“…Based on measurements and finite-element computations 20 on the mechanical behavior of the fan (e.g. blade eigenfrequencies), it has been concluded that the noise dedicated to the rotor is predominantly airborne noise.…”
A combined acoustic-aerodynamic diagnostics methodology is presented herein, involving the phased array microphone technique and an in-house rotating source identifier processing algorithm. The methodology makes possible the industrial onsite investigation of unducted or short-ducted rotor-only axial fans. Its application is illustrated in a case study of a free-inlet, free-exhausting fan, surveyed from the upstream direction. A methodology is outlined for guaranteeing the appropriateness of averaging of the phased array microphone records. A technique is proposed for eliminating the ambiguity of the angular position of the rotor noise sources. A complete set of noise source maps of various frequencies have been evaluated. Noise sources, such as tip leakage flow and suction side blade boundary layer flow, have been identified. Empirical correlations have been pointed out between indicators of aerodynamic noise and aerodynamic loss along the blade span. Such correlations may contribute to redesign guidelines for simultaneous reduction of noise and improvement of efficiency at prescribed fan performance.
“…Based on measurements and finite-element computations 20 on the mechanical behavior of the fan (e.g. blade eigenfrequencies), it has been concluded that the noise dedicated to the rotor is predominantly airborne noise.…”
A combined acoustic-aerodynamic diagnostics methodology is presented herein, involving the phased array microphone technique and an in-house rotating source identifier processing algorithm. The methodology makes possible the industrial onsite investigation of unducted or short-ducted rotor-only axial fans. Its application is illustrated in a case study of a free-inlet, free-exhausting fan, surveyed from the upstream direction. A methodology is outlined for guaranteeing the appropriateness of averaging of the phased array microphone records. A technique is proposed for eliminating the ambiguity of the angular position of the rotor noise sources. A complete set of noise source maps of various frequencies have been evaluated. Noise sources, such as tip leakage flow and suction side blade boundary layer flow, have been identified. Empirical correlations have been pointed out between indicators of aerodynamic noise and aerodynamic loss along the blade span. Such correlations may contribute to redesign guidelines for simultaneous reduction of noise and improvement of efficiency at prescribed fan performance.
“…the blade tip of axial fans. 5,6 Similar vibrations can be observed in vehicle parts such as airplane wings or a fuselage. [1][2][3][4] Figure 1.…”
Section: Introductionmentioning
confidence: 65%
“…On the other hand, AE of lower frequency (0.1–1.0 kHz) can also be generated by, e.g., the blade tip of axial fans. 5,6 Similar vibrations can be observed in vehicle parts such as airplane wings or a fuselage. 1–4 Figure 1.Numerical descriptors of a burst type AE signal in the time domain.…”
In general, the aim of acoustic emission (AE) assisted tensile tests of composite materials is to identify and characterize the damage and failure modes of the specimens. This paper presents a fiber-bundle-cells (FBC) based statistical model, which provides a possible solution to the problem of characterizing the mechanical and failure behavior of the material. The model, based on the results of mechanical tests and AE measurements, decomposes the measured AE event number and tensile force-load time processes into components corresponding to the different damage modes. The AE events belonging to different failure modes are described by inhomogeneous Poisson point processes, while failures are modeled with the breakage of fibers as elementary parts of the sample. Hence damage modes can be characterized with the number fraction, and the tensile strength and signal energy distributions of the components. Moreover, the variation of the number fraction of the intact or damaged fibers as a function of the load time can be calculated and depicted as well. As reliability function or a kind of damage map, it reveals the mechanical load-bearing ability of the material tested. The applicability of the model is demonstrated by compact tension testing and the comparison of short and long glass fiber reinforced VERTON PP sheets and injection molded wood fiber reinforced PP composites.With the help of fast Fourier transformation (FFT), the frequency components of the single AE signals can be characterized by the amplitude or power spectrum in the frequency domain. Their description parameters are the bandwidth (B), the mean frequency (fm), the frequency of the maximum amplitude component (fP), and the amplitude (AP) or energy (UP) of the maximum peak. Similar parameters can be obtained from the mean spectra of a series of AE signals, and histograms and/or cross-plots can be calculated from the data of single AE signals. 2,4 Special techniques can provide information in both the time and the frequency domain, resulting in time-dependent spectrum statistics. 2,4 The classic method is the short time FFT (SFFT) technique, which uses a moving time window. In the case of wavelet transformations (WT), AE signals are analyzed with the help of various wavelet components. The signal transformations mentioned above provide additional or in itself usable information about the sources and failure modes in the frequency domain [21][22][23][24][25][26] or both in the time and the frequency domains [27][28][29][30][31][32][33][34][35][36] . In the latter case, wavelet transformations based on Haar 28 or Gabor 27, 33 wavelet components as well as Hilbert-Huang transform 30 can be used to decompose the AE signals and calculate the energy of the components. [27][28][29][30][31][32][33][34][35][36] In the case of thin plates, modal analysis can be applied to study and separate extensional and flexural Lamb waves. 37,38 The finite element method (FEM) can give a good base for wave discrimination. 38 The localization of a single AE signal in the spe...
“…This corresponds to roughly 300 Hz frequency. Based on measurement and simulation the first 10 natural frequencies of the investigated fan are between 100 and 200 Hz [22]. Therefore, in the case of the vortex wandering frequency scales with the blade tip velocity then a lower rotational speed may induce vibrations as well.…”
The natural unsteadiness of turbomachinery tip leakage vortex has been reported in both measurement and simulation. The visualization and quantification of this vortex wandering are challenging using pathlines or simple vortex identification methods. In the present paper, the tip leakage vortex wandering is investigated in the case of a low-speed fan's unsteady Reynoldsaveraged Navier-Stokes simulation. The wandering motion of the leakage vortex is detected by Banks' vorticity predictor -pressure corrector vortex core line localization method. Despite the low computational costs, the simulation predicted the complex vortical structure present in the blade passage. The vortex detection algorithm proved successful in finding the tip leakage vortex, but it failed to follow the core line upon heavy disturbance in the flow. The extent and the frequency of the vortex wandering were determined.
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