B. W. van Oudheusden

Silicon thermal flow sensors

Abstract This paper discusses the design and application of silicon thermal flow sensors. It is shown how the transduction path in the complete measurement system, going from flow variable to electrical signal, results in three fundamental problems, which can be associated with, respectively, the mechanical, thermal and electrical signal domains. These aspects are further discussed in relation to the silicon flow-sensor research performed at the Electronic Instrumentation Laboratory.

PIV-based pressure measurement

The topic of this article is a review of the approach to extract pressure fields from flow velocity field data, typically obtained with particle image velocimetry (PIV), by combining the experimental data with the governing equations. Although the basic working principles on which this procedure relies have been known for quite some time, the recent expansion of PIV capabilities has greatly increased its practical potential, up to the extent that nowadays a time-resolved volumetric pressure determination has become feasible. This has led to a novel diagnostic methodology for determining the instantaneous flow field pressure in a non-intrusive way, which is rapidly finding acceptance in an increasing variety of application areas. The current review describes the operating principles, illustrating how the flow-governing equations, in particular the equation of momentum, are employed to compute the pressure from the material acceleration of the flow. Accuracy aspects are discussed in relation to the most dominating experimental influences, notably the accuracy of the velocity source data, the temporal and spatial resolution and the method invoked to estimate the material derivative. In view of its nature of an emerging technique, recently published dedicated validation studies will be given specific attention. Different application areas are addressed, including turbulent flows, aeroacoustics, unsteady wing aerodynamics and other aeronautical applications.

Integrated thermopile sensors

Abstract This paper is about integrated silicon thermopiles and their applications in silicon sensors. After a short description of the thermoelectric effect and its use in silicon thermopiles, some attention is devoted to the design of micromachined structures for implementing thermal sensors. The various sensing principles based on thermal effects are discussed next. Finally, an impression is given of some of the recently developed silicon-thermopile sensors which implement these sensing principles.

Three-dimensional vortex organization in a high-Reynolds-number supersonic turbulent boundary layer

Tomographic particle image velocimetry was used to quantitatively visualize the three-dimensional coherent structures in a supersonic (Mach 2) turbulent boundary layer in the region between y/? = 0.15 and 0.89. The Reynolds number based on momentum thickness Re? = 34000. The instantaneous velocity fields give evidence of hairpin vortices aligned in the streamwise direction forming very long zones of low-speed fluid, consistent with Tomkins & Adrian (J. Fluid Mech., vol. 490, 2003, p. 37). The observed hairpin structure is also a statistically relevant structure as is shown by the conditional average flow field associated to spanwise swirling motion. Spatial low-pass filtering of the velocity field reveals streamwise vortices and signatures of large-scale hairpins (height > 0.5?), which are weaker than the smaller scale hairpins in the unfiltered velocity field. The large-scale hairpin structures in the instantaneous velocity fields are observed to be aligned in the streamwise direction and spanwise organized along diagonal lines. Additionally the autocorrelation function of the wall-normal swirling motion representing the large-scale hairpin structure returns positive correlation peaks in the streamwise direction (at 1.5? distance from the DC peak) and along the 45° diagonals, which also suggest a periodic arrangement in those directions. This is evidence for the existence of a spanwise–streamwise organization of the coherent structures in a fully turbulent boundary layer.

Three-dimensional instantaneous structure of a shock wave/turbulent boundary layer interaction

An experimental study is carried out to investigate the three-dimensional instantaneous structure of an incident shock wave/turbulent boundary layer interaction at Mach 2.1 using tomographic particle image velocimetry. Large-scale coherent motions within the incoming boundary layer are observed, in the form of three-dimensional streamwise-elongated regions of relatively low- and high-speed fluid, similar to what has been reported in other supersonic boundary layers. Three-dimensional vortical structures are found to be associated with the low-speed regions, in a way that can be explained by the hairpin packet model. The instantaneous reflected shock wave pattern is observed to conform to the low- and high-speed regions as they enter the interaction, and its organization may be qualitatively decomposed into streamwise translation and spanwise rippling patterns, in agreement with what has been observed in direct numerical simulations. The results are used to construct a conceptual model of the three-dimensional unsteady flow organization of the interaction.

Influence of wing kinematics on aerodynamic performance in hovering insect flight

The influence of different wing kinematic models on the aerodynamic performance of a hovering insect is investigated by means of two-dimensional time-dependent Navier–Stokes simulations. For this, simplified models are compared with averaged representations of the hovering fruit fly wing kinematics. With increasing complexity, a harmonic model, a Robofly model and two more-realistic fruit fly models are considered, all dynamically scaled at Re = 110. To facilitate the comparison, the parameters of the models were selected such that their mean quasi-steady lift coefficients were matched. Details of the vortex dynamics, as well as the resulting lift and drag forces, were studied. The simulation results reveal that the fruit fly wing kinematics result in forces that differ significantly from those resulting from the simplified wing kinematic models. In addition, light is shed on the effect of different characteristic features of the insect wing motion. The angle of attack variation used by fruit flies increases aerodynamic performance, whereas the deviation is probably used for levelling the forces over the cycle.

Design, aerodynamics and autonomy of the DelFly

One of the major challenges in robotics is to develop a fly-like robot that can autonomously fly around in unknown environments. In this paper, we discuss the current state of the DelFly project, in which we follow a top-down approach to ever smaller and more autonomous ornithopters. The presented findings concerning the design, aerodynamics and autonomy of the DelFly illustrate some of the properties of the top-down approach, which allows the identification and resolution of issues that also play a role at smaller scales. A parametric variation of the wing stiffener layout produced a 5% more power-efficient wing. An experimental aerodynamic investigation revealed that this could be associated with an improved stiffness of the wing, while further providing evidence of the vortex development during the flap cycle. The presented experiments resulted in an improvement in the generated lift, allowing the inclusion of a yaw rate gyro, pressure sensor and microcontroller onboard the DelFly. The autonomy of the DelFly is expanded by achieving (1) an improved turning logic to obtain better vision-based obstacle avoidance performance in environments with varying texture and (2) successful onboard height control based on the pressure sensor.

High-sensivity 2-D flow sensor with an etched thermal isolation structure

Abstract This paper describes a two-dimensional thermal flow sensor, fabricated by silicon planar technology and subsequent micromachining. Using a two-step etching process, a thermally isolated floating-membrane structure has been formed in the chip. Flow is measured by detecting flow-induced temperature differences in two directions on the heated membrane, and this principle allows directional flow measurements over the full range of 360°. When compared to a wafer-thick silicon flow sensor of similar dimensions, this new structure possesses a much higher sensitivity, and an improved offset and time response behaviour. The response time for the behaviour of the average membrane temperature is found to be of the order of 150 ms, while for the temperature-difference measurement mode the response time is estimated to be only 14 ms. First experiments show a typical error of 3° in predicted flow angle, and 5% in flow velocity.

Silicon thermal flow sensor with a two-dimensional direction sensitivity

The design and testing of a new type of silicon sensor to measure the magnitude and direction of a fluid flow is described. The measurement principle is based on the detection of flow-induced temperature differences on a hot sensing surface. By measuring temperature differences in two directions perpendicular to each other, a full 360 degrees sensitivity can be obtained. The sensor is made of a silicon IC, on which the flow-induced temperature differences are measured with integrated thermopiles. The use of an IC sensor allows the realisation of a compact multicomponent sensor, which can easily be combined with electronic circuitry. Calibration measurements are presented for air flow.

Integrated flow friction sensor

Abstract The new application of integrated flow sensors is presented, namely the measurement of wall shears stress (the friction force that a flow exerts on the surface of an object). To this end, a thermal flow sensor chip is bonded on a ceramic carrier substrate that has its back side exposed to the flow and is mounted in the surface on which the wall shear stress is to be measured. As the sensor chip has no direct contact with the flow, simple wire bonding can be used. The relation between the flow rate and the total heat loss is measured, as well as the temperature difference on the chip in the direction of the flow. A novel feature with respect to earlier designs is the use of integrated thermopiles for the measurement of the on-chip temperature difference. Measurements are reported for turbulent boundary-layer flow.