Direct numerical simulation of heat transfer from a cylinder immersed in the production and decay regions of grid-element turbulence

2018 ◽  
Vol 847 ◽  
pp. 452-488 ◽  
Author(s):  
I. Paul ◽  
G. Papadakis ◽  
J. C. Vassilicos
Keyword(s):  
Heat Transfer ◽  
Numerical Simulation ◽  
Boundary Layer ◽  
Nusselt Number ◽  
Direct Numerical Simulation ◽  
Stagnation Point ◽  
Turbulent Wake ◽  
Production Region ◽  
Grid Element ◽  
Non Gaussian

The present direct numerical simulation (DNS) study, the first of its kind, explores the effect that the location of a cylinder, immersed in the turbulent wake of a grid-element, has on heat transfer. An insulated single square grid-element is used to generate the turbulent wake upstream of the heated circular cylinder. Due to fine-scale resolution requirements, the simulations are carried out for a low Reynolds number. Three locations downstream of the grid-element, inside the production, peak and decay regions, respectively, are considered. The turbulent flow in the production and peak regions is highly intermittent, non-Gaussian and inhomogeneous, while it is Gaussian, homogeneous and fully turbulent in the decay region. The turbulence intensities at the location of the cylinder in the production and decay regions are almost equal at 11 %, while the peak location has the highest turbulence intensity of 15 %. A baseline simulation of heat transfer from the cylinder without oncoming turbulence was also performed. Although the oncoming turbulent intensities are similar, the production region increases the stagnation point heat transfer by 63 %, while in the decay region it is enhanced by only 28 %. This difference cannot be explained only by the increased approaching velocity in the production region. The existing correlations for the stagnation point heat transfer coefficient are found invalid for the production and peak locations, while they are satisfied in the decay region. It is established that the flow in the production and peak regions is dominated by shedding events, in which the predominant vorticity component is in the azimuthal direction. This leads to increased heat transfer from the cylinder, even before vorticity is stretched by the accelerating boundary layer. The frequency of oncoming turbulence in production and peak cases also lies close to the range of frequencies that can penetrate the boundary layer developing on the cylinder, and therefore the latter is very responsive to the impinging disturbances. The highest Nusselt number along the circumference of the cylinder is shifted 45 degrees from the front stagnation point. This shift is due to the turbulence-generating grid-element bars that result in the prevalence of intense events at the point of maximum Nusselt number compared to the stagnation point.

The Effect of a Turbulent Wake on the Stagnation Point: Part II — Heat Transfer Results

1992 ◽  
Author(s):  
Anthony J. Hanford ◽  
Dennis E. Wilson
Keyword(s):  
Heat Transfer ◽  
Experimental Data ◽  
Boundary Layer ◽  
Nusselt Number ◽  
Stagnation Point ◽  
Turbulent Wake ◽  
Energy Spectra ◽  
Incoming Flow ◽  
Two Dimensional ◽  
Energy Equations

A phenomenological model is proposed which relates the effects of freestream turbulence to the increase in stagnation point heat transfer. The model requires both turbulence intensity and energy spectra as inputs to the unsteady velocity at the edge of the boundary layer. The form of the edge velocity contains both a pulsation of the incoming flow and an oscillation of the streamlines. The incompressible unsteady and time-averaged boundary layer response is determined by solving the momentum and energy equations. The model allows for arbitrary two-dimensional geometry, however, results are given only for a circular cylinder. The time-averaged Nusselt number is determined theoretically and compared to existing experimental data.

Direct numerical simulation of a turbulent jet impinging on a heated wall

2015 ◽  
Vol 764 ◽  
pp. 362-394 ◽  
Author(s):  
T. Dairay ◽  
V. Fortuné ◽  
E. Lamballais ◽  
L.-E. Brizzi
Keyword(s):  
Heat Transfer ◽  
Numerical Simulation ◽  
Nusselt Number ◽  
Direct Numerical Simulation ◽  
Temperature Fields ◽  
Heated Wall ◽  
Small Scale ◽  
Turbulent Regime ◽  
High Heat Transfer ◽  
Radial Evolution

AbstractDirect numerical simulation (DNS) of an impinging jet flow with a nozzle-to-plate distance of two jet diameters and a Reynolds number of 10 000 is carried out at high spatial resolution using high-order numerical methods. The flow configuration is designed to enable the development of a fully turbulent regime with the appearance of a well-marked secondary maximum in the radial distribution of the mean heat transfer. The velocity and temperature statistics are validated with documented experiments. The DNS database is then analysed focusing on the role of unsteady processes to explain the spatial distribution of the heat transfer coefficient at the wall. A phenomenological scenario is proposed on the basis of instantaneous flow visualisations in order to explain the non-monotonic radial evolution of the Nusselt number in the stagnation region. This scenario is then assessed by analysing the wall temperature and the wall shear stress distributions and also through the use of conditional averaging of velocity and temperature fields. On one hand, the heat transfer is primarily driven by the large-scale toroidal primary and secondary vortices emitted periodically. On the other hand, these vortices are subjected to azimuthal distortions associated with the production of radially elongated structures at small scale. These distortions are responsible for the appearance of very high heat transfer zones organised as cold fluid spots on the heated wall. These cold spots are shaped by the radial structures through a filament propagation of the heat transfer. The analysis of probability density functions shows that these strong events are highly intermittent in time and space while contributing essentially to the secondary peak observed in the radial evolution of the Nusselt number.

Direct Numerical Simulation of Radial Convection in a Cylindrical Annulus With and Without Rotation

2019 ◽  
Author(s):  
Diogo B. Pitz ◽  
William R. Wolf
Keyword(s):  
Heat Transfer ◽  
Numerical Simulation ◽  
Nusselt Number ◽  
Kinetic Energy ◽  
Direct Numerical Simulation ◽  
Centrifugal Acceleration ◽  
Radial Acceleration ◽  
Turbulent Fluctuations ◽  
Parallel Disks ◽  
Cylindrical Annulus

Abstract In rotating systems with temperature gradients, convection may occur due to gravitational or centrifugal effects. In cases where rotation is strong enough so that the centrifugal acceleration is higher than gravity, the flow is induced by centrifugal buoyancy and gravitational effects can be neglected. The problem of flow induced by centrifugal buoyancy in a cylindrical annulus has been used as a canonical setup to investigate industrial configurations, such as buoyancy-driven flows occurring in gas turbine secondary air systems, as well as geophysical flows, such as convection in the core of planets and the global circulation of the atmosphere. Due to the constraints imposed by the Taylor-Proudman theorem, such flows are quasi-homogeneous along the axial direction, and heat transfer as well as turbulent fluctuations tend to be suppressed by the action of the Coriolis force. Previous work has demonstrated that when the annulus is bounded by parallel disks, boundary layers scaling consistently with laminar Ekman layers are formed near each of the disks, even though the flow is purely buoyancy-induced. Also, the Nusselt number measured on the outer cylindrical surface has been shown to scale with the Rayleigh number as in natural convection between horizontal plates. In the present work we use direct numerical simulation (DNS) to investigate buoyancy-induced flow in an air-filled cylindrical annulus bounded by two adiabatic parallel disks, with and without rotation around the axis. In both cases the outer cylindrical surface is at a higher temperature than the inner one, so that a radial acceleration directed outwards induces an unstable stratification. In the case with rotation, the flow is induced by the centrifugal acceleration in the radial direction, and Coriolis forces are considered. For the case without rotation, the Coriolis terms are suppressed in the calculations, whereas the radial acceleration is the same as in the rotating case. Statistics are obtained and compared in the two cases, including the time-averaged Nusselt number, mean temperature profiles, velocity and temperature fluctuations, as well as terms of the turbulent kinetic energy equation. By analysing such statistics, the extent to which rotation suppresses heat transfer and turbulent fluctuations, as well as the contribution of each term to the turbulent kinetic energy budget, can be assessed.

Direct Numerical Simulation and Large Eddy Simulation of Laminar Separation Bubbles at Moderate Reynolds Numbers

2014 ◽  
Vol 136 (6) ◽  
Author(s):  
Francois Cadieux ◽  
Julian A. Domaradzki ◽  
Taraneh Sayadi ◽  
Sanjeeb Bose
Keyword(s):  
Heat Transfer ◽  
Numerical Simulation ◽  
Boundary Layer ◽  
Direct Numerical Simulation ◽  
Separation Bubble ◽  
Leading Edge ◽  
Transition To Turbulence ◽  
Laminar Separation Bubble ◽  
Laminar Separation ◽  
Large Eddy

Flows over airfoils and blades in rotating machinery for unmanned and microaerial vehicles, wind turbines, and propellers consist of different flow regimes. A laminar boundary layer near the leading edge is often followed by a laminar separation bubble with a shear layer on top of it that experiences transition to turbulence. The separated turbulent flow then reattaches and evolves downstream from a nonequilibrium turbulent boundary layer to an equilibrium one. Typical Reynolds-averaged Navier–Stokes (RANS) turbulence modeling methods were shown to be inadequate for such laminar separation bubble flows (Spalart and Strelets, 2000, “Mechanisms of Transition and Heat Transfer in a Separation Bubble,” J. Fluid Mech., 403, pp. 329–349). Direct numerical simulation (DNS) is the most reliable but is also the most computationally expensive alternative. This work assesses the capability of large eddy simulations (LES) to reduce the resolution requirements for such flows. Flow over a flat plate with suitable velocity boundary conditions away from the plate to produce a separation bubble is considered. Benchmark DNS data for this configuration are generated with the resolution of 59 × 106 mesh points; also used is a different DNS database with 15 × 106 points (Spalart and Strelets, 2000, “Mechanisms of Transition and Heat Transfer in a Separation Bubble,” J. Fluid Mech., 403, pp. 329–349). Results confirm that accurate LES are possible using O(1%) of the DNS resolution.

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