A laser velocity gradient and vorticity probe for turbulent shear flows

A laser-based probe for the direct, non-intrusive measurement of temporally- and spatially-resolved velocity gradient and vorticity was developed and demonstrated. The optical probes developed were used in turbulent two-stream mixing layers as well as laminar and turbulent boundary layers for the measurement of several velocity gradients and spanwise vorticity. The measurement technique is based on the collection and direct heterodyning of coherent light scattered from particles in two adjacent locations allowing the determination of a velocity gradient. The beat frequency of the heterodyned light gives the difference in the Doppler shift and is proportional to the velocity difference of two points. This frequency is analyzed using a conventional laser Doppler velocimetry (LDV) signal processor (in the present, a burst spectrum analyzer). The angle between the laser beam and the direction of the scattered light determines the measured component of velocity. Therefore, a component of the vorticity vector is measured by using two sets of transmitting and collecting optics, focused at a single location, along with two LDV processors. The technique is non-intrusive, straightforward and relatively inexpensive since it uses standard, off-the-shelf optical and electronic components. It also allows the development of a compact probe with integrated instrumentation. In the present research, the time-frozen measurements of ∂u/∂ y, ∂v/∂x, ∂ v/∂y in the boundary and mixing layers were carried out using the laser velocity gradient (LVG) probe. Time-frozen measurements of the spanwise component of vorticity were also carried out using the laser vorticity probe (LVP) which is composed of, essentially, two sets of LVG probes. LVG and LVP measurements are compared to LDV measurements and data available in the literature. These measurements demonstrated the viability of the new measurement technique as a research tool. Analyses and several systematic measurements were also performed to determine the data rates and the intrinsic measurement uncertainty associated with the technique.