Spatially explicit characterization of boreal forest gap dynamics using multi-temporal lidar data

Understanding a disturbance regime such as gap dynamics requires that we study its spatial and temporal characteristics. However, it is still difficult to observe and measure canopy gaps extensively in both space and time using field measurements or bi-dimensional remote sensing images, particularly in open and patchy boreal forests. In this study, we investigated the feasibility of using small footprint lidar to map boreal canopy gaps of sizes ranging from a few square meters to several hectares. Two co-registered canopy height models (CHMs) of optimal resolution were created from lidar datasets acquired respectively in 1998 and 2003. Canopy gaps were automatically delineated using an object-based technique with an accuracy of 96%. Further, combinatorics was applied on the two CHMs and the delineated gaps to provide information on the area of old and new gaps, gap expansions, new random gap openings, gap closure due to lateral growth of adjacent vegetation or due to vertical growth of regeneration. The results obtained establish lidar as an excellent tool for rapidly acquiring detailed and spatially extensive short-term dynamics of canopy gaps.     

Theoretical Prediction of Pressure and Temperature of an Aluminized High Explosive in Underwater Explosion

The blast wave propagation in underwater explosion was studied. The shock propagation in water medium was different from that in air. The blast effect in water lasted longer and offered resistance to the expansion of hot gases and release of energy. A theoretical analysis of the expansion of blast wave in water was carried out and numerical results for pressures and temperatures were obtained as functions of distance and time by analytically solving the governing equations. The initial peak pressures of blast waves, which were required for theoretical analysis were calculated using the blast wave theory. Underwater blasts with different weights (0.045, 0.5, and 1.0 kg) of the aluminized high explosive HBX‐3 were conducted to record pressure as a function of distance and time from the blast point. Theoretical results were compared with experimental data and empirical data for HBX‐3 from literature. Since the measurement of pressure and temperature at close proximity of point of detonation is difficult, theoretical modeling of underwater blast is of significant importance.     

Nonbuckling Vertical Four‐Point Bend Setup for Measuring Edge Strength of Thin Glass

The four‐point bend test is widely used for testing the edge strength of glass specimens. Vertical four‐point bend (V4PB) setup involves orienting the specimen in a vertical fashion. However, thin specimens tend to buckle when using V4PB setup. In this study, a finite element model was built to simulate the buckling that occurs during V4PB testing. To validate this model, experiments were conducted on the existing setup. Upon validation, the finite element model was used to propose an improved setup. Then, experiments were carried out on the improved setup to verify that it worked as intended. The primary advantage of the proposed setup is that the standard ASTM [ASTM (2007), C158–02] four‐point bending formula can continue to be used to calculate the stress from the applied force.