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Effects of free-stream turbulence and three-dimensional roughness on boundary layer transition
KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences (SCI), Engineering Mechanics. (SMBC)ORCID iD: 0000-0003-2186-9277
2022 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

External disturbances such as free-stream turbulence (FST), and isolated three-dimensional roughness are strong disturbance sources to the laminar boundary layers (BLs), which can lead to a rapid transition to turbulence. The transition process eventuates to increase in skin-friction coefficient and heat transfer rate and hence, both of the aforementioned disturbance sources have practical importance. The current thesis is an experimental work, with investigations carried out in low-turbulence wind-tunnels to study the influence of these disturbance sources on boundary layer transition. Today, in FST transition, it is known that the turbulence intensity and the streamwise integral length scale in the free stream are the two influential characteristics that decide the transition onset, location and the extent. Unsteady, elongated streaks in the streamwise direction dominate this scenario, whose amplitudes and spanwise scales are set by the FST conditions prevalent at the leading edge (LE). In reality, a LE is unavoidable and the influence of the inherent LE pressure gradient region on BL transition was always doubted and not investigated in detail. The first part of the current thesis explores the FST transition scenario for a wide range of FST conditions and pressure gradients providing an input to the future transition prediction models. An important result in this thesis is that the entire energy spectrum needs to be known if an accurate prediction of the transition onset is desired, i.e. the LE condition in terms of characteristic length scale and turbulence intensity is not sufficient. In the second part, isolated roughness-induced transition is investigated thoroughly by changing the roughness height in micrometer precision at various diameters. In the previous experimental studies, the investigations were performed by altering the free-stream velocity at a fixed aspect ratio and hence modifying the base flow. In contrast, here, the aspect ratio of the roughness element is altered in an extensive range and the influence of the aspect ratio on the roughness Reynolds number that causes transition is studied without affecting the base flow. Instabilities that occur prior to the transition onset were examined in detail by performing flow visualization experiments. Moreover, interaction of secondary disturbances like Tollmien-Schlichting waves with the roughness was investigated.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2022. , p. 61
Series
TRITA-SCI-FOU ; 2022:18
Keywords [en]
boundary-layer transition, intermittency, free-stream turbulence, electret microphones, isolated roughness, sinuous instability, varicose instability
National Category
Fluid Mechanics
Research subject
Engineering Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-312756ISBN: 978-91-8040-229-3 (print)OAI: oai:DiVA.org:kth-312756DiVA, id: diva2:1659925
Public defence
2022-06-10, U1, Brinellvägen 26, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

QC 220523

Available from: 2022-05-24 Created: 2022-05-23 Last updated: 2025-02-09Bibliographically approved
List of papers
1. Laminar-turbulent state estimation in intermittent flows based on wall-pressure fluctuations
Open this publication in new window or tab >>Laminar-turbulent state estimation in intermittent flows based on wall-pressure fluctuations
(English)Manuscript (preprint) (Other academic)
Abstract [en]

In transitional boundary layers, the onset, development and end of transition are quantitatively derived from the intermittency factor γ. The present work originates from the need to establish an intermittency detection method using signals measured using an electret microphone recessed in a cavity behind a pinhole on a flat plate. This method uses a miniature microphone to detect changes to the noise received from the boundary layer. For this method, there is no requirement of amplitude or phase calibration and traversing time is absent, unlike conventional hot-wire based measurements. The tests conducted in Reynolds numbers 1×105 to 8×105 demonstrate that the obtained γ-distributions has a universal shape. To prove this, three grids with varied solidity levels were mounted in the test section of the wind tunnel resulting indifferent turbulence intensities and integral length scales and the results prove that this method is very robust. Most of the previous work has been done on turbulent boundary layers which are briefly reviewed in this report. Preliminary design criteria for mounting the condenser microphones on the flat plate and the intermittency estimation procedure are discussed in detail. It is shown that the diameter of the microphone has a visible influence on the measured γ-distributions.

Keywords
intermittency, boundary-layer transition, electret microphone
National Category
Fluid Mechanics
Identifiers
urn:nbn:se:kth:diva-312482 (URN)
Note

QC 20220530

Available from: 2022-05-19 Created: 2022-05-19 Last updated: 2025-02-09Bibliographically approved
2. Leading-edge pressure gradient effect on boundary layer receptivity to free-stream turbulence
Open this publication in new window or tab >>Leading-edge pressure gradient effect on boundary layer receptivity to free-stream turbulence
2022 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 935, article id A30Article in journal (Refereed) Published
Abstract [en]

Free-stream turbulence (FST) induced boundary layer transition is an intricate physical process that starts already at the leading edge (LE) with the LE receptivity process dictating how the broad spectrum of FST scales is received by the boundary layer. The importance of the FST integral length scale, apart from the turbulence intensity, has recently been recognized in transition prediction but a systematic variational study of the LE pressure gradient has still not been undertaken. Here, the LE pressure gradient is systematically varied in order to quantify its effect on the transition location. To this purpose, we present a measurement technique for accurate determination of flat-plate boundary layer transition location. The technique is based on electret condenser microphones which are distributed in the streamwise direction with high spatial resolution. All time signals are acquired simultaneously and post-processed giving the full intermittency distribution of the flow over the plate in a few minutes. The technique is validated against a similar procedure using hot-wire anemometry measurements. Our data clearly shows that the LE pressure gradient plays a decisive role in the receptivity process for small integral length scales, at moderate turbulence intensities, leading to variations in the transitional Reynolds number close to 40 %. To our knowledge, this high sensitivity of LE pressure gradient to transition has so far not been reported and our experiments were therefore partly repeated using another LE to ensure set-up independence and result repeatability.

Place, publisher, year, edition, pages
Cambridge University Press (CUP), 2022
Keywords
boundary layer receptivity, transition to turbulence
National Category
Fluid Mechanics
Identifiers
urn:nbn:se:kth:diva-308646 (URN)10.1017/jfm.2022.19 (DOI)000750189400001 ()2-s2.0-85124690664 (Scopus ID)
Note

QC 20220215

Available from: 2022-02-15 Created: 2022-02-15 Last updated: 2025-02-09Bibliographically approved
3. Transition to turbulence in boundary layers subjected to free-stream turbulence
Open this publication in new window or tab >>Transition to turbulence in boundary layers subjected to free-stream turbulence
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Challenges in discerning the intricacies of boundary layers buffeted by free-stream turbulence (FST) over a flat-plate have a renewed interest in the recent years. The FST transition scenario is dominated by unsteady streamwise streaks with spanwise scales that are dependent on the initial conditions at the LE. Many old transition studies show that the transition Reynolds number is simply a function the turbulence intensity (Tu). The current parameter variation study is the largest, performed in a low turbulence wind tunnel by varying the FST conditions namely, Tu in between [1%-7%] and the FST integral length scale in the range of [4.3-35.4] mm as well as the free-stream velocity in [6-15] m/s. Here, we present experimental results from 83 unique FST cases. The results confirm more recent studies that the transitional Reynolds number is not only a function of Tu but also the integral Reynolds number based on the integral length scale and the free-stream speed and that the transitional Reynolds number shows a twofold effect with respect to integral length scale. There is a threshold Tu where an increase in integral length scale goes from advancing to postponing transition, which is in agreement with recent experimental findings. Furthermore, our results also agree with previous studies that the FST condition at the LE have a strong influence on the spanwise wavelength of the streaks. We show that the averaged streak spacing, normalized by the FST integral length scale, follows a simple empirical power-law built on the FST Reynolds number.

Keywords
boundary-layer transition, free-stream turbulence, electret microphone, hot-wire probe
National Category
Engineering and Technology Fluid Mechanics
Identifiers
urn:nbn:se:kth:diva-312477 (URN)
Note

QC 20220531

Available from: 2022-05-19 Created: 2022-05-19 Last updated: 2025-02-09Bibliographically approved
4. A comparative study of experiments with numerical simulations of free-stream turbulence transition
Open this publication in new window or tab >>A comparative study of experiments with numerical simulations of free-stream turbulence transition
(English)Manuscript (preprint) (Other academic)
Abstract [en]

To date, very few careful and direct comparisons between experiments (EXP) and direct numerical simulations (DNS) have been published on free-stream turbulence (FST)induced boundary-layer transition, whilst there exist numerous published works on the comparison of canonical turbulent boundary layers. The primary reason being that the former comparison is vastly more difficult to carry out, simply because all known transition scenarios have large energy gradients and are extremely sensitive to surrounding conditions. From the existing literature, it is clear that the FST induced boundary-layer transition scenario in its very simplest case, i.e. for a zero-pressure gradient flow, we are still raising questions on the receptivity process, and we do not seem to have reached a consensus on the break-down process of streamwise streaks into turbulent spots. In this paper we present a detailed comparison between our EXP and available DNS data of this complex transition scenario at a turbulence intensity level of about Tu = 3% and an FST Reynolds number of about 67. The most important factors that need to be considered if one wants to compare DNS with EXP, or one EXP/DNS case with another EXP/DNS case for that matter, are elucidated. Through careful matching of these factors, we show that the agreement between EXP and DNSis satisfying and that all these factors must be important for describing the transition scenario appropriately.

National Category
Fluid Mechanics
Identifiers
urn:nbn:se:kth:diva-312491 (URN)
Note

QC 20220524

Available from: 2022-05-19 Created: 2022-05-19 Last updated: 2025-02-09Bibliographically approved
5. Isolated Roughness and Tollmien-Schlichting Waves in Boundary-Layer Transition
Open this publication in new window or tab >>Isolated Roughness and Tollmien-Schlichting Waves in Boundary-Layer Transition
2022 (English)In: Iutam Laminar-Turbulent Transition / [ed] Sherwin, S Schmid, P Wu, X, Springer Nature , 2022, Vol. 38, p. 193-202Conference paper, Published paper (Refereed)
Abstract [en]

An experimental investigation aimed at detecting the critical roughness Reynolds number (Re-kk,Re-tr) both with and without Tollmien-Schlichting (T-S) waves is described in this paper. As a novel technique to examine Re-kk,Re-tr systematically, we employed isolated, cylindrical roughnesses with automatically adjustable height of micro-meter precision. The experiment was performed using hot-wire anemometry in flat-plate boundary layers developing under close to zero-pressure-gradient conditions in a low-turbulence level wind tunnel. The present data for Re-kk,Re-tr without T-S waves confirmed previous results and showed a strong correlation between the roughness aspect ratio and root Re-kk,Re-tr. Controlling the roughness height while keeping the free-stream velocity fixed revealed noteworthy hysteresis for Re-kk,Re-tr. As expected the critical roughness Reynolds number decreased with the presence of T-S waves. The necessary T-S wave amplitude needed for transition became smaller with increasing the roughness height with a sudden drop when approaching the critical roughness height without T-S waves.

Place, publisher, year, edition, pages
Springer Nature, 2022
Series
IUTAM Bookseries, ISSN 1875-3507
National Category
Fluid Mechanics
Identifiers
urn:nbn:se:kth:diva-304762 (URN)10.1007/978-3-030-67902-6_16 (DOI)000709087600016 ()2-s2.0-85112602527 (Scopus ID)
Conference
9th IUTAM Symposium on Laminar-Turbulent Transition, SEP 02-06, 2019, Imperial Coll London, London, ENGLAND
Note

Part of proceedings: ISBN 978-3-030-67902-6, ISBN 978-3-030-67901-9

QC 20211112

Available from: 2021-11-12 Created: 2021-11-12 Last updated: 2025-02-09Bibliographically approved
6. Instabilities in the wake of a cylindrical roughness element: a flow visualization study
Open this publication in new window or tab >>Instabilities in the wake of a cylindrical roughness element: a flow visualization study
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The instability mechanism behind a geometrically simple roughness element continues to be a challenging topic in fluid mechanics. Significant advances towards fully understanding the phenomena in the wake of a roughness have been made in the recent years, but in order to be able to predict the type of instability in any given flow configuration, further research is required. This is of particular interest, as these instabilities dictate the transition to turbulence and thus are significant for large-scale effects such as skin friction drag. A smoke-flow visualization study with large variation of parameters (aspect ratios in the range of 0.25–7), enabled by a cylindrical roughness element that is connected to a linear traverse, has been performed in order to broaden the understanding of this type of flow. Results show good agreement with previous investigations and give further insights into the stability properties. Unexpected results arise especially for particularly low as well as very high roughness aspect ratios. For low aspect ratios, no global instability is detected even at the highest Reynolds numbers, whereas high aspect ratios suggest a delay in the onset of instability. Furthermore, information regarding the dominating frequency can be extracted from the visualization images. Results show a new scaling of the frequency as the velocity is increased. Additionally, the dominating frequency in the wake can be predicted well using a Strouhal number in a certain flow regime.

Keywords
cylindrical roughness, flow visualization, instabilities, sinuous mode, varicose mode
National Category
Fluid Mechanics
Identifiers
urn:nbn:se:kth:diva-312488 (URN)
Note

QC 20220530

Available from: 2022-05-19 Created: 2022-05-19 Last updated: 2025-02-09Bibliographically approved

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Mamidala, Santhosh Babu

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  • ieee
  • modern-language-association-8th-edition
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  • nn-NO
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  • Other locale
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