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The measurement of ventilation rates is crucial in understanding buildings’ performances, but can be a rather complex task due to the time-dependency of wind and buoyancy forces, which are responsible for the pressure differences that induce air movement across the envelope. Thus, assessing air change rate through one-time measurements during brief periods of time may not be a reliable indicator. In this paper, the variability in the measurement of ventilation rates using the decay technique was evaluated. To that end, two compartments of a typical single-family detached dwelling were selected as a case study and 132 tests were performed, considering two different boundary conditions (door closed and door open). This work allowed the large variability of the results to be highlighted, as the coefficient of variation ranged from 20% to 64%. Wind speed had a key effect on the results, especially because during the measurements indoor–outdoor temperature differences were not so significant. The possibility of using occupant-generated carbon dioxide as tracer gas was also analyzed, but problems of cross-contamination were identified.

The increasing energy concerns across the European Union enhanced the importance of buildings ventilation and pointed out the reduction of energy for ventilation as one of the targets. The measurement of ventilation and leakage rates are therefore necessary to understand buildings performance, especially in mild climate countries where often dwellings ventilation is adventitious and highly influenced by user behaviour. In mild climate countries, natural ventilation is the most common strategy. The air movement is therefore a combined result of pressure differences produced by wind and buoyancy forces, which are both time-dependent. Under these circumstances, one-time measurements of the air change rate, made over short periods, may not be an accurate indicator. An experimental campaign for the evaluation of the air change rate in a Portuguese dwelling was conducted over a period of two months. During this period, tracer gas measurements were performed in two compartments: (R1) living room; and (R2) bedroom. In each, two different boundary conditions were tested: (i) door closed; and (ii) door opened. A total of 132 tests were carried out. The decay technique was used for the quantification of the air change rate. Simultaneously, indoor and outdoor conditions were continuously monitored. The results highlighted the importance of performing several measurements as some variability was found, especially for the open door scenarios. Moreover, the importance of wind speed was also confirmed as the main trigger mechanism in naturally ventilated buildings, especially in the mid-season period where the indoor/outdoor temperature difference is not significant. [ABSTRACT FROM AUTHOR]

The decay motion as well as the harmonic excited roll motion are established techniques to estimate roll damping for ships. This paper compares the advantages and disadvantages of both techniques and focuses on their applicability. Different analysis methods for both techniques to determine the nonlinear roll damping moment are investigated with the aim of developing an accurate estimation approach without additional filtering, curve fitting and offset manipulation of the recorded time series. Damping coefficients of both techniques are compared for available experiments of the benchmarking post panamax container ship model Duisburg Test Case (DTC). Reasons for deviations are investigated, and the importance of an accurate estimation of the current nonlinear hydrostatic roll moment will be shown. A method for the determination of the nonlinear hydrostatic moment during a harmonic excited roll motion test is introduced. Different approximations of roll damping based on series expansion are investigated. Limitations of the widely used linear-quadratic and linear-cubic function approaches are discussed based on the results. [ABSTRACT FROM AUTHOR]