Dominik Raps

Simulating the Freezing of Supercooled Water Droplets Impacting a Cooled Substrate

To study ice adhesion at the droplet scale, a strategy is presented to simulate the impact and solidification of a supercooled water droplet on a cooled substrate. Upon impact, nucleation is assumed to occur instantaneously, and properties of the droplet are chosen to account for the nucleation process. Simulations are performed in ANSYS Fluent using a coupled volume-of-fluid and level-set method to capture the air–water interface, and an enthalpy-porosity method is used to capture the liquid–solid interface. Calibration of a simulation parameter Amush is performed in order to match experimental data for different ideal surface types and surface temperatures. The simulation strategy successfully predicts the overall droplet response for several droplet impact conditions.

Evaluation of Roughness Effects on Ice Adhesion

In this study, the adhesion strength of ice to bare aluminum substrates with different values of surface roughness is investigated. A thin glaze ice accumulation is formed on the substrate surface within an icing wind tunnel facility. The adhesion strength between the ice and the metal samples is measured by means of a permanent magnet shaker. Ice de-bonding occurs in the interface between the ice and the aluminum. The experimental results reveal that the ice adhesion strength is significantly dependent on the degree of surface roughness of the respective aluminum substrate.

Mono-Dispersed Water Droplet Delivery in a Small-Scale Refrigerated Wind Tunnel

A numerical investigation of several droplet delivery tubes designed for a small-scale refrigerated wind tunnel is presented. A hybrid RANS/LES strategy was employed to model the resulting unsteady, turbulent flow in the tunnel. In terms of delivering droplets to the tunnel airflow, numerical results show that a composite tube design, with a pressure relief slot and an inner tube, was preferable to a simple tube design with no slot and no inner tube. Alternative tube cross sections selected to reduce the turbulent wake were also investigated. The results demonstrated that using an NACA0030 airfoil as the cross section of the droplet delivery tube significantly reduced scatter in the droplet trajectories thereby improving reliability of droplet delivery to the test section.

Laboratory Investigation into Anti-contamination Coatings for Mitigating Insect Contamination with Application to Laminar Flow Technologies

Insect contamination on aircraft leading edge surfaces can result in premature transition of the boundary layer, leading to an increase in skin friction drag and fuel consumption. An evaluation of candidate anti-contamination coatings was undertaken. Coatings were characterized before impact testing. Surface energy was quantified by dynamic contact angle analysis and surface roughness measured using a profilometer. Superhydrophobic coatings showed a reduction in contamination when compared to the higher surface energy specimens tested. The surface topography and chemistry, in particular the sliding angle of a coating, were found to have a significant influence on the effectiveness of a coating. Insect residue areas were theoretically predicted using high-speed liquid droplet theory and compared to experimentally obtained results. Tests with different insect species were conducted to investigate the effect of insect size and type on the effectiveness of the coatings and the evaluation procedure. Good correlations were obtained between the two test facilities used. The effect of substrate temperature on insect impact dynamics and adhesion was also evaluated.

Effects of Surface Characteristics and Droplet Diameter on the Freezing of Supercooled Water Droplets Impacting a Cooled Substrate

Reducing the accretion of ice on aerodynamic surfaces remains a challenge. Certain surface types have been hypothesized to reduce ice buildup. In this paper, a previously developed numerical method is employed to investigate the effects of droplet size and surface characteristics on the solidification of a supercooled water droplet as it impacts a cooled surface at high speed. Upon impact, nucleation is assumed to occur instantaneously, and properties of the droplet are chosen to account for the nucleation process. Simulations are performed in ANSYS Fluent using a coupled Volume of Fluid and Level-Set method to capture the air-water interface and an Enthalpy-Porosity method to capture the liquidsolid interface. We consider high-speed Supercooled Large Droplet impacts on hydrophilic, hydrophobic, and superhydrophobic surfaces. Results show that superhydrophobic surfaces may not be icephobic for larger diameter droplets.

A Comparison of VOF Simulations with Experimental Data for Droplet Impact on a Dry Surface

A numerical investigation was performed to determine the suitability of two different computational fluid dynamics codes – ANSYS Fluent 12.0 and OpenFOAM® 2.0 – for predicting the outcome of droplet impact on dry, hydrophobic and superhydrophobic surfaces. Axisymmetric simulation results were compared with experimental data available in the literature and data obtained at EADS-IW. Good quantitative agreement between experimental data and simulation results was observed only for predictions made using Fluent for a highly viscous, glycerin droplet on a hydrophobic surface. In general, the maximum spread was over predicted and the retraction phase was not well modeled for the limited number of cases considered.