Advances in microfluidic and nanofluidic technologies have been paralleled by advances in methods for direct optical measurement of transport phenomena on these scales. A variety of methods for microscale flow visualization have appeared and evolved since the late 1990s. These methods and their applications to date are reviewed here in detail, and in context of the both the fundamental phenomena they exploit and the fundamental phenomena they are applied to measure. Where possible, links to macroflow visualization methods are established, and the physical mechanisms underlying these methods are explained.

Microscale flow visualization

A canonical flat-plate turbulent boundary layer with Re θ = 4590 is exposed to a favourable mean streamwise pressure gradient sufficient to cause relaminarization. The favourable pressure gradient is generated by a linear contraction, yielding a peak value of the acceleration parameter of K = 4.4 × 10 −6 which is sustained for approximately 13 local boundary layer thicknesses. The relaminarization process is characterized by an extensive series of mean flow and turbulence measurements obtained at several representative streamwise locations. In anticipation of the loss of standard log-law behaviour, the local wall shear stress is directly measured using the oil-film interferometry technique. Mean flow measurements show a systematic variation in the Karman and additive constants with applied streamwise strain rate. The series of measurements also indicate an apparent decoupling of the outer and near-wall regions of the accelerating boundary layer. In accord with this, conditional measurements show that fourth-quadrant sweep events are virtually eliminated, while much less frequent but larger-amplitude near-wall second-quadrant ejection events remain. The reduction in fourth-quadrant sweep events is matched by an observed increase in near-wall third-quadrant events. The consequent reduction in near-wall Reynolds stress correlation and associated cross-stream momentum transport results in a large reduction in c f for the relaminarized flow.

An experimental investigation of a highly accelerated turbulent boundary layer

An Ultrasound Doppler Velocity Profile method (UVP method) has been developed for fluid mechanical measurements in physics and engineering. The principle of the method is straightforward; echography and Doppler effect. This method was originally developed in medical engineering to measure blood flow. However, due to various constraints arising from its direct application to the human body, the measurement method never realized its full potentiaL We subsequently extended this method to non-medical flow measurements and developed specific application systems. Although the initial motivation was to find a new flow measurement method for liquid metal flows, the method itself was found to be quite useful to flow measurements in general and additionally through years of use has gradually become accepted as a tool to study the physics and engineering of fluid flow. In the sections to follow we \vill describe the measurement method and review the applications to date.

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Ultrasonic Doppler method for velocity profile measurement in fluid dynamics and fluid engineering

Granular flows occur widely in nature and industry, yet a continuum description that captures their important features is yet not at hand. Recent experiments on granular materials sheared in a cylindrical Couette device revealed a puzzling anomaly, wherein all components of the stress rise nearly exponentially with depth. Here we show, using particle dynamics simulations and imaging experiments, that the stress anomaly arises from a remarkable vortex flow. For the entire range of fill heights explored, we observe a single toroidal vortex that spans the entire Couette cell and whose sense is opposite to the uppermost Taylor vortex in a fluid. We show that the vortex is driven by a combination of shear-induced dilation, a phenomenon that has no analogue in fluids, and gravity flow. Dilatancy is an important feature of granular mechanics, but not adequately incorporated in existing models.

A dilation-driven vortex flow in sheared granular materials explains a rheometric anomaly.

Control of the coupled double pendulums system

Properties of a relaminarizing turbulent boundary layer under a favorable pressure gradient

An Experiment on a Taylor Vortex Flow in a Gap with a Small Aspect Ratio (Instability of Taylor Vortex Flows)

Both Taylor vortex flows in a symmetric or asymmetric system exhibit various patterns (cell modes). They can be classified by the process of cell formation, the number of cells and the direction of flow for the cell, into primary modes or secondary modes, and normal modes or anomalous modes. Following the previous report in which flows in a symmetric end condition were classified, in the present work, for flows in an asymmetric end condition, the Reynolds number at which a secondary mode bifurcates into another mode is experimentally investigated, and the bifurcation of the Taylor vortex flows in an asymmetric system when the Reynolds number is gradually decreased is presented in a bifurcation diagram.

An experiment on a taylor vortex flow in a gap with a small aspect ratio bifurcation of flows in an asymmetric system

An experiment on a taylor vortex flow in a gap with a small aspect ratio :bifurcation of flows in a symmetric system

Coherent structures in the near field of a three-dimensional jet have been investigated. Experiments were carried out for a free jet issuing from a square nozzle using a water channel. Instantaneous velocity profiles were obtained in the axial and radial directions by using an ultrasonic velocity profile (UVP) monitor. Axial variations of dominant time-scales of vortex structures were examined from one-dimensional wavelet spectra. Wavenumber-frequency spectra were calculated by two-dimensional Fourier transform along the axial direction in a mixing layer, and it was found that a convective velocity of flow structures was nearly constant independently of their scales in space and time. Coherent structures in the axial direction were investigated in terms of proper orthogonal decomposition (POD). Eigenfunctions are similar to a sinusoidal wave, and reconstructed velocity fields by the lower-order and higher-order POD modes demonstrate large-scale and smaller-scale coherent structures, respectively.

Shintaro Yamashita

Papers

Properties of a relaminarizing turbulent boundary layer under a favorable pressure gradient

An Experiment on a Taylor Vortex Flow in a Gap with a Small Aspect Ratio (Instability of Taylor Vortex Flows)

An experiment on a taylor vortex flow in a gap with a small aspect ratio bifurcation of flows in an asymmetric system

An experiment on a taylor vortex flow in a gap with a small aspect ratio :bifurcation of flows in a symmetric system

The ultrasonic velocity profile measurement of flow structure in the near field of a square free jet