We investigate electrokinetic behavior of colloidal particles in the vicinity of a solid-liquid interface. Colloidal liquids are expected to be used as thermal transport media for heat transfer applications such as nanofluids and phase change emulsions. They contain submicrometer-sized particles in liquid, and electrokinetic behavior of the solute particles should play an important role in the heat transfer between solid-liquid interfacing boundaries. However, experimental investigation of the behavior remains difficult due to the required spatial resolution beyond diffraction limit. We developed a measurement system based on laser Doppler principle using an interference of evanescent waves generated at total internal reflections of incident lasers at a solid wall. The system was developed for the measurement of velocities of colloidal particles at an interfacing boundary of colloidal liquid and a solid wall. The system has a unique advantage of a high spatial resolution in the direction perpendicular to the boundary due to the short penetration depth of an evanescent wave in the range of a few hundred nanometers. The principle and performance of the measurement system were investigated using a scanning probe in the measurement volume. We experimentally confirmed the validity of the measurement and characterized the uncertainty of velocity measurement. The system was further applied in a series of measurements of alumina particles dispersed in water in a square-shaped cell under induced electric fields. The measured velocities are proportional to the field strengths at different particle concentrations. The linear relationship is consistent with theoretical predictions, which demonstrates the feasibility of the system for the measurement of velocities of colloidal particles in the near wall region.

Coronary artery disease (CAD) is among the foremost causes for human death worldwide. This study aims at investigating the performance of different boundary condition model types to characterize CAD functional significance. In addition, alternate models to estimate FFR using any different combination of boundary conditions at inlet and outlet were analyzed. In the first type of boundary condition, an outflow resistance model is used combined with a fixed pressure at inlet. In the second model of boundary conditions, constant pressure values are imposed at the domain inlet and outlet/s sections. In the third model, a zero diffusion flux is applied at outlet with a pre-specified flow rate at inlet. Numerical simulations performed on healthy and stenosed idealized and physiological arterial models revealed the superiority of the first type of boundary condition to directly capture the level of ischemia in diseased arteries. However, in this model, special numerical treatment at the outflow boundary is needed to dampen pseudo numerical reflections entering the computational domain. Alternative simple methods are developed to tackle the problem incurred in the second and third types of boundary condition types. The alternate models are effective for carrying extensive parametric studies with minimal computational effort. The new developed methods allow results generated via generic simulations under any specified boundary condition type to correctly estimate CAD functional significance. The obtained surrogate models account for the effects of the patient-specific physiologic parameters and can be easily incorporated without modifying existing CFD codes. Moreover, where it is unfeasible to experimentally incorporate the downstream effects of a given diseased arterial segment, an important aspect the alternative models provide is that they can be easily adopted by experimentalists through building in-vitro arterial models to assess the functional significance of the obstruction caused by the disease and its relation to any given patient specific physiologic parameter.

In the present work, a sharp interface immersed boundary method using ghost-cells is implemented in a traditional 5th-order WENO scheme for shock capturing. To avoid non-physical negative density and pressure involved in high order conservative schemes, a cut-off flux limiter is introduced to enforce the positivity-preserving property, which allows to simulate the flow near vacuum and with strong discontinuities. The combined approach was first benchmarked by a steady oblique shock reflection problem with an exact solution from the oblique shock theory. Then, the algorithm was tested for singularity treatment by another classical problem of shock interaction with a forward facing step. For problems with moving boundaries, the new approach was validated against well-tested numerical solutions of a prescribed moving cylinder problem and a shock-lifting-off-cylinder problem. The agreement in all the benchmark studies shows the accuracy and capability of the current algorithm, in its simple form, to simulate the shockwave interacting with stationary obstacles or moving structures, where the structures′ motion can be either prescribed or determined by fully-coupled dynamics with surrounding flows. Finally, the approach was applied to investigate the impact of shock waves on a deformable body modeled by spring-linked plates, with different spring constants being considered.

Blast waves are generated upon release of a large amount of energy in few milliseconds. Upon release, these high pressure waves propagate rapidly and interact with human head and lead to severe traumatic brain injury (TBI). Understanding the mechanics of blast flow would allow us to develop effective tools to protect the head against these shockwaves. Military helmets are known as the most effective tool for protecting the soldier’s head against blast threats. However, due to the complicated nature of the shockwave development and propagation, as well as its interaction with head and helmet, the efficiency of helmets is still in question. The major problem with using helmets under blast loading is the entrapment of blast shockwaves inside the helmet gap and its reflection from the interior of the helmet’s shell. Moreover, development of an amplified pressure region at the opposite side of the incoming blast waves, referred to as the underwash effect of helmets has raised some concerns. To this end, we performed a computational fluid dynamics (CFD) analysis to better understand the mechanism of the blast shockwave interaction with head, as well as the effect of the helmet on the alteration of flow mechanics. The compressible, turbulent blast flow was simulated in ANSYS CFX by releasing the air from a high-pressure domain into the low-pressure one (at ambient pressure). The un/protected heads were exposed to an identical blast overpressure of 520 kPa in a frontal open blast scenario. Pressure contours and velocity profiles were recorded at several time instances for both unprotected and helmeted heads. Our primary results revealed that the change of the flow momentum inside the helmet gap, the reunion of the blast flow inside the gap as well as the development of adverse pressure gradient (and hence recirculating flow region) at the rear side of the head are the major reasons leading to this adverse phenomenon.

Hydrocyclone separators are widely used in various industrial applications in the oil and mining industries to sort, classify and separate solid particles or liquid droplets within liquid suspensions. Often, studies in the literature have investigated idealized and simplified geometries, which are also typically scaled down to very small sizes. In this study, the two phase flow system inside a transparent acyclic model with actual milling circuit cyclone hydraulics was investigated computationally and experimentally. The diameter and height of the hydrocyclone are 12.7 cm and 94 cm, respectively. In many industrial applications, a single phase flow system in a hydrocyclone is a rarity, since nearly all cyclones have an underflow which is open to atmosphere, and therefore an air core is present along the central axis. In this study, the flow field with an air core present has been investigated. The computational modelling was conducted using Star CCM+, a commercial Computational Fluid Dynamics (CFD) software package. Large Eddy Simulation (LES) and the Volume of Fluid multiphase model was used. Additionally, the computational studies also focused on the prediction of the dimensions of the air core, which were measured experimentally. The tests were conducted in the Reynolds number range of 20,000–150,000 and 9000–67,800 for the water and NaI solution respectively. The model hydrocyclone was made of optically transparent acrylic plastic with flat, smooth outer surfaces so that there were no reflections, distortions, or obstructions. Refractive index matching, to minimize refraction effects, between the test fluid and acrylic test piece was achieved using a test liquid of sodium iodide aqueous solution (63.3% NaI by weight). Images of the flow field with the air core were taken using a Canon DSLR camera. A comparison between the experimental data and the computational results were made in the r-z plane. The experimental results and the computational results will be discussed in this paper.

In this study, we conducted experiments using a shock tube to investigate how the thickness of soft reflecting material affects the reflected shock wave of oblique shock reflection. Results indicated that transition to Mach reflection (MR) is delayed when the reflecting surface is silicone, compared with that when the surface is rigid. Regarding the influence of reflecting material thickness, no remarkable difference was observed between 1mm and 10mm thickness, due to the trade-off of reflecting wedge angle and material thickness. No significant difference was observed regarding the influence of silicone on wave configuration.

An experimental study was carried out to investigate the turbulent flow field of confined vortical flow in a narrow annulus. The test section consists of two concentric annuli that are separated by a tube with four equally spaced rectangular apertures. Water is passing from the inner annulus to the outer annulus and the main region of interest is near aforementioned rectangular holes. Flow velocity was measured using a stereoscopic particle image velocimetry (PIV) system. The small size of the outer annulus required development of multiple improvements to the PIV system. Stereoscopic calibration within the curved and narrow annulus required development of a calibration method that ensured accurate alignment of the laser sheet to provide an accurate polynomial mapping of the physical coordinates to the pixel location on the ICCD chips of the cameras. Macro lenses were used to provide the desired field of view. Test section dimensions required accurate vertical repositioning of the cameras and the laser within the measurement plane. This was implemented using a two-axis positioning system. Reflections within the outer annulus required development of a polyurethane based paint that reflected the laser light with a shifted wavelength. The paint survived high shear stresses and pressure gradients during testing. Three dimensional flow field was assembled based on measurements taken, providing insight into the behavior of the flow as it exits the aperture and mixes in the outer annulus. A vortex core was identified. The results of measurements are compared to CFD results and serve as a tool validating the CFD model. Both the CFD and the measured results predict similar trends in flow behavior, showing strong radial and tangential components of velocity at the plane coincident with the edge of the hole.

The current study investigates the flow field near a surface with a micro-PIV system using a square tube to enhance optical access. Measurements of velocity fields and eddy structures near the wall of tubes are important to the design of in-tube surface geometries. In experimental fluid mechanics, particle image velocimetry (PIV) is now a common way to measure velocity. However, PIV measurements near walls require efforts to deal with low particle density, high shear gradient and wall reflection. The current paper discusses a PIV measurement technique utilized to observe flow dynamics in near-wall regions. PIV uncertainty analysis is discussed in this study. The experimental results are compared with previous results for validation.

Foreign object strikes are one of the major aviation incidents that cause tremendous risk to both the aircraft and passengers, the adverse implications of which can ripple through the aviation industry. The demand for air transportation has been on the rise, leading to foreign object ingestion into the engines becoming one of the most threatening scenarios. In January 2009, Flight 1549 took off from LaGuardia Airport in New York City and struck a flock of Canadian Geese during takeoff. Both engines ingested birds, resulting in mid-air catastrophic engine failure. Fortunately, none of the damaged engine components penetrated into the cabin and the aircraft successfully completed an emergency landing on the Hudson River without incurring any casualties. In this work, explicit finite element strategies have been adopted to model the Fluid-Solid Interactions (FSI) present in a bird ingestion scenario. Taking into account the fluidic composition of bird bodies, a proper methodology to model the Fluid-Solid Interaction was implemented. The investigations were aimed to understand the significance of impact force histograms in an accurately represented model, analyze shockwave propagations and reflections during the impact window, and perform parametric studies to highlight a superior representation for the Fluid-Solid Interaction (FSI). The paper presents a modern explicit finite element methodology adopted to accurately model bird ingestion into a complex turbomachinery forward section and analyze subsequent failure sequencing of the relevant structural components within the system.

Particle image velocimetry (PIV) has transformed fluid mechanics research in recent years. PIV also holds the possibility to transform fluids engineering undergraduate education with the ability of students to get hands-on experience in visualization of real flow fields. One barrier to use of PIV is the cost of a system. Research grade systems are often over $100,000 and inaccessible to many undergraduate students. Recent availability of low-cost high-frame-rate digital cameras, lasers, and public domain software offer potential accessibility for PIV for many labs at academic institutions. In this paper we describe the development of a PIV system for under $1000 including specific components and their costs. In our lab this system is currently being used for several liquid flow experiments including obtaining the flow field in and near small bifurcations. Although we are using the system for liquid flows, it may be used for gas flows as well. One issue that is addressed is the cost of flow seeding — this has been solved by using pulverized green algae as seed particles with a strong reflection by a 532 nm green laser. The system is small and portable and is useful for observing flow in locations that are not of direct interest for experiments, but may have a bearing on flow conditions in experimental measurements; such as upstream of test sections. We describe some examples of how we have used the lost-cost PIV system in our lab and how it can be used for fluids engineering education and research. The current research application of this system is performing loss coefficient calculations in a test section using the energy dissipation.

Rotodynamic pumps designed by traditional methods have persisted at their current levels of maturity, vis-à-vis performance and efficiency, for decades now. These historical statistics of performance have been formulated as industry standard charts of efficiency based on specific speed and flow-rate (Fig. 1). Through the last decade, research activity aimed at challenging and surpassing these historical performance levels, via the application of various optimization methods and tools, has been slowly but surely building up. The work presented in this paper is one such effort to enhance the performance of a pump, with the impeller blade of a medium specific speed centrifugal pump chosen as its subject, the blade being the heart of any turbo-machine. This study is somewhat unique in the respect that intuitive, high-level features were used to optimize the blade design rather than inanimate variables. A radical and proprietary blade development scheme allowed a 75% reduction in the number of parameters needed to fully describe the blade, compared to conventional blade layout and development schemes. A software suite comprised of an intimate blend of proprietary and commercial tools, for design, performance simulation and optimization, constituted the engine for this study. A template was generated with all software linked in a seamless bi-directional communication loop for hands-off execution of the optimization study (Fig. 3). A Design of Experiments (DoE) scheme is used to generate several hundred design points corresponding to different geometry and flow configurations. Computational Fluid Dynamics (CFD) software was used to simulate the performance and yield integrated output parameters, viz., head, power, and efficiency, for each design point at multiple flow-rates. Data from the simulated DoE table was then used to generate an n -dimensional Response Surface (RS), which established pre-validated relationships of the output to the input parameters. Finally, a Multi Objective Genetic Algorithm (MOGA) technique was used to extract the most efficient designs satisfying other weighted goals or objectives. The weighting process forces some objectives to be sacrificed to a certain extent in favor of others, which is a good reflection of reality, since a trade-off between different objectives is an essential element of any design process. The predictions from the optimizer were then subjected to a final round of CFD simulations for validation. Two discrete application examples were used as test cases for this study, using the optimization template described in the foregoing. This multi objective optimization approach resulted in a performance improvement of 2 efficiency points over a mature baseline blade design, developed using an advanced proprietary design code. Traditional CFD-assisted design enhancements were found to provide less than 1 point of efficiency improvement over this baseline design.

This paper examines cavitation phenomena generated by a 500 kHz, high-intensity focused ultrasonic (HIFU) beam, with pressure amplitude in the focal zone of up to 19 atm, in quiescent water and at the exit of a jet. The pressure field and cavitation are visualized using high-speed digital in-line holography. The spatial distribution of the acoustic pressure is determined form the variations in the water density and refractive index. A partial standing wave is generated by the reflection of the sound from the wall of the test chamber. Several cavitation phenomena are observed. At low to moderate sound levels, bubbly layers form in the pressure nodes of the standing wave, in the periphery of the focal zone. At high sound levels, clouds of vapor bubbles are generated in the antinodes, and migrate in the direction of the acoustic beam at speeds in the 1–4 m/s range. Both the cloud size and velocity oscillate, with the size peaking in the nodes and the velocity in the antinodes. A model for the cloud dynamics shows that the periodic velocity variation is dominated by the balance between the primary Bjerknes force and the drag. The secondary Bjerknes force involving interactions among the bubbles within the cloud is a likely cause for the size oscillations.

Particles on the plate vibrated by ultrasonic were gathered on the node of vibration plate. Similar to this phenomenon, if two plates are set with parallel, a plate is vibrated by ultrasonic and acoustic pressure distribution is produced between the vibration and reflection plate, nano-sized particles might be collected from the air flow and air is cleaned. Although ultrasonic was applied for the liquid and remove particles, but this has not been researched for air. Then the purpose of this study is to calculate the acoustic pressure distribution between the vibration and reflection plate by using CFD, and investigate the nano-sized particle motion in acoustic pressure distribution. When standing wave between plates is calculated, acoustic wave generated by ultrasonic vibration was reflected at the reflection plate. In the case of CFD, standing wave could not be calculated because these waves disclaim each other and wave generated by a plate doesn’t reach at the other plate. Therefore acoustic wave is also produced at reflection plate, duplicate these waves from vibration and reflection plates, and standing wave between plates were obtained. About investigation for the nano-sized particle motion in acoustic pressure distribution, particles were injected into calculation domain. Particles which have the same diameter and density with influenza virus were applied in this calculation. Examination of particle’s motion and efficient condition on dust catch were calculated in case of changing parameter, for example, amplitude and frequency of vibration plate, velocity of flowing particles, concentration of particles in the flow and so on.

Computational fluid dynamics (CFD) is one of available methods to quantitatively evaluate the treatments of cardiovascular disease. However, currently, applications of this technology to cardiac surgery are few due to the complexity of performing physiological simulation. Here, we used CFD to study the outcome of the Norwood surgical procedure for palliating hypoplastic left heart syndrome in a 33-month-old child. The Reynolds number for post-surgical flow calculated at the peak of systole was about 4000, consistent with turbulent flow. During diastole, by contrast, the flow reduced to low speed, suggesting the strong transition flow from systole to diastole. Therefore, to improve the simulation of transitional flow, we determined that time step intervals of 10 −5 second were best in using the k –ε turbulence model. We also develop a new boundary condition to simulate blood pressure wave reflection from peripheral vessels in order to physiologically capture pressure recovery and correctly obtained flow through each arch-branch and flow pattern in the coronary. Then we computed time-varying energy losses, local pressure, and wall shear stress at the anastomosis to evaluate the surgical outcome. The results suggest the time step and boundary conditions that take account of pressure wave reflection improve simulation of cardiovascular flow.

The influence of acoustic pressure and flexural vibration on friction reduction effect by ultrasonic has been investigated in this study. Then the relationship between friction reduction of particles by each effect and the particle density has been shown by using of ultrasonic. Additionally, the maximum of particle density which could receive the friction reduction effect by the acoustic pressure has been expressed. In short, when the particle density was large, the influence of friction reduction effect by the acoustic pressure decreased and the influence of friction reduction effect by the flexural vibration grew. In this study, two plates were set parallel to each other. The dried particles were scattered on the lower plate, and the ultrasonic was applied. Then, the entire equipment setup was tilted slowly until the scattered particles began to move, and the friction coefficient was measured. Then, influence of acoustic pressure and flexural vibration were evaluated. In order to evaluate the reduction effect by acoustic pressure and flexural vibration, firstly, the distribution of acoustic pressure between reflection plate and the vibration plate have been measured. As a result, it was clarified that acoustic pressure distribution became the almost same whether ultrasonic was applied for the upper or lower plate, and the reflection plate vibrated little. Therefore it was possible to divide the influence of acoustic pressure and flexural vibration on the friction reduction.

In this study, we have examined a crystallization effect of colloidal dispersion induced by the various elongational flows. Extremely strong electrostatic repulsion makes a crystal structure called ‘colloid crystal’. A colloid crystal has hundreds of nano-meters in grating scale and it reflects the visible light due to the Bragg diffraction. It has the potential to become different photonic devices such as an inexpensive photonic device and a planar laser source, but it requires the evolution of the process of making a single-crystal with external stimulus. The methods using flow operation described in this study are expected to the crystallization action of a colloidal dispersion. In the experiment, 2 types of the flow have been examined. The flows have a contraction or an expansion part between two parallel plates separated by 0.1 mm gap and it cause deformations of a contraction or an extension for the colloid. We have evaluated the crystallization effects by a spectroscopic observation of visible-lights reflection on the flow region. As a result, while expansion flows have no crystallization effect, contraction flows have shown it.

Multiphase flow in microchannels is frequently encountered in microdevices. Predicting the behavior of the multiphase flow has been difficult. The dynamics of liquid films inside of capillaries is important to microfluidic flows. At high flow rates with gas-liquid flows, the dynamics of liquid films on the interior surfaces of the microchannels can have a global affect on device operation. Measurement via optical microscopy of velocities in these liquid films and interface dynamics has been hampered by the inability to correctly image near dyanamic interfaces due to optical reflection and very short time scales of the flow. A novel technique for optically measuring liquid film thicknesses within circular microchannels has been developed. A swept-field confocal microscopy unit has been modified to capture the reflected light by replacing the dichroic mirror, commonly used in fluorescence confocal microscopy, with a 50/50 beam splitter. “Optical slicing” in the x-y plane, coupled with precision z-axis stepping allows for the detection of the gas-liquid interface by blocking out-of-focus light.

It is known theoretically [1–3] that infinitely long fluid loaded plates in mean flow exhibit a range of unusual phenomena in the ‘long time’ limit. These include convective instability, absolute instability and negative energy waves which are destabilized by dissipation. However, structures are necessarily of finite length and may have discontinuities. Moreover, linear instability waves can only grow over a limited number of cycles before non-linear effects become dominant. We have undertaken an analytical and computational study to investigate the response of finite, discontinuous plates to ascertain if these unusual effects might be realized in practice. Analytically, we take a “wave scattering” [2,4] — as opposed to a “modal superposition” [5] — view of the fluttering plate problem. First, we solve for the scattering coefficients of localized plate discontinuities and identify a range of parameter space, well outside the convective instability regime, where over-scattering or amplified reflection/transmission occurs. These are scattering processes that draw energy from the mean flow into the plate. Next, we use the Wiener-Hopf technique to solve for the scattering coefficients from the leading and trailing edges of a baffled plate. Finally, we construct the response of a finite, baffled plate by a superposition of infinite plate propagating waves continuously scattering off the plate ends and solve for the unstable resonance frequencies and temporal growth rates for long plates. We present a comparison between our computational results and the infinite plate theory. In particular, the resonance response of a moderately sized plate is shown to be in excellent agreement with our long plate analytical predictions.

Experimental flow visualization in porous media is often conducted using optical techniques such as PIV and PTV for velocity field estimation and LIF for concentration field measurements. The porous bed is made optically accessible to laser light and imaging by matching refractive indices of the liquid phase to that of the solid matrix, including the channel walls. The methods most commonly employed to match the refractive indices have been to maximize the transmitted intensity through the bed or to rely on refractometers for measurement of the liquid and solid phases. Refractometers with sensitivity of 0.001 could still cause refraction problems in a porous bed, while accuracy and sensitivity of transmission based methods are limited by the camera resolution and noise scattered by impurities and stray light caused by reflections at interfaces. Both these methods fail to provide uncertainty estimates for particle position determination due to slight refractive index mismatching. This work presents a method for assessing the matching of refractive indices that relies on measuring distortion of a target when imaged through a porous bed. The target used is a grid of 250 μm dots irradiated with light at the necessary wavelength at which refractive indices are to be matched. Two principle types of distortion are quantified, distortion of the image centroid due to interface refraction and intensity distortion within the image for index mismatching as low as 0.0005.

Many micromachines use rotating shafts and other moving parts which carry a load and need fluid bearings for support. Most of them operate with air or water as the lubricating fluid. The present study analyzes air microbearing represented as an eccentric cylinder rotating in a stationary housing. The fluid mechanics and operating characteristics of microbearing are different from their larger cousins. The small length-scale may invalidate the continuum approximation in Navier-Stokes equations, and slip flow, rarefaction, compressibility and other unconventional effects may have to be taken into account. Surface effects dominate in small devices due to a high surface-volume ratio. In this study, two-dimensional eccentric-shaft journal microbearings with different eccentricities are simulated by direct simulation Monte Carlo (DSMC) code incorporated with a Volume-CAD software. The diffuse reflection model and Cercignani-Lampis-Lord (CLL) model are applied to model the molecule-surface interaction by considering the accommodation coefficients of shaft wall and housing wall separately. The distribution of mean free path in the flow field indicates that the continuum model may break down and it is necessary to carry our molecular modeling. Calculation results show that at high eccentricity and high accommodation coefficient on the housing wall (ACO) the flow may develop a recirculation region. However, the accommodation coefficient on shaft wall (ACI) does not have any effect on the occurrence of recirculation and the size of recirculation zone. There is antisymmetry of the pressure about a vertical axis, which produces a pressure force on the shaft wall. The influence of ACI to isobars is larger than that of ACO. The shear stress profile on shaft wall is big at low ACI. At the region of short clearance between the shaft wall and housing wall, it is also influenced by the surface condition of housing wall and may even change its direction at low ACO. The pressure profile is reduced in amplitude as the ACI increases, but it is enhanced a little with the increase of ACO. The ACO has great impact on the viscous force in the case of big eccentricity. With the increase of ACI, the viscous force decreases. The pressure force is high at large eccentricity. The influence of ACO to pressure force is insignificant, but the pressure force fall is enormous when ACI increases, especially for large eccentricity. The total force decreases markedly at high eccentricity when ACI increases. The ACO almost has no impact on the total force. The torque increases with ACO, but decreases with ACI. The eccentricity also has great impact on the torque, and the microbeaing may have large torque at high eccentricity. The method developed in this paper would be very useful for designing and evaluating journal microbearing.