Blade Passage Frequency Research Articles

An experimental modal decomposition method to compute sound power of multi-dimensional acoustic waves from turbocharger compressors

The automotive turbocharger compressor in the present experimental study features a ported shroud casing treatment, which is known to elevate tonal noise at the blade-pass frequency (BPF) while reducing broadband whoosh noise and providing higher boost pressures at low mass flow rates. The high operating rotational speeds of such modern turbocharger compressors push the BPF to ranges where acoustic wave propagation is multi-dimensional within the compressor ducting. Simultaneously propagating acoustic modes at the BPF result in strong circumferential and axial variation of in-duct sound pressure levels. This poses a challenge for acoustic characterization and comparison of different hardware since typical measurement techniques do not consider the sensitivity of acoustic pressure to the measurement location. The current work utilizes a steady-flow turbocharger gas stand with a unique rotating compressor inlet duct fitted with multiple wall-mounted dynamic pressure transducers capable of performing a modal decomposition of the acoustic field. The decomposition is done using time-resolved acoustic pressure measurements from different orientations of the rotating inlet duct during steady compressor operation. The resulting modal amplitudes are then used to determine the sound power level, a quantity that is independent of the acoustic pressure measurement locations. Therefore, in addition to revealing the modal content of noise at the compressor inlet, the rotating inlet duct experimental setup better characterizes the acoustic field with sound power levels across the operating flow range at various compressor rotational speeds.

An air handling equipment manufacturer’s perspective on tonal noise measurement and application

In order to reliably predict sound from HVAC equipment, including tonal noise, sound power data needs to be determined for each noise producing component in the equipment. Though there are accepted standards for determining broad band sound in resolutions no finer than one-third octaves, there is a general lack of component data regarding tonal characteristics. This noise produced by the HVAC system in indoor environments is often an important source of masking noise improving speech privacy and the overall acoustical environment. The presence of prominent tones that are propagated into the space is counter to creating comfortable acoustical environments. The primary HVAC noise source is the fan and all fans produce tones at the blade pass frequency and harmonics. There is currently no accepted standard method of test for the tonality of fan noise. This paper proposes a standard method that can consistently be used to determine the tonality of fan noise, which can in turn be used to determine if when combined with other self gen noise generated by the air distribution system to determine the prominence of tones that will propagate to the occupied space.

Low-frequency acoustic rotorcraft classification

Rotorcraft emit unique acoustic signatures tied primarily to their blade pass frequencies. In this study, we examine the ability of artificial neural network machine learning methods to successfully classify several distinct types of rotorcraft based on their low-frequency acoustic signatures. Time series, power spectral density, and Welch power spectral density input types are examined, corresponding to convolutional neural networks for the first input type, and multi-layer perceptrons for the latter two input types. A K-fold cross-validation is used to initially assess the generalization performance of the various models, along with confusion matrices for a deeper dive into the model performance. Ablation studies are discussed for insights into model behavior. These insights are used to showcase further enhanced neural network models with increased robustness and accuracy.

Tonal noise case study: Locating the source of tones on HVAC equipment

The overall noise level measured from chiller units is controlled by primarily tonal noises from (1) the fans, (2) the compressors, and (3) the associated pipework. Three large chiller units were measured using AHRI 370 methodology to quantify the overall sound level spectra and A-weighted sound level from different chiller units in a range of environmental conditions. Whilst this method can identify a general area of tonal emission to within a couple of meters across a unit’s surface that is emitting discrete tonal noises, the method is limited when there are multiple potential tonal sources. Sound intensity imagery was used on two units to quantify the overall sound power emanating from the source, but also to offer a more advanced method of localizing the source of discrete tones to aid in the application of mitigation treatments. Results presented include visually displaying and quantifying the location of noise emission from fan blade pass frequencies and associated harmonics, localization of problematic tones emanating from individual pipework junctions and valves, and the reductions in sound power emissions achieved from lagging compressors and pipework.

A Review of Fluid-Induced Excitations in Centrifugal Pumps

This paper describes the related research work in the field of fluid-induced vibration of centrifugal pumps conducted by many researchers. In recent years, all walks of life have put forward higher demands for the vibration performance of pumps which drives the investigation on the root cause of pump vibration and the development of guidelines for the design of low-vibration pumps. Fluid-induced excitation is the most important and significant source of pump vibration. Understanding its generation mechanism and dominant characteristics is important for developing low-vibration pump design methodology. This paper starts with the analysis of unsteady flow in the centrifugal pump and summarizes unsteady flow characteristics such as jet–wake structure, secondary flow, and rotational stall in the operating pump. Based on the understanding of the unsteady flow structure in the pump, the fluid-induced excitation mechanism and its characteristics based on the investigation of unsteady pressure pulsation and excitation forces in the pump are summarized. For the pump operating at nominal flow rate, the excitation at blade passing frequency (BPF) dominates and related suppression methods are classified and summarized to provide reference for the design of a low-vibration pump.

Transition of amplitude–frequency characteristic in rotor–stator interaction of a pump-turbine with splitter blades

With the increase in the designed net head and output capacity, the operational stability of the pump turbine, which is the core component of a pumped storage power station, requires further investigation. The originality of the current research lies in the analysis of the pressure pulsation characteristics in the vaneless space of a pump-turbine runner designed with splitter blades. Under the designed net head and high-flow-rate conditions, the dominant frequency of the pressure pulsation in the vaneless space corresponds to half the total number of runner blades, 4fn. As the flow rate decreases, a blade-passing frequency of 8fn corresponding to the total number of runner blades was observed. Subsequently, the amplitude of 8fn increased, which gradually became the dominant frequency, whereas that of 4fn diminished. The contribution of current research is that the transition of the amplitude-frequency characteristic of the rotor-stator interaction of pump-turbine runners with splitter blades is revealed, which is significantly different from the analysis of equal-blade pump-turbines in the existing literature. For pumped storage power stations equipped with pump-turbine runners with splitter blades, it is necessary to consider the dominant frequency in the vaneless space to avoid the adverse effect of the transition of the dominant frequency of the rotor–stator interaction on the structural response of the station.

Numerical Simulation of Cavitation Characteristics of a Centrifugal Pump Based on an Improved ZGB Model

We modified the Zwart–Gerber–Belamri (ZGB) cavitation and RNG turbulence models based on their rotational motion characteristics, as well as simulating the phenomenon of small fluctuations in rotational speed due to the action of hydraulic excitation force, to increase the precision of numerical simulations of the cavitation characteristics of centrifugal pumps. According to the theory behind the enhanced model, the pressure gradient in the impeller runner changes uniformly, and the cavitation bubble initially manifests itself at the front edge of the blade’s suction. With the reduction in the effective margin, changes in the impeller flow channel’s pressure gradient increased, the blade’s suction-side cavitation area expanded, and the flow field’s internal disturbance enhanced, resulting in hydraulic loss to the centrifugal pump, considerably reduced operating performance, and the blade pressure side by the cavitation-affected area becoming smaller. The blade’s suction-surface pressure–load curve can be divided into a low-pressure cavitation zone, a pressure fluctuation zone, and a pressure stabilization zone. The pressure–frequency-domain diagram of each monitoring point has a discrete distribution, and the main frequency is the blade-passing frequency ƒ = 293 Hz and its n times frequency. With the deepening of the cavitation, the amplitude of the main frequency in the low-frequency band showed a trend of increasing and then decreasing, and the amplitude of the high-frequency band showed an increasing trend. When we applied the ZGB cavitation model based on the improved rotational motion characteristics to our numerical calculation of the centrifugal pumps’ cavitation characteristics, our calculated cavitation performance curve was closer to the experimental value than the ZGB model, and when the centrifugal pump’s cavitation degree was increased, the area covered by the vacuole area was 1/4 of the suction side of the vane.

Numerical and Experimental Investigations of Axial Flow Fan with Bionic Forked Trailing Edge

To improve the performance of the aerodynamic properties and reduce the aerodynamic noise of an axial flow fan in the outdoor unit of an air conditioner, this study proposed a bionic forked trailing-edge structure inspired by the forked fish caudal fin and implemented by modifying the trailing edge of the prototype fan. The effect of the bionic forked trailing edge on the aerodynamic and aeroacoustic performance was investigated experimentally, and detailed analyses of the blade load and internal vortex structures were performed based on large-eddy simulations (LES). It is shown that the bionic forked trailing edge could effectively adjust the blade load distribution, reduce the pressure difference between the pressure side and suction side near the trailing edge of the blade tip region, and weaken the intensity and influence range of the inlet vortex (IV) and the tip leakage vortex (TLV). The discrete noise caused by the vortices in the rotor tip area was also reduced, particularly at the blade passing frequency (BPF) and its harmonic frequency. The experimental results confirmed the existence of an optimal bionic forked trailing-edge structure, resulting in the maximum power-saving rate γ of 7.5% and the reduction of 0.3 ~ 0.8 dB of aerodynamic noise, with an included angle θt of 13.5°. The detailed analysis of the internal vortex structures provides a good reference for the efficiency improvement and noise reduction of axial flow fans.

Numerical Investigation of Cavitation Flow Characteristics in a Hydrodynamic Levitated Micropump with Eccentric Rotation

In this study, the anti-cavitation performance and cavitation flow characteristics in a hydrodynamic levitated micropump were investigated based on numerical simulation and experiment. The cavitation characteristic curves and the development process of cavitation in the levitated micropump was firstly analyzed. Special emphasis was put on the effects of eccentricity on the anti-cavitation performance. The results show that as the eccentricity increases, the critical cavitation number gradually decreases, indicating that the eccentric rotation is beneficial to improve the anti-cavitation ability of the levitated micropump. The coupling effects between the radial force on the impeller and cavitation were also numerically studied. With the decrease of cavitation number, the radial force on the impeller gradually declines at first, then has a sudden increase and finally reduces with fluctuation. The drop of the radial force will lead to the decrease of eccentricity, resulting in the deterioration of cavitation further. In addition, the unsteady pressure pulsation was analyzed. The predominant frequencies of pressure pulsation are the blade passing frequency (BPF) and the harmonic frequency of BPF under both noncavitation and critical cavitation. Under critical cavitation, the amplitude of BPF has a drop, while the amplitude of low frequency less than BPF becomes larger.

S-shaped characteristics of pump turbine with large guide vane opening by experimental and numerical analysis

Pumped storage hydropower is perhaps the most reliable solution to the energy crisis, but its development and application are limited by hydraulic instability, such as S-shaped characteristics. In this paper, based on the S-shaped region pressure fluctuation test of a model pump turbine, the connection between the guide vane opening and S-shaped pressure fluctuation is established, and a guide vane opening of 28 mm is selected for numerical analysis to expand the discussion of the S-shaped pressure fluctuation characteristics to large guide vane openings. The frequency components of the vaneless space are extracted by fast Fourier transform analysis, and the rotational stall frequency is identified. Among them, the blade passage frequency and its multiplier are induced by the rotor–stator interaction, and the low-frequency component of pressure fluctuations is induced by the rotational stall and vortex rope. Based on this, the internal flow and flow rate along the S-shaped characteristic curve and their correlation with the rotational stall are analyzed. In addition, the percentage of and variation in the entropy production rate in the S-shaped region are also analyzed.

Effects of non-uniform elbow inflow on the unsteady flow and energy development characteristics of a centrifugal pump

A non-uniform inflow caused by an elbow structure is one of the main reasons for the low energy transformation efficiency of a centrifugal pump. Prototype experiments for the centrifugal pump are used to analyze the transformation rule of pump energy characteristics under the uniform inflow and non-uniform elbow inflow. The non-uniform elbow inflow increases the hydraulic losses in the centrifugal pump to reduce the pump head and operation efficiency. The pump efficiency decreases by 8% under the design flow condition. The large eddy simulation with a shear-modified eddy viscosity model is applied to simulating flow excitation characteristics and energy development features. The non-uniform inflow increases the impact loss at an impeller inlet and produces unsteady flow excitation in the impeller passage. The elbow inflow affects the pressure fluctuation frequency, and the main frequency is twice the blade-passing frequency (250 Hz). The high entropy production regions are mainly concentrated at the suction surface of the blade and the interface of impeller and volute, coinciding with the unsteady flow areas in the pump. The asymmetrical entropy yield distribution in the elbow inflow pump is intensified because of the increase in the flow loss caused by the non-uniform inflow.

Numerical flow noise simulation of an axial fan with a Lattice-Boltzmann solver

In this paper, a transient and compressible solver based on the Lattice-Boltzmann Method/Very Large Eddy Simulation (LBM/VLES) approach is employed to predict unsteady flow physics and flow-induced noise generation of a low-pressure axial fan. Aerodynamic and aeroacoustic measurements provided by the European Acoustics Association (EAA) benchmark platform are used for validation purposes. Boundary and design operating conditions are applied to the numerical model to replicate the experimental setup. Simulation and experimental data are compared, showing an excellent agreement in terms of fan efficiency with less than 1% deviation, as well as broadband and tonal noise within 0.7 dBA in terms of overall sound pressure level. An advanced post-processing analysis is performed to shed light on the noise generation mechanism in tip clearance. It is observed that both fine random turbulent structures and large coherent vortices are generated in the tip gap. The continuous impingement of the fine turbulence with the following blades and the blade itself is responsible for the radiation of broadband noise, while the interaction between the large coherent tip vortices, spinning at a lower angular velocity with respect to the fan shaft, and the following blades leads to the generation of narraowband peaks at sub-harmonics of the blade-passing frequencies. Finally, a beamforming analysis further confirms that the main noise sources are located in the blade tip clearance and tip regions.

CFD Analysis of a Hydrostatic Pressure Machine with an Open Source Solver

The open source and freely available fluid flow solver named caffa3d was adapted to simulate the performance of a medium-scale Hydrostatic Pressure Machine (HPM) with straight radial blades, which had been previously tested in a laboratory facility. A fully detailed explanation of the code caffa3d is not intended to be included in this paper, but some of its main characteristics are mentioned for completeness. In addition, convergence and grid sensitivity analysis were performed in order to assess the adequacy of the model. Evolution of instantaneous power over a few turns of the HPM shows typical blade pass frequency for all operating discharges and another oscillatory phenomena at rotating frequency for higher discharges. The Power—Discharge and Efficiency—Discharge curves obtained from the simulations present a good correlation with the experimental curves, up to the discharge value corresponding to the maximum power of the HPM. The comparison error in power and efficiency remained below 6% for discharge lower than 97.8 L/s. For higher discharge, the flow through the HPM becomes very unsteady, with big eddy structures, underfilling of the buckets and recirculation down to the entry of the channel, significantly reducing the generated power. This behaviour was also observed during the previous experiments. In the present work, the foundations for the study of other types of turbines with caffa3d are laid.

Investigating Pressure Fluctuations on Marine Vessel Rudders: Numerical Results from Propeller–Rudder–Hull Interactions

Yuan, H.; Li, F.; Yan, Q.; Zhang, W., and Hu, J., 2023. Investigating pressure fluctuations on marine vessel rudders: Numerical results from propeller–rudder–hull interactions. Journal of Coastal Research, 39(2), 284–295. Charlotte (North Carolina), ISSN 0749-0208. Severe pressure fluctuations induced by propeller wake enhance rudder vibrations. In the present study, the pressure fluctuations on the rudder are numerically investigated by using k-ω Menter's shear stress transport turbulence model. The pressure fluctuations on the port and starboard sides of the rudder are compared in time and frequency domains. Furthermore, the impact of the advance coefficient on the displacement of propeller wake and pressure fluctuations is also discussed. The numerical results indicate that the propeller wake displacement is oriented upward and downward at the port and starboard sides, respectively. The most intense pressure fluctuation occurs at the propeller blade passage frequency. As a result of vortex displacement, the suction-side fluctuations are weaker than those on the pressure side at frequencies of 100–300 Hz over an advance coefficient range of J = 0.75–0.925. The displacement of the propeller wake weakens when the advance coefficient decreases.

Unsteady Study on the Influence of the Angle of Attack of the Blade on the Stall of the Impeller of the Double-Suction Centrifugal Pump

In order to clearly show the influence on the rotating stall of the impeller of a double-suction centrifugal pump, this paper, using the numerical simulation method of Shear Stress Transform (SST), analyzes the effects of different inlet angles of the blade on hydraulic performance, internal flow field and pressure pulsation in the impeller. The results show that the small angle of attack of the blade inlet scheme can effectively suppress the impeller rotation stall and that the design point head and efficiency are increased by 6.4% and 5.7% respectively. This paper, using turbulence intensity to determine the generation of rotating stall, proposed that the average of turbulence intensity exceeding 2% is a necessary condition for the generation of rotating stall and discovered that the standard deviation of the big angle of attack of the scheme is always greater than that of the small angle being analyzed by the impeller pressure pulsation. The basic critical frequencies of blade inlet pressure pulsation with components of a low frequency is dominated by the impeller rotating frequency F0 and its harmonic frequencies 2F0, and 3F0, but the basic critical frequencies of blade outlet pressure pulsation is governed by Blade Passing Frequency (BPF). The research results can provide some theoretical support for stall research and hydraulic performance optimization of a double-suction pump.

Comparative studies on air borne noise and flow induced noise of a double suction centrifugal pump

The Flow Induced Noise (FIN) of a centrifugal pump directly affects the Air Borne Noise (ABN) of the same. But, the information regarding focus on ABN of a pump in relation to its FIN is very scarce. This paper presents the comparative study performed on the ABN and FIN of a double suction centrifugal pump. Experiments were conducted on a double suction pump of specific speed (ns) 19 to measure ABN. Free field noise measurements for ABN, aligning with the industrial standards were conducted using FFT analyzer for 3 speeds such as 990 rpm, 1200 rpm, and 1450 rpm. For each speed, the normalized flow ratio (QQn) covered were 0.4, 0.6, 0.8, 1 and 1.2 for measurements. For FIN prediction, transient numerical simulations were conducted for the same pump at 990 & 1450 rpm. Detached Eddy Simulation (DES) and Fast Fourier Transform (FFT) were used to understand the noise generation at the blade passing frequency (BPF) and the variation of discharge flow recirculation in impeller outlet at different speeds. Moreover, for FIN, a generalized quadratic equation with parabolic form is derived for a speed of 1450 rpm. Also, the derived equation at 1450 rpm has been used to develop a relation to predict the FIN at other speeds. Further, the derived FIN relations are used to predict the ABN at different speeds by expressing ABN in terms of FIN for particular speed. Overall, the developed FIN function is useful in predicting the FIN in dB at BPF of a double suction pump for impeller Reynolds number and specific speeds in ranges 2.7∗106≤Re≤8.2∗106 and 19≤ns≤52 respectively with error ranging from +7.5 % to −7.5 %. Empirical correlation is also developed to predict the FIN with error ranging from +6 % to −6%, where FIN was the function of structural (geometrical) parameters and hydraulic (fluid and flow) parameters of the pump. Also, the methodology is used to predict ABN at BPF from the calculated FIN for that particular speed with error ranging from −2.5 % to +2.5 %.

Vibration analysis of a high-pressure multistage centrifugal pump

High-pressure multistage centrifugal pumps have been widely used in modern industry and required low vibration and noise. In this study, modal analysis of the rotor system of a seven-stage centrifugal pump was carried out numerically by introducing fluid force to ensure that the centrifugal pump would not resonate. A vibration test bench was established to investigate the characteristics with flow rates of 0.8Qd, 1.0Qd, and 1.2Qd, and the vibration data of ten measuring points were collected. The period of the vibration at the bearing was found to be around 20 ms and the period was related to the shaft frequency (SF) and the blade passing frequency (BPF). The vibration of the pump casing was mainly determined by the SF, two times the SF, and two times the BPF. Mechanical motion is the main factor causing pump vibration, and fluid unstable motion is also an important cause.

Low-frequency amplification of propeller tonal noise due to the scattering by a compact rigid cylinder

The present work addresses the scattering of the tonal noise of a low-speed propeller by a rigid cylinder, as a generic configuration representative of installed marine propellers. Both propeller and cylinder axes are parallel to each other. The diameters of the propeller and of the cylinder are much smaller than the acoustic wavelengths, as well as the propeller-cylinder distance. This corresponds to a compact regime of diffraction. Only the hydrodynamic tonal noise of the propeller at the first multiples of the blade-passing frequency is considered, assuming rigid blades, in a two-dimensional formulation. The direct and scattered sound fields are expressed in terms of spinning modes, with respect to the propeller and cylinder axes, respectively. Use is made of the exact Green’s function of the cylinder for the Helmholtz equation. The modes of orders ±1 are found the only efficient ones in the direct field, whereas higher-order modes rapidly decay. Yet, in the presence of the cylinder, higher-order modes are scattered into the contra-rotating mode of order 1 in the reference frame of the cylinder, with a strong amplification. A simple experiment, performed in air but with Helmholtz numbers typical of marine applications, confirms these results as key features of the asymptotic Green’s function of the cylinder. The same modal behavior is reproduced as closed-form simple expressions from a low-frequency approximation of the Green’s function. The results show that the installation effect is crucial for the tonal noise of marine propellers at very low frequencies.

Inlet Gap Effect on Tonal Noise Generated from a Voluteless Centrifugal Fan

In this study, three voluteless centrifugal fans are compared for their aeroacoustic performances. The tonal noise is predicted by coupling the IDDES with Formulation 1A of Farassat. The sources of the tonal noise at the blade passing frequency (BPF) are identified. It is found that the sources are related to the fan inlet gap, which introduces higher velocity intensities and turbulent fluctuations interacting with the blade leading edge. By redesigning the gap, the tonal noise at the BPF is reduced effectively.

Computational study of boundary layer effects on stochastic rotor blade vortex shedding noise

This work entailed the use of lattice-Boltzmann simulations to investigate the effects of different sUAS rotor blade surface treatments, volumetric grid resolutions, and surface roughness heights on broadband noise prediction. Aerodynamic results showed trailing edge flow separation and a mispredicted laminar separation bubble for the laminar-to-turbulent transitional wall-function case and the fully-turbulent wall-function case as well as a transition front for the laminar-to-turbulent transitional wall-function case. It was seen that the current version of PowerFLOW (6-2021) cannot predict turbulent structures associated with turbulent wall-bounded flow unless a triggering mechanism (e.g., boundary layer trip) is employed, possibly explaining aerodynamic force and broadband noise underprediction. Tonal noise results showed thickness (i.e., geometry dependent) noise dominance at out-of-plane observers located above the rotor plane for the fundamental blade passage frequency and at all observer locations for the second harmonic of the blade passage frequency. Extrapolation techniques were also successfully used over broadband noise results to separate regions of underpredicted broadband noise caused by inadequate spatial resolution from other underpredicted regions caused by the misprediction of flow physics using the current computational strategy.