Invited Speaker

(In Honour to Professor Stan Dennis) Experimental Visualization as an Aid for Pointing out Typical Fundamentals in Flow Behaviours

Universit\'e de Poitiers

Poitiers, France

Flow visualization is widely recognized as a very useful tool to help researchers in understanding flow behaviours and in detecting the influencing parameters, as well in industrial domain as in fundamental research, and this, for a very long time. Indeed, the first elaborated techniques, based on this princip, have been proposed at the end of the previous century an, since then, a constant effort has been devoted for perfecting these techniques and elaborating new ones especially with the benefit of the rapid development of computer possibilities. The method permits to render visible what, normally, is not perceptible to our eyes. So, it is a precious tool to point out flow structures, like vortices, and to help interpreting blind flow measurements, like hot wires, laser velocimetry... Now, with modern developments, flow visualization has became itself a means for a quantitative flow approach.

The present lecture develops the following points

First, a short retrospect, based on selected known pictures, recalls how flow visualization is used in industrial domain and in fundamental research, pointing out, for this latter domain, its help in:

\begin{itemize}

detection of new phenomena

support and complement (both qualitative and quantitative) for numerical calculations (see as examples the fruitful collaboration with S. C. R. Dennis)

reconstruction of flow topology

\end{itemize}

The second part is devoted to the presentation of recent researchs related to starting recirculating flows. A more special attention is paid to starting nominally 2D-wakes, in the Reynolds number range covering transition, namely, $60\le Re \le 600$; different geometries of cross-body shapes are considered successively:

\begin{itemize}

semicircular-shell

other typical cross-section geometries: circular - perpendicular flat plate - semicircular in reversed arrangement.

\end{itemize}

In each case, the way B\'enard-K\'arm\'an vortices form and detach from the selected bodies have been studied and some typical examples are presented. Furthermore, interest is focused on the onset and development with time of body-end induced three-dimentionalities and, for $Re < Re_{transition}\approx 200$, intrinsic three-dimensionalities. The shell case, for which well formed Taylor-G\"ortler-vortex patterns have been captured, is shown to be very suitable to point out these different sorts of 3D phenomena.

The influence of the cross-body shape on the phenomena observed in the shell case is presented and the important role of the upstream-face geometry is deduced.

Finally, primary results related successively to:

\begin{itemize}

control of starting wakes by a splitter plate, with an application to the semicircular body wake in the range $60\le Re \le 600$,

starting recirculation flows in variously shaped cavities, for $1000\le Re \le 3200$,

\end{itemize} are briefly presented.

In conclusion, proposition is made to calculate the new examples of 3D-flows presented, with a confrontation of the results at the next Canadian Symposium on Fluid Dynamics!

\begin{center} {\bf An example of fruitful collaboration between experimental visualization and calculation in pointing out fundamental wake topological behaviour. }

\end{center}

Transposition process between the closure-saddle points of two counter-rotating vortices behind an impulsively started rotating and translating circular cylinder (adapted from Coutanceau et al., Atlas of Visualization III, 1997): Schematics deduced from experimental visualization.

{\bf numerical calculation (Dennis \& Badr)}

{\noindent Streamline patterns numerically generated for $Re=500$ and $\alpha$(speed ratio)=0.5}