| A reduction in the friction between the surface of moving objects and the fluid willresult in higher effciency, lower fuel consumption and less emission. Fast-swimming sharkand dolphin provide people with excellent bionic drag-reduction templates. The researchersare inspired by the skins of shark and dolphin to carry out researches about the effect ofbionic non-smooth and compliant structures on the friction reduction. It fully displays thatdesigning and preparing bionic drag-reduction materials have a great potentiality and abroad application prospect.According to the structural characteristics of shark skin and dolphin skin, polyurethane(PU) elastomer with excellent designability and regulation, a good moulding processabilitywas selected as the matrix in this study to prepare homogenic (single layer) and anisotropic(double layer) PU sheets. Then the microstructure of shark skin was constructed on thesurfaces of homogenic and anisotropic PU sheets. The drag-reduction properties of aboveprepared PU sheets were measured by self-made rotating drag-measure instrument, and thedrag-reduction efficiency at the rotational speed of398rpm was used to evaluate thedrag-reduction properties of the PU sheets. Besides, the effect of the structure and themechanical properties of PU sheets on the drag-reduction property was consequentlystudied. The concrete content of this study as follows:The homogenic (single layer) PU sheets with different thickness and differentproperties were prepared from polyether PU prepolymers M1, M2and chain extenders4,4’-Diamino-3,3’dichlorodiphenylmethane (MOCA),1,4-Butanediol (1,4-BD) by thecasting method. Their mechanical properties and drag-reduction properties werecharacterized. According to the above results, the relationship between drag-reductionproperty and the thickness, the compliant coefficient α of PU sheets was analyzed. On thisbasis, the anisotropic (double layer) PU sheets S1, S2and S3were designed and prepared,their drag-reduction properties were measured and analyzed. And the preliminary study onthe drag-reduction mechanism of the compliant PU sheets was carried out. The resultsshowed that for the homogenic (single layer) PU sheets with thickness in the range of0.5~3.0mm, the ones with2.0mm thick had a better drag-reduction effect. When the elastic modulu of the homogenic PU sheets with2.0mm thick was in the range of5~10MPa andthe value of α was around1.1, the PU sheets had a good drag-reduction effect, whosedrag-reduction efficiency at the rotational speed of398rpm were up to16.7%. Comparingwith the corresponding homogenic (single layer) PU sheets, the double layers of theanisotropic PU sheets S1and S2showed obvious interfaces, and their drag-reductionproperties had no improvement. But the anisotropic (double layer) PU sheet S3without anobvious interface showed a better drag-reduction property than the correspondinghomogenic PU sheets, and its drag-reduction efficiency at the rotational speed of398rpmwas up to17.0%. This suggested that a good transition of the layers of the anisotropic PUsheets would fully perform drag-reduction functions of different layers, which was one ofthe keys for the excellent drag-reduction effect.Furthermore, the structure of shark skin was constructed on the surfaces of homogenicand anisotropic PU sheets by the method of combining the microreplication with the casting.And the homogenic PU sheets with shark-skin surface M1-D-B, M2-D-B and theanisotropic PU sheets with shark-skin surface S1-B, S3-B were successfully prepared. Theresults of the drag-reduction measure and analysis showed that the drag-reductionefficiencies at the rotational speed of398rpm of M1-D-B and M2-D-B were16.0%and20.7%respectively, and S1-B and S3-B were7.6%and24.6%respectively. Theirdrag-reduction efficiencies were higher than the corresponding compliant PU sheets withoutshark-skin surface. This suggested that the shark-skin structure on the compliant PU sheetshelped to improve the drag-reduction property of the compliant PU sheets. |
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