The Dynamics of Shock Dispersion and Interactions in Supersonic Freestreams with Counterflowing Jets

Abstract

An active flow control concept using counterflowing jets to significantly modify the external flowfields and strongly weaken or disperse the shock-waves of supersonic and hypersonic vehicles to reduce the aerothermal loads and wave drag was investigated. Experiments were conducted in a trisonic blow-down wind-tunnel, complemented by pre-test computational fluid dynamics (CFD) analysis of a 2.6% scale model of Apollo capsule, with and without counterflowing jets, in Mach 3.48 and 4.0 freestreams, to assess the potential aerothermal and aerodynamic benefits of this concept. The model was instrumented with heat flux gauges, thermocouples and pressure taps, and employed five counterflowing jet nozzles (three sonic and other two supersonic with design Mach numbers of 2.44 and 2.94) and nozzle exit diameters ranging from 0.25 to 0.5 inch. Schlieren data show that at low jet flow rates of 0.05 and 0.1 lb,/sec, the interactions result in a long penetration mode (LPM) jet, while the short penetration mode (SPM) jet is observed at flow rates greater than 0. Ilb,/sec., consistent with the pre-test CFD predictions. For the LPM, the jet appears to be nearly hlly-expanded, resulting in a very unsteady and oscillatory flow structure in which the bow shock becomes highly dispersed such that it is no longer discernable. Higher speed camera Schlieren data reveal the shock to be dispersed into striations of compression waves, which suddenly coalesce to a weaker bow shock with a larger standoff distance as the flow rate reached a critical value. The pronounced shock dispersion could significantly impact the aerodynamic performance (L/D) and heat flux reduction of spacecrafk in atmospheric entry and re-entry, and could also attenuate the entropy layer in hypersonic blunt body flows. For heat transfer, the results show significant reduction in heat flux, even giving negative heat flux for some of the SPM interactions, indicating that the flow wetting the model is cooling, instead of heating the model, which could significantly impact the requirements and design of thermal protection system. These findings strongly suggest that the application of counterflowing jets as active flow control could have strong impact on supersonic and hypersonic vehicle design and performance. IntroductionOne of the technical challenges in space exploration and interplanetary missions is controlled entry and re-entry into planetary and Earth atmospheres, which requires the dissipation of considerable kinetic energy as the spacecrafk decelerates and penetrates the atmosphere. As such, effective heat load management of stagnation points and acreage heating remain a technological challenge and pose significant risk, especially for human missions.

A Survey of Supersonic Retropropulsion Technology for Mars Entry, Descent, and Landing

“…Moeckel [6], [7] also observed flow separation on the forebody with the same configuration. Both effects were later observed in other experiments with a supersonic retropropulsion nozzle along the centerline of a blunt entry body [15], [18]- [21], [23]- [28], [34]- [35], [37], [39]- [41].…”

“…This behavior is shown by Mach number contours in Figure 4 [21]. A stable flowfield occurs when the bow shock is close to the body, and the jet flow does not penetrate the bow shock; in this case, the flowfield structure is not oscillating.…”

“…Experimental results for low thrust coefficients consistently show significant increases in the total axial force coefficient (summation of aerodynamic drag and thrust) for peripheral nozzle locations [15], [25]- [26]. In contrast, aerodynamic drag reduction was observed for centerline nozzle locations [15], [18]- [21], [23]- [25], [27]- [28], [34]- [35], [37], [39]- [41] . For high thrust coefficients, all nozzle configurations contributed substantially to the effective total axial force on the vehicle, though by thrust contributions only (no aerodynamic drag contribution) [15], [18], [20]- [21], [23]- [24], [26]- [27].…”

“…Although the majority of the literature is focused on deceleration applications, the aerothermal effects of supersonic retropropulsion, the development of test scaling parameters, and the capabilities of computational analysis have also been explored [15], [19], [21], [34]- [45]. Both experimental and computational work show the aerothermal effects of retropropulsion to be important, with the potential for heat transfer to the body to be doubled when combustion products are injected into the shock layer.…”

“…For high thrust coefficients, all nozzle configurations contributed substantially to the effective total axial force on the vehicle, though by thrust contributions only (no aerodynamic drag contribution) [15], [18], [20]- [21], [23]- [24], [26]- [27]. Additionally, the stability of the flowfield and resulting aerodynamic effects were found to be strongly dependent on the ratio of total pressure between the retropropulsion and the freestream [15], [18]- [21], [23]- [28], [34]- [35], [37], [39]- [41].…”