Turbulent shear flow over complex surfaces

Abstract

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 =
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 =
(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.