Numerical Investigation of Flow Oscillation in the Diffuser and Test Chamber of a High Altitude Test Simulator
Document Type : Original Article
Authors
1
Space Transportation Research Institute, Iranian Space Research Center
2
Department of Aerospace engineering,, Sharif university of Technology, Tehran, Iran
3
Department of Aerospace Engineering, Sharif University of Technology, Azadi Street, P.O. Box: 11365-11155, Tehran, Iran
Abstract
Keywords
Article Info
The present research has been conducted to numerical simulation and phenomenology of the pressure fluctuations and instability of the diffuser with the area ratio of the diffuser inlet to the nozzle outlet of 1.91. The numerical simulation has been done with both steady and unsteady methods for the nozzle with an expansion ratio of 30. Validation of the numerical solution has been done with the help of the experimental test data of the high-altitude test simulator available in the Space transportation research institute. Investigations indicate that two types of start occur in the system, stable and unstable start. In the stable start process, which occurs in high motor pressure tests, the flow is supersonic over much of the length of the diffuser. On the contrary and in the unstable start, the flow detection and study shows that the flow is not supersonic in most of the length of the diffuser and the thickness of the boundary layer in the supersonic region is very thick. This causes the downstream disturbances of the diffuser to be transferred to the upstream. These disturbances are mainly caused by unstable structures of turbulence separation in the flow. The fluctuation of the separation zones in the diffuser causes the fluctuation of the internal shock train structure. The change in the structure of the shocks as well as the instabilities created in the shock train and their movement upstream creates a flow separation zone at the entrance of the vacuum chamber and mass suction occurs inside the chamber, which results in the diffuser leaving the starting condition. As the time increases and the pressure of the vacuum chamber increases, the momentum of the exiting flow from the nozzle dominates again and creates a quasi-starting state in the diffuser, and this repetition loop continues intermittently.
Article Info
The present research has been conducted to numerical simulation and phenomenology of the pressure fluctuations and instability of the diffuser with the area ratio of the diffuser inlet to the nozzle outlet of 1.91. The numerical simulation has been done with both steady and unsteady methods for the nozzle with an expansion ratio of 30. Validation of the numerical solution has been done with the help of the experimental test data of the high-altitude test simulator available in the Space transportation research institute. Investigations indicate that two types of start occur in the system, stable and unstable start. In the stable start process, which occurs in high motor pressure tests, the flow is supersonic over much of the length of the diffuser. On the contrary and in the unstable start, the flow detection and study shows that the flow is not supersonic in most of the length of the diffuser and the thickness of the boundary layer in the supersonic region is very thick. This causes the downstream disturbances of the diffuser to be transferred to the upstream. These disturbances are mainly caused by unstable structures of turbulence separation in the flow. The fluctuation of the separation zones in the diffuser causes the fluctuation of the internal shock train structure. The change in the structure of the shocks as well as the instabilities created in the shock train and their movement upstream creates a flow separation zone at the entrance of the vacuum chamber and mass suction occurs inside the chamber, which results in the diffuser leaving the starting condition. As the time increases and the pressure of the vacuum chamber increases, the momentum of the exiting flow from the nozzle dominates again and creates a quasi-starting state in the diffuser, and this repetition loop continues intermittently.
Keywords
Subjects
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Volume 5, Issue 1 - Serial Number 9
September 2025Pages 18-30
- Receive Date 10 June 2024
- Revise Date 10 July 2024
- Accept Date 15 February 2025