Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session A20: Turbulence: Thermal Flows |
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Chair: Luciano Castillo, Texas Tech University Room: 208 |
Sunday, November 22, 2015 8:00AM - 8:13AM |
A20.00001: The Power Law and Log-law Behaviors of the Accelerated Thermal Turbulent Boundary Layer Luciano Castillo, Guillermo Araya, Fazle Hussain DNS of spatially evolving thermal turbulent boundary layers with strong favorable pressure gradient (FPG) is performed by employing a multi-scale dynamic approach for generating realistic inflow turbulent information. Results reveal that the thermal fluctuation, $\theta$' and the Reynolds shear stress, $<$$u'v'$$>$ , both exhibit a logarithmic behavior in the meso-layer region (e.g., $30 \leq y^+ \leq 300$). The thickness of the log-region increases in the flow direction and with the strength of the acceleration. Moreover, the mean temperature profiles do not exhibit a log behavior even in the ZPG region, rather they show a power law. Furthermore, the maxima of the streamwise heat flux, $<$$u'\theta'$$>$, increases linearly in the FPG region but remains constant in the ZPG region. On the contrary, the wall-normal heat flux remains frozen over the ZPG and acceleration regions. Meanwhile, $v'$, $w'$, and $<$$u'v'$$>$ continue to decay along the flow direction. However, a surprising result is observed in the $\theta'$ and $<$$u'\theta'$$>$ components which change from constant in ZPG to linear rise as the FPG increases. This increase occurs in spite the fact that turbulence production is drastically reduced in the accelerated region. [Preview Abstract] |
Sunday, November 22, 2015 8:13AM - 8:26AM |
A20.00002: Connections between density, wall-normal velocity, and coherent structure in a heated turbulent boundary layer Theresa Saxton-Fox, Stanislav Gordeyev, Adam Smith, Beverley McKeon Strong density gradients associated with turbulent structure were measured in a mildly heated turbulent boundary layer using an optical sensor (Malley probe). The Malley probe measured index of refraction gradients integrated along the wall-normal direction, which, due to the proportionality of index of refraction and density in air, was equivalently an integral measure of density gradients. The integral output was observed to be dominated by strong, localized density gradients. Conditional averaging and Pearson correlations identified connections between the streamwise gradient of density and the streamwise gradient of wall-normal velocity. The trends were suggestive of a process of pick-up and transport of heat away from the wall. Additionally, by considering the density field as a passive marker of structure, the role of the wall-normal velocity in shaping turbulent structure in a sheared flow was examined. Connections were developed between sharp gradients in the density and flow fields and strong vertical velocity fluctuations. [Preview Abstract] |
Sunday, November 22, 2015 8:26AM - 8:39AM |
A20.00003: Influence of wall roughness and thermal coductivity on turbulent natural convection Paolo Orlandi, Sergio Pirozzoli, Matteo Bernardini We study turbulent natural convection in enclosures with conjugate heat transfer. The simplest way to increase the heat transfer in this flow is through rough surfaces. In numerical simulations often constant temperatures are assigned on the walls, but this is an unrealistic condition in laboratory experiments. Therefore, in the DNS, to be of help to experimentalists, it is necessary to solve the heat conduction in the solid walls together with the turbulent flow between the hot and the cold walls. Here the cold wall, $0.5h$ tick is smooth, and the hot wall has 2D and 3D rough elements of thickness $0.2h$ above a solid layer $0.3h$ tick. The simulation is performed in a bi-periodic domain $4h$ wide. The Rayleigh number varies from $10^6$ to $10^8$. Two values of the thermal conductivity, one corresponding to copper and the other ten times higher were assumed. It has been found that the Nusselt number behaves as $Nu=\alpha Ra^\gamma$, with $\alpha$ increasing with the solid conductivity and depending of the roughness shape. 3D elements produce a heat transfer greater than 2D elements. An imprinting of the flow structures on the thermal field inside the walls is observed. The one-dimensional spectra at the center, one decade wide, agree with those of forced isotropic turbulence. [Preview Abstract] |
Sunday, November 22, 2015 8:39AM - 8:52AM |
A20.00004: How different is Buoyant Turbulence from Isotropic Turbulence? Julien Claret, Guillaume Blanquart This work seeks to establish a new approach for simulating variable density turbulent buoyant flows by extraction of the anisotropic component inherent to buoyant flows. This anisotropy is known to be the main difficulty when simulating buoyant flows. We perform for this an \emph{a priori} analysis using the DNS data from [Caroll, Blanquart TCFD (2015)]. The anisotropy is quantified first through variance of the velocity field, two-point autocorrelation and energy spectra. The main observation is that for buoyant flows the velocity has a dependency on density shown by a non null conditional mean whereas this is not observed In isotropic flows. This allows to decompose Homogeneous Buoyant Turbulence (HBT) into three terms. The first corresponds to the conditional mean velocity on density that contains -but is not reduced to- the small scales anisotropy. The second term corresponds to the mean velocity averaged only in the direction of gravity which contains large scale anisotropy. The final term corresponds to a field of Homogeneous Isotropic Turbulence (HIT). This decomposition allows the reduction of the problem to the study of an HIT field which is well known. It also sheds light onto the development of a new Sub-Grid Scale (SGS) to simulate flows driven by buoyancy. [Preview Abstract] |
Sunday, November 22, 2015 8:52AM - 9:05AM |
A20.00005: Skin friction field and thermal plume formation in turbulent convection Joerg Schumacher, Vinodh Bandaru, Anastasiya Kolchinskaya, Janet Scheel, Kathrin Padberg-Gehle The dynamics in the thin boundary layers of temperature and velocity is the key to a deeper understanding of turbulent transport of heat and momentum in thermal convection. The velocity gradient right at the heated plate of a Rayleigh-B\'{e}nard convection cell forms the two-dimensional skin friction field and is related to the formation of thermal plumes in the boundary layer right above the plate. Our analysis is based on a direct numerical simulation of Rayleigh-B\'{e}nard convection in a closed cylindrical cell of aspect ratio $\Gamma=1$ and focused on the critical points of the skin friction field. We identify triplets of critical points, which are composed of two unstable nodes and a saddle between them, as the characteristic building block of the skin friction field. Isolated triplets as well as networks of triplets are detected. The majority of the ridges of line-like thermal plumes coincide with the unstable manifolds of the saddles. From a dynamical Lagrangian perspective, thermal plumes are formed together with an attractive hyperbolic Lagrangian Coherent Structure of the skin friction field. We discuss the differences from the skin friction field in turbulent channel flows from the perspective of the Poincar\'{e}-Hopf index theorem for two-dimensional vector field. [Preview Abstract] |
Sunday, November 22, 2015 9:05AM - 9:18AM |
A20.00006: The effect of turbulent fluctuations on the relaxation of thermal non-equilibrium Sualeh Khurshid, Diego Donzis In many engineering and natural systems, the microscopic behavior of constituent molecules can affect the macroscopic behavior of the flow. This interaction is significant when the two phenomena have commensurate time scales. We study the effect of turbulence on the relaxation of thermal non-equilibrium (TNE), in particular vibrational energy relaxation, using direct numerical simulation (DNS). First order effects are observed in the evolution of both vibrational energy and turbulence. For example, the rate of decay of kinetic energy is accelerated and temperature fluctuations are amplified. Analytic expressions for equilibrium vibrational energy, $E_v^*$, and characteristic relaxation time scale, $\tau_v$, are compared against DNS data and used to understand features of the decay. This decay can be divided into two regimes, one dominated by TNE exchanges in time scales of the order of $\tau_v$ followed by a turbulence decay. Between the two regimes, some vibrationally hot flows become cold before reaching equilibrium. This reflects an aspect of the strong coupling between turbulence and TNE in both regimes. Compressiblity effects, quantified by turbulent Mach number ($M_t$), are also discussed. [Preview Abstract] |
Sunday, November 22, 2015 9:18AM - 9:31AM |
A20.00007: Construction of a Non-Equilibrium Thermal Boundary Layer Facility Drummond Biles, Alireza Ebadi, Allen Ma, Christopher White A thermally conductive, electrically heated wall-plate forming the bottom wall of a wind tunnel has been constructed and validation tests have been performed. The wall-plate is a sectioned wall design, where each section is independently heated and controlled. Each section consists of an aluminum 6061 plate, an array of resistive heaters affixed to the bottom of the aluminum plate, and a calcium silicate holder used for thermal isolation. Embedded thermocouples in the aluminum plates are used to monitor the wall temperature and for feedback control of wall heating. The wall-plate is used to investigate thermal transport in both equilibrium and non-equilibrium boundary layers. The non-equilibrium boundary layer flow investigated is oscillatory flow produced by a rotor-stator mechanism placed downstream of the test section of the wind tunnel. [Preview Abstract] |
Sunday, November 22, 2015 9:31AM - 9:44AM |
A20.00008: Scaling of co-spectra in grid turbulence with a mean cross-stream temperature gradient Carla Bahri, Gilad Arwatz, Marcus Hultmark, Michael Mueller Scaling of grid turbulence with a constant mean cross-stream temperature gradient is investigated using a combination of theoretical predictions and DNS. Conditions for self-similarity of the governing equations and particularly the scalar co-spectrum are investigated, which reveals necessary conditions for self-similarity to exist. These conditions provide a theoretical framework for scaling of the temperature flux spectrum, which offers new insights into the interaction of the turbulent velocity field with the scalar field. One necessary condition, predicted by the theory, is that the co-spectrum must vary as $\propto ^2$ for a self-similar solution to exist. DNS results are used to validate the theoretical predictions and good collapse of the co-spectrum is observed, which validates the self-similarity theory. [Preview Abstract] |
Sunday, November 22, 2015 9:44AM - 9:57AM |
A20.00009: ABSTRACT WITHDRAWN |
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