• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.24 no.3
• /
• pp.467-476
• /
• 2000

An unsteady numerical simulation was performed for locally-forced separated and reattaching flow over a backward-facing step. The local forcing was given to the separated and reattaching flow by means of a sinusoidally oscillating jet from a separation line. A version of the $k-{\varepsilon}-f_{\mu}$ model was employed, in which the near-wall behavior without reference to distance and the nonequilibrium effect in the recirculation region were incorporated. The Reynolds number based on the step height (H) was fixed at $Re_H=33000$, and the forcing frequency was varied in the range $0{\leq}St_H{\leq}2$. The predicted results were compared and validated with the experimental data of Chun and Sung. It was shown that the unsteady locally-forced separated and reattaching flows are predicted reasonably well with the $k-{\varepsilon}-f_{\mu}$ model. To characterize the large-scale vortex evolution due to the local forcing, numerical flow visualizations were carried out.

• Journal of Advanced Marine Engineering and Technology
• /
• v.30 no.5
• /
• pp.597-607
• /
• 2006

This paper presents a numerical simulation of the behavior of the LiBr solution droplets and falling films in horizontal tube banks of absorber. The model developed here accounts for the details of the droplets formation and impact process for absorption on horizontal tubes including the heat transfer from solution film to the tube wall. Especially. the characteristic of unsteady behavior of solution flow has been investigated. Flow visualization studies shown that the solution droplets and falling films have some of the complex characteristics. It is found that. with the numerical conditions similar to the operating condition of an actual absorption chiller/heater, the outlet solution temperature and heat flux from solution film to the tube wall have a stable periodic behavior with time. The solution droplets and falling films in horizontal tube banks of absorber is a periodic unsteady flow. The results from this model are compared with previous experimental observation taken with a high-speed digital video camera and shown good agreement.

• Proceedings of the Korea Committee for Ocean Resources and Engineering Conference
• /
• 2003.10a
• /
• pp.198-203
• /
• 2003

Unsteady two-dimensional nonlinear waves which are generated by the numerical wavemaker of plunging type are numerically simulated using a finite difference method in rectangular grid system. Two-dimensional Navier-Stokes equations and the continuity equation are used for the computations. Irregular leg lengths and stars are employed near the boundaries of body and free surface to satisfy the boundary conditions. Marker-density function method is adopted for the simulation of wave breaking phenomena, and the computations are carried out with various wave amplitudes and two section shapes of wavemaker. The computation results are compared with other existing computational and experimental results, and the agreement between the experimental data and the computation results is good.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.20 no.9
• /
• pp.2973-2980
• /
• 1996

Secondary instability in an obstructed channel is investigated using direct numerical simulation. Flow geometry under consideration is a plane channel with two-dimensional thin obstacles mounted symmetrically in the vertical direction and periodically in the streamwise direction. Flow separation occurs at the tip of the sharp obstacles. As a basic flow, we consider an unsteady periodic solution which results from Hopf bifurcation. Depending on the Reynolds number, the basic flow becomes unstable to three-dimensional disturbances, which results in a chaotic flow. Numerical results obtained are consistent with experimental findings currently available.

• Journal of the Korean Society of Propulsion Engineers
• /
• v.1 no.1
• /
• pp.8-23
• /
• 1997

Steady and unsteady numerical simulations are conducted for the comparison with the experiments performed to investigate the ram accelerator flow field by using an expansion tube facility in Stanford University. Wavier-Stokes equations for chemically reacting flows are analyzed by fully implicit and time accurate numerical methods with Jachimowski's detailed chemistry model for hydrogen-air combustion involving 9 species and 19 reaction steps. Although the steady state numerical simulation shows a good agreement with the experimental schlieren and OH PLIF images for the case of $2H_2$＋$O_2$＋$17N_2$ fails in reproducing the combustion region behind the shock intersection point shown in the case of $2H_2$＋$O_2$＋$12N_2$ mixture. Therefore, an unsteady numerical simulation is conducted for this case and the result shows all the detailed flow stabilization process. From the result of unsteady numerical simulation, the experimental result seems to be an instantaneous state during the flow stabilization process. The combustion behind the shock intersection point is the result of a normal detonation formed by the intersection of strong oblique shocks that exist at early stage of the stabilization process. At final stage, the combustion region behind the shock intersection point disappears and the steady state result is retained. The time required for stabilization of the reacting flow in the model ram accelerator is found to be very long in comparison with the experimental test time.

• 한국연소학회:학술대회논문집
• /
• 2005.10a
• /
• pp.294-301
• /
• 2005

The instabilities in rocket engines and gas turbine combustors due to the interaction between the fluid flow (acoustics) and the heat transfer (thermal energy) are called thermoacoustic or combustion instabilities. Almost all analysis assumes constant hot section temperature for Modern mathematical analysis of acoustic oscillations in Rijke type devices. However, it is impossible to predict whether a system is stable or not because the flame or heater response model can have a dramatic effect on predicted growth rates. In this study, A standard ${\kappa}-{\varepsilon}$ turbulent model and hybrid combustion model(eddy breakup model and chemical reaction) were used. After steady solution was gotten, unsteady calculation is simulated by perturbating on pressure boundary. As a result, we obtained the relationship of equivalence ratio and frequency by numerical simulation, and they are comparable to the experimental result. In addition, in spite of these results, there are limitations of using turbulent and combustion model in simulation method of thermoacoutic instability

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.27 no.10
• /
• pp.1401-1411
• /
• 2003

A two-dimensional direct numerical simulation was performed to investigate the flame structure of CH$_4$$N_2$-air counterflow nonpremixed flame interacting with a single vortex. The detailed transport properties and a modified 16-step augmented reduced mechanism based on Miller and Bowman's detailed chemistry were adopted in this simulation. The characteristic vortex and chemical time scales were introduced to quantify and investigate the extinction phenomenon during a flame-vortex interaction. The results showed that fuel- and air-side vortex cause an unsteady extinction. In this case, the flame interacting with a vortex was extinguished at much larger scalar dissipation rate than steady flame. It was also found that the air-side vortex extinguished a flame more rapidly than the fuel-side vortex. Furthermore, it was noted that the degree of unsteady effect experienced by a flame can be investigated by comparing the above two characteristic time scales, and this analysis could give an appropriate reason for the results of the previously reported experiment.

• Journal of the Korean Society of Safety
• /
• v.33 no.2
• /
• pp.132-137
• /
• 2018

Compared with general rivers, fluctuations of the water level and the river bed are severe in the tidal river. In hydro-dynamic aspect, such fluctuation gives different river-bed data to us according to observing period. The time-dependent river-bed data and pre-estimation of the Manning's roughness coefficient which is the key factor of numerical modelling induces uncertainty of evaluating the design flood level. Thus it is necessary to pay more attention to calculate the flood level at tidal rivers than at general rivers. In this study, downstream of the Imjin River where is affected by tide of the West Sea selected as a study site. From the numerical modelling, it was shown that the unsteady simulation gave considerable mitigation of the water level from the starting point to 15 km upstream compared to the steady simulation. Either making a detention pond or optional dredging was not effective to mitigate the flood level at Gugok - Majung region where is located in the downstream of the Imjin River. Therefore, a more sophisticated approach is required to evaluate the design flood level estimation before constructive measures adopted in general rivers when establishing the flood control plan in a tidal river.

• KSCE Journal of Civil and Environmental Engineering Research
• /
• v.44 no.3
• /
• pp.329-336
• /
• 2024

In the case of multi-function weirs installed in Korea, the free hydraulic jump or the submerged hydraulic jump is occurred depending on the height of the gate opening and the tailwater level when the sluice gate of the movable weir is partially opened. In this study, the submerged hydraulic jump for the flows under the sluice gate were simulated and the mean flow, turbulence statistics, and relative water depth are investigated using numerical simulation. For numerical simulation, the unsteady Reynolds-averaged Navier-Stokes equation, volume of fluid method, and k-ωSST turbulence model were used. The numerical model was validated using the results of other researchers' previously performed experiments, and it was investigated that the numerical model appropriately simulates the two-phase flow in the hydraulic jump. In addition, the distribution of mean flow, turbulence statistics, and the length of recirculation region was investigated.

• Proceedings of the KSME Conference
• /
• 2003.11a
• /
• pp.706-711
• /
• 2003

A numerical method is presented for computing unsteady incompressible two-phase flows with immersed solids. The method is based on a level set technique for capturing the phase interface, which is modified to satisfy a contact angle condition at the solid-fluid interface as well as to achieve mass conservation during the whole calculation procedure. The modified level set method is applied for numerical simulation of bubble deformation in a micro channel with a cylindrical solid block and liquid jet from a micro nozzle.