A pilot study to test a method to enhance the turbulence of a water flow in a laboratory flume

A pilot study to test a method to enhance the turbulence of a water flow in a laboratory flume

Suastika, I.K.

Civil Engineering and Geosciences

Hydraulic Engineering

1998-11-01

In this report we present some results of a laboratory pilot study which has been conducted to test a method to artificially enhance the turbulence of a water flow in a laboratory flume. The purpose of this test is to provide data which will be used to verify whether the method is suitable for the study of the dissipative effects of turbulence on surface waves. In order to study the dissipative effects of turbulence on the surface waves, it is essential to enhance the turbulent intensity of the flow. On the other hand, it is desirable to enhance the shear rates of mean flow, and to generate turbulence of relatively large length scales. A strong interaction between the turbulent motion and the surface waves might be expected in cases where the turbulent length scale is of the same order as the wave length of the surface waves. In this study, we investigate the feasibility of a method to intensify the turbulence of the flow and to establish larger structures ("turbulence") in the flow of length scale of order of the surface waves wave length. For this purpose, we disturb the flow in the flume of smooth walls by constructing a number of rectangular plates at the bottom of the flume. As first estimates, we use plates of height of about 20 percent of the water depth and width of about 20 percent of the flume width. The plates were fastened at the bottom on two rails by using T-shaped chords. It is possible to adjust the orientation of a particular plate by rotating it around the horizontal part of the T-chord and by rotating the T-chord itself on the rail around its vertical leg. Further, it is possible to adjust the distance between two subsequent plates on the rail. However, once the plates have been fastened on the rails, it is not possible to change their orientation and position during a particular test. In this series of experiments, we measured the flow velocity at a number of measurement points in a cross section of the flume. From the velocity measurement, a number of flow parameters are determined: mean velocity,standard deviation(r.m.s.-value of the velocity fluctuation),lateral shear stress and mean rate of shear. From the same measurement, we determine also the auto-correlation function of the flow velocity at each measurement point. In the analysis, these parameters and the plot of the auto-correlation function determined from the measurement in the presence of the plates are compared with those determined from a reference measurement, that is the measurement in the absence of the plates. CONCLUSIONS 1. The presence of the plates in the flume changes the mean longitudinal flow velocity; a decrease near the bottom and an increase at higher elevations. 2. The increase of turbulent intensity caused by the vertical plates is very much larger in the locations near the bottom, slightly smaller in the locations near the free surface and, over the whole cross section, very much larger than that caused by the tilted plates. 3. The increase of magnitude of the Reynolds stress caused by the vertical plates is larger than that caused by the tilted plates. 4. The increase of mean rates of shear caused by the vertical plates is smaller in the locations at higher elevations and larger in the locations near the bottom than that caused by the tilted plates. 5. The plates do not introduce turbulence of larger time scales.

turbulence
enhancement
laboratory study
water flow
flume
dissipative effects

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