Submerged jets in short cylindrical flow vessels

On different numerical inverse Laplace methods for solute transport problems

In this paper, a two-dimensional axisymmetric numerical simulation model was developed for optimization of double (coaxial) tube vertical ground heat exchangers (GHEs) in cooling mode. Details of the heat transfer rates and pressure drops for each model are presented and analyzed. The results of the numerical study of optimization of double tube vertical GHEs have been done by considering heat transfer rates and pressure drops. The effect of different inlet and outlet tube diameters, and mass flow rates were numerically investigated. Effect of the different materials on heat transfer and longtime operation also discussed. The double tube vertical GHEs are more effective in laminar flow condition considering balance between heat transfer and pressure drop. The results indicate that since in laminar flow condition, pressure drop is not significantly high, it is possible to reduce the inlet and outlet diameter of double tube GHEs if double tube GHEs operate in laminar flow condition. The heat transfer rate decreased only 17% but diameter of the inlet tube can be reduced from 130 mm to 40 mm with fixed outlet diameter 20 mm. Heat transfer rate can also be enhanced by reducing the outlet tube diameter for a fixed inlet tube diameter. Long time operation suggested the possibility of installation of multiple double tube GHE at 2.0 m apart.

Fouling and fouling mitigation on heated metal surfaces

Staggered circular cylinders immersed in a uniform planar shear flow

An experimental study was conducted on the flow of aqueous solutions of detergent through mesh screens to mimic cloth washing. Pressure losses across the mesh screens were measured for water, dilute polymer and several aqueous detergent solutions. A reduction of pressure losses was observed for the flow of aqueous solutions of low molecular weight surfactants such as Laurylether (AE), Laurylbenzene-sulfonic acid-sodiumsalt (LAS), Benzalkonium-chloride (BC) Sodium-dodecyl-sulfate (SDS), and Hexadecyltrimetyl-ammonium-bromide (CTAB), but not for the high molecular weight polymers like Polyethylene-oxide (PEO18) and Polyacrylamide (PAA), through mesh screens. A flow visualization experiment was carried out to observe the flow pattern upstream and downstream of the mesh screen. Photographic images revealed that, instead of an expected large converging flow from the upstream section into the screen opening as in orifice flow, the bulk of the liquid entering the screen aperture took the form of a liquid column of similar diameter as the inlet tube. Based on this observation, a flow model, which led to a new set of definitions of Reynolds number and drag coefficient, was proposed. Good correlations of drag coefficient and Reynolds number were obtained for all test solutions, and the drag reduction phenomenon was manifested for detergent aqueous solutions.

Propulsive capability of manta rays' flapping pectoral fins has inspired many to incorporate these fins as propulsive mechanisms for autonomous underwater vehicles. In particular, geometrical factors such as sweep angle have been postulated as being influential to these fins' propulsive capability, specifically their thrust generation. Although effects of sweep angle on static/flapping wings of aircrafts/drones have been widely studied, little has been done for underwater conditions. Furthermore, the findings from air studies may not be relatable to the underwater studies on pectoral fins because of the different Reynolds number (compared to the flapping wings) and force generation mechanism (compared to the static wings). This paper aims to establish a relationship between the sweep angle and thrust generation. An experiment was conducted to measure the thrust generated by 40 fins in a water channel under freestream and still water conditions for chord Reynolds number between 2.2 × 104 and 8.2 × 104. The fins were of five different sweep angles (0 deg, 10 deg, 20 deg, 30 deg, and 40 deg) that were incorporated into eight base designs of different flexibility characteristics. The results showed that the sweep angle (within the range considered) may have no significant influence on these fins' thrust generation, implying no significant effects on thrust under uniform flow condition and on the maximum possible thrust under still water. Overall, it can be concluded that sweep angle may not be a determinant of thrust generation for flapping pectoral fins. This knowledge can ease the decision-making process of design of robots propeled by these fins.

It is difficult to predict the gas transfer performance in oxygenators theoretically. Because there have not been a method which could calculate nonlinear blood-gas reactions and dealt blood and gas layer simultaneously. We have been trying to calculate blood-gas transfer with a CFD method involving nonlinear hemoglobin-gas reactions. In this study, we analyzed distribution of gaseous concentrations and gas transfer performance of a partial oxygenator model. Referenced whole oxy-genator has a rectangular formed hollow fiber bundle. Partial oxygenator model had a part of blood flow section and the same length of primary blood flow direction (30 mm) of whole oxygenator's fiber bundle, and was made of a blood layer, gas layers and staggered cylindrical hollow fibers (225 μm outer diameter). Our method adopted O2 and CO2 reaction models derived on nonlinear relationship equations for source terms in mass transfer equations between each concentration and reaction source. In a result, a state of nonlinear distributions could be observed under Reynolds number=3.9. If a blood flow section of this model converted into that of whole oxygenator in condition of uniform blood flow, 0, and CO, transfer rates were 71.1 mL/min and 80.0 mL/min, respectively at 1 UL/min gas flow, and 71.1 mL/min and 83.7 mL/min, respectively at 3 L/min gas flow in case of 1 L/min blood flow. This calculated results showed an enough performance with reality.

Numerical simulation of vortex-induced vibration of a full-scale flexible riser at supercritical Reynolds number in uniform and linear shear flow

Utilizing passive methods is an efficient way in order to improve the thermal performance of heat exchangers. The novelty of the present paper is that two kinds of passive techniques for heat transfer enhancement, including a corrugated tube equipped with a coiled wire as turbulator, are combined in the present study. Heat transfer and pressure drop in a corrugated tube with a wire coil are analyzed numerically by a commercial CFD software. The range of the considered Reynolds number is 5000–20,000, which guarantees that the flow regime is fully turbulent. The effects of two different geometrical parameters and of two flow operating parameters, namely the Reynolds and Prandtl numbers, are numerically investigated. The conservative equations for momentum are closed using the k-e realizable model. Results indicate that the geometrical and fluid flow parameters have significant effects on the thermal performance of the system. When one increases the pitch ratio of the wire coil (P/d), the heat transfer enhancement ratio NuER decreases. The maximum decrease is obtained for P/d = 5 with − 7.78% at Re = 20,000 in comparison with the baseline case P/d = 2. The maximum increase in the performance evaluation criterion (PEC) is observed at Re = 5000: + 11.16% and + 17.4% for P/d = 3 and P/d = 5, respectively. The configuration with a roughness height equal to e = 3 mm exhibits the best thermal performance with a maximum enhancement in terms of NuER of + 3.3% at Re = 10,000 in comparison with the baseline case e = 0.5 mm. By considering the PEC number, increasing the parameter e has a negative effect on the thermal performance of the proposed system.

In the present investigation the distribution of mean velocity, static pressure and total pressure are experimentally studied on a C-shape diffuser of 40° angle of turn with an area ratio of 1.284 and centerline length was chosen as three times of inlet diameter. The experimental results then were numerically validated with the help of Fluent and then a series of parametric investigations are conducted with same centre line length and inlet diameter but with different area ratios varying from 1.25 to 2.0 and angle of turn 30° to 75°. The measurements were taken at Reynolds number 2.25 × 105 based on inlet diameter and mass average inlet velocity. Predicted results of coefficient of mass averaged static pressure recovery (48%) and coefficient of mass averaged total pressure loss (12%) are in good agreement with the experimental results of coefficient of mass averaged static pressure recovery (45%) and coefficient of mass averaged total pressure loss (11%) respectively. Standard k-e model in fluent solver was chosen for validation. From the parametric investigation it is observed that for the increase in area ratio from1.25 to 2.0, static pressure recovery increases sharply but with the increase of angle of turn pressure recovery decreases steadily. The coefficient of total pressure loss almost remains constant with the change in area ratio and angle of turn for similar inlet conditions.

Moonpools are holes in the middle of a ship's hull and through its deck. In this study, the flow mechanisms in moonpools in calm water were investigated. Star-CCM+ software was used to calculate the flow within a two-dimensional (2D) moonpool under uniform flow conditions. A 2D moonpool numerical calculation model was constructed, and the convergence analysis results and experimental data were used to verify the accuracy of the calculated results. Flow field and free surface fluctuations are the main factors that cause the drag force of the moonpool. The drag force increases with the flow velocity, and the drag coefficient presents different trends at different stages. With an increase in the Froude number, Fr, the 2D moonpool enters the frequency lock-in state of the piston and sloshing natural frequency. Additionally, increasing the moonpool length results in a nonlinear change in the flow state.

The effect of parapets on the wind loading of low-rise buildings has been examined in the past, primarily under conditions of uniform flow not representing the variation of the mean speed with height and the turbulence characteristics of the natural wind. This paper presents the results of a recent experimental study carried out in a boundary layer wind tunnel in order to determine the wind loads on low-rise buildings of different configurations with and without parapets. Data show that the addition of parapets reduces the magnitude of roof peak suction coefficients with values greater than about 1.75 for open country and about 3.0 for suburban exposures, whereas lower suction values remain unaffected or slightly increase. On the other hand, parapets tend to increase the mean suction locally close to roof corners. The implication of the data presented for current Standards and Codes of Practice is analyzed and suggestions are made.

수공구조물 설계 또는 하상의 유동 특징을 나타내기 위해선 마찰속도 산정이 필수적이다. 하지만 기존의 개수로 내의 마찰속도 공식인 Log 방법이나 Power방법은 벽면 근처에서 경계층 조건 혹은 다양한 수리적 특성이 지속적으로 변화하기 때문에 마찰속도의 산정에 어려움이 있다. 따라서 본 연구에서는 매개변수를 최소화하고 오차를 줄일 수 있는 새로운 마찰속도 공식을 제안하였다. 제안한 공식에 대한 신뢰성과 정확성을 검증하기 위하여 엔트로피매개변수M을 각각의 수로마다 산정을 하여 Dr.Song이 산정한 3가지 방법(Clauser 방법, <TEX>$\sqrt{gRI}$</TEX> 방법, 레이놀즈 응력법)으로 산정한 마찰속도와 비교하였다. 그 결과 가속부등류 조건의 경우 <TEX>$R^2=0.9621$</TEX> 감속부등류 조건의 경우 <TEX>$R^2=0.9274$</TEX> 등류흐름 조건의 경우 <TEX>$R^2=0.8865$</TEX>로 매우 높은 정확성으로 등류조건뿐만 아니라 부등류 조건에서도 거의 일치하였다. In order to demonstrate the flow properties of the river bed and the design of hydraulic structures, the estimation of friction velocity is essentially required. However, existing friction velocity equations such as Log method and Power law have trouble to estimate the friction velocity because a boundary condition and various hydraulic properties are changed constantly in near the wall. In the present study, therefore, a new friction velocity equation that can minimize the parameters and reduce an error was suggested. To verify accuracy and reliability for the proposed equation, Clauser method, <TEX>$\sqrt{gRI}$</TEX> method, reynolds stress method by Dr. Song were compared with the proposed method by estimated entropy parameter M for each channel. Consequently, the results show that uniform flow condition as well as non-uniform flow condition with highly accuracy nearly matched in case of accelerating non-uniform condition of <TEX>$R^2=0.9621$</TEX>, Decelerating Non Uniform condition of <TEX>$R^2=0.9274$</TEX>, Uniform condition of <TEX>$R^2=0.8865$</TEX>.

Laminar flow and heat transfer in a finned tube annulus

Buckling of cold-stretched cylindrical vessels under external pressure: Experimental and numerical investigation