Bert Johansson

Silicon based flow sensors used for mean velocity and turbulence measurements

Small and directional sensitive silicon based sensors for velocity measurements have been designed and fabricated using microelectronic technology. Single-chip as well as double-chip sensors for the determination of mean velocity and turbulent stresses have been developed. To determine the performance of these silicon sensors, comparisons with conventional hot-wire sensors were done in a well-defined two-dimensional turbulent flat plate boundary layer at a constant Reynolds number of 4.2 · 106. All the silicon sensors were found to have a spatial and frequency resolution that makes them suitable for turbulence measurements. In the studied flow field the measured profiles of mean velocities and Reynolds stresses of all silicon sensors show the same accuracy as corresponding hot-wire measurements. The silicon sensors are also shown to operate with good resolution even when the temperature of the heated part of the chip is reduced considerably.

Turbulence modification by particles in a horizontal pipe flow

Measurements were made of turbulence intensities and turbulent energy spectra in a fully developed, turbulent air-particle pipe flow. The influence of the particles on the turbulence was studied. Measurements were made with spherical particles and particles with a large aspect ratio (pulp fibres). There is a significant change in turbulence intensity at higher particle concentrations with loading ratios of m = 0.1 and 0.03. The measurements show that the turbulence intensity increases close to the centre of the pipe while the turbulence intensity decreases close to the pipe wall for the spherical particles. These results are in agreement with earlier measurements found in the literature. For the fibres, the turbulence intensity decreases over the whole pipe cross-section. Fibre flocs, however, give variations in the mean velocity that result in the production of turbulence in the lower part of the channel.

A Sensor Based on Silicon Technology for Turbulence Measurements

A very small and highly sensitive flow velocity sensor has been designed and fabricated using silicon microelectronic technology. To determine the performance of this silicon sensor, comparisons with a conventional hot-wire sensor were made in a well defined two-dimensional turbulent flat-plate boundary layer at a constant Reynolds number of 4.2*106. The silicon sensor was found to have a spatial and frequency resolutions that make it suitable for turbulence measurements. In the investigated flow field the new silicon sensor measures profiles of the mean velocity and the turbulence intensity with accuracy as the hot wire.

Turbulence measurements using sensors based on silicon technology

A small and highly sensitive flow velocity sensor was designed and fabricated using silicon microelectronic technology. To determine the performance of this silicon sensor, comparisons with a conventional hot-wire sensor were made in a well-defined two-dimensional turbulent flat-plate boundary layer at a constant Reynolds number of 1.2*10/sup 6/. The silicon sensor was found to have a spatial and frequency resolution that makes it suitable for turbulence measurements. In the investigated flow field the silicon sensor measures profiles of the mean velocity and the turbulence intensity with the same accuracy as the hot wire. The results of the flat-plate measurements were used for the development of new sensor configurations that allow the measurement of turbulent stresses and velocity and temperature correlations.<<ETX>>

A fast simple hot-wire method of determining the mean velocity vector of complex three-dimensional flows

A fast and simple method of determining the mean velocity vector of complex three-dimensional flow fields is outlined. Straight and slanted single hot-wires are rotated in two perpendicular planes. This method increases the angular resolution, which is of importance in flow situations where one of the velocity components dominates and the other changes rapidly from one point to another. The method was calibrated in a wind tunnel and assessed in the internal flow field at the outlet of a fan in a defroster channel. It is shown that the hot-wire method yields good agreement with corresponding flow visualizations determined using a textile thread, and an integration of the measured mean flow yields a flow rate which agrees within a few percent with corresponding direct measurements on an orifice plate.

Measurement of the Enstrophy and the Vorticity Vector in a Plane Cylinder Wake Using a Spectral Method

A method to determine derivative moments using only two slanted hot-wires and a spectral method has been employed for a complete determination of the enstrophy and the vorticity vector. The general idea behind the method is to minimize the measuring volume by use of only two hot-wires and to post-process the measured data. All measurements were conducted in the far wake of a cylinder at a Reynolds number of 1840. Comparisons between the spectral method and conventional measuring techniques are made, and the results justifies the approximation between enstrophy and dissipation. For the vorticity vector it was found that the components of the normal and lateral directions were considerably larger than the stream-wise ones.

A silicon transducer for the determination of wall-pressure fluctuations in turbulent boundary layers

Small and sensitive silicon sensors for turbulent wall-pressure fluctuation measurements have been designed and fabricated using microelectronic technology. For the detection of the pressure fluctuations piezoresistive gauges are placed on a diaphragm and the resistance of these gauges depends on the stresses in the diaphragm. For the determination of the performance of these pressure transducers comparisons with conventional microphones were carried out in a well-defined two-dimensional boundary layer. Power spectra from the silicon pressure transducer revealed a slope in the inertial sublayer corresponding approximately to the 1/3-law of Kolmogorov, and the normalized wall-pressure fluctuations agreed well with other direct measurements.

Reynolds stress measurements using direction-sensitive double-chip silicon sensors

A small direction-sensitive double-chip silicon-based sensor has been designed and fabricated using microelectronic technology. To determine the performance of this sensor the Reynolds stresses in a two-dimensional flat plate boundary layer were determined at a Reynolds number of 4.2*106. Comparisons with conventional hot-wire sensors were made showing that the double-chip sensor was able to determine the turbulent stresses to the same accuracy as a cross hot wire.

An investigation of the turbulence field in the three-dimensional wall jet

A three-dimensional wall jet (3DWJ) is created when a jet of finite extension is ejected close to a wall, forming a complex and highly turbulent flow field, which spreads rapidly in the lateral direction. It is difficult to capture the 3DWJ in numerical simulations, especially the large lateral spreading rate, and as this flow case is of significant importance in many industrial applications, e.g. film cooling of gas turbine blades, a better understanding of the flow is required in order to improve its modeling.

Determination of Derivative Moments using Two Slanted Hot-Wires and a Spectral method

A method to determine derivative moments using slanted hot-wires and a spectral method has been developed. The general idea is to minimize the measuring volume by using only two hot-wires, and a post-processing of the measurement data. It is shown that the spectral method is equivalent to conventional determination of correlations. The method developed has been tested in the self-preserving region of a plane wake by computing profiles of the Reynolds stresses as well as the derivative moments in the expression for the total energy dissipation. One term in this expression, which with conventional methods requires two triple-wires or four cross-wires, has been determined using the spectral method.