Optical Methods (flow visualization, fluid density, concentration and temperature)

Optical Methods

(flow visualization, fluid density, concentration and temperature)

Václav Uruba

CTU Prague, AS CR

VISUALISATION

Visualisation

• Surface particles

• Flow particles

– Volume particles distribution + light sheet – Volume illumination + localized particles

• Fluid density

Visualisation

• We see pathlines and not streamlines

• Laminar: pathlines  streamlines

• Turbulent: pathlines  streamlines

Flow Visualizations

• On a solid surface

– Oil flow – Sublimation – Sand erosion – Wall tufts – Oil dots – Oil streaks – Wall dye steaks

• On a free surface (water table)

– Surface bubbles

– Hydraulic analogy (surface waves) – Hydraulic analogy (Water colors)

• In a 3D space

– Tufts in air (grid, pole) – Smoke in air

– Colored smoke (smoke wire) – Laser sheet & smoke

– Dye in water: color streaks, or all water – Hydrogen bubbles in water

Oil Flow

“surface streamlines” , i.e. surface shear stress lines top view – VKI L2-A wind tunnel, 28cm x 28cm, 40 m/s

Coloured Oil Flow

“surface streamlines”, separation in front of cylinder on flat plate

Sand Erosion for Urban Microclimate

Sand erosion zones , i.e. zones of high intensity ground wind top view – VKI L1-B & L2-B wind tunnels

Courtesy of D.Olivari, ~1990 VKI L1-B wind tunnel, 3 m x 2m

Courtesy of J. Van Beeck, ~2006

VKI L2-B wind tunnel, 35 cm x 35 cm

Surface Tufts

Tufts indicating wind direction

air intake in turbine engine pod VKI L1-A wind tunnel, 3m diam, 60 m/s

Surface Tufts

Tufts direction indicating position of stagnation point

VKI L1-A wind tunnel, 3m diam, 60 m/s Leading Edge of AEA B-737 wing

Oil Streaks

Oil streaks indicating velocity

Hypersonic delta wing VKI Longshot tunnel, 42cm diam, Mach = 14

Coloured Oil Streaks

Coloured oil streaks indicating flow pattern

Squealer-tip turbine blade, VKI CT-2 cascade facility

Courtesy of T. Hofer & T. Arts, 2009

Water Tunnel – Coloured Streaks

Coloured wall streaks – Triangular fin on flat plate VKI Water Tunnel, 12cm x 24 cm - 0,1 m/s

Water Table – Surface Soap Bubbles

VKI Christmas postcard, test by M. Carbonaro, ~1978 Turbulent unsteady wake in vortex-shedding flowmeter

Water Table – Surface Soap Bubbles

Turbulent wakes downstream water collectors in cooling towers

Supercritical Water Table – Surface Waves

Choked and non-choked hypersonic intake

Real-life “Water Table” – Surface Waves

Flood in Milano, “Corriere della Sera” sept.2010 Front bow shock, side shocks at wheels and wake recompression shocks

Tufts in air

Hand-held stick with wool tuft,

to explore flow field

Smoke Streaks

Flow across open door due to temperature

difference between rooms

Smoke Streaks

For advertising purposes only…obvious streamline behaviour

Smoke plumes

Forced-draft cooling tower plumes VKI L1-A wind tunnel, 3m diam.

Smoke Wire – Coloured Smoke Sheets

Ref. C. Sieverding & P. Van Den Bossche, J. Fluid Mechanics, vol 134, 1983 Secondary Flows in Turbine Cascade - VKI special set-up

Laser Sheet Enhanced Smoke Flow

Vortical wake behind a delta wing

VKI L-7 Wind Tunnel, 16cm x 16 cm, ~20 m/s

Dye Streaks in Water

Bluff protuberance on flat plate – landing gear pod VKI Water Channel 12 cm x 24 cm, ~2 cm/s

Dye Streaks in Water

Bluff protuberance on flat plate – landing gear pod VKI Water Channel 12 cm x 24 cm, ~2 cm/s

Dye Streaks in Water

Bluff protuberance on flat plate – landing gear pod VKI Water Channel 12 cm x 24 cm, ~2 cm/s

Dye Streaks in Water Tunnel

Body vortices on leeward side of missile at incidence – side view VKI Water Tunnel, 12 cm x 24 cm, ~5 cm/s

Dye Streaks in Water Tunnel

Body vortices on leeward side of missile at incidence – top view VKI Water Tunnel, 12 cm x 24 cm, ~5 cm/s

Fluorescent Dye

Streak & Laser Sheet

Karman vortex street behind circular cylinder

VKI Water Tunnel, 12cm x 24 cm,

~5 cm/s

Hydrogen Bubbles in Water

Wakes behind prongs of a scaled-up hot wire probe VKI Water Channel, 12cm x 24 cm, 10cm/s

Hydrogen Bubbles in Water

“Time-Lines” of periodically excited transitional boundary layer VKI Water Tunnel, 12cm x 24 cm, ~5cm/s

Vibrating ribbon 85Hz, time-lines 30 Hz

DENSITY EVALUATION

Density evaluation

• For variable density flows

– High velocities – compressibility – Temperature fields

• Methods

– Shadowgraph – Schlieren

– Interferometry

Refraction index

• Density  Refraction index

• Clausius-Mosotti:

Refraction index

Density Gladstone-Dale constant

Air: K = 0,226 cm³/g

1

n   K 

Shadowgraph

Dvořák 1880 Density 2^nd spatial derivative

Schlieren

Density 1^st spatial derivative Foucalt 1859, Toepler 1864

Mach-Zehnder interferometer

Density distribution Zehnder 1891, Mach 1892

Schlieren

LASER SHEET TECHNIQUES

Laser sheet techniques

• Particle Image Velocimetry (PIV)

• Laser-Induced Fluorescence (PLIF)

• Laser-Induced Incancescence (LII)

• Interferometic Particle Imaging (IPI)

• Rayleigh Thermometry

LASER INDUCED FLUORESCENCE

LIF, PLIF

Planar Laser Induced Fluorescence

Laser-Induced Fluorescence

Excited State

Ground State

Photon

Absorption

Excited Molecule

Emission

Fluorescence

• Species selective measurements (OH, formaldehyde, fuel tracers, etc.)

Laser-Induced Fluorescence

0 0,2 0,4 0,6 0,8 1,0

200 250 300 350 400 450 500 550

Wavelength

Fluorescence spectrum

N or m a lis ed in te ns ity

Bandpass filter Laser

line

600

Absorption spectrum

Detected

LIF

Residual

laser light

Planar Laser-Induced Fluorescence

Light Sheet Optics

Detector

Flame

Combustion LIF system

CCD Camera

Burner

Nd:YAG Laser

Dye Laser Sheet

Optics UV Camera

Lens Optical

Filter

Image Intensifier

PLIF

instantaneous

mean

variance

LIF in Combustion

• Gas with chemical reactions

• Production of radicals

• Qualitative concentration of radical – OH

– CH – NO – etc

• Concentration of larger molecules/tracers – Formaldehyde

– Acetone – etc

LASER-INDUCED INCANCESCENCE

LII

Laser-Induced Incancescence

• Soot particles are heated up by laser radiation

• The increased particle temperature results in increased emission of Plank radiation

Time (ns)

Size decreases

LII intensity (a.u.)

0 100 200 300 400 500

Laser diagnostics in an IC engine

Quantitative LII

Soot-volume-fraction in a Diesel engine

Work done at Combustion Physics, Lund University, Sweden

Soot volume fration (ppm) • Soot formation at different EGR rates

• Soot formation at different piston bowl geometries

INTERFEROMETIC PARTICLE

IMAGING

IPI

• Interferometric detection of light scattered and refracted from individual particles

• Evaluation of particles size

• Could be combined with PIV

IPI principle

Focused image Defocused image

Processed Validation Focused

Defocused

RAYLEIGH THERMOMETRY

What is Rayleigh scattering?

• Elastic scattering of light

- Mie scattering: D_particle >> l - Rayleigh scattering: D_particle < l

• All molecules contribute to the Rayleigh scattering, thus species specific

measurements are not possible.

• Blue light is scattered more efficiently than red light

Rayleigh Thermometry

• The Rayleigh signal is dependent on:

- Laser intensity

- Scattering cross section - Number density

• If species composition and

pressure are known in the gas the gas temperature can be

determined from imaging of the Rayleigh scattering.

Rayleigh Thermometry results

Takes into account:

• Scattering cross-section

• Pressure

• Laser pulse energy

A last good advice...