• Journal of computational fluids engineering
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• v.13 no.1
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• pp.57-62
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• 2008

Laminar flows over a cube and a cuboid (cube extended in the streamwise direction) are numerically investigated for the Reynolds numbers between 50 and 350. First, vortical structures behind a cube and lift characteristics are scrutinized in order to understand the variation in vortex shedding characteristics with respect to the Reynolds number. As the Reynolds number increases, the flow over a cube experiences the steady planar-symmetric, unsteady planar-symmetric, and unsteady asymmetric flows. Similar to the sphere wake, the planar-symmetric flow over a cube can be divided into two different regimes: single-frequency regime and multiple-frequency regime. The former has a single frequency due to regular shedding of vortices with the same strength in time, while the latter has multiple frequency components due to temporal variation in the strength of shed vortices. Second, the effect of the length-to-height ratio of the cuboid on the flow characteristics is investigated for the Reynolds number of 270, at which planar-symmetric vortex shedding takes place behind a cube. With the ratio smaller than one, the flow over the cuboid becomes unsteady asymmetric flow, whereas it becomes steady flow for the ratios greater than one. With increasing the ratio, the drag coefficient first decreases and then increases. This feature is related to the flow reattachment on the side faces of the cuboid.

• Journal of the Korean Society of Visualization
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• v.1 no.1
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• pp.64-74
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• 2003

An experimental study was carried out to investigate the effect of periodic blowing and suction on a turbulent boundary layer. Particle image velocimetry (PIV) was used to probe the characteristics of the flow. The local forcing was introduced to the boundary layer via a sinusoidally-oscillating jet issuing from a thin spanwise slot. Three forcing frequencies (f$^{+}$=0.044, 0.066 and 0.088) with a fixed forcing amplitude (A$^{+}$=0.6) were employed at $Re_{=690. The effect of the forcing angles ($\alpha$=60$^{\circ}$ , 90$^{\circ}$ and 120$^{\circ}$ ) was investigated under the fixed forcing frequency (f$^{+}$=0.088). The PIV results showed that the wall region velocity decreases on imposition of the local forcing. Inspection of phase-averaged velocity profiles revealed that spanwise large-scale vortices were generated in the downstream of the slot and persist further downstream. The highest reduction in skin friction was achieved at highest forcing frequency (f$^{+}$=0.088) and a forcing angle of $\alpha$=120$^{\circ}$. The spatial fraction of the vortices was examined to analyze the skin friction reduction.

• Korea-Australia Rheology Journal
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• v.19 no.4
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• pp.211-219
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• 2007

Here we demonstrate complex transient behavior of viscoelastic liquid described numerically with the Leonov model in straight and contraction channel flow domains. Finite element and implicit Euler time integration methods are employed for spatial discretization and time marching. In order to stabilize the computational procedure, the tensor-logarithmic formulation of the constitutive equation with SUPG and DEVSS algorithms is implemented. For completeness of numerical formulation, the so called traction boundaries are assigned for flow inlet and outlet boundaries. At the inlet, finite traction force in the flow direction with stress free condition is allocated whereas the traction free boundary is assigned at the outlet. The numerical result has illustrated severe forward-backward fluctuations of overall flow rate in inertial straight channel flow ultimately followed by steady state of forward flow. When the flow reversal occurs, the flow patterns exhibit quite complicated time variation of streamlines. In the inertialess flow, it takes much more time to reach the steady state in the contraction flow than in the straight pipe flow. Even in the inertialess case during startup contraction flow, quite distinctly altering flow patterns with the lapse of time have been observed such as appearing and vanishing of lip vortices, coexistence of multiple vortices at the contraction comer and their merging into one.

• 한국가시화정보학회:학술대회논문집
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• 2005.12a
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• pp.58-63
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• 2005

Three-dimensional vortical structures in the wake behind a road vehicle has been visualized with the help of two-dimensional PIV measurement data. A three-dimensional velocity field has been reconstructed from several sectional measurement data in the x-y, y-z and z-x planes. Isovorticity surface observed by stacking only the sectional data in each plane, does not show the vortical structures within the recirculation region but represents only the strong shear flows. Thus, in the present study, the velocity component normal to the x-y plane is obtained by interpolating those velocities in the z-x plane. Then, a $\lambda_{2}$-definition which captures the local pressure minimum or vortex core, is applied to visualize the vortices in the recirculation region. The final results represent a successful configuration for the three-dimensional vortices.

• Journal of the Korean Institute of Navigation
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• v.9 no.2
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• pp.43-63
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• 1985

As a circular cylinder has a comparatively simple shape and becomes a basic problem for flows around other various shapes of bodies, the problem of two-dimensional viscous flow around the circular cylinder has been investigated, both theoretically and experimentally. But not a few problems are left unsolved. It is well known that the calculations are successfully made with the approximations of Stokes or Oseen for very low Reynolds numbers, but as Reynolds number is increased, Oseen's approximations as well as Stokes's ones become more and more remote from the exact solution of the Navier-Stokes equations. Therefore, in this paper, the authors transform the Navier-Stokes equations into the finite difference equations in the steady two-dimensional viscous flow at Reynolds number up to 45, and then solve the solution of the Navier-Stokes equations numerically. Also, the authors examine the accuracy of the solution by means of flow visualization with aluminum powder. The main results are as follows; (1) The critical Reynolds number at which twin vortices begin to form in the rear of the circular cylinder is found to be 6 in the experiment and 4 in the numerical solution. (2) As Reynolds number is increased, it is proved that the ratio of the length of the twin vortices to the diameter is grown almost linearly, both experimentally and numerically. (3) Separation angle is also increased according to reynolds number. But it is found that it would converge into 101.3 degrees, both experimentally and numerically.

• Journal of computational fluids engineering
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• v.16 no.1
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• pp.83-89
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• 2011

Laminar flow over a cube near a plane wall is numerically investigated in order to understand the effects of the cube-wall gap on the flow characteristics as well as the drag and lift coefficients. The main focus is placed on the three-dimensional vortical structures and its relation to the lift force applied on the cube. Numerical simulations are performed for the Reynolds numbers between 100 and 300, covering several different flow regimes. Without a wall nearby, the flow at Re=100 is planar symmetric with no vortical structure in the wake. However, when the wall is located close to the cube, a pair of streamwise vortices is induced behind the cube. At Re=250, the wall strengthens the existing streamwise vortices and elongates them in the streamwise direction. As a result, the lift coefficients at Re=100 and 250 increase as the cube-wall gap decreases. On the other hand, without a wall, vortex shedding takes place at Re=300 in the form of a hairpin vortex whose strength changes in time. The head of hairpin vortex or loop vortex, which is closely related to the lift force, seems to disappear due to the nearby wall. Therefore, unlike at Re=100 and 250, the lift coefficient tends to decrease more or less as the cube approaches the wall.

• Journal of computational fluids engineering
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• v.17 no.2
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• pp.93-99
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• 2012

The present study focuses on the case of developing flow with in a channel containing a long array of sinusoidal waves (2a/${\lambda}$=0.1, ${\lambda}$=h, ${\lambda}$ is the wavelength, 2a is the wave height, h is the mean channel depth) at the bottom wall. The Reynolds number defined with channel height, h and the mean velocity, U, is Re=6,700. The channel is sufficiently long such that transition is completed and the flow is fully developed over the downstream half of the channel. For the case of an incoming steady flow with no resolved turbulence, the instantaneous flow fields in the transition region are characterized by the formation of arrays of highly-organized large-scale hairpin vortices whose dimensions scale with that of the roughness elements. The paper explains the mechanism for the formation of these arrays of hairpin vortices and shows these eddies play the primary role in the formation of the large-scale streaks of high and low velocity over the wavy wall region. The presence of resolved turbulence in the incoming flow, reduces the streamwise distance needed for the streaks to develop over the wavy region, but does not affect qualitatively the transition process. In the fully-developed region, isolated and trains of large-scale hairpins play an important role in the dynamics of the streaks over the wavy wall.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.24 no.10
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• pp.1343-1350
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• 2000

The evolutionary structure of tip vortices has been investigated with a two-dimensional LDV system for a plain and a slotted blade, respectively. To analyze the effect of slots which bypasses a part of main stream into the tip face, velocity profiles, vortex sizes, their displacements and turbulence intensities during one revolution of the rotor were measured by the phase averaging process. For the comparison of circumferential velocity components of the plain blade and the slotted blade, the peak values of the slotted blade were lower than those of the plain blade, and axial velocity components of the slotted blade were considerably larger than those of the plain blade. The slotted rotor blade enlarged the core size and made the vortex delayed compared with those of the plain blade at the same wake ages. Turbulence profiles had peaks inside the core radii and decayed gradually in the radial direction of vortex coordinate. Also, using a quasi 3-D LDV measurement technique the budget of turbulence kinetic energy was analyzed in radial direction of the vortex core.

• Journal of Mechanical Science and Technology
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• v.16 no.9
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• pp.1137-1146
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• 2002

An experiment has been conducted on the flow and heat transfer characteristics of film coolant injected from a row of five holes with compound angle orientations of 35$^{\circ}$ inclination angle and 45$^{\circ}$ orientation angle. The Reynolds number based on the mainstream velocity and injection hole diameter 3.58${\times}$10$^4$. Three-dimensional velocity, film cooling effectiveness and heat transfer coefficient data are presented at three different mass flux ratios of 0.5, 1.0 and 2.0. Flow entrainment has been found between the vortices generated by adjacent injectants. The injectant with compound angle orientation entrains not only the mainstream boundary layer flow but also the adjacent injectant. Because of the flow entrainment, the injectant. With compound angle orientation is characterized by a single vortex while two bound vortices are usually observed in the case of simple angle injection. The strength of the secondary flow depends strongly on the mass flux ratio, which shows significant influence on the film cooling effectiveness and heat transfer coefficient.

• International Journal of Aeronautical and Space Sciences
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• v.1 no.1
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• pp.1-12
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• 2000

The wake geometries of a two-bladed rotor in axial flights using a time-marching free-wake method without a non-physical model of the far wake are calculated. The computed free-wake geometries of AH-1G model rotor in climb flight are compared with the experimental visualization results. The time-marching free-wake method can predict the behavior of the tip vortex and the wake roil-up phenomena with remarkable agreements. Tip vortices shed from the two-bladed rotor can interact with each other significantly. The interaction consists of a turn of the tip vortex from one blade rolling around the tip vortex from the other. Wake expansion of wake geometries in radial direction after the contraction is a result of adjacent tip vortices begging to pair together and spiral about each other. Detailed numerical results show regular pairing phenomenon in the climb flights, the hover at high angle of attack and slow descent flight too. On the contrary, unstable motions of wake are observed numerically in the hover at low angle of attack and fast descent flight. It is because of the inherent wake instability and blade-vortex-interaction rather then the effect of recirculation due to the experimental equipment.