Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session R32: Turbulent Boundary Layers VI |
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Chair: Joel Weitzman, Stanford University Room: 2020 |
Tuesday, November 25, 2014 1:05PM - 1:18PM |
R32.00001: Laboratory investigation of a thermally stable boundary layer subject to a step change in wall temperature Tyler Van Buren, Owen J. Williams, Alexander J. Smits Thermally stable boundary layers with a step change in boundary condition are seen in industrial applications (e.g. plate heat exchangers) as well as in nature (e.g. onshore breezes). Previous studies indicate that bulk indicators of stability are often insufficient to describe the local state of turbulence because the local flow strongly depends on its upstream history. Experiments were conducted to gain further insight into these flows. A low-speed wind tunnel was used to generate a rough-wall boundary layer at up to $Re_\theta=1500$. After a development length of approximately $22\delta$, the downstream half of the tunnel wall was heated, creating a step change in wall temperature of up to $135^\circ C$. Particle image velocimetry and a thermocouple rake were used to measure the fluctuating velocity field and mean temperature profile at three locations downstream of the step change. We examine the rate of growth of the internal boundary layer and the corresponding evolution of the turbulent stresses in relation to changes in mean local stratification. [Preview Abstract] |
Tuesday, November 25, 2014 1:18PM - 1:31PM |
R32.00002: The role of large scale motions on passive scalar transport Suranga Dharmarathne, Guillermo Araya, Murat Tutkun, Stefano Leonardi, Luciano Castillo We study direct numerical simulation (DNS) of turbulent channel flow at Re$_{\tau}=394$ to investigate effect of large scale motions on fluctuating temperature field which forms a passive scalar field. Statistical description of the large scale features of the turbulent channel flow is obtained using two-point correlations of velocity components. Two-point correlations of fluctuating temperature field is also examined in order to identify possible similarities between velocity and temperature fields. The two-point cross-correlations betwen the velocity and temperature fluctuations are further analyzed to establish connections between these two fields. In addition, we use proper orhtogonal decompotion (POD) to extract most dominant modes of the fields and discuss the coupling of large scale features of turbulence and the temperature field. [Preview Abstract] |
Tuesday, November 25, 2014 1:31PM - 1:44PM |
R32.00003: Development of Turbulence Downstream of a Submerged Aquatic Canopy Francisco Zarama, Robert Zeller, Jeffrey Koseff Submerged aquatic canopies are present throughout nature in the form of seagrasses, corals, and other sessile organisms. The turbulence generation mechanisms of these systems have been explored, but the effect of the turbulent signature on the flow downstream is poorly understood. Moreover, the drag created by these canopies will create a lingering velocity deficit. These lasting effects of the canopy may have profound effects on downstream mixing, sediment dynamics, and propagule transport. The present study focuses on the adjustment of turbulence and flow characteristics downstream of a model canopy. Specifically, this work examines the evolution of mean velocity, TKE and Reynolds stresses using both acoustic velocimetry and 2D particle image velocimetry. To determine the dependencies of the downstream evolution, these experiments vary canopy height and current velocity. As expected, preliminary results show the flow evolves in distinct phases with changes in the mean statistics happening early, while the turbulence statistics adjust further downstream. Moreover, the mean flow takes a monotonic path towards the steady-state boundary layer, but the Reynolds stresses and TKE grow and spread towards the surface and bottom boundary before relaxing to the steady-state. [Preview Abstract] |
Tuesday, November 25, 2014 1:44PM - 1:57PM |
R32.00004: Self-Similarity of Wakes in Wave-Driven Canopy Flow Robert Zeller, Francisco Zarama, Joel Weitzman, Jeffrey Koseff Wave-driven flow within a canopy is characterized by complex spatial heterogeneity caused by element wakes. Capturing this variability is difficult in numerical simulations and laboratory experiments because of computational cost and measurement access restrictions, respectively. In light of these issues, one way to account for horizontal variability is to assume that element wakes are self-similar. However, self-similarity depends on two conditions that are not necessarily satisfied in wave-driven canopy flows: 1) the wakes must be quasi-steady and 2) the wakes must be 2-D. In this study, phase-averaged particle image velocimetry measurements within a rigid canopy were used to evaluate the assumption of self-similarity. It was found to predict some flow statistics more accurately than others. In addition, the accuracy was found to be dependent on both the Keulegan-Carpenter number and the vertical location within the canopy. At low Keulegan-Carpenter number, the quasi-steady condition was violated because the wakes did not have time to develop. Near the top of the canopy, the 2-D assumption was violated because of the influence of the mixing layer. [Preview Abstract] |
Tuesday, November 25, 2014 1:57PM - 2:10PM |
R32.00005: Wind shear induced wave-turbulence interaction in quiescent water in presence of flexible protruding obstacles Tirtha Banerjee, Marian Muste, Gaby Katul PIV experiments involving air-water and flexible oscillating components have been conducted and a spectral analysis framework regarding data analysis involving noise, wave and turbulence separation has been presented. The experiments reveal that wave and turbulence effects are simultaneously produced at the air-water interface and the nature of their coexistence is found to vary between different flow parameters including water level, mean wind speed, obstacle density and flexibility. For deep water levels, signature of fine-scaled inertial turbulence is found at deeper layers of the water system. The wave action appears stronger close to the air-water interface and damped by the turbulence deeper inside the water system. As expected, wave action is found to be dominated in a certain frequency range driven by the wind forcing, while it is also diffused to lower frequencies by means of (wind-induced) oscillations of the obstacles. Existence of a counter-current flow and its switching to fully forward flow in the direction of the wind under certain combinations of flow parameters has also been observed. The relative importance of wave and turbulence to the overall energy, degree of anisotropy in the turbulent energy components, and momentum transport mechanisms are also quantified. [Preview Abstract] |
Tuesday, November 25, 2014 2:10PM - 2:23PM |
R32.00006: Wind-tunnel experiments of turbulent flow over a surface-mounted 2-D block in a thermally-stratified boundary layer Wei Zhang, Corey Markfort, Fernando Port\'e-Agel Turbulent flows over complex surface topography have been of great interest in the atmospheric science and wind engineering communities. The geometry of the topography, surface roughness and temperature characteristics as well as the atmospheric thermal stability play important roles in determining momentum and scalar flux distribution. Studies of turbulent flow over simplified topography models, under neutrally stratified boundary-layer conditions, have provided insights into fluid dynamics. However, atmospheric thermal stability has rarely been considered in laboratory experiments, e.g., wind-tunnel experiments. Series of wind-tunnel experiments of thermally-stratified boundary-layer flow over a surface-mounted 2-D block, in a well-controlled boundary-layer wind tunnel, will be presented. Measurements using high-resolution PIV, x-wire/cold-wire anemometry and surface heat flux sensors were conducted to quantify the turbulent flow properties, including the size of the recirculation zone, coherent vortex structures and the subsequent boundary layer recovery. Results will be shown to address thermal stability effects on momentum and scalar flux distribution in the wake, as well as dominant mechanism of turbulent kinetic energy generation and consumption. [Preview Abstract] |
Tuesday, November 25, 2014 2:23PM - 2:36PM |
R32.00007: Turbulent Channel Flow Measurements with a Nano-scale Thermal Anemometry Probe Sean Bailey, Brandon Witte Using a Nano-scale Thermal Anemometry Probe (NSTAP), streamwise velocity was measured in a turbulent channel flow wind tunnel at Reynolds numbers ranging from $Re_\tau=500$ to $Re_\tau=4000$. Use of these probes results in the a sensing-length-to-viscous-length-scale ratio of just 5 at the highest Reynolds number measured. Thus measured results can be considered free of spatial filtering effects. Point statistics are compared to recently published DNS and LDV data at similar Reynolds numbers and the results are found to be in good agreement. However, comparison of the measured spectra provide further evidence of aliasing at long wavelengths due to application of Taylor's frozen flow hypothesis, with increased aliasing evident with increasing Reynolds numbers. In addition to conventional point statistics, the dissipative scales of turbulence are investigated with focus on the wall-dependent scaling. Results support the existence of a universal pdf distribution of these scales once scaled to account for large-scale anisotropy. [Preview Abstract] |
Tuesday, November 25, 2014 2:36PM - 2:49PM |
R32.00008: Analysis of Wall Models for Internal Combustion Engine Simulations Using High-speed Micro-PIV Measurements Peter Ma, Tim Ewan, Christopher Jainski, Andreas Dreizler, Louise Lu, Volker Sick, Matthias Ihme The performance of internal combustion engines (IC-engine) is affected by the thermo-viscous boundary layer region. Computational models for the prediction of engine performance typically rely on wall functions to overcome the need for resolving the boundary layer structure. The objective of this contribution is to assess some of the assumptions on the wall functions under realistic operating conditions in a motored engine. Crank angle resolved high-resolution micro particle image velocimetry ($\mu $-PIV) measurements were conducted previously in a spark-ignition direct-injection single cylinder engine. Data analysis is performed to assess the inner structure of the boundary layer. Using these measurements, the performance of a hierarchy of wall models, including the wall function model, which is commonly used in RANS and LES IC-engine simulations, and three hybrid RANS/LES wall models with increasing fidelity are investigated. It is shown that all four models provide adequate predictions if the first grid-point is located in the viscous sublayer; the wall function model has consistently underpredicted the shear velocity if the first grid-point is located outside the viscous sublayer, however the other three hybrid wall models all give reasonable results in this region. [Preview Abstract] |
Tuesday, November 25, 2014 2:49PM - 3:02PM |
R32.00009: Time-resolved PIV measurement of a developing turbulent boundary layer on a towed plate JungHoon Lee, YongSeok Kwon, Jason Monty, Nicholas Hutchins Time-resolved particle image velocimetry (TRPIV) is used to investigate the development of a zero-pressure-gradient turbulent boundary layer from trip to a high Reynolds number state. The unique experimental facility consists of a 5m long flat plate towed through a 60 x 2 x 2 m tow tank at speeds of up to 1 m/s. Windows in the side of the tank enable the evolution of the turbulent boundary layer along the towed plate to be captured using a stationary TRPIV system. This provides a unique view of a spatially and temporally evolving turbulent boundary layer from inception at the trip up to $Re_\tau = 3000$ (near the trailing edge of the plate). Here $Re_\tau = \delta U_\tau/\nu$ is the K\'arm\'an number where $\delta$ is boundary layer thickness, $U_\tau$ is wall-shear velocity, and $\nu$ is kinematic viscosity. In this frame of reference, evolving large-scale features with convection velocities close to the freestream appear nominally stationary within the field of view, enabling us to document the origins and evolution of these features. An analysis of instantaneous convection velocity associated with low- and high-speed structures reveals differences in the trajectory and local convection velocity of these features as the turbulent boundary layer develops along the flat plate. [Preview Abstract] |
Tuesday, November 25, 2014 3:02PM - 3:15PM |
R32.00010: Turbulence structures in a strongly decelerated boundary layer Ayse G. Gungor, Yvan Maciel, Mark P. Simens The characteristics of three-dimensional intense Reynolds shear stress structures (Qs) are presented from a direct numerical simulation of an adverse pressure gradient boundary layer at $Re_\theta=1500-2175$. The intense Q2 (ejections) and Q4 (sweeps) structures separate into two groups: wall-attached and wall-detached structures. In the region where turbulent activity is maximal, between $0.2\delta$ and $0.6\delta$, $94\%$ of the structures are detached structures. In comparison to canonical wall flows, the large velocity defect turbulent boundary layers are less efficient in extracting turbulent energy from the mean flow. There is, furthermore, much less turbulence activity and less velocity coherence near the wall. Additionally, the wall-detached structures are more frequent and carry a much larger amount of Reynolds shear stress. [Preview Abstract] |
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