Low Reynolds Number Region Research Articles

A novel micro-mixer with a quasi-active rotor: fabrication and design improvement

In the present paper, a novel micro-mixer with a quasi-active rotor for micro-plant applications is proposed and design considerations for the improvement of the mixing performance of the proposed micro-mixer are derived. The proposed micro-rotor mixer combines an active micro-mixer with a passive micro-mixer. The micro-rotor, which is a moving part of an active micro-mixer, is added to the micro-chamber of a passive micro-mixer. The micro-rotor was rotated by inflows tangential to the chamber, causing strong perturbations. Two models of the micro-rotor mixer were fabricated with six layers of photosensitive glass which were individually fabricated and thermally bonded together. Improvement in the design of the micro-rotor mixer was achieved after the fabrication and experimental testing of the first model. In the design of the second model, the channel width and the rotor diameter were diminished and the number of rotor blades was increased from four to five. Through these design improvements, the micro-rotor started rotating at a lower Reynolds number; the rotor rotated at Re 1000 in the first model, whereas it did so at Re 200 in the second model. The mixing efficiency values of the micro-mixers were measured using an image analysis method. In the results, the mixing performance was dominated by molecular diffusion in the low Reynolds number region. On the other hand, convection flows such as twisted flows and Coanda flows dominated the mixing performance in the higher Reynolds number region. In the upper Reynolds number region, the micro-rotor was rotated and strong perturbations were induced. The mixing efficiency values of the micro-rotor mixers were found to exceed 90% when the rotor was rotated.

Turbulence Model Behavior in Low Reynolds Number Regions of Aerodynamic Flowfields

The behaviors of the widely used Spalart-Allmaras and Menter shear-stress transport turbulence models at low Reynolds numbers and under conditions conducive to relaminarization are documented. The flows used in the investigation include 2-D zero-pressure-gradient flow over a flat plate from subsonic to hypersonic Mach numbers, 2-D airfoil flow from subsonic to supersonic Mach numbers, 2-D subsonic sink flow, and 3-D subsonic flow over an infinite swept wing (particularly its leading-edge region). Both models exhibit a range over which they behave transitionally even with inflow values set to cause immediate growth of the turbulence quantities, in the sense that the flow is neither laminar nor fully turbulent, but these behaviors are different: the shear-stress transport model typically has a well-defined transition location, whereas the Spalart-Allmaras model does not. Both models are predisposed to delayed activation of turbulence with increasing freestream Mach number. Also, both models can be made to achieve earlier activation of turbulence by increasing their freestream levels, but too high a level can disturb the turbulent solution behavior. The technique of maintaining freestream levels of turbulence without decay in the shear-stress transport model, introduced elsewhere, is shown here to be useful in reducing grid dependence of the model's transitional behavior. Both models are demonstrated to be incapable of predicting relaminarization; eddy viscosities remain weakly turbulent in accelerating or laterally strained boundary layers for which experiment and direct simulations indicate turbulence suppression. The main conclusion is that these models are intended for fully turbulent high Reynolds number computations, and using them for transitional (e.g., low Reynolds number) or relaminarizing flows is not appropriate. Competing models which fare better in these areas have not been identified.

0124 ヒービング運動翼の境界層を含む壁近傍に発達する渦流れ(OS1-5 噴流,後流および剥離流れの流動解析と応用,オーガナイズドセッション)

Many studies on unsteady flow at low Reynolds number regions have been carried out with experimental and numerical approaches. Especially, the flow field behind unsteady airfoil has attracted significant attentions in bio-hydrodynamics for the need to understand the propulsion mechanisms of aquatic animals, birds and insects in recent years. Moreover, Micro-Air-Vehicle (MAV) and micro flight robots with these mechanisms are also being developed actively. We have visualized detailed wake structures behind rigid and elastic unsteady airfoils using PIV measurements. However, impacts of elasticity on a flow field and differences from a flow field in the wake of a rigid airfoil have not been clarified yet. The purpose of the present study is to clarify the detailed vortex flow structure near the wall of the heaving airfoil, NACA0010. The vortex flow developed near the wall of the heaving airfoil was clarified using particular airfoil with some pinholes.

Detailed Wake Structure behind an Elastic Airfoil

An unsteady flow in a low Reynolds number region attracts attention in recent years. The authors measured vortex flow in the wake of the rigid flat plate, rigid NACA0010, elastic flat plate and elastic NACA0010 with heaving motions and evaluated accelerating flows induced by the vortex flow patterns and vortex flows in the wake of the airfoils. Furthermore, we measured dynamic thrusts acting on these airfoils with heaving motions and clarified their relations with vortex flows. Thrust producing vortex streets same as those of the airfoils can be formed in the wake of the elastic flat plate with heaving motions by giving an elasticity to the latter half of the flat plate. The dynamic thrusts that act on the rigid NACA0010, elastic flat plate and elastic NACA0010 are strongly dependent on the Strouhal number based on the trailing edge amplitude regardless of heaving amplitudes, heaving frequencies, airfoil shapes and elastic deformations.

Surface Pressure Distributions on 4% Circular Arc Airfoil at Low Reynolds Number

V ERY small aircraft, called micro air vehicles (MAVs), are of high interest because electronic equipment can be miniaturized to allow for the easy manufacture of a vehicle whose entire mass is only a few dozen grams. Small-sized MAVs operate at chord Reynolds numbers below 1:0 10. Because of the influence of viscosity, thin and sharp leading edge airfoils with thickness ratios less than about 5%offer better aerodynamic characteristics than thick and blunt edge airfoils with Reynolds numbers lower than 1:0 10 [1]. Mueller has measured the aerodynamic forces acting on a circular arc airfoil for MAVs; however, compared with thick and blunt edge airfoils, very little literature is available for thin and sharp leading-edge airfoils [1,2], which indicates that more research needs to be done for the low Reynolds number region. Cosyn and Vierendeels [3] numerically studied the low Reynolds number aerodynamics of a flat plate and an S5010 airfoil, which is an airfoil with a 10% thickness ratio. They have also pointed out that low Reynolds number flows exhibit complex flow phenomena, such as laminar separation, which was described by Mueller and DeLurier [4]. For the design and manufacture of MAVs, it is important to know the details of their aerodynamic characteristics, such as surface pressure distribution. However, there is no space inside of the circular arc airfoil for plumbing the pipes from the static pressure port to the pressure transducer. Thus, as far as the authors know, there are no experimental data measuring surface pressure distributions over a circular arc airfoil except for [2], which used luminescent, pressuresensitive paint. In this study, to measure the surface static pressure of the circular arc airfoil, wemade a 4% cambered-airfoil sectionmodel with a 1% thickness ratio by soldering copper pipes. We also conducted surface flow visualizations using the oil flow technique. The present experimental results at a chord Reynolds number of Re 62; 000 are expected to provide useful information for understanding the flowfield of the circular arc airfoil at a low Reynolds number aswell as for confirming the accuracy of numerical estimations concerning a circular arc airfoil.

A numerical investigation on the fluid flow and heat transfer in the confined impinging slot jet in the low Reynolds number region for different channel heights

The numerical solution is obtained for unsteady two-dimensional fluid flow and heat transfer in a confined impinging slot jet using the finite volume method. In order to consider the effect of Reynolds number and height ratio on the flow and temperature fields in the channel, the numerical simulations were performed for different Reynolds numbers of 50–500 and different height ratios of 2–5. The critical Reynolds number, beyond which the flow and thermal fields change their state from steady to unsteady, depends on the Reynolds number and height ratio. The unsteadiness gives a big impact on the flow and temperature fields and as a result the pressure coefficient, skin friction coefficient and Nusselt number in the unsteady region show different characteristics from those in the steady region.

Vortex structure and heat transfer in a diffusion combustion field with circulating flow

A numerical study was performed to examine the vortical structures for combusting flow from a two-dimensional bluff-body slot burner in the transitional recirculating flow. The numerical results support the flow visualisation results for C3H8 combustion by Chin and Tankin [Chin, L.P. and Tankin, R.S. (1991) 'Vortical structures in a 2-d vertical bluff-body burner', Combustion Science and Technology, Vol. 80, pp.207?229]. We identify the fuel effect on the dynamic vortical structures inside the flame for C3H8 and CH4 fuels. The Reynolds number for the inner slot fuel flow is varied from 10?250 at a fixed Reynolds number of 400 for the outer slot air flow. As a result of the study on a two-dimensional bluff-body burner, it was possible to reproduce the dynamic motions of vortical structures obtained by Chin and Tankin visualisation experiment. In the low Reynolds number region, vortex structures in the recirculating region gives large effect in the flame form. In addition, it was confirmed that independently of the fuel variety, the flow at the bluff-body aft comes into three shape varieties.

1934 ヒービング運動する弾性翼に働く非定常推進力(S21-6 噴流,後流及びはく離流れの流動解析と応用(6),21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

An unsteady flow in a low Reynolds number region attracts attention in recent years. The authors measured vortex flow in the wake of the elastic flat plate and elastic NACA0010 with heaving motions. Furthermore, we measured dynamic thrusts acting on these airfoils with heaving motions and clarified their relations with vortex flows. By giving an elasticity to the latter half of the flat plate, thrust-producing vortex streets same as those of the airfoils can be formed in the wake of the elastic flat plate with heaving motions. The dynamic thrusts that act on the elastic flat plate and elastic NACA0010 are strongly dependent on the Strouhal number based on the trailing edge amplitude regardless of heaving amplitudes, heaving frequencies, airfoil shapes and elastic deformations.

Influence of the Different Wall Oscillation Modes on the Heat Transfer Enhancement

We studied the influence of the different wall oscillation modes on the heat transfer characteristics of a two-dimensional channel numerically. In this study, the following two channels with different wall oscillation modes were used. One is a two-dimensional channel bounded by the fixed wall and the transversely oscillating wall (channel A) and the other is a two-dimensional channel whose upper and lower wall oscillate transversely in the same manner (channel B). The fully implicit finite difference method was used for the analysis of the conservation equations and the time-dependent coordinate transformations were applied to solve the moving boundary problem. The calculated results are summarized as follows. (1) The wall oscillation takes a remarkable effect on the heat transfer enhancement in a low Reynolds number region for each channel. (2) If we must avoid the increase in the pressure loss, the channel B is suitable for this requirement compared to the channel A.

Comparison of vertical and horizontal slot die coatings

AbstractComparison of vertical and horizontal slot coatings was carried out experimentally using aqueous glycerin solutions with varying viscosity on a laboratory‐scale slot coater. The maximum coating speeds achieved in vertical slot coating were found to be higher than those of horizontal slot coating for low‐viscosity solutions. The coating defects observed below the minimum wet thickness changed from ribbing to air entrainment at a critical solution viscosity of around μ = 75 mPa s. Above this critical viscosity, a lower minimum wet thickness was obtained for the horizontal coating but not for vertical coating. The difference between vertical and horizontal coating was mainly the length of the coating bead. The dimensionless minimum wet thickness was correlated as a function of Reynolds number. There existed three regions of minimum wet thickness depending on Reynolds number. Initially, the dimensionless wet thickness increased in the low Reynolds number region (Region I), then reached a plateau in the intermediate Re region (Region II), finally dropped off sharply in the high Re region (Region III). The boundaries of these three regions were identified by two critical Reynolds numbers, i.e., Re = 1 and 20. Only Regions I and II of the wet thickness could be found for horizontal coating, while mainly Regions II and III were obtained for vertical coating, depending on the viscosity of the coating solution. POLYM. ENG. SCI., 47:1927–1936, 2007. © 2007 Society of Plastics Engineers

피칭익에서 박리되는 와류의 거동

Most of experimental visualizations and numerical results on the flow field separated form a leading edge around an unsteady foil show a continuous streakline from the leading edge and large reverse flow between the streakline and the suction surface. However, they have not exactly clarified yet the dynamic behavior of vortices separated from the leading edge because separation around an unsteady foil is very complicated phenomenon due to many parameters. In the present study the flow fields around pitching foils have been visualized by using a Schlieren method with a high speed camera in a wind tunnel at low Reynolds number regions. It has been observed that small vortices are shed discretely from the leading and trailing edge and that they stand in line on the integrated streakline of separation shear layer. By counting vortices in the VTR frames it was clarified that the number of vortex shedding from the leading and trailing edge during one pitching cycle strongly depends on the non-dimensional pitching rate. Futhermore, the vortices moving up to the leading edge on the suction surface of the pitching foil are visualized. They play an important role to balance the number of vortex shedding from both edges.

Experimental and numerical studies of liquid flow and heat transfer in microtubes

Experimental and numerical studies were conducted to reveal the flow and heat transfer characteristics of liquid laminar flow in microtubes. Both the smooth fused silica and rough stainless steel microtubes were used with the hydraulic diameters of 50–100 μm and 373–1570 μm, respectively. For the stainless steel tubes, the corresponding surface relative roughness was 2.4%, 1.4%, 0.95%. The experiment was conducted with deionized water at the Reynolds number from 20 to 2400. The experimental data revealed that the friction factor was well predicted with conventional theory for the smooth fused silica tubes. For the rough stainless steel tubes, the friction factor was higher than the prediction of the conventional theory, and increased as the surface relative roughness increased. The results also confirmed that the conventional friction prediction was valid for water flow through microtube with a relative surface roughness less than about 1.5%. The experimental results of local Nusselt number distribution along the axial direction of the stainless steel tubes do not accord with the conventional results when Reynolds number is low and the relative thickness of the tube wall is high. The numerical study reveals that the large ratio of wall thickness over tube diameter in low Reynolds number region causes significant axial heat conduction in the tube wall, leading to a non-linear distribution of the fluid temperature along the axial direction. The axial heat conduction effect is gradually weakened with the increase of Reynolds number and the decrease of the relative tube wall thickness and thus the local Nusselt number approaches the conventional theory prediction.

On the heat transfer characteristics of heat sinks: Influence of fin spacing at low Reynolds number region

This study examines the thermal–hydraulic performance of heat sinks having plate, slit, and louver fin patterns. Comparison of the associated heat transfer performance and the effect of fin spacing are made. The results indicate that the enhanced fin patterns like louver or slit fin operated at a higher frontal velocity and at a larger fin spacing is more beneficial than that of plain fin geometry. The heat transfer performance of louver fin is usually better than that of slit fin but accompanies with higher pressure drops. However, it is found that the pressure drops for slit fin is comparable to the louver fin geometry when the fin spacing is reduced to 0.8 mm. This is associated with the appreciable rise of entrance/exit loss (form drag) caused by the slit fin geometry. The test results also reveal a significant drop of heat transfer performance at a low Reynolds number and at a small fin spacing, or the so-called “maximum” phenomenon of Colburn j factor. This is applicable to all the tested geometries. By a careful examination of the test results, it is concluded that this phenomenon is related to the developing/fully developed flow characteristics. In fact, the maximum point occurred roughly at x + = 0.1 where fully developed and developing flow is separated.

810 低レイノルズ数域における多孔板の流体抵抗に及ぼす穴形状の影響(OS6-1 電子機器の熱設計と課題(1),オーガナイズドセッション:6 電子機器の熱設計と課題)

810 低レイノルズ数域における多孔板の流体抵抗に及ぼす穴形状の影響(OS6-1 電子機器の熱設計と課題(1),オーガナイズドセッション:6 電子機器の熱設計と課題)

Dynamic Behavior of Vortices and Dynamic Forces around a Pitching Airfoil

In the present study flow patterns around pitching airfoils such as a NACA 65-0910 and BTE have been visualized by three kinds of dyes in a water tunnel at low Reynolds number region, Re=4.0×103.Moreover, the dynamic lift and drag acting on them have been measured by a six-axes sensor.At the angle of attack α=16±6 deg., where the separation occurs under the stationary condition, and at higher non-dimensional pitching rate, the vortex with clockwise rotation shed from the leading edge reattaches on the suction surface and the re-circulation region was formed on the suction surface. It interferes with the other vortex with counter clockwise rotation shed from the trailing edge. They form a cloud of vortices behind five chords length distant from the trailing edge.The dynamic lift is much higher than the stationary one because of the re-circulation region on the suction surface. The dynamic behavior of vortices on the suction surface influences the dynamic lift strongly.

607 多段曲がり流路における粘弾性流体の二次流れに関する実験的研究(2)(OS6-2 非ニュートン流体の流動現象,OS6 非ニュートン流体の流動現象,オーガナイズドセッション)

Recently, the elastic turbulence of non-Newtonian fluids with curved channels has been reported in the very low Reynolds number region. In this paper, we deal with the secondary flow of viscoelastic fluids in multiple curvilinear pipes in the higher Reynolds number region where the centrifugal force is not negligible. As viscoelastic fluids, polyacrylamide (PAA) solutions of 0.025, 0.05 and 0.1 wt% were used. The number of curves was varied from 6 to 60. We found that the reversal secondary flow promoted the fluid mixing for PAA 0.05 and 0.1 wt%.

B101 波動テイラー渦流れの変調

Experiments studying on the transition of the flow between concentric cylinders with the inner cylinder rotating and the outer one at rest have been made by means of flow visualization. In this study, an inner conical mirror has been used to observe the overall flow pattern at all times. Attention is focused on the effect of aspect ratio on flow regime and modulation. It has been found that the number of travelling azimuthal waves increases and modulation occurs at lower Reynolds number region with a decrease in the aspect ratio.

1418 高分子流体の球間隙における流れの特性(S30-4 複雑流体の流動現象(4),S30 複雑流体の流動現象)

The flow configurations in a gap between coaxially rotating inner spheres and stationary outer sphere were studied experimentally for flows with glass fiber suspended polymeric fluid. In the present study, relatively low concentration polyacryamid-water solutions with glass fiber suspension are used as test fluids. In experiment, two spherical gap sizes are adopted, and Reynolds number, aspect rate of glass fiber were changed accordingly. The frictions torque with inner sphere was measured. From measurement results of friction torque, polymeric fluids follow the power-law model in low Reynolds's number region in two spherical gap ranges. If the aspect ratio becomes bigger, the suspension tends to deport from the power-law model approximation.

611 繊維懸濁系高分子流体の球間隙における流れの特性(2)(OS6-3 非ニュートン流体の流動現象,OS6 非ニュートン流体の流動現象,オーガナイズドセッション)

The flow configurations in a gap between coaxially rotating inner spheres and stationary outer sphere were studied experimentally for flows with glass fiber suspended polymeric fluid. In the present study, relatively low concentration polycryamid-water solutions with glass fiber suspension are used as test fluids. In experiment, two spherical gap sizes are adopted, and Reynolds number, aspect rate of glass fiber were changed accordingly. The frictions torque with inner sphere was measured. From measurement results of friction torque, polymeric fluids follow the power-law model in low Reynolds's number region in two spherical gap ranges. If the aspect ratio becomes bigger, the suspension tends to depart from the power-law model approximation.

A numerical study of laminar convective heat transfer in microchannel with non-circular cross-section

Three-dimensional numerical simulations of the laminar flow and heat transfer of water in silicon microchannels with non-circular cross-sections (trapezoidal and triangular) were performed. The finite volume method was used to discretize the governing equations. Numerical results were compared with experimental data available in the literature, and good agreements were achieved. The effects of the geometric parameters of the microchannels were investigated, and the variations of Nusselt number with Reynolds number were discussed from the field synergy principle. The simulation results indicate that when the Reynolds numbers are less than 100, the synergy between velocity and temperature gradient is much better than the case with Reynolds number larger than 100. There is an abrupt change in the intersection angle between velocity and temperature gradient around Re = 100 . In the low Reynolds number region the Nusselt number is almost proportional to the Reynolds number, while in the high Reynolds number region, the increasing trend of Nusselt number with Reynolds number is much more mildly, which showed the applicability of the field synergy principle. In addition, for the cases studied the fully developed Nusselt number for the microchannels simulated increases with the increasing Reynolds number, rather than a constant.