Andrea Cattanei

Further Study of the Test Configuration for Axial Flow Fans Noise Measurements

The present paper concerns the effect of the test configuration on the noise generated by axial flow fans employed for automotive cooling systems. Different test configurations may be employed depending on the design phase. Discrepancies are often observed between the results of different tests, resulting in difficulties in transferring and generalizing the measured SPL. In the present case the effect of the mounting system (free-discharge wooden panel versus test plenum with zero and design downstream pressure) on the irregular growth of the tonal noise during speed ramps is considered. The spectral decomposition method applied to the measured SPL spectra has been used to obtain the propagation function which has been compared with the scaled SPL at the blade passage harmonics. The comparison shows that the fluctuations of the SPL during the speed ramps depend on the acoustics of the surrounding environment and not on variations in the incoming flow field.

Experimental study of the effect of the rotor-stator gap variation on the tonal noise generated by low-speed axial fans

In the present paper the effect of the rotor stator axial gap on tonal noise generated by axial flow fans employed for automotive cooling systems is studied. A fan equipped with either a 18 vanes stator or a 3 struts one, positioned at several axial gaps, is tested in a hemianechoic chamber during rotational speed ramps. To analyze the acoustic pressure properties three different quantities have been employed: the velocity scaled SPL at the BPF harmonics, the phase of the acoustic pressure at the BPF harmonics and the propagation function obtained by means of the spectral decomposition. The useful properties of these quantities and their joint use are first analyzed. Then they are employed for two purposes: investigating propagation effects and source strength fluctuations and identifying contributions to tonal noise due to random sources (large scale inlet turbulent structures) and due to deterministic sources (aerodynamic rotor-stator interaction). The results of the study show that the stator causes unexpected propagation effects which affect the measured SPL. These effects depend on the axial gap and on the considered BPF harmonic and they also affect the feasibility of a SPL velocity scaling.

An Experimental Study of the Effects of the Test Configuration on the Noise Generated by Axial Flow Fans

The present paper concerns the effect of the test configuration on the noise generated by axial flow fans employed for automotive cooling systems. During the development of a fan, a number of noise tests are usually taken on different configurations, whose characteristics depend on the design step. Often, significant discrepancies are observed between the results of different tests, resulting in difficulties in transferring and generalizing the measured SPL level spectra. Such differences regard both the broad band noise and the tonal one. In the present paper the effects of the following aspects are considered: the rotor balancing method, the geometrical tolerances, the rotor mounting system on simplified configurations (with and without downstream stator) and the production configuration (a stationary plastic shroud) without heat exchangers. The balancing method affects the high frequency part of the SPL spectrum, due to the broad band noise, while the mounting system may affect the OASPL and the low frequency part of the SPL spectrum, due to the tonal noise. The geometrical tolerances do not affect the generated noise.

Psychoacoustic Optimization of the Spacing of Propellers, Helicopter Rotors, and Axial Fans

The paper deals with the minimization of the subjective annoyance (that is, of the quality of the received sound rather than of its acoustic power) due to tonal noise generated by subsonic low-solidity axial-flow rotors. The employed method is based on the uneven spacing of the blades, is typically applied to open or nearly open rotors, and is based on acoustic interference. Thus, linear propagation of the acoustic waves is a prerequisite for its application. An optimization problem is formulated for which the objective function is derived from a psychoacoustic metric, the decision variables are the blades angular positions, the sound pressure level spectrum generated by a single blade is a parameter, and the constraints are the rotor balancing and the minimum angular distance between adjacent blades. A further intuitive criterion, based on lowering the highest peaks, is also considered. The solution of the problem is critical due to the presence of a very large number of minima in the objective function,...

An experimental investigation of the spectral properties of the tonal noise generated by axial flow fans with and without rotor-stator interaction

The present paper is aimed at studying the spectral properties of the tonal noise generated by axial flow fans. The phase of the acoustic pressure FFT is considered as a means to integrate the usual plots of SPL at BPF harmonics. Four different configurations of a fan, obtained combining an upstream bar and a downstream stator in presence of large inlet turbulent structures, have been tested in a hemi-anechoic chamber. In order to vary the strength of the acoustic source, measurements are taken during rotational speed ramps at low angular acceleration. The joint analysis of SPL and phase at BPF harmonics shows that it is possible to distinguish between tonal noise due to inlet turbulence from tonal noise due to rotor-stator aerodynamic interaction. Furthermore, information about the presence of important propagation effects, which result in a non linear phase trend, may also be gained.

Experimental study and velocity scaling of the tip-leakage noise generated by low-speed axial flow-fans

The present paper reports an experimental study of the tip-leakage noise generated by axial-flow fans employed for automotive cooling systems. Two operating points and three different configurations, with and without tip-leakage flow, have been considered. The measurements have been taken in a hemi-anechoic chamber at constant rotational speed and during speed ramps. Different noise components have been identified and the properties of the generating mechanisms have been separated from the acoustic propagation effects. To this aim, the acoustic response function of the test configuration has been computed employing the spectral decomposition method, and then it has been compared with the constant-Strouhal number SPL. The low frequency broadband part of the spectra and the peaks related to the tip-leakage flow are affected by the same propagation effects but show a different dependence on the rotational speed. The scaling exponent also depends on the operating point.

Analysis of the SPL Spectra Generated by Axial Flow Fans Working under Similarity Conditions

In the present paper a technique based on the similarity theory is shown which is aimed to study the trailing edge noise generated by axial flow fans. Given a set of SPL data measured on rotors of different diameter, once the analitycal structure of the SPL power spectrum is assumed, the technique allows to determine the dependence of the spectrum on the rotational speed and on the diameter. Such a technique has a general usefulness since it allows to verify the main features of measured and predicted SPL spectra. In the present case it has been developed to test trailing edge noise prediction methods and, possibly, to develop new semi-empirical correlations. After a description of the basic theory, the technique is applied to preliminary measurements taken in a hemi-anechoic chamber. The application shows that, for the present rotors, the measured OASPL follows the expected scaling law and slightly decreases as the Reynolds number is increased. Such a result needs to be confirmed by further investigations taken under more controlled inflow conditions. Nomenclature a0 = speed of sound D = rotor tip diameter f = frequency

Experimental study of the trailing edge noise generated by axial flow fans

An experimental rotor has been expressly realized to study the trailing edge noise generated by axial flow fans and has then been tested in a hemi-anechoic chamber. The rotor blades have has radially stacked profiles based on a NACA 0012 airfoil, resulting in a simple geometry, which prevents complex flows and eases comparisons with noise predictions. Several working points and rotational speeds have been considered yielding a complete data base. The SPL spectra have been scaled according to the similarity rules and then analyzed resulting in typical trends: a broad peak at low blade loading, which grows and moves towards low frequency as the blade loading increases. Finally, the experimental SPL spectra have been compared with those predicted by the Brooks, Pope and Marcolini (BPM) method. The comparisons show a good agreement at the lower blade loadings and rotational speeds but the agreement becomes poorer as both parameters increase. The effect of the rotational speed was unexpected and seems to indicate that, for the present application, some of the BPM correlations should be modified.

Aerodynamic noise from cooling and HVAC systems features important to land vehicles in urban traffic conditions

The present paper deals with the aerodynamic noise generated by cooling and HVAC systems employed in land vehicles. After a brief description of the physical origin of aerodynamic noise, the features important to system engineers dealing with fans installation and operation are considered. First, the main scaling laws useful to estimate the sound level spectrum are reported. Then, the main characteristics of the sound pressure level spectrum are related to the typical aerodynamic generating mechanisms and to the quality of the perceived sound, and some guidelines for noise reduction are listed. Finally, a method for the employ of results obtainable from laboratory tests is proposed which aims to identify the origin of annoying fluctuations in the received noise.

An Experimental Investigation of the Unsteady Flow in a Two-Stage Low-Pressure Research Turbine

Results of an experimental investigation of the unsteady flow in a two-stage low-pressure large-scale research turbine are presented. Velocity and turbulence measurements were performed upstream and downstream each blade row at midspan by means of a two-sensor hot-wire probe. Results show a complex unsteady flow with remarkable wake generated and potential flow interaction effects.