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The concept of bound states in the continuum and leaky resonances is utilized in the design of a reactive silencer that can effectively suppress significant spectral lines while maintaining a low-pressure drop within the flow duct and does not require additional installation space. By adjusting the geometrical parameters of thin plates that are embedded in a waveguide, quasi-bound states (or leaky resonances) can be achieved. An optimization algorithm is employed to fine-tune these parameters, and this process is illustrated through two specific examples. The resulting design is validated through numerical simulations that account for the effects of low Mach number flow. The investigations showed that it is possible to design a spectral silencer with low-pressure drop based on the chosen approach. By combining several leaky resonances, stopbands were created with a transmission loss of up to 17 dB in a frequency range of 10 Hz. [ABSTRACT FROM AUTHOR]

Advances in wind-plant control have often focused on more effectively balancing power between neighboring turbines. Wake steering is one such method that provides control-based improvements in a quasi-static way, but this does little to fundamentally change the wake recovery process, and thus, it has limited potential. This study investigates use of another control paradigm known as dynamic wake control (DWC) to excite the mutual inductance instability between adjacent tip-vortex structures, thereby accelerating the breakdown of the structures. The current work carries this approach beyond the hypothetical by applying the excitation via turbine control vectors that already exist on all modern wind turbines: blade pitch and rotor speed control. The investigation leverages a free-vortex wake method (FVWM) that allows a thorough exploration of relevant frequencies and amplitudes of harmonic forcing for each control vector (as well as the phase difference between the vectors for a tandem configuration) while still capturing the essential tip-vortex dynamics. The FVWM output feeds into a Fourier stability analysis working to pinpoint candidate DWC strategies suggesting fastest wake recovery. Near-wake length reductions of >80% are demonstrated, although without considering inflow turbulence. Analysis is provided to interpret these predictions considering the presence of turbulence in a real atmospheric inflow. [ABSTRACT FROM AUTHOR]

Being able to predict ship and marine propulsion noise is an important issue for naval architectures and the international maritime community. The main objective of this paper is the numerical investigation on the noise propagation by the high skew marine propeller working in a non-uniform inflow via RANS solver in the broadband frequency range. The pressure fluctuations were monitored at three points on the propeller blade, then by using the FFT operator we computed the blade passing frequency (BPF) for different propeller loading conditions. Based on these pressure pulses and adopting the Fowcs Williams-Hawking model we calculated noise radiated at the monitoring points. The results showed the BPF and noise level increased by increasing the load on the blades and we also observed that the noise generated at the leading edge was greater than at other points. Furthermore, the study of pressure fluctuations showed the propeller tip has more pressure variations in one revolution than other regions of the propeller surface.

This paper presents efficient procedures to model multistage flows using Fourier-based methods. The Favre-averaged nonlinear harmonic method is extended to compute multistage flows by introducing rotor-rotor or stator-stator interactions. A unified treatment to transfer disturbances through blade-row interfaces is also formulated. The performance of the proposed method is demonstrated in a multistage compressor and turbine with different levels of flow nonlinearity. The computed solutions using the proposed method matches well with unsteady Reynolds-averaged Navier-Stokes (RANS) simulations but require far smaller computational resources. A method is proposed to reduce the computational cost of clocking studies even further by building reduced models from existing Fourier modes. The method allows the flowfield for a required clocking position to be constructed by postprocessing the Fourier modes computed for a single arbitrary clocking configuration. This technique is demonstrated by predicting compressor efficiency variation with respect to rotor blade clocking and wall temperature distributions in a turbine hot streak migration problem. Excellent agreement with unsteady RANS simulations is found. The proposed methods become less accurate in the presence of large flow nonuniformity but the produced solutions still show significant improvement over mixing plane solutions. [ABSTRACT FROM AUTHOR]

Investigations and observations on fluid flow and performance characteristics (numerically and experimentally) and sound generation (experimentally) of single and double outlet squirrel cage fans are performed in this study. The main objective is to survey the performance of double outlet fans and the effect of two volute tongues as main sound sources. Fan performance and sound experiments are conducted using an in-duct experimental setup. The efficiency and pressure curves show that each outlet channels of the double outlet fan operates similar to a single outlet one. As a criterion for evaluating the sound generation, total sound pressure level (SPL) and the noise component at blade passing frequency (BPF) in the power spectrum are considered. A comparison between the total sound pressure levels of the fans shows that in both of them the BPF noise increases with flow rate, while higher SPL is found for the double outlet one. An exactly-higher velocity jet/wake flow from the rotor in double outlet fan is responsible for the higher BPF noise.

The demands to apply transonic centrifugal compressor have increased in the advanced gas turbine engines. Various techniques are used to increase the aerodynamic performance of the centrifugal compressor. The effects of the inclined leading edges in diffuser vanes of a transonic centrifugal compressor on the flow-field unsteadiness and noise generation are investigated by solving the compressible, three-dimensional, transient Navier–Stokes equations. Diffuser vanes with various inclination angles of the leading edge from shroud-to-hub and hub-to-shroud are numerically modeled. The results show that the hub-to-shroud inclined leading edge improves the compressor performance (2.6%), and the proper inclination angle is effective to increase the stall margin (3.88%). In addition, in this study, the transient pressure variations and radiated noise prediction at the design operating point of the compressor are emphasized. The influences of the inclined leading edges on the pressure waves were captured in time/space domain with different convective velocities. The pressure fluctuation spectra are calculated to investigate the tonal blade passing frequency (BPF) noise, and it is shown that the applied inclination angles in the diffuser blades are effective, not only to improve the aerodynamic performance and stall margin, but also to reduce the BPF noise (7.6 dB sound pressure level reduction). Moreover, it is found that the diffuser vanes with inclination angles could suppress the separation regions and eddy structures inside the passages of the diffuser, which results in reduction of the overall sound pressure level and the broadband noise radiated from the compressor. [ABSTRACT FROM AUTHOR]

Investigations and observations on fluid flow and performance characteristics (numerically and experimentally) and sound generation (experimentally) of single and double outlet squirrel cage fans are performed in this study. The main objective is to survey the performance of double outlet fans and the effect of two volute tongues as main sound sources. Fan performance and sound experiments are conducted using an in-duct experimental setup. The efficiency and pressure curves show that each outlet channels of the double outlet fan operates similar to a single outlet one. As a criterion for evaluating the sound generation, total sound pressure level (SPL) and the noise component at blade passing frequency (BPF) in the power spectrum are considered. A comparison between the total sound pressure levels of the fans shows that in both of them the BPF noise increases with flow rate, while higher SPL is found for the double outlet one. An exactlyhigher velocity jet/wake flow from the rotor in double outlet fan is responsible for the higher BPF noise. [ABSTRACT FROM AUTHOR]

Flow around the Guide vane! (GV) in Francis turbine differs with the shape of hydrofoils. The difference in the pressure of fluid travelling 10 pressure side and suction side of GV contributes to flow behavior. This study presents the numerical technique using alternative clearance gap method to predict the flow around GV and its consequent effect on turbine performance. GV profile has a significant effect on the performance of the turbine with sediment contained flui i flow. In this paper, symmetrical NACA 0012 and cambered NACA 2412, NACA 4412 hydrofoils are studied introducing 0 mm, 2 mm, and 4 mm clearance gaps. Vortex filament can be seen when fluid leaves the clearance gap due to the leakage flow occurring through the gap. The intensity of vortex leaving clearance gap rises with an increase in the size of the clearance gap. However, in the case of asymmetrical GV profile, the velocity of fluid travelling along the vortex compared to that of symmetrical hydrofoil is lower. In case of low specific speed Francis turbines, this vortex is found to be a major reason to erode the runner surface d ie to high velocity of a sand particle travelling with them. With the alternative clearance gap approach, this paper compares the pressure pulsation downstream o f GVs contributed by leakage flow for three NACA profiles, whose frequency is half of blade passing frequency. [ABSTRACT FROM AUTHOR]

A large-scale, vaned-voluted centrifugal pump can be applied as the key component in water-transfer projects. Pressure pulsation will be an important factor in affecting the operation stability. This paper researches the propagation and spatial distribution law of blade passing frequency (BPF) and its harmonics on the design condition by numerical simulation. Experimental and numerical monitoring is conducted for pressure pulsation on four discrete points in the vaneless region, which shows that the BPF is dominant. The pulsation tracking network (PTN) is applied to research propagation law and spatial distribution law. It provides a reference for frequency domain information and visualization vaned diffuser. The amplitude of BPF and its harmonics decays rapidly in the vaneless region. BPF and BPF’s harmonics influence each other. BPF has local enhancement in the vaneless region when its harmonics attenuate. In the vaned diffuser, the pulsation amplitude of BPF attenuates rapidly, but the local high-pressure pulsation amplitude can be found on the vane blade concave side because of obstruction and accumulation of the vaned diffuser. In the volute, the pulsation amplitude of BPF is low with the decelerating attenuation. This study provides an effective method for understanding the pressure pulsation law in turbomachinery and other engineering flow cases.

The demands to apply transonic centrifugal compressor have increased in the advanced gas turbine engines. Various techniques are used to increase the aerodynamic performance of the centrifugal compressor. The effects of the inclined leading edges in diffuser vanes of a transonic centrifugal compressor on the flow-field unsteadiness and noise generation are investigated by solving the compressible, three-dimensional, transient Navier–Stokes equations. Diffuser vanes with various inclination angles of the leading edge from shroud-to-hub and hub-to-shroud are numerically modeled. The results show that the hub-to-shroud inclined leading edge improves the compressor performance (2.6%), and the proper inclination angle is effective to increase the stall margin (3.88%). In addition, in this study, the transient pressure variations and radiated noise prediction at the design operating point of the compressor are emphasized. The influences of the inclined leading edges on the pressure waves were captured in time/space domain with different convective velocities. The pressure fluctuation spectra are calculated to investigate the tonal blade passing frequency (BPF) noise, and it is shown that the applied inclination angles in the diffuser blades are effective, not only to improve the aerodynamic performance and stall margin, but also to reduce the BPF noise (7.6 dB sound pressure level reduction). Moreover, it is found that the diffuser vanes with inclination angles could suppress the separation regions and eddy structures inside the passages of the diffuser, which results in reduction of the overall sound pressure level and the broadband noise radiated from the compressor.

Vertical axis wind turbines (VAWT) are a source of renewable energy and are used for both industrial and domestic purposes. The study of noise characteristics of a VAWT is an important performance parameter for the turbine. This study focuses on the development of a linear microphone array and measuring acoustic signals on a cambered five-bladed 45 W VAWT in an anechoic chamber at different tip speed ratios. The sound pressure level spectrum of VAWT shows that tonal noises such as blade passing frequencies dominate at lower frequencies whereas broadband noise corresponds to all audible ranges of frequencies. This study shows that the major portion of noise from the source is dominated by aerodynamic noises generated due to vortex generation and trailing edge serrations. The research also predicts that dynamic stall is evident in the lower Tip speed ratio (TSR) region making smaller TSR values unsuitable for a quiet VAWT. This paper compares the results of linear aeroacoustic array with a 128-MEMS acoustic camera with higher resolution. The study depicts a 3 dB margin between two systems at lower TSR values. The research approves the usage of the 8 mic linear array for small radius rotary machinery considering the results comparison with a NORSONIC camera and its resolution. These observations serve as a basis for noise reduction and blade optimization techniques.

Pressure fluctuation is the primary factor that affects the stability of turbomachines. The goal of the present work is to explore the propagation of pressure fluctuations in a two-stage double-suction centrifugal pump. The pressure fluctuation characteristics of each component of a two-stage double-suction centrifugal pump are simulated under four typical flow rates based on the SST k-ω turbulence model. It is shown that the pressure fluctuation frequency at blade passing frequency and its first harmonic is the same at the suction chamber, the leading edge, and the middle of the first-stage impeller, which is different from the rotor–stator interaction. Moreover, the uneven impeller inlet flow distribution will produce fluctuations with rotation frequency and its harmonics at the leading edge of the impellers in both stages. Finally, broadband frequency is found at the trailing edge of the impellers in both stages associated with the first harmonic of the rotation frequency, especially under the part load condition. The large size backflow vortex appears in the blade flow channel leading to the low-pressure zone between the impeller, the tongue, and the start of the partition. That is why the pressure drops significantly twice in one rotation period when the blades pass through the tongue and the start of the partition.

Three-dimensional, compressible, unsteady Navier–Stokes equations are solved to investigate the flow field of a transonic centrifugal compressor with high compression ratio. The computational domain is consisted of an inlet bell mouth and an impeller with splitter blades, followed by a two-dimensional wedge vaned diffuser. The numerical method was validated by comparing the results with those of experiments in terms of aerodynamic compressor performance and flow field within the compressor passages. A detailed analysis of instantaneous and time-averaged flow field was conducted in the impeller and diffuser passages. The present study focuses on the pressure fluctuations and entropy production within the impeller and diffuser passages at the compressor design point. It is shown that the interaction between the impeller and diffuser blades leads to unsteadiness at the interface region and a pulsating behavior within the diffuser passages. Pressure waves with different convective velocities, generated by the impeller–diffuser interaction and pseudo-periodic unsteady separation bubbles, are captured in the time/space domain along the diffuser blade surfaces. The pressure fluctuation spectra were evaluated to analyze the noise characteristics of the centrifugal compressor as the main source of blade passing frequency noise. It is expected that the current unsteady Reynolds-Averaged Navier–Stokes (URANS) approach can be used as a tool for the prediction of unsteady flow and compressor noise characteristics with a proper turbulence model and well-resolved grids. [ABSTRACT FROM AUTHOR]

Three-dimensional, unsteady Navier-Stokes equations are numerically solved to investigate the turbulent flows in a stirred vessel. The computational domain consists of an unbaffled, cylindrical vessel with a pitched-blade turbine impeller. An Eulerian-Eulerian multiphase flow model is applied to determine the shape of the free-surface vortex core. This numerical method is validated by comparing its results with laser Doppler velocimetry measurements in terms of velocity distribution and turbulence kinetic energy profiles at different positions. In the present study, URANS approach with a hybrid zonal turbulence model, k − ω SST and SST-SAS, is used to predict the unsteady pressure and velocity fluctuations within the vessel. Pressure and inward-outward radial velocity waves are generated by the impeller rotation and are captured in the time/space domain close to the impeller trailing edge. The pressure and velocity spectra are computed to characterize the blade passing frequency as the main source of unsteadiness in the turbulent flow within the vessel. The results indicate that the current URANS approach with a proper turbulence model and well-resolved grids can be used as a predictive tool for the flow field and large turbulence scales in the stirred tanks. [ABSTRACT FROM AUTHOR]

Cavitation in pumps causes destructive consequences and occurs at certain operating circumstances. Therefore, it must be detected and prevented. The main thrust of the present work is detection and investigation of cavitation in a centrifugal pump experimentally, by monitoring the flow characteristics as well as the vibration spectrum. It is an attempt to declare the discrete signature of cavitation at various strengths on the vibration spectrum. The experimental results on a centrifugal pump at a rotational speed of 2850 rpm and absolute values of suction pressures from 10 to 78 kPa, showed that the inception of cavitation is declared by sudden deterioration in the hydraulic characteristics, decrease in the efficiency and increase in the overall vibration level. As the cavitation strength increases, the vibration level decreases and then increases sharply. Cavitation can be detected by online monitoring of the vibration amplitude of the discrete frequencies corresponding to the pump rotational speed and the first blade passing. As the pump operation moves from no-cavitation to cavitation case, the vibration amplitude of the rotational speed frequency increases while it decreases for the first passing frequency. Also, the cavitation in pumps signs the vibration spectrum by appearance of high energy at high frequencies. [ABSTRACT FROM PUBLISHER]

Copyright of Journal of Drainage & Irrigation Machinery Engineering / Paiguan Jixie Gongcheng Xuebao is the property of Editorial Department of Drainage & Irrigation Machinery Engineering and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Abstract: Pressure pulsations excited by a centrifugal turbomachinery such as compressor, fan or pump at the blade passing frequency may cause severe noise and vibrations in piping system. Therefore, the practical evaluation method of pressure pulsations is strongly recommended. In particular, the maximum pressure amplitude under the resonant conditions should be appropriately evaluated. In this study, a one-dimensional excitation source model for a compressor or pump is introduced based on the equation of motion, so as to incorporate the non-linear damping proportional to velocity squared in the total piping system including the compressor or pump. The damping characteristics of the compressor or pump are investigated by using the semi-empirical model. It is shown that the resistance coefficient of the compressor or pump depends on the Reynolds number that is defined using the equivalent velocity of the pulsating flow. The frequency response of the pressure amplitude and the pressure distribution in the piping system can be evaluated by introducing the equivalent resistance of the compressor or pump and that of piping system. In particular, the relation of the maximum pressure amplitude in piping system to the location of the excitation source under resonant conditions can be evaluated. Finally, the reduction of the pressure pulsations by use of an orifice plate is discussed in terms of the pulsation energy loss. [Copyright &y& Elsevier]

Abstract: In a 210MWe gas turbine, compressor blades in stage 3 failed and caused large scale collateral damage to the plant. The plant outage resulted in heavy loss to the generation company. The blade fractured at root of the airfoil due to high cycle fatigue as evidenced by beach marks and microscopic striations on the leading edge of the blade. Corrosion related pitting was also noticed on the leading edge fillet region on many blades of stage 3 and also on many blades of other stages. The Campbell diagram indicated possibility of high vibratory stresses at the leading edge due to excitation of the fundamental mode of the blade. Due to large number of startups caused due to technical reasons, it was estimated that the response of the blade could have initiated crack in the pitted region. It was further estimated that after the startup, crossing of the other fundamental modes at the synchronous speed could have propagated the crack to failure under normal operating stress. On the two other working units of similar make and capacity, it was necessary to assess the health of the compressors blades and take necessary action if warranted. The paper provides failure analysis of the failed blade and a new approach based on casing vibration analysis for assessing health of compressor blades in two operating plants. The innovative approach identified abnormalities in both the operating units. In the immediate plant outage, the compressor blades were examined by ultrasonic technique and cracks were detected in several blades at the root. The unit was thus saved from yet another major mishap. [Copyright &y& Elsevier]

A large-scale, vaned-voluted centrifugal pump can be applied as the key component in water-transfer projects. Pressure pulsation will be an important factor in affecting the operation stability. This paper researches the propagation and spatial distribution law of blade passing frequency (BPF) and its harmonics on the design condition by numerical simulation. Experimental and numerical monitoring is conducted for pressure pulsation on four discrete points in the vaneless region, which shows that the BPF is dominant. The pulsation tracking network (PTN) is applied to research propagation law and spatial distribution law. It provides a reference for frequency domain information and visualization vaned diffuser. The amplitude of BPF and its harmonics decays rapidly in the vaneless region. BPF and BPF's harmonics influence each other. BPF has local enhancement in the vaneless region when its harmonics attenuate. In the vaned diffuser, the pulsation amplitude of BPF attenuates rapidly, but the local high-pressure pulsation amplitude can be found on the vane blade concave side because of obstruction and accumulation of the vaned diffuser. In the volute, the pulsation amplitude of BPF is low with the decelerating attenuation. This study provides an effective method for understanding the pressure pulsation law in turbomachinery and other engineering flow cases. [ABSTRACT FROM AUTHOR]

Vertical axis wind turbines (VAWT) are a source of renewable energy and are used for both industrial and domestic purposes. The study of noise characteristics of a VAWT is an important performance parameter for the turbine. This study focuses on the development of a linear microphone array and measuring acoustic signals on a cambered five-bladed 45 W VAWT in an anechoic chamber at different tip speed ratios. The sound pressure level spectrum of VAWT shows that tonal noises such as blade passing frequencies dominate at lower frequencies whereas broadband noise corresponds to all audible ranges of frequencies. This study shows that the major portion of noise from the source is dominated by aerodynamic noises generated due to vortex generation and trailing edge serrations. The research also predicts that dynamic stall is evident in the lower Tip speed ratio (TSR) region making smaller TSR values unsuitable for a quiet VAWT. This paper compares the results of linear aeroacoustic array with a 128-MEMS acoustic camera with higher resolution. The study depicts a 3 dB margin between two systems at lower TSR values. The research approves the usage of the 8 mic linear array for small radius rotary machinery considering the results comparison with a NORSONIC camera and its resolution. These observations serve as a basis for noise reduction and blade optimization techniques. [ABSTRACT FROM AUTHOR]

Pressure fluctuation is the primary factor that affects the stability of turbomachines. The goal of the present work is to explore the propagation of pressure fluctuations in a two-stage double-suction centrifugal pump. The pressure fluctuation characteristics of each component of a two-stage double-suction centrifugal pump are simulated under four typical flow rates based on the SST k-ω turbulence model. It is shown that the pressure fluctuation frequency at blade passing frequency and its first harmonic is the same at the suction chamber, the leading edge, and the middle of the first-stage impeller, which is different from the rotor–stator interaction. Moreover, the uneven impeller inlet flow distribution will produce fluctuations with rotation frequency and its harmonics at the leading edge of the impellers in both stages. Finally, broadband frequency is found at the trailing edge of the impellers in both stages associated with the first harmonic of the rotation frequency, especially under the part load condition. The large size backflow vortex appears in the blade flow channel leading to the low-pressure zone between the impeller, the tongue, and the start of the partition. That is why the pressure drops significantly twice in one rotation period when the blades pass through the tongue and the start of the partition. [ABSTRACT FROM AUTHOR]