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Experimental studies in jet flows and zero pressure-gradient turbulent boundary layers
KTH, School of Engineering Sciences (SCI), Mechanics.ORCID iD: 0000-0002-1663-3553
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
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.

Place, publisher, year, edition, pages
Stockholm: KTH , 2009. , viii, 100 p.
Series
Trita-MEK, ISSN 0348-467X ; 2009:08
Keyword [en]
Axisymmetric jet, swirling jet, coaxial jet, heated jet, zero pressuregradient turbulent boundary layer, wall-bounded turbulent flows, overlap region, passive scalar mixing, hot-wire anemometry, spatial resolution.
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-10448ISBN: 978-91-7415-369-9 (print)OAI: oai:DiVA.org:kth-10448DiVA: diva2:217489
Public defence
2009-06-12, D3, Lindstedtsvägen 5, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Note
QC 20100820Available from: 2009-06-04 Created: 2009-05-14 Last updated: 2010-08-20Bibliographically approved
List of papers
1. An experimental study of the near-field mixing characteristics of a swirling jet
Open this publication in new window or tab >>An experimental study of the near-field mixing characteristics of a swirling jet
2008 (English)In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 80, no 3, 323-350 p.Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
Netherlands: Springer, 2008
Keyword
Swirling jet, passive scalar mixing, hot-wire anemometry, heated jet
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-10447 (URN)10.1007/s10494-007-9126-y (DOI)000254248800003 ()2-s2.0-43449084090 (Scopus ID)
Note
QC 20100820Available from: 2009-05-14 Created: 2009-05-14 Last updated: 2017-12-13Bibliographically approved
2. On the passive control of the near-field of coaxial jets by means of vortex shedding
Open this publication in new window or tab >>On the passive control of the near-field of coaxial jets by means of vortex shedding
2008 (English)In: Int. Conf. on Jets, Wakes and Separated Flows, ICJWSF-2, September 16-19, 2008, Technical University of Berlin, Germany, 2008, 1-16 p.Conference paper, Published paper (Refereed)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-24180 (URN)
Note
QC 20100820Available from: 2010-08-20 Created: 2010-08-20 Last updated: 2010-08-20Bibliographically approved
3. Preliminary studies on acoustic excitation in axisymmetric transitional jet flows
Open this publication in new window or tab >>Preliminary studies on acoustic excitation in axisymmetric transitional jet flows
(English)Manuscript (preprint) (Other academic)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-24181 (URN)
Note
QC 20100820Available from: 2010-08-20 Created: 2010-08-20 Last updated: 2010-08-20Bibliographically approved
4. Low Reynolds number Zero Pressure-Gradient Equilibrium Turbulent Boundary-Layer Experiments
Open this publication in new window or tab >>Low Reynolds number Zero Pressure-Gradient Equilibrium Turbulent Boundary-Layer Experiments
(English)Manuscript (preprint) (Other academic)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-24182 (URN)
Note
QC 20100820Available from: 2010-08-20 Created: 2010-08-20 Last updated: 2010-08-20Bibliographically approved
5. On the determination of the wall position in wall-bounded turbulent flows
Open this publication in new window or tab >>On the determination of the wall position in wall-bounded turbulent flows
(English)Manuscript (preprint) (Other academic)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-24183 (URN)
Note
QC 20100820Available from: 2010-08-20 Created: 2010-08-20 Last updated: 2010-08-20Bibliographically approved
6. On spatial resolution issues related to time-averaged quantities using hot-wire anemometry
Open this publication in new window or tab >>On spatial resolution issues related to time-averaged quantities using hot-wire anemometry
2010 (English)In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 49, no 1, 101-110 p.Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
Springer, 2010
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-24184 (URN)10.1007/s00348-009-0808-1 (DOI)000278923400010 ()2-s2.0-77953810763 (Scopus ID)
Note
Uppdaterad från manuskript till artikel: 20100820 QC 20100820Available from: 2010-08-20 Created: 2010-08-20 Last updated: 2017-12-12Bibliographically approved

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