Pressure Fluctuation Patterns Research Articles

Interstage transmission and differential analysis of pressure fluctuations in multistage centrifugal pump-as-turbine

Pumps as turbines (PATs) are used in petroleum and chemical industries to recover high-pressure residual energy. Multistage PATs allow for a wider energy recovery interval and wider range of applications. However, because multistage centrifugal pumps were not originally designed for turbine conditions, complex pressure fluctuations occur, impacting the stable operation and performance of multistage PAT. Pressure fluctuation is essentially a wave, and by analogy with the wave intensity definition, pressure fluctuations were quantified using the pressure wave energy flow density, and the pressure fluctuation patterns at different stages were investigated. The findings reveal significant differences in the intensity of pressure fluctuations at different locations within the multistage PAT. Specifically, the pressure fluctuation intensity is significantly higher from the second to the final stage, compared to the first stage. The difference in inlet flow conditions is the main reason for this difference in pressure fluctuations between stages. The inlet inflow from the second to the final stage is subject to rotational effects that exacerbate the difference in pressure fluctuation intensity between stages. Pressure fluctuations are found to be negatively related to the distance from the source of fluctuations and positively related to the flow state. Different flow conditions and interaction regions of the impeller affect the distribution of pressure fluctuation intensity and the distribution of pressure fluctuation energy across different frequency domains within the guide vanes. The main source of fluctuations in the shaft frequency and four times the shaft frequency is the impeller inlet interaction region, whereas the fluctuations in the blade passing frequency originate from the impeller outlet interaction region. This paper provides a reference for improving the stable operation of multistage PATs.

The complex unsteady flow exploration of a contra-rotating rotor under inlet distortion

This article explores the relationship between stall types and speed ratios in contra-rotating boost stages, discovering stationary circumferential stall cells and dual stall cell phenomena, and examines the relationship between stall characteristics and inlet distortions. These stall characteristics of counter-rotating boost stages are markedly different from the single stall patterns of traditional compressor rotor-stator structures, highlighting the complexity of the flow field in counter-rotating stages. The study uses the experimental methods to investigate different speed ratios under both uniform and distorted inlet conditions, conducting both accelerating and decelerating stall tests, with each stall condition analyzed using data from 25 s. The main focus is on the stall types, frequencies, and development patterns under different distortion conditions at various speed ratios, filling a research gap in counter-rotating boost stages. It explores the unstable flow characteristics of counter-rotating boost stages under distorted conditions, particularly the stall disturbance characteristics and pressure fluctuation patterns at the blade tip during stall entry and recovery under distortions. Comparing distortions with uniform inlet conditions provides an intuitive understanding of the influence of various types of distortions on wall pressure and stall in the flow field. Researchers can use this knowledge to design casings or blowing devices to suppress stall disturbances, thereby achieving stall prevention and enhancing stability.

Blood Pressure Fluctuation Pattern Associated with 90-Day Ischemic Stroke and Transient Ischemic Attack After Total Knee Arthroplasty-A Retrospective Study.

Blood pressure (BP) fluctuation has been demonstrated to be closely associated with stroke. However, at present, no attention is paid to the BP fluctuation following total knee arthroplasty (TKA). The aim of this study is to investigate what is BP fluctuation patterns in stroke and transient ischemic attack (TIA) patients after primary TKA, compare clinical characteristics among different BP fluctuation patterns and determine whether it could predict the occurrence of a 90-day stroke. A retrospective study was conducted from February 1, 2011 to February 15, 2021. Stroke or TIA patients within 90 days after TKA were included. Data from a hospitalization management system were analyzed, including demographics, smoking status, comorbidities, medications, surgical variables, and 90-day stroke, mortality, and readmission rates. BP fluctuation patterns were defined based on the average BP in the first 3 days after TKA. A logistic regression analysis model was used to identify risk factors for 90-day stroke or TIA following TKA. Of the 1687 patients who underwent primary TKA, 4.0% (68/1687) experienced a 90-day stroke or TIA. A total of 63 patients met the inclusion criteria, with an average age of 67.5 years and 55.6% (35/63) female. The number of stroke or TIA patients in each BP fluctuation pattern (T1-T5) was 6 (9.5%), 5 (7.9%), 4 (6.3%), 13 (20.6%), and 35 (55.6%) respectively. Patients with a T5 pattern tended to be older and had a greater reduction in average systolic blood pressure (SBP) 3 days after surgery compared to other patterns. The T5 pattern was associated with a higher occurrence of stroke and a lower occurrence of TIA. Patients with a T5 pattern also had a longer length of stay within 90 days following TKA. After adjusting for confounding factors, average SBP fluctuation ≥40 mmHg and the T5 pattern of BP fluctuation remained risk factors for 90-day stroke or TIA after TKA. BP fluctuation patterns in the early postoperative period are associated with 90-day stroke and TIA following primary TKA.

Experimental investigation on the unsteady surface pressure fluctuation patterns over an airfoil

This article presents a comprehensive mapping of wall-pressure fluctuations over an airfoil under three different inflow conditions to shed light on some basic assumptions taken for granted for the recent aeroacoustic and aerodynamics experimental studies and in the noise prediction models. Unsteady and steady pressure measurements were performed over a heavily instrumented airfoil, which was exposed to smooth inflow, grid-generated turbulent inflow, and a smooth inflow with a tripping tape over the airfoil to explore the unsteady response of the airfoil for a broad range of angles of attack, 0°≤α≤20°. The results are presented in terms of non-dimensional pressure coefficient, root mean square non-dimensional pressure coefficient, frequency-energy content pattern map at isolated frequencies for the entire airfoil, and spectra of frequency-energy content at selected transducer locations. The results show that the unsteady airfoil response patterns for the tripped boundary layer and turbulence ingestion cases show a dramatic difference compared to the airfoil response patterns of the smooth inflow conditions. The response patterns differ across angles of attack, frequency, and between both sides of the airfoil. The results suggest a three-region pattern for the smooth inflow case, a two-region pattern for the tripped boundary layer case, and a two-region pattern for the turbulence ingestion case. Moreover, the results indicate that the presence of tripping may provide a flow with necessary statistical characteristics for the experimental rigs representing the full-scale application; however, it may misrepresent the frequency-dependent nature of the boundary layer.

Analysis of tip leakage flow unsteadiness in a transonic turbine cascade using data-driven modal decomposition methods

The evolution of tip leakage flow and subsequent vortical structures is accompanied by inherent unsteadiness. This paper presents a novel characterization for the unsteady dynamics of turbine tip flow using data-driven, i.e., equation-free modal decomposition methods, which are applied to the hybrid Reynolds-averaged Navier–Stokes/large eddy simulation data at a transonic condition. By combining these techniques, the identified eigen-tuples (eigenvalues, eigenvectors, and time evolution) are well determined, and the differences between the obtained patterns (modes) are also pronounced. The snapshot proper orthogonal decomposition (POD) analysis can isolate the large-scale fluctuating structures that populate the rear part of the suction-side, which is mainly attributed to the shock-induced vortex instability. Similar to the turbulence cascade, macro-scale coherent structures that correspond to the tip leakage vortex shedding phenomena and the subsequently transitional and noisy parts closely related to the dissipation can be well derived by a quadruple reconstruction. Three dynamic mode decomposition (DMD) variants including the amplitude selecting-DMD method, the DMD with criterion method, and the sparsity promoting (SP)-DMD method are also compared in extracting dominant modes from the periodic tip flow, and the SP-DMD method which can distill modes of broadband frequencies and low dissipation is proved to be more conducive to representing and reconstructing the complex tip flow. Additional spectral-kernel-based POD (SPOD) analysis that can identify the similar primary unsteadiness frequencies as the DMD method is also encompassed in this study. Specifically, although it manifests that a physical resemblance of the pattern of pressure fluctuations to tip eddy unsteadiness can be captured by all these approaches, the behavior of small-scale vortical interaction downstream of the trailing edge can be clearly isolated with the intrinsic Karman-type vortex layer shedding process via DMD and SPOD approaches, which also demonstrates that these two techniques are more favorable to decomposing the complex tip flows into uncoupled single-frequency coherent structures compared to the conventional POD method. On this basis, resulting modes of velocity components have been accounted for verifying their contributions to the turbulent kinetic energy fields. The ensuing observations can offer a glimpse of the complex dynamics in the tip region, which also sheds light on features previously masked by conventional analysis approaches.

Effects of Continuous Positive Airway Pressure Therapy on Nocturnal Blood Pressure Fluctuation Patterns in Patients with Obstructive Sleep Apnea.

This retrospective study was designed to evaluate the effects of continuous positive airway pressure (CPAP) therapy, a well-established treatment for obstructive sleep apnea (OSA), on nocturnal blood pressure fluctuations (NBPFs) during rapid eye movement (REM) and non-REM sleep, and to evaluate the NBPF patterns in patients with OSA. We included 34 patients with moderate-to-severe OSA who underwent polysomnography using pulse transit time before and at 3–6 months after CPAP therapy. Nocturnal BP and NBPF frequency in REM and non-REM sleep were investigated, as well as NBPF pattern changes after receiving CPAP therapy. CPAP therapy resulted in significant reductions in the apnea–hypopnea index (AHI), arousal index, nocturnal systolic and diastolic BP, and NBPF frequency in REM and non-REM sleep (all p < 0.01). A higher AHI before CPAP resulted in lower nocturnal systolic BP (r = 0.40, p = 0.019) and NBPFs (r = 0.51, p = 0.002) after CPAP. However, 58.8% of patients showed no change in NBPF patterns with CPAP therapy. CPAP therapy significantly improved almost all sleep-related parameters, nocturnal BP, and NBPF frequency in REM and non-REM sleep periods, but NBPF patterns showed various changes post-CPAP therapy. These results suggest that factors other than OSA influence changes in NBPF patterns.

Blood pressure fluctuation pattern and stroke outcomes in acute ischemic stroke.

Blood pressure (BP) fluctuates widely during the acute phase of stroke. Compared to single BP assessment, patterns of BP over time may have greater power in predicting stroke outcome. This study aims to investigate the effect of BP fluctuation patterns on stroke outcomes in acute ischemic stroke (IS) patients. IS patients within 24 h of onset registered in the BOSS registry between 2012 and 2014 were analyzed. Fluctuation of BP was predefined as the change trend in systolic BP (SBP) from Day 1 to Day 7 after onset and was used to divide patients into groups with sustained high SBP (≥160 mmHg) during the first 7 days (C1); rapid (C2: within the first 2 days) or delayed (C3: after 2 days) decline from high (≥160 mmHg) to low (<160 mmHg); consistently low SBP (C4); and elevation from low to high (C5). The primary stroke outcome was defined as a modified Rankin Scale score ≥3 at 3 months after onset. Of 1,095 IS patients, C1 (n = 90) had the highest risk of poor outcome (23.3%), while C2 (n = 198, risk = 11.6%) and C4 (n = 650, risk = 12.2%) had the lowest risk. C2 and C4 had a significant reduction in poor outcome risk when compared to C1, even after adjustment for average BP and BP variability (BPV) during the first 7 days (adjusted odds ratio[OR]C2 = 0.32, 95% CI: 0.12-0.80; ORC4 = 0.37, 95% CI: 0.14-0.97). The BP fluctuation pattern in the acute phase of IS might be a useful predictive parameter for functional outcome independent of average BP and BPV.

Investigation of Unsteady Aerodynamic Excitation on Rotor Blade of Variable Geometry Turbine

To investigate the aerodynamic excitations in variable geometry turbines, the full three-dimensional viscous unsteady numerical simulations were performed by solving N-S equations based on SAS SST method. The aerodynamic excitations at varied expansion ratios with six different vane stagger angles that cause the unsteady pressure fluctuation on the rotor blade surface are phenomenologically identified and quantitatively analyzed. The blade pressure fluctuation levels for turbines with different vane stagger angles in the time and frequency domain are analyzed. As the results suggest, the blade excitation mechanisms are directly dependent on the operating conditions of the stage in terms of vane exit Mach numbers for all test cases. At subsonic vane exit Mach numbers the blade pressure fluctuations are simply related to the potential filed and wake propagation; at transonic conditions, the vane trailing edge shock causes additional disturbance and is the dominating excitation source on the rotor blade, and the pressure fluctuation level is three times of the subsonic conditions. The pressure fluctuation energy at subsonic condition concentrates on the first vane passing period; pressure fluctuation energy at higher harmonics is more prominent at transonic conditions. The variation of the aerodynamic excitations on the rotor blade at different vane stagger angles is caused by the varied expansion with stator and rotor passage. The aerodynamic excitation behaviors on the rotor blade surface for the VGT are significantly different at varied vane stagger angle. Spanwise variation of the pressure fluctuation patterns on is also observed, and the mechanism of the excitations at different spans is not uniform.

Unsteady flow behavior of a steam turbine control valve in the choked condition: Field measurement, detached eddy simulation and acoustic modal analysis

Abstract The unsteady flow behavior of a steam turbine control valve in the choked condition, which occurs during the warming-up process of a steam turbine, was studied through complementary techniques, including field measurements of the valve spindle’s vibration behavior, detached eddy simulation (DES) of the unsteady flow field, and acoustic modal analysis of the valve chamber. Three peak frequencies at St = 0.044, St = 0.17, and St = 0.59 were identified from field measurements of the vibration behavior. Subsequently, the unsteady flow fields in the control valve were determined from DES and then analyzed using the state-of-the-art data-driven proper orthogonal decomposition (POD) method and cross-correlation analysis, which extracted the dominant unsteady flow behavior in relation to the valve spindle’s vibrations. The findings demonstrated that the valve spindle’s lateral force fluctuations at St = 0.019 occurred due to the alternating oscillations of the annular wall-attached jet, which resulted from the first two POD modes occupying 25% of the turbulent fluctuation energy. The valve spindle’s axial force fluctuations at St = 0.043 were attributed to the synchronous oscillations of the annular wall-attached jet, which resulted from the third, fourth, and fifth POD modes occupying 15% of the turbulent fluctuation energy. Finally, through acoustic modal analysis that compared the pressure fluctuations extracted from the DES results, the axial acoustic mode in the valve’s cavity at St = 0.19 was found to be associated with the valve spindle’s axial force fluctuations at St = 0.174, while the first circumferential acoustic mode of the valve diffuser at St = 0.61 was found to be associated with the valve spindle’s lateral force fluctuations at St = 0.62. This confirmed the potential coupling between the acoustic mode pattern and pressure fluctuation pattern. These complementary techniques were demonstrated to be effective methodologies for the flow-induced vibration behavior and intensive acoustics in valves.

Mechanism of Nonsynchronous Blade Vibration in a Transonic Compressor Rig

This paper presents a numerical study on blade vibration for the transonic compressor rig at the Technische Universität Darmstadt (TUD), Darmstadt, Germany. The vibration was experimentally observed for the second eigenmode of the rotor blades at nonsynchronous frequencies and is simulated for two rotational speeds using a time-linearized approach. The numerical simulation results are in close agreement with the experiment in both cases. The vibration phenomenon shows similarities to flutter. Numerical simulations and comparison with the experimental observations showed that vibrations occur near the compressor stability limit due to interaction of the blade movement with a pressure fluctuation pattern originating from the tip clearance flow. The tip clearance flow pattern travels in the backward direction, seen from the rotating frame of reference, and causes a forward traveling structural vibration pattern with the same phase difference between blades. When decreasing the rotor tip gap size, the mechanism causing the vibration is alleviated.

Relationship between volute pressure fluctuation pattern and tonal noise generation in a squirrel-cage fan

In this work, the pressure fluctuation pattern in the volute of a squirrel-cage fan is analyzed. Also studied is how this pattern is modified when a slight geometry change is introduced in the volute tongue. The study has been carried out on a commercial machine, used in automotive air conditioning units. A three-dimensional and unsteady numerical simulation of the flow in the complete machine has been carried out using the commercial code FLUENT. In this way, the pressure fluctuations in some locations near the volute wall have been obtained. The results of this numerical simulation have been compared to the sound pressure level spectra radiated by the fan, measured in a ducted test installation at the laboratory. The tendencies of the sound pressure levels measured at the blade passing frequency show a good correlation with the amplitudes of pressure fluctuations obtained numerically near the volute wall.

레벨셋법을 이용한 2 차원 사각 탱크 내부의 슬로싱 특성에 관한 수치적 연구

The sloshing phenomena in a two-dimensional rectangular tank are investigated using a level set method based on finite volume method. The code validations are performed by comparing between the present results and previous numerical results, which gives a good agreement. We present the streamlines pattern, free surface shape, maximum free surface elevation and pressure fluctuation patterns in the tank under the pitch and surge motions with various frequencies. These two different motions cause the different flow structures in the tank. The time variations of surface elevation and pressure at the different locations in the tank strongly depend on the exciting frequency of tank moving.

PRESSURE FLUCTUATION AND FLOW REGIMES OF AIR-WATER FLOW IN A SMALL TUBE

Previous studies have shown that flow regimes of gas-liquid flow in small tubes and channels are related to pressure drop fluctuations along the flow passages, and some flow regimes exhibit characteristic pressure fluctuation patterns. This has led to the possibility of using pressure fluctuations to identify flow patterns. This article presents a test program to show experimentally the relationship of pressure fluctuations to gas bubbles, liquid slugs, and waves on a gas-liquid interface in small tubes and channels. In the experiment, a high-speed video camera was used to examine flow mixture passing a pressure tap. Pressure fluctuation at the tap was measured simultaneously using a pressure transducer. The results have shown that high-frequency pressure fluctuation is related to gas bubbles, liquid slugs, or waves on a gas-liquid interface passing the pressure tap. It has found that low-frequency pressure waves are related to flow regimes in the test tube. This research has provided further information for flow regime identification using pressure fluctuation traces.

Fluidization and slugging in large-particle systems

Square-nose slugging that occurs with large particles in relatively small-diameter fluidized beds shows certain similarities with the fluidization behaviour in a fluidized bed coal combustion system with closely packed heat-exchanger tubes. In the present investigation, square-nose slugging is studied in fluidized beds of 0.1 and 0.15 m I.D. with coarse sand and alumina particles, at ambient conditions. Recording of pressure fluctuations was used to analyse the fluidization behaviour. A remarkable change in the pressure fluctuation pattern occurs at the transition from normal fluidization to slugging: a more regular signal with a narrowed frequency spectrum is found. In the square-nose slugging regime, the pressure fluctuations seem to be caused by the disintegration of a rising solids slug, followed by the raining down of the particles. Experimental evidence for this mechanism was found in the behaviour of the magnitude of the pressure fluctuations as a function of operating variables. The frequency of square-nose slugging increases with approximately the square root of the bed diameter and appears to be independent of the type of particles used. The slug frequency decreases slightly with the gas velocity between about 0.8 and 1.8 m.s −1, and is inversely proportional to the stationary bed height between 0.15 and 0.4 m.

The visualization of pressure distribution along a plate by jet impingement with a holographic method.

A holographic visualization of the pressure on a plate impinged by an axisymmetric turbulent air jet has been carried out. A subdivided silicon rubber sheet located on the plate was transformed with a pressure of the impinging jet and its displacement was measured by the holographic interferometer. The deformation of the rubber plate has no effect on the flow field because of the small wall displacement of order on the 10-5 m. Fringe patterns of the interferometry correspond to the instantaneous isobaric lines, which correlate well to the ones measured with a semiconducter pressure transducer. On the other hand, when the plate image deformed by an impinging jet is used as the original image, fringe patterns indicate pressure fluctuations. These pressure fluctuation patterns were converted to electric signals by a ccd-camera and microcomputer, and analyzed by a digital image processing method. The flow characteristics of the impinging jet were clarified by this visualization method.

Some Features of the Unsteady Pressure Field in Transonic Airfoil Buffeting

Comparisons are made between the unsteady transonic flowfields of two airfoils: a Whitcomb supercritical airfoil, and a conventional NACA 0012 section. Wind tunnel experiments on these airfoils included penetration into buffeting as a result of high cl and/or high M^. Fluctuating surface pressure, lift, and shock location were measured on both airfoils. Two-point pressure cross-correlations were used to determine coherence and propagation direction of pressure fluctuation patterns on the upper surface of each airfoil. Between the uppersurface shock and the trailing edge (a region with intense pressure fluctuations), pressure disturbances propagated upstream in attached flow, but traveled downstream when extensive separation existed. In the latter case, convection velocities were found to be frequency dependent. Another cross-correlation, relating surfacepressure fluctuations to unsteady lift, was employed to establish which regions of the pressure fields were of primary importance in producing buffeting forces. While many of the principal features of the pressure/lift cross-correlations were common to both airfoils, some specific differences were found. For example, the supercritical airfoil exhibited less periodicity in its cross-correlation. This result was attributed to the flattopped, aft-cambered shape of the supercritical airfoil section, which reduced the coupling between shock oscillations and lift fluctuations.