A three-dimensional serpentine microchannel design with a "C shaped" repeating unit is presented in this paper as a means of implementing chaotic advection to passively enhance fluid mixing. The device is fabricated in a silicon wafer using a double-sided KOH wet-etching technique to realize a three-dimensional channel geometry. Experiments using phenolphthalein and sodium hydroxide solutions demonstrate the ability of flow in this channel to mix faster and more uniformly than either pure molecular diffusion or flow in a "square-wave" channel for Reynolds numbers from 6 to 70. The mixing capability of the channel increases with increasing Reynolds number. At least 98% of the maximum intensity of reacted phenolphthalein is observed in the channel after five mixing segments for Reynolds numbers greater than 25. At a Reynolds number of 70, the serpentine channel produces 16 times more reacted phenolphthalein than a straight channel and 1.6 times more than the square-wave channel. Mixing rates in the serpentine channel at the higher Reynolds numbers are consistent with the occurrence of chaotic advection. Visualization of the interface formed in the channel between streams of water and ethyl alcohol indicates that the mixing is due to both diffusion and fluid stirring.

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Passive mixing in a three-dimensional serpentine microchannel

Introduction to heat transfer

By L I Sedov London: Cleaver-Hume Press Ltd Pp xvi + 363 Price 105s This is really two separate books within the same pair of covers First of all Chapters 1-3, some 145 pages, are devoted to the discussion of similarity and dimensional, methods and their application to a variety of problems in mechanics and fluid mechanics

http://iopscience.iop.org/article/10.1088/0031-9112/11/6/008/pdf

Similarity and Dimensional Methods in Mechanics

This article reviews stability and laminar-turbulent transition in high-speed boundary-layer flows, emphasizing qualitative features of the disturbance spectrum leading to new mechanisms of receptivity and instability. It is shown that the extension of subsonic and low-supersonic stability concepts and transition prediction methods to hypersonic speeds is not straightforward. The discussion focuses on theoretical models providing insights into the physics of instability and helping make proper decisions on transition control strategies.

Transition and Stability of High-Speed Boundary Layers

https://engineering.purdue.edu/~aae519/BAM6QT-Mach-6-tunnel/reviewpapers/cone-scram-blt-review-pas-feb04.pdf

Hypersonic laminar–turbulent transition on circular cones and scramjet forebodies

It is well known that the high levels of noise present in conventional hypersonic ground-test facilities cause transition to occur earlier than in e ight. Flight measurements of incoming noise are reviewed and compared with measurements in ground-test facilities, of both conventional and quiet design, at hypersonic and high supersonic speeds. The low noise present in e ight is apparently the reason for the very large transition Reynolds numbers sometimes measured in e ight, when roughness, crosse ow, and other factors are controlled. Design will usually involve consideration of the trend in transition when a parameter is varied. The effect of facility noise on these trends is reviewed. In some cases, the trend of conventional-tunnel data is opposite to the trend in quiet-tunnel data. Thus, transition measurements in conventional ground-test facilities are not reliable predictors of e ight performance, except perhaps in special cases.

Effects of High-Speed Tunnel Noise on Laminar-Turbulent Transition

The effect of roughness on hypersonic boundarylayer transition has been studied for three primary purposes: to trip a laminar layer to turbulence, to determine whether naturally occurring roughness is expected to cause early transition, and to determine the largest allowable roughness that will not affect the location of transition. Roughness is often divided into two classes: isolated roughness in which each protuberance can be considered separately, and distributed roughness similar to sandpaper, in which the roughness elements are many and are not considered separately. The effects of roughness on hypersonic transition are reviewed, considering the physics of the process, known parametric effects, some of the common correlations, and a few case studies. The three or more modes by which roughness can affect transition are outlined. At hypersonic edge Mach nunbers, it requires very large roughness heights to affect transition. Various correlations are often used to estimate the effect of roughness; several of these are described, although none provide good agreement with all the data.

Effects of Roughness on Hypersonic Boundary-Layer Transition

Published e ight data for boundary-layer transition at hypersonic speeds are surveyed. The survey is limited to measurements reported in the open literature and carried out at hypersonic and high-supersonic speeds, on vehicles for which ablation is believed to be negligible or small. The emphasis is on work that may be suitable for validation of advanced transition-estimation methods that are based on simulation of the physical mechanisms, such as e N , the parabolized stability equations, and direct numerical simulations. Brief discussions are presented for each report. Known comparisons to the advanced simulation methods are also presented. Nomenclature Me = Mach number at the boundary-layer edge Res = Reynolds number at transition onset, based on arc length from the leading edge and local conditions at the boundary-layer edge ReT = Reynolds number at transition onset, based on arc length and conditions at the boundary-layer edge ReT/ft = unit Reynolds number per foot, at the boundary-layer edge at transition onset Reµ = Reynolds number at transition, usually onset, based on momentum thickness and conditions at the boundary-layer edge Te = temperature at the boundary-layer edge Tr = recovery temperature at the wall Tw = wall temperature xT = arc length to transition onset, from the nose µc = cone half-angle, deg

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Flight Data for Boundary-Layer Transition at Hypersonic and Supersonic Speeds

Conventional hypersonic wind tunnels and shock tunnels sufier from high levels of freestream ∞uctuations which are typically one to two orders of magnitude above ∞ight levels. These freestream ∞uctuations are generally dominated by acoustic noise radiated from the turbulent boundary layers on the nozzle walls. Although the efiects of the noise are often small, and so can be neglected, this noise often has a dramatic efiect on laminar-turbulent transition on models, and it can have an signiflcant efiect on other phenomena as well. Quiet-∞ow hypersonic tunnels with low noise levels comparable to ∞ight have therefore been sought for more than 50 years. The development of these tunnels is reviewed. The largest efiort was carried out at NASA Langley from the late 1960’s through the middle 1990’s, and resulted in successful quiet tunnels at Mach 3.5 and 6, although the Mach 6 tunnel was later decommissioned. The effort was then carried forward at Purdue University from 1990 through the present, leading to the nowsuccessful Boeing/AFOSR Mach-6 Quiet Tunnel.

Steven P. Schneider

Papers

Hypersonic laminar–turbulent transition on circular cones and scramjet forebodies

Effects of High-Speed Tunnel Noise on Laminar-Turbulent Transition

Effects of Roughness on Hypersonic Boundary-Layer Transition

Flight Data for Boundary-Layer Transition at Hypersonic and Supersonic Speeds

Development of Hypersonic Quiet Tunnels