LDV is a technique to measure the velocity of a flow based on the measurement of light scattering caused by particles in the flow. It is a very important and useful technique because it is a noncontact, non-invasive method that can be used for a wide variety of flows. Direct measurements of the kinematics of the fluid motion can be obtained with this method. LDV has offered an incomparable experimental tool to analyse complex aerodynamic flows whose exploration by material probes (hot wires, multi-hole pressure probes) was complicated, hazardous, or impossible. The benefit is that one can observe the smallest of flows surrounding any obstructions; which may not appear when imaging particles within a flow field. It is possible to investigate in great detail 3D, separated, highly fluctuating flows in a velocity range extending from low subsonic conditions to high supersonic flow Mach numbers.
The flow has to be seeded with particles capable to follow the flow such that their movement reflects the motion of the flow well. The particle movement information is collected using two overlapped focused laser beams. The beams form an interference pattern at the intersection. The particles passing through this region scatter light with an intensity that fluctuates at a frequency proportional to the Doppler shift between the incident and scattered light and from this data it is possible to measure the velocity of the particles. Usually one laser colour is used for each velocity vector dimension. Particle flow analysis through LDV are used in a wide variety of applications in science as well as in industry, including aerodynamics in avionics, fuel injection and combustion studies in automotive, inject diagnostics in semiconductor manufacturing and soot measurements in environmental monitoring. The laser source suitable for such applications needs to have long and stable coherence length, low noise and good power stability.
LDV measurements exhibit good accuracy and a high level of repeatability. It is a compliment to PIV and is used for non-intrusive, time-resolved (kHz), point measurements of local fluid velocities and their fluctuations.
We hoped you enjoyed our webinar “Laser Doppler Velocimetry (LDV) – Fundamentals & Applications” as much as we did! If you weren’t able to make it, catch our replay.
See our speaker, Dr. William D. Bachalo’s biography here.
Download Dr. William D. Bachalo’s presentation here.