Pressure Fluctuation Intensity Research Articles

Numerical investigation of influence of the clocking effect on the unsteady pressure fluctuations and radial forces in the centrifugal pump with vaned diffuser

The relative position between the diffuser vane and the volute tongue (clocking effect) has a great effect on the performance of the single-stage centrifugal pump, which however, is often ignored by designers. In this paper, the influence of clocking effect on the unsteady pressure fluctuation in a centrifugal pump and on the radial force of impeller is investigated. The hydrodynamic performance of the centrifugal pump with vaned diffuser was experimentally measured. Numerical simulation based on the Reynolds-averaged Navier–Stokes (RANS) combined with the SST k-w turbulence model was used to obtain hydrodynamic performance of the centrifugal pump. The numerical results of the hydrodynamic performance were in agreement with the experimental data. The results show that the clocking effect has great influence on the pressure fluctuation and on the unsteady radial force imposed on the impeller. When the diffuser vane approaches the volute tongue, the pressure fluctuation intensity in volute is relatively lower. Meanwhile, relatively larger radial force on the impeller and the lower efficiency are obtained when the diffuser vane is near the volute tongue. Thus, it is suggested that the volute tongue should be located near the middle of two diffuser vanes to obtain better performance.

Use of pressure to mitigate gas bypassing in fluidized beds of FCC catalyst particles

Deep gas fluidized beds of low-fines fluid catalytic cracking (FCC) catalyst particles can have severe gas maldistribution due to gas bypassing. Tests were conducted in a 0.6-m-diameter unit using 3.2% and 4% fines less than 44μm FCC catalyst particles to determine the influence of system pressure on gas bypassing in a fluidized bed of 3.66m static bed height. The freeboard pressure was varied up to 207kPag (30psig). Differential pressure fluctuations were measured at four locations around the column, bubble void fraction was measured at two opposite locations close to the column wall, and radial bubble void fraction profiles were measured at axial elevations of 0.9 and 1.52m. At no or low pressures, gas bypassing was present in the bed. With gas bypassing, differential pressure fluctuation intensities were significantly different around the column, significantly higher bubble void fractions were measured close to the inner wall on one side of the column than on the opposite side, and the radial bubble void fraction profiles were not symmetrical about the column axis. Increasing the system pressure weakened the intensity of gas bypassing. Gas bypassing disappeared at a freeboard pressure of about 100 to 140kPag (15 to 20psig).

Experimental investigation of the wall dynamics of the A-pillar vortex flow

The influence of the A-pillar vortex on the wall flow of the side window of a car is investigated experimentally using a 30° dihedron model. The measurement of the unsteady pressure at the wall provides a map of the pressure fluctuation intensity, and a spectral analysis is performed to track the dominant frequencies of the wall pressure fluctuations. The wall flow generated by the vortex structure that develops parallel to the side-wall is characterised by means of particle image velocimetry (PIV). Its structure is analysed and compared to cross-sections of the A-pillar vortex in order to identify the different separation and reattachment lines. A comparison of the field of turbulent kinetic energy obtained by PIV with the map of the pressure fluctuations shows a correlation between the structure of the A-pillar vortex and the pressure fluctuations. It is found that the dominant wall pressure fluctuations are located at the secondary separation line, whereas the primary reattachment line does not show any significant pressure variations, that the A-pillar vortex will not naturally break down and that discrete vortices may be associated with the pressure fluctuations.

Numerical Analysis of the Clocking Effect on the Pressure Fluctuation in the Centrifugal Pump with Vaned Diffuser

In order to study the clocking effect on the unsteady pressure fluctuations in the centrifugal pump with a vaned diffuser, the three-dimensional unsteady numerical simulations are undertaken to analyze the inner flow characteristics with 5 different positions of diffuser by solving the RANS equations with SST k-ω turbulence model under 60% and 162% of the design conditions. The pressure fluctuation intensity distributions caused by the clocking effect in the diffuser and the volute are obtained. The results show that the clocking effect of diffuser on pressure fluctuation intensity due to the rotor-stator interaction is obvious. In the diffuser, the highest pressure fluctuation intensities occur at the throat area and the pressure side near the leading edge. The pressure fluctuation intensity propagates and weakens along the diffuser passage. Pressure fluctuation intensity distribution due to the clocking effect is irregular significantly. Under the partload condition in the diffuser, when the diffuser position is C3, the maximum value of the pressure fluctuation intensity is 0.44; when the diffuser positions are C2 and C4, the maximum values are 0.24 and 0.22, respectively. Under the overload condition in the diffuser, when the diffuser position is C2, the maximum value of the pressure fluctuation intensity is 0.87; when the diffuser positions are C1 and C4, the maximum values are 0.72 and 0.77, respectively. The pressure fluctuation intensity is much smaller than that in the diffuser under the partload and overload operating conditions. The relative position C4 can decrease the pressure fluctuation intensity and the result can provide a reference to the setting relative position of the diffuser to the volute.

Cavitation erosion of the calcium carbonate deposits

The paper presents the research of erosion effects caused by acoustic cavitation on metallic and enamelled specimens coated with calcium carbonate - scale. The deposition of calcium carbonate was performed under controlled laboratory conditions. The deposition took place at the same integral conditions to the specimens of different surface roughness. Cavitation as a cause of erosion was generated in a vessel with ultrasonic excitation. Simultaneously with the monitoring of cavitation above the specimens, the destruction of scale on the surface of specimens was analysed. The results of experiments indicate a characteristic effect of cavitation intensity on the destruction of scale surface layer. The intensity of cavitation erosion is proportional to the voltage on the sonotrode or the intensity of pressure fluctuations, and depends on the roughness of the surface of specimens, on which the scales were deposited.

Numerical investigation of the vortex core precession in a model hydro turbine with the aid of hybrid methods for computation of turbulent flows

Numerical modeling of the unsteady flow in the draft tube of the test bench hydro turbine is conducted. The hybrid RANS-LES methods for modeling turbulent flows are compared. The intensity and frequency of pressure fluctuations, which are induced by the vortex core precession under the runner, and the integral characteristics are considered. An analysis of the synchronous and asynchronous parts of pressure fluctuations is done; the generating and influence of the synchronous component of fluctuations are considered. The vortex core interaction with the draft tube elbow is considered.

Numerical investigation of tandem-cylinder noise reduction using plasma-based flow control

AbstractThe noise of flow over tandem cylinders at $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}{\mathit{Re}}_D= 22\, 000$ and its reduction using single dielectric barrier discharge (SDBD) plasma actuators are simulated numerically both to confirm and extend experimental results. The numerical approach is based on large-eddy simulation (LES) for the turbulent flow field, a semi-empirical plasma actuation model, and Lighthill’s theory for acoustic calculation. Excellent agreement between LES and experimental results is obtained for both the baseline flow and flow with plasma control in terms of wake velocity profiles, turbulence intensity, and frequency spectra of pressure fluctuations on the downstream cylinder. The validated flow-field results allow an accurate acoustic analysis based on Lighthill’s equation, which is solved using a boundary-element method. The effectiveness of plasma actuators for reducing noise is clearly demonstrated. In the baseline flow, the acoustic field is dominated by the interaction between the downstream cylinder and the upstream wake. Through suppression of vortex shedding from the upstream cylinder, the interaction noise is reduced drastically by the plasma flow control, and the vortex-shedding noise from the downstream cylinder becomes equally important. At a free-stream Mach number of 0.2, the peak sound pressure level is reduced by approximately 16 dB. This suggests the viability of plasma actuation for active aeroacoustic control of airframe noise.

Numerical prediction of 3-D periodic flow unsteadiness in a centrifugal pump under part-load condition

Numerical simulation and 3-D periodic flow unsteadiness analysis for a centrifugal pump with volute are carried out in whole flow passage, including the impeller with twisted blades, the volute and the side chamber channels under a part-load condition. The pressure fluctuation intensity coefficient (PFIC) based on the standard deviation method, the time-averaged velocity unsteadi- ness intensity coefficient (VUIC) and the time-averaged turbulence intensity coefficient (TIC) are defined by averaging the results at each grid node for an entire impeller revolution period. Therefore, the strength distributions of the periodic flow unsteadiness based on the unsteady Reynolds-averaged Navier-Stokes (URANS) equations can be analyzed directly and in detail. It is shown that under the 0.6Qdes. condition, the pressure fluctuation intensity is larger near the blade pressure side than near the suction side, and a high fluctuation intensity can be observed at the beginning section of the spiral of the volute. The flow velocity unsteadiness intensity is larger near the blade suction side than near the pressure side. A strong turbulence intensity can be found near the blade suction side, the impeller shroud side as well as in the side chamber. The leakage flow has a significant effect on the inflow of the impeller, and can increase both the flow velocity unsteadiness intensity and the turbulence intensity near the wall. The accumulative flow unstea- diness results of an impeller revolution can be an important aspect to be considered in the centrifugal pump optimum design for ob- taining a more stable inner flow of the pump and reducing the flow-induced vibration and noise in certain components.

Research on numerical wave tank based on the improved moving-particle semi-implicit method with large eddy simulation

Moving-particle semi-implicit (MPS) method is a new mesh-free numerical method based on Lagrangian particle. In this paper, MPS method is applied to the study on numerical wave tank. For the purpose of simulating numerical wave, we combine the MPS method with large eddy simulation (LES) which can simulate the turbulence in the flow. The intense pressure fluctuation is a significant shortcoming in MPS method. So, we improve the original MPS method by using a new pressure Poisson equation to ease the pressure fluctuation. Divergence-free condition representing fluid incompressible is used to calculate pressure smoothly. Then, area-time average technique is used to deal with the calculation. With these improvements, the modified MPS-LES method is applied to the simulation of numerical wave. As a contrast, we also use the original MPS-LES method to simulate the wave in a numerical wave tank. The result shows that the new method is better than the original MPS-LES method.

Statistical analysis of pressure fluctuations during unsteady flow for low-specific-speed centrifugal pumps

A three-dimensional transient numerical simulation was conducted to study the pressure fluctuations in low-specific-speed centrifugal pumps. The characteristics of the inner flow were investigated using the SST k-ω turbulence model. The distributions of pressure fluctuations in the impeller and the volute were recorded, and the pressure fluctuation intensity was analyzed comprehensively, at the design condition, using statistical methods. The results show that the pressure fluctuation intensity increases along the impeller streamline from the leading edge to the trailing edge. In the impeller passage, the intensity near the shroud is much higher than that near the hub at the inlet. However, the intensity at the middle passage is almost equal to the intensity at the outlet. The pressure fluctuation intensity is the highest at the trailing edge on the pressure side and near the tongue because of the rotor-stator interaction. The distribution of pressure fluctuation intensity is symmetrical in the axial cross sections of the volute channel. However, this intensity decreases with increasing radial distance. Hence, the pressure fluctuation intensity can be reduced by modifying the geometry of the leading edge in the impeller and the tongue in the volute.

Mechanics and response of a surface rock block subjected to pressure fluctuations: A plucking model and its application

Plucking (removal of rock blocks) is often the dominant mechanism for producing a scour hole on riverbeds comprised of heavily jointed rock masses being subjected to pressure fluctuations from a jet flow. This paper explores the mechanics and response of a surface block subjected to pressure fluctuations. First a particle-flow simulation was conducted to demonstrate how repeated pressure fluctuations are able to gradually remove rock bridges in discontinuities surrounding a rock block, if a pressure fluctuation's intensity is substantial. As a consequence, these weak planes may become fully persistent. The block's uplift speed then depends on the pressure differences on the opposite (horizontal) faces, and the frictional resistance of the lateral discontinuities. This paper proposes a theoretical framework to model the mechanics and response of a rock block subjected to a sinusoidal pressure fluctuation. This model can be applied to estimate the development of a scour hole, through plucking, during a specific flood event. An example demonstrates the applicability of the proposed approach in predicting the potential depth of the scour hole.

Numerical Analysis of Pressure Fluctuation Intensity for a Low-Speed Sewage Centrifugal Pump

Sewage centrifugal pumps can operate at different flow rates with variable rotating speeds to meet different performance requirements and achieve certain energy consumption criteria in pipeline systems. In this paper, the unsteady pressure field has been investigated numerically by CFD calculation to evaluate the transient pressure variation in a single-blade pump running with low rotating speeds. Based on the CFD results, the pressure fluctuation intensity was analyzed quantitatively by defining the standard deviation of the pressure fluctuation of a revolution period. The analysis of the results shows that the flow rate can influence the fluctuation intensity distribution obviously, and the same intensity distribution law can be found in the volute domain for the same flow rates with different rotating speeds. Obvious asymmetrical distributions can be observed in both front and back side chamber channels, and the pressure fluctuation intensity in back side chamber is weaker than in front side chamber. This work can obtain the knowledge of unsteady pressure phenomenon for typical flow rates in the low-speed running single-blade pump and can provide basis to optimize and to obtain more reliable pump at variable rotating speeds.

Numerical Investigation of Periodically Unsteady Pressure Field in a High Power Centrifugal Diffuser Pump

Pressure fluctuations are the main factors that can give rise to reliability problems in centrifugal pumps. The periodically unsteady pressure characteristics caused by rotor-stator interaction have been investigated by CFD calculation in a residual heat removal pump. Side chamber flow effect is also considered for the simulation to accurately predict the flow in whole flow passage. The pressure fluctuation results in time and frequency domains were considered for several typical monitoring points in impeller and diffuser channels. In addition, the pressure fluctuation intensity coefficient (PFIC) based on standard deviation was defined on each grid node for entire space and impeller revolution period. The results show that strong pressure fluctuation intensity can be found in the gap between impeller and diffuser. As a source, the fluctuation can spread to the upstream and downstream flow channels as well as the side chamber channels. Meanwhile, strong pressure fluctuation intensity can be found in the discharge tube of the circular casing. In addition, the obvious influence of operational flow rate on the PFIC distribution can be found. The analysis indicates that the pressure fluctuations in the aspects of both frequency and intensity can be used to comprehensively evaluate the unsteady pressure characteristics in centrifugal pumps.

Numerical Investigation of Pressure Fluctuation in Centrifugal Pump Volute Based on SAS Model and Experimental Validation

This paper presents an investigation of pressure fluctuation of a single-suction volute-type centrifugal pump, particularly volute casing, by using numerical and experimental methods. A new type of hybrid Reynolds-averaged Navier-Stokes/Large Eddy Simulation, referred to as the shear stress transport-scale-adaptive simulation (SAS) model, is employed to study the unsteady flow. Statistical analysis method is adopted to show the pressure fluctuation intensity distribution in the volute channel. A test rig for pressure pulsation measurement is built to validate the numerical simulation results using eight transient pressure sensors in the middle section of the volute wall. Results show that the SAS model can accurately predict the inner flow field of centrifugal pumps. Radial force acting on the impeller presents a star distribution related to the blade number. Pressure fluctuation intensity is strongest near the tongue and shows irregular distribution in the pump casing. Pressure fluctuation is distributed symmetrically at the cross-section of the volute casing because the volute can eliminate the rotational movement of the liquid discharged from the impeller. Blade passing frequency and its multiples indicate the dominant frequency of the monitoring points within the volute, and the low-frequency pulsation, particularly in the shaft component, increases when it operates at off-design condition, particularly with a small flow rate. The reason is that the vortex wave is enhanced at the off-design condition, which has an effect on the axle and is presented in the shaft component in the frequency domain.

S0510502 ヘリカルモードを用いた超音速ジェットのマッハ波放射方向制御([S051-05]噴流,後流およびはく離流れ現象の探求と先端的応用(5))

The three-dimensional time dependent compressible Navier-Stokes equations are numerically solved to study acoustic emission nature in a supersonic round jet at high convective Mach number (A/=2.0) using high-order compact upwind schemes. Three cases for Jet Mach number M=2.Q are presented. These cases are the jet flow forced by a pair of first helical modes(m=±1), second helical modes(m=±2) and third helical modes(w=±3), respectibery. The numerical results provide new physical insights into acoustic wave generation mechanisms in a supersonic round jet. The numerical results show that the mode phase velocity affect the emission intensity of pressure fluctuation from a supersonic jet.

Nonlinear fluid-structure interaction of an elastic panel in an acoustically excited two-dimensional inviscid compressible fluid

The focus of this paper is on the asymptotic investigation of the nonlinear fluid-structure interaction of an acoustically excited clamped panel immersed in an inviscid compressible fluid. A multiple-scales analysis of the corresponding two-dimensional unsteady potential flow initial-boundary-value-problem is employed to investigate both primary resonance and a 3:1 internal resonance between the panel fifth and ninth modes. Validation of the asymptotic structural response and the fluid pressure shows good agreement with numerical solution of a weakly nonlinear panel in a quadratic Euler field. The results shed light on the intricate acoustic interaction bifurcation structure which exhibits coexisting bi-stable periodic solutions, and quasiperiodic response reflecting spatially periodic modal energy transfer for both panel and fluid. This behavior is found to occur for panel excitation by finite level acoustic pressure waves that can be a crucial factor for design of high integrity structural systems required for aviation or space where light structures are exposed to intensive acoustic pressure fluctuations.

Removal of Fine Particles on a Wall by High-Frequency Turbulence Added Air Flow

A cleaning technique to remove particles of several micro-meter diameter from a surface under dry environmental conditions is greatly needed in the manufacturing processes of LCDs. However, it is usually difficult to remove the fine particles by simple airflow because the particles adhere to the surface by strong forces. For this reason, a cleaning device equipped with a special nozzle is used in the actual industrial process. The nozzle has triangular cavities to add strong high-frequency fluctuations to the airflow. To clarify the effect of this fluctuation on particle removal quantitatively, we measured the airflow velocity, pressure fluctuation on a surface, and removal ratio for four types of nozzles: two varieties cavity nozzles and two straight nozzles with different lengths. The correlation between the intensity of pressure or velocity fluctuation and removal ratio for the cavity nozzles suggests that the turbulent fluctuation added by the cavity contributes to particle removing.

1143 ACOUSTIC LOADING ON A PLATE BEHIND A BACKWARD FACING STEP

A backward-facing step is used as a model problem for acoustic fatigue. The vortices generated in the wake of the step impose a time varying load on a flexible plate located downstream, parallel to the flow. 3D LES computations are carried out to simulate these coherent structures and their effect on the plate. In addition, the plate response to the simulated load is analysed using finite elements. The results are compared against measurements carried out at the University of Cincinnati [1]. The pressure fluctuation intensity on the measurement plate is found to be sensitive to the inlet boundary condition used, while the mean flow and the general spectral characteristics of the load on the plate are not sensitive to the inlet BC. Unfortunately, the plate response is over-predicted, also when the pressure fluctuation intensity compares well to the measurements.

Numerical Analysis of Three-Dimensional Unsteady Turbulent Flow in Circular Casing of a High Power Centrifugal Diffuser Pump

For high power centrifugal pump which is usually used in high risk applications, circular casing structure has been adopted to increase the reliability of the pump. This special casing structure can make the flow more complex and cause huge hydraulic losses. In this paper, the periodically unsteady turbulent flow in the circular casing of a high power centrifugal diffuser pump has been investigated numerically by CFD calculation. The velocity distributions in different positions were analyzed, and the pressure fluctuation results in time and frequency domains were considered. Finally, the pressure fluctuation intensity coefficient (PFIC) and peak pressure coefficient (PPC) based on statistical thinking were defined on each grid node and the results were analyzed quantitatively to check the time accumulation effect of the unsteady flow phenomenon. The results show that strong flow unsteadiness can be found near diffuser outlet positions of the circular channel and in discharge tube. In addition, the PPC and PFIC results can supply a new angle of view to evaluate the pressure variation, and it is a good supplement to the traditional unsteady pressure analysis method in time and frequency domains.

Analysis of Pressure Field for Flow Past a Circular Cylinder with a Slit

The signal strength, linearity, stability and measurement range of vortex flowmeter are primarily decided by the shape and geometrical parameters of bluff body. A circular cylinder with a slit was found to produce better vortex signal properties by experiments. Based on previous research, pressure distributions of flow past a circular cylinder are studied by numerical simulation. It is found that the intensity of pressure fluctuation caused by vortex shedding of the circular cylinder with a slit is stronger than that of circular cylinder and 0.2d downstream from the center line of slit at one side of cylinder is found to be an optimal position to detect the vortex signal, where d denotes the diameter of the cylinder. Finally the evolution process of vortex shedding is analyzed to explain why the intensity of pressure varies at different positions.