Strong Pressure Fluctuations Research Articles

Experimental study of the transmission of vibrations and noise of an hydrofoil excited by cavitation in a hydrodynamic tunnel

Ship propellers are responsible for radiated noise in the near and far environment. One of the main sources of noise from propellers is due to the cavitation phenomenon that occurs on each blade. This phenomenon produces strong pressure fluctuations, which in turn generate noise that propagates in the ocean but also inside the boat. The strong wall pressure fluctuations excite the hull structure, which then transmits the noise inside the ship. The transmission is problematic for passenger ships and cruise liners, for whom silence is synonymous with quality. Efforts are being made to improve sound absorption and insulation, but the phenomenon of noise transmission from propellers to the hull via strong pressure fluctuations is still poorly understood. The aim of the paper is to simplify the problem studying a hydrofoil excited by a controlled flow and subjected to different states of cavitation. Vibration and acoustic measurements will be carried out to study the transmission of noise and vibration through the tunnel wall in order to be correlated to the different states of cavitation.

Evolution of clusters of turbulent reattachment due to shear layer instability in flow past a circular cylinder

We perform large eddy simulations of flow past a circular cylinder for the Reynolds number (Re) range, 2×103≤Re≤4×105, spanning subcritical, critical, and supercritical regimes. We investigate the spanwise coherence of the flow in the critical and supercritical regimes using complex networks. In these regimes, the separated flow reattaches to the surface in a turbulent state due to the turbulence generated by the shear layer instability. In the early critical regime, the turbulent reattachment does not occur simultaneously at all span locations. It occurs incoherently along the span in clusters. We treat strong surface pressure fluctuations due to the shear layer instability as extreme events and construct time-varying spatial proximity networks where links are based on synchronization between events. This analysis unravels the underlying complex spatiotemporal dynamics by enabling the estimation of characteristics of clusters of turbulent reattachment via the concept of connected components. In the critical regime, the number and size of the clusters increase with the increase in Re. At higher Re in the supercritical regime, they coalesce to form bigger clusters, resulting in increase in spanwise coherence of turbulent reattachment. We find that the size and number of clusters govern the variation of the time-averaged coefficient of drag (C¯D) in the critical and supercritical regimes. C¯D exhibits power-law distribution with the largest cluster size (C¯D∝S¯CL−25) and the most probable cluster size [C¯D∝E(SC)−25].

Flow boiling heat transfer and pressure fluctuation characteristics in a divergent channel with inclined downward-facing heater

The core catcher cooling system is used in nuclear power plant to mitigate the core meltdown accident through flow boiling heat transfer. In order to better understand the flow boiling characteristics in the system, experimental research was conducted on an inclined channel. The flow channel is oriented 15° with a divergent downward-heating surface. The two-phase flow behavior and pressure fluctuations were capture by a high-speed camera and pressure transducer. The inlet mass flux and subcooling effects on flow boiling characteristics were deeply investigated. The inlet mass flux and subcooling conditions are within 100–400 kg/m2s and 1–30 K, respectively. The results showed that the boiling curves shift to the higher wall superheat with the increase of mass flux, and the effect of subcooling is weak. The bubble dynamic images by the high-speed camera showed that the bubbles continue to grow and coalesce during the sliding process along the wall. Large bubble blankets are formed periodically upon the boiling surface beyond a heat flux threshold, and they would undergo a rapid condensation process after departing the heating area, which leads to the liquid reversal and accompanied by strong pressure fluctuations. Further, beyond a certain subcooling and heat flux, two-phase flow instability phenomenon appears, which is characterized by complete condensation of bubble blanket accompanied by pressure shocks, meanwhile, the heat transfer coefficient decreases and the wall superheat increases sharply. The bubble blanket frequency could be obtained by the Fast Fourier Transform analysis of pressure, which is positively correlated with the heat flux and is negatively correlated with the mass flux and subcooling. Furthermore, a dimensionless correlation of reduction factor (RF) was proposed, which directly related the bubble blanket characteristics to the thermal–hydraulic parameters.

Exploring the Mechanism of Strong-Pressure Fluctuation under Partial Load in the Turbine Mode of Pump Turbines for Hydro and Marine Power Storage

As a core component of pumped storage power plants for hydro and marine power storage, this paper investigates the mechanism of pressure pulsation fluctuations under different load conditions to improve the efficiency and operational stability of the storage units. The results of a combination of experiments and numerical simulations showed that the pressure pulsation fluctuations in the pump turbine under strong and weak loads were significantly different at different monitoring points. The three-dimensional flow lines diagram of the pump turbine unit from the CFD numerical simulation showed that the flow line of the pump turbine was relatively chaotic and the vortex existed under weak load conditions. Pressure clouds and flow lines in the cross-section and longitudinal section of the pump turbine are shown. Both showed high-pressure values and a chaotic flow line with a vortex under weak load conditions. To a certain extent, it revealed the pressure pulsation fluctuation mechanism of the pump turbine and provides some guidance for solving practical problems in engineering.

Spatiotemporal Characteristics and Pressure Fluctuations of Internal Flow in a High-Speed Centrifugal Blower for Vacuum Cleaner at Low Flow-Rate Conditions

The steady and unsteady characteristics of the internal flow in a high-speed centrifugal blower are studied by computational fluid dynamics (CFD) approach at low flow rates. It is demonstrated that as the flow rate decreases, the separation of flow in the blade passage becomes serious, and separated vortexes always occur on the suction surface of the blade which gradually expand and block the passage. The stall cells move downstream and generate vortices at the exit of the passage, resulting serious loss to the performance of the blower. Q-criteria is used to analyze the flow field and explore the evolution of the vortex structure in the impeller. It is further found that strong pressure fluctuations are caused by the rotating stall in the impeller. At the stall conditions, the instability characteristics are particularly obvious. At flow rates of 0.65Qn and 0.47Qn, the pressure fluctuation in the blade passage is dominated by the blade passing frequency, while a lower frequency dominates at 0.26Qn. Moreover, the flow on the suction surface of impeller blades fluctuates substantially. The characteristics of steady flow and unsteady flow can clearly explain the internal flow of centrifugal blower for vacuum cleaners at low-flow conditions, which can be widely used in various engineering designs of vacuum cleaners.

Stress Characteristic Analysis of Pump-Turbine Head Cover Bolts during Load Rejection Based on Measurement and Simulation

It is not uncommon for pump-turbine units in pumped storage power plants to experience load rejections due to the sudden disconnection of the generator from the power grid. Load rejection can suddenly increase the rotating speed of the pump-turbine and cause strong pressure fluctuations in the flow passage of the pump-turbine unit. During load rejection, the strong pressure fluctuations caused by the water hammer effect can cause strong structural vibrations, high stresses and even damage to the turbine runner, head cover, stay ring, bottom ring, head cover bolts and bottom ring bolts. In order to study, in detail, the flow-induced stress characteristics of the prototype pump-turbine unit, and the pressure variations during load rejection in a high-head pumped storage power plant were measured first. Then the measured data were used to set up computational fluid dynamics (CFD) simulations in the entire flow passage of the prototype pump-turbine and to calibrate the simulation results. The calculated pressure distributions in the flow passage during load rejection were exported and mapped on the finite element model of the stationary structures of the pump-turbine unit so that the flow-induced stresses on the head cover, stay ring, bottom ring, head cover bolts and bottom ring bolts can be calculated. The results of the analysis show that the maximum stresses in the head cover bolts and bottom ring bolts are located on the rounded corner of the bolt near the stay ring and that the stresses in the bolts vary with time during load rejection. The maximum stresses of the head cover bolts are higher than the maximum stresses of the bottom ring bolts, and the maximum stresses of the bolts are above two-thirds of the yield strength of the bolt material. It is recommended to use larger nominal diameter bolts to avoid damage to the connecting bolts of the pump-turbine unit.

Bladder paraganglioma: basic characteristics and new perspectives on perioperative management.

Paraganglioma and pheochromocytoma are rare neuroendocrine tumors with severe metabolic and cardiovascular complications. Bladder PGLs are rare, and their clinical management is not precise. Here, we discuss the basic characteristics and perioperative management of bladder PGLs. We retrospectively reviewed 20 bladder PGL cases diagnosed at Sun Yat-sen University Cancer Center. Case notes were reviewed, clinical presentations, therapies, and outcomes were collected, and data analysis was performed. Ten male and ten female patients with a median age of 47.5years (range 14-69years) were included. Most patients (65%) had no symptoms, and PGL was detected incidentally during medical checkups. All patients were treated surgically; 4 (20%) underwent transurethral resection of bladder tumor (TURBT), and 16 (80%) underwent partial cystectomy. Strong intraoperative blood pressure fluctuations were observed in 13 patients (65%). Two patients who were treated preoperatively with α-receptor blockers also experienced severe intraoperative blood pressure fluctuations. Postoperative measurements of troponin I were available for 3 patients, and all were significantly elevated. All patients were diagnosed with bladder PGL on postoperative pathological examination. The median follow-up time was 51months (range 2-147months), and 2 patients were lost to follow-up at 1 and 3months; 16 (88.9%) survived without recurrence, 2 patients (11.1%) experienced recurrence, and 1 patient died. Most bladder paragangliomas are easily mistaken for bladder urothelial carcinoma, and robust hemodynamic instability during surgery might be a challenge for urologists. Postoperative monitoring of troponin I, regardless of the presence of clinical symptoms, is recommended for patients with bladder PGL.

Unconventional relaxation of hydrodynamic modes in anharmonic chains under strong pressure fluctuations

We develop an effective numerical scheme to capture hydrodynamic (HD) modes in general classical anharmonic chains. This scheme is based on the HD theory suggested by Ernst–Hauge–van Leeuwen, which takes full role of pressure fluctuations into account. With this scheme we show that the traditional pictures given by the current nonlinear fluctuating HD theory are valid only when the system’s pressure is zero and the pressure fluctuations are weak. For nonvanishing pressure, the HD modes can, however, respond to small and large pressure fluctuations and relax in some distinct manners. Our results shed new light on understanding thermal transport from the perspective of HD theory.

Computational Modeling of Aortic Stenosis With a Reduced Degree-of-Freedom Fluid-Structure Interaction Valve Model.

In this study, a novel reduced degree-of-freedom (rDOF) aortic valve model is employed to investigate the fluid-structure interaction (FSI) and hemodynamics associated with aortic stenosis. The dynamics of the valve leaflets are determined by an ordinary differential equation with two parameters and this rDOF model is shown to reproduce key features of more complex valve models. The hemodynamics associated with aortic stenosis is studied for three cases: a healthy case and two stenosed cases. The focus of the study is to correlate the hemodynamic features with the source generation mechanism of systolic murmurs associated with aortic stenosis. In the healthy case, extremely weak flow fluctuations are observed. However, in the stenosed cases, simulations show significant turbulent fluctuations in the ascending aorta, which are responsible for the generation of strong wall pressure fluctuations after the aortic root mostly during the deceleration phase of the systole. The intensity of the murmur generation increases with the severity of the stenosis, and the source locations for the two diseased cases studied here lie around 1.0 inlet duct diameters (Do) downstream of the ascending aorta.

회전 억제용 플로팅 링 실을 사용한 7톤급 터보펌프의 실매질 시험 고주파 신호 분석

A floating ring seal in turbopump is frequently employed to control the leakage through the secondary flow passage. Depending on its shape and operating conditions of the turbopump, the floating ring seal can have excursive motions and such motions induce strong pressure fluctuations, wear, and frictional oxidation due to rubbing between impeller and pump casing. It affects pump inlet and outlet pressures resulting in unstable operation or, in severe cases, failure of the turbopump. In this study, high-frequency signals from an accelerometer and pressure fluctuation sensors installed on the LOX pump were measured. Frequency spectrums and Root Mean Square (RMS) values were calculated in the real-propellant test for 7-tonf turbopump using two types of floating ring seal, the normal floating ring seal and the anti-rotating floating ring seal. The specific instability frequencies were investigated, and visual inspection was conducted to estimate the structural and hydrodynamic stabilities for 7-tonf turbopump. In consequence, the anti-rotating floating ring seal is able to effectively reduce the unstable pressure fluctuations and the wear compared to the normal floating ring seal.

STATISTICAL CHARACTERISTICS OF PRESSURE FLUCTUATION IN SHOCK WAVE AND TURBULENT BOUNDARY LAYER INTERACTION

Shock wave and turbulent boundary layer interaction widely exists in the internal and external flow of high-speed aircraft. The aerodynamic performance and flight safety of aircraft are seriously affected by the strong pressure fluctuation in the interaction region. To investigate statistical characteristics of fluctuating pressure, the interaction between an incident shock of 33.2° and a spatially developed Mach 2.25 turbulent boundary layer is analyzed by means of direct numerical simulation (DNS). The numerical results have been carefully validated against with previous experiment and DNS at similar flow conditions in terms of mean velocity profile, turbulence intensity and wall pressure distribution. Statistics at the wall and in the outer layer, including fluctuation intensity, power spectral density, two-point correlation and space-time correlation, are quantitatively compared. The differences between them are analyzed in detail. It is found that the effect of the shock interaction on the wall-pressure fluctuation and the fluctuating pressure in the outer layer are utterly different. Based on the analysis of the power spectra density, the fluctuations in the separated region are both characterized by the low-frequency content, but in the reattachment region, the peak frequency of outer pressure fluctuations quickly shifts to higher frequency, with the low-frequency energy of wall-pressure fluctuation still being predominant. It is identified that the two-point correlations of pressure fluctuation at the wall and in the outer layer are both more elongated in the spanwise direction than that in the streamwise direction. The integral scale at the wall is generally increased, while the one in the outer layer increases sharply after passing the shock and then gradually decreases. The analysis of space-time correlation indicates that the iso-correlation contours are similar to the elliptical distribution and the convection velocity deduced by the correlation is dramatically decreased. Downstream of the interaction, the convection velocity in the outer layer is higher than that of wall-pressure fluctuation.

Spatiotemporal correlation and control of wall pressure fluctuations induced by subsonic cavity flows

Strong wall pressure fluctuations are generated mostly by a high-speed flow passing over a cavity and should be accurately predicted and controlled for the structural design of aircraft. In this work, a spatiotemporal analysis was performed to estimate wall pressure fluctuations and reveal the mechanism of flow control by a sawtooth-like vortex generator installed on the leading edge or by the use of a rounded trailing edge in suppressing wall pressure fluctuations. Both the cavity tones and overall sound pressure level were analyzed under different flow speeds and flow control approaches. To investigate the power transfer in the temporal and spatial domains, power spectral density contours were drawn and discussed. For the mode separation, the phased array technique with an improved beamforming algorithm was adopted to calculate the wavenumber map and then to separate the acoustic modes from the convective modes by utilizing the spatial correlation of the wall pressure fluctuations. The mode separation results show that the convective modes contribute mainly to the low-frequency pressure fluctuations, while the acoustic modes contribute to both the low-frequency and high-frequency pressure fluctuations. Flow control by the rounded trailing edge approach is more effective than that by the serrated structure approach in reducing the cavity acoustic environment, which enables wall pressure fluctuations to transfer from the high-frequency range to the low-frequency range compared with the “no control” case.

Short- and long-term agreement and reproducibility of 48-hours intraocular pressure measurements in glaucoma patients

BackgroundGlaucomatous eyes often show strong intraocular pressure (IOP) fluctuations and individual measurements at different time points are necessary for personalized therapy. To survey IOP variations 48-hours diurnal and nocturnal IOP measurements were performed on two consecutive days. Aims of this study were to investigate the short-term repeatability of 48-hours measurements within one patient’s IOP profile and long-term repeatability between two separate IOP profiles of the same patient.MethodsA retrospective cohort study was performed evaluating data of 90 glaucoma patients in a German university medical center between 2006 and 2013. All patients underwent two separate diurnal IOP profiles of 48 h. IOP was measured at 8 am, 2 pm, 6 pm, 9 pm using Goldmann applanation tonometry and at 12 midnight using Perkins tonometry in supine position on two consecutive days. Intraclass correlation coefficients (ICC) were calculated to evaluate agreement for the same time points (each time point agreement) and for consecutive measurements within the IOP profiles (between time point agreement). ICC ≤ 0.4 was defined as poor agreement, 0.4–0.75 as moderate and ≥ 0.75 as excellent. Differences between time points were investigated by Bland Altman plots.ResultsEach time point measurements of profile 1 showed moderate to excellent agreement (ICCs 0.62–0.93). There was a moderate to excellent agreement for measurements between time points of profile 1 (ICCs day one 0.57–0.86, day two 0.71–0.90). Profile 2 was performed at a median interval of 12.0 months (quartiles 11.0 to 21.0). Each time point agreements within profile 2 showed ICCs from 0.23 to 0.81. It showed moderate to excellent agreement for changes between time points (ICCs 0.53–0.94). Day two demonstrated ICCs from 0.74 to 0.88. Long term IOP repeatability (over both pressure profiles) showed moderate to good agreement (ICCs 0.39–0.67).ConclusionsShort and long-term agreement of IOP measurements evaluated by diurnal IOP profiles is moderate to good. Due to mostly moderate agreements, which we believe represent IOP fluctuations, we conclude that it is necessary to perform 48-hours IOP profiles to gain a better overview of the individual IOP fluctuations.

Comparisons of vortical flow and cavitation inside a Francis turbine with different draft tubes

It is well known that strong pressure fluctuation will occur in the draft tube of a Francis turbine operated under part-load operation. The pressure fluctuation is usually due to the vortex rope as well as cavitation. However, the vortical flow and pressure field are dependent on the geometry of draft tube of Francis turbine. In this study, hydraulic performance and pressure fluctuation have been tested for a model Francis turbine. The internal flow for the turbine is investigated numerically. It is confirmed that the numerical results predict reasonably the hydraulic performance and the pressure fluctuation due to vortex rope at part-load operation.For a better understanding of the vortical flow in the model turbine, a conical draft tube is used instead of the conventional elbow draft tube. The vorticity transport equation is applied to analyze the vortex evolution inside Francis turbine based on the unsteady flow simulation. The flow mechanism of the vortex rope and pressure fluctuation with different draft tubes is discussed. It is shown that both cavitation and the bending structure influence the vortex rope, which will cause a significant difference in the distribution of pressure fluctuation.

Influence of Forward Skew Blade Angle on Positive Slope Characteristics of Mixed Flow Pumps

Under the part-load conditions, the flow instability in a mixed-flow pump could lead to strong pressure fluctuations and vibrations, posing a great threat to the safety of the system. The current paper investigates effect of the forward skew blade angle on the positive slope characteristic in a high specific speed mixed flow pump using Reynolds averaged Navier-Stokes (RANS) equations coupled with the SST k-ω model. The simulation results are compared with the experimental data and show good accordance. In the present study, five types of the mixed-flow pumps with different skew blade angles are prepared based on the conventional pump. It is found that with the increasing of the skew angle, the positive slope region becomes wider and moves towards a deeper part-load region. Further investigations are carried out by calculating the impeller head. Results depict that compared with the dynamic pressure head H d, the static pressure head H st plays more significant role in the formulation of the positive slope characteristic (PSC). Besides, the relative static pressure head H rel drops dramatically while the pump operates under the positive slope region, which matches with the pump performance well. Finally, analysis on the spanwise velocity profiles are further investigated in this paper, and results indicate that the flow accumulated near the impeller hub is one of the reasons to alleviate the head drop and shift the positive slope region to a deeper part-load condition.

A Numerical Investigation on the Rapid Turbulent Filtration Combustion under High Background Pressure

ABSTRACT A numerical investigation of the rapid turbulent filtration combustion within a packed pebble bed is conducted at a fixed superficial velocity of 0.5 m/s, equivalence ratio of 1.0 (methane/air) under two background pressures of 1.0 and 0.5 MPa. Turbulence is modeled with the detached eddy simulation (DES) method. The interaction between turbulence and chemical reaction is considered through the fast reaction method, i.e., burning velocity model (BVM). To validate the DES method, the relative turbulence intensities in a 3D staggered cylinders are verified previously which are in excellent agreement with the experimental data (no reaction). With the confidence in the DES method, the turbulent combustion characteristics in a 3D packed pebble bed are studied in detail, including pressure fluctuation, spatial inhomogeneities of velocity and temperature fields, and evolution of the flame surface. Numerical results show that a strong pressure fluctuation arises after ignition due to thermal expansion contributing significantly to the velocity and temperature fluctuations for the case of 1.0 MPa, whereas it is relatively moderate for the case of 0.5 MPa. Though the evolution of the flame surface both can be divided into two stages for the two cases, i.e., initial-rapid spreading stage, and stable propagating stage, there is a slight difference between them. Specifically, it closely resembles to an atomic blast for the former, while it is similar to a volcanic eruption for the later. Undergoing this stage, about 20 ms, the wrinkle and fragmented flames or flamelets creep over the pebbles through the thin layers close to the walls. Statistical stability is identified in the process of flame propagating upstream. Turbulent flame speeds obtained in this study agree well with the experimental data, especially for the case of 1.0 MPa.

Numerical noise prediction and source identification of a realistic landing gear

Noise predictions of realistic landing gear configurations are obtained combining high-fidelity CFD simulations and the Ffowcs Williams–Hawkings (FWH) acoustic analogy. The aeroacoustic prediction of such a complex aeronautical system depends on geometry fidelity and the ability of the mesh and numerical method to resolve important flow features responsible for noise generation. To understand the role of small components in the far-field noise predictions, different setups are analyzed including a detailed full complexity landing gear and a simplified, yet realistic, configuration. For the latter case, two different setups in terms of mesh resolution and topology are analyzed. For the finer mesh, refinement is applied in regions with strong pressure fluctuations and also close to edges where shear layers develop flow instabilities. We also compare noise predictions obtained by a landing gear installed on a full aircraft with those computed only for a setup with the bottom half of the fuselage. An assessment of the solutions obtained from solid and permeable FWH surfaces is presented for different configurations of permeable surfaces. One objective of this work consists in the identification and analysis of noise sources in the landing gear. For this task, we first employ acoustic analogy to individual components of the landing gear to identify potential sources of tonal noise. Then, proper orthogonal decomposition is applied to identify mechanisms of noise generation at specific frequencies. It is shown that turbulent coherent structures are responsible for tonal noise generation by the towing link, wheel cavities and landing gear compartment. For the external cavity of the wheel, a Rossiter mode is excited and leads to resonance. We demonstrate that removing small-scale components of the full complexity geometry affects the flow characteristics inside the landing gear compartment and, consequently, its noise emission. In this case, the presence of small scale components leads to finer turbulent eddies that generate more noise at higher frequencies. These finer scales also reduce the coherence of larger turbulent structures, reducing amplitude of the low frequency band of the spectrum.

Effect of the flow upstream the impeller inlet on flow instability of a centrifugal pump

Under part-load conditions, flow instability phenomena in a hydraulic machinery could lead to strong pressure fluctuations and vibrations, posing a great threat to the safety of the units. This paper analysed unsteady flow in a centrifugal pump (specific speed of 39 min−1·m3s− 1·m) with different inlet geometries under part-load conditions. An advanced turbulence model, PANS (partially averaged Navier-Stokes), acting as a bridge from RANS to DNS, is applied for numerical simulations. The results indicated the pump with a straight inlet pipe reached higher values of pump head and efficiency near the design point than the test pump with a non-straight inlet pipe. According to the characteristic curves, hump phenomenon was observed at 0.75ϕbep for the pump having a straight inlet pipe, while it occurred at 0.5ϕbep for the test pump. Under the condition of a low flow rate i.e. ϕ=0.5ϕbep, there is strong pre-swirling flow upstream impeller inlet for the test pump, which contributes to the propagation of rotating stall cells; But energy loss in the impeller inlet of the pump with straight inlet pipe is slightly higher than that in the test pump due to the presence of the stationary stall cells, which causes large energy loss in the impeller and the passage upstream impeller inlet. Further, rotating stall cells are captured in the impeller in both pumps under the condition of ϕ=0.75ϕbep.

Noise control of subsonic flow past open cavities based on porous floors

In implicit large eddy simulations, a porous material was used to suppress the noise produced by a flow past an open cavity. The base case has a Mach number and Reynolds number based on the cavity depth ReD = 105. Strong pressure fluctuations were produced. The solid cavity floor was replaced by a porous material to suppress high-pressure oscillations. With a porous floor, both the pressure fluctuations inside the cavity and noise in the near field were substantially suppressed. Four controlled cases with different porosities were considered. For low porosities, the control was better with increasing porosity. The control was best when the porosity was about 11.2%, and the maximum noise reduction was more than 10 dB. As the porosity was further increased from 11.2% to 19.27%, the control effect was decreased slightly. A porous floor can produce effects of suction and injection, which alter the structures of the recirculation and the shear layer. The control is mainly influenced by the strength of the suction effect. With control, the shear layer has less energetic smaller structures, and the interactions between the shear layer and recirculation inside the cavity are weakened. The vortex–edge impingements are mitigated, and thus, the acoustic feedback is lower, which decreases the self-sustained oscillations and the noise. Basically, the noise reduction mechanisms of the four controlled cases are similar. Our results suggest that a porous cavity floor is an effective method of noise control. However, an appropriate porosity must be chosen.

Experimental research on pressure fluctuation characteristics of high-head Francis turbine under part load conditions

Pressure fluctuation at partial load operation is an obvious and harmful phenomena in Francis turbines. It could be induced by rotor stator interaction and cavitation vortex rope in draft tube and must be minimized during the design of Francis turbine. To reduce the pressure fluctuation at partial load operation, accurate assessment of the cause of this phenomena is crucial. In this paper, a reduced-scale physical model of a high head Francis turbine is used as the research object, and the test technique is applied to investigate the excitation source of the pressure fluctuation at two partial load operation conditions. Based on experimental data, it can be concluded that the amplitude of pressure fluctuation in the draft tube significant is increased with the decrease of the power of turbine, which is indicated that the stability of turbine deteriorated sharply with the power reduced. But the dominant frequency of the pressure fluctuation is almost the same. And the strong pressure fluctuation in the draft tube can be transmitted to the upstream components, which are affected the pressure fluctuation caused by rotor stator interaction.