Laminar flow control of a flat plate boundary layer using dielectric barrier discharge plasma

Abstract

The drag developed on an object as it moves through a fluid comprises of a number of components arising from various and differing fluid phenomena. For aerodynamic bodies such as aircraft, one of the most dominant components of the total drag force is that arising from shear interactions between the surface of the object and the fluid. In steady, cruise conditions this shear-induced skin friction drag can account for almost 50% of the total drag force on the body and hence this is the reason much interest surrounds the minimisation of this component. Laminar Flow Control (LFC) is the field of aerodynamics focused on minimising skin friction, or viscous drag. The viscosity of a fluid, and the shear interactions between the layers of fluid and the aerodynamic body give rise to a boundary layer, a region of fluid with diminished fluid velocity and momentum. Laminar Flow Control aims to minimise the momentum deficit within the boundary layer by manipulating the flow within and encouraging favourable flow conditions to exist and be maintained. In essence, Laminar Flow Control attempts to maintain laminar flow within the boundary layer, improving the stability of the flow, delaying the onset of turbulence and the formation of a turbulent boundary layer that develops significantly more drag than an equivalent laminar structure. A number of techniques exist for controlling and maintaining laminar flow within a boundary layer. Examples include compliant surfaces, acoustic arrays and suction, and all share the common trait of complexity, which to date has limited the application of such systems in the real world. In the search for simpler Laminar Flow Control technology, attention has been turned towards Dielectric Barrier Discharge (DBD) plasma actuators as a possible alternative. Through the formation of a small volume of plasma, these actuators are capable of producing an electrostatic body force that can couple with the surrounding air and bring about a jetting effect without the addition of mass. This jetting effect, if controlled effectively, can potentially favourably augment a boundary layer flow and lead to a delay in transition. The work discussed in this thesis represents a contribution to the field of DBD-based Laminar Flow Control. The aim was to further investigate the potential of plasma actuators for improving the hydrodynamic stability of a boundary layer and hence contribute to the limited published data pertaining to this field. The research involved the development of a DBD-based LFC system in which plasma actuators were used to augment the most fundamental of boundary layer flows, the flat plate, Blasius-type. By measuring the augmentation to the velocity profile of the boundary layer brought about by the LFC system, the stability of the flow was able to be investigated and hence the feasibility of the technology determined. The plasma actuators utilised in this research were designed such that control could be achieved over the shape of the induced jetting profile. To minimise adverse interactions with boundary layer flows, the plasma actuators were designed so that the magnitude and position of the maximal induced jetting velocity could be controlled. After consultation of the literature, novel actuators utilising orthogonally arranged electrodes were conceived and tested in a parametric study. Through variation of the distance to which the exposed electrode sat proud above the surface of the actuator, in addition to variation of the applied voltage, it was found that the desired control over the induced jet could be attained, leading to the identification of two mechanisms through which the DBD-based LFC system could be tuned. The details of the design and development of these orthogonal actuators and the effect of the electrode height on the jetting characteristics of the devices can be found in Gibson et al. (2009a) and Gibson et al. (2009b). After identifying suitable and novel actuator arrangements, a tuning strategy was conceived to hasten the development of the LFC system. Rather than implementing the actuators and measuring the response of the boundary layer to the plasma first, Linear Stability Theory was instead used to identify desirable boundary layer augmentation objectives for the LFC system. Linear Stability Analyses (LSAs) were performed on a number of idealised boundary layer flows, obtained from curve fitting analytical functions to published DBDaugmented boundary layer data, as well as from boundary layer theory. The LSAs were conducted using an Orr-Sommerfeld Equation solver developed as part of this research, which utilises a finite differencing scheme. The outcome of this comparison process was that the developed DBD-based LFC system was used to attempt to augment the boundary layer such that the flow attained an asymptotic suction velocity profile, which would give the boundary layer a limit of stability almost two orders of magnitude greater than that of the base flow, and hence significant robustness to transition. The conceived DBD-based LFC system was implemented into a Blasiusv type boundary layer which was formed over the Flat Plate Rig (FPR) designed and developed as part of this research. Initially a single actuator was utilised, positioned just upstream of the location of the critical Reynolds Number (limit of stability) of the flow. Due to the design of the FPR and the actuators utilised, it was possible to study the response of the layer to the plasma with and without a mild suction effect, introduced through a 5mm wide slot that was required for operation of the actuator. This mild suction effect was measured to be approximately 4Pa, and by itself was found to be insufficiently strong enough to augment the flow such that it attained the characteristics of a boundary layer with uniform wall suction. With the FPR, measurements of the velocity profile of the boundary layer with and without flow control were made around the critical Reynolds Number location of the flow (80000 < Rex < 120000), which allowed the changes to the stability of the flow to be studied. As discussed in Gibson et al. (2012) the initial results of the DBD-based LFC system showed that the plasma was adversely affecting the stability of the flow. Subsequent tuning of the system was therefore performed through variation of the applied voltage of the actuator. From this tuning it was found that an actuator operated with an applied voltage of 19.0kVpp (referred to as a low-voltage actuator) in conjunction with the mild suction effect, produced boundary layer characteristics akin to those of a flow exposed to uniform wall suction. In addition, an actuator operated with an applied voltage of 21.4kVpp (referred to as a high-voltage actuator) was found to adversely affect the stability, even more so in the absence of the mild suction effect. The single low-voltage actuator was found to be able to maintain uniform wall suction-like characteristics for 50mm beyond the trailing edge of the encapsulated electrode. This finding pertaining to the use of the low-voltage actuator highlighted the potential of a single DBD device to develop uniform wall suction-like characteristics with only a mild suction effect through a single slot, and hence in a less complex fashion than conventional suction systems. An attempt was made to maintain the favourable benefits of the single, low-voltage actuator by using two such actuators placed in series. However, the effect of this combined double-actuator/suction system differed only slightly from the suction-only system (with two slots instead of one), meaning that in this configuration, the use of the plasma was somewhat superfluous. Hence it could be concluded from the results of the research that a single low-voltage actuator operated in conjunction with a mild suction effect is more effective as a LFC system than a single mild-suction slot, but a combined double-low-voltage actuator/suction system is no better than a simpler and less energy consuming double-mild-suction slot system. It is, however, anticipated that through the undertaking of future works, utilising additional actuators that have undergone further tuning, a LFC even more effective than the double suction slot system tested in this research will ultimately be developed.