A method of tracking ensembles of particle images

A method for tracking ensembles of particle images of a digital PIV record by making use of the minimum quadratic difference (MQD) technique is described. The similarity of this method with correlation schemes used for the evaluation of PIV is discussed. The method is exemplified with two applications that are also evaluated by auto- and cross-correlation, respectively.Receipt of a fellowship from Deutscher Akademischer Austauschdienst (DAAD) for L. C. Gui is gratefully acknowledged.     

Investigations of the measurement accuracy of stereo particle image velocimetry

With a stereo PIV system, in order to perform reliable measurements of the three velocity components in liquid flow, it is mandatory to minimise the errors made in determining the 2D displacement vectors and the viewing direction of each of the two cameras. We present a method for determining the viewing direction in the angular displacement stereo system by means of a digital imaging procedure such that the direct measurement of geometrical parameters of the set-up is avoided. This makes the method particularly useful for measurements through the transparent walls confining the liquid flow. A third order polynomial used for calibrating the stereo system is shown to provide more accurate results than imaging functions of lower order. Further improvement of the evaluation accuracy is obtained with the application of an artificial neuronal network, but at the expense of considerably increasing the computation time. A comparison of the evaluation results obtained with the operational procedures presented in this paper with those generated with another method that is applicable to liquid flow (Soloff et al. 1997) shows, that the present procedures can be considered as a viable alternative to existing methods.     

Density-sensitive whole-field flow measurement by optical speckle photography

An optical whole-field, line-of-sight method is surveyed that allows the measurement of the refractive deflection angles of light transmitted through a flow with density changes. The method uses the principles of speckle photography. In comparison to classical optical methods such as interferometry, a much higher number of data values can be obtained from one record without the need of interpolating between experimental data. This high signal density is particularly advantageous for analyzing turbulent flows with density fluctuations. Such an analysis is, so far, restricted to the existence of axisymmetric turbulence. A number of applications to this type of turbulent flow are reported, and proposals are made for future developments that will allow this speckle technique to be applied to the study of turbulent shear flow.     

Flow separation in laminar natural convection

Theoretical and experimental studies of flow separation in laminal natural convection are presented. Since classical boundary-layer theory cannot determine separation on curved walls in natural convection, two extensions of the classical boundary-layer theory are discussed: boundary-layer theory of higher order and double-deck theory. Both theories are applied to experiments on a vertical flat plate with humps.     

Direct interferometric measurement of streaming birefringence

The two refractive indices in the flow of a colloidal birefringent liquid are measured separately by means of a Mach-Zehnder interferometer. For a quantitative evaluation of the resulting interferograms it is not necessary to linearize the respective equations relating the refractive index distribution to the deformation velocity in the flow. Therefore it becomes possible to perform velocity measurements in the non-Newtonian flow range. An additional measurement of the mean flow rate enables one to determine the velocity field without the need of a calibration of the observed interference fringes.     

A novel PIV technique for measurements in multiphase flows and its application to two-phase bubbly flows

A new experimental procedure for performing simultaneous, phase-separated velocity measurements in two-phase flows is introduced. Basically, the novel particle image velocimetry (PIV) technique is a combination of the three most often used PIV techniques in multiphase flows: PIV with fluorescent tracer particles, shadowgraphy, and the digital phase separation with a masking technique. The combination of these three independent measurement techniques is achieved by shifting the background intensity of a PIV recording to a higher, but uniform gray value level. In order to combine the advantages of these multiphase-PIV methods, a new PIV set-up was developed. With this set-up the velocity distributions of the two phases are measured simultaneously with only one b/w camera. This experimental set-up is aimed at providing a means for characterizing the modification of turbulence in the liquid phase by bubbles. This phenomenon is often called "pseudo-turbulence".     

The erosion of dust by a shock wave in air: initial stages with laminar flow

The erosion of dust by a shock wave in air and by the subsequent air flow was investigated theoretically and experimentally. The paths of single particles were calculated for the initial state of erosion when the flow in the shock tube boundary layer was still laminar. High-speed cinematographic experiments performed with a shock tube yielded mapping of the development of the dust cloud. From the agreement between the measured height of the cloud and the calculated height of flight of the particles one can conclude that the assumed model for the motion of the particles adequately describes the removal of particles from the wall.