Numerical Study of Sharp Leading Edge Performance

Introduction

Air-breathing, hypersonic vehicles are being considered for use as reusable launch vehicles and other trans-atmospheric aircraft. Among the primary technology issues concerning the aerodynamics of such vehicles is the need to minimize drag and heating. One promising approach to these problems involves the use of sharp leading edges for lifting surfaces and engine inlets obtained using power-law shapes [1]. Prior theoretical and analytical studies have focused attention on different flight regimes. For example, O'Brien and Lewis [2] considered very low altitude, inviscid conditions by solving the Euler equations, while Santos and Lewis [3] applied the direct simulation Monte Carlo method (DSMC) [4] to study high altitude, rarefied conditions. The present work has as its goal a more comprehensive analysis, using various numerical methods, of the aerodynamic performance of sharp leading edges across the full range of conditions experienced in a representative flight trajectory. The assessment will involve comparison of heat transfer, drag force, and shock standoff distance, for a variety of on- and off-design points.

To first order, the trajectory of an air-breathing hypersonic vehicle is designed to provide pressures inside the scramjet propulsion system to be a sizeable fraction of atmospheric pressure to achieve efficient combustion. For the present numerical studies, we use a typical air-breathing vehicle trajectory provided by Bertin [5]. For this trajectory, Fig. 1 shows the velocity and dynamic pressure as a function of altitude. Note that, as the vehicle ascends through the atmosphere, the Knudsen number increases from the continuum regime (10-4) through the transition regime into rarefied flow (10-1). While the region of maximum heat flux (around 35 km) has a Knudsen number in the near-continuum regime (about 10-3), the local radius of curvature on a sharp leading edge will be smaller than the gross dimension of 1 cm thus increasing the local Knudsen number. Hence, the overall assessment of sharp leading edges for such vehicles requires both continuum and rarefied flow analyses. In our work, the continuum regions will be analyzed using the Navier-Stokes equations and the rarefied regions will be computed using the DSMC technique.


Preliminary Results

We have elected to start our analysis at the high altitude portion of the trajectory due to the existence of prior computational results by Santos and Lewis [3]. The leading edge geometry considered is a simple power-law form given in Ref. 3 as:

y = a xn

where x and y are the two-dimensional coordinates, and we consider the case of n = 0.5 with a maximum height of 1 cm. The general purpose DSMC code MONACO [6] is applied to this leading edge at an altitude of 70 km where the free stream conditions on the trajectory shown in Fig. 1 are: r = 8.3×10-5 kg/m3, U = 7,470 m/s and T = 220 K. The wall temperature is fixed at 880 K and fully diffuse reflection is assumed. In Fig. 2, the final computational mesh adapted to the flow field and contours of translational temperature are shown. Note the very large peak temperature generated by the near orbital speed of the trajectory at this altitude. These preliminary computations ignore chemistry, but such effects will be included in the final results. In Fig. 3, profiles along the surface of Stanton number and skin friction coefficient are shown.

In the final paper, the results of many such computations will be compiled and compared in terms of total drag, total and peak heating, and shock standoff distance. A number of basic parameters will be varied including leading edge geometry, flight conditions (altitude, on/off design), and flow modeling.


Acknowledgments

Financial support for this work is provided by the NASA-URETI on Reusable Launch Vehicles, Grant NCC-3989. Helpful discussions with Professor Mark J. Lewis are gratefully acknowledged.

References

[1] Mason, W.H. and Lee, J., "Aerodynamically Blunt and Sharp Bodies," Journal of Spacecraft and Rockets, Vol. 31, 1994, pp. 378-382.

[2] O'Brien, T.F. and Lewis, M.J., "Power-Law Shapes for Leading-Edge Blunting With Minimal Shock Standoff," Journal of Spacecraft and Rockets, Vol. 36, 1999, pp. 653-658.

[3] Santos, W.F.N. and Lewis, M.J., "Power-Law Shaped Leading Edges in Rarefied Hypersonic Flow," Journal of Spacecraft and Rockets, Vol. 39, 2002, pp. 917-925.

[4] Bird, G.A., Molecular Gas Dynamics and the Direct Simulation of Gas Flows, Oxford University Press, 1994.

[5] Bertin, J.J., Hypersonic Aerothermodynamics, AIAA Press, 1994, p. 2.

[6] Dietrich, S. and Boyd, I.D., "Scalar and Parallel Optimized Implementation of the Direct Simulation Monte Carlo Method," Journal of Computational Physics, Vol. 126, 1996, pp. 328-342.

Figure 1. Typical trajectory of a hypersonic, air-breathing vehicle [5]

 

Figure 2. Illustration of adapted computational mesh and translational temperature (K) field from DSMC computations of flow around a sharp leading edge at 70 km altitude

 

Figure 3. Surface properties for DSMC computations of flow around a sharp leading edge at 70 km altitude.

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