Coherent structures in wall turbulence transport momentum and provide a means of producing turbulent kinetic energy. Above the viscous wall layer, the hairpin vortex paradigm of Theodorsen coupled with the quasistreamwise vortex paradigm have gained considerable support from multidimensional visualization using particle image velocimetry and direct numerical simulation experiments. Hairpins can autogenerate to form packets that populate a significant fraction of the boundary layer, even at very high Reynolds numbers. The dynamics of packet formation and the ramifications of organization of coherent structures (hairpins or packets) into larger-scale structures are discussed. Evidence for a large-scale mechanism in the outer layer suggests that further organization of packets may occur on scales equal to and larger than the boundary layer thickness.

Hairpin vortex organization in wall turbulencea)

Heat transfer surface enhancement through the use of longitudinal vortices: a review of recent progress

In order to gain a better insight into flow, vortical and turbulence structure and their correlation with the local heat transfer in impinging flows, we performed large-eddy simulations (LES) of a round normally impinging jet issuing from a long pipe at Reynolds number Re = 20000 at the orifice-to-plate distance H = 2D, where D is the jet-nozzle diameter. This configuration was chosen to match previous experiments in which several phenomena have been detected, but the underlying physics remained obscure because of limitations in the measuring techniques applied. The instantaneous velocity and temperature fields, generated by the LES, revealed interesting time and spatial dynamics of the vorticity and eddy structures and their imprints on the target wall, characterized by tilting and breaking of the edge ring vortices before impingement, flapping, precessing, splitting and pairing of the stagnation point/line, local unsteady separation and flow reversal at the onset of radial jet spreading, streaks pairing and branching in the near-wall region of the radial jets, and others. The LES data provided also a basis for plausible explanations of some of the experimentally detected statistically-averaged flow features such as double peaks in the Nusselt number and the negative production of turbulence energy in the stagnation region. The simulations, performed with an in-house unstructured finite-volume code T-FlowS, using second-order-accuracy discretization schemes for space and time and the dynamic subgrid-scale stress/flux model for unresolved motion, showed large sensitivity of the results to the grid resolution especially in the wall vicinity, suggesting care must be taken in interpreting LES results in impinging flows.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/BEE751344193DF18D01C0ABC9E3FFB43/S002211200700955Xa.pdf/div-class-title-vortical-structures-and-heat-transfer-in-a-round-impinging-jet-div.pdf

Vortical structures and heat transfer in a round impinging jet

ExHFT for fourth generation heat transfer technology

Effect of nozzle geometry on local convective heat transfer to a confined impinging air jet

Experimental Heat Transfer, Fluid Mechanics, and Thermodynamics 1993.

Free Convection From a Vertical Plate With Discontinuous Wall Temperature

Publisher Summary This chapter focuses on the diffusion of a fully turbulent, circular buoyant jet discharged into a surrounding ambient of the same fluid. Two-dimensional trajectories are included, wherein any ambient flow is taken as parallel to the horizontal component of jet velocity. Some of the jet and/or ambient characteristics considered include buoyancy effects arising from density differences between the jet and the ambient; ambient density stratification, arising from vertical nonuniformity of temperature and/or concentration in the ambient; ambient flow conditions with respect to the jet, of differing magnitude and orientation relative to the jet; and initial jet discharge characteristics, including direction of momentum. The chapter compares the predictions of various entrainment models over a Froude number range F = 1–200, in terms of trajectories, growth, and decay; and compares flowing ambient models with each other and with some data. The chapter investigates the effects of the most simple and the more realistic conditions of stratified quiescent ambients. These comparisons are made to assess the models, relative to each other and to the very sparse data, and to show the effects of more realistic stratification modeling.

The Diffusion of Turbulent Buoyant Jets

An experimental investigation of natural convection liquid immersion cooling of a three by three array of rectangular protrusions in an enclosure is presented. The heated elements geometrically simulated 20 pin dual-inline electronic packages and were mounted on a plexiglass substrate, which formed one vertical wall of a dielectric liquid filled rectangular enclosure. The remaining vertical boundaries of the enclosure were insulated, while the top and bottom were maintained at prescribed temperatures using individual heat exchanger plates. Protrusion surface temperatures in steady state are reported for a range of power dissipation levels for three fluorinert liquids spanning a Prandtl number range from about 20 to 1400. The influence of enclosure top and bottom surface boundary conditions and its width on element temperatures is investigated. Non-dimensional heat transfer results are empirically correlated. Changes in component temperatures due to partial powering of the array are also measured.

M. D. Kelleher

Papers

Experimental Heat Transfer, Fluid Mechanics, and Thermodynamics 1993.

Free Convection From a Vertical Plate With Discontinuous Wall Temperature

The Diffusion of Turbulent Buoyant Jets

Natural Convection Heat Transfer From an Array of Rectangular Protrusions in an Enclosure Filled With Dielectric Liquid

Design and Optimization of Air-Cooled Heat Exchangers