Original articles
Numerical assessment of the flowfield features at the exit of Scirocco plasma wind tunnel nozzle

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Abstract

The simulation of the aerothermal environment that a space vehicle experiences during the atmospheric descent is mandatory for the design of thermal shields of re-entry vehicles. In this framework, both numerical and experimental design approaches, as computational fluid dynamics and plasma wind tunnel simulations, are fundamental. Numerical and engineering analyses of the non-equilibrium flow of Scirocco plasma wind tunnel nozzle, aimed to get test chamber predictions, are described in this paper. A data interpolation code, named SINDA, developed at CIRA in order to support the experimental test activities performed in Scirocco, is presented. SINDA is able to evaluate several flowfield features at the nozzle exit sections, starting only from a limited set of numerical rebuilding of facility tests and using a spline based algorithm. Comparisons between numerical data and SINDA results show good agreement between each other.

Introduction

The simulation of the aerothermal environment (e.g., pressure, heat flux and integrated heat load conditions) that a space vehicle has to withstand during the atmospheric descent is of essential interest for the design of thermal shields of re-entry vehicles [8]. This can be done, in a collaborative design approach, by involving both numerical and experimental approaches by means of computational fluid dynamics (CFD) and plasma wind tunnel (PWT) simulations, respectively [1]. For instance, as far as the atmospheric gas flows through the shock wave which envelops the planetary entering vehicle, it is heated to high temperatures and its composition changes as a result of thermo-chemical processes that take place within the shock layer [2]. So, from the experimental point of view, it can be simulated only in a high enthalpy facility by using very energized flows. On the other hand, when a PWT test is performed, it is of upmost importance to be able to compute the flowfield inside the nozzle, in order to ascertain the upstream conditions in front of the model being studied. To date, the world powerful PWT facility is Scirocco, located at Centro Italiano Ricerche Aerospaziali (CIRA). It is a hypersonic PWT based on an electric arc heater (EAH), with a maximum power of 70 MW. This facility allows to simulate experimentally the aero-thermal environment experienced by a space vehicle during an atmospheric entry [13]. For instance, plasma tests enable to verify the thermo-structural resistance characteristics of the materials and structures used for the thermal protection system (TPS) of space vehicles. Air plasma, produced by means of an EAH, expands through a conical nozzle and, finally, shots onto the test article. The plasma jet can reach, at the nozzle exit, up to about 2 m in diameter, a total temperature of 10,000 K, and a Mach number up to 10 [6].

In this framework a design environment, named SINDA, has been developed at the Aerothermodynamic and Space Propulsion Laboratory, aiming researchers to assess the flowfield features at the exit of Scirocco nozzle, thus supporting PWT experimental test campaigns being performed at CIRA. Hence, a numerical analysis of the chemically and vibrationally non-equilibrium flow that takes place in the PWT nozzle is also discussed.

Scirocco tests are very expensive (e.g., both facility and test preparation), so in order to reduce the number of runs needed for facility regulation, CFD simulations are mandatory to predict the flowfield features in the test chamber. Extensive numerical support, however, is not a practicable approach since, during design activities, the number of numerical simulations to be performed is too high and they cannot be executed in a reasonable time. Therefore, it is mandatory to consider a simpler methodology. To this aim, SINDA basing on the heritage of a limited set of numerical rebuilding of facility tests and from previous PWT testing campaigns, throughout an interpolation technique, allows to quickly evaluate several flowfield features at the nozzle exit section, for assigned Scirocco running conditions.

Section snippets

The Scirocco facility

The Scirocco PWT is an electric arc heated facility capable of producing a low pressure–high enthalpy hypersonic flow of large dimension in scale up to 1:1 with a test duration up to 30 min. A scheme of the facility is shown in Fig. 1[4]. Four different nozzle configurations are available, to achieve the desired flow conditions in the test chamber and to match model size. The four nozzles (named as C, D, E and F) differ from one another for the length and for the exit section diameter (see Fig. 2

Scirocco test chamber flow assessment

Generally speaking, a ground-based test focuses on duplication of free flight conditions experienced by a space vehicle during re-entry. Hence, considering that the facility performance envelope is recognized through a total pressure (P0)–total enthalpy (H0) chart, experimental testing campaigns demand the assessment, before the experiments, of the reservoir conditions (P0, H0) (e.g., facility regulation) required to duplicate the needed flowfield features in the test chamber. As a matter of

Governing equations and numerical simulation of the Scirocco nozzle flowfield

The flow in the nozzle starting from the reservoir conditions has been simulated numerically by means of the CFD code H3NS, developed at the Aerospace Propulsion and Reacting Flow Laboratory of CIRA [12]. It solves the Reynolds Averaged Navier–Stokes equations for a thermo-chemical non-equilibrium flow since, in order to accurately predict the nozzle flowfield, it is necessary to take into account for both physical and chemical effects of high enthalpy expanding flows [5], [14]. The system of

SINDA: a spline based algorithm

The polynomial approximation through a limited number of points is usually the simplest and efficient way to interpolate data. On the other side, the polynomial approximation through a large number of points should be avoided, due to the oscillations that increase as n (degree of the polynomial) grows. This difficulty can be prevented by approximating the function locally between each couple of points, with distinct polynomials of low degree (splines), and imposing that the whole built function

Concluding remarks

An interpolation technique, named SINDA, able to assess the flowfield conditions in the test chamber of the Scirocco facility of the Centro Italiano Ricerche Aerospaziali has been presented and described in this paper. SINDA is a spline-based algorithm able to provide, for particular Scirocco running conditions, several flowfield features at the nozzle exit only starting from a limited set of CFD rebuilding of facility tests, without recurring to the challenging and time consuming integration

Acknowledgements

The authors would like to thank Dr. Alfonso Matrone, Dr. Marco Marini and Dr. Andrea Mastellone of CIRA for their helpful suggestions in the progress of the present work.

This work has been partially funded by PRORA (Italian Program of Aerospace Research).

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