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 A18: Turbulence Measurements I |
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Chair: Mitul Luhar, USC Room: 400 |
Saturday, November 23, 2019 3:00PM - 3:13PM |
A18.00001: Resolvent-mode-based Reconstruction of Wall-bounded Turbulent Flows From Non-time-resolved PIV Measurements C Vamsi Krishna, Mengying Wang, Maziar Hemati, Mitul Luhar Turbulent flows are characterized by broadband spatio-temporal fluctuations, which makes the acquisition of fully-resolved velocity measurements challenging. The goal of this study is to use a physics-based model---projecting the velocity field onto resolvent modes---to reconstruct velocity field from non-time resolved 2D PIV measurements in turbulent channel flow. The resolvent modes are generated via a gain-based decomposition of the governing equations, ensuring physical consistency. A large database of resolvent modes is generated. The Forward Regression with Orthogonal Least Squares algorithm is then used to identify the dominant resolvent modes and to calibrate their amplitude and phase. After calibration, the velocity field can be reconstructed at arbitrary spatiotemporal resolution using the weighted resolvent modes. The weighted resolvent modes also enable estimation of out-of-plane components of velocity and pressure. For proof-of-concept tests of this method, we use DNS data of turbulent channel flow from the Johns Hopkins Turbulence Database. Reconstruction error is quantified and compared with previous studies using Rapid Distortion Theory and Taylor’s Hypothesis for reconstruction. [Preview Abstract] |
Saturday, November 23, 2019 3:13PM - 3:26PM |
A18.00002: An eigen-ensemble-variational algorithm for identifying scalar sources from remote measurements in turbulent environments Qi Wang, Tamer Zaki The ability to identify the location and intensity of a scalar source in turbulent environment from remote measurements is obfuscated by the stochasticity of turbulent eddies and by diffusion. An algorithm is proposed to solve this inverse problem, which relies on estimating the left and right singular vectors of the scalar impulse-response system, or its eigen-sources and eigen-measurements. The projection of the true source onto an eigen-source is proportional to the projection of the sensor signal onto the corresponding eigen-measurement, and the proportionality is given by the singular value. When only the sensor signal is available, the unknown source is identified by requiring that it accurately reproduces this proportionality. A pre-requisite of the algorithm is knowledge of the eigen-spectrum of the system, which can be available from historical data or approximated using proper orthogonal decomposition of the observation matrix from an ensemble of trial sources. We demonstrate that using only five ensemble members, the source location and intensity are predicted with less than 10\% error, and we quantify the effect of sensor noise. Furthermore, the algorithm utilizes forward simulations only and can be easily adopted with expanding time horizon of measurement. [Preview Abstract] |
Saturday, November 23, 2019 3:26PM - 3:39PM |
A18.00003: On the dynamics of Reynolds stresses in the tip region of axial turbomachines Huang Chen, Yuanchao Li, David Tan, Joseph Katz Experiments examining the flow in the tip regions of axial turbomachines have been performed in a refractive index matched facility, enabling unobstructed optical access. Stereo PIV measurements in closely spaced planes provide high-resolution 3D distributions of the strain rate and Reynolds stress tensors in the rotor passage. In areas dominated by large vortical structures, such as the tip leakage vortex (TLV) and the backflow vortex that propagates circumferentially, the strain and Reynolds stress tensors are poorly correlated, and the measured eddy viscosity fluctuates from large negative to positive values. In the vortex cores, a substantial fraction of the unsteady motion involves large-scale structures. The turbulence is highly anisotropic and inhomogeneous, with an anisotropy tensor shifting from 1D to 2D to 3D over small distances. Ingestion of turbulence across the tip gap enhances the turbulence production and increases the Reynolds stresses substantially. However, the distributions of Reynolds stresses depend on their history, hence affected by advection and diffusion in addition to local production and dissipation rates. The TLV breaks up in the aft part of the passage generating a broad area with elevated turbulence, but with lower peak values. [Preview Abstract] |
Saturday, November 23, 2019 3:39PM - 3:52PM |
A18.00004: Time-resolved tomographic PIV measurements of a turbulent shear layer flow past an open cavity Jose Moreto, Xiaofeng Liu Characterization of the pressure-related turbulence terms including pressure--rate-of-strain, pressure diffusion and velocity--pressure-gradient tensor in the Reynolds stress transport equation in canonical turbulent flows is of critical importance for calibrating and improving turbulence models for RANS (Reynolds-Averaged Navier Stokes) based flow simulation. Recent work of Liu and Katz (2018) based on planar-PIV shows the complex nature of the pressure related terms and their substantial impact on the dynamics of turbulence transport throughout a shear layer flow past an open cavity. They also demonstrate the need for a full three-dimensional characterization of the pressure-related turbulence transport terms around the cavity trailing corner. To address this need, time-resolved tomographic PIV measurements of the 3D pressure-related terms in the Reynolds stress transport budget for a turbulent shear layer past a cavity is being carried out. The curl-free property of the pressure gradient is used to control the quality of the measured pressure gradient, and the continuity equation is used to control the quality of the velocity measurement. The incoming flow quality is fully characterized so as to facilitate CFD simulation. Detailed velocity and Reynolds stress profiles and turbulence spectrum at selected locations, as well as preliminary tomo-PIV data of the flow field just above the cavity trailing corner, will be presented. [Preview Abstract] |
Saturday, November 23, 2019 3:52PM - 4:05PM |
A18.00005: Automatic identification and characterization of bursting periods in a turbulent velocity field Roni Hilel Goldshmid, Dan Liberzon A new automatic method for accurate detection of bursting periods in single-point velocity field records is presented. Identification of bursting periods is made by locating an instantaneous increase in the normalized ``instantaneous'' TKE dissipation rate, obtained using moving window averaging. Use of the record rms and average values for normalization eliminate the need to define flow specific thresholds, and hence decouples burst identification from the generation mechanism. This potentially makes the method universally applicable across various types of turbulent flows. The method performance is examined using a dataset of buoyancy driven turbulent boundary layer flow. Turbulent statistics of the identified non-bursting periods show distinguished similarity to those expected in canonical turbulence, while the identified bursting period statistics differ significantly. To examine the bursting period generation mechanism, statistical findings of temperature fluctuations are examined, and additional tests are provided to assist in identification of the generation mechanism. Examination of the flow field scalar variations in connection with turbulent bursting periods can assist in further understanding of bursting generation and scalar transfer processes. [Preview Abstract] |
Saturday, November 23, 2019 4:05PM - 4:18PM |
A18.00006: Lagrangian acceleration time scales in anisotropic turbulence Romain Volk, Peter Huck, Nathanael Machicoane We present experimental Lagrangian measurements of tracer particle acceleration auto-correlation functions in an anisotropic and inhomogeneous flow spanning the typical range of experimentally accessible Reynolds numbers. The large scale forcing of the flow creates a stagnation point topology where straining motion governs the anisotropic velocity and acceleration fluctuations. We show that the time scales of the acceleration components remain anisotropic at high Reynolds numbers and that they are related to the dissipative time scale by the Lagrangian structure function scaling constants $C_0$ and $a_0$. The scaling relation proposed herein is supported by observations using experimental Lagrangian trajectory data sets and analytical calculations using a jointly-Gaussian two-time stochastic model. Examination of acceleration power spectra show that acceleration fluctuations become isotropic in the dissipative range which suggests that the acceleration time scale is not only determined by small scales, but also by large and anisotropic scales whose contributions are substantial, even in the high Reynolds number limit. [Preview Abstract] |
Saturday, November 23, 2019 4:18PM - 4:31PM |
A18.00007: Inter-scale energy budget in a von K\'{a}rm\'{a}n mixing tank Anna N. Knutsen, Pawel Baj, Nicholas A. Worth, James R. Dawson, John M. Lawson, Eberhard Bodenschatz The inter-scale energy budget in the center region of a von K\'{a}rm\'{a}n mixing tank has been investigated based on fully resolved measurement data from volumetric and stereo PIV experiments at \textit{Re}$_{\lambda }=$199. The K\'{a}rm\'{a}n-Howarth equation generalized for non-isotropic, inhomogeneous turbulence (sometimes referred to as the K\'{a}rm\'{a}n-Howarth-Monin-Hill equation) is used to map the full energy transfer, and determine the importance of the different mechanisms transporting energy. The results show that the mean flow, despite its small magnitude relative to the turbulent fluctuations, plays a significant role in the local inter-scale energy transfer. This result stands out from similar investigations of other types of flows, where the mean flow has been found to have a negligible role in transporting energy across scales. This large contribution is caused by the strong gradients in all directions of the mean flow, which is a special feature of the von K\'{a}rm\'{a}n flow. The isotropy of the various terms is also evaluated, and strong local variations of the different terms in scale space are observed even down to very small separations, highlighting the importance of using planar or volumetric data when energy transfer is studied. [Preview Abstract] |
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