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Pressure- and Velocity-Fields at a Porous Gravel Bed

Instantaneous shear and pressure forces acting on outer grains are the driving mechanisms for destabilisation processes of a mobile bed. Beyond their influence on the stability of a river bed, turbulent structures are also a determining factor for colmation processes and mass transfer in the transition between main flow and subsurface interstitial flow. To describe these mechanisms there is a need for synoptic measurements of velocity- and pressure fields in the main flow as well as within the pore flow within the bed that is interacting with the main flow.

A novel experimental setup enabling a synoptic investigation of the flow and pressure fields within and above a gravel layer has been used. This setup enables synchronous measurements of pressure and velocity within three artificial pores and in open channel flow. A 2-D Particle Image Velocimetry (PIV) system measuring in streamwise vertical or horizontal planes above the gravel layer was used, synchronized with a pressure sensor array of 16 miniaturized piezometric pressure sensors (MPPS). This setup enables the visualization and quantification of instantaneous, simultaneous velocity and pressure fields. Results under stable bed conditions show alternating large-scale wedge-like structures of uniform momentum, inclined at an angle of 10 − 25° to the bed in flow direction. The measurements show a significant pressure drop in regions where fluid with high speed interacts with fluid of lower velocity in the sense of a sweep event. This characteristic pressure drop can lead to the initial lift that is needed for the entrainment of single grains.


Project Director: Prof. Gerhard H. Jirka, Ph.D.
Research Associates:

Dipl.-Ing.Martin Detert

Sketch of MPPS pickup (mm). (click to enlarge.)

Array of MPPS fixed on a grid. (click to enlarge.)

Structures of turbulence, visualisation by tracer particles. (click to enlarge.)

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Detert, M., Hydrodynamic processes at the water-sediment interface of streambeds. doctoral thesis, University of Karlsruhe, downloadable at: http://digbib.ubka.uni-karlsruhe.de/volltexte/1000008267

Detert, M., Weitbrecht, V., Jirka, G.H., Simultaneous Velocity and Pressure Measurements using PIV and Multi Layer Pressure Sensors Array in Gravel Bed Flows,  In: Proceedings of HMEM, Lake Placid 09/2007, E. Cohen et al. (Eds.)

Detert, M., Klar, M., Wenka, T., Jirka, G.H., Pressure- and Velocity-Measurements Above and Within a Porous Gravel Bed at the Threshold of Stability, In: Gravel-Bed Rivers 6, Habersack, H., Piégay, H., Rinaldi, M. (Eds.), Elsevier, 2007

Detert, M., Weitbrecht V., Jirka, G. H., Druck- und Geschwindigkeitsmessungen in und über rauer Sohle, In: Grenzschicht Wasser und Boden, Workshop 09.03.2005, Grabe, J. (Ed.), TUHH, Hamburg, 2005

Detert, M., Jirka, G.H., Klar, M., Jehle, M., Jähne, B., Köhler, H.-J., Wenka, T., Pressure Fluctuations Within Subsurface Gravel Bed Caused by Turbulent Open-Channel Flow, In: Proceedings of the Conference River-Flow2004, Greco, M., Carravetta, A., Della Morte, R. (Eds.), Napoli, Balkema, Rotterdam, 2004

Klar, M., Jehle, M., Jähne, B., Detert, M., Jirka, G.H., Köhler, H.-J., Wenka, T., Simultaneous 3-D PTV and Micro-Pressure Sensor Measurement as Equipment for Subsurface Gravel Layer Flow Analysis. In: Proceedings of the Conference River-Flow2004, Greco, M., Carravetta, A., Della Morte, R. (Eds.), Napoli, Balkema, Rotterdam, 2004

Detert, M., Lang, C., Jirka, G.H., Weitbrecht, V., Projekt Sohlstabilität, Bericht Phase III: Druckfluktuationen im Interstitial bei Belastung durch die Hauptströmung bis zur einsetzenden Destabilisierung, Institutsbericht Nr. 808, Inst. f. Hydromech., Univ. Karlsruhe, 2004

Klar, M., Jehle, M., Jähne, B., Detert, M., Jirka, G.H., Köhler, H.-J., Wenka, T., Analysis of Subsurface Gravel Layer Flow caused by Turblent Open Channel Flow using 3D-PTV and Pressure Sensor Techniques, In: BAW Workshop Boden- und Sohl-Stabilität - Betrachtungen an der Schnittstelle zwischen Geotechnik und Wasserbau, BAW, Karlsruhe, 2004

Detert, M., Kühn, G., Klar, M., Interaction between Turbulent Open-Channel Flow and Pressure Fluctuations in Subsurface Gravel Layer, In: Proceedings of Sedimentations and Sediment Transport, Gyr/ Kinzelbach, Monte Veritá, 2002, Kluwer Academic Publishers, 2003

Lang, C., de los Santos Ramos, F. & Kühn, G. (2001), Projekt Sohlstabilität, Bericht Phase II: Untersuchungen zur Stabilität poröser Gerinnesohlen,Institutsbericht Nr. 782, Inst. f. Hydromech., Univ. Karlsruhe.

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