Unsteady Flow of a Non-Linear Viscoelastic Fluid in Pipes

Flow of a viscoelastic fluid in round pipes is analyzed for the case where the pressure gradient is oscillatory with varying amplitude. The fluid is modelled according to Phan-Thien-Tanner’s constitutive equation. The analysis is carried out by using the perturbation method in which a material parameter is considered small. Velocity field and other kinematic and dynamic variables are evaluated for a range of relevant parameters. The results are compared with the base Newtonian and linear Maxwell flows. The effect of the PTT model in these type of flows is highlighted.

Flow Control Using Active Dimples

In this study, a novel concept of using active dimples for flow control is introduced. It is widely known that dimples on a golf ball dramatically reduce its aerodynamic drag. They are much more effective than surface roughness since the hollow spherical shape produces cavity flow, thus the drag coefficient remains relatively constant at higher Reynolds numbers. It has also been shown by previous studies that by use of circular-arc grooves or dimples, the separation point on a cylinder could be regulated and drag reduced due to the re-circulation occurring in the dimpled surface. Another approach to flow separation that uses the concept of momentum-flux change by near-wall manipulation is an active one, such as synthetic jets or acoustical excitation. The long-term goal of this study is to merge these two approaches and create a continuous smart surface that would have active depressions, which would then be actuated at desired frequencies and conform to a desired shape for optimal results. Current investigation had only touched the tip of the iceberg of this new and unexplored field. In order to begin to comprehend the complexity of the fluid mechanics of the active dimples, a dual focus had been outlined in this study. The first focused on the investigation of a single active dimple on a flat plate, while the latter investigated the effect of a row consisting of such devices on a circular cylinder. The main factors of interest are optimal actuation frequency and dimple positioning relative to the freestream.

Micro-Pneumatic Logic

Advances in silicon membrane and microvalve technology continue to be made. Microvalves utilizing membranes have always encompassed the attribute of an on-off switch, thereby suggesting a logic element, although their main application has been arguably as a proportional flow control device. Recently, an analogy has been suggested between a microelectronic, p-channel MOSFET, and a microvalve. The analogy includes a qualitative comparison between the flow vs. pressure relationship for the microvalve, and the current vs. voltage relationship for the MOSFET. It also includes a simple, small-signal frequency analysis of microvalve flow, based on a ‘saturation’ flow behavior chosen arbitrarily to be similar to that in a MOSFET. In this work, a quantitative and rigorous model for the flow vs. pressure relationship for a microvalve is presented. The model couples the mechanical behavior of a silicon membrane, with the fluid mechanical behavior facilitated by the membrane’s motion. The model is substantiated by measurements. The model is compared by analogy to the related MOSFET model equations. The pneumatic model is then applied to both a normally-closed microvalve, and a normally-closed poppet valve. The normally-closed microvalve is analogous to a p-MOSFET. The normally-closed poppet microvalve is analogous to an n-MOSFET. By appropriate physical coupling of these two devices, a fully complementary pneumatic NOR gate results. The quantitative pneumatic flow model is applied to this structure, and the logic transfer function is obtained. The ramifications of the results for scaled, micro-pneumatic logic devices will be discussed.

Measurement of Velocity Profiles of Laminar to Turbulent Water Flows in a Tube Using Particle Image Velocimetry

The transition from laminar to turbulent in-tube flow is studied in this paper. Water flow in a glass tube with an inside diameter of 21.7 mm was investigated by two methods. First, a dye visualization test using a setup similar to the 1883 experiment of Osborne Reynolds was conducted. For the dye visualization, Reynolds numbers ranging from approximately 1000 to 3500 were tested and the transition from laminar to turbulent flow was observed between Reynolds numbers of 2500 and 3500. For the second method, a particle image velocimetry (PIV) system was used to measure the velocity profiles of flow in the same glass tube at Reynolds numbers ranging from approximately 500 to 9000. The resulting velocity profiles were compared to theoretical laminar profiles and empirical turbulent power-law profiles. Good agreement was found between the lower Reynolds number flow and the laminar profile, and between the higher Reynolds number flow and turbulent power-law profile. In between the flow appeared to be in a transition region and deviated some between the two profiles.

Transport Mechanisms in Electrokinetic Nanoscale Channels

We investigate electrokinetic transport in nanometer-scale fluidic channels. Our study includes numerical studies of nanofluidic transport of both charged and uncharged analytes in conditions of finite Debye layer thickness and high zeta potentials. The models are based on continuum mass transport and field theory. We also perform an experimental parametric study using etched nanoscale channels. Experimental results agree with model predictions and show that bulk electrokinetic transport in nanoscale channels depends strongly on the shape and size of the EDL and on the effects of transverse electrophoretic migration.

Pressure Drop Measurements for Air Flow Through Open-Cell Aluminum Foam

Wind-tunnel pressure drop measurements for airflow through two samples of forty-pore-per-inch commercially available open-cell aluminum foam were undertaken. Each sample’s cross-sectional area perpendicular to the flow direction measured 10.16 cm by 24.13 cm. The thickness in the flow direction was 10.16 cm for one sample and 5.08 cm for the other. The flow rate ranged from 0.016 to 0.101 m3/s for the thick sample and from 0.025 to 0.134 m3/s for the other. The data were all in the fully turbulent regime. The pressure drop for both samples increased with increasing flow rate and followed a quadratic behavior. The permeability and the inertia coefficient showed some scatter with average values of 4.6 × 10−8 m2 and 2.9 × 10−8 m2, and 0.086 and 0.066 for the thick and the thin samples, respectively. The friction factor decayed with the Reynolds number and was weakly dependent on the Reynolds number for Reynolds number greater than 35.

AC Electrokinetics for Biosensors

We develop here tools for speeding up binding in a biosensor device through augmenting diffusive transport, applicable to immunoassays as well as DNA hybridization, and to a variety of formats, from microfluidic to microarray. AC electric fields generate the fluid motion through the well documented but unexploited phenomenon, Electrothermal Flow, where the circulating flow redirects or stirs the fluid, providing more binding opportunities between suspended and wall-immobilized molecules. Numerical simulations predict a factor of up to 8 increase in binding rate for an immunoassay under reasonable conditions. Preliminary experiments show qualitatively higher binding after 15 minutes. In certain applications, dielectrophoretic capture of passing molecules, when combined with electrothermal flow, can increase local analyte concentration and further enhance binding.

Feasibility Study of a MEMS Viscous Rotary Engine Power System (VREPS)

In this paper is presented an analytic, theoretical and numerical study of the Viscous Rotary Engine Power System (VREPS). In addition, a proposed process flow for the fabrication of the VREPS using DRIE of silicon is described. The design premise of the VREPS is to derive mechanical power from the surface viscous shearing forces developed by a pressure driven flow present between a rotating disk or annulus and a stationary housing. The resulting motion of the rotating disk or annulus is converted into electrical power by using an external permanent magnet, embedded nickel-iron magnetic circuits, and an external switched magnetic pole electric generator similar to the design proposed by M. Senesky for the UC Berkeley micro-Wankel Engine [1]. This paper will examine the power output, isentropic efficiency, and operating characteristics of the disk and annular viscous turbines using the lubrication approximation and the Creeping Flow Equations (Stokes Flow). The viscous turbine is optimized for maximum isentropic efficiency using MATLAB numerical optimization routines. Finally, a unique triple-wafer micro-fabrication process for VREPS is presented. The proposed design consists of a 250 μm thick, 3.4 mm OD / 2.4 mm ID annular rotor with embedded magnetic poles and four 10 μm driving channels on each side of the rotor. Electrical power is generated with a switched magnetic pole generator, external permanent magnet, and integrated magnetic circuits. Calculations with water predict an output power of 825 mW at an isentropic efficiency of 25% using a pressure drop of 5 MPa cross the device.

Micro Pulsatile Jets for Thrust Optimization

Thrust optimization of micro-synthetic pulsatile jets is studied. Cylindrical cavities with a small circular orifice at one end, and a vibrating diaphragm at the other are used for thrust generation. The governing parameters are identified and the tradeoffs between electrostatic, piezoelectric, and electromagnetic actuation methods are investigated. Optimization of the micro jets requires a solution that gives maximum diaphragm displacement while minimizing voltage. The size of the orifice diameter is chosen to maintain a formation number of 4, at which the length of an expelled slug of fluid from the exit orifice is four times the diameter of orifice. This relationship maximizes the circulation and impulse in the leading vortex rings generated by the actuator. To examine the effects of cavity dimensions, a number of actuators are constructed out of aluminum with various cavity diameter, cavity height, and orifice diameters. Piezoelectric disks bonded to brass shims are used for actuation. The jets are tested in air at various actuation voltage and wave-shape functions. Maximum thrust generation is achieved at the resonant frequency of the cavity. Hot wire anemometry is used to further characterize the jet flow field. An investigation into electrostatic, piezoelectric, and electromagnetic diaphragm actuation methods revealed that electromagnetic actuation provides the maximum diaphragm displacement using a constant voltage.

Test Facility for Multiphase Pumps in Serial and Parallel Operation

Multiphase twin screw pumps are investigated experimentally and theoretically. A test facility with two screw pumps that can be continuously operated with water, oil and air mixtures is used to measure the delivering characteristics and the influence of recirculation on pump performance. Unsteady flow regimes like slug flow often occur in natural oil and gas processing. Therefore the operational behavior of the multiphase pumps is investigated for this case. In the development of new oil fields multiphase pumps are installed not only individually but also in pipeline networks. Serial operation of two srew pumps is investigated for steady and unsteady flow conditions. In addition to experiments a model to calculate the volumetric efficiency of the pumps is presented. The calculated results are compared to measured ones.