The impact of freely suspended particles on laminar boundary layers

Following recent volatility in fuel markets, laminar flow technologies are again being considered as an option for increasing aircraft efficiencies in order to cope with the forecasted growth of air traffic and to simultaneously reduce emissions. However, some aspects on the operation of such aircraft are not fully understood, such as the impact of ice crystals in cirrus cloud. This leads to uncertainties in regard to fuel planning for laminar flow aircraft, and would prevent the optimal benefits accruing in flight operations. The latest published in-flight experimental research, from the 1980s, resulted in information that is too sparse and incomplete to allow for conclusive deductions. A computational analysis, which has been set up in accordance to Northrop X-21 flight tests, revealed major discrepancies regarding some of the basic assumptions made for the original analytical study concerning the impact of ice particles on laminar flow. Recent laboratory experiments succeeded to demonstrate the effect of freely suspended particles. These experiments validated both the double pitot approach, which was originally used, as well as hot films as eligible candidates for the detection of the phenomena; however, the former approach is believed to produce an incomplete picture. Even though sensitivity studies on the development of the critical limits could be undertaken, definite statements will require more detailed investigation allowing for a better controllability of the particles' injection. Furthermore, it is believed that greater insight into the basic phenomenon of how freely suspended particles deteriorate an initially laminar boundary layer will only be gained from capturing the flow field around a single particle close to its surface impingement location. For this purpose, experiments are in progress involving smoke flow visualisation and a high speed camera. The difficulties connected with reproducing the effect in a wind tunnel with realistic parameters might require further support from numerical analyses.