Studying the Coanda effect


As part of the continued application of modern techniques in the design and development of the improved class P2, The A1 Steam Locomotive Trust has commissioned Frewer & Co Engineers Ltd of Dorking, Surrey to examine the impact of the Coanda effect on the new locomotive. Following the successful partnership with Frewer & Co Engineers Ltd announced in March 2021 where Frewer undertook the Computational Fluid Dynamics (CFD) analysis of the cylinder block steam passageways, a further project has been commenced.

The second of the original Gresley class P2s No. 2002 Earl Marischal, was completed at Doncaster Works five months after No. 2001 Cock o ’the North in October 1934. Unlike No. 2001, which was equipped with Lentz-type rotary-cam actuated poppet valve-gear supplied by the Associated Locomotive Equipment Company, No. 2002 was fitted with Walschaerts valve gear as used on the Gresley class A1 and A3 ‘Pacifics’. As a consequence, No. 2002 was documented as suffering from issues with exhaust steam obscuring the view of the driver. This problem was initially addressed by fitting additional smoke deflectors to No. 2002 and was finally resolved with the fitment of a Gresley class A4-style streamlined front-end to the class P2s. The final four Gresley class P2s, No. 2003 – No. 2006, were fitted with the streamlined front-end from new in 1936, No. 2002 in 1936 and No. 2001 in 1938 when also rebuilt with Walschaerts valve gear.

In order to ensure that No. 2007 does not suffer from the same problem with the original Gresley class P2 boiler casing design, a CFD study of the influence of the Coanda effect on the exhaust steam over the casing is currently underway.

The Coanda effect is the tendency of a fluid to follow a flat or curved surface. In the case of the class P2, a crosswind travelling over the casing will ‘stick’ to the casing and draw the exhaust steam down over the surface obscuring the Driver’s view of the railway ahead. The study aims to attempt to replicate the documented problem in the simulation environment. If this is achieved, design changes can then be tested to investigate how to prevent this problem occurring with No. 2007 – the intention being to find a means of influencing the air and steam flow that does not change the design outline of the locomotive’s boiler casing.

Ben McDonald, Group Director of Engineering, A1SLT, commented, “The commissioning of Frewer to undertake a Computational Fluid Dynamics analysis of the influence of the Coanda effect on the exhaust steam over the casing is part of the continued application of modern techniques in the design and development of our improved class P2, completing the work of Sir Nigel Gresley from the 1930s.”

Martha Cooper, Head of Advance Analysis Division, Frewer, added, “We are very excited to be contributing to this project, and it is a unique opportunity to demonstrate how modern analysis tools and capabilities can be applied to a classic design, inspiring the next generation of engineers. We have found it really interesting to work closely with A1SLT and appreciate the expertise and attention to detail, both in Gresley’s original and in the modern improvements to the design.”

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