Dimitris Nikitopoulos - Academia.edu (original) (raw)

Papers by Dimitris Nikitopoulos

Research paper thumbnail of A Study of Square and Circular Coaxial Jet Flows

APS, Nov 1, 1998

Results from experiments in flows ensuing from circular and square coaxial nozzles with parallel ... more Results from experiments in flows ensuing from circular and square coaxial nozzles with parallel sides are presented. The co-flow-jet Reynolds number is 19,000 and the inner-to-outer jet velocity ratio is 0.3. Visualizations and local measurements have indicated mixing enhancement when square nozzles are used compared to the circular ones. This is largely attributed to the difference in initial velocity profiles between these two configurations. The existence of axis-switching, a phenomenon observed in single non-axisymmetric nozzles, is not clearly evident from visualizations and measurements in the square coaxial nozzles. Both coaxial nozzle flows exhibit four distinct regions in the near field flow development. The evolution of each region and its characteristics are outlined using flow visualization combined with local measurements of the axial velocity field. Scaling of mean and fluctuation velocity profiles as well as spectral aspects of the coaxial flow are also examined and discussed.

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Research paper thumbnail of Design, Characterization, and Verification of a Closed Loop Wind Tunnel With Linear Cascade and Upstream Wake Generator

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Research paper thumbnail of Comparisons between circular and square coaxial jet flows

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Research paper thumbnail of Visualization and DPIV measurements of non-circular coaxial jet flows

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Research paper thumbnail of One Component Two Phase Flow in Horizontal and Vertical Ducts: Some Basic Considerations

OSTI OAI (U.S. Department of Energy Office of Scientific and Technical Information), Dec 15, 1983

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Research paper thumbnail of Structure of square coaxial nozzle flows

Flow visualization and measurement results are presented for flows in square coaxial nozzles with... more Flow visualization and measurement results are presented for flows in square coaxial nozzles with parallel sides, and the corresponding geometry of axisymmetric coaxial nozzles, at a co-flow-jet Reynolds number of 19,000 and at two inner-to-outer jet velocity ratio values (0.22 to 0.3). The presence of large-scale periodic structures is evident near the origin of the shear layers, although not as clear as in lower Reynolds number flows. The internal unmixed regions are reduced in size with decreasing velocity ratio. Visualizations and local measurements have indicated mixing enhancement when square nozzles are used compared to the axisymmetric ones. This is largely attributed to the difference in initial velocity profiles between these two configurations. The existence of axis-switching, a phenomenon observed in single nonaxisymmetric nozzles, is not clearly evident from visualizations and measurements in the square coaxial nozzle. Four distinct regions in the development of the coaxial nozzle flows have been identified on the basis of the measured trends. Scaling of mean velocity and turbulence profiles has also been examined for the three shear layers formed in the near field of axisymmetric and square coaxial flows.

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Research paper thumbnail of Visualization and two-color DPIV measurements of flows in circular and square coaxial nozzles

Experiments in Fluids, Jul 1, 2001

High-resolution, reactive Mie scattering laser-sheet visualizations, two-color digital particle i... more High-resolution, reactive Mie scattering laser-sheet visualizations, two-color digital particle image velocimetry (DPIV) and thermal anemometry measurements in flows generated by equivalent coaxial circular and square jets are presented. Visualization results were obtained for three square, coaxial configurations, and a reference circular coaxial nozzle, at two Reynolds numbers of the outer jet (19,000 and 29,000) and for inner-to-outer jet velocity ratios

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Research paper thumbnail of Evaluation of micromilled metal mold masters for the replication of microchip electrophoresis devices

Microfluidics and Nanofluidics, Jun 7, 2006

High-precision micromilling was assessed as a tool for the rapid fabrication of mold masters for ... more High-precision micromilling was assessed as a tool for the rapid fabrication of mold masters for replicating microchip devices in thermoplastics. As an example, microchip electrophoresis devices were hot embossed in poly(methylmethacrylate) (PMMA) from brass masters fabricated via micromilling. Specifically, sidewall roughness and milling topology limitations were investigated. Numerical simulations were performed to determine the effects of additional volumes present on

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Research paper thumbnail of Transport of single molecules through nanochannels: A novel approach to DNA sequencing

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Research paper thumbnail of Fabrication of 2.5D Rock-Based Micromodels With High Resolution Features

Volume 10: Micro- and Nano-Systems Engineering and Packaging, 2015

Fabrication of 2.5D rock-based micromodels with high resolution features is presented using SU-8 ... more Fabrication of 2.5D rock-based micromodels with high resolution features is presented using SU-8 multi-layer lithography and nickel electroforming for nickel molds. Processes associated with SU-8 were carefully optimized by the use of the vacuum contact, the use of UV filter, and controls of UV exposure doses and baking times. The use of SU-8 MicroSpray enabled the easy fabrication of multi-layers of SU-8, while exhibiting some total thickness variations. The thirteen layered SU-8 samples showed reliable patterning results for features at 10 and 25 μm resolutions, and minor pattern distortions of features at the 5 μm resolution. Flycutting method employed in multi-layer lithography of SU-8 yielded accurate total thickness control within ±1.5 μm and excellent pattern formation for all of 5, 10, and 25 μm features. Electroforming of nickel was optimized with electroplating bath composition and electroplating parameters such as current density to realize the high resolution nickel mold...

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Research paper thumbnail of Bubble Size Effects on Dispersed Phase Motion in Vertical Bubbly Pipe Flow

Volume 1: Fora, Parts A, B, C, and D, 2003

Three-dimensional measurements of bubble size, and velocity are presented along with the associat... more Three-dimensional measurements of bubble size, and velocity are presented along with the associated statistics for dilute bubbly flow in a vertical pipe. These measurements were obtained through a combination of precise index-of-refraction matching and a stereoscopic high-speed imaging system. An image-processing algorithm has been developed and used to extract instantaneous bubble size, shape, velocity and trajectory information and statistics corrected for magnification errors (maximum of 2%). The experiments have been conducted at a Reynolds of 14,600 in vertical pipe-flow and a volumetric gas flow ratio of 0.29%. Bubble sizes ranged from sub millimeter to several millimeters. The results indicate that bubbles less than approximately 20 wall units with very low ratios of rise velocity-to-liquid-average-velocity occupy the full breadth of the test section, while bubbles in the range between 20 and 60 wall units with ratios of rise-velocity-to-liquid-average-velocity greater than 0.5 are efficiently trapped by the wall and are almost exclusively found in the inner wall region (y+ < 30). Bubbles larger than 60 wall units with ratios of rise-velocity-to-liquid-average-velocity less than 0.5 are found throughout the pipe cross-section although with a strong preference to the wall neighborhood. This bubble behavior is put in perspective considering four mechanisms of bubble migration: (a) turbulent dispersion due to near-wall large scale structure, (b) inviscid transverse forces (Magnus), (c) viscous transverse forces (Saffman), and (d) unsteady transverse forces because of vortex shedding and bubble shape deformation.© 2003 ASME

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Research paper thumbnail of Design, Characterization, and Verification of a Closed Loop Wind Tunnel With Linear Cascade and Upstream Wake Generator

Volume 5B: Heat Transfer, 2015

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Research paper thumbnail of Multiphase Digital Particle Image Velocimetry in a Dispersed, Bubbly, Axisymmetric Jet

Volume 2: Symposia and General Papers, Parts A and B, 2002

A particle image velocimetry (PIV) technique developed for application to two-phase flows is pres... more A particle image velocimetry (PIV) technique developed for application to two-phase flows is presented and validated. The technique is capable of simultaneously measuring carrier and bubble phase velocities on a plane. The validation experiments have been conducted in a vertical upwards, two-phase (water-air) bubbly jet flow at a Reynolds numbers of 5,673 and 11,345 and low bubble concentration matching the experiments of Stanley and Nikitopoulos (1998). Comparisons with measurements obtained by Stanley and Nikitopoulos (1998) using Phase Doppler Analysis (PDA) experiments indicate that the agreement between the two techniques is very satisfactory (deviations of the order of 5%) for both single-phase and two-phase jet carrier-flow velocities. In addition, bubble phase velocity measurements obtained from backlit visualizations of the bubbly jet flow using the bubble-tracking method of Fiedler et al. (2001) are successfully compared to those obtained from PIV. The PIV study confirms t...

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Research paper thumbnail of Control of Internal Stress for High Quality Nickel Large Area Mold Inserts

Volume 12: Micro and Nano Systems, Parts A and B, 2009

Metallic large area mold inserts (LAMIs) are essential for the replication of polymer microfluidi... more Metallic large area mold inserts (LAMIs) are essential for the replication of polymer microfluidic devices. Successful molding of micro- or nanoscale features over large areas is dependent on improving the dimensional control of the mold inserts, particularly those fabricated by electrodeposition using the LIGA or UV-LIGA processes. A systematic approach to controlling the internal stress of the nickel deposits, which was essential for predicting the final flatness of the LAMIs prior to electroplating, was carried out. The internal stress of the nickel deposits from a nickel sulfamate solution was estimated using a bent strip stress measurement method after maintaining electroplating chemicals and conditions and reducing contamination. Over-electroplating of the nickel LAMIs was performed on SU-8 electroplating molds on 150 mm diameter Si wafers. Detailed characterization of the nickel LAMIs to determine the relationship between the overall flatness of the LAMIs and the internal str...

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Research paper thumbnail of Bubble Behavior in the Shear Layer of a Bubbly Jet

Physics of Fluids, 2002

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Research paper thumbnail of Mass-Transfer Distribution in Rotating, Two-Pass, Ribbed Channels with Vortex Generators

Journal of Thermophysics and Heat Transfer, 2001

ABSTRACT

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Research paper thumbnail of An Experimental Investigation of Pure-Substance, Adiabatic Two-Phase Flow in a Vertical Pipe

Journal of Energy Resources Technology-transactions of The Asme, Mar 1, 1994

Measurements of pressure drop, temperture, and average void fraction are presented for adiabatic,... more Measurements of pressure drop, temperture, and average void fraction are presented for adiabatic, vertical-upwards, two-phase flow of Refrigerant 114 in a pipe. An experimental method has been developed according to which the evolution of flow states occurring in long pipes can be realized in a test section of limited length. The experiments cover the range of the flow from flashing to near choking. The measurements indicate existence of macroscopic thermodynamic equilibrium, except in the immediate neighborhood of flashing. Compressibility due to phase change is shown to play a very important role in the development of the flow. Three regions are recognized based on the measured energetics of the flow. Each region is dominated by potential energy changes, dissipation, and kinetic energy changes, respectively. The evolution of the flow is governed by hydrostatic effects in the initial region after flashing and by high, phase-change-induced kinetic energy increases far downstream as the flow approaches choking. In the intermediate region, viscous, inertial and gravitational effects play a role of comparable importance. The interfacial and wall shear forces have also been calculated from the measurements. The former dominate the initial regions of the flow, while the latter are strongest at high vapor contents.

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Research paper thumbnail of Simulation and design of micromixers for microfluidic devices

ABSTRACT

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Research paper thumbnail of A Parallel Code for DNS of Complex, Unsteady, Incompressible Turbulent Flows

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Experimental Assessment of Fluid Velocity to Control Gas Accumulation and Removal While Drilling ?Horizontal? Wells

Bookmarks Related papers MentionsView impact

Research paper thumbnail of A Study of Square and Circular Coaxial Jet Flows

APS, Nov 1, 1998

Results from experiments in flows ensuing from circular and square coaxial nozzles with parallel ... more Results from experiments in flows ensuing from circular and square coaxial nozzles with parallel sides are presented. The co-flow-jet Reynolds number is 19,000 and the inner-to-outer jet velocity ratio is 0.3. Visualizations and local measurements have indicated mixing enhancement when square nozzles are used compared to the circular ones. This is largely attributed to the difference in initial velocity profiles between these two configurations. The existence of axis-switching, a phenomenon observed in single non-axisymmetric nozzles, is not clearly evident from visualizations and measurements in the square coaxial nozzles. Both coaxial nozzle flows exhibit four distinct regions in the near field flow development. The evolution of each region and its characteristics are outlined using flow visualization combined with local measurements of the axial velocity field. Scaling of mean and fluctuation velocity profiles as well as spectral aspects of the coaxial flow are also examined and discussed.

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Design, Characterization, and Verification of a Closed Loop Wind Tunnel With Linear Cascade and Upstream Wake Generator

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Comparisons between circular and square coaxial jet flows

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Visualization and DPIV measurements of non-circular coaxial jet flows

Bookmarks Related papers MentionsView impact

Research paper thumbnail of One Component Two Phase Flow in Horizontal and Vertical Ducts: Some Basic Considerations

OSTI OAI (U.S. Department of Energy Office of Scientific and Technical Information), Dec 15, 1983

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Structure of square coaxial nozzle flows

Flow visualization and measurement results are presented for flows in square coaxial nozzles with... more Flow visualization and measurement results are presented for flows in square coaxial nozzles with parallel sides, and the corresponding geometry of axisymmetric coaxial nozzles, at a co-flow-jet Reynolds number of 19,000 and at two inner-to-outer jet velocity ratio values (0.22 to 0.3). The presence of large-scale periodic structures is evident near the origin of the shear layers, although not as clear as in lower Reynolds number flows. The internal unmixed regions are reduced in size with decreasing velocity ratio. Visualizations and local measurements have indicated mixing enhancement when square nozzles are used compared to the axisymmetric ones. This is largely attributed to the difference in initial velocity profiles between these two configurations. The existence of axis-switching, a phenomenon observed in single nonaxisymmetric nozzles, is not clearly evident from visualizations and measurements in the square coaxial nozzle. Four distinct regions in the development of the coaxial nozzle flows have been identified on the basis of the measured trends. Scaling of mean velocity and turbulence profiles has also been examined for the three shear layers formed in the near field of axisymmetric and square coaxial flows.

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Visualization and two-color DPIV measurements of flows in circular and square coaxial nozzles

Experiments in Fluids, Jul 1, 2001

High-resolution, reactive Mie scattering laser-sheet visualizations, two-color digital particle i... more High-resolution, reactive Mie scattering laser-sheet visualizations, two-color digital particle image velocimetry (DPIV) and thermal anemometry measurements in flows generated by equivalent coaxial circular and square jets are presented. Visualization results were obtained for three square, coaxial configurations, and a reference circular coaxial nozzle, at two Reynolds numbers of the outer jet (19,000 and 29,000) and for inner-to-outer jet velocity ratios

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Evaluation of micromilled metal mold masters for the replication of microchip electrophoresis devices

Microfluidics and Nanofluidics, Jun 7, 2006

High-precision micromilling was assessed as a tool for the rapid fabrication of mold masters for ... more High-precision micromilling was assessed as a tool for the rapid fabrication of mold masters for replicating microchip devices in thermoplastics. As an example, microchip electrophoresis devices were hot embossed in poly(methylmethacrylate) (PMMA) from brass masters fabricated via micromilling. Specifically, sidewall roughness and milling topology limitations were investigated. Numerical simulations were performed to determine the effects of additional volumes present on

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Transport of single molecules through nanochannels: A novel approach to DNA sequencing

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Fabrication of 2.5D Rock-Based Micromodels With High Resolution Features

Volume 10: Micro- and Nano-Systems Engineering and Packaging, 2015

Fabrication of 2.5D rock-based micromodels with high resolution features is presented using SU-8 ... more Fabrication of 2.5D rock-based micromodels with high resolution features is presented using SU-8 multi-layer lithography and nickel electroforming for nickel molds. Processes associated with SU-8 were carefully optimized by the use of the vacuum contact, the use of UV filter, and controls of UV exposure doses and baking times. The use of SU-8 MicroSpray enabled the easy fabrication of multi-layers of SU-8, while exhibiting some total thickness variations. The thirteen layered SU-8 samples showed reliable patterning results for features at 10 and 25 μm resolutions, and minor pattern distortions of features at the 5 μm resolution. Flycutting method employed in multi-layer lithography of SU-8 yielded accurate total thickness control within ±1.5 μm and excellent pattern formation for all of 5, 10, and 25 μm features. Electroforming of nickel was optimized with electroplating bath composition and electroplating parameters such as current density to realize the high resolution nickel mold...

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Bubble Size Effects on Dispersed Phase Motion in Vertical Bubbly Pipe Flow

Volume 1: Fora, Parts A, B, C, and D, 2003

Three-dimensional measurements of bubble size, and velocity are presented along with the associat... more Three-dimensional measurements of bubble size, and velocity are presented along with the associated statistics for dilute bubbly flow in a vertical pipe. These measurements were obtained through a combination of precise index-of-refraction matching and a stereoscopic high-speed imaging system. An image-processing algorithm has been developed and used to extract instantaneous bubble size, shape, velocity and trajectory information and statistics corrected for magnification errors (maximum of 2%). The experiments have been conducted at a Reynolds of 14,600 in vertical pipe-flow and a volumetric gas flow ratio of 0.29%. Bubble sizes ranged from sub millimeter to several millimeters. The results indicate that bubbles less than approximately 20 wall units with very low ratios of rise velocity-to-liquid-average-velocity occupy the full breadth of the test section, while bubbles in the range between 20 and 60 wall units with ratios of rise-velocity-to-liquid-average-velocity greater than 0.5 are efficiently trapped by the wall and are almost exclusively found in the inner wall region (y+ < 30). Bubbles larger than 60 wall units with ratios of rise-velocity-to-liquid-average-velocity less than 0.5 are found throughout the pipe cross-section although with a strong preference to the wall neighborhood. This bubble behavior is put in perspective considering four mechanisms of bubble migration: (a) turbulent dispersion due to near-wall large scale structure, (b) inviscid transverse forces (Magnus), (c) viscous transverse forces (Saffman), and (d) unsteady transverse forces because of vortex shedding and bubble shape deformation.© 2003 ASME

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Design, Characterization, and Verification of a Closed Loop Wind Tunnel With Linear Cascade and Upstream Wake Generator

Volume 5B: Heat Transfer, 2015

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Multiphase Digital Particle Image Velocimetry in a Dispersed, Bubbly, Axisymmetric Jet

Volume 2: Symposia and General Papers, Parts A and B, 2002

A particle image velocimetry (PIV) technique developed for application to two-phase flows is pres... more A particle image velocimetry (PIV) technique developed for application to two-phase flows is presented and validated. The technique is capable of simultaneously measuring carrier and bubble phase velocities on a plane. The validation experiments have been conducted in a vertical upwards, two-phase (water-air) bubbly jet flow at a Reynolds numbers of 5,673 and 11,345 and low bubble concentration matching the experiments of Stanley and Nikitopoulos (1998). Comparisons with measurements obtained by Stanley and Nikitopoulos (1998) using Phase Doppler Analysis (PDA) experiments indicate that the agreement between the two techniques is very satisfactory (deviations of the order of 5%) for both single-phase and two-phase jet carrier-flow velocities. In addition, bubble phase velocity measurements obtained from backlit visualizations of the bubbly jet flow using the bubble-tracking method of Fiedler et al. (2001) are successfully compared to those obtained from PIV. The PIV study confirms t...

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Control of Internal Stress for High Quality Nickel Large Area Mold Inserts

Volume 12: Micro and Nano Systems, Parts A and B, 2009

Metallic large area mold inserts (LAMIs) are essential for the replication of polymer microfluidi... more Metallic large area mold inserts (LAMIs) are essential for the replication of polymer microfluidic devices. Successful molding of micro- or nanoscale features over large areas is dependent on improving the dimensional control of the mold inserts, particularly those fabricated by electrodeposition using the LIGA or UV-LIGA processes. A systematic approach to controlling the internal stress of the nickel deposits, which was essential for predicting the final flatness of the LAMIs prior to electroplating, was carried out. The internal stress of the nickel deposits from a nickel sulfamate solution was estimated using a bent strip stress measurement method after maintaining electroplating chemicals and conditions and reducing contamination. Over-electroplating of the nickel LAMIs was performed on SU-8 electroplating molds on 150 mm diameter Si wafers. Detailed characterization of the nickel LAMIs to determine the relationship between the overall flatness of the LAMIs and the internal str...

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Bubble Behavior in the Shear Layer of a Bubbly Jet

Physics of Fluids, 2002

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Mass-Transfer Distribution in Rotating, Two-Pass, Ribbed Channels with Vortex Generators

Journal of Thermophysics and Heat Transfer, 2001

ABSTRACT

Bookmarks Related papers MentionsView impact

Research paper thumbnail of An Experimental Investigation of Pure-Substance, Adiabatic Two-Phase Flow in a Vertical Pipe

Journal of Energy Resources Technology-transactions of The Asme, Mar 1, 1994

Measurements of pressure drop, temperture, and average void fraction are presented for adiabatic,... more Measurements of pressure drop, temperture, and average void fraction are presented for adiabatic, vertical-upwards, two-phase flow of Refrigerant 114 in a pipe. An experimental method has been developed according to which the evolution of flow states occurring in long pipes can be realized in a test section of limited length. The experiments cover the range of the flow from flashing to near choking. The measurements indicate existence of macroscopic thermodynamic equilibrium, except in the immediate neighborhood of flashing. Compressibility due to phase change is shown to play a very important role in the development of the flow. Three regions are recognized based on the measured energetics of the flow. Each region is dominated by potential energy changes, dissipation, and kinetic energy changes, respectively. The evolution of the flow is governed by hydrostatic effects in the initial region after flashing and by high, phase-change-induced kinetic energy increases far downstream as the flow approaches choking. In the intermediate region, viscous, inertial and gravitational effects play a role of comparable importance. The interfacial and wall shear forces have also been calculated from the measurements. The former dominate the initial regions of the flow, while the latter are strongest at high vapor contents.

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Simulation and design of micromixers for microfluidic devices

ABSTRACT

Bookmarks Related papers MentionsView impact

Research paper thumbnail of A Parallel Code for DNS of Complex, Unsteady, Incompressible Turbulent Flows

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Experimental Assessment of Fluid Velocity to Control Gas Accumulation and Removal While Drilling ?Horizontal? Wells

Bookmarks Related papers MentionsView impact