An experimental investigation of viscous aspects of propeller blade flow

TECHNISCHE _UNIVERSTEIX Laboratorium voor

Stheepshydromechanict

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Mekelweg 2,2628 CD Delft

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THE CATHOLIC UNIVERSITY OF AMERICA

AN EXPERIMENTAL INVESTIGATION OF VISCOUS ASPECTS OF _{PROPELLER BLADE FLOW}

A DISSERTATION

Submitted to the Faculty of The School of Engineering and Architecture Of The Catholic University of America In Partial Fulfillment of _{the Requirements}

For the Degree Doctor of Philosophy

by

Stuart Dodge Jessup

Washington, D.C. 1989

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This dissertation was approved by Mario Casarella, Ph.D. as Director, and Y.C. Whang, Ph.D and Thomas T. Huang, Ph.D. as Readers.

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TABLE OF CONTENTS Page LIST OF FIGURES iv LIST OF TABLES viii ACKNOWLEDGEMENTS ix LIST OF SYMBOLS CHAPTER 1 - INTRODUCTION 1.1 Background 1

1.2 Classification of Viscous Flow _Regions

3

1.3 Review of Previous Experimental _{Investigations}

7

1.4 Objective of Research

11

CHAPTER 2 - EXPERIMENTAL TECHNIQUE

15 2.1 Facility 15 2.2 Propellers Tested 17 2.3 LDV System 19

2.4 Traverse Systems and the Tunnel Window Insert 24 2.5 LDV Measurement Configurations

27

2.6 Computerized Data Acquisition ₂₈

2.7 Measurement Accuracy

32

2.8 Measurement Matrix and Test Conditions CHAPTER 3 - EXPERIMENTAL RESULTS AND _ANALYSIS

65

3.1 Upstream and Downstream Time _{Average Flow}

65

3.2 Upstream Time Varying Flow ₉₀

3.3 Blade Surface Pressure Distributions ₁₀₉

3.4 Blade Boundary Layer Flow ₁₂₁

3.4.1 Results at r/R.,.0.7

122

3.4.2 Results at r/R-0.9 and r/R-0.95 ₁₂₄

3.4.3 Results at r/R-0.975 Near the Tip Vortex 126 3.5 Global Blade Wake Flow

154

3.5.1 General Wake Flow ₁₅₅

3.5.2 Tip Flow

157 3.5.3 Hub Flow

159

3.5.4 Downstream Vorticity Field ₁₆₀

3.5.5 Downstream Wake Trajectory ₁₆₂

3.6 Detailed Blade Wake Flow ₂₀₂

3.6.1 Mid-Span Trailing Edge Flow ₂₀₂

3.6.2 General Wake Characteristics ₂₀₄

3.6.3 Radial Variation in Blade Wake Parameters ₂₀₅ 3.6.4 Downstream Variation of Blade _{Wake Parameters:}

Blade Wake Decay ₂₀₉

3.6.5 Tip Vortex ₂₁₂

CHAPTER 4 - SUMMARY AND CONCLUSIONS ₂₃₆

4.1 Summary of Results ₂₃₆ 4.2 Conclusions ₂₄₀ REFERENCES 247 iii .0.o .... ... ... -,..---..., ..._{...,--... x} . 1 ... . ... .... ...,. ... ... ... ... ... ...-...,... , . ... ...

LIST OF FIGURES

Page 1-1 Comparison of Velocity Distributions Downstream of Similar

Propellers, X/R=0.33, r/110.8 13

1-2 Propeller 4119 Looking From Upstream 14

1-3 Propeller 4119 Looking From the Side 14

2-1 DTRC 24 Inch Water Tunnel 37

2-2 DTRC 24 Inch Water Tunnel Test Section 38

2-3 Velocity Distribution Across Test Section Center 39

2-4 Propellers 4119 and 4842 40

2-5 Hub and Shaft Details for Propellers Tested 41

2-6 Open Water Test Results for Propeller 4119 42

2-7 Open Water Test Results for Propeller 4842 43

2-8 Hub and Shaft Details for Propellers Tested in Open Water 44

2-9

LDV System Optics Board 45

1-10 Blue and Violet Beam Standard Optics Path 45

2-11 Green Beam Fiber Optic Probe Optical Path 46

2-12 Optical Path for One Component Configuration 46

2-13 Traverse System for Two Component Standard Optics 47

2-14 LDV Optics Board and Traverse Systems 48

2-15 Schematic of Fiber Optic Probe Traverse 50

2-16 Velocity and Coordinate Description for Three Component

Measurement 50

2-17 Tunnel Configuration for Three Component Measurement 51 2-18 Alternative Measurement of Axial and Radial Velocity

Using Standard Lens Optics 52

2-19 Measurement of Streamwise Blade Boundary Layer 52

2-20 Measurement of Radial Blade Boundary Layer Using

Alternative Optics Configuration 53

2-21 Tunnel Configuration for Radial Blade Boundary Layer

Measurement 54

2-22 Data Acquisition System 55

2-23 Shaft Encoded Data Acquisition Program 55

2-24 LDV Measurement Test Matrix for Propeller 4119 61

2-25 LDV Measurement Matrix for Propeller 4842 64

3-1 Axial Velocity Distribution About Propeller 4119,

Hub Only 73

Tangential Velocity Distribution About Propeller 4119,

... .... .. ... ... . ... .... ... , .... ... . , ....

...

...

3-3 _{Calculated Axial Velocity Distributions About}

Propeller 4842, Hub Only ₇₄

3-4 _{Time Average Flow Ahead of Propeller 4119}

75

3-5 Upstream Tangential Velocity Distributions ₇₇

3-6 _{Axial Velocity Distribution Upstream of Propeller 4842}

77

3-7 Axial Velocity Downstream of Propeller 4119 ₇₈

3-8 Three Component Mean Velocity Downstream of _{Propellers 79}

3-9 Downstream Radial Flow Angles ₈₂

3-10 Measured Circulation Distributions ₈₃

3-11 _{Hub Boundary Layer Flow, Propeller 4119 84}

3-12 _{Comparison of Measured and Calculated Time Average}

Velocity Downstream of Propellers ₈₅

3-13 _{Comparison of Measured and Calculated Circulation}

88 3-14 _{Tangential Velocity Downstream of Propeller Showing}

Blade Wake ₈₉

3-15 _{Typical Shaft Encoded Velocity Distribution}

Ahead of Propeller 4119 ₉₄

3-16 _{Velocity Vector Representation of the Flow Field}

Upstream Near the Leading Edge of Propeller 4119 ₉₅ 3-17 _{Velocity Distributions Ahead of Propeller 4119}

96 3-18 _{Velocity Distributions Ahead of Propeller 4842}

99 3-19 _{Near-Hub Axial Velocity and Hub Boundary Layer}

Distributions Upstream of Propeller 4119 ₁₀₂

3-20 _{Radial Velocity at Inner Radii, Upstream of Propeller 4119} 105 3-21 _{Radial Velocity Distributions Within the Hub Boundary Layer 106} 3-22 _{Comparison of Measured and Calculated Time Varying Velocity}

Upstream of Propeller 4119 ₁₀₇

3-23 _{Typical Blade to Blade Flow Through Propeller}

Plane

From Inertial Frame of Reference ₁₁₃

3-24 _{velocity Vector Field Through Blades at 0.7}

Radius

From Moving Blade Frame of Reference ₁₁₄

3-25 _{Streamwise Pressure Distribution at 0.7 Radius Derived}

From Blade Surface Velocity Measurements ₁₁₅

3-26 _{Blade Surface Radial Velocity Distributions With}

Comparison to Panel Method Calculations ₁₁₆

3-27 _{Comparison of Pressure Distribution Derived From Total}

and Streamwise Blade Surface Velocities ₁₁₇

3-28 _{Comparison of Measured and Calculated Pressure}

Distributions ₁₁₈

3-29 _{Typical Blade Boundary Layer Measurement Represented in}

the Inertial Frame of Reference ₁₃₁

3-30 _{Streamwise Boundary Layer At 0.7 Radius of Propeller 4119 132} 3-31 _{Shape Factor and Displacement Thickness at 0.7 Radius}

of Propeller 4119 ₁₃₅ . ... ,... .... ... -...,, ... , -. _. ... ... ... .... , . . .

3-32 _{Streamwise Turbulence Intensity on the Suction Side}

at 0.7 Radius of Propeller 4119 ₁₃₆

3-33 _{Suction Side Radial Boundary Layers at 0.7 Radius}

on Propeller 4119 ₁₃₈

3-34 _{Blade Boundary Layers at 0.9 Radius of Propeller 4119 139} 3-35 _{Shape Factor and Displacement Thickness at 0.9 Radius}

of Propeller 4119 ₁₄₁

3-36 _{Blade Boundary Layers at 0.95 Radius of Propeller 4119 142} 3-37 _{Shape Factor and Displacement Thickness at 0.95 Radius}

of Propeller 4119 ₁₄₄

3-38 _{External Velocity Vectors and Streamlines in the Tip}

Region of Propeller 4119 _.145

3-39 _{Suction Side Streamwise Boundary Layers at 0.975 Radius}

of Propeller 4119 ₁₄₆

3-40 _{External Streamwise Velocity on the Suction Side}

of Propeller 4119 at 0.975 Radius 147

3-41 External Radial Velocity on the Suction Side

of Propeller 4119 at the 0.975 Radius ₁₄₈

3-42 Streamwise Turbulence Intensity on the Suction Side

of Propeller 4119 at the 0.975 Radius ₁₄₉

3-43 _{Streamwise Boundary Layers on the Pressure Side at 0.975}

Radius 149

3-44 _{Shape Factor and Displacement Thickness at 0.975 Radius}

of Propeller 4119 ₁₅₀

3-45 _{Composite of Radial Velocity Distribution at the Tip of}

Propeller 4119 ₁₅₁

3-46 Estimated Tip Vortex Trajectory 153

3-47 Typical Fixed Frame Velocity Field Downstream

of Propeller 4119 ₁₆₆

3-48 Moving Frame Pitchline Coordinate System 167

3-49 _{Typical Velocity Field Downstream of Propeller 4119}

Represented in the Pitchline Coordinate System 168

3-50 Fixed Frame Velocity Fields Downstream of Propeller 4119

at X/R=0.328 169

3-51 Streamwise Pitchline Velocity at X/R=0.328 171

3-52 Cross Plane Vector Field Downstream of Propeller 4119 at

X/R-0.328 172

3-53 Moving Frame Velocity Fields at X/R-0.951 175

3-54 Pitchline Cross Plane Vector Field at X/R=0.951 178

3-55 Pitchline and Radial Velocity Distributions Near the

Tip of Propeller 4119 at X/R-0.328 179

3-56 Surface Representation of Pitchline Velocity Near the Tip

at X/R=0.3t8 180

Velocity Distributions Through the Tip Vortex Core at

.. .. . , ... ...

3-58 _{Pitchline Cross Plane Vector Field Near the Tip}

at X/R-0.328 ₁₈₁

3-59 _{Surface Representation of Pitchline Velocity Near the Tip}

at X/R=0.951 ₁₈₂

3-60 _{Velocity Distributions Through the Tip Vortex Core at} X/R-0.951

182 3-61 _{Pitchline Cross Plane Vector Field Near the Tip}

at X/R=0.951 ₁₈₃

3-62 _{Surface Representation of Pitchline Velocity}

Near the Hub.. 184 3-63 _{Pitchline Cross Plane Vector Fields Near}

the Hub ₁₈₅

3-64 _{Pitchline Vorticity Distribution in the Wake}

187

3-65 _{Surface Representation of Pitchline Vorticity}

in the Wake.. 188 3-66 _{Contours of Pitchline Vorticity in the}

Wake 189

3-67 _{Distribution of Pitchline Circulation in}

the Wake 192

3-68 _{Variation of Tip Vortex Parameters in the Wake}

194 3-69 _{Radial Location of Tip Vortex}

195 3-70 _{Determination of Downstream Wake Pitch}

196 3-71 _{Angular Position of Wake With Downstream}

Distance 197

3-72 _{Tip Vortex Pitch Trajectory}

197

3-73 _{Distribution of Downstream Wake Pitch for}

Propeller 4119 198 3-74 _{Contours of Pitchline Vorticity in the Near Wake of}

Propeller 4842 ₁₉₉

3-75 _{Propeller Wake Fields}

200 3-76 _{Velocity Vector Field at the Tailing Edge}

at 0.7 Radius 214 3-77 _{Total Velocity and Flow Angle at the Trailing Edge}

at 0.7 Radius

215 3-78 _{Derived Pressure Distribution at the Trailing Edge}

at 0.7 Radius

216 3-79 _{Fixed Frame Velocity Distributions}

Through the Blade Wake 217 3-80 _{Moving Frame Pitchline Velocity Distribution}

Through Blade Wake ₂₁₉

3-81 _{Comparison of Turbulence Intensity From Direct and} Derived Measurement

221 3-82 _{Mean Pitchline Blade Wake Profiles}

222 3-83 _{Pitchline Velocity Fields Near the Tip}

223 3-84 _{Radial Distribution of Blade Wake Momentum Thickness}

225 3-85 _{Radial Distribution of Blade Section Drag}

Coefficients 225

3-86 _{Downstream Variation of Blade Wake Characteristics}

at

0.7 Radius ₂₂₆

3-87 _{Downstream Variation of Blade Wake Characteristics} at 0.9 Radius ₂₃₁ vii ... ... . ... , ...

3-88 Decay of Wake Centerline Velocity With Downstream Distance. .233

3-89 Wake Spreading With Downstream Distance 233

3-90 Velocity Distributions Through Tip Vortex Core 234

3-91 Variation of Radial Velocity Through Tip Vortex With

Downstream Distance 235

LIST OF TABLES

2-1 Geometry of DTRC Propeller 4119 242

2-2 Geometry of DTRC Propeller 4842 243

2-3 Thickness and Camber Distribution for DTRC Propellers

4119 and 4842 244

2-4 Load Data For Propellers 4119 and 4842 245

2-5 Specifications of 3 Component LDV System 246

... ...

... ....

:=1

ACKNOWLEDGEMENTS

This thesis effort could not have been possible without _{the many}

contributions I have received _{from colleagues and friends. To them I wish} to express my sincere appreciation.

I would like to express appreciation to my many colleagues at the _David

Taylor Research Center who have assisted me in the development of my

career in the area of marine propellers. Mr. Robert Boswell trained _{me in} the practical art of propellers during my early years at DTRC and has been a continued inspiration in all areas of investigation. Dr. Terry Brockett supported me in the early development of propeller flow measurement using Laser Doppler Velocimetry and motivated me to pursue this advanced degree. Also, I would like to thank Dr. William Morgan and Mr. Justin McCarthy for

their consistant support of my work in the propeller area.

I _{would like to acknowledge the support I received from DTRC that}

permitted me to complete my dissertation. Work performed at DTRC was

primarily funded under the Navy's Independent Research (IR) _{program. My} University program was supported by the DTRC Extended Term Training program which provided me a two year period to persue my academic goals.

The results to be presented would have been difficult to obtain without the efforts of Dr. David Frye in development of the _{data acquisition system} for the IBM PC. The work of Dr. Frye _{represents possibly the best LDV data} collection software in existence today.

I wish to express appreciation to my dissertation _{director, Professor}

Mario Casarella. Dr. Casarella supervised

_{my work with patience and}

perseverance in an area that was somewhat new to him. He asisted in the organization of the effort and followed it through _{to completion.}

I wish to thank Dr.s Huang and Whang for taking the time to review the dissertation. Their comments improved the work and helped _{to formulate the}

important conclusions from the results.

Finally, _{I would like to thank my family for their support though the} long process that led to a finished thesis. My _{absence for many weekends}

was difficult. Hopefully, in their eyes, the completion of my thesis was worth their sacrifice.

LIST OF SYMBOLS blade section chord length.

total blade section Pressure coefficient, 1-(VB/VR)2-(Vr/VR)2 (Cv)s streamwise blade pressure coefficient, 1-(VB/VR)2

propeller diameter blade section camber

maximum camber of blade section fM

shape factor, boundary layer or wake displacement thickness divided by momentum thickness,

_We

iT blade section rake, axial displacement of blade section relative to blade generator line, positive aft

advance coefficient, V/nD blade number

thrust coefficient, thrust/(1/2pn2D4) Kt

K torque Coefficient, torque/(1/2pn2D5) blade section Pitch

radius

tip radius of propeller

Rn Reynolds number, (VR C)/ v, if not specified, VR based on 0.7R (RMS) root mean square value, equivalent to turbulence intensity

distance from trailing edge, along blade pitch helix blade section thickness

tm maximum blade section thickness

V freestream axial velocity aproaching propeller, advance speed VB streamwise velocity in the angle direction relative to the

moving blade coordinate system, in the wake,

= 0

VB = -V

21-Inrcos0

= (Vt)m cos

0 +

Vx sin.

'TN lateral velocity perpendicular to the streamwise 0 direction relative to the moving blade coordinate system

VN = (Vt)m sins - Vx

cos0p

Vr radial velocity, positive outward from propeller shaft VR resultant inflow velocity to blade section,

VR= (V2 + (21Tnr)2)1/2

Vt tangential velocity, positive in the direction of propeller rotation

(Vt)m tangential velocity in the blade moving frame (Vt)m Mar - Vt

Vz vertical velocity, positive upward

Vi velocity measured in (4) direction, in fixed frame of reference

WB _{vorticity in the moving streamwise pitchline coordinate system,}

WB - VN - 1 aVr + avN

rsino

ae

or

X _{axial distance from propeller centerline, positive downstream} also distance along chord line from leading edge _{of blade section}

Xc fraction of chord

horizontal distance from propeller axis, positive to the right looking upstream

vertical distance from propeller axis, positive up streamwise displacement thickness

circulation fluid density

kinematic viscosity blade angular position

streamwise momentum thickness

es

projected skew angle, degrees

0 _{LDV velocity measurement direction, angle from vertical, positive} upstream

propeller blade section pitch angle

ABSTRACT

An experimental investigation of the laminar/turbulent flow in the vicinity of a rotating propeller blade was conducted using laser doppler

velocimetry. Details of the flow were measured to assess the viscous

features relative to classical potential theory and wing flow.

Three-dimensional velocity component measurements were made of the propeller blade boundary layer and wake using laser doppler velocimetry with a phase averaging technique to account for blade rotation.

The propeller blade flow was characterized by streamwise and radial boundary layer profiles. Laminar boundary layers were initiated at the leading edge with transition to turbulence occurring at the mid-chord of the blade. The midspan streamwise boundary layer resembled typical two-dimensional behavior. The radial boundary layer exhibited large outward flow near the wall in regions of laminar flow which was reduced after

transition. The outer blade boundary layer edge velocities along the blade were predicted by potential theory implying no significant viscous-invicid

interactions. The tip vortex initially formed at the blade tip and

convected over the blade surface locally distorting the blade surface

boundary layer.

The propeller turbulent wake was dominated by individual blade wakes, hub and tip vortices. The radial attached boundary layer at the trailing

edge of the blade convected into the wake, and produced significant

outward, radial flow at the wake centerlines causing a redistribution of

the classical sheet vortex. However, in the streamline direction, the

measured wakes followed typical two-dimensional turbulent wake decay laws. Tip vortex roll-up was almost complete at the blade trailing edge, causing

a reduction of the vortex sheet strength near the tip relative to

moderately loaded propeller theory. With increasing downstream distance, the vortex and the blade wake system diverged through mutual induction and

locally decayed and dispersed through turbulent dissipation.

This investigation of propeller flows supports and improves current empirical propeller wake models that incorporate distinct tip vortices and

deformed vortex wake sheets. It is proposed that improvements in

performance prediction could be made by considering the dispersion of the wake sheet, measured blade section drag coefficients, and modification of

1

CHAPTER 1

INTRODUCTION 1.1 Background

Propellers have been the primary form of self-propulsion for ships for over 100 years. Aircraft have almost exclusively relied on propellers for propulsion until recent use of jet propulsion. The development of modern

aircraft propellers or prop fans has shown significant improvements in efficiency over jet propulsion and these designs are being adopted for the

next generation of air transports. Propellers produce thrust by the

production of lift from the individual blades, and therefore are part of

the more general class of lifting surfaces which also include wings,

rudders, and sails. The propeller lifting surface is developed along a

helical surface whereas a wing is developed along a planar surface.

Marine propellers generally operate _{at the stern within the wake of} the ship. This provides improved efficiency but significantly complicates the propeller hydrodynamics. The blade inflow varies significantly as the propeller rotates, producing unsteady lift and thrust. Also, a complicated interaction occurs between the propeller and the ship's wake. Through the years, ship and propeller designers have overcome this complex situation

through experience, good design practices , and model testing. These

efforts resulted in efficient propulsion with acceptable vibration

characteristics. Propeller theory is used to provide the basic

understanding to develop new propeller designs with a minimum of model

testing and preliminary design iterations.

Modern propeller theory is primarily based on potential theory. Vortex

lattice and panel methods are currently used in propeller design and

analysis. A review paper by Kerwin(1986) summarizes the present _{state of} the art.

Kerwin and his colleagues at MIT have formulated lifting surface

computational procedures for propeller design and analysis with increasing

sophistication since the early 1960s. These numerical methods have been

primarily based on vortex lattice models, representing each blade as a

condition was enforced on the meanline surface _{to arrive at the strength of} the vortex lattice elements. Blade thickness _{was represented in a} two-dimensional stripwise sense. Early procedures only modeled the blades, and incorporated a simple blade wake model where the vortex segments continued downstream at _{a specified pitch angle with no wake contraction.}

Current procedures, specifically the analysis _{program, PSF-2, written by Greeley}

and Kerwin (1982) incorporate _{a more sophisticated wake model drawing} on

empirical data to include the tip vortex _{roll-up and iterative pitch}

alignment in a transition wake along with a set of hub and tip vorticies in

the ultimate wake. Wang (1985) added the hub to _{the propeller vortex}

lattice model. The most recent propeller computational program developed at MIT by Kerwin(1987) is _{a panel method that includes the propeller blades}

and hub, and incorporates the _{same wake model as Greeley. This improved}

code overcomes the shortcomings of the previous _{vortex lattice methods in} properly modeling the thickness of the blades especially near the hub and blade leading edges.

Other panel codes have been developed in _{recent years with some}

variation to the latest MIT program. Hess and Valarezo(1985) _{developed a} propeller and hub panel code that utilized a simplified downstream wake of a pure helix in the transitional wake and _{a semi-infinite cylindrical} wake as an ultimate wake model. Vaidanathan (1984) _{and his colleages have} also developed a panel code for marine propellers. The wake model for this code is relatively simple with the wake sheets convected _{back along pure} helices. This computational program, VSAERO, has been used _{at DTRC by Yang} (1987,1988) to _{correlate with existing propeller blade measurements by}

Jessup (1986). These comparisons showed excellent correlation, _{except in} the tip region, when the hub was included in the model.

All of these programs should perform similarly when properly modeling the blades and hub for propellers operating in uniform inflows. Variations

will occur with paneling distributions near the edges of the blade,

different application of a Kutta condition, and various representations of the propeller wake. The accuracy of potential flow predictions has now

reached a level in which, for marine propellers of moderate or light

loading, _{viscous flow interaction effects warrant consideration for any} significant improvements in state-of-the-art propeller flow predictions.

"41

Compressible Euler solvers have already been developed by Yamamoto (1986) and Agarwal(1986) in the aerodynamic field for prop fans and helicopter

rotors. An Euler solver for incompressible flow for marine application may permit correct consideration of inflow vorticity and vorticity convection

in the wake, but presently no working programs _{exist. Propeller-hull}

interaction, and the complexities of the propeller inflow require this consideration.

Few viscous flow codes have been developed for marine propellers.

Groves (1981,1984) developed a momentum integral boundary layer

computational procedure for rotating blades, and later, a finite difference procedure. A partially parabolic procedure by Stern et al.(1986) represents the blades as body force distributions. Viscous field solutions including detailed blade flow have not yet been developed.

1.2 Classification of Viscous Flow Regions

The various viscous/inviscid interaction regions of the propeller flow can be identified as the flow approaches and passes through blades of the propeller. For moderately loaded propellers, _{regions of the flow can be}

identified as predominately viscous or potential flow.

The viscous

dominated regions will be discussed relative to the issues addressed in

this research investigation.

Propeller Inflow

For the case of a propeller operating in uniform inflow, _{the flow is} purely inviscid potential flow. The idealized case of purely uniform inflow only occurs in the model testing stage of ship design. Each model propeller is tested over a range of operating conditions in open water, driven from a

long extended shaft in a towing basin. Results from these _{open water tests}

are used to evaluate model propulsion. Therefore understanding the

propeller flow in this configuration is important. However, this flow condition is the simplest to predict.

Hub flows for even the simplest case of uniform inflow should be

considered. General hub shapes will produce a potential flow perturbation that will influence the blade inflow. This effect can easily be predicted with hub modeling. The boundary layer flow over a spinning hub _{can provide} a complicated inflow to the root sections of the blades. _{The hub inflow} boundary layer can be significant for the case of a propeller driven by

3

the shaft which can extend far upstream. An example _{would be a propeller} driven from upstream in a water tunnel. In this case the hub boundary layer can extend up to 10Z of the tip radius. This could have profound _{effects on}

horseshoe vortex generation about the hub, and general secondary flow about the root of the blade.

Most marine propellers do not operate in this flow regime. _{In the}

case of submarines and merchant ships,

the inflow

is highly three

dimensional and viscous, with the propeller almost entirely _{inbedded in the}

hull boundary layer. Most high speed naval surface _{ships drive their}

propellers with inclined shafts open to the flow below the _{hull. In this} case, the propeller operates in a primarily potential flow field, but _the shaft inclination causes the propeller to experience _{unsteady operation.} The shaft and the supporting struts generate localized wakes _{that convect} into the propeller.

These practical configurations cause complex unsteady flow _phenomena and are beyond the considerations of this fundamental _{investigation. This} basic study of the physical aspects of the viscous flow in the near wall regions of the blade and wake will address the idealized inflow of the open water test. This minimizes inflow uncertainty so that dominant real flow effects can be more easily isolated.

Blade Leading Edge Flows

Real flow effects begin at the blade leading edge. A stagnation point

can occur at some point along the leading edge with stagnation lines

extending from the point inboard and outboard along the edge. Flow from

these lines begins the boundary layer development on each side of the

blade. Most marine propellers are designed to operate, in the average sense, at ideal angle of attack. For this case the boundary layer develops

in a strong favorable pressure gradient typical of 2-d stagnation point flow, and no flow separation occurs. When the propeller is operating off design , high velocities around the leading edge followed by an unfavorable

pressure gradient will cause laminar flow separation. At conditions far

from design, complete separation over the entire blade can occur, while for less extreme loadings, the flow reattaches developing a turbulent boundary layer. Studies of this flow have been performed primarily due to interest in leading edge cavitation. Kuiper(1981) has studied the scaling of blade

layer. The detailed mechanics of this flow are unknown due to the small scale of typical model propellers. Greeley (1982) attempted to model the leading edge flow with a local tip solution which allowed vorticity _{to be}

shed from the leading edge simulating flow separation.

_{Swept wing}

experiments, reviewed by Greeley showed strong effects of Reynolds _number and leading edge swept angle.

The present investigation was restricted to propeller operation at design condition only. For this case, over most of the blade, _{the leading}

edge flow is void of separation and initial boundary layer flows can be assumed as stagnation flow. Detailed measurements at the leading _{edge are} very difficult with typical model propeller geometry. Limited positioning

accuracy, surface reflection, and measurement volume size restrict

measuring flows around the tip in detail.

Mid-Span Blade Flows

The mid-span regions of the blade, when void of _{separation, are}

characterized by three dimensional boundary layer flow. _{This flow regime is} perhaps the simplest, fortunately, since there the majority of the blade load is generated. Propeller rotation affects the boundary layer flow along

with the general potential flow field.

_{This flow was calculated by}

Groves(1984) for simple propeller geometry _{Blade flow separation usually}

occurs towards the trailing edge of the blade. The flow separation _is a

complex three-dimensional phenomenon.

Trailing Edge Flows

Trailing edge flows comprise the merging of the blade boundary layer flows on either side of the blade. In potential _{flow theory, this flow is}

addressed with the Kutta condition. In reality the blade boundary layers at the trailing edge are relatively thick, _{as compared to the trialing edge} thickness and the condition of constant _{pressure at the trailing may not}

hold. Propeller blades often have discontinuous chisel shapes at the trailing edge which produce local flow separation at the discontinuity.

This feature complicates the trailing edge flow. The _{propeller tested has} continuous rounded trailing edges producing a relatively _{simple flow. The} LDV measurements performed in this investigation _{provide good detail in} this region.

Tip Flows

Tip flows are one of the most complicated flow regions on the blade. Near the tip the _{leading edge flow at design condition will produce a} stagnation line out along the leading edge. Flow will be attached around the leading edge. Radial flows increase as the tip is approached with flow tending to go around the tip. At some point the radial flow around the tip cannot remain attached through the adverse pressure gradients around the

tip and the flow separates at the tip. At that point the tip vortex

originates.

The development of the tip vortex seems to be greatly dependent on the blade geometry in the tip region. For blades with zero chordlength at the tip, the vortex starts at the outermost tip and travels over the suction side of the blade moving inboard. For blades with significant skew, the tip is moved downstream relative to the inboard blade sections, altering the tip vortex blade flow interaction. Also, blades with finite chordlength at the tip will generally cause the tip vortex separation to occur at the tip trailing edge. The effect of tip geometry will be addressed to some extent

in this investigation.

After initial vortex formation at

the point of blade surface

separation, the "roll-up" process begins where vorticity from the blade "vortex sheet" convects and rolls up into the tip vortex. This process is

highly viscous at the vortex core center, and primarily a general flow convection process away from the core. Understanding of this process for a typical marine propeller will also be examined from the experimental data.

Mid-Span Wake Flows

The wake flow in the mid-span of the blade is characterized by the

three dimensional convection of

the

blade wakes downstream of

the

propeller. The blade wakes decay and spread as they convect downstream. The radial velocity distribution which was generated by the three dimensional flow over the blades is important in distorting the sheet and convecting

vorticity toward the tip and hub.

This flow was measured

in this

investigation and will be reported in considerable detail.

7

blade hub intersection. This flow is also _{very complicated involving hub} boundary layer inflow, the blade boundary layer, _{possible flow separation} at the intersection, _{and vortex development along and downstream of the} blade. This flow was measured to some extent, with detailed flow evaluated

primarily in the wake. Again, the configuration

_{tested utilized an}

essentially infinite hub downstream. Therefore, _{the formation of a single} hub vortex does not occur, but a separate vortex associated with each blade was observed.

1.3 Review of Previous Experimental Investigations

Previous flow field studies on propellers _{can be distinctly separated}

by the development of Laser Doppler Velocimetry. Early work on marine

propellers focused on the tracking of tip

_{vortex trajectories when}

cavitation occurred. Lifting surface calculation procedures had been developed during the 1960's,

_{but limited knowledge existed}

_{for the}

downstream wake of the blades. Most lifting surface models assumed that the wake was convected downstream at the induced pitch angle determined _{from a}

lifting line model of the propeller.

At MIT, Loukakis(1971) made the first _{attempt to measure and predict} the structure of the downstream wake.

Propellers were operated in a water tunnel and the tip vortex _{cavitation was generated to track its pitch and} radius in the wake. Calculations _{of the tip vortex trajectory}

were made

from a model of helical _{vortex lines. Loukakis observed}

significant

contraction of the tip vortices downstream of the propeller. He believed that a relatively short transition wake region occurred where the _trailing vortex sheet rolled up and contracted, _{followed by an ultimate wake region} made up of individual tip vortices of constant radius and pitch. Loukakis

also observed a very complex transition region where wakes from _different blades combined. This qualitative _{understanding of the flow was entirely}

based on observations of various forms of cavitation generated by the propeller.

Later, Kerwin(1976) and Cummings(1976) _{extended Loukakis's work by}

performing further measurements of

_{the tip vortex radius and pitch}

downstream of the propeller. The tip _{vortex pitch near the blades and far} downstream was significantly less than the moderately loaded lifting line

model predicted. This apparently verified that tip vortex rollup was

important in modeling the flow.

The first laser doppler velocimetry measurements about a marine

propeller were made by Min(1978) and provided further insights into the

propeller wake. The LDV system at the MIT water tunnel was a one component forward scatter system measuring axial velocity ahead of and behind the

propeller operating in the tunnel. A tracker was used to process the

measurements at a sufficiently high enough data rate so as to assume a continuous analog signal that could be waveform averaged via a once per

revolution trigger on the shaft. These measurements are considered landmark

in providing the first quantitative time varying velocity measurements

about an operating propeller. The measurement spatial resolution was

limited, but it was sufficient to draw numerous conclusions concerning the

flow and the validity of the potential flow model. Min(1978) determined

distributions of slipstream radius, tip vortex pitch, and extent of roll-up for a series of propellers with varying skew. These wake characterizations were derived with crude techniques and limited measurements of the details of the flow however,they have become standard empirical settings in the MIT wake model. Min also demonstrated the ability of the MIT potential flow

codes to predict the potential flow regions of the flow upstream and

downstream outside the slipstream of the propeller. Finally, Min identified

the viscous wakes shed by the blades which led other investigators to

measure the wakes in more detail.

The next propeller flow field study was also performed at MIT by

Kobayashi(1981). This work was an extension of Min's study where the

viscous wake downstream of the blades was measured with some detail. The

single component LDV system was used, but by rotating the optics and

repeating measurements at the top and side of the propeller disk, three

components of velocity were obtained in a non-simultaneous fashion.

Measurements were made downstream of the propeller to derive the viscous drag of the blade sections. From the measurements, the vorticity in the

wake was calculated by making four measurements about a central point to calculate velocity gradients at the central point. From the vorticity, and assumptions about the flow , section drag coefficients were obtained.

Kobayashi (1981) developed computerized data acquisition to replace

the analog wave averaging system. This system collected data through an

analog to digital converter which assumed continuous signals from the LDV

the tracker will not follow the flow. This problem may have effected

Kobayashi's results, especially for the blade wakes.

As a check on this effect of data acquisition _{was made}

with Kobayashi's results

using

similar propeller geometries,

and

measurement locations downstream of the propeller. The propeller in this

study was a three bladed unskewed propeller, DTRC Propeller 4119, while

Kobayashi's results were obtained with DTRC Propeller 4381, a five bladed

unskewed propeller. Both measurements were made at approximatey 0.33R

downstream of the propeller centerline at 0.8R radius. Some variation _in blade wake characteristic will occur due to differences in blade _section Reynolds number,and helical distance from the blade trailing edges, _but

this comparision is considered reasonable. The comparison is shown in Figure 1-1. Note that the sign conventions used are those of Kobayashi for comparison purposes. The velocity variations with blade angular position are similar, but are better defined through the blade wake in the _present study. The largest discrepancy occurs in the tangential velocity which contains the major component of the streamwise blade wake deficit. _{It is} unclear what effect the smoothing out of the wake gradients would have, but some reduction in calculated vorticity would be expected. Similar vorticity calculations were made in this investigation.

Despite this problem with Kobayashi's data collection _{process, these}

measurements were the first three component flow field

_measurements

downstream of the propeller. The primary results showed a complex wake

field with significant radial velocity variations _{in the wake which in}

potential flow models is represented by a vortex sheet. This implied that complete wake roll-up does not occur in the near wake, but persists well downstream.

After Kobayashi's work, _{numerous other investigators developed LDV}

systems for flow measurements about operating propellers. Billet(1987) conducted LDV measurements about marine propellers with _{the inclusion of} turbulence measurements derived from the rocs _{of the ensemble average of} data collected at each discrete anglular position of the propeller. High

turbulence regions were shown in the wakes

of

the blades. Similar

measurements were made by Cenedese(1985) in a water tunnel. Kotb and

Schetz(1986) measured the wake flow behind a propeller operating in a

nonuniform

shear flow in air. The measurements were made with hot wires

stress measurements. Turbulence was also shown to be localized at the blade wakes but higher levels occurred near the hub and tip associated with the tip and hub vortex structure. With the hot wires, no data could be obtained inside the blade passage. Other LDV measurements about aeroplane propellers have been conducted by Serafini(1981), Lepicovsky(1984) and _{Sundar (1985).}

All of these experimental investigations have identified the blade _wake, hub, and tip regions as complications in the flow with high turbulence, _but detailed wake measurements had not been made. Jessup(1984) _{measured the} streamwise blade boundary layers on a marine propeller demonstrating _the

similarity of the blade flow mid-span to two-dimensional blade _section

flow. Radial or wake flows were not measured. Recent

_{studies by}

Schetz(1988) obtained wake measurements on a marine propeller operating in &wind tunnel using hot wire techniques.

The field of turbomachinery also has studied the flows about turbine blades. The field is quite extensive, but noteworthy studies _{are the work} of Lakshminarayana(1974,1982) and Reynolds(1979). Measurements _{were made as} early as 1974 of the blade row wakes using stationary hot wire probes. In 1979, the wakes were measured with a probe that rotated with the blade row. In 1982, measurements were made of the blade boundary layers with the

rotating probe. These measurements included three components of velocity, turbulence and Reynolds stress. These results have been used to develop viscous flow codes most recently by Warfield and Lakshminarayana(1987).

Unfortunately, the turbomachinery problem is significantly different from the open propeller. Some of the differences are the suppression of tip flow, high freestream turbulence, higher blade loading, number of blades, and upstream and downstream blade interactions.

There are a variety of other related viscous flows in which detailed experimental data exists. These include two-dimensional airfoils, rotating bodies , rotating disks, vortex flows, wings, and wing body juncture flows.

Data has been obtained in the wake region of two dimensional airfoils characterizing the mean and turbulence quantities. Hah(1980) measured the wake behind a NACA 0012 airfoil at various angles of attack with analysis of the decay of the mean and turbulent quantities with distance downstream. Yu(1981) made similar measurements for a NACA 63-012 airfoil concentrating

on the near wake less than 101 chord downstream. In each case, Reynolds stress was measured. These studies can provide a baseline of comparison

Data obtained on rotating bodies provides _{insights to the blade}

boundary layer characteristics. Lakshminarayana _{(1972) measured mean and} limited turbulent quantities on a blade of large chord rotating in an annulus. These measurements were _{very idealized, being an extension of} rotating disk.

1.4 Research Objective

The objective

of

_{this research was to determine experimentally the} detailed viscous/inviscid

_flow

_{about the blade}

_of

_{an operating propeller.} This study addressed the case

_of

_{a propeller operating in a uniform inflow} with low levels of freestream turbulence. _{Propeller loading was relatively} low, typical of a marine propeller, _{so that strong viscous effects were} restricted to blade and hub boundary _{flow and the individual blade wakes} convected downstream. For this _{case the interesting aspects of the}

_flow

become the three dimensional blade _{boundary layers, the tip vortex,}

near hub vortex, and wake

_flow

_{downstream of the blades. Most}

_of

the flow field near the blades remains effectively inviscid, _{interacting with the regions}

of strong vorticity and turbulence.

The propeller flow field _{was measured with a three component laser}

doppler velocimetry (LDV) system about a model propeller operating _in a

water tunnel, as shown in Figure HT and 1-3. The propeller was driven from

downstream operating in a uniform inflow at its design condition. LDV

measurements were phase averaged to the _{angular position of the rotating} propeller to obtain mean and flucuating velocity distributions relative to the individual blades. Measurements were obtained upstream, downstream, and through the propeller disk. The _{measurement resolution was sufficient}

to define the blade boundary layers

_and

_{blade wakes. A three bladed marine}

propeller model of conventional _{geometry was used for this study.}

Some limited measurements were also performed on a model representing a typical modern marine propeller design.

The results of these experiments _{were analyzed from the following}

perspectives. From a scientific point of view, the characteristics of

rotating blade flows are unique because of the general three dimensionality of the flow, the effects of rotation, and the helical nature

of

the wake. Comparisons of the

flow

_{can be made with low aspect ratio wings, line} vortex flow, rotating disk flow, and two dimensional _{airfoil flows.}

A second perspective for analysis is from a propeller designer's point

of view. Modern propeller design relies heavily on numerical procedures to

design full scale geometry and predict performance. Most computational methods are based on potential flow , where an account of viscous effects

is through simple account of blade drag. Results from this experimental

investigation can improve the empirical corrections to present potential flow codes. Also, experimental data quantifying the detailed viscous nature

of the flow will help in the future development of full Navier Stokes solutions to complex propeller flow.

0.2

VrAt Vt/V Vx/V

-0.4

0.0

0.6

1.5

0.9

13 l 1 \

BLADE ANGULAR POSITION

Figure 1-3 Comparison of

Velocity Distributions Downstream of

Similar

Propellers, X/R0.33, r/R.-0.8 _ ... ...

PROP 4119__\

Z

_\

..-.."

--

.-

\

_-\

PROP 4381. KOBAYASHI(1981)

FIJ74

Figure 1-2 Propeller 4119 Looking From Upstream

15

CHAPTER 2

EXPERIMENTAL 'TECHNIQUE

2.1 Facility

The facility used for these experiments _{is the David Taylor Research} Center (DTRC) 24 inch Variable Pressure Water Tunnel. The water tunnel was constructed in _{1940 primarily for performing}

cavitation tests on model propellers. A schematic of the tunnel is _{shown in Figure 2-1. The tunnel}

has an open test section in which _{the flow enters and exits an enlarged}

test section chamber producing a _{24 inch diameter flow with}

a peripheral shear layer extending to relatively _{still water within the chamber.}

The test section is shown in Figure 2-2.

Propellers can be driven from _{upstream or downstream shafting which}

extend out of the tunnel to dynamometers _{and motor drives. The upstream}

drive utilizes a 25 HP dynamometer while the downstream drive can be driven by a 150 HP or 10 HP dynamometer _{system depending on the propeller size.}

The 150 HP system is driven through _{a rigid 2.25 inch diameter shafting}

system. This system was selected for these _{experiments because of its high} torsional stiffness required for the _{measurement of the propeller angular} position _{The disadvantage of this arrangement}

was the low accuracy of the thrust and torque measurements resulting from operating at approximately 5% of the rated loading of the _dynamometer.

The quality of the mean flow in the test section is considered _average

when compared to typical modern flow facilities. Figure 2-3 shows the

distribution of mean axial

_{velocity,Vx' and vertical velocity,}

Vz' horizontally across the center of the test section at the nominal test flow speed of _{11 feet/second (f/s). The axial}

velocity shows a retardation of

the flow in the center of the test _{section, which appears to be due to}

potential flow effects of the upstream contraction and entrance nozzle. The vertical velocity distribution shows a slight upward component of the flow, largest at the tunnel center. The cause of this is unknown but could be due to the asymmetry of the test chamber _{about the mid-horizontal plane. The}

inflow can be considered uniform to within 1%, which is sufficient for the experiments to be described.

The present impeller drive system controls the impeller rotation speed to a relatively low accuracy of 1%. The water speed in the test section

varied linearly with impeller rotation speed producing a similar 12

accuracy on tunnel speed. A typical time history of the impeller RPM shows a constant rpm for a period of 5 to 30 minutes followed by a 1% drop in RPM followed by a slow increase back to the original value over a 5 to 10

minute period. This behavior caused problems when performing automated

measurement traverses across the test section. During the test a high

resolution encoder and counter was installed on the impeller so that its rotation speed could be monitored. During runs, the impeller speed was either manually adjusted to maintain constant flow in the test section or measurements were discarded if the impeller speed changed more that .25% of

its nominal value.

The quality of the turbulence characteristics of the test section flow is relatively poor when compared to other flow facilities. At typical test flow speeds of 8 to 11 f/s the free stream turbulence was measured to be at best 1.6% of the freestream velocity. This value was relatively high when compared to other modern facilities.

Fortunately, for these tests addressing propeller flows, the proper representation of turbulence quantities should be made from the moving

blades frame of reference. The effective flow speed across a specified blade section is referred to as the resultant velocity, VR,

VR = ( V2 +(211-nr)2)0*5

V - advance speed or in this case tunnel flow speed ms, propeller rotation speed, rps

r - radius

Also, with the advance coefficient, J V/nD, the ratio,

VR/V -

el

+ (U(r/R)/J)2)°'5

freestream turbulence in the blade moving frame is reduced _{to 0.67%, which} is relatively low.

2.2 Propellers Tested

Two model propellers were used for this study. Most measurements were

made with DTRC Propeller 4119, _{a three bladed propeller of relatively}

simple geometry previously displayed in Figures 1-1 and 1-2. Limited

measurements were also made with DTRC Propeller 4842, a five bladed, _skewed propeller representing a modern marine propeller geometry. Figure 2-4 shows photographs of these two propellers including the fairwater, _{hub and shaft} geometry used during these tests. As mentioned earlier, _{these propellers} were driven from downstream. The tabulated design _{geometry for these two}

propellers is presented in Tables 2-1 ,2-2 ,and 2-3. _{The hub and shaft} details for the two propellers is shown in Figure 2-5.

Propeller 4119 is a 12 inch diameter, three bladed _{propeller of}

relatively simple geometry. The propeller was designed by Denny (1968), for

uniform inflow as a double thickness version _{of Propeller 4118. The}

propeller was designed with state of the art _{lifting surface techniques at} that time. _{The design condition of the propeller occurs}

at an advance coefficient ,J 0.833 with a thrust coefficient, Kt _{of 0.154. At that}

condition, when operated in uniform flow, _{the blade sections are designed}

to operate at shock free entry, _{ideal angle of attack. This produces}

a

relatively well behaved leading edge flow _{where the forward half of the} chord sees a favorable pressure gradient _{in respect to the blade boundary}

layer growth. The propeller has _{a near optimum load distribution which}

tends to load the hub and tip region higher that a typical modern marine propeller. The propeller has relatively constant pitch with no skew or rake as shown in Table 2-1. This results in a relatively straight blade which is very conducive to LDV measurements of the surface flow _{on the blade. LDV}

measurements can be made of the entire _{blade surface using the optical}

configuration to be be discussed later in this report. Propeller 4119 is a relatively old propeller which was hand filed from a monobloc casting using templates and gages. Therefore the _{accuracy of model was checked using a} coordinate axis automated measuring machine and found to sufficiently match the design geometry.

Propeller 4842 is _{a five bladed propeller with characteristics of a}

typical modern marine propeller. A large hub diameter _{and nonlinear skew}

17

-distribution is indicative of a typical controllable pitch propeller. _The propeller was numerical control (nc) machined with monobloc construction and therefore represents a state of the art high _{accuracy model. The radial} load distribution is somewhat reduced at the hub and tip from optimum. The

design advance coefficient is J=0.905 with a relatively high thrust coefficient of 0.305. The propeller is a wake adapted design meaning that the specified inflow is not uniform but a specified distribution associated with a given ship. For this case, the radial variation of axial inflow is slight, increasing approximately 10% from hub to tip. The propeller blade sections were .designed for ideal angle of attack in the specified wake. Testing or analyzing this propeller with a uniform inflow, for this case

should not significantly alter the blade surface flow from the design case.

Both of the tested propellers were designed with no hub model.

Therefore the existance of the hub may alter the blade section inflow significantly for the hub configurations shown in Figure 2-5. The effect of the hub on the measured flow will be discussed in a later section of this

report. The ability to design the hub and propeller within a lifting

surface design code has been demonstrated by Wang(1985).

Both of the propellers have been tested in open water for thrust and torque characteristics over a range of advance coefficients. These tests

were performed in the MAC towing basin. The results of these tests are

shown in Figures 2-6 and 2-7. Also shown is the comparison of the measured results to the design values at design J. The hub configurations for these tests were somewhat different than those shown in Figure 2-5 and are shown

in Figure 2-6. Table 2-4 summarizes these load results. The open water

tests are conducted with and without the blades on the hub of the

propeller. The data shown in Figures 2-6, and 2-7, and Table 2-4 represents

the measured loads of the propeller minus the measured loads of the hub

alone.

The open water test data show fairly good agreement with the design

values. The design values represent the calculated thrust and torque from

lifting line calculations. The final propeller pitch and camber

distribution is calculated from lifting surface procedures to produce the design thrust and torque. If the the propeller design inflow is reasonably uniform, as it is for these propellers, then the openwater results should

openwater results will be effected by the hub and shaft _{configuration. For} small hubs, traditionally these effects _{are believed to be small, but for}

large hub propellers like Propeller 4842, these effects could be

significant.

Two countering effects of hub influence _{the resulting loads in the} open water configuration. One is the image effect of the blades attached to the hub. Wang(1985) showed that inclusion of _{the hub will cause the blade} sections near the root to generate additional _{loading. The other effect is} the perturbation of the incoming flow _{as in passes around the hub. As an} example, the _{ellipsoidal hub of Propeller 4842 will cause a relatively} large augmentation of the axial inflow _{to the hub. This tends to decrease} the loading of the root sections of the propeller. These two effects oppose each other and could cancel depending on the shape of the hub and the blade geometry near the root.

The effects discussed above _{are not considered viscous related}

, but

must be considered when attempting to sort out the real flow performance of

the these propellers. The _{open water results are considered the}

most accurate load characteristics, since the measurements in the 24 inch water tunnel were made at a small fraction of the load rating of the _dynamometer.

2.3 LDV System

The LDV system used was a TSI Inc. _{dual beam on-axis backscatter} system that was configured as a one, _{two and three component system for}

various portions of the test. The _{configuration was changed depending on}

whether measurements were being made _{near the blade surface or in the wake.} The first two components of measurement were made using conventional _lens optics entering the tunnel from _{the side, while the third}

component was obtained using a TSI fiber optic probe _{mounted at the bottom of the tunnel} test section.

The requirements for the measurements _{of the flows near the operating} propeller were to maintain small measuring dimensions relative to the large flow gradients near the blades. _{Measurement volume size on the order of}

0.003" or 0.075= was necessary _{to resolve the blade boundary layers and}

wake details. A system to measure the streamwise velocities near the blades

had been developed earlier by Jessup(1984) to measure _{streamwise blade} boundary layers utilizing the green 514 rim wavelength line of an ion-argon 2 watt laser. In 1985 an experiment _{was conducted to measure radial blade}

boundary layers using the one component system. In certain aspects, these

measurements were superior to the present study and will be discussed.

The system was later expanded to two components using the green and blue

(0.488nm) lines to measure axial and tangential velocities about the

propeller. The final configuration used for these tests expanded the system further to three components using the violet (0.476 nm) line and a fiber optic probe. The final system will be described first.

Laser and Beam Separation

The three component system was a modified and expanded TSI Inc. System 9100-7 four beam, two color LDV system. The standard configuration of a

green and blue two color arrangement was expanded to a green beam fiber

optic one component system and a blue and violet beam system analogous to

the original green and blue 9100-7 system. Figure 2-9 shows a simple

schematic of part

of

the LDV breadboard arrangement. The optics were

tightly pack on a 36"x 60" aluminum surface honeycomb sandwiched

breadboard. This was necessary to fit the system within the observation

room attached to the test section of the tunnel. A 2 Watt Lexel Ion-Argon Laser was used, operating in multiline mode. The beam was color separated

into multiple beams using standard TSI hardware . The green beam was

directed into the first set of optic modules for the fiber optic probe. The blue and violet beams were directed into the standard 9100-7 optics train as shown in Figure 2-9.

The etalon temperature control was used to adjust the distribution of power between the three beams from the factory setting for green ,blue,

and voilet of

78, 14, and 8 74 respectively to 60, 29, and 11 74

respectively. This provided more power for the weakest violet beam. Part of the reasoning for the present arrangement was due to the large attenuation

of power through the fiber optic system. Therefore, the green beam was

selected for the fiber optic probe because it contained the largest

fraction of power. The green transmission beams were attenuated through the

fibers to 1/4 the incoming power. This maintained relatively equivalent power for all three components. Another reason for the beam selection was the order of the beam split permitted the use of standard available mirrors used within the color separator box.

collection rates that was annoying at times. _{In the future, this problem} should be avoided and may require running _{without the etalon, as has been} done before without serious problems.

Standard Two Component Optics Path

Figure 2-10 shows a schematic of _{standard optics path for the two} color, blue violet optics. The blue beam entered the center of the modules and was split into two coherent beams _{in the vertical plane. The violet}

beam entered to the outside of _{the blue beam and was displaced to the}

center of the module after the blue beam was split. The violet beam passed through a polarization rotator which _{oriented the beam's polarity to the} vertical plane. _{The violet beam was then split into}

two beams in the

horizontal plane. One of the blue _{beams was frequency shifted through}

a

bragg cell which permitted the blue _{beam set measuring vertical (or for} this configuration, tangential) _{velocity to distinguish flow direction.}

A beam steering module was used to steer the four beams to cross at the same point. The beam stop blocked the _{extraneous beams that exit from the brag} cell. The 22 mm beam _{spacers reduced the beam separation from 50}

mm to 22 mm for entrance into the beam expander. _{The beam expander expanded the beam} diameter from typically 1 _{mm by a factor of 2.27 and returned the}

beam

separation to 50 mm. The four beams _{passed through two tranverse}

system mirrors to the final focusing _{lens. Measurements were made}

with 250 and 309 mm focal length lenses.

The unusual feature of _{the standard optics path was the use of}

single receiving module to receive _{the returned scattered light}

from both

blue and violet beam sets and direct it to a single _{photomultiplier.}

Separation of the doppler signals from each component was performed _through

band pass filtering. The blue _{beam, vertical component was frequency}

shifted by 40 Mhz,

_{and high pass filtered before}

the signals were

downshifted for processing. This removed crosstalk from the the violet beam doppler signals. The doppler signals _{from the the violet beams low}

pass

filtered the blue beam signals _{using the counter processor filters.}

For this configuration, the filtering mirror normally used with the _receiving module was removed. Originally, _{a color separator incorporating}

a dichroic mirror was procured to separate the _{blue and violet received light,}

similar to a TSI model 9145, but the _{separator suffered from very}

poor blue line

transmission and was not used. The single _{receiving module arrangement}

proved to work very successfully. The only disadvantage was crosstalk that occurred with the green line from the fiber optic probe. This was _overcome by offsetting the beam crossing of the green beam from the other components by typically 0.2 mm.

Fiber Optic Component Path

The fiber optic probe optical path was configured as _{a standard TSI} one component fiber optic probe system as shown in Figure 2-11. The green line beam entered the optic modules through a polarizing rotator to set the

polarization to the vertical. The beam splitter split the beam into two

beams in the horizontal plane. One beam was frequency shifted through the brag cell module. The beams extended through a spacer separating the extraneous shifted beams, and avoiding their interference with the main

beam at the fiber coupler. The two beams passed through a beam translator which provided lateral adjustment of the beams into the fiber coupler. The 9262 optical coupler provided final lateral adjustment and focusing of the beams into the two transmitting beam single fiber cables. These cables entered a single waterproof jacketed cable which connected to the probe. Within the probe, the transmitting beams were focused and exited a flat

glass window to intersect 180 mm (7.09") from the probe end (in water). Receiving optics within the probe focused the received scattered light back through a third optical fiber connected to a standard TSI photo multiplier.

Performance of the System

The overall performance of the three component system was less that optimum due to the performance of the fiber optic probe. Originally it was believed that the performance of each component would be well matched, but when tested the fiber optic probe performance was well below the standard two component system. Table 2-5 summarizes the measuring volume sizes and

relative signal to noise ratio, SNR. The standard optics resulted in

measuring volume dimensions 2 to 3 times smaller than the fiber optic

probe. Also the relative SNR was four to eight times higher for the

standard optics.

The use of

the fiber optic probe permitted simultaneous three

component measurements. The standard lens optics measuring axial and

tangential velocities utilized the 250 ram lens. With this arrangement the fiber optic probe measured the third component, the radial velocity. The

optics to measure axial and radial velocity. This arrangement permitted

potentially higher quality radial velocity measurements over the fiber

optic probe, but measurement sets had to be repeated to obtain three

component measurements. Further discussion of these tradeoffs will be made in a later section when also considering the orientation of the measuring volume relative to the highest flow gradients, and extent of _measurement

domain.

One and Two Component Systems

For some of the test period the three component system was

reconfigured as a one or two component system. Configuration as a two

component system was easily performed by simply blocking the green line

within the color separator box for the fiber optic probe and operating the standard lens optics alone.

The one component setup was used to measure velocity very close to the rotating blade surface within the blade boundary layer. This required that the plane in which the transmission beams were contained be rotated about the optical axis, and this plane be tangential to the blade surface. The optical path for this configuration is shown in Figure 2-12. One _additional feature of this arrangement was the rotating mounts used at each end of _the optical path. These permitted the modules between them to rotate about _the optical axis. This would change the measurement _{direction within a plane} perpendicular to the optical axis. Also shown in this configuration _is a

field stop module,which was used to spatially filter the received light _by focusing it through a pinhole aperture. This module _{was marginally useful} in improving the signal quality when the measuring volume _{was close to a}

solid surface, i.e.,

the blade or hub.

Further discussion of this

configuration relative to the blade geometry will be discussed in the next section.

LDV signal processing

The doppler signals output from the LDV system were processed with TSI 1990C counter processors. The counters were operated in single measurement

per burst mode validating measurements with eight cycles through the doppler burst. The counters

are digital output devices which upon

validation of the measurement generate a data ready pulse and 12 bit

mantissa and 4 bit exponent digital output of the period of the eight