• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.20 no.8
• /
• pp.2601-2610
• /
• 1996

Surface protrusions have been attached on a cylinder surface to reduce the flow-induced structural vibration by controlling the wake flow. Wind tunnel tests on the near wake of a circular cylinder with surface protrusions were carried out to investigate the flow characteristics of the controlled wake. Three experimental models were used in this experiment; one plain cylinder of diameter D and two cylinders wrapped helically by three small wires of diameter d=0.075D with pitches of 5D and 10D, respectively. Free stream velocity was ranged to have Reynolds number from 5000 to 50,000. Streamwise and vertical velocity components of the wake were measured by a hot-wire anemometry. The spanwise velocity component measured by a one-component fiber optic LDV revealed that time-averaged wake field has a nearly two-dimensional structure. It was found that the surface protrusions elongate the vortex formation region, which decrease the vortex shedding frequency. The suppression of vortices caused by the surface protrusions increases the velocity deficit in the center of wake region.

• 한국전산유체공학회:학술대회논문집
• /
• 2009.04a
• /
• pp.97-103
• /
• 2009

A numerical study of the flow characteristics inside a U-type circular tube is carried out in this paper. The numerical simulations carried out by using a Navier-Stokes code which is commercially available. Before detailed numerical simulations, validation of present numerical approach is made by comparing numerical solutions with experimental data. Numerical simulations are performed to study the effect of curvature on the flow characteristics inside a U-type tube. Numerical solutions show that a significant effect on the secondary flow structure in the cross section of the tube, especially in the curved section is shown when the curvature ratio, ratio of curvature to tube diameter, is smaller than about 3.5. As the curvature ratio decreases below 3.5, a counter rotating vortex is found below the primary vortex in the cross section of the tube. Another dramatic change of the flow structure is the formation of streamwise separation zone when the curvature ratio is decreased below 1.25.

• Transactions of the Korean Society of Mechanical Engineers
• /
• v.16 no.7
• /
• pp.1391-1397
• /
• 1992

Improved power law flow model was suggested for the calculation of wake flow characteristics around the three dimensional ship stern in case of the formation of bilge vortex in the direction of stern. In comparison with the power law and Coles flow model, the flow velocity calculated based on this study was delayed around the boundary of inner layer and outer layer in reverse flow. More accurate results was obtained with this improved power law flow model by the velocity calculation around ship stern. Accuracy was validated with the comparison of other calculation results and experimental datas.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.24 no.11
• /
• pp.1540-1548
• /
• 2000

Near-wake flow field downstream of a circular cylinder in the wake-transition regime where fine-scale secondary vortices have a spanwise wavelength of around one diameter has been studied by means of phase-averaging from cinematic PIV data. A cross-correlation algorithm in conjunction with the FFT(Fast Fourier Transform)analysis and an offset correlation technique is used for obtaining the velocity vectors. Which the help of very high sampling rate compared to the shedding frequency, it is possible to obtain phase-averaged flow fields although the shedding is not forced but natural. Phase -locked three-dimensional vortical structures are reconstructed form the phase-averaged data in one x-y(cross-sectional) and several z-x(spanwise-streamwise)planes. In this process of phase-averaging in a z-x plane, a technique to freeze the secondary vortices relative to the centerline is applied. The formation process of the secondary vortices is shown by considering spatial relations between the primary Karman and the secondary vortices and their temporal evolutions.

• International Journal of Automotive Technology
• /
• v.5 no.3
• /
• pp.155-164
• /
• 2004

The direct injection gasoline spray-wall interaction was characterized inside a heated pressurized chamber using various visualization techniques, including high-speed laser-sheet macroscopic and microscopic movies up to 25,000 frames per second, shadowgraph, and double-spark particle image velocimetry. Two hollow cone high-pressure swirl injectors having different cone angles were used to inject gasoline onto a heated plate at two different impingement angles. Based on the visualization results, the overall transient spray impingement structure, fuel film formation, and preliminary droplet size and velocity were analyzed. The results show that upward spray vortex inside the spray is more obvious at elevated temperature condition, particularly for the wide-cone-angle injector, due to the vaporization of small droplets and decreased air density. Film build-up on the surface is clearly observed at both ambient and elevated temperature, especially for narrow cone spray. Vapor phase appears at both ambient and elevated temperature conditions, particularly in the toroidal vortex and impingement plume. More rapid impingement and faster horizontal spread after impingement are observed for elevated temperature conditions. Droplet rebounding and film break-up are clearly observed. Post-impingement droplets are significantly smaller than pre-impingement droplets with a more horizontal velocity component regardless of the wall temperature and impingement angle condition.

• Journal of computational fluids engineering
• /
• v.14 no.3
• /
• pp.105-114
• /
• 2009

A numerical study of the flow characteristics inside a U-type circular tube is carried out in this paper. The numerical simulations carried out by using a Navier-Stokes code which is commercially available. Before detailed numerical simulations, validation of present numerical approach is made by comparing numerical solutions with experimental data. Numerical simulations are performed to study the effect of curvature on the flow characteristics inside a U-type tube. Numerical solutions show that a significant effect on the secondary flow structure in the cross section of the tube, especially in the curved section is shown when the curvature ratio, ratio of curvature to tube diameter, is smaller than about 3.5. As the curvature ratio decreases below 3.5, a counter rotating vortex is found below the primary vortex in the cross section of the tube. Another dramatic change of the flow structure is the formation of streamwise separation zone when the curvature ratio is decreased below 1.25.

• Journal of Advanced Marine Engineering and Technology
• /
• v.21 no.4
• /
• pp.430-436
• /
• 1997

Time-dependent Flow characteristics of a two-dimensional lid-driven square cavity flow of six high Reynolds numbers, $10^4 2{\times}lO^4., 3{\times}l0^4, 5{\times}lO^4, 7.5{\times}lO^4$ and $10^5$ were investigated. A convection conservative difference scheme based upon SOLA to maintain the nearly 2nd-order spatial accuracy was adopted on irregular grid formation. Irregular grid number is $80{\times}80$ and its minimum size is about 1/400 of the cavity height(H) and its maximum is about 1/53 H. The result shows that at Re= $10^4$, periodic migration of small eddies appearing in corner separation region and its temporal sinusoidal fluctuation are represented. And another critical Reynolds number which shows the commencement of flow randomness emerging from the periodic fluctuation is assumed to be around Re= $1.5{\times}10$. At five higher Reynolds numbers ($2{\times}lO^4., 3{\times}l0^4, 5{\times}lO^4, 7.5{\times}lO^4$ and $10^5$), an organizing structure of four consecutive vortices similar to a Moffat vortex at two lower corners is revealed from time-mean flow patterns.

• International Journal of Railway
• /
• v.7 no.4
• /
• pp.100-108
• /
• 2014

A numerical analysis method for predicting aerodynamic noise at inter-coach space of high-speed trains, validated by wind-tunnel experiments for limited speed range, is proposed. The wind-tunnel testing measurements of the train aerodynamic sound pressure level for the new generation Korean high-speed train have suggested that the inter-coach space aerodynamic noise varies approximately to the 7.7th power of the train speed. The observed high sensitivity serves as a motivation for the present investigation on elucidating the characteristics of noise emission at inter-coach space. As train speed increases, the effect of turbulent flows and vortex shedding is amplified, with concomitant increase in the aerodynamic noise. The turbulent flow field analysis demonstrates that vortex formation indeed causes generation of aerodynamic sound. For validation, numerical simulation and wind tunnel measurements are performed under identical conditions. The results show close correlation between the numerically derived and measured values, and with some adjustment, the results are found to be in good agreement. Thus validated, the numerical analysis procedure is applied to predict the aerodynamic noise level at inter-coach space. As the train gains speed, numerical simulation predicts increase in the overall aerodynamic sound emission level accompanied by an upward shift in the main frequency components of the sound. A contour mapping of the aerodynamic sound for the region enclosing the inter-coach space is presented.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.20 no.6
• /
• pp.2014-2022
• /
• 1996

Flow characteristics of the wake behind a finite circular cylinder(FC) mounted on a flat plate was experimentally investigated. Three finite cylinder models having aspect ratio (length to diameter ratio, L/D) of 6,10 and 13 were tested in this study. Wake velocity was measured by a hot-wire anemometry at Reynolds number of 20,000, and the results were compared with those of two-dimensional circular cylinder. As a result, the free-end effect on the wake structure becomes more dominant with decreasing the aspect ratio(L/D) of the finite cylinder. Invisid flow entrained into the wake region decreases the turbulence intensity and periodicity of the vortex shedding due to existence of the free end. From spectral analysis and cross correlation of the velocity signals, vortices having 24Hz frequency characteristics are found in the down wash flow just behind the free end. There exists very complicated flow near the free end due to interaction between the entrained flow and streamwise vortices. Vortex formation region is destroyed significantly in the near wake and shows quite different wake structures from those of 2-D cylinder.

• Membrane Journal
• /
• v.7 no.2
• /
• pp.95-109
• /
• 1997

The effects of varing axial flow rate and solute concentration on the performance of both module sets made by different methods for active layer formation were compared and determined. All experiments were conducted simultaneously at the same transmembrane pressure and energy consumption per membrane area. In every comparative run between the presence of Dean vortices in a helical module and absence of such vortices in a linear module from the first module set, the solution fluxes and permeabilities were higher, and in some cases substantially higher for the vortex flow. With pure water, the permeabilities of both modules from the second module set were different and the flux in a linear module was 150% higher than in the helical module. This explained both module membranes were totally different.