Validation of airflow measurement in ducts using Laser Doppler Anemometry and Computational Fluid Dynamics modelling

The problem of airflow measurement is of interest to Building Services Engineers to allow for the effective commissioning and validation of predictive procedures. The velocity distribution in a square section duct (400 mm x 400 mm) was investigated using a Laser Doppler Anemometer (LDA) to determine the velocity distribution and volumetric flow rate in the system, and to compare it with CFD and theoretical predictions (for both square and circular sections). The procedure has revealed a number of practical issues involved in the measurement of such air flows involving an LDA, including boundary flow measurement issues, and the consistency of results. A standard Computational Fluid Dynamics (CFD) package was also used to model the same flow regime, and agreement was obtained with the LDA results for a range of flow rates. Of particular interest was the detailed distribution of modelled and measured velocities across the duct, and the ways in which these compared with the commonly assumed one-seventh power law relationship for turbulent flows. The detailed nature of the observations made allowed investigation of the suitability of power laws for the circular case, and enabled assessment of whether an alternative exponent or method of predicting such flows would be more appropriate in air flow modelling. The study shows the comparison of velocity distribution in a square duct and theoretical similarly sized circular duct.