Talk:Transition to turbulence
Transition is the dynamical process(es) via which a flow field changes from the laminar state to the turbulent one. It is a complex problem mathematically and analytical solutions can only be obtained for very few simple situations. Physical and numerical experiments are then needed to close the gap. The field is technologically important because of the potential to advance or delay transition. Early transition results in more vigorous mixing and resistance to boundary layer separation, which leads to lower pressure drag. Delaying transition keeps the skin-friction drag from the phenomenally high levels associated with the turbulent state.
Two prominent researchers in the field authored Scholarpedia’s entry on Transition. It is written from a dynamical systems theory viewpoint. Other aspects such as transition control or physics are not emphasized here. Although relatively well written, the piece still requires some editing for spelling, grammar, etc. Encyclopedias typically also provide a different level of editing to ensure that all entries are for the most part free of jargon and, as a result, accessible to the non-specialists. This was not done in this case. I presume that is why Scholarpedia is free and McGraw-Hill Encyclopedia of Science and Technology, say, is not.
The entry is free of technical errors, and is acceptable albeit with a slight concern. Who would benefit from reading the brief treatise? If it is the expert in this particular field, then he/she presumably already knows that stuff. If it is a scientist or engineer who is new to transition and even fluid mechanics, then this entry is not accessible to him or her. Writing the piece in terse bullet format does not help either. Witness as one example the sentence: “The simplest examples are the forward fork bifurcation between steady states and the forward Hopf bifurcation between a steady state and an oscillatory state.” Who would know what a forward fork bifurcation is? I can understand this writing style in a journal paper, but an encyclopedia entry?
Mohamed Gad-el-Hak, Virginia Commonwealth University
Reviewer B (formerly C)
The Scholarpedia article "Transition to turbulence" is written by two leading experts the field. Turbulence highly enhances the mixing properties of the fluid with applications in various domains. Understanding theoretically the transition to turbulence, and its control, is thus very important. Concerning the transition control, the authors could give some light in particular situations, as in closed systems.
At some point it should be emphasized that the theoretical viewpoint uses dynamical system concept, to be pointed to other Scholarpedia's entries, as well as for other technical notions involved in this article.
From the physical viewpoint, I have some suggestions:
The authors could maybe write, at a basic level, something about the occurrence of turbulence whenever the energy input exciting motions in the fluid overcomes the viscous resistance provided by the fluid, as measured/represented by the Reynolds number.
In the part "Scenarios in open flows", to give an idea of the importance of the symmetry breaking in the transition to turbulence, the example of a flow past a cylinder (wake) could be use with the loss of several symmetries as the Reynold number is increased (left-right, up-down, time translation, translation in the spanwise direction).
It could also be noted that in free turbulent flows, like turbulent boundary layers, one of the main feature is the sharp boundary between rotational and irrotational regions.
There are some typo errors.