Title: Visualizing instantaneous Reynolds stresses in turbulent open channel flow: Implications to incipient motion of bed material
by Ahmet Ozan Celik
Criteria for the initiation of bed material motion in rivers and waterways have largely been based on the comparison between mean flow properties and the critical values associated with those properties. The critical values represent the threshold for incipient motion. However, turbulence causes intense variations in flow properties. Instantaneously the extreme values are responsible for the bed material to initiate. Therefore, average quantities in fact do not represent the capability of the flow for initiating the motion.
For a better understanding of the effect of highly random turbulenct flow characteristics on particle entrainment, experimental tools were equipped for synchronized measurements of flow velocity with a 2D LDV system, and particle entrainment (by means of a laser based), in turbulent resolving frequencies (www.hydraulicslab.cee.vt.edu/im).
One of the important observations during the preliminary experiments is addressing the inadequacy of the existing threshold conditions. Such that, in addition to the inability of mean quantities in describing the phenomenon, not all low frequency and high magnitude turbulence fluctuations are triggering the test particle dislodgement. This means that there is still a need for a better definition for threshold conditions. This phenomenon could be described through a visual investigation of the extreme flow events near the bed. The purpose of this project is to explore and visualize the instantaneous behavior of the turbulent flow responsible for particle entrainment using large experimental data sets.
Design surface plots of Reynolds shear stresses (on u«-w« plane ) that will visualize the locations where entrainment takes place.
Animation of Reynolds shear stress surface stretching with time (on u«-w« plane ), to see the burst episodes coupling with entrainment.
Generate overlapped graphs of Reynolds stress components as well as total stress components to depict the associations of both stress components through entrainment process.
Design 3D plots of Reynolds stress components and duration of the corresponding bursts to evaluate the coupling of extreme events with respect to their durations at the instants of entrainment.
PV-Wave and Matlab for the graphics, Matlab for calculations and HTML for web based project presentation.