NOTES from WORKSHOPS
Interest of data Analysis in Transient Convective Heat Transfer and
From Transient to Chaotic Behaviour in Convective Flows
J. Padet & F. Arinc
During the Symposium, two workshops were held. These were: "Interest of Data Analysis in Transient
Convective Heat Transfer", and "From Transient to Chaotic Behaviour in Convective Flows. The first
workshop was held following the lecture presented by Colette Padet. The second one concluded the
meeting and was introduced by Roger Martin who presented an experimental investigation realized with
a mixed convection flow of water in a horizontal pipe heated on a part of its length. In this experiment,
irregular fluctuations of temperature occur on the pipe wall, which are due to chaotic structures appearing
inside the flow. The ensuing discussions and the ideas expressed by A. Bejan, B.L. Bhatt, L. Estel,
H. Herwig, R. Martin, J.K. Nieuwenhuizen, C. Padet, J. Padet, I. Pop, J.B. Saulnier, and A. Ungut are
presented here.
During the discussions following the first workhop, all agreed that there is a strong need for reliable and
physically clear (and as easy to handle as possible) packages of data analysis. For instance, in many
practical applications, one may seek to identify a marginal effect in a complex transient phenomenon:
usually, the trend that people is looking for is hidden within noise and other unrelated effects.
The comment on the idea of being able to extract information about the mean behaviour (B. Bhatt) was
that it has relevance also to the twophase flow phenomena. The stochastic fluctuations, inherent to any
twophase flow, interact with the system dominated time dependent phenomena as in externally imposed
transients in the study of transient response characteristics.
Separation of scales is identified as another important problem, for instance, in turbulent unsteady flows.
It was expressed that scales and trends are also needed in twophase flows in order to improve the
modeling of movements for the dilute phase. Nevertheless, a preliminary scale analysis gives a basic
benefit since it places theory ahead of experiment. But, separation of scales based on data analysis is
useful too for verification of a theory as well as for giving basic information when it is missing. So, the
method proposed by Colette Padet and JeanMarie Roche could be useful in such problems, and is
complementary to the wavelet analysis approach which provides a similar information in the frequency
domain. It would be interesting to extend the method to the twodimensional analysis where there is a
similar need.
As far as the second topic is concerned, dissipative or chaotic structures in fluids are generally
investigated with stationary boundary conditions. But, it appears from the papers by S. Kakaç, R. Martin,
A.A. Merrikh and L. Estel, that they can be generated, modified, or destroyed by timedependent boundary
conditions. Moreover, the question is strongly linked to the problem of scales and consequently to data
analysis. For example, on the onset of Benard convection (or in other transient conditions) where
convection can be associated with a comparison of time scales, the system opts for the shorter
communication time across the fluid layer. Why should it be 'in a hurry!?
Another important question is to improve the precision of the relation of unstable and chaotic
behaviours. All these problems offer a large field for further investigations.
It was noticed at last that multidisciplinary approaches, as illustrated by the meeting of data analysis and
transient convective heat transfer, can be very fruitful and must be encouraged, despite the fact that
publications on these topics are more difficult because they are often considered by the reviewers as
"not enough mathematical" or "not enough physical papers". They serve at least one purpose which is an
increased understanding of the fundamental phenomena or mechanisms.
