Abstract
This article describes the development and demonstration of a non-intrusive method for the quantitative determination of speed of air movement along the ground and inside an isolated subsurface structure, a type of confined space. Natural ventilation occurs continuously and reduces risk to entrants from contact with a hazardous atmosphere. One of the most important parameters still undetermined was the speed of air movement during the process. Small puffs of artificial “smoke” were used to visualize air movement. Tracker, an open-source physics program, provided the capability to analyze this movement. Measurement of air speed requires access to individual frames in the video, capability to move forward and backward, and the means to manipulate the image to highlight the “smoke”. Background subtraction, control of brightness and contrast, and conversion of color to greyscale were essential for obtaining these measurements. Measurements for a single opening indicated that flow along the ground was borderline turbulent (Reynolds number ~3000) and in the opening and inside the airspace, within the bounds of laminar flow (Reynolds number <2250). Video obtained during this work showed behavior observable in laboratory studies of Helmholtz resonators. Results provide the basis for a larger study of the ventilation process to facilitate design improvements.
Highlights
The subsurface infrastructure contains many types of structures
Quantitative determination of airspeed inside subsurface structures accessed through manhole covers is one of the most important remaining gaps in the information base concerning ventilation induced by natural forces
This article describes the development and use of a potential simple and practical, non-obtrusive method for quantifying the average speed of air moving along the ground and inside an isolated subsurface structure
Summary
The subsurface infrastructure contains many types of structures. Many of these structures satisfy requirements for classification as confined spaces. Many people have died in confined spaces, mainly because of the development of atmospheric hazards [1]. Ventilation of these structures is an important subject for research and other enquiry to prevent injury and death to passersby and people who enter and work in these structures. Confined spaces are spaces in which people do not normally work and are not equipped with amenities for occupancy. Confined spaces become workspaces when opened for access. Access and egress often are difficult because of the size and location of the opening for entry and can inhibit the provision of first aid and other emergency response services
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