On the Drag Efficiency of Counterjets in Low Supersonic Flow

Abstract

A series of nose-cones with and without mechanical aerospikes or counterjets (fluidic aerospikes) were computed, and the resulting flowfields and drag forces compared. In particular, shapes that were close to aerodynamically optimal were also considered. The incoming flowfield was a Ma = 2.5 flow at atmospheric conditions. Different nozzle exit conditions were tried in order to obtain both short and long penetration modes. The study revealed that the gains from counterjets were of some interest for blunt bodies, but moderate at best for the more aerodynamical shapes. The reseach on counterjets now spans more than six (!) decades. Two types of flowfields have been identified, depending on the 'penetration length' of the counterjet into the incoming flowfield. The so- called short penetration mode (SPM) is characterized by a close proximity of the detached shock from the counterjets exit, and is seen as steady, stable, and therefore relatively easy to control. The other mode is the so-called long penetration mode (LPM), where the counterjet pushes the detached shock of the incoming flow much further upstream. This mode is highly unsteady, and its control may pose an engineering challenge. Both experimental and numerical results indicate that under favourable circumstances counterjets can reduce drag, thermal loading and shock signature of vehicles flying at supersonic speeds for blunt bodies. This last point is important: most (if not all) of the studies to date considered very blunt bodies (e.g. re-entry capsules or aeroshells) and not aerodynamically optimal shapes. While most of the papers cited above were concerned with the physics of the flowfields generated by aerospikes or counterjets, as well as local pressures and heat transfer rates, few have actually reported a net reduction in drag. Das et al. 10 reported at most a 15% reduction. Golovitchev et al. 15 reported an almost 40% drag reduction, albeit without taking into consideration the energy expended for combustion. The present study arose from the desire to conduct a sled-test experiment with realistic (in size and shape), aerodynamic nose-cones at moderate supersonic speeds (Ma=O(2.5)). The aim was to see what flowfields and loads could be expected so as to instrument optimally the experiment. To this end, a series of nose-cones were computed, and the resulting flowfields and drag forces compared. In particular, shapes that were close to aerodynamically optimal were also considered. The study revealed that the gains from counterjets were of some interest for blunt bodies, but moderate at best for the more aerodynamical shapes.