Bubbly drag reduction (BDR) is a technique used to reduce the drag force experienced by a ship's hull as it moves through water. The basic idea behind BDR is to inject tiny air bubbles into the turbulent boundary layer that forms along the hull, which can significantly reduce the skin friction drag. However, the behavior of these bubbles and their interaction with the turbulent flow is complex and not yet fully understood.
In a study presented at the 10th International Symposium on Ultrasonic Doppler Methods for Fluid Mechanics and Fluid Engineering, researchers from the University of Tokyo used an innovative measurement system called the UVP-DUO to investigate the behavior of void waves in a bubbly two-phase turbulent boundary layer beneath a flat-bottom model ship during drag reduction.
The UVP-DUO is a state-of-the-art ultrasonic velocity profiler developed by Met-Flow, a leading manufacturer of advanced flow measurement instruments. This system uses ultrasound to measure the velocity of fluid particles in a non-invasive and highly accurate way, allowing researchers to obtain detailed information about the flow field in a wide range of applications.
In this study, the UVP-DUO was used to measure the velocity field of the turbulent boundary layer beneath the model ship, both with and without the injection of air bubbles. The system was mounted on a traverse mechanism that allowed it to be moved along the length of the ship, providing measurements at multiple locations.
The researchers found that the injection of air bubbles into the boundary layer resulted in the formation of void waves, which are oscillations in the volume fraction of the bubbles. These waves propagate along the boundary layer and can significantly affect the drag reduction performance.
Using the UVP-DUO, the researchers were able to measure the velocity field of the turbulent flow and the void fraction of the bubbles simultaneously, providing valuable insights into the behavior of the void waves and their interaction with the turbulent flow.
The study found that the void waves naturally stand out when drag reduction is enhanced through the local spatial gradient of the void fraction. This suggests that the formation and propagation of void waves may play an important role in the drag reduction mechanism of BDR.
Overall, the study demonstrates the power of the UVP-DUO as a tool for investigating complex flow phenomena in bubbly two-phase turbulent boundary layers. By providing highly accurate and detailed measurements of the velocity field and void fraction, this system is helping researchers to better understand the behavior of void waves and their impact on drag reduction performance.
In addition to its applications in BDR, the UVP-DUO is also used in a wide range of other industries and research fields, including environmental monitoring, process control, and biomedical engineering. With its advanced features and versatile design, this system is an essential tool for anyone seeking to measure and analyze fluid flow with high accuracy and precision.
Read drag reduction publications below from our Digital Library.
Studying drag reduction with UVP-DUO: exploring void waves in bubbly two-phase turbulent boundary layers with the UVP-DUO