The Experts below are selected from a list of 9 Experts worldwide ranked by ideXlab platform

Peter F. Pelz - One of the best experts on this subject based on the ideXlab platform.

  • On the Kinematics of Sheet and Cloud Cavitation and Related Erosion
    Advanced Experimental and Numerical Techniques for Cavitation Erosion Prediction, 2014
    Co-Authors: Peter F. Pelz, T. Keil, Gerhard Ludwig
    Abstract:

    The influence of flow parameters such as cavitation number and Reynolds number on the cavitating cloud behavior and aggressiveness is analysed in an experimental work. The focused geometry is a convergent-divergent nozzle with a given radius of curvature at the minimum cross section. By means of a high-speed camera the kinematics of cloud cavitation is visualized. The shape of the cloud is a horse shoe (U-shaped) with two legs ending at the material surface which is in agreement with the Helmholtz Vortex Theorem. Indeed it is worthwhile to look at the cavitation cloud as a ring Vortex whose second half is a mirror Vortex within the material. Due to the convection flow, the legs of the Vortex are elongated and hence the rotational speed of the Vortex core will increase. Thus cavitation bubbles will concentrate within the legs of the Vortex and that behavior is observed in the cavitation experiments. The aggressiveness of the cloud is quantified by using soft metal inserts adapted on the nozzle geometry. The interpretation of the plastic deformation, called pits, is done with a 2-dimensional optical measurement system, which is developed to scan large and curved surfaces. In this way damage maps are obtained. Consequently dimensional analysis is used to analyse and generalize the experimental results. Thus a critical Reynolds number is found for the transition from sheet to cloud cavitation. Further an upper limit for the Strouhal number exists for the given geometry. A physical model for the critical Reynolds number is given elsewhere [1]. Also a model for the dynamics of the observed stretched cloud with circulation is published by Buttenbender and Pelz [2].

  • THE INFLUENCE OF IMPOSED STRAIN RATE AND CIRCULATION ON BUBBLE AND CLOUD DYNAMICS
    2012
    Co-Authors: Johannes Buttenbender, Peter F. Pelz
    Abstract:

    Within the presented work the effects of rotation and strain on the bubble dynamics inside a cavitating cloud is analyzed and discussed. To follow this task, the mixture of cavitation bubbles and liquid is treated as a continuous medium. With the Helmholtz Vortex Theorem in mind a torus is considered to be preferable cloud geometry. In fact analyzing cloud cavitation horse show cloud structures are dominant and motivate the torus shape we focus on. The flow inside the cloud is treated quasi one-dimensional, the flow outside is modeled by a potential flow. The excitation of the cloud is carried out dynamical by a pressure distribution at infinity and/or kinematical by imposing a circulation and/or a temporal strain distribution. The research was performed in two steps: In a first step the influence of the time scales and void fraction on bubble growth, cloud breathing, cloud collapse delay, and work done on the cloud are considered. Based on a simplification of the governing equations the effects are divided in those of homogeneous, all bubbles in the cloud are equal sized in each time step, and heterogeneous bubble interaction. In a second step the influences of circulation and strain on these quantities have been analyzed. It was found that the circulation enhances the bubble growth in the center of the cloud especially for low void fractions. As consequences the cloud breathing (expansion of the cloud) and the work done on the cloud increased in comparison to the circulation free case, the collapses are delayed; the shocks moving to the center are attenuated. These effects depending on the level of circulation and therefor the density reduction in the cloud core region. The circulation, if the excitation time is high enough, strengths the acoustic load of the environment, represented by the far field pressure, and the maximum compression of the bubbles in the center. An imposed strain results in a density-specific vorticity production and therefor works like an increased circulation but independent of interaction and excitation time if the cloud related strain rate is constant. To bring it to a point: Strain and circulation increase the damage potential.

  • A Physical Model for the Tip Vortex Loss: Experimental Validation and Scaling Method
    Volume 8: Turbomachinery Parts A B and C, 2012
    Co-Authors: Sascha Karstadt, Peter F. Pelz
    Abstract:

    Losses through secondary flows occur in every turbomachine. Between the rotating blades and the casing of a turbomachine there is a secondary flow through the tip clearance caused by the pressure difference between the pressure and the suction side of the blade. This tip leakage flow is not involved in the work done by the rotating blades hence it reduces the aerodynamic efficiency. The flow through the tip clearance rolls up to a spiral Vortex on the suction side of the blade and induces drag. Size and circulation of this Vortex, according to the Helmholtz Vortex Theorem, depend on the bound Vortex and the width of the tip clearance. Examinations of this structure lead to an idea of describing the tip Vortex loss with analytical methods. Therefore an analytical approach is made regarding mainly the circulation at the blade tips.The method is discussed critically in the context of known loss models. It is shown to be a good summary of earlier methods. Since no explicit geometry data of the turbomachine is needed, it is much easier to use. The most important aspect is the excellent agreement with measurements performed at the Chair of Fluid Systems Technology. In total eleven different fan configurations are measured and analyzed in regard to their tip clearance losses. The measurements are performed at a test rig located at the laboratory of the Chair of Fluid Systems Technology at Technische Universitat Darmstadt. Additionally further published measurement data is used to validate the method.Copyright © 2012 by ASME

Johannes Buttenbender - One of the best experts on this subject based on the ideXlab platform.

  • THE INFLUENCE OF IMPOSED STRAIN RATE AND CIRCULATION ON BUBBLE AND CLOUD DYNAMICS
    2012
    Co-Authors: Johannes Buttenbender, Peter F. Pelz
    Abstract:

    Within the presented work the effects of rotation and strain on the bubble dynamics inside a cavitating cloud is analyzed and discussed. To follow this task, the mixture of cavitation bubbles and liquid is treated as a continuous medium. With the Helmholtz Vortex Theorem in mind a torus is considered to be preferable cloud geometry. In fact analyzing cloud cavitation horse show cloud structures are dominant and motivate the torus shape we focus on. The flow inside the cloud is treated quasi one-dimensional, the flow outside is modeled by a potential flow. The excitation of the cloud is carried out dynamical by a pressure distribution at infinity and/or kinematical by imposing a circulation and/or a temporal strain distribution. The research was performed in two steps: In a first step the influence of the time scales and void fraction on bubble growth, cloud breathing, cloud collapse delay, and work done on the cloud are considered. Based on a simplification of the governing equations the effects are divided in those of homogeneous, all bubbles in the cloud are equal sized in each time step, and heterogeneous bubble interaction. In a second step the influences of circulation and strain on these quantities have been analyzed. It was found that the circulation enhances the bubble growth in the center of the cloud especially for low void fractions. As consequences the cloud breathing (expansion of the cloud) and the work done on the cloud increased in comparison to the circulation free case, the collapses are delayed; the shocks moving to the center are attenuated. These effects depending on the level of circulation and therefor the density reduction in the cloud core region. The circulation, if the excitation time is high enough, strengths the acoustic load of the environment, represented by the far field pressure, and the maximum compression of the bubbles in the center. An imposed strain results in a density-specific vorticity production and therefor works like an increased circulation but independent of interaction and excitation time if the cloud related strain rate is constant. To bring it to a point: Strain and circulation increase the damage potential.

Gerhard Ludwig - One of the best experts on this subject based on the ideXlab platform.

  • On the Kinematics of Sheet and Cloud Cavitation and Related Erosion
    Advanced Experimental and Numerical Techniques for Cavitation Erosion Prediction, 2014
    Co-Authors: Peter F. Pelz, T. Keil, Gerhard Ludwig
    Abstract:

    The influence of flow parameters such as cavitation number and Reynolds number on the cavitating cloud behavior and aggressiveness is analysed in an experimental work. The focused geometry is a convergent-divergent nozzle with a given radius of curvature at the minimum cross section. By means of a high-speed camera the kinematics of cloud cavitation is visualized. The shape of the cloud is a horse shoe (U-shaped) with two legs ending at the material surface which is in agreement with the Helmholtz Vortex Theorem. Indeed it is worthwhile to look at the cavitation cloud as a ring Vortex whose second half is a mirror Vortex within the material. Due to the convection flow, the legs of the Vortex are elongated and hence the rotational speed of the Vortex core will increase. Thus cavitation bubbles will concentrate within the legs of the Vortex and that behavior is observed in the cavitation experiments. The aggressiveness of the cloud is quantified by using soft metal inserts adapted on the nozzle geometry. The interpretation of the plastic deformation, called pits, is done with a 2-dimensional optical measurement system, which is developed to scan large and curved surfaces. In this way damage maps are obtained. Consequently dimensional analysis is used to analyse and generalize the experimental results. Thus a critical Reynolds number is found for the transition from sheet to cloud cavitation. Further an upper limit for the Strouhal number exists for the given geometry. A physical model for the critical Reynolds number is given elsewhere [1]. Also a model for the dynamics of the observed stretched cloud with circulation is published by Buttenbender and Pelz [2].

Sascha Karstadt - One of the best experts on this subject based on the ideXlab platform.

  • A Physical Model for the Tip Vortex Loss: Experimental Validation and Scaling Method
    Volume 8: Turbomachinery Parts A B and C, 2012
    Co-Authors: Sascha Karstadt, Peter F. Pelz
    Abstract:

    Losses through secondary flows occur in every turbomachine. Between the rotating blades and the casing of a turbomachine there is a secondary flow through the tip clearance caused by the pressure difference between the pressure and the suction side of the blade. This tip leakage flow is not involved in the work done by the rotating blades hence it reduces the aerodynamic efficiency. The flow through the tip clearance rolls up to a spiral Vortex on the suction side of the blade and induces drag. Size and circulation of this Vortex, according to the Helmholtz Vortex Theorem, depend on the bound Vortex and the width of the tip clearance. Examinations of this structure lead to an idea of describing the tip Vortex loss with analytical methods. Therefore an analytical approach is made regarding mainly the circulation at the blade tips.The method is discussed critically in the context of known loss models. It is shown to be a good summary of earlier methods. Since no explicit geometry data of the turbomachine is needed, it is much easier to use. The most important aspect is the excellent agreement with measurements performed at the Chair of Fluid Systems Technology. In total eleven different fan configurations are measured and analyzed in regard to their tip clearance losses. The measurements are performed at a test rig located at the laboratory of the Chair of Fluid Systems Technology at Technische Universitat Darmstadt. Additionally further published measurement data is used to validate the method.Copyright © 2012 by ASME

T. Keil - One of the best experts on this subject based on the ideXlab platform.

  • On the Kinematics of Sheet and Cloud Cavitation and Related Erosion
    Advanced Experimental and Numerical Techniques for Cavitation Erosion Prediction, 2014
    Co-Authors: Peter F. Pelz, T. Keil, Gerhard Ludwig
    Abstract:

    The influence of flow parameters such as cavitation number and Reynolds number on the cavitating cloud behavior and aggressiveness is analysed in an experimental work. The focused geometry is a convergent-divergent nozzle with a given radius of curvature at the minimum cross section. By means of a high-speed camera the kinematics of cloud cavitation is visualized. The shape of the cloud is a horse shoe (U-shaped) with two legs ending at the material surface which is in agreement with the Helmholtz Vortex Theorem. Indeed it is worthwhile to look at the cavitation cloud as a ring Vortex whose second half is a mirror Vortex within the material. Due to the convection flow, the legs of the Vortex are elongated and hence the rotational speed of the Vortex core will increase. Thus cavitation bubbles will concentrate within the legs of the Vortex and that behavior is observed in the cavitation experiments. The aggressiveness of the cloud is quantified by using soft metal inserts adapted on the nozzle geometry. The interpretation of the plastic deformation, called pits, is done with a 2-dimensional optical measurement system, which is developed to scan large and curved surfaces. In this way damage maps are obtained. Consequently dimensional analysis is used to analyse and generalize the experimental results. Thus a critical Reynolds number is found for the transition from sheet to cloud cavitation. Further an upper limit for the Strouhal number exists for the given geometry. A physical model for the critical Reynolds number is given elsewhere [1]. Also a model for the dynamics of the observed stretched cloud with circulation is published by Buttenbender and Pelz [2].