The first Symposium Transsonicum took pl.ace in Aachen thirteen years ago during a period of decreasing governmental. and industrial. support for transonic flow research. Since then, there has been a strong revival. in interest in transonic flow research so that the number of partici pants at the second symposium remained about the same as at the first even in spite of tight financial. means and Limited governmental. support. During both meetings the number of participants reached the upper Limit of the number desirabl.e for such a symposium. Participants came from aU over the worl.d and there was a weU bal.anced distribution of participants from aU countries interested in transonic flow research. The discussions - mostLy conducted in EngLish - were stimul.ating and there was a great deal. of interest in the l.ectures as was shown by the good attendance even during the l.ast session on Saturday morning.

"Symposium Transsonicum" was founded by Klaus Oswatitsch four decades ago when there was clearly a need for a systematic treatment of flow problems in the higher speed regime in aeronautics. The first conference in 1962 brought together scientists concerned with fundamental problems involving the sonic flow speed regime. Results of the conference provided an understanding of some basic tran sonic phenomena by proposing mathematical methods that allowed for the de velopment of practical calculations. The "Transonic Controversy" (about shock free flows) was still an open issue after this meeting. In 1975 the second symposium was held, by then there was much understanding in how to avoid shocks in a steady plane flow to be designed, but still very little was known in unsteady phenomena due to a lack of elucidating experiments. A third meeting in 1988 reflected the availability oflarger computers which allowed the numerical analysis of flows with shocks to a reasonable accuracy. Because we are trying to keep Oswatitsch's heritage in science alive especially in Gottingen, we were asked by the aerospace research community to organize another symposium. Much had been achieved already in the knowledge, techno logy and applications in transonics, so IUT AM had to be convinced that a fourth meeting would not just be a reunion of old friends reminiscing some scientific past. The scientific committee greatly supported my efforts to invite scientists ac tively working in transonic problems which still pose substantial difficulties to ae rospace and turbomachinery industry.

Continuing the tradition of the IUTAM Symposia TRANSSONICA, this review of the numerical simulation and physical modelling of transonic flows presents new developments in the fields of computational and experimental aerodynamics. A major topic of the symposium proceedings is the evaluation of present numerical analysis techniques with respect to transonic aerodynamics. In the field of experimental aerodynamics, the high Reynolds number effect and the interference-free testing in transonic wind tunnels are of special interest.

The Illiac IV was the first large scale array computer. As the fore runner of today's advanced computers, it brought whole classes of scientific computations into the realm of practicality. Conceived initially as a grand experiment in computer science, the revolutionary architecture incorporated both a high level of parallelism and pipe lining. After a difficult gestation, the Illiac IV became operational in November 1975. It has for a decade been a substantial driving force behind the develooment of computer technology. Today the Illiac IV continues to service large-scale scientific aoolication areas includ ing computational fluid dynamics, seismic stress wave propagation model ing, climate simulation, digital image processing, astrophysics, numerical analysis, spectroscopy and other diverse areas. This volume brings together previously published material, adapted in an effort to provide the reader with a perspective on the strengths and weaknesses of the Illiac IV and the impact this unique computa tional resource has had on the development of technology. The history and current status of the Illiac system, the design and architecture of the hardware, the programming languages, and a considerable sampling of applications are all covered at some length. A final section is devoted to commentary.