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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 summarized the recent progress for the cavitation study in the hydraulic machinery including turbo-pumps, hydro turbines, etc., and identified the 1-D analysis method, which is identified to be very useful for engineering applications regarding the cavitating flows in inducers, turbine draft tubes, etc.
Abstract: This paper mainly summarizes the recent progresses for the cavitation study in the hydraulic machinery including turbo-pumps, hydro turbines, etc.. Especially, the newly developed numerical methods for simulating cavitating turbulent flows and the achievements with regard to the complicated flow features revealed by using advanced optical techniques as well as cavitation simulation are introduced so as to make a better understanding of the cavitating flow mechanism for hydraulic machinery. Since cavitation instabilities are also vital issue and rather harmful for the operation safety of hydro machines, we present the 1-D analysis method, which is identified to be very useful for engineering applications regarding the cavitating flows in inducers, turbine draft tubes, etc. Though both cavitation and hydraulic machinery are extensively discussed in literatures, one should be aware that a few problems still remains and are open for solution, such as the comprehensive understanding of cavitating turbulent flows especially inside hydro turbines, the unneglectable discrepancies between the numerical and experimental data, etc.. To further promote the study of cavitation in hydraulic machinery, some advanced topics such as a Density-Based solver suitable for highly compressible cavitating turbulent flows, a virtual cavitation tunnel, etc. are addressed for the future works.

296 citations

Journal ArticleDOI
TL;DR: In this paper, a 3D Lagrangian Coherent Structures (LCS) was used to analyze the dynamics of cavitation-vortex interactions in the Delft twisted hydrofoil.

145 citations

Journal ArticleDOI
TL;DR: In this paper, a series of experimental observations around hydrofoils are carried out in the cavitation tunnel of the China Ship Scientific Research Center (CSSRC) to illustrate the spatial-temporal evolution of the cloud cavity in detail.

97 citations

Journal ArticleDOI
TL;DR: In this article, the authors investigate ventilated cavitating flow structures with special emphasis on vortex shedding dynamics via combining experimental and numerical methods, and the results show that the flow patterns can be classified into two principally different categories: structures mainly with vortex shedding (namely Benard-Karman vortex street; Benard Karman vortex Street with vortex filaments and Aligned vortices) and relatively stable structures (such as Aligned Vortices with Re-entrant jet; Re-enterrant jet and Stable supercavity).

70 citations

Journal ArticleDOI
TL;DR: In this article, the authors assess the prediction of impact load spectra of cavitating flows, i.e., the rate and intensity distribution of collapse events based on a detailed analysis of flow dynamics.
Abstract: Cavitation erosion is the consequence of repeated collapse-induced high pressure-loads on a material surface. The present paper assesses the prediction of impact load spectra of cavitating flows, i.e., the rate and intensity distribution of collapse events based on a detailed analysis of flow dynamics. Data are obtained from a numerical simulation which employs a density-based finite volume method, taking into account the compressibility of both phases, and resolves collapse-induced pressure waves. To determine the spectrum of collapse events in the fluid domain, we detect and quantify the collapse of isolated vapor structures. As reference configuration we consider the expansion of a liquid into a radially divergent gap which exhibits unsteady sheet and cloud cavitation. Analysis of simulation data shows that global cavitation dynamics and dominant flow events are well resolved, even though the spatial resolution is too coarse to resolve individual vapor bubbles. The inviscid flow model recovers increasingly fine-scale vapor structures and collapses with increasing resolution. We demonstrate that frequency and intensity of these collapse events scale with grid resolution. Scaling laws based on two reference lengths are introduced for this purpose. We show that upon applying these laws impact load spectra recorded on experimental and numerical pressure sensors agree with each other. Furthermore, correlation between experimental pitting rates and collapse-event rates is found. Locations of high maximum wall pressures and high densities of collapse events near walls obtained numerically agree well with areas of erosion damage in the experiment. The investigation shows that impact load spectra of cavitating flows can be inferred from flow data that captures the main vapor structures and wave dynamics without the need for resolving all flow scales.

68 citations

References
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Journal ArticleDOI
15 Mar 2013-Wear
TL;DR: In this article, a thin aluminum foil was attached to the surface of a transparent Venturi section using two sided transparent adhesive tape and two high speed cameras were used to record cavitation structures and surface of the foil.

67 citations

Journal ArticleDOI
25 Aug 2008-Wear
TL;DR: In this paper, Bachert et al. investigated the phenomenon of pit clustering within the incubation period and the influence of the already eroded surface on appearance of new pits, and the possibility of self-amplification of the erosion due to the presence of small deformations (pits).

63 citations

Journal ArticleDOI
TL;DR: Structural integrity and long-term durability of heat valve prosthesis are becoming of central importance in moving toward treatment of lower risk populations and emerging technologies and newer generations of devices seem promising in dealing with these matters.
Abstract: Transcatheter aortic valve implantation (TAVI) has been increasingly recognized as a curative treatment for severe aortic stenosis (AS). Despite important improvements in current device technology and implantation techniques, specific complications still remain and warrant consideration. Vascular complications and peri-procedural neurological events were the first concerns to emerge with this new technology. Recently, significant post procedural para-valvular leak has been shown to be more frequent after TAVI than after surgical aortic valve replacement (SAVR), and its potential association with worse long-term prognostic has raised concerns. In moving toward treatment of lower risk populations, structural integrity and long-term durability of heat valve prosthesis are becoming of central importance. Emerging technologies and newer generations of devices seem promising in dealing with these matters.

60 citations

Journal ArticleDOI
29 Oct 2009-Wear
TL;DR: A semi-empirical model has been developed at the University of Southampton which incorporates dynamic Hertzian contact mechanics to model the damage during particle impact and accommodates the effect of erodent deforming the surface leading to an increased corrosion activity as mentioned in this paper.

56 citations

Book
19 Jun 2009
TL;DR: Numerical techniques for direct and large-eddy simulations as discussed by the authors can be applied to practical problems of flow, turbulence, and combustion, and the boundary conditions for DNS and LES, along with time integration methods.
Abstract: [Publisher's description] Compared to the traditional modeling of computational fluid dynamics, direct numerical simulation (DNS) and large-eddy simulation (LES) provide a very detailed solution of the flow field by offering enhanced capability in predicting the unsteady features of the flow field. In many cases, DNS can obtain results that are impossible using any other means while LES can be employed as an advanced tool for practical applications. Focusing on the numerical needs arising from the applications of DNS and LES, Numerical Techniques for Direct and Large-Eddy Simulations covers basic techniques for DNS and LES that can be applied to practical problems of flow, turbulence, and combustion. After introducing Navier–Stokes equations and the methodologies of DNS and LES, the book discusses boundary conditions for DNS and LES, along with time integration methods. It then describes the numerical techniques used in the DNS of incompressible and compressible flows. The book also presents LES techniques for simulating incompressible and compressible flows. The final chapter explores current challenges in DNS and LES. Helping readers understand the vast amount of literature in the field, this book explains how to apply relevant numerical techniques for practical computational fluid dynamics simulations and implement these methods in fluid dynamics computer programs.

56 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.