Measurements in Fluid Mechanics 058:180:001 (ME:5180:0001) Time & Location: 2:30P - 3:20P MWF 218 MLH Office Hours: 4:00P – 5:00P MWF 223B-5 HL Instructor: Lichuan Gui

2 Lecture 16. Flow visualization with marker techniques

3 Flow visualization Marker techniques Optical techniques - Foreign material added to the fluid flow - Based on refraction index changes - Foreign energy (e.g. heat or electric discharge) added to the fluid flow - Compressible gas flow

4 Flow visualization

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6 Flow visualization with marker techniques Tufts Trailing vortices behind an inclined delta-wing as visualized by a tuft screen - short pieces of yarn, string, or other flexible material - fastened one end on surface or thin support in flow - used in both gaseous and liquid flows - indicate flow direction, separation region, flow instability - performance affected by density, stiffness, and length tuft screen - tufts mounted on mesh of thin wires tuft wand - tufts mounted on a thin rod pin tufts - tufts attached to thin needles - tufts off surface:

7 Flow visualization with marker techniques Surface marking methods - visualize flow direction and flow patterns near solid surfaces Oil-streak method - applicable to both gas and liquid flows - surface coated with a paint consisting of a pigment suspended in mineral oil - pigment coagulates and is deposited on surface in form of small lumps forming short streaklines - commonly used pigment: TiO 2, china clay, lamp black, copier toner, and fluorescent chrysene - commonly used oil: kerosene, diesel oil, and light oil - special coatings used to visualize pressure and temperature distributions Oil-dot method - similar to oil-streak method, for surface with complex shapes Oil-film method - works with interferometry to measure wall shear stress

8 Flow visualization with marker techniques Dyes - visualize liquid flow with injecting dye solution or producing colored region in fluid Dye injection - dye solutions injected into liquid stream through wall orifices or tubes in midstream - dyed stream should be injected at local speed of the flow - dye solution should match the density the fluid Vortex flow behind a yawed cylinder visualized with mixture of ink, milk and alcohol Example: Dye lines in water flow

9 Flow visualization with marker techniques Dyes - visualize liquid flow with injecting dye solution or producing colored region in fluid Electrochemical methods - chemical reaction triggered by electric current Thymol blue (pH indicator) technique flow generated by a rotating cylinder - orange-yellow in acidic environment (pH electric current in water releases hydrogen ions at cathode and leaves OH ions to increase pH vortex street behind a cylinder Tellurium wire techniqueElectrolytic precipitation Others:

10 Flow visualization with marker techniques Dyes - visualize liquid flow with injecting dye solution or producing colored region in fluid Photolytic methods - working fluid contains photochromic indicator - temporarily capable of absorbing photons in red-green range after illuminated by ultraviolet light - visible dark blue region appears when illuminated by white light UV laser Blue laser - photochromic substances can be dissolved in alcohol, kerosene, and other solvents, but not in water - photochromic substances return to original state in a few seconds

11 Flow visualization with marker techniques Hydrogen bubbles Consecutive rows of hydrogen bubbles indicating Velocity profiles a flat plat boundary layer Electric current causes electrolysis in water: Thin metallic wire of  m used as cathode, much thick carbon rod as anode Hydrogen bubbles release to water flow from the thin cathode

12 Flow visualization with marker techniques Smoke, mists, and fogs Common methods for generating smoke Aerosol generation Burning vaporization and condensation Smoke lines around an airfoil model in a wind-tunnel

13 Flow visualization with marker techniques Solid markers, bubbles, and droplets Oil drops in laminar pipe flow Tracer particles in water flow Particle images obtained with stroboscopic illumination Propeller Turbulence Photographed in Stroboscopic Light as Water Passes the Torpedo

14 Homework - Questions and Problems: 2 on page Read textbook on page Due on 10/03