Showing papers by "Franz Durst published in 2022"

Central rebound jet of a droplet normal impact on a confined thin liquid film

Abstract: The phenomenon of droplet impact on a confined thin liquid film is encountered in a variety of industrial applications. Despite exhaustive research, the central rebound jet (CRJ) and its pinch-off are still far from being understood owing to their strong randomness and uncertainty in secondary pinch-off droplet numbers. This study investigated experimentally the CRJ and its pinch-off formed by the normal impact of a single droplet on a confined thin liquid film. The dynamic evolution of CRJ formation and its pinch-off are discussed for three typical Weber numbers ( We). Its morphology was analyzed by focusing on the effects of We and film thickness, and a qualitative comparison on CRJ height was made with the previous results. The critical thresholds of CRJ pinch-offs are characterized, and a novel concise prediction method was developed. The results show that the increase in dome diameter is caused not only by CRJ rising, but also by its fallback. Pinch-off heights of the CRJ usually lead to a critical threshold of We (or K), decreasing with increase in film thickness. The CRJ maximum height increases with increase in Fr and shows a power function. An active region of liquid film thickness taking a Gaussian normal distribution were found for CRJ formation and its pinch-off. The film thickness has significant influence on CRJ height in the active region, but little outside this region. A novel concise equation for predicting CRJ pinch-off and its droplet numbers was further obtained by an inverse multiple power-law function.

Laminar-to-Turbulent Transition of Pipe Flows Triggered by Wall-Mounted, Ring-Type Obstacles

Abstract: The literature stresses the inherent stability of laminar pipe flows with parabolic velocity profiles and this paper refers to the relevant publications summarizing this work. To cause such flows to turn into their turbulent state requires laminar pipe flows to be triggered externally. Ring-type, wall-mounted obstacles can be used for this purpose, and investigations in this area are of particular interest to the authors’ work, summarized in this paper. In the investigations presented here, however, a special triggering technique was employed that allowed laminar pipe flows to be exposed to obstacle disturbances for only about 30 ms. Individual puffs and slugs could be produced in this way. Comparisons with fixed wall-mounted obstacles showed that the properties of both types of turbulent slugs were the same. Theoretical derivations are described to provide the required obstacle height as a function of the Reynolds number, to trigger fully developed laminar pipe flows to turn into their turbulent state. Corresponding experimental investigations were also performed as described. Very good agreement between the theoretical and experimental results was obtained. All this demonstrates that a relatively simple ‘ad hoc theory’ can derive the required height of ring-type, wall-inserted obstacles to trigger laminar pipe flows with parabolic velocity profiles to turn turbulent. Other ways to trigger laminar pipe flows to turn turbulent were also investigated by employing blowers and plenum chambers and varying the lengths and diameters of pipes. It is demonstrated, in a somewhat qualitative way, that the maintenance of laminar pipe flows requires all components of a test rig to be matched to each other to maintain pipe flows laminar.