The Experts below are selected from a list of 79452 Experts worldwide ranked by ideXlab platform

Ivan Marusic - One of the best experts on this subject based on the ideXlab platform.

  • uniform momentum zones in turbulent Boundary Layers
    Journal of Fluid Mechanics, 2016
    Co-Authors: Charitha De Silva, N Hutchins, Ivan Marusic
    Abstract:

    Structural properties of regions of uniform streamwise momentum in turbulent Boundary Layers are examined using experimental databases obtained from particle image velocimetry. This investigation employs a large range of Reynolds numbers, spanning more than an order of magnitude ( ), enabling us to provide a detailed description of uniform momentum zones as a function of Reynolds number. Our analysis starts by examining the identification criterion used by Adrian et al. (J. Fluid Mech., vol. 422, 2000, pp. 1–54) to report the presence of uniform momentum zones in turbulent Boundary Layers. This criterion is then applied to show that a zonal-like structural arrangement is prevalent in all datasets examined, emphasising its importance in the structural organisation. Streamwise velocity fluctuations within the zones are observed to be small but they are bounded by distinct step changes in streamwise momentum which indicate that shear Layers of intense vorticity separate each zone. A log-linear increase in the number of these zones with increasing Reynolds number is revealed, together with an increase in the thicknesses of zones with increasing distance from the wall. These results support a hierarchical length-scale distribution of coherent structures, which generate zonal-like organisation within turbulent Boundary Layers. Interpretation of these findings is aided by employing synthetic velocity fields generated using a model based on the attached eddy hypothesis, which is described in the work of Perry and co-workers. Comparisons between the model and experimental results show that a hierarchy of self-similar structures leads to population densities and length-scale distributions of uniform momentum zones that closely adhere to those observed experimentally in this study.

  • evolution of zero pressure gradient Boundary Layers from different tripping conditions
    Journal of Fluid Mechanics, 2015
    Co-Authors: Ivan Marusic, Kapil Chauhan, V Kulandaivelu, N Hutchins
    Abstract:

    In this paper we study the spatial evolution of zero-pressure-gradient (ZPG) turbulent Boundary Layers from their origin to a canonical high-Reynolds-number state. A prime motivation is to better understand under what conditions reliable scaling behaviour comparisons can be made between different experimental studies at matched local Reynolds numbers. This is achieved here through detailed streamwise velocity measurements using hot wires in the large University of Melbourne wind tunnel. By keeping the unit Reynolds number constant, the flow conditioning, contraction and trip can be considered unaltered for a given Boundary layer’s development and hence its evolution can be studied in isolation from the influence of inflow conditions by moving to different streamwise locations. Careful attention was given to the experimental design in order to make comparisons between flows with three different trips while keeping all other parameters nominally constant, including keeping the measurement sensor size nominally fixed in viscous wall units. The three trips consist of a standard trip and two deliberately ‘over-tripped’ cases, where the initial Boundary Layers are over-stimulated with additional large-scale energy. Comparisons of the mean flow, normal Reynolds stress, spectra and higher-order turbulence statistics reveal that the effects of the trip are seen to be significant, with the remnants of the ‘over-tripped’ conditions persisting at least until streamwise stations corresponding to $Re_{x}=1.7\times 10^{7}$ and $x=O(2000)$ trip heights are reached (which is specific to the trips used here), at which position the non-canonical Boundary Layers exhibit a weak memory of their initial conditions at the largest scales $O(10{\it\delta})$ , where ${\it\delta}$ is the Boundary layer thickness. At closer streamwise stations, no one-to-one correspondence is observed between the local Reynolds numbers ( $Re_{{\it\tau}}$ , $Re_{{\it\theta}}$ or $Re_{x}$ etc.), and these differences are likely to be the cause of disparities between previous studies where a given Reynolds number is matched but without account of the trip conditions and the actual evolution of the Boundary layer. In previous literature such variations have commonly been referred to as low-Reynolds-number effects, while here we show that it is more likely that these differences are due to an evolution effect resulting from the initial conditions set up by the trip and/or the initial inflow conditions. Generally, the mean velocity profiles were found to approach a constant wake parameter ${\it\Pi}$ as the three Boundary Layers developed along the test section, and agreement of the mean flow parameters was found to coincide with the location where other statistics also converged, including higher-order moments up to tenth order. This result therefore implies that it may be sufficient to document the mean flow parameters alone in order to ascertain whether the ZPG flow, as described by the streamwise velocity statistics, has reached a canonical state, and a computational approach is outlined to do this. The computational scheme is shown to agree well with available experimental data.

  • evolution of zero pressure gradient Boundary Layers from different tripping conditions
    Journal of Fluid Mechanics, 2015
    Co-Authors: Ivan Marusic, Kapil Chauhan, V Kulandaivelu, N Hutchins
    Abstract:

    In this paper we study the spatial evolution of zero-pressure-gradient (ZPG) turbulent Boundary Layers from their origin to a canonical high-Reynolds-number state. A prime motivation is to better understand under what conditions reliable scaling behaviour comparisons can be made between different experimental studies at matched local Reynolds numbers. This is achieved here through detailed streamwise velocity measurements using hot wires in the large University of Melbourne wind tunnel. By keeping the unit Reynolds number constant, the flow conditioning, contraction and trip can be considered unaltered for a given Boundary layer’s development and hence its evolution can be studied in isolation from the influence of inflow conditions by moving to different streamwise locations. Careful attention was given to the experimental design in order to make comparisons between flows with three different trips while keeping all other parameters nominally constant, including keeping the measurement sensor size nominally fixed in viscous wall units. The three trips consist of a standard trip and two deliberately ‘over-tripped’ cases, where the initial Boundary Layers are over-stimulated with additional large-scale energy. Comparisons of the mean flow, normal Reynolds stress, spectra and higher-order turbulence statistics reveal that the effects of the trip are seen to be significant, with the remnants of the ‘over-tripped’ conditions persisting at least until streamwise stations corresponding to and trip heights are reached (which is specific to the trips used here), at which position the non-canonical Boundary Layers exhibit a weak memory of their initial conditions at the largest scales , where is the Boundary layer thickness. At closer streamwise stations, no one-to-one correspondence is observed between the local Reynolds numbers ( , or etc.), and these differences are likely to be the cause of disparities between previous studies where a given Reynolds number is matched but without account of the trip conditions and the actual evolution of the Boundary layer. In previous literature such variations have commonly been referred to as low-Reynolds-number effects, while here we show that it is more likely that these differences are due to an evolution effect resulting from the initial conditions set up by the trip and/or the initial inflow conditions. Generally, the mean velocity profiles were found to approach a constant wake parameter as the three Boundary Layers developed along the test section, and agreement of the mean flow parameters was found to coincide with the location where other statistics also converged, including higher-order moments up to tenth order. This result therefore implies that it may be sufficient to document the mean flow parameters alone in order to ascertain whether the ZPG flow, as described by the streamwise velocity statistics, has reached a canonical state, and a computational approach is outlined to do this. The computational scheme is shown to agree well with available experimental data.

  • pressure gradient effects on the large scale structure of turbulent Boundary Layers
    Journal of Fluid Mechanics, 2013
    Co-Authors: Zambri Harun, Jason P. Monty, Romain Mathis, Ivan Marusic
    Abstract:

    Research into high-Reynolds-number turbulent Boundary Layers in recent years has brought about a renewed interest in the larger-scale structures. It is now known that these structures emerge more prominently in the outer region not only due to increased Reynolds number (Metzger & Klewicki, Phys. Fluids , vol. 13(3), 2001, pp. 692–701; Hutchins & Marusic, J. Fluid Mech. , vol. 579, 2007, pp. 1–28), but also when a Boundary layer is exposed to an adverse pressure gradient (Bradshaw, J. Fluid Mech. , vol. 29, 1967, pp. 625–645; Lee & Sung, J. Fluid Mech. , vol. 639, 2009, pp. 101–131). The latter case has not received as much attention in the literature. As such, this work investigates the modification of the large-scale features of Boundary Layers subjected to zero, adverse and favourable pressure gradients. It is first shown that the mean velocities, turbulence intensities and turbulence production are significantly different in the outer region across the three cases. Spectral and scale decomposition analyses confirm that the large scales are more energized throughout the entire adverse pressure gradient Boundary layer, especially in the outer region. Although more energetic, there is a similar spectral distribution of energy in the wake region, implying the geometrical structure of the outer layer remains universal in all cases. Comparisons are also made of the amplitude modulation of small scales by the large-scale motions for the three pressure gradient cases. The wall-normal location of the zero-crossing of small-scale amplitude modulation is found to increase with increasing pressure gradient, yet this location continues to coincide with the large-scale energetic peak wall-normal location (as has been observed in zero pressure gradient Boundary Layers). The amplitude modulation effect is found to increase as pressure gradient is increased from favourable to adverse.

  • a parametric study of adverse pressure gradient turbulent Boundary Layers
    International Journal of Heat and Fluid Flow, 2011
    Co-Authors: Jason P. Monty, Zambri Harun, Ivan Marusic
    Abstract:

    There are many open questions regarding the behaviour of turbulent Boundary Layers subjected to pressure gradients and this is confounded by the large parameter space that may affect these flows. While there have been many valuable investigations conducted within this parameter space, there are still insufficient data to attempt to reduce this parameter space. Here, we consider a parametric study of adverse pressure gradient turbulent Boundary Layers where we restrict our attention to the pressure gradient parameter, β, the Reynolds number and the acceleration parameter, K. The statistics analyzed are limited to the streamwise fluctuating velocity. The data show that the mean velocity profile in strong pressure gradient Boundary Layers does not conform to the classical logarithmic law. Moreover, there appears to be no measurable logarithmic region in these cases. It is also found that the large-scale motions scaling with outer variables are energised by the pressure gradient. These increasingly strong large-scale motions are found to be the dominant contributor to the increase in turbulence intensity (scaled with friction velocity) with increasing pressure gradient across the Boundary layer.

Philipp Schlatter - One of the best experts on this subject based on the ideXlab platform.

N Hutchins - One of the best experts on this subject based on the ideXlab platform.

  • uniform momentum zones in turbulent Boundary Layers
    Journal of Fluid Mechanics, 2016
    Co-Authors: Charitha De Silva, N Hutchins, Ivan Marusic
    Abstract:

    Structural properties of regions of uniform streamwise momentum in turbulent Boundary Layers are examined using experimental databases obtained from particle image velocimetry. This investigation employs a large range of Reynolds numbers, spanning more than an order of magnitude ( ), enabling us to provide a detailed description of uniform momentum zones as a function of Reynolds number. Our analysis starts by examining the identification criterion used by Adrian et al. (J. Fluid Mech., vol. 422, 2000, pp. 1–54) to report the presence of uniform momentum zones in turbulent Boundary Layers. This criterion is then applied to show that a zonal-like structural arrangement is prevalent in all datasets examined, emphasising its importance in the structural organisation. Streamwise velocity fluctuations within the zones are observed to be small but they are bounded by distinct step changes in streamwise momentum which indicate that shear Layers of intense vorticity separate each zone. A log-linear increase in the number of these zones with increasing Reynolds number is revealed, together with an increase in the thicknesses of zones with increasing distance from the wall. These results support a hierarchical length-scale distribution of coherent structures, which generate zonal-like organisation within turbulent Boundary Layers. Interpretation of these findings is aided by employing synthetic velocity fields generated using a model based on the attached eddy hypothesis, which is described in the work of Perry and co-workers. Comparisons between the model and experimental results show that a hierarchy of self-similar structures leads to population densities and length-scale distributions of uniform momentum zones that closely adhere to those observed experimentally in this study.

  • evolution of zero pressure gradient Boundary Layers from different tripping conditions
    Journal of Fluid Mechanics, 2015
    Co-Authors: Ivan Marusic, Kapil Chauhan, V Kulandaivelu, N Hutchins
    Abstract:

    In this paper we study the spatial evolution of zero-pressure-gradient (ZPG) turbulent Boundary Layers from their origin to a canonical high-Reynolds-number state. A prime motivation is to better understand under what conditions reliable scaling behaviour comparisons can be made between different experimental studies at matched local Reynolds numbers. This is achieved here through detailed streamwise velocity measurements using hot wires in the large University of Melbourne wind tunnel. By keeping the unit Reynolds number constant, the flow conditioning, contraction and trip can be considered unaltered for a given Boundary layer’s development and hence its evolution can be studied in isolation from the influence of inflow conditions by moving to different streamwise locations. Careful attention was given to the experimental design in order to make comparisons between flows with three different trips while keeping all other parameters nominally constant, including keeping the measurement sensor size nominally fixed in viscous wall units. The three trips consist of a standard trip and two deliberately ‘over-tripped’ cases, where the initial Boundary Layers are over-stimulated with additional large-scale energy. Comparisons of the mean flow, normal Reynolds stress, spectra and higher-order turbulence statistics reveal that the effects of the trip are seen to be significant, with the remnants of the ‘over-tripped’ conditions persisting at least until streamwise stations corresponding to $Re_{x}=1.7\times 10^{7}$ and $x=O(2000)$ trip heights are reached (which is specific to the trips used here), at which position the non-canonical Boundary Layers exhibit a weak memory of their initial conditions at the largest scales $O(10{\it\delta})$ , where ${\it\delta}$ is the Boundary layer thickness. At closer streamwise stations, no one-to-one correspondence is observed between the local Reynolds numbers ( $Re_{{\it\tau}}$ , $Re_{{\it\theta}}$ or $Re_{x}$ etc.), and these differences are likely to be the cause of disparities between previous studies where a given Reynolds number is matched but without account of the trip conditions and the actual evolution of the Boundary layer. In previous literature such variations have commonly been referred to as low-Reynolds-number effects, while here we show that it is more likely that these differences are due to an evolution effect resulting from the initial conditions set up by the trip and/or the initial inflow conditions. Generally, the mean velocity profiles were found to approach a constant wake parameter ${\it\Pi}$ as the three Boundary Layers developed along the test section, and agreement of the mean flow parameters was found to coincide with the location where other statistics also converged, including higher-order moments up to tenth order. This result therefore implies that it may be sufficient to document the mean flow parameters alone in order to ascertain whether the ZPG flow, as described by the streamwise velocity statistics, has reached a canonical state, and a computational approach is outlined to do this. The computational scheme is shown to agree well with available experimental data.

  • evolution of zero pressure gradient Boundary Layers from different tripping conditions
    Journal of Fluid Mechanics, 2015
    Co-Authors: Ivan Marusic, Kapil Chauhan, V Kulandaivelu, N Hutchins
    Abstract:

    In this paper we study the spatial evolution of zero-pressure-gradient (ZPG) turbulent Boundary Layers from their origin to a canonical high-Reynolds-number state. A prime motivation is to better understand under what conditions reliable scaling behaviour comparisons can be made between different experimental studies at matched local Reynolds numbers. This is achieved here through detailed streamwise velocity measurements using hot wires in the large University of Melbourne wind tunnel. By keeping the unit Reynolds number constant, the flow conditioning, contraction and trip can be considered unaltered for a given Boundary layer’s development and hence its evolution can be studied in isolation from the influence of inflow conditions by moving to different streamwise locations. Careful attention was given to the experimental design in order to make comparisons between flows with three different trips while keeping all other parameters nominally constant, including keeping the measurement sensor size nominally fixed in viscous wall units. The three trips consist of a standard trip and two deliberately ‘over-tripped’ cases, where the initial Boundary Layers are over-stimulated with additional large-scale energy. Comparisons of the mean flow, normal Reynolds stress, spectra and higher-order turbulence statistics reveal that the effects of the trip are seen to be significant, with the remnants of the ‘over-tripped’ conditions persisting at least until streamwise stations corresponding to and trip heights are reached (which is specific to the trips used here), at which position the non-canonical Boundary Layers exhibit a weak memory of their initial conditions at the largest scales , where is the Boundary layer thickness. At closer streamwise stations, no one-to-one correspondence is observed between the local Reynolds numbers ( , or etc.), and these differences are likely to be the cause of disparities between previous studies where a given Reynolds number is matched but without account of the trip conditions and the actual evolution of the Boundary layer. In previous literature such variations have commonly been referred to as low-Reynolds-number effects, while here we show that it is more likely that these differences are due to an evolution effect resulting from the initial conditions set up by the trip and/or the initial inflow conditions. Generally, the mean velocity profiles were found to approach a constant wake parameter as the three Boundary Layers developed along the test section, and agreement of the mean flow parameters was found to coincide with the location where other statistics also converged, including higher-order moments up to tenth order. This result therefore implies that it may be sufficient to document the mean flow parameters alone in order to ascertain whether the ZPG flow, as described by the streamwise velocity statistics, has reached a canonical state, and a computational approach is outlined to do this. The computational scheme is shown to agree well with available experimental data.

  • inclined cross stream stereo particle image velocimetry measurements in turbulent Boundary Layers
    Journal of Fluid Mechanics, 2005
    Co-Authors: N Hutchins, William Hambleton, Ivan Marusic
    Abstract:

    © Cambridge University Press. Hutchins, N., Hambleton, W. T., & Marusic, I. (2005). Inclined cross-stream stereo particle image velocimetry measurements in turbulent Boundary Layers. Journal of Fluid Mechanics, 541, 21-54. http://www.jfm.damtp.cam.ac.uk/

Ricardo Vinuesa - One of the best experts on this subject based on the ideXlab platform.

Alexandra Bobke - One of the best experts on this subject based on the ideXlab platform.