• Journal of Advanced Marine Engineering and Technology
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• v.39 no.6
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• pp.614-619
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• 2015

In this study, a dynamics model was developed to predict the motion performance of an Autonomous Underwater Vehicle (AUV). The dynamics model includes basic dynamic state variables of the hull and force terms to determine the motion of the AUV. The affecting terms for the forces are hydrostatic force, added mass, hydrodynamic damping, lift and drag forces. The force terms can be calculated using analytical and Computational Fluid Dynamics methods. For the underwater motion simulation, a simple PD controller was used. Also, the AUV was tested in a water tank and near sea for the partial verification of the fluid drag force coefficients and way-point tracking motions.

• Journal of Ocean Engineering and Technology
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• v.10 no.1
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• pp.75-80
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• 1996

The importance of the impact force on the vertical offshore circular structure member in the surf zone due to the breaking wave has been recognized recently. In this paper characteristics of breaking wave forces and the corresponding estimation procedures for them are investigated. For the characterization of the wave forces, three parts, drag force, inertia force, impact force are categorized and identified, respectively. Among them the impact force is maimly studied and the concise form of the force is proposed with the application scheme for the design of offshore circular structure member. The resulting form porposed here for impact force is well coincided with former research results by other people. Except the impact force, so called Morison equation can be employed for the common offshore structure design. The drag force and inertia force are represented as convertionally for the profile except the breaking part. In the numerical example, for thpical sea condition and the member size, the proposed procedures for the breaking wave forces calculation are demonstrated. It is found that the impact force is the most deminant one comparing with inertia and drag forces in the surf zone.

• Proceedings of the KSME Conference
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• 2001.06e
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• pp.253-258
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• 2001

Control of drag force on a circular cylinder using a detached splitter plate is numerically studied for laminar flow. A splitter plate with the same length as the cylinder diameter(d) is placed horizontally in the wake region. Its position is described by the gap ratio(G/d), where G represents the gap between the cylinder base point and the leading edge of the plate. The drag varies with the gap ratio; it has the minimum value at a certain gap ratio for each Reynolds number. The drag sharply increases past the optimum gap ratio; this seems to be related to the sudden change in the bubble size in the wake region. This trend is consistent with the experimental observation currently available in case of turbulent flow. It is also found that the net drag coefficient significantly depends on the variation of base suction coefficient.

• Proceedings of the Korean Society of Marine Engineers Conference
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• 2001.05a
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• pp.129-134
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• 2001

The drag reduction of the cylinder having square dimpled surface was studied by the measuring the drag force acting on the cylinder. The level of the drag reduction was changed by the arrangement shape of the square grooves and Reynolds number. The drag of the cylinder was reduced about 28% with proper arrangement of square grooves. The flow field around the cylinder having grooves at the minimum drag was visualized by using post color ink in order to see the influence of the grooves. In this case, the separation points were sifted rearward and the wake region was smaller than that of the smooth cylinder.

• Journal of Advanced Marine Engineering and Technology
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• v.26 no.2
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• pp.233-239
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• 2002

The drag reduction of the cylinder having square dimpled surface was studied by the measuring the drag force acting on the cylinder. The level of the drag reduction was changed by the arrangement shape of the square grooves and Reynolds number. The drag of the cylinder was reduced about 28% with proper arrangement of square grooves. The flow field around the cylinder having grooves at the minimum drag was visualized by using post color ink in order to see the influence of the grooves. In this case, the separation points were silted rearward and the wake region was smaller than that of the smooth cylinder.

• Transactions of the Korean Society of Automotive Engineers
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• v.14 no.2
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• pp.194-201
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• 2006

Roof-fairing and deflector system have been used on heavy-duty trucks to minimize aerodynamic drag force not only for driving stability of the truck but also for energy saving by reducing the required driving power of the vehicle. In this study, a numerical simulation was carried out to see aerodynamic effect of the drag reducing device on the model vehicle. Drag and lift force generated on the five different models of the drag reducing system were calculated and compared them each other to see which type of device is efficient on the reduction of driving power of the vehicles quantitatively. An experiment has been done to see airflow characteristics on the model vehicles. Airflow patterns around the model vehicles were visualized by smoke generation method to compare the complexity of airflow around drag reducing device. From the results, the deflector systems(Model 5,6) were revealed as a better device for reduction of aerodynamic drag than the roof-fairing systems(Model 2,3,4) on the heavy-duty truck and it can be expected that over 10% of brake power of an engine can be saved on a tractor-trailer by the aerodynamic drag reducing device at normal speed range($80km/h{\sim}$).

• Journal of computational fluids engineering
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• v.7 no.3
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• pp.1-8
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• 2002

The counter jet flow which is injected against the free stream at stagnation region of blunt body for improvement of aerodynamic performance has been studied by using upwind Navier-Stokes method. The variations of drag force and upwind forward penetration depth due to changes in the stagnation thermodynamic properties of counter jet flow such as total pressure, Mach number, and total temperature have been studied. The results show that the changes in the stagnation pressure and Mach number have large effects on the wall pressure and drag force, but the total temperature does not affect the wall pressure and drag force.

• 유체기계공업학회:학술대회논문집
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• 2006.08a
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• pp.397-400
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• 2006

In this study, we experimentally investigate the possibility of skin-friction drag reduction by series of transverse cavities in a turbulent boundary layer flow. The effects of cavity depth (d), cavity length (l) and cavity spacing (s) on the skin friction drag are examined in the range of $Re_{\theta}\;=\;4030\;{\sim}\;7360$, $d/{\theta}_0\;=\;0.13\;{\sim}1.03$, l/d = 1 ~ 4 and s/d = 5 ~ 20. We perform experiments for twenty different cavity geometries and directly measure total drag force using in-house force measurement system. In most cases, the skin friction drag is increased. At several cases, however, small drag reduction is obtained. The variation of the skin ftiction drag is more sensitive to the cavity length than to the cavity depth or cavity spacing, and drag is reduced at $s/l\;{\geq}\;10$ and $l/{\theta}_0\;{\leq}\;0.26$ irrespective of the cavity depth. At $l/\bar{\theta}_0\;=\;0.13$ and s/l = 10, maximum 2% drag reduction is achieved. When the skin friction drag is reduced, there is little interaction between the flows inside and outside cavity, and the flow changed by the cavity is rapidly recovered at the following crest. A stable vortex is formed inside a cavity in the case of drag reduction. This vortex generates negative skin friction drag at the cavity bottom wall. Although there is form drag due to the cavity itself, total drag is reduced due to the negative skin friction drag.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.41 no.3
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• pp.171-181
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• 2017

Two-phase cross flow exists in many shell-and-tube heat exchangers such as condensers, evaporators, and nuclear steam generators. The drag force acting on a tube bundle subjected to air/water flow is evaluated experimentally. The cylinders subjected to two-phase flow are arranged in a normal square array. The ratio of pitch to diameter is 1.35, and the diameter of the cylinder is 18 mm. The drag force along the flow direction on the tube bundles is measured to calculate the drag coefficient and the two-phase damping ratio. The two-phase damping ratios, given by the analytical model for a homogeneous two-phase flow, are compared with experimental results. The correlation factor between the frictional pressure drop and the hydraulic drag coefficient is determined from the experimental results. The factor is used to calculate the drag force analytically. It is found that with an increase in the mass flux, the drag force, and the drag coefficients are close to the results given by the homogeneous model. The result shows that the damping ratio can be calculated using the homogeneous model for bubbly flow of sufficiently large mass flux.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.29 no.2
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• pp.149-159
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• 2016

In this study, the solitary wave-induced tsunami force acting on an onshore bridges in coastal area was numerically modelled by means of TWOPM-3D based on Navier-Stokes solver and VOF method which can track free surface effectively. The validity of numerical analysis was verified by comparing the experimental tsunami bore force acting on vertical wall and column structure. In particular, the characteristics of tsunami force with the changing tsunami intensity were surveyed through numerical experiments. The availability of 3-dimensional numerical analysis was reviewed through the comparison between the existing numerical results and design criteria for each drag force coefficient by applying Morison equation considering only drag force. As reasonable and high-precision estimation method of tsunami force, it was suggested to apply the estimation method taking drag and inertial force into consideration at the same time.