Visco-elastic flow past circular cylinders mounted in a channel: experimental measurements of velocity and drag

Abstract Experimental results are described of a flow in a rectangular channel with a width of 20 mm and a height of 0.16 m. In the symmetry plane of this channel we can mount one or two cylinders each with a diameter of 10 mm. In the case of the two cylinders they are displaced with respect to each other in the streamwise direction. In this set-up, the velocity field is measured by means of Laser Doppler Velocimetry (LDV) under various flow conditions and along various traverses. First, a reference experiment is carried out with a Newtonian fluid. For this Newtonian case we have also carried out a numerical computation which can be used to test the accuracy of our measuring system. The computational results agree excellently with the experimental data. The computations also show that the flow in the channel cannot be considered as 2D. After this a series of experiments are discussed which have been carried out with visco-elastic polymer fluids. Material parameters of the fluids, such as the shear viscosity and the first normal stress difference, are obtained with help of rheometric measurements. For the visco-elastic polymer fluids our observations indicate differences with the Newtonian case, especially, in the wake of the cylinders when the velocity is increased. Eventually, at some critical velocity elastic instabilities occur. Finally, we note that, while in a Newtonian fluid the drag varies linearly with the flow rate, this is no longer the case in a visco-elastic fluid, where we find that the drag increases with the square of the flow rate.