The attenuation of sweep events within the turbulent boundary layer over a flat plate using a micro-cavity array

Abstract

For most aeronautical applications skin friction drag is a significant issue for efficient operation. It is estimated that 49% of the total drag an aircraft experiences is due to skin friction drag and a small reduction of 5% would currently result in an annual saving of $3.3 billion US dollars. The large Reynolds number at which aerospace applications typically operate results in a turbulent boundary layer, which causes a large increase in shear stresses and a subsequent increase in skin friction drag. The key culprits for the large shear stresses are the turbulent boundary layer structures that form once the boundary layer transitions from the initial laminar state, the most influential of which are the coherent structures. These structures pump fluid into (sweep) and away (ejection) from the near wall region and generate the shear stresses. Hence the aim of this research is to manipulate the turbulent boundary layer to reduce the effect of the aforementioned coherent structures. Specific attention has been applied to the passive application of a micro-cavity array as a potential control technique to attenuate the coherent structures. The micro-cavity array consists of a cavity arranged flush with the surface, underneath of which is a backing cavity similar to the design of a Helmholtz resonator. As a passive control technique, this device aims to have the advantages of an easy implementation and the absence of an external power source, with targeted control of the coherent structures commonly achieved by active systems. The micro-cavity array aims to capture and dampen the sweep events, therefore reducing the strength of both the ejection and sweep events due to their high dependence on one another. As such the present work assesses the ability of an array of micro-cavities to reduce the turbulent properties of a fully developed boundary layer. Previous results from the flow excited Helmholtz resonator and a two-dimensional square cavity on a flat plate have confirmed the potential of the micro-cavity array. Both techniques achieved successful control of the boundary layer and attenuation of the coherent structures. However these applications had limitations at higher Reynolds numbers and as a result a smaller device such as the micro-cavity is proposed and forms the basis of this thesis. Being of smaller size, the shear layer is hypothesised not to break apart while traversing the small orifices of the micro-cavity, which occurs for the larger flow excited Helmholtz resonator and results in an adverse pressure gradient and an undesirable increase in the disturbances and viscous drag in the boundary layer. To investigate the potential of the proposed micro-cavity array, the device has been thoroughly examined experimentally at a range of Reynolds numbers (1195 < Reθ < 3771). Experiments were predominately focused on identifying the potential of the micro-cavity, whilst evaluating the impact of the orifice distribution along the cavity array and the effect of other geometric parameters, including the length of the cavity array and the backing volume. Measurements were made using a single hot-wire and a constant temperature hot-wire anemometry system downstream of the cavity arrays, with all results being compared against canonical boundary layer profiles to record the effect of the micro-cavity array. These experiments demonstrated the success of the micro-cavity array in controlling the turbulent boundary layer and identified the mechanism causing the recorded attenuation of the boundary layer. The results showed that the optimal orifice diameter must be equal to a value of approximately 60 times the viscous length scale. This resulted in a maximum reduction in the turbulence and sweep intensities of 13% and 14%, respectively. The results demonstrated that for a cavity orifice diameter less than 20 times the viscous length scale, the sweep events are restricted and no events are captured by the array. Additionally, if the diameter of the orifice exceeds 145 times the viscous length scale, separation of the shear layer was observed, causing an increase in the turbulence energy production in the near wall region. The volume of the backing cavity was also shown to be a very important characteristic in the design of the micro-cavity array, while the orifice length of the cavity array had negligible effect in modifying the reduction of the turbulent energy by the cavity array. The maximum reduction in turbulence generation occurred when the backing volume was as large as possible, which reduced the reactive impedance of the microcavity device. However, the sweep intensity reduction reached a limiting value as the volume increased. The reduction in turbulent energy was also shown to occur irrespective of whether the individual cavity arrays shared a common backing volume or had individual backing volumes. Consequently a strongly supported finding is that the cavity array weakens the sweep intensity of the captured sweep events by damping the energy of the events through the friction losses in the cavity array and also in the large volume of the backing cavity. This results in a reduction in the strength of the bursting events responsible for the shear stresses in the near wall region. The body of work presented here is only the beginning of the development of the knowledge required for this area of work. The results of this study demonstrate an improved understanding of the micro-cavity array as a potential flow control device for the turbulent boundary layer in the future and as such requires further investigation.