Intrusive Measurement Techniques

Even nowadays the flow is still diagnosed by solid probes such as Pitot probes, thermocouples and hot wires in particular, it has been crucial in not only capturing the mean flow, but also in providing further insight into coherent structures. Despite more sophisticated measurement techniques, in many cases their use is justified by their accuracy, reliability, cost and simplicity. The main drawback are their intrusive effects, leading to scepticism during the measurement of shear layer developed by a separated flow and are more delicate in transonic flow. Besides, to avoid vibration of the probe the struts and mounts have the tendency to be voluminous and this introduces solid blockage which has an impact on the desired pressure distribution. Also, during spatial characterisation of the flow, complex and expensive traverse system is usually required and the displaced position of the strut could again impair with the local and overall pressure gradient. During the measurement in low pressure turbines and hypersonic flows, the probes need to be able to withstand very high temperature without disintegrating or deforming due to thermal expansions. An operational limit is usually set at approximately 1200 K and above this temperature more exotic material or sometimes cooling systems are required, which increases cost and complexity of the technique. In confined experimental facilities where reduced scale, miniature models are tested, deformation and vibration of the probes introduces large uncertainty in terms of positioning of the probe and hence compromising significantly the end result.