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Quanhua Sun - One of the best experts on this subject based on the ideXlab platform.

  • Computational Analysis of High-Altitude Ionization Gauge Flight Measurements
    2016
    Co-Authors: Quanhua Sun, J H Clemmons, Iain D. Boyd, Chunpei Cai, J H Hecht
    Abstract:

    The rarefied, three-dimensional flows experienced during the turbulent oxygen mixing experiment (TOMEX) at altitudes between 85 and 143 km are simulated using the direct simulation Monte Carlo (DSMC) method. The present study focuses on ionization gauge measurements obtained by TOMEX. The payload is, thus, modeled in detail, and the simulations employ complex meshes. The simulations show that a Bow Shock Wave is generated in front of the payload at low altitude that becomes diffusive at higher altitudes. When the altitude increases, the pressure in the channels of the ionization gauge and the pressure variation around the payload are both decreased. The DSMC results agree very well with data predicted by compressible flow theory and free molecular theory when applicable. Comparison between the DSMC results and the TOMEX flight data shows generally good agreement. Nomenclature H = altitude K n = Knudsen number k = Boltzmann constant M = Mach number m = mass of molecules n = number density p = pressure s = normalized velocity T = temperature U, V, W = velocity in x, y, or z direction x, y, z = physical coordinates γ = ratio of specific heats Subscripts and Superscripts N2 = molecular nitrogen O = atomic oxygen O2 = molecular oxygen x, y, z = component in x, y, or z direction 0 = stagnation point 3 = component normal to surface ∞ = freestream condition I

  • vibrational translational energy exchange models for the direct simulation monte carlo method
    Physics of Fluids, 1999
    Co-Authors: Pradeep Vijayakumar, Quanhua Sun, Iain D. Boyd
    Abstract:

    The model which controls the distribution of energy among the different molecular modes is a crucial component of accurate simulation of nonequilibrium rarefied flows. Two new models for the direct simulation Monte Carlo method that govern energy redistribution between the translational and vibrational modes are presented here. The first model is a modified form of the phenomenological Borgnakke–Larsen model. The probability of inelastic collision is evaluated using the relative velocity of collision. The second energy exchange model considered in this study is the multiple quantum-step transition model. The process of vibrational relaxation occurs through transitions between the different energy levels, allowing jumps of more than one level. Probabilities of activation and deactivation which depend on the relative velocity are used here. The new models are compared with existing schemes for several conditions. Significant differences are found for the vibrational energy distribution function computed in a hypersonic Bow-Shock Wave.

Iain D. Boyd - One of the best experts on this subject based on the ideXlab platform.

  • Computational Analysis of High-Altitude Ionization Gauge Flight Measurements
    2016
    Co-Authors: Quanhua Sun, J H Clemmons, Iain D. Boyd, Chunpei Cai, J H Hecht
    Abstract:

    The rarefied, three-dimensional flows experienced during the turbulent oxygen mixing experiment (TOMEX) at altitudes between 85 and 143 km are simulated using the direct simulation Monte Carlo (DSMC) method. The present study focuses on ionization gauge measurements obtained by TOMEX. The payload is, thus, modeled in detail, and the simulations employ complex meshes. The simulations show that a Bow Shock Wave is generated in front of the payload at low altitude that becomes diffusive at higher altitudes. When the altitude increases, the pressure in the channels of the ionization gauge and the pressure variation around the payload are both decreased. The DSMC results agree very well with data predicted by compressible flow theory and free molecular theory when applicable. Comparison between the DSMC results and the TOMEX flight data shows generally good agreement. Nomenclature H = altitude K n = Knudsen number k = Boltzmann constant M = Mach number m = mass of molecules n = number density p = pressure s = normalized velocity T = temperature U, V, W = velocity in x, y, or z direction x, y, z = physical coordinates γ = ratio of specific heats Subscripts and Superscripts N2 = molecular nitrogen O = atomic oxygen O2 = molecular oxygen x, y, z = component in x, y, or z direction 0 = stagnation point 3 = component normal to surface ∞ = freestream condition I

  • vibrational translational energy exchange models for the direct simulation monte carlo method
    Physics of Fluids, 1999
    Co-Authors: Pradeep Vijayakumar, Quanhua Sun, Iain D. Boyd
    Abstract:

    The model which controls the distribution of energy among the different molecular modes is a crucial component of accurate simulation of nonequilibrium rarefied flows. Two new models for the direct simulation Monte Carlo method that govern energy redistribution between the translational and vibrational modes are presented here. The first model is a modified form of the phenomenological Borgnakke–Larsen model. The probability of inelastic collision is evaluated using the relative velocity of collision. The second energy exchange model considered in this study is the multiple quantum-step transition model. The process of vibrational relaxation occurs through transitions between the different energy levels, allowing jumps of more than one level. Probabilities of activation and deactivation which depend on the relative velocity are used here. The new models are compared with existing schemes for several conditions. Significant differences are found for the vibrational energy distribution function computed in a hypersonic Bow-Shock Wave.

Nicolas Rochuon - One of the best experts on this subject based on the ideXlab platform.

  • Effect of Unsteadiness on the Performance of a Transonic Centrifugal Compressor Stage
    Journal of Turbomachinery, 2009
    Co-Authors: Isabelle Trébinjac, Pascale Kulisa, Nicolas Bulot, Isabelle Trébinjac, Nicolas Rochuon
    Abstract:

    Numerical and experimental investigations were conducted in a transonic centrifugal compressor stage composed of a backswept splittered unshrouded impeller and a vaned diffuser. The characteristic curves of the compressor stage resulting from the unsteady simulations and the experiments show a good agreement over the whole operating range. On the contrary, the total pressure ratio resulting from the steady simulations is clearly overestimated. A detailed analysis of the flow field at design operating point led to identify the physical mechanisms involved in the blade row interaction that underlie the observed shift in performance. Attention was focused on the deformation in shape of the vane Bow Shock Wave due its interaction with the jet and wake flow structure emerging from the impeller. An analytical model is proposed to quantify the time-averaged effects of the associated entropy increase. The model is based on the calculation of the losses across a Shock Wave at various inlet Mach numbers corresponding to the moving of the jet and wake flow in front of the Shock Wave. The model was applied to the compressor stage performance calculated with the steady simulations. The resulting curve of the overall pressure ratio as a function of the mass flow is clearly shifted towards the unsteady results. The model in particular enhances the prediction of the choked mass flow.

  • Effect of Unsteadiness on the Performance of a Transonic Centrifugal Compressor Stage
    Volume 6: Turbomachinery Parts A B and C, 2008
    Co-Authors: Isabelle Trébinjac, Pascale Kulisa, Nicolas Bulot, Nicolas Rochuon
    Abstract:

    Numerical and experimental investigations were conducted in a transonic centrifugal compressor stage composed of a backswept splittered unshrouded impeller and a vaned diffuser. The characteristic curves of the compressor stage resulting from the unsteady simulations and the experiments show a good agreement over the whole operating range. On the contrary, the total pressure ratio resulting from the steady simulations is clearly overestimated. A detailed analysis of the flow field at design operating point led to identify the physical mechanisms involved in the blade row interaction that underlie the observed shift in performance. Attention was focused on the deformation in shape of the vane Bow Shock Wave due its interaction with the jet and wake flow structure emerging from the impeller. An analytical model is proposed to quantify the time-averaged effects of the associated entropy increase. The model is based on the calculation of the losses across a Shock Wave at various inlet Mach numbers corresponding to the moving of the jet and wake flow in front of the Shock Wave. The model was applied to the compressor stage performance calculated with the steady simulations. The resulting curve of the overall pressure ratio as a function of the mass flow is clearly shifted towards the unsteady results. The model in particular enhances the prediction of the choked mass flow.Copyright © 2008 by ASME

J H Hecht - One of the best experts on this subject based on the ideXlab platform.

  • Computational Analysis of High-Altitude Ionization Gauge Flight Measurements
    2016
    Co-Authors: Quanhua Sun, J H Clemmons, Iain D. Boyd, Chunpei Cai, J H Hecht
    Abstract:

    The rarefied, three-dimensional flows experienced during the turbulent oxygen mixing experiment (TOMEX) at altitudes between 85 and 143 km are simulated using the direct simulation Monte Carlo (DSMC) method. The present study focuses on ionization gauge measurements obtained by TOMEX. The payload is, thus, modeled in detail, and the simulations employ complex meshes. The simulations show that a Bow Shock Wave is generated in front of the payload at low altitude that becomes diffusive at higher altitudes. When the altitude increases, the pressure in the channels of the ionization gauge and the pressure variation around the payload are both decreased. The DSMC results agree very well with data predicted by compressible flow theory and free molecular theory when applicable. Comparison between the DSMC results and the TOMEX flight data shows generally good agreement. Nomenclature H = altitude K n = Knudsen number k = Boltzmann constant M = Mach number m = mass of molecules n = number density p = pressure s = normalized velocity T = temperature U, V, W = velocity in x, y, or z direction x, y, z = physical coordinates γ = ratio of specific heats Subscripts and Superscripts N2 = molecular nitrogen O = atomic oxygen O2 = molecular oxygen x, y, z = component in x, y, or z direction 0 = stagnation point 3 = component normal to surface ∞ = freestream condition I

  • computational analysis of high altitude ionization gauge flight measurements
    Journal of Spacecraft and Rockets, 2006
    Co-Authors: J H Clemmons, J H Hecht
    Abstract:

    The rarefied, three-dimensional flows experienced during the turbulent oxygen mixing experiment (TOMEX) at altitudes between 85 and 143 km are simulated using the direct simulation Monte Carlo (DSMC) method. The present study focuses on ionization gauge measurements obtained by TOMEX. The payload is, thus, modeled in detail, and the simulations employ complex meshes. The simulations show that a Bow Shock Wave is generated in front of the payload at low altitude that becomes diffusive at higher altitudes. When the altitude increases, the pressure in the channels of the ionization gauge and the pressure variation around the payload are both decreased. The DSMC results agree very well with data predicted by compressible flow theory and free molecular theory when applicable. Comparison between the DSMC results and the TOMEX flight data shows generally good agreement.

Nicolas Bulot - One of the best experts on this subject based on the ideXlab platform.

  • Analysis of the flow in a transonic centrifugal compressor stage from choke to surge
    2011
    Co-Authors: Nicolas Bulot, N Buffaz
    Abstract:

    Abstract: Numerical and experimental investigations were conducted in a transonic centrifugal compressor stage composed of a backswept splittered unshrouded impeller and a vaned diffuser. Unsteady three-dimensional simulations were performed with the code elsA that solves the turbulent-averaged Navier–Stokes equations, at three operating points: choked flow, peak effi-ciency, and near surge. Numerical results were validated with experimental data coming from laser Doppler anemometry and unsteady pressure measurements. This article focuses on the change in flow structures when the operating point moves from choke to surge. The main changes in the impeller consist in an enlargement of the wake (of the jet-wake flow structure) and an increase in the exit time-averaged flow angle. Consequently, in the diffuser passage, the main flow trajectory moves towards the vane pressure side, and the boundary layer separation transfers from pressure side to suction side. The interaction between the vane Bow Shock Wave and the impeller blade leads to pressure Waves þ, which propagate in the diffuser passage. These pressure Waves generate alternately opposite and favourable pressure gradients, which drive the boundary layers to periodic separa-tion. From choke to surge, the intensity of the pressure Waves þ increases. The interaction also leads to subsonic pockets D, which are torn out from the vane-leading edge Bow Shock and swept along the vane suction side. The induced change in the Shock shape and location combined with the severe hub/suction side corner separation are thought to be at the origin of the surge inception

  • Effect of Unsteadiness on the Performance of a Transonic Centrifugal Compressor Stage
    Journal of Turbomachinery, 2009
    Co-Authors: Isabelle Trébinjac, Pascale Kulisa, Nicolas Bulot, Isabelle Trébinjac, Nicolas Rochuon
    Abstract:

    Numerical and experimental investigations were conducted in a transonic centrifugal compressor stage composed of a backswept splittered unshrouded impeller and a vaned diffuser. The characteristic curves of the compressor stage resulting from the unsteady simulations and the experiments show a good agreement over the whole operating range. On the contrary, the total pressure ratio resulting from the steady simulations is clearly overestimated. A detailed analysis of the flow field at design operating point led to identify the physical mechanisms involved in the blade row interaction that underlie the observed shift in performance. Attention was focused on the deformation in shape of the vane Bow Shock Wave due its interaction with the jet and wake flow structure emerging from the impeller. An analytical model is proposed to quantify the time-averaged effects of the associated entropy increase. The model is based on the calculation of the losses across a Shock Wave at various inlet Mach numbers corresponding to the moving of the jet and wake flow in front of the Shock Wave. The model was applied to the compressor stage performance calculated with the steady simulations. The resulting curve of the overall pressure ratio as a function of the mass flow is clearly shifted towards the unsteady results. The model in particular enhances the prediction of the choked mass flow.

  • Effect of Unsteadiness on the Performance of a Transonic Centrifugal Compressor Stage
    Volume 6: Turbomachinery Parts A B and C, 2008
    Co-Authors: Isabelle Trébinjac, Pascale Kulisa, Nicolas Bulot, Nicolas Rochuon
    Abstract:

    Numerical and experimental investigations were conducted in a transonic centrifugal compressor stage composed of a backswept splittered unshrouded impeller and a vaned diffuser. The characteristic curves of the compressor stage resulting from the unsteady simulations and the experiments show a good agreement over the whole operating range. On the contrary, the total pressure ratio resulting from the steady simulations is clearly overestimated. A detailed analysis of the flow field at design operating point led to identify the physical mechanisms involved in the blade row interaction that underlie the observed shift in performance. Attention was focused on the deformation in shape of the vane Bow Shock Wave due its interaction with the jet and wake flow structure emerging from the impeller. An analytical model is proposed to quantify the time-averaged effects of the associated entropy increase. The model is based on the calculation of the losses across a Shock Wave at various inlet Mach numbers corresponding to the moving of the jet and wake flow in front of the Shock Wave. The model was applied to the compressor stage performance calculated with the steady simulations. The resulting curve of the overall pressure ratio as a function of the mass flow is clearly shifted towards the unsteady results. The model in particular enhances the prediction of the choked mass flow.Copyright © 2008 by ASME