Predicted Pressure Fluctuations Research Articles

Numerical investigation of the impact of cavitation on the pressure fluctuations in a Francis turbine at deep part load conditions

Hydropower plants can play an important role to stabilize the electrical grid when the share of volatile renewable energies increases. This goes hand in hand with a more flexible operation of the turbines and longer operation in off-design operating points. The increased pressure fluctuations that occur at off-design conditions may reduce the lifetime of the turbines. Consequently, this should be considered within the design process to develop efficient but also robust turbines. To derive suitable simulation setups that can be used in the design process, it is crucial to have a deep understanding of specific flow phenomena. As pressure fluctuations are of main importance in terms of fatigue, it is important that simulations can capture them well. Within this study, a deep part load operating point of a Francis turbine at model scale is investigated. By using three different meshes the influence of mesh refinement on the predicted pressure fluctuations is analyzed. Furthermore, by means of single- and two-phase simulations it is investigated how the pressure fluctuations are affected by the occurrence of cavitation. The results show only minor differences between the different meshes, whereas significant differences are observed between a single- and two-phase treatment.

Modeling of nonlinear viscoelastic creep of a polyethylene pipeline during water hammer

ABSTRACT In this research, the nonlinear behavior of a viscoelastic pipe during water hammer is investigated. To this end, the creep coefficients of the multiple element Kelvin–Voigt model have been determined for different flow rates and then they are incorporated to derive a holistic relation for the creep compliance. A linear interpolation scheme has been adopted to arrive at the holistic generalized Kelvin–Voigt model as a function of flow rates. The proposed relation is then applied in the established mathematical model and corresponding numerical solution to predict pressure fluctuations. The proposed approach in the application of the creep functions could capture the nonlinear material behavior during transients. Specifically, it revealed a higher creep behavior for higher loading rates (Joukowsky pressures) which was consistent with the experimental observations. Several numerical and experimental results were analyzed and investigated and to demonstrate the performance of the proposed approach.

A Turbulence Model Assessment for Deep Part Load Conditions of a Francis Turbine

Today, hydropower plants received increased attention as they have the ability to stabilize the electrical grid. However, this is accompanied by more start-stop sequences as well as operation in off-design conditions. One big task is to develop improved runner designs that can resist the increased pressure fluctuations. In the scope of this study, a Francis turbine is simulated for a deep part load operating point with the use of the SAS model and the more recently developed SBES model with ANSYS CFX. Both turbulence models are hybrid RANS-LES models. The simulation results are compared against model test data from a measurement at high cavitation number. For the evaluation of the pressure fluctuations comparison is furthermore made against experimental data at low cavitation number. All in all, SAS and SBES model predict similar flow characteristics. Both, the velocity profiles in the PIV measurement plane and the predicted pressure fluctuations, show only minor differences. The comparison against experimental data shows that the amplitude of the broadband frequency spectrum, which results from the occurrence of vortices, is overestimated at the investigated locations on the runner blades.

Computational Fluid Dynamics (CFD) simulation of liquid column separation in pipe transients

The present study deals with the computational simulation of transient events of water hammer and column separation in a water pipeline system. Three-dimensional CFD simulations based on the Finite-Volume (FV) approach are performed to predict pressure fluctuations and to visualize liquid column separation/rejoining caused by the sudden closure of a valve located at the upstream end of the pipe. Explanation of vaporous cavitation phenomena is also presented. The Volume of Fluid (VOF) model and Schnerr-Sauer cavitation model are used to describe the multiphase flow and the transient vaporous cavitation, respectively. Moreover, the shear-stress transport (SST) turbulence model is applied to model the Reynolds stresses using the Boussinesq hypothesis. Present results are compared with available experimental and numerical results from the literature. The comparisons show that the present method gives adequate results. Also, the 3D model adopted is deemed physically superior to the existing 1D models as it removes the restriction of the 1D models that vapor cavities, when formed, fill the whole cross-section of the pipe without radial variation. In addition, 1D models are not able to predict the stratification effect due to density variation of the two phases. Consequently, the 3D model can better visualize the phenomenon of liquid column separation/rejoining in pipes than 1D models.

An advanced numerical framework for the simulation of flow induced vibration for nuclear applications

In this work, an advanced solver for the simulation of Fluid-Structure Interaction (FSI) problems for nuclear applications is presented. The numerical framework, called as NRG-FSIFOAM, uses a Finite Volume (FV) fluid solver and a Finite Element (FE) structural solver coupled together with a partitioned approach. State-of-the-art implicit coupling algorithms are used in order to increase the accuracy and the stability of the solver for low solid-to-fluid density ratio, i.e. strongly coupled, FSI problems. Furthermore, the solver offers significant advantages with respect to traditional approaches for the solution of turbulent flows because it is equipped with an advanced algorithm to predict pressure fluctuations without the necessity to perform high fidelity simulations. This approach, called as Pressure Fluctuation Model (PFM), uses a stochastic model to generate velocity fluctuations that satisfy the mean turbulent quantities; the corresponding fluctuating pressure field is computed from the Poisson’s equation. The pressure fluctuation field complements the mean pressure field computed with URANS models and it is then used as a part of the external forcing for the structural problem. This solver is first tested against benchmark FSI problems with laminar flows. Subsequently, it is used to simulate flow induced vibrations of cylindrical fuel rods in turbulent water and liquid metal flows and to simulate the vibrations of a cantilever beam in turbulent water flow.

Study on the Accuracy of RANS Modelling of the Turbulent Flow Developed in a Kaplan Turbine Operated at BEP. Part 2 - Pressure Fluctuations

The aim of the paper is to investigate the limitations of unsteady Reynolds-averaged Navier-Stokes (RANS) simulations of the flow in an axial turbine. The study is focused on modelling the pressure pulsations monitored on the runner blades. The scanned blade geometry renders the meshing process more difficult. As the pressure monitor points are defined on the blade surface the simulation relies on the wall functions to capture the flow and the pressure oscillations. In addition to the classical turbulence models, a curvature correction model is evaluated aiming to better capture the rotating flow near curved, concave wall boundaries. Given the limitations of Reynolds-averaged Navier-Stokes models to predict pressure fluctuations, the Scale Adaptive Simulation-Shear Stress Transport (SAS-SST) turbulence model is employed as well. The considered test case is the Porjus U9, a Kaplan turbine model, for which pressure measurements are available in the rotating and stationary frames of reference. The simulations are validated against time-dependent experimental data. Despite the frequencies of the pressure fluctuations recorded on the runner blades being accurately captured, the amplitudes are considerably underestimated. All turbulence models estimate the correct mean wall pressure recovery coefficient in the upper part of the draft tube.

Mitigating flow induced vibration in heater radiant coil

Vibration of process heater tubes in a fired heater can cause fretting-wear damage of the tubes at the locations of contact points with the supports. For the reboiler of a naphtha splitter in a naphtha hydrotreating unit, a scenario of fretting-wear damage was observed exactly at contact areas between the top return bends and the hanger supports, which likely indicated constant rubbing between them during vibration. A root-cause analysis of this tube vibration problem was carried out through a combined study of process simulation, Computational Fluid Dynamics (CFD) and vibration analysis. Results from CFD simulations revealed dual phase flow inducing pressure fluctuations inside the radiant tube. The predicted pressure fluctuations were further analyzed using Fast Fourier Transform (FFT) to identify the dominant frequency of pressure fluctuations. Some of the resulting dominant frequencies were within 20% band of the estimated natural frequency of the tube, which could lead to resonance mode. This predicted resonant vibration matched with the locations of severe grooving, as reported in the heater inspection report. A scenario of mitigating this resonance mode has also been presented through decreasing feed flow rates to the radiant tube coils and installing additional support at the mid-height of the radiant tube coils.

Prediction of pressure fluctuation on a vehicle by Large Eddy Simulation

The objective of this study is to predict accurately interior aeroacoustics noise of a car for a wide range of frequency between 100 Hz and 4 kHz. One-way coupled simulations of computational fluid dynamics (CFD), structural analysis and acoustical analysis were performed to predict interior aeroacoustics noise. We predicted pressure fluctuations on the outer surfaces of a test car by computing unsteady flow around the car as the first step. Secondary, the predicted pressure fluctuations were fed to the subsequent structural analysis to predict vibration accelerations on the inner surfaces of the test car. Finally, acoustical analysis was performed to predict sound fields in the test car by giving vibration accelerations computed by the structural analysis as the boundary conditions. In this paper, we focus on the unsteady flow computations, which is the first step of the coupled simulations. Large Eddy Simulation (LES) was performed to predict the pressure fluctuations on the outer surfaces of the test car. We used the computational mesh composed of approximately 5 billion hexahedral grids with a spatial resolution of 1.5 mm in the streamwise and spanwise directions to resolve the dynamics of the small vortices in the turbulence boundary layer. Predicted and measured pressure fluctuation at several sampling points on the surface of the test car were compared and they matched well in a wide range of frequency up to 2 kHz.Copyright © 2015 by JSME

Numerical study of pressure fluctuations transfer law in different flow rate of turbine mode in a prototype pump turbine

Numerical simulation using SST k-w turbulence model was carried out, to predict pressure fluctuation transfer law in turbine mode. Three operating points with different mass flow rates are simulated. The results of numerical simulation show that, the amplitude and frequency of pressure fluctuations in different positions are very different. The transfer law of amplitude and frequency of pressure fluctuations change with different position and different mass flow rate. Blade passing frequency (BPF) is the first dominant frequency in vaneless space, while component in this frequency got smaller in the upstream and downstream of vaneless space when the mass flow is set. Furthermore triple blade passing frequency (3BPF) component obtained a different transfer law through the whole flow passage. The amplitude and frequency of pressure fluctuations is also different in different circumference position of vaneless space. When the mass flow is different, the distribution of pressure fluctuations in circumference is different. The frequency component of pressure fluctuations in all the positions is different too.

Experimental and numerical analysis of pressure pulses characteristics in a Francis turbine with partial load

This study experimentally and numerically investigates the pressure pulses characteristics and unsteady flow behavior in a Francis turbine with partial load. Unsteady wall pressure measurements with partial load condition are performs to investigate thoroughly pressure fields in the spiral case, runner head cover and straight draft tube dynamically. The unsteady Reynolds- averaged Navier-Stokes equations with the k-ωbased SST turbulence model were used to model the unsteady flow within the entire flow passage of the Francis turbine. The dominate frequency of the predicted pressure pulses at runner inlet agree with the experimental results in the head cover. The influence of the blade passing frequency causes the simulated peak-to-peak amplitudes in the runner inlet to be larger than in the head cover. The measured and predicted pressure pulses at different positions along the runner are comparable. The predicted pressure fluctuations in the draft tube agree well with the experimental results. However the peak-to-peak amplitudes in the spiral case are not as well predicted so the calculation domain and the inlet boundary conditions need to be improved. At the most unstable operating condition, the pulse in the flow passage are due to the rotor-stator interference (RSI) between the runner and the guide vanes, the blade channel vortex in the runner blade passage and the vortex rope in the draft tube. The unsteady flow patterns in the turbine, including the blade channel vortex in the runner and the helical vortex rope in the draft tube, are classified numerically.

Numerical analysis of unsteady flow under high‐head operating conditions in Francis turbine

PurposeThe purpose of this paper is to describe how the hydraulic performance and pressure fluctuations in the entire flow passage of a Francis turbine were predicted numerically for the highest head. The calculations are used to partition the turbine operating regions and to clarify the unsteady flow behavior in the entire flow passage including the blade channel vortex in the runner and vortex rope in the draft tube.Design/methodology/approachThree‐dimensional unsteady numerical simulations were performed for a number of operating conditions at the highest head. The unsteady Reynolds‐averaged Navier‐Stokes equations with the k‐ω based SST turbulence model were solved to model the unsteady flow within the entire flow passage of a Francis turbine.FindingsThe predicted pressure fluctuations in the draft tube agree well with the experimental results at low heads. However the peak‐to‐peak amplitudes in the spiral case are not as well predicted so the calculation domain and the inlet boundary conditions need to be improved. The unsteady simulation results are better than the steady‐state results. At the most unstable operating condition of case a0.5h1.26, the pulse in the flow passage is due to the rotor‐stator interference between the runner and the guide vanes, the blade channel vortex in the runner blade passage and the vortex rope in the draft tube.Originality/valueThis study investigates the characteristics of the dominant unsteady flow frequencies in different parts of the turbine for various guide vane openings at the highest head. The unsteady flow patterns in the turbine, including the blade channel vortex in the runner and the helical vortex rope in the draft tube, are classified numerically, and the turbine operating regions are partitioned to identify safe operating regions.

Nonlinear Characterization of Pressure Fluctuations in Fluidized Beds

Nonlinear time series analysis techniques were applied to predict pressure fluctuation data in fluidized beds in two different hydrodynamic states. The method of delays was used to reconstruct the ...

A comparative study of four‐field and fully compressible magnetohydrodynamic turbulence in the solar wind

A four‐field system of equations has been recently derived from the compressible magnetohydrodynamic (MHD) equations to describe turbulence in the solar wind. These equations apply to a plasma permeated by a spatially varying mean magnetic field when the plasma beta is of the order unity or less. In the presence of spatial inhomogeneities, the four‐field equations predict pressure fluctuations of the order of the Mach number of the turbulence, as observed by Helios 1 and 2. It is shown that inhomogeneities that occur only in the form of pressure‐balanced structures cannot account for density fluctuations as large as the Mach number of the turbulence. Numerical predictions from the four‐field and compressible MHD equations are compared using the same initial condition. The comparison shows that the predictions of the four‐field model are qualitatively consistent with the predictions of the compressible MHD model and suggests that the four‐field model is a viable model of compressible turbulence in the solar wind.

Prediction of wind noise radiated from passenger cars and its evaluation based on auralization

Abstract This paper describes the prediction of radiated wind noise from automobiles and its reduction. The Lighthill acoustic analogy was employed to estimate the wind noise at far field. In order to compute accurate pressure fluctuations, which play a role as the noise sources in the analogy, an overlapped grid system is used to calculate flow fields in detail with a finite-difference method. This approach was applied to predict the wind noise radiated from a door mirror and a front pillar. Measured data are compared with the computed results of pressure fluctuations on the side-window surface. The predicted pressure fluctuations agree well with the experimental results. Then, shapes of the front pillar and the door mirror were modified based on the computed results so as to reduce the wind noise. The effects of the modification were proved by additional experiments.

Estimating Wave Heights from Pressure Measured in Sand Bed

Comparison of predicted with observed attenuation of pressure fluctuations shows that wave heights can be estimated with observations from a pressure sensor that is buried a known depth in fine sand. The attenuation of pressure fluctuations within the sand bed under unbroken shoaling waves, bores in the surf zone, and swash near the shoreline was measured with vertical stacks of buried pressure sensors. The attenuation increased with increasing frequency and depth below the bed surface, consistent with previous observations under nonbreaking waves in deeper water and with model predictions based on poro-elastic theory. In the limit of an infinitely deep soil skeleton that is much more compressible than the pore fluid, the predicted pressure fluctuations decrease exponentially with increasing burial depth, and the attenuation is independent of the sediment properties. For the fine-grained sand beds considered here, this exponential limit accurately predicts the observed attenuation.

Vibration and pressure fluctuation in a flexible hydraulic power systemon an aircraft

A full fluid–structural coupled model has been established to predict pressure fluctuation and pipe dynamic responses in a hydraulic power system. This model includes the coupled effects of fluid flow–structural stiffness and damping. A solution procedure combining the method of characteristics (MOC) and the finite element method (FEM) was used to solve the coupled equations. The method was then applied to an example aircraft hydraulic pipe system. Comparison of the predicted results with those of de-coupled model demonstrated that a fluid–structure interaction (FSI) model is necessary in studies of the dynamics of a flexible pipe system.

VORTEX SIMULATION IN RADIAL FLOW IMPELLERS OF CENTRIFUGAL PUMPS AT PART LOADS

The phenomena of vortex formation in relative flow field of a radial flow centrifugal pump, running at zero and part loads of the pump discharge, has been investigated. The flow field is described by two dimensional Poisson's equation, which has been solved numerically, in difference form, by relaxation technique. A physical model for the vortex formation andshedding mechanisms, at part loads, has been introduced. The effect of vortex shedding, from the impeller, on the pressure fluctuation at the pump exit has been also discussed. Experimental investigation has shown a satisfactory agraement between the measured and the theoretically predicted pressure fluctuations at part flow rates. At high flow rates, the viscosity effect on the vortex simulation model, has to be considered.

The response of railway passengers to pressure fluctuations

This paper describes a study carried out on behalf of Deutsche Bundesbahn (West German Railways) by British Rail in collaboration with the Institute of Consumer Ergonomics. The aim was to evaluate subjective responses to predicted pressure fluctuations that are likely to occur in vehicles that will run on a new railway line under construction. The study showed that most subjects found the pressure fluctuations acceptable but there was a significant proportion of subjects who found the most severe pressure fluctuations uncomfortable.

Nonlinear Motion of an Air-Cushion Vehicle over Waves

The nonlinear behavior of an air-cushion vehicle (ACV) during coupled pitch and heave motion over a sinusoidal wave is studied. The vehicle is of the plenum-chamber type with a transverse stability skirt, which is a contributor to the nonlinearily. Further nonlinearity results from the contact of the flexible skirt with the surface. The airflow is considered incompressible, and water compliance is neglected. Numerical computations have been made for the craft response in head seas as a function of encounter frequency for a set of Froude numbers and skirt configurations. The motion is found to be highly damped, and strongly dependent on the depth of the stability skirt, though not on its longitudinal position. Weak peaks in the response curves are displayed, their strength and location being a function of the wave height. The effect of skirt contact forces is found to be negligible on the craft motion. The predicted pressure fluctuations can be large and, at sufficiently high frequencies, result in subatmospheric pressures in the cushion.