PREFACE TO FLUID MECHANICS@ INTERNAL FLOW SERIES Design engineers concerned with plant and equipment face an ever-increasing demand for products with a performance which must be substantiated under stringent conditions of cost and environment. The customers for their products are becoming better educated in the ways of achieving@ and what can be achieved by@ competitively designed equipment. Assuming the skill of the designer@ no other single factor can contribute more to the economic preparation of a satisfactory@ competitive design than the data upon which that design is based and the speed with which those data can be found and applied. Fluid flow data@ in common with those relating to many other disciplines of concern to the engineer in industry@ are widely scattered and when found are of variable quality and relevance to industrial application. The engineering designer does not@ generally speaking@ have the facilities required to collect and evaluate all the data available which may be applicable to each aspect of the work. Even with the desire to undertake such a task it is unlikely that the necessary time could be allowed to prepare the own data ??working tool' at the expense of time spent in the application of knowledge and experience to the particular job in hand. In any case@ the customer will increasingly wish to view the design@ not against some locally derived data@ but against what can be demonstrated to be the best available data whose integrity is underwritten by a significant cross section of the engineering community concerned with the derivation and application of that data. It is ESDU's purpose to provide data in this and many other fields. ESDU has prepared this information for both the designer and equipment user and@ by serving the needs of many@ accomplishes the task at much lower cost than could apply to individuals performing it on their own behalf. Qualified staff collect and evaluate the existing data. Irrelevant or unreliable data are discarded: those remaining are correlated and presented as an authoritative working tool for engineering applications. Technical guidance on individual Data Items is given by Working Parties of specialists drawn from industry@ national laboratories and the universities together with other leading specialists. New work is prepared under the guidance of the Fluid Mechanics@ Internal Flow Panel.