Experimental studies of supersonic cavities with passive flow control

Chang, Ting Tsung (2026) Experimental studies of supersonic cavities with passive flow control. PhD thesis, University of Glasgow.

Abstract

In this research, the open-type supersonic cavity flow is experimentally investigated. The cavity of the aspect ratio L/D = 4 is studied both qualitatively by flow visualization and quantitatively by pressure sensitive paint (PSP) and pressure measurement. The cavity is under supersonic conditions, Mach number of 1.65, and the Reynolds number based on the cavity length is of the order of 730000. At supersonic speed, the pressure difference between the front and rear walls is significant. This leads to a high aerodynamic form drag when the cavity is exposed to the free stream. Moreover, high frequency acoustic oscillations in the cavity would cause structural fatigue on the airframe. Thus, it is of interest to recognize the mean surface pressure distribution in the cavity and the dynamic of the acoustic oscillations in the cavity under various conditions and the effective way to control fluid dynamic behavior.

Visualizations from Schlieren and oil flow well picture the macro-structure of the supersonic cavity flow. The quasi-steady compression wave at the leading edge, and bow shock like structure are observed in the cavity flow. However, the pressure measurements by the pressure transducer and PSP reveal that the low-pressure region in the first half section of the cavity, which is a typical behavior in the backward-facing step in the supersonic flow, is the result of flow expansion. The pressure rises rapidly as it approaches the rear wall due to the flow recompresion (bow shock). The pressure is generally higher on the rear wall because of direct impingement of the shear layer. There is a local minimum measured in the middle of the rear wall that is caused by the downward acceleration of the flow entrainment. This is supported by the separation line observed on the rear wall by oil flow. The relatively high pressure fluctuation is also measured on the rear wall. By the Schlieren images, the variation of the bow shock structure is the main reason for the intense pressure fluctuation. Characteristic acoustic oscillations, Rossiter modes, are observed in the power spectral density spectrum. However, in the time-frequency domain, it shows that these oscillations are not periodic (theoretical assumption), but intermittent in time. Modes occur intensively within a relatively short time frame.

Passive control methods do not significantly change shock structures based on Schlieren images, but oil flow indicates that vortex structures inside the cavity are altered. The rear wall vortex moves away from the rear wall with a decreasing ramp angle. The separation line on the ramp moves toward the floor with the decreasing ramp angle, suggesting more flow entrainment. This consequently leads to a lower pressure and a more stable environment in the cavity. The probability density function (PDF) offers solid evidence showing less pressure fluctuation. Nevertheless, a significant pressure difference is induced by passive control. Aerodynamically, this has an unfavorable result: high form drag. In terms of acoustic oscillation suppression, the tonal amplitude of the principal Rossiter modes is significantly reduced. Special cases are also investigated to demonstrate the possibility of acoustic resonance amplification as the cavity is properly designed. It is necessary to note that such amplification is incorporated with the transition from Rossiter modes(fluid dynamic) to pure acoustic modes.

Parametric studies on the effect of the finite width on the supersonic cavity flow are conducted. The change due to the finite width is explicit. From the streak lines, an additional flow structure is observed along the junction of the rear wall and the side wall, suggesting that the flow escapes from the cavity. This leads to a surface pressure that is lower than that in the fullspan cavity. Moreover, slightly asymmetric streak lines are observed on the rear wall, indicating a skewed shear layer impingement on the rear wall in the finite-width cavity. The asymmetric behavior is more pronounced as the passive control method is applied. The pressure measurement on the rear wall surface provides solid evidence for this interpretation. From the frequency spectrum, the switch of the dominant mode from the second to the third mode as the width decreases is observed. The ratio of the two scalograms of the full-span and finite-width cavities shows a measurable energy change between modes. However, the switch of the dominant mode is not strongly related to the asymmetric behavior in the flow field. The general dynamic mode remains essentially the wake mode.

In addition to the geometry of the cavity, an investigation is also conducted to study the effect of the external installation placed in the cavity. The effects of volume and geometry are compared. There is a decreasing trend for the sound pressure level and the tonal amplitudes with increasing occupied volume of the installations. Furthermore, these installations lead to a steady and constant pressure distribution within the cavity. The geometry of these installations merely affects the pressure distribution or the frequency spectrum. When the volume of these installations doubles, although the trend of decreasing sound pressure level continues, the behavior of Rossiter modes starts to change. It is assumed that the height of the artifice alters the formation of the shear layer, and consequently the behavior of the associated mode is changed. PDF of modulated signals by the CWT show that the feedback loop of the dominant oscillation mode is enhanced.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Supported by National Chung-Shan Institute of Science and Technology (NCSIST). Due to third party copyright issues the electronic version of this thesis is not available for viewing.
• Subjects: T Technology > TJ Mechanical engineering and machinery; T Technology > TL Motor vehicles. Aeronautics. Astronautics
• Colleges/Schools: College of Science and Engineering > School of Engineering
• Funder's Name: National Chung-Shan Institute of Science and Technology (NCSIS)T
• Supervisor's Name: Kontis, Professor Konstantinos
• Date of Award: 2026
• Depositing User: Theses Team
• Unique ID: glathesis:2026-86247
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 21 Sep 2026 11:45
• Last Modified: 21 Sep 2026 11:47
• Thesis DOI: 10.5525/gla.thesis.86247
• URI: https://theses.gla.ac.uk/id/eprint/86247