S.G. Nychas

Aspects of Flow Structure During a Cylinder Wake-Induced Laminar/Turbulent Transition

The process of a strong wake-induced laminar/turbulent transition on a flat-plate boundary layer was investigated from the point of view of the occurring coherent structures. The wake was generated by a horizontal cylinder positioned in the freestream and upstream of the flat plates leading edge. An X-type hot-wire probe measured the stream wise and normal-to-the-wall velocity components, whereas a gradient hot-wire probe, located in the cylinders wake and at the same stream wise position as the X-type hot-wire probe, detected the passage of the von Karman vortices. From the simultaneously acquired velocity and gradient signals, the intermittency factor distribution and cross-correlation functions, combined with a quadrant-splitting analysis, were computed. Furthermore, an ensemble-average technique was applied to the signals. The analysis of the signals revealed that during the transition a secondary vortical structure occurs near the wall. Turbulent kinetic energy transfer to and from the wall during the transition was evaluated

The behaviour of a conductivity probe in electrolytic liquid/solid suspensions

A conductivity probe having one electrode with a small surface (the sensing electrode) can be used for the measurement of the instantaneous local concentration during liquid mixing in an electrolytic solution. The presence of solid particles in the mixing field causes fluctuations of the electrolyte concentration signal, contaminating in this way the concentration measurement. The influences of the volume fraction and size of the solid particles on the electrolyte-concentration signal are investigated. The data indicate how to estimate the effect of the presence of particles in the specification of the electrolyte concentration. Furthermore, a method which allows the measurement of the mean electrolyte concentration at high solids volume fractions in electrolytic liquid/solid suspensions is proposed.

Drifting behaviour of a conductivity probe

Conductivity probes, as they are proposed by Gibson and Schwarz, are suitable for local concentration measurements in electrolyte turbulent flows. The dependence of the drifting behaviour in NaCl-solutions of such probes from various factors, such as ion concentrations in tap water, temperature, space resolution, etc., has been quantified. Knowing this behaviour errors can be minimized during concentration measurements.

Validation study of the dispersion Lagrangian particle model DIPCOT over complex topographies using different concentration calculation methods

In this paper, the Lagrangian particle dispersion model DIPCOT is evaluated by simulating two dispersion experiments over highly complex topographies (the TRANSALP90 and the ETEX experiments). The resulting predicted concentrations were compared with the experimental ones using some well-known performance indices and various types of plots. Furthermore the model results are compared to the statistical performance of other dispersion models that have been applied at the ETEX experiment. The effect of different concentration calculation methods on the predicted concentrations is examined, by comparing the results of five methods: a box counting method with fix dimensions, and four types of concentration estimation density kernels. According to the statistical evaluation, the agreement between the predicted and the measured concentration is affected by the concentration calculation method; the overall behaviour of the model, however, is reasonably good.

Boundary Layer Transition Induced by a Von Karman Vortex Street Wake

A study has been made of the process of laminar to turbulent transition induced by a von Karman vortex street wake, in the boundary layer on a flat plate. The boundary layer developed under zero pressure gradient conditions while the vortex street was generated by a cylinder positioned in the free stream. Hot-wire measurements over a range of Strouhal frequencies and free stream velocities were used for the identification of the transition onset. From the analysis of the experimental data, two different transition mechanisms known in the literature as ‘strong’ wake and ‘weak’ wake induced transition, could be identified. It was established that, the onset of the strong von Karman wake induced transition process was a function of the free stream velocity, the position of the cylinder with respect to the plate, the cylinder diameter, the drag coefficient and the minimum velocity in the developing wake at the streamwise position of the onset of the boundary layer transition. The end of the strong wake induced transition, was defined at the streamwise distance, where the wake of the cylinder met the wall. A correlation for the prediction of the onset of the transition was developed

Scalar transport in a quasi two-dimensional turbulent wake interacting with a boundary layer

Abstract An experimental study of heat transport in the interaction region between the wake of a cylinder and a turbulent boundary layer is presented in this work. The cylinder was placed parallel to a flat plate and normal to the flow. Its position was selected above the boundary layer edge, so that the lower part of the wake was interacting with the boundary layer. Heat was supplied to the boundary layer flow by means of a line heat source. Presence of surface roughness on the cylinder resulted in the deviation of the velocity power spectrum scaling region from the −5/3 power law which is characteristic of three-dimensional turbulence. Point measurements of the instantaneous values of two velocity components and temperature have been taken using hot-wire anemometry. Two turbulent scalar flux components and a Reynolds shear stress component have been obtained directly from the experimental data. Results were assessed in conjunction with the periodic coherent structures in the wake (a von Karman vortex street) using a phase-averaging technique that provided a clear picture of the heat transport procedures involved.

Validation of DIPCOT-II model based on the Kincaid experiment

This study presents the performance of the Lagrangian particle dispersion model DIPCOT-II (DIsPersion over COmplex Terrain) at the Kincaid field experiment. The validation of the model is performed on the basis of the maximum arcwise and near centreline concentrations using the bootstrap re-sampling procedure and the variation of the model residuals.

A fast digital technique for calibration of hot-wires over a wide temperature range

This work describes a calibration technique for hot-wire anemometry which is accurate, fast, easy to implement, and covers extended temperature and velocity ranges. The novel feature of the technique is the collection of large quantities of data during a calibration experiment, by recording and analysis of the dynamic response of hot-wires to gradual heating. This approach differs from static methods, where the response of hot-wires to only one, constant value of temperature and velocity is obtained per calibration experiment.

Validation of the Demokritos dispersion modelling system based on the Indianapolis experiment

To assess the ability of a model to simulate atmospheric dispersion, its performance must be tested using data from real field experiments. This paper presents the results of a validation study of the atmospheric dispersion model Dispersion over Complex Terrain (DIPCOT) against data from the Indianapolis field experiment. Three different modules of the dispersion code are examined a puff model, a random walk model based on random displacement, and a stochastic model based on the Langevin equation. The results of the three modules are statistically and qualitative compared with the maximum arcwise concentrations and the near centreline concentrations.

Conditional analysis of turbulent heat transport in a quasi two-dimensional wake interacting with a boundary layer

Abstract A conditional averaging technique has been used for the analysis of experimental data obtained from heat flux measurements in the region of interaction of a cylinder wake with a boundary layer. The circular cylinder was placed normal to the flow and parallel to a flat plate, just outside of the boundary layer. The surface of the cylinder was intentionally rough, following our observation that in this way the velocity power spectrum in the intermediate wake could be forced to acquire some quasi two-dimensional turbulence characteristics, namely to display a scaling region deviation from the −5/3 power law which is characteristic of homogeneous, three-dimensional turbulence. The streamwise and the normal (to the plate) velocity components, as well as the temperature at the same position, have been recorded simultaneously, using hot wire anemometry. Time variations of the relevant Reynolds shear stress and heat fluxes were evaluated directly from the experimental data. These results were then conditionally averaged according to the quadrant splitting analysis technique. It was found that in the boundary layer region, a particular fluid motion (‘ejections’, in the quadrant splitting analysis terminology) dominate the flow. That motion is the main carrier of hot fluid from the boundary layer to the wake.