Induced Pressure Fluctuations Research Articles

Study of the effects of modified draft tube with inclined conical diffuser on draft tube and upstream region

Various countermeasures including geometry optimization have been proposed and proved to have a huge impact on flow filed within the draft tube in order to mitigate the vortex rope and the induced pressure fluctuations for Francis turbine under part load operation. However, the effect of these approaches on the hydro-dynamics in upstream region still remains unclear, which is of great significance for the overall performance of the unit. This study aims to explore the influences of modified draft tube with inclined conical diffuser on the pressure fluctuations in whole flow passage. The results reveal that Generation-1 and Generation-2 draft tubes are effective in alleviating pressure fluctuations resided in the draft tube, but the former one would trigger a low-frequency pressure fluctuation with higher amplitude in the runner zone. In addition, the modified draft tube has a very limited effect on the high-frequency pressure fluctuations in the guide vane and vaneless areas. To eliminate the adverse effect of inclined conical diffuser, a design with a transitional section is put forward to smoothly connect the runner zone and inclined conical diffuser. This further developed Generation-2 draft tube presents a reasonably good performance in terms of improving flow stabilities, i.e. alleviating the pressure fluctuations not only in draft tube but also in upstream region such as runner zone. This study provides reference for obtaining a better mitigating effect on pressure fluctuations in the whole turbine flow passage.

Active Control of the Vortex Induced Pressure Fluctuations in a Hydro Turbine Model via Axial and Radial Jets at the Crown Tip

Active Control of the Vortex Induced Pressure Fluctuations in a Hydro Turbine Model via Axial and Radial Jets at the Crown Tip

Study on the Unsteady Flow Characteristics of a Pump Turbine in Pump Mode

Extensive research has been conducted on the performance of pump turbines, particularly focused on understanding the generation mechanism of S-shaped characteristics. However, there has been a lack of research on unsteady flow characteristics in hump characteristics with small guide vane openings. This study focuses on the hump characteristics of a pump turbine in pump mode. The unsteady numerical simulation method is used along with experimental testing to examine the internal flow characteristics and induced pressure fluctuations under pump operating conditions. The results indicate that flow separation occurs in the impeller when the flow rate decreases to the valley operating condition, and recirculation flow occurs near the impeller inlet at the partial flow rate. Moreover, the unstable flow on the positive slope exhibits a low-frequency characteristic of 0.15fn. The pressure fluctuation from the hub to shroud areas of the guide vane region diminishes sequentially. Notably, distinct vortex structures emerge at the draft tube cone section under the valley operating condition. These structures extend toward the elbow section of the draft tube as the flow rate decreases. This phenomenon generates low-frequency pressure fluctuation originating from the primary frequency of the vortex and dean vortex on the surface, located at 0.4 D of the draft tube under conditions of low flow rate.

Runner cone drilling and slotting to reduce vortex rope - induced pressure fluctuations in a Francis turbine

Pressure fluctuations caused by helical vortex rope in the draft tube is one of the main reasons for the instability of Francis turbines. Previous studies showed that drilling holes on runner cone can reduce the amplitude of pressure fluctuation. In this study, based on the runner cone which has been drilled one hole, two kinds of new runner cones are designed by means of drilling two more holes (2 holes runner cone) and drilling a slot (2 holes and 1 slot runner cone). Pressure fluctuation and flow pattern observation experiments as well as numerical simulations at partial load operation condition in a Francis turbine were carried out for the three types of runner cones. Analyses were performed to reveal the generation mechanism of the low frequency pressure fluctuations. The results showed that low frequency fluctuations are related to the cyclical rotation of helical vortex rope. Holes and slots on the runner cone have little impact on the efficiency of the Francis turbine. Slotting on the runner cone can reduce the amplitude of pressure fluctuation induced by vortex rope to a maximum of 29.41%. Hence, proper slotting on the runner cone can effectively reduce the low-frequency fluctuation caused by vortex rope in Francis turbines. These findings provide a new idea to improve the hydraulic instability of Francis turbines induced by vortex rope.

Numerical study on cavitation flow and induced pressure fluctuation characteristics of centrifugal pump

In order to reveal the variation law between cavitation process and pressure pulsation characteristics of centrifugal pump, the internal flow characteristics and pressure pulsation characteristics of centrifugal pump during cavitation development are studied in this paper. First, select the standard k- ε with good coincidence between external characteristics and test data under rated conditions model for calculation after compare the standard k- ε, RNG k- ε and SST k- ω. Then, based on the unsteady calculation results, the characteristics of the internal flow before and after the cavitation critical point, the frequency domain characteristics of the pressure pulsation on the volute wall and at the impeller inlet monitoring points in different cavitation stages are compared and analyzed. Finally, root mean square (RMS) is used to study the change trend of pressure fluctuation energy intensity with cavitation process. The research results show that the prediction accuracy of the characteristic points of RMS change curve of pressure fluctuation for cavitation is higher than that of the traditional 3% head drop point. Using the energy characteristics of pressure fluctuation can more accurately and timely determine the occurrence of cavitation.

Experimental evaluation of wind induced pressure fluctuations in cavity shaped open volumetric air receivers

In this study wind induced surface pressure fluctuations during operation of an open volumetric cavity receiver were investigated in wind tunnel experiments. Due to the process design of the open volumetric receiver, surface pressure fluctuations impact the receiver mass flow which is crucial for cooling the highly irradiated receiver. For this study a model of a solar tower was constructed under the concept of similitude which allows wind tunnel experiments at Reynolds numbers up to 13.17 × 106. Transient pressure measurements were conducted under 17 different flow directions to evaluate the impact of separation induced pressure fluctuations. The experimental design further allows a rotation of the model to evaluate the impact of changes in wind direction on the surface pressure at the receiver. For cases under a constant wind direction, a critical angle range could be identified in head on wind with a horizontal inclination of 45.0 to 78.8° relative to the central receiver axis. The results further show that surface pressure fluctuations are more sensitive to changes in wind direction as the amplitudes with maximum 20Pa for a Reynolds number of 13.17 × 106 are about one order of magnitude higher compared to the cases with a constant flow direction. Furthermore, a frequency analysis shows the shielding effect of the cavity design for side-wind as no peaks in the pressure spectrum appear. For flow directions within the critical range of 45.0 to 78.8° a main vortex shedding frequency under constant Strouhal numbers of 0.25 to 0.50 can be observed.

Thermally Induced Pressure Fluctuations in Single-Phase Fluid-Saturated Porous Media Described by the Fluctuation Dissipation Theorem

Thermally Induced Pressure Fluctuations in Single-Phase Fluid-Saturated Porous Media Described by the Fluctuation Dissipation Theorem

Study on the flow pattern and pressure fluctuation in a vertical volute centrifugal pump with vaned diffuser under near stall conditions

The vertical volute centrifugal pump is widely used in the long-distance water division project. Flow separation and vortex phenomenon can be observed in the vaned diffuser under near stall operating conditions. This kind of unsteady flow structures would result in the instability of the pump unit which affect the safety and reliability of the unit operation. In this paper, the unsteady flow patterns and induced pressure fluctuation in a vertical volute centrifugal pump with vaned diffuser were simulated based on the SAS turbulence model and refined mesh under near stall conditions. The research results show that the SST turbulence model combined with the selected grid can be validated in the CFD uncertainty analysis during the steady calculation based on the ITTC relevant procedures. The performance results predicted by the SAS turbulence model show quite good agreement with the experimental data, and the prediction errors were less than 5%. The 3D flow matching between the flow angle at the impeller blade outlet and the incidence angle of the guide vane at different spans would induce the stall inception of the pump. The flow patterns of the pump under near stall conditions were revealed combined with vorticity and streamline distributions. When the pump unit was operated under near stall conditions, the main frequency changed from the blade passing frequency 7fn to 0.9fn. It was found that the main frequency was mainly affected by the large-scale vortices in the vaned diffuser, and the main vortices in the vaned diffuser appear at every 0.9 time of the period of one revolution of impeller.

Experimental investigation of unsteady attached cavitating flow induced pressure fluctuation

Experimental investigation of unsteady attached cavitating flow induced pressure fluctuation

Numerical and Experimental Analysis of the Noise Generated in a Ducted Cavity Working in Various Conditions

Flow over a cavity or a gap may induce pressure fluctuations that are emitted as sound waves and perceived by a human as noise. This phenomenon may occur in different kinds of industrial machines or in everyday life devices, e.g., cars. For this reason, it is important to predict the flow conditions that intensify or attenuate the noise. This research paper presents the numerical and experimental analysis of the pressure fluctuations in a deep, ducted cavity. The experimental test stand made it possible to investigate the flow over a cavity with air velocity in the range of 30–80 m/s. The pressure fluctuations were measured using miniature microphones located in the duct and the cavity wall and processed with LabView software. The phenomena were also analysed using the computational fluid dynamics (CFD) technique. The several modelling approaches were tested and validated against the experimental data. The highest sound pressure levels were obtained for 40 and 70 m/s. The sound frequency increased with the flow velocity.

A Bayesian approach for the separation of the acoustic and the correlated aerodynamic wall pressure fluctuations.

When performing measurements with wall-installed microphone array, the turbulent boundary layer that develops over the measuring system can induce pressure fluctuations that are much greater than those of acoustic sources. It then becomes necessary to process the data to extract each component of the measured field. For this purpose, it is proposed in this paper to decompose the measured spectral matrix into the sum of matrices associated with the acoustic and aerodynamic contributions. This decomposition exploits the statistical properties of each pressure field. On the one hand, assuming that the acoustic contribution is highly correlated over the sensors, the rank of the corresponding cross-spectral matrix is limited to a finite number. On the other hand, the correlation structure of the aerodynamic noise matrix is constrained to resemble a Corcos-like model, with physical parameters estimated within the separation procedure. This separation problem is solved by a Bayesian inference approach, which takes into account the uncertainties on each component of the model. The performance of the method is first evaluated on wind tunnel measurements and then on a particularly noisy industrial measurement setup: microphones flush-mounted on the fuselage of a large aircraft.

Influence of Shock Wave on Loss and Breakdown of Tip-Leakage Vortex in Turbine Rotor with Varying Backpressure

In modern turbine rotors, tip-leakage flow is a common phenomenon that accounts for about 1/3 of the stage loss. Studies show that as the imposed load increases, a shock wave appears in the tip region, which causes a significant interference on the leakage vortex. In the present study, numerical simulations are carried out to investigate the influence of the shock wave on the loss and breakdown of the tip-leakage vortex. The obtained results indicate that with no effective control on the flow, the loss of the leakage vortex has an approximate exponential growth up to about 10 times as the outlet Mach number increases from 0.67 to 1.15 and the corresponding proportion in the total loss increases sharply to 30.2%. It is found that the stagnation position of the breakdown changes with the backpressure and the amplitude of variation along the axial direction is up to 0.13 Cx. It is inferred that the breakdown of the leakage vortex core may be affected by the periodical passing of downstream blade and the induced pressure fluctuation may result in additional vibration in this rotor blade. The leakage vortex is unstable in supersonic flow with a shock wave and it may transfer to a flow with a low-velocity bubble in its core region. It is concluded that the leakage vortex breakdown mainly originates from interferences of the shock wave, while the internal cause of such breakdown is the centrifugal instability of the vortex.

An experimental study to investigate novel physical mechanisms that enhance viscoelastic polymer flooding and further increase desaturation of residual oil saturation

This study uses a combination of shear rheometry and core-flooding using viscoelastic polymers to understand better the enhanced oil sweep efficiency after residual oil saturation is achieved by conventional water-flood. This work addresses the question of anomalous (enhanced) desaturation of oil by water-flooding using polymer and which has been widely reported since 2008. A mechanism to explain the enhanced desaturation is developed. Berea sandstone was saturated with synthetic oil (34mPa.s @ 200 C) at set reservoir conditions (2000 psi, 900 C). It was water-flooded from initial oil saturation (Soi=76.21 %, 47.5 ml) to residual oil saturation (Sorw=40.43 %, 25.2 ml) where oil cut was zero using brine (33390 ppm). The core was subject to further flooding using inelastic Newtonian 85 wt % Glycerol flooding until zero oil cut (Sorgly=36.90 %, 23.0 ml), followed by viscous Non-Newtonian 1720 ppm Xanthan Gum flooding (Sorx=34.33 %, 21.4 ml), followed by 6000 ppm viscoelastic FLOPAAM 3230 (Sor3230=34.33 %, 21.4 ml, zero oil cut) and ended by 2000 ppm FLOCOMB 6525 (Sor6525=33.21 %, 20.7 ml). It was found an additional 3.27% OOIP was recovered by the elastic turbulence effect of high Mw viscoelastic polymer-flooding below critical Capillary number, having the Deborah number, NDe=2.13. Since the late 1960s, EOR researchers have developed different continuum and pore-scale viscoelastic models for quantifying the viscoelastic polymer-flooding effects on Sor. From the literature, research articles conclude that Sor reduction depends upon the flux rate as well as on reservoir wettability, brine salinity, reservoir permeability, polymer elasticity, Mw of viscoelastic polymer and oil viscosity. We have come to the conclusion in this paper that despite these noteworthy Sor reduction explanations, the mechanism of quantified Sor reduction by viscoelastic effect is as yet an unresolved mechanism that requires further investigation. We conclude that the proposed elastic turbulence mechanism has played an important role in the occurrence of increased pore-level induced pressure fluctuations, which in turn increases velocity rate fluctuations and hence additional desaturation of immobile residual oil.

Achievements in Observation and Prediction of Cavitation: Effect and Damage on the Ship Propellers

Cavitation erosion is the most avoided problems caused by the bubble cavitation inception. Other frequent problems occurring are vibration and noise because of the cavitation induced pressure fluctuation on the hull. This paper aims to review the cavitation phenomenon on ship propellers, which have been performed in the previous research. Several techniques to obtain reliable observation and prediction of cavitation inception, related to the adverse effect on material damages are discussed through advance experimental laboratory and numerical modeling. Then, prediction and observation of cavitation behavior on the full scale and model scale, cavitation flow behind hull conditions, and geometry modification of the skew propeller are thoroughly discussed.

Effects of balanced skew and biased skew on the cavitation characteristics and pressure fluctuations of the marine propeller

In order to study the relationship between ship propeller cavitation and induced pressure fluctuations, two propellers with different skew patterns are designed. The RANS method and the Schneer & Sauer cavitation model were used to solve the cavitation flow of the HSP-Ⅱpropeller and the results were in agreement with the measured data. At the cavitation condition, the balanced skew propeller shows a smaller cavitation volume than the biased skew propeller; for monitoring points above the propeller, skew type has little effect on the pressure fluctuations, the balanced propeller shows slightly lower pressure fluctuations. For a clockwise propeller (from the stern), the pressure fluctuations on the starboard are stronger than the port side. Finally, the fast Fourier transform (FFT) of the pressure signal shows that whether it is cavitation or full wetted flow, the point above the propeller contains first 3 harmonics of blade passing frequency (BPF), while the point in the wake shows only 1BPF at cavitation condition.

Effect of induced pressure fluctuation in reservoir of a pulse tube cryocooler

Pulse Tube Cryocoolers are used extensively in aerospace applications, environmental studies, and medical applications. It has several advantages of which some are very low vibration and long life. The performance of Pulse tube cryocooler can be enhanced by using a combination of inertance tube and reservoir. For practical applications the pulse tube cryocooler should be compact in size. This could be achieved by reducing the volume of the reservoir. This paper analyses the feasibility of introducing a reverse fluctuation inside the reservoir to minimize reservoir volume. Reverse fluctuation could be introduced by using the back volume of hermetically sealed linear compressor. To ascertain the same a numerical model was developed and simulated using Ansys Fluent. The effect of pressure variation in the reservoir and its equivalent performance was also simulated. The results suggest that the use of bounce space has a better results at lower volume and equivalent performance on increasing volume when compared to the cryocooler with reservoir.

Extended validation of a graph-based modelling methodology for high-pressure networks

Various applications use high-pressure pumps for waterjet machining. A cutting head generates the waterjet, which is capable to cut hard and brittle materials. These cutting heads induce pressure fluctuations with respect to an a priori unknown switching pattern. This paper applies a graph-based modelling methodology on complex high-pressure networks representing different waterjet facilities. For that, network sections are assigned to homogeneous segments, each representing a local pressure state. Segments are subsequently interconnected along their fluid flow paths, resulting in a lumped parameter model. In this way, the graph-based methodology offers a flexible modelling to cope with various network topologies. The resulting mathematical models of different waterjet facilities have been verified for specific cutting head switching patterns. Measurements on a test bench show that a desired accuracy is obtained over a wide operating range from 0 to 2.45 l/min, even for a fast switching period of 0.5 seconds.

Acoustogenic Probes: A New Frontier in Photoacoustic Imaging

Photoacoustic imaging (PAI) is a powerful imaging modality capable of mapping the absorption of light in biological tissue via the PA effect. When chromophores are optically excited, subsequent energy loss in the form of heat generates local thermoelastic expansion. Repeated excitation from a pulsed laser induces pressure fluctuations that propagate through tissue and can be detected as ultrasound waves. By combining ultrasonic detection with optical excitation, PAI enables high-resolution image acquisition at centimeter depths. PAI is also relatively inexpensive and relies on safe, nonionizing excitation light in the near-infrared window, making it an attractive alternative to other common biomedical imaging modalities. Research in our group is aimed at developing small-molecule activatable probes that can be used for analyte detection in deep tissue via PAI. These probes contain reactive triggers that undergo a selective chemical reaction in the presence of specific stimuli to produce a spectral change that can be observed via PAI. Chemically tuning the absorbance profile of the probe and the reacted product such that they are both within the PA imaging window enables ratiometric imaging when each species is irradiated at a specific wavelength. Ratiometric imaging is an important design feature of these probes as it minimizes error associated with tissue-dependent signal fluctuations and instrumental variation. In this Account, we discuss key properties for designing small-molecule PA probes that can be applied for in vivo studies and the challenges associated with this area of probe development. We also highlight examples from our group including probes capable of detecting metal ions (Cu(II)), reactive nitrogen species (NO), and oxygen tension (hypoxia). Each of these targets can be sensed using a modular design strategy based on influencing the electronic and spectral properties of a NIR-absorbing dye platform. We demonstrate that ideal small-moleculePA probes have high molar absorptivity, low fluorescence quantum yields, and selective triggers that can reliably report on a single analyte in a complex biological setting. Probes must also be highly chemo- and photostable to enable long-term imaging studies. We show that these PA probes react rapidly and selectively and can be utilized for deep-tissue imaging in mouse models of various diseases. Overall, these examples represent a new class of biomedical imaging tools that seek to enable high-resolution molecular imaging capable of improving diagnostic methods and elucidating new biological discoveries. We anticipate that the combination of small-molecule PA probes with new PAI technology will enable noninvasive detection of analytes relevant to disease progression and mapping of tissue microenvironments.

Role of Wave‐Induced Diffusion and Energy Flux in the Vertical Transport of Atmospheric Constituents in the Mesopause Region

AbstractVertical constituent transport is characterized by solving the continuity equation for a minor species in the presence of waves, turbulence, molecular diffusion, and chemical sources and sinks. Random vertical displacement fluctuations induced by waves contribute to vertical mixing of the atmosphere similar to turbulence. The effective wave diffusivity is proportional to the sensible heat flux. Because waves induce pressure fluctuations, diffusivity can also be expressed in terms of the wave energy and enthalpy fluxes. Waves induce strong down gradient mixing in regions where dissipation is significant and weak up gradient mixing where dissipation is negligible. At midlatitudes wave diffusivity is maximum in the upper mesosphere near 88 km where the annual mean is ~130 m2/s. Waves also enhance molecular diffusion of reactive species in regions of slow chemical production and reduce eddy mixing in regions of fast chemical production. The enhancement/reduction factor is proportional to the perturbed lapse rate variance and varies between 15 and 45% at mesopause heights. Wave‐induced chemical transport is related to the chemical production/loss rates and temperature fluctuation variance. The vertical transport of atomic oxygen, meteoric Fe, and meteoric Na is assessed. Analytical predictions compare favorably with models of the chemical loss of O and the meteoric influxes of Fe and Na and with lidar measurements of the vertical fluxes of Fe and Na. Predicted seasonal variations of wave diffusivity at 97.5 km compare favorably with the total diffusivity inferred from satellite observations of neutral density and the O/N2 ratio in the thermosphere.

Experimental investigation of frequency and amplitude of density wave oscillations in vertical upflow boiling

Historically, study of two-phase flow instabilities has been arguably one of the most challenging endeavors in heat transfer literature due to the wide range of instabilities systems can manifest depending on differences in operating conditions and flow geometry. This study utilizes experimental results for vertical upflow boiling of FC-72 in a rectangular channel with finite inlet quality to investigate Density Wave Oscillations (DWOs) and assess their potential impact on design of two-phase systems for future space missions. High-speed flow visualization image sequences are presented and used to directly relate the cyclical passage of High and Low Density Fronts (HDFs and LDFs) to dominant low-frequency oscillations present in transient pressure signals commonly attributed to DWOs. A methodology is presented to determine frequency and amplitude of DWO induced pressure oscillations, which are then plotted for a wide range of relevant operating conditions. Mass velocity (flow inertia) is seen to be the dominant parameter influencing frequency and amplitude of DWOs. Amplitude of pressure oscillations is at most 7% of the time-averaged pressure level for current operating conditions, meaning there is little risk to space missions. Reconstruction of experimental pressure signals using a waveform defined by frequency and amplitude of DWO induced pressure fluctuations is seen to have only moderate agreement with the original signal due to the oversimplifications of treating DWO induced fluctuations as perfectly sinusoidal in nature, assuming they occur at a constant frequency value, and neglecting other transient flow features. This approach is nonetheless determined to have potential value for use as a boundary condition to introduce DWOs in two-phase flow simulations should a model be capable of accurately predicting frequency and amplitude of oscillation.