Why Make Measurements? Why Make Measurements? Optical Measurement - PowerPoint PPT Presentation

Why Make Measurements? Why Make Measurements? Optical Measurement Optical Measurement Techniques Techniques • Turbulent flows are the rule, not the exception. • Turbulent flows are the rule, not the exception. • Practical turbulent flows are VERY difficult to • Practical turbulent flows are VERY difficult to ME637 -Particle Transport, ME637 -Particle Transport, simulate using DNS. simulate using DNS. Deposition and Removal II Deposition and Removal II • Verification of turbulent flow models. • Verification of turbulent flow models. Jeffrey A. Taylor Jeffrey A. Taylor Optical Techniques: Optical Techniques: Optical Techniques: Optical Techniques: Advantages Advantages Disadvantages Disadvantages • Non-intrusive (seed, but no probes in the flow • Non-intrusive (seed, but no probes in the flow field) field) • Expensive!! • Expensive!! • Robust (no particle collection on probe) • Robust (no particle collection on probe) • Fragile optics. • Fragile optics. • High accuracy (accuracy is predictable) • High accuracy (accuracy is predictable) • Seeding... Seeding... Seeding... • Seeding... Seeding... Seeding... • High precision (Very little drift) • High precision (Very little drift) • Small measurement volume • Small measurement volume 1

What Are My Options and What Are My Options and How do LDA & PDA How do LDA & PDA Who Sells Them? Who Sells Them? Systems Work? Systems Work? • A pair of coherent laser beams intersect, • A pair of coherent laser beams intersect, forming a fringe pattern in the measurement forming a fringe pattern in the measurement volume. volume. • DANTEC -- LDA, PDA, PIV, PLIF, IPI • As a seed particle passes through the fringe • DANTEC -- LDA, PDA, PIV, PLIF, IPI • As a seed particle passes through the fringe • TSI -- LDA, PDA, PIV • TSI -- LDA, PDA, PIV pattern, the light reflected from the particle pattern, the light reflected from the particle • VioSense -- LDA, Shear Stress, PIV • VioSense -- LDA, Shear Stress, PIV pulsates. pulsates. • The pulsating light is measured by a • LAVision -- PIV, PLIF, IPI • LAVision -- PIV, PLIF, IPI • The pulsating light is measured by a photodetector. photodetector. • The frequency of the pulsating light and the • The frequency of the pulsating light and the fringe spacing is used to compute a velocity. fringe spacing is used to compute a velocity. LDA: u = d/t LDA: Equations LDA: u = d/t LDA: Equations Fringe Spacing Fringe Spacing Frequency of Pulse Frequency of Pulse Velocity Velocity Figure courtesy of DANTEC Measurement Technology Figure courtesy of DANTEC Measurement Technology 2

LDA: System LDA: System LDA: Directional Ambiguity LDA: Directional Ambiguity Configurations Configurations Forward Scatter Forward Scatter • Particles moving forward or backwards will • Particles moving forward or backwards will Difficult to align Difficult to align Lower power requirements Lower power requirements produce a pulsating wave with identical produce a pulsating wave with identical frequencies. frequencies. • An accousto-optical modulator (Bragg Cell) • An accousto-optical modulator (Bragg Cell) can be used to oscillate the fringes in the can be used to oscillate the fringes in the Back Scatter Back Scatter measurement volume. measurement volume. Fiber optic LDA systems Fiber optic LDA systems • Velocity is calculated by subtracting the • Velocity is calculated by subtracting the make alignment a non-issue make alignment a non-issue Larger power requirements modulator frequency from the measured Larger power requirements modulator frequency from the measured frequency. frequency. Figure courtesy of DANTEC Measurement Technology Figure courtesy of DANTEC Measurement Technology LDA: Multiple Components LDA: Multiple Components LDA: Seed Particles LDA: Seed Particles of Velocity? of Velocity? Particle Frequency Response Particle Frequency Response • A different color, λ , is used for measuring • A different color, λ , is used for measuring each velocity component. each velocity component. • Each beam is then separated into three • Each beam is then separated into three colors: colors: green: λ = 514.5 nm green: λ = 514.5 nm blue: λ = 488 nm blue: λ = 488 nm purple: λ = 476.5 nm purple: λ = 476.5 nm • A single probe can be used for 2 components • A single probe can be used for 2 components • A second probe is necessary for 3 components • A second probe is necessary for 3 components Table courtesy of DANTEC Measurement Technology Table courtesy of DANTEC Measurement Technology 3

PDA: Phase - Diameter PDA: Phase - Diameter Phase Dopper Anemometry Phase Dopper Anemometry Relationship Relationship • A particle scatters light from two incident laser • A particle scatters light from two incident laser beams beams • Both scattered waves interfere in space and • Both scattered waves interfere in space and create a beat signal with a frequency which is create a beat signal with a frequency which is proportional to the velocity of the particle proportional to the velocity of the particle • Two detectors receive this signal with different • Two detectors receive this signal with different phases phases • The phase shift between these two signals is • The phase shift between these two signals is proportional to the diameter of the particle proportional to the diameter of the particle Figure courtesy of DANTEC Measurement Technology Figure courtesy of DANTEC Measurement Technology Preconditions of PDA PDA: General Set-up Preconditions of PDA PDA: General Set-up • Beam intersection angle • Beam intersection angle • Optical access to the measurement area • Optical access to the measurement area θ θ • Sphericity of particles • Sphericity of particles • Scattering angle ϕ • Scattering angle ϕ • Homogeneity of particle medium • Homogeneity of particle medium • Elevation angle ψ • Elevation angle ψ • Refractive indices of the particle and the • Refractive indices of the particle and the • Polarization • Polarization continuous medium must usually be known continuous medium must usually be known (parallel or perpendicular to scattering plane) (parallel or perpendicular to scattering plane) • Particle size between 0.5 µm and several mm • Particle size between 0.5 µm and several mm • Shape and size of • Shape and size of • Max. particle concentration is limited • Max. particle concentration is limited detector aperture detector aperture Figure courtesy of DANTEC Measurement Figure courtesy of DANTEC Measurement Technology Technology 4

How does PIV Work? PIV: System Configuration How does PIV Work? PIV: System Configuration • Seed particles are uniformed dispersed • Seed particles are uniformed dispersed throughout a flow. throughout a flow. • Two images are acquired, separated by a • Two images are acquired, separated by a short period of time, t. short period of time, t. • Spatial correlation between image pair is used • Spatial correlation between image pair is used to determine a shift, s, in the particle locations. to determine a shift, s, in the particle locations. • Velocity is computed as, v = s/t • Velocity is computed as, v = s/t Figure courtesy of DANTEC Measurement Technology Figure courtesy of DANTEC Measurement Technology Visualization vs. Visualization vs. PIV: Data Flow PIV: Data Flow Measurement? Measurement? Figure courtesy of DANTEC Measurement Technology Figure courtesy of DANTEC Measurement Technology 5

Stereo PIV: Error Reduction Stereo PIV: Error Reduction Make measurements of flow through the measurement Make measurements of flow through the measurement plane. plane. Figure courtesy of DANTEC Measurement Technology Figure courtesy of DANTEC Measurement Technology 6