The Experts below are selected from a list of 354 Experts worldwide ranked by ideXlab platform
Mo Samimy - One of the best experts on this subject based on the ideXlab platform.
-
the impulse response of a high speed jet forced with localized arc filament plasma actuators
2012Co-Authors: Aniruddha Sinha, Mo Samimy, Hind Alkandry, Martin Kearneyfischer, Tim ColoniusAbstract:We present experimental and theoretical analyses of the response of high-speed, high-Reynolds-number, round jets to impulsive forcing with arc-filament-plasma actuators. The impulse response is obtained with forcing Strouhal numbers, based on the nozzle Exit Diameter and Exit center line velocity, less than 0.1. The resulting phase-averaged near-field pressure signature displays a compact wave with a positive peak preceding a negative one, indicative of a large scale structure in the shear layer of the jet. Scaling laws derived by operating the jet at four subsonic Mach numbers are used to distinguish this hydrodynamic component of the phase-averaged jet response from the direct actuator noise. As the forcing frequency increases, the compact waves in the near-field pressure signal overlap each other, indicating interaction of the growing seeded structures. For this regime, the phase-averaged response is approximately replicated by linear superposition of the impulse response, thereby demonstrating the quasi-linearity of structure interaction. A novel application of linear parabolized stability theory yields a successful model of the impulse response.
-
acoustic and flow fields of an excited high reynolds number axisymmetric supersonic jet
2010Co-Authors: Mo Samimy, Martin Kearneyfischer, Jinhwa Kim, Aniruddha SinhaAbstract:An axisymmetric perfectly expanded Mach 1.3 jet, with a Reynolds number based on the nozzle Exit Diameter (ReD )o f 1.1 × 10 6 and turbulent boundary layer at the nozzle Exit, was excited using localized arc filament plasma actuators over a wide range of forcing Strouhal numbers (StDF ). Eight actuators distributed azimuthally were used to excite azimuthal modes m = 0–3. Far-field acoustic, flow velocity and irrotational near-field pressure were probed with a three-fold objective: (i) to investigate the broadband far-field noise amplification reported in the literature at lower speeds and ReD using excitation of m = 0 at low StDF ; (ii) to explore broadband far-field noise suppression using excitation of m = 3 at higher StDF ; and (iii) to shed some light on the connection between the flow field and the far-field noise. The broadband far-field noise amplification observed is not as extensive in amplitude or frequency range, but still sufficiently large to be of concern in practical applications. Broadband far-field noise suppression of 4–5 dB at 30 ◦ polar angle peak frequency, resulting in approximately 2 dB attenuation in the overall sound pressure level, is achieved with excitation of m =3 at StDF ∼ 0.9. Some of the noteworthy observations and inferences are (a) there is a strong correlation between the far-field broadband noise amplification and the turbulence amplification; (b) far-field noise suppression is achieved when the jet is forced with the maximum jet initial growth rate frequency thus limiting significant dynamics of structures to a shorter region close to the nozzle Exit; (c) structure breakdown and dynamic interaction seem to be the dominant source of noise; and (d) coherent structures dominate the forced jet over a wide range of StDF (up to ∼ 1.31) with the largest and most organized structures observed around the jet preferred mode StDF .
-
control of a high reynolds number mach 0 9 heated jet using plasma actuators
2009Co-Authors: Martin Kearneyfischer, Jinhwa Kim, Mo SamimyAbstract:The results of particle image velocimetry (PIV) measurements in a high subsonic, heated, jet forced using localized arc filament plasma actuators (LAFPAs) show that LAFPAs can consistently produce significant mixing enhancement over a wide range of temperatures. These actuators have been used successfully in high Reynolds number, high-speed unheated jets. The facility consists of an axisymmetric jet with different nozzle blocks of Exit Diameter of 2.54 cm and variable jet temperature in an anechoic chamber. The focus of this paper is on a high subsonic (Mj=0.9) jet. Twelve experiments with various forcing azimuthal modes (m=0, 1, and ±1) and temperatures (To/Ta=1.0, 1.4, and 2.0) at a fixed forcing Strouhal number (StDF=0.3) have been conducted and PIV results compared with the baseline results to characterize the effectiveness of LAFPAs for mixing enhancement. Centerline velocity and turbulent kinetic energy as well as jet width are used for determining the LAFPAs’ effectiveness. The characteristics of l...
-
active control of high speed and high reynolds number jets using plasma actuators
2007Co-Authors: Mo Samimy, Jinhwa Kim, Jeff Kastner, Igor V Adamovich, Yurii UtkinAbstract:Localized arc filament plasma actuators are used to control an axisymmetric Mach 1.3 ideally expanded jet of 2.54 cm Exit Diameter and a Reynolds number based on the nozzle Exit Diameter of about 1.1×10 6 . Measurements of growth and decay of perturbations seeded in the flow by the actuators, laser-based planar flow visualizations, and particle imaging velocimetry measurements are used to evaluate the effects of control. Eight actuators distributed azimuthally inside the nozzle, approximately 1 mm upstream of the nozzle Exit, are used to force various azimuthal modes over a large frequency range ( St DF of 0.13 to 1.3). The jet responded to the forcing over the entire range of frequencies, but the response was optimum (in terms of the development of large coherent structures and mixing enhancement) around the jet preferred Strouhal number of 0.33 ( f = 5 kHz), in good agreement with the results in the literature for low-speed and low-Reynolds-number jets. The jet (with a thin boundary layer, D /θ ∼ 250) also responded to forcing with various azimuthal modes ( m = 0 to 3 and m = ±1, ±2, ±4), again in agreement with instability analysis and experimental results in the literature for low-speed and low-Reynolds-number jets. Forcing the jet with the azimuthal mode m = ±1 at the jet preferred-mode frequency provided the maximum mixing enhancement, with a significant reduction in the jet potential core length and a significant increase in the jet centreline velocity decay rate beyond the end of the potential core.
-
molecular tagging velocimetry measurements in supersonic microjets
2002Co-Authors: Walter R Lempert, Naibo Jiang, Subin Sethuram, Mo SamimyAbstract:The application of acetone-based molecular tagging velocimetry (MTV) is demonstrated in sonic and supersonic jets produced by a 1-mm-Exit-Diameter nozzle. Measurements are performed in the static pressure range 1.3‐53 mbar, with spatial resolution of approximately 10 πm. The statistical uncertainty (2ae) in velocity is found to be of order 6 ‐10 m/s, approximately independent of e owe eld pressure. Acetone laser-induced e uorescence temporal decay curves were also obtained, with 1/ e lifetime found to range from » 200 ns at 1.3 mbarto less than 50 ns at 24 mbar. These relatively short lifetimes were nonetheless sufe cient to obtain MTV data over the entire pressure range.
Vaclav Sobolik - One of the best experts on this subject based on the ideXlab platform.
-
PIV and Electrodiffusion diagnostics of flow field, wall shear stress and mass transfer beneath three round submerged impinging jets
2016Co-Authors: Kodjovi Sodjavi, Amina Meslem, Brice Montagné, Pierre Bragança, Paul Byrne, Cédric Degouet, Vaclav SobolikAbstract:This paper reports on measurements of velocities, wall shear rates and mass transfer in an impinging round jet issued from a round nozzle. The effect of the nozzle shape on transfer phenomena was investigated. A round orifice perforated either on a flat plate (RO/P) or on a hemispherical surface (RO/H) was compared to a reference convergent nozzle (CONV). All the nozzles have the same Exit Diameter D. The Exit volumetric flow rate was also conserved and led to the same Reynolds number based on the Exit bulk velocity, Reb = 5620. The nozzle-to-wall distance was constant and equal to 2D. The Particle Image Velocimetry technique (PIV) was used to capture the jet flow field. The limitations of the PIV technique in the vicinity of the target disc are addressed by using the electrodiffusion technique (ED) to obtain the wall shear rate distribution. The ED technique was extended for the measurement of local mass transfer distribution and global mass transfer on the target disc. The whole velocity field, wall shear rates and mass transfer in the three impinging round jets were compared. It was shown that at constant volumetric flow rate, the use of an orifice nozzle not only improves wall shear rate, but also increases local and global mass transfer. The global mass transfer on a target disc of a 3.2 D Diameter is 25% and 31% higher for RO/H and RO/P nozzles, respectively, compared to the reference CONV nozzle. The orifice nozzles generate narrower Exit profiles relatively to the convergent nozzle. The vena contracta effect in orifice jets, more intense with RO/P than with RO/H, generates an increase of the Exit centerline velocity. The hemispherical surface of RO/H nozzle stretches the flow at the Exit and somewhat attenuates the vena contracta effect. The characteristic scale representation of the data confirms the origin of the observed differences between the three jets. A link between the wall shear stress and the mass transfer is revealed. The wall shear rate and the mass transfer are in a close relationship with the near field flow features, themselves affected by the nozzle geometry. Time-resolved tomographic PIV technique reveals that the wall shear rate fluctuation is related to the dynamics of the jet coherent structures. The instantaneous PIV fields indicates the formation of secondary vortices in the region where a secondary peak in local mass transfer emerges. The level of this secondary peak is sensitive to the nozzle shape. The higher is the jet acceleration, the more intense is the level of the secondary peak
-
flow dynamics and mass transfer in impinging circular jet at low reynolds number comparison of convergent and orifice nozzles
2013Co-Authors: Amina Meslem, Vaclav Sobolik, Flori Ode, Kodjovi Sodjavi, Yassine Zaouali, Ilinca Nastase, Cristiana CroitoruAbstract:Abstract Electrodiffusion and Particle Image Velocimetry (PIV) measurements were made in two impinging jets, the first Exiting from a convergent nozzle and the second from a square-edged orifice nozzle having same Exit Diameter. The Reynolds number, based on nozzle Diameter and Exit bulk-velocity, was equal to 1360 in each flow. The wall-shear rate and mass transfer generated by orifice impinging jet on a flat plate are up to 42% and 18% respectively higher than in its counterpart convergent nozzle jet. The transfer feature on the impingement surface has a close relationship with near field flow dynamics, itself affected by flow conditions at the nozzle Exit. The orifice jet flow generates larger in size, well-defined and vigorous primary Kelvin–Helmholtz (K–H) structures with comparison to the convergent nozzle jet. These differences were associated with differences in initial velocity profiles and in the resulting flow development. The vena contracta in the orifice jet generates a thinner shear layer and an increase of the Exit mean velocity relative to Exit bulk-velocity. Organized structures in the two flows are educed using DMD and POD applied to PIV measurements. The dominant frequency of each flow related to the K–H instability, captured using DMD analysis of PIV fields, is also obtained by studying energy spectra of electrodiffusion signals. A quantitative measure of the kinetic energy (KE) distribution in different POD modes reveals that for reconstruction of 80% of the total KE, 7 modes are required for the orifice impinging jet and 42 modes for the convergent impinging jet. This comparison confirms the larger degree of flow organization in orifice jet before impinging on the target wall and explains its performance in the resulting mass transfer on the wall surface.
Krishna Mahesh - One of the best experts on this subject based on the ideXlab platform.
-
dynamics and mixing of vortex rings in crossflow
2008Co-Authors: Rajes Sau, Krishna MaheshAbstract:Direct numerical simulation is used to study the effect of crossflow on the dynamics, entrainment and mixing characteristics of vortex rings issuing from a circular nozzle. Three distinct regimes exist, depending on the velocity ratio (ratio of the average nozzle Exit velocity to free-stream crossflow velocity) and stroke ratio (ratio of stroke length to nozzle Exit Diameter). Coherent vortex rings are not obtained at velocity ratios below approximately 2. At these low velocity ratios, the vorticity in the crossflow boundary layer inhibits roll-up of the nozzle boundary layer at the leading edge. As a result, a hairpin vortex forms instead of a vortex ring. For large stroke ratios and velocity ratio below 2, a series of hairpin vortices is shed downstream. The shedding is quite periodic for very low Reynolds numbers. For velocity ratios above 2, two regimes are obtained depending upon the stroke ratio. Lower stroke ratios yield a coherent asymmetric vortex ring, while higher stroke ratios yield an asymmetric vortex ring accompanied by a trailing column of vorticity. These two regimes are separated by a transition stroke ratio whose value decreases with decreasing velocity ratio. For very high values of the velocity ratio, the transition stroke ratio approaches the ‘formation number’. In the absence of trailing vorticity, the vortex ring tilts towards the upstream direction, while the presence of a trailing column causes it to tilt downstream. This behaviour is explained. In the absence of crossflow, the trailing column is not very effective at entrainment, and is best avoided for optimal mixing and entrainment. However, in the presence of crossflow, the trailing column is found to contribute significantly to the overall mixing and entrainment. The trailing column interacts with the crossflow to generate a region of high pressure downstream of the nozzle that drives crossflow fluid towards the vortex ring. There is an optimal length of the trailing column for maximum downstream entrainment. A classification map which categorizes the different regimes is developed.
-
direct numerical simulation of round turbulent jets in crossflow
2007Co-Authors: Suma Muppidi, Krishna MaheshAbstract:Direct numerical simulation is used to study a round turbulent jet in a laminar crossflow. The ratio of bulk jet velocity to free-stream crossflow velocity is 5.7 and the Reynolds number based on the bulk jet velocity and the jet Exit Diameter is 5000. The mean velocity and turbulent intensities from the simulations are compared to data from the experiments by Su & Mungal (2004) and good agreement is observed. Additional quantities, not available from experiments, are presented. Turbulent kinetic energy budgets are computed for this flow. Examination of the budgets shows that the near field is far from a state of turbulent equilibrium – especially along the jet edges. Also – in the near field – peak kinetic energy production is observed close to the leading edge, while peak dissipation is observed toward the trailing edge of the jet. The results are used to comment upon the difficulty involved in predicting this flow using RANS computations. There exist regions in this flow where the pressure transport term, neglected by some models and poorly modelled by others, is significant. And past the jet Exit, the flow is not close to established canonical flows on which most models appear to be based.
Martin Kearneyfischer - One of the best experts on this subject based on the ideXlab platform.
-
the impulse response of a high speed jet forced with localized arc filament plasma actuators
2012Co-Authors: Aniruddha Sinha, Mo Samimy, Hind Alkandry, Martin Kearneyfischer, Tim ColoniusAbstract:We present experimental and theoretical analyses of the response of high-speed, high-Reynolds-number, round jets to impulsive forcing with arc-filament-plasma actuators. The impulse response is obtained with forcing Strouhal numbers, based on the nozzle Exit Diameter and Exit center line velocity, less than 0.1. The resulting phase-averaged near-field pressure signature displays a compact wave with a positive peak preceding a negative one, indicative of a large scale structure in the shear layer of the jet. Scaling laws derived by operating the jet at four subsonic Mach numbers are used to distinguish this hydrodynamic component of the phase-averaged jet response from the direct actuator noise. As the forcing frequency increases, the compact waves in the near-field pressure signal overlap each other, indicating interaction of the growing seeded structures. For this regime, the phase-averaged response is approximately replicated by linear superposition of the impulse response, thereby demonstrating the quasi-linearity of structure interaction. A novel application of linear parabolized stability theory yields a successful model of the impulse response.
-
acoustic and flow fields of an excited high reynolds number axisymmetric supersonic jet
2010Co-Authors: Mo Samimy, Martin Kearneyfischer, Jinhwa Kim, Aniruddha SinhaAbstract:An axisymmetric perfectly expanded Mach 1.3 jet, with a Reynolds number based on the nozzle Exit Diameter (ReD )o f 1.1 × 10 6 and turbulent boundary layer at the nozzle Exit, was excited using localized arc filament plasma actuators over a wide range of forcing Strouhal numbers (StDF ). Eight actuators distributed azimuthally were used to excite azimuthal modes m = 0–3. Far-field acoustic, flow velocity and irrotational near-field pressure were probed with a three-fold objective: (i) to investigate the broadband far-field noise amplification reported in the literature at lower speeds and ReD using excitation of m = 0 at low StDF ; (ii) to explore broadband far-field noise suppression using excitation of m = 3 at higher StDF ; and (iii) to shed some light on the connection between the flow field and the far-field noise. The broadband far-field noise amplification observed is not as extensive in amplitude or frequency range, but still sufficiently large to be of concern in practical applications. Broadband far-field noise suppression of 4–5 dB at 30 ◦ polar angle peak frequency, resulting in approximately 2 dB attenuation in the overall sound pressure level, is achieved with excitation of m =3 at StDF ∼ 0.9. Some of the noteworthy observations and inferences are (a) there is a strong correlation between the far-field broadband noise amplification and the turbulence amplification; (b) far-field noise suppression is achieved when the jet is forced with the maximum jet initial growth rate frequency thus limiting significant dynamics of structures to a shorter region close to the nozzle Exit; (c) structure breakdown and dynamic interaction seem to be the dominant source of noise; and (d) coherent structures dominate the forced jet over a wide range of StDF (up to ∼ 1.31) with the largest and most organized structures observed around the jet preferred mode StDF .
-
control of a high reynolds number mach 0 9 heated jet using plasma actuators
2009Co-Authors: Martin Kearneyfischer, Jinhwa Kim, Mo SamimyAbstract:The results of particle image velocimetry (PIV) measurements in a high subsonic, heated, jet forced using localized arc filament plasma actuators (LAFPAs) show that LAFPAs can consistently produce significant mixing enhancement over a wide range of temperatures. These actuators have been used successfully in high Reynolds number, high-speed unheated jets. The facility consists of an axisymmetric jet with different nozzle blocks of Exit Diameter of 2.54 cm and variable jet temperature in an anechoic chamber. The focus of this paper is on a high subsonic (Mj=0.9) jet. Twelve experiments with various forcing azimuthal modes (m=0, 1, and ±1) and temperatures (To/Ta=1.0, 1.4, and 2.0) at a fixed forcing Strouhal number (StDF=0.3) have been conducted and PIV results compared with the baseline results to characterize the effectiveness of LAFPAs for mixing enhancement. Centerline velocity and turbulent kinetic energy as well as jet width are used for determining the LAFPAs’ effectiveness. The characteristics of l...
Kodjovi Sodjavi - One of the best experts on this subject based on the ideXlab platform.
-
PIV and Electrodiffusion diagnostics of flow field, wall shear stress and mass transfer beneath three round submerged impinging jets
2016Co-Authors: Kodjovi Sodjavi, Amina Meslem, Brice Montagné, Pierre Bragança, Paul Byrne, Cédric Degouet, Vaclav SobolikAbstract:This paper reports on measurements of velocities, wall shear rates and mass transfer in an impinging round jet issued from a round nozzle. The effect of the nozzle shape on transfer phenomena was investigated. A round orifice perforated either on a flat plate (RO/P) or on a hemispherical surface (RO/H) was compared to a reference convergent nozzle (CONV). All the nozzles have the same Exit Diameter D. The Exit volumetric flow rate was also conserved and led to the same Reynolds number based on the Exit bulk velocity, Reb = 5620. The nozzle-to-wall distance was constant and equal to 2D. The Particle Image Velocimetry technique (PIV) was used to capture the jet flow field. The limitations of the PIV technique in the vicinity of the target disc are addressed by using the electrodiffusion technique (ED) to obtain the wall shear rate distribution. The ED technique was extended for the measurement of local mass transfer distribution and global mass transfer on the target disc. The whole velocity field, wall shear rates and mass transfer in the three impinging round jets were compared. It was shown that at constant volumetric flow rate, the use of an orifice nozzle not only improves wall shear rate, but also increases local and global mass transfer. The global mass transfer on a target disc of a 3.2 D Diameter is 25% and 31% higher for RO/H and RO/P nozzles, respectively, compared to the reference CONV nozzle. The orifice nozzles generate narrower Exit profiles relatively to the convergent nozzle. The vena contracta effect in orifice jets, more intense with RO/P than with RO/H, generates an increase of the Exit centerline velocity. The hemispherical surface of RO/H nozzle stretches the flow at the Exit and somewhat attenuates the vena contracta effect. The characteristic scale representation of the data confirms the origin of the observed differences between the three jets. A link between the wall shear stress and the mass transfer is revealed. The wall shear rate and the mass transfer are in a close relationship with the near field flow features, themselves affected by the nozzle geometry. Time-resolved tomographic PIV technique reveals that the wall shear rate fluctuation is related to the dynamics of the jet coherent structures. The instantaneous PIV fields indicates the formation of secondary vortices in the region where a secondary peak in local mass transfer emerges. The level of this secondary peak is sensitive to the nozzle shape. The higher is the jet acceleration, the more intense is the level of the secondary peak
-
flow dynamics and mass transfer in impinging circular jet at low reynolds number comparison of convergent and orifice nozzles
2013Co-Authors: Amina Meslem, Vaclav Sobolik, Flori Ode, Kodjovi Sodjavi, Yassine Zaouali, Ilinca Nastase, Cristiana CroitoruAbstract:Abstract Electrodiffusion and Particle Image Velocimetry (PIV) measurements were made in two impinging jets, the first Exiting from a convergent nozzle and the second from a square-edged orifice nozzle having same Exit Diameter. The Reynolds number, based on nozzle Diameter and Exit bulk-velocity, was equal to 1360 in each flow. The wall-shear rate and mass transfer generated by orifice impinging jet on a flat plate are up to 42% and 18% respectively higher than in its counterpart convergent nozzle jet. The transfer feature on the impingement surface has a close relationship with near field flow dynamics, itself affected by flow conditions at the nozzle Exit. The orifice jet flow generates larger in size, well-defined and vigorous primary Kelvin–Helmholtz (K–H) structures with comparison to the convergent nozzle jet. These differences were associated with differences in initial velocity profiles and in the resulting flow development. The vena contracta in the orifice jet generates a thinner shear layer and an increase of the Exit mean velocity relative to Exit bulk-velocity. Organized structures in the two flows are educed using DMD and POD applied to PIV measurements. The dominant frequency of each flow related to the K–H instability, captured using DMD analysis of PIV fields, is also obtained by studying energy spectra of electrodiffusion signals. A quantitative measure of the kinetic energy (KE) distribution in different POD modes reveals that for reconstruction of 80% of the total KE, 7 modes are required for the orifice impinging jet and 42 modes for the convergent impinging jet. This comparison confirms the larger degree of flow organization in orifice jet before impinging on the target wall and explains its performance in the resulting mass transfer on the wall surface.