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Journal ArticleDOI

Visualisation and les simulation of cavitation cloud formation and collapse in an axisymmetric geometry

TL;DR: In this paper, the authors present a large-scale visualization and large-dydydy simulation of cavitation inside the apparatus used for surface erosion acceleration tests and material response monitoring.
About: This article is published in International Journal of Multiphase Flow.The article was published on 2015-01-01 and is currently open access. It has received 75 citations till now. The article focuses on the topics: Cavitation & Vortex shedding.

Summary (3 min read)

1. INTRODUCTION

  • The present contribution aims to provide more insight to the details of the cavitation sheet/cloud developing in a purpose build device that has been previously used for extensive cavitation erosion measurements [15] .
  • In this paper the same apparatus is used to visualise the cavitating flow in an effort to correlate the observed cavitation erosion locations with the location of cavitation development.
  • The next section of the paper gives a short description of the experimental apparatus used, followed by a brief description of the computational model; then the presentation of the obtained results follows while the most important conclusions are summarised at the end.

2. EXPERIMENTAL SETUP

  • The test section is placed in a closed circuit comprised by different equipment: centrifugal pump, heat exchanger, test section, electromagnetic flowmeter.
  • The flow through the system is measured using an electromagnetic flow meter.
  • A heat exchanger allows maintaining the water temperature constant.
  • The maximum operating pressure of the circuit is 40bars, which corresponds to a mean velocity of 65m/s at the turn located at the nozzle exit, calculated at the peripheral surface of the cylinder with height 2.5mm and radius 8mm.
  • The pressurization of the system is supported by means of a balloon located downstream the test section.

3. Flow structure and post processing methods

  • This error decreases with increasing radial distance and becomes smaller than 5% in the area of cloud collapse.
  • The second image post-processing method employed estimation of averages along the circumferential direction.
  • This time, the mean of the binary values along each arc has been obtained for every time instant recorded; this can be simply expressed as 1 0 1.

4. CFD Simulations

  • Regarding the vapour distribution in the normal direction between .
  • On average, the vapour distribution from the CFD results shows a stronger vapour presence near the upper disk and this could possibly.

5.Spatial mean cavitation distribution and

  • Having described the flow development the authors now proceed to the presentation of the results obtained from the post-processing of the collected image shows the spatial distribution of the temporal mean cavitation presence and its standard deviation for selected operating conditions of the middle of the picture.
  • A general remark from Fig. 12is that there is a slight asymmetry in the time averaged results.
  • As shown in geometry of the device is not entirely axi pipes that are positioned every 90degress.
  • The effect of cavitation number, effect on the extent of cavitation.
  • The erosion sites 2 and 3, as shown in the previous coincide well with the estimated cloud length, which effectively indicates the area where the cavitation cloud collapses.

Spatial mean cavitation distribution and cloud length

  • Having described the flow development the authors now proceed to the presentation of the results obtained processing of the collected images.
  • The first series of results is presented in shows the spatial distribution of the temporal mean cavitation presence and its standard deviation for selected operating conditions of Table 1 .
  • This zone coincides well with the area of high standard deviation values.
  • For te' condition and it does not extend further in the radial direction.
  • The erosion sites 2 and 3, as shown in the previous Fig. 1b he estimated cloud length, which effectively indicates the area where the cavitation cloud collapses.

6. Temporal development of cavitation cloud and shedding frequency at location of collapse

  • From these plots, it is possible to evaluate the frequency of the cloud appearance/disappearance and collapse.
  • The f cloud has been evaluated by the fast Fourier transformation (FFT)and it is shown in Fig. 15a for all operating conditions tested.
  • Direct comparison of the temporal frequencies between the experiment and CFD is not straightforward, since the flow field was not to past observations of cavitation cloud shedding (for example, [18] ) the clouds at high σ are small and detach/collapse frequently, whereas at low σ the clouds grow and collapse at a slower rate.
  • Finally, Fig. 15b shows the Strouhal number, estimates using as length scale the cloud length and the mean velocity of the flow at this location.
  • It is noticeable that the Strouhal number seems to be relatively constant for all tested conditions.

7. DISCUSSION OF RESULTS

  • 6) The detached cavity transforms into a bubble cloud and moves downstream following the flow.
  • Re-entrant jet is formed, (4) the re-entrant jet detaches the cavity from the adjacent wall, (5) the vapour cavity is entirely detached from the parent cavity at the turn, (6) the detached vapour cavity follows the flow and travels downstream, while it may expand even more due to vorticity.
  • The growth and collapse of the vapour clouds is speculated to be linked to the compressibility effects that occur in cavitating flows.
  • As mentioned above, a series of vapour cavities is shed from the primary inception site at the turn.
  • As the bubble cloud is travelling downstream, pressure is recovering and vorticity is dissipating due to viscosity.

8. CONCLUSIONS

  • From the CFD results, it becomes apparent that a mechanism similar to vortex shedding occurs.
  • Indeed, significant vorticity is generated downstream the turn, organizing in vortex tubes and manifesting as filament-like structures.
  • This was confirmed by the experimental observations, since similar thread like structures have been captured during the image acquisition process.
  • The LES/RANS methodology used in the present work shows that there could be a secondary nucleation site downstream the turn; this site cannot be predicted by pure RANS methodologies but, at the present, cannot be confirmed by experimental observations, thus requiring further examination.
  • It is noticeable that the erosion free zone, located in-between two erosion zones, coincides with the areas where CFD predictions indicated as the secondary cavitation inception site.

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Citations
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Journal ArticleDOI
TL;DR: In this paper, the authors investigated the effects of air injection on the characteristics of two different unsteady sheet/cloud cavitation shedding mechanisms, namely re-entrant flow mechanism and bubbly shock propagation mechanism, in the convergent-divergent channel.

20 citations

Journal ArticleDOI
TL;DR: In this paper, the authors make quantitative measurements of the vapor distribution in a cavitating nozzle, owing to the strong scattering of visible light at gas-liquid boundaries and wall boundaries.
Abstract: Making quantitative measurements of the vapor distribution in a cavitating nozzle is difficult, owing to the strong scattering of visible light at gas–liquid boundaries and wall boundaries, and the...

17 citations


Cites background from "Visualisation and les simulation of..."

  • ...Substantial effort continues to be made in the development of numerical models for cavitating nozzle flows [7, 8, 9, 10, 11]....

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Journal ArticleDOI
TL;DR: In this paper, a comprehensive investigation on cavitating flow and cavitation-induced erosion was performed experimentally in an orifice plate system, where three image post-processed approaches were applied to analyze the test data, in order to obtain the cavitation characteristics.
Abstract: A comprehensive investigation on cavitating flow and cavitation-induced erosion was performed experimentally in an orifice plate system. Three image post-processed approaches were applied to analyze the test data, in order to obtain the cavitation characteristics. The cavitating flow pattern was studied by high speed images. In one cavitation developing period, there could be three distinct cavitation clouds, whereas the second one is not fully developed. The first image post-processing approach was applied to obtain the mean value and standard deviation distribution, which indicate the erosion area may cover almost all the cavitation developing route and the most vulnerable erosion area locates near the cavitation collapse site. It is coincides with the erosion tests analyzed through the pit-count algorithm approach. The cavitation circulation frequency was invested via PSD analysis approach. It shows that the frequency linearly decreasing with decreasing cavitation number. Additionally, the cavitation intensity effect on cavitation erosion was quantitatively studied based. It is found that the damages are strongly enhanced when increasing the flow velocity. Moreover, the growth rate of eroded pits number is actually stepwise instead of linear (similar to our previous work in a venturi tube), which supports the idea that the cloud cavitation collapse is the primary reason for erosion. The present approaches applied here shows good potential ability of investigating cavitating flows and can be utilized for other apparatus.

17 citations

Journal ArticleDOI
TL;DR: In this paper, the notch flow characteristics to form the large vapor cavity and its surge instability characteristics are discussed by experimental and numerical analysis, and it is found that, instead of the vena contracta flow, but the notch vortex flow creates the more suitable low pressure condition for cavitation inception, with the helical-stream-trend to form a cavity spiral shape with clear vapor-liquid interface.

17 citations

Journal ArticleDOI
TL;DR: In this article, high-fidelity simulations provide a very detailed insight into the vortex generation in the injector nozzle; strings appear within the time scales that are relevant for fast injection events (on the order of 0.1 milliseconds) and their generation seems mostly related to the flow pattern in the sac.

16 citations

References
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Book
01 Oct 2013
TL;DR: In this paper, the fundamental physical processes involved in bubble dynamics and the phenomenon of cavitation are described and explained, and a review of the free streamline methods used to treat separated cavity flows with large attached cavities is provided.
Abstract: This book describes and explains the fundamental physical processes involved in bubble dynamics and the phenomenon of cavitation. It is intended as a combination of a reference book for those scientists and engineers who work with cavitation or bubble dynamics and as a monograph for advanced students interested in some of the basic problems associated with this category of multiphase flows. A basic knowledge of fluid flow and heat transfer is assumed but otherwise the analytical methods presented are developed from basic principles. The book begins with a chapter on nucleation and describes both the theory and observations of nucleation in flowing and non-flowing systems. The following three chapters provide a systematic treatment of the dynamics of the growth, collapse or oscillation of individual bubbles in otherwise quiescent liquids. Chapter 4 summarizes the state of knowledge of the motion of bubbles in liquids. Chapter 5 describes some of the phenomena which occur in homogeneous bubbly flows with particular emphasis on cloud cavitation and this is followed by a chapter summarizing some of the experiemntal observations of cavitating flows. The last chapter provides a review of the free streamline methods used to treat separated cavity flows with large attached cavities.

2,994 citations

Journal ArticleDOI
TL;DR: In this paper, the authors show that the zero-shear-stress points on the surface and on the obstacle must be such that the sum of the nodes and the saddles of the saddle must satisfy
Abstract: In flows around three-dimensional surface obstacles in laminar or turbulent streamsthere are a number of points where the shear stress or where two or more component,s of the mean velocity are zero. In the first part of this paper we summarize and extend the kinematical theory for the flow near these points, particularly by emphasizing the topological classification of these points as nodes or saddles. We show that the zero-shear-stress points on the surface and on the obstacle must be such that the sum of the nodes ΣN and the sum of the saddles Σs satisfy \[ \Sigma_N -\Sigma_S = 0. \] If the obstacle has a hole through it, such as a passageway under a building, \[ \Sigma_N -\Sigma_S =-2. \] If the surface is a junction between two pipes, \[ \Sigma_N -\Sigma_S =-1. \] We also consider, in two-dimensional plane sections of the flow, the points where the components of the mean velocity parallel to the planes are zero, both in the flow and near surfaces cutting the sections. The latter points are half-nodes N′ or half-saddles S′. We find that \[ (\Sigma_N +{\textstyle\frac{1}{2}}\Sigma_{N^{\prime}}-(\Sigma_{S^{\prime}}+{\textstyle\frac{1}{2}}\Sigma_{S^{\prime}}) = 1-n, \] where n is the connectivity of the section of the flow considered.In the second part new flow-visualization studies of laminar and turbulent flows around cuboids and axisymmetric humps (i.e. model hills) are reported. A new method of obtaining a high resolution of the surface shear-stress lines was used. These studies show how enumerating the nodes and saddle points acts as a check on the inferred flow pattern.Two specific conclusions drawn from these studies are that: for all the flows we observed, there are no closed surfaces of mean streamlines around the separated flows behind three-dimensional surface obstacles, which con-tradicts most of the previous suggestions for such flows (e.g. Halitsky 1968);the separation streamline on the centre-line of a three-dimensional bluff obstacle does not, in general, reattach to the surface.

682 citations

Journal ArticleDOI
TL;DR: In this article, a simulation of turbulent cavitation in a Venturi-type section was performed by two-dimensional computations of viscous, compressible, and turbulent cavitating flows.
Abstract: Unsteady cavitation in a Venturi-type section was simulated by two-dimensional computations of viscous, compressible, and turbulent cavitating flows. The numerical model used an implicit finite volume scheme (based on the SIMPLE algorithm) to solve Reynolds-averaged Navier-Stokes equations, associated with a barotropic vapor/liquid state law that strongly links the density variations to the pressure evolution. To simulate turbulence effects on cavitating flows, four different models were implemented (standard $k-\varepsilon$ RNG; modified $k-\varepsilon$ RNG; $k-\omega$ with and without compressibility effects), and numerical results obtained were compared to experimental ones. The standard models $k-\varepsilon$ RNG and $k-\omega$ without compressibility effects lead to a poor description of the self-oscillation behavior of the cavitating flow. To improve numerical simulations by taking into account the influence of the compressibility of the two-phase medium on turbulence, two other models were implemented in the numerical code: a modified $k-\varepsilon$ model and the $k-\omega$ model including compressibility effects. Results obtained concerning void ratio, velocity fields, and cavitation unsteady behavior were found in good agreement with experimental ones. The role of the compressibility effects on turbulent two-phase flow modeling was analyzed, and it seemed to be of primary importance in numerical simulations.

380 citations

Journal ArticleDOI
TL;DR: In this paper, a 2D numerical model is proposed to simulate unsteady cavitating flows in 2D geometries, where the mixture of liquid and vapour is considered as a single fluid with variable density.
Abstract: A 2D numerical model is proposed to simulate unsteady cavitating flows The Reynolds-averaged Navier–Stokes equations are solved for the mixture of liquid and vapour, which is considered as a single fluid with variable density The vapourization and condensation processes are controlled by a barotropic state law that relates the fluid density to the pressure variations The numerical resolution is a pressure-correction method derived from the SIMPLE algorithm, with a finite volume discretization The standard scheme is slightly modified to take into account the cavitation phenomenon That numerical model is used to calculate unsteady cavitating flows in two Venturi type sections The choice of the turbulence model is discussed, and the standard RNG k–emodel is found to lead to non-physical stable cavities A modified k–emodel is proposed to improve the simulation The influence of numerical and physical parameters is presented, and the numerical results are compared to previous experimental observations and measurements The proposed model seems to describe the unsteady cavitation behaviour in 2D geometries well Copyright © 2003 John Wiley & Sons, Ltd

271 citations

Frequently Asked Questions (11)
Q1. What is the effect of the vorticity on the edge of the bubble cloud?

Once the surrounding pressure force counteracts the vorticity 587 induced centrifugal force, the edge of the bubble cloud, approximately at a radial distance of 25mm 588 from the axis of symmetry, starts to collapse. 

The 531 effect of Reynolds number is primarily linked to the shedding frequency of the cavitation structures; 532 indeed, when considering a constant cavitation number σ, at low back pressures (which also 533 corresponds to low Reynolds number) the shedding frequency is lower. 

The increase of the Reynolds 550 number with the corresponding increase of the velocity increases the rate of generation of vortices and 551 consequently the frequency of shedding of the resulting cavitating structures. 

The 77 studies were conducted in a cavitating Venturi nozzle section, where part of the nozzle was covered 78 by a thin aluminium foil; this enabled the rapid accumulation of erosion pits and allowed the 79 observation of the erosion development, since the rest nozzle walls were transparent. 

It seems that the increase of the 31 Reynolds number leads to a reduction of the collapse frequency; it is believed that this effect is due to 32 the agglomeration of vortex cavities, which causes a decrease of the apparent frequency. 

Due to the aforementioned detrimental 51 effects of cavitation on hydraulic equipment, most of experimental research has focused over the 52 years on methods with which cavitation damage could be quantified and linked to measurable 53material properties. 

Due to the complexity of the flow 22 field, direct observation of the flow structures was not possible, however vortex shedding is inferred 23 from relevant simulations performed for the same conditions. 

One notable example of a DNS 90 study of the collective bubble collapse is the recent work of[20], where the authors employed massive 91 parallelism to simulate a cluster of 15,000 bubbles collapsing near a wall, utilizing a grid with size of 92 13 trillion cells. 

In each plot, the feed inlet area corresponds to the circle in oncentrically with it, a thin zone has been plottedreflections have prevented the collectionsymmetric at the outlet of the disks gap; there are four outlet e a disturbance in the velocityposition of ~25mm. 

The cavitation cloud was found to change location rather transiently and non-axisymmetrically 196 despite the steady-state operation and the axisymmetric geometry utilized; a typical sequence of the 197 cloud formation and development is shown in Fig. 

The contribution of the pressure wave propagation within the 642 bubbly medium is possibly a driving factor on the development of discrete cavitation clouds, but in 643 the present cannot be taken into account, due to limitations of the modelling capabilities of the 644 software used.