Relation Between Sound Sources and Vortical Structures in Isotropic Compressible Turbulence

We investigate the relation between vortical structures and sound source in isotropic compressible turbulence by direct numerical simulations with various turbulent Mach numbers. The sound source is obtained numerically from the Lighthill equation. As a first step, we study the sound source from the Reynolds stress, which is the dominant source in flows at low Mach numbers. We investigate, especially, sound source structures around the “coherent fine scale eddies” [1–4] to lead a universal conclusion of sound generation mechanism from the fine scale structures in supersonic flows. We find that the sound source structures around the coherent fine scale eddies show some distorted structures only in high Mach number flows because shocklets appear around the fine scale eddies in those flows. This change in sound source structures around the coherent fine scale eddies does not appear in low and moderate Mach number cases.

Using Shear and DC Electric Fields to Manipulate and Self-Assemble Dielectric Particles on Microchannel Walls

Manipulating suspended neutrally buoyant colloidal particles of radii a = O(0.1 μm–1 μm) near solid surfaces, or walls, is a key technology in various microfluidics devices. These particles, suspended in an aqueous solution at rest near a solid surface, or wall, are subject to wall-normal “lift” forces described by the DLVO theory of colloid science. The particles experience additional lift forces, however, when suspended in a flowing solution. A fundamental understanding of such lift forces could therefore lead to new methods for the transport and self-assembly of particles near and on solid surfaces. Various studies have reported repulsive electroviscous and hydrodynamic lift forces on colloidal particles in Poiseuille flow (with a constant shear rate γ̇ near the wall) driven by a pressure gradient. A few studies have also observed repulsive dielectrophoretic-like lift forces in electroosmotic (EO) flows driven by electric fields. Recently, evanescent-wave particle tracking has been used to quantify near-wall lift forces on a = 125 nm–245 nm polystyrene (PS) particles suspended in a monovalent electrolyte solution in EO flow, Poiseuille flow, and combined Poiseuille and EO flow through ∼30 μm deep fused-silica channels. In Poiseuille flow, the repulsive lift force appears to be proportional to γ̇, a scaling consistent with hydrodynamic, vs. electroviscous, lift. In combined Poiseuille and EO flow, the lift forces can be repulsive or attractive, depending upon whether the EO flow is in the same or opposite direction as the Poiseuille flow, respectively. The magnitude of the force appears to be proportional to the electric field magnitude. Moreover, the force in combined flow exceeds the sum of the forces observed in EO flow for the same electric field or in Poiseuille flow for the same γ̇. Initial results also imply that this force, when repulsive, scales as γ̇1/2. These results suggest that the lift force in combined flow is fundamentally different from electroviscous, hydrodynamic, or dielectrophoretic-like lift. Moreover, for the case when the EO flow opposes the Poiseuille flow, the particles self-assemble into dense stable periodic streamwise bands with an average width of ∼6 μm and a spacing of 2–4 times the band width when the electric field magnitude exceeds a threshold value. These results are described and reviewed here.

Free Flow Isoelectric Focusing in a Microfluidic Device

In recent years, there are growing interests in the use of free flow isoelectric focusing (FFIEF). In FFIEF, a thin sheath of laminar flow is introduced perpendicular to the direction of the applied electric field for continuous separation of proteins and charged species. This technique is especially useful in microfluidic device since the electrophoretically separated bands do not have to be mobilized for detection or further analysis. In this study, a mathematical model is developed to simulate free flow isoelectric process in microfluidic devices considering electroneutrality and incompressibility of electrolytes. Our mathematical model is based on mass, momentum and charge conservation equations. A finite volume based numerical scheme is implemented to simulate two dimensional FFIEF in a microfluidic chip. Simulation results indicate that pH gradient forms as samples flow downstream and proteins can be separated effectively using this technique. A new design of microfluidic chip is proposed for separation for cardiac troponin I from serum albumin using FFIEF technique.

The Effect of Longitudinal Core Flow on Trailing Vortex Stability

The current experimental study investigates the effect of longitudinal core flow on the formation and structure of a trailing vortex. The vortex is generated using four airfoils connected to a central hub through which a jet flow is added to the vortex core. Time averaged vorticity, circumferential velocity, and turbulent kinetic energy are studied. The statistics of vortex wandering are identified and corrections applied to the vorticity distribution. The vortex generator used in this study was built on the basis of the design described by Beninati et al. [1]. It uses four NACA0012 airfoils connected to a central hub. The wings orientation can be adjusted such that each contributes to a strong trailing vortex on the center of the test section. The vortex generator also had the capability to deliver an air jet directed longitudinally through a hole in the hub at the joint of the airfoils. Tests were done without the jet and with the air jet at jet velocities of 10 and 20 m/s. Planar PIV was used to measure the velocity field in the vicinity of the vortex core. The measurements were taken at 3 chords behind the vortex generator.

Hydraulic Design of Inlet Guide Vane and its Full Flow Passage Numerical Simulation on Centrifugal Pump

In order to effectively improve hydraulic performance of centrifugal pump on off-conditions, the hydraulic design of inlet guide vane (IGV) was completed by adopting two dimensional theory in-house code based on one kind of IS series of centrifugal pump, which can achieve pre-whirl regulation of centrifugal pump. During design process the trailing edge of vane is assumed as equal velocity moment condition, and the distribution of vane setting angle along meridional streamline is also given as a quartic function firstly, the camber line is then drawn by point-by-point integration method and thickened at both sides along circumferential direction. With local vortex dynamics diagnosis theory, the optimal improvement of vane space shape can be finished by adjusting the design parameters of vane setting angle distribution coefficient ap. The full flow passage numerical simulations of centrifugal pump with IGV device are completed to analyze the influence of pre-whirl regulation on hydraulic performance of centrifugal pump under various pre-whirl angles. The results show that the pre-whirl regulation can improve the hydraulic performance of centrifugal pump on off-conditions. Under the positive pre-whirl regulation conditions, the best efficient point shift to small flow rate zone, and under the negative pre-whirl regulation conditions it moves to large flow rate zone. Compared with the pump without IGV device at the same flow rate condition of 0.8Q (Q the design flow rate), the hydraulic efficiency of centrifugal pump with IGV device improves obviously and reaches up to 1.43%. Meanwhile compared with that installed with the straight vanes designed based on the traditional theory, the inner flow field of centrifugal pump with the designed vanes improves and the overall hydraulic efficiency of centrifugal pump is somewhat increased.

Experimental Characterization of a Modified Airlift Pump

This paper presents an experimental investigation on a modified airlift pump. Experiments were undertaken as a function of air-water flow rate for two submergence ratios (ε=0.58 and 0.74), and two different riser geometries (i) straight pipe with a constant inner diameter of 19 mm and (ii) enlarged pipe with a sudden expanded diameter of 19 to 32 mm. These transparent vertical pipes, of 1 m length, were submerged in a transparent rectangular tank (0.45×0.45×1.1 m3). The compressed air was injected into the vertical pipe to lift the water from the reservoir. The flow map regime is established for both configurations and compared with previous studies. The two phase air-water flow structure at the expansion region is experimentally characterized. Pipeline geometry is found to have a significant influence on the output water flow rate. Using high speed photography and electrical conductivity probes, new flow regimes, such as “slug to churn” and “annular to churn” flow, are observed and their influence on the output water flow rate and efficiency are discussed. These experimental results provide fundamental insights into the physics of modified airlift pump.

Transient Two-Phase Flow Model for High Viscous Liquids Over Hilly Terrain

This paper presents the detailed development of a multiphase model to predict the behavior of terrain-induced slugging, influenced by the viscous effects and hilly terrain. Currently, high viscosity heavy crude oil represents most of the available fossil resources. This crude flows inside long and expensive pipelines, usually over hilly terrain, causing the formation of slug flow. A very common flow pattern produces critical effects on pipelines in terms of modelling, mechanical stress, induced oscillations, fatigue, production losses, and other negative effects for the system. An accurate characterization of this pattern may give critical data for the mechanical design of piping systems and provide valuable information for the downstream process equipment selection. At present, most of the existing models to predict the behavior of slug flow neglect relevant parameters such as the effect of liquid viscosity and the effect of topographic terrain profile. The objective of this study is to present a mechanistic fluid model to determine the behavior of slug flow affected by the hilly terrain using viscous fluids. The model predicts the four stages of slug flow proposed by Schmidt et al. [1], and extends these stages to hilly terrain systems. The model is valid for a wide range of fluid viscosities and considers a range of pipe inclinations between 0° and 90°. Model validation with available literature and experimental data, shows a maximum deviation of 6%.

Turbulence Measurements in the Corner Wall Jet

The corner wall jet is similar to the standard three-dimensional wall jet with the exception that one half of the surface has been rotated counter-clockwise by 90 degrees. The corner wall jet investigated here is formed using a long round pipe with a Reynolds number of 159,000. Contours of the mean and turbulent flow field were measured using hot-wire anemometry. The results indicate that the ratio of lateral to vertical growth in the corner wall jet is approximately half of that in a standard turbulent three-dimensional wall jet.

Efficient Airload Determination for Bluff Body Aeromechanics

A continuous-rotation testing technique is applied to capture the variation of aerodynamic loads with attitude on objects of arbitrary shape. The technique converts the problem of measuring static air loads at various attitudes into a periodic problem. Phase-resolved ensemble-averaging is used to capture load variations with arbitrarily fine azimuthal resolution. The airload variations are obtained in closed form as discrete Fourier series. Experiments on a cylinder model of equal length and diameter were used to study the ability to capture asymmetries, and resolve support interference issues. A closed cuboid is used to correlate with prior work. A flat plate with a central cylindrical load, and a porous box are also studied. Free-swing tests using rigid tethers fixed to a pitch-yaw-roll gimbal mount are used to derive dynamic behavior in a free stream. The cylinder results showed the ability to resolve the effect of minor geometric asymmetries on airloads. The flat plate at 10 degrees pitch shows strong differences in dynamics between cases with a rounded versus squared-off edge facing the freestream. The porous box shows the differences between cases with and without one side blocked.

Computational Modeling and Simulations of Isothermal Plane (Linear) Air Jet Velocity Profile for Slot Diffusers

In the present work, computational modeling and simulations of isothermal plane (linear) air jet velocity profile for slot diffusers are performed. Plane air jets are formed by linear slots or rectangular openings with a large aspect ratio. Numerical simulations are performed using the commercial computational fluid dynamics (CFD) code ANSYS FLUENT. Three plane air jet flow simulations will be investigated such as free plane (linear) jets, attached jets, and air flow through a slot diffuser in a room setting. The purpose of simulating the free plane jet through a slot diffuser is to study the behavior of jet velocity profile that is not blocked by side walls or ceilings. The jet velocity profile is modified when obstructed by the walls and the air jet desires to attach to the surfaces along its path. For this reason, attached jet simulations through a slot diffuser will be conducted. The CFD study of plane air jet flows will eventually be extended to jet flows through a slot diffuser to a room to investigate the fluid flow behavior that enters a room under a ceiling. In addition, effects of two-equation turbulence models such as standard, renormalization group (RNG), and realizable k-ε on the CFD simulations will be investigated. Predicted velocity profiles and decays of free plane jet through a slot diffuser will be validated with a semi-empirical model [1]. Predicted velocity profiles of attached jet simulations will also be compared with a semi-empirical expression [2]. The slot diffuser air flow simulations will be compared with experimental data by the work of Chen and Srebric [3]. All simulations will be conducted at a specified inlet air velocity. The effects of grid resolution are also examined. It is established that the standard k-ε turbulence model best simulates attached and free jet flows. The standard k-ε turbulence model is applied to a room setting under isothermal conditions. The results are compared with non-isothermal experimental data [3]. It is shown that temperature which is a passive scalar has less influence on the flow pattern at a high air velocity than at a low air velocity in a room setting.