An experimental and numerical investigation of drag reduction in a turbulent boundary layer using a rigid rodlike polymer

Abstract

The drag reduction in a zero pressure gradient (ZPG) turbulent boundary layer (TBL) using a rigid rodlike polymer was experimentally and numerically investigated. Using injection of the rigid polysaccharide scleroglucan, drag reductions of approximately 10–15 % were observed, with three distinct drag reduction regimes: a non-Newtonian flow region near the injector, followed by a region of nearly constant drag reduction, and finally a region of negligible drag reduction. Decreasing the effective rotary Peclét number reduced the drag reduction effectiveness. Increasing the concentration did not improve the drag reduction, but instead shifted the spatial development of the drag reduction further downstream. A complementary direct numerical simulation of the ZPG TBL using the rigid rod constitutive equation was performed at a matching inlet Reynolds number. The simulation assumed a homogeneous concentration distribution and used estimated effective parameters for the rodlike additive. Spatial evolution of the fiber stresses is rapid and develops asynchronously with the flow structure. The simulated turbulence statistics and experimental measurements at a position 23 boundary layer thicknesses downstream compare favorably, with the primary differences due to the concentration distribution assumed in the simulation.

Enhancement of velocity contrasts by shear-thinning solutions flowing in a rough fracture

“…For a dilute polymer solution, the relaxation time τ may be estimated as the inverse of the value of the shear rateγ 0 corresponding to the upper limit of the Newtonian plateau. This leads to respective values τ = 38 s and 13 s for the 1000 ppm and 500 ppm solutions (a relaxation time τ 10 s of the same order of magnitude, although lower, has been obtained for 1000 ppm scleroglucan solutions from G and G measurements [28]). In the present geometry, the characteristic time associated to the flow field may be taken equal to the transit time over the correlation length of the aperture in the direction the flow: this length has been taken equal in Section 2.1 to the value δ c 0.5 mm of the lag distance for which the normalized correlation function Γ/(2σ 2 e ) is equal to 0.5 (See Fig.…”

Shape effects on dynamics of inertia-free spheroids in wall turbulence

“…Inertia effects on particle dynamics were substantial for St > 1, whereas shape effects were significant for St ≤ 1. The orientation of inertia-free rods St = 0 has been addressed by means of alternative approaches based on a Fokker-Planck-type equation 19 or second-moments of the fiber orientation vector 20 and a detailed comparison between these two methods was made by Gillissen et al 21 This letter presents results for inertia-free spheroidal particles suspended in a turbulent channel flow. Although the inertia-free spheroids passively translate along with the fluid, the particle orientation and rotation strongly depend on the particle shape.…”

On the performance of the moment approximation for the numerical computation of fiber stress in turbulent channel flow

“…1 Similar but less profound effects have also been observed using stiff polymers 2 and macroscopic slender particles. 3 An extensive overview with respect to drag reduction can be found in Ref.…”