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  • 1751.
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Experimental studies in jet flows and zero pressure-gradient turbulent boundary layers2009Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    This thesis deals with the description and development of two classical turbulent shear flows, namely free jet and flat plate turbulent boundary layer flows. In both cases new experimental data has been obtained and in the latter case comparisons are also made with data obtained from data bases, both of experimental and numerical origin. The jet flow studies comprise three parts, made in three different experimental facilities, each dealing with a specific aspect of jet flows. The first part is devoted to the effect of swirl on the mixing characteristics of a passive scalar in the near-field region of a moderately swirling jet. Instantaneous streamwise and azimuthal velocity components as well as the temperature were simultaneously accessed by means of combined X-wire and cold-wire anemometry. The results indicate a modification of the turbulence structures to that effect that the swirling jet spreads, mixes and evolves faster compared to its non-swirling counterpart. The high correlation between streamwise velocity and temperature fluctuations as well as the streamwise passive scalar flux are even more enhanced due to the addition of swirl, which in turn shortens the distance and hence time needed to mix the jet with the ambient air. The second jet flow part was set out to test the hypothesis put forward by Talamelli & Gavarini (Flow, Turbul. & Combust. 76), who proposed that the wake behind a separation wall between two streams of a coaxial jet creates the condition for an absolute instability. The experiments confirm the hypothesis and show that the instability, by means of the induced vortex shedding, provides a continuous forcing mechanism for the control of the flow field. The potential of this passive mechanism as an easy, effective and practical way to control the near-field of interacting shear layers as well as its effect towards increased turbulence activity has been shown. The third part of the jet flow studies deals with the hypothesis that so called oblique transition may play a role in the breakdown to turbulence for an axisymmetric jet.For wall bounded flows oblique transition gives rise to steady streamwise streaks that break down to turbulence, as for instance documented by Elofsson & Alfredsson (J. Fluid Mech. 358). The scenario of oblique transition has so far not been considered for jet flows and the aim was to study the effect of two oblique modes on the transition scenario as well as on the flow dynamics. For certain frequencies the turbulence intensity was surprisingly found to be reduced, however it was not possible to detect the presence of streamwise streaks. This aspect must be furher investigated in the future in order to understand the connection between the turbulence reduction and the azimuthal forcing. The boundary layer part of the thesis is also threefold, and uses both new data as well as data from various data bases to investigate the effect of certain limitations of hot-wire measurements near the wall on the mean velocity but also on the fluctuating streamwise velocity component. In the first part a new set of experimental data from a zero pressure-gradient turbulent boundary layer, supplemented by direct and independent skin friction measurements, are presented. The Reynolds number range of the data is between 2300 and 18700 when based on the free stream velocity and the momentum loss thickness. Data both for the mean and fluctuating streamwise velocity component are presented. The data are validated against the composite profile by Chauhan et al. (Fluid Dyn. Res. 41) and are found to fulfil recently established equilibrium criteria. The problem of accurately locating the wall position of a hot-wire probe and the errors this can result in is thoroughly discussed in part 2 of the boundary layer study. It is shown that the expanded law of the wall to forth and fifth order with calibration constants determined from recent high Reynolds number DNS can be used to fix the wall position to an accuracy of 0.1 and 0.25 l_ * (l_* is the viscous length scale) when accurately determined measurements reaching y+=5 and 10, respectively, are available. In the absence of data below the above given limits, commonly employed analytical functions and their log law constants, have been found to affect the the determination of wall position to a high degree. It has been shown, that near-wall measurements below y+=10 or preferable 5 are essential in order to ensure a correctly measured or deduced absolute wall position. A  number of peculiarities in concurrent wall-bounded turbulent flow studies, was found to be associated with a erroneously deduced wall position. The effect of poor spatial resolution using hot-wire anemometry on the measurements of the streamwise velocity is dealt with in the last part. The viscous scaled hot-wire length, L+, has been found to exert a strong impact on the probability density distribution (pdf) of the streamwise velocity, and hence its higher order moments, over the entire buffer region and also the lower region of the log region. For varying Reynolds numbers spatial resolution effects act against the trend imposed by the Reynolds number. A systematic reduction of the mean velocity with increasing L+ over the entire classical buffer region and beyond has been found. A reduction of around 0.3 uƬ, where uƬ is the friction velocity, has been deduced for L+=60 compared to L+=15. Neglecting this effect can lead to a seemingly Reynolds number dependent  buffer or log region. This should be taken into consideration, for instance, in the debate, regarding the prevailing influence of viscosity above the buffer region at high Reynolds numbers. We also conclude that the debate concerning the universality of the pdf within the overlap region has been artificially complicated due to the ignorance of spatial resolution effects beyond the classical buffer region on the velocity fluctuations.

  • 1752.
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Experimental study of passive scalar mixing in swirling jet flows2006Licentiatavhandling, monografi (Annet vitenskapelig)
    Abstract [en]

    Despite its importance in various industrial applications there is still a lack of experimental studies on the dynamic and thermal field of swirling jets in the near-field region. The present study is an attempt to close this lack and provide new insights on the effect of rotation on the turbulent mixing of a passive scalar, on turbulence (joint) statistics as well as the turbulence structure.

    Swirl is known to increase the spreading of free turbulent jets and hence to entrain more ambient fluid. Contrary to previous experiments, which leave traces of the swirl generating method especially in the near-field, the swirl was imparted by discharging a slightly heated air flow from an axially rotating and thermally insulated pipe (6 m long, diameter 60 mm). This gives well-defined axisymmetric streamwise and azimuthal velocity distributions as well as a well-defined temperature profile at the jet outlet. The experiments were performed at a Reynolds number of 24000 and a swirl number (ratio between the angular velocity of the pipe wall and the bulk velocity in the pipe) of 0.5.

    By means of a specially designed combined X-wire and cold-wire probe it was possible to simultaneously acquire the instantaneous axial and azimuthal velocity components as well as the temperature and compensate the former against temperature variations. The comparison of the swirling and non-swirling cases clearly indicates a modification of the turbulence structure to that effect that the swirling jet spreads and mixes faster than its non-swirling counterpart. It is also shown that the streamwise velocity and temperature fluctuations are highly correlated and that the addition of swirl drastically increases the streamwise passive scalar flux in the near field.

  • 1753.
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Low Reynolds number Zero Pressure-Gradient Equilibrium Turbulent Boundary-Layer ExperimentsManuskript (preprint) (Annet vitenskapelig)
  • 1754.
    Örlü, Ramis
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    On the determination of the wall position in wall-bounded turbulent flowsManuskript (preprint) (Annet vitenskapelig)
  • 1755.
    Örlü, Ramis
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Alfredsson, P. Henrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik.
    An experimental study of the near-field mixing characteristics of a swirling jet2008Inngår i: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 80, nr 3, s. 323-350Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The present experimental investigation is devoted to the mixing charac- teristics of a passive scalar in the near-field region of a moderately swirling jet issuing from a fully developed axially rotating pipe flow. Instantaneous streamwise and azimuthal velocity components as well as the temperature were simultaneously accessed by means of a combined X-wire and cold-wire probe. The results indicate a modification of the turbulence structures to that effect that the swirling jet spreads, mixes and evolves faster compared to its non-swirling counterpart. The high correlation between streamwise velocity and temperature fluctuations as well as the streamwise passive scalar flux are even more enhanced due to the addition of swirl, which in turn shortens the distance and hence time needed to mix the jet with the ambient air.

  • 1756.
    Örlü, Ramis
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Alfredsson, P. Henrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    On spatial resolution issues related to time-averaged quantities using hot-wire anemometry2010Inngår i: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 49, nr 1, s. 101-110Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The effect of spatial resolution on streamwise velocity measurements with single hot-wires is targeted in the present study, where efforts have been made to distinguish between spatial resolution and Reynolds number effects. The basis for measurements to accurately determine the mean velocity and higher order moments is that the probability density distribution is measured correctly. It is well known that the turbulence intensity is increasingly attenuated with increasing wire length. Here, it is also shown (probably for the first time) that besides the probability density distribution and hence the higher order moments, even the mean velocity is affected, albeit to subtle extent, but with important consequences in studies of concurrent wall-bounded turbulence.

  • 1757.
    Örlü, Ramis
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Kalpakli Vester, Athanasia
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. KTH, Skolan för industriell teknik och management (ITM), Centra, Competence Center for Gas Exchange (CCGEx).
    Flow visualization of an oblique impinging jet: vortices like it downhill, not uphill2015Inngår i: Journal of Visualization, ISSN 1343-8875, E-ISSN 1875-8975Artikkel i tidsskrift (Fagfellevurdert)
  • 1758.
    Örlü, Ramis
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Malizia, Fabio
    Cimarelli, Andrea
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Talamelli, Alessandro
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW. Univ Bologna, Italy.
    The influence of temperature fluctuations on hot-wire measurements in wall-bounded turbulence2014Inngår i: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 55, nr 7, s. 1781-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    There are no measurement techniques for turbulent flows capable of reaching the versatility of hot-wire probes and their frequency response. Nevertheless, the issue of their spatial resolution is still a matter of debate when it comes to high Reynolds number near-wall turbulence. Another, so far unattended, issue is the effect of temperature fluctuations-as they are, e. g. encountered in non-isothermal flows-on the low and higher-order moments in wall-bounded turbulent flows obtained through hot-wire anemometry. The present investigation is dedicated to document, understand, and ultimately correct these effects. For this purpose, the response of a hot-wire is simulated through the use of velocity and temperature data from a turbulent channel flow generated by means of direct numerical simulations. Results show that ignoring the effect of temperature fluctuations, caused by temperature gradients along the wall-normal direction, introduces-despite a local mean temperature compensation of the velocity reading-significant errors. The results serve as a note of caution for hot-wire measurements in wall-bounded turbulence, and also where temperature gradients are more prevalent, such as heat transfer measurements or high Mach number flows. A simple correction scheme involving only mean temperature quantities (besides the streamwise velocity information) is finally proposed that leads to a substantial bias error reduction.

  • 1759.
    Örlü, Ramis
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Inflow length and tripping effects in turbulent boundary layers2011Inngår i: 13TH European Turbulence Conference (ETC13): Wall-Bounded Flows And Control Of Turbulence, Institute of Physics Publishing (IOPP), 2011, s. 022018-Konferansepaper (Fagfellevurdert)
    Abstract [en]

    A recent assessment of available direct numerical simulation (DNS) data from turbulent boundary layer flows [Schlatter & Orlu, J. Fluid Mech. 659, 116 (2010)] showed surprisingly large differences not only in the skin friction coefficient or shape factor, but also in their predictions of mean and fluctuation profiles far into the sublayer. For the present paper the DNS of a zero pressure-gradient turbulent boundary layer flow by Schlatter et al. [Phys. Fluids 21, 051702 (2009)] serving as the baseline simulation, was re-simulated, however with physically different inflow conditions and tripping effects. The downstream evolution of integral and global quantities as well as mean and fluctuation profiles are presented and results indicate that different inflow conditions and tripping effects explain most of the differences observed when comparing available DNS. It is also found, that if transition is initiated at a low enough Reynolds number (based on the momentum-loss thickness) Re-theta < 300, all data agree well for both inner and outer layer for Re-theta > 2000; a result that gives a lower limit for meaningful comparisons between numerical and/or wind tunnel experiments.

  • 1760.
    Örlü, Ramis
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Segalini, Antonio
    Alfredsson, Henrik
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Talamelli, Alessandro
    KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    On the passive control of the near-field of coaxial jets by means of vortex shedding2008Inngår i: Int. Conf. on Jets, Wakes and Separated Flows, ICJWSF-2, September 16-19, 2008, Technical University of Berlin, Germany, 2008, s. 1-16Konferansepaper (Fagfellevurdert)
  • 1761.
    Örlü, Ramis
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Segalini, Antonio
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Alfredsson, P. Henrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Talamelli, Alessandro
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Passive control of mixing in a coaxial jet2008Inngår i: Proc. 7th Int. ERCOFTAC Symp. on Engineering Turbulence Modelling and Measurements (ETMM7), 2008, s. 450-455Konferansepaper (Fagfellevurdert)
    Abstract [en]

    An experimental investigation regarding interacting shear layers in a coaxial jet geometry has been performed. The present paper confirms experimentally the theoretical result by Talamelli and Gavarini (2006), who proposed that the wake behind the separation wall between the two stream of a coaxial jet creates the condition for an absolute instability. This instability, by means of the induced vortex shedding, may provide a continuous forcing mechanism for the control of the flow field. The potential of this passive mechanism as an easy, effective and practical way to control the near-field of interacting shear layers has been demonstrated.

  • 1762.
    Örlü, Ramis
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Segalini, Antonio
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Stabilitet, Transition, Kontroll. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Alfredsson, P. Henrik
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Strömningsfysik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Talamelli, Alessandro
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Turbulence Enhancement in Coaxial Jet Flows by Means of Vortex Shedding2009Inngår i: PROGRESS IN TURBULENCE III / [ed] Peinke, J.; Oberlack, M.; Talamelli, A., 2009, Vol. 131, s. 235-238Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Over the past decades a variety of passive and active flow control mechanisms have been tested and applied in a variety of canonical as well as applied flow cases. An example for the latter is the coaxial jet flow, which has mainly been investigated regarding the receptivity to active flow control strategies (see e.g. [1]), probably due to the multitude of parameters characterising the complex flow field [2]. Physical and numerical experiments (see e.g. [3] and [5]) have established that the vortical motion in coaxial jet flows is dominated by the vortices emerging from the outer shear layer. The frequency of these vortices is related to the Kelvin- Helmholtz instability as predicted by linear stability analysis for single jets. The vortices in the inner shear layer, on the other hand, are trapped in the spaces left free between two consecutive outer shear layer vortices, and are therefore sharing the frequencies of the most amplified modes of the outer shear layer and do not relate to the values one would expect from linear stability analysis. This fact has become known as the “locking phenomenon”, which describes the mutual interaction of both shear layers. Nevertheless it is believed that only the outer shear layer is able to significantly control the evolution of the inner shear layer [7], which may explain the focus of control strategies on the outer shear layer.

  • 1763.
    Örlü, Ramis
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Vinuesa, Ricardo
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Vila, C. S.
    DIscetti, S.
    Ianiro, A.
    Schlatter, Philipp
    KTH, Skolan för teknikvetenskap (SCI), Mekanik. KTH, Skolan för teknikvetenskap (SCI), Centra, Linné Flow Center, FLOW.
    Towards canonical adverse-pressure-gradient turbulent boundary layers2017Inngår i: 10th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2017, International Symposium on Turbulence and Shear Flow Phenomena, TSFP10 , 2017, Vol. 3Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The present investigation focuses on the concerted investigation of pressure gradient and streamwise curvature effects on turbulent boundary layers. In particular, a number of direct and large-eddy simulations covering a wide range of pressure gradient parameters and streamwise histories on flat and curved surfaces is performed and will be compared with wind-tunnel experiments utilising hot-wire anemometry and particle image velocimetry that overlap and extend the Reynolds number range. Results are aimed at isolating the effects of pressure gradients, streamwise curvature and streamwise (pressure gradient) histories as well as Reynolds number, which have traditionally inhibited to draw firm conclusions from the available data.

  • 1764.
    Östberg, Martin
    KTH, Skolan för teknikvetenskap (SCI), Centra, VinnExcellence Center for ECO2 Vehicle design.
    Modelling Tools for Quieter Vehicles: Effective Vibro-Acoustical Modelling of Rotationally Symmetric Structures Consisting of Visco-Elastic and Poro-Elastic Media2010Licentiatavhandling, med artikler (Annet vitenskapelig)
  • 1765.
    Östberg, Martin
    KTH, Skolan för teknikvetenskap (SCI), Farkost och flyg, MWL Strukturakustik.
    Modelling tools for quieter vehicles: Waves in poro– and visco–elastic continua2012Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    New modelling tools intended to contribute to the development of components for quieter vehicles are developed. The tools are based on continuum models for wave propagation in poro– and visco–elastic media. By using geometric attributes of the studied components, the computational cost may be radically decreased compared to traditional methods. By assigning known analytical functions for one or two of the spatial directions, the spatial dimension of the remaining numerical problem is reduced. This reduction of spatial dimensions is performed in two di↵erent ways. The first one treats wave propagation in infinitely extended homogeneous and hollowed cylindrical rods, or wave guides, consisting of visco–elastic media. The wave solutions obtained are then used to model rubber vibration isolators of finite length by mode–matching the fields to the radial boundary conditions of interest. The second one is a method for modelling rotationally symmetric multilayered structures consisting of poro–elastic, elastic and fluid domains. By using a harmonic expansion for the azimuthal spatial dependence, the original three–dimensional problem is split up into several, much smaller, two– dimensional ones, radically decreasing the computational load.

    Moreover, using a mixed measurement/modelling approach, the audible frequency range characteristics of a viscous damper from a truck is studied, illustrating the influence of the rubber bushings by which it is attached to surrounding structures.

    The modelling approaches presented in this thesis are intended as tools aiding the design process of new vehicles, enabling new technology striving for more sustainable vehicle concepts. More specifically, the tools aim to improve the modelling of sound and vibration properties which are often penalised when seeking new, more sustainable vehicle designs.

  • 1766.
    Österlund, Jens, M.
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Johansson, Arne, V.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Nagib, Hassan, M.
    Illinois insitute of Technology.
    Comment on "A note on the intermediate region in turbulent boundary layers" [Phys. Fluids 12, 2159 (2000)]2000Inngår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 12, s. 2360-2363Artikkel i tidsskrift (Fagfellevurdert)
  • 1767.
    Österlund, Jens, M.
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Johansson, Arne, V.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Nagib, Hassan, M.
    Massachusetts Institute of Technology.
    Hites, Michael, H.
    A note on the overlap region in turbulent boundary layers2000Inngår i: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, nr 12, s. 1-4Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Two independent experimental investigations of the behavior of turbulent boundary layers with increasing Reynolds number were recently completed. The experiments were performed in two facilities, the Minimum Turbulence Level (MTL) wind tunnel at Royal Institute of Technology (KTH) and the National Diagnostic Facility (NDF) wind tunnel at Illinois Institute of Technology (IIT). Both experiments utilized oil-film interferometry to obtain an independent measure of the wall-shear stress. A collaborative study by the principals of the two experiments, aimed at understanding the characteristics of the overlap region between the inner and outer parts of the boundary layer, has just been completed. The results are summarized here, utilizing the profiles of the mean velocity, for Reynolds numbers based on the momentum thickness ranging from 2500 to 27 000. Contrary to the conclusions of some earlier publications, careful analysis of the data reveals no significant Reynolds number dependence for the parameters describing the overlap region using the classical logarithmic relation. However, the data analysis demonstrates that the viscous influence extends within the buffer region to y+≈200, compared to the previously assumed limit of y+≈50.Therefore, the lowest Reθ value where a significant logarithmic overlap region exists is about 6000. This probably explains why a Reynolds number dependence had been found from the data analysis of many previous experiments. The parameters of the logarithmic overlap region are found to be constant and are estimated to be κ=0.38, B=4.1 and B1=3.6 (δ=δ95).

  • 1768.
    Österlund, Jens M.
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Lindgren, Björn
    KTH, Skolan för teknikvetenskap (SCI), Mekanik.
    Johansson, Arne
    KTH, Skolan för teknikvetenskap (SCI), Mekanik, Turbulens.
    Flow structures in zero pressure-gradient turbulent boundary layers2003Inngår i: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 22, nr 4, s. 379-390Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The near-wall region of zero-pressure gradient turbulent boundary layers was studied through correlation- and other two- point measurements over a wide range of Reynolds numbers. The requirements of high spatial resolution were met by use of a MEMS-type of hot-film sensor array together with a small, in-house built hot-wire probe. Streak-spacing and characteristics of buffer region shear-layer events were studied. At high Reynolds numbers the motions that are of substantially larger scale than the streaks have a significant influence on the near-wall dynamics. By removing such scales through high-pass filtering a streak spacing was recovered that is close to that found in low Reynolds number flows. The frequency of occurrence of shear- layer events was found to scale with a mixed time scale, in analogy with earlier findings in channel flow, again indicating the increasing relative influence of large scales with increasing Reynolds number.

33343536 1751 - 1768 of 1768
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