Non-cavitating Conditions Research Articles

Reversibility of Functional and Structural Changes of Lysozyme Subjected to Hydrodynamic Flow

In this initial study, the effect of hydrodynamic flow on lysozyme structure and function was investigated using a microchannel device. Protein was subjected to bubbly cavitation as well as noncavitating flow conditions at pH 4.8 and 25 °C. Interestingly, time course analyses indicated that the secondary structure content, the hydrodynamic diameter, and enzymatic activity of lysozyme were unaffected by cavitation. However, noncavitating flow conditions did induce a decrease of the hydrodynamic diameter. The corresponding structural change was subtle to the extent that bioactivity was marginally suppressed. Moreover, native diameter and bioactivity could be fully restored following a brief period of ultrasonication. The findings encouraged further study of various hydrodynamic flow conditions in order to better ascertain the potential risks and benefits of invasive hydrodynamic cavitation in medicine. The results also served to highlight the counter-intuitive notion that proteins need not necessarily be denatured in high-shear media, risks that typically correlate well with forcefully agitated solutions.

An experimental investigation of the effect of foul release coating application on performance, noise and cavitation characteristics of marine propellers

A number of ship propellers have been coated with anti-fouling paint, namely foul release, to improve propeller efficiency as a result of dramatic increases in fuel prices. This has recently been coupled with tougher environmental regulations forcing the shipping community to review their anti-fouling policies. With the introduction of a new generation antifouling coatings, it is now possible to apply these coatings not only to keeping the hull free from bio-fouling, but also on propellers to improve their performance. Within the above context an investigation into effect of the coatings on the hydrodynamic performances of marine propellers were carried out. This paper reports on effects of foul release coatings on the efficiency, cavitation and noise characteristics of a marine propeller at model scale. The paper includes the details of these tests conducted in Emerson Cavitation Tunnel of Newcastle University and discusses paint application, surface characterisation issues and analysis of test results for coated and uncoated conditions. The results indicated that coating may not cause any performance reduction when applied correctly. Cavitation development on blades was slightly reduced by the coating and the coating of blades reduced noise level in non-cavitating condition, but increased in developed cavitation condition, requiring further investigation.

A 3-D Potential Based Boundary Element Method for Modelling and Simulation of Marine Propeller Flows

AbstractThe present article focuses on the modelling and simulation of marine propeller flows in port basins motivated by the increasing challenges on ports for future generations of container ships. In the future, the cargo handling in ports will rise and ship dimensions will increase. The induced unsteady pressures on quay walls caused by the propeller slipstream of huge ships is far higher than by smaller units. The employed method aims to accurately predict operational loads induced by marine propeller flows on quay walls in ports.In the first part of the article, the mathematical model based on potential flow theory is derived and the numerical approach is introduced. The numerical method used in this work is a three-dimensional first-order panel method which is implemented into the in-house simulation tool panMARE. In addition, a cavitation model has been developed and implemented in panMARE in order to include the influence of sheet cavitation effects on propeller's performance and propeller induced pressure pulses on quay walls, which is crucial for the overall accuracy of the attainable results.In the second part of this work, the proposed cavitation model is validated and an application case is introduced to investigate the capabilities of the developed method. Hereby, a marine propeller is simulated close to a quay wall both in cavitating and in non-cavitating conditions.

Influence of physical fuel properties on the injection rate in a Diesel injector

This paper presents an experimental investigation of the influence of fuel density and fuel viscosity on the flow characteristics generated from a high pressure Diesel injector, equipped with conical orifices. For this purpose, mass flow rate measurements were performed with nine different fuels over operating conditions from 30 to 180MPa of injection pressure and from 1 to 9MPa of back pressure. Fuel viscosity was varied from 0.6 to 7mm2/s and fuel density from 683 to 876kg/m3 at the operating temperature. Fuel viscosity induces a decrease of up to 10% in the discharge coefficient at low pressure difference but for higher pressure difference, fuel density remains the only fuel property driving the mass flow rate. The discharge coefficient is also a function of the pressure difference at any given Reynolds number under non-cavitating conditions. A correlation is suggested to estimate the discharge coefficient taking into account the fuel properties and the operating conditions.

Dynamic Response to Global Oscillation of Propulsion Systems with Cavitating Pumps

A time-domain model was developed to evaluate the dynamic response of pumping systems in the accelerating environment of rockets with a focus on cavitation. The model was first verified by comparing the results with measurements in ground-based tests of an LE-7A rocket engine. In these tests, various resonances occurred and levels of pump cavitation or incorporation of an accumulator altered them. The model results simulated the test data well, matching both the frequency and the amplitude. The test and model results also demonstrated the stability of the LE-7A propulsion system within nonaccelerating environments. Then, the model was used to examine the response of the propulsion system in accelerating frames; sinusoidal vehicle oscillations over a range of frequencies were explored. Under noncavitating conditions, the pressure amplitudes within the propulsion system did not substantially exceed the quasi-static acceleration head response ρah. However, under cavitating conditions (σ = 0.02), the same accelerations produced violent responses with pressure and flow amplitudes about 2 orders of magnitude greater than in noncavitating conditions. The obvious conclusion is that vehicle oscillations can cause substantial pressure and flow amplitudes, particularly when the pump is cavitating, even if the ground-based tests and the calculations in static frames indicate stable and well-behaved responses.

Modeling of the Subgrid-Scale Pressure Distribution in Turbulent Mixing Layer

Attention was focused on the statistical properties of pressure fluctuation, which is caused by fine-scale vortices in turbulent flow. The aim is improvement in the prediction of cavitation inception due to fine-scale turbulence vortices, which are usually in subgrid-scale (SGS) in Large-eddy simulation (LES). We conducted a finely-resolved direct numerical simulation (DNS) of a spatially-developing turbulent mixing layer in cavitating and non-cavitating (single-phase) conditions. The result under cavitating condition suggested that low-pressure region corresponding to the core of fine-scale vortices could become an origin of cavitation. We applied filtering technique to DNS database under non-cavitating condition to model the low-pressure region of subfilter vortices observed in result under cavitating condition. Filtering volume is corresponding to that in the probable LES. In fully developed turbulence, proportional relation was found out between intensity of SGS pressure fluctuation and turbulent kinetic energy. In addition, Gaussian profile reasonably approximated instantaneous pressure distribution within a filtering volume. We therefore think the possibility of cavitation inception can be accurately estimated by using the intensity of SGS kinetic energy in couple with filtered pressure field.

ANALYSIS OF DIESEL SPRAY ATOMIZATION BY MEANS OF A NEAR-NOZZLE FIELD VISUALIZATION TECHNIQUE

In this paper, a high-resolution visualization technique has been used in order to characterize Diesel spray structure in the near-nozzle field. For this purpose, three conical nozzles with different diameters have been used to relate nozzle geometry and spray behavior in non-cavitating conditions. As a first step, geometrical and hydraulic characterization of the nozzles has been performed. The analysis of the images obtained has shown that spray aperture varied significantly during the first millimeters of the spray, until reaching to the stabilized value of spray cone angle, which is the one usually characterized by other visualization techniques. Furthermore, the structure of the liquid−gas interface presented an oscillatory behavior, which is related to primary atomization process. These oscillations have been quantified and statistically correlated with nozzle geometry and operating conditions by means of three different non-dimensional numbers: Reynolds number, air/fuel density ratio, and Ohnesorge number.

Influence of biofuels on the internal flow in diesel injector nozzles

In this paper, the behavior of the internal nozzle flow of a standard diesel fuel has been compared against a biodiesel fuel (soybean oil) at cavitating and non-cavitating conditions, using a homogeneous equilibrium model. The model takes into account the compressibility of both phases (liquid and vapour) and use a barotropic equation of state which relates pressure and density to calculate the growth of cavitation. Furthermore, turbulence effects have been introduced using a RNG k - ε model. The comparison of both fuels in a real diesel injector nozzle has been performed in terms of mass flow, momentum flux, effective velocity at the outlet and cavitation appearance. The decrease of injection velocity and cavitation intensity for the biodiesel noticed by numerical simulation at different injection conditions, predict a worse air–fuel mixing process.

A numerical study of steady and unsteady cavitation on a 2d hydrofoil

The steady and unsteady cavitation phenomena on a 2D NACA0015 hydrofoil predicted by the multiphase RANS code FLUENT are studied in this paper. Besides a numerical sensitivity study of the non-cavitating condition, the present investigation focuses on two cavitation numbers: σ=16 (steady cavitating flow) σ=1.0 (with dynamic shedding). With a modified SST K – ω turbulence model, a periodic shedding is revealed: the main sheet cavity breaks up by the re-entrant jet and a cloudy cavity forms and is convected with the downstream flow. Finally, the experience with FLUENT has been used to discuss the general ability of multiphase RANS codes to predict the cavitation erosion risk.

RANS Simulations of Supercavity Flows

This work aims at evaluating the capacity and limitations of conventional Reynolds-averaged Navier-Stokes (RANS) techniques to numerically simulate supercavity flows. The configuration is that of a two-dimensional (2D) symmetrical supercavitating wedge investigated experimentally by Michel (1974). Mesh effect is studied in detail under noncavitating conditions. The computational grid is refined in the region where cavitation develops in order to accurately track the supercavity. The effect of tunnel height is also analyzed, and the height finally chosen was large enough to simulate an infinite flow-field. The cavitating flow is treated as a homogeneous mixture of variable density. To account for vaporization and condensation, an additional continuity equation for the vapor (or the liquid) is solved with an appropriate source term expressing mass transfer between the two phases. The effect of nuclei concentration on the vaporization rate and then on the development of the supercavity is investigated. Results obtained using two different RANS codes are compared. They are also compared with experimental data and with inviscid solutions including a nonlinear boundary element method. They concern in particular cavity length and shape, pressure distribution, and drag coefficient. Under unsteady conditions, a special attention is paid to the characteristic growth time of the supercavity following a sudden pressure drop, from noncavitating conditions.

Cavitating behaviour analysis of Darrieus-type cross flow water turbines

The aim of this paper is to study the cavitating behaviour of bare Darrieus-type turbines. For that, the RANS code CAVKA, has been used. Under non-cavitating conditions, the power coefficient and the thrusts calculated with CAVKA are compared to experimental values obtained in the LEGI hydrodynamic tunnel. Under cavitating conditions, for several cavitation numbers, the numerical power coefficients and vapour structures are compared to experimental ones. Different blade profiles and camber lines are also studied for non-cavitating and cavitating conditions.

An implicit low-diffusive HLL scheme with complete time linearization: Application to cavitating barotropic flows

A numerical method for generic barotropic flows is presented, together with its application to the simulation of cavitating flows. A homogeneous-flow cavitation model is indeed considered, which leads to a barotropic state equation. The continuity and momentum equations for compressible flows are discretized through a mixed finite-element/finite-volume approach, applicable to unstructured grids. P1 finite elements are used for the viscous terms, while finite volumes for the convective ones. The numerical fluxes are computed by shock-capturing schemes and ad-hoc preconditioning is used to avoid accuracy problems in the low-Mach regime. A HLL flux function for barotropic flows is proposed, in which an anti-diffusive term is introduced to counteract accuracy problems for contact discontinuities and viscous flows typical of this class of schemes, while maintaining its simplicity. Second-order accuracy in space is obtained through MUSCL reconstruction. Time advancing is carried out by an implicit linearized scheme. For this HLL-like flux function two different time linearizations are considered; in the first one the upwind part of the flux function is frozen in time, while in the second one its time variation is taken into account. The proposed numerical ingredients are validated through the simulations of different flow configurations, viz. the Blasius boundary layer, a Riemann problem, the quasi-1D cavitating flow in a nozzle and the flow around a hydrofoil mounted in a tunnel, both in cavitating and non-cavitating conditions. The Roe flux function is also considered for comparison. It is shown that the anti-diffusive term introduced in the HLL scheme is actually effective to obtain good accuracy (similar to the one of the Roe scheme) for viscous flows and contact discontinuities. Moreover, the more complete time linearization is a key ingredient to largely improve numerical stability and efficiency in cavitating conditions.

Internal Flow of a Two-Bladed Helical Inducer at an Extremely Low Flow Rate

Abstract The attachment of inducer upstream of main impeller is an effective method to improve the suction performance of turbopump. However, various types of cavitation instabilities are known to occur even at the designed flow rate as well as in the partial flow rate region. The cavitation surge occurring at partial flow rates is known to be strongly associated with the inlet back flow. In the present study, in order to understand the detailed structure of internal flow of inducer, we firstly carried out the experimental and numerical studies of non-cavitating flow, focusing on the flow field near the inlet throat section and inside the blade passage of a two bladed inducer at a partial flow rate. The steady flow simulation with cavitation model was also made to investigate the difference of flow field between in the cavitating and non-cavitating conditions. Keywords : Inducer, Internal flow, Cavitation, Laser Doppler velocimetry measurement, Computational fluid dynamics 1. Introduction

翼端渦の解像を考慮したＣＦＤ によるプロペラキャビテーションの数値シミュレーション

Marine propellers are operating in the unsteady ship wake flow, which causes the repeat of cavitation occurrence and disappearance. Such cavitation causes the serious vibration and erosion problems. Therefore, the decrease of the erosion risk is one of the very important issues in the propeller design. Especially, latest propeller design for high efficiency is based on detail estimation of cavitation performance. It is necessary to estimate the process of cavitation disappearance related to both blade cavitation and tip vortex cavitation near the very local tip region for the evaluation of the erosion risk.Recently, RANS method becomes the common useful design tool for the analysis of the flow field around marine propellers, and the application to the cavitation simulation is highly expected. It is thought that it becomes an effective method for the evaluation of the erosion risk, if it may simulate the blade cavitation and the tip vortex cavitation in detail.First of all, numerical simulation of the tip vortex of the DTMB P4119 propeller in non-cavitating condition was performed in order to examine the effectiveness of the adaptive mesh generation. Numerical results of flow field behind the propeller were compared with LDV measurement results.Finally, the cavitation simulation including tip vortex for Seiun Maru .I propellers was performed by and the cavitation patterns were compared with the experiments.

Large Eddy Simulation of the Dynamic Response of an Inducer to Flow Rate Fluctuations

Abstract A Large Eddy Simulation (LES) of the flow in an inducer is carried out under flow rate oscillations. The present study focuses on the dynamic response of the backflow and the unsteady pressure performance to the flow rate fluctuations under non-cavitation conditions. The amplitude of angular momentum fluctuation evaluated by LES is larger than that evaluated by RANS. However, the phase delay of backflow is nearly the same as RANS calculation. The pressure performance curve exhibits a closed curve caused by the inertia effect associated with the flow rate fluctuations. Compared with simplified one dimensional evaluation of the inertia component, the component obtained by LES is smaller. The negative slope of averaged performance curve becomes larger under unsteady conditions. From the conservations of angular momentum and energy, an expression useful for the evaluation of unsteady pressure rise was obtained. The examination of each term of this expression show that the apparent decrease of inertia effects is caused by the response delay of Euler’s head and that the increase of negative slope is caused by the delay of inertial term associated with the delay of backflow response. These results are qualitatively confirmed by experiments.

Cause of Cavitation Instabilities in Three Dimensional Inducer

Alternate blade cavitation, rotating cavitation and cavitation surge in rocket turbopump inducers were simulated by a three dimensional commercial CFD code. In order to clarify the cause of cavitation instabilities, the velocity disturbance caused by cavitation was obtained by subtracting the velocity vector under non-cavitating condition from that under cavitating condition. It was found that there exists a disturbance flow towards the trailing edge of the tip cavity. This flow has an axial flow component towards downstream which reduces the incidence angle to the next blade. It was found that all of the cavitation instabilities start to occur when this flow starts to interact with the leading edge of the next blade. The existence of the disturbance flow was validated by experiments.

J0501-3-6 低流量における平板ヘリカルインデューサの内部流れに関する研究(流体機械の研究開発におけるEFD/CFD(3))

The attachment of inducer upstream of main impeller is an effective method to improve the suction performance of turbopump. However, various types of cavitation instabilities are known to occur even at the designed flow rate as well as in the partial flow rate region. The cavitation surge occurring at partial flow rates is known to be strongly associated with the inlet back flow. In the present study, in order to understand the detailed structure of internal flow of inducer, we firstly carried out the experimental and numerical studies of non-cavitating flow, focusing on the flow field near the inlet throat section and inside the blade passage of a two bladed inducer at a partial flow rate. The steady flow simulation with cavitation model was also made to investigate the difference of the flow field between in the cavitating and non-cavitating conditions.

An Experimental Study of Cavitation Detection in a Centrifugal Pump Using Envelope Analysis

Cavitation represents one of the most common faults in pumps and could potentially lead to a series of failure in mechanical seal, impeller, bearing, shaft, motor, etc. In this work, an experimental rig was setup to investigate cavitation detection using vibration envelope analysis method, and measured parameters included sound, pressure and flow rate for feasibility of cavitation detection. The experiment testing included 3 operating points of the centrifugal pump (B.E.P, 90% of B.E.P and 80% of B.E.P). Suction pressure of the centrifugal pump was decreased gradually until the inception point of cavitation. Vibration measurements were undertaken at various locations including casing, bearing, suction and discharge flange of the centrifugal pump. Comparisons of envelope spectrums under cavitating and non-cavitating conditions were presented. Envelope analysis was proven useful in detecting cavitation over the 3 testing conditions. During the normal operating condition, vibration peak synchronous to rotational speed was more pronounced. It was however during cavitation condition, the half order sub-harmonic vibration component was clearly evident in the envelope spectrums undertaken at all measurement locations except at the pump bearing. The possible explanation of the strong sub-harmonic (½ of BPF) during cavitation existence in the centrifugal pump was due to insufficient time for the bubbles to collapse completely before the end of the single cycle.

High-Performance Partially Cavitating Hydrofoils

Partial cavitation reduces hydrofoil wetted area and friction, but usually with a significant drag penalty associated with unsteady cavity dynamics. A design concept for a high-performance partially cavitating hydrofoil that centers on the suppression of these oscillations by tuning the local pressure gradient in the region of cavity closure is described. The design algorithm is based on ideal fluid theory. An example of this design is the OK-2003 hydrofoil, which is derived from the NACA-0015 geometry. The concept is verified through water tunnel experiments. It is found that under partial cavitation, a two-dimensional hydrofoil exhibits up to a 100% increase in the lift-to-drag ratio compared to its noncavitating conditions. The substantial increase in this ratio relative to the ratio exhibited by the initial NACA-0015 hydrofoil remains significant within a 3 deg range of angle of attack and under a variation of cavitation number of about ±0.2. It was also found that this new hydrofoil design reduces hydrodynamic force pulsations under partially cavitating conditions to operational levels typical of noncavitating flow. In order to further verify the design benefits, the effects of cross flow have been studied with a swept hydrofoil of the same cross section. It was found that outstanding performance of the designed cavitating hydrofoil is retained with 15 deg sweep.

Near-Field Flow Measurements of a Cavitating Jet Emanating From a Crown-Shaped Nozzle

The effect of a crown-shaped nozzle on cavitation is studied experimentally in the near-field of a 25 mm diameter (D) water jet at ReD=2×105 using particle image velocimetry (PIV) and high speed shadowgraphy recorded with a 5000 fps digital camera. The objectives are to passively control the jet flow structure and to examine its consequences on the physical appearance of cavitating bubbles. The experiments are performed in a closed-loop facility that enables complete optical access to the near-nozzle region. The cavitating and noncavitating mean velocity fields are obtained up to three nozzle diameters downstream and compared to those of a companion round nozzle. PIV measurements are taken in two distinct azimuthal planes passing through the tip and bottom points of the crown nozzle edge. The data include shear layer momentum thickness and vorticity thickness, spanwise vorticity distribution and streamwise normal Reynolds stress. Significant deviation from an axisymmetric shear layer is observed in the noncavitating flow consistently up to one diameter downstream, after which identical asymptotic conditions are achieved in both round and crown-shaped nozzles. Maximum magnitudes of spanwise vorticity and streamwise normal Reynolds stress are the highest downstream of the nozzle tip edges under noncavitating conditions. Significant modifications in trends and magnitudes are observed for the shear layer momentum thickness under cavitating conditions up to one diameter downstream. Qualitative flow visualization reveals that bubble growth occurs at different conditions depending on azimuthal location. Bubbles, in the form of elongated filaments, are the dominant structures produced downstream of the valley edges of the nozzle with an inclination of 45 deg with respect to the direction of the flow, and are observed to persist with significant strength up to two diameters downstream. These filaments are stretched between periodic larger-scale, spanwise bubbly clusters distorted in the shape of the nozzle outlet. The tip edges produce cavitating bubbles under conditions similar to that of a classical round nozzle. In summary, it was demonstrated that passive control of turbulent structures in the jet does impact the cavitation process.