245MI - Architettura NavaleHydrodynamic Cavitation

Docente: Prof. Mitja MorgutUniversità degli Studi di TriesteDipartimento di Ingegneria e ArchitetturaVia A. Valerio 10, 34127 TriesteEdificio C5 - Stanza C5_2.44mail: mmorgut@units.it

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Historical aspects

Architettura Navale

I In 1754 Euler guessed the conditions that might cause the appearance ofcavitation in rotating flow machinery and its effects on their performances

I The intensive study of cavitation phenomena did not start earlier than atthe end of the nineteenth century.The growing interest to cavitation started as a reaction to the disastrousperformances of several (screw-driven) steamships.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Historical aspects

Architettura Navale

I In 1754 Euler guessed the conditions that might cause the appearance ofcavitation in rotating flow machinery and its effects on their performances

I The intensive study of cavitation phenomena did not start earlier than atthe end of the nineteenth century.The growing interest to cavitation started as a reaction to the disastrousperformances of several (screw-driven) steamships.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Historical aspects

Architettura Navale

I In 1754 Euler guessed the conditions that might cause the appearance ofcavitation in rotating flow machinery and its effects on their performances

I The intensive study of cavitation phenomena did not start earlier than atthe end of the nineteenth century.

The growing interest to cavitation started as a reaction to the disastrousperformances of several (screw-driven) steamships.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Historical aspects

Architettura Navale

I In 1754 Euler guessed the conditions that might cause the appearance ofcavitation in rotating flow machinery and its effects on their performances

I The intensive study of cavitation phenomena did not start earlier than atthe end of the nineteenth century.The growing interest to cavitation started as a reaction to the disastrousperformances of several (screw-driven) steamships.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

The famous case - Turbinia

Architettura Navale

Turbinia is the first Parson’s turbine ship

I In 1894 during the trials the designed speed was not achieved by Turbinia.It achieved just under 20 knots.In fact, despite Euler’s intuition one century earlier, at that time it was notclear yet, that the poor performances were related to the extremecavitation phenomenon developed on the blades which caused the socalled thrust breakdown.

I In the case of Turbinia however, Parson was able to overcome the problemof cavitation by enlarging the blade area, increasing the number ofpropellers per shaft from one (original design) to three mounted in tandem.

I With the new arrangement of propellers the vessel was able to absorb allthe power at the correct shaft speed and was also able to reach theremarkable trial speed of 32.75 knots.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

The famous case - Turbinia

Architettura Navale

Turbinia is the first Parson’s turbine ship

I In 1894 during the trials the designed speed was not achieved by Turbinia.It achieved just under 20 knots.

In fact, despite Euler’s intuition one century earlier, at that time it was notclear yet, that the poor performances were related to the extremecavitation phenomenon developed on the blades which caused the socalled thrust breakdown.

I In the case of Turbinia however, Parson was able to overcome the problemof cavitation by enlarging the blade area, increasing the number ofpropellers per shaft from one (original design) to three mounted in tandem.

I With the new arrangement of propellers the vessel was able to absorb allthe power at the correct shaft speed and was also able to reach theremarkable trial speed of 32.75 knots.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

The famous case - Turbinia

Architettura Navale

Turbinia is the first Parson’s turbine ship

I In 1894 during the trials the designed speed was not achieved by Turbinia.It achieved just under 20 knots.In fact, despite Euler’s intuition one century earlier, at that time it was notclear yet, that the poor performances were related to the extremecavitation phenomenon developed on the blades which caused the socalled thrust breakdown.

I In the case of Turbinia however, Parson was able to overcome the problemof cavitation by enlarging the blade area, increasing the number ofpropellers per shaft from one (original design) to three mounted in tandem.

I With the new arrangement of propellers the vessel was able to absorb allthe power at the correct shaft speed and was also able to reach theremarkable trial speed of 32.75 knots.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

The famous case - Turbinia

Architettura Navale

Turbinia is the first Parson’s turbine ship

I In 1894 during the trials the designed speed was not achieved by Turbinia.It achieved just under 20 knots.In fact, despite Euler’s intuition one century earlier, at that time it was notclear yet, that the poor performances were related to the extremecavitation phenomenon developed on the blades which caused the socalled thrust breakdown.

I In the case of Turbinia however, Parson was able to overcome the problemof cavitation by enlarging the blade area, increasing the number ofpropellers per shaft from one (original design) to three mounted in tandem.

I With the new arrangement of propellers the vessel was able to absorb allthe power at the correct shaft speed and was also able to reach theremarkable trial speed of 32.75 knots.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

The famous case - Turbinia

Architettura Navale

Turbinia is the first Parson’s turbine ship

I In 1894 during the trials the designed speed was not achieved by Turbinia.It achieved just under 20 knots.In fact, despite Euler’s intuition one century earlier, at that time it was notclear yet, that the poor performances were related to the extremecavitation phenomenon developed on the blades which caused the socalled thrust breakdown.

I In the case of Turbinia however, Parson was able to overcome the problemof cavitation by enlarging the blade area, increasing the number ofpropellers per shaft from one (original design) to three mounted in tandem.

I With the new arrangement of propellers the vessel was able to absorb allthe power at the correct shaft speed and was also able to reach theremarkable trial speed of 32.75 knots.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

The famous case - Turbinia

Architettura Navale

Turbinia is the first Parson’s turbine ship

I In 1894 during the trials the designed speed was not achieved by Turbinia.It achieved just under 20 knots.In fact, despite Euler’s intuition one century earlier, at that time it was notclear yet, that the poor performances were related to the extremecavitation phenomenon developed on the blades which caused the socalled thrust breakdown.

I In the case of Turbinia however, Parson was able to overcome the problemof cavitation by enlarging the blade area, increasing the number ofpropellers per shaft from one (original design) to three mounted in tandem.

I With the new arrangement of propellers the vessel was able to absorb allthe power at the correct shaft speed and was also able to reach theremarkable trial speed of 32.75 knots.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Definition

Architettura Navale

I Cavitation is a general fluid mechanics phenomenon that can occur whenever aliquid is used in a machine which induces pressure and velocity fluctuations in thefluid.

I Thus, it can occur in propellers, pumps and turbines for instance

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Definition

Architettura Navale

I Cavitation is a general fluid mechanics phenomenon that can occur whenever aliquid is used in a machine which induces pressure and velocity fluctuations in thefluid.

I Thus, it can occur in propellers, pumps and turbines for instance

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Definition

Architettura Navale

I Cavitation is a general fluid mechanics phenomenon that can occur whenever aliquid is used in a machine which induces pressure and velocity fluctuations in thefluid.

I Thus, it can occur in propellers, pumps and turbines for instance

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Hydrodynamic cavitation

Architettura Navale

I The cavitation appearance is caused by variations in local flow values (pressure,velocity) induced by the geometry of the system

I Depending on the quality of the liquid (presence of nuclei and solid particles), thepressure field in the cavitation zone, as well as on the geometry configuration androughness of solid-boundaries, it can take different forms.

Examples of hydrodynamic cavitation over a hydrofoil

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Hydrodynamic cavitation

Architettura Navale

I The cavitation appearance is caused by variations in local flow values (pressure,velocity) induced by the geometry of the system

I Depending on the quality of the liquid (presence of nuclei and solid particles), thepressure field in the cavitation zone, as well as on the geometry configuration androughness of solid-boundaries, it can take different forms.

Examples of hydrodynamic cavitation over a hydrofoil

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Hydrodynamic cavitation

Architettura Navale

I The cavitation appearance is caused by variations in local flow values (pressure,velocity) induced by the geometry of the system

I Depending on the quality of the liquid (presence of nuclei and solid particles), thepressure field in the cavitation zone, as well as on the geometry configuration androughness of solid-boundaries, it can take different forms.

Examples of hydrodynamic cavitation over a hydrofoil

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Hydrodynamic cavitation

Architettura Navale

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Hydrodynamic cavitation

Architettura Navale

I The different possible cavitation patterns can be classified as:

I Fixed or Attached or Sheet CavitationI Travelling (Bubble) CavitationI Cloud CavitationI Vortex cavitation

I Some cavitation patterns can be a combination of several types.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Hydrodynamic cavitation

Architettura Navale

I The different possible cavitation patterns can be classified as:I Fixed or Attached or Sheet CavitationI Travelling (Bubble) CavitationI Cloud CavitationI Vortex cavitation

I Some cavitation patterns can be a combination of several types.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Hydrodynamic cavitation

Architettura Navale

I The different possible cavitation patterns can be classified as:I Fixed or Attached or Sheet CavitationI Travelling (Bubble) CavitationI Cloud CavitationI Vortex cavitation

I Some cavitation patterns can be a combination of several types.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Hydrodynamic cavitation

Architettura Navale

I Fixed or Attached or Sheet Cavitation appears as a region of detachedflow filled with vapour. Its shape is usually under steady/quasi-steadyconditions or changes relatively slowly and/or periodically.If the cavity extents well beyond the body, it is called supercavitation.Fixed cavities typically form downstream steep decreasing pressuregradients such is the leading edge of a body.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Hydrodynamic cavitation

Architettura Navale

I Fixed or Attached or Sheet Cavitation appears as a region of detachedflow filled with vapour. Its shape is usually under steady/quasi-steadyconditions or changes relatively slowly and/or periodically.

If the cavity extents well beyond the body, it is called supercavitation.Fixed cavities typically form downstream steep decreasing pressuregradients such is the leading edge of a body.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Hydrodynamic cavitation

Architettura Navale

I Fixed or Attached or Sheet Cavitation appears as a region of detachedflow filled with vapour. Its shape is usually under steady/quasi-steadyconditions or changes relatively slowly and/or periodically.If the cavity extents well beyond the body, it is called supercavitation.

Fixed cavities typically form downstream steep decreasing pressuregradients such is the leading edge of a body.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Hydrodynamic cavitation

Architettura Navale

I Fixed or Attached or Sheet Cavitation appears as a region of detachedflow filled with vapour. Its shape is usually under steady/quasi-steadyconditions or changes relatively slowly and/or periodically.If the cavity extents well beyond the body, it is called supercavitation.Fixed cavities typically form downstream steep decreasing pressuregradients such is the leading edge of a body.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Hydrodynamic cavitation

Architettura Navale

I Travelling (Bubble) Cavitation is characterized by cavities or bubbleswhich form in the liquid and travel with the liquid as they expand andcollapse. The bubbles appear in low-pressure regions of the liquid as aresult of fast growing of nuclei present in the liquid and then subsequentlycollapse when they are convected to higher pressure regions. This type ofcavitation most commonly occurs on hydrofoils with a small angle ofattack.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Hydrodynamic cavitation

Architettura Navale

I Travelling (Bubble) Cavitation is characterized by cavities or bubbleswhich form in the liquid and travel with the liquid as they expand andcollapse. The bubbles appear in low-pressure regions of the liquid as aresult of fast growing of nuclei present in the liquid and then subsequentlycollapse when they are convected to higher pressure regions. This type ofcavitation most commonly occurs on hydrofoils with a small angle ofattack.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Hydrodynamic cavitation

Architettura Navale

I Cloud Cavitation appears as a mist or cloud of very small bubbles.Frequently it is formed by the periodic break up of the sheet cavity.This type of cavitation usually causes engineering problems such as erosion,noise and vibrations, especially when bubbles collapse near the surfaces.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Hydrodynamic cavitation

Architettura Navale

I Cloud Cavitation appears as a mist or cloud of very small bubbles.Frequently it is formed by the periodic break up of the sheet cavity.

This type of cavitation usually causes engineering problems such as erosion,noise and vibrations, especially when bubbles collapse near the surfaces.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Hydrodynamic cavitation

Architettura Navale

I Cloud Cavitation appears as a mist or cloud of very small bubbles.Frequently it is formed by the periodic break up of the sheet cavity.This type of cavitation usually causes engineering problems such as erosion,noise and vibrations, especially when bubbles collapse near the surfaces.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Hydrodynamic cavitation

Architettura Navale

I Vortex cavitation occurs in cores of vortices which form in regions of highshear and can appear as travelling cavities or as fixed cavity.This type of cavitation is often present on a propeller blade. In thisparticular case it looks like a vapour-gas rope that extents from propellertip far away in the flow field. Moreover it is often observed behindpropeller as well as turbine hubs.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Hydrodynamic cavitation

Architettura Navale

I Vortex cavitation occurs in cores of vortices which form in regions of highshear and can appear as travelling cavities or as fixed cavity.

This type of cavitation is often present on a propeller blade. In thisparticular case it looks like a vapour-gas rope that extents from propellertip far away in the flow field. Moreover it is often observed behindpropeller as well as turbine hubs.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Hydrodynamic cavitation

Architettura Navale

I Vortex cavitation occurs in cores of vortices which form in regions of highshear and can appear as travelling cavities or as fixed cavity.This type of cavitation is often present on a propeller blade. In thisparticular case it looks like a vapour-gas rope that extents from propellertip far away in the flow field. Moreover it is often observed behindpropeller as well as turbine hubs.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

The cavitation number

Architettura Navale

I The susceptibility of a flow to cavitate is commonly identified by thecavitation number, σ.

I In general, for the hydrofoil case:

σ =PREF − PV(T)

0.5ρLV2

for the propeller

σn =PREF − PV(T)

0.5ρL(nD)2)

I The lower is the value of σ the larger is the risk of the appearance of asevere cavitation phenomenon.

In the above equations PREF is reference the pressure, PV(T) is the vapour pressure at the operatingtemperature of the fluid T, ρL and V are density and free-stream velocity of the working liquid. n and D are thepropeller rotational speed and diameter, respectively.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

The cavitation number

Architettura Navale

I The susceptibility of a flow to cavitate is commonly identified by thecavitation number, σ.

I In general, for the hydrofoil case:

σ =PREF − PV(T)

0.5ρLV2

for the propeller

σn =PREF − PV(T)

0.5ρL(nD)2)

I The lower is the value of σ the larger is the risk of the appearance of asevere cavitation phenomenon.

In the above equations PREF is reference the pressure, PV(T) is the vapour pressure at the operatingtemperature of the fluid T, ρL and V are density and free-stream velocity of the working liquid. n and D are thepropeller rotational speed and diameter, respectively.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

The cavitation number

Architettura Navale

I The susceptibility of a flow to cavitate is commonly identified by thecavitation number, σ.

I In general, for the hydrofoil case:

σ =PREF − PV(T)

0.5ρLV2

for the propeller

σn =PREF − PV(T)

0.5ρL(nD)2)

I The lower is the value of σ the larger is the risk of the appearance of asevere cavitation phenomenon.

In the above equations PREF is reference the pressure, PV(T) is the vapour pressure at the operatingtemperature of the fluid T, ρL and V are density and free-stream velocity of the working liquid. n and D are thepropeller rotational speed and diameter, respectively.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Hydrodynamic cavitation

Architettura Navale

The effect of cavitation on an airfoil section pressure distribution

I When cavitation occurs on the blade (airfoil) section the non-cavitatingpressure distribution is modified with increasing significance as thecavitation number decreases.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Cavitation-induced damage

Architettura Navale

I In the majority of casese it seeems that the cavitation erosion is related to theaction of travelling bubbles which either pass around the airfoil, pass around afixed cavity or break off from a fixed cavity.

I From the above figure (left) it is possible to note that the collapse mechanismgenerates a set of of shock pressure contours.

I The above figure (right) shows that if the collapse mechanism take place close toa solid boundary surface a microjet is formed which is directed towards thesurface.

I The cavity rebound activity is also important.Rebound is the regrowth of the vapour phase of the cavityIt seems that this mechanism can provide an important contribution to thedamaging process originating from the microjet and pressure wave attack.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Cavitation-induced damage

Architettura Navale

I In the majority of casese it seeems that the cavitation erosion is related to theaction of travelling bubbles which either pass around the airfoil, pass around afixed cavity or break off from a fixed cavity.

I From the above figure (left) it is possible to note that the collapse mechanismgenerates a set of of shock pressure contours.

I The above figure (right) shows that if the collapse mechanism take place close toa solid boundary surface a microjet is formed which is directed towards thesurface.

I The cavity rebound activity is also important.Rebound is the regrowth of the vapour phase of the cavityIt seems that this mechanism can provide an important contribution to thedamaging process originating from the microjet and pressure wave attack.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Cavitation-induced damage

Architettura Navale

I In the majority of casese it seeems that the cavitation erosion is related to theaction of travelling bubbles which either pass around the airfoil, pass around afixed cavity or break off from a fixed cavity.

I From the above figure (left) it is possible to note that the collapse mechanismgenerates a set of of shock pressure contours.

I The above figure (right) shows that if the collapse mechanism take place close toa solid boundary surface a microjet is formed which is directed towards thesurface.

I The cavity rebound activity is also important.Rebound is the regrowth of the vapour phase of the cavityIt seems that this mechanism can provide an important contribution to thedamaging process originating from the microjet and pressure wave attack.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Cavitation-induced damage

Architettura Navale

I In the majority of casese it seeems that the cavitation erosion is related to theaction of travelling bubbles which either pass around the airfoil, pass around afixed cavity or break off from a fixed cavity.

I From the above figure (left) it is possible to note that the collapse mechanismgenerates a set of of shock pressure contours.

I The above figure (right) shows that if the collapse mechanism take place close toa solid boundary surface a microjet is formed which is directed towards thesurface.

I The cavity rebound activity is also important.Rebound is the regrowth of the vapour phase of the cavityIt seems that this mechanism can provide an important contribution to thedamaging process originating from the microjet and pressure wave attack.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Cavitation-induced damage

Architettura Navale

I In the majority of casese it seeems that the cavitation erosion is related to theaction of travelling bubbles which either pass around the airfoil, pass around afixed cavity or break off from a fixed cavity.

I From the above figure (left) it is possible to note that the collapse mechanismgenerates a set of of shock pressure contours.

I The above figure (right) shows that if the collapse mechanism take place close toa solid boundary surface a microjet is formed which is directed towards thesurface.

I The cavity rebound activity is also important.Rebound is the regrowth of the vapour phase of the cavityIt seems that this mechanism can provide an important contribution to thedamaging process originating from the microjet and pressure wave attack.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Cavitation-induced damage

Architettura Navale

I In the majority of casese it seeems that the cavitation erosion is related to theaction of travelling bubbles which either pass around the airfoil, pass around afixed cavity or break off from a fixed cavity.

I From the above figure (left) it is possible to note that the collapse mechanismgenerates a set of of shock pressure contours.

I The above figure (right) shows that if the collapse mechanism take place close toa solid boundary surface a microjet is formed which is directed towards thesurface.

I The cavity rebound activity is also important.Rebound is the regrowth of the vapour phase of the cavityIt seems that this mechanism can provide an important contribution to thedamaging process originating from the microjet and pressure wave attack.

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Cavitation-induced damage

Architettura Navale

I Erosion damage often starts with a surface deterioration or roughening

I This is followed by a plastic deformation of the surface and the surfaceappears as a orange peel.

I This deformation of the surface is essentially caused by a large-scale andessentially random bombardament of the surface by microjet impact andpressure waves from collapsing bubbles

I The erosion damage first occurs in the vicinity of the cavity collapseregions

I The travelling or shed cavities that are normally considered responsible forerosion (within the limits of present understanding)

I Thus, bubble or cloud cavitation, rather than stable sheet cavitation, aremost responsable for material erosion attack.

I The speed with which cavitation erosion take place is variable

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Cavitation-induced damage

Architettura Navale

I Erosion damage often starts with a surface deterioration or rougheningI This is followed by a plastic deformation of the surface and the surface

appears as a orange peel.

I This deformation of the surface is essentially caused by a large-scale andessentially random bombardament of the surface by microjet impact andpressure waves from collapsing bubbles

I The erosion damage first occurs in the vicinity of the cavity collapseregions

I The travelling or shed cavities that are normally considered responsible forerosion (within the limits of present understanding)

I Thus, bubble or cloud cavitation, rather than stable sheet cavitation, aremost responsable for material erosion attack.

I The speed with which cavitation erosion take place is variable

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Cavitation-induced damage

Architettura Navale

I Erosion damage often starts with a surface deterioration or rougheningI This is followed by a plastic deformation of the surface and the surface

appears as a orange peel.I This deformation of the surface is essentially caused by a large-scale and

essentially random bombardament of the surface by microjet impact andpressure waves from collapsing bubbles

I The erosion damage first occurs in the vicinity of the cavity collapseregions

I The travelling or shed cavities that are normally considered responsible forerosion (within the limits of present understanding)

I Thus, bubble or cloud cavitation, rather than stable sheet cavitation, aremost responsable for material erosion attack.

I The speed with which cavitation erosion take place is variable

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Cavitation-induced damage

Architettura Navale

I Erosion damage often starts with a surface deterioration or rougheningI This is followed by a plastic deformation of the surface and the surface

appears as a orange peel.I This deformation of the surface is essentially caused by a large-scale and

essentially random bombardament of the surface by microjet impact andpressure waves from collapsing bubbles

I The erosion damage first occurs in the vicinity of the cavity collapseregions

I The travelling or shed cavities that are normally considered responsible forerosion (within the limits of present understanding)

I Thus, bubble or cloud cavitation, rather than stable sheet cavitation, aremost responsable for material erosion attack.

I The speed with which cavitation erosion take place is variable

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Cavitation-induced damage

Architettura Navale

I Erosion damage often starts with a surface deterioration or rougheningI This is followed by a plastic deformation of the surface and the surface

appears as a orange peel.I This deformation of the surface is essentially caused by a large-scale and

essentially random bombardament of the surface by microjet impact andpressure waves from collapsing bubbles

I The erosion damage first occurs in the vicinity of the cavity collapseregions

I The travelling or shed cavities that are normally considered responsible forerosion (within the limits of present understanding)

I Thus, bubble or cloud cavitation, rather than stable sheet cavitation, aremost responsable for material erosion attack.

I The speed with which cavitation erosion take place is variable

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Cavitation-induced damage

Architettura Navale

I Erosion damage often starts with a surface deterioration or rougheningI This is followed by a plastic deformation of the surface and the surface

appears as a orange peel.I This deformation of the surface is essentially caused by a large-scale and

essentially random bombardament of the surface by microjet impact andpressure waves from collapsing bubbles

I The erosion damage first occurs in the vicinity of the cavity collapseregions

I The travelling or shed cavities that are normally considered responsible forerosion (within the limits of present understanding)

I Thus, bubble or cloud cavitation, rather than stable sheet cavitation, aremost responsable for material erosion attack.

I The speed with which cavitation erosion take place is variable

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Nuclei content and roughness

Architettura Navale

I The fluid nuclei content and the wall roughness of a given device areimportant because they can influence not only the extent but also thestructure of the cavity.

Effect of the nuclei content. Lower nuclei content (left), higher nuclei content (right).

Effect of the leading edge surface roughness. Lower roughness (left), higher roughness (right).

Historical aspects Definition Hydrodynamic cavitation Cavitation-induced damage Nuclei content and roughness

Nuclei content and roughness

Architettura Navale

I The fluid nuclei content and the wall roughness of a given device areimportant because they can influence not only the extent but also thestructure of the cavity.

Effect of the nuclei content. Lower nuclei content (left), higher nuclei content (right).

Effect of the leading edge surface roughness. Lower roughness (left), higher roughness (right).