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Mamidala, Santhosh B.ORCID iD iconorcid.org/0000-0003-2186-9277
Publications (10 of 13) Show all publications
Weingärtner, A., Mamidala, S. B. & Fransson, J. H. M. (2023). Application of Miniature Vortex Generators for Boundary Layer Transition Delay. In: : . Paper presented at AIAA SCITECH 2023. National Harbor, MD & Online, Article ID AIAA 2023-0097.
Open this publication in new window or tab >>Application of Miniature Vortex Generators for Boundary Layer Transition Delay
2023 (English)Conference paper, Published paper (Other academic)
Abstract [en]

The efficacy of spanwise velocity gradients (SVGs) to attenuate the growth of disturbances in a laminar boundary layer (BL) has been an active topic of research for more than 20 years. It was shown both numerically and in wind-tunnel experiments that modulating a laminar BL with steady, alternating high- and low- speed streaks can delay the transition to turbulence and hence lead to a reduction in skin friction drag. In this project, the feasibility of this mechanism to delay transition in a real-world application is explored. Wind-tunnel experiments are performed in order to design miniature vortex generators (MVGs) intended to be used on the fuselage of an aircraft during flight with the goal to delay natural transition in a realistic environment. Blade-type MVGs with a height lower than the BL thickness have been found to be effective in generating steady and stable SVGs in a BL that can accomplish transition delay.

Place, publisher, year, edition, pages
National Harbor, MD & Online: , 2023
National Category
Fluid Mechanics
Identifiers
urn:nbn:se:kth:diva-357092 (URN)
Conference
AIAA SCITECH 2023
Note

QC 20241211

Available from: 2024-12-03 Created: 2024-12-03 Last updated: 2025-02-09Bibliographically approved
Weingärtner, A., Mamidala, S. B. & Fransson, J. H. M. (2023). Application of Miniature Vortex Generators for Boundary Layer Transition Delay. In: : . Paper presented at AIAA SCITECH 2023.
Open this publication in new window or tab >>Application of Miniature Vortex Generators for Boundary Layer Transition Delay
2023 (English)Conference paper, Published paper (Other academic)
Abstract [en]

The efficacy of spanwise velocity gradients (SVGs) to attenuate the growth of disturbances in a laminar boundary layer (BL) has been an active topic of research for more than 20 years. It was shown both numerically and in wind-tunnel experiments that modulating a laminar BL with steady, alternating high- and low- speed streaks can delay the transition to turbulence and hence lead to a reduction in skin friction drag. In this project, the feasibility of this mechanism to delay transition in a real-world application is explored. Wind-tunnel experiments are performed in order to design miniature vortex generators (MVGs) intended to be used on the fuselage of an aircraft during flight with the goal to delay natural transition in a realistic environment. Blade-type MVGs with a height lower than the BL thickness have been found to be effective in generating steady and stable SVGs in a BL that can accomplish transition delay.

National Category
Fluid Mechanics
Identifiers
urn:nbn:se:kth:diva-357086 (URN)
Conference
AIAA SCITECH 2023
Note

QC 20241205

Available from: 2024-12-03 Created: 2024-12-03 Last updated: 2025-02-09Bibliographically approved
Weingärtner, A., Mamidala, S. B. & Fransson, J. H. M. (2023). Instabilities in the wake of an isolated cylindrical roughness element. Journal of Fluid Mechanics, 960, Article ID A8.
Open this publication in new window or tab >>Instabilities in the wake of an isolated cylindrical roughness element
2023 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 960, article id A8Article in journal (Refereed) Published
Abstract [en]

The instability mechanism behind a geometrically simple cylindrical roughness element continues to be a challenging topic in fluid mechanics. Considerable progress has been made in understanding the phenomena in recent years, but more research is needed to predict the temporal nature and spatial structure of the dominant instability in a given flow configuration. 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 a large variation of parameters, featuring a cylindrical roughness element connected to a linear traverse, has been performed. Results show good agreement with previous investigations and provide further insights into the stability properties, revealing several unexpected effects. For a low roughness aspect ratio ?, no global instability is detected even at the highest roughness Reynolds number Rekk, whereas a high aspect ratio indicates a delay in the onset of instability. From the acquired visualizations, we constructed the, so far, richest instability diagram of the wake behind an isolated roughness element in the Rekk-? space, sampled in the same measurement campaign. Furthermore, information regarding the dominant frequency in the wake can be extracted from the visualization images. Our results suggest a new scaling of the frequency as the velocity is increased. Finally, it is shown that the dominant frequency in a certain flow regime can be well predicted using a Strouhal number based on the cylinder diameter and the roughness velocity.

Place, publisher, year, edition, pages
Cambridge University Press (CUP), 2023
Keywords
absolute/convective instability, boundary layer stability, transition to turbulence
National Category
Fluid Mechanics
Identifiers
urn:nbn:se:kth:diva-326491 (URN)10.1017/jfm.2023.171 (DOI)000960921300001 ()2-s2.0-85151528259 (Scopus ID)
Note

QC 20230503

Available from: 2023-05-03 Created: 2023-05-03 Last updated: 2025-02-09Bibliographically approved
Mamidala, S. B., Weingärtner, A. & Fransson, J. H. M. (2022). A comparative study of experiments with numerical simulations of free-stream turbulence transition. Journal of Fluid Mechanics, 951, Article ID A46.
Open this publication in new window or tab >>A comparative study of experiments with numerical simulations of free-stream turbulence transition
2022 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 951, article id A46Article in journal (Refereed) Published
Abstract [en]

To date, very few careful and direct comparisons between experiments 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 is 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. This paper presents a detailed comparison between new experiments and available DNS data of the complex FST transition scenario in a flat plate boundary layer at turbulence intensity level about Tu = 3 % and FST Reynolds number about Re-fst = 67. The leading edge (LE) pressure gradient distribution and the full energy spectrum at the LE are identified as the two most important parameters for a satisfying comparison. Matching the LE characteristic FST parameters is not enough as previously thought, which is illustrated by setting up two experimental FST cases with about the same FST integral parameters at the LE but with different energy spectra. Finally, an FST boundary layer penetration depth (PD) measure is defined using DNS, which suggests that the PD grows with the downstream distance and stays around 20 % of the boundary layer thickness down to transition onset. With this result, one cannot rule out the significance of the continuous FST forcing along the boundary layer edge in this transition scenario, as indicated in previous studies.

Place, publisher, year, edition, pages
Cambridge University Press (CUP), 2022
Keywords
boundary layer receptivity, boundary layer stability, transition to turbulence
National Category
Fluid Mechanics
Identifiers
urn:nbn:se:kth:diva-322229 (URN)10.1017/jfm.2022.883 (DOI)000882992800001 ()2-s2.0-85142057771 (Scopus ID)
Note

Not duplicate with DiVA 1659161

QC 20221205

Available from: 2022-12-05 Created: 2022-12-05 Last updated: 2025-02-09Bibliographically approved
Mamidala, S. B. (2022). Effects of free-stream turbulence and three-dimensional roughness on boundary layer transition. (Doctoral dissertation). Stockholm: KTH Royal Institute of Technology
Open this publication in new window or tab >>Effects of free-stream turbulence and three-dimensional roughness on boundary layer transition
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
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:nbn:se:kth:diva-312756 (URN)978-91-8040-229-3 (ISBN)
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
Hara, S., Mamidala, S. B. & Fransson, J. H. M. (2022). Isolated Roughness and Tollmien-Schlichting Waves in Boundary-Layer Transition. In: Sherwin, S Schmid, P Wu, X (Ed.), Iutam Laminar-Turbulent Transition: . Paper presented at 9th IUTAM Symposium on Laminar-Turbulent Transition, SEP 02-06, 2019, Imperial Coll London, London, ENGLAND (pp. 193-202). Springer Nature, 38
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
Mamidala, S. B., Weingärtner, A. & Fransson, J. (2022). Leading-edge pressure gradient effect on boundary layer receptivity to free-stream turbulence. Journal of Fluid Mechanics, 935, Article ID A30.
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
Gholamisheeri, M., Durovic, K., Mamidala, S. B., Fransson, T., Hanifi, A. & Henningson, D. S. (2022). THE INTERACTION BETWEEN AN ISOLATED ROUGHNESS ELEMENT AND FREE-STREAM TURBULENCE. In: ECCOMAS Congress 2022: 8th European Congress on Computational Methods in Applied Sciences and Engineering. Paper presented at 8th European Congress on Computational Methods in Applied Sciences and Engineering, ECCOMAS Congress 2022, Oslo, Norway, Jun 5 2022 - Jun 9 2022. Scipedia, S.L.
Open this publication in new window or tab >>THE INTERACTION BETWEEN AN ISOLATED ROUGHNESS ELEMENT AND FREE-STREAM TURBULENCE
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2022 (English)In: ECCOMAS Congress 2022: 8th European Congress on Computational Methods in Applied Sciences and Engineering, Scipedia, S.L. , 2022Conference paper, Published paper (Refereed)
Abstract [en]

Control and delay of the laminar-turbulent transition is a key parameter in reducing skin friction and drag. The flow characteristics, surface roughness, and environmental noise can affect the onset of transition. The present work investigates, numerically and experimentally, the interaction of the free-stream turbulence (FST) and an isolated cylindrical roughness element, and the resulting impact on the transition onset in a flat-plate boundary layer. High-fidelity direct numerical simulations (DNS) are performed for a roughness element immersed in the boundary layer over a flat plate with an asymmetrical leading edge, with and without FST. The numerical results are compared to hot-wire anemometry measurements performed in the Minimum Turbulence Level wind tunnel at KTH. The initial numerical and experimental results show that in the absence of FST, for the chosen flow parameters, high-and low-speed streaks are generated downstream of the roughness element while the flow remains laminar and globally stable. When FST is added, the spanwise spacing of the streaky structures changes and the transition location of the boundary layer moves upstream. It was found that the aspect ratio of the streaky structures does not vary significantly.

Place, publisher, year, edition, pages
Scipedia, S.L., 2022
Keywords
Free-stream Turbulence, Roughness Induced Transition
National Category
Fluid Mechanics
Identifiers
urn:nbn:se:kth:diva-333434 (URN)10.23967/eccomas.2022.269 (DOI)2-s2.0-85146939028 (Scopus ID)
Conference
8th European Congress on Computational Methods in Applied Sciences and Engineering, ECCOMAS Congress 2022, Oslo, Norway, Jun 5 2022 - Jun 9 2022
Note

QC 20230802

Available from: 2023-08-02 Created: 2023-08-02 Last updated: 2025-02-09Bibliographically approved
Fransson, J. H. M., Mamidala, S. B., Fallenius, B. E. G., Mårtensson, H. & Wallin, F. (2018). An experimental setup for idealised studies on transition to turbulence on a generic compressor outlet guide vane. In: Proceedings of the ASME Turbo Expo: . Paper presented at Volume 2A-2018, 2018ASME Turbo Expo 2018: Turbomachinery Technical Conference and Exposition, GT 2018, Oslo, Norway, 11 June 2018 through 15 June 2018;. ASME Press, Volume 2A-2018, 2018
Open this publication in new window or tab >>An experimental setup for idealised studies on transition to turbulence on a generic compressor outlet guide vane
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2018 (English)In: Proceedings of the ASME Turbo Expo, ASME Press, 2018, Vol. Volume 2A-2018, 2018Conference paper, Published paper (Refereed)
Abstract [en]

The understanding of flow phenomena in turbomachinery has come far with respect to three-dimensional flow patterns and pressure distributions. Much is due to improved measurements and a continuously evolving fidelity in computational fluid dynamics (CFD). Turbulence and transition in boundary layers are two classical areas where improvements in modeling are desired and where experimental validation is required. Apart from this, fundamental improvements in efficiency can be obtained by developing experimental resources where technologies affecting transition can be studied. The reduction in friction drag can be considerable if the transition to turbulence can be delayed. An experimental setup in an idealized configuration has been designed and built with the objective to study transition on a very large-scale guide vane profile at low speed. The purpose of the rig is to enable high quality fundamental studies of technologies to delay transition, but also to see how effects of manufacturing or other constraints may affect the boundary layer. In the present paper we report the first validation of the experimental setup, by comparing the first test results to CFD calculations performed during the rig design, i.e. no post-calculations with experimental data as input to the simulations have been done yet. The pressure distribution is in line with the design intent, which is a good indicator that the tunnel design is suitable for the intended purpose. At last we report some velocity measurements performed in the wake and we calculate the total drag based on the wake velocity deficit for various Reynolds numbers and with and without turbulence tripping tape. We illustrate that a two dimensional tripping around 7% of the chord from the leading edge can increase the total drag by 50% with respect to the reference case without tripping tape.

Place, publisher, year, edition, pages
ASME Press, 2018
Series
Proceedings of the ASME Turbo Expo
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-238428 (URN)10.1115/GT2018-76507 (DOI)000456493600032 ()2-s2.0-85054023604 (Scopus ID)9780791850992 (ISBN)
Conference
Volume 2A-2018, 2018ASME Turbo Expo 2018: Turbomachinery Technical Conference and Exposition, GT 2018, Oslo, Norway, 11 June 2018 through 15 June 2018;
Note

QC 20181105. QC 20200113

Available from: 2018-11-05 Created: 2018-11-05 Last updated: 2022-06-26Bibliographically approved
Mamidala, S. B., Weingärtner, A. & Fransson, J. H. M.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
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ORCID iD: ORCID iD iconorcid.org/0000-0003-2186-9277

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