Direct Numerical Simulation Code Research Articles

DNS for flow separation control around an airfoil by pulsed jets

Direct numerical simulation (DNS) for flow separation and transition around a NACA-0012 airfoil with an attack angle of 4° and Reynolds number of 100,000 has been reported in our previous paper. The details of flow separation, formation of the detached shear layer, Kelvin–Helmholtz instability (inviscid shear layer instability) and vortex shedding, interaction of nonlinear waves, breakdown, and re-attachment are obtained and analyzed. The power spectral density of pressure shows the low frequency of vortex shedding caused by the Kelvin–Helmholtz instability still dominates from the leading edge to trailing edge. Based on our understanding on the flow separation mechanism, we try to reveal the mechanism of the flow separation control using blowing jets and then optimize the jets. DNS simulations for flow separation control by blowing jets (pulsed and pitched and skewed jets) are reported and analyzed. The effects of different unsteady blowing jets on the surface at the location just before the separation points are studied. The length of separation bubble is significantly reduced (almost removed) after unsteady blowing technology is applied. The mechanism of early transition caused by the blowing jets was found. A blowing jet with K–H frequency, sharp shape function (very small mass blowing), pitching and skewing obtained the best efficiency based on the increase of the ratio of lift over drag and decrease of blowing mass flow. In this work, a DNS code with high-order accuracy and high-resolution developed by the computational fluid dynamics group at University of Texas at Arlington is applied.

A Coupling Scheme for Direct Numerical Simulations with an Acoustic Solver

A code for direct numerical simulations (DNS) has been coupled with an acoustic solver for aeroacoustic applications. The DNS code is intended to compute the noise generation while the acoustic far-field is evaluated using the linearized Euler equations. Both codes run on different computers on which they show their best computational performance. As a test case, we show a pressure pulse emitting from the DNS domain and being propagated by the acoustic solver.

Numerical simulation of flow behind active vortex generators with direct forcing immersed boundary method

The immersed boundary method with discrete direct forcing approach was combined with a direct numerical simulation (DNS) code to study the time-dependent response of boundary-layer flow over a flat plate to the active vortex generators (AVG) that consist of a pair of deployable circular wing lip type blades driven by a pre-defined duty cycle. The DNS code solves the three-dimensional Navier–Stokes equations for compressible flow in general curvilinear coordinates using a fully implicit LU-SGS method. A fourth-order finite difference scheme is used to compute the spatial derivatives. The underlying curvilinear mesh near the vortex generator is designed properly with a large portion of the immersed boundary intersecting with the grid nodes to reduce the number of near-boundary nodes that require interpolation. The current immersed boundary technique was tested and verified on backward-facing step flow and flow past a circular cylinder. Numerical experiments have shown that the discrete direct forcing approach is very effective. In numerical simulation of flow behind the AVG, the computational results are comparable to the experimental data.

Electrohydrodynamics of yeast cells in microchannels subjected to travelling electric fields

While travelling wave dielectrophoresis (twDEP) offers a promising method for the control of micro-sized particles suspended in liquids, particularly when the motion of particles along the length of the channel is sought without having to pump the liquid itself, it leads to a variety of complex dynamical regimes which need to be clearly understood for the design of efficient microfluidic devices targeting particular functions. In this paper, we describe the various dynamical regimes in terms of the forces acting on the particles, i.e. the conventional dielectrophoretic and twDEP force and torque, the viscous drag exerted by the fluid on the particle and the electrostatic and hydrodynamic particle–particle interactions. We also explore the variation of the dynamical regimes in three different configurations typical of microfluidic channels whose electrodes are embedded in the bottom wall. The first two configurations have different, i.e. aligned and staggered, electrode geometries, and the third configuration consists of aligned electrodes but energized at different potentials. For these purposes, we use our direct numerical simulation code based on the distributed Lagrange multiplier method for solving the equations of motion for both the fluid and the individual particles, and the point dipole model to compute the electrostatic forces. The model particles are chosen so as to have mechanical and electrical properties of yeast cells suspended in an aqueous solution. It is found that the motion of the particles not only depends significantly on the Clausius–Mossotti factor, which is a function of the electric properties of the fluid and the particles, but also on the specific configuration considered. Particularly, the spinning of particles plays a crucial role in the particle translations and interactions, but the direction of such spinning motion depends on the particular device configuration used.

Direct numerical simulation of jets in cross-flow

Direct numerical simulation (DNS) of jets in cross-flow (JICF) has been carried out in this study, aiming for the investigation of vortex structure formation and evolution process associated with JICF. A recently developed DNS code is used, which solves three-dimensional (3D) compressible unsteady Navier–Stokes (NS) equations using high-order finite differences and multi-block structure grid treatment for complex geometry. Jet flow from a square duct, perpendicular to the mainstream flow, is introduced and the flow Reynolds number is 100 based on the jet duct diameter (D) and free-stream quantities. Two-dimensional (2D) calculations using various jet to free-stream velocity ratio (R = V jet/V free) reveals different vortex patterns and a further 3D study continues focusing on a velocity ratio of R = 2, for which complex vortex structure is produced. It is found from the 3D simulation that a counter-rotating vortex pair (CRVP) forms immediately after the jet exits, as observed from the experimental test and reproduced by other numerical simulations. The CRVP originates from the near wall viscous layer and its core position moves away from the wall as it evolves downstream. For the condition simulated, the CRVP is finally weakened (due to viscous diffusion) at about 1.6D downstream from the centre of the jet exit. No asymmetric CRVP has been observed, which was reported by other researchers for high-Reynolds number simulations.

Transient simulation of reacting radiating flows

Laminar methane–air diffusion flame was simulated by coupling a method of lines based parallel direct numerical simulation code with a radiation code based on method of lines solution of discrete ordinates method. The predictions of the code are validated against experimental data as well as numerical results of the same code without radiation model. Comparisons show that incorporation of radiation code to the computational fluid dynamics code results in a significant improvement in the predicted temperatures. Transient results exhibit the physically expected trends. The coupled code is a promising tool for the simulation of transient reacting radiating flows.

Performance Analysis of a Linux Pc Cluster Using a Direct Numerical Simulation of Fluid Turbulence Code

In this paper, we have tested the parallel performance of an Intel Xeon-based Linux PC cluster using a finite element code for direct numerical simulation (DNS) of incompressible fluid turbulence. The parallel performance of the PC cluster, which used up to 64 2.8 GHz processors, was evaluated by comparing three scales of DNS trial runs consisting of 3.3, 5.8, and 10.1 million elements. Subroutines of different natures were contrasted to investigate the scalability of the DNS code. For DNS calculation of sufficiently large scale, the subroutines showed reasonable parallel efficiency. Doubling the number of processors reduced the CPU time by about 40%. Of particular interest was the CPU time required by the two subroutines handling interprocessor communication that was fairly constant within the range of processors tested. PC Linux clusters are thus affordable platforms, compared with more expensive supercomputers, to conduct large-scale scientific computing for fluid turbulence research.

Performance analysis of a parallel finite element solution to the direct numerical simulation of fluid turbulence on Linux PC clusters

The parallel performance of a direct numerical simulation (DNS) computer code was tested on three different sets of Linux PC clusters including Intel^(R) Pentium^(R) (P) III 800MHz, P III 1GHz, and Xeon(TM) 2.8GHz. The DNS code is used to simulate incompressible fluid turbulence and is developed by the Galerkin finite element method (FEM) with equal-order interpolating polynomials. It employs domain decomposition for data structure together with data parallelism for matrix assemblies and operations. This paper discusses the scalability of the code and contrasts the code performance on the Linux PC clusters.

Direct numerical simulation of turbulence/radiation interaction in premixed combustion systems

An important fundamental issue in chemically reacting turbulent flows is turbulence/radiation interaction (TRI); TRI arises from highly nonlinear coupling between temperature and composition fluctuations. Here, a photon Monte Carlo method for the solution of the radiative transfer equation has been integrated into a turbulent combustion direct numerical simulation (DNS) code. DNS has been used to investigate TRI in a canonical configuration with systematic variations in optical thickness. The formulation allows for nongray gas properties, scattering, and general boundary treatments, although in this study, attention has been limited to gray radiation properties, no scattering, and black boundaries. Individual contributions to emission and absorption TRI have been isolated and quantified. Of particular interest are intermediate values of optical thickness where, for example, the smallest hydrodynamic and chemical scales are optically thin while the largest turbulence scales approach an optically thick behavior. In the configuration investigated, the temperature self-correlation contribution (emission) is primarily a function of the ratio of burned-gas temperature to unburned-gas temperature, and is the dominant contribution to TRI only in the optically thin limit. Even in the most optically thin case considered, the absorption coefficient–Planck function correlation and absorption coefficient–intensity correlation are not negligible. At intermediate values of optical thickness, contributions from all three correlations are significant.

Three-dimensional direct simulations and structure of expanding turbulent methane flames

Direct numerical simulations (DNS) are ideally suited to investigate in detail turbulent reacting flows in simple geometries. For an increasing number of applications, detailed models must be employed to describe the chemical processes with sufficient accuracy. Despite the huge cost of such simulations, recent progress has allowed the direct numerical simulation of turbulent premixed flames while employing complete reaction schemes. We briefly describe our own developments in this field and use the resulting DNS code to investigate more extensively the structure of premixed methane flames expanding in a three-dimensional turbulent velocity field, initially homogeneous and isotropic. This situation typifies, for example, the initial flame development after spark ignition in a gas turbine or an internal combustion engine. First investigation steps have been carried out at low turbulence levels on this same configuration in the past Symposium, and we build on top of these former results. Here, a considerably higher Reynolds number is considered, the simulation has been repeated twice in to limit the possibility of spurious, very specific results, and several complementary post-processing steps are carried out. Characteristic features concerning the observed combustion regime are presented. We then investigate in a quantitative manner the evolution of flame surface area, global stretch-rate, flame front curvature, flame thickness, and correlation between thickness and curvature. The possibility of obtaining reliable information on flame front curvature from two-dimensional slices is checked by comparison with the exact procedure.

Scalability of hybrid programming for a CFD code on the Earth Simulator

The Earth Simulator (ES) is an SMP cluster system. There are two types of parallel programming models available on the ES. One is a flat programming model, in which a parallel program is implemented by MPI interfaces only, both within an SMP node and among nodes. The other is a hybrid programming model, in which a parallel program is written by using thread programming within an SMP node and MPI programming among nodes simultaneously. It is generally known that it is difficult to obtain the same high level of performance using the hybrid programming model as can be achieved with the flat programming model. In this paper, we have evaluated scalability of the code for direct numerical simulation of the Navier–Stokes equations on the ES. The hybrid programming model achieves the sustained performance of 346.9 Gflop/s, while the flat programming model achieves 296.4 Gflop/s with 16 PNs of the ES for a DNS problem size of 256 3. For small scale problems, however, the hybrid programming model is not as efficient because of microtasking overhead. It is shown that there is an advantage for the hybrid programming model on the ES for the larger size problems.

Numerical simulation of high Schmidt number flow over a droplet by using moving unstructured mesh

A direct numerical simulation code for flow over a droplet, which rises freely, deforms in shape, and dissolves into a continuous phase, was developed. A two-phase flow field was discretised by a cell-centred arbitrary Lagrangian–Eulerian finite volume formulation by using three-dimensional hybrid unstructured meshes, which consist of triangular prisms near interface for the resolution of viscous boundary layer and tetrahedrons in the other part. Since mass boundary layer is much thinner than that of momentum for high Schmidt number problems, very-thin-layer cells are generated within one layer of the prisms attached to the interface only for solving mass transfer. This can be claimed as a novelty of the present simulation method. From the simulation results, the mechanisms of shape deformation and/or oscillation, trajectory morphology, and vortex shedding are elucidated and correlated to mass transfer at moderately high Reynolds number.

Two- versus three-dimensional direct simulations of turbulent methane flame kernels using realistic chemistry

Direct numerical simulations (DNS) are ideally suited to investigated in detail turbulent reacting flows in simple geometries. When considering such problems as pollutant emission or stability limits, detailed models must generally be employed to describe the chemical processes with sufficient accuracy. Due to the huge cost of such simulations, they have been mostly restricted to two-dimensional configurations up to now, leading to unsolved questions concerning the generality of the obtained results. We have recently developed a three-dimensional DNS code leading to reasonable computing times, thanks to the low-Mach-number approximation and to a new chemistry reduction technique. This code is used here to investigate the evolution of premixed methane/air flame kernels placed in a homogeneous isotropic turbulence field. This situation typifies the initial flame development after spark ignition in a gas turbine or an internal combustion engine. Laminar reference computations are carried out in one and two dimensions and are compared with turbulent results obtained in two and three dimensions. Evolution of flame surface area, stretch rate, and flame front curvature are in particular presented and show considerable differences between two-dimensional and three-dimensional computations. The interest of repeating the computations to increase the statistical validity of the results is demonstrated in two dimensions, but is not sufficient to explain the discrepancy obtained with the three-dimensional computation. Further three-dimensional simulations are nevertheless needed to quantify more precisely the observed changes (slower increase of the equivalent radius, higher stretch rate, and curvature shifted toward positive values).

Curvature effects on flame kernels in a turbulent environment

This paper presents results from the direct numerical simulation (DNS) of flame kernels in various turbulent environments. The flames are fully premixed and propagate through a field of decaying isotropic turbulence. The DNS code solves the fully compressible reacting flow equations using high-order explicit finite differences in space and a third-order explicit Runge-Kutta method in time. Results have been obtained for three-dimensional time evolution of initially laminar flame kernels. The normalized turbulence intensity u ′/S L was varied from moderately low values that produce only minor flame-wrinkling effects to larger values that produce severe wrinkling. When the turbulence intensity is large in comparison with the laminar flame speed, holes begin to appear in the flame surface and evidence of local flame breakaway is observed. Flame kernels have significant mean curvature, and it is the curvature and other geometrical properties of the flame surface that form the main focus of the present work. Simulation results are presented for a number of different turbulence Reynolds numbers corresponding to computational grid sizes with up to 384 3 points, and a detailed analysis provides statistical data on flame curvature and local shape effects. Turbulence intensity is found to have a major influence on the extent and character of the observed flame wrinkling. In particular, effects due to the mean curvature of the kernel are shown to diminish strongly with increasing turbulence intensity.

Comparison of active control techniques over a dihedral plane

This work is devoted to the numerical comparison of four active control techniques in order to increase the pressure recovery generated by the deceleration of a slightly compressible viscous flow over a dihedral plane. It is performed by the use of vortex generator jets and intrusive sensors. The governing equations, the two-dimensional direct numerical simulation code and the flow configuration are first briefly recalled. Then, the objective of the control is carefully displayed, and the uncontrolled flow described. The main part of this work deals with the explanation, the implementation and the comparison of four active control strategies: closed loop control, adaptative control, physical ramp control and sub-optimal control. Each of these techniques is of different nature, and results are very formative to understand what is important – or less – to make the control efficient.

K-1340 T字形合流管内の流れに関する研究 : 数値解析によるアーチ渦の検討(S15-12 管内のはく離)(S15 噴流・はく離流れの計測と制御)

Numerical analyses for a turbulence mixing water experiment, were carried out using a direct numerical simulation code DINUS-3. From the calculated results, it was confirmed that the numerical method is applicable to the evaluation of three flow patterns, i.e., an adhesion flow, a deflection flow and an impinging flow, which were measured in the experiment. Also, arched vortex characteristics agreed well with the experimental results in the adhesion flow condition.

Direct numerical simulation of turbulent premixed flames using intrinsic low-dimensional manifolds

In order to carry out numerical simulations taking into account detailed chemistry effects with a realistic computing cost, we describe in this work a new method to generate intrinsic low-dimensional manifold (ILDM) databases. This method is based on a continuation procedure to determine the manifold. We use in practice manifolds with two chemical dimensions in this work. Validations are performed for a homogeneous reaction system and one-dimensional laminar premixed CO/H2/air flames. Computing times are reduced by a factor of 20 compared to the detailed chemistry case. We also check two different projection methods used to project the diffusion term on the manifold. We observe that a simple perpendicular projection is appropriate for our configuration. The final part of this paper describes the implementation of the ILDM method in the direct numerical simulation code parcomb. A comparison with corresponding detailed chemistry results is shown, proving that this reduction technique is appropriate for the investigation of turbulent combustion problems. The CPU time is reduced by a factor of 2.6 when using the ILDM method.

Direct numerical simulation of an impinging jet into parallel disks

A direct numerical simulation code with cylindrical geometry has been developed. A direct numerical simulation (DNS) of an impinging round jet into parallel disks is performed for a Reynolds number of 10,000 based on the nozzle exit velocity and the nozzle diameter (D). Mean flow variables, turbulent intensity, pressure distribution and turbulent kinetic energy budgets are obtained at various radial locations. The present DNS results are in fairly good agreement with the two‐dimensional PTV measurements by Nishino and co‐workers in 1996. Some flow features of this impinging round jet regarding a turbulent transition process are discussed.

Perturbation dynamics in viscous channel flows

Plane viscous channel flows are perturbed and the ensuing initial-value problems are investigated in detail. Unlike traditional methods where travelling wave normal modes are assumed as solutions, this work offers a means whereby arbitrary initial input can be specified without having to resort to eigenfunction expansions. The full temporal behaviour, including both early-time transients and the long-time asymptotics, can be determined for any initial small-amplitude three-dimensional disturbance. The bases for the theoretical analysis are: (a) linearization of the governing equations; (b) Fourier decomposition in the spanwise and streamwise directions of the flow; and (c) direct numerical integration of the resulting partial differential equations. All of the stability criteria that are known for such flows can be reproduced. Also, optimal initial conditions measured in terms of the normalized energy growth can be determined in a straightforward manner and such optimal conditions clearly reflect transient growth data that are easily determined by a rational choice of a basis for the initial conditions. Although there can be significant transient growth for subcritical values of the Reynolds number, it does not appear possible that arbitrary initial conditions will lead to the exceptionally large transient amplitudes that have been determined by optimization of normal modes when used without regard to a particular initial-value problem. The approach is general and can be applied to other classes of problems where only a finite discrete spectrum exists (e.g. the Blasius boundary layer). Finally, results from the temporal theory are compared with the equivalent transient test case in the spatially evolving problem with the spatial results having been obtained using both a temporally and spatially accurate direct numerical simulation code.

Estimation numérique du bruit rayonné par une turbulence homogène et isotrope, à l'aide d'un code de simulation directe et d'un code de simulation des grandes échelles

Lighthill's acoustic analogy is combined with a numerical simulation of homogeneous and isotropic turbulence in order to evaluate the farfield noise. Two different codes are used: a Large Eddy Simulation code and a Direct Numerical Simulation code. First, the form of the acoustic analogy to be used is analyzed. Then, the acoustic pressure is computed for several realizations of the turbulent field. Finally, nondimensional acoustic power is obtained from an ensemble average of acoustic pressure. The results computed with the two simulations are shown to be in good agreement