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Dynamics and stability of locomotion for actively powered simplest walkers

Safak, K.K.

Conference Object | 2004 | Advances in Bioengineering, BED , pp.195 - 196

In this paper we explore methods to achieve actively powered walking on level ground using a simple 2D walker model. The walker is activated either by applying equal joint torques at hip and ankle, or by an impulse applied at toe-off immediately before heel-strike, or by the combination of both. We show that activating the walker by equal joint torques at hip and ankle on level ground is equivalent to the dynamics of the passive walker on a downhill slope. We calculate the stability of the gait cycle by an analytical approximation to the Jacobian of the walking map. Results indicate that short-period gait cycle always has an unstabl . . .e eigenvalue, whereas stability of the long-period gait cycle depends on the selection of initial stance angle. Copyright © 2004 by ASME Daha fazlası Daha az

Computational investigation of vortex structure in the corrugated channel

Unal, E. | Sorguven, E. | Ahn, H.

Conference Object | 2013 | ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)07:00:00 , pp.195 - 196

Characteristics of the flow on a corrugated wall are investigated using computational fluid dynamics. Steady and unsteady k-w simulations and large eddy simulations (LES) are performed. Results are compared with the experimental data obtained via particle image velocimetry (PIV). RANS simulations predict a coherent vortex motion, which is contained within the cavity and has little effect on the outer flow. RANS results demonstrate a mainly 2-dimensional flow and underestimate the pressure loss and friction coefficient by about 25%, compared to LES results. LES results show that the flow is highly turbulent, 3-dimensional, unsteady, . . .and there is a strong interaction between the flow inside the corrugations and the bulk flow. Both LES and PIV represent that the separation and reattachment points vary spatially and temporally, resulting in a thick boundary layer and high friction coefficient. Sudden ejection of the flow from the cavity to the outer flow is also observed in the instantaneous snapshots taken from LES and PIV. These flow ejections prevail in the time-averaged PIV results, indicating a mean inflow towards the cavity near the side walls and a mean outflow from the cavity at the channel center. Even though instantaneous LES results show similar flow bursts, timeaveraged LES results even out the inflow and outflow from the cavities, yielding no net flow in and out of the cavities. Copyright © 2013 by ASME Daha fazlası Daha az

Experimental investigation of vortex structure in the corrugated channel

Unal, E. | Ahn, H. | Sorguven, E. | Gul, M.Z.

Conference Object | 2013 | ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)07:00:00 , pp.195 - 196

Vortex structure in a corrugated channel has been studied with a PIV system measuring two-dimensional velocity fields at different locations and Reynolds numbers. The geometry of corrugation under investigation is the two-dimensional reflection of the circular cross-sectional stainless-steel flex pipe. The results show that turbulence caused by the corrugated wall affects the whole flow field in the channel even at low Reynolds number. The bulk flow field is rather chaotic in the entire channel. Moreover, the velocity vectors show significant interaction between the flow in the groove and the bulk flow. Vortex generated from the gro . . .ove is very unstable and intermittent, and the vortex is not confined within the groove even at low Reynolds number. Vortex in the groove either migrates out of the groove without breaking up, or causes bursting flow from the groove to the bulk. In addition, intermittent and time-mean flow reversals are observed near the crest of the corrugation at low Reynolds number. Though the channel design is intended to be two-dimensional, flow structures in the groove appear to be three-dimensional at high Reynolds number while two-dimensional at low Reynolds number. Copyright © 2013 by ASME Daha fazlası Daha az

Experimental investigation on the condensation efficiency of humid air in a cross-flow condenser

Ahn, H. | Gul, B. | Sahin, Y. | Hartoka, O.

Conference Object | 2013 | ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)8 B , pp.195 - 196

The condensation of steam in the presence of air has been investigated experimentally in the cross-flow flat-plate singlechannel condenser. In particular, the condensation efficiency which is defined by the ratio of heat released during the condensation process (the amount of latent heat) to the total heat extracted from the mixture of vapor and non-condensable gas (the sum of latent and sensible heats) is examined as a function of the air-steam mixture temperature and humidity at inlet and the flow rates of the air-steam mixture and cooling air. The preliminary results are obtained with the operating condition of the air-steam mixt . . .ure flow at 70°C and 80, 85 and 90% relative humidity at inlet. The most notable result is that the condensation efficiency evidently decreases with the increase of the cooling air flow rate. With both mixture and cooling flow rates kept constant, the condensation efficiency increases, as expected, with the increasing air-steam mixture humidity at the inlet. On the other hand, the air-steam mixture flow rate appears to have little effect on the condensation efficiency. Copyright © 2013 by ASME Daha fazlası Daha az

Turbulent heat and mass transfer inside open and closed cavities

Şeker, D. | Egrican, N.

Conference Object | 2007 | ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)8 , pp.1457 - 1466

ASME ASME 2007 International Mechanical Engineering Congress and Exposition, IMECE 2007 -- 11 November 2007 through 15 November 2007 -- -- 112020

Computational noise prediction of a centrifugal fan

Sorguven, E. | Dogan, Y. | Bayraktar, F. | Arslan, E.

Conference Object | 2007 | ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)8 , pp.1427 - 1432

In this study, computational aeroacoustics methods are employed to analyze the flow and the noise emission in a centrifugal fan. Unsteady flow inside the centrifugal fan is predicted with large eddy simulation. Acoustic sources are computed based on the results of the time-dependent flow simulation. The turbulent pressure fluctuations on the blades and on the volute of the fan are used as the source terms in the acoustic analogy of Ffowcs Williams and Hawkings. Propagation, diffraction and scattering of the acoustic sources inside the volute are computed with the boundary element method. Numerically obtained sound pressure level dis . . .tribution in narrow band frequency spectrum is compared with experimental measurements at certain microphone points. The numerical and experimental sound intensity maps are also compared to validate the numerical prediction of directivity. Computational results agree well with the experimental data and provide an insight of the noise emission mechanisms. Copyright © 2007 by ASME Daha fazlası Daha az

Experimental investigation of the parameters influencing refrigerant migration in a refrigeration system

Kocaturk, S. | Guldali, Y. | Egrican, A.N.

Conference Object | 2007 | ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)8 , pp.1409 - 1419

Refrigerant migration, which is characterized by the pressure equalization between condenser and evaporator during compressor off period, is one of the most effective cycling losses phenomena with the cause of 5-15% increasing rates in the energy consumption of a refrigeration system. In this study, an experimental facility was set up to investigate the parameters influencing refrigerant migration. A freezer cabinet with wire&tube evaporator and R600a and R134a reciprocating compressors were used. For the detailed control of various condensing parameters (condensing temperature, pressure, condensing rate in the condenser) a double-t . . .ube water-cooled condenser was designed. The condenser was constructed to allow the application of various lengths. Temperature and pressure values on various points of the system, refrigerant mass-flow rate, compressor power and consumed energy values were measured simultaneously. Condensing pressure and condensing rate in the condenser, condenser length, cycling frequency, refrigerant type used in the system, and ambient temperature were investigated as parameters influencing refrigerant migration. Refrigerant migration during the off period was blocked with a solenoid valve that is located before the inlet of the expansion device, and energy consumptions and COP values were mainly compared for the opened-valve and closed-valve conditions for each controlled experimental parameter. Copyright © 2007 by ASME Daha fazlası Daha az

Flow simulation and optimization of a left ventricular assist device

Sorguven, E. | Ciblak, N. | Okyar, A.F. | Akgun, M.A. | Egrican, A.N. | Safak, K.K. | Kucukaksu, S.

Conference Object | 2007 | ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)8 , pp.1401 - 1407

Artificial assist devices offer a promising treatment option for patients with congestive heart failure, especially when the patient is not eligible for heart transplantation. In order to develop a left ventricular assist device an interdisciplinary research, involving engineering and medical research teams, is conducted. The left ventricular assist device investigated in this study is the MicroMed DeBakey VAD [1], an axial blood pump that provides flow from the left ventricle to the aorta. The geometry of this baseline design is generated via parametric modeling. An optimization surface around the baseline design is formed by using . . . the design of experiments method. Accordingly, eighty parameter sets and the corresponding CAD models are created. Flow through these pumps is simulated at the operation point. Flow data are evaluated to predict the pump performance, blood damage and bearing friction. An axial pump, closer to the optimum, is found that provides 8635 Pa pressure increase at a flow rate of 6 l/min and a rotational speed of 10000 rpm. Pressure head of the selected pump is 18% higher and blood damage is 4% less than the baseline design. Copyright © 2007 by ASME Daha fazlası Daha az

Frtcoils: General purpose simulation software for design and prediction of thermal and hydraulic performance of finned-tube compact heat exchangers

Tun, M.N.N. | Egrican, N.

Conference Object | 2007 | ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)8 , pp.1355 - 1360

This paper presents computer software developed for rating and optimum selection of finned circular tubes compact heat exchangers with various coil geometries. The software is developed to use as a computing tool for commercial and R&D purposes in FRITERM A.S, an original equipment manufacturer (OEM) of finned tube heat exchangers. Finned-tube heat exchangers are highly utilized in refrigeration and process industries and heat transfer and pressure drop calculations are very important to manufactures and design engineers. For this purpose, a simulation and design software to predict the performance of finned-tube heat exchangers is . . .presented. In finned-tube coils fin side fluid is air and tube side fluid can be water, oil, glycol water solution mixture and refrigerants. The analysis and rating of coils at dry and wet operating conditions are presented. Design and the most suitable selections of coils at the given parameters and design constraints from many different coil geometries are also performed in the software. User-friendly objectoriented programming C# is applied in developing the software. The software is developed in modular basic. Six modules are developed: Heating Coils, Cooling Coils, Condenser Coils, Steam Coils, Heat Recovery Coils and Evaporator (DX) Coils. REFPROP is also integrated in the software and all fluids' thermal and transport properties are obtained from REFPROP. Heat transfer and pressure drop correlations available from literature are evaluated with recommendations. Simulated results are verified against experimental results. Copyright © 2007 by ASME Daha fazlası Daha az

Discharge of granular materials from hoppers with various exit geometries

Ahn, H. | Yilmaz, E. | Yilmaz, M. | Bugutekin, A.

Conference Object | 2007 | ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)8 , pp.1421 - 1426

The discharge of glass beads from axisymmetric hoppers was experimentally investigated. Two types of hoppers were employed: one was of conical hoppers with different angles and the other was nozzle-shaped. The result showed that, for a conical hopper with a hopper angle greater than 45°, the hopper angle had little effect on the discharge coefficient. On the other hand, for a hopper angle less than 45°, the discharge coefficient rapidly increased with decreasing hopper angle. The present data agreed well with other experimental data available in the literature. The discharge coefficient of the nozzle-shaped hopper was measured to be . . . as considerably high as that of a conical hopper with the hopper angle of 10°. Preliminary investigation on velocity profiles at hopper exits was conducted by examining the traces of particles in digital photographs. Visual observation on the degree of spread of the jets of particles coming out of hoppers with different hopper angles indicated that the velocity profile at the exit for a narrow hopper was more uniform than that for a wide hopper. Copyright © 2007 by ASME Daha fazlası Daha az

Experimental investigation on pressure drop in corrugated pipes

Ahn, H. | Uslu, I.

Conference Object | 2013 | ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)07:00:00 , pp.1421 - 1426

The characteristics of pressure drop in corrugated pipes were experimentally studied in both straight and helically coiled configurations. The present study employed the stainless-steel pipes with the corrugation of circular cross section, which are widely used in boilers and pipe systems between solar panels and boilers. The diameters of corrugated pipes were 20.4, 25.4, 34.5 and 40.5 mm. The corrugated pipe, approximately 10 m in length, was configured either in the straight manner or in the helical coil with the helix diameter of 0.43 or 0.64 m. Water stored in a tank was fed into a corrugated pipe by a pump while the flow rate w . . .as controlled by a control valve. The friction factors of the pipes remain constant over the range of Reynolds number from 4,000 to 50,000, indicating that the flow in the pipe was fully turbulent. When the pipe was straightly configured, the friction factors were measured to be 0.070, 0.075, 0.12 and 0.22 for the diameter of 20.4, 25.4, 34.5 and 40.5 mm, respectively. Thus the present study showed that the friction factors increased with the increasing diameter of the pipe. This result is clearly contrary to a rare experimental result available in the literature. On the other hand, as expected, the friction factor for the helically coiled configuration was higher than that of the straight configuration with the same tube diameter, and the configuration of the smaller helix diameter yielded the larger friction factor. The reason for the increasing friction factor with the increasing pipe diameter remains to be explored further. Copyright © 2013 by ASME Daha fazlası Daha az

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