Abstract
This thesis describes the investigation of the dynamics of turbulent shear flows over nonsmooth 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 TaylorCouette flow over a riblet surface. The TaylorCouette 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 socalled 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 TaylorCouette 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 quasisteady 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 scalingfactor 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 nearwall turbulent flow was examined by applying inhouse produced viscoelastic coatings with three different stiffnesses. Two typical flowsurface interaction regimes were identified; the oneway coupled regime and the twoway coupled regime. The oneway 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 oneway 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 = 0.700.80 Ub, indicating a strong correlation with the highintensity pressure fluctuations in the turbulent boundary layer away from the wall. The twoway 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 wavetrains with high amplitudes originating from the initial surface undulations caused by the pressure fluctuations in the turbulent boundary layer (i.e. oneway 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 wavetrain with high amplitudes. The wavetrains dominate the coating surface incrementally with increasing bulk velocity and propagate with a wave velocity of cw = 0.170.18 Ub. The 1way/2way regime transition is estimated to occur around ζrms > δv/2. The turbulent flow along the slowmoving wavetrains resembles the classical phenomenon of a turbulent flow over a rigid wavy surface, with a local acceleration and deceleration of the fluid. When the wavetrains start to dominate the coating surface, a linear correlation determines the abovementioned downward shift Δu+, based on the wallnormal velocity component dζ/dt. No frictional drag reduction under turbulent flow conditions was found in this study with this type of viscoelastic compliant coatings.
Original language  English 

Qualification  Doctor of Philosophy 
Awarding Institution 

Supervisors/Advisors 

Award date  2 Oct 2020 
Print ISBNs  9789464161526 
DOIs  
Publication status  Published  2020 
Keywords
 Turbulent flow
 Compliant wall
 riblets
 Drag reduction
 fluidstructureinteraction
 Taylor–Couette flow