During the sea-level start-up of liquid rocket engines, the nozzle is highly overexpanded and an internal flow separation takes place, characterized by a shock wave boundary layer interaction (SWBLI) which causes the shedding of vortical structures and the unsteadiness in the shock wave position. This produces dynamic side-loads, which reduce the safe life of the engine and could lead to a failure of the nozzle structure.
The main aim of this research project is to investigate the flow separation unsteadiness in an over-expanded transonic nozzle flow through large-scale numerical simulations. The spectral features of the separation shock when the recirculation bubble is open to the external ambient will be explored through an advanced analysis based on wavelet transform. This characterization will contribute to the basic understanding of the shock induced separation in internal flows of typical rocket engines at start-up, potentially allowing to predict and control the level of side loads. In such a way it will be possible to design larger area ratio nozzle, increasing the nozzle performance and reducing the costs of the access to space.
The second objective of the project is to provide a comparison between a large eddy simulation (LES) and a Detached Eddy Simulation (DES) of the same flow, in order to assess the validity of the DES approach for this kind of flow. This task will be carried out by means of simulations performed with a fully validated in house flow solver based on Finite Volume (FV) approach, which takes advantage of a kinetic energy preserving formulation.
If the ambitious objectives here proposed are fulfilled, the project will bring significant advances in a crucial aerospace research area and will have a strong impact from the technological point of view.
The outcomes of the project are expected to have a strong impact from the viewpoint of fundamental science, bringing advances to a critical research area for both computational and theoretical fluid dynamics, shedding some light on the understanding of the key mechanisms involved in shock boundary layer interactions.
One of the main results expected from this research is the frequency characterization of the shock induced flow separation unsteadiness and the individuation of the main sources of disturbance. This characterization is very important from (i) a theoretical point of view, since it can give a contribution to the basic understanding of the shock wave/turbulent boundary layer interaction physics in supersonic flows with separation in propulsive nozzles, and also from (ii) a technological point of view, since a better knowledge of this phenomenon can help in predicting and possibly control the level of side loads. In such a way it will be possible to design larger area ratio nozzle, increasing the nozzle performance and reducing the costs of the access to space.
As far as shock wave/boundary layer interaction (SWBI) is concerned, it has to be said that a lot of papers can be found in literature in the last decades. The main finding that seems to be achieved is the idea that the shock unsteadiness has common features across a broad range of flows [1]. Less agreement can be found on the causes of the unsteadiness. According to Poggie [2], the shock system behaves as a selective amplifier of large scale disturbance in the incoming flow, while according to Touber [3] the low-frequency unsteadiness is an intrinsic properties of the coupled shock/boundary layer dynamical system. All these literature address the problem of closed recirculation bubbles, caused by shocks impinging on a flat plate or by compression ramps. Instead, the flow physics of the shock induced separation with a recirculating region open to the ambient air (typical of over-expanded nozzles), has received less attention and very few papers can be found in literature [4]. This phenomenon, consisting in fluctuating pressure loads and pulsating recirculating flows, should be carefully addressed by researchers and rocket nozzle designers, since the fundamental flow physics has not yet been completely understood. The comprehension of the behavior of over-expanded nozzles during the start-up will help in the development of flow control systems for the mitigation of side loads, allowing a safer design or a design with improved performances. It must be remembered, that the increase of the nozzle performance is considered one of the key factor to improve the propellant need of the launcher first stages and consequently to reduce the cost of the access to space.
Another important expected result is the assessment of the performance of the hybrid RANS/LES methods for this kind of flows. In recent litertaure [4] it is argued that the main sources of unsteadiness come from the subsonic separated flow and not from the attached supersonic boundary layer upstream of the shock. In such a case, hybrid methods could be well suited to study this kind of flow, allowing a reduction of the computational cost with respect to LES models and to afford the high Reynolds number of this kind of flows of industrial interest.
References.
[1] Clemens N.T., Narayanaswamy V., ¿Low-Frequency Unsteadiness of Shock Wave/Turbulent Boundary Layer Interactions¿, Annu. rev. Fluid. Mech., 46, 469-462, 2014.
[2] Poggie J., Bisek N.J., Kimmel R.L., Stanfield S.A., "Spectral Characteristics of Separation Shock Unsteadiness", AIAA Journal, Vol. 53, No. 1, 2015.
[3] Touber E., SANDHAM N.D. "Low-order stochastic modelling of low-frequency motions in reflected shock-wave/boundary-layer interactions". Journal of Fluid Mechanics, 671, pp 417-465, 2011.
[4] Olson B.J., Lele S.K., "A mechanism for unsteady separation in over-expanded nozzle flow", Phys. Fluids Vol. 25, issue 11, 2013.