Liquid metal coolants have a significant role in the design of advanced fusion reactors. There is... more Liquid metal coolants have a significant role in the design of advanced fusion reactors. There is a need for an investigation of the thermal behavior of the liquid metal in working reactor environment, such as when fluid flow at low Prandtl number (Pr) with a buoyancy effect, is subjected to a magnetic field. In the present study, a direct numerical simulation (DNS) for a low Pr number fluid flow resulting in turbulent heat transfer with buoyancy effect under a magnetic field has been carried out between two vertical plates kept at different temperatures. In this simulation, the values of the Hartmann number (Ha) were 0 and 6, Pr number was 0.06 and Grashof numbers were 6.4 × 10 5 , 9.6 × 10 5 , and 1.6 × 10 6 . The turbulent quantities of the parameters such as the mean temperature, turbulent heat flux, and temperature variance were obtained by direct numerical simulation (DNS). The Reynolds number (Re) for channel flow based on friction velocity averaged by both walls, viscosity, and channel half-width was set to be constant as Re * = 150. A uniform magnetic field was applied in a direction perpendicular to the walls of the channel. The profiles of mean velocity and velocity fluctuations became asymmetric, and the tendency was enhanced with the increasing buoyancy effect. However, by the application of a magnetic field the tendency decreased. In other words, thermal transport between the walls became weak due to the magnetic effect.
International Journal of Heat and Mass Transfer, 2020
Make-to-order firms use different strategies, such as dynamic pricing and due date management, to... more Make-to-order firms use different strategies, such as dynamic pricing and due date management, to influence their performance. In these strategies, orders are segmented into classes based on their sensitivity to lead time and price. Quoting different prices and lead times to different classes of customer can increase a firm's profit and its capacity utilization. Most research in this area does not consider the effects of production constraints on price and lead time decisions. In this paper, we consider the role of flexibility in dynamically choosing the price, lead time and segmentation of customers in make-to-order environments with limited production capacity and multi-period horizon under a stochastic demand function. To reflect the dynamic variations of a system's conditions, we propose a Multi-stage Stochastic Programming (MSP) method to jointly determine prices, lead time and production. Furthermore, we assume that demand is a linear function of price, lead time and time. Through numerical analyses, we indicate the benefits of dynamic pricing and lead time decisions, based on different customer classes in various environments.
Suppressed wake transition and vortex lock-on phenomena in a perturbed flow past a circular cylin... more Suppressed wake transition and vortex lock-on phenomena in a perturbed flow past a circular cylinder at Re = 220 1 JUNG YUL YOO, SOO HYEON KIM, JOONG HUN BAE, Seoul National University -Direct numerical simulation (DNS) is performed to investigate the change of vortex structure and lock-on phenomena in the wake of a circular cylinder disturbed by sinusoidal perturbation at the Reynolds number of 220 (A-mode instability regime). The sinusoidal perturbation, of which the frequency is near twice the natural shedding frequency, is superimposed on the free stream velocity. The power spectra of the streamwise velocity signal and orthogonal enstrophy are investigated to analyze the change of vortex structure. It is observed that the wake transition behind the circular cylinder can be suppressed due to the sinusoidal perturbation. And this change causes the jump of the Strouhal number from the value corresponding to A-mode instability regime to the value corresponding to retarded wake transition regime (extrapolated from laminar shedding regime) in the Strouhal-Reynolds number relationship. As a result, the vortex shedding frequency is locked on the perturbation frequency depending not on the natural shedding frequency but on the modified shedding frequency.
Effects of large density variation on strongly heated internal air flows
Physics of Fluids, 2006
Direct numerical simulation (DNS) is employed to examine the effects of large density variation o... more Direct numerical simulation (DNS) is employed to examine the effects of large density variation of strongly heated air flowing in a vertical pipe on turbulent heat and momentum transfer. The predictions of the heat transfer and skin friction coefficients as well as the mean velocity and temperature profiles are in excellent agreement with the existing experimental data. Like some previous studies on heated air flows, the present study shows that the flow is laminarized with heat flux because a large reduction occurs in turbulence intensity. Unlike the velocity fluctuations, however, the thermal turbulence intensities such as the normalized density and temperature fluctuations remain as relatively insensitive to the heating conditions. Moreover, it is observed that the mean velocity and temperature profiles become dissimilar to each other during the process of laminarization with heating. In the present study we elucidate these anomalous behaviors of the strongly heated air flows wit...
Direct numerical simulation of heated CO2 flows at supercritical pressure in a vertical annulus at Re=8900
Physics of Fluids, 2008
The present study is concerned with thermal physics of the fluid at supercritical pressure (SCP) ... more The present study is concerned with thermal physics of the fluid at supercritical pressure (SCP) where many singular phenomena are observed in turbulent heat transfer due to severe property variations of the fluid. Direct numerical simulation is conducted for upward annular flows of CO2 at a pressure of 8MPa with a constant-heat-flux boundary condition imposed on the inner wall. All simulations are made at the inlet bulk Reynolds number of 8900 with particular attention being paid to the structure of the heated boundary layers at SCP. It is shown that most singular phenomena at SCP occur when the pseudocritical temperature arises between the heated wall and bulk fluid temperatures. The mean velocity profile near the heated wall shows no logarithmic distribution in the inertial subrange because a large reduction in the Reynolds shear stress occurs in the viscous region. The computational flow visualization reveals that alternating low- and high-speed streaks in the viscous region are...
Direct numerical simulation of turbulent supercritical flows with heat transfer
Physics of Fluids, 2005
Turbulent heat transfer to CO2 at supercritical pressure flowing in heated vertical tubes is inve... more Turbulent heat transfer to CO2 at supercritical pressure flowing in heated vertical tubes is investigated using direct numerical simulation at the inlet Reynolds number Re0=5400, which is based on inlet bulk velocity and tube diameter. Temperature range within the flow field covers the pseudocritical region, where very significant fluid property variations are involved. Both upward and downward flows are considered. The wall temperature distribution shows well-known heat transfer deterioration characterized by the localized peak in upward flows, while no such anomaly is observed in downward flows. The deterioration occurs at the region where turbulence is attenuated significantly, and is followed by the enhancement with restoration of turbulence caused by complicated interactions with a buoyancy effect. Further investigation of turbulence statistics indicates that ρux″ux″¯, ρux″ur″¯, and ρux″h″¯ are significantly affected by their respective buoyancy production terms due to ρ′ux′¯, ...
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Papers by Joong Hun Bae