The Busemann Air Intake for Hypersonic Speeds DOI: http://dx.doi.org/10.5772/intechopen.82736

Problems of viscous losses and flow starting are eased by use of wavecatcher technology, providing leading edge truncation and sweep and by the fact that high adverse pressure gradients occur in the inviscid core flow rather than in the wall surface boundary layers. Hypersonic intakes that utilize axisymmetric compressive

A preferred geometry for a scramjet combustor is a duct with a circular cross section because of its superior ability to withstand both heat and pressure loads. Frictional losses are also at a minimum for such a duct since a cylinder has the smallest surface area for a given cross-sectional area. This leads to a cylindrical (axially symmetric) geometry as being desirable also for the intake that is attached to the front of the combustor duct. The same circular exit geometry for the intake is demanded by a gas turbine engine, in this case because the axial compressor face is circular. Towards these ends, it is pertinent to study an axisymmetric flow and it is entirely fortuitous that axisymmetric, conical, Taylor-Maccoll flow provides a streamtube shape [1, 2] that satisfies the above intake design requirements, both geometric/structural as well as aerodynamic [3]. In recognition of Adolph Busemann's pioneering work [1] on such streamtube shapes, they are called

basic flows with specified entrance and exit shapes have received attention because of their high performance (capability and efficiency) and analytical

Hypersonic Vehicles - Past, Present and Future Developments

Busemann flows and Busemann intakes. References [1–18, 24] all concern

The reduction of Mach number, in the various basic flows, is accomplished by one or more fluid mechanical mechanism: (a) compressive flow turning; (b) flow convergence with area contraction and compression in a converging passage and (c) flow deflection through an oblique shock. Flow turning and contraction are isentropic processes leading to no loss in efficiency. Flow deflection through an oblique shock entails an entropy increase—a loss in intake efficiency. If shocks are needed to deflect or re-direct the flow then they should be as weak as possible, occurring at the lowest possible Mach number (e.g., Busemann shock). Planar flow turning by Prandtl-Meyer-type flow requires much turning to accomplish a significant Mach number reduction, so that, after P-M turning, strong shocks are required to re-direct the flow back to the freestream direction for the combustor. On the other hand, isentropic Mach number reduction by area contraction leads to a rapid streamwise Mach number reduction when the flow is axial. In such flows, Busemann flow being typical, there is comparatively little flow turning towards the center line, the compression being accomplished by area contraction and, as a result, there is no need for much deflection (re-turning) by a shock at the exit. Also, since there is considerable Mach number reduction in the converging flow, the terminal shock faces a reduced Mach number. This weaker terminal shock minimizes efficiency losses. The axial flow intakes derive their high efficiency from the axial convergence, being only little degraded by flow deflection through the terminal shock. The axial Busemann intakes have been mistakenly labeled as "inward turning" even when part of their converging flow is turning outward, away from the axis. We suggest dropping the "inward turning inlet" terminology in favor of "axial flow intake" or "converging flow intake," because their fundamental and characterizing distinction is axial convergence. It is precisely the lack of much "inward turning" that leads to the high performance of Busemann intakes. It would be better to use the flow-related and meaningful concepts of turning, convergence and deflection to characterize intake flow types in general. Isentropic turning, as in

1.3 Intake flow processes and inward/outward flows

simplicity [4–24].

Busemann flow.

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P-M flow, may have to be used where variable geometry demands the use of planar flow. The resulting flow turning, away from the flight direction, has to be compensated by lossy oblique shock deflections. On the other hand, flow convergence, such as occurs in sink flow, is an effective mechanism because it is isentropic and involves no flow turning. Deflection occurs through an oblique shock; it is nonisentropic and it should be used only when there is no other possibility of orienting the flow. It should not be used to reduce the Mach number. A practical flow, such as Busemann [1], incorporates all three of these aerodynamic mechanisms as they interactively contribute to intake performance.

The three modes of compression are illustrated in the Prandtl-Meyer intake, the Oswatitch intake and the Busemann intake (Figure 2). The Prandtl-Meyer intake obtains performance by isentropic turning through the compression fan, followed by deflection through the oblique shock; there is no convergence. The Oswatitch intake has flow divergence and turning followed by deflection through a shock. The Busemann intake has turning and convergence followed by shock deflection. Three intake models were designed to reduce the Mach number from 8.33 to 4.8 with a static pressure ratio of 26.8. All three intakes were tested in a gun tunnel [11] at Mach 8.33 and it was found that, for the same amount of contraction, the inviscid total pressure recoveries of the Busemann, Oswatitch and Prandtl-Meyer intakes were 0.983, 0.763 and 0.763. Experimental total pressure recoveries were 0.484, 0.485 and 0.240. The reason for the differences stems from the fact that the surface area and consequently the viscous losses, were greatest for the Prandtl-Meyer intake. Sidewalls, needed to contain the planar Prandtl-Meyer flow, did not preserve the intake's efficiency but contributed to the surface area and viscous losses. The lack of an extensive leading edge and attendant viscous flow contributed to the efficiency of the Oswatitch intake. These results illustrate the superiority of axial over planar basic flows where it is the Mach number reduction, achieved by convergence, that leads to the high performance of the Busemann intake.

Figure 2. Schematics of three intakes tested in a gun tunnel at Mach 8.33 [11].
