Daniel Haller

Fluid Dynamics and Transfer Processes in Bended Microchannels

Abstract The understanding of the flow processes in microchannels and micromixers is essential for the design of microfluidic devices like microreactors or analytical equipment. We have performed a systematic numerical CFD-study of mixing and mass transfer in sharp 90° bends and heat transfer in T-joints to obtain a detailed insight into the flow patterns and corresponding transfer processes in a wide range of Reynolds numbers. With increasing flow velocity, the straight laminar flow starts to form symmetrical vortices in the bend, at the entrance of the mixing channel, and in T-joints. The vortices enhance the transport processes like heat and mass transfer in the channels significantly. The influence of the geometry and the flow conditions is shown by an analytical estimation of the relevant forces. The appearance of convective transport processes is used for the definition of microflows, which are controlled by viscous forces and diffusive transfer processes.

Piezo-Polymer-Composite Unimorph Actuators for Active Cancellation of Flow Instabilities Across Airfoils

This article presents a smart device for active cancellation of flow instabilities. An array of two piezo unimorph actuators fabricated in piezo-polymer-composite technology is combined with a thin silicone membrane to mimic a movable wall with a closed surface. By locally displacing the thin membrane, a surface wave is generated that interferes with naturally occurring flow instabilities within the boundary layer of an airfoil. Using flow sensors and an intelligent control enables a destructive interference and therefore, an attenuation of natural flow instabilities. This leads to a delay of transition. The boundary layer remains laminar which means drag is reduced. Within the next pages, the setup of the device with actuators, membrane, sensors, and control is introduced. The main focus of this article is on actuator design, modeling, and implementation for wind tunnel experiments. Results of actuator characterization are presented. The non-linear behavior of the piezoactuator (harmonic distortions and impact of high electric fields) is investigated in detail. This study concludes with the results obtained in wind tunnel experiments which prove the functionality of the presented approach. A maximal attenuation of natural occurring flow instabilities of 80% is achieved.

Cymbal type Piezo-Polymer-Composite actuators for active cancellation of flow instabilities on airfoils

This paper presents the design and fabrication of a Cymbal type piezo actuator in Piezo-Polymer-Composite (PPC) technology. The purpose of the developed actuators is, in combination with a full setup including flow sensors, a digital control system, and an elastic membrane, an active manipulation of boundary layer disturbances at airfoils. The goal is to reduce the friction drag on an airfoil by delaying the transition from a laminar to a turbulent boundary layer. The presented results of wind tunnel experiments will prove the successful accomplishment of this task. Boundary layer instabilities, the cause for the transition, are dampened by more than 80 %.

A piezo-actuated closed loop MEMS system for active delay of transition

This paper presents the design, fabrication and application of a highly integrated closed loop MEMS system for an active control of aerodynamic flow instabilities across airfoils. A special piezo-polymercomposite (PPC) technology was used for the fabrication of powerful piezo-microactuators that were integrated with hotwire flow sensors and a digital control system. This work shows first wind tunnel experiments that prove the principal suitability of the developed device for dampening disturbances in the boundary layer. The local amplitudes of natural disturbances, so called Tollmien-Schlichting (TS) waves, have been reduced by 42%.

Assessing the elastostriction and the electrostriction parameter of bulk PZT ceramics

For high electrical loads, the electromechanical characteristics of PZT actuators are impacted by nonlinearities. This work presents a procedure, derived from nonlinear analysis, to assess separately the electrostriction and the elastostriction parameters of bulk PZT ceramics. A detailed investigation has shown that the electrostriction is dominant in actuator applications, while the elastostriction affects sensor applications. Further, the relationship between electrostriction and polarization in PZT material is discussed.

Fluid Dynamics and Transfer Processes in Bended Micro Channels

The understanding of the flow processes in microchannels and micro mixers is essential for the design of micro fluidic devices like micro reactors or analytical equipment. We have performed a systematic numerical CFD-study of mixing and mass transfer in sharp 90° bends and heat transfer in T-joints to obtain a detailed insight into the flow patterns and corresponding transfer processes in a wide range of Reynolds numbers. With increasing flow velocity the straight laminar flow starts to form symmetrical vortices in the bend, at the entrance of the mixing channel, and in T-joints. The vortices enhance the transport processes like heat and mass transfer in the channels significantly. The influence of the geometry and the flow conditions is shown by an analytical estimation of the relevant forces. The appearance of convective transport processes is used for the definition of microflows which are controlled by viscous forces and diffusive transfer processes.Copyright

A dynamic linearization concept for piezoelectric actuators

We present a linearization circuit based on a capacitive Wheatstone bridge that is able to set a desired polarization in a piezoactuator. The system is meant to be used for dynamic actuation in a broad frequency range. A general nonlinear model for piezoactuators is presented in which two nonlinear sub-systems are cascaded: the electric-field-to-polarization (E-P) and the polarization-to-strain (P-x) blocks. The inversion of the latter sub-system in combination with the linearization bridge results in a reduction of up to 19 dB of the harmonic distortion of the actuators mechanical displacement.

Development and Fabrication of Active Microstructures for Wave Control on Airfoils

Transition of an airfoil’s boundary layer from the laminar to the turbulent flow regime increases the overall drag of an airplane significantly. The major origin of this transition are Tollmien-Schlichting (TS) waves. Similar to the dolphin’s skin, a system that is capable to dampen TS waves locally is proposed here. A surface wave can interfere destructively with TS waves and thus delay transition towards the edge of the airfoil. For this purpose, an actuator array is combined with a thin membrane. The presented actuators were developed and improved continuously so that all requirements for the dampening of TS waves are fulfilled. Several actuators are cascaded in a compact manner and combined with a membrane and an array of sensors. The system has proven in wind tunnel experiments to be capable of dampening TS waves successfully and delaying transition.

Investigation on Actuator Arrays for Active Wave Control on a 2D Airfoil

This paper describes different actuators for delaying laminar-turbulent transition by active boundary layer control. Since transition on an unswept 2D wing is initiated by Tollmien-Schlichting instabilities, friction drag can be reduced by attenuation of these waves. The performed experiments are based on earlier investigation with single spanwise as well as streamwise cascaded actuation. As a consequent step and essential part of this work actuator arrays consisting of slots and oscillating surface membranes for spatially distributed actuation were developed. A single spanwise sensor actuator system was applied to a glider, because flow instabilities of free atmosphere can not be simulated in wind tunnel experiments. In this way the method of active TS wave attenuation could successfully be verified under actual flight conditions.