ON THE NECESSITY OF APPLYING A ROTATION TO INSTANTANEOUS VELOCITY MEASUREMENTS IN RIVER FLOWS

✍ Scribed by ROY, ANDRÉ G.; BIRON, PASCALE; SERRES, BERNARD DE

• Publisher: John Wiley and Sons
• Year: 1996
• Tongue: English
• Weight: 738 KB
• Volume: 21
• Category: Article
• ISSN: 0360-1269
• DOI: 10.1002/(sici)1096-9837(199609)21:9<817::aid-esp618>3.0.co;2-4

✦ Synopsis

In studies on river channel flow turbulence, it is often the case that the measured mean vertical velocity is different from zero, indicating that the frame of reference of the current meter is not parallel to the flow streamline. This situation affects the estimate of Reynolds shear stress in the streamwise and vertical planes and consequently the analysis of the flow turbulent structure. One way to solve this problem is to correct data by applying a rotation and this is reviewed in the first part of the paper. However, in fluvial geomorphology, the studied flow is often complex and streamlines may exhibit significant changes from one point of measurement to the other. In this context, applying a rotation complicates the situation more than it simplifies it. The second part of this paper examines the question of velocity data correction in complex flows using a field example of the turbulent boundary layer over a very rough gravel bed and a laboratory example taken from flow at a river channel confluence. In both cases, velocity vectors are spatially variable. In the first case, errors in the Reynolds shear stress estimates are relatively low (ranging from -13 to 7 per cent/deg) while in the second case, they are much larger (-200 to 164 per cent/deg). The significance of these errors on the interpretation of turbulence statistics in river channel flows is discussed. We propose that corrections should be applied in all clear cases of sensor misalignment and when the frame of reference changes spatially and temporally. However, no corrections should be used where different flow velocity vector orientations, not sensor misalignment, are responsible for the mean vertical velocity differing from zero.