Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session S11: Experimental Techniques: Quantitative Flow Visualization III |
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Chair: Tadd Truscott, Utah State University Room: 3B |
Tuesday, November 26, 2019 10:31AM - 10:44AM |
S11.00001: ABSTRACT WITHDRAWN |
Tuesday, November 26, 2019 10:44AM - 10:57AM |
S11.00002: Towards quantification of biologically generated turbulence through 3D scanning particle image velocimetry Matthew Fu, Isabel Houghton, John Dabiri The role of biologically generated turbulence in scalar transport and ocean mixing remains inadequately understood. Though the turbulent scales created by a single swimmer might be limited to those of the individual organism, recent work has suggested that the larger sizes associated with the aggregates of vertically migrating swimmers can introduce mixing scales relevant to the surrounding water column. Quantifying this process requires that the entire flow field and the full range of mixing scales, from the size of the aggregate to below that of the individual animal, must be resolved. While there have significant advancements in volumetric velocity measurements, the spatial resolution associated with these techniques is typically insufficient for exploring common species of vertically migrating swimmers. Here, we present a scanning particle image velocimetry apparatus for quantifying three-dimensional configurations of vertically migrating swimmers and their volumetric, three-component velocity fields and demonstrate its use on the vertical migrations of brine shrimp \textit{Artemia salina.} [Preview Abstract] |
Tuesday, November 26, 2019 10:57AM - 11:10AM |
S11.00003: Simultaneous measurements of large-scale flow dynamics and small-scale turbulent properties by a hybrid method combining cross-correlation and optical-flow schemes Ye Tian, Zhen-Yuan Gao, Shi-Di Huang, Tianshu Liu Particle image velocimetry (PIV) has been established a standard technique based on cross-correlation scheme for velocity field measurement. While PIV holds advantages in the measurements of large-scale flow dynamics, its accuracy in measuring small-scale turbulent quantities is always unsatisfactory due to limited spatial resolution. On the other hand, optical flow method is born with the capability of extracting high-resolution velocity field up to one vector per pixel [J. Fluid Mech. (2008), vol. 614, pp. 253--291]. In this work, we utilize a hybrid method, combining cross-correlation and optical flow schemes, to analyze the PIV images of global field measured in turbulent Rayleigh-BĂ©nard system. Both the large-scale flow structures and small-scale turbulent properties are obtained simultaneously. It is found that the time-averaged velocity fields are consistent with the results obtained by standard PIV technique. Furthermore, the kinetic energy dissipation and structure functions obtained from present method are compared with those obtained from standard PIV method. [Preview Abstract] |
Tuesday, November 26, 2019 11:10AM - 11:23AM |
S11.00004: Time-resolved turbulent velocity field reconstruction using a long short-term memory (LSTM) Zhiwen Deng, Yingzheng Liu, Kyung Chun Kim This paper focuses on the time-resolved turbulent flow reconstruction from discrete point measurements and non-time-resolved PIV measurements using an artificial intelligence framework based on LSTM. To this end, an LSTM-based proper orthogonal decomposition (POD) model is proposed to establish the relationship between velocity signals and time-varying POD coefficients obtained from non-TR-PIV measurements. An inverted flag flow at Re$=$6,200 was experimentally measured using TR-PIV for the construction of training and testing datasets and for validation. Two different time-step configurations were employed to investigate the robustness and learning ability of the LSTM-based POD model: a single-time-step structure and a multi-time-step structure. The results demonstrate that the LSTM-based POD model has great potential for time-series reconstruction since it can successfully recover the temporal revolution of POD coefficients with remarkable accuracy, even in high-order POD modes. In addition, a relative error reconstruction analysis was conducted to compare the performance of different time-step configurations further, and the results demonstrated that the POD model with multi-time-step structure provided better reconstruction of the flow fields. [Preview Abstract] |
Tuesday, November 26, 2019 11:23AM - 11:36AM |
S11.00005: Extending "Postage-Stamp PIV" to MHz Rates for Measurement of Turbulent Velocity Spectra Steven Beresh, Russell Spillers, Melissa Soehnel, Seth Spitzer Previously, time-resolved particle image velocimetry using a pulse-burst laser demonstrated direct measurement of turbulent velocity spectra at very high frequencies without frozen turbulence assumptions. By accepting reduced laser pulse energy and confining the measurement to a field of view of only 128 x 120 pixels, sequences of 4000 images at 400 kHz were acquired, giving rise to the moniker of "postage-stamp PIV." Still, the data reached frequency limitations of approximately 120 kHz due to noise interference. To raise the effective frequency response, the increased speeds of the most recently available cameras obtained data at a sampling rate of 990 kHz, but require overcoming drawbacks in these cameras that can have unfortunate properties for accurate PIV measurements. The increased framing rate oversamples the data and therefore a conventional image-pair correlation can be replaced by multiple-frame image interrogation. The increased accuracy of multiple-frame methods lowers the noise floor and therefore reveals higher-frequency content showing deviation from the theoretical -5/3 power law of the inertial subrange, likely because the turbulent fluctuations have not reached isotropy. [Preview Abstract] |
Tuesday, November 26, 2019 11:36AM - 11:49AM |
S11.00006: Wall Tracking Method for High Resolution Boundary Layer Measurements on a Wing-Fuselage Model David Jeon, Christian Willert, Damian Hirsch, Morteza Gharib Turbulent boundary layer experiments pose a problem with the large range of length scales that need to be resolved. Measurement techniques tend to focus at either end of this scale. Techniques like holographic PIV primarily measure the region nearest to the wall. Those like conventional PIV typically only resolve down to the log layer. A variation on PIV has been developed by one of us (Willert), where a long-range micro PIV setup is used to resolve down to sub-layer scales. This type of micro PIV setup was used on a wing-fuselage model to measure the boundary layer on the model at Re$_{\mathrm{\theta }}$ up to 2700, from y$^{\mathrm{+}}\approx $1 in the sub-layer nearly to the outer edge of the boundary layer. In addition, wall shear stress can be computed both using the profile in the sub-layer and the Clauser method through the log-layer and compared against each other. The greatest problem faced with this technique is compensating for apparent wall motion. For example, at the conditions where we can resolve down to y$^{\mathrm{+}}\approx $1, the apparent wall motion is on the order of \textpm 10 wall units. After compensation, this drops below 0.1 wall units. Therefore, the wall tracking method used is critical to get data close to the wall. [Preview Abstract] |
Tuesday, November 26, 2019 11:49AM - 12:02PM |
S11.00007: Turbulence characteristics of flow through an open cell metal foam. Youngwoo Kim, Chanhee Moon, Kyung Chun Kim Random nature of stochastic foam provides favorable geometrical properties for thermal applications such as large specific surface area and high porosity. However, notwithstanding intensive investigations for the last two decades, hydrodynamic characteristics of stochastic foam are still poorly understood. Current 3-D printing technology supports printing of transparent complex structures. This study investigates turbulence characteristics of a stochastic foam. Using micro-tomography and stereo-lithography, a transparent stochastic foam was printed. Quantitative flow visualization was performed using refractive index matching technique and time-resolved particle image velocimetry. Mixing and turbulence characteristics were discussed. Mechanical mixing and high turbulence in the stochastic foam is beneficial for thermal applications, but large wake area behind the struts cause high pressure drop. On the basis of the results, a new concept of turbulence due to complex geometry can be named as ``structure generated turbulence (SGT)''. [Preview Abstract] |
Tuesday, November 26, 2019 12:02PM - 12:15PM |
S11.00008: Smartphone PIV David Armijo, Lori Caldwell, Sarbajit Mukherjee, Vladimir Kulyukin, Angela Minichiello, Tadd Truscott We are developing a smartphone app that performs particle image velocimetry (PIV). This app is called Mobile Instructional Particle Image Velocimetry (miPIV). The intent is to increase the availability of PIV systems to high school and undergraduate students. PIV is used to measure the velocity within a flow field by illuminating neutrally buoyant particles with a laser sheet and recording the motion with a video camera. In miPIV the particles are illuminated with a laser pointer spread into a laser sheet, and the camera is a smartphone. Here, the work is advanced by integrating the image capture, pre-processing, PIV calculations, and vector output post-processing onto a mobile device. The Java app will be compatible with any smartphone running Android 5.0 Lollipop (API 21) or higher. The system accuracy is benchmarked by comparing a free stream flow field miPIV output with a lab grade system. Results indicate that the system performs well where the illumination quality is high and the flow speeds are below the motion blur limit. Even when these perfect scenarios are not met, students can see flow structures and average velocities that reveal information about the flow field in a way that can inspire them to further study fluid mechanics and its applications [Preview Abstract] |
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