Transitional Flow on Model Propellers and Their Influence on Relative Rotative Efficiency

Why some propellers have unexpectedly low value of relative rotative efficiency, new paper

ηR is the relative rotative efficiency of a propeller that reflects the torque difference for a propeller working in the open water and in the behind conditions. It is obtained during the scaling of model test results with the standard ITTC-78 method. A research article, published in the Journal of Marine Science and Engineering (JSME), presents our new findings and a thorough explanation about the cause of low ηR-value for the low blade area propellers. Model testing and a RANS method with a transition model were used in this work to study the near-wall flow characteristics in open water and behind conditions for three different propeller designs. Apart from the difference in Reynolds number (Rn) scale effect between the open water and the behind conditions, the paper points out that flow separation in the behind condition could be another significant reason for the drop of ηR for some propellers. The latter phenomenon is associated with the design philosophy of propellers. 

- "With the awareness of the potential problems with the ITTC-78 method, we encourage the continuing effort in the ITTC committees and member society to develop new scaling methods for unconventional propellers. Some initial work has been started at SSPA", says Ph.D. Sofia Werner, Manager Strategic Research Hydrodynamics.

The paper

Title: Transitional Flow on Model Propellers and Their Influence on Relative Rotative Efficiency
Authors: Da-Qing Li, Per Lindell and Sofia Werner 
Published: Journal of Marine Science and Engineering. Available online since 25 November 2019.
Link: www.mdpi.com/2077-1312/7/12/427

Abstract: Unexpected low value of the relative rotative efficiency ηR is sometimes noted when scaling the towing tank model-test result with the ITTC-78 method to obtain the propulsive efficiency factors of propellers. The paper explains the causes of this phenomenon. The boundary layer state of three propellers was studied by a paint test and a RANS method. The paint tests showed that the propellers in behind conditions at low Reynolds number (Rn) are covered mainly with laminar flow, which is different from open water tests conducted at a high Rn. Apart from that a moderate difference in Rn between the open water and the self-propulsion test may lead to a low ηR value, the paper points out that flow separation in behind conditions could be another significant reason for the drop of ηR for some propellers. Therefore, two factors will lead to an unexpected decrease of ηR: (1) A slightly lower open water torque interpolated from an open water test carried out at a high Rn and (2) a slightly higher torque in a self-propulsion test due to laminar flow separation near the trailing edge. The phenomenon is caused by the Rn scaled effect and closely associated with design philosophy like the blade section profile, the chord length, and chordwise load distribution. 

The Journal of Marine Science and Engineering (JMSE)

An international peer-reviewed open access journal which provides an advanced forum for studies related to marine science and engineering. The journal aims to provide scholarly research on a range of topics, including ocean engineering, chemical oceanography, physical oceanography, marine biology, marine geosciences and coastal engineering www.mdpi.com/journal/jmse

Illustrations

Near-wall boundary layer flow pattern on a propeller blade, obtained with the paint test (a), by the intermittency transition model (b)-(c), and by SST k-ω turbulence model (d).
 
PhD Da-Qing Li, researcher at SSPA, presented the paper at SMP'19, Sixth International Symposium on Marine Propulsors, in Rome, Italy.