On the analysis of the contact angle for impacting droplets using a polynomial fitting approach

ractical considerations on the measurement of the dynamic contact angle and the spreading diameter of impacting droplets are discussed in this paper. The contact angle of a liquid is commonly obtained either by a polynomial or a linear fitting to the droplet profile around the triple phase point. Previous works have focused on quasi-static or sessile droplets, or in cases where inertia does not play a major role on the contact angle dynamics. Here, we study the effect of droplet shape, the order of the fitting polynomial, and the fitting domain, on the measurement of the contact angle on various stages following droplet impact where the contact line is moving. Our results, presented in terms of the optical resolution and the droplet size, show that a quadratic fitting provides the most consistent results for a range of various droplet shapes. As expected, our results show that contact angle values are less sensitive to the fitting conditions for the cases where the droplet can be approximated to a spherical cap. Our experimental conditions include impact events with liquid droplets of different sizes and viscosities on various substrates. In addition, validating past works, our results show that the maximum spreading diameter can be parameterised by the Weber number and the rapidly advancing contact angle

Assessment of common turbulence models under conditions of temporal acceleration in a pipe

In this paper, transient flow in a pipe at Reynolds numbers (based on bulk velocity and diameter) ranged from 7000 to 45200 is numerically simulated using four common turbulence models. The models considered are the Baldwin-Lomax algebraic model, the k-e model with wall correction of Lam and Bremhorst, the k-w model and the k-e-v2 model of Durbin. The results of these models are compared with those of the recent experiments reported in the literature. The predicted velocity and delay period using the models compared well with measured values for short and long ramp-up flow excursions. The delay period of the calculated turbulence kinetic energy close to the pipe centerline is around 4 sec which agrees with the experiments. The k-e-v2 model was found to provide the best results compared to the measured data in the region away from the wall. At the end of the excursion near the wall, however, the results of this model differs from those of the experiments

Study of in-flight and impact dynamics of non-spherical particles from HVOF guns

High velocity oxygen fuel thermal spray has been widely used to deposit hard composite materials such as WC-Co powders for wear-resistant applications. Unlike gas atomized spherical powders, WC-CO powders form a more complex geometry. The knowledge gained from the existing spherical powders on process control and optimization may not be directly applicable to WC-Co coatings. This paper is the first to directly examine nonspherical particle in-flight dynamics and the impingement process on substrate using computational methods. Two sets of computational models are developed. First, the in-flight particles are simulated in the CFD-based combusting gas flow. The particle information prior to impact is extracted from the CFD results and implemented in a FEA model to dynamically track the impingement of particles on substrate. The morphology of particles is examined extensively including both spherical and nonspherical powders to establish the critical particle impact parameters needed for adequate bonding. <br/