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Understanding Fluid Flows
• Dynamics causing the flow (forces)– Pressure, buoyancy, inertia, viscous
forces, surface forces…. Etc.
• Effects of these forces on the flow (Flow and fluid properties)– Velocity, acceleration, vorticity– Viscosity, temperature, scalar
concentration, density …. Etc.
Why Flow Visualization
• Understanding flow phenomenon• Verifying model or theory results• Easier measurements for designing• To get a priori knowledge of
solution
What we need to visualize?
• Stream lines– a line, tangent to which at any given instant
is the velocity vector at that point
• Streak lines– locus of particles which have passed through
a prescribed point during a specified time interval
• Path lines– locus of points traversed by a given fluid
particle during some specified time interval
• Time lines– line joining different adjacent points at any
instant of time
Methods of flow visualization
• Gas flows– Smoke visualization– Particle– Shadowgraphs and Schlieren
• Liquid flows– Dye– Shadowgraphs and Schlieren– Particles– Bubbles– Fluorescence, phosphorescence
Some gas flow visualization images
Smoke visualization
Top- flow past aerofoilSide- laminar smoke jet
Jets and Plumes
•Top left- flow below an ice cube in water
•Top right- near field of a jet
•Side- jet and flame jet
Measurements in Flows
• Velocity– Conventional methods
• Pitot tubes• Hotwire anemometers
– Local steady state measurements
• Vane anemometers
– Non intrusive methods• Laser Doppler Velocimetry• Particle Image Velocimetry• Particle Tracking Velocimetry
• Concentration– Conventional
• Sample analyzers– Collecting samples by introducing samplers in
the flow
• Conductivity probes– Depends on the conductivity of the scalar being
used
– Optical methods• PH Sensitive dyes• Light extinction• Scattering• Fluorescence methods.
If = IeCL,
Where = Quantum efficiency of the dye = Attenuation coefficientL =Spatial extent of illuminated volume under consideration
•Temperature
–Thermister probes
– Change in resistance with temperature
–Thermocouples
– Voltage produced at junction of two metals depends on temperature
–Temperature Sensitive Paints
•Density
–From Absorption/ scattering/refractive index
–Capacitance/vibrations