Numerical simulation of a convergent divergent supersonic nozzle flow
Creators: name: Ekanayake, Sudharshani
Publisher: name: RMIT University
Year: 2024

description: Supersonic fluid flow through a two dimensional Convergent Divergent (CD) nozzle with angle variation added to the divergent section of the nozzle using ANSYS CFX. . Flow undergoes many forms of unique phenomena, including ﬂow separation, unsteadiness, ﬂow mixing, turbulence, Shock Induced Boundary Layer (SIBL) separation and Mach Shock Diamonds, when gas expanded through a CD nozzle. Some of these phenomena lead to energy loss, thereby reducing the overall thrust generated by the nozzle. The thrust loss due to shock waves and boundary layer separations in nozzle flows remains poorly understood, hence failed to reach maximum potential of an engine.<br><br> In this research, two nozzle conﬁgurations, with two symmetric and two asymmetric geometry shapes are investigated. Asymmetric is introduced by adding contraction angles at the divergent section. Numerical analysis is focused on the inﬂuence of the nozzle geometry, the Nozzle Area Ratio (NAR) and Nozzle Pressure Ratio (NPR) on the ﬂow properties downstream (divergent section) and the external (jet plume) region of the nozzle. Capturing the boundary layer ﬂow characteristics under strong adverse pressure gradients is of particular interest of this study. The NAR is varied to investigate the gas ﬂow direction and speed, for the asymmetric nozzle in Underexpanded conditions at high NPRs. The two symmetric model conﬁgurations, NAR 1.5 and NAR 1.66, have a divergent angle at the throat of 2.801 and 3.89 degrees respectively. The two asymmetric geometry conﬁgurations, NAR 1.14 and 1.21, consist of a divergent angle of 2.801 degrees at the throat with contraction angle variations in the divergent section. ANSYS CFX is used to solve time-dependent RANS equations for supersonic two dimensional (2D) nozzle ﬂow with Shear Stress Transport (SST) turbulence model. The NPRs for both symmetric and asymmetric are varied between 1.27 – 12.0 under the sea-level condition for the steady state solutions.<br><br>It is found that -under low NPRs, the SST model has transcended in performance to capture internal shocks, as well as boundary layer separation, re-circulation zones, shear layer stresses caused by strong adverse pressure gradients accurately in axis-symmetric nozzles. Typical Lambda shocks associated to internal ﬂow separation is not observed for the asymmetry type model, NAR 1.21. Asymmetric nozzles have produced higher Mach number values - than that of symmetric - nozzles.<br><br>The research has also found that when varying the divergent section of the asymmetric nozzles, the ﬂow path is vectored away from the nozzle axis line at high NPRs. The ﬂow is signiﬁcantly oﬀset in the desired direction and is considerably diﬀerent from the traditional Mach Diamond shock pattern observed in symmetric nozzle shapes at the jet plume region. Asymmetric nozzle geometries have a major contribution towards the size of the Mach disks and Diamond shock patterns within the jet plume region. Varying the angle of the top and the bottom walls has a signiﬁcant eﬀect on the exhaust ﬂow direction. This could be implemented in the future high speed nozzle designs.