University Events

Description: EAS Doctoral Dissertation Defense by Shabnam Mohammadshahi

Date: Tuesday, July 23, 2024

Time: 11:00 a.m.

Topic: Experimental Study of the Stability of Super-Hydrophobic Surface in Turbulent Flow

Location: LIB 314

Abstract:
The hydrodynamic skin friction in turbulent flows contributes to 60-70% of the total drag of most surface and subsurface vessels. Super-hydrophobic surface (SHS) is a new passive method to reduce the friction drag in turbulent flows, due to its ability to trap a thin layer of gas (or plastron) 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 and thereby 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 this challenge, this thesis has made three contributions. First, we developed a simple method to fabricate SHSs with controlled roughness heights based on superimposing nanosized hydrophobic silica particles on top of the sandpapers. The surface roughness was controlled by 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. 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. Second, we studied the impact of surface roughness on the stability and drag reduction of SHS fabricated on sandpapers in turbulent flows. Multiple SHSs with different roughness heights were tested in a turbulent channel flow facility. We found a strong correlation between drag reduction and krms+=krms/δv, where δv is the viscous length scale and krms is the root-mean-square roughness height. For krms+<1, drag reduction was independent of krms+ and was nearly a constant (~47%) as increasing Reynolds number. For 1<krms+<2, less drag reduction was observed due to the roughness effect. And for krms+>2, the SHSs caused an increase in drag. We also found that surface roughness influenced the trend of gas depletion. As increasing Reynolds number, the gas fraction (φg) is reduced gradually for SHSs with large krms, but reduced rapidly and maintained as a constant for SHSs with small krms. Last, we investigated the effects of texture size and texture shape on the stability of SHS consisting of transverse grooves in turbulent flows. We systematically varied the groove width (g), texture height (h), and texture wavelength (λ) in the range of 200 to 800 µm. The experiments were performed in a turbulent channel flow facility, and the status of the gas layer on SHS was imaged by reflected-light microscopy. We found that as increasing Reynolds number, the SHS experienced a sudden wetting transition from the Cassie-Baxter state to the Wenzel state. A metastable state where the liquid partially filled the grooves was not observed. We found that the wetting transition was delayed or occurred at a higher Reynolds number as increasing h and reducing g, which indicates that a larger energy barrier between the Cassie-Baxter state and Wenzel state led to a more stable interface. The trend between g and the critical Reynolds number Recr for wetting transition was well captured by theoretical models based on the force balance at the gas-liquid interface. We also showed that grooves with a T-shape geometry maintained a more stable plastron in turbulent flows.


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.