Alexander Duchmann

PIV-based estimation of DBD plasma-actuator force terms

PIV measurements in close proximity to dielectric-barrier discharge plasma actuators have been conducted to quantify the momentum transfer of the plasma to the surrounding air flow. Based on this data a comparative analysis of six existing approaches to estimate the induced body force is presented. Integral methods calculate an integral value for the actuator force based on the momentum balance equation. Insight into the spatial distribution of the body force is provided by differential methods, which are based either on the Navier-Stokes equations or on the vorticity equation. It is demonstrated that the intensity as well as the domain of the force increase with increasing operating power levels. Emphasis is also placed on the issue of self-induced drag. It is shown that 30% of the induced momentum is consumed by wall friction. All results are validated with previously obtained balance force data and luminosity analysis of the identical actuators.

On the classification of dielectric barrier discharge plasma actuators: A comprehensive performance evaluation study

The increasing popularity and maturity of plasma actuators for many flow control applications requires a common standard for plasma actuator performance evaluation. In the present work, a comprehensive comparative study of existing and new evaluation measures is presented, based on results from identical plasma-actuator configurations. A power-flow diagram is introduced that covers the entire range of power stages from the energy source to the flow-control success. All individual power stages are explained, existing controversial definitions are clarified, and an evaluation guideline is applied to previously obtained data. Finally, the defined systematic analysis is applied to the results of a recently conducted plasma-actuator in-flight experiment.

Linear Stability Analysis for Manipulated Boundary-Layer Flows using Plasma Actuators

This paper presents the implementation of a method for linear stability analysis (LSA) and its application to investigate transitional boundary-layer flows affected by dielectric-barrier discharge (DBD) actuators. These flow-control devices are used to influence the process of boundary-layer transition by electrohydrodynamic coupling of momentum to the surrounding fluid molecules. The boundarylayer profile and its stability characteristics are changed. Linear stability analysis is applied to numerical and experimental data and helps to understand the effective mechanisms of these flow-control actuators when applied for transition control. Amplification rates in the linear growth stage are diminished and the critical as well as the local Reynolds number are affected by DBD actuation, leading to considerable delay of transition.

Dielectric Barrier Discharge Plasma Actuators for In-Flight Transition Delay

A single dielectric barrier discharge plasma actuator is employed for flow control on the pressure side of a natural laminar flow wing section under free-flight conditions. A full-sized motorized glider is equipped with the flow-control device and data-acquisition hardware to quantify the impact of the actuator on boundary-layer transition. A transition delay of approximately 3% chord is achieved, quantified by microphone and hot-wire measurements. Simultaneously, the influence of the variable ambient conditions on the flow-control performance is characterized. A closed-loop control algorithm enables constant actuator performance, despite varying humidity, temperature, and density throughout the test flights. The energy efficiency of the flow-control approach is estimated, providing a positive outlook for further improvements and a net benefit of transition control. Finally, a numerical procedure is presented to quantitatively estimate the effect of dielectric barrier discharge actuation on boundary-layer...

In-flight Transition Delay with DBD Plasma Actuators

A single dielectric barrier discharge plasma actuator is employed for flow control on the pressure side of a natural laminar flow wing section under free-flight conditions. A full sized motorized glider is equipped with the flow control device and data acquisition hardware to quantify the impact of the actuator on boundary-layer transition. A transition delay of approximately 3% chord is achieved, quantified by microphone and hot-wire measurements. Simultaneously, the influence of the variable ambient conditions on the flow control performance is characterized. A closed-loop control algorithm enables constant actuator performance, despite varying humidity, temperature and density throughout the test flights. The energy efficiency of the flow control approach is estimated, providing a positive outlook for further improvements and a net benefit of transition control.

Characterization of Tollmien-Schlichting Wave Damping by DBD Plasma Actuators Using Phase-Locked PIV

A phase averaged optical measurement technique is used for characterization of externally excited boundary-layer disturbances. Two-dimensional velocity fields of flat-plate boundary-layer flow are recorded via Particle Image Velocimetry to resolve the effect of Dielectric Barrier Discharge plasma actuators on Tollmien-Schlichting waves. These are artificially excited by a vibrating disturbance source flush-mounted with the surface of the flat plate. A delay-generator is used to trigger the image acquisition on predetermined phase angles of the excitation frequency. Phase averaging unveils the temporal and spatial evolution of the boundary-layer disturbances. Characteristics of the excited waves are quantified and compared to linear stability theory. The phase-locked technique allows the evaluation of plasma actuation on the excited waves. A reduction of the wave amplitude by 26% was recorded for a continuously operated, single Dielectric Barrier Discharge actuator. A comprehensive investigation of the actuator effect under variable flow velocities and pressure gradients is presented in this manuscript.

Closed-Loop Performance Control of Dielectric-Barrier-Discharge Plasma Actuators

The counteraction of changing environmental conditions (i.e., changes of pressure level and airflow speed) on the resulting plasma-actuator performance is demonstrated in the present work. The impact of these changing (fluctuating and/or transient) airflow conditions on the performance of dielectric-barrier-discharge plasma actuators is suppressed using a novel closed-loop performance-control procedure. The goal of controlling a preset plasma-actuator performance online and in situ is achieved and successfully demonstrated. This novel approach represents the first step toward optimal-discharge-based flow control because, beyond the common purpose of favorably manipulating the airflow, any advanced dielectric-barrier-discharge-based flow-control system will necessarily require an appropriate closed-loop performance control of the discharge device.

Influence of air flow on the performance of DBD Plasma Actuators

In the present work the effect of the airflow on the performance of dielectric-barrier discharge (DBD) plasma-actuators is investigated experimentally at low and moderate velocities. This velocity range appears relevant for take-off and approach scenarios of many aerodynamic lifting surfaces, but is also essential for plasma-assisted control of unmanned aerial vehicles or internal flows. In order to analyze the actuator performance, luminosity measurements have been carried out simultaneous with the recording of the relevant electrical parameters. A performance drop of about 10% is identified for the entire measured parameter range at a flow speed of M = 0.15 (U1 = 50m/s). This insight is of particular importance, since the plasma-actuator control authority is reduced even at this modest speed level. From the combined analysis the conclusion is drawn that the decreased electrical performance PA correlates with a decreasing peak intensity b G for increasing airflow velocities. Two non-dimensional scaling numbers , i.e. �PA and K, are proposed to characterize and quantify the air flow influence. It is demonstrated that these numbers span a universal performance drop diagram for the entire range of investigated operating parameters.

Linear Stability Analysis of DBD Boundary-Layer Flow-Control Experiments and Simulations

Linear stability theory is applied to analyze boundary-layer data from flow-control experiments with dielectric barrier discharge plasma actuators. The hydrodynamic stability of laminar boundary-layer flow along a flat plate subject to an adverse pressure gradient is enhanced by the induced momentum from a dielectric barrier discharge. Changes in the velocity distribution are measured with laser Doppler anemometry while a traversable hot-wire probe quantifies the macroscopic effect on the transition location. The observed connection between the altered velocity distribution and the delayed transition is supported by a stability analysis of experimental boundary-layer profiles. A numerical procedure to study the impact plasma actuation has on hydrodynamic stability is presented to enable parametric studies and optimization of flow-control applications.

Closed-Loop Performance Control of DBD Plasma Actuators

The impact of fluctuating airflow conditions on the performance of dielectric barrier discharge (DBD) plasma actuators is suppressed using a novel closed-loop performance control procedure. The goal of controlling the plasma actuator performance online and in-situ is achieved and successfully demonstrated. This novel approach represents a first step towards optimal discharge based flow control, since beyond the common purpose of favorably manipulating the airflow, any advanced DBD-based flow control system will necessarily require an appropriate closed-loop performance control of the discharge device.