Showing papers in "Journal of Fluids Engineering-transactions of The Asme in 2019"

Spatial-temporal evolution of tip leakage vortex in a mixed flow pump with tip clearance

Abstract: Tip clearance in pump induces tip leakage vortex (TLV), which interacts with the main flow and leads to instability of flow pattern and decrease of pump performance. In this work, the characteristics of TLV in a mixed-flow pump are investigated by the numerical simulation using shear stress transport (SST) k–ω turbulence model with experimental validation. The trajectory of the primary tip leakage vortex (PTLV) is determined, and a power function law is proposed to describe the intensity of the PTLV core along the trajectory. Spatial–temporal evolution of the TLV in an impeller revolution period T can be classified into three stages: splitting stage, developing stage, and merging stage. The TLV oscillation period TT is found as 19/160 T, corresponding to the frequency 8.4 fi (fi is impeller rotating frequency). Results reveal that the TLV oscillation is intensified by the sudden pressure variation at the junction of two adjacent blades. On analysis of the relative vorticity transport equation, it is revealed that the relative vortex stretching item in Z direction is the major source of the splitting and shedding of the PTLV. The dominant frequency of pressure and vorticity fluctuations on the PTLV trajectory is 8.4 fi, same as the TLV oscillation frequency. This result reveals that the flow instability in the PTLV trajectory is dominated by the oscillation of the TLV. The blade number has significant effect on pressure fluctuation in tip clearance and on blade pressure side, because the TLV oscillation period varies with the circumferential length of flow passage.

Cavitation–Vortex–Turbulence Interaction and One-Dimensional Model Prediction of Pressure for Hydrofoil ALE15 by Large Eddy Simulation

Abstract: The cavitating flow around the asymmetric leading edge (ALE) 15 hydrofoil is investigated through large eddy simulation with the modified Schnerr–Sauer cavitation model, which considers the effect of noncondensable gas. The statistical average velocity profiles obtained by simulation and experimentation show good agreement. The time evolution of cavity shape shows that cavity growth and separation start from the short side and spread toward the long side due to a side-entrant jet. The variation frequency of the cavity length of ALE15 hydrofoil at the long side is 163.93 Hz, and the cavitation shedding frequency at the short side is 306.67 Hz, which is about twice the value of the former. The filtered vorticity transport equation is employed to investigate the cavitation–vortex–turbulence interaction. Results indicate that vortex stretching is the major promoter of cavitation development, and vortex dilatation links vapor cavity and vortices. Baroclinic torque is noticeable at the liquid–vapor interface, and turbulent stress is related to cavitation inception. Moreover, a one-dimensional model for predicting pressure fluctuation is proposed, and results show that the model can effectively predict cavitation-induced pressure fluctuation on a hydrofoil, even on a three-dimensional ALE15 hydrofoil.

Influence of Guide Ring on Energy Loss in a Multistage Centrifugal Pump

Abstract: Multistage centrifugal pumps are highly efficient and compact in structure. Pump efficiency can be improved by an effective understanding of hydraulic behavior and energy loss, however, the traditional hydraulic loss evaluation method does not readily reveal the specific locations of energy loss in the pump. In this study, a guide ring was imposed in multistage pumps, and an entropy production theory was applied to investigate irreversible energy loss of a multistage pump with and without guide ring. Detailed distributions of energy losses in the pumps were calculated to determine the respective entropy production rates (EPRs). The EPR values as calculated are in close accordance with actual hydraulic loss values in the pumps. EPR values were higher in the multistage pump with the guide ring than the pump without a guide ring under part-load flow conditions (0.2Qd). However, the vortex flow in the pump was weakened (or eliminated) by the guide ring as flow rate increased; this reduced energy loss in the chambers. Flow passing the chamber was stabilized by the guide ring, which decreased shock and vortex loss in the chamber and guide vane. Under both designed flow condition and overload conditions, the EPR values of the guide ring-equipped multistage pump were lower than those without the guide ring. Furthermore, minimum efficiency index (MEI) values were also calculated for the two chamber structures; it was found that overall efficiency of pump with guide ring is better than that without.

Investigation of the Motion of Bubbles in a Centrifugal Pump Impeller

Abstract: Centrifugal pumps operate below their nominal capacity when handling gas–liquid flows. This problem is sensitive to many variables, such as the impeller speed and the liquid flow rate. Several works evaluate the effect of operating conditions in the pump performance, but few bring information about the associated gas–liquid flow dynamics. Studying the gas phase behavior, however, can help understanding why the pump performance is degraded depending on the operating condition. In this context, this paper presents a numerical and experimental study of the motion of bubbles in a centrifugal pump impeller. The casing and the impeller of a commercial pump were replaced by transparent components to allow evaluating the bubbles' trajectories through high-speed photography. The bubble motion was also evaluated with a numerical particle-tracking method. A good agreement between both approaches was found. The numerical model is explored to evaluate how the bubble trajectories are affected by variables such as the bubble diameter and the liquid flow rate. Results show that the displacement of bubbles in the impeller is hindered by an increase of their diameter and impeller speed but facilitated by an increase of the liquid flow rate. A force analysis to support understanding the pattern of the bubble trajectories was provided. This analysis should enlighten the readers about the dynamics leading to bubble coalescence inside an impeller channel, which is the main reason behind the performance degradation that pumps experience when operating with gas–liquid flows.

Comparison of DDES and URANS for Unsteady Tip Leakage Flow in an Axial Compressor Rotor

Abstract: Tip leakage vortex (TLV) has a large impact on compressor performance and should be accurately predicted by computational fluid dynamics (CFD) methods. New approaches of turbulence modeling, such as delayed detached eddy simulation (DDES), have been proposed, the computational resources of which can be reduced much more than for large eddy simulation (LES). In this paper, the numerical simulations of the rotor in a low-speed large-scale axial compressor based on DDES and unsteady Reynolds-averaged Navier–Stokes (URANS) are performed, thus improving our understanding of the TLV dynamic mechanisms and discrepancy of these two methods. We compared the influence of different time steps in the URANS simulation. The widely used large time-step makes the unsteadiness extremely weak. The small time-step shows a better result close to DDES. The time-step scale is related to the URANS unsteadiness and should be carefully selected. In the time-averaged flow, the TLV in DDES dissipates faster, which has a more similar structure to the experiment. Then, the time-averaged and instantaneous results are compared to divide the TLV into three parts. URANS cannot give the loss of stability and evolution details of TLV. The fluctuation velocity spectra show that the amplitude of high frequencies becomes obvious downstream from the TLV, where it becomes unstable. Last, the anisotropy of the Reynolds stress of these two methods is analyzed through the Lumley triangle to see the distinction between the methods and obtain the Reynolds stress. The results indicate that the TLV latter part in DDES is anisotropic, while in URANS it is isotropic.

Affinity Law Modified to Predict the Pump Performance for Different Viscosities using the Morrison Number

Abstract: The goal of this study is to provide pump users a simple means to predict a pump's performance change due to changing fluid viscosity. During the initial investigation, it has been demonstrated that pump performance can be represented in terms of the head coefficient, flow coefficient, and rotational Reynolds number with the head coefficient data for all viscosities falling on the same curve when presented as a function of ф*Rew−a. Further evaluation of the pump using computational fluid dynamics (CFD) simulations for wider range of viscosities demonstrated that the value of a (Morrison number) changes as the rotational Reynolds number increases. There is a sharp change in Morrison number in the range of 104

Investigation on Combined Air and Water Injection in Francis Turbine Draft Tube to Reduce Vortex Rope Effects

Abstract: In this paper, the effect of water, air, and their combined injection from two different injection points is studied in order to reduce vorticity effects in a draft tube of prototype turbine working at three operating points. The flow from spiral case to the end of draft tube is simulated using the shear stress transport k–ω turbulence and two-phase models. Using an appropriate validation method, acceptable results were obtained under the noninjection condition. To determine suitable number of points and inlet flow rate for air injection as well as the appropriate nozzle diameter for air and water injection, a new method which considers the ratio of total loss to the pressure recovery factor is used, in addition to using the traditional method which calculates the total loss in the draft tube. Comparing results of the three types of injections shows air injection in the operating range greater than 70% of turbine design flow rate, is much more effective than water injection or the combination of air and water injection. However, in the operating range below 70%, either water or air injections are not suitable, but combination of these two fluids can improve system performance.

Multi-Objective Shape Optimization on the Inlet Pipe of a Vertical Inline Pump

Abstract: Vertical inline pump is a single-stage single-suction centrifugal pump with a bent pipe before the impeller, which is usually used where installation space is a constraint. In this paper, with three objective functions of efficiencies at 0.5 Qd, 1.0 Qd, and 1.5 Qd, a multi-objective optimization on the inlet pipe of a vertical inline pump was proposed based on genetic algorithm with artificial neural network (ANN). In order to describe the shape of inlet pipe, the fifth-order and third-order Bezier curves were adopted to fit the mid curve and the trend of parameters of cross sections, respectively. Considering the real installation and computation complexity, 11 variables were finally used in this optimization. Latin hypercube sampling (LHS) was adopted to generate 149 sample cases, which were solved by CFD code ANSYS cfx 18.0. The calculation results and design variables were utilized to train ANNs, and these surrogate models were solved for the optimum design using multi-objective genetic algorithm (MOGA). The results showed the following: (1) There was a great agreement between numerical results and experimental results; (2) The ANNs could accurately fit the objective functions and variables. The maximum deviations of efficiencies at 0.5 Qd, 1.0 Qd, and 1.5 Qd, between predicted values and computational values, were 1.94%, 2.35%, and 0.40%; (3) The shape of inlet pipe has great influence on the efficiency at part-load and design conditions while the influence is slight at overload condition; (4) Three optimized cases were selected and the maximum increase of the efficiency at 0.5 Qd, 1.0 Qd, and 1.5 Qd was 4.96%, 2.45, and 0.79%, respectively; and (5) The velocity distributions of outflow in the inlet pipe of the three optimized cases were more uniform than the original one.

Optimal Control of Part Load Vortex Rope in Francis Turbines

Abstract: The mitigation of the precessing vortex core developing in the draft tube of Francis turbines operating under part load conditions is crucial to increase the operation flexibility of these hydraulic machines to balance the massive power production of intermittent energy sources. A systematic approach following the optimal control theory is, therefore, presented to control this vortical flow structure. Modal analysis characterizes the part load vortex rope as a self-sustained instability associated with an unstable eigenmode. Based on this physical characteristic, an objective function targeting a zero value of the unstable eigenvalue growth rate is defined and subsequently minimized using an adjoint-based optimization algorithm. We determine an optimal force distribution that successfully quenches the part load vortex rope and sketches the design of a realistic control appendage.

Research on Improving the Dynamic Performance of Centrifugal Pumps With Twisted Gap Drainage Blades

Abstract: Through numerical simulation and experiments analysis, it is indicated that the hydraulic and anticavitation performance of a centrifugal pump with twisted gap drainage blades based on flow control theory can be significantly improved under certain operating conditions. In order to introduce the technology of gap drainage to practical applications, we put forward the parameter formulas of the twisted gap drainage blade to design three-dimensional new type blade, which are also proved to be effective for enhancing the dynamic characteristics of the centrifugal pump. Furthermore, a practical centrifugal pump is redesigned to be a twisted gap drainage impeller with the same structure size as the original impeller, and the nonlinear hybrid Reynolds-averaged Navier–Stokes (RANS)/large eddy simulation (LES) method is employed to simulate the hydraulic dynamic characteristics. Numerical simulation results show that the hydraulic performance and dynamic characteristics of the redesigned impeller centrifugal pump are significantly enhanced. In experiments, the twisted gap drainage blades structure not only remarkably improves the hydraulic performance and the pressure pulsation characteristics of the centrifugal pump but also reduces the vibration intensity.

Mechanism of fast transition of pressure pulsations in the vaneless space of a model pump-turbine during runaway

Abstract: The pressure pulsations in the vaneless space of pump-turbines are extremely intense and always experience rapid time variations during transient scenarios, causing structural vibrations and even more serious accidents. In this study, the mechanism behind the rapid time variations of the vaneless space pressure pulsations in a model pump-turbine during runaway was analyzed through three-dimensional (3D) numerical simulations. These results show that the high-frequency pressure pulsation components originating from rotor–stator interactions (RSI) are dominant during the whole process. These components fluctuate significantly in frequency when the working point goes through the S-shaped region of the characteristic curve, with the amplitudes increasing. Meanwhile, some low-frequency pulsations are also enhanced and become obvious. These features can be attributed to the transitions of the inter blade vortex structures (IBVSs) to the forward flow vortex structures (FFVSs) and the back flow vortex structures (BFVSs) at the impeller entrance, when the pump-turbine operates in the region with S-shaped characteristics. The FFVSs mainly cause decreases in frequency and introduce low-frequency pulsations, while the BFVSs are responsible for the unstable fluctuations. These findings contribute to the understanding of how transient flow patterns evolve and may provide new ideas about avoiding severe pressure pulsations caused by rotating stalls in the pump-turbine during transient scenarios.

Numerical Investigation of Added Mass and Hydrodynamic Damping on a Blunt Trailing Edge Hydrofoil

Abstract: Added mass and hydrodynamic damping play significant roles in fluid-structure interaction (FSI) in hydraulic turbines. Added mass can reduce natural frequencies, while hydrodynamic damping could result in a higher amplitude decay speed of the vibration. In order to quantify the added mass and hydrodynamic damping of a three-dimensional (3D) NACA 0009 hydrofoil with a blunt trailing edge, a two-way FSI simulation method was employed. The effects of grid scale, time-step, turbulence model, exciting force, and numerical damping on the calculation accuracy of the two-way FSI numerical simulation were analyzed in great detail through comparison with the previously published experimental data. Hydraulic force was obtained by using a transitional shear stress transport model at the flow region of the Reynolds number ReL = 0.2 × 106–2 × 106. The vortex shedding frequency, the natural frequency of the first-order bending mode in water, and the hydrodynamic damping ratio obtained from the numerical simulations agree well with the experimental data, with maximum deviations in 6.12%, 4.53%, and 8.82%, respectively. As the flow velocity increases, the natural frequency may not significantly change, while the added mass coefficient gradually increases, considering the effect of added stiffness. Above the first-order bending mode lock-in region, the results indicate that the first-order bending mode hydrodynamic damping ratio increases linearly with velocity. The present numerical achievements offer a higher level of accuracy for predicting the added mass and hydrodynamic damping characteristics of a hydrofoil.

Numerical Simulation on Pump Transient Characteristic in a Model Pump Turbine

Abstract: Pump performance characteristics of pump turbines in transient processes are significantly different from those in steady processes. In the present paper, transient processes of a flow rate that increased and decreased in the pump mode of a model pump turbine were simulated through unsteady simulations using the shear stress transport (SST) k–ω turbulence model. The numerical results reveal that there is a larger hysteresis loop in the performance characteristics of the increasing and decreasing directions of the flow rate compared with those of steady results. Detailed discussions are carried out to determine the generation mechanism of obvious hysteresis characteristics using the methods of entropy production and continuous wavelet analysis. Analyses show that the states of the backflow at the draft tube outlet and the vortices in the impeller and guide/stay vanes are promoted or suppressed owing to the acceleration and deceleration of the fluid. This contributes to the difference in pump performance characteristics of the pump turbine.

Experimental Investigation of Nozzle Spacing Effects on Characteristics of Round Twin Free Jets

Abstract: The objective of this paper is to investigate the effects of nozzle spacing on the mean velocity and higher-order turbulent statistics of free twin round jets produced from sharp contraction nozzles. The experiments were performed in an air chamber where four nozzle spacing ratios, S/d = 2.8, 4.1, 5.5, and 7.1, were investigated at a fixed Reynolds number of 10,000. A planar particle image velocimetry (PIV) system was used to conduct the velocity measurements. The results show that downstream of the potential core, a reduction in spacing ratio leads to an earlier and more intense interaction between the jets, indicated by enhanced half-velocity width spread rate in the inner shear layers and a significant rise of turbulent intensities and vorticity thickness along the symmetry plane. A reduction in spacing ratio, however, confines the ambient fluid entrainment along the inner shear layers leading to a reduced core jet velocity decay rate. The closer proximity of the jets also leads to the decrease of Reynolds stresses in the inner shear layers but not in the outer shear layers. The Reynolds stress ratios along the jet centerline reveal the highest anisotropy in the potential core region.

Cylindrical smoothed particle hydrodynamics simulations of water entry

Abstract: This paper presents a smoothed particle hydrodynamics (SPH) modeling technique based on the cylindrical coordinates for axisymmetrical hydrodynamic applications, thus to avoid a full three-dimensional (3D) numerical scheme as required in the Cartesian coordinates. In this model, the governing equations are solved in an axisymmetric form and the SPH approximations are modified into a two-dimensional cylindrical space. The proposed SPH model is first validated by a dam-break flow induced by the collapse of a cylindrical column of water with different water height to semi-base ratios. Then, the model is used to two benchmark water entry problems, i.e., cylindrical disk and circular sphere entry. In both cases, the model results are favorably compared with the experimental data. The convergence of model is demonstrated by comparing with the different particle resolutions. Besides, the accuracy and efficiency of the present cylindrical SPH are also compared with a fully 3D SPH computation. Extensive discussions are made on the water surface, velocity, and pressure fields to demonstrate the robust modeling results of the cylindrical SPH.

Experimental Quantification of Vent Mechanism Flow Parameters in 18650 Format Lithium Ion Batteries

Abstract: Lithium ion batteries have a well-documented tendency to fail energetically under various abuse conditions. These conditions frequently result in decomposition of the electrochemical components within the battery resulting in gas generation and increased internal pressure which can lead to an explosive case rupture. The 18650 format cell incorporates a vent mechanism located within a crimped cap to relieve pressure and mitigate the risk of case rupture. Cell venting, however, introduces additional safety concerns associated with the flow of flammable gases and liquid electrolyte into the environment. Experiments to quantify key parameters are performed to elucidate the external dynamics of battery venting. A first experiment measures the vent burst pressure. Burst vent caps are then tested with a second experimental fixture to measure vent opening area and discharge coefficient during choked-flow venting, which occurs during battery failure. Vent opening area and discharge coefficient are calculated from stagnation temperature, stagnation pressure, and static pressure measurements along with compressible-isentropic flow equations and conservation of mass. Commercially sourced vent caps are used with repeated tests run to quantify repeatability and variability. Validation experiments confirmed accuracy of opening area and discharge coefficient measurement. Further, trials conducted on vent caps from two sources demonstrate the potential for variation between manufacturers.

Drone Scale Coaxial Rotor Aerodynamic Interactions Investigation

Abstract: Coaxial rotor uninhabited aerial vehicles (UAVs) are compact compared to single rotor UAVs of comparable capacity. At the low Reynolds numbers (Re) where they operate, the simplifying assumptions from high Re rotor aerodynamics are not valid. The low Re coaxial rotor flowfield is studied including aerodynamic interactions and their effect on performance. The evolution of the wake is captured using high-speed stereo particle image velocimetry (SPIV). Improvement of upper rotor performance due to viscous swirl recovery from the lower rotor is discovered and then verified by analyzing PIV data. Interesting vortex–vortex sheet interactions are observed under the coaxial rotor affecting wake structure spatially and temporally. A qualitative model explaining the observed wake interaction phenomena is presented. Comparison with the performance of high Re rotors shows higher profile and induced drag at low Re for the same thrust coefficient.

Dynamic Behaviors of Re-Entrant Jet and Cavity Shedding During Transitional Cavity Oscillation on NACA0015 Hydrofoil

Abstract: Transitional cavity shedding is known as the stage of attached cavitation with high instability and distinct periodicity. In this study, we experimentally investigated the dynamic characteristics of transitional cavity (0.8≤L/c<1) shedding on NACA0015 hydrofoil with high-speed video observation and synchronous pressure measurement. In the partial cavity (0.4

Characteristics of the Flow Past a Wall-Mounted Finite-Length Square Cylinder at Low Reynolds Number With Varying Boundary Layer Thickness

Abstract: Direct numerical simulation (DNS) is performed to investigate the modes of shedding of the wake of a wall-mounted finite-length square cylinder with an aspect ratio (AR) of 7 for six different boundary layer thicknesses (0.0–0.30) at a Reynolds number of 250. For all the cases of wall boundary layer considered in this study, two modes of shedding, namely, anti-symmetric and symmetric modes of shedding, were found to coexist in the cylinder wake with symmetric one occurring intermittently for smaller time duration. The phase-averaged flow field revealed that the symmetric modes of shedding occur only during instances when the near wake experiences the maximum strength of upwash/downwash flow. The boundary layer thickness seems to have a significant effect on the area of dominance of both downwash and upwash flow in instantaneous and time-averaged flow field. It is observed that the near-wake topology and the total drag force acting on the cylinder are significantly affected by the bottom-wall boundary layer thickness. The overall drag coefficient is found to decrease with thickening of the wall boundary layer thickness.

On the applicability of cavitation erosion risk models with a URANS Solver

Abstract: In the maritime industry, cavitation erosion prediction becomes more and more critical, as the requirements for more efficient propellers increase. Model testing is yet the most typical way a propeller designer can, nowadays, get an estimation of the erosion risk on the propeller blades. However, cavitation erosion prediction using computational fluid dynamics (CFD) can possibly provide more information than a model test. In the present work, we review erosion risk models that can be used in conjunction with a multiphase unsteady Reynolds-averaged Navier-Stokes (URANS) solver. Three different approaches have been evaluated, and we conclude that the energy balance approach, where it is assumed that the potential energy contained in a vapor structure is proportional to the volume of the structure, and the pressure difference between the surrounding pressure and the pressure within the structure, provides the best framework for erosion risk assessment. Based on this framework, the model used in this study is tested on the Delft Twist 11 hydrofoil, using a URANS method, and is validated against experimental observations. The predicted impact distribution agrees well with the damage pattern obtained from paint test. The model shows great potential for future use. Nevertheless, it should further be validated against full scale data, followed by an extended investigation on the effect of the driving pressure that leads to the collapse.

Oil/Water Emulsions Stabilized by Nanoparticles of Different Wettabilities

Abstract: A state-of-the-art, portable dispersion characterization rig (P-DCR) is used to investigate the effect of nanoparticles (NP) on oil-water emulsion formation and stabilization. Spherical silica NP of different wettabilities were used to investigate their effect on separation kinetics of solid stabilized emulsions in terms of solid particle concentration, wettability, initial dispersion phase, water-cut, and shearing time. The main findings of the study include the following: NP, even at concentrations as low as 0.005% or 0.01% (by weight), can significantly increase separation time of oil/water emulsions from a few minutes to several hours or even days. The P-DCR is recommended as an effective inline tool to measure emulsion stability in the field.

Relaminarization of Pipe Flow by Means of 3D-Printed Shaped Honeycombs

Abstract: Based on a novel control scheme, where a steady modification of the streamwise velocity profile leads to complete relaminarization of initially fully turbulent pipe flow, we investigate the applicability and usefulness of custom-shaped honeycombs for such control. The custom-shaped honeycombs are used as stationary flow management devices which generate specific modifications of the streamwise velocity profile. Stereoscopic particle image velocimetry and pressure drop measurements are used to investigate and capture the development of the relaminarizing flow downstream these devices. We compare the performance of straight (constant length across the radius of the pipe) honeycombs with custom-shaped ones (variable length across the radius) and try to determine the optimal shape for maximal relaminarization at minimal pressure loss. The optimally modified streamwise velocity profile is found to be M-shaped, and the maximum attainable Reynolds number for total relaminarization is found to be of the order of 10,000. Consequently, the respective reduction in skin friction downstream of the device is almost by a factor of 5. The break-even point, where the additional pressure drop caused by the device is balanced by the savings due to relaminarization and a net gain is obtained, corresponds to a downstream stretch of distances as low as approximately 100 pipe diameters of laminar flow.

Numerical Modeling of Stall and Poststall Events of a Single Pitching Blade of a Cycloidal Rotor

Abstract: In the present work, a numerical study is carried out to compare the performance of seven turbulence models on a single pitching blade of cycloidal rotor operating in deep dynamic stall regime at moderate Reynolds number. The investigated turbulence models were: (i) kω-shear stress transport (SST), (ii) kω-SST with γ, (iii) transition SST (γ–Reθ), (iv) scale adaptive simulation (SAS), (v) SAS coupled with transition SST, (vi) SAS with γ, and (vii) detached eddy simulation (DES) coupled with transition kω-SST. The wake vortices evolution and shedding analysis are also carried out for the pitching blade. The performance of the investigated turbulence models is evaluated at various critical points on the hysterias loop of lift and drag coefficients. The predictions of the investigated turbulence models are in good agreement at lower angle of attack, i.e., αu ≤ 20 deg. The detailed quantitative analysis at critical points showed that the predictions of SAS and transition SST-SAS turbulence models are in better agreement with the experimental results as compared to the other investigated models. The wake vortices analysis and fast Fourier transport analysis showed that the wake vortex characteristics of a pitching blade are significantly different than those for the low amplitude oscillating blade at the higher reduced frequency.

Investigation of the Integrated Model of Side Chamber, Wear-Rings Clearance, and Balancing Holes for Centrifugal Pumps

Abstract: The characteristics of radial pressure distribution inside side chamber and leakage flow through balancing holes of centrifugal pumps are very important for accurately predicting the axial thrust produced by a balancing system. Therefore, a rapid and sufficiently accurate calculation method is required. An integrated model describing the characteristics of radial pressure distribution inside side chamber and leakage flow rate through balancing holes was established. In this model, the correction coefficient of liquid pressure at side chamber entrance and the discharge coefficient of balancing holes were obtained by experiment. The IS 80-50-315 type single-suction, single-stage, and cantilevered centrifugal pump with the structure of double wear-rings and balancing holes was employed as a model to investigate the characteristics of internal liquid flow of the balancing system. Under different pump flow rates, pump performance curves, radial pressure distribution inside side chamber, and leakage flow rate through balancing holes were examined with different balancing hole diameters. Afterward, the experimental data were compared with the predicted results. The comparisons showed a reasonable consistency. Consequently, according to detailed pump dimensions and operating conditions, the integrated model in current form is recommended for centrifugal pumps, which have low specific speed and are with balancing systems of double wear-rings and balancing holes. This can be used for the prediction of radial pressure distribution inside side chamber and leakage flow rate through balancing holes and can be used both at the design stage and nondesign stage.

Numerical Investigation of the Clocking Effect Between Inducer and Impeller on Pressure Pulsations in a Liquid Rocket Engine Oxygen Turbopump

Abstract: The clocking positions between the inducer and the impeller have a certain impact on the performance of the high-speed centrifugal pump, which however, is often ignored by designers. In the present study, three-dimensional numerical simulation based on detached eddy simulation method is adopted to evaluate the influence of this clocking effect on unsteady pressure pulsations in a full-scale liquid rocket engine oxygen turbopump. A new omega vortex identification method is introduced to clarify the internal correlation between unsteady flow structures and pressure pulsations and to shed comprehensive light on the formation mechanism of this clocking effect. Results show that the clocking effect has little influence on the unsteady pressure field in inducer passages while it significantly affects the rotor–stator interaction (RSI) effect leading to the alteration of the pressure spectra in RSI region, diffuser and volute diffuser pipe. The components at the inducer blade passing frequency in the pressure spectra are remarkably suppressed and the total pressure pulsation energy in these regions is decreased by an average of 13.94%, 12.94%, and 34.65%, respectively, when the inducer blade trailing edges are located in the middle of two adjacent impeller blades. The vortex analysis in the specific region reveals that the pressure pulsations in RSI region and the downstream regions are closely associated with the unsteady vortex shedding from the diffuser blades and the formation of the clocking effect is precisely due to different processes of the periodic vortex shedding from the diffuser blade pressure surfaces.

An Unsteady Separated Stagnation-Point Flow Towards a Rigid Flat Plate

Abstract: This paper discusses an unsteady separated stagnation-point flow of a viscous fluid over a flat plate covering the complete range of the unsteadiness parameter β in combination with the flow strength parameter a (>0). Here, β varies from zero, Hiemenz's steady stagnation-point flow, to large β-limit, for which the governing boundary layer equation reduces to an approximate one in which the convective inertial effects are negligible. An important finding of this study is that the governing boundary layer equation conceives an analytic solution for the specific relation β = 2a. It is found that for a given value of β (≥0) the present flow problem always provides a unique attached flow solution (AFS), whereas for a negative value of β the self-similar boundary layer solution may or may not exist that depends completely on the values of a and β (<0). If the solution exists, it may either be unique or dual or multiple in nature. According to the characteristic features of these solutions, they have been categorized into two classes—one which is AFS and the other is reverse flow solution (RFS). Another interesting finding of this analysis is the asymptotic solution which is more practical than the numerical solutions for large values of β (>0) depending upon the values of a. A novel result which arises from the pressure distribution is that for a positive value of β the pressure is nonmonotonic along the stagnation-point streamline as there is a pressure minimum which moves toward the stagnation-point with an increasing value of β > 0.

Assessment of Computational Fluid Dynamic for Surface Combatant 5415 at Straight Ahead and Static Drift β = 20 deg

Abstract: Collaboration is described on assessment of computational fluid dynamics (CFD) predictions for surface combatant model 5415 at static drift β = 0 deg and 20 deg using recent tomographic particle image velocimetry (TPIV) experiments. Assessment includes N-version verification and validation to determine the confidence intervals for CFD solutions/codes, and vortex onset, progression, instability, and turbulent kinetic energy (TKE) budget analysis. The increase in β shows the following trends. Forces and moment increase quadratically/cubically, and become unsteady due to shear layer, Karman and flapping instabilities on the bow. Wave elevation becomes asymmetric; its amplitude increases, but the total wave elevation angle remains same. The vortex strength and TKE increase by about two orders of magnitude, and for large β, the primary vortices exhibit helical mode instability similar to those for delta wings. Forces and moment for both β and wave elevation for β = 0 deg are compared within 4% of the data, and are validated at 7% interval. Wave elevation for β = 20 deg, and vortex core location and velocities for both β are compared within 9% of the data, and are validated at 12% interval. The vortex strength and TKE predictions show large 70% errors and equally large scatter and are not validated. Thus, both errors and scatter need reduction. TKE budgets show transport of turbulence into the separation bubble similar to canonical cases, but pressure transport is dominant for ship flows. Improved CFD predictions require better grids and/or turbulence models. Investigations of solution-adaptive mesh refinement for better grid design and hybrid Reynolds-averaged Navier-Stokes/large eddy simulation models for improved turbulent flow predictions are highest priority.

Effect of Oil Viscosity on Self-Excited Noise Production Inside the Pilot Stage of a Two-Stage Electrohydraulic Servovalve

Abstract: The occurrence of self-excited noise felt as squealing noise is a critical issue for an electrohydraulic servovalve that is an essential part of the hydraulic servocontrol system. Aiming to highlight the root causes of the self-excited noise, the effect of oil viscosity on the noise production inside a two-stage servovalve is investigated in this paper. The pressure pulsations' characteristics and noise characteristics are studied at three different oil viscosities experimentally by focusing on the flapper-nozzle pilot stage of a two-stage servovalve. The cavitation-induced and vortex-induced pressure pulsations' characteristics at upstream and downstream of the turbulent jet flow path are extracted and analyzed numerically by comparing with the experimental measured pressure pulsations and noise characteristics. The numerical simulations of transient cavitation shedding phenomenon are also validated by the experimental cavitation observations at different oil viscosities. Both numerical simulations and experimental cavitation observations explain that cavitation shedding phenomenon is intensified with the decreasing of oil viscosity. The small-scale vortex propagation with the characteristic of generating, growing, moving, and merging is numerically simulated. Thus, this study reveals that the oil viscosity affects the transient distribution of cavitation and small-scale vortex, which, in turn, enhances the pressure pulsation and noise. The noise characteristics achieve a good agreement with pressure pulsation characteristics showing that the squealing noise appears accompanied by the flow field resonance in the flapper-nozzle. The flow-acoustic resonance and resulting squealing noise possibly occurs when the amplitude of the pressure pulsations near the flapper is large enough inside a two-stage servovalve.

Numerical Simulation of Drag Reduction in Microgrooved Substrates Using Lattice-Boltzmann Method

Abstract: We study drag reduction of a uniform flow over a flat surface due to a series of rectangular microgrooves created on the surface. The results reveal that making grooves on the surface usually leads to the generation of secondary vortices inside the grooves that, in turn, decreases the friction drag force and increases the pressure drag force. By increasing the thickness of the grooves to the thickness of the obstacle, the pressure drag increases due to the enhancement of the generated vortices and the occurrence of separation phenomenon and the friction drag reduces due to a decrease of the velocity gradient on the surface. In addition, by increasing the grooves depth ratio, the pressure drag coefficient decreases and the friction drag coefficient increases. However, the impact of the pressure drag coefficient is higher than that of the friction drag coefficient. From a specific point, increasing the groove depth ratio does not effect on decreasing the total pressure drag of the plate. Therefore, creating the grooves in flat surfaces would reduce the total drag coefficient of the plate if the thickness of the grooves does not exceed a specific size and the depth of the grooves is chosen to be sufficiently large. The lattice-Boltzmann method (LBM) is used and the optimal reduction of the drag coefficient is calculated. It is found that for the width ratio equal to 0.19 and the groove depth ratio equal to 0.2548, about 7% decrease is achieved for the average total drag.

Numerical and Experimental Investigation of the Cavitating Flow Within Venturi Tube

Abstract: Hydrodynamic cavitation represents complex physical phenomenon undesirably affecting operation as well as lifespan of many hydraulic machines from small valves to the large hydro power plants. On the other hand, the same phenomenon and its concomitants such as pressure pulsations can be exploited in many positive ways. One of them which seems to be very promising and perspective is the cavitation utilization for reduction of the microorganisms such as cyanobacteria within large bulks of water. Mutual effect of the swirl induced by the upstream mounted generator and flow constriction in converging–diverging nozzle has been experimentally investigated. The analysis of the hydraulic losses in the wide range of the cavitation regimes has been done as well as the investigation of the pipe wall acceleration induced by the fluctuations of the cavitating structures. The dynamics of the cavitation was studied using the proper orthogonal decomposition (POD) of the captured video records. The main scope of this paper is numerical investigation complementing the experimental results. The multiphase simulations were carried out using the OpenFOAM 1606+ and its interPhaseChangeFoam solver. The present study focuses on computational fluid dynamics results of the cavitating velocity field within the nozzle and analysis of the loss coefficient within the nozzle. The results of the numerical analysis were utilized for the further discussion of the experimental results.