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Unsteady flows in a channel with oblique plates have been numerically investigated. The oblique plates as disturbance promoters are installed at the upper wall. Unsteady characteristics are examined for Re = 350 and the plate angles of a = 60^{。}- 120^{。}. The flow fields represent three-dimensional features variously as the plate angle varies. From frequency analysis, it is noted that the disturbed flow by the oblique plates has peculiar unsteady modes. As the flow is more unstable, multiple frequencies are appeared.

Many researchers have paid much attention to the unsteady flow of a channel having various obstacles such as fins, ribs, baffles or plates [

From the previous studies [

In the present study, the unsteady flow for the channel having a wall with periodic plates is investigated. For a channel with various obstacles, as the Reynolds number increases, the two-dimensional steady flow becomes a time-periodic and chaotic flow. This can be explained by a bifurcation theory. An example is Hopf bifurcation where a steady state bifurcates into a time-periodic state. Compared with other obstacles, the plate has a geometrical advantage to realize various flow conditions using one configuration. The channel flow with plates can be diversely changed depending on the inclination of plates. So, this study attempts a parametric study to examine the effects of geometrical variation on the unsteady flow characteristics for the channel flow with plates. The plate interval is 2 H and the plate length is 0.4 H. The inclination angle of the plates are changed for

The flow is two-dimensional with constant properties. The x-axis is taken in the flow direction and the y-axis is perpendicular to the flow direction. The flow is assumed to be laminar and incompressible. Buoyancy induced effects are neglected. The governing equations for continuity, momentum and temperature are given by

where

The spatial discretization is performed with the fourth-order compact scheme. The viscous term and the other terms are evaluated by the fourth-order central difference. A nonstaggered grid arrangement is adopted and the momentum interpolation techniqueis employed to avoid pressure-velocity decoupling. The PISO algorithm is employed for pressure-velocity coupling [

conditions are specified on the upper and lower walls of the channel. The lengths of the oblique plates d is 0.4 H. Periodicity lengths

In order to examine the three-dimensional heat transfer for channel with oblique plates, two-dimensional simulations is performed to analyze basic transition process from the steady state to unstable state. For all selected tilt angle, the relationship between the flow frequency and the tilt angle is studied.

The presence of periodic oblique plates in a channel wall makes the flow unstable. When the flow evolves from a steady state to time-dependent self-sustained periodic flows, the critical Reynolds number (

In order to determine the unsteady flow characteristics, i.e., main fundamental frequencies and harmonics. As the tilt angle increases, Fourier power spectra of u-velocity and u-and v-phase portrait for

According to the above analysis, the change of the tilt angle brings significant change in flow characteristic. In order to detect the 3D flow, the two-dimensional flow field is expanded in the spanwise direction.

mensional T-S wave grows with time and becomes unstable to three-dimensional perturbation. The two curves are identical before the appearance of the 3D disturbance, due to the secondary instability which induces the three-dimensional flow is not prevailing compared with the basic flow. After the different initial time, however, the secondary instability becomes big enough to distort the basic flow, the two curves deviate both in magnitude and in phase. 3D perturbation is infinitesimal for

Unsteady flow and heat transfer in a channel with oblique plates have been numerically investigated. The inclination angle of the plates is changed in a range of

As the tilt angle changes, the flows evolve from steady state to periodic state with one primary frequency and then to quasi periodic with two primary frequencies and their linear combinations (

the transition from the periodic state to chaotic state is obtained quickly. For the geometric condition having multiple frequencies, the flow becomes chaotic not turbulent, because coherent vortices are not produced.

Yinxiao Zhan,Tae Seon Park, (2015) Unsteady Flows Characteristics in a Channel with Oblique Plates. Journal of Applied Mathematics and Physics,03,974-979. doi: 10.4236/jamp.2015.38119