ON THE DRAG COEFRCIENT OF TRIANGULAR PRORLES Giovanni Menduni* MECCANICA 23 (1988), 147-155 SOMMARIO. Si espongono i.risuttati di un lavoro sperimen- tale volto allo studio del coefficiente di resistenza di un pro- filo triangolare con geometria analoga a quella delle dune fluvialL Gli esperimenti sono condotti in una galleria del vento a bassa velocit?~, misurando direttamente la forza totale esercitata dalla corrente su uno dei profili e provvedendo poi a sottrarre la forza di attrito superficiale stimata. L 'anda- mento del coefficiente di attrito d studiato in funzione del numero di Reynolds. I risultati sono po discussi alia luce di alcuni recenti contributi bibliografici. SUMMARY. Results of a laboratory study on the determi- nation of the drag coeffieien t of triangular profiles simulating a train of sedimentary dunes are here presented. The study is carried out in a low speed wind tunnel by performing a direct measurement of the total drag force acting on one of t!ze profiles and then subtracting the skin shear force to obtain the net pressure drag force. Drag coefficient beha- vior is studied as a function of the profile Reynolds number. Results are then discussed using some recent criteria available in literature. 1. INTRODUCTION The resistance to open channel flow over a cohesionless granular bed is a problem of great technical importance. The mutual interaction between flow, sediment transport and occurrence of various types of bed forms make the physical phenomenon a very complex one and has focused the attention of many researches in the past years. Among the other aspects, bed form geometry has been widely studied in the past few years especially for what is concerns the dune regime, the most common bed form occurrence in nature. This is because the change in bed configuration strongly affects the flow resistance and sedi- ment discharge, introducing additional shear stress due to form drag. Knowledge of dune characteristics is thus of fundamental importance to understand the general behavior of an alluvional channel. Most among the contributions in literature follow an empirical approach substantially based on the analysis of experimental data for deriving relationships between geo- metric characteristics of bedforms and the flow parameters; among the others is here worth to mention Garde et A1. [1], Yalin [2], Vanoni et A1. [3], Ranga Raju et A1. [4], * Associate Professor, Istituto di Idraulica, Politecnico di Milano, Piazza Leonardo da Vinci, 32,1-20133 Milano. Yalin et A1. [5], Van Rijn [6], Menduni & Paris [7]. Other experimental and/or theoretical works (Raudkivi [8], Vittal et AI. [9]), Menduni & Paris [10]) follow, a more <<mechanical>> approach focusing the attention on the energy dissipation downstream the dune crest studying the behavior of the drag coefficient of dunes, defined by the expression 1 (1) Po =--.A. .p.u 2 2 (where F D is the drag force exclusively due to the pressure drop across the dune, A is the cross sectional area of the dune, CD is the drag coefficient, # is the fluid mass density and U is a reference velocity as a function of the flow para- meters. Particularly the work of Menduni & Paris [10] shows how a relationship between Co and U, .A Re, - , (2) v dune Reynolds number (where A is the form height, v the Kinematic viscosity of fluid, u. is the total shear velocity) can be used to derive an efficient predictor for dune steep- ness. The aim of the present paper is to show results of a wind tunnel experimental study for the determination of the drag coefficient of dune-like triangular profiles by a direct measurement of the force exerted by the air flow on the profile. Results are discussed following the approaches proposed by Menduni & Paris [10] and Vittal & A1. [9]. 2. EXPERIMENTAL SET-UP AND PROCEDURE 2.1. Wind tunnel A low speed plexiglas walls wind tunnel has been em- ployed whose technical specifications are reported in Table 1. Table 1. Wind tunnel specifications Location: Hydraulic Laboratory, University of Bologna, Italy. Type: Open circuit, low speed Test section type: Closed Test section shape: Rectangular, .12 x .60 (rn) Test section area: .072 (In2) Test zone length: 3.1 (m) Maximum flow velocity: 70 (m/s) Type of Engine: Asyncronous 380 V/AC Power: 11 kW 23 (1988) 147