Session X22: Quantitative Flow Visualization II: PIV, PTV, PLIF

Progress on molecular tagging velocimetry (MTV) for wall shear stress measurements in water

Measuring the instantaneous wall shear stress (WSS) in high-Reynolds-number wall-bounded turbulent flows is challenging greatly due to the small spatial and temporal scales involved. Molecular tagging velocimetry (MTV) is a non-intrusive optical diagnostic that gives continuous velocity profiles and is hence well-suited to measure velocity gradients. A novel photobleaching scheme, combined with Talbot-effect structured illumination, overcomes the traditional limitations of spatio-temporal resolution of MTV techniques in water. The approach has been demonstrated in a turbulent channel flow with viscous length scales on the order of 10 μm. The technique's performance is assessed with various pairs of common pulsed neodymium-doped lasers (351-355 nm and 527-532 nm) and cameras. Special attention is paid to making the MTV system compact and remotely operated to allow for the upcoming in-situ deployment from within a model in very large water facilities.   

Two-Photon Laser-Induced Fluorescence for Temperature and Density of Atomic Oxygen in a High-Temperature, Atmospheric Pressure Plasma Torch

High-temperature plasma sources are becoming an increasingly useful tool for nanomaterial synthesis and testing of hypersonic materials, however experimental measurements that probe the thermochemical environment are lacking. To address this lack of experimental data, we present temperature and density of atomic oxygen (O) in the plume of a high-temperature (~6000K) inductively-coupled plasma torch using two-photon laser-induced fluorescence (TALIF). Temperature will be extracted from two techniques: by fitting to the Boltzmann populations of the ground triplet states and from the Doppler Broadening of the absorption lines, with their results compared. The number densities are extracted by first taking the TALIF signal to the photoionization limited regime (thus creating a quenching-independent measurement), and calibrating to a known reference. To extend this technique to material testing, the temperature and number density of atomic oxygen in the boundary layer of an ablating graphite sample are also investigated. The effects of photoionization and saturation on the TALIF signal are assessed in both cases and included in the model.   

Towards Hydroxyl Tagging Velocimetry Integrated With Fourier Integral Microscopy Of High-Speed Flow Around A Modified NACA Airfoil

As a first-of-its-kind experimental campaign in a high-speed (up to Re = 4,600,000 based on the chord) anechoic wind tunnel over a modified NACA 0019-94 airfoil, Hydroxyl tagging velocimetry (HTV) was deployed in June 2022. In a nutshell, a two-laser system was used to create ('write' step) a continuous line of tracers and subsequently 'read' them. An excimer laser operating at 193 nm was used to 'write' the taglines, afterwards a tunable dye laser 'read' those lines at 281.905 nm and a sCMOS camera equipped with an image intensifier captured the flow phenomena at 308 nm. The 'write'/'read' time interval was optimized between 30 μs to 160 μs for the three different speeds studied: 10, 20 and 30 m/s. However, some improvement opportunities were identified to enhance the overall measurement technique. One of them is employing a low-glare laser delivery window for the betterment of post-processing of the images especially in the viscous sublayer region. Another major change is a better optical assembly to achieve better resolution. On top of this, integration of a Fourier integral microscopy (FIMic) approach with the HTV technique is underway which will allow us to extract 3D flow information in micro-scale from 2D data.   

New Compact Design for A Fourier Lightfield Microscope

New methods for the acquisition and processing of 3D microscopic samples have been in constant development in recent years. An approach of growing interest is Integral Imaging Microscopy or IMic. Recent work has sought to utilize the stereoscopic capabilities of IMic for direction measurements in applied fields such as naval hydrodynamics. This growing demand for IMic, and further plenoptic imaging techniques, has revealed a new need for compact configurations of these systems. In previous work, we demonstrated a ~200cm Fourier Integral Microscope(FIMic) with improved spatial resolution compared to traditional IMic systems by imaging the Fourier plane of an infinity-corrected objective. A new, compact 10cm Fourier Lightfield microscope design is presented, capable of >12μm lateral resolution, a Depth of Field of 0.53mm, and a Field of View of 3.06mm. Successful 3D reconstructions of various scenes captured by the compact imager are presented using the Shift & Multiply reconstruction algorithm in addition to a Richardson-Lucy Deconvolution approach. Further, we demonstrate the reconstructions of photobleached signals in a Low Reynolds number flow via Molecular Tagging Velocimetry.   

Soft sensor for measuring instantaneous wall stress distributions in turbulent wall flows

Obtaining wall stress distribution in turbulent flow have overarching implications. Past technologies based on piezo-electric materials resulted in costly and bulky sensors with low frequency responses and measurement sensitivities. Enabled by our recent success in synthesizing wrinkle-free nm metallic thin film encased in polymers (i.e. flexible mirror), we are developing a technique capable of measuring non-intrusively and simultaneously pressure and shear stress distributions at high spatial resolutions. To achieve such capabilities, we create an array of polymer filled mWells in a substrate with only their top surfaces open to flows. The nm-scale 3D deformations within each mWell (pixel), caused independently by its interactions with local flow shear and pressure, is obtained by measuring remotely deformation of a nm thin film mirror embedded horizontally within each mWell by microscopic interferometry. Using interfacial jamming by nanoparticles, we have successfully fabricated miniaturized polymer encased flexible mirrors and microscopic interferometry. Initial assessment shows the sensitivity of ~4Pa and response frequency of 170 kHz. Additional validations are ongoing using our benchtop shear facility.   

Effect of Low-Frequency Oscillations on the Performance of Closed-Loop Pulsating Heat Pipe

The pulsating heat pipe (PHP) is a simple heat exchange device, which differs from conventional heat pipes in terms of construction and flow behavior and it has no wick structures inside. The present study focuses on experimentally investigating and exploring the effects of low-frequency oscillations on the thermal performance of the pulsating heat pipe. To understand the thermal performance of the closed loop PHP, a copper capillary tube of seven turns with an internal diameter of 2mm and an external diameter of 3mm with evaporator and condenser sections of length 80 mm each, adiabatic section of length 126mm has been designed. Hot and cold fluids are circulated by using temperature baths in evaporator and condenser sections separately, by varying evaporator (45ºC-60ºC) and condenser temperatures (10ºC-20ºC). The CLPHP performance has been studied for the filling ratio of 50% (n-pentane as working fluid) in bottom mode heating mode subjected to transverse oscillations by mounting it on a linear motor and operating at a frequency range of 0-3Hz at an amplitude of 1mm and 3mm respectively. The results show thermal resistance increases with the increase in the evaporator temperature. Thermal resistance shows a decreasing trend expectably with an increase in heat input. The external forcing triggers the oscillations of the slug-plug flow. But there is no substantial change in the heat input, however, heat input increases with the amplitude of forcing and thus reduces the thermal resistance.   

Development of a a new temporal method of phosphorescence of ZnO particules applied to temperature measurments in water.

In numerous experimental studies, it is necessary to measure instantaneous temperature in laminar or turbulent water flow in order to study mixing or heat transfer between anisothermal flow and a wall for instance.

These experimental measurements are necessary to validate developments of new turbulence models or to validate L.E.S simulations for instance in case of high turbulent flow. instantaneous 2D fields of temperature can be obtained non intrusive metrology from images issued from Fluorescence or Phosphorescence emission consecutive to Laser / particule interactions. Two kind of techniques can be used: spectral integration of total or filtered wavelengthes method, called spectral methods and time resolved emission method.

This study deals with experimental development of a novel technique issued from temporal emission of ZnO phosphorescence by using two cameras synchronized with 266 nm UV LASER. Experimental setup is proposed and calibration curve with Temperature sensibility is studied in the field of 20 - 35 °C in a in house calibration cell.

Another 2D instantaneous temperature fields metrologies are applied like classical spectral ZnO phosphorescence emission and B Rhodamine fluorescence emission. All of these techniques are using 266 nm Laser emission and visible light intensified camera.

A comparison of thesensibility and precision of these 3 techniques are then proposed and discussed to show that temporal method apply to ZnO phosphorescence with 2 cameras show a better sensitity of temperature measurment.   

Interpretation of Omnidirectional Integration using the Green's Function: Inversion from Error-contaminated Pressure Gradient to Pressure FIelds

Accurate and efficient measurement of the pressure field is crucial in fluid mechanics. This research introduces a novel method for reconstructing the instantaneous pressure field from error-embedded pressure gradient data using the Green's function of the Laplacian operator as the convolution kernel. The method establishes a mathematical connection of the Green's Function Integral (GFI) with the omnidirectional integration (ODI) for pressure reconstruction, but with improved computational simplicity. The GFI method is applied to simple canonical and isotropic turbulence flows in two-dimensional and three-dimensional domains, respectively, while considering uncertainty quantification through eigenanalysis. Results demonstrate that the accuracy of GFI is comparable to ODI, yet with significantly improved computational efficiency. This method presents a promising approach for the reconstruction of not only pressure field, but also other types of scalar potential quantities, such as density, temperature, stream function, velocity potential, etc., from their error-contaminated gradient measurement data in various fluid mechanics applications.   

Error propagation analysis in the continuous limit of the "Omni-Directional Integral" and a novel pressure reconstruction method based on Helmholtz-Hodge decomposition and Curl-free Radial Basis Functions

Reconstructing pressure fields from image velocimetry measurements commonly involves one of two general strategies: 1) solving the pressure Poisson equation (PPE), and 2) recovering pressure directly from measured pressure gradients (e.g., the omni-directional integral (ODI) methods). ODI methods attempt a finite ensemble reconstruction of the pressure field on a discrete mesh satisfying the path independence property (PIP) of the line integral for a scalar field and perform well when white noise is present in the measured pressure gradient. By invoking the Helmholtz-Hodge Decomposition (HHD), which extracts the curl-free components of any vector field, respecting the PIP exactly, our rigorous error propagation analysis on ODI and HHD demonstrates that the continuous limit of ODI is to apply HHD to a measured pressure gradient. We also propose a novel direct HHD-based pressure field reconstruction strategy that offers the following advantages: 1) effective processing of scattered and structured PTV/PIV data using radial basis functions with curl-free kernels, 2) complete elimination of divergence-free bias in measurements, resulting in superior accuracy compared to PPE and ODI, and 3) avoidance of ensemble practices and more than a 100-fold reduction in computational cost compared to conventional ODI methods.   

Pressure Reconstruction from Time-resolved Tomographic PIV Data Using CFD Technique

One common method to reconstruct the pressure field form experimental velocity field is to solve the pressure Poisson equation. The Poisson equation for pressure can be solved numerically (e.g., Finite Difference and Finite Volume Method) or analytically (e.g., Green-function-based integration). Standard CFD solvers (FVM based solvers) offer powerful and efficient tools that can be used to compute the pressure field from PIV velocity field. Unlike the FDM based solvers, these solvers can easily handle unstructured grid and complex geometry. In this work, the pressure field for synthetic jet impingement is reconstructed from time resolved, tomo-PIV data using a CFD technique. A standard solver (pisoFOAM), available within the popular open source CFD package OpenFOAM, is modified and used to interpolate the experimental velocimetry data on the CFD grid and solve the pressure Poisson equation. The modified solver can handle different combination of mixed boundary conditions and various forms of the governing equation. The reconstructed pressure fields are compared with a pressure sensor and with the pressure field computed with a FDM based solver.   

Modified (n + 1)D Laplacian for smooth pressure reconstruction based on time-resolved Velocimetry: from analysis to experiments

Smooth reconstruction of pressure fields from time-resolved image velocimetry involves two general strategies: 1) suppressing the noise of the measured velocity field and/or 2) applying some smoother to the pressure reconstruction. For example, the 4D pressure solver in DaVis 10, LaVision, employed the idea of the latter category by introducing the “modified Poisson equation”: ▽²p+ξ²∂²p/∂t²=f(u), where p is the pressure field, u is the velocity field measured by time-resolved image velocimetry.  The tunable parameter ξ² is used to control the solver's smoothing behavior by adjusting the diffusivity in time. However, the selection of ξ² is not trivial until the features of the modified Laplacian are known. This work focuses on investigating the fundamental properties of the modified Poisson equation. We first provide rigorous analysis, with numerical and experimental validation, to prove that this solver can smooth the computed pressure by setting a large enough ξ², and the error in the reconstructed pressure is bounded. We also discuss the potential drift of pressure due to the partitioning in time, which is an optional strategy used in LaVision's current 4D solver to reduce computational costs, with theoretical analysis backed by numerical and experimental tests. Our analysis and validation not only show that careful choice of the parameters (e.g., ξ²) is needed for smooth and accurate pressure field reconstruction but provide guidelines for parameter tuning when similar time-resolved solvers are used.   

A Novel Microfluidic Pressure Sensor for the Study of Capillary Pressure and Multiphase Flow in Porous Media

Pressure quantification and control at the microscale is crucial to numerous applications involving fluid mechanics problems. Despite recent advancement of technologies, microscale pressure measurement such as in microfluidic devices and multiphase flow is still not trivial. In particular, capillary pressure is a defining variable porous medium flows and plays a key role in flow modeling and prediction. It, however, has rarely been directly measured at the pore level, primarily due to its small length scale and the high spatial and temporal resolutions it would require. To that end, we design and fabricate an on-chip sensor using soft lithography with a thin polydimethylsiloxane (PDMS) membrane that deflects subject to pressure changes in the fluid flow, which is in turn quantified optically using astigmatic particle tracking. Combined with a microscope and a high-speed camera, the microfluidic pressure allows us to perform in-situ measurements of capillary pressure within individual pore spaces, providing valuable insight into the pore scale mechanisms. Furthermore, the sensor also enables a detailed quantification of the well-known Haines jumps from a pressure perspective, which is previously not possible.