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Mie scattering

Mie scattering is elastic scattered light of particles that have a diameter similar to or larger than the wavelength of the incident light. The Mie signal is proportional to the square of the particle diameter.
 
Mie scattering is much stronger than Rayleigh scattering and, therefore, a potential source of interference for this weaker light scattering process. There is a strong angular dependency of the scattered intensity especially for smaller particles which has to be considered for successful Mie imaging experiments. Mie scattering is often used to measure flow velocities applying Particle Image Velocimetry (PIV).

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Particle Image Velocimetry (PIV)

What is PIV?

Particle image velocimetry is an optical method of flow visualization used in education and research. It is used to obtain instantaneous velocity measurements and related properties in fluids.

In Particle Image Velocimetry (PIV) light scattering particles are added to the flow. A laser beam is formed into a light sheet illuminating seeding particles twice with a short time interval Δt. In 2D-PIV the scattered light is recorded onto two consecutive frames of a high resolution digital camera. 

Stero Particle Image Velocity (PIV)

In Stereo-PIV two cameras at different observation angles are used to measure also the third (out-of-plane) component of the flow velocity in the light sheet.


For velocity calculation the particle image of each camera is subdivided into small interrogation windows. The average particle displacement within an interrogation window is determined by cross-correlation followed by the localisation of the correlation peak. From the known time difference Δt and the measured displacement in each direction the velocity components are calculated. Perspective correction, distortion compensation and image mapping of the two views are taken care of by Self-Calibration procedures. Advanced multi-pass image deformation techniques are used for higher accuracy and spatial resolution.

Spatial & Temporal Derivatives

From one velocity field a range of spatial derivatives can be calculated such as vorticity and shear stress. Ensemble statistics provide additional information like turbulent kinetic energy or Reynolds stresses. Time-resolved velocity fields recorded with high-frame-rate cameras and high frequency lasers allow for deeper dynamic insights about flow field evolution, fluid element trajectories, acceleration and turbulence statistics.

Tomographic PIV

From one velocity field a range of spatial derivatives can be calculated such as vorticity and shear stress. Ensemble statistics provide additional information like turbulent kinetic energy or Reynolds stresses. Time-resolved velocity fields recorded with high-frame-rate cameras and high frequency lasers allow for deeper dynamic insights about flow field evolution, fluid element trajectories, acceleration and turbulence statistics.

Contact us to find out how PIV can aid in your current & future researches.

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