Turbulent Heat Transfer

Fortunately the X-15 structure was designed conservatively to cope with turbulent heating. Hypersonic turbulent heating rates were recognized from the outset as an area of special importance for flight measurements because of the weakness of the available semi-empirical prediction methods and the almost complete lack of reliable hypersonic wind-tunnel data for turbulent flow. It should not have been any cause for real surprise, therefore, when the X-15 flight results of 1961 (ref. 16) showed a marked departure from the predictions available at that time, averaging about 35 percent below the van Driest and the Eckert T' predictions (refs. 17, 18) for the low wall temperature conditions of these flights (fig. 6).

Figure 6 - line graph chart of turbulent heat-transfer data for wing of X-15
Figure 6. Turbulent heat-transfer data for wing
of X-15 compared with prediction method.

At first this result was received with disbelief by most fluid mechanics specialists, who are by nature skeptical of flight measurements. The result has been thoroughly substantiated., however, by repeated measurements at several locations on the airplane together with local flow f ield surveys to aid in analysis of the data. The important highlighting of this weakness of the prediction methods by the X-15 stimulated comprehensive studies in a variety of ground facilities. New cold-wall data have been obtained which fully confirm the X-15 results. Improved prediction methods are being sought and new investigations of the structure of the cooled turbulent boundary layer are in progress (ref. 19, for instance). This is an excellent example of one of the greatest values of an exploratory research airplane - the highlighting of an important problem and the stimulating of ground-based research for its solution.

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