Experimental studies of the effects of hydrophobic coatings on flow separation around 3D bluff bodies

This study explores how hydrophobic coating influences the structure of the flow separation at the rear end of 3D bluff bodies. Two 3D representative bluff bodies, namely the standard Ahmed body (SAB), which features flow separation and reattachment at its slant surface, and the elliptical Ahmed body (EAB), which exhibits fully separated flow, are employed. The bluff bodies were coated with Ultra Ever Dry hydrophobic paint. Experiments were carried out on pairs of coated and uncoated SABs and EABs in a water tunnel utilizing time-resolved and standard particle image velocimetry (PIV) at a Reynolds number of 4.3 × 10⁴ based on the model height. The results show that hydrophobic coatings influence the flow features of these bluff bodies. For the SAB, the coating alters the slant separation bubble, increasing the reattachment length by 80% and reducing the shear stress. The Strouhal number on the slant surface of the SAB also increases, with a dominant value of <i>S</i> _<i>t</i> = 0.24. Proper orthogonal decomposition (POD) analysis shows dominant Strouhal numbers of <i>S</i> _<i>t</i> = 0.36 and <i>S</i> _<i>t</i> = 0.48 for the first and second POD modes, respectively. Additionally, dynamic mode decomposition (DMD) analysis identifies a dominant Strouhal number of <i>S</i> _<i>t</i> = 0.3 in the wake. Conversely, the EAB, which already has a fully separated flow, is less affected by the coating. The wake recirculation length is reduced by 6%. Strouhal numbers on the coated EAB's slant surface range from 0.40 to 0.55, and those in the wake vary from 0.25 to 0.85. The POD analysis does not reveal dominant Strouhal numbers in the EAB's wake, while the DMD analysis indicates a dominant Strouhal number of <i>S</i> _<i>t</i> = 0.013, pointing to energetic modes due to the fully separated flow. These findings demonstrate that hydrophobic coatings affect the flow characteristics of 3D bluff bodies differently, depending on their inherent flow separation properties.