University Events

Description: EAS Doctoral Proposal Defense by Shabnam Mohammadshahi

Date: Tuesday, December 19, 2023

Time: 10am

Topic: Drag Reduction and Plastron Stability of Super-Hydrophobic Surfaces in Turbulent Flows

Location: SENG 108

Abstract:

The hydrodynamic skin friction in turbulent flows contributes to 60-70% of the total drag of most surface and subsurface vessels. Applying Super Hydrophobic Surface (SHS) is a new passive method to reduce the friction drag in turbulent flows, due to its ability to trap a layer of gas bubbles (or plastrons) within the surface micro-structures. However, the application of SHS in real engineering systems, e.g., marine vessels, is still a challenge for the reason that the SHS may lose the gas bubbles and the drag-reducing property under turbulent flows. It is unclear what is the optimal surface texture for achieving sustained drag reduction by SHS. To address these challenges, this dissertation proposal has three contributions: (i) development of a simple method to fabricate SHSs with different surface roughness; (ii) evaluation of the impact of surface roughness on the friction drag of SHS in turbulent flows; and (iii) development of novel optical methods to investigate the flow-induced deformation of the gas-liquid interface on SHS.

First, we developed a novel method to fabricate SHSs with different roughness heights based on superimposing nanosized hydrophobic silica particles on top of the sandpapers. The surface roughness was changed by simply using sandpapers of different grit sizes. We found that the coated sandpapers with grit sizes of 240, 400, 800, 1000, and 1500 exhibited super-hydrophobicity, while other coated sandpapers with grit sizes of 60, 120, and 600 did not show superhydrophobicity for the reason that the Cassie-Baxter state was not stable. The fabricated SHS remained in the partial Cassie-Baxter state at the highest pressure (2.4 atm), although the percentage of surface area covered by gas reduces with increasing pressure. Then, the drag-reducing properties of fabricated SHSs were measured in a fully developed turbulent channel flow facility. The mean flow velocity varied from 0.45 to 4 m/s, and the Reynolds number, based on the channel height and mean flow velocity varied from 2,900 to 24,500. The wall friction and drag reduction of the SHSs were measured based on the pressure drops in the fully developed region. We found the slip length of SHS reduces as increasing Reynolds number due to the increase of roughness effect and the reduction of surface area covered by gas. A maximum 47% drag reduction was obtained by the fabricated SHSs in turbulent flows. Moreover, we found that the drag reduction of SHS increases with reducing the roughness height (i.e., increasing the grit size of the sandpaper).

Finally, we developed novel optical methods to measure the gas-liquid interface on SHS in turbulent flows. Understanding the dynamics of the interface on SHS is critical to design robust SHS that can survive in strong turbulent flows. For interface on non-transparent SHS (e.g., coated sandpapers), we used a reflected light microscope. For an interface on transparent SHS (e.g., patterned PDMS surface), we used a Reflection Interference Contrast Microscopy which has the capability to resolve 3D interface shape. Preliminary data of the interface status in turbulent flows have been obtained, which showed a reduction of surface area covered by gas as increasing Reynolds number. Future work of this Ph.D. thesis will include velocity measurement by particle-image-velocimetry and interface deformation measurement by high-speed imaging.

ADVISOR(S):
Dr. Hangjian Ling, Department of Mechanical Engineering
(hling1@umassd.edu)

COMMITTEE MEMBERS:
Dr. Banafsheh Seyedaghazadeh, Dept of Mechanical Engineering
Dr. Caiwei Shen, Dept of Mechanical Engineering
Dr. Geoffrey W. Cowles, Dept of Marine Science & Technology

NOTE:
All EAS Students are ENCOURAGED to attend.