ignition problem, improving ignition models and experimental validation.
The surface topology of a solid subjected to destructive environments is often difficult to quantify. In thermal environments, the size and shape of the solid changes as it pyrolyzes, ablates, warps, or chars. Quantitative descriptions of such responses are valuable for data reporting and model validation. In this work, a three-dimensional scanner is evaluated for non-destructive material analysis. The scans spatially resolve the response of materials to a high-heat-flux environment. To account for the effect of distortion induced in thin materials, back-side scans of the sample are used to characterize the displacement of the bulk material. Data spanning the area of the sample, rather than using a net or average quantity, enhances the evaluation of the crater formed by the incident flux. The 3D reconstruction of the sample also provides the ability to perform volumetric calculations. The data obtained from this methodology may be useful for characterizing materials exposed to a variety of destructive environments.
Nuclear weapon airbursts can create extreme radiative heat fluxes for a short duration. The radiative heat transfer from the fireball can damage and ignite materials in a region that extends beyond the zone damaged by the blast wave itself. Directed energy weapons also create extreme radiative heat fluxes. These scenarios involve radiative fluxes much greater than the environments typically studied in flammability and ignition tests. Furthermore, the vast majority of controlled experiments designed to obtain material response and flammability data at high radiative fluxes have been performed at relatively small scales (order 10 cm2 area). A recent series of tests performed on the Solar Tower at the National Solar Thermal Test Facility exposed objects and materials to fluxes of 100 – 2,400 kW/m2 at a much larger scale (≈1 m2 area). This paper provides an overview of testing performed at the Solar Tower for a variety of materials including aluminum, fabric, and two types of plastics. Tests with meter-scale objects such as tires and chairs are also reported, highlighting some potential effects of geometry that are difficult to capture in small-scale tests. The aluminum sheet melted at the highest heat flux tested. At the same flux, the tire ignited but the flames were not sustained when the external heat flux was removed; the damage appeared to be limited to the outer portion of the tire, and internal pressure was maintained.