“The answer is blowin in the wind – case study of a perforated roof screen
Time: 2:00 pm
Author: Anthony Nash
Abstract ID: 2072
A recently-completed building was fitted with a roof screen fabricated from perforated sheet metal panels having U-shaped upturned flanges. When wind impinges on the panels, complex tone clusters are generated, leading to complaints from the occupants. The unusual character of the tonal spectrum is reminiscent of a film sound effect intended to simulate a hovering extraterrestrial spacecraft. After some preliminary (but inconclusive) field investigations, it was decided to test samples of the perforated panel in a large commercial wind tunnel where the speed and angle of the airstream could be controlled. Tones generated in the tunnel were found to occur in groups or clusters these are most pronounced when the airstreams angle of incidence is close to grazing. Gradually increasing airspeed caused the frequency of the tones to jump from one cluster to the next higher cluster. The physical principles of the tone-generating mechanism are not fully understood; however, it appears that structural resonances in the panel flanges are excited by air flowing over the perforate. Some form of a positive structural-acoustical feedback loop is involved since a) the frequencies within each tone cluster are quite stable and, b) damping the panel flanges extinguishes the tones.
Acoustical analysis of sound generated by synthetic jet actuators
Time: 1:40 pm
Author: Song Wang
Abstract ID: 2851
Piezoelectrically driven synthetic jet actuators (SJA) are useful in various applications such as flow control, heat transfer and camera lens cleaning. This paper aims to better understand the fundamental sound generation mechanisms of synthetic jet actuators and investigate methods for the noise reduction and vibration control. The SJAs tested in this paper are driven by sinusoidal signals at frequencies ranging between 100 and 600 Hz, and can produce pulsated air jets at high velocity, up to 100 m/s. The sound generated by these devices, generally tonal and rich in harmonics, was modeled as the superposition of two monopoles associated with the breathing mode of the diaphragm and of the pulsated jet. Component analyses showed that the two monopoles cancelled each other partially depending on their amplitudes and phase relationship. A computational aeroacoustic model of the SJAs was built using PowerFLOW, a computation fluid dynamic simulation software. Simulation results were compared with jet velocities measured with a hot-wire anemometer and flow patterns were analyzed. Active and passive control methods were investigated, and a sound quality analysis was performed in order to reduce the overall radiated sound power and improve sound quality.
An experimental investigation of turbulent flow over two-dimensional obstacles on a flat plate
Time: 2:20 pm
Author: Shivam Sundeep
Abstract ID: 2711
Aeroacoustic and aerodynamic characteristics of the turbulent boundary layer encountering a large obstacle are experimentally investigated in this paper. Two-dimensional obstacles with a square and a semi-circular cross-section mounted on a flat plate are studied in wind tunnel tests, with particular interests in the shear layer characteristics, wall pressure fluctuations, and far-field noise induced by the obstacles. Synchronized measurements of the far-field noise and the wall pressure fluctuations were conducted using microphone arrays in the far-field and flush-mounted in the plate, respectively. Additionally, the streamwise and wall-normal velocity fluctuations behind the obstacle were measured using the X-wire probe. The measured velocity profiles, spectra, and wall pressure spectra are compared, showing that the rectangular obstacle has a significant impact on both the turbulent flow and far-field noise. The large-scale vortical structures shed from the obstacles can be identified in the wall pressure spectra, the streamwise velocity spectra, and the wall pressure coherence analysis. Within the shear layer, the pairing of vortices occurs and the frequency of the broadband peak in the velocity spectra decreases as the shear layer grows downstream. Further eddy convective velocities of large-scale vortical structures inside the shear layer were analyzed based on the wall pressure fluctuations.
Empirical prediction of flight effect on subsonic coaxial-jet noise by introducing an adjusted flight velocity term
Time: 2:40 pm
Author: Incheol Lee
Abstract ID: 2719
The effect of forward flight on jet noise is difficult to quantify through flyover tests since only the total noise is measured in a full-scale flyover test, and the contribution of the jet noise is difficult and sometimes nearly impossible to identify. Thus, most studies on the flight effect have been carried out through model-scale experiments with a single-stream jet simulator in a free jet facility. In this paper, the effect of forward flight was captured by using an adjusted flight velocity term (?V) to describe jet velocity in a new prediction of coaxial-jet noise. The new jet noise prediction method assumes that there are three components: primary, secondary, and mixed components with no filter functions. The coefficient ? is determined by a thorough investigation of the model-scale data gained from an experiment in the anechoic wind tunnel of ONERA. The value of ? is 1 for the primary component, 0.5 for the secondary component, and a linear function of the angle for the mixed component. The simple adjustment of the flight velocity successfully embodied the effect of forward flight at all angles, with no separate velocity exponent or an additional term.