Session G21: Experimental Techniques: Data Analysis, Bias and Uncertainty

Focus enhancement in long schlieren imaging systems using corrector lenses

Schlieren imaging is a widely implemented refractive imaging technique utilized to visualize flow fields around objects. Implementation of schlieren imaging may require an extended test section in which the parallel light test section is longer than the focal length of the schlieren optics. Such a configuration introduces a limited region in the test section which is in focus. Here, corrector lenses varying in focal length from -1000 to -100 mm were introduced before the camera lens altering the focus location. The resulting focus range achieved with the corrector lenses was studied using lens type and z-type schlieren imaging systems having test sections longer than the schlieren optics. Lens type imaging systems having schlieren optics of different focal lengths were also compared. The location of optimal focus for the different imaging configurations was determined through implementation of an automated image processing routine. The experimentally determined focus range was compared to the focusing plane calculated using the thin lens equation. The thin lens equation is in good agreement with the experimental results, however a better fit was achieved for corrector lenses with a larger negative focal length. The agreement between the experiments and the thin lens approximation yielded design equations for implementing the corrector lenses in a given schlieren system.   

Plot Generating AI

They say pictures are worth a thousand words, and in the experimental research the equivalent might be a figure is worth a thousand data points. Our objective is to build a generative adversarial network (GAN) that can generate "good" plots. Where a "good" plot is defined as displaying correlation, physically interpretable and meaningful, and following the principle of parsimony. We propose three metrics for defining what makes a "good" plot. Then with an initial random guess we can optimize the initial guess until we arrive at the nearest "good" plot; this plot is defined as a potential candidate. Iterating many times we can create a sample of candidates. The sample is combined with actual plots gathered from literature and then used in the creation of the generator, a convolutional neural network, and the discriminator, a deconvolutional neural network.   

Construction and characterization of a large jet grid for the Twente Water Tunnel

A new jet grid has been developed for generating turbulence in the Twente Water Tunnel to facilitate experimental investigations into multiphase flows at high Taylor-Reynolds numbers. Our innovative system features 112 individually controllable jets pointing parallel to the main flow direction arranged in a square staggered grid. The jets fire with a nozzle exit velocity of around 10 m/s. The jet grid measures 45 by 45 centimeters, and boasts various advancements, such as the capability to independently control the mean flow and turbulence generation and a novel firing protocol, based on OpenSimplex noise. This type of gradient noise provides spatial and temporal coherence, allowing fine-tuning of turbulence downstream from the grid. We have experimentally investigated various jetting protocols using LDV and CTA measurements. We will present results of these validation measurements to demonstrate that the resulting turbulence is homogeneous and isotropic and to quantify the turbulence levels.   

Novel Single-Camera Light Field Imager for Three-Dimensional Measurements

Typical optical setups for three dimensional measurements require several cameras which can be cost prohibitive or even impossible due to limited optical access or space requirements. One alternative is the use of a “plenoptic” or “light field” technique which allows three-dimensional imaging using a single camera by measuring both the spatial and angular information of collected light. These methods have previously been demonstrated for both tomographic PIV and scalar field measurements. Recently, an alternative light field technique has gained popularity in microscopy, dubbed “Fourier Light Field Microscopy” or FLFM the technique directly captures multiple perspective views. By using relatively few views the the angular information is sampled more sparsely which directly leads to higher spatial resolution compared to other plenoptic techniques. This method has been demonstrated for fluid mechanics with 3D molecular tagging velocimetry. Here we present a novel optical setup based on the principles of FLFM to be used for larger scales. With our setup we image a volume up to 14x14x4 mm with seven perspective views. The maximum angularity between views is 28 degrees which is sufficient to perform tomographic PIV with good quality. Directly capturing multiple views also allows the images to be easily split and treated individually so that existing tomography software designed for multiple cameras can be used. We process PIV images acquired with our setup using the commercial software DaVis and demonstrate that the reconstructed volume has low ghost particle intensity.