J. Chiva

UNSTEADY FORCES ON A CIRCULAR CYLINDER AT CRITICAL REYNOLDS NUMBERS

It is well known that the flow past a circular cylinder at critical Reynolds number combines flow separation, turbulence transition, reattachment of the flow, and further turbulent separation of the boundary layer. The transition to turbulence in the boundary layer causes the delaying of the separation point and an important reduction of the drag force on the cylinder surface known as the drag crisis. In the present work, large-eddy simulations of the flow past a cylinder at Reynolds numbers in the range 2.5 × 105-6.5 × 105 are performed. It is shown how the pressure distribution changes as the Reynolds number increases in an asymmetric manner, occurring first on one side of the cylinder and then on the other side to complete the drop in the drag up to 0.23 at Re = 6.5 × 105. These variations in the pressure profile are accompanied by the presence of a small recirculation bubble, observed as a small plateau in the pressure, and located around ϕ = 105∘ (measured from the stagnation point). This small recir...

Optimising the Termofluids CFD code for petascale simulations

ABSTRACT This paper presents some recent efforts carried out on the expansion of the scalability of TermoFluids multi-physics Computational Fluid Dynamics (CFD) code, aiming to achieve petascale capacity for a single simulation. We describe different aspects that we have improved in our code in order to efficiently run it on 131,072 CPU-cores. This work has been developed using the BlueGene/Q Mira supercomputer of the Argonne Leadership Computing Facility, where we have obtained feedback at the targeted scale. In summary, this is a practical paper showing our experience at reaching the petascale paradigm for a single simulation with TermoFluids.

Coherent Structures in a Flow Past a Circular Cylinder at Critical and Super-Critical Reynolds Numbers

It is well known that the wake topology in the flow past a circular cylinder remains almost unchanged up to Reynolds number \(\sim 10^5\) Williamson (Annu Rev Fluid Mech 28(1), 477–539 (1996)) [1]. Then, at \(Re\sim 2\times 10^5\) major changes take place entailing flow separation, turbulence transition in the detached shear layers, reattachment of the flow and further separation of the boundary layer. In the present work, large-eddy simulations of the flow past a cylinder at Reynolds numbers in the range \(2.5\times 10^5{-}10^6\) are performed. This range includes both critical and super-critical Reynolds numbers (J Fluid Mech 10(3), 345–356 (1961)) [2]. Contradicting results about the wake configuration and structures are found in the literature.

DNS OF FALLING DROPLETS IN A VERTICAL CHANNEL

This paper presents Direct Numerical Simulation (DNS) of the falling motion of single and multiple deformable drops in a vertical channel. A systematic study of the wall effect on the motion of single drop is performed for Eötvös number (0.5≤Eo≤5), Morton number (10−3≤M≤10-8), and confinement ratio CR = 2. Second, the gravity-driven motion of multiple drops and their interactions are studied in a periodic vertical channel for CR = 4. These simulations are performed using a multiple marker level-set methodology, integrated in a finite-volume framework on a collocated unstructured grid. Each droplet is described by a level-set function, which allows capturing multiple interfaces in the same control volume, avoiding the numerical merging of the droplets. Numerical algorithms for fluid motion and interface capturing have been developed in the context of the finite-volume and level-set methodology, surface tension is modeled by means of the continuous surface force approach, and the pressure-velocity coupling is solved using a fractional-step projection method. DNS of single drop shows that they migrate to the symmetry axis of the channel when the Reynolds number is low, following a monotonic approach or damped oscillations according to the dimensionless parameters. If Eötvös number increases, stronger oscillations around the symmetry axis are observed. Simulations of multiple drops show that the collision of two drops follows the drafting-kissing tumbling (DKT) phenomenon. Deformable drops do not collide with the wall, whereas DKT phenomenon in the droplet swarm leads to the formation of groups which move through the center of the channel.

Numerical simulation of roughness effects on the flow past a circular cylinder

In the present work large eddy simulations of the flow past a rough cylinder are performed at a Reynolds number of Re = 4.2 × 105 and an equivalent sand-grain surface roughness height ks = 0.02D. In order to determine the effects of the surface roughness on the boundary layer transition and as a consequence on the wake topology, results are compared to those of the smooth cylinder. It is shown that surface roughness triggers the transition to turbulence in the boundary layer, thus leading to an early separation caused by the increased drag and momentum deficit. Thus, the drag coefficient increases up to CD ≈ 1.122 (if compared to the smooth cylinder it should be about CD ≈ 0.3 — 0.5). The wake topology also changes and resembles more the subcritical wake observed for the smooth cylinder at lower Reynolds numbers than the expected critical wake at this Reynolds number.

On the Large-Eddy Simulations of the Flow Past a Cylinder at Critical Reynolds Numbers

The flow past a circular cylinder is associated with different types of instabilities which involve the wake, the separated shear layers and the boundary layer.

Direct Numerical Simulation of Low-Mach Turbulent Natural Convection Flow in an Open Cavity of Aspect Ratio 4

Natural convection heat transfer in cavities has been studied extensively in the literature due to its relevance to many engineering areas such as low temperature solar collectors, design of buildings, cooling of nuclear reactors, etc.