Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session E14: Experiments: Turbulence |
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Chair: John Eaton, Stanford University Room: 27B |
Sunday, November 18, 2012 4:45PM - 4:58PM |
E14.00001: Transition to Turbulence in Oscillatory Flow for Pulse Tube Cryocoolers Meghan McNulty, Benjamin Jewell, Thomas Fraser Pure (zero-mean) oscillatory flows are less studied than the pulsatile flows found in biology, yet they are frequently used in low-temperature cooling systems, like the pulse tube cryocooler (PTC). PTCs have high potential for extended lifespans and reductions in size, weight, and power compared to other cryocoolers, but advancements of the technology have been hampered by the lack of knowledge of the working fluid's behavior. While design guides assume laminar flow in the pulse tube, an evaluation of PTCs in literature using the Womersley parameter $\alpha $\textit{=a($\omega $/$\nu )$}$^{1/2}$ and the oscillatory Reynolds number \textit{Re}$_{\delta }$\textit{=U(2/$\omega \nu )$}$^{1/2}$ indicates the probability of transitional or turbulent flow. Because PTC operation relies on thermal stratification of the oscillating internal gas, turbulent mixing will significantly reduce performance. We quantify the fluid flow within a PTC under near-operational conditions using planar particle image velocimetry (PIV) and calculate the first full-field velocity measurements that provide insight to the presence of transitional or turbulent flow and the physics that underlie experimentally observed ``streaming effects.'' [Preview Abstract] |
Sunday, November 18, 2012 4:58PM - 5:11PM |
E14.00002: Time-Resolved, Two-Dimensional Imaging of Scalar Mixing in Turbulent Gas-Phase Jets Michael Papageorge, Jeffrey Sutton The objective of this work is to examine the dynamics of scalar mixing in turbulent, gas-phase jets using kHz-rate laser diagnostics. The research is underpinned by a new High Energy Pulse Burst Laser System (HEPBLS), which is capable of delivering more than 150 high-energy ($>500mJ$) pulses with repetition rates exceeding 10 kHz. The unique system allows for the extension of traditionally low repetition-rate planar laser techniques such as Rayleigh scattering and Planar Laser-Induced Fluorescence (PLIF) to high-speed imaging applications. In this study, two turbulent jets with Reynolds number equal to $10,000$ and $15,000$ (based on jet diameter) are used to study time-dependent scalar mixing and dissipation processes. Temporally-resolved, two-dimensional images of the mixture fraction and scalar dissipation rate fields are obtained at axial positions of $\frac{x}{D} = 10$ to $\frac{x}{D} = 40$, revealing the highly transient mixing topology within turbulent jets. Averaged results are validated against similar imaging techniques at low repetition rates and known turbulent scaling laws. In addition to ``real time'' visualization, the scalar mixing dynamics are characterized with temporal and spatial statistics. [Preview Abstract] |
Sunday, November 18, 2012 5:11PM - 5:24PM |
E14.00003: On investigating wall shear stress in two-dimensional plane turbulent wall jets Faraz Mehdi, Gunnar Johansson, Christopher White, Jonathan Naughton Mehdi \& White [Exp Fluids 50:43--51(2011)] presented a full momentum integral based method for determining wall shear stress in zero pressure gradient turbulent boundary layers. They utilized the boundary conditions at the wall and at the outer edge of the boundary layer. A more generalized expression is presented here that uses just one boundary condition at the wall. The method is mathematically exact and has an advantage of having no explicit streamwise gradient terms. It is successfully applied to two different experimental plane turbulent wall jet datasets for which independent estimates of wall shear stress were known. Complications owing to experimental inaccuracies in determining wall shear stress from the proposed method are also discussed. [Preview Abstract] |
Sunday, November 18, 2012 5:24PM - 5:37PM |
E14.00004: Turbulent Dispersion of Film Coolant in a Turbine Vane Cascade Sayuri Yapa, Christopher Elkins, John Eaton Gas turbine engines operate at peak temperatures in excess of the material limits because the high pressure turbine nozzles and buckets are film cooled. The nozzle vanes of the first stage turbine use the most cooling air because they are exposed directly to the high temperature combustor exhaust. Existing turbine analysis assumes a uniform temperature at the rotor inlet. However, the coolant does not mix completely with the mainstream flow before impinging on the turbine rotor, and the coolant streaks create variations in temperature along the leading edge of the downstream turbine blades. 3D velocity and concentration measurements are made using magnetic resonance (MR) imaging techniques to study turbulent mixing in a realistic film-cooled nozzle vane cascade. A scalar mixing analogy for thermal diffusion is used in which a chemical contaminant plays the role of temperature. In a typical experiment, the mainstream flow is water and the film coolant is a copper sulfate solution. The concentration of copper sulfate measured anywhere in the flow is a surrogate for normalized temperature. The turbulent scalar diffusivity in the scalar transport equation can be estimated from the MR data and used to improve computational fluid dynamics models. [Preview Abstract] |
Sunday, November 18, 2012 5:37PM - 5:50PM |
E14.00005: Turbulent inflow and wake of a marine hydrokinetic turbine, including effects of wave motion Toby Dewhurst, Matthew Rowell, Judson DeCew, Ken Baldwin, Rob Swift, Martin Wosnik A research program to investigate the spatio-temporal structure of turbulent flows relevant to marine hydrokinetic (MHK) energy conversion, including turbulent inflow and turbine wakes, has been initiated at UNH. A scale model MHK turbine was deployed from a floating platform at two open water tidal energy test sites, one sheltered (Great Bay Estuary, NH) and one exposed (Muskeget Channel, MA). The inflow upstream of the turbine under test was characterized using an acoustic Doppler Velocimeter (ADV) and an acoustic Doppler current profiler (ADCP), which vary considerably in temporal and spatial resolution as well as practical applicability in this environment. The turbine was operated at previously determined peak efficiency for a given tidal current. The wake of the turbine was measured with a second, traversing ADV during ramp-up and at peak tidal current velocities, at two to six shroud diameters downstream. An inertial motion unit installed near the turbine hub is used to correct for platform motion. A platform-mounted wave-staff and an independently taut-moored pressure sensor were used to measure wave climate. Together, these data are used to validate theoretical and tank model results for utilizing surface-based platforms for MHK turbine deployments. [Preview Abstract] |
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