• The Transactions of the Korean Institute of Electrical Engineers D
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• v.51 no.9
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• pp.431-437
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• 2002

Low hemolysis is one of the key factors in the production of successful rotary blood pumps. It is, however, difficult to identify the areas where hemolysis occurs. Computational fluid dynamics(CFD) analysis enables the engineer to predict hemolysis on a computer Fluid dynamics in five different axial flow pumps was analyzed 3-dimensionally using CFD software. The impeller was rotated at a speed which supplied a flow of 5L/min at a pressure difference of 100mmHg. Changes in the turbulent kinetic energy along streamlines through the pumps were computed. Reynolds' shear stress( (equation omitted) ) was calculated using the turbulent kinetic energy. Hemolysis was evaluated based on Reynolds'shear stress and its exposure time(t) : dHb/Hb=3.62$\times$10$^{-5}$ $t^{0.785}$$\tau$$^{2.416}$ . Hemolysis of the pumps was measured in vitro using fresh bovine blood to which citrate phosphate dextrose was added to prevent clotting. A pump flow of 5L/min was maintained at a pressure difference of 100mmHg for 3h. The normalized index of hemolysis(NIH) as measured. Reynolds' shear stress was high behind the impellers. The measured NIH and the calculated hemolysis(dHb/Hb) shoed a good correlation; NIH=0.0003(dHb/Hb) (r=0.90, n=6) in the range of NIH between 0.003 and 1.1. CFD analysis can predict the in vitro results of hemolysis as well as the areas where hemolysis occurs.ysis occurs.

• Journal of the Korean Society of Marine Environment & Safety
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• v.18 no.4
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• pp.352-359
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• 2012

An experiment was carried out to figure out the turbulence flow characteristics in the wake of the transom stern's 2-dimensional section by 2-frame grey level cross correlation PIV method at Re= $3.5{\times}10^3$, Re= $7.0{\times}10^3$. The angles of transom stern are $45^{\circ}$(Model "A"), $90^{\circ}$(Model "B") and $135^{\circ}$(Model "C") respectively. The depth of wetted surface is 40mm from free surface. Strong turbulence intensity appears at the interaction between the flow separation of the bottom of a model and the free surface. This study provides statistic flow information such as turbulence intensity, Reynolds stress and turbulence kinetic energy. Model C type (Raked transom) has low Reynolds stress and turbulence kinetic energy.

• Wind and Structures
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• v.4 no.3
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• pp.177-196
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• 2001

At present the most popular turbulence models used for engineering solutions to flow problems are the $k-{\varepsilon}$ and Reynolds stress models. The shortcoming of these models based on the isotropic eddy viscosity concept and Reynolds averaging in flow fields of the type found in the field of Wind Engineering are well documented. In view of these shortcomings this paper presents the implementation of a non-linear model and its evaluation for flow around a building. Tests were undertaken using the classical bluff body shape, a surface mounted cube, with orientations both normal and skewed at $45^{\circ}$ to the incident wind. Full-scale investigations have been undertaken at the Silsoe Research Institute with a 6 m surface mounted cube and a fetch of roughness height equal to 0.01 m. All tests were originally undertaken for a number of turbulence models including the standard, RNG and MMK $k-{\varepsilon}$ models and the differential stress model. The sensitivity of the CFD results to a number of solver parameters was tested. The accuracy of the turbulence model used was deduced by comparison to the full-scale predicted roof and wake recirculation zone lengths. Mean values of the predicted pressure coefficients were used to further validate the turbulence models. Preliminary comparisons have also been made with available published experimental and large eddy simulation data. Initial investigations suggested that a suitable turbulence model should be able to model the anisotropy of turbulent flow such as the Reynolds stress model whilst maintaining the ease of use and computational stability of the two equations models. Therefore development work concentrated on non-linear quadratic and cubic expansions of the Boussinesq eddy viscosity assumption. Comparisons of these with models based on an isotropic assumption are presented along with comparisons with measured data.

• Wind and Structures
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• v.24 no.6
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• pp.579-611
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• 2017

Bora is a strong, usually dry temporally and spatially transient wind that is common at the eastern Adriatic Coast and many other dynamically similar regions around the world. One of the Bora main characteristics is its gustiness, when wind velocities can reach up to five times the mean velocity. Bora often creates significant problems to traffic, structures and human life in general. In this study, Bora velocity and near-ground turbulence are studied using the results of three-level high-frequency Bora field measurements carried out on a meteorological tower near the city of Split, Croatia. These measurements are analyzed for a period from April 2010 until June 2011. This rather long period allows for making quite robust and reliable conclusions. The focus is on mean Bora velocity, turbulence intensity, Reynolds shear stress and turbulence length scale profiles, as well as on Bora velocity power spectra and thermal stratification. The results are compared with commonly used empirical laws and recommendations provided in the ESDU 85020 wind engineering standard to question its applicability to Bora. The obtained results report some interesting findings. In particular, the empirical power- and logarithmic laws proved to fit mean Bora velocity profiles well. With decreasing Bora velocity there is an increase in the power-law exponent and aerodynamic surface roughness length, and simultaneously a decrease in friction velocity. This indicates an urban-like velocity profile for smaller wind velocities and a rural-like velocity profile for larger wind velocities. Bora proved to be near-neutral thermally stratified. Turbulence intensity and lateral component of turbulence length scales agree well with ESDU 85020 for this particular terrain type. Longitudinal and vertical turbulence length scales, Reynolds shear stress and velocity power spectra differ considerably from ESDU 85020. This may have significant implications on calculations of Bora wind loads on structures.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.26 no.6B
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• pp.573-581
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• 2006

This paper presents a turbulence modeling of the open-channel flows over smooth-rough bed strips. A Reynolds stress model is used for the turbulence closure. The simulated mean flow and turbulence structures are compared with the previously reported experimental data. Comparisons reveal that the developed Reynolds stress model successfully predicts the mean flow and turbulence structures of open-channel flows over smooth-rough bed strips. The computed flow vectors show cellular secondary currents, of which the upflow occurs over the smooth bed strip and the downflow over the rough bed strip. It is found that the cellular secondary currents affect the mean flow and turbulence structure. A budget analysis of the streamwise vorticity equation is also carried out to investigate the mechanism by which the secondary currents are generated.

• Journal of Advanced Marine Engineering and Technology
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• v.20 no.5
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• pp.58-67
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• 1996

In the present study, the pressure distribution, wall shear stress distribution and friction factor of developing turbulent pulsating flows are investigated theoretically and experimentally in the entrance region of a square duct. The pressure distribution for turbulent pulsating flows are in good agreement with the theoretical values. The time-averaged pressure gradients of the turbulent pulsating flows show the same tendency as those of turbulent steady flows as the time-averged Reynolds number $(Re_{ta})$ increase. Mean shear stresses in the turbulent pulsating flow increase more in the inlet flow region than in the fully developed flow region and approach to almost constant value in the fully developed flow region. In the turbulent pulsating flow, the friction factor of the quasi-steady state flow $({\lambda}_{q, tu})$ follow friction factor's law in turbulent steady flow. The entrance length of the turbulent pulsating flow is not influenced by the time-averaged Reynolds number $(Re_{ta})$ and it is about 40 times as large as the hydraulic diameter.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.30 no.2B
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• pp.179-189
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• 2010

This paper presents a numerical investigation of vorticity generation in fully vegetated open-channel flows. The Reynolds stress model is used for the turbulence closure. Open-channel flows with rough bed-smooth sidewalls and smooth bed-rough sidewalls are simulated. The computed vectors show that in channel flows with rough bed and rough sidewalls, the free-surface secondary currents become relatively smaller and larger, respectively, compared with that of plain channel flows. Also, open-channel flows over vegetation are simulated. The computed bottom vortex occupies the entire water depth, while the free-surface vortex is reduced. The contours of turbulent anisotropy and Reynolds stress are presented with different density of vegetation. The budget analysis of vorticity equation is carried out to investigate the generation mechanism of secondary currents. The results of the budget analysis show that in plain open-channel flow, the production by anisotropy is important in the vicinity of the wall and free-surface boundaries, and the production by Reynolds stress is important in the region away from the boundaries. However, this rule is not effective in vegetated channel flows. Also, in plain channel flows, the vorticity is generated mainly in the vicinity of the free-surface and the bottom, while in vegetated channel flows, the regions of the bottom and vegetation height are important to generate the vorticity.

• Journal of the Korean Society of Visualization
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• v.15 no.3
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• pp.27-33
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• 2017

Direct numerical simulations of turbulent channel flows with moving wall conditions on the top wall are performed to examine the effects of the moving wall on the turbulent characteristics. The moving wall velocity only applied to the top wall with the opposite direction to the main flow is systematically varied to reveal the sustained-mechanism for turbulence. The turbulence statistics for the Couette-Poiseuille flow, such as mean velocity, root mean square of the velocity fluctuations, Reynolds shear stress and pre-multiplied energy spectra of the velocity fluctuations, are compared with those of canonical turbulent channel flows. The comparison suggests that although the turbulent activity on the top wall increases with increasing the Reynolds number, that on the bottom wall decreases, contrary to the previous finding for the canonical turbulent channel flows. The increase of the turbulent energy on the top wall is attributed to not only the increase of the Reynolds number but also elongation of the logarithmic layer due to increase of the wall layer on the top wall. However, because the logarithmic layer is shortened on the bottom wall due to the decrease of the wall layer, the turbulence energy on the bottom wall decreases despite of the increase of the Reynolds number.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2007.11a
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• pp.840-846
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• 2007

This paper investigates the dynamic behavior of a HDD spindle system due to the imperfect roundness of a rotating shaft. The shaft of a spindle motor rotates with eccentricity by the unbalanced mass of the rotating part. The eccentricity generates the run-out of a spindle motor which results in the eccentric motion of a rotating part. Roundness of a shaft affects this motion which limits the memory capacity of a HDD. This research proposes a modified Reynolds equation for the coupled journal and thrust FDBs to include the variable film thickness due to the roundness. Finite element method is used to solve the Reynolds equation for the pressure distribution. Reaction forces and friction torque are obtained by integrating the pressure and shear stress, respectively. The dynamic behavior is determined by solving the equations of a motion of a HDD spindle system in six degrees of freedom with the Runge-Kutta method to characterize the motion of a rotating part. This research shows that the roundness of a rotating shaft causes the excitation frequency with integer multiple of a rotating frequency.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2007.05a
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• pp.852-858
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• 2007

This paper investigates the dynamic characteristics of a tilted HDD spindle system with fluid dynamic bearings (FDBs). Tilting motion of a HDD spindle system may be caused by improper manufacturing tolerance, such as imperfect cylindricity between shaft and sleeve of FDBs, imperfect perpendicularity between shaft and thrust as well as the gyroscopic moment of the unbalanced mass of the rotating part. Tilting motion may result in the instability of the HDD spindle system and it may increase the disk run-out to limit memory capacity. This research proposes a modified Reynolds equation for the coupled journal and thrust FDBs to include the variable film thickness due to the cylindricity and the perpendicularity. Finite element method is used to solve the Reynolds equation for the pressure distribution. Reaction forces and friction torque are obtained by integrating the pressure and shear stress, respectively. The dynamic behavior is determined by solving the equations of a motion of a HDD spindle system in six degrees of freedom with the Runge-Kutta method to study whirling and tilting motions. This research shows that the cylindricity and the perpendicularity increase the tilting angle and whirl radius of the rotor.