• Korean Journal of Hydrosciences
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• 제1권
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• pp.49-60
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• 1990

Manning's roughness factor to flow in sand-bed channels may be divided into the grain roughness factor nd the form roughness factor. The grain roughness factor may be dedermined by using Keulegan's formula. By using available experimental data, it was found there is a unique relationship between the form roughness and the hydraulic radius to sediment particle size ratio for a given value of the Froude number. The form roughness and the bed form may be determined by using this unique relationship. The technique for engineering applications of the results appears to be quite simple.

• International Journal of Fluid Machinery and Systems
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• 제2권4호
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• pp.353-362
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• 2009

In the process of turbine modernizations, the investigation of the influences of water passage roughness on radial flow machine performance is crucial and validates the efficiency step up between reduced scale model and prototype. This study presents the specific losses per component of a Francis turbine, which are estimated by CFD simulation. Simulations are performed for different water passage surface roughness heights, which represents the equivalent sand grain roughness height. As a result, the boundary layer logarithmic velocity profile still exists for rough walls, but moves closer to the wall. Consequently, the wall friction depends not only on roughness height but also on its shape and distribution. The specific losses are determined by CFD numerical simulations for each component of the prototype, taking into account its own specific sand grain roughness height. The model efficiency step up between reduced scale model and prototype value is finally computed by the assessment of specific losses on prototype and by evaluating specific losses for a reduced scale model with smooth walls. Furthermore, surveys of rough walls of each component were performed during the geometry recovery on the prototype and comparisons are made with experimental data from the EPFL Laboratory for Hydraulic Machines reduced scale model measurements. This study underlines that if rough walls are considered, the CFD approach estimates well the local friction loss coefficient. It is clear that by considering sand grain roughness heights in CFD simulations, its forms a significant part of the global performance estimation. The availability of the efficiency field measurements provides an unique opportunity to assess the CFD method in view of a systematic approach for turbine modernization step up evaluation. Moreover, this paper states that CFD is a very promising tool for future evaluation of turbine performance transposition from the scale model to the prototype.

• 한국수자원학회논문집
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• 제46권12호
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• pp.1209-1220
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• 2013

본 연구에서는 내성천 하류 구간을 대상으로 검보정된 1차원 수치모형을 이용하여 흐름모의를 수행함으로써 추정된 구간 조도계수에 대해 분석하였다. 또한 실측 및 모의된 수리조건을 이용하여 향석 지점에서의 하상형태 예측을 수행하였으며 사립조도에 의한 흐름저항 계수 값을 고려하여 총 흐름저항 조도계수를 산정하였다. 수치모의에 의해 추정된 구간 조도계수와 사립조도 및 하상형태에 의한 흐름저항 계수를 추정한 결과 값을 상호 비교 분석하였으며 그 결과, 저수류 영역 흐름에서는 사립조도 및 하상형태에 의한 흐름저항 외에 식생, 만곡도, 사주 등의 기타 요인들에 의한 영향이 크게 반영되어 수치모의 상의 조도계수 값이 사립조도 및 하상형태에 의해 추정 가능한 조도계수 범위보다 크게 산정되는 것으로 나타났다. 그러나 $500m^3/s$ 이상의 천이구간 및 고수류 영역에서는 사립조도 및 하상형태 예측에 의한 조도계수 범위에 수치모의에 의해 검증된 Manning 조도계수가 포함되는 것으로 나타났다.

• 한국군사과학기술학회지
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• 제25권4호
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• pp.364-371
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• 2022

Surface state change of ammunition generated during the storage period increases the surface roughness and this affects the flight of ammunition, but there are no research results quantitatively indicating this. In this study, the drag force for each Mach number of howitzer shells was calculated through CFD to which the surface data of the howitzer shell was applied, and analysis of trajectory was performed using drag force values as an input of the 4th Runge-Kutta method, and the degree of decrease of the maximum range caused by the surface roughness of the howitzer shell was estimated. As a result, it was confirmed that the maximum range of howitzer shell with high surface roughness was 1.12 % shorter than that of howitzer shell without roughness. It was confirmed that the effect of surface roughness on the trajectory is not negligible.

• Geomechanics and Engineering
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• 제12권3호
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• pp.399-415
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• 2017

A series of direct shear tests were conducted to investigate the frictional properties of the interface between structures and the filling soil of Chongqing airport fourth stage expansion project. Two types of structures are investigated, one is low carbon steel and the other is the bedrock sampled from the site. The influence of soil water content, surface roughness and material types of structure were analyzed. The tests show that the interface friction and shear displacement curve has no softening stage and the curve shape is close to the Clough-Duncan hyperbola, while the soil is mainly shear contraction during testing. The interface frictional resistance and normal stress curve meets the Mohr-Coulomb criterion and the derived friction angle and frictional resistance of interface increase as surface roughness increases but is always lower than the internal friction angle and shear strength of soil respectively. When surface roughness is much larger than soil grain size, soil-structure interface is nearly shear surface in soil. In addition to the geometry of structural surface, the material types of structure also affects the performance of soil-structure interface. The wet interface frictional resistance will become lower than the natural one under specific conditions.

• 한국지반공학회:학술대회논문집
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• 한국지반공학회 2010년도 추계 학술발표회
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• pp.1182-1189
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• 2010

The mechanical behavior of a granular material is governed by the applying effective stresses and its skeletal structure which is considered to be the packing of particles giving overall density and degree of anisotropic. Factors that affect soil packing are the particle size, size distribution and shape, and the arrangement of grain contact. Soil particle size and shape are the most important factor, but difficult to quantify. In this study, 2D Fourier analysis is applied to quantify the shape of granular particles. Jumunjin sand was used in the experiment and particle images are captured using an optical microscope. The results showed that three lower order Fourier descriptor are closely related with roundness, sphericity of the granular particle. Also statistical approach is used to determine roundness, form factor, elongation ratio, roughness of Jumunjin sand.

• Journal of Mechanical Science and Technology
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• 제20권7호
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• pp.1077-1088
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• 2006

This paper describes a numerical investigation on the performance deteriorations of a low speed, single-stage axial turbine due to use of rough blades. Numerical calculations have been carried out with a commercial CFD code, CFX-Tascflow, by using a modified wall function to implement rough surfaces on the stator vane and rotor blade. To assess the stage performance variations corresponding to 5 equivalent sand-grain roughness heights from a transition ally rough regime to a fully rough regime, stage work coefficient and total to static efficiency were chosen. Numerical results showed that both work coefficient and stage efficiency reduced as roughness height increased. Higher surface roughness induced higher blade loading both on the stator and rotor which in turn resulted in higher deviation angles and corresponding work coefficient reductions. Although, deviation angle changes were small, a simple sensitivity analysis suggested that their contributions on work coefficient reductions were substantial. Higher profile loss coefficients were predicted by higher roughness heights, especially on the suction surface of the stator and rotor. Furthermore sensitivity analysis similar to the above, suggested that additional profile loss generations due to roughness were accountable for efficiency reductions.

• 자원환경지질
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• 제39권6호
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• pp.719-738
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• 2006

표층퇴적물의 특성과 멀티빔 후방산란 음압의 상관관계를 알아보기 위하여 사퇴가 발달되어 있는 황해 동부해역에서 Simrad EM950 멀티빔 수심기와 그랩을 이용하여 후방산란 음압 자료와 표층퇴적물 시료를 취득하였다. 연구지역은 분급이 양호한 세립사로 덮여있는 서쪽 사퇴 지역과 분급이 불량하고, 자갈과 패각편을 포함하는 중사가 우세하게 덮여 있는 동쪽의 사퇴가 없는 지역으로 나뉜다. 두 지역의 입도차이는 두드러지지 않으나, 후방산란 음압의 변화는 뚜렷한 차이를 보인다. 연구지역의 멀티빔 후방산란 음압은 표층 퇴적물의 입도와 높은 상관관계를 보일 뿐만 아니라 탄산염 함량과도 깊은 관계를 갖는 것으로 나타났다. 상대적으로 높은 탄산염 패각편의 함량은 해저면의 거칠기와 입도의 증가를 야기 시킨다. 동쪽의 사퇴가 없는 지역에서 높게 나타나는 후방산란 음압 값은 조립한 입도와 상대적으로 높은 탄산염 패각편 함량의 영향을 반영하는 것으로 여겨진다.

• 유체기계공업학회:학술대회논문집
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• 유체기계공업학회 2005년도 연구개발 발표회 논문집
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• pp.405-412
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• 2005

Numerical and experimental investigations were conducted to assess the aerodynamic performance of several centrifugal compressors. In order to impose an appropriate physics at the interface between impeller and vaned diffuser numerically, two different techniques, frozen rotor and stage models, were applied and the simulation results were compared with the corresponding prototype test data. An equivalent sand-grain roughness height was utilized in the present computational study to consider a relative surface roughness effect on the stage performance simulated. From a series of investigations, it was found that the stage model is more suitable than the frozen rotor scheme for the steady interactions between impeller and diffuser in turbocompressor applications. It is supposed that the solution by frozen rotor scheme is inclined to overrate the non-uniformity of the flow fields. The predicted aerodynamic performance accounting for surface roughness effect shows favorable agreement with experimental data. Simulations based on the aerodynamically smooth surface assumption tend to overestimate the stage performance.

• 한국음향학회지
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• 제22권8호
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• pp.652-659
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• 2003

사질로 이루어진 평탄한 경계면에서 수평입사각 (25°, 40°, 65°, 80°)에 따른 고주파(40∼120 kHz) 반사손실 측정 실험을 수조 내에서 수행하였다. 5×5×5 m 크기의 수조는 바닥을 두께가 0,5m 이고 평균입도가 0.5 ø인 사질 퇴적물로 채웠으며 퇴적층 경계면은 평탄하게 조성하였다. 측정된 주파수별 수평입사각에 따른 반사손실은 고주파 해저면 반사손실 모델인 APL-UW 모델 (Mourad & Jackson, 1989)과 비교하였다. 60 kHz 이하 주파수의 경우 모델과 실측치가 거의 일치하였으나 70 kHz 이상의 경우에는 주파수가 증가함에 따라 2∼3dB씩 증가하는 결과를 보였다. 70 kHz 이상의 경우 모델과 실측치 간의 차이는 모델에서 다루지 않는 거칠기 (입도)의 수직 크기 때문이며 고주파로 갈수록 산란이론의 레일리 인자 (Rayleigh parameter)의 값이 증가하여 거칠기에 의한 산란효과를 포함하기 때문이다. 따라서 평탄한 해저면일지라도 사질과 수층으로 이루어진 경계면에서의 반사손실모델은 입도분포의 신뢰구간 내에서 갖는 거칠기 영향에 의한 주파수의 종속성을 고려하여야한다.