• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.21 no.9
• /
• pp.1149-1164
• /
• 1997

In this study, the applicability of the RNG k-.epsilon. model to the analysis of the complex flows is studied. The governing equations based on a non-orthogonal coordinate formulation with Cartesian velocity components are used and discretized by the finite volume method with non-staggered variable arrangements. The predicted results using the RNG k-.epsilon. model of three complex flows, i.e., the flow over a backward-facing step and a blunt flat plate, the flow around a 2D model car are compared to these from the standard k-.epsilon. model and experimental data. That of the unsteady axisymmetric turbulent flow within a cylinder of reciprocating model engine including port/valve assembly and the spray characteristics within a chamber of direct injection model engine are compared to these from the standard k-.epsilon. model and experimental data. The results of reattachment length, separated eddy size, average surface pressure distribution using the RNG k-.epsilon. model show more reasonable trends comparing with the experimental data than those using the modified k-.epsilon. model. Although the predicted rms velocity using the modified k-.epsilon. model is lower considerably than the experimental data in incylinder flow with poppet valve, predicted axial and radial velocity distributions at the valve exit and in-cylinder region show good agreements with the experimental data. The spray tip penetration predicted using the RNG k-.epsilon. model is more close to the experimental data than that using the modified k-.epsilon. model. The application of the RNG k-.epsilon. model seems to have some potential for the simulations of the unsteady turbulent flow within a port/valve-cylinder assembly and the spray characteristics over the modified k-.epsilon. model.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.26 no.4
• /
• pp.578-586
• /
• 2002

Flow structures around a rectangular prism have been investigated by using a PIV(Particle Image Velocimetry) technique. A thick turbulent boundary layer was generated by using spires arid roughness elements. The boundary layer thickness, displacement thickness and momentum thickness were 650mm, 117.4mm and 78mm, respectively. The ratio between the model height(40mm) and the boundary layer thickness H/$\delta$, was 0.06. The Reynolds number based on the free stream velocity and the height of the model was 7.9$\times$10$^3$. The PIV measurements were performed at three different wall normal planes. Three recirculation regions at forward facing step, top of the roof and backward facing step are clearly seen and show three dimensional features. Dramatic changes of flow patterns are observed in the wake regions in the different spanwise wall normal planes. Instead of reattachment and recirculation zone, rising streamlines are depicted at the normal planes near the side wall due to the interaction with a rising horse shoe vortex. The peak of turbulent kinetic energy occurs at the separation bubble on top of the roof and the magnitude is 2.5 times higher compared with that of the wake region.

• Clinics in Shoulder and Elbow
• /
• v.8 no.2
• /
• pp.63-67
• /
• 2005

Osteochondritis dissecans is a common cause of lateral elbow pain in throwing athletes and gymnasts between the age of 10 and 15 years. The pain frequently is insidious and progressive in nature and is relieved with rest in most cases. Compressive forces at the radiocapitellar joint along with a tenuous blood supply to the region may play a role in the etiology of this condition. Treatment is directed according to presenting symptoms, radiographic findings and status of the involved segment. Surgical management involves either an excision of a loose lesion and removal of loose bodies or reattachment of an osteochondral fragment. The role of subchondral penetration of debridement of the defect after fragment excision is unclear. Whether to excise and debride or to fix an unstale fragment is a highly controversial topic. The clinician should recognize osteochondritis dissecans of the elbow as a potentially disabling condition where the prognosis for return to sport is guarded

• Journal of Biomedical Engineering Research
• /
• v.23 no.4
• /
• pp.281-286
• /
• 2002

In this study, blood flow in a carotid artery supplying blood to the human's brain has been numerically simulated to find out how the blood flow affects the genesis and the growth of atherosclerosis and arterial thrombosis. Velocity Profiles and hemodynamic parameters have been investigated for the carotid arteries with three different stenoses under physiological flow condition. Blood has been treated as Newtonian and non-Newtonian fluid. To model the shear thinning properties of blood for non-Newtonian fluid, the Carreau-Yasuda model has been employed. The result shows that the wall shear stress(WSS) increases with the development of stenosis and that the wall shear stress in Newtonian fluid is highly evaluated compared with that in non-Newtonian Fluid. Oscillatory shear index has been employed to identify the time-averaged reattachment point and this point is located farther from the stenosis for Newtonian fluid than for non-Newtonian fluid The wall shear stress gradient(WSSG) along the wall has been estimated to be very high around the stenosis region when stenosis is developed much and the WSSG peak value of Newtonian fluid is higher than that of non-Newtonian fluid.

• Transactions of the Korean Society of Mechanical Engineers
• /
• v.11 no.3
• /
• pp.444-453
• /
• 1987

In the present study, a new computational closure model is proposed in order to contain physical models in the k- and .epsilon.- equations. The time scale of the third-order diffusive transport of turbulent kinetic energy in a curved streamline flow field is assumed as a function of a velocity time scale and a curvature time scale, the latter being derived from the analogy between buoyancy and streamline curvature effects on turbulence. The curvature time scale is represented by a combination of Brunt-Vaisala frequency of the curvature instability and the velocity time scale. Besides the modification of diffusive transport time scale, the destruction term in the dissipation rate equation is modeled to incorporate the streamline curvature effect on the dissipation rate of turbulent kinetic energy as a function of the ratio between velocity time scale and curvature time scale. The new curvature dependent 2-equation model is found to yield very good prediction accuracy for the various turbulent recirculating flows. Particurarly, the recovery of the mean velocity profile in the redeveloping region after the reattachment is correctly simulated by the present model.

• Journal of the Korean Society for Aeronautical & Space Sciences
• /
• v.49 no.1
• /
• pp.41-52
• /
• 2021

During atmospheric ascent of a launch vehicle, airborne acoustic loads act on the vehicle and its effect becomes pronounced at transonic speed. In the present study, acoustic loads acting on a hammerhead launch vehicle at a transonic speed have been analyzed using ��-ω SST based IDDES and the results including mean Cp, Cprms, and PSD are compared to available wind-tunnel test data. Mesh dependency of IDDES results has been investigated and it has been concluded that with an appropriate turbulence scale-resolving computational mesh, the characteristic flow features around a transonic hammerhead launch vehicle such as separated shear flow at fairing shoulder and its reattachment on rear body as well as large pressure fluctuation in the region of separated flow behind the boat-tail can be predicted with reasonable accuracy for engineering purposes.

• Wind and Structures
• /
• v.34 no.1
• /
• pp.15-27
• /
• 2022

This work comes within the framework of the "Benchmark on the Aerodynamics of a Rectangular Cylinder" that investigates a rectangular cylinder of length-to-depth ratio equal to 5. The present study reports and discusses velocity fields acquired using planar Particle Image Velocitmetry for several angles of attack and Reynolds numbers. In particular, for a cylinder depth-based Reynolds number of 2 × 10⁴ and zero incidence angle, the flow features along the lateral (parallel to the freestream) upper and lower surfaces of the cylinder are reported. Using first and second order statistics of the velocity field, the main flow features are discussed, especially the size and location of the time-averaged flow structures and the distribution of the Reynolds stresses. The variation of the flow features with the incidence is also studied considering angles of attack up to 6°. It is shown that the time-averaged flow is fully detached for incidence higher than 2°. For an angle of attack of 0°, the effects of the Reynolds number varying between 5 × 10³ and 2 × 10⁴ are investigated looking at flow statistics. It is shown that the time-averaged location of the reattachment point and the shape and position of the time-averaged main vortex are mostly constant with the Reynolds number. However, the size of the inner region located below the time-averaged shear layer and just downstream the leading edge corner appears to be strongly dependent on the Reynolds number.

• Journal of the Korean Society of Propulsion Engineers
• /
• v.18 no.1
• /
• pp.1-7
• /
• 2014

In this study, the effect of velocity ratio and particle size on the flow mixing characteristics in the secondary combustor was investigated. Both PIV(Particle Image Velocimetry) technique and LES(Large Eddy Simulation) were applied. Two sizes of Polystyrene PIV seeding particle of 5 and $50{\mu}m$, and three velocity ratios of 5, 3, and 1.5 were considered. Results showed that the mixing of two air streams created reattachment and recirculation regions. The size of the recirculation region was decreased as the velocity ratio increased. For the larger particle cases, due to the increased momentum by the larger particles, the size of the recirculating regions were larger than that of the smaller particle cases and the effect of the velocity ratio was not as significant as in the smaller particle case.

• Journal of Ocean Engineering and Technology
• /
• v.10 no.3
• /
• pp.138-152
• /
• 1996

Many studies of heat transfer on the swirling flow or unswirled flow in a abrupt pipe expansion are widely carried out. The mechanism is not fully found evidently due to the instabilities of flow in a sudden change of the shape and appearance of turbulent shear layers in a recirculation region and secondary vortex near the corner. The purpose of this study is to obtain data through an experimental study of the swirling flow and heat transfer downstream of an abrupt expansion in a circular pipe with uniform heat flux. Experiments were carried out for the turbulent flow nd heat transfer downstream of an abrupt circular pipe expansion. The uniform heat flux condition was imposed to the downstream of the abrupt expansion by using an electrically heated pipe. Experimental data are presented for local heat transfer rates and local axial velocities in the tube downstream of an abrupt 3:1 & 2:1 expansion. Air was used as the working fluid in the upstream tube, the Reynolds number was varied from 60, 00 to 120, 000 and the swirl number range (based on the swirl chamber geometry, i.e. L/d ratio) in which the experiments were conducted were L/d=0, 8 and 16. Axial velocity increased rapidly at r/R=0.35 in the abrupt concentric expansion turbulent flow through the test tube in unswirled flow. It showed that with increasing axial distance the highest axial velocities move toward the tube wall in the case of the swirling flow abrupt expansion. A uniform wall heat flux boundary condition was employed, which resulted in wall-to-bulk temperatures ranging from 24.deg. C to 71.deg. C. In swirling flow, the wall temperature showed a greater increase at L/d=16 than any other L/d. The bulk temperature showed a minimum value at the pipe inlet, it also exhibited a linear increase with axial distance along the pipe. As swirl intensity increased, the location of peak Nu numbers was observed to shift from 4 to 1 step heights downstream of the expansion. This upstream movement of the maximum Nusselt number was accompanied by an increase in its magnitude from 2.2 to 8.8 times larger than fully developed tube flow values.

• Wind and Structures
• /
• v.34 no.1
• /
• pp.115-125
• /
• 2022

The high-Reynolds number flow around a rectangular cylinder, having streamwise to crossflow length ratio equal to 5 is analyzed in the present paper. The flow is characterized by shear-layer separation from the upstream edges. Vortical structures of different size form from the roll-up of these shear layers, move downstream and interact with the classical vortex shedding further downstream in the wake. The corresponding mean flow is characterized by a recirculation region along the lateral surface of the cylinder, ending by mean flow reattachment close to the trailing edge. The mean flow features on the cylinder side have been shown to be highly sensitive to set-up parameters both in numerical simulations and in experiments. The results of 21 Large Eddy Simulations (LES) are analyzed herein to highlight the impact of the lateral mean recirculation characteristics on the near-wake flow features and on some bulk quantities. The considered simulations have been carried out at Reynolds number Re=DU_∞/ν=40 000, being D the crossflow dimension, U_∞ the freestream velocity and ν the kinematic viscosity of air; the flow is set to have zero angle of attack. Some simulations are carried out with sharp edges (Mariotti et al. 2017), others with different values of the rounding of the upstream edges (Rocchio et al. 2020) and an additional LES is carried out to match the value of the roundness of the upstream edges in the experiments in Pasqualetto et al. (2022). The dimensions of the mean recirculation zone vary considerably in these simulations, allowing us to single out meaningful trends. The streamwise length of the lateral mean recirculation and the streamwise distance from the upstream edge of its center are the parameters controlling the considered quantities. The wake width increases linearly with these parameters, while the vortex-shedding non-dimensional frequency shows a linear decrease. The drag coefficient also linearly decreases with increasing the recirculation length and this is due to a reduction of the suctions on the base. However, the overall variation of C_D is small. Finally, a significant, and once again linear, increase of the fluctuations of the lift coefficient is found for increasing the mean recirculation streamwise length.