Fluid Inertial Effects Research Articles

Influence of the Reynolds number on chaotic mixing in a spatially periodic micromixer and its characterization using dynamical system techniques

A micromixer is a key component of various microfluidic systems, such as microreactors and μ-total analysis systems. One important strategy for passive mixer design is to generate chaotic advection using channel geometry, which usually has spatially periodic structures. In this paper, the influence of the Reynolds number on chaotic mixing in such mixers is studied with three mixer models. Characterization of the mixer with dynamical system techniques is also studied. The influence of fluid inertial effects on the occurrence of chaotic advection is first discussed. It is found that at low Re(Re < 1), the flow could become reversible in the mixer, which raises the difficulty to generate chaotic advection. In this case, specific fluid manipulations, such as stretching and folding processes, are necessary. This study also proposes a characterization method using Lyapunov exponent (λ) and Poincaré mapping information, which allows us to analyze the mixing performance of the mixer with one single mixer unit. Results show that it objectively reflects the dynamical properties of the mixers, such as being globally chaotic, partially chaotic or stable. So it can be used as an analytical tool to differentiate, evaluate and optimize various chaotic micromixers.

Unsteady Calibration of Fast-Response Pressure Probes, Part 1: Theoretical Studies

Recent advances in miniaturization of pressure transducers, including microelectromechanical systems technology, have provided miniature, high-bandwidth pressure transducers and transducer arrays well suited for fast-response, multihole probes. The miniature size of these arrays enables a design in which the transducers are embedded in, or close to, the probe tip while maintaining a tip diameter of 1‐2 mm. Although the frequency response of such a probe is excellent, there are several unresolved issues pertaining to fluid inertia-related unsteady aerodynamic effects on probe calibration. As a result of these effects, when the probe is used in a dynamically changing flowfield a quasi-steady probe calibration can no longer be used to resolve the instantaneous velocity vector. We present theoretical analysis of these effects and formulate methods of calibrating multihole, fast-response probes for use in unsteady flows in order to accurately resolve the instantaneous velocity vector. The main objective of the theoretical analysis is to study and quantify the dependence of the probe measured pressures on the magnitude of the flow inertial effects in terms of properly formulated nondimensional parameters, as well as allow a direct comparison of theory and experiment. Among the contributions of this work, it was particularly interesting to find that proper definition of the nondimensional, flow-angle-dependent pressure coefficients renders them insensitive to unsteady effects. This has the significant ramification that the steady calibration of these coefficients can be also used in unsteady flowfields for the calculation of the flow angles. Calculation of the velocity magnitude, however, requires quantification of and correction for the fluid inertial effects.

Effect of viscosity on retention time and hydrodynamic lift forces in sedimentation/steric field-flow fractionation

Particles entrained in fluid flow between parallel bounding walls tend to be driven by hydrodynamic forces, acting perpendicular to the direction of flow, towards certain equilibrium positions between the walls. Sedimentation field-flow fractionation is a technique that is well suited to the measurement of these forces, particularly in the regions close to the walls. Forces much stronger than those due to fluid inertial effects are commonly observed in the near-wall regions. A study is presented here of the influence of carrier fluid viscosity on these hydrodynamic lift forces. The carrier viscosity is varied via two different methods: (1) various ternary mixtures of water, glycerol, and ethanol were used that varied in viscosity while being of constant density and (2) the temperature of the FFF system was raised, changing the carrier viscosity while not significantly altering its density. The near-wall lift forces are shown to be dependent on fluid viscosity. An empirical equation describing the apparent dependence is presented.

Cavitation and Air Entrainment Effects on the Response of Squeeze Film Supported Rotors

This paper analyzes the effects of air entrainment and cavitation on the synchronous response of squeeze film supported rigid rotors. The fluid film force coefficients are obtained from experimental measurements corresponding to a wide spectrum of operating conditions. These conditions include regimes in which air entrainment effects are dominant. Other conditions where vapor cavitation and fluid inertial effects are dominant are included for comparison. The effects of air entrainment are shown to produce a nonlinear response representative of a softening spring effect not previously known to exist in squeeze film dampers.

Fluid Inertial Effects on the Load Capacity of Spherical Spiral Groove Bearings

回転体の高速化に伴い流体によって潤滑された軸受の特性は,流体の慣性力によって影響を受ける。本論文では,球面スパイラル・グループ軸受の負荷容量に及ぼす慣性力の影響を検討している。その結果,慣性力によって負荷容量が著しく減少すること,また,グループ面回転の場合と平滑面回転の場合では減少量が異なること,そして軸荷重一定の系では浮上量に上限のあることなどを明らかにしている。

Inertial Considerations in Parallel Circular Squeeze Film Bearings

A second order regular perturbation solution for squeeze film flow of a Newtonian fluid between circular parallel surfaces is presented. All the inertial terms are included in the momentum equation. Experimental measurements of pressure in a squeeze film at high squeeze rates are shown to be in good agreement with the solution. The solution is particularly useful in assessing the extent to which fluid inertial effects would cause the classical lubrication theory to be in error for any particular squeeze film. The inertial effects are shown to be functions of two readily determined dimensionless parameters, the squeeze Reynold’s number Res and the squeeze acceleration number A.