Change search
Link to record
Permanent link

Direct link
BETA
Alternative names
Publications (10 of 253) Show all publications
Morra, P., Sasaki, K., Hanifi, A., Cavalieri, A. V. G. & Henningson, D. S. (2020). A realizable data-driven approach to delay bypass transition with control theory. Journal of Fluid Mechanics, 883, Article ID A33.
Open this publication in new window or tab >>A realizable data-driven approach to delay bypass transition with control theory
Show others...
2020 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 883, article id A33Article in journal (Refereed) Published
Abstract [en]

The current work presents a realizable method to control streaky disturbances in boundary layer flows and delay transition to turbulence by means of active flow control. Numerical simulations of the nonlinear transitional regime in a Blasius boundary layer are performed where streaks are excited in the boundary layer by means of a high level of free-stream turbulence. The occurring disturbances are measured by means of localized wall-shear-stress sensors and damped out using near-wall actuators, which resemble ring plasma actuators. Each actuator is powered by a time-varying signal whose amplitude is computed by processing signals from the sensors. The processed signal is the result of two control laws: the linear quadratic Gaussian regulator (LQG) and the inverse feed-forward control technique (IFFC). The use of the first control method, LQG, requires a state-space representation of the system dynamics, so the flow is described by means of a linear time-invariant operator that captures only the most relevant information of the dynamics and results in a reduced-order model (ROM). The ROM is computed by means of the eigensystem realization algorithm (ERA), which is based on the impulse responses of the real system. Collecting such impulse responses may be unfeasible when considering free-stream turbulence because of the high dimensionality of the input forcing needed for a precise description of such a phenomenon. Here, a new method to identify the relevant system dynamics and generate the needed impulse responses is proposed, based on additional shear-stress measurements in an upstream location. Transfer functions between such measurements and other downstream sensors are obtained and allow the derivation of the ERA system, in a data-driven approach that would be realizable in experiments. Finally, in order to discuss the advantages of the LQG based on the ROM and analyse its performance, the implemented LQG is compared to the IFFC, which consists of wave cancellation. The work (i) presents a systematic and straightforward way to deal with high-dimensional disturbances in order to build ROMs for a feasible control technique, and (ii) shows that even when considering practical constraints, such as the type and size of actuators and sensors, it is possible to achieve at least as large delay of bypass transition as that obtained in more idealized cases found in the literature.

Place, publisher, year, edition, pages
CAMBRIDGE UNIV PRESS, 2020
Keywords
boundary layer control, drag reduction, transition to turbulence
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-267149 (URN)10.1017/jfm.2019.793 (DOI)000508121500033 ()
Note

QC 20200217

Available from: 2020-02-17 Created: 2020-02-17 Last updated: 2020-02-17Bibliographically approved
von Deyn, L. H., Forooghi, P., Frohnapfel, B., Schlatter, P., Hanifi, A. & Henningson, D. S. (2020). Direct Numerical Simulations of Bypass Transition over Distributed Roughness. AIAA Journal, 58(2), 702-711
Open this publication in new window or tab >>Direct Numerical Simulations of Bypass Transition over Distributed Roughness
Show others...
2020 (English)In: AIAA Journal, ISSN 0001-1452, E-ISSN 1533-385X, Vol. 58, no 2, p. 702-711Article in journal (Refereed) Published
Abstract [en]

Bypass transition in a boundary layer subjected to freestream turbulence and distributed surface roughness is studied numerically. The distributed surface roughness is reproduced with an immersed boundary technique, and the freestream turbulence is artificially generated by a superposition of eigenmodes of the Orr-Sommerfeld and Squire equations. Both an undisturbed laminar inflow and a disturbed inflow with freestream turbulence are studied. In either case a parametric study on the effects of the roughness size and density is carried out. The simulations reveal that the presence of roughness induces streaks in the laminar flow. When the freestream is turbulent, both roughness height and density show an impact on the onset of transition. The superposition of surface roughness and freestream turbulence causes amplified streaks. As a result, the streak instability occurs earlier within the boundary layer. The results show good qualitative and quantitative agreement to both experimental and numerical studies available in the literature.

Place, publisher, year, edition, pages
AMER INST AERONAUTICS ASTRONAUTICS, 2020
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-269475 (URN)10.2514/1.J057765 (DOI)000513533200015 ()
Note

QC 20200310

Available from: 2020-03-10 Created: 2020-03-10 Last updated: 2020-03-10Bibliographically approved
Chauvat, G., Peplinski, A., Henningson, D. S. & Hanifi, A. (2020). Global linear analysis of a jet in cross-flow at low velocity ratios. Journal of Fluid Mechanics, 889, Article ID A12.
Open this publication in new window or tab >>Global linear analysis of a jet in cross-flow at low velocity ratios
2020 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 889, article id A12Article in journal (Refereed) Published
Abstract [en]

The stability of the jet in cross-flow is investigated using a complete set-up including the flow inside the pipe. First, direct simulations were performed to find the critical velocity ratio as a function of the Reynolds number, keeping the boundary-layer displacement thickness fixed. At all Reynolds numbers investigated, there exists a steady regime at low velocity ratios. As the velocity ratio is increased, a bifurcation to a limit cycle composed of hairpin vortices is observed. The critical bulk velocity ratio is found at approximately for the Reynolds number , above which a global mode of the system becomes unstable. An impulse response analysis was performed and characteristics of the generated wave packets were analysed, which confirmed results of our global mode analysis. In order to study the sensitivity of this flow, we performed transient growth computations and also computed the optimal periodic forcing and its response. Even well below this stability limit, at , large transient growth ( in energy amplification) is possible and the resolvent norm of the linearized Navier-Stokes operator peaks above . This is accompanied with an extreme sensitivity of the spectrum to numerical details, making the computation of a few tens of eigenvalues close to the limit of what can be achieved with double precision arithmetic. We demonstrate that including the meshing of the jet pipe in the simulations does not change qualitatively the dynamics of the flow when compared to the simple Dirichlet boundary condition representing the jet velocity profile. This is in agreement with the recent experimental results of Klotz et al. (J. Fluid Mech., vol. 863, 2019, pp. 386-406) and in contrast to previous studies of Cambonie & Aider (Phys. Fluids, vol. 26, 2014, 084101). Our simulations also show that a small amount of noise at subcritical velocity ratios may trigger the shedding of hairpin vortices.

Place, publisher, year, edition, pages
CAMBRIDGE UNIV PRESS, 2020
Keywords
boundary layer stability, jets
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-269438 (URN)10.1017/jfm.2020.85 (DOI)000514704100001 ()2-s2.0-85081086529 (Scopus ID)
Note

QC 20200316

Available from: 2020-03-16 Created: 2020-03-16 Last updated: 2020-03-16Bibliographically approved
Sasaki, K., Morra, P., Cavalieri, A. V. G., Hanifi, A. & Henningson, D. S. (2020). On the role of actuation for the control of streaky structures in boundary layers. Journal of Fluid Mechanics, 883, Article ID A34.
Open this publication in new window or tab >>On the role of actuation for the control of streaky structures in boundary layers
Show others...
2020 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 883, article id A34Article in journal (Refereed) Published
Abstract [en]

This work deals with the closed-loop control of streaky structures induced by free-stream turbulence (FST), at the levels of 3.0% and 3.5 %, in a zero-pressure-gradient transitional boundary layer, by means of localized sensors and actuators. A linear quadratic Gaussian regulator is considered along with a system identification technique to build reduced-order models for control. Three actuators are developed with different spatial supports, corresponding to a baseline shape with only vertical forcing, and to two other shapes obtained by different optimization procedures. A computationally efficient method is derived to obtain an actuator that aims to induce the exact structures that are inside the boundary layer, given in terms of their first spectral proper orthogonal decomposition (SPOD) mode, and an actuator that maximizes the energy of induced downstream structures. All three actuators lead to significant delays in the transition to turbulence and were shown to be robust to mild variations in the FST levels. Integrated total drag reductions observed were up to 21% and 19% for turbulence intensity levels of 3.0% and 3.5 %, respectively, depending on the considered actuator. Differences are understood in terms of the SPOD of actuation and FST-induced fields along with the causality of the control scheme when a cancellation of disturbances is considered along the wall-normal direction. The actuator optimized to generate the leading downstream SPOD mode, representing the streaks in the open-loop flow, leads to the highest transition delay, which can be understood due to its capability of closely cancelling structures in the boundary layer.

Place, publisher, year, edition, pages
CAMBRIDGE UNIV PRESS, 2020
Keywords
boundary layer control, drag reduction, transition to turbulence
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-267150 (URN)10.1017/jfm.2019.893 (DOI)000508121500034 ()
Note

QC 20200217

Available from: 2020-02-17 Created: 2020-02-17 Last updated: 2020-02-17Bibliographically approved
Beneitez Galan, M., Duguet, Y., Schlatter, P. & Henningson, D. S. (2019). Edge tracking in spatially developing boundary layer flows. Journal of Fluid Mechanics, 881, 164-181
Open this publication in new window or tab >>Edge tracking in spatially developing boundary layer flows
2019 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 881, p. 164-181Article in journal (Refereed) Published
Abstract [en]

Recent progress in understanding subcritical transition to turbulence is based on the concept of the edge, the manifold separating the basins of attraction of the laminar and the turbulent state. Originally developed in numerical studies of parallel shear flows with a linearly stable base flow, this concept is adapted here to the case of a spatially developing Blasius boundary layer. Longer time horizons fundamentally change the nature of the problem due to the loss of stability of the base flow due to Tollmien-Schlichting (TS) waves. We demonstrate, using a moving box technique, that efficient long-time tracking of edge trajectories is possible for the parameter range relevant to bypass transition, even if the asymptotic state itself remains out of reach. The flow along the edge trajectory features streak switching observed for the first time in the Blasius boundary layer. At long enough times, TS waves co-exist with the coherent structure characteristic of edge trajectories. In this situation we suggest a reinterpretation of the edge as a manifold dividing the state space between the two main types of boundary layer transition, i.e. bypass transition and classical transition.

Place, publisher, year, edition, pages
Cambridge University Press, 2019
Keywords
boundary layer stability, nonlinear dynamical systems, transition to turbulence, Aerodynamics, Boundary layer flow, Boundary layers, Dynamical systems, Parallel flow, Shear flow, Trajectories, Turbulence, Basins of attraction, Blasius boundary layer, Boundary layer stabilities, Boundary layer transitions, Classical transition, Subcritical transition, Tollmien-Schlichting waves, Atmospheric thermodynamics, boundary layer, fluid dynamics, fluid flow, nonlinearity
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-263766 (URN)10.1017/jfm.2019.763 (DOI)000506237100008 ()2-s2.0-85074285559 (Scopus ID)
Note

QC 20191112

Available from: 2019-11-12 Created: 2019-11-12 Last updated: 2020-01-29Bibliographically approved
Negi, P., Hanifi, A. & Henningson, D. S. (2019). Global Stability of rigid-body-motion fluid-structure-interaction problems.
Open this publication in new window or tab >>Global Stability of rigid-body-motion fluid-structure-interaction problems
2019 (English)Report (Other academic)
Abstract [en]

A rigorous derivation and validation for linear fluid-structure-interaction (FSI) equations for a rigid-body-motion problem is performed in an Eulerian framework. We show that the “added-stiffness” terms arising in the formulation of Fanion et al. (2000) vanish at the FSI interface in a first-order approximation. Several numerical tests with rigid-body motion are performed to show the validity of the derived formulation by comparing the time evolution between the linear and non-linear equations when the base flow is perturbed by identical small-amplitude perturbations. In all cases both the growth rate and angular frequency of the instability matches within 0.1% accuracy. The derived formulation is used to investigate the phenomenon of symmetry breaking for a rotating cylinder with an attached splitter-plate. The results show that the onset of symmetry breaking can be explained by the existence of a zero-frequency linearly unstable mode of the coupled fluid-structure-interaction system. Finally, the structural sensitivity of the least stable eigenvalue is studied for an oscillating cylinder, which is found to change significantly when the fluid and structural frequencies are close to resonance.

Publisher
p. 38
Series
TRITA-SCI-RAP ; 2019:007
National Category
Fluid Mechanics and Acoustics Aerospace Engineering
Research subject
Aerospace Engineering
Identifiers
urn:nbn:se:kth:diva-262856 (URN)
Funder
Swedish National Infrastructure for Computing (SNIC)
Note

QC 20191025. QC 20191030

Available from: 2019-10-23 Created: 2019-10-23 Last updated: 2019-10-30Bibliographically approved
Quaranta, H. U., Brynjell-Rahkola, M., Leweke, T. & Henningson, D. S. (2019). Local and global pairing instabilities of two interlaced helical vortices. Journal of Fluid Mechanics, 863, 927-955
Open this publication in new window or tab >>Local and global pairing instabilities of two interlaced helical vortices
2019 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 863, p. 927-955Article in journal (Refereed) Published
Abstract [en]

We investigate theoretically and experimentally the stability of two interlaced helical vortices with respect to displacement perturbations having wavelengths that are large compared to the size of the vortex cores. First, existing theoretical results are recalled and applied to the present configuration. Various modes of unstable perturbations, involving different phase relationships between the two vortices, are identified and their growth rates are calculated. They lead to a local pairing of neighbouring helix loops, or to a global pairing with one helix expanding and the other one contracting. A relation is established between this instability and the three-dimensional pairing of arrays of straight parallel vortices, and a striking quantitative agreement concerning the growth rates and frequencies is found. This shows that the local pairing of vortices is the driving mechanism behind the instability of the helix system. Second, an experimental study designed to observe these instabilities in a real flow is presented. Two helical vortices are generated by a two-bladed rotor in a water channel and characterised through dye visualisations and particle image velocimetry measurements. Unstable displacement modes are triggered individually, either by varying the rotation frequency of the rotor, or by imposing a small rotor eccentricity. The observed unstable mode structure, and the corresponding growth rates obtained from advanced processing of visualisation sequences, are in good agreement with theoretical predictions. The nonlinear late stages of the instability are also documented experimentally. Whereas local pairing leads to strong deformations and subsequent breakup of the vortices, global pairing results in a leapfrogging phenomenon, which temporarily restores the initial double-helix geometry, in agreement with recent observations from numerical simulations.

Place, publisher, year, edition, pages
CAMBRIDGE UNIV PRESS, 2019
Keywords
vortex flows, vortex instability, vortex interactions
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-245126 (URN)10.1017/jfm.2018.904 (DOI)000458504100001 ()2-s2.0-85060944108 (Scopus ID)
Note

QC 20190315

Available from: 2019-03-15 Created: 2019-03-15 Last updated: 2019-06-11Bibliographically approved
Brynjell-Rahkola, M. & Henningson, D. S. (2019). Numerical realization of helical vortices: application to vortex instability. Theoretical and Computational Fluid Dynamics
Open this publication in new window or tab >>Numerical realization of helical vortices: application to vortex instability
2019 (English)In: Theoretical and Computational Fluid Dynamics, ISSN 0935-4964, E-ISSN 1432-2250Article in journal (Refereed) Published
Abstract [en]

The need to numerically represent a free vortex system arises frequently in fundamental and applied research. Many possible techniques for realizing this vortex system exist but most tend to prioritize accuracy either inside or outside of the vortex core, which therefore makes them unsuitable for a stability analysis considering the entire flow field. In this article, a simple method is presented that is shown to yield an accurate representation of the flow inside and outside of the vortex core. The method is readily implemented in any incompressible Navier–Stokes solver using primitive variables and Cartesian coordinates. It can potentially be used to model a wide range of vortices but is here applied to the case of two helices, which is of renewed interest due to its relevance for wind turbines and helicopters. Three-dimensional stability analysis is performed in both a rotating and a translating frame of reference, which yield eigenvalue spectra that feature both mutual inductance and elliptic instabilities. Comparison of these spectra with available theoretical predictions is used to validate the proposed baseflow model, and new insights into the elliptic instability of curved Batchelor vortices are presented. Furthermore, it is shown that the instabilities in the rotating and the translating reference frames have the same structure and growth rate, but different frequency. A relation between these frequencies is provided.

Place, publisher, year, edition, pages
Springer, 2019
Keywords
Elliptic instability, Helical vortices, Mutual inductance instability, Vortex dynamics
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-268590 (URN)10.1007/s00162-019-00509-8 (DOI)2-s2.0-85075875978 (Scopus ID)
Note

QC 20200330

Available from: 2020-03-30 Created: 2020-03-30 Last updated: 2020-03-30Bibliographically approved
Morra, P., Semeraro, O., Henningson, D. S. & Cossu, C. (2019). On the relevance of Reynolds stresses in resolvent analyses of turbulent wall-bounded flows. Journal of Fluid Mechanics, 867, 969-984, Article ID PII S0022112019001964.
Open this publication in new window or tab >>On the relevance of Reynolds stresses in resolvent analyses of turbulent wall-bounded flows
2019 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 867, p. 969-984, article id PII S0022112019001964Article in journal (Refereed) Published
Abstract [en]

The ability of linear stochastic response analysis to estimate coherent motions is investigated in turbulent channel flow at the friction Reynolds number Re-r = 1007. The analysis is performed for spatial scales characteristic of buffer-layer and large-scale motions by separating the contributions of different temporal frequencies. Good agreement between the measured spatio-temporal power spectral densities and those estimated by means of the resolvent is found when the effect of turbulent Reynolds stresses, modelled with an eddy-viscosity associate with the turbulent mean flow, is included in the resolvent operator. The agreement is further improved when the flat forcing power spectrum (white noise) is replaced with a power spectrum matching the measures. Such a good agreement is not observed when the eddy-viscosity terms are not included in the resolvent operator. In this case, the estimation based on the resolvent is unable to select the right peak frequency and wall-normal location of buffer-layer motions. Similar results are found when comparing truncated expansions of measured streamwise velocity power spectral densities based on a spectral proper orthogonal decomposition to those obtained with optimal resolvent modes.

Place, publisher, year, edition, pages
CAMBRIDGE UNIV PRESS, 2019
Keywords
turbulent boundary layers
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-249766 (URN)10.1017/jfm.2019.196 (DOI)000463073000001 ()2-s2.0-85063881975 (Scopus ID)
Note

QC 20190429

Available from: 2019-04-29 Created: 2019-04-29 Last updated: 2019-04-29Bibliographically approved
Brynjell-Rahkola, M., Hanifi, A. & Henningson, D. S. (2019). On the stability of a Blasius boundary layer subject to localised suction. Journal of Fluid Mechanics, 871, 717-741
Open this publication in new window or tab >>On the stability of a Blasius boundary layer subject to localised suction
2019 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 871, p. 717-741Article in journal (Refereed) Published
Abstract [en]

In this study the origins of premature transition due to oversuction in boundary layers are studied. An infinite row of circular suction pipes that are mounted at right angles to a flat plate subject to a Blasius boundary layer is considered. The interaction between the flow originating from neighbouring holes is weak and for the parameters investigated, the pipe is always found to be unsteady regardless of the state of the flow in the boundary layer. A stability analysis reveals that the appearance of boundary layer transition can be associated with a linear instability in the form of two unstable eigenmodes inside the pipe that have weak tails, which extend into the boundary layer. Through an energy budget and a structural sensitivity analysis, the origin of this flow instability is traced to the structures developing inside the pipe near the pipe junction. Although the amplitudes of the modes in the boundary layer are orders of magnitude smaller than the corresponding amplitudes inside the pipe, a Koopman analysis of the data gathered from a nonlinear direct numerical simulation confirms that it is precisely these disturbances that are responsible for transition to turbulence in the boundary layer due to oversuction.

Place, publisher, year, edition, pages
CAMBRIDGE UNIV PRESS, 2019
Keywords
boundary layer stability, transition to turbulence
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-264150 (URN)10.1017/jfm.2019.326 (DOI)000493076600011 ()2-s2.0-85066907512 (Scopus ID)
Note

QC 20191209

Available from: 2019-12-09 Created: 2019-12-09 Last updated: 2019-12-09Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0001-7864-3071

Search in DiVA

Show all publications