Effect of boattail angles on the flow pattern on an axisymmetric afterbody surface at low speed

An investigation has been conducted in the Langley pilot transonic cryogenic tunnel to determine the effects of varying Reynolds number on boattail drag at subsonic speeds. Six boattailed cone-cylinder nacelle models were tested with the jet exhaust simulated by a cylindrical sting. Reynolds number was varied from about 2.6 million to 132 million by changing model length and unit Reynolds number. Boattail pressure coefficient distributions show that increasing Reynolds number tends to make the pressure coefficients in the expansion region more negative and the pressure coefficients in the recompression region more positive. These two effects were compensating and as a result there was little or no effect of Reynolds number on the pressure drag of the isolated boattails.

The effect of Reynolds number on boattail drag

In the absence of a rational theory to predict all aspects of the base drag problem of projectiles and other bodies and the influence of base injection on base drag, a number of semi-empirical correlations have been developed based upon various models of the base flow phenomena to gain an understanding of the experimental results. Several theoretical models are also available which analyze in detail one or more aspects of the base flow phenomena. This report presents a bibliography on the subject of base flows and some of the models and correlations attempted to explain the different aspects of such flows. It is clear that further systematic experimental studies are required if a general analytical model is to be developed even for particular classes of projectiles with injection.

Base flow phenomena with and without injection: experimental results, theories, and bibliography

Flow behavior around axisymmetric boattail surface was studied by numerical methods A wide range of boattail angles from 0° to 24° was investigated to find the drag trend of the model Numerical simulation was validated by experimental results with the same flow conditions Results showed that the use of boattail model always has a positive effect on drag reduction Total drag showed minimum value at boattail model of around 14° Length of the recirculation after body decreases with increasing boattail angle up to 14° and then becomes constant at higher angle The trend of boattail pressure drag showed similar to previous studies for high-speed flow However, base drag showed different trend to previous observation The base drag showed to be the most important parameter to determine drag trend of the model The effect of flow fields around boattail on pressure distribution and drag is discussed in detail

Effect of boattail angle on near-wake flow and drag of axisymmetric models: a numerical approach

Until the mid-seventies there were many variable density transonic wind tunnels which had been calibrated at only one total pressure. Today this still holds for some tunnels. If Reynolds number, i.e., total pressure, is varied in such tunnels during a test, small systematic errors in freestream Mach number M0 do then result. In M0-sensitive measurements this causes exaggerated (pseudo) Reynolds number trends. For the examples of afterbody pressure drag, transonic shock location, and transonic maximum lift, it is shown that up to 100% of these trends can be attributed to the lack of a full Reynolds number calibration. The remaining few percent must be due to true Reynolds number effects and to additional, yet unknown, systematic errors.

Calibration-related pseudo-Reynolds number trends in transonic wind tunnels

A flight and wind tunnel investigation was conducted to determine the effects of Reynolds number on the installed boattail drag of an underwing nacelle. Tests were run on a modified F-106B aircraft and 0.05 and 0.22 scale wind tunnel models. Tests were conducted at Mach numbers of 0.6 and 0.9 and over a 16 to 1 range of Reynolds numbers. Highest drag was obtained at intermediate Reynolds numbers corresponding to about the lowest flight values and that of the 0.22 scale model. Significantly lower drag was obtained at both higher and lower Reynolds numbers.

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B. J. Blaha

Papers

Flight and wind tunnel investigation of the effects of Reynolds number on installed boattail drag at subsonic speeds