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Wave Drag Reduction by Pivoting Aero-Spikes

Background

Conventional aero-spike devices mounted in front of blunt bodies can help to protect the dome and to reduce the wave drag at supersonic speeds. It is well known, that the conventional fixed spikes aligned along the longitudinal axis are counterproductive at relatively high angles of attack. The main goal of the current investigation was the examination of an innovative pivoting-spike concept (see Fig.1).

Sketch of the test model with proposed self-aligning pivoting-spike concept

Measuring Method

This concept is based on a self-aligning spike-device, which uses the weathercock principle for passive control of its optimal alignment in the flow at wide range of angles of attack and yaw. The wind tunnel tests were carried out in the Ludwieg Tube Facility RWG-DNW at DLR Göttingen using a simplified axially symmetric hemisphere-cylinder model with variation of the free-stream Mach number between 2 and 5 at a row of model's angle of attack a  from 0 to 30 degrees. Three model configurations were investigated: reference blunt body without a spike (left), the blunt body equipped with a conventional fixed spike (center), and the blunt body with a novel aero-spike aligned into the flow (right). The standard shadowgraph technique for flow visualization, the force measurements an internal 6-component balance, as well as the quantitative Infrared Thermography for heat-flux measurements were applied for these tests.

The results show a very stable behaviour and distinct drag reductions over the whole a -range. Typical wave structures at 20-degree incidence are shown on shadowgrams in Fig. 2 for the fixed and aligned flat-edged spikes in comparison with that of the reference blunt body without a spike. Opposite to the effect of the conventional fixed spike, the separation zone around the aligned spike remains quite symmetrical, causing weaker shock waves in the presence of the pivoting spike not only upstream of the hemisphere but also along the wave-fronts far from the interaction zones.

Fig. 2: Effect of spikes on the shock wave structure at Mach 2 and a = 20°: reference body without a spike(left), body with a conventional fixed spike (center), and body with an aligned spike of the same length (right)

The results of force and moment measurements shown in Fig. 3 demonstrate that the aligned spikes are responsible for the higher performance of blunt bodies than the conventional spikes. The pivoting configuration shows, in comparison, a very stable behaviour and substantial drag reductions in the entire a-range. For example, at a = 25° and Mach 5 the forebody drag was found to be about approximately 43% smaller, the lift-to-drag ratio about approximately 116% bigger, as well as the destabilizing pitching moment about approximately 34% smaller than the ones of the reference body without a spike.

Fig. 3: Effect of spikes on the drag reduction and lift-over-drag ratio from wind-tunnel measurements at Mach 5

Moreover, the aligned spike caused a smoother heat flux distribution at significantly lower heating levels. For the test configurations investigated at Mach 5 , for example, at a = 15° the maximal value of Stanton number obtained for the aligned spike is up to 70% smaller than the ones of the conventional spiked body.

Publications on this topic

• Schülein E. (2008) “Wave Drag Reduction Approach for Blunt Bodies at High Angles of Attack: Proof-of-Concept Experiments”, AIAA Paper 2008-4000, 4th AIAA Flow Control Conference, Seattle, Washington (USA), 2008-06-23 - 2008-06-26, 13p
• Schülein E. (2009) “Wave Drag Reduction Concept for Blunt Bodies at High Angles of Attack”, Shock Waves, Proc. of the 26th International Symposium on Shock Waves (ISSW26), K.Hannemann - F.Seiler (Eds.), Vol.2, pp.1315-1320, Springer