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Where are fluid mechanicians?

Zhigang Suo's picture

iMechanica has just passed the milestone of 1000 registered users, and showed no sign of slowing down. Despite all the enthusiasms among a growing number of active users, you might have noticed that iMechanica is missing a powerful community: the community of fluid mechanicians.

Watching my colleagues Howard Stone, Michael Brenner and L. Mahadevan, I find the field of fluid mechanics just as exciting as the field of solid mechanics. The exploration of flow in small devices, as well as in cells and tissues, has just opened new opportunities. There are plenty of other challenges in fluid mechanics at all size scales.

Phenomena like fluid-solid interactions (e.g., the flutter of a flag) and deformation of soft matters (e.g., liquid crystals and hydrogels) have reminded us of a simple fact: the division of disciplines is all in our head, and has no more significance than chopping a large body of knowledge into chunks convenient for teaching and learning. But such division makes little sense when you explore new phenomena in nature and in technology.

I'd like to start this new forum topic to explore

  1. historical reasons of this divergence of the communities, and
  2. ways to use iMechanica to bring the two communities back together.

It will be great if you can share your expertise and thoughts on this topic.

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Henry Tan's picture

Explosion engineering is a field that needs the combination of both fluid mechanics (shockwave in air) and solid mechanics (structure responses). Seems few people here are interested in this field.It is definitly not a hot point like nano- and bio- mechanics, but actually more interesting and unsolved mechanics problems there. And more exciting applications.

Hassan Aref's picture

Zhigang has raised an important issue.

Probably the simplest explanation is that the increased specialization in the field of mechanics, as in so many other fields, has created subgroups who really don't talk as much as they should, and who consequently don't know one another. Many fluid mechanicians probably don't know about the iMechanica initiative and, since it was taken by a solid mechanician, may think that it is not for them. Add to that the simple fact that we're all busy and - maybe - that is the end of the "mystery".

But, of course, there is probably more to it.

Let me mention a couple of things that I think contribute to the "polarization" other than the purely human nature aspects of "birds of a feather" wanting to "flock together".

We gravitate to different professional societies. The American Physical Society's Division of Fluid Dynamics is really the focal point for fluid mechanics researchers in the US and its annual meetings are a big draw. Other professional societies, such as AIAA, ASME, AIChE, SES, etc., also have some fluid mechanics represented, but not in the same concentrated way that APS/DFD does. There's an immediate coupling between APS/DFD and Physics of Fluids, a premier fluid mechanics journal. The major fluid mechanics prizes belong to APS/DFD. And so on.

We don't publish in the same journals. Fluid mechanics has been blessed with having a relatively small number of of very high quality journals that attract some of the very best papers in the field. Journal of Fluid Mechanics has been pre-eminent for a long time, due in large measure to the outstanding leadership and dedication provided by its founding editor G. K. Batchelor. Physics of Fluids is today of comparable stature thanks in no small part to the dedicated work of A. Acrivos who served as editor for a number of years and really brought that journal upward and forward. (No slight intended to current editors, but the contributions tend to be more visible in hindsight.) The third important journal or serial is Annual Review of Fluid Mechanics, again lovingly shepherded by Milton van Dyke. It has provided a great focal point for our field for many years.

The "mixed" journals, I would argue, such as J. Appl. Mech., have never attained similar standing or impact in the field of fluid mechanics, whereas I believe they are very well regarded in solid mechanics. So, even when it comes to publishing, the two subfields meet only on occasion.

And, of course, in our everyday lives we deal with curricular matters, admit graduate students, review faculty candidates, and conduct seminars largely independently. It is the sad consequence of specialization that I mentioned before. One might even encounter the odd fluid mechanics faculty member who believes that having a graduate student take a solids course is a waste of time and vice versa...

There is some positive news. Organizations such as the US National Committee on Theoretical & Applied Mechanics and IUTAM do still try to promote a cohesive view of mechanics. They realize that there is more that binds us together than there are things driving us apart. In essence we all work on a description of phenomena where matter is treated as a continuum, or where that viewpoint is maintained for as long as it makes sense (i.e., until nanoscience and -technology take over). More than that, I think we share an interest in a scientific methodology, a range of analytical, numerical and experimental techniques, and probably a philosophical outlook on how to do science and engineering.

So, Zhigang's notion of finding ways to bridge the gaps between the two communities are right on the money. Within IUTAM I know that we are seeing more and more symposia and sessions at congresses that don't fit neatly into the rubrics "solid", "fluid" or "dynamics". The subjects that sit at the interstices of these fields, or that bridge them, are all very, very exciting - and rather demanding. The new mechanics curriculum probably ought to include subjects such as statistical physics, quantum mechanics, and topological methods, just to mention a few of my personal favorites. The programs and schools that are early adopters of such cross-cutting subjects will, in my view, stand to gain a lot in terms of attracting the best and the brightest graduate students, funding, and the opportunity to make seminal discoveries. (And I haven't even mentioned integrating biology into the curriculum...)

Clearly, Zhigang hit one of my hot buttons... The demise of the TAM department at Illinois, where we were realizing some of the ideal integration I have described above was a sad, sad story, both at a personal level (because I had so much time and effort involved in it) and at a professional level. One reason it happened, I am afraid, was that there was not a strong voice from the US mechanics community opposing the move. Imagine, for example, what would happen if a university wanted to shut down its mathematics department (you will recall an actual instance of this in the not too distant past). Mechanics as a science is, in my view, somewhat under siege generally in the US and around the world. Mechanics as a service discipline will, no doubt, continue to thrive. For those fluid mechanicians who believe that mechanics (the science) deserves its academic home in every serious research university, there is actually much to gain from finding our intellectual partners in solid mechanics.

Hassan Aref
Reynolds Metals Professor
Department of Engineering Science & Mechanics
Virginia Tech, Blacksburg, VA 24061, USA

Niels Bohr Visiting Professor
Center for Fluid Dynamics
Technical University of Denmark, Lyngby, DK-2800, Denmark

Zhigang Suo's picture

Dear Hassan:

Your long and perceptive comments will go a long way to steer this discussion. iMechanica (and the Internet in general) may have provided us a handle to deal with this problem of divergence.

Reading your comments also reminded me of an early post of mine, What's wrong with Applied Mechanics. Looking it over today, I felt that I might have pushed a single idea too hard, but my heart was in the right place. In any case, the post has not been scrutinized by many fellow mechanicians. (It was dated on 4 September 2009, before iMechanica officially went online, when we had fewer than 20 users, many recruited from my family and my research group.)

Xiao-Yan Gong's picture

I can't agree more with Professor Aref.  Educated in the Mechanics Department at Beijing University (BTW, it is a history too since last year), I found that it's fascinating I adopted myself into a mechanical engineer and gave up my childhood dream of being a professor.  I also found that fluid-structural interaction might be considered as an interdiscplinary topic from many perspectives.  When asking where are the fluid mechanicians, are we by default already excluded them from our solid mechanics world? 

Mechanics in medicine has needs for effective fluid-structural interactive (FSI) analysis and testing methods to reach the level of "predictive medicine".  For instances, Zhigang's example of the flutter of a flag was faced in heart valve industry where flutter of the valve leaflets were observed.  Circulation system are blood pipelines that involve FSI, the need is there, unfortunately we mechanicians have not stand tall enough to effectively address them.

MichelleLOyen's picture

Certainly we have seen an explosion of modeling of fluid-solid interactions in biomechanics in recent years, both with the emphasis on poroelasticity instead of (or in addition to) viscoelasticity and with the direct modeling of fluid-solid interactions, frequently using "physics based modeling" approaches.  I would guess much of this work is right at the stage of about-to-be-published since the topic has been so dominant at recent conferences but not yet really taken over in the archival jouranls.

Konstantin Volokh's picture

The separation of the fluid and solid mechanics can be traced back to the 50s of the past century. In their influential course on theoretical physics Landau and Lifshitz wrote in the foreword to the Continuum Mechanics volume (2nd edition),

"The solution of clearly formulated mathematical problems described by linear PDEs plays the key role in the theory of elasticity. Thus, the theory of elasticity is essentially a part of the so-called mathematical physics.

The nature of hydrodynamics is significantly different. Its equations are nonlinear and their direct solution is available rarely. Because of that, the development of hydrodynamics is tied to experiments. The latter draws hydrodynamics together with other parts of physics."

Eventually, Hydrodynamics and Elasticity were separated in two volumes in the later editions.

L. Mahadevan's picture

In a broader context, the question raised is one of education of ourselves and our students. Problems in nature and technology, as we know well, do not come neatly packaged into "quantum, statistical, classical and continuum,..." any more than they come packaged into "biological, chemical, physical, mechanical, electrical, ... " categories. It is us humans who incapable as we are of comprehending the larger whole who break up the questions into bite sized pieces ... but rarely are able to put back the pieces into a whole.

Any one who has walked in and through the no-man's land between disciplines  knows both the thrill  and the dismay; the former from discovering new problems, the latter from the realization of how little we understand.

If we really wanted to emphasize the unity of the subject, an approach that seems to work well in the life sciences may be worth thinking about. Is there room to think seriously about a multi-authored text (perhaps electronic ) that harnesses our individual expertise and interests to educate the next generation.  I know of no place where there is a systematic introduction to the subject that emphasizes the unity rather than the division of the subjects and yet remains attached to the ground - i.e. focuses on explaining observable phenomena in ways that are "as simple as possible, but no simpler" to quote AEinstein. Continuum mechanics where taught can and does spend time very usefully on generalities and attempts at a unified approach, but rarely focuses on the solution of problems or the consideration of experimental and experiential phenomena that perplex and challenge us. While specific courses focus on the latter at the expense of the generalities that connect  different subjects and thus the ability to tackle new questions. Similar statements could be made of classical, statistical and quantum mechanics that have homes in mathematics, chemistry, physics, chemical engineering and materials science.... each has both a formal and an intuitive aspect to it that should have much interplay; sometimes they do, often they do not.

What if we complemented our educational approach  by using unusual phenomena i.e. those not in accord with "intuition" (defined as a moving target deliberately, as MPlanck once said - "in physics, one never understands anything, one simply gets used to it" !) to motivate the subject in addition to spending time on the usual generalities and formalisms - similar to "case studies" which could be industrial problems, applications motivated by other disciplines etc. ? And emphasized the areas of ignorance as much as what we do understand.

Indeed this approach could do well in an even broader context that looks at collective phenomena in general - of which hydrodynamics, thermodynamics, condensed phase behavior and indeed large parts of molecular and cellular biology and physiology are specific examples.  For example,  in hydrodynamics  there is a rather nice CDROM produced by Cambridge University Press titled "Multi-media fluid mechanics" that is a collection of movies and tutorials that highlight a variety of phenomena, and an excellent book (in French) by Guyon, Petit and Hulin titled "Ce qui disent les fluides" that is a modern version of the phenomenal "An album of fluid mechanics" by Van Dyke that excited a generation of curious kids about the subject (self included). At a more advanced level "Perspectives in Fluid Dynamics" put together by Batchelor and colleagues talks about the vistas that this wonderful subject affords. And there is  in addition an internet site called ""

Is there room for an "Album of materials"  ? Or a multi-media "Condensed phases of matter" ? If they exist, it would be good to know about them ...

The closest I know of anything with this flavor are the two books by J. E. Gordon titled "Structures: Or why things don't fall down" and "The new science of strong materials", and the very nice "Gases, liquids and solids" by D Tabor, as well as Pippard's "Classical Thermodynamics". Are there other such books at a similar level ? In Physics, there is of course the Feynman Lectures; in Mathematics there is the 3 volume series by Alexandrov, Kolmogorov and Lavrentiev "Mathematics: its content, methods and meaning" and Courant and Robbins 'What is mathematics" - all of which try to present a unified view of their respective subjects...

L. Mahadevan, Engineering and Applied Sciences,
Organismal and Evolutionary Biology, and
Systems Biology
Harvard University and Harvard Medical School

Zhigang Suo's picture

The need for a modern synthesis of our field has also been touched on in a previous thread of discussion initiated by Roberto Ballarini.

Perhaps we should talk about possible methods to make this synthesis. A list of good books will be an excellent starting point. Certainly books (electronic or print) by individuals and multiple authors will still be a method of choice by many people. A few audacious ones have also experimented with new authoring tools like Wiki. While Wikipedia is successful for many topics, I have not found good examples of wikibooks in our field.

One possible topic for further discussion is possible approach to a modern synthesis. What are the new tools? How are people motivated to do it? What kind of synthesis will be useful to us and to future generations?

It's better for a fluid mechanician to know something of solid mechanics, and vice versa.

Roberto Ballarini's picture

To me, Imechanica's has made a very important contribution to mechanics by bringing together a very large number of individuals that demonstrate not only their scientific abilities, but also their generosity. The signs of this wonderful disposition include posted unpublished books, lecture notes, and the willingness to answer questions raised by colleagues across the globe.

It is for this reason that I believe the time is right for attempting at least one of the activities proposed by Professor L. Mahadevan; the continuing development of multi-author treatments of various topics in mechanics. On a personal note, I have learned much by reading Zhigang's notes on statistical mechanics and fracture mechanics and William Nix's book on thin films. I have incorporated parts of these treatments into my own courses. There is no doubt in my mind that the highest quality text could be achieved by interweaving, for example, Zhigang's notes with those of others that have spent much of their life enjoying learning fracture mechanics (I am confident that I could contribute to such an endeavour).

So, what about an "open source" text of fracture mechanics (and other topics)?  We all have different takes on the subjects, and we all have contributions to make towards teaching the subjects to future generations. Our efforts would prove to be synergistic.

N. Sukumar's picture

I echo Roberto's sentiments.  There are quite a few lecture notes on fracture that have been posted, and all of them are particularly well-written. Hence, it would be very beneficial and of significant value if a single version of a fracture mechanics text can emerge through iMechanica.  It would become the graduate-level text of choice for courses on the subject and also as a first reference for those inclined towards research in fracture mechanics. This would provide the seeds for other such book-projects on topics of interest to mechanicians.

I know this website casually. But I found that it was good. Zhigang said there were less persons in Fluid mechanics, therefore I decided join in.

 I just search about application of fracture mechanics in Fluid. But it seems that there are very little information. All of us regard it strange that fracture might exist in Fluid! But why not!

Good idea and possibly something that could extend to many areas of engineering. Is the latest edition of whatever statics or mechanics of materials book you use for undergraduate teaching really worth the over $100 per copy cost?

However, to implement open source books would take a lot of coordination and some real work - don't underestimate the time commitment. I would guess that it would take resources beyond a group of people pitching together.

I am working on Fluid mechanics. I'm also interested in solid mechanics. There must be interest things between them. 

I was searching for a forum for people involved in fluid dynamics since a long time. However, expect CFD discussion boards, there is almost nothing seriously going on in the WWW. Today, through a blog of a researcher, I came accross this nice site. As a PhD students one is exposed to a number of graduate courses and hence forced to get in contact with students of different areas, which in fact is enriching. However locally one is quite restricted. Through conferences (APS, EFMC, ETC etc) one is more or less restricted to people in fluid dynamics and maybe thermodynamics and heat transfer, but not so often to solid mechanics. I haven't found yet much intersection areas with solid mechanics, but any intersection will be fruitful I believe.

Hello, Ramis,

I share the same feeling with you. I didn't major in solid mechanics. But I would like to know what they are doing, and what are the topics and difficulties in this fields. Maybe it would helpful for us in research. It's nice to have a 'curious' heart.



Prof. Suo, I have a naive idea.  Maybe we can write a letter of invitation to
professors around the world, in the field of theoretical or experimental
mechanics, solid or fluid, to invite them to join imechanica, in your own name
or in the name of imechanica.

Zhigang Suo's picture

Here is a letter of invitation that you can e-mail to other mechanicians to publicize iMechanica.

One important group of people that imechanica shall attract are fluid-structure interaction researchers.  I graduated with PhD training only on fluid dynamics, but later on I gained my interests on solid mechanics while working in applied mathematics department. I believe that many researchers of this group are working in departments of mechanical, civil, aerospace engineering as well as applied mathematics.

Fluid and solid equations could be solved all together in an unified approach ( using the Finite-Volume method which is more popular in CFD. This convinces me more that imechanica in the future would include people from both solid and fluid sides, and even applied mathematicans.

I am recently involved with  one interesting conference ( As a reviewer of Computers and Structures, it appears to me that more and more fluid researchers are sending good-quality manuscripts to participate this conference and publish fluid mechanics papers in this well-known solid mechanics journal. In terms of unifying the divergence between solid and fluid, the solid mechanicians are already pushing harder than the fluid side.

I work on both fluid and solid mechnics and most probably will be working on FSI problems in future. I am here, lets see if imechanica helps this youngster on his way through or not.

In the states of matter, liquid and solid are often considered adjacent states, but sometimes there are mesophases between them. For instance, there are miniscule moving parts in a LCD that effect an optical switch for display purposes. Jerald Ericksen, in his Timosheko acceptance address reproduced here in iMechanica, points out that a mechanical couple, driven by an electrical field, turns the switch. Students that aspire to be more than technicians will study principles of applied physics that pertain to all states of matter. 

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