NUMERICAL SIMULATION OF GAS DYNAMICS AND ACOUSTICS OF ROCKET LIFT-OFF

It is known that high-level acoustic loads can damage the vehicle payloads. This problem makes the noise level reduction during the lift-off highly relevant. It leads to the necessity of the accurate prediction of acoustic loads. This goal can be achieved by using both experimental and numerical methods. Experimental investigation is very expensive, making it acceptable mainly for small-scale models, whereas the accuracy of numerical simulation is limited by plenty of factors, e.g. approximation order, computational costs, etc. So small-scale experiments can be used for the verification of numerical models, and numerical simulations should be applied for small-scale experiments scaling on real-size equipment. High-power acoustic waves are emitted by supersonic turbulent jets going out from the vehicle nozzles. State-of-the-art CFD techniques, like LES or DES are known to reproduce many non-linear effects with acceptable accuracy in the region of interest (near-field in proximity to jets). However, acoustic waves propagation to far-field and their interaction is very time-consuming problem. Diffusive properties of the numerical schemes implemented in many general purpose CFD codes operating with unstructured grids like OpenFOAM demand very high mesh resolution. For example, full three-dimensional CFD simulation of rocket jets on launch pad together with sound wave propagation require thousands of billions cells. An alternative way is hybrid model usage. Region of interest is split into two subregions called near-field and far-field. Full CFD simulation is performed only in the near-field region where fluid flow is essentially non-linear. In the far-field the convective motion of the fluid is negligible. So, acoustic waves propagation can be described by linear second-order wave equation with constant coefficients. This equation can be solved using for example, boundary integral equation methods. It allows to reduce dimension of the problem to be solved numerically, which decreases overall computational cost. In the present research the following techniques are used. 1) CFD model is used for the simulation of supersonic turbulent jet and flow in the near-field. CFD equations are approximated with the implicit Finite Volume Method (FVM) and hybrid scheme for subsonic/supersonic flows. 2) Wave propagation of acoustic disturbances in far-field is performed using Boundary Element Method (BEM). This approach allows taking into account waves reflection from launch pad walls, launch tower and other buildings and structures. In order to preserve uniqueness of solution for all wave numbers, combined form of exterior mixed problem is used.