Samuel Raben

Adaptive gappy proper orthogonal decomposition for particle image velocimetry data reconstruction

This work presents a novel method for replacing erroneous measurements in digital particle image velocimetry (DPIV) data using an adaptive reconstruction with gappy proper orthogonal decomposition (POD). Previous studies have shown that gappy POD can be used to replace erroneous data with high accuracy. Conventional gappy POD methods employ a spatially constant number of modes for reconstructing the missing information across the entire field. In contrast, the method presented herein proposes a locally adaptive criterion that allows for determination of the optimum number of POD modes required for the reconstruction of each replaced measurement. This reconstruction produces higher accuracy results using more POD modes than with previous POD methods. The new method was compared against commonly utilized techniques for DPIV vector replacement, namely Kriging, bootstrapping and basic interpolation, as well as previously presented POD reconstruction techniques. The results showed that the adaptive gappy POD reconstruction provides higher accuracy and robustness.

Local Scour at Bridge Piers: The Role of Reynolds Number on Horseshoe Vortex Dynamics

This paper reports a comprehensive study of the major scour agent around bridge piers: the turbulent horseshoe vortex. The intricate and inherently unsteady characteristics of the junction flow are captured within a series of scaled laboratory experiments. We applied the state-of-the art Time-Resolved Digital Particle Image Velocimetry (TR-DPIV) technique to measure the velocity field at the centerline plane of symmetry upstream of a cylindrical model. Three levels of Reynolds numbers (Re D ) based on the obstacle diameter were studied: 26,000, 48,000 and 117,500. We evaluated the effect of this factor based on the time-averaged analyses of velocity and vorticity. Basic statistical analysis of the fluctuating velocity components provided insight to the physical mechanism that governs the behavior of the horseshoe vortex at the aforementioned levels of Re D .

Time Resolved DPIV in a Transonic Turbine Cascade

This paper presents an extension of existing Proper Orthogonal Decomposition (POD) based methods for gappy data reconstruction to allow its use with experimental data without a priori knowledge of the ‘True’ solution, and applies it time-resolved DPIV data taken in a transonic turbine cascade. The method is based on minimizing the residual of a divergence criterion, in order to determine the optimum number of modes required for reconstruction. This serves as a convergence criterion for termination of Venturi and Karniadakis’s iterative Gappy POD method. Gappy flow fields were created using DNS data from a near wall turbulence simulation. Gappyness levels of 5%, 11%, 20%, 50% and, 80% were created with gap sizes 3×3, 7×7, 11×11, and arbitrary N×M vector spaces. The method is shown to closely predict the optimum required modes for a minimum-error reconstruction, and the errors associated with this convergence criterion are shown to be on or below the error associated basic DPIV velocity uncertainty measurements. Finally, this method is tested on gappy experimental data from a transonic turbine cascade. The resulting reconstructed flow fields demonstrate clearly observable vortical structures and dominant frequencies.Copyright

Spatiotemporal Development of Transitional Wall Jets

Wall jets have many applications in engineering ranging from active flow control to film cooling. A typical wall jet is characterized by both spatial and temporal variation. Here we use Time Resolved-Digital Particle Image Velocimetry (TR-DPIV) to deliver high spatially and temporally resolved investigation of wall jets across a wide range of Reynolds numbers (150–10,000). We employ Proper Orthogonal Decomposition (POD) to post-process the data and generate low-order models describing the underlying physics. The results show the presence of near wall structures forming at the jet exit and convecting downstream directly influencing the transition to turbulence. Using the time coefficients associated with the POD modes, the frequency content of the individual modes is determined and the mechanism of energy transfer between the modes is quantified. This study provides the first spatiotemporally resolved experimental investigation of the transition to turbulence of a rectangular wall jet.Copyright