Simon J. Illingworth
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View article: Streak generation in viscosity-stratified wall-bounded flows
Streak generation in viscosity-stratified wall-bounded flows Open
Streamwise elongated flow structures, or streaks, dominate wall-bounded flows. We show that the renowned lift-up mechanism, that generates streaks, is significantly altered by viscosity stratification. We additionally identify a novel path…
View article: Nonlinear estimation in turbulent channel flows
Nonlinear estimation in turbulent channel flows Open
We design a nonlinear estimator for channel flows at $$Re_{\tau }=180$$ and 590. The nonlinear estimator uses a linear estimator structure based on the linearised Navier–Stokes equations and explicitly calculates the nonlinear forc…
View article: Mode-to-mode nonlinear energy transfer in turbulent channel flows
Mode-to-mode nonlinear energy transfer in turbulent channel flows Open
We investigate nonlinear energy transfer for channel flows at friction Reynolds numbers $Re_{\tau }=180$ and $590$ . The key feature of the analysis is that we quantify the energy transferred from a source mode to a recipient mode, with ea…
View article: Nonlinear estimation in turbulent channel flows
Nonlinear estimation in turbulent channel flows Open
We design a nonlinear estimator for channel flows at $Re_{\tau}=180$ and $590$.The nonlinear estimator uses a linear estimator structure based on the linearised Navier-Stokes equations and explicitly calculates the nonlinear forcing from t…
View article: Nonlinear estimation in turbulent channel flows
Nonlinear estimation in turbulent channel flows Open
We design a nonlinear estimator for channel flows at $Re_τ=180$ and $590$. The nonlinear estimator uses a linear estimator structure based on the linearised Navier-Stokes equations and explicitly calculates the nonlinear forcing from the e…
View article: Mode-to-mode nonlinear energy transfer in turbulent channel flows
Mode-to-mode nonlinear energy transfer in turbulent channel flows Open
We investigate nonlinear energy transfer for channel flows at friction Reynolds numbers of $Re_τ=180$ and $590$. The key feature of the analysis is that we introduce a new variable, which quantifies the energy transferred from a source mod…
View article: Use of eddy viscosity in resolvent analysis of turbulent channel flow
Use of eddy viscosity in resolvent analysis of turbulent channel flow Open
The predictions obtained from resolvent analysis with and without an eddy viscosity model for turbulent channel flow at Reτ=550 are compared to direct numerical simulation data to identify the scales and wave speeds for which resolvent ana…
View article: On the use of eddy viscosity in resolvent analysis of turbulent channel flow
On the use of eddy viscosity in resolvent analysis of turbulent channel flow Open
The predictions of resolvent analysis for turbulent channel flow are evaluated for a friction Reynolds number of Retau = 550. In addition to the standard resolvent operator with kinematic viscosity, a resolvent operator augmented with the …
View article: Resolvent-based approach for $$\pmb {H_2}$$-optimal estimation and control: an application to the cylinder flow
Resolvent-based approach for $$\pmb {H_2}$$-optimal estimation and control: an application to the cylinder flow Open
We consider estimation and control of the cylinder wake at low Reynolds numbers. A particular focus is on the development of efficient numerical algorithms to design optimal linear feedback controllers when there are many inputs (disturban…
View article: Navier-Stokes–based linear model for unstably stratified turbulent channel flows
Navier-Stokes–based linear model for unstably stratified turbulent channel flows Open
We use the linearized Navier-Stokes equations to study the large-scale flow structures in unstably stratified turbulent channel flows. The impulse response of the linear operator at bulk Richardson numbers from Ri_b = 0.001 to Ri_b = 1.0 a…
View article: Optimal sensor and actuator placement for feedback control of vortex shedding
Optimal sensor and actuator placement for feedback control of vortex shedding Open
We consider linear feedback control of the two-dimensional flow past a cylinder at low Reynolds numbers, with a particular focus on the optimal placement of a single sensor and a single actuator. To accommodate the high dimensionality of t…
View article: Linear-model-based estimation in wall turbulence: improved stochastic forcing and eddy viscosity terms
Linear-model-based estimation in wall turbulence: improved stochastic forcing and eddy viscosity terms Open
We use Navier–Stokes-based linear models for wall-bounded turbulent flows to estimate large-scale fluctuations at different wall-normal locations from their measurements at a single wall-normal location. In these models, we replace the non…
View article: Resolvent-based approach for H2-optimal estimation and control: an application to the cylinder flow
Resolvent-based approach for H2-optimal estimation and control: an application to the cylinder flow Open
We consider estimation and control of the cylinder wake at low Reynolds numbers. A particular focus is on the development of efficient numerical algorithms to design optimal linear feedback controllers when there are many inputs (disturban…
View article: Energy transfer mechanisms and resolvent analysis in the cylinder wake
Energy transfer mechanisms and resolvent analysis in the cylinder wake Open
We investigate energy transfer mechanisms for vortex shedding in the two-dimensional cylinder wake at a Reynolds number of Re=100. In particular, we focus on a comparison of the energy transfer in the true flow field to that predicted by r…
View article: Energy transfer in turbulent channel flows and implications for resolvent modelling
Energy transfer in turbulent channel flows and implications for resolvent modelling Open
View article: Large-scale structures predicted by linear models of wall-bounded turbulence
Large-scale structures predicted by linear models of wall-bounded turbulence Open
The objective of this article is to determine for which scales stochastic forcing of the linearized Navier-Stokes equations, recast as the resolvent operator, is sufficient to reproduce second-order statistics in turbulent channel flow. Ou…
View article: Model-based estimation of vortex shedding in unsteady cylinder wakes
Model-based estimation of vortex shedding in unsteady cylinder wakes Open
This paper considers single-sensor estimation of vortex shedding in cylinder\nwakes at $Re=100$ in simulations and at $Re=1036$ in experiments. A model based\non harmonic decomposition is developed to capture the periodic dynamics of\nvort…
View article: Coherent large-scale structures from the linearized Navier–Stokes equations
Coherent large-scale structures from the linearized Navier–Stokes equations Open
The wall-normal extent of the large-scale structures modelled by the linearized Navier–Stokes equations subject to stochastic forcing is directly compared to direct numerical simulation (DNS) data. A turbulent channel flow at a friction Re…
View article: Linear control of coherent structures in wall-bounded turbulence at Re$_τ= 2000$
Linear control of coherent structures in wall-bounded turbulence at Re$_τ= 2000$ Open
We consider linear feedback flow control of the largest scales in an incompressible turbulent channel flow at a friction Reynolds number of Re$_τ$ = 2000. A linear model is formed by linearizing the Navier-Stokes equations about the turbul…
View article: Linear control of coherent structures in wall-bounded turbulence at Re$_\tau = 2000$
Linear control of coherent structures in wall-bounded turbulence at Re$_\tau = 2000$ Open
We consider linear feedback flow control of the largest scales in an incompressible turbulent channel flow at a friction Reynolds number of Re$_{\tau}$ = 2000. A linear model is formed by linearizing the Navier-Stokes equations about the t…
View article: Model-Based Estimation of Vortex Shedding in Unsteady Cylinder Wakes
Model-Based Estimation of Vortex Shedding in Unsteady Cylinder Wakes Open
This paper considers single-sensor estimation of vortex shedding in cylinder wakes at $Re=100$ in simulations and at $Re=1036$ in experiments. A model based on harmonic decomposition is developed to capture the periodic dynamics of vortex …
View article: Linear estimation of coherent structures in wall-bounded turbulence at Re<sub> <i>τ</i> </sub> = 2000
Linear estimation of coherent structures in wall-bounded turbulence at Re<sub> <i>τ</i> </sub> = 2000 Open
The estimation problem for a fully-developed turbulent channel flow at Reτ = 2000 is considered. Specifically, a Kalman filter is designed using a Navier–Stokes-based linear model. The estimator uses time-resolved velocity measurements at …
View article: Frequency domain and time domain analysis of thermoacoustic oscillations with wave-based acoustics
Frequency domain and time domain analysis of thermoacoustic oscillations with wave-based acoustics Open
Many thermoacoustic systems exhibit rich nonlinear behaviour. Recent studies show that this nonlinear dynamics can be well captured by low-order time domain models that couple a level set kinematic model for a laminar flame, the $G$ -equat…