On the re-entrant jet of a supercavitating body

On the re-entrant jet of a supercavitating bodyOn the re-entrant jet of a supercavitating body

Ya-dong Wang

Northwest Polytech Univ,Xi’an Shaanxi, P.R.China, 710072

Proceedings of the 8Proceedings of the 8thth International Symposium on International Symposium on CavitationCavitationCAV2012 – Abstract No. 82CAV2012 – Abstract No. 82August 14-16, 2012, August 14-16, 2012, SingaporeSingapore 

Author Introduction

Ya-dong Wang

College of Marine, Northwestern Polytechnical University, Xi’an, Shaanxi, China

Email: roby868@ 163.com

Northwest Polytech Univ,Xi’an Shaanxi, P.R.China, 710072

Structure of Presentation

1. Introduction

2. Experiment setup

3. Results and discussion

4. Conclusions

1. Introduction

Various methods have been setup to study cavitating flow, they mainly fall to:

1. Potential method with modifications

2. Multiphase CFD simulation

3. Experiment technique

1. Introduction

Flow near the cavity closure region is the most complicated in cavitating flow. Several closure models have been setup:

Riaboushinsky Model

Efros Model

Pressure Recovery Model

1. Introduction

The Efros closure form, which assumes there is a re-entrant jet in the cavity closure region, is frequently observed in experimental studies:

Wosnik (2003)

Our experiment results

1. Introduction

According to the previous investigations, the re-entrant jet can induce the instabilityinstability of cavitation. The unstable cavity results in the undetermined dynamic undetermined dynamic featurefeature, which is a main problem of supercavitating vehicle design.

Therefore, the characteristics of re-entrant jet for a supercavitating body should be studied in the following aspects:

1. Main feature of re-entrant jet in cavitating flow

2. The factors affecting the re-entrant jet

3. Ways to restrain the adverse effects

2. Experiment Setup

Experiment was conducted in the high speed water tunnel of NPU, using the ventilation method to obtain supercavity, a series of instruments to acquire desired data.

2.1 Experiment Scheme

The experiment configuration is mainly composed of the measure and control system, ventilation system and the image capture system.

Compressed air

PwV

Pressure transducer array

Pc

M.C. system

Water tunnel control

V

Camera

Attack angle adjusting system

2.2 High Speed Water Tunnel in NPU

The water tunnel is a recirculating, closed jet facility with pressure regulation and is capable of velocities in excess of 18m/s.

Working Section Size: Φ400×2000mm, three side observations

2.3 Test Models

We mainly conducted three kinds of experiments: Pressure in closure region measurement Effects of head shape and attack angle to re-entrant jet Re-entrant jet restraining

2.3 Test Models

Pressure in closure region measurement:• Obtain the pressure distribution in the re-entrant jet

start location• An array of pressure transducers

2.3 Test Models

Effects to re-entrant jet:• Three model heads with different nose diameters• Same full model length• Adjust inflow angle

Φ29mm Φ25mm Φ20mm Full length: 385mmCylinder section diameter: 50mm

2.3 Test Models

Re-entrant jet restraining:• Two rings with different diameters• Arrange in the middle of test model

Φ55mm Φ60mm

2.4 Test Instruments

MICRO pressure transducer KULITE pressure transducer

ALICAT gas mass-flow-rate controller MEGA SPEED MS75K high speed camera

3. Results and Discussion

Through groups of experiments, we successfully got the pressure data, re-entrant jet images and the effect of re-entrant restraining method.

3.1 Pressure in Closure Region

Pressure distribution in the re-entrant jet start location

1 2 3 4 5 6 7 885

90

95

100

105

110

115

Pressure Monitor No.

P(K

Pa)

0

-1

1

-2

2No.1 ------ No.10

Pressure peak in the closure line

Vc

model

cavity

re-entrant jetγ VbVf

3.1 Pressure in Closure Region

Comparisons between upwind and downwind sides

0 2 4 6 885

90

95

100

105

110

115

Pressure Monitor No.

P(K

Pa)

-0.5

0.5

0 2 4 6 885

90

95

100

105

110

115

Pressure Monitor No.

P(K

Pa)

-1.0

1.0

0 2 4 6 885

90

95

100

105

110

115

Pressure Monitor No.

P(K

Pa)

-1.5

1.5

0 2 4 6 885

90

95

100

105

110

115

Pressure Monitor No.

P(K

Pa)

-2.0

2.0

3.2 Re-entrant jet effect to cavity feature

Cavities in different σc

Cavities generated by different cavitators Cavities of different lengths

3.2 Re-entrant jet effect to cavity feature

Cavities in different attack angles

Re-entrant jet disturbances are much weaker in upwind sides

Model 1

Model 2

Model 3Close line attached to model surface

Close out of model surface

3.3 Re-entrant jet induced instability

Two forms of cavity instability

Unstable cavity closure line

Unstable cavity shape

StretchedCompressed

Next period

Vertical disturbance

3.4 Re-entrant jet control method

Comparisons of model with/without re-entrant jet restraining ring

Front cavity is more clear and stable

3.4 Re-entrant jet control method

Comparisons of model with/without re-entrant jet restraining ring

Ф55mm ring Ф60mm ring

betterworse

best

4. Conclusions

1. A pressure peak exists in the re-entrant jet formation area;

2. Strength of re-entrant jet becomes greater as σc increases. Cavity closure location also plays a role in deciding re-entrant jet influence to cavity;

3. Re-entrant jet can induce the instability of cavity in cavity length and shape, and the cycle time is indeterminate;

4. Re-entrant jet can only be fully prevented if the blocking ring fits well with local cavity, improper designed blocking ring may worsen the disturbance of re-entrant jet to cavity.

Northwest Polytech Univ,Xi’an Shaanxi, P.R.China, 710072