1. Direct Measurement of Wall Shear Stress in Single- and Multiphase Flows
Valery Sheverev, Lenterra Inc.
and
Bruce Brown, Srdjan Nesic, Ohio University
presentation at the
Institute for Corrosion and Multiphase Technology
Ohio University
Athens, OH
October 11, 2011 ` 1

2. Scope
We report measurements of wall shear stress taken at five installations of the Institute for Corrosion and Multiphase Technology, using the first of its kind Lenterra RealShearTM sensor.
Single-phase flows
1” pipe flow loop (Britol 50T oil, flow rate < 9 gpm)
Thin Channel Flow Cell #1(water, flow rates < 25 gpm)
Thin Channel Flow Cell #2(water, flow rate < 80 gpm)
Multi-phase flows
Standing slug system
Moving slug installation

3. Presenter - Lenterra, Inc.
Privately-owned emerging company, a provider of innovative sensor instrumentation based on its proprietary technologies.
Located in the New Jersey Institute of Technology (NJIT) Enterprise Development Center (incubator) in Newark, NJ.
$2.6M in SBIR (Small Business Innovation Research) grants from federal agencies (NSF, DOE, NASA)
Sales started in 2011
3 patents granted, 4 pending
Enterprise Development Center,
Newark, NJ
Founder and current president: Valery A Sheverev
Industry Professor of Physics, Polytechnic Institute of New York University 3

4. Flow Force on the Wall u Pressure, scalar quantity Flowx z y
Pressure,
scalar quantity
Wall shear stress (WSS), vector in x,z plane u
Sensing area
Wall
Slope=du/dy|y=0 y x
WSS, , is a product of velocity gradient (shear rate) near the wall,
and dynamic viscosity of the fluid,  :
Therefore WSS is an indirect measure of dynamic viscosity of the fluid or shear rate. 4

5. WSS Measurement – Needs in Oil and Gas Industry
The breadth of applications in an industry that utilizes flows is evident when one considers the common use of pressure transducers and the fact that shear stress characterizes flow action much better than pressure
Flow Assurance
Single- and multiphase simulations – WSS is a critical parameter in most models
Characterization of multiphase flows (slug effects etc.)
Direct detection of high and low viscosity components, especially in harsh environments such as high pressure and low temperature of deep water energy production
Corrosion Analysis
Direct relation: WSS ↑ → Corrosion Rate ↑ , via mass transfer
Corrosion inhibitors testing: WSS ↑ → removes inhibitor from the wall → Corrosion Rate↑
Important parameter in corrosion models 5

6. WSS Measurement – History
Indirect measurements: WSS is inferred, through a set of assumptions, from another flow property, such as streamwise velocity or heat transfer rate measured at or near the wall
Require a model of the flow near the wall and knowledge of flow parameters such as temperature and viscosity
Examples:
hot-wire/film-based anemometry – quite rude estimate, no temporal resolution
laser-based near-wall flow velocity measurement
Laser-based or Particle Image Velocimetry (PIV) methods are well developed but they work only in transparent single-phase fluids (water, air) 6

7. WSS Measurement – Direct Methods
Floating element
Wall y x
Direct - measure motion of a floating element,
positioned flush within the wall.
Floating element displacement measured by :
Electrical techniques
Piezoelectric – shear deformation of a PZT element
Capacitor-based - floating element is one of the capacitor plates - shift of the floating element changes the capacitance
Drawbacks:
- Susceptibility to electromagnetic interference
- Narrow temperature range
- Difficult to separate WSS from normal force (pressure)
Optical techniques
Variety of imaging or resonant methods (such as Fabry-Perot interferometers) to monitor floating element
- not durable - require delicate alignment of the resonator
- the resonator to be optically clean - difficult to sustain
All earlier direct techniques not robust enough for use in-field 7

8. Lenterra WSS Measurement Technology
Lenterra’s solution: Combine floating element with mechanical cantilever with micro-optical strain gage that are durable and not affected by the flow
- Preferred type of optical strain gage: Fiber Bragg Grating (FBG)
Floating element
Wall
Fiber-optical strain gages
Cantilever
Optical fibers
Sensor enclosure
Optical strain gage (FBG) versus:
resistive strain gages – not nearly sensitive
semiconductor strain gages – sensitivity comparable but narrow temperature range
both types require delicate electronics (preamplifier) embedded in the probe
Robust: Materials used in our sensor: stainless steel + glass 8

9. Micro Optical Strain Gage - Fiber Bragg Grating
FBGs are periodic structures of varying refractive index embedded in optical fibers.
FBG is attached to the cantilever. When the cantilever bends in response to shear stress, the FBG is strained which shifts its optical spectrum.
By interrogating FBG with a light source, this strain (and therefore WSS) can be measured by tracking the shift in the resonant wavelength.
Force Dl
Dl ~ Force 9

10. Temperature Compensation
Strain shifts FBG spectrum, but so does temperature
Solution: use two FBGs attached to opposite sides of cantilever
Temp Strain
FBG 1 (strain due to applied force increases spectral shift due to temperature)
Temp Strain
FBG 2 (strain decreases temperature shift)
Differential signal (shift of FBG 1 spectrum less shift of FBG 2 spectrum) is independent from temperature 10

11. Spectrum Measurement and WSS Computation
Using laser diode (LD):
Monochromatic light from a tunable
laser is directed to FBGs and reflected
light is recorded by photodiode
As the laser frequency is tuned, reflection
spectra (reflected light intensity versus
wavelength) are recorded for FBG1 & FBG2
Shift in the resonant wavelengths (Dl = lFBG1 - lFBG2) calculated
WSS is found from τw = kDl where k is the calibration coefficient
Sensors are calibrated by applying a varying mechanical force F to the tip of the cantilever and measuring Dl, τw = F/A (A-area of floating element)
k it is determined by
Properties of FBGs
Area of the floating element
Elastic modulus, length and diameter of the cantilever
Scanning laser
FBG
Photodiode
Fibers 11

12. Lenterra RealShear™ Sensor - Probe
The RealShear ™ sensor
1/4″-80 threaded housing
FBGs are attached (currently glued) to cantilever
Detailed Specifications are found at 12

13. Lenterra RealShear™ Sensor – Complete Measurement System
Includes :
A probe with connecting fibers
Controller combining optical components and data acquisition electronics
Computer
Measurement Software

14. Sensitivity to Off-Axis Shear Stress
RealShear™ sensors have a bidirectional response and can be used to find flow direction at the wall. 14

15. Multi-Phase Detection: Two Disks Apparatus
Apparatus: rotating disk testbed
Fluid - glycerin (viscosity 900 cP)
Lower disk rotating at 122 RPM
Smaller tooth gap 0.9 mm
Wider tooth gap 1.2 mm
Two full rotations shown
Plot on right: Air bubbles are induced behind teeth
The RealShear ™ sensor

16. Single-Phase Tests at OU: WSS in a 1”Metal Pipe
Test section: 1 m long
Sensor floating element flush with the inner wall.
Laminar Flow (Remax=130)
Fluid: Britol 50T oil 16

17. Single-Phase Tests at OU: WSS in a 1”Metal Pipe
WSS vs. Flow Rate
Fluid: Britol 50T oil
Viscosity (μ =185 mPa-s) - measured after completion of tests using a falling ball viscometer.
Estimated data: analytical solution of Navier-Stokes equations for fully developed flow in cylindrical pipe.
10% uncertainty in the estimated data is due to uncertainty in viscosity and flow rate
U - averaged velocity (from flow rate), R - pipe radius 17

18. Single-Phase Tests at OU – WSS in a 1”Metal Pipe
Instantaneous Signal 18

19. Single-Phase Tests at OU – WSS in a 1”Metal Pipe
Instantaneous Signal-Detail 19

20. Ohio University - Institute for Corrosion and Multiphase Technology Thin Channel Flow Cell (TCFC) (3mm x 100mm x 600mm)
Sensor’s
sensitive surface

21. Single-Phase Tests at OU - Thin Channel Flow Cell #1
Instantaneous Signal
Random excitation of mechanical oscillations of cantilever by eddies of turbulent flow 21

22. Single-Phase Tests at OU - Thin Channel Flow Cell #1
WSS vs. Flow Rate
Fluid: water
Room Temperature
Flow Rate measured by a flowmeter
Estimated: from Darcy–Weisbach equation using Darcy friction factor (found from a Moody diagram, assuming a particular roughness),
U is averaged velocity (from flow rate)
ρ is fluid density
f is Darcy friction factor
Measurements arcing downwards – Why?
Bruce Brown: top plate of Plexiglas bows up at higher flow rate, increases channel height 22

23. Single-Phase Tests at OU - Thin Channel Flow Cell #2
Time evolution
Water
Room Temp.
Diff. Pressure 76 psi
Measurement rate 10 kS/s 23

24. Single-Phase Tests at OU - Thin Channel Flow Cell #2
Time evolution - detail
Separate turbulences are readily observable 24

25. Single-Phase Tests at OU - Thin Channel Flow Cell #2
WSS vs. Flow Rate
Re=
System pressure 40 psig - total pressure is system pressure + differential pressure
Differential pressure between standard ports was directly measured
No saturation of WSS at higher flow rates (compare with TCFC #1)
Estimate: same model as with TCFC #1 data 25

26. Multi-Phase Tests at OU – Standing Slug (Water-Air)
4” Plexiglas pipe
Sensor flush with the wall
Sensor inserted here
Adapter 26

27. Multi-Phase Tests at OU – Standing Slug (Water-Air)
Adapter 27

28. Multi-Phase Tests at OU – Standing Slug (Water-Air)
Clear indication of slug influence on WSS
Instantaneous values of WSS in the slug are several times higher those observed in the upstream region
◘ Maximum measured WSS<100 Pa 28

29. Multi-Phase Tests at OU – Moving Slug (Water-Air)
Sensor at the bottom of the pipe, flush with the wall.
Water superficial velocity 0.3 m/s
Gas superficial velocity 3.6 m/s
No characteristic features caused by moving slugs observed
Thus WSS due to slug < 4 Pa 29

30. What Did We Learn From the First Tests?
WSS was measured from few Pa to over 1 kPa, at pressures <50
WSS on the pipe wall in laminar flow was measured systematically somewhat higher than predicted
Possibly due to calibration that does not include pressure difference across the floating element
Average values of WSS in two TCFCs (turbulent flows) are in the reasonable range of expected values
Standing water-air slug produced slightly higher instantaneous WSS (under 100 Pa).
Moving water-air slug showed no increase in WSS (<4 Pa)
High measurement rate allows to observe details of the turbulent flow (eddies striking the floating element) that are however masked by mechanical oscillations of the cantilever
Last observation lead to a new combined Wall Shear Stress - Corrosion Sensor concept: 30

31. Advanced Corrosion Sensor Concept Based on Lenterra’s Sensor Design
“When you say the corrosion sensor is flush mounted, make sure it is flush” – words of a professional.
Local fluid turbulence created by a protruding sensor can have a major impact on the damage mechanisms and the rate of damage
Srdjan Nesic: “Lenterra’ WSS sensor can be modified to directly measure corrosion rate!”
V. Sheverev and S. Nesic “Methods And Devices For Monitoring Interaction Between A Fluid And A Wall” – PPA filed June 21, 2011.

32. State of the Art– Tuning Fork Corrosion Sensor
Mechanical oscillator - tuning fork tines attached to a diaphragm that is driven by a piezoceramic or another driver
Resonance frequency
f0=(1/2π)√(k/m) m =system mass, k= stiffness
Corrosion rate is incurred from change in f0
Problems:
Tuning forks needs to be immersed in the fluid – how to make it flush with the wall?
Some experiments resulted in decrease in f0 when loss of material occurred. Why?
Answer: Corrosion affects not only the tip, but also the base – not only m is reduced, but k as well - f0 may change in any direction

33. Corrosion Sensor – New Concept
Basic design similar to RealShear sensor
Oscillator consists of a cantilever and floating element – the oscillator system known as “cantilever with a tip mass”
Only the outer surface of floating element is corrodible (for example coupon inserted)
Due to corrosion, only mass of the floating element reduces changing m, stiffness k not changed since it is determined by cantilever only
The sensor is flush – minimal gaps are needed for detection of oscillation

34. Passive Excitation-Example
FFT Spectra
Signal from TCFC #1

35. Thank You
Any Questions? 35