Turbulent shear flow over complex surfaces: An experimental study

This thesis describes the investigation of the dynamics of turbulent shear flows over non-smooth surfaces. The research was conducted in two parts, related to the experimental facility used in combination with the applied functional surface. The first part describes the experiments of a turbulent Taylor-Couette flow over a riblet surface. The Taylor-Couette facility proves to be an accurate measurement device to determine the frictional drag of surfaces under turbulent flow conditions. Sawtooth riblets are applied on the inner cylinder surface and have the ability to reduce the total measured drag by 5.3% at Res=4.7x104.  Under these conditions, a small shift is observed in the azimuthal velocity profile that indicates the change in the net system rotation, which on its turn affects the quantity of drag change, the so-called rotation effect. A model based on the angular momentum balance is proposed and quantifies the drag change due to the rotation effect. Using the total measured drag change, the model accurately predicts the velocity shift in the azimuthal direction. In addition to the steady operational conditions, periodically driven Taylor-Couette flows were investigated by modulating the velocity between the two cylinders as a sinusoidal function, while maintaining RΩ = 0. The main scaling parameters are the shear Reynolds number Res, the oscillation Reynolds number Reosc and the Womersley number Wo, such that the required power to overcome the frictional drag becomes equal to <Pd> = f(Res,Reosc, Wo). Large velocity amplitudes A = Reosc/Res > 0.10 induce the growth of frictional drag due to the additional turbulent fluctuations. The required power to overcome the frictional drag is given by <Pd> = <Pd,0>(f(A)+ K*Wo4A2). The first term represents the analytical quasi-steady state solution with the accompanying velocity modulation, while the second term involves the magnitude of the boundary acceleration with the associated velocity fluctuation, where K* is the conditional scaling-factor between the additional drag and the dimensionless acceleration. Riblets are still able to reduce the frictional drag under small accelerations of the periodically driven boundaries, but the effect declines drastically or even enhances the frictional drag when the boundary acceleration becomes more significant.   The second part of this thesis describes the assessment of the applied water tunnel and the interactional behavior between a compliant coating and a turbulent boundary layer flow in the tunnel. In the assessment of the water tunnel, the Clauser chart method showed to be a suitable procedure to quantify the local wall shear stress τw. The interaction between a compliant wall and the near-wall turbulent flow was examined by applying in-house produced visco-elastic coatings with three different stiffnesses.  Two typical flow-surface interaction regimes were identified; the one-way coupled regime and the two-way coupled regime. The one-way coupled regime is valid when the turbulent flow initiates moderate coating surface deformation, while the fluid flow remains undisturbed. All of the three coatings exhibited the one-way coupled interactional behavior, where the surface modulations ζ were smaller than the viscous sublayer thickness δv and scale with the turbulent pressure fluctuations over the coating shear modulus, i.e. ζrms ~ prms/|G*|. In this regime, the surface waves have the propagation velocity in the order of cw = 0.70-0.80 Ub, indicating a strong correlation with the high-intensity pressure fluctuations in the turbulent boundary layer away from the wall. The two-way coupled regime has only been observed for the coating with the lowest shear modulus when Ub > 4.5 m/s, indicating significant surface deformation accompanied by additional fluid motions (u',v') and an increase in the local Reynolds stresses. The velocity profile shifts downwards Δu+ in the log region, which verifies the drag increase due to the significant surface undulations. The visualizations of the surface deformation showed the formation of wave-trains with high amplitudes originating from the initial surface undulations caused by the pressure fluctuations in the turbulent boundary layer (i.e. one-way coupling). When these early surface undulations start to protrude the viscous sublayer, the turbulent flow is capable of transfering more energy towards the coating and initiates the wave-train with high amplitudes. The wave-trains dominate the coating surface incrementally with increasing bulk velocity and propagate with a wave velocity of cw = 0.17-0.18 Ub. The 1-way/2-way regime transition is estimated to occur around ζrms > δv/2. The turbulent flow along the slow-moving wave-trains resembles the classical phenomenon of a turbulent flow over a rigid wavy surface, with a local acceleration and deceleration of the fluid. When the wave-trains start to dominate the coating surface, a linear correlation determines the abovementioned downward shift Δu+, based on the wall-normal velocity component dζ/dt. No frictional drag reduction under turbulent flow conditions was found in this study with this type of visco-elastic compliant coatings.