• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.49 no.6
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• pp.457-464
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• 2021

To analyze the buffet phenomenon that causes serious vibration loads on a satellite launch vehicle, the pressure fluctuations on a hammerhead launch vehicle at transonic speeds are predicted by coupling CFD analysis and semi-empirical methods. From the RANS simulation, shock oscillation region, separation region, and separation reattachment region are identified, and the boundary layer thickness, the displacement thickness, and flow properties at boundary layer edge are calculated. The pressure fluctuations and power spectra on the hammerhead fairing are predicted by coupling RANS results and semi-empirical methods considering spatial distribution, and compared with the experimental data.

• Transactions of the Korean Society of Mechanical Engineers
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• v.14 no.5
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• pp.1300-1310
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• 1990

Naphthalene sublimation technique is employed to investigate the mass transfer processes from a square cylinder at various Reynolds numbers and various angles of attack. Distribution of the local mass transfer coefficients on each face of the cylinder changes dramatically with the angle of attack. Such variation of local mass transfer rates closely connected with the complex flow phenomena such as stagnation, acceleration, separation, reattachment and vortex shedding. The average Sherwood number has a minimum value at 12.deg.-13.deg., and a maximum value at a=20.deg.-25.deg. A comparison of present mass transfer measurement with other heat transfer measurements, using the heat/mass transfer analogy, shows good agreement in average transfer rates, same trend but notable differences in local values. Therefore, naphthalene sublimation technique can be adopted to explore heat transfer processes in the complex flow situations, which is considered to be hardly possible with the conventional heat transfer measurements.

• Proceedings of KOSOMES biannual meeting
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• 2005.11a
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• pp.153-158
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• 2005

The turbulent shear flaw around a surface-mounted vertical wall was investigated using the two-frame PIV(CACTUS 3.1) system. From this study, it is revealed that at least 500 instantaneous velocity field data are required for ensemble average to get reliable turbulence statistics, but only 200 field data are sufficient for the time-averaged mean velocity information The flow has an unsteady recirculation region post vertical wall with bottom gap, followed by a slow relaxation to the fiat-plate boundary layer flow. The time-averaged reattachment length estimated from the streamline distribution is about x/H=3H. The large eddy structure in the separated shear layer seems to have signification influence on the development of the separated shear layer and the reattachment process.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2002.04a
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• pp.35-40
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• 2002

A numerical analysis based on two-dimensional and three-dimensional incompressible Navier-Stokes equations has been carried out for double-circular-arc compressor cascades and the results are compared with available experimental data at various incidence angles. The 2-D and 3-D computational codes based on SIMPLE algorithm adopt pressure weighted interpolation method for non-staggered grid and hybrid scheme for the convertive terms. Turbulence modeling is very important for prediction of cascade flows, which are extremely complex with separation and reattachment by adverse pressure gradient. In this paper k-$\varepsilon$ turbulence model with wall function is used to increase efficiency of computation times.

• Journal of the Korean Society of Propulsion Engineers
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• v.6 no.3
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• pp.29-36
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• 2002

A numerical analysis based on two-dimensional and three-dimensional incompressible Wavier-Stokes equations has been carried out for double-circular-arc compressor cascades and the results are compared with available experimental data at various incidence angles. The 2-D and 3-D computational codes based on SIMPLE algorithm adopt pressure weighted interpolation method for non-staggered grid and hybrid scheme for the convective terms. Turbulence modeling is very important for prediction of cascade flows, which are extremely complex with separation and reattachment by adverse pressure gradient. Considering computation times, $\kappa$-$\varepsilon$ turbulence model with wall function is used.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.44 no.2
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• pp.99-107
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• 2016

Test research on Infra-Red Thermography(IRT) technique in a supersonic wind tunnel has been conducted. Inadvertent technical difficulties and their solutions associated with the technique in running of the facility were examined. Two flow conditions at Mach number of 3 and 4 were considered. A double compression ramp model, that replicates realistic high-speed vehicle configuration, was used as test model. The present IR data were compared with shadowgraph visualization images and laminar computational fluid dynamics(CFD) results. It has been shown that the IRT technique can be used in quantifying various fluid dynamic features such as flow transition, separation and three-dimensional phenomena around the double compression ramp model.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.49 no.1
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• pp.41-52
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• 2021

During atmospheric ascent of a launch vehicle, airborne acoustic loads act on the vehicle and its effect becomes pronounced at transonic speed. In the present study, acoustic loads acting on a hammerhead launch vehicle at a transonic speed have been analyzed using ��-ω SST based IDDES and the results including mean Cp, Cprms, and PSD are compared to available wind-tunnel test data. Mesh dependency of IDDES results has been investigated and it has been concluded that with an appropriate turbulence scale-resolving computational mesh, the characteristic flow features around a transonic hammerhead launch vehicle such as separated shear flow at fairing shoulder and its reattachment on rear body as well as large pressure fluctuation in the region of separated flow behind the boat-tail can be predicted with reasonable accuracy for engineering purposes.

• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.4
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• pp.775-786
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• 1992

Studies are made of the turbulent separation bubble in a two-dimensional semi-infinite blunt plate aligned to a uniform free stream when the oncoming free stream contains a pulsating component. The discrete-vortex method is applied to simulate this flow situations because this approach is effective to represent the unsteady motions of turbulent shear layer and the effect of viscosity near the solid surface. The two key external paramenters in the free stream, i.e., the amplitude of pulsation, A, and the frequency parameter St[=fH/ $U_{1}$], are dealt with in the present numerical computations, A particular frequency gives a minimum reattachment which is related to the drag reduction and the most effective frequency is dependent on the most amplified shedding frequency. The turbulent flow structure is scrutinized. A comparison between the unperturbed flow and the perturbed at the particular frequency of the minimum reattachment length of the separation bubble suggests that the large-scale structure is associated with the shedding frequency and the flow instabilities.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.26 no.12
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• pp.1674-1680
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• 2002

An experimental study was made of a large-scale vortical structure over a backward-facing step. The Reynolds number based on the step height was R $e_{H}$ =33,000. To recognize the large-scale vortex, three components of velocity were measured. The measurements were performed in the recirculation zone (x/H=4.0) and the reattachment zone(x/H=7.5). To measure the wall pressure fluctuations in a turbulent flow over a backward-facing step, a 32-channel microphone array was installed beneath the wall in the streamwise and spanwise directions. From the measured pressure field, the size of large-scale vortex was obtained. As a detailed study, a conditionally-averaging technique was employed to characterize the coherent structure of the large-scale vortex. To see the relationship between the flow field and the relevant spatial mode of the pressure field, the spatial box filtering (SBF) was examined. A cross-correlation between velocity and pressure fluctuations was performed to identify the structure and the length scale of the large-scale vortex.x.

• 한국전산유체공학회:학술대회논문집
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• 1995.10a
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• pp.37-42
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• 1995

In the present study, an improved version of 4-equation low-Reynolds-number 4-equation model is proposed. The equations of the temperature variance ($k_{\theta}$) and its dissipation rate(${\varepsilon}_{\theta}$) are solved, in concert with the equations of the turbulent kinetic energy(k) and its dissipation rate(${\varepsilon}$). In the present model, the near-wall effect and the non-equilibrium effect are fully taken into consideration. The validation of the model is then applied to the turbulent flow behind a backward-facing step and the flow over a blunt body. The predicted results of the present model are compared and evaluated with the relevant experiments.