*3-D Particle Imagery*

Typical mean velocity field measured with 3-D PIV.

Velocimetry is a laser-based fluid flow diagnostic technique used at the Matched-Index-of-Refraction Flow Facility (MIR Lab) at Idaho National Laboratory (INL) in Idaho Falls, Idaho. The technique is used primarily to validate computer simulation codes that are being developed to design, analyze and improve the safety of advanced nuclear energy systems such as the Next Generation Nuclear Plant (NGNP).

Several different velocimetry techniques are used at the MIR Lab. Laser Doppler velocimetry (LDV) is a technique that measures the instantaneous velocity at a specific point in a flow field that is described by the size of the measurement volume produced by the intersection of two different wavelength laser beams. By measuring the flow field at a number of these points an entire boundary layer profile can be displayed. All three velocity components can be measured with a 3-D system. Also, the velocity fluctuations in all three components and the average velocity at each point can determined with LDV.

*Nuclear Applications*

Mean velocity vector field of a model of a typical lower plenum for a very high temperature gas-cooled nuclear reactor measured with 3-D PIV.

A second technique is particle image velocimetry (PIV). PIV measures the instantaneous 2-D velocity at the same time at many points in a flow field that are illuminated by a double-pulsed laser. A typical result for a velocity field could contain as many as 12,000 mean velocity vectors. Each of these vectors represents the average of the two instantaneous velocity components collected by the system. Typically between 750 and 1000 images are collected from which these averages are calculated. The system also determines the instantaneous velocity field for each image-pair, the RMS fluctuations, average and turbulent kinetic energy and all six Reynolds stress components of the flow.

*Validating Models*

A more sophisticated technique of is stereoscopic PIV. Stereo PIV measures the instantaneous three-dimensional velocity field. This system can determine all of the velocity measurements obtained by standard PIV but with the addition of the third velocity component.
High speed PIV is another advanced stereoscopic PIV technique. This system can measure all three velocity components at very high frame rates. High speed PIV is very valuable in determining the temporal behavior of transient or unsteady flows.

The MIR Lab is exploring the addition of planar laser induced fluorescence (PLIF). This is a stereoscopic method of measuring the concentrations and velocity fields and flow visualization of multiphase or multispecies flows.

*Measuring Velocity*