Internal Pressure Modelling for Low-Rise Buildings in Tornadic Winds

Internal pressures play a large role in the failure of wood-frame houses as the loss of the entire roof section becomes much more likely once the envelope of the building has been breached. Many studies have used internal pressure modelling to simulate internal pressures in structures from atmospheric boundary layer (ABL) winds, however, relatively little work has been done on this subject using tornadic winds. The objective of this study is to explore internal pressure modelling issues for tornadoes. The first part of the study uses a computational internal pressure model to simulate tornadic internal pressures of a low-rise structure; the second part uses the same model to estimate failure wind speeds of a flexible garage door, one of the critical failure modes of these structures. The internal pressure model is able to reasonably simulate measured internal pressures in tornadic winds, although not quite as well as in ABL winds. The modelled internal pressure coefficients are mostly within 0.1 of measured internal pressure coefficients, which is similar to uncertainty bounds. When comparing ABL and tornadic building pressures, some differences are found in the mean pressures at oblique directions and the pressure distributions for normal wind directions. An analysis of the spectra of the theoretical model equation terms reveals that a lack of internal volume scaling in the tornadic tests also contributes to the differences from ABL tests. The same theoretical model also shows that net loads on garage doors are typically reduced to 34-46% of the external pressure applied from the wind due to the internal pressure developed in the garage from the fluctuating opening size during loading. When these results are combined with experimental net pressures of garage door failures, the resulting range of expected failure wind speeds are 130-265 km/h

NDM-525: EFFECTS OF TORNADO WIND SPEEDS ON CONCRETE ROAD BARRIERS

Wind speeds can be difficult to measure during tornadoes due to their destructive nature. They pose a significant threat to lives and infrastructure in many parts of Canada and the U.S. The Enhanced-Fujita scale focuses on estimating these wind speeds by observing damage to different types of buildings, but significantly less research has been performed on the damage of other structures. Learning more about the effects of high wind speeds on these structures will help improve the ease and accuracy of future tornado classification. A wind tunnel study was performed at the Boundary Layer Wind Tunnel Laboratory of Western University. The study focusses on estimating the wind speeds that cause overturning in a standard 32” concrete “Jersey” barrier. On April 27, 2014, an EF4 Tornado struck Mayflower, Arkansas, and among the damage, several of these concrete barriers were blown over during the storm. The goal of this study was to find the overturning wind velocity and compare it to other damage in this event. This study was performed by placing a 1:8 scale-model of these barriers in a wind tunnel at a variety of orientations and wind speeds. Through analysis, it was determined that an instantaneous wind velocity of 4.55 to 4.85 m/s would cause overturning. These values correspond to an instantaneous wind speed of 340-360 km/h at full scale. It was estimated that the 3-second gust (used for EF rating) was 300-320 km/h, which sits at the top of the 267-322 km/h classification range for an EF4 tornado

Wind Speed Estimates for Garage Door Failures in Tornadoes

Severe wind events, such as tornadoes, pose a significant threat to lives and infrastructure in many locations around the world. Residential buildings are the structures most affected by these events, since they are widespread and often not designed to withstand severe loading. For the wood-frame, low-rise houses typical of North America, once the envelope of the building has been breached, such as through the failure of a garage door, the loss of the entire roof structure becomes much more likely. One of the issues with garage doors is their flexibility; as they begin to deflect under wind load, relatively large openings allow air flow into the internal volume. As a result of these positive pressures on the garage door, positive pressures are transferred into the internal volume, subsequently reducing the net wind load on the door. The objectives of this study are to determine failure net pressures of garage doors through experimental testing, and to combine those results with internal pressure models including the effects of garage door flexibility in order to estimate the failure wind speeds of garage doors. Six garage doors of various types are tested, and the failure wind speeds acquired through the internal pressure model are compared to the Enhanced Fujita Scale. Experimental testing found the failure net pressures of the garage doors to be between 0.42 and 1.75 kPa. With the internal pressure model showing that the net load on the garage doors is typically reduced to 34–46% of the external pressure, the resulting range of expected failure wind speeds obtained was 130–265 km/h. This range is found to encompass and exceed the expected failure wind speeds in the EF-Scale of 130–185 km/h, which would only be applicable for the weaker range of garage doors

Plasma Science in Planetary Entry

Spacecraft entering a planetary atmosphere dissipate a great deal of energy into the surrounding gas. In the frame of reference of the vehicle, the atmospheric gas suddenly decelerates from hypersonic (Mach ~5-50) to subsonic velocities. The kinetic energy of the gas is rapidly converted to thermal and chemical energy, forming a bow shock behind which a plasma with energies on the order of one electron volt (eV) is produced. The resulting shock layer relaxes from strong thermal non-equilibrium that is translationally hot but internally cold and un-ionized toward a thermochemically equilibrated plasma over a distance of a few centimeters. Composition is dependent upon the planetary atmosphere Air for Earth, CO2/N2 for Mars and Venus, N2/CH4 for Titan and H2/He/CH4 for Saturn, Neptune and Jupiter. Typical velocities of entry may range from 3-7 km/s (4-25 MJ/kg) for Titan/Mars, 8-14 km/s (30-100 MJ/kg) for Earth/Venus, and 25-40 km/s (300-800 MJ/kg) for outer planets. The equilibrium plasmas produced from these conditions are highly dissociated (up to and above 99%) and ionized (0.1- 15%), with temperatures from 7,000-15,000K and pressures from 0.1-1.0 bar. Understanding the behavior of these plasmas the way in which they approach equilibrium, how they radiate, and how they interact with materials is an active area of research necessitated by requirements to predict and test the performance of thermal protection systems (TPS) that enable spacecraft to deliver scientific instruments, and people, to foreign worlds and back to Earth. The endeavor is a multi-physics problem, with key processes highlighted in Fig. 1. This white paper describes the current state of the art in simulating shock layer plasmas both computationally and in ground test facilities. Gaps requiring further research and development are identified

Physics-Based Modeling of Meteor Entry and Breakup

A new research effort at NASA Ames Research Center has been initiated in Planetary Defense, which integrates the disciplines of planetary science, atmospheric entry physics, and physics-based risk assessment. This paper describes work within the new program and is focused on meteor entry and breakup. Over the last six decades significant effort was expended in the US and in Europe to understand meteor entry including ablation, fragmentation and airburst (if any) for various types of meteors ranging from stony to iron spectral types. These efforts have produced primarily empirical mathematical models based on observations. Weaknesses of these models, apart from their empiricism, are reliance on idealized shapes (spheres, cylinders, etc.) and simplified models for thermal response of meteoritic materials to aerodynamic and radiative heating. Furthermore, the fragmentation and energy release of meteors (airburst) is poorly understood. On the other hand, flight of human-made atmospheric entry capsules is well understood. The capsules and their requisite heatshields are designed and margined to survive entry. However, the highest speed Earth entry for capsules is less than 13 km/s (Stardust). Furthermore, Earth entry capsules have never exceeded diameters of 5 m, nor have their peak aerothermal environments exceeded 0.3 atm and 1 kW/cm2. The aims of the current work are: (i) to define the aerothermal environments for objects with entry velocities from 13 to greater than 20 km/s; (ii) to explore various hypotheses of fragmentation and airburst of stony meteors in the near term; (iii) to explore the possibility of performing relevant ground-based tests to verify candidate hypotheses; and (iv) to quantify the energy released in airbursts. The results of the new simulations will be used to anchor said risk assessment analyses. With these aims in mind, state-of-the-art entry capsule design tools are being extended for meteor entries. We describe: (i) applications of current simulation tools to spherical geometries of diameters ranging from 1 to 100 m for an entry velocity of 20 km/s and stagnation pressures ranging from 1 to 100 atm; (ii) the influence of shape and departure of heating environment predictions from those for a simple spherical geometry; (iii) assessment of thermal response models for silica subject to intense radiation; and (iv) results for porosity-driven gross fragmentation of meteors, idealized as a collection of smaller objects. Lessons learned from these simulations will be used to help understand the Chelyabinsk meteor entry up to its first point of fragmentation