Velocity Measurement of Fluid Flows by Ryan Holman November 9, 2004 IMG Brown Bag Lectures

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Velocity Measurement of Fluid Flows byRyan
HolmanNovember 9, 2004IMG Brown Bag Lectures
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Outline

• Introduction
• Flow velocity measurement
• Laser Doppler Anemometry (LDA)
• Particle Image Velocimetry (PIV)
• Results
• Summary

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Introduction

• Fluid flows arise in myriad practical
  applications of everyday life
• Aerodynamic surfaces (airfoils, boats,
  submarines, automobiles, )
• Pipe flows (water lines, hydraulic lines, )
• Bio-fluid flows (circulatory system)
• Fluid machinery (pumps, fans, )
• Lubricated machine parts
• Geophysical flows (atmosphere, ocean currents,
  rivers, )
• How to characterize fluid flows?

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Introduction

• Flow visualization
• Useful qualitative technique
• Identify flow structures
• How to quantify flow behavior?

Flow direction
Flow direction
Magnesium cuttings in oil
Dye in water
Images from Van Dyke, An Album of Fluid Motion
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Introduction

• Identical flows

CFD
Flow vis
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Introduction

• Governing equations of fluid flows
• Mass
• Momentum
• assuming Newtonian fluid, incompressible flow,
  constant viscosity, Stokes hypothesis
• Energy
• assuming incompressible flow, constant thermal
  conductivity
• Energy can be decoupled from continuity and
  momentum

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Introduction

• Solution of governing equations generally results
  in a velocity profile
• Non-linear, 2nd order PDE exact solutions only
  available for a very limited number of cases
  (after making key assumptions)
• Resort to other techniques
• Computational fluid dynamics (CFD)
• Experimental velocity measurements

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Flow Velocity Measurement

• Why are experimental velocity measurements
  important?
• Quantify flows over aerodynamic surfaces
• Determine forces acting on bodies due to fluid
  flows (control volume approach)
• Shear stress distribution in a flow, wall shear
  stress
• Vorticity field
• Validate CFD predictions, theoretical treatments

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Flow Velocity Measurement

• Velocity measurement techniques
• Intrusive
• Pitot total-static pressure probe
• Hot wire anemometry
• Non-Intrusive
• Laser Doppler Anemometry (LDA)
• Particle Image Velocimetry (PIV)
• Require seed particles
• Particle velocity measured, NOT flow velocity

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Flow Velocity Measurement
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Flow Velocity Measurement

• Seed particle considerations
• Large enough to scatter enough light to be
  detected
• Small enough to track the flow
• Neutrally buoyant
• Not condense quickly

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LDA

• Coherent beams from a laser source intersect to
  form a probe volume with optical fringes
• The intersection occurs at the beam waist such
  that the fringes are plane

fringes
Probe volume
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LDA

• Spacing between fringes df is known
• As a particle moves through the probe volume with
  velocity u, light is scattered
• The scattered light varies in intensity as the
  particle passes through the fringes at the
  Doppler frequency
• Measuring fD allows for computation of u

light wavelength
beam half-angle
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LDA

• Due to Gaussian intensity of the beams, probe
  volume is an ellipsoid
• Probe volume dimensions computed from the beam
  waist diameter df
• Typical dimensions 59 by 58 by 523 microns

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LDA

• Scattered light detected by a photomultiplier
  tube (PMT)
• Converts light intensity into current, a Doppler
  burst
• Processor converts current to voltage

• DC component called Doppler pedestal
• periods equals fringes of the probe volume
• Computing FFT of the burst yields the Doppler
  frequency fD and hence velocity

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LDA Theory of Operation

• Directional ambiguity system cannot distinguish
  between positive and negative frequencies

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LDA

• Bragg cell serves to eliminate directional
  ambiguity as well as the beam splitter
• Introduces a fixed frequency shift to one of the
  beams

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LDA

• Fixed frequency f0 causes probe volume fringes to
  move in one direction
• Negative velocities can be measured down to

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LDA

• Particles scatter light from the probe volume in
  all directions, including back toward probe
• In backscatter mode, scattered light is collected
  at the probe and transferred to the PMTs
• In off-axis scatter mode, separate receiving
  optics are positioned to acquire scattered light
  for the PMTs
• Advantages and disadvantages to both methods

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LDA

• Backscatter mode
• Advantage simplicity transmitting and
  receiving optics contained in one assembly
• Disadvantage Intensity of scattered light can be
  low, velocity computed for entire probe volume
  elongated football
• Probe volume dimensions 59 by 58 by 523 microns

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LDA

• Off-axis mode
• Advantage Can position optics to maximize
  intensity, and/or slice through the probe volume
• Disadvantage Difficult to align, minimum probe
  volume slice corresponds to minimum intensity
  (Mie scattering)
• Probe volume dimensions 58 by 59 by O(50) microns

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LDA

• Off-axis optics positioned at 90 to maximize
  spatial resolution
• Scattered light intensity is high enough to
  adequately detect Doppler bursts despite Mie
  scattering

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LDA

• Allows for velocity measurements very near to a
  solid surface
• Direction cosine matrix must be applied to
  obtain the streamwise and cross-stream velocity
  components

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LDA
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PIV

• CCD camera acquires 2 images in quick succession
• Image is divided into interrogation windows

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PIV

• Limited by minimum achievable time between image
  pairs and camera spatial resolution
• Particles should not travel more than half the
  interrogation window
• Large dt for slow velocity, small dt for high
  velocity
• Outlier rejection performed on vector field
• Several vector fields averaged to reduce noise in
  measurements

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PIV
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Results

• PIV results for synthetic jet flow

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Results

• S12, Re18, flow region 1-2 (no jet formation)
• Error bars omitted for clarity
• x/d 0.08

axial velocity
radial velocity
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Results

• Quantitative comparison of uncertainty (excludes
  bias errors)

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Summary

• Velocity measurement yields useful information on
  fluid flows
• LDA, PIV non-intrusive techniques for velocity
  measurement
• Require seeding particles
• High spatial resolution
• Accurate