• Ocean Systems Engineering
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• v.3 no.3
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• pp.167-180
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• 2013

Hydrodynamic characteristics of a bluff cylinder oscillating along transverse direction in steady flow were experimentally investigated at Reynolds number of $2{\times}10^5$. The effects of non-dimensional frequency, oscillating amplitude and Reynolds number on drag force, lift force and phase angle are studied. Vortex shedding mechanics is applied to explain the experimental results. The results show that explicit similarities exist for hydrodynamic characteristics of an oscillating cylinder in high and low Reynolds number within subcritical regime. Consequently, it is reasonable to utilize the test data at low Reynolds number to predict vortex induced vibration of risers in real sea state when the Reynolds numbers are in the same regime.

• International Journal of Fluid Machinery and Systems
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• v.1 no.1
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• pp.101-108
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• 2008

The effect of Reynolds number on the performance of a regenerative pump was examined in a low Reynolds number range in experiment. The head of the regenerative pump increased at low flow rates and decreased at high flow rates as the Reynolds number decreased. The computation of the internal flow was made to clarify the cause of the Reynolds number effect. At low flow rates, the head is decreased with increasing the Reynolds number due to the decrease of the shear force exerted by the impeller caused by the increase of leakage and hence local flow rate. At higher flow rates, the head is increased with increasing the Reynolds number with decreased loss at the inlet and outlet as well as the decreased shear stress on the casing wall.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.14 no.4
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• pp.92-99
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• 2015

The deformation of injection molding is seriously affected by injection molding conditions, such as melt and mold temperature and injection and holding pressure. In these conditions, the mold temperature is controlled by flowing coolant, which can be classified by the Reynolds number in the mold-cooling channel. In this study, the deformation of the automotive side molding according to the variation of the Reynolds number in the coolant was simulated by Moldflow. In the results, as the Reynolds number was increased, the mold cooling was also increased. However, when the Reynolds number exceeded a certain range, the mold cooling was not increased further. In addition to the Moldflow verification, the mold cooling by the coolant was simulated by CFX. The CFX results confirmed that the Reynolds number significantly influenced the mold cooling. The coolant, which has a high Reynolds number value, quickly cooled the mold. However, the coolant, which has a low Reynolds number value, such as 0 points, hardly cooled the mold. In an injection molding experiment, as the Reynolds number was high, the deformation of the moldings was reduced. The declining tendency of the deformation was similar to the Moldflow results.

• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.10
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• pp.1940-1954
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• 1992

Fine grid computations were attempted to analyze the turbulent flows in the near wall low Reynolds number region and the numerical analyses were incorporated by a finite-volume discretization with full find grid system and low Reynolds number k-.epsilon. model was employed in this region. For the improvement of low Reynolds number k-.epsilon. model, modification coefficient of eddy viscosity $f_{\mu}$ was derived as a function of turbulent Reynolds number $R_{+}$ and nondimensional length $y^{+}$ from the concept of two length scales of dissipation rate of turbulent kinetic energy. The modification coefficient $f_{\epsilon}$ in .epsilon. transport equation was also derived theoretically. In the turbulent kinetic energy equation, pressure diffusion term was added in order to consider low Reynolds number region effect. The main characteristics of this low Reynolds number k-.epsilon. model were founded as : (1) In high Reynolds number region, the present model has limiting behavior which approaches to the high Reynolds number model. (2) Present low Reynolds number k-.epsilon. model dose not need additional empirical constants for the transport equations of turbulent kinetic energy and dissipation of turbulent kinetic energy in order to consider wall effect. Present low Reynolds number turbulence model was tested in the pipe flow and obtained improved results in velocity profiles and Reynolds stress distributions compared with those from other k-.epsilon. models.s.s.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2008.03a
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• pp.195-201
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• 2008

In the jet engines on the aircrafts cruising at high altitude over 20 km and subsonic speed, the Reynolds number in terms of the compressor blades becomes very low. In such an operating condition with low Reynolds number, it is widely reported that total pressure loss of the air flow through the compressor cascades increases dramatically due to separation of the boundary layer and the secondary-flow. But the detail of flow mechanisms causes the total pressure loss has not been fully understood yet. In the present study, two series of numerical investigations were conducted to study the effects of Reynolds number on the aerodynamic characteristics of compressor cascades. At first, the incompressible flow fields in the two-dimensional compressor cascade composed of C4 airfoils were numerically simulated with various values of Reynolds number. Compared with the corresponding experimental data, the numerically estimated trend of total pressure loss as a function of Reynolds number showed good agreement with that of experiment. From the visualized numerical results, the thickness of boundary layer and wake were found to increase with the decrease of Reynolds number. Especially at very low Reynolds number, the separation of boundary layer and vortex shedding were observed. The other series, as the preparatory investigation, the flow fields in the transonic compressor, NASA Rotor 37, were simulated under the several conditions, which corresponded to the operation at sea level static and at 10 km of altitude with low density and temperature. It was found that, in the case of operation at high altitude, the separation region on the blade surface became lager, and that the radial and reverse flow around the trailing edge become stronger than those under sea level static condition.

• Journal of KSNVE
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• v.7 no.5
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• pp.861-869
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• 1997

An approximate analytical method has been developed for estimating hydrodynamic forces acting on oscillating inner cylinder in concentric annulus. When the rigid inner cylinder executes translational oscillation, fluid inertia and damping forces on the oscillating cylinder are generated by unsteady pressure and viscous skin friction. Considering the dynamic-characteristics of unsteady viscous flow and the added mass coefficient of inviscid fluid, these hydrodynamic forces including viscous effect are dramatically simplified and expressed in terms of oscillatory Reynolds number and the geometry of annular configuration. Thus, the viscous effect on the forces can be estimated very easily compared to an existing theory. The forces are calculated by two models developed for relatively high and low oscillatory Reynolds numbers. The model for low oscillatory Reynolds number is suitable for relatively high ratio of the penetration depth to annular space while the model for high oscillatory Reynolds number is applicable to the case of relatively low ratio. It is found that the transient ratio between two models is approximately 0.2~0.25 and the forcea are expressed in terms of oscillatory Reynolds number, explicity. The present results show good agreements with an existing numerical results, especially for high and low penetration ratios to annular gap.

• Journal of computational fluids engineering
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• v.18 no.2
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• pp.1-8
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• 2013

In nuclear power plant, the reactor cooling system has maintained high-Reynolds-number flow above 1E+07 to cool a heat generated by the reactor. To minimize uncertainty for flow calibration, it is necessary to simulate the high Reynolds' number flow. Y-connection is selected to connect four (4) parallel high flow circulation pumps for maintaining the high flow rate. This paper describes the characteristics for Y-connection by computer flow simulation. It was confirmed through the results that the pressure loss of the Y-connection was lower than that of T-connection. Also as the connection angle of Y-connection was small, as the pressure loss was low.

• Transactions of the Korean Society of Mechanical Engineers
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• v.17 no.6
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• pp.1596-1608
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• 1993

Low Reynolds number second moment turbulence model which be applicable to the fine gird near the wall region was developed. In this model, turbulence model coefficients in the pressure strain model of the Reynolds stress equation was expressed as functions of turbulence Reynolds number $R_{t}\equivk^{2}/(\nu\varepsilon)).$ In the derivation procedure of the present low Reynolds number algebraic stress model, Laufer's near wall experimental data on Reynolds stresses were curve fitted as functions of R$_{t}$ and the resulting simultaneous equations of the model coefficients were solved by using the boundary conditions at wall and high Reynolds number limiting conditions. Predicted Reynolds stresses and dissipation rate of turbulent kinetic energy etc. in the 2 dimensional parallel, plane channel flow and pipe flow were compared with the preditions obtained by employing the Launder-Shima model, standard algebraic stress model and several experimental data. Results show that all the Reynolds stresses and dissipation rate of turbulent kinetic energy predicted by the present low Reynolds number algebraic stress model agree better with the experimental data than those predicted by other algebraic stress models.

• Journal of Power System Engineering
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• v.13 no.5
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• pp.11-17
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• 2009

A numerical study of unsteady mixed convection in a cavity with high viscous fluid is presented. Finite volume method was employed for the discretization and PISO algorithm was used for calculating pressure term. The parameters governing the problem are the Rayleigh number ($10^3\;{\leq}\;Ra\;{\leq}\;10^5$), the Reynolds number (0 < Re $\leq$ 1), and the aspect ratio (0.5 $\leq$ AR $\leq$ 2). The fluid used is silicon oil, a high prandtl number fluid, Pr = 909.1. The results show velocity vectors and temperature distributions. It is found that the periodic flows in a cavity are observed at very low Reynolds numbers, and the period of periodic flow decreases with increasing Reynolds and Rayleigh numbers, and increases with increasing aspect ratio. Also, the Reynolds number range of periodic flow increases with increasing Rayleigh numbers and aspect ratio.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2008.03a
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• pp.202-212
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• 2008

A three-dimensional computation was conducted to understand effects of the low Reynolds number on the loss characteristics in a transonic axial compressor, Rotor67. As a gas turbine becomes smaller in size and it is operated at high altitude, the operating condition frequently lies at low Reynolds number. It is generally known that wall boundary layers are thickened and a large separation occurs on the blade surface in axial turbomachinery as the Reynolds number decreases. In this study, it was found that the large viscosity did not affect on the bow shock at the leading edge but significantly did on the location and the intensity of the passage shock. The passage shock moved upstream towards leading edge and its intensity decreased at the low Reynolds number. This change had large effects on the performance as well as the internal flows such as the pressure distribution on the blade surface, tip leakage flow and separation. The total pressure rise and the adiabatic efficiency decreased about 3% individually at the same normalized mass flow rate at the low Reynolds number. In order to analyze this performance drop caused by the low Reynolds number, the total pressure loss was scrutinized through major loss categories such as profile loss, tip leakage loss, endwall loss and shock loss.