title: Directed energy deposition flow control for high speed intake applications
Creators: name: Russell, Andrew
Publisher: name: University of Glasgow
Year: 2020
Subjects: subject: supersonic aerodynamics, flow control, experimental.

description: One of the key enabling technologies for high speed airframes is the propulsion system. Some of the important factors in any widely used propulsion system are reliability and efficiency. Both of these performance indicators are areas that can still be improved for a popular high speed propulsion system, the Ramjet/Scramjet. Regarding the propulsion system efficiency, one component that has a significant impact is the intake. Specifically the intake efficiency, often quantified by the total pressure loss, can significantly influence the overall system performance. Now considering the reliability of the system, a phenomenon known as unstart can present issues for a Ramjet/Scramjet during its operation. This phenomenon refers to the ejection of the shock structure, known as a shock train, from within the internal duct in the ramjet/Scramjet called the isolator. This results in a bow shock upstream of the propulsion system intake leading to significant drag increases and thrust decreases due to reductions in intake mass flow. This work aims to propose methods that can tackle both of the issues highlighted above. The goal is to use energy deposition based flow control methods to improve the system performance. To achieve this, the first step was the improvement of the available wind tunnel facilities. Through the careful examination and numerical simulations of the existing wind tunnel design, key issues are identified and solved methodically. This included the re-design of the nozzle; both the subsonic and supersonic sections of the nozzle are carefully designed. The diffuser is also re-designed to ensure it is suitable for the desired test section Mach number. Finally, the location of the wind tunnel relative to the vacuum tank is examined and changes are made in order to reduce the conductance present within the system to a level at which the desired mass flow rate could be achieved, and the wind tunnel can operate at the desired flow conditions. It has previously been shown that the inclusion of a cavity within the isolator can improve the unstart margin. However the introduction of a cavity will result in an increase in the drag of the overall propulsion system. Therefore the aim of this work, with regard to improving the unstart performance of the propulsion system, is to examine how energy deposition flow control can be used to control a supersonic cavity flow. This research will examine the use of nanosecond, high voltage dielectric barrier discharge (ns-DBD) plasma actuators to control the cavity flow. However before this, the interaction of the cavity with the baseline internal duct flow was examined. This was in order to classify the scale of cavity that could be included in a duct before the influence on the flow was too large for the duct to start. Through Schlieren imaging, it is shown that increasing cavity scale led to an increase in the cavity shear layer which, in turn, became a limiting factor in the ability of the supersonic duct flow to start successfully. It is suggested that increasing cavity length is the dimension with the largest impact on the starting of the duct flow. This work allowed the cavity being examined for flow control applications to be appropriately scaled so as to still allow the supersonic duct flow to start correctly. The second topic that is necessary before the cavity flow control can be examined is the charaterisation of the ns-DBD actuators. The key goal of this section of research is to identify the combination of actuators parameters that resulted in the strongest flow control influence, measured in this case by the generated pressure wave strength. This characterisation process is conducted through the image processing of the captured Schlieren images during the actuator operation. It is found that the only parameters investigated with significant impact on the actuator control authority are electrode length and dielectric thickness. Shorter electrodes and thinner dielectric barriers are found to be the most successful, for the same electrical input signal used and the same dielectric material. Whilst characterising the actuators for control authority, the electrical efficiency of the actuators is also investigated using electrical measurements, quantitative Schlieren and infrared thermography. The quantitative Schlieren and electrical measurements were found to give accurate results, however they did not present any clear conclusions regarding how the geometric parameters investigated influenced the electrical efficiency. The infrared thermography was unsuccessful due to the small temperature variations being measured and the relatively large uncertainty of the equipment used. Following the characterisation of the ns-DBD plasma actuators, they were applied as flow control to the supersonic cavity flow. Laser vibrometry, Schlieren and oil flow are used to investigate the impact that these actuators have on the baseline supersonic cavity flow. It is found that the actuators available are not able to provide any measureable flow control authority on the baseline cavity flow. The other performance indicator highlighted above is the efficiency of the intake. This study focuses on the improvement of intake total pressure recovery through the use of laser energy deposition flow control. The aim of the flow control is to mitigate the separation observed on the external compression ramp and, as a result, mitigate the negative impact of the buzz instability, commonly observed in supersonic intakes. It is shown that the use of laser energy deposition can provide an improvement in total pressure recovery and does reduce the impact of these buzz instability modes. It is also illustrated how, for a particular set of example conditions, this could improve the overall fuel consumption of a given Ramjet propulsion system.