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 H23: Boundary Layers: General |
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Chair: David Buchta, Johns Hopkins University Room: 605 |
Monday, November 25, 2019 8:00AM - 8:13AM |
H23.00001: Turbulent superstructures in a zero pressure gradient turbulent boundary layer for the Mach number range 0.3 $-$ 3.0 Matthew Bross, Sven Scharnowski, Christian J. K\"{a}hler Meandering high- and low-momentum flow motions often called superstructures in turbulent boundary layers (TBLs) can extend up to several boundary layer thicknesses and contain a large portion of the layer's turbulent kinetic energy. However, compared to the extensive number of incompressible investigations much less is known about the structural characteristics for compressible TBLs. Therefore, in this investigation TBLs on a flat plate over a range of Reynolds numbers and Mach numbers are considered in order to investigate the effect of compressibility on superstructures. Measurements are performed in the Trisonic Wind Tunnel Munich (TWM) for 0.3 $<$ Ma $<$ 3.0 and a friction Reynolds number of 2700 $<$ Re$_{\tau}$ $<$ 14800 or 19800 $<$ Re$_{\delta_2}$ = $\rho_e u_e\theta^*/\mu_{\mbox{\scriptsize{w}}}$ $<$ 40800. Velocity fields are recorded using planar particle image velocimetry methods (PIV and stereo-PIV) in three perpendicular planes. Using multi-point statistical and spectrogram methods it was found that the streamwise wave lengths associated with superstructures in the log-law layer slightly increase with Mach number and a distinct increase in the spanwise spacing of these structures was found for the supersonic cases when compared to the subsonic and transonic cases [Preview Abstract] |
Monday, November 25, 2019 8:13AM - 8:26AM |
H23.00002: Optimal interpretation of measurements for enhanced-fidelity prediction of hypersonic boundary layer transition David Buchta, Tamer Zaki High-speed boundary-layer transition is extremely sensitive to the freestream disturbances which are often uncertain, thus compromising the accuracy of model predictions. To enhance the fidelity of simulations, we directly infuse them with measurements data. Our methodology is general and can be adopted with any simulation technique, e.g. parabolized stability equations or direct numerical simulations. An ensemble variational (EnVar) optimization is performed, whereby we determine the upstream flow that optimally reproduces the measurements. The cost functional can account for our relative confidence in the model and the measurements, and judicious choice of the ensemble members improves convergence and reduces the prediction uncertainty. We demonstrate our data-augmented simulations for boundary-layer transition at Mach 4.5. Without prior knowledge of the freestream condition and using only wall measurements from an independent computation (true flow), all the relevant inflow disturbances are identified, and their amplitudes and phases are accurately predicted. We can then evaluate the entire flow field, beyond the original limited wall measurements. Our predicted flow compares favorably to the true “unknown” state, and discrepancies are analyzed in detail. [Preview Abstract] |
Monday, November 25, 2019 8:26AM - 8:39AM |
H23.00003: Velocity-Vorticity Correlation Structure (VVCS) in Transitional Compressible Turbulent Boundary Layer Jun Chen, Shi-Yao Li, Zhen-Su She, Fazle Hussain Velocity-vorticity correlation structure (VVCS) is used to measure the geometry of vortices in the numerically simulated compressible boundary layer (BL) at Ma=2.25, 4.5, and 6.0. Wall normal vorticity represented by $VVCS_{12}$ corresponds to the low-speed streaks, flanked by counter-rotating streamwise vortices identified by $VVCS_{11}$. The ratio of spanwise size to spanwise spacing of $VVCS_{12}$ decreases from 3 to unity during transition to turbulence, indicating low-speed streaks gradually populating in $x$. During transition, correlation coefficient, R$_{ij}$ near the wall decreases at first then increases up to 0.55, indicating that the well-arranged hairpins break up into streamwise vortices, as observed in visualization of this region. At each $y$, the length of spanwise vortices identified by VVCS$_{13}$ decreases fast during transition at all $Ma$s, indicating thickening of BL, while becoming nearly invariant in $x$ in the well-developed region, consistent with a self-similar turbulent BL predicted by our theory (She2018JFM). For both $VVCS_{11}$ and $VVCS_{12}$ the spanwise size equals the transverse size in the developed flow region. These results affirm the predominant role of the multi-layered structure and suggest new possibilities for control of turbulent flow. [Preview Abstract] |
Monday, November 25, 2019 8:39AM - 8:52AM |
H23.00004: The post-singularity structure in the boundary-layer flow induced by a rotating sphere Jim Denier, Sophie Calabretto The flow induced by a rotating sphere has long held the interest of fluid mechanists. This simple flow provides a paradigm for the study of colliding boundary layers; in the case of the sphere the boundary-layer collision occurs when fluid is advected from the poles to the equator. The collision process is the physical manifestation of a finite-time singularity in the boundary-layer equations. Here we will present new experimental and computational results which demonstrate that the collision process results in a smooth separation of flow within the boundary layer at the equator and into a radial jet. Our results demonstrate that existing theories for the post-singularity structure do not provide an accurate description of the flow. [Preview Abstract] |
Monday, November 25, 2019 8:52AM - 9:05AM |
H23.00005: The Leading Edge theory: a new insight into the laminar Boundary Layer Mohammad Gabr The flow properties at the leading edge of a flat plate represent a singularity to the Blasius laminar boundary layer equations; by applying the diffusion equation where the velocity of a moving flat plate in a stationary fluid is diffused to the far field, the leading edge velocity profiles are studied. Experimental observations as well as the theoretical analysis show an exact Gaussian distribution curve as the original starting profile of the laminar flow.To conclude; the main key results are as follows:\newline [1] The velocity profiles at the leading edge of a flat plate are Gaussian Curves that grow in space and time; whereas the Blasius velocity profile is a part of the general Gaussian curves solution.\newline [2]A new method to calculate the friction drag is successfully tested, based on the displacement area of the leading edge velocity profile.\newline [3] In order to obtain the final physical proof of the new theory it is recommended to carry out experiments using an ultra-thin flat plate moving in a stationary fluid and measuring the velocity profiles at the leading edge with different measurement techniques. [Preview Abstract] |
Monday, November 25, 2019 9:05AM - 9:18AM |
H23.00006: Boundary-layer flow of air over a falling soap film Yuna Hattori, Rory Cerbus, Julio Barros Jr., Pinaki Chakraborty A falling soap film is a well-known experimental setup to realize two-dimensional flows in a laboratory. The soap film is invariably embedded in ambient air, which, in turn, is set to motion due to the falling film. We experimentally measure the velocity profile in the air using Particle Tracking Velocimetry (PTV). We find that the measured velocity profile conforms well to theoretical predictions using boundary-layer approximation. We discuss some implications of our results on the modeling of geophysical flows. [Preview Abstract] |
Monday, November 25, 2019 9:18AM - 9:31AM |
H23.00007: Characterization of Turbulent Friction Drag Over Biofilm Like Surfaces James Gose, Elizabeth Callison Biofilms are complex ever-changing colonies of bacteria and microbes that form on wetted surfaces. When biofilm formation occurs at flow boundaries, the associated friction drag is increased, which can adversely affect the performance of hydrodynamic systems. The production of increased frictional drag of biofilms is not well understood; however, it is often attributed to the surface roughness, compliance, and the presence of undulating streamers in the flow boundary. Recent studies on living biofilms and rigid analogs have helped to address this very issue with acknowledgement that the biofilm evolution over time as it is exposed to flow raised additional questions. In this study, it is proposed to characterize biofilm like surfaces that have a more stable formation (i.e. non-evolving, such as faux fur), and their own three-dimensionally printed rigid replicas in a controlled environment. The surfaces will be evaluated in the Skin-Friction Flow Facility at the University of Michigan using streamwise pressure drop measurements, planar particle image velocimetry, and in situ measurement of the biofilm replacement with a line scanner. Comparisons of the resistance curves for the rigid replicas and biofilm like surfaces will be discussed and flow measurements will be presented. [Preview Abstract] |
Monday, November 25, 2019 9:31AM - 9:44AM |
H23.00008: Augmenting Restricted Nonlinear Turbulence to Capture Scale Separation Benjamin Minnick, Dennice Gayme The restricted nonlinear (RNL) dynamics comprises a streamwise constant mean flow interacting with a dynamically restricted perturbation field. This model aims to capture key features of wall-bounded turbulence with reduced computational expense. Constraining the nonlinear interactions leads to a reduced representation in which the RNL dynamics are supported by a small number of streamwise varying modes (non-zero streamwise Fourier coefficients) interacting with the streamwise mean, which captures statistical features of low Reynolds turbulence. To correctly capture momentum transfer at moderate Reynolds numbers where scale separation emerges, the streamwise varying modes must correspond to the small wave lengths associated with the outer-layer peak of the surrogate dissipation spectra. At higher Reynolds numbers where there is a clear separation of scales, we expect the model to require additional large scales consistent with triadic interactions of the small scales. Here, we simulate these interactions by including a large scale streamwise varying mode which is permitted to interact nonlinearly with small scale modes. We demonstrate that this augmented RNL (ARNL) model reproduces key flow statistics for Reynolds numbers as high as $Re_{\tau} \approx 5200$. [Preview Abstract] |
Monday, November 25, 2019 9:44AM - 9:57AM |
H23.00009: Convolutional neural networks for identifying coherent turbulent structures Eric Jagodinski, Siddhartha Verma Identifying coherent structures and extreme events is key to understanding the physics of near-wall turbulent flows. Deep convolutional neural networks were originally developed for image recognition, but have immense potential for extracting features in multi-dimensional vector fields. Here, velocity fields extracted from Direct Numerical Simulations (DNS) of a periodic turbulent channel flow were input into a convolutional neural network in order to identify coherent structures and extreme events. The domain was reduced to a Minimal Flow Unit (MFU), as done in Jimenez (1991), and features were labeled for training the neural network using thresholding. Once trained, the network is able to correctly classify coherent structures and extreme events in the flow field. [Preview Abstract] |
Monday, November 25, 2019 9:57AM - 10:10AM |
H23.00010: Dynamics and structure of backflow events in turbulent channels Jose Cardesa-Duenas, Jason Monty, Julio Soria, Min Chong A statistical description of flow regions with negative streamwise velocity is provided based on simulations of turbulent plane channels in the range $547\leq Re_{\tau}\leq 2003$. It is found that regions of backflow are attached and their density per surface area - in wall units - is an increasing function of $Re_{\tau}$. Their size distribution along the three coordinates reveals that, even though in the mean they appear to be circular in the wall-parallel plane, they tend to become more elongated in the spanwise direction after reaching a certain height. Time-tracking of backflow regions in a $Re_{\tau}=934$ simulation showed they convect downstream at the mean velocity corresponding to $y^{+}\approx 12$, they seldom interact with other backflow events, their statistical signature extends in the streamwise direction for at least 300 wall units and they result from a complex interaction between high and low spanwise vorticity regions far beyond the viscous sublayer. This could explain why some statistical aspects of these near-wall events do not scale in viscous units; they are dependent on the $Re_{\tau}$-dependent dynamics further away from the wall. [Preview Abstract] |
Monday, November 25, 2019 10:10AM - 10:23AM |
H23.00011: Turbulent inflow information generation for compressible boundary layers Guillermo Araya, Kenneth Jansen In this study, the dynamic rescaling-recycling approach for incompressible flows (J. Fluid Mech., 670, pp. 581-605, 2011) is extended to compressible spatially-developing turbulent boundary layers (SDTBL) for turbulent inflow conditions. Since the inlet boundary layer thickness is fixed, the inlet friction velocity is computed based on a power function of the momentum thickness, where the power exponent is calculated "on the fly" according to the flow solution downstream. Thus, there is no need of an empirical correlation as in other recycling methodologies. Additionally, the Morkovin's Strong Reynolds Analogy (SRA) is used in the rescaling process of thermal fluctuations. The methodology is validated in a suite of Direct Numerical Simulation (DNS) of isothermal Zero-Pressure Gradient (ZPG) boundary layers at a Mach number of 2.86. The Reynolds number range is approximately 400-800, based on the freestream density, momentum thickness, freestream velocity and wall viscosity. Low/high order flow statistics are compared with wind tunnel experiments from the literature. Focus is given to the assessment of wall temperature on the thermal transport phenomena and the dynamics of thermal turbulent coherent structures. [Preview Abstract] |
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