V. S. R. Somandepalli

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

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 Peclet 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...

Streamwise development of turbulent boundary-layer drag reduction with polymer injection

Zero-pressure-gradient turbulent boundary-layer drag reduction by polymer injection has been studied with particle image velocimetry. Flow fields ranging from low to maximum drag reduction have been investigated. A previously developed technique – the (1 − y /δ) fit to the total shear stress profile – has been used to evaluate the skin friction, drag reduction and polymer stress. Current results agree well with the semi-log plot of drag reduction vs . normalized polymer flux which has been used by previous workers and can be used as a guide to optimize the use of polymer from a single injector. Detailed flow-field statistics show many special features that pertain to polymer flow. It is shown that the mean velocity responds quickly to the suddenly reduced wall shear stress associated with polymer injection. However, it takes a much longer time for the entire Reynolds shear stress profile to adjust to the same change. The Reynolds shear stress profiles in wall units can be higher than unity and this unique feature can be used to further judge whether the flow is in equilibrium. The streamwise evolution of drag reduction magnitude is used to divide the flow into three regions: development region; steady-state region; and depletion region. The polymer stress is estimated and found to be proportional to drag reduction in the depletion region, but not necessarily so in the other regions. The interaction between injected polymer and turbulent activity in a developing boundary-layer flow is dependent upon the flow history and it produces an equally complex relationship between polymer stress and drag reduction. The stress balance in the boundary layer and the dynamical contribution of the various stresses to the total stress are evaluated and it is seen that the polymer stresses can account for up to 25% of the total stress. This finding is in contrast to channel flows with homogeneous polymer injection where the polymer stress is found to account for up to 60% of the total stress.

Concentration flux measurements in a polymer drag-reduced turbulent boundary layer

The drag-reducing action of dilute solutions of long-chain polymers in a flat-plate turbulent boundary layer is studied using particle imaging velocimetry (PIV) and planar laser induced fluorescence (PLIF). The results are used to characterize and quantify the spatial distribution of the injected polymer solution and the downstream development of the DR along the flat plate. The two techniques were used simultaneously to document and study the spread of the injected polymer solution and the resulting changes in the structure and statistics of the turbulence in the boundary layer. The PLIF images provide a qualitative and quantitative measure of the dispersion of the injected polymer solution. The mean and root mean square (r.m.s.) concentration profiles obtained using PLIF showed that the polymer greatly suppressed the turbulent dispersion in the near-wall region. The quantitative concentration measurements across the boundary layer, combined with simultaneous velocity measurements, are used to obtain concentration flux measurements in the boundary layer and are used to study the effect of the turbulence on the dispersion of the injected polymer. The variation of the fluxes with concentration of the injected polymer solutions and with increasing downstream distance is also studied and documented. The action of the polymer is to reduce the streamwise fluxes in the boundary layer, the suppression increasing with concentration. Further, the fluxes are also used to estimate the turbulent Schmidt number ( Sc T ) for the drag-reduced flow. For the polymer injection experiments, the Sc T are all greater than unity with the highest magnitude measured to be around 6, with the magnitude increasing with increasing concentration of the injected solutions. However, for each experiment, the estimated Sc T decreases along the length of the flat plate reflecting the loss of polymer effectiveness.

Boundary Layer Studies on Polymer Drag Reduction Using PIV and PLIF

The structure of turbulence in a drag reduced flat plate boundary layer has been studied with particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF). Drag reduction was achieved by injection of a solution of water-soluble polymer through a spanwise slot near the leading edge of the flat plate. Velocity and concentration data were obtained using PIV and PLIF, respectively, in planes parallel to the wall (x-z plane) and perpendicular to the wall (x-y plane). Measurements of velocity, vorticity and streak spacing were obtained and trends analyzed. For increasing drag reduction, damping of streak oscillations, suppression of streak splitting and merging, streak stabilization and coarsening of the low speed streaks was observed. PLIF measurements of the injected polymer solution showed that regions of high polymer concentration are correlated with the low speed streaks. PIV measurements in the x-y plane showed that there are significant differences in the statistics of turbulence between boundary layers with polymer injection and channel flow with an ocean of polymer.Copyright

Determination of Total Shear Stress and Polymer Stress Profiles in Drag Reduced Boundary Layer Flows With Polymer Injection

Two methods of recovering the entire total shear stress profile from incomplete velocity data in turbulent boundary layers are presented and validated for both DNS simulations and experimental measurements. The first method, an exponential-polynomial curve fit, recovers the whole total shear stress profile well by using the data from the outer part of the boundary layer (y/δ > 0.3). However, this curve fit is sensitive to the quality of the data. The second method, a new (1−y) weighted straight line fit, which is very simple and accurate, has been applied to current experiments of drag reduction in zero pressure gradient turbulent boundary layers with polymer injection. The total shear stress profile obtained from this fit is used to estimate the contribution of the polymer stress to the total shear stress.Copyright