• Transactions of the Korean Society for Noise and Vibration Engineering
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• 제22권1호
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• pp.53-60
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• 2012

To reduce the vibration in industrial settings, the viscous fluid dampers have been widely used. Since the damper shows a viscoelastic behavior, many methods to predict the behavior have been investigated. But the methods did not consider a change of damper size that is important factor for practical design engineer. In this study, to predict a change of damper ability with respect to a change of damper size, the dynamic experiment were conducted with fixed aspect ratio and gap. The damping coefficient at zero frequency was computed through theoretical and experiment approach in order to fit the experimental results using fractional derivative Maxwell model.

• Transactions of the Korean Society of Mechanical Engineers B
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• 제22권9호
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• pp.1208-1216
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• 1998

The present numerical study investigates the effect of a secondary flow on the heat transfer in order to delineate the mechanism of laminar heat transfer enhancement of a viscoelastic fluid in rectangular ducts. The second normal stress generating a secondary flow is modeled by adopting the Reiner-Rivlin constitutive equation and the calculated secondary flow showed good agreement with experiments. The primary velocity U as well as the pressure drop were not affected by the secondary flow in rectangular ducts, whose order of magnitude is less than 0.1% of the primary velocity. The small magnitude of the secondary flow, however, affect moderately the temperature fields. The calculated Nusselt numbers with secondary flow show 50% heat transfer enhancement over those of a purely viscous non-Newtonian fluid, which are considerably lower than the experimental values. Therefore, we conclude that there should be an additional heat transfer enhancement mechanism involved in the viscoelastic fluid such as temperature-dependence.

• The Korean Journal of Rheology
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• 제6권1호
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• pp.49-59
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• 1994

높은 전기장 하에서 다분산, 비구형 실리카/실리콘 오일 현탁액의 전기유변 (electrorheological, ER)현상을 살펴 보았다. 전기유변유체는 전기장 방향으로 사슬구조를 형 성하는 특성을 보이며 이것이 유변물성의 향상에 기여하는 것으로 알려졌다. 동적(dynamic) 상태 실험에서 전기장 하의 실리카 현탁액은 매우낮은 임계변형율(${\gamma}$c =0.1%)이상에서 비선 형 점탄성을 보였다. 저장탄성계수(G＇)는 변형율 변화에 손실탄성계수(G＂)는 매질의 점도 에 더 민감한 의존성을 보였다. 또한 겉보기 항복응력은 입자의 부피분율과 전기장에$\Phi$1.9E1.4 의 의존성을 보였는데 부피분율에 대한 의존성이 큰 이유는 0.1 이상의 부피분율에서 복합 사슬 구조 내의 입자들 간의 상호 정전효과가 지배적으로 나타나기 때문이라고 생각된다. 정상상태 실험에서는 부피분율이 크거나 높은 전기장 하에서 전단속도가 0.1sec-1 정도 이하 로 감소함에 따라 전단응력이 급겨히 증가하는 현상을 보였다. 그러므로 본질적인 동적 항 복응력을 얻기 위해서는 매우 낮은 전단속도 영역의 특이한 응력거동을 고려해야한다. 큰 전단속도 하에서는 hydrodynamic interaction의 영향으로 전단속도의 증가에 따라 전단응력 이 증가하였다. 이같은 전단응력의 거동을 계단전단실험으로 확인하였다.

• Transactions of the Korean Society of Mechanical Engineers A
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• 제22권2호
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• pp.302-310
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• 1998

In the accompanying paper, part I, we have presented the physical modeling and the associated numerical analysis of injection molding process with a compressible viscoelastic fluid model. In part II, the effects of compression stage in the injection/compression molding process are presented. Numerical results showed that the injection/compression molding process reduced birefringence as compared with the injection molding process. In this respect, one can conclude that the injection/compression molding process is more suitable for manufacturing the precise optical products than the injection molding process. In the distribution of birefringence, the effect of packing procedure in injection/compression molding process was found to be similar to that in injection molding process. From the numerical results, we found that birefringence becomes smaller as the melt temperature gets higher and the closing velocity of the mold gets smaller with the flow rate and the mold temperature affecting the birefringence insignificantly. As far as the distribution of density is concerned, the flow rate, the melt temperature, and the closing velocity of the mold had insignificant effect on the distribution of density in comparison with the mold temperature.

• Polymer(Korea)
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• 제25권4호
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• pp.536-544
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• 2001

We investigate, in the viewpoint of mathematical stability, the validity of the time-strain separability hypothesis employed in polymer viscoelasticity on the basis of experimental results. There have been suggested two distinct stability criteria such as Hadamard related to quick response and dissipative stability conditions, and in the limit of high deformation rate we have proved that separable constitutive equations are either Hadamard or dissipative unstable. The fact that the separability is not valid in the short time region in stress relaxation experiments exactly coincides with the results of our analysis. Therefore, since the application of the separability hypothesis incurs thermodynamic inconsistency as well as mathematical instability, such application should be avoided in the formulation of constitutive equations. In addition, careful attention should be paid to the limit of its validity even in experiments. It is also proved that there is neither theoretical nor physical validity of using the damping function.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• 제2권1호
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• pp.37-48
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• 1990

The drag and heat transfer reduction phenomena and degradation effects of drag reducing polymer solutions which are known as the viscoelastic fluids are investigated experimentally for the turbulent circular tube flows. Two stainless steel tubes are used for the experimental flow loops. Aqueous solutions of Polyacrylamide Separan AP-273 with concentrations from 300 to 1000 wppm are used as working fluids. Flow loops are set up to measure the friction factors and heat transfer coefficients of test tubes in the once-through system and the recirculating flow system. Test tubes are heated by power supply directly to apply constant heat flux boundary conditions on the wall. Capillary tube viscometer and falling ball viscometer are used to measure the viscous characteristics of fluids and the characteristic relaxation time of a fluid is determined by the Powell-Eyring model. The order of magnidude of the thermal entrance length of a drag reducing polymer solution is close to the order of magnitude of the laminar entrance length of Newtonian fluids. Dimensionless heat transfer coefficients of the viscoelastic non-Newtonian fluids may be represented as a function of flow behavior index n and newly defined viscoelastic Graetz number. As degradation continues viscosity and the characteristic relaxation time of the testing fluids decrease and heat transfer coefficients increase. The characteristic relaxation time is used to define the Weissenberg number and variations of friction factors and heat transfer coefficients due to degradation are presented in terms of the Weissenberg number.

• Transactions of the Korean Society of Automotive Engineers
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• 제6권6호
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• pp.113-131
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• 1998

The objective of the present study is to investigate the characteristics of the dividing flow in the laminar flow region. Using glycerine water solution(wt43%) for Newtonian fluid and the polymer of viscoelastic fluid(500wppm) for non-Newtonian fluid, this research investigates the flow state of the dividing tube in steady laminar flow region of the two dimensional dividing tube by measuring the effect of Reynolds number, dividing angle, and the flow rate ratio on the loss coefficient. In T- and Y-type tubes, the loss coefficients of the Newtonian fluid decreases in constant rate when the Reynolds number is below 100. The effect of the flow rate ratio on the loss coefficients is negligible. But when the Reynolds number is over 100, the loss coefficient with various flow rate ratios approach an asymptotic value. The loss coefficient of the non-Newtonian fluid for different the Reynolds number shows the similar tendency of the Newtonian fluid. And when the Reynolds number is over 300, the loss coefficient is approximately 1.03 regardless of flow rate ratio or the dividing angle. The aspect ratio does hardly influence the reattachment length and the loss coefficient of both Newtonian and non Newtonian fluid. The loss coefficient decreases as the Reynolds number increases. The loss coefficient of Newtonian fluid is larger than that of non-Newtonian fluid.

• Korean Journal of Food Science and Technology
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• 제35권2호
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• pp.188-194
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• 2003

천마식품가공시 기초자료로 활용될 수 있는 천마 추출농축액의 리올로지 특성을 조사하였다. 천마의 열수 및 50% 에탄올 추출물운 각각 5, 10, 15, 및 20 Brix와 10, 20, 30, 40 및 50 Brix로 농축하여 제조하였다. 열수 및 50% 에탄올 농축액의 정상유동특성과 동적점탄성을 조사하였다. Power law model로 구한 n에 의하면 모든 열수 농축액들은 전형적인 의가소성$(n=0.484{\sim}0.802)$유체의 흐름특성을 보였고, 50% 에탄올 농축액들은 뉴튼유체$(n=0.875{\sim}1.078)$에 가까운 흐름특성을 나타내었다. 열수 및 50% 에탄올 농축액의 ${\eta}_{app}$는 온도가 증가함에 따라 감소하는 경향을 나타내었다. Herschel-Bulkley model에 의한 열수 및 50% 에탄올 농축액의 C는 각각 $0.030{\sim}1.255$ Pa와 $0.007{\sim}0.065$ Pa이었다. ${\eta}_{app}$에 대한 온도 및 농도의 영향은 arrhenius식으로 해석하였다. 열수 및 50% 에탄올 농축액의 $E_a$는 농도가 증가함에 따라 각각 $3.340{\sim}4.620{\times}10^3\;J/mol{\cdot}kg$과 $1.6289{\sim}18.6699{\times}10^3\;J/mol{\cdot}kg$의 범위로 나타났다. 진동수(angular frequency, ${\omega}$)가 증가할수록 저장탄성률(storage modulus, G#)과 손실탄성률(loss modulus, G@)은 일반적으로 증가하는 경향을 나타냈다. 50% 에탄올 농축액의 경우 모든 진동수에서 G#이 G@보다 높게 나타나 전형적인 저분자용액의 흐름특성을 보였다. 그러나 열수농축액의 경우 G@이 G#보다 높았고 0.5 rad/sec에서 교차점을 형성하였으며 전형적인 고분자 용액의 흐름특성을 나타내었다.

• Transactions of the Korean Society of Mechanical Engineers
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• 제12권5호
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• pp.1189-1196
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• 1988

Thermal entrance lengths of turbulent tube flow for viscoelastic polymer solutions are investigated experimentally in the recirculating flow system with tubes of inside diameters 8.5mm(L/D=710) and 10.3mm(L/D=1158), respectively. In the present system, the hydrodynamic and thermal boundary layers develop simultaneously from the beginning of the test section. To provide the boundary condition of constant heat flux at the wall, the test tubes are heated directly by electricity. The polymer solution used in the current study is 1000 wppm aqueous solution of polyacrylamide(Separan AP-273). The apparent viscosity of the polymer solutions circulating in the flow system are measured by the capillary tube viscometer at regular time intervals. Thermal entrance lengths vary due to the rate of degradation. The entrance lengths of degraded polymer solutions are about 500~600 times the diameter. However, the entrance lengths of fresh polymer solutions are greater than the lengths of the test tubes used in this study suggesting that thermal entrance lengths for viscoelastic polymer solutions are greater than 1100 tube times the diameters. Friction factor is almost insensitive to the degradation, but the heat transfer $j_{H}$-factor is affected seriously by degradation. Based on the present experimental data of fresh solutions a correlation for the heat transfer $j_{H}$-factor is presented.ted.

• Journal of Oral Medicine and Pain
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• 제15권1호
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• pp.9-25
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• 1991

타액의 유동학적 성질은 타액의 윤활작용 및 구강내 경조직, 연조직 보호작용에 영향을 미친다. 그러므로 타액의 유동성을 잘 이해하면, 구강내 기능적 불편감의 평가와 인공타액의 개발에 필요한 중요한 정보를 얻을 수 있다. 저자는 전단율 변화에 따른 타액점도 변화의 연령 및 성별에 따른 차이를 알고자, 각 연령층별로 구강건조증으로 고통받은 병력이 없으며 타액채취시 약물을 복용하고 있지 않은 남녀 각 20명씩 총 240명을 대상으로 자극시 분비된 타액의 점도를 cone-and-plate 형태의 점도계를 이용하여 전단율 11.3에서부터 450.0(/sec )사이에서 측정하였다. 또, 20대 남녀 각각 20명의 경우, 자극시 분비 된 전타액의 점도와 함께 비자극시 분비된 전타액, 자극시 분비된 이하선 타액, 자극시 분비된 악하선 설하선 혼합타액의 점도를 같은 방법으로 측정한 결과 다음과 같은 결론을 얻었다. 1. 타액은 점탄성의 성질을 가지는 non-Newtonian 유체로서 점도와 전단율 사이의 관계는 점근선으로 표시될 수 있었다. 2. 자극시 분비된 전타액의 점도는 연령이 높을수록 증가된 양상을 보였으나, 남자의 경우 50대 이상에서 가장 높고 여자의 경우 30대에서 가장 높았다. 3. 자극시 분비된 전타액의 점도는 20대, 30대, 40대에서 여자가 남자보다 높았다. 4. 타액 점도는 자극시 분비된 악하선 설하선 혼합타액에서 가장 높았으며, 비자극시 분비된 전타액, 자극시 분비된 전타액, 자극시 분비된 이하선 타액 순이었다.