Tip Clearance Noise Research Articles

The UNSW tip clearance flow noise test rig

A new test rig to measure the far-field sound, surface pressure fluctuations and flow-field in tip clearance flows has been designed at UNSW. The test rig is a part of the UNSW anechoic wind tunnel (UAT) and has been designed to allow studies of the tip clearance flow and sound radiation across a vast parameter space—tip clearance height, Reynolds number, angle of attack, and the tip geometry. This paper will present and discuss the design, characterization, and capabilities of this new test rig, along with some far-field sound results for a tip clearance flow formed by placing a stationary cambered airfoil adjacent to a wall. The measurements were performed for 51 different clearance heights at free-stream velocity of 30 m/s. The geometric angle of attack in the measurements was 5.6 and the clearance height Reynolds number ranged between 2000 and 102,000. The far-field sound was measured using a 64-microphone spiral phased array and the array output was beamformed to reveal the tip clearance noise sources. It is expected that the test rig will aid in a better understanding of the sound radiated by tip clearance flows which may be used to devise noise control strategies in the future.

Tip flow on rotating instability on an axial compressor with different tip clearances

Rotating instability (RI) as an unsteady phenomenon usually occurs at off-design conditions in compressors, featuring a broadband hump in the pressure spectra below the blade passing frequency (BPF). RI phenomenon could not only cause deterioration to the performance, but also lead to tip clearance noise or non-synchronous vibration to the compressor. In this paper a detailed experiment was carried out in a low-pressure single-rotor compressor. By replacing the casing the relative tip clearance size was changed and the time-resolved wall pressure with a distinct RI signature was thus measured under several different conditions through the Kulite pressure transducers both circumferentially and chordwise installed. The temporal-spatial distribution of RI signature evolution with the flow coefficient is interrogated, and its propagation characteristics are also investigated through the cross-correlation method. Moreover, the quasi-instantaneous contour measured by the chordwise transducers shows no distinct flow structure which could be connected to the RI signature, i.e., the prestall disturbance. Therefore, a quasi-SFT method is developed to identify different disturbance counts and the corresponding relationship between the disturbance count per revolution and the azimuthal mode order could thus be strictly established. The prestall disturbance is found to originate from the mid-chord positions as RI initiates and travel upstream as the reduction of flow coefficient, and eventually reaches the leading edge as rotating stall occurs. Furthermore, the location of prestall disturbances is proven to be independent on the blade position. These findings describe the tip flow field when the RI mode orders are relatively high, and provide new insights for the shear layer instability theory as RI mechanism.

Mechanism of Axial Compressor Tip Clearance Noise Based on Taylor–Couette Flow Rotating Instability

Abstract Prediction of the tip clearance noise (TCN) of axial compressors is essential to accurately determine the noise level of the compressor, because it is left unattenuated during propagation. The existing studies on numerical methods are inadequate for accurate determination of the TCN owing to the limited understanding of the physical phenomenon. The most convincing explanation is that the TCN is generated because of the instability in shear waves in the tip clearance region. In this study, the rotating instability theory was developed further by modeling the physical phenomenon of the instability as a Taylor–Couette (T–C) flow. The TCN sources were obtained using T–C flow modeling, and duct noise analysis was performed to obtain the TCN at various locations and frequencies. The TCN predictions were compared with the existing experimental results and noise results obtained using FW–H. Results showed that the T–C flow modeling is sufficiently accurate for predicting the frequencies, wavenumber distributions, and SPL of the tip clearance noise of axial compressors.

Axial fan tip clearance noise: Experiments, Lattice–Boltzmann simulation, and mitigation measures

The effect of tip clearance in an axial fan on its aerodynamic and aeroacoustic performance is investigated experimentally as well as via a Lattice–Boltzmann flow simulation method. An increase in tip clearance degrades fan pressure rise and efficiency, but also increases significantly the overall sound power emitted by the fan. A large tip clearance causes a clear structure of well distinguishable unsteady vortices which interact with neighboring blades and hence produce an increase in broadband sound. Moreover, if, compared to the design flow rate, there is a moderate flow rate reduction, the local tip vortex systems of all individual blade tips form a circumferentially coherent flow structure, resulting in distinct humps of sound pressure in the acoustic far field. By means of a rigid ring-type protrusion fixed to the inner casing wall, the generation of the tip clearance vortices and slowly rotating coherent flow structures could be suppressed. As a consequence, the sound emitted by the fan is substantially reduced.

Rotating coherent flow structures as a source for narrowband tip clearance noise from axial fans

Noise from axial fans typically increases significantly as the tip clearance is increased. In addition to the broadband tip clearance noise at the design flow rate, narrowband humps also associated with the tip flow are observed in the far-field acoustic spectra at lower flow rate. In this study, both experimental and numerical methods are used to shed more light on the noise generation mechanism of this narrowband tip clearance noise and provide a unified description of this source. Unsteady aeroacoustic predictions with the Lattice-Boltzmann Method (LBM) are successfully compared with experiment. Such a validation allows using LBM data to conduct a detailed modal analysis of the pressure field for detecting rotating coherent flow structures which might be considered as noise sources. As previously found in ring fans the narrowband humps in the far-field noise spectra are found to be related to the tip clearance noise that is generated by an interaction of coherent flow structures present in the tip region with the leading edge of the impeller blades. The visualization of the coherent structures shows that they are indeed part of the unsteady tip clearance vortex structures. They are hidden in a complex, spatially and temporally inhomogeneous flow field, but can be recovered by means of appropriate filtering techniques. Their pressure trace corresponds to the so-called rotational instability identified in previous turbomachinery studies, which brings a unified picture of this tip-noise phenomenon for the first time.

Numerical Investigation on the Characteristics of Flow-Induced Noise in a Centrifugal Blower

Abstract In the present study, a computational analysis of the flow in a centrifugal blower is carried out to predict a performance and to explain noise characteristics of the blower. Unsteady, 3D Navier-Stokes equations were solved with k- e turbulence model using CFX software. CFD results were compared with the experimental data that is acquired from an experiment conducted with the same blower. The pressure fluctuation in the blower was transformed into the frequency domain by Fourier decomposition to find the relationship between flow behaviors and noise characteristics. Sound pressure level (SPL) which is obtained from wall pressure fluctuation at impeller outlet represents relative overall sound level of the blower well. Sound spectra show that there are some specific peak frequencies at each mass flow rate and it can be explained by flow pattern. Keywords : centrifugal blower, wall pressure fluctuation, flow-induced noise. 1. Introduction The centrifugal blower for a fuel cell electric vehicle (FCEV) is known as a significant part of the source of noise because of its very fast rotating speed and high pressure operating condition. It is known that the cause of noise is strongly related with the flow in a blower, that is to say flow-induced noise. There have been some efforts to understand characteristics of the noise of a blower and to reduce it. An experimental study to explain the sound generation mechanisms by wall pressure fluctuations has been existed and the relation of tip clearance noise and rotating instability signals is discussed (Raitor et al. [2008]). In the field of CFD study, it is well known that large eddy simulation (LES) is more accurate than unsteady Reynolds averaged Navier-Stokes (uRANS) method for predicting features of flow-induced noise (Kim et al. [2010]). But LES must pay very expansive cost. In this study, an attempt to get reasonable prediction of noise characteristics while using uRANS is made. And these CFD results are compared with experimental data (Jeong et al. [2012]). Based on an assumption that wall pressure fluctuation is related to the generation of flow-induced noise , wall

Experimental Research on the Vibration and Noise Characteristics of the Marine Diesel Engine Turbocharger

An experimental study on the spectrum analysis and sound intensity identification of vibration and noise source was carried out. The study result represents that the primary reasons leading to the volute vibration of turbocharger, whose vibration energy focus on the frequency range of 0~8Kz, are the volute internal rotor shaft frequency of compressor and its harmonic vibration excitation and rotating unsteady air flow excitation. The volute radiation noise is primarily produced by the structural vibration caused by tip clearance noise and rotor shaft frequency and its harmonics. The noise energy of the turbocharger distributes at a wide range of 1k~8kHz and the sound power almost increases linearly with its rotating speed, which increases by about 20dB as the rotating speed increased from 50% to rated speed.

Effect of skewed blades on tip-clearance for different flow rates in axial fans

Tip-clearance noise induced by tip-leakage flow in axial flow fans, with the blades skewed circumferentially at angles of ±8.3°, has been investigated under off-design conditions by numerical simulation and experiments. Aerodynamic and aeroacoustic performances have been measured. Tip-leakage flow has been investigated by Navier-Stokes flow simulations at off-design conditions. The characteristics of noise sources in the tip-clearance region, obtained numerically, have been analyzed using vortex sound theory. Synergy between the tip-leakage vortex and the noise source in the tip region is discussed. Moreover, the noise source in the tip region has been measured by dynamic pressure sensors mounted in the casing wall near the blade tip and the acoustic far field has been obtained simultaneously as a function of flow rate. The frequency characteristic of the tip noise component is analyzed by measured results. The characteristics of noise source reveal that tip-clearance noise is associated with skewed blades, which are found to be effective in controlling tip-leakage flow and in eliminating the noise source in the tip-clearance region at off-design conditions. Forward-skewed blade are more effective than backward-skewed blades in reducing noise

1205 軸流圧縮機における旋回不安定擾乱の解析(OS12-2 流体機械の諸問題,オーガナイズドセッション)

The rotating instability has been investigated at a near-stall condition in an axial flow compressor rotor with large tip clearance by EFD and CFD. This flow phenomenon has disturbances that move relative to the rotor blades and cause high amplitude rotor blade vibrations and tip clearance noise. Furthermore, it has closely related to rotating stall inception. In the present study, it has been found that rotating instability disturbances propagate in cyclic process of strong breakdown and weak breakdown of the tip leakage vortex.

Experimental validation of a model of fan trailing-edge noise

Broadband noise is a major part of the noise radiated by industrial and domestic fans. For low-speed fans broadband noise contribution to the overall A-weighted noise level is often much higher than that of the tonal noise at the blade passage frequency and its harmonics. One of the main mechanisms of broadband noise is trailing-edge noise, which is due to the convection of the turbulent flow past the blade trailing edge. This paper presents results of a research program in progress, which aims at validating a model that predicts broadband noise levels of rotating blades. Trailing-edge noise prediction is made using an analytical model deduced from an extension of Amiet's formulation. The input data to the model are the frequency spectrum and the spanwise correlation length scale of the wall pressure fluctuations on the blade suction side close to the trailing edge. This model was successfully validated on single fixed airfoils at different angles of attack in an anechoic wind tunnel. The input data were measured with wall pressure transducers on the airfoil suction side. In the present study, the validity of the trailing-edge noise model is assessed for a 800-mm axial flow fan which is used without casing in order to avoid the tip clearance noise contribution. Wall pressure fluctuations are measured with small pressure transducers flush mounted on two rotating blades, using a slip ring to transmit the pressure signals to the frequency analyser. A comparison of measured and predicted far-field sound pressure spectra at different observation angles is presented for two blade pitch angles 20° and 30°. The prediction proves to be quite good at 30° but strongly underestimates fan noise levels over a large frequency range at the lower blade angle. In this case another sound source related to the blade tip vortex is detected.

Sound generation in centrifugal compressors

An experimental study is described to explore the dominant sound generation mechanisms of the spectral components governing the overall noise level of centrifugal compressors. At the design speed with supersonic flow conditions in the rotor blade channels, blade tone noise and buzz-saw noise are the main contributors. On the inlet, rotor-alone noise is the main source while rotor–stator interaction noise dominates on the outlet side in case of vaned outlet diffusers. Over a large range of rotor speeds with subsonic flow conditions, radial compressor noise is dominated by tip clearance noise which is produced by the secondary flow through the gap between rotor blade tips and the casing wall which in turn gives rise to the rotating instability phenomena observed earlier in axial-flow machines.

Tip clearance noise of axial flow fans operating at design and off-design condition

The noise due to tip clearance (TC) flow in axial flow fans operating at a design and off-design conditions is analyzed by an experimental measurement using two hot-wire probes rotating with the fan blades. The unsteady nature of the spectra of the real-time velocities measured by two hot-wire sensors in a vortical flow region is investigated by using cross-correlation coefficient and retarded time of the two fluctuating velocities. The results show that the noise due to TC flow consists of a discrete frequency noise due to periodic velocity fluctuation and a broadband noise due to velocity fluctuation in the blade passage. The peak frequencies in a vortical flow are mainly observed below at four harmonic blade passing frequency. The discrete frequency component of velocity fluctuation at the off-design operating conditions is generated in vortical flow region as well as in reverse flow region. The peak frequency can be an important noise source when the fans are rotated with a high rotational speed. The authors propose a spiral pattern of velocity fluctuation in the vortical flow to describe the generation mechanism of the peak frequency in the vortical flow. In addition, noise increase due to TC flow at low flow rate condition is analyzed with relation to the distribution of velocity fluctuation due to the interference between the tip leakage vortex and the adjacent pressure surface of the blade.

Active control of the aerodynamic performance and tonal noise of axial turbomachines

Two projects on axial fans are presented in this paper. The first aims at improving the aerodynamic performance and reducing the tip clearance noise caused by the rotating instability. This is achieved by injecting air into the radial gap between the impeller blade tips and the fan casing. Steady air injection with small mass flows results in remarkable reductions in the noise level along with improved aerodynamic performance. Larger injected mass flows give significant improvements in the aerodynamic performance at the expense of an increased overall noise level. The second project uses flow control actively to reduce the blade passage frequency (BPF) level. Experimental results are presented for steady jets injected into the main flow and cylindrical rods at axial positions downstream of the impeller blades. The method is successful for higher-order mode sound fields where the BPF level is reduced by up to 20.5 dB.

Experimental Study of Tip Clearance Losses and Noise in Axial Turbomachines and Their Reduction

An experimental study is described to investigate the negative effects of the tip clearance gap on the aerodynamic and acoustic performance of axial turbomachines. In addition to the increased broadband levels reported in the literature when the tip clearance is enlarged, significant level increases were observed within narrow frequency bands below the blade passing frequency. Measurements of the pressure and velocity fluctuations in the vicinity of the blade tips reveal that the tip clearance noise is associated with a rotating flow instability at the blade tip, which in turn is only present under reversed flow conditions in the tip clearance gap. A turbulence generator inserted into the tip clearance gap is found to be effective in eliminating the tip clearance noise and in improving the aerodynamic performance.

ROTATING BLADE FLOW INSTABILITY AS A SOURCE OF NOISE IN AXIAL TURBOMACHINES

An experimental study is presented to investigate the aeroacoustic generation mechanism of the tip clearance noise in axial turbomachines. In addition to the increased broadband levels reported in the literature when the tip clearance is enlarged, significant level increases were observed within narrow frequency bands below the blade passing frequency. Measurements of the pressure fluctuations at the casing wall just upstream of the entrance plane of the impeller and on the rotating blades reveal that the tip clearance noise is associated with a rotating blade flow instability at the blade tip which in turn is only present under reversed flow conditions in the tip clearance gap. The rotating instability is interpreted as a rotating source or vortex mechanism which moves relative to the blade row at a fraction of the impeller shaft speed, similar to the cell(s) of rotating stall. A model for the generation of the narrow-band tip clearance noise is presented.

Control of tip-vortex noise of axial flow fans by rotating shrouds

Noise and performance tests were conducted on a low tip speed, half-stage, axial flow fan such as is used in automotive applications. The purpose of the tests was to determine the characteristics, relative importance, and the methods of control of the tip clearance noise of the fan. The fan was 356 mm in diameter and had eight equally spaced blades. The noise measurements were made in both free field and reverberant field environments and the fan backpressure and speed were varied during the tests. An acenaphthene coating on the blades was used to determine the regions of laminar and turbulent flow and smoke was used to visualize the blade-tip flow patterns. Tip clearance noise was shown to contribute up to 15 dB in overall noise level over the entire fan operating range for tip clearances of the order of 3–4% of the blade chord and larger. Large clearances result in a significant (∼10%) drop in fan peak efficiency. Essentially, tip clearance noise is due to interactions of the blade tip vortex with the trailing edge of the blade and with the leading edge of the following blade. A properly contoured, rotating ring-shroud attached to the fan tips was found to be a practical method of controlling tip clearance noise. For example, with a clearance of 6·5% of the chord length, the noise level with the ring-shroud was as much as 12 dB lower than in the fixed shroud case with the same clearance.