A novel system to image and reconstruct a 3-dimensional map of the refractive index based on the diffraction of light through a transparent sample is presented. This method is tested and validated on computer-generated data sets. The proposed system is an advanced variation of an imaging technique used in engineering for the study of aerodynamics. This method, which is termed Reference Image Topography, is used to reconstruct the water/air interface of the free surface in fluid dynamics studies. This surface profile is reconstructed by comparing an image of a random pattern viewed through the transparent free surface against a reference image, to determine the change in the refractive index caused by changes in the height. The proposed system is highly sensitive and capable of imaging intricate features in the transparent sample that are of low contrast when imaged with other imaging methods. For each projection, the change in direction of the light passing through the sample when placed in between the light source and the imaging system, can be related to the line integral for the change in refractive index across the sample. Utilizing multiple projections, a 3- dimensional map of the refractive index of the sample is reconstructed with computed tomography.
An approach is studied for side-wall heat transfer enhancement in the magnetohydrodynamic flow of fluid in a rectangular duct that is damped by a strong transverse magnetic field. The mechanism employs the rotational oscillation of a cylinder placed inside the duct to encourage vortex shedding, which promotes the mixing of fluid near a hot duct wall with cooler fluid in the interior. The effectiveness of the heat transfer enhancement is investigated over a wide range of oscillation amplitudes and forcing frequencies. The motivation for exploring this mechanism is inspired by the transient growth response of this flow, which indicates that the optimal disturbances feeding the vortex shedding process are localized near the cylinder, and are characterized by an asymmetrical disturbance with respect to the wake centreline. The results show that a considerable increase in heat transfer from the heated channel wall due to rotational oscillation of the cylinder can be achieved, with the maximum enhancement of more than 30% over a zone extending 10d downstream of the cylinder. As the angular velocity amplitude of oscillation is increased, the range of oscillation frequencies for effective enhancement is widened, and the frequency at which the peak Nusselt number occurs is shifted slightly to lower frequencies. As the amplitude is increased, the formation of strong discrete wake vortices draws fluid from the wall boundary layers into the wake, enhancing heat transfer. The effect of oscillation amplitude on the distribution of local Nusselt number \documentclass[12pt]{minimal}\begin{document}$\textit {Nu}_w$\end{document}Nuw along the heated wall is significant. With an increase in Reynolds number, scope for additional heat transfer enhancement is possible.
The link to the online abstract of this manuscript, accepted in Phys. Rev. Fluids, is this https URL.
A subcritical route to turbulence via purely quasi-two-dimensional mechanisms, for a quasi-two-dimensional system composed of an isolated exponential boundary layer, is numerically investigated. Exponential boundary layers are highly stable, and are expected to form on the walls of liquid metal coolant ducts within magnetic confinement fusion reactors. Subcritical transitions were detected only at weakly subcritical Reynolds numbers (at most $\approx 70$% below critical). Furthermore, the likelihood of transition was very sensitive to both the perturbation structure and initial energy. Only the quasi-two-dimensional Tollmien-Schlichting wave disturbance, attained by either linear or nonlinear optimisation, was able to initiate the transition process, by means of the Orr mechanism. The lower initial energy bound sufficient to trigger transition was found to be independent of the domain length. However, longer domains were able to increase the upper energy bound, via the merging of repetitions of the Tollmien-Schlichting wave. This broadens the range of initial energies able to exhibit transitional behaviour. Although the eventual relaminarization of all turbulent states was observed, this was also greatly delayed in longer domains. The maximum nonlinear gains achieved were orders of magnitude larger than the maximum linear gains (with the same initial perturbations), regardless if the initial energy was above or below the lower energy bound. Nonlinearity provided a second stage of energy growth by an arching of the conventional Tollmien-Schlichting wave structure. A streamwise independent structure, able to efficiently store perturbation energy, also formed.
The design of vortex promoters in a heated-wall duct is often limited by the considerations of practicality, especially in complex systems such as fusion blankets. The present study investigates the use of current injection to invoke a street of vortices in quasi-two-dimensional high transverse magnetic field magnetohydrodynamic duct flows to enhance instability behind a cylinder. The intent is to generate intensive flow vorticity parallel to a magnetic field downstream of a field-aligned cylinder. Electric current enters the flow through an electrode embedded in one of the Hartmann walls, radiates outward, imparting a rotational forcing around the electrode due to the Lorentz force. The quasi-two-dimensional nature of these flows then promotes a vortical rotation across the interior of the duct with axis aligned to the magnetic field. The hot and cold walls are parallel to the magnetic field. Electric current amplitude and pulse width, excitation frequency and electrode position are systematically varied to explore their influences on the convective heat transport phenomenon. This investigation builds on a recommendation from previous work of Bühler ( J. Fluid Mech. , vol. 326, 1996, pp. 125–150) dedicated to understanding of the flow stability in a similar configuration. This study provides supportive evidence for the use of current injection as an alternative to the conventional mechanically actuated turbuliser, with heat transfer almost doubled for negligible additional pumping power requirements.
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