Laser Doppler Velocity and Vorticity Measurements in Turbulent Shear Layers.
A Laser Doppler Velocimeter (LDV) system was developed to measure the instantaneous spanwise vorticity, -(omega)(,z), in a turbulent shear layer. It was necessary to design and fabricate the LDV optics and processing electronics, as no commercially available LDV systems met the specifications of measuring the velocity of four closely spaced points to the requisite accuracy. Measurements were also made of the instantaneous u, v, u', v' and -u'v'. The instantaneous vorticity was processed to obtain an estimate of its probability density function, from which the mean and rms values were estimated. It was also possible to separate the irrotational fraction of the flow (-(omega)(,z) (DBLTURN) 0) from the rotational (intermittent) fraction of the flow (-(omega)(,z) (NOT=) 0). The development of the intermittency profiles, based on vorticity, as a function of the downstream distance from the splitter plate was studied. A notable feature is that the vorticity is found to have values opposite the mean sense of rotation, i.e., -(omega)(,z)(t) < 0, a significant fraction of the time. Additionally, a detailed study was performed to evaluate the approximation of -(PAR -DIFF)v/(PAR-DIFF)x, in terms of various local temporal derivatives (PAR-DIFF)v/u(y)(PAR-DIFF)t. The optimum choice for u(y) can be found and is influenced by the relative local convection velocities of the small and large scale structures.
- Pub Date:
- Physics: Fluid and Plasma