An attempt at estimating the instantaneous velocity field of a nonstationary flow from velocity measurement at a few points

Abstract

Generally speaking, velocity measurement devices such as hot-wire and LDV (laser Doppler velocimeter), which may be called Eulerian type flow measurement devices, can record temporally continuous and precise velocity variations. If the flow is laminar, steady or unsteady, or in a transition stage to turbulence, repeated measurements will yield the whole image of the flow. However, even with an array of these devices, the image of the instantaneous flow field cannot be obtained, if the flow is turbulent. Usually, to detect the structure of turbulence conditional ensemble averages are taken. Because of the arbitrariness of the conditions imposed, the result may be different from the true ones. In this paper, it is attempted to reconstruct the instantaneous image of the whole flow field from velocities detected instantaneously at sparse points. The first step is the spatial interpolation of the velocity field by the method of "virtual load" proposed by Hino. The second step is to regulate them by the mass conservation technique which is used in the field of meteorology. The usefulness of the method has been checked by numerical simulations. An example of analysis on the instantaneous feature of a large eddy structure is given.

Reconstruction of a quasi-instantaneous image of coherent structures from hotwire signals obtained by a multi-point simultaneous measurement system

“…The applicability of the MASCON model to the estimation of the whole flow field for laboratory-scale flows has been tested, and some improvements made by Hino et al (1991) for both simulation data and wind tunnel data, and a satisfactory degree of success has been achieved. An example of the effectiveness of the foregoing method is given in fig.…”

“…They applied the conditional sampling technique under low-speed conditions; the ensemble-averaged configuration they revealed is a pair of vortices aligned in the direction of the mean flow, which they calIed a "banana vortex". Unfortunately, as was perceived earlier and pointed out, for instance, by Hino et al (1991), the conditional sampling and ensemble averages with one detector probe for investigating 3-D configurations of coherent motions have a fatal defect in that whole detected signals are averaged without proper discrimination except for a threshold condition, such as a threshold Reynolds stress or a threshold velocity. Consequently, the results obtained by hotwire measurements give a picture of a vortex pair symmetric with regard to the plane on which a detector probe is located, since vortices with different configurations such as asymmetric ones with stronger left and weaker right, or vice versa, will occur with uniform probability.…”