Effect Of Oscillation Frequency Research Articles

Effect of oscillation frequency on wall shear stress and pressure drop in a rectangular channel for heat transfer applications

The exploitation of flow unsteadiness in microchannels is a potentially useful technique for enhancing cooling of future photonics systems. Pulsation is thought to alter the thickness of the hydrodynamic and thermal boundary layers, and hence affect the overall thermal resistance of the heat sink. While the mechanical and thermal problems are inextricably linked, it is useful to decouple the parameters to better understand the mechanisms underlying any heat transfer enhancement. The current work characterises the behaviour of the wall shear stress and pressure gradient with frequency, using experimental particle image velocimetry (PIV) measurements and the analytical solution for oscillatory flow in a two-dimensional rectangular channel. Both wall shear stress and pressure gradient are augmented with frequency compared to steady flow, though the pressure gradient increases more significantly as a result of growing inertial losses. The three distinct regimes of unsteadiness are shown to display unique relationships between the parameters pertinent to heat transfer and should therefore be considered independently with respect to thermal enhancement capability. To this end, the regime boundaries are estimated at Womersley number Wo = 1.6 and 28.4 in a rectangular channel, based on the contribution of viscous and inertial losses.

Experimental visualization of gas-liquid-solid three-phase flow during reciprocating motion

In the current work, a high-speed camera was employed to capture the flow patterns of nanofluids inside a simplified piston gallery at various crank angles. The heat transfer mechanisms of gas-liquid-solid three-phase flow (air, base fluid, and nanoparticles) during reciprocating motion was explored. The effects of oscillation frequency, nanofluid filling ratio, and nanoparticle concentration were also discussed. The addition of nanoparticles into a base fluid enhances the heat and mass transfer processes effectively. Of all the three determining factors of the flow field during the reciprocating motion, the oscillation frequency plays a major role, the nanoparticle concentration plays a minor role and the nanofluid filling ratio lies in between.

Experimental Investigation of Flow Over a Transversely Oscillating Square Cylinder at Intermediate Reynolds Number

This paper presents an experimental study of flow over a square cylinder oscillating in transverse direction. The Reynolds number selected for present study is 485. Limited study has also been made for two other Reynolds numbers, namely, 295 and 775. The objective of the present study is to modify the near-wake flow structure using actuation of the cylinder for possible reduction in drag force. Transverse oscillations to the cylinder are provided using electromagnetic actuators. The flow field is investigated using two-dimensional (2D)-particle image velocimetry (PIV) system, hotwire anemometer (HWA), as well as flow visualization techniques. The effect of oscillation frequency and the amplitude on parameters like Strouhal number, drag coefficient, recirculation length, power spectrum, and Reynolds stress are studied. It is observed that the recirculation length is reduced significantly with increase in forcing frequency, and consequently drag coefficient is also reduced. For a constant forcing frequency, the vortex strength is reduced with the increase in the amplitude. Further, variation of instantaneous spanwise vorticity shows that separated shear length decreases with increase in forcing frequency. As a result, vortices are moved closer to the cylinder. These phenomena affect the forces acting on the cylinder. Lock-on is also observed at a frequency close to the vortex shedding frequency of the stationary cylinder.

Experimental visualization of gas–liquid two-phase flow during reciprocating motion

The piston-cooling system is one of the critical elements that impact on the thermal management of engines. At present, the use of a cooling gallery inside the piston head makes it possible to optimize the heat extraction and control the piston temperature effectively. However, owing to the small space in piston galleries and their complicated structure, it is rather difficult experimentally to perform accurate visual observations of the internal cooling oil and study the heat transfer characteristics. In the current work, a high-speed camera was employed to capture the flow patterns of water and air inside a simplified piston gallery at various crank angles. The heat transfer mechanisms in gas–liquid two-phase flow during reciprocating motion were explored. The effects of oscillation frequency and water filling ratio were discussed. An effective method was developed to achieve a full understanding of the flow and heat transfer characteristics of cooling medium inside a piston cooling gallery. This could provide an effective method to optimize the thermal management of engines in future.

Characteristics of overexpanded nozzle flows in imposed oscillating condition

A computation study is performed to investigate the effect of imposed oscillation of nozzle pressure ratio (NPR) on the flow structure in a two-dimensional, non-axisymmetric supersonic converging–diverging nozzle. In this study, the overexpanded flow conditions are considered which are dominated by the shock-induced boundary-layer interaction and corresponding free shock separation. The computational results are well validated with the available experimental measurements. Results showed that the internal flow structure of the nozzle is dependent on the process of change of pressure ratio during the oscillation. Distinct flow structures are observed during increasing and decreasing processes of the change of pressure ratio even when the nozzle is at the same NPR. Irreversible behaviors in the locations of free shock separation, Mach stem, and the strength of Mach stem are observed at the same NPRs during this oscillation. However, the nozzle thrust performance does not show the same order of irreversibility as in the cases of shock structures. Further, the effect of oscillation frequency is explored on this irreversible behavior.

Effect of oscillation frequency on wind turbine airfoil dynamic stall

At the same oscillation amplitude, Reynolds Number, mean angle of attack, the dynamic stall characteristics of the NREL S809 airfoil undergoing sinusoidal pitch oscillations of different oscillation frequencies were investigated with modified k-ω SST turbulence model of CFD solution for two-dimensional numerical simulation. The predicted lift, drag coefficients and moment coefficients were compared with the Ohio State University wind tunnel test results, which showed a good agreement. The birth, development and breaking off of eddies were analyzed through streamline distribution around airfoil and the influence of oscillation frequencies on dynamic stall characteristics was also described and analyzed in detail, which enrich the database of dynamic stall characteristics needed by the quantization of oscillation frequencies on dynamic characteristics and prove that sliding mesh method is reliable when dealing with dynamic stall problems.

Surface hardening of AISI 4340 steel by laser linear oscillation scanning

This study investigated the surface hardening of AISI 4340 steel by linear oscillation scanning with a fibre laser. Various frequencies and amplitudes of oscillation were used under laser power of 2020 and 3020 W. Microscopic evaluation was done, and the effect of oscillation frequency on the hardened depth was examined. Hardness profiles were measured along the centre of the irradiated track toward the feeding direction of the laser, across the irradiated width and into the depth below the irradiated surface. The homogeneity of hardness and hardened depth with different processing parameters was investigated. The hardness profiles were compared with the results obtained with conventional single track hardening.

Effects of Oscillation Frequency on the Tribological Properties of Self-Lubrication Spherical Plain Bearings with PTFE Woven Liner

The working principle and performance characteristics of a self-made oscillation tribo-tester were introduced. Under different oscillation frequency and pressure conditions, the friction coefficient and linear wear of spherical plain bearings with polytetrafluoroethylene (PTFE) woven liner were investigated. By using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), the wear mechanism of the spherical plain bearing was analyzed. The results indicate that with the oscillation frequency increases, the friction coefficient of spherical plain bearings decreased while the wear loss increased. In the high frequency of 4.8Hz, the reduction ranges of the friction coefficient is not obvious with the contact pressure increased, but the effect of contact pressure on the wear of the spherical plain bearing is great. During the experiment, adhesive wear, abrasive wear and spalling wear took place on the surfaces of woven liners.

Impact of temperature on the linear viscoelastic region of wood

The impact of temperature on the linear viscoelastic region, which is characterized by critical strain, of Chinese fir ( Cunninghamia lanceolata (Lamb.) Hook.) was investigated at various temperatures between –100 and 220 °C for specimens with a moisture content of approximately 0.6%. The effect of oscillation frequency on the linear viscoelastic region under various constant temperatures was also examined. The results indicated that critical strain generally decreased with increasing temperature except at –80, –20, 40, 120, and 220 °C. These five exceptions were attributed to the occurrence of relaxation processes. With an increase in testing frequency from 1 to 20 Hz, the critical strain decreased slightly at all temperatures. It is suggested that the stored elastic energy and yield stress, which were obtained at critical strain, could be indicators to predict wood mechanical performance.

Motion Sickness with Fully Roll-Compensated Lateral Oscillation: Effect of Oscillation Frequency

During lateral acceleration, the addition of an appropriate roll motion can improve comfort, but some combinations of lateral and roll motion increase motion sickness. To determine how motion sickness caused by lateral oscillation fully compensated by roll oscillation (so subjects feel no lateral acceleration) depends on the frequency of oscillation and compare sickness with that caused by uncompensated lateral oscillation. A total of 160 subjects (8 groups of 20) were exposed for 30 min to fully roll-compensated sinusoidal lateral oscillation at one of 8 frequencies (0.05, 0.08, 0.125, 0.16, 0.20, 0.315, 0.5, 0.8 Hz). A further 60 subjects (3 groups of 20) were exposed to lateral oscillation (at 0.315, 0.5, or 0.8 Hz) to allow comparison of sickness with that caused by uncompensated lateral oscillation at frequencies not previously studied. Subjects rated symptoms at 1-min intervals. With fully roll-compensated lateral oscillation, illness ratings tended to increase with increasing frequency of oscillation from 0.05 to 0.2 Hz (with peak lateral velocity, +/- 1.0 m x s(-1)) and tended to decrease from 0.315 to 0.8 Hz (with peak lateral jerk, +/- 1.96 m x s(-3)). Roll compensation significantly reduced the duration before subjects developed nausea. Motion sickness is increased by roll oscillation used to compensate fully for low-frequency lateral oscillation. In general, when roll oscillation is combined with low-frequency lateral oscillation, motion sickness cannot be predicted from either the roll oscillation or the lateral oscillation alone. The dependence of motion sickness on the frequency of oscillation is broadly similar for pure lateral oscillation and 100% roll-compensated lateral oscillation.

Gas Holdup and Gas-Liquid Mass Transfer Investigations in an Oscillatory Flow in a Baffled Column

Gas holdup and gas-liquid mass transfer were investigated in a vertical baffled column. Pure carbon dioxide (C02) was used as the dispersed phase and tap water was used as the continuous phase. Gas holdup and mass transfer rate of C02 were measured under semi-batch condition, while the liquid phase was measured in batch mode. Gas holdup was estimated as the volume fraction of the gas in the two-phase mixture in the column. Mass transfer was expressed in terms of the liquid-side volumetric mass transfer coefficient (kLa). The effects of oscillation frequency, oscillation amplitude and gas flow rate on gas holdup andmass transfer were also determined. The results showed that a significant increase in gas holdup and mass transfer could be achieved in an oscillatory baffled column compared to a bubble column. Gas holdup and mass transfer were correlated as a function of power density and superficial gas velocity. Keywords: gas holdup, mass transfer coefficient, power density, superficial gas velocity

Rheology of Asphaltene−Toluene/Water Interfaces

The stability of water-in-crude oil emulsions is frequently attributed to a rigid asphaltene film at the water/oil interface. The rheological properties of these films and their relationship to emulsion stability are ill defined. In this study, the interfacial tension, elastic modulus, and viscous modulus were measured using a drop shape analyzer for model oils consisting of asphaltenes dissolved in toluene for concentrations varying from 0.002 to 20 kg/m(3). The effects of oscillation frequency, asphaltene concentration, and interface aging time were examined. The films exhibited viscoelastic behavior. The total modulus increased as the interface aged at all asphaltene concentrations. An attempt was made to model the rheology for the full range of asphaltene concentration. The instantaneous elasticity was modeled with a surface equation of state (SEOS), and the elastic and viscous moduli, with the Lucassen-van den Tempel (LVDT) model. It was found that only the early-time data could be modeled using the SEOS-LVDT approach; that is, the instantaneous, elastic, and viscous moduli of interfaces aged for at most 10 minutes. At longer interface aging times, the SEOS-LVDT approach was invalid, likely because of irreversible adsorption of asphaltenes on the interface and the formation of a network structure.

Evaluation of Four Microkeratome Models: Quality and Reproducibility of Cut Edge and Cut Surface as Determined by Scanning Electron Microscopy

To compare the quality and reproducibility of cuts produced by four automatic microkeratomes: Amadeus, Hansatome, Summit-Krumeich-Barraquer (SKBM), and Supratome. For the adjustable models (Amadeus and SKBM), the effects of oscillation frequency and blade feed rate were determined. Eight cuts (flap thickness 160 to 180 microm) were made with each microkeratome at each parameter setting and examined using a scanning electron microscope. Quality of the cut surface was scored for each flap according to four criteria, and results were averaged to give a maximum possible score of 64. Quality of the cut edges was scored from 1 (blurred indistinct edge) to 3 (sharp and clearly visible edge). The four microkeratomes performed similarly on overall quality of the cut surface (range: 78% to 88% of theoretical maximum). The Hansatome scored highest (88%), followed by SKBM (86%; 1.0 mm/s blade feed and 7000 rpm oscillation), Amadeus (84%; 2.5 mm/s, 8000 rpm), Supratome (84%; 16.6 mm/s, 12,500 rpm), Amadeus (81%; 3.0 mm/s and 13,000 rpm), and SKBM (78%; 1.5 mm/s, 14,000 rpm). Performance differences were clearer for cut edge quality. The Amadeus produced cuts of the highest quality in 62.5% of cases (at both settings), compared with only 12.5% of cases with the Hansatome, 25% of cases with the Supratome and SKMB (1.0 mm/s, 7000 rpm), and in no cases with the SKBM at 1.5 mm/s and 14,000 rpm. Overall, high frequency with low blade feed rate is desirable for reproducible sharp edges and cut area quality. However, at very high frequencies (e.g., 14,000 rpm with the SKBM) tissue is displaced, producing an uneven cut area. Oscillation/feed quotients of 2000 to 4000 (rpm/mm x s) are a good compromise. Sharp edges are believed to reduce the risk of comeal erosion, introduction of epithelial cells into the interfacial region, and scarring during wound healing.

Gas-LiquidMassTransferin Continuous Oscillatory Flow BaaledColumnina

Gas-liquid mass transfer in continuous oscillatory flow was conducted in a vertical baffled column. Pure carbon dioxide (C02) was used as the dispersed phase and tap water was used as the continuous phase. The mass transfer rate of C02 measured under continuous operation was expressed in terms of the liquid-side volumetric mass transfer coefficient (kLa) and was calculated using a stationary method. The effects of oscillation frequency, oscillation amplitude, and flow rates on mass transfer were also determined. The results showed that a significant increase in mass transfer could be achieved in oscillatory flow in a baffled column comp.:lfed to that in a bubble column. The mass transfer in continuous oscillatory flow in a baffled column was not affected by the liquid flow rate in the range tested. Then, kLQwas correlated as a function of power density and superficial gas velocity.

A Study of Bubble Velocity and Bubble Residence Time in a Gassed Oscillatory Baffled Column: Effect of Oscillation Frequency

In this paper, we report an experimental evaluation of velocity and residence time of single bubbles of different sizes in a gassed oscillatory baffled column (OBC) using a high-speed imaging technique. This work is particularly concentrated on the effect of oscillation frequency. The results show that the axial bubble velocities span several orders of magnitude with respect to the mean at an oscillatory frequency greater than 2 Hz. The ratio of the axial to radial bubble velocity components approaches unity at elevated oscillation frequencies. Combining the high-speed imaging with the high-resolution digital particle image velocimetry, we are able to examine the influence of the fluid flow on the bubble motion within the OBC.We also report the direct measurement of residence time of the bubbles within a baffled cell.

Experimental Study of Flocculation of Bentonite and Alcaligenes Eutrophus in a Batch Oscillatory Baffled Flocculator

An experimental investigation is reported of the flocculation of bentonite and Alcaligenes eutrophus in a batch oscillatory baffled flocculator (OBF), where fluid mixing is achieved by eddies that are generated when fluid passes through a set of equally spaced stationary orifice baffles. Periodically formed vortices can be controlled by a combination of operational and geometrical parameters, such as, oscillation frequency, oscillation amplitude, baffle diameter and baffle spacing. The effect of oscillation frequency and amplitude in the OBF on the percentage of flocculation of both bentonite and Alcaligenes eutrophus was examined, the floc sizes at various operational conditions observed and the strain rates in the OBF measured using a digital particle image velocimetry (DPIV) technique. The results show that the oscillation amplitude is the dominant factor in influencing the percentage of flocculation. The measured strain rate was linked with the percentage of flocculation per ppm polymer dose and the results compared with those obtained in conventional stirred tank flocculators.

1911 急拡大部を持つノズルからの自励振動噴流による衝突熱伝達(続報)

Whistler nozzle has a simple geometry, consisting of a constant diameter straight pipe nozzle and the sudden expansion of a collar which can slide over the pipe nozzle. With a certain sliding length of the collar, audible self-oscillation can be generated in the jet flow. This oscillatory excitation can enhance the entrainment of the surroundings and disturb the potential core of the jet. In the previous flow visualization experiment, comparison between the excited and non-excited cases revealed that the oscillation drastically modifies the vortical structures of the jet, which results in the change of the distribution patterns of the impingement heat transfer. In the present study, more detail experiments were made to see how the effects of oscillation frequency, jet velocity and impingement distance on the heat transfer distributions are. It was found that the jet flow structure changes with Strouhal number, most dominant parameter based on the frequency, and has great influence on the distributions. The transition of the distribution pattern in the case of the excited case was changed more quickly with an increase in the impinging distance, compared with the non-excited case.

Efficiency of mechanical trituration of amalgam. II. Effects of some variables.

The performance criterion of success time has been employed to determine the effects of oscillation frequency, amplitude, mercury content and the mass of the capsule charge on the relative efficiency of the trituration process. The effects of frequency and amplitude can be satisfactorily explained in terms of the impact energy of the mix on the capsule endwall; this includes the contribution of the resulting rise in temperature. However, attempts to measure the temperature of the capsule contents during operating revealed experimental difficulties which may lead to erroneous estimates if thermocouples or similar devices are used. The mass of the amalgam mix has a distinct effect on trituration efficiency, and is most apparent at lower mercury contents. The consequence of this is that "double spills" or larger mixes will require less time to reach the same state as their smaller counterparts. At minimal mercury contents there is the distinct danger of overtrituration as a result. Mercury content is relatively more important than mix mass, and suggests that variations in dispensers and preproportioned capsules may have considerable importance in the uniformity of the resulting mix. Amalgamators may be described by a "power number", having the same dimensions as mW/g, being the product (frequency in Hz)3/(amplitude in m)2, which is thus a guide to the maximum power capable of being expended on the trituration process assuming 100 percent efficiency. Success time provides a measure of that efficiency. The use of the success time in studies of "difficult" variables in the trituration process has been indicated, and these will form the basis of future work.