Experimental Study of Cavitating Flow

ABSTRACT

With the increase in cruising speed, underwater vehicles may encounter a significant phenomenon known as cavitating flow, which can greatly affect their performance. In this talk, I will provide an overview of various experimental facilities used to study cavitating flow, based on a detailed literature survey, and discuss their respective advantages and limitations. I will also address main challenges in constructing such facilities and in characterizing cavitating flows.

Additionally, I will present our in-house development of an air-assisted launching system in a water tank, share some experimental results, outline its current limitations, and propose ideas for enhancing its capability to generate more valuable data for numerical validation and to advance our understanding of cavitating flow phenomena.

Design Trade-offs in Building Porosity: A Parametric Analysis of Vertical Placement and Geometry for Urban Ventilation

ABSTRACT

This study investigates the critical influence of building porosity's vertical placement and geometric configuration on street canyon airflow. While porosity is increasingly advocated for urban ventilation, a systematic understanding of how these design parameters affect performance is lacking. The purpose of this research is to decouple these effects through a parametric computational fluid dynamics (CFD) study. A Reynolds-Averaged Navier-Stokes (RANS) model, validated against experimental data, was used to analyze a series of idealized building configurations.

Two sets of simulations were conducted. The first compared identical porous openings placed at six different vertical levels, from the ground floor upwards. The second investigated various opening geometries at a fixed mid-building level. Principal results reveal a stark difference in performance based on vertical position. Ground-level porosity was found to be most effective for preserving robust pedestrian-level wind flow. In contrast, mid- and upper-level porosity consistently degraded near-ground conditions relative to the ground-level case, with some configurations causing significant velocity deficits. Furthermore, for a fixed vertical level, the specific geometry of the opening was also shown to be a highly sensitive parameter.

The major conclusion is that the vertical location of porosity is the primary determinant of pedestrian-level ventilation. A "one-size-fits-all" approach to porosity design is ineffective; the optimal solution is highly dependent on the targeted ventilation objective (e.g., pedestrian comfort vs. upper-level air exchange).