Noise Generation in Ventilation Systems by the Interaction of Airflow with Duct Discontinuities: Part 2 Take-offs

Earlier work on devising generalised noise prediction techniques for ventilation system elements based upon simple bend configurations has been extended to the study of the noise generated by branch take-offs. These configurations are more complex than simple bends as there is a divergence of the airflow. The simple semi-empirical technique proposed by Oldham and Ukpoho, based upon a modified version of the Nelson and Morfey equations, has been employed to collapse the experimental data. A good collapse of normalised data is seen for each component. Furthermore, a single collapsed data curve is proposed for each main/branch diameter ratio for all circular junctions. The resulting curves were found to be very similar for most configurations examined and to have a form similar to those of the bends considered in the earlier work. The general form is characterized by a region for which the Strouhal number is less than two over which the value of 20log[K(St)] falls at a rate of approximately 40 dB per decade. For Strouhal numbers greater than two the curve is approximately linear and falls with increasing Strouhal number at a rate of approximately 65 dB per decade. The results indicate that a method based upon pressure loss characteristics could form the basis of a technique for predicting the airflow-generated noise due to branch take-offs in circular ductwork. However, it is not proven that a single universal curve for the prediction of the airflow generated noise due to any duct component exists, but this work lends support to the theory that a range of component specific curves might exist and provide a means of accurately predicting airflow generated noise in ventilation systems.