Session J16: Experimental Techniques: Velocimetry

Analysis of flow field around magnetic-responsive soft materials using tomographic particle image velocimetry

The hydrodynamics induced by deformation of magnetic-responsive soft materials are investigated and the resultant flow field is analyzed using tomographic particle image velocimetry (tomo-PIV). Magnetic-responsive composite arms incorporated with a multifunctional joint design are actuated by an external magnetic oscillating field with a period of roughly 1 sec. The tomo-PIV system quantifies the surrounding aqueous flow at 250 frames per second. The effects of asymmetric multimodal actuation (i.e., folding versus bending) on the flow field are compared with the single actuation mode, by creating an asymmetric joint at the mid-point of the magnetic material. In addition, the effect of non-symmetric time interval of positive/negative magnetic field generation is investigated on the flow field around the magnetic material. Analysis of the kinematics of the soft material, as well as the velocity, vorticity and pressure of the flow field surrounding the magnetic material indicate that the kinematics of the non-symmetric time interval of magnetic field provides sufficient time for the flow to become steady between each upward and downward stroke motions. Thus, the peak values of vorticity and pressure decrease to smaller magnitudes compared to the symmetric case. In addition, the asymmetric joint at the mid-point of the magnetic material arm affects the flow in a way that the peak values of both vorticity and pressure are larger in the downward motion (rigid body motion, i.e. folding) compared to the upward motion (elastic deformation, i.e. bending).   

A quadruple-frame volumetric PTV system using RGB cameras and telecentric lenses

Novel processing algorithms (e.g., Shack-The-Box, STB) for particle tracking have been developed and have largely improved the tracking accuracy and efficiency; however, such algorithm needs to utilize at least four consecutive frames, which calls for the need for ultra-high-speed cameras and high-power illumination source. In this study, we take a different approach to provide hardware support for the STB algorithm by using four prism-based multi-sensor RGB cameras, and an in-house designed RGB LED light array. This system can provide quadruple-frame realizations using cost-effective hardware. In addition, we employ telecentric lenses to reduce projection error as well as simplify the calibration process. We demonstrate this technique in a simplified cerebral aneurysm model and discuss the pros and cons of this system compared to other volumetric PTV systems.

    

Measurement and Comparison of Flow Fields in the Wake of a Bluff Body using Tomo-PIV, LPTV, and Two-pulse LPTV

Volumetric velocimetry techniques, including tomographic particle image velocimetry (Tomo-PIV), Lagrangian Particle Tracking Velocimetry (LPTV), and two-pulse LPTV, are becoming more common to investigate complex high Reynold's number flows. Despite their popularity, the relative merits, de-merits, and applicability of these methods to investigate specific fluid flow problems is not fully known. In this study, time-averaged and time-resolved velocity measurements of the wake behind a bluff body at Re ~110k is measured using Tomo-PIV, LPTV, and two-pulse LPTV. The effect of processing settings such as Motion Tracking Enhancement (MTE) for tomo-PIV, and particle track length, data assimilation settings for LPTV and two-pulse LPTV on the measured flow fields is investigated. Mean velocity fields and Reynold's shear stresses are compared for time-averaged measurements. For time-resolved measurements, terms in the incompressible vorticity transport equation from the measured flow field is computed and compared.   

Mean velocity and TKE measurements in a refrigerator machine room using magnetic resonance velocimetry

Improving the energy efficiency and flow noise of a refrigerator is critical to manufacturers. To this end, it is essential to comprehend the flow distribution and turbulent kinetic energy (TKE) characteristics of the machine room of the refrigerator. We analyzed the characteristics of the mean velocity and TKE obtained by magnetic resonance velocimetry (MRV) with a 1/2 scale-down, 3D-printed model of a refrigerator machine room. As a result, we proposed an improved geometry resulting in a better flow distribution and a higher flow rate across a condenser and a lower TKE in the machine room. We will discuss the results of flow visualization in detail as well as the MRV’s capabilities in measuring the 3-dimensional, 3-component velocity and TKE for highly complex and opaque geometries.

    

Rapid cm-scale 3D PIV/PTV with plenoptic macro photography

Multi-camera-based flow description in Eulerian and Lagrangian frames of reference at mm and cm scales is usually challenging due to shallow depth of field (DOF) from macro lenses and calibration procedures. We will describe a rapid 3D particle tracking system developed for cm-scale volume applications. With the use of a microlens array (MLA) in the intermediate plane between the macro lens and camera sensor, light field information can be obtained by a single camera cage system without stereo calibration. A fast ray intersection and cloud point classification method with GPU acceleration is applied to reconstruct 3D particle position in a uniformly discretized image space and then mapped to non-uniform objective space based on macro lens properties. With atomic operation within the onboard memory of GPU, the processing time per image can be under one second. Lagrangian particle tracking Crocker-Grier method is used for particle linking to characterize time-resolved flow field.

    

Flow Visualization on Wire-Wrapped 37-pin Rod Bundle for Sodium-cooled Fast Reactor by Magnetic Resonance Velocimeter

Sodium-cooled Fast Reactor (SFR) is paid attention to as one of the 4th generation reactor concepts. It uses liquid sodium as a coolant and has the advantage of being able to operate under atmospheric pressure and using spent nuclear fuel. The fuel assembly uses helically wire-wrapped rods for enhanced mixing. However, the effects of such wire and rod bundle on the flow characteristics are hardly uncovered by flow visualization technique due to complicated flow geometry. Using Magnetic Resonance Velocimetry (MRV), we performed quantitative measurements of 3 dimensional (3D), 3 component (3C) mean velocities in a 3D-printed flow model of wire-wrapped 37-pin rod bundle with an aqueous solution as a working fluid. The MRV results were validated by comparing the flow distribution by subchannels against other independent experimental data. The flow was successfully resolved by MRV to show secondary flows behind the wires. We will discuss the effects of the wire-wrapped structure on the global flow, non-unform flow distributions caused by helical wires, etc. For example, we found that the subchannel region of a locally low flowrate is rotated with a 90-degree phase lag of wire rotation. In addition, we were able to observe a global swirling motion due to the helical wire structure.

    

High-Speed Laboratory Tomographic X-ray Particle Tracking Velocimetry

Many flows are opaque due to the nature of the media or multiple refractive interfaces, making them difficult to measure with visible light techniques such as PIV and PTV. In a series of experiments, we show that advances in X-ray sources and imaging hardware are enabling rapid improvement in X-ray Particle Image Velocimetry and Particle Tracking Velocimetry (XPIV/XPTV). Using XPTV, we measure both Poiseulle pipe flow and a Taylor bubble at frame rates O(1kHz) with 50μm diameter tracers – two orders of magnitude faster the previous state of the art. Furthermore, we take advantage of the high-speed X-ray imaging setup to capture tomographic XPTV of both flows. We individually discuss the impact of the new tracer particles we designed, brighter liquid metal jet X-ray sources, and energy-resolving photon counting detectors (PCDs).

We further expand on the potential of PCDs in X-ray flow visualization. PCDs can nominally attain a higher signal-to-noise ratio than scintillating detectors. Most importantly, many new PCDs have two or more photon energy thresholds, enabling experimentalists to track multiple species at once, whether it be tracer particles, solvent concentration, or multiple fluid phases. This capability opens up a world of opportunity for X-ray flow visualization.   

Pressure from PIV for oscillating internal flow

Most PIV-based pressure field reconstructions pertain to steady, external flows. For such cases, the measurement domain is extended to include well-behaved boundary conditions. This approach is not applicable to internal flows of practical interest, where the choice of measuring or prescribing boundary conditions is limited by experimental constraints. In this work, the pressure field for an impinging synthetic jet is computed from time-resolved tomo-PIV data. The pressure field is reconstructed by 4D, 3D, and 2D Poisson solvers and the results are compared to a dynamic pressure transducer. The pressure field computed by the 3D solver shows random temporal fluctuations that can be attributed to PIV measurement noise and limitations in the boundary condition implementation. The 4D solver uses information from the temporal gradient of the pressure to mitigate these fluctuations; however, this yields a non-physical temporal drift. With mostly computed boundary conditions, the 2D solver also produces random fluctuations. Significant improvement of the reconstructed pressure field is possible by adjusting the type and implementation of the boundary conditions. Eliminating the squared source terms in the governing equation of the 2D Poisson solver further improves the pressure field.   

Development of underwater stereo-PIV systems for the characterization of high Reynolds number separated flow around a 6:1 prolate spheroid

The presence of three-dimensional boundary layers, pressure gradients, and open separation poses a considerable challenge for turbulence models, particularly when it comes to prediction of dynamic loading on the bodies that generate such flows. This difficulty is due in large part to incomplete understanding of the mechanisms responsible for the onset and sustainment of separation.  As such, measurements of both the near-wall and separated regions of a representative flow offer the potential to advance modeling capability for this important class of flows.  To this end, we describe the development of several submersible stereo-PIV systems designed to characterize the flow around a 3 meter chord length 6:1 prolate spheroid across a range of attack angles.  One such system is to be contained within the model itself and will target the near-wall flow and wall shear stress, while the other will be mounted downstream and will target the lee side wake.  Discussion of the internal system includes a strategy for imaging the flow through the curved wall of the body, a consideration necessitated by the differing refractive indices of common viewing window materials and the flow medium.

    

High speed PIV measurements in water hammer.

An experimental study addressing the challenge to measure the relaxation coefficient between vapor and liquid phase in very fast phenomena such as water hammers is presented. A titanium projectile encapsulating a sapphire tube containing water is accelerated and impacts a metal wall creating a water hammer. A custom-built compressed air system is used to accelerate the projectile to speeds up to 30 m/s and create the impact leading to the water hammer. The system guarantees repeatability of the impact speed within 0.1% to allow to perform statistics on the collected data. The sapphire tube, being transparent to green and UV light allows the deployment of non-intrusive laser-based diagnostic techniques such as Particle Image Velocimetry, Shadowgraphy, and Fiber Optic Hydrophone pressure measurements. These imaging techniques are deployed at a very high repetition rate (> 300 kHZ) to visualize the water hammer shock wave traveling through the projectile, while the fiber optic hydrophone measures the pointwise pressure with a bandwidth of 10 MHz. Simultaneous Shadowgraphy, PIV, and pressure measurements during the water hammer are presented and discussed.

    

Construction of the vortex-surface field from tomographic particle image velocimetry data of flow past a vortex generator

We extend the vortex-surface field (VSF), a Lagrangian-based flow diagnostic method, to experimental data of the tomographic particle image velocimetry (Tomo-PIV). The boundary-constraint method is applied to construct the VSF from the instantaneous Tomo-PIV velocity field in the wake flow of a ramp vortex generator (VG) at a moderate Reynolds number. Under finite experimental noises, the VSF construction has satisfactory errors, showing the applicability of the VSF to visualize Tomo-PIV data. From a Lagrangian viewpoint, the VSF is used to elucidate the formation and evolution of coherent structures in the VG wake. The initially planar vortex surfaces consisting of undisturbed vortex lines in the laminar boundary layer are first lifted as the flow past over the VG. Subsequently, the bulge-like outer vortex surfaces in the near wake of VG generate a strong shear layer, and the near-wall inner vortex surface downstream to VG is lifted by the streamwise vortices formed from the lateral VG edges. Further downstream, the outer vortex surfaces break up into arch- or hairpin-like structures due to the Kelvin--Helmholtz instability. The geometric deformation of vortex surfaces is quantified by conditional means of the VSF gradient.