Drag Reduction In Turbulent Flow Research Articles

A Bio-inspired Approach to Heavy Vehicle Drag Reduction in Turbulent Flow using Shark Dermal Denticles

A Bio-inspired Approach to Heavy Vehicle Drag Reduction in Turbulent Flow using Shark Dermal Denticles

In situ monitor of superhydrophobic surface degradation to predict its drag reduction in turbulent flow

In situ monitoring is the most insightful technique to examine superhydrophobic surface degradation as it provides real-time information on the liquid–solid interface in a continuous, noninvasive manner. Using reflecting-pixel intensity, we introduced a simple method to characterize in situ the air-plastron over a superhydrophobic surface in a turbulent channel flow. Prior to the turbulent experiments, a no-flow hydrostatic test was carried out to determine a critical absolute pressure under which the surfaces are able to maintain the air layer for a prolonged period of time. Pressure-drop and velocity measurements were conducted in a series of turbulent flow tests. Resulting from the coupling effects of normal and shear stresses over the plastron, the air layer was progressively lost with flow time which caused the drag ratio (i.e., the friction factor ratio between superhydrophobic and smooth surfaces) to increase. Meanwhile, the average pixel intensity also increased with time and exhibited a consistent trend with the drag ratio evolution. At a fixed near-wall y/h location (within the viscous sublayer), the velocity increased with time since the shear stress increased. However, a velocity measurement at the center of the channel exhibited a decrease, consummate with an overall downward shift of the velocity profile. Both pressure-drop and velocity results were observed to be correlated with the average pixel intensities of the images captured over the surfaces, and therefore, this is a suitable proxy measure of the plastron. This technique is confirmed to be valid for monitoring the air layer and, hence, predicting the consequent loss of drag reduction.

Effect of Rigid Xanthan Gums (RXGs) on Flow and Pressure Drops to Improve Drag Reduction Rates in Horizontal Pipe Flow

Drag reduction in turbulent flow may be significantly reduced by adding tiny quantities of fiber, polymer, and surfactant particles to the liquid. Different drag-reduction agents have proven to be effective in enhancing the flowability of the liquid when added. This study investigated the potential of decreasing the drag, turbulent flow, and pressure drop in horizontal pipe flow by using a mixture of modified xanthan gums (XGs). Xanthan gums are an environmentally friendly natural polymer complex. They can be extracted from xanthan gum plants and utilized to formulate different concentrations of complexes. The flowability of the xanthan gum was experimentally investigated in a 1-m-long pipe by using addition concentrations of 300 to 950 ppm, an inner diameter of 0.254 inches, and four different flow rates. The results revealed that the pressure drop was reduced considerably with an increase in the concentration of the additives. The mixture (xanthan gums plus water) resulted a favorable reduction in the pressure, which reached 65% at a concentration of 950 ppm. The results of the computational fluid dynamic simulation using the COMSOL simulator showed a change in the fluid velocity profiles, which became more parabolic. This occurred because of an increase in the mean fluid velocity due to the addition of the drag-reducing polymers.

Polymer and surfactant flows through a periodically constricted tube

The flow of three non-Newtonian fluids, comprising polymer and surfactant additives, in a periodically constricted tube (PCT) are experimentally compared. The radius of the tube walls is sinusoidal with respect to the streamwise direction. The three fluids are aqueous solutions of flexible polymers, rigid biopolymers and surfactants, which are typically used for drag-reduction in turbulent flows. Steady shear viscosity measurements demonstrate that rigid and flexible polymer solutions are shear-thinning, while surfactant solutions have a Newtonian and water-like shear viscosity. Capillary driven extensional rheology demonstrates that only flexible polymer solutions produce elastocapillary thinning. Particle shadow velocimetry is used to measure the velocity of each flow within the PCT at five Reynolds numbers spanning roughly 0.5 to 300. Relative to the Newtonian flows, rigid polymer solutions exhibit a blunt velocity profile. Flexible polymer solutions demonstrate a distinct chevron-shaped velocity contour and zones of opposing vorticity when the Deborah number exceeds 0.1. Using the vorticity transport equation, it is revealed that the opposing vorticity zones are coupled with a non-Newtonian torque. The PCT reveals that the surfactant solutions have similar non-Newtonian features as flexible polymer solutions – those being a chevron velocity pattern, opposing vorticity and a finite non-Newtonian torque. This observation is of practical importance since conventional shear and extensional rheometric measurements are not capable of demonstrating non-Newtonian features of the surfactant solutions. The investigation demonstrates that the PCT serves as a viable geometry for showing the non-Newtonian traits of dilute surfactant solutions.

Multistage transitions in viscoelastic turbulent flow: Vortex structures and self-sustaining mechanisms

Viscoelastic dilute polymer solutions are known to display drag reduction in turbulent flows. At constant Re and as the fluid becomes more elastic (increasing Wi), the flow undergoes transitions between several stages of different behaviors. The first is the onset of drag reduction, before which the turbulent friction factor remains indistinguishable from its Newtonian limit. The second is the transition from low-extent (LDR) to high-extent drag reduction (HDR), which increasing evidences have shown to be a qualitative transition between different types of turbulent dynamics. The third is the convergence to the maximum drag reduction (MDR) asymptote, after which the friction factor becomes constant. Although increasing drag reduction with increasing Wi is intuitively expected, transitions between these stages are accompanied by drastic changes in flow statistics and structural patterns, indicating fundamental differences in the underlying turbulent dynamics. Mechanistic understanding into these transitions is far from complete. Our latest work has offered fresh insights into both the LDR-HDR and HDR-MDR transitions. For the former, we developed a new vortex analysis method, called VATIP, and applied it to analyze the differences between the conformation statistics in LDR and HDR regimes. The result links the occurrence of HDR to the dominance of a new turbulence self-sustaining mechanism. For the latter, although a full answer to the MDR problem remains elusive, our analysis of the so-called elastoinertial turbulence (EIT) challenges the long-held presumption of MDR being an ultimate flow state that is no longer affected by polymers. The finding calls for a new direction of thought in constructing a theoretical explanation for MDR.

An implicit conformation tensor decoupling approach for viscoelastic flow simulation within the monolithic projection framework

The highly nonlinear nature of the system governing equations makes it difficult to simulate viscoelastic flows efficiently. In this paper, an implicit decoupling approach is proposed for the viscoelastic flow simulation with a monolithic projection method. The decoupling approach can be derived from the approximate block factorization at the matrix level, which has been successfully applied into the Newtonian flow simulations. In our work, we extend this approach to decouple the pressure, conformation tensor and velocity from the viscoelastic flow system sequentially. Firstly, the pressure-conformation tensor-velocity decoupling is realized by a two-step approximate factorization. By combining the conformation tensor and velocity together at the first-step factorization, the original efficient pressure Poisson solver for the Newtonian flow simulation can be directly adopted in the present framework for non-Newtonian flow simulation. Secondly, the conformation tensor components are further decoupled from each other by the present component-decoupling approach. Then all quantities, including pressure, conformation tensor and velocity, can be resolved without iteration. Additionally, the accuracy corrector is proposed to access the prior scheme limiter-estimation problem introduced by the implicit high-resolution scheme. We numerically demonstrate that all quantities preserve the second-order accuracy in time and space as expected by the theoretical splitting errors. Finally, several different types of canonical benchmark flow examples are conducted to validate the present solver, including laminar flow, turbulent drag-reducing flow and the rotation of a spherical particle immersed in a sheared viscoelastic fluid.

Understanding effect of fluid salinity on polymeric drag reduction in turbulent flows of slickwater fluids

Produced waters are brines drawn during the oil and gas production. These fluids are often re-injected into unconventional reservoirs along with certain additives and proppants in order to induce/sustain fractures and boost oil/gas production. The additives may transform these fluids into drag-reducing slickwater fluids that are pumped at high flowrates (turbulent conditions) to transport proppants at low viscosities. The drag-reducer polymeric additives in the slickwater fluids facilitate high flowrates by reducing drag during the flow. The present work investigated the influence of fluid salinity/hardness on polymer dynamics and its impact on the ability of the polymer to alleviate drag. The study focuses on two synthesized drag-reducing anionic acrylamide (APAM) polymers: first is a copolymer of acrylamide and acrylic acid (Base-APAM), and the second is a ter-polymer of acrylamide, acrylic acid and a functional group (Functionalized-APAM). The polymers were characterized using capillary viscometry, NMR and degree of ionicity.For evaluation of the drag reduction performance of the above polymers, a laboratory flow-loop was built to mimic the turbulent flow conditions in the wellbore. The experimental evaluation was conducted for the fluids with added polymers and range of fluid salinities to encompass plausible variations in the field. Additionally, the fluids’ viscosity performance was evaluated using conventional oil-field viscometers. The obtained experimental data was used to build a semi-empirical correlation that predict the polymer relaxation defined by Weissenberg number i.e., Wi, modelled as a function of fluid salinity and polymeric structure. The drag reduction performance may be derived based on the Re-Wi parameter space, and a correlation for the studied range of parameters has been constructed. The experimental data and models also evidently depicted that the synthesized ter-polymer provides a superior salt-tolerance compared to the co-polymer for the range of parameters studied.

Effects of actuation mode on plasma-induced spanwise flow oscillations

Two different plasma actuation strategies for producing near-wall flow oscillations, namely the burst-modulation and beat-frequency mode, are characterized with planar particle image velocimetry in quiescent air. Both concepts are anticipated to work as non-mechanical surrogates of oscillating walls aimed at turbulent flow drag reduction, with the added benefit of no moving parts, as the fluid is purely manipulated by plasma-generated body forces. The current work builds upon established flow-control and proof-of-concept demonstrators, as such, delivering an in-depth characterization of cause and impact of the plasma-induced flow oscillations. Various operational parameter combinations (oscillation frequency, duty cycle and input body force) are investigated. A universal performance diagram that is valid for plasma-based oscillations, independent of the actuation concept is derived. Results show that selected combinations of body force application methods suffice to reproduce oscillating wall dynamics from experimental data. Accordingly, the outcomes of this work can be exploited to create enhanced actuation models for numerical simulations of plasma-induced flow oscillations, by considering the body force as a function of the oscillation phase. Furthermore, as an advantage over physically displaced walls, the exerted body force appears not to be hampered by resonances and therefore remains constant independent of the oscillation frequency. Hence, the effects of individual parameter changes on the plasma actuator performance and fluid response as well as strategies to avoid undesired effects can be determined.

Stability and drag reduction in turbulent flow of skin with quasi-periodic elastic supports

In order to reduce the flow resistance on the surface of ships, the skin with quasi-periodic elastic supports is designed based on the flexible skin drag reduction technology inspired by dolphin skin. In this paper, the stability characteristic equation of fluid-structure coupling system is established based on small perturbation theory and solved by MATLAB. The results indicate that the skin with quasi-periodic elastic supports has the ability of maintaining stability of turbulent boundary layer when the structural parameters are reasonable. In addition, the drag reduction performance of the skin with quasi-periodic elastic supports is studied by immersion boundary method. The analysis results demonstrate that the skin with quasi-periodic elastic supports can show better drag reduction performance than the skin with periodic elastic supports when the structural parameters q and the support spacing L are rationally set. In a certain speed range, the relative drag reduction rate is up to 72.16%. These optimal distribution parameter data of the skin under certain speed conditions obtained in this paper provide a reference for the practical application and intelligent optimization design of the skin with quasi-periodic elastic supports.

Numerical simulation of drag reduction by microbubbles in a vertical channel

Drag reduction in turbulent flows is a topic of great interest for both internal and external flows, and gas microbubbles interacting with the boundary layer of a liquid flow is one of the ways to obtain such effect. This technique presents the advantage that microbubbles can be easily added and removed from the liquid phase, and does not cause contamination. However, the underlying mechanism responsible for drag reduction is still not fully understood. The present work seeks to shed some more light and help to understand more of such subject. The mutual interactions between microbubbles and turbulence in a vertical upward channel flow using Direct Numerical Simulation with an Euler–Lagrange approach and two-way coupling were examined. The effects of different volume fractions and microbubble sizes at Reτ=150 were analyzed. The liquid-microbubble drag, the weight-buoyancy, lift, and added mass forces were taken into account to represent the interaction between liquid flow and gas microbubbles. Experimental and simulation data from literature are utilized to validate the single- and two-phase flow simulations of the present work. For the same pressure gradient, the addition of microbubbles changed the velocity profile, significantly impacting the flow in the viscous sublayer, and increasing the overall velocity of the flow, which, in turn, point to drag reduction by the addition of the microbubbles. Also, the simulations enabled the identifications of a direct relationship between the drag reduction and the void fraction, and an inverse relationship between the drag reduction and the bubble size.

Statistical interpretation of LDA measurements in naturally developing turbulent drag-reducing flow using invariant theory

Turbulent drag reduction by an extremely diluted aqueous solution of polyacrylamide was studied in a fully developed plane channel flow under non-tripped flow conditions allowing the natural development of turbulence presumably from the initially laminar like a flow state. Using laser-Doppler anemometry, all components of the Reynolds stress tensor were measured. This permitted the determination of the turbulence anisotropy and independent invariants of the anisotropy tensor. Introducing a measure for the departure of the Reynolds stress tensor from the statistically axisymmetric state that ensures laminarity, fundamental insights relating the origin of turbulence and turbulent drag reduction were analyzed in light of the measured results and existing databases of direct numerical simulations. The chief mechanism relevant to the onset of turbulence and turbulent drag reduction deduced from analytic considerations involving only kinematic constraints was confirmed by the experimental data. Parametrization of the drag reduction effect in terms of the Deborah number based on turbulence quantities evaluated at the wall supports the deduction of the polymer-turbulence interaction in terms of the elastic properties of the polymer and the near-wall turbulence structure.

Precise prediction of the drag reduction efficiency of polymer in turbulent flow considering diameter effect

One important issue for drag reduction of turbulent flow with polymers is the pipe diameter effect. Although this scale-up effect has been extensively investigated, there are still debates. For instance, when using the traditional scaling law to predict the friction factor in pipe flow of wide pipe diameters, the relative errors between experimental data and predicted values can be up to 1000%. This paper develops a new method to predict the friction factor in pipe flow applicable to a wide range of pipe diameters. The new method developed in this study transforms the Reynolds number (Re) vs friction factor (f) to log10(Ref) vs 1/f in the Prandtl–von Karman coordinate, where the latter two parameters are correlated on straight lines. We find that the slope and intercept of these straight lines can be predicted by empirical correlations involving diameters and polymer concentrations. Thus, if Re and f from a pipe flow of small diameter are known, these two correlations for slope and intercept can predict the cases for large pipe diameters. With our method, most relative errors between the predicted values and experimental data are within 20%, much better than the traditional scaling law. Finally, we point out the physical meaning of the slope and intercept, and make it clear why the previous scaling law becomes not reliable when it is employed to predict the friction factor for pipe flow with a too wide range of pipe diameters.

Prediction and New Insight for the Drag Reduction of Turbulent Flow with Polymers and Its Degradation Mechanism.

A physicochemical understanding of the mechanism of turbulent flow drag reduction with polymer and its degradation is of great interest from both science and industry perspectives. Although the correlation based on the Fourier series has been proposed to predict the drag reduction and its degradation, its physical meaning was not clear until now. This letter aims to clarify this issue. We develop a comprehensive model to predict the drag reduction and degradation of polymers in turbulent flow from a chemical thermodynamics and kinetics viewpoint. We demonstrate that the Fourier series employed to predict the drag reduction and its degradation is due to the viscoelastic property of drag-reducing polymer solution, and the phase angle in the model, in physical nature, represents the hysteresis of the polymer in turbulent flow. Besides, our new insight of drag reduction with flexible polymers can also explain why a maximum drag reduction in rotational flow appears before degradation happens.

Experimental Investigation of Drag Reduction in Turbulent Flow Using Biological and Synthetic Macromolecules: A Comparative Study

It was shown that the concept of drag-reducing in the pipe flow with the aid of macromolecules is of great importance in practical engineering applications. In this study, the drag-reducing the performance of three biological macromolecules including guar gum (GG), xanthan gum (XG), and carboxymethyl cellulose (CMC) was compared with three synthetic macromolecules including polyethylene oxide (PEO), polyacrylamide (PAM), and polyacrylic acid (PAA). Results showed that all the macromolecules enhanced the DR% except for GG. DR% for almost all of the macromolecules deteriorated with increasing fluid flow rate. On the other hand, DR% enhanced with increasing the pipe diameter for the synthetic polymers but this effect is not obvious for biological polymeric solutions. Maximum DR was 44%, which occur at 1000 ppm concentration of XG at 30 °C and flow rate of 6 l/min and diameter ½ inch. Finally, a new correlation was developed for the prediction of friction coefficient based on the Prandtl-Karman relation with the newly adjusted slope which is a linear function of polymer concentration. This correlation was in excellent agreement with the experimental data.

Speed dependence of integrated drag reduction in turbulent flow with polymer injection

Drag reduction is vitally important for marine applications, and a promising means of reducing the turbulent frictional drag in external flow is polymer injection. In this paper, the integrated drag reduction rate (DR) of polymer injection in turbulent flow is investigated using pressure measurements. As the inlet speed varies 1.0–3.5 m/s, the speed dependence of DR is derived, DR increases with speed at an equivalent normalized polymer flux, and a maximum value of 50.0% is obtained. It is postulated that the injected polymer interacts with the turbulence much more efficiently as the speed increases, which is similar to the drag-reducing behavior of homogeneous polymer flows. The DR performance in polymer-injected flows reported in this paper not only indicates the existence of speed dependence at moderate speeds, but also supplies the reported speed dependence in higher speed tests.

Cooperative drag reduction in turbulent flows using polymer additives and superhydrophobic walls

The injection of long-chain polymer additives and the water-repellent (or superhydrophobic) texturing of submerged solid walls, have both evolved independently over the years into effective, stand-alone methods for frictional drag reduction in wall-bounded turbulent flows. Experiments performed in turbulent Taylor-Couette flow demonstrate that the two techniques, when combined carefully, result in an additive effect, producing significant enhancements in the overall level of drag reduction achieved.

Polymers and Plastrons in Parallel Yield Enhanced Turbulent Drag Reduction

Despite polymer additives and superhydrophobic walls being well known as stand-alone methods for frictional drag reduction in turbulent flows, the possibility of employing them simultaneously in an additive fashion has remained essentially unexplored. Through experimental friction measurements in turbulent Taylor–Couette flow, we show that the two techniques may indeed be combined favorably to generate enhanced levels of frictional drag reduction in wall-bounded turbulence. We further propose an additive expression in Prandtl–von Kármán variables that enables us to quantitatively estimate the magnitude of this cooperative drag reduction effect for small concentrations of dissolved polymer.

Persistence–of–straining and polymer alignment in viscoelastic turbulence

Drag reduction in turbulent flows by the addition of polymers is a phenomenon that has a variety of applications in the engineering. The highly nonlinear interactions between the turbulent flow and the polymers in the polymeric solution still impose challenges to the understanding of this phenomenon. Efforts to address this problem come mainly from two sources: experiments and direct numerical simulations. In the present work we investigate through direct numerical simulations the roles of the non–persistence–of–straining tensor and the covariant strain acceleration tensor from the statistical and their alignment with the conformation tensor. In this regard, we provide a better picture of the relative orientation of this quantity.

Reinforcement learning for bluff body active flow control in experiments and simulations.

We have demonstrated the effectiveness of reinforcement learning (RL) in bluff body flow control problems both in experiments and simulations by automatically discovering active control strategies for drag reduction in turbulent flow. Specifically, we aimed to maximize the power gain efficiency by properly selecting the rotational speed of two small cylinders, located parallel to and downstream of the main cylinder. By properly defining rewards and designing noise reduction techniques, and after an automatic sequence of tens of towing experiments, the RL agent was shown to discover a control strategy that is comparable to the optimal strategy found through lengthy systematically planned control experiments. Subsequently, these results were verified by simulations that enabled us to gain insight into the physical mechanisms of the drag reduction process. While RL has been used effectively previously in idealized computer flow simulation studies, this study demonstrates its effectiveness in experimental fluid mechanics and verifies it by simulations, potentially paving the way for efficient exploration of additional active flow control strategies in other complex fluid mechanics applications.

Drag Reduction in Turbulent Flows by Polymer and Fiber Additives

This article provides a review of the recent progress in understanding and predicting additives-induced drag reduction (DR) in turbulent wall-bounded shear flows. We focus on the reduction in friction losses by the dilute addition of high-molecular weight polymers and/or fibers to flowing liquids. Although it has long been reasoned that the dynamical interactions between polymers/fibers and turbulence are responsible for DR, it was not until recently that progress was made in elucidating these interactions in detail. Advancements come largely from numerical simulations of viscoelastic turbulence and detailed measurements in turbulent flows of polymer/fiber solutions. Their impact on current understanding of the mechanics and prediction of DR is discussed, and perspectives for further advancement of knowledge are provided.