3D DNS Research Articles

Stability of the non‐isothermal flow between co‐ and counter‐rotating disks

AbstractThe 3D DNS computations are performed to study the isothermal and not‐isothermal transitional flow between two co‐ and counter‐rotating disks enclosed by two rotating cylinders. Computations have been performed for the wide range of Reynolds numbers and different end‐wall conditions including throughflow. For chosen examples 3D LES computations are performed. The flow is described by 3D Navier‐Stokes, energy and continuity equations. Numerical computations are based on a pseudospectral Chebyshev‐Fourier method. The time scheme is semi‐implicit and second‐order accurate. For LES computations, a Lagrangian dynamic sub‐grid model is used. In this paper we studied the spatial structures, which appeared in different stages of the lam.‐turb. transition process. We considered cavities of the aspect ratio 9 and curvature parameters Rm=1.5–3.0. Influence of the thermal Rossby number on lam.‐turb. transition process was also investigated. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Experimental and numerical investigation of turbulent natural convection in a large air-filled cavity

We investigate experimentally and numerically the turbulent natural convection flow that develops in a differentially heated cavity of height H=1 m, width W= H and depth D=0.32 H, submitted to a temperature difference between the active vertical walls equal to 15 K resulting in a characteristic Rayleigh number equal to 1.5 × 10 9. In the experiment temperature is measured by 25 μm micro-thermocouples and velocity by a Laser Doppler Anemometer. Both 2D and 3D LES and 3D DNS are performed. DNS uses a Chebyshev spectral method and LES a finite volume method with a local subgrid diffusivity model. Numerical simulations are performed for both adiabatic conditions and experimentally measured temperature on the horizontal walls. Time-averaged quantities and turbulent statistics in the median vertical plane are presented and compared.

Transient heat transfer of deforming droplets at high Reynolds numbers

A numerical study of heat transfer enhancement due to the deformation of droplets at high Reynolds numbers is described. The two phase‐flow has been computed with a 3D DNS program using the volume‐of‐fluid method. The droplets are deformed because of the surrounding gas stream especially due to a sudden rise of flow velocity from zero to Ui. As the governing non‐dimensional parameter the Weber number of the droplets has been varied between 1.3 and 10.8 by assuming different surface tensions at Reynolds numbers between 360 and 853. The dynamical behavior of the droplets as a function of the Weber and the Ohnsorge number are in good agreement with experimental results from the literature. At the highest Reynolds number Re=853, a significant dependency of Nu on We has been found. The comparison of a Nusselt number computed with the real surface area with a Nusselt number computed with the spherical surface area shows that the heat transfer increases not only due to the droplet motion but also due to the larger surface area of the deformed droplet.

Numerical Evaluation Of The Influence Of Fuel Generation On The Geometry Of A Diffusion Flame: Implications To Micro-gravity Fire Safety

Improvement in the understanding of the structure of flames in micro-gravity is a critical issue for spacecraft fire safety. A numerical study of the influence of a gas diffusion flame on a flow of air over a flat plate in micro-gravity environment is presented in this paper. 3D DNS simulations with a mixture fraction model for combustion have been performed with an adaptation of the Fire Dynamics Simulator code (FDS) developed at NIST. A particular flow speed representative of ventilation in spacecrafts and a fuel injection speed comparable to those induced by pyrolysis is used to illustrate the effect of the sample geometry, the fuel injection and thermal expansion of the flow characteristics. This work is benchmarked with experiments conducted on board of the CNES Airbus-300 and with the numerous observations available in the literature. It was observed that a leading edge can cause velocity overshoots and that fuel injection can have a significant effect in propagating the pressure perturbations downstream. Nevertheless, the calculations show that thermal expansion affects the flow in a much more dramatic way, thus becomes the most important parameter to be considered when evaluating the precision of analytical models.

Studies on stability and dynamics of a swirling jet

The temporal instability and nonlinear evolution of the swirling jet near a nozzle exit are studied by both normal-mode method and three dimensional direct numerical simulation (3D DNS). It is found that the swirl enhances the maximum linear growth rates for negative helical modes, while decreases the growth rate for axisymmetric mode. Numerical simulations show that the evolution in early stage is compared well with the linear stability theory. In nonlinear stage, the swirl promotes the breakup of 3D large scale organized structures in the flow into small eddies.