Shahram Pouya

Vortices in the wake of a femtosecond laser filament.

We report on the experimental observation of fluid flow caused by propagation of femtosecond filaments in dry air. We find that the ionization of the medium deposits a non-negligible amount of heat, which creates vortices in a semi-confined glass cylinder. We confirm the influence of thermal gradients on vortex formation by the use of a heated wire in a similar configuration.

Fluid flow vorticity measurement using laser beams with orbital angular momentum

Vorticity is one of the most important dynamic flow variables and is fundamental to the basic flow physics of many areas of fluid dynamics, including aerodynamics, turbulent flows and chaotic motion. We report on the direct measurements of fluid flow vorticity using a beam with orbital angular momentum that takes advantage of the rotational Doppler shift from microparticles intersecting the beam focus. Experiments are carried out on fluid flows with well-characterized vorticity and the experimental results are found to be in excellent agreement with the expected values. This method allows for localized real-time determination of vorticity in a fluid flow with three-dimensional resolution.

A multi-time-delay approach for correction of the inherent error in single-component molecular tagging velocimetry

A technique is proposed to correct the bias error associated with one-component molecular tagging velocimetry (1c-MTV). 1c-MTV is typically used to capture the velocity component normal to a single or multiple lines of tagged fluid molecules with very high spatial resolution. However, the measurements are affected by an inherent error, which arises from the presence of a velocity component parallel to the lines. In the present study, a Taylor-series-based approach is used to derive a general mathematical expression for this error. The derived expression, which is validated using simulated MTV measurements in laminar channel flow with suction/blowing, is used as the basis for introducing a correction method involving the acquisition of MTV images at multiple time delays. To examine the effectiveness of this ‘multi-time-delay’ approach for correcting 1c-MTV data, an experiment is conducted whereby a known bias error is deliberately imposed on measurements in laminar channel flow. The corrected measurements agree with the true velocity profile to better than 2%, thus validating the correction method.

Characterization of vorticity in fluids by a spatially shaped laser beam

Measuring the speed and direction of vortices is of great importance in fluid dynamics. We report on the use of a CW laser beam with a superposition of Laguerre-Gaussian (LG) modes generated by a phase mask imprinted on a two-dimensional spatial light modulator. The shaped beam is then guided and scattered of a sample which is rotated; the rotational frequency is extracted from spectral analysis of the scattered light. This method allows for virtually real-time determination of vorticity characterization in a fluid.