Takahiro Tsukahara

DNS of turbulent Couette flow with emphasis on the large-scale structure in the core region

A series of direct numerical simulation (DNSs) of the turbulent plane Couette flow were performed with various box lengths, L x = 24h, 32h, 45h and 64h, where h is the channel height. The Reynolds numbers Re w (= U w h/ν) of 3000 and 8600 were chosen, where U w is the relative wall speed between top and bottom walls and ν is the kinematic viscosity. In the core region, the meandering of the large-scale structure (LSS) has been captured with a long box size (L x ≥ 32h). Correspondingly, the streamwise two-point correlation decreases and becomes negative at half the box length. The effect of the computational box size on the statistics, e.g. turbulence intensities, is also examined. Significant Reynolds-number dependence is observed in the streamwise LSS. For the higher Reynolds number, both the visualized instantaneous flow field and the pre-multiplied energy spectra show that the LSS can be present, with a finite wavelength of 21–32h in the streamwise direction and a spanwise spacing of 2.1–2.5h. The structure in the wall-normal direction is also discussed using the two-dimensional two-point correlations.

Flow regimes in a plane Couette flow with system rotation

Flow states in plane Couette flow in a spanwise rotating frame of reference have been mapped experimentally in the parameter space spanned by the Reynolds number and rotation rate. Depending on the ...

DNS study on viscoelastic effect in drag-reduced turbulent channel flow

The effect of viscoelastic contribution on wall-bounded turbulent flow of drag-reducing surfactant solution is investigated through direct numerical simulations (DNS). A series of DNS on turbulent channel flow is performed for different rheological properties at two different Reynolds numbers. It is found that high drag reduction is achieved by suppressing the turbulent contribution for the high Weissenberg number, and/or by decreasing the viscosity ratio and the effective viscosity. A highly drag-reduced turbulent flow at a high Reynolds number is mainly caused by the viscoelastic effect in the elastic layer, whereas the outer layer flow hardly affects the drag reduction. Moreover, we focus on the viscoelastic contribution term in the budget of Reynolds stress and its relation to the local flow pattern. It is shown that in the near-wall region of the highly drag-reduced flow a positive work done by viscoelastic stress is closely associated with vortex stretching that produces turbulent kinetic energy from stored elastic energy, whereas a negative one causes vortex compression.

Turbulence stripe in transitional channel flow with/without system rotation

We report direct numerical simulations and experiments conducted in two types of plane channel flows—plane Poiseuille flow and plane Couette flows without/with system rotation—considering the subcritical-transition regime with using large aspect-ratio computational domains and channels. Both flows give rise to coexisting laminar and turbulent equilibrium regions in the form of oblique stripes, which are tilted by a certain angle with respect to the mean flow. When subjected to a stabilizing rotation, the Couette turbulence is locally quenched and exhibits the stripe pattern as similar to non-rotating cases. In this context, the turbulence stripe can be seen as an intrinsic phenomenon in the reverse transition in a channel flow.

Proposal of Damping Function for Low-Reynolds-Number - Model Applicable in Prediction of Turbulent Viscoelastic-Fluid Flow

A low-Reynolds-number k- model applicable for viscoelastic fluid was proposed to predict the frictional-drag reduction and the turbulence modification in a wall-bounded turbulent flow. In this model, an additional damping function was introduced into the model of eddy viscosity, while the treatment of the turbulent kinetic energy (k) and its dissipation rate () is an extension of the model for Newtonian fluids. For constructing the damping function, we considered the influence of viscoelasticity on the turbulent eddy motion and its dissipative scale and investigated the frequency response for the constitutive equation based on the Giesekus fluid model. Assessment of the proposed model’s performance in several rheological conditions for drag-reduced turbulent channel flows demonstrated good agreement with DNS (direct numerical simulation) data.

Comparison of heat-transfer reduction in drag-reduced turbulent channel flows with different fluid and thermal boundary conditions

Direct numerical simulations of drag-reduced viscoelastic turbulent channel flow with heat transfer were carried out for four kinds of rheologically different fluids, that is, with different values of Weissenberg number and viscosity ratio. Two different thermal boundary conditions were considered. We present the budget of temperature variance and the relationship between the Heat-Transfer Reduction (HTR) and the Drag Reduction (DR) for each rheologically-different fluid. A case with a low viscosity ratio was found to give rise to high DR, with relatively low HTR compared with that obtained with a high Weissenberg number, suggesting dissimilarity between the heat and momentum transports.

DNS of turbulent plane Couette flow with emphasis on turbulent stripe

Reverse transition of ‘turbulence → laminar’ in wall-bounded shear flows remains poorly understood and few studies have been reported. A plane Couette flow (CF) is conceptually one of the simplest non-trivial fluid dynamics systems, where the flow is solely driven by the shear. This flow is linearly stable for all Reynolds numbers, but experiences direct transition to turbulence through the development of localized perturbations (cf. [1] for a discussion). Above some threshold, turbulence is sustained with a complex behavior characterized by laminar-turbulent co-existence in the form of eturbulent stripe. The transition, though well-described experimentally [2], is far from being completely elucidated. On the other hand, a direct numerical simulation (DNS) of intermittently turbulent flows is challenging and requires a huge computational domain so that another strategy has been necessary. For instance, existing numerical studies are limited within frameworks of a semi-realistic model [3] or of a tilted geometry with a minimal domain [4]. However, owing to the recent development of computers, DNS of the subcritical CF is now possible to be performed.

Effect of Large-Scale Structures upon Near-Wall Turbulence

Direct numerical simulation (DNS) of a turbulent boundary layer has been carried out to examine the effect of the large-scale structures (LSSs) upon the near-wall turbulence. It is found by comparison with the DNS data of the Poiseuille and the Couette flows that the spanwise scales of LSSs are different among the canonical wall turbulences when normalized with the channel half width or the boundary layer thickness, and that the effect of LSSs on the near-wall turbulence depends upon the spanwise scales. We introduce a Reynolds number based on the spanwise scales, and suggest that the new Reynolds number dependence of the nearwall turbulence can be analyzed irrespective of the flow configurations at least at low-to-moderate Reynolds numbers.

DNS of Rotating Turbulent Plane Poiseuille Flow in Low Reynolds- and Rotation-Number Ranges

We performed a series of direct numerical simulation (DNS) of the rotating turbulent channel flow in low Reynolds-number and rotation-number ranges, in order to investigate presence or absence of turbulent stripe, i.e., large-scale laminarturbulent banded pattern, in the rotating Poiseuille flow and to elucidate the transitional process in terms of structures including longitudinal roll cells induced by the Coriolis force. As a result, we found that the turbulent stripe occured only in flows with weak (or no) spanwise system rotations and coexisted with the roll cell under a limited condition. We investigated relationships between the dominant-structure alterations and the changes of rotation-number dependencies of statistics.

DNS on turbulent heat transfer of viscoelastic fluid flow in a plane channel with transverse rectangular orifices

Heat-transfer characteristics of a viscoelastic turbulence past rectangular orifices were investigated in the context of the reduction effects of fluid elasticity on drag and heat transfer. To simulate the fully-developed channel flow through transverse orifices located periodically at intervals of 6.4 times channel height, we imposed periodic conditions at the upstream and downstream boundaries. To discuss the dissimilarity between the velocity and thermal fields, the molecular Prandtl number was set to be 1.0 and any temperature dependence of the fluid and rheological properties was not considered. In the present condition, the ratio of the reduction rates in drag and heat transfer was found to be 2.8:1.0, revealing that the present flow configuration is better than a smooth channel for avoiding the heat-transfer reduction. This phenomenon was attributed to the sustainment of the quasi-streamwise vortex downstream of the reattachment point despite the absence of strong spanwise vortices emanating from the orifice edge in the viscoelastic fluid. The longitudinal vortices behind the reattachment point caused a high turbulent heat flux and increased the local Nusselt number.