The Experts below are selected from a list of 426 Experts worldwide ranked by ideXlab platform
Norsarahaida Amin - One of the best experts on this subject based on the ideXlab platform.
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Unsteady boundary layer flow induced by accelerating motion near the Rear Stagnation Point in a micropolar fluid
International Journal of Fluid Mechanics Research, 2009Co-Authors: Yian Yian Lok, Norsarahaida Amin, I. PopAbstract:The unsteady boundary layer flow of a micropolar fluid induced by a two-dimensional body, which is started impulsively from rest, is studied in this paper. The variation with time t of the external stream V(t) is assumed to be of the form V(t) 1 - exp(-atm), where a = 0 means a coefficient of acceleration and m is an arbitrary integral value. The problem is formulated for the flow at the Rear Stagnation Point on an infinite plane wall. Numerical solutions of the unsteady boundary layer equations are obtained using an implicit finite-difference scheme known as the Keller's box method. Results are given for the velocity and microrotation profiles, as well as for the dimensionless time elapsed before the boundary layer begins to separate from the wall. It is found that the dimensionless time elapsed before separation takes place is lower for a micropolar fluid (K 0) than for a Newtonian fluid (K 0), where K denotes the micropolar or material parameter.
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Unsteady boundary layer flow of a micropolar fluid near the Rear Stagnation Point of a plane surface
International Journal of Thermal Sciences, 2003Co-Authors: Yian Yian Lok, Norsarahaida AminAbstract:The growth of the boundary layer flow of a viscous and incompressible micropolar fluid started impulsively from rest near the Rear Stagnation Point of a two-dimensional plane surface is studied theoretically. The transformed non-similar boundary-layer equations are solved numerically using a very efficient finite-difference method known as Keller-box method. This method may present well-behaved solutions for the transient (small time) solution up to the separation boundary layer flow. Numerical results are given for the reduced velocity and microrotation profiles, as well as for the skin friction coefficient when the material parameter K takes the values K=0 (Newtonian fluid), 0.5, 1, 1.1, 1.5, 2, 2.5 and 3 with the boundary condition for microrotation n=0 (strong concentration of microelements) and n=1/2 (weak concentration of microelements), respectively. Important features of these flow characteristics are shown on graphs and in table
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unsteady boundary layer flow of a micropolar fluid near the forward Stagnation Point of a plane surface
International Journal of Engineering Science, 2003Co-Authors: Yian Yian Lok, P Phang, Norsarahaida AminAbstract:The growth of the boundary layer flow of a viscous and incompressible micropolar fluid started impulsively from rest near the Rear Stagnation Point of a two-dimensional plane surface is studied theoretically. The transformed non-similar boundary-layer equations are solved numerically using a very efficient finite-difference method known as Keller-box method. This method may present well-behaved solutions for the transient (small time) solution up to the separation boundary layer flow. Numerical results are given for the reduced velocity and microrotation profiles, as well as for the skin friction coefficient when the material parameter K takes the values K=0 (Newtonian fluid), 0.5, 1, 1.1, 1.5, 2, 2.5 and 3 with the boundary condition for microrotation n=0 (strong concentration of microelements) and n=1/2 (weak concentration of microelements), respectively. Important features of these flow characteristics are shown on graphs and in tables
Yian Yian Lok - One of the best experts on this subject based on the ideXlab platform.
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Unsteady boundary layer flow induced by accelerating motion near the Rear Stagnation Point in a micropolar fluid
International Journal of Fluid Mechanics Research, 2009Co-Authors: Yian Yian Lok, Norsarahaida Amin, I. PopAbstract:The unsteady boundary layer flow of a micropolar fluid induced by a two-dimensional body, which is started impulsively from rest, is studied in this paper. The variation with time t of the external stream V(t) is assumed to be of the form V(t) 1 - exp(-atm), where a = 0 means a coefficient of acceleration and m is an arbitrary integral value. The problem is formulated for the flow at the Rear Stagnation Point on an infinite plane wall. Numerical solutions of the unsteady boundary layer equations are obtained using an implicit finite-difference scheme known as the Keller's box method. Results are given for the velocity and microrotation profiles, as well as for the dimensionless time elapsed before the boundary layer begins to separate from the wall. It is found that the dimensionless time elapsed before separation takes place is lower for a micropolar fluid (K 0) than for a Newtonian fluid (K 0), where K denotes the micropolar or material parameter.
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Unsteady boundary layer flow of a micropolar fluid near the Rear Stagnation Point of a plane surface
International Journal of Thermal Sciences, 2003Co-Authors: Yian Yian Lok, Norsarahaida AminAbstract:The growth of the boundary layer flow of a viscous and incompressible micropolar fluid started impulsively from rest near the Rear Stagnation Point of a two-dimensional plane surface is studied theoretically. The transformed non-similar boundary-layer equations are solved numerically using a very efficient finite-difference method known as Keller-box method. This method may present well-behaved solutions for the transient (small time) solution up to the separation boundary layer flow. Numerical results are given for the reduced velocity and microrotation profiles, as well as for the skin friction coefficient when the material parameter K takes the values K=0 (Newtonian fluid), 0.5, 1, 1.1, 1.5, 2, 2.5 and 3 with the boundary condition for microrotation n=0 (strong concentration of microelements) and n=1/2 (weak concentration of microelements), respectively. Important features of these flow characteristics are shown on graphs and in table
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unsteady boundary layer flow of a micropolar fluid near the forward Stagnation Point of a plane surface
International Journal of Engineering Science, 2003Co-Authors: Yian Yian Lok, P Phang, Norsarahaida AminAbstract:The growth of the boundary layer flow of a viscous and incompressible micropolar fluid started impulsively from rest near the Rear Stagnation Point of a two-dimensional plane surface is studied theoretically. The transformed non-similar boundary-layer equations are solved numerically using a very efficient finite-difference method known as Keller-box method. This method may present well-behaved solutions for the transient (small time) solution up to the separation boundary layer flow. Numerical results are given for the reduced velocity and microrotation profiles, as well as for the skin friction coefficient when the material parameter K takes the values K=0 (Newtonian fluid), 0.5, 1, 1.1, 1.5, 2, 2.5 and 3 with the boundary condition for microrotation n=0 (strong concentration of microelements) and n=1/2 (weak concentration of microelements), respectively. Important features of these flow characteristics are shown on graphs and in tables
Gennaro Cardone - One of the best experts on this subject based on the ideXlab platform.
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the von karman street behind a circular cylinder flow control through synthetic jet placed at the Rear Stagnation Point
Journal of Fluid Mechanics, 2020Co-Authors: Carlo Salvatore Greco, Gerardo Paolillo, Tommaso Astarita, Gennaro CardoneAbstract:The present paper aims at establishing the synthetic jet technology capabilities in controlling the von Karman street behind a circular cylinder. The circular cylinder, placed in an open-circuit wind tunnel, presents a slot in its Rear position, through which the synthetic jet is issued. The Reynolds number, based on the circular cylinder diameter and the free-stream velocity, is equal to 4600 and the von Karman street is characterized, in the baseline configuration (i.e. without synthetic jet), by a shedding frequency of 16.2 Hz. Several synthetic jet operating conditions are tested. Therefore, the effects of the momentum coefficient (.
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SYNTHETIC JET-BASED CONTROL OF A CYLINDER WAKE
2017Co-Authors: Carlo Salvatore Greco, Gerardo Paolillo, Tommaso Astarita, Gaetano Pascarella, Gennaro CardoneAbstract:Synthetic jets are zero-net mass flux flows that have been recently used in a wide range of flow control applications, among which the control of bluff body wakes. An optimal control is based on an appropriate tuning of the involved parameters, which implies knowledge of the effects of such parameters on the flow field. The present work reports on an experimental investigation of the active open-loop control of a cylinder wake at Reynolds number equal to 4980 using a slot synthetic jet located at the Rear Stagnation Point. Three values of f+ (0.48, 0.96 and 1.92) are considered and, for each of them, Cµ is varied among as many different values (4.6%, 9.3% and 18.5%). The flow field in the cylinder wake is found to be highly sensitive to the variations of f+ and Cµ. By increasing Cµ. and/or f+ the extent of the separated flow region behind the cylinder is observed to decrease. Indeed, at low f+ and/or high Cµ, periodic momentary interruptions of the vortex shedding are observed, while, for higher values of f+ and/or lower values of Cμ, a strong interaction between von Kármán vortices and synthetic jet vortex pairs occur
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CONTROL OF VON KÁRMÁN VORTEX SHEDDING THROUGH SYNTHETIC JET TECHNOLOGY: MOMENTUM COEFFICIENT AND NON-DIMENSIONAL FREQUENCY EFFECTS
2017Co-Authors: Gerardo Paolillo, Carlo Salvatore Greco, Tommaso Astarita, Gaetano Pascarella, Gennaro CardoneAbstract:Synthetic jets have been recently employed in several fields of application, such as flow control and thermal management. A typical synthetic jet device consists of a cavity bounded by an orifice plate and an oscillating membrane. The oscillation of this membrane allows synthetizing the jet from the ambient in which the device is embedded, without the need of an external piping. This feature involves simple design and small size and, thus, makes synthetic jet devices very appealing in flow control applications. The present work reports on the experimental investigation of the flow field over a circular cylinder controlled by a two-dimensional synthetic jet located at the Rear Stagnation Point. Time-resolved particle image velocimetry (PIV) measurements are carried out in an aspirated wind tunnel at a cylinder Reynolds number equal to about 4500 in order to assess the influence of the momentum coefficient Cμ and the ratio of the excitation frequency to the natural shedding frequency f+ on the wake flow structure. Cμ is varied between 0 (case with no flow control) and 20%, while f+ is varied between 0.5 and 2, thus covering a wide range of operating conditions. The time evolution of the velocity fields is studied by means of an innovative decomposition, based on Fourier analysis and relying exclusively on the PIV data, which allows separating the periodic coherent fluctuations at different frequencies (namely, synthetic jet and shedding frequencies) from the uncoherent turbulent one. The present results show that the flow topology of the cylinder wake is highly sensitive to the variations of Cμ and f+. The increase of both Cμ and f+ leads to a reduction of the cylinder wake extension, although with different mechanisms. The increase of Cμ at low values of f+ results in a longer fluid stroke during the synthetic jet ejection, which momentarily interrupts the vortex shedding; in the following suction phase, the wake reverts to the shedding mode, which is observed to occur in a symmetrical configuration at high values of Cμ. At the highest investigated values of f+, the synthetic jet consists essentially of discrete vortex pairs (not followed by trailing jets) and the increase of Cμ results in a stronger interaction of these structures with the separated shear layers and the von Kármán vortices. In these operating conditions (high values of Cμ and f+), the vortex shedding is observed to synchronize with the synthetic jet, i.e., the shedding frequency is shifted to a subharmonic of the excitation frequency. Also the velocity fluctuations strictly depend on the two investigated control parameters: they are observed to increase with Cμ, while the increase of f+ causes strong fluctuations in both the coherent and the uncoherent components of the velocity field within the very near wake region
Jinjun Wang - One of the best experts on this subject based on the ideXlab platform.
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modification of a circular cylinder wake with synthetic jet vortex shedding modes and mechanism
European Journal of Mechanics B-fluids, 2014Co-Authors: Li Hao Feng, Jinjun WangAbstract:Abstract The wake behind a circular cylinder is modified by a synthetic jet positioned at the front Stagnation Point. The flow field is measured with a time-resolved particle image velocimetry (PIV) system, and the proper orthogonal decomposition (POD) and λ c i methods are used to analyze the vortex dynamics. The synthetic jet vortex pair is induced near the exit orifice periodically and then moves upstream. The interaction between the synthetic jet and the oncoming flow gives rise to an envelope formed upstream of the circular cylinder, which acts as a virtual aerodynamic shape. It is found that the envelope can be categorized into the periodic closed envelope and the quasi-steady open envelope, leading to different shedding modes for the wake around the circular cylinder. In the present investigation, six kinds of vortex shedding modes under synthetic jet control have been classified as natural Karman vortex mode, bistable state mode I, symmetric mode, bistable state mode II, antisymmetric mode with shortened vortex formation length, vortex generation close to the Rear Stagnation Point. The vortex dynamics analysis indicates that the wake vortex trajectory, vortex circulation, and convection velocity at the vortex core all exhibit regular variations for these typical shedding modes. The formation mechanisms for these shedding modes have been further revealed, which present some novel formation processes in comparison with the natural Karman vortex street. Moreover, the effects of the synthetic jet momentum coefficient and excitation frequency on the control are also compared, which suggests that the type of the front envelope is most important for the vortex shedding modes.
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Circular cylinder vortex-synchronization control with a synthetic jet positioned at the Rear Stagnation Point
Journal of Fluid Mechanics, 2010Co-Authors: Li Hao Feng, Jinjun WangAbstract:The flow over a circular cylinder controlled by a two-dimensional synthetic jet positioned at the mean Rear Stagnation Point has been experimentally investigated in a water channel at the cylinder Reynolds number Re = 950. This is an innovative arrangement and the particle-image-velocimetry measurement indicates that it can lead to a novel and interesting phenomenon. The synthetic-jet vortex pairs induced near the exit convect downstream and interact with the vorticity shear layers behind both sides of the cylinder, resulting in the formation of new induced wake vortices. The present vortex synchronization occurs when the excitation frequency of the synthetic jet is between 1.67 and 5.00 times the natural shedding frequency at the dimensionless stroke length 99.5. However, it is suggested that the strength of the synthetic-jet vortex pair plays a more essential role in the occurrence of vortex synchronization than the excitation frequency. In addition, the wake-vortex shedding is converted into a symmetric mode from its original antisymmetric mode. The symmetric shedding mode weakens the interaction between the upper and lower wake vortices, resulting in a decrease in the turbulent kinetic energy produced by them. It also has a significant influence on the global flow field, including the velocity fluctuations, Reynolds stresses and flow topology. However, their distributions are still dominated by the large-scale coherent structures.
Carlo Salvatore Greco - One of the best experts on this subject based on the ideXlab platform.
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the von karman street behind a circular cylinder flow control through synthetic jet placed at the Rear Stagnation Point
Journal of Fluid Mechanics, 2020Co-Authors: Carlo Salvatore Greco, Gerardo Paolillo, Tommaso Astarita, Gennaro CardoneAbstract:The present paper aims at establishing the synthetic jet technology capabilities in controlling the von Karman street behind a circular cylinder. The circular cylinder, placed in an open-circuit wind tunnel, presents a slot in its Rear position, through which the synthetic jet is issued. The Reynolds number, based on the circular cylinder diameter and the free-stream velocity, is equal to 4600 and the von Karman street is characterized, in the baseline configuration (i.e. without synthetic jet), by a shedding frequency of 16.2 Hz. Several synthetic jet operating conditions are tested. Therefore, the effects of the momentum coefficient (.
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SYNTHETIC JET-BASED CONTROL OF A CYLINDER WAKE
2017Co-Authors: Carlo Salvatore Greco, Gerardo Paolillo, Tommaso Astarita, Gaetano Pascarella, Gennaro CardoneAbstract:Synthetic jets are zero-net mass flux flows that have been recently used in a wide range of flow control applications, among which the control of bluff body wakes. An optimal control is based on an appropriate tuning of the involved parameters, which implies knowledge of the effects of such parameters on the flow field. The present work reports on an experimental investigation of the active open-loop control of a cylinder wake at Reynolds number equal to 4980 using a slot synthetic jet located at the Rear Stagnation Point. Three values of f+ (0.48, 0.96 and 1.92) are considered and, for each of them, Cµ is varied among as many different values (4.6%, 9.3% and 18.5%). The flow field in the cylinder wake is found to be highly sensitive to the variations of f+ and Cµ. By increasing Cµ. and/or f+ the extent of the separated flow region behind the cylinder is observed to decrease. Indeed, at low f+ and/or high Cµ, periodic momentary interruptions of the vortex shedding are observed, while, for higher values of f+ and/or lower values of Cμ, a strong interaction between von Kármán vortices and synthetic jet vortex pairs occur
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CONTROL OF VON KÁRMÁN VORTEX SHEDDING THROUGH SYNTHETIC JET TECHNOLOGY: MOMENTUM COEFFICIENT AND NON-DIMENSIONAL FREQUENCY EFFECTS
2017Co-Authors: Gerardo Paolillo, Carlo Salvatore Greco, Tommaso Astarita, Gaetano Pascarella, Gennaro CardoneAbstract:Synthetic jets have been recently employed in several fields of application, such as flow control and thermal management. A typical synthetic jet device consists of a cavity bounded by an orifice plate and an oscillating membrane. The oscillation of this membrane allows synthetizing the jet from the ambient in which the device is embedded, without the need of an external piping. This feature involves simple design and small size and, thus, makes synthetic jet devices very appealing in flow control applications. The present work reports on the experimental investigation of the flow field over a circular cylinder controlled by a two-dimensional synthetic jet located at the Rear Stagnation Point. Time-resolved particle image velocimetry (PIV) measurements are carried out in an aspirated wind tunnel at a cylinder Reynolds number equal to about 4500 in order to assess the influence of the momentum coefficient Cμ and the ratio of the excitation frequency to the natural shedding frequency f+ on the wake flow structure. Cμ is varied between 0 (case with no flow control) and 20%, while f+ is varied between 0.5 and 2, thus covering a wide range of operating conditions. The time evolution of the velocity fields is studied by means of an innovative decomposition, based on Fourier analysis and relying exclusively on the PIV data, which allows separating the periodic coherent fluctuations at different frequencies (namely, synthetic jet and shedding frequencies) from the uncoherent turbulent one. The present results show that the flow topology of the cylinder wake is highly sensitive to the variations of Cμ and f+. The increase of both Cμ and f+ leads to a reduction of the cylinder wake extension, although with different mechanisms. The increase of Cμ at low values of f+ results in a longer fluid stroke during the synthetic jet ejection, which momentarily interrupts the vortex shedding; in the following suction phase, the wake reverts to the shedding mode, which is observed to occur in a symmetrical configuration at high values of Cμ. At the highest investigated values of f+, the synthetic jet consists essentially of discrete vortex pairs (not followed by trailing jets) and the increase of Cμ results in a stronger interaction of these structures with the separated shear layers and the von Kármán vortices. In these operating conditions (high values of Cμ and f+), the vortex shedding is observed to synchronize with the synthetic jet, i.e., the shedding frequency is shifted to a subharmonic of the excitation frequency. Also the velocity fluctuations strictly depend on the two investigated control parameters: they are observed to increase with Cμ, while the increase of f+ causes strong fluctuations in both the coherent and the uncoherent components of the velocity field within the very near wake region
