03-05-2011, 09:43 AM
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Abstract
A numerical study has been carried out to investigate the full flow path and aerodynamic characteristics of a hypersonic vehicle ata 7.0 free stream Mach number. Results indicate that the inlet started and unstarted operations have remarkable effects on the flow patternof the full flow path. When the inlet operates in a started mode, the transverse pressure gradient generated by the forebody alters theair captured characteristics and the entering flow quality of the inlet. Furthermore, the expansion process of the nozzle jet flow is obviouslyaffected by the external flow field around the afterbody with the cross section shape transiting from a near rectangle at the exit ofthe nozzle to a near triangle at the tail of the vehicle. When the inlet operates in an unstarted mode, the aerodynamic instability can beobserved in the full flow path of the vehicle. Due to the oscillation of the external compressed shock wave and nozzle jet flow, theaerodynamic characteristics of the vehicle vary periodically with the lift-drag ratio changing from 0.25 to 2.09. Finally, by comparing tothe experimental data, the reliability of the CFD is verified.
Keywords: hypersonic vehicle; full flow path; inlet start; inlet unstart; unsteady; lift-drag ratio
1 Introduction*
The insufficient net thrust obtained by hypersonicvehicles poses a formidable challenge to thedesign of various aerodynamic components in thepropulsion system and the airframe. One promisingtechnology that is capable of meeting the challengeis the airframe/propulsion integration. Since the1960s, a lot of research studies have been dedicatedto the integration of the propulsion system with thehypersonic airframe, including the integration of theinlet with the forebody, the integration of the nozzlewith the afterbody, the optimization of the full flowpath, the effects of the scramjet operating states onthe aerodynamics of the airframe and so on[1-8]. Unfortunately,there are few papers that focus on theeffects of the inlet started/unstarted condition on the flow pattern of the full flow path and the aerodynamiccharacteristics of the vehicle.It is well known that hypersonic inlets shouldalways operate in a started mode in the whole flightenvelope. However, because of the immaturity ofthe current design methods and the uncertainties ofthe engine thrust regulations as well as the complexflight conditions, the unstarted condition of the inletoccurs inevitably during the development of hypersonicair-breathing propulsion system. According toRefs.[9-12], a prominent feature of unstarted hypersonicinlets is of an unsteady flow which canthreaten the stable operation of the propulsion systemand even lead to unsteady aerodynamic forceand moment on the vehicle. Moreover, the unsteadyaerodynamic characteristics will bring troubles tothe flight control and the structure safety of the vehicle.Therefore, from both the academic and thepractical point of view, it arises an urgent need forthorough investigation on the aerodynamic characteristicswhile the inlet is either in started or in unstartedmode. Recently, the influences of the inletstarted/unstarted operations on the vehicle aerodynamiccharacteristics were experimentally studiedby Ref.[13]. Some useful conclusions were drawn,but the complex phenomena still remain unclearbecause the pressure measurement adopted was ofsteady type and the FPS (frames per second) of theCCD camera was insufficient.In order to gain a deep insight into the complexphenomena, the integrated flow field in Ref.[13] hasbeen numerically studied in an unsteady manner inthis paper. With the aid of CFD (short for computationalfluid dynamics) technology, this paper acquiresan intensive knowledge of the aerodynamiccharacteristics and the flow patterns of the full flowpath while the inlet operates either in a started or inan unstarted state. Besides, the reliability of theCFD method is also verified by the experimentaldata in Ref. [13].
2 Methodology
2.1 Experimental setup
The internal and external configurations of thehypersonic vehicle(NX-1) are presented in Fig.1and Fig2. The whole flow path consists of an inlet,an isolator, a combustor and a nozzle. The inlet islocated at the aft part of the fuselage and the distancefrom the apex of the fuselage to the leadingedge of the cowl is 253 mm. The shock-on-lip Machnumber of the inlet is 7.0. The outside compressionof the inlet is realized through three ramp surfaceswith 5°, 6° and 3° turning angles respectively. Thetotal area contraction ratio of the inlet is 7.0 with theinternal one being 2.2. Downstream the throat of theinlet is followed by a constant cross-sectional ductwith a length 4 times the throat height (H0), whichserves as the isolator. During the experiment, thethroat height can be altered to obtain the inletstarted or unstarted condition. The combustor has asmall section of constant area succeeded by a ductwith a 2° divergence angle. The injection holes andrecessed cavities are not taken into account. Followingthe combustor, an expanding duct with divergenceangles of 16° and 7° acts as a nozzle. Furthermore,a set of typical points are taken as follows:point O—the apex of the fuselage, point A—theentrance of the internal duct, point B—the exit ofthe isolator, point C—the entrance of the nozzle,and point D—the exit of the internal duct.