Oksana Stalnov

Large trucks drag reduction using active flow control

Aerodynamic drag is the cause for more than two-thirds of the fuel consumption of large trucks at highway speeds. Due to functionality considerations, the aerodynamic efficiency of the aft-regions of large trucks was traditionally sacrificed. This leads to massively separated flow at the lee-side of truck-trailers, with an associated drag penalty of at least a third of the total aerodynamic drag. Active Flow Control (AFC), the capability to alter the flow behavior using unsteady, localized energy injection, can very effectively delay boundary layer separation. By attaching a compact and relatively inexpensive “add-on” AFC device to the back side of truck-trailers (or by modifying it when possible) the flow separating from it could be redirected to turn into the lee-side of the truck, increasing the back pressure, thus significantly reducing drag. A comprehensive and aggressive research plan that combines actuator development, computational fluid dynamics and bench-top as well as wind tunnel experiments was performed. The research uses an array of 15 newly developed Suction and Oscillatory Blowing actuators housed inside a circular cylinder attached to the aft edges of a generic 2D truck model. Preliminary results indicate a net drag reduction of 10% or more.

Evaluation of active flow control applied to wind turbine blade section

A feasibility study for implementing active flow control (AFC) methods to improve the performance of wind turbines was performed. The experimental effort investigated the impact of zero-mass-flux (ZMF) piezofluidic actuators attempting to control boundary layer separation from thick airfoils that are suitable for wind turbine rotor blades. It was demonstrated that the ZMF actuators can replace passive vortex generators that are commonly used for boundary layer separation delay, without the inherent drag penalty that the passive devices impose. It has been shown that ZMF fluidic actuators are suitable for flow control in wind turbine application due to the fact that they are adjustable for wider Reynolds number range, while vortex generators are tuned to perform well in one design point. It was demonstrated that AFC can effectively double the maximum lift of this airfoil at low Reynolds numbers. A possible application is a significant reduction of the turbine start-up velocity. It was also found that even ...

Identifying noisy and quiet modes in a jet

In the current jet noise study, an empirical modal decomposition is proposed which distills the noisy and quiet modes of the flow field. In particular, the POD of flows is generalised for an optimal resolution of the far-field noise as opposed to a least-order representation of the hydrodynamic fluctuation level. This decomposition technique, which we call ‘most observable decomposition (MOD)’, is based on a linear cause-eect relationship between the hydrodynamics (cause) and the far-field acoustics (observed eect). In the current study, this relationship is identified from a linear stochastic estimation between the flow field and the far-field pressure — taking into account the propagation time of sound. We employ MOD to turbulent jet noise at Ma = 0.9, Re = 3600 using CFD/CAA data from RWTH Aachen. While more than 350 POD modes are necessary to capture only 50% of the flow fluctuation energy, a mere 24 MOD modes resolve 90% of the far-field acoustics. Evidently, far-field noise acts as filter which ‘sees’ only a low-dimensional subspace of the flow and ‘ignores’ silent subspaces which contain a large amount of fluctuation energy. The MOD methodology yields ‘least-order’ representations of any other observable as well — assuming a linear relationship between flow and observable.

Roll control via active flow control: from concept to flight

This paper describes a series of experiments that enabled a flight demonstration of roll control without moving control surfaces. That goal was achieved using a wing with a partial span Glauert-type airfoil, characterized by an upper-surface boundary-layer separation from the two-thirds chord location at all incidence angles. The flow over that region was proportionally controlled using zero-mass-flux unsteady excitation emanating from piezofluidic actuators. The control was applied to one wing at a time, resulting in gradual suppression of the boundary-layer separation, increased lift, and reduced drag, leading to a coordinated turning motion of the small electric drone. The extensive multidisciplinary study (starting from the actuator adaptation, the airfoil integration, and the two dimensional wind-tunnel tests) led to the selection of a configuration for the flight demonstrator. Further development of a lightweight wing and piezofluidic actuators, along with a compact, lightweight, energy-efficient electronic drive system, was followed by full-scale wind-tunnel tests and three successful flight tests. It was flight demonstrated that active flow control can induce roll moments that are sufficient to control the vehicle flight path during cruise, as well as during landing. A linear model was used to predict the roll motion of the active-flow controlled drone, with reasonable agreement to the flight-test data. The current study resulted in several pioneering (to the best of our knowledge)achievements that should pave the way to further integration of active-flow-control methods in flight vehicles for hingeless flight attitude and flight-path control, as well as improved performance and increased reliability with lower observability.

Analysis of scattering from an acoustic cloak in a moving fluid

This work develops a theoretical framework for acoustic cloak scattering analysis in a low speed non-stationary fluid that is simply described as a potential flow. The equivalent sound source induced by the moving fluid local to the cloak is analytically constructed and is then estimated using Born approximation. The far-field scattering can thereafter be obtained using the associated Greens function of the convected wave equation. The results demonstrate that the proposed analytical approach, which might be helpful in the design and evaluation of cloaking systems, effectively elucidates key characteristics of the relevant physics. In addition, it can be seen that, in a moving fluid, the so-called convected cloaking design achieves better cloaking performance than the classical cloaking design.

Experimental Validation of Sensor Placement for Control of a D-Shaped Cylinder Wake

The effectiveness of a sensor configuration, based on body mounted sensors, for feedback flow control of a D-shaped cylinder wake is investigated experimentally. The research is aimed at suppressing unsteady loads resulting from the von Karman vortex shedding in the wake of bluff bodies at a Reynolds number range of 100-1000. A low-dimensional Proper Orthogonal Decomposition (POD) procedure was applied to the stream-wise and cross-stream velocities in the near wake field obtained using Particle Image Velocimetry (PIV) with steady state vortex shedding. The data was collected from the unforced condition, which served as a baseline, as well as during influence of forcing but within the “lock-in” region. The design of sensor number and placement was based on data from a laminar direct numerical simulation of the Navier Stokes equations. A Linear Stochastic Estimator (LSE) was employed to map the surface mounted sensor signals to the temporal coefficients of the reduced order model of the wake flow field in order to provide accurate yet compact estimates of the low-dimensional states. For a three sensor configuration, results show that the root mean square estimation error of the first two cross-stream modes is within 20 – 40% of the PIV generated POD time coefficients. This level of error is acceptable for a moderately robust controller required to close the loop, based on previous investigation.

On Amplitude Scaling of Active Separation Control

Various scaling options for the effects of excitation magnitude on the lift alternation due to zero-mass-flux periodic excitation for boundary layer separation control are examined. Physical scaling analysis leads to five amplitude parameters. The different scaling laws are examined using experimental data acquired at low Reynolds numbers and various angles of attack. The results indicate that both the velocity ratio and the momentum coefficient, commonly used for amplitude scaling of separation control applications, do not scale the current data-set. For 2D excitation with a Strouhal number of order unity, a Reynolds weighted momentum coefficient provides reasonable scaling. For 3D excitation with a Strouhal number greater than 10, the Reynolds scaled momentum coefficient, the Strouhal scaled velocity ratio and the newly defined vorticity-flux coefficient, all provide good scaling. The airfoil incidence variations are accounted for by using the velocity at the boundary layer edge at the actuation location, rather than the fixed free-stream velocity as a velocity scale. The main finding of this study is that the Reynolds number scaled momentum coefficient provides good amplitude scaling for the entire current data set.

Prediction of Broadband Trailing-Edge Noise Based on Blake Model and Amiet Theory

This paper extends previously published TNO-Blake methods to predict airfoil broadband self-noise due to interaction of a turbulent boundary-layer with a sharp trailing edge. The method presented herein combines Blake’s model to predict surface pressure fluctuations and Amiet model to predict trailing edge noise emission. The Blake method is based on the solution of the Poisson equation as an integral of the mean-shear turbulent source interaction source, over the entire boundary layer thickness. A recent advances in description of streamwise turbulent intensity are employed. Surface pressure spectra are measured with remote microphones and compared to the prediction. In the next step, the wavenumberfrequency spectrum as predicted by the TNO model was utilized as an input to the Amiet model to evaluate the far-field trailing edge noise.

Leading edge serrations for the reduction of aerofoil separation self-noise

This paper presents an experimental investigation into the use of LE serrations for the reduction of trailing edge self-noise, at least for the NACA-65 aerofoil family. It is shown that the leading edge serrations are able to reduce the self-noise in a low frequency range at small and negative angles of attack. The exact mechanism of this reduction is still not completely discovered, but the LE serrations are discovered able to modulate the mean velocity field and turbulent velocity spectrum in that range of frequencies, as well as to dampen the effect of the angle of attack on the pressure field and to reduce its coherence. We emphasise that this paper represents work in progress and further investigations are still necessary in order to completely understand the dynamics behind this reduction.

On the Estimation of Time Dependent Lift of a European Starling (Sturnus vulgaris) during Flapping Flight

We study the role of unsteady lift in the context of flapping wing bird flight. Both aerodynamicists and biologists have attempted to address this subject, yet it seems that the contribution of unsteady lift still holds many open questions. The current study deals with the estimation of unsteady aerodynamic forces on a freely flying bird through analysis of wingbeat kinematics and near wake flow measurements using time resolved particle image velocimetry. The aerodynamic forces are obtained through two approaches, the unsteady thin airfoil theory and using the momentum equation for viscous flows. The unsteady lift is comprised of circulatory and non-circulatory components. Both approaches are presented over the duration of wingbeat cycles. Using long-time sampling data, several wingbeat cycles have been analyzed in order to cover both the downstroke and upstroke phases. It appears that the unsteady lift varies over the wingbeat cycle emphasizing its contribution to the total lift and its role in power estimations. It is suggested that the circulatory lift component cannot assumed to be negligible and should be considered when estimating lift or power of birds in flapping motion.