Frequency Of Pressure Fluctuations Research Articles

Oscillatory behaviors of multiple shock waves to upstream disturbances

The oscillatory response of multiple shock waves to upstream disturbances in a supersonic flow is studied numerically in a constant area rectangular duct. The flow is accelerated through a nozzle with an exit Mach number of 1.75 and continues in the constant area duct, where multiple shock waves are formed. To investigate the effect of upstream disturbance on shock oscillations, three parameters are varied systematically: upstream turbulent intensity, frequency of upstream pressure fluctuation, and amplitude of upstream pressure fluctuation. The wall shear stress variation along the duct length provides the location of separation and reattachment points in the flow field. The wall pressure frequency spectra were used to investigate the low-frequency unsteadiness in shock oscillations. The power spectral density of the wall static pressure and the probability density function (PDF) of shock location are analyzed, and the results suggest that as the upstream turbulent intensity is increased, the dominant frequency of oscillation is increased and the shock oscillations become more symmetrical. As the upstream disturbance frequency is increased, the shock oscillations become more symmetrical and follow the Gaussian curve closely. The shock wave oscillates with the same upstream excitation frequency when the upstream disturbance amplitude is increased. At large values of upstream disturbance amplitude, the PDF shows a large deviation from the Gaussian, and the rms amplitude of shock oscillation increases monotonously. At higher amplitudes of upstream disturbance excitation, the traces of shock train leading-edge location display path-dependence characteristics.

Flow-Induced Vibration of a Reversed U-Shaped Jumper Conveying Oil-Gas Two-Phase Flow

Subsea jumpers connecting the underwater wellhead and nearby manifold commonly undergo flow-induced vibration (FIV) due to the spatially frequent alteration in the flow direction, velocity, pressure and phase volume fraction of the oil–gas two-phase flow, potentially leading to fatigue damage. This paper reports the numerical results of the FIV of a reversed U-shaped jumper excited by gas–liquid two-phase flow, which evolves from the initial slug flow with a fixed gas–liquid ratio of 1:2 when transporting through the jumper. The FIV response and flow pattern evolution are examined with a gas flow rate of Qg = 4–12 kg/s and a liquid flow rate of QL = 96–288 kg/s. When the gas–liquid flow passes through the jumper, the flow regime subsequently presents the slug flow, bubble flow, churn flow and imperfect annular flow. The out-of-plane response frequency coincides with the pressure fluctuation frequency for the four connecting bends, suggesting the fluid–structure interaction (FSI). Nevertheless, the vibration displacement is limited with the maximum value less than 0.0014D (where D is the jumper diameter) in the present considered flow rate range.

Oil–water two-phase flow-induced vibration of a cylindrical cyclone with vortex finder

Cylindrical cyclones play an important role in oil–water separation and sewage treatment in the petroleum industry. Here, we describe the characteristics of vibration induced by a two-phase rotational flow in a cylindrical cyclone. The cyclone operating parameters together with a dimensional analysis and multiphase flow numerical simulation were used to understand the flow field characteristics. The frequency and amplitude of pressure fluctuation were obtained by measuring pressure changes at points on the axis of the device. It shows that the pressure in a cylindrical cyclone varies periodically during separation and that fluctuation frequency and amplitude are related to the inlet velocity and flow split ratio. The effect of the overflow split ratio on the pressure fluctuation frequency is negligible, but increasing the overflow split ratio will cause greater fluctuation of the flow. For a cylindrical cyclone, the pressure fluctuation frequency can be calculated from the inlet velocity. Adjusting the inlet velocity and the overflow split ratio changes the mechanical response of the structure. The results of a modal analysis show that the structural vibration response is consistent with the response state of the lowest point of the internal central-vortex pressure and that both are in approximate circular motion. Furthermore, the frequency of pressure fluctuation induced by the flow is close to the intrinsic frequency of the structure with a single bottom constraint, which can cause unwanted resonance easily. Therefore, an appropriately added constraint on a cylindrical cyclone should be taken into consideration to avoid the resonance frequency.

Analysis of flame instability in low-emission tower-type gas turbine combustor with multi-stage methane injection

A low-emission tower-type coaxial-staged combustor with a pilot stage, first main stage and second main stage is simulated in this study. The flow field, flame structure, vortex-mixing interaction, and flame instabilities are predicted with large eddy simulation and partially stirred reactor model. Heat radiation is not considered in the simulations. The results show that the maximum of the averaged length and width of the inner recirculation zone are approximately 249.8 and 71.3 mm, respectively. The distributions of the fuel from various stages in the combustor are analyzed based on the respective mixture fractions. The pressure and heat release rate (HRR) in the combustor fluctuate periodically with a frequency of about 373 Hz. Both HRR and pressure fluctuations increase and then decrease with time, and there is a phase difference of 46.1° between them. Due to the combustion instabilities, the flame structures change periodically, including attached flame and lifted V-shaped flame, in one HRR oscillation cycle. Moreover, the swirling flame length also varies periodically with time. As the pilot stage and first main stages are turned-off, the frequency of HRR fluctuations and pressure fluctuations, as well as the flame root dynamics, are significantly affected. It is also found that the effects of the fuel ratio of the first and second main stages have a significant influence on the flame root dynamics, e.g., different degrees of flashback observed in different cases.

The effect of wake vortex on the unsteady characteristics of tip leakage flow in a high-loading counter-rotating compressor

The effect of upstream wake vortex on the unsteady characteristics of downstream tip leakage flow (TLF) in a high-loading counter-rotating compressor, have been revealed. For the strength of the tip leakage vortex (TLV) is stronger than that of the upstream wake vortex and the TLF spills from the leading edge (LE) of the neighbor blade under near stall (NS) condition, the wake vortex can only transport bypass the interface, formed by the TLF and main flow, just below the tip. The TLV is separated into several parts by the wake vortex such that it weakened rapidly and the dominant frequency of the static pressure fluctuation near the trailing edge (TE) is about 1 relative blade passing frequency (RBPF) under the NS condition. When the extent of TLV breakdown is enough, the TLV is responsible for the dominant frequency near the LE. Therefore, there are two dominant frequencies (one is located near LE, the other is located near TE) in the tip region of the downstream supersonic rotor under NS condition. It’s harmful for the stable operation of compressors due to lower frequency resulting from TLV. At small tip clearance size, affected by the upstream wake vortex, when the static pressure fluctuation caused by the TLV at the LE of the downstream rotor just appeared, the momentum ratio increases. And it locates above the limit line (applied to judge the static pressure fluctuation caused by the self-excitation of the TLF), this means the wake vortex is beneficial to suppress the self-excitation of the TLF.

On low-frequency unsteadiness in swept shock wave–boundary layer interactions

We derive a scaling law for the characteristic frequencies of wall pressure fluctuations in swept shock wave/turbulent boundary layer interactions in the presence of cylindrical symmetry, based on analysis of a direct numerical simulations database. Direct numerical simulations in large domains show evidence of spanwise rippling of the separation line, with typical wavelength proportional to separation bubble size. Pressure disturbances around the separation line are shown to be convected at a phase speed proportional to the cross-flow velocity. This information is leveraged to derive a simple model for low-frequency unsteadiness, which extends previous two-dimensional models (Piponniau et al., J. Fluid Mech., vol. 629, 2009, pp. 87–108), and which correctly predicts growth of the typical frequency with the sweep angle. Inferences regarding the typical frequencies in more general swept shock wave/turbulent boundary layer interactions are also discussed.

Effects of oscillation frequency on the flow characteristics of a centrifugal pump impeller with sinusoidal flow rate

This study uses large eddy simulation to investigate the flow characteristics of a centrifugal pump impeller with sinusoidal flow rate and constant rotational speed. Five different oscillation frequencies ( f/f0 = 1.0, 1.2, 1.5, 2.0, and 3.0, where f0 indicates one-ninth of the rotational speed) are selected to isolate the influence of oscillation frequency on the flow characteristics. Studies show that the pressure deviation between dropping and rising stages concentrates at the impeller inlet at instantaneous 0.5 Qd ( Qd is the design load), and its value increases with the increase in oscillation frequency. The lowest period of many flow characteristics, including pressure fluctuation and head, equals to that of flow rate. The differences in pressure fluctuation between adjacent channels decreases with the increase in oscillation frequency. The first and second dominant frequencies of pressure fluctuation are mainly affected by the oscillation frequency in the nonstall passage, and they have an important relationship with the stall vortex in the stall passage. With the increase in oscillation frequency, the amplitude of lowest order head pulsation increases, and the high order head pulsation gradually weakens. The average head is approximately 2.36 m.

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.

Investigation on Pressure Fluctuation of the Impellers of a Double-Entry Two-Stage Double Suction Centrifugal Pump

Double-entry two-stage double-suction centrifugal pumps with high flow rates and high heads are used in some large water supply applications. The pressure fluctuation of the impeller is a key factor influencing the vibration in centrifugal pumps. In this paper, the pump is simulated and verified by experiments, and the pressure fluctuation distribution of two stage impellers is obtained. The study on the time domain and frequency domain of the two-stage impellers demonstrates that the pressure fluctuation of the first-stage single-suction impeller is affected by the twin volute. At 1.0 Q, the dominant frequency on the blade suction side and pressure side is twice the rotation frequency. The main frequency of pressure fluctuations at the outlet side of the blade at a low flow rate is higher than that at the design flow rate. Pressure fluctuations in the second-stage impeller are influenced by the inter-stage passage. The dominant frequency amplitudes grow incrementally along the streamlined direction. In the second-stage double-suction impeller, the dominant frequency amplitude at 0.6 Q is approximately twice that of 1.0 Q. Research in this paper can guide the design and operation of a two-stage pump.

The low frequency pressure pulsation and control of the open-jet wind tunnel

An open jet wind tunnel has low-frequency pressure pulsation in common wind speed range due to its unique structural form, which seriously damages the quality of flow field in the test section. The low-frequency pressure fluctuation performance and control mechanism of Jilin University open jet and return flow wind tunnel are investigated by experiments and numerical simulation. The results show that the low-frequency pressure fluctuation is a narrow pulse phenomenon that only occurs in certain intervals, and several velocity intervals may be found in the same wind tunnel. The reliability of the numerical simulation is verified by comparing the peak frequency and amplitude of pressure fluctuation in numerical simulation and wind tunnel tests. A simplified model similar to and amplifying the phenomenon is established. The flow structure and vortex evolution are analyzed via detached eddy simulation. In the test section, large-scale shedding vortices are formed at the nozzle exit, introducing periodic pulsating instantaneous velocity and acting with the collector to form an edge-feedback. This acoustic feedback forms resonance with the pipeline circuit, resulting in poor flow field quality. In accordance with the mechanism of nozzle jet, two methods of controlling pulsation are proposed: spoiler and flow-follow device. The study shows that the effects of two methods are abrupt, and the frequency of pressure pulsation is changed. The spoiler destroys the complete structure of vortex ring in free jet and develops into a complementary double vortex ring structure, which is highly sensitive to size factors. The flow-follow device supplements the velocity loss of the free jet at the nozzle and develops into a double vortex ring with master–slave structure in the middle of the test section. Its vibration reduction effect is greatly affected by the flow velocity. It takes effect in an appropriate range where the flow velocity is higher than the nozzle velocity. If the follow velocity is extremely low, the flow-follow device cannot change the original jet structure. If the follow velocity is extremely high, the momentum of the fan will be greatly reduced, the flow field will be unstable, and another order of pulsation may be induced. This work lays a solid foundation for further understanding the aerodynamic characteristics and optimization mechanism of open jet wind tunnel.

Height Effect on Interactions between the Hydraulic Fracture and Natural Fractures

Since natural fractures have shorter heights, it is necessary to incorporate the height difference in the mechanical behavior of hydraulic fracture crossing cemented natural fractures at different angles. However, the impact of fracture height growth on the mechanical behavior of intersection of natural fracture and hydraulic fracture is not yet fully understood at present. In this study, we use adaptive cohesive zone methods to investigate the impact of fracture height on the mechanical behaviors of a propagating hydrofracture crossing natural fractures at different intersection angles. The fracture patterns and mechanical mechanisms for crack propagation crossing with cemented natural fractures are discussed. The increase of fracture height could restrain the crack propagation along the southwest orientation of the natural fracture, but it hardly affects the propagation along the northeast orientation of the natural fracture. A lot of bands appear at a relatively larger formation thickness. The stress shadow between adjacent cohesive layers increases as the formation height increases, which could promote crack initiation and propagation between the adjacent joints or natural fractures. The cracking zone is related to the position of the natural fracture or joint sets with respect to the advancing hydrofracture. Key factors, including the formation height, tensile strength of cemented natural fractures, and their intersection angle with the growing hydrofracture propagation, are investigated in details. In addition, the pressure fluctuation frequency increases as the fracture height increases due to the strong hydraulic and natural fracture interaction. This study provides a new perspective for the development of complex fracture network patterns in cemented formations.

Gas lifted well performance evaluation based on operational parameters’ fluctuations

The paper describes the gas lifted well performance evaluation ability based on operational parameters’ fluctuations analysis. Experimental laboratory research showed that the optimum gas-liquid lift performance correlates with minimal fluctuation of flowrate and pressures. Also, trials in the «Neft Dashlary» oilfield were conducted. Gas lifted well’s flowing tubing head pressure fluctuations were evaluated at different lift gas injection rates – below, above, and at the optimum gas injection rates. The results of field trials show that at optimum gas lifter well performance, the tubing head pressure fluctuation amplitude reduction and frequency increase is observed. This is in line with theoretical prediction and lab experiments conducted. It is shown that gas lifted and naturally flowing wells’ performance quick evaluation is doable during normal operation. The introduced method allows timely detection of well performance deviation from optimal. Necessary measures can be taken to optimize the wells without conducting additional surveillance. Keywords: well; gas lift; flow rate; operational parameters; well performance.

Transient aerodynamic performance of bidirectional impulse turbine in reciprocating airflows for hydrokinetic energy conversion systems

As a part of hydrokinetic energy conversion systems, bidirectional impulse turbines are used in oscillating water columns (OWCs) to extract wave energy. Transient numerical simulations were implemented to predict the unsteady behaviors of reciprocating airflow. Firstly, the effects of blade thickness, blade rotation angle, and blade incident angle on the aerodynamics performance were investigated, and results shows that blade thickness has a slightly influence on turbine efficiency under steady-state condition, however, both the decrease of the rotation angle and the incident angle significantly increased the turbine efficiency. An efficiency peak of 57% was achieved with α = -6° and a flow coefficient of 1.0. Secondly, and pressure fluctuations of an impulse turbine were analyzed under sinusoidal flow, which shows that the energy loss of the turbine is attributed to the vortex generated near the downstream guide vanes. It is also shown that the pressure fluctuation is mainly caused by the unsteady incident wave, moreover, the pressure fluctuation frequency of blade surface is near to the incident wave frequency or a multiple of the incident wave frequency.

Experimental Study of Hydrodynamic Pressures Acting on a Submerged Gate

Vortical flow formed by a submerged hydraulic jump may produce significant hydrodynamic lift and drag pressures on a gate beneath the hydraulic jump. In this study, an experimental setup equipped with multi-pressure sensors was used to measure fluctuating impact pressures on the submerged gate for different flow conditions characterized by the inlet Froude number and submergence factor (S). Time-averaged and instantaneous pressure coefficients are evaluated based on simultaneous measurements of wall pressures at multiple locations, including those at the lip and downstream face of the gate. In particular, instantaneous lift pressure coefficients are observed to be independent of the submergence ratio for S > 0.6. It is found that low Froude number flows produce high surface pressure fluctuations and the dominant frequency of pressure fluctuations shifts to higher frequency as the Froude number increases. Pressure measurements for the free hydraulic jump suggest that the power spectra of lift pressure fluctuations are devoid of any significant energy level, whereas the resulting power spectra of the submerged flow exhibits significant energy level in the frequency range of 14-19 Hz. The proposed measurement system can be used for the in situ identification of hydrodynamic pressures acting on the gates in irrigation canals.

Pressure Fluctuation Characteristics Analysis of Centrifugal Pump as Turbine in Its Start-Up Process

In order to study pressure fluctuation characteristics of a centrifugal pump as turbine (PAT) in its start-up process, Fluent was used to do numerical simulation of the PAT start-up process. Time and frequency domain analyses were performed on the data acquired at different positions and different heads. The results show that a large number of low-pressure areas and strong vortexes are formed within the impeller at the initial time of start-up. With the increase in rotating speed, the vortexes rapidly decrease and are concentrated on the blade non-working face. The pressure fluctuation amplitude is the maximum at the start-up initial instant; with increase in rotating speed, it reduces rapidly at the volute spiral part. The pressure fluctuation number within one impeller rotation cycle is consistent with blade number, and the dominant frequency of pressure fluctuation is 6 times the impeller rotational frequency. The dominant frequency amplitude of radial pressure fluctuation increases with decrease in radial size in the volute. The pressure fluctuation in the impeller is much more intensive than that in the volute, and its maximum dominant frequency amplitude occurs in the middle of the impeller towards the inner edge. With the increase in head, the number of pressure fluctuation and the amplitude of dominant frequency increase at the same time during the start-up process.

Numerical Simulation of Unsteady Cavitation Flow in a Low-Specific-Speed Centrifugal Pump with an Inducer

Cavitation is an undesirable phenomenon in the pumps. In this paper, unsteady cavitation flow in a low-specific-speed centrifugal pump with an inducer is investigated based on the full cavitation model and standard k-ε turbulence model using the commercial software PumpLinx. The numerical results of external performance curve and cavitation performance curve of design condition agree well with that of the experiment. The bubbles in the inducer mainly appear on the outer blade leading edge. The regions of larger vapor volume fraction in the inducer and impeller increase with the decreasing of NPSHa. The regions of larger vapor volume fraction in the inducer expand from the outer edge of the blade inlet to the hub on suction surface. Under very low NPSHa, the impeller may be filled with bubbles in the passage and the pump head drops drastically, and there exist distinct back flows near the suction surface of the blade and impeller outlet. Affected by pre-swirl of inducer outlet and the circle flow of impeller blade inlet, the amplitude of pressure fluctuation near the impeller inlet is obviously larger than that at the inlet in the inducer. The dominant frequency of pressure fluctuation for four monitoring points is shaft frequency. Compared with the non-cavitation flows, the maximum amplitudes of pressure pulsation increase for cavitating flow.

Numerical simulation of unsteady flow characteristics in a centrifugal pump during runaway process caused by power failure

Abstract Centrifugal pump is widely used in agricultural irrigation, petrochemical, mining, metallurgy and other industries. A special centrifugal pump with elbow pipe is researched in this paper. The flow rate and rotating speed change rapidly, especially in elbow pipe, causing serious pressure fluctuation and complex vorticity in the centrifugal pump. A user-defined function to solve the angular momentum equation was calculated the pump rotational speed. Unsteady numerical simulation with SST turbulence model is carried out and Short time Fourier transform (STFT) and vorticity strength are used to analyse the pressure fluctuation and vortexes respectively. The results indicate that the rotational speed first decreases to zero and then reversely increases to runaway speed. Large area vorticity appeared at the elbow pipe when the high amplitude frequency of pressure fluctuation increased. The change law of main frequency of pressure fluctuation at the elbow pipe is consistent with the rotational speed change ruler, which is equal to blade passing frequency (BPF). The main source of pressure fluctuation was rotor stator interaction. This study’s results provide a meaningful reference about pump runaway process caused by power failure, leading to the prevention of dangerous situations, for safe operations of the pumping unit.

Development of a mathematical model to monitoring the velocity of subsidence of charge material column in the blast furnace based on the parameters of gas pressure in the furnace tract

A problem of estimating the velocity of subsidence of a column of charge materials using non-contact methods was considered. This is important because the level of furnace charge materials and the velocity of their subsidence are main indicators of melting intensity determining the furnace productivity. The design of a blast furnace and its blast path were described and existing methods and means of controlling the velocity of charge materials in the blast furnace were analyzed. A mathematical model was presented for estimating the velocity of subsidence of charge materials in a blast furnace based on the magnitude and fluctuations of gas pressure along the furnace shaft height. The model is based on the fact that the furnace gases rise up in the furnace shaft through elementary channels in the column of charge materials consisting of a combination of capacitances and resistances. Volume of capacities and values of resistance of elementary channels are constantly changing. This changes hydraulic resistance to gas movement in the blast furnace. The system of differential equations describes the dependence of the amplitude of pressure fluctuations on the amplitude of change in coefficients of resistance and frequency of pressure fluctuations on the frequency of change in coefficients of resistance. The experimental data on velocity of the column of charge materials and fluctuations in the pressure differential in the furnace were processed and their significant relationship was shown to confirm the previous theoretical study results. To assess the model adequacy, the simulation method was used. The results of the simulation model work were confirmed by experimental data. The developed mathematical model can be introduced into production. This will make it more economical and safer through better and more predictable control and improved flexibility in operation under different production conditions.

Research on Abnormal Vibration and Vibration Reduction Measures of a Natural Gas Compressor Station: A Case Study of the JYG Compressor Station

As an important medium transmission element, the pipeline has a very important application in the chemical industry, electric power industry, and petroleum industry. However, the pipeline vibration problem has seriously restricted the large-scale development of equipment, reduced the reliability of equipment operation, and even caused serious accidents. Therefore, it is very important to analyze the dynamic characteristics of these vibrations and to reduce the impact of vibrations within pipelines. We focus on the problem of abnormal vibrations within the JYG compressor station pipeline and use a 3D calculation model of the launcher pipeline of west–east gas pipeline III to perform simulation analysis using Fluent to find the cause of the abnormal vibrations. Our results show that the fluid pressure fluctuation in the pipeline is the main factor for the abnormal vibration of the launcher pipeline in the JYG compressor station. The main causes of the vibrations are excessive fluid flow and high flow velocity. Also, by comparing and analyzing the natural frequency of the pipeline system and the pressure fluctuation frequency of the vortex core in the pipeline, we found that the pressure fluctuation frequency is close to the low-order natural frequency of the pipeline system, which is prone to resonance. In this paper, three vibration reduction schemes of the JYG compressor station are suggested and verified. The efficiency of vibration reduction is 68–94%, which can be effectively applied to the outgoing pipeline of the west–east gas pipeline III to solve the abnormal vibration problem of the pipeline.

Flow Stability in a Miniature Centrifugal Pump under the Periodic Pulse Flow

The flow behavior inside a miniature centrifugal pump, under a periodic pulse flow rate, was studied by means of numerical simulation. For a given incoming periodic pulse flow with a sine wave, the performance of the centrifugal pump was investigated in the section with increasing flow and the section with decreasing flow, and the special points of the flow rate and the periodic flow were identified. Further, the energy gradient method and the Q-criterion were adopted to analyze the internal vertical structure and flow stability. It was found that the regions with large variations in velocity and total pressure were mainly located at the leading edge of the suction surface and the middle area of the pressure surface of the blades. Irregular pressure fluctuation frequency under the periodic pulse flow was shown; this was mainly concentrated in the low-frequency zones close to the impeller’s rotational frequency. In addition, for the same flow rate in the periodic pulse flow, the pressure frequency fluctuation for the increasing flow rate section was higher than that observed for the decreasing flow rate section. It was found that the most unstable sections appeared in the first half-period of the flow rate variation (large flow rate), according to the distributions of the Q criteria of the vortex and the energy gradient function K. In this section, motions of strong vortices led to large gradients of the mechanical energy.