Experimental verification and comparison of non-intrusive methods for detecting laminar-turbulent transition on a natural laminar flow axisymmetric body

Stewart, Ian (1997) Experimental verification and comparison of non-intrusive methods for detecting laminar-turbulent transition on a natural laminar flow axisymmetric body. University of Southampton, Doctoral Thesis.

Abstract

The use of natural laminar flow technology for the reduction of drag has long been known to be successful for aerodynamic and hydrodynamic bodies, but the degree of this success has been under discussion for many years. Part of the problem may be attributed to the difficulties associated with measuring the regime of the boundary layer flow over the body. Many sensors used for such measurements have been only minimally intrusive, however, the boundary layer of natural laminar flow vehicles may be critically stable. This has resulted in a lack of hydrodynamic data for natural laminar flow underwater vehicles, since by placing such a sensor on the surface of the vehicle, the flow within the boundary layer may be altered, and a subsequent alteration in hydrodynamic forces occurs.

This study was undertaken to develop and compare sensors suitable for determining whether the boundary layer on a natural laminar flow vehicle is laminar or turbulent, whilst remaining totally non-intrusive to the flow. Flush-mounted hot-film anemometry, pulsed-Doppler ultrasound, piezoelectric film, and flush-mounted microphones were assessed. Fully embedded microphones were selected as the most suitable sensor system and placed on an unfinned 1 metre towing-tank model for further validation.

A support sting for the model was specifically designed to be non-detrimental to the flow, to enable comparisons with previous researcher's work using similarly shaped vehicles. However, a vibrational response of the sting to the fluid flow prevented the target Reynolds number to be achieved. The flow separated from the vehicle prematurely, causing a large wake, thus producing larger hydrodynamic forces than those of previous researchers.

The microphones, however, were used to explain the hydrodynamic behaviour of the vehicle in this study, and were demonstrated as suitable for a prototype sensor for distinguishing between laminar and turbulent boundary layer flow on a natural laminar flow body, providing the far-field background noise remains constant in frequency content.

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