Numerical processing of flow-visualization pictures – measurement of two-dimensional vortex flow

Abstract

A new system has been developed for estimating experimentally some of the principal physical variables of fluid flows, through flow-visualization and image-processing techniques. Distributions of stream function, vorticity and pressure are calculated by this system with reasonable accuracy for two examples of two-dimensional flow: namely unsteady twin-vortex flow behind a circular cylinder accelerated impulsively to constant speed, and Kármán vortices behind a circular cylinder moving at constant speed. A detailed explanation of the image-processing technique and the numerical calculation process is given first, and then some consideration is given to calculated results in these two types of flow. Comparison shows that some results of the unsteady twin-vortex experiment coincide well with those of previously published experimental investigations and theoretical calculations. Errors introduced at each stage of this system are estimated in some detail.

Tractor Propeller-Pylon Interaction, Part I: Characterization of Unsteady Pylon Loading

“…For the propeller-slipstream plane, pressure fields were computed from the velocity fields by numerically integrating the incompressible Navier-Stokes equations following a finite-difference approach. 20 For this purpose, the Poisson solver outlined in Ref. 19 A SAFEX Twin Fog DP generator with SAFEX Inside Nebelfluid was used for flow seeding.…”

Large-scale flow structures in particle–wall collision at low Deborah numbers

“…6 we show the modulus of the pressure gradient, defined as |∇p| = (∂p/∂x) 2 + (∂p/∂z) 2 . We have computed the pressure gradients starting from the instantaneous velocity field following the procedure suggested by Imaichi and Ohmi [22]. We acknowledge some limitation of the above computation, in particular we disregard the time dependence of the velocity fields, so that we employ the steady two-dimensional Navier-Stokes equations as in [22].…”

Particle Tracing: Revisited

“…However, in order to visualize two components of the velocity in a three-dimensional flow field, illumination of a thin but finite cross-section of the flow is required. A sheet of light can be generated by passing a white light beam through a slit (Wiese Nielson [7], Imaichi and Ohmi [8]). The practical difficulties of collimating the light sheet for uniform thickness or generating a thin light sheet from an incoherent light source can be solved using lasers.…”

“…The photographic recording time is normally controlled by the illumination period. In conventional modem particle tracing methods, either the photographic negative or positive is digitized by digitizing tables ( [8], [9]) or video frame grabbers [10]. The digitizing process is controlled by a host computer.…”

“…The methods that use digitizing tables and human operators to recognize traces and transform them to the digital form do not fall in the category of automatic particle tracing methods. These efforts suffer from the fact that recognizing process depends on the operator's judgement, who should bear in mind the flow direction and use some undefined criterion to identify the position and the length of each trace ( [7] & [8], among others).…”