Fluid Dynamics

FLUID DYNAMICS

In fluid dynamics, high-speed imaging provides an indispensable tool to measure and to visualize the complex movement of liquids, gases and plasmas in motion. The movement of liquids and gases is generally referred to as flow, a concept that describes how fluids behave and how they interact with their surrounding environment. Flow can be either steady or unsteady, laminar or turbulent. Laminar flows are smoother, while turbulent flows are more unstable. The study of liquid flow is called hydrodynamics. While liquids comprise of a variety of substances including oils and chemicals, the most common liquid is water. Most applications for hydrodynamics involve managing the flow of these types of liquids.

The flow of gas, commonly referred to as aerostatics, has many similarities to the flow of liquid, however it is important to note that it also has some differences. First, gas is compressible, and liquids are generally considered incompressible. Second, gas flow is hardly affected by gravity. The most common gas is air. Wind can cause air to move around various structures, including building and it can also be forced to move by fans or pumps.

Photron high-speed cameras have been designed to meet the requirements of specialized imaging techniques employed in fluid dynamics including Particle Image Velocimetry (PIV), Laser Induced Fluorescence (LIF) and others.

For years, high-speed imaging has been utilized in the following industries for Fluid Dynamics research and analysis: Automotive, Aerospace, Biotech and Medical, Marine Propulsion, and Electronics.

Effects of Nozzle-Lip Length on Reduction of Transonic Resonance in 2D Supersonic Nozzle

It is known that the transonic resonance takes place, in divergent section of supersonic nozzle, similarly to the longitudinal acoustic resonance of a conical section with one end closed and the other end open. And the “conical section” is similar to the separation zone between shock wave and nozzle exit in divergent part of supersonic nozzle. The present paper describes an experimental work to investigate a reduction of transonic resonance by change the lip length of 2–Dimensional converging-diverging nozzle. In this study, the nozzle pressure ratio varied in the range between 1.4 and 2.2 as shock-containing flow conditions. And a Schlieren optical system was used to visualize the flow fields. Especially, by using a high-speed video camera, we obtained the shock position at that moment. And acoustic measurements were employed to compare the sound spectra level of each experimental case. And it was found that the transonic resonance was decreased when a large separation zone located at the side, where a nozzle-lip attached to nozzle exit additionally. In this case, the amplitude of shock oscillation and wall static pressure oscillation were also decreased.

Ultrafast imaging method to measure surface tension and viscosity of inkjet-printed droplets in flight

In modern drop-on-demand inkjet printing, the jetted droplets contain a mixture of solvents, pigments and surfactants. In order to accurately control the droplet formation process, its in-flight dynamics, and deposition characteristics upon impact at the underlying substrate, it is key to quantify the instantaneous liquid properties of the droplets during the entire inkjet-printing process. An analysis of shape oscillation dynamics is known to give direct information of the local liquid properties of millimeter-sized droplets and bubbles. Here, we apply this technique to measure the surface tension and viscosity of micrometer-sized inkjet droplets in flight by recording the droplet shape oscillations microseconds after pinch-off from the nozzle. From the damped oscillation amplitude and frequency, we deduce the viscosity and surface tension, respectively. With this ultrafast imaging method, we study the role of surfactants in freshly made inkjet droplets in flight and compare to complementary techniques for dynamic surface tension measurements.

Single-Phase and Boiling Flow in Microchannels with High Heat Flux

A cooling system for high heat flux applications is examined using microchannel evaporators with water as the working fluid and boiling as the heat transfer mechanism. Experimental studies are performed using single channel microevaporators allowing for better control of the flow mechanics unlike other investigations where multiple, parallel, flow channels can result in a non-uniform distribution of the working fluid. High-speed flow visualizations are performed in conjunction with heat transfer and pressure drop measurements to support the quantitative experimental data. Flow patterns associated with a range of boundary conditions are characterized and then presented in the form of novel flow regime maps that intrinsically reflect the physical mechanisms controlling two-phase pressure distributions and heat transfer behavior. Given the complexity associated with modeling of boiling heat transfer and the lack of a universal model that provides accurate predictions across a broad spectrum of flow conditions, flow regime maps serve as a valuable modeling aid to assist in targeted modeling over specific flow regimes. This work represents a novel and original contribution to the understanding of boiling mechanisms for water in microchannels.

Optical-flow-based background-oriented schlieren technique for measuring a laser-induced underwater shock wave

The background-oriented schlieren (BOS) technique with the physics-based optical flow method (OF-BOS) is developed for measuring the pressure field of a laser-induced underwater shock wave. Compared to BOS with the conventional cross-correlation method that is also applied for particle image velocimetry (here called PIV-BOS), by using the OF-BOS, the displacement field generated by a small density gradient in water can be obtained at the spatial resolution of one vector per pixel. The corresponding density and pressure fields can be further extracted. It is demonstrated in particular that the sufficiently high spatial resolution of the extracted displacement vector field is required in the tomographic reconstruction to correctly infer the pressure field of the spherical underwater shock wave. The capability of the OF-BOS method is critically evaluated based on synchronized hydrophone measurements. Special emphasis is placed on direct comparison between the OF-BOS and PIV-BOS methods.