City Research Online

Abstract

Wake-boundary layer interactions are common in multi-element aerofoils and turbomachines. The laminar-turbulent transition mechanism in such systems, occurring due to the interaction of upstream wake with the downstream boundary layer, has been the subject of several studies reported in the literature. Nevertheless, most of the research carried out in the literature has used a circular cylinder as the upstream wake generating body, while most real applications experience upstream disturbances originating from an aerofoil wake. Further, the wake characteristics of a circular cylinder and aerofoil are entirely different; this, in turn, might result in different transitional characteristics. Thus, to reveal the mechanisms in wake boundary layer interactions in multi-element aerofoils, it is necessary to reproduce the evolving flow field as realistically as possible. In this context, the present research addresses the interaction problem considering a simplified but realistic system involving an aerofoil and a flat plate. Such an experimental setup is new and has not be employed before for transition studies. An experimental investigation has been carried out in the low turbulence Gaster wind tunnel at City, University of London. Here, a NACA 0014 aerofoil at zero degree angle of attack is used to generate an upstream wake and a flat plate is used as the downstream surface, resulting in an interaction of the aerofoil wake with the initially laminar boundary layer on the flat plate. All the measurements in the present experiments have been carried out using hot-wire anemometry technique to obtain the mean velocity profiles and two-point correlations. Using the new data, a skewness based technique has been developed to differentiate the laminar, turbulent and transition zones in the flow. In addition, a rational procedure for determining the intermittency is proposed with the aim to overcome the subjectivity involved in the methods proposed in the literature. The results obtained from the proposed method have been validated with the existing dual-slope method. The core part of the investigation is the interaction of the upstream aerofoil wake and the downstream boundary layer on the flat plate. In particular, the laminarturbulent transition characteristics due to the above interaction has been studied in the pre transitional zone in order to understand the initial disturbance growth and further establish the transition mechanism. An important contribution lies in the study of the effect of gap between the aerofoil and the flat plate on the transitional characteristics. From the measurements in the pre-transitional zone, it is observed that the streamwise disturbance growth is exponential. In addition, the streamwise power spectrum confirms that the spatial growth of the disturbance is similar to that of a T-S wave, which is further verified using linear stability analysis. Certainly, such spatial amplification is characteristic of natural transition. On the other hand, in the same pre-transitional region, two features typical of by-pass transition are observed: a non-modal disturbance profile in the wall-normal direction and the presence of a longitudinal streaky structure. These measurements in the pre-transitional zone reveal the presence of mixed behaviour, with characteristics of both natural and bypass transition, which is entirely different to that of the cylinder wake-boundary layer interaction and to free-stream turbulence induced transition. Another interesting observation is that the spanwise scale of the streaky structure appears to be much larger (> 10δ) than that predicted by transient growth theory, and it varies with the gap between the aerofoil and the flat plate. On the whole, the aerofoil-wake boundary layer interaction seems to result in a different type of transition mechanism which possesses features associated with both natural and bypass transition.

Publication Type:	Thesis (Doctoral)
Subjects:	Q Science > Q Science (General) Q Science > QA Mathematics T Technology > TJ Mechanical engineering and machinery
Departments:	Doctoral Theses School of Science & Technology School of Science & Technology > Engineering

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