Drag Enhancement Research Articles

Wake Flow of Dandelion Pappus

There are two kinds of seeds that fly by wind, pappose seeds and winged seeds. The former is known to make parachuting flights, which utilize the drag force acting on the pappus. Each flying seed is designed to have a small rate of descent and to float in the air current as long as possible, according to their environmental conditions. The common dandelion is a classic example of a wind-dispersed plant and facilitates flight by the pappus, which is composed of numerous filaments and gaps. The spatial structures in high porosity pappus are presumed to the key to drag enhancement and long-distance dispersal. In the present study, in order to investigate the relationship between the spatial characteristics of pappus with high porosity and flow behavior around the pappus, smoke flow visualization was performed by changing the porosity of the pappus. The drag coefficient decreases with increasing the porosity of the pappus. However, even if the same porosity, the aerodynamic characteristics were changed by the spatial structure of the pappus.

Alterations in dynamics of counterjet injection technique in presence of real gas effects

Large wave drag is common problem encountered for flights driving at high speeds. It should be dealt with utmost care for better design of long range flights cruising at high speeds. Studies found in the literature only deals with computations or experiment that are carried at lower stagnation temperature or considering perfect gas flow. Thus, the present work incorporates the real gas effects on various flow features like Mach disk diameter, Mach disk location and recirculation pressure. Further the effect of various freestream Mach number (M∞) and freestream total enthalpy (Ho) are observed on wall pressure for blunt body hemisphere considering real gas reactions by means of in house perfect and real gas solvers at hypersonic speeds. Pressure ratio of opposing jet is fixed as 15 while temperature ratio is fixed as 1. Results reveals that the pressure on the object lowers with the increase in M∞ especially upstream of maximum surface pressure point both for the reacting as well as the perfect solver. Surface pressure is found to be much higher at elevated freestream total enthalpy in comparison of smaller freestream total enthalpy for reacting solver while surface pressure almost shows similar trend at all Freestream total enthalpy for perfect solver case. Surface pressure for perfect solver is found to be lower than surface pressure for reacting solver for all the cases studied and this reduction is significant at higher M∞. Drag on body is rising with Freestream total enthalpy but this rate of drag enhancement diminishes at higher M∞.

Relaminarization of spanwise-rotating viscoelastic plane Couette flow via a transition sequence from a drag-reduced inertial to a drag-enhanced elasto-inertial turbulent flow

Direct numerical simulation of spanwise-rotation-driven flow transitions in viscoelastic plane Couette flow from a drag-reduced inertial to a drag-enhanced elasto-inertial turbulent flow state followed by full relaminarization is reported for the first time. Specifically, this novel flow transition begins with a drag-reduced inertial turbulent flow state at a low rotation number $0\leqslant Ro \leqslant 0.1$ , and then transitions to a rotation/polymer-additive-driven drag-enhanced inertial turbulent regime, $0.1\leqslant Ro \leqslant 0.3$ . In turn, the flow transitions to a drag-enhanced elasto-inertial turbulent state, $0.3\leqslant Ro \leqslant 0.9$ , and eventually relaminarizes at $Ro=1$ . In addition, two novel rotation-dependent drag enhancement mechanisms are proposed and substantiated. (1) The formation of large-scale roll cells results in enhanced convective momentum transport along with significant polymer elongation and stress generated in the extensionally dominated flow between adjacent roll cells at $Ro\leqslant 0.2$ . (2) Coriolis-force-generated turbulent vortices cause strong incoherent transport and homogenization of significant polymer stress in the bulk via their vortical circulations at $Ro=0.5 - 0.9$ .

Settling of highly porous and impermeable particles in linear stratification: implications for marine aggregates

The settling velocity of porous particles in linear stratification is affected by the diffusive exchange between interstitial and ambient water. The extent to which buoyancy and interstitial mass adaptation alters the settling velocity depends on the ratio of the diffusive and viscous time scales. We conducted schlieren experiments and lattice Boltzmann simulations for highly porous (95 %) but impermeable spheres settling in linear stratification. For a parameter range that resembles marine porous particles, ‘marine aggregates’, i.e. low Reynolds numbers ($0.05\leq \textit {Re}\leq 10$), intermediate Froude numbers ($0.1\leq \textit {Fr}\leq 100$) and Schmidt number of salt ($\textit {Sc}=700$), we observe delayed mass adaptation of the interstitial fluid due to lower-density fluid being dragged by a particle that forms a density boundary layer around the particle. The boundary layer buffers the diffusive exchange of stratifying agent with the ambient fluid, leading to an enhanced density contrast of the interstitial pore fluid. Stratification-related drag enhancement by means of additional buoyancy of dragging lighter fluid and buoyancy-induced vorticity resembles earlier findings for solid spheres. However, the exchange between density boundary layer and pore fluid substantially increases stratification drag for small$\textit {Fr}$. To estimate the effect of stratification on marine aggregates settling in the ocean, we derived scaling laws and show that small particles ($\leq$0.5 mm) experience enhanced drag which increases retention times by 10 % while larger porous particle (>0.5 mm) settling is dominated by delayed mass adaptation that diminishes settling velocity by 10 % up to almost 100 %. The derived relationships facilitate the integration of stratification-dependent settling velocities into biogeochemical models.

Drag Reduction Using Biomimetic Sharkskin Denticles

This paper explores the use of sharkskin in improving the aerodynamic performance of aerofoils. A biomimetic analysis of the sharkskin denticles was conducted and the denticles were incorporated on the surface of a 2-Dimensional (2D) NACA0012 aerofoil. The aerodynamic performance including the drag reduction rate, lift enhancement rate, and Lift to Drag (L/D) enhancement rate for sharkskin denticles were calculated at different locations along the chord line of the aerofoil and at different Angles of Attack (AOAs) through Computational Fluid Dynamics (CFD). Two different denticle orientations were tested. Conditional results indicate that the denticle reduces drag by 4.3% and attains an L/D enhancement ratio of 3.6%.

Thermal transport in CuCr2X4 ( X=S , Se, Te): Experiment and ab initio calculations

We present a combined experimental and theoretical study of the longitudinal and transverse thermoelectric properties of ferrimagnetic spinels $\mathrm{Cu}{\mathrm{Cr}}_{2}{X}_{4}$ ($X=\mathrm{S}$, Se, Te). The thermoelectric power of all studied phases is positive and consists of two contributions; an almost linear diffusive thermopower observed at high temperature, and a strong enhancement below ${T}_{C}$ attributed to magnon drag. The diffusive part of thermopower and resistivity decreases from $X=\mathrm{S}$ to Te, whereas the magnon drag enhancement is the biggest for $X=\mathrm{Se}$, which has also the highest ${T}_{C}$. The thermopower was calculated by DFT method using GGA, $\mathrm{GGA}+U, \mathrm{GGA}+\text{oeeHyb}$, and mBJ potentials. A good agreement with the experimental thermopower was achieved using GGA potential for $X=\mathrm{S}$ and with mBJ potential for $X=\mathrm{Te}$. The mBJ potential, which was designed for $sp$-type semiconductors, better describes the valence bands of $\mathrm{Cu}{\mathrm{Cr}}_{2}{\mathrm{Te}}_{4}$ that exhibit stronger $sp$-orbitals character than sulfide and selenide. The anomalous Nernst effect (ANE) is negative at room temperature for all phases, the highest absolute value $\ensuremath{\sim}1.5 \ensuremath{\mu}\mathrm{V}$/K is observed for $X=\mathrm{Te}$ and Se around room temperature, whereas ANE for $X=\mathrm{S}$ is much smaller. A sign change of ANE to positive is observed at 285 K for $X=\mathrm{S}$ and at 65 K for $X=\mathrm{Se}$. The trend of anomalous Nernst conductivity is reproduced by Berry phase calculations if the renormalization of the bands due to the strong correlation effects is taken into account.

Energy balance in lubricated drag-reduced turbulent channel flow

Abstract

Retention of rising droplets in density stratification

In this study, we present results from experiments on the retention of single oil droplets rising through a two-layer density stratification. These experiments confirm the significant slowdown observed in past literature of settling and rising particles and droplets in stratification, and are the first experiments to study single liquid droplets as opposed to solid particles. By tracking the motion of the droplets as they rise through a stratified fluid, we identify two timescales which quantitatively describe this slowdown: an entrainment timescale, and a retention timescale. The entrainment timescale is a measure of the time that a droplet spends below its upper-layer terminal velocity and relates to the length of time over which the droplet's rise is affected by entrained dense fluid. The retention time is a measure of the time that the droplet is delayed from reaching an upper threshold far from the density transition. Both timescales are found to depend on the Froude and Reynolds numbers of the system, Fr $=U_u/(Nd)$ and Re $=\rho_u U_u d/\nu$. We find that both timescales are only significantly large for Fr $\lesssim1$, indicating that trapping dynamics in a relatively sharp stratification arise from a balance between drop inertia and buoyancy. Finally, we present a theoretical formulation for the drag enhancement $\Gamma$, the ratio between the maximum stratification force and the corresponding drag force on the droplet, based on a simple force balance at the point of the velocity minimum. Using our experimental data, we find that our formulation compares well with recent theoretical and computational work by Zhang et al. [J. Fluid Mech. 875, 622-656 (2019)] on the drag enhancement on a solid sphere settling in a stratified fluid, and provides the first experimental data supporting their approach.

Electron-phonon drag enhancement of transport properties from a fully coupled ab initio Boltzmann formalism

We present a combined treatment of the non-equilibrium dynamics and transport of electrons and phonons by carrying out \textit{ab initio} calculations of the fully coupled electron and phonon Boltzmann transport equations. We find that the presence of mutual drag between the two carriers causes the thermopower to be enhanced and dominated by the transport of phonons, rather than electrons as in the traditional semiconductor picture. Drag also strongly boosts the intrinsic electron mobility, thermal conductivity and the Lorenz number. Impurity scattering is seen to suppress the drag-enhancement of the thermal and electrical conductivities, while having weak effects on the enhancement of the Lorenz number and thermopower. We demonstrate these effects in \textit{n}-doped 3C-SiC at room temperature, and explain their origins. This work establishes the roles of microscopic scattering mechanisms in the emergence of strong drag effects in the transport of the interacting electron-phonon gas.

Dynamics of a reactive spherical particle falling in a linearly stratified fluid

The behavior of a melting sphere, sinking in a linearly stratified fluid, is carefully investigated using an experimental setup. The melting of a sphere induces a large enhancement of the drag coefficient. A stratification drag enhancement for high Reynolds numbers is also observed. Internal gravity waves generated by wake turbulence redistribute the kinetic energy into the stratified fluid over a long period.

Polymer-induced flow relaminarization and drag enhancement in spanwise-rotating plane Couette flow

Abstract

Statistic estimation of cell compressibility based on acoustophoretic separation data

We present a new experimental method that measures the compressibility of phenotype-specific cell populations. This is done by performing statistical analysis of the cell counts from the outlets of an acoustophoresis chip as a function of the increasing actuator voltage (i.e. acoustic energy density) during acoustophoretic separation. The theoretical separation performance curve, henceforth, Side-Stream Recovery (mathrm {SSR}), vs the piezo-actuator voltage (V) is derived by moment analysis of a one-dimensional model of acoustophoresis separation, accounting for distributions of the cell or microparticle properties and the system parameters (hydrodynamics, radiation force, drag enhancement, and acoustic streaming). The acoustophoretic device is calibrated with polymer microbeads of known properties by fitting the experimental mathrm {SSR} with the theoretical mathrm {SSR}, in which the acoustic energy density is considered proportional to the squared voltage, i.e. E_mathrm {ac}^{}=alpha ,V^2_{}. The fitting parameter alpha for the calibration procedure is the device effectivity, reflecting the efficiency in performing acoustophoretic microparticle displacement. Once calibrated, the compressibility of unknown cells is estimated by fitting experimental mathrm {SSR} cell data points with the theoretical mathrm {SSR} curve. In this procedure, the microparticle compressibility is the fitting parameter. The method is applied to estimate the compressibility of a variety of cell populations showing its utility in terms of rapid analysis and need for minute sample amounts.

Flow and heat transfer in drag-reducing polymer solution flow through the corrugated tube and circular tube

Flow and heat transfer characteristics of xanthan gum (XG) solutions are experimentally studied to compare the drag reduction (DR) and heat transfer reduction (HTR) behavior in the corrugated tube with that in the circular tube. Effects of XG concentration (C) and bulk velocity (v) on drag enhancement (DE), heat transfer enhancement (HTE) and comprehensive performance index (ξ) in the corrugated tube are also examined. The results show that the DR in the corrugated tube increases and then decreases until becomes negative with increasing bulk velocity under the intense turbulent disturbance of the corrugated wall, which is different from the DR behavior in the circular tube, and the onset of DR is found to occur earlier. In addition, the variation trend of HTR is consistent with that of DR’s absolute value, which is closely related to the interaction between drag reduction property and high viscosity property of XG solutions. Furthermore, the HTE and ξ increase with increasing XG concentration and decreasing bulk velocity, and the maximum HTE and ξ can be up to 25 and 1600, which proves the corrugated tube has a specific applicability in terms of energy saving and heat transfer enhancement for high-concentration XG solutions at low velocity.

BTO/P(VDF-TrFE) Nanofiber-based Artificial Lateral Line Sensor with Drag Enhancement Structures

A flexible Artificial Lateral Line (ALL) sensor is presented in this paper, featuring a barium titanate/polyvinylidene fluoride-trifluoroethylene [BTO/P(VDF-TrFE)] nanofiber mat, a hydrogel cupula, and a constriction structure in the canal.. The excellent piezoelectric performance of the BTO/P(VDF-TrFE) nanofiber, superior to that of a pristine P(VDF-TrFE) nanofiber, helps improve the sensitivity of the ALL sensor. The hydrogel cupula imitating the cupula in a fish lateral line system enhances the ALL sensitivity through a material-induced drag enhancement mechanism. The constriction mimics the diminution structure found in fish canal lateral line systems, endowing the canal ALL sensor with enhanced sensitivity through a structure-induced drag enhancement mechanism. The contributions of the hydrogel cupula and constriction structure in offering an enhanced sensing performance are studied experimentally, in comparison with conventional ALL sensors. The constriction structure in the canal helps modify the frequency response of the canal ALL sensor, i.e., attenuating low frequencies while amplifying high frequencies. The proposed biomimetic flow sensor is expected to aid in the development of smart skins for underwater robotics applications.

Bubbly drag reduction using a hydrophobic inner cylinder in Taylor–Couette turbulence

In this study we experimentally investigate bubbly drag reduction in a highly turbulent flow of water with dispersed air at $5.0 \times 10^{5} \leq \text{Re} \leq 1.7 \times 10^{6}$ over a non-wetting surface containing micro-scale roughness. To do so, the Taylor-Couette geometry is used, allowing for both accurate global drag and local flow measurements. The inner cylinder - coated with a rough, hydrophobic material - is rotating, whereas the smooth outer cylinder is kept stationary. The crucial control parameter is the air volume fraction $\alpha$ present in the working fluid. For small volume fractions ($\alpha < {4}\,\%$), we observe that the surface roughness from the coating increases the drag. For large volume fractions of air ($\alpha \geq 4\,\%$), the drag decreases compared to the case with both the inner and outer cylinders uncoated, i.e. smooth and hydrophilic, using the same volume fraction of air. This suggests that two competing mechanisms are at place: on the one hand the roughness invokes an extension of the log-layer - resulting in an increase in drag - and on the other hand there is a drag-reducing mechanism of the hydrophobic surface interacting with the bubbly liquid. The balance between these two effects determines whether there is overall drag reduction or drag enhancement. For further increased bubble concentration $\alpha = {6}\,\%$ we find a saturation of the drag reduction effect. Our study gives guidelines for industrial applications of bubbly drag reduction in hydrophobic wall-bounded turbulent flows.

Forced convective heat transfer of nonlinear viscoelastic flows over a circular cylinder at low Reynolds inertia regime

In this paper, the forced convective heat transfer of viscoelastic fluid flow around a circular cylinder at a low Reynolds number is studied numerically. The effect of viscous dissipation on the problem is modeld which is the main innovative aspect of present study. The Phan–Thien–Tanner (PTT) model is used as the nonlinear constitutive equation. To avoid divergence and stabilize the numerical process in high elastic cases, the log-conformation approach is used. The results indicated that the Nusselt number (Nu) monotonically increases with increasing elasticity number (El), retardation ratio (β), Prandtl number (Pr), and Brinkman number (Br) in a wide range. Both drag reduction (for El < 0.025) and drag enhancement (for El > 0.025) were seen in the numerical results. In high elastic flow regime (El > 10), due to the increased storing nature of viscoelastic fluid compared to the dissipative nature, drag coefficient (CD) and Nu become constant, which causes the fluid to behave like a Newtonian fluid. The temperature-dependent properties were also investigated for all of fluid property, and the large effects of viscosity, and relaxation time were seen in hydrodynamic and thermal parameters. The contribution of normal stresses in viscous dissipation (VD) is much lower than shear stress, due to the lower values of corresponding components in deformation tensor. The elasticity influences the heat transfer by two mchanisms of convection empowering as an indirect way and viscous dissipation as a direct way. Increasing El, β, and ξ increases Nu via the convection mechanism (even at Br = 0), while Nu is decreased with ε. However, Nu increases with the variations of all the parameters, in presence of viscous dissipation. It was also seen that the heat transfer direction changes for Br > 90. Finally, the effects of El, β, ε, ξ, Pr, and Br on the distribution of stress components, velocity, temperature, Nu, and CD were investigated. Two accurate correlations were proposed for the variations of CD and Nu against El, which were validated for a wide range of El.

The correspondence between drag enhancement and vortical structures in turbulent Taylor–Couette flows with polymer additives: a study of curvature dependence

We report direct numerical simulation results that clearly elucidate the mechanism that leads to curvature dependence of drag enhancement (DE) in viscoelastic turbulent Taylor–Couette flow. Change in the angular momentum transport and its inherent link to transitions in vortical flow structures have been explored to depict the influence of the curvature of the flow geometry on DE. Specifically, it has been demonstrated that a transition in vortical structures with increasing radius ratio leads to weakening and elimination of the small-scale Görtler vortices and development and better organization (occupying the entire gap) of large-scale Taylor vortices as also evinced by the patterns of angular momentum current. The commensurate change in DE and its underlying mechanism are examined by contributions of convective flux and polymeric stress to the angular momentum current. The present finding paves the way for capturing highly localized elastic turbulence structures in direct numerical simulation by increasing geometry curvature in traditional turbulent curvilinear flows.

Sedimentation of a small sphere in stratified fluid

An extensive theoretical and numerical study of gravitational settling of small particles in a stratified fluid was performed. Adopting the integral equation on surface traction, we derived the drag enhancement due to stratification at low Reynolds and Peclet numbers, and confirmed with simulations.

Underwater superoleophobicity of a superhydrophilic surface with unexpected drag reduction driven by electrochemical water splitting

In a fluid transport system, fouling and drag force are key issues because they cause additional expenditure for cleaning and fuel consumption, respectively. Various surface treatments, especially wettability control, have been explored to solve the related problems. However, surface modification has inherent limitations because it has conflicting effects on the anti-fouling and drag force. A superhydrophilic surface has an excellent anti-fouling property underwater because of its excellent oleophobicity, but it also causes drag enhancement due to its high affinity towards water. On the other hand, the superhydrophobic surface shows a drag-reducing effect due to the formation of an air layer underwater, but at the same time, it is easily contaminated by oil due to its high oleophilicity. Herein, we suggest a novel method to overcome these conflicting effects for a superhydrophilic surface for anti-fouling and drag enhancement by employing electrochemical gas generation. Due to its large effective surface area and underwater superaerophobicity, the superhydrophilic surface is ideal for use as an electrocatalyst for electrochemical water splitting (EWS). The small and fine air bubbles (~30 μm sizes) generated on a superhydrophilic electrode surface form an air layer, which lowers the fluid density near the surface and induces a drag reduction efficiency of ~26.5% compared to the flat surface. This value is comparable to the superhydrophobic surface, which has a high drag reduction efficiency. The current study presents an original idea that combines the oleophobicity and superaerophobicity of a superhydrophilic surface for realizing EWS-bubble induced anti-fouling and drag reduction effects simultaneously, which has not been reported before.

Turbulent drag reduction in Taylor-Couette flows using different super-hydrophobic surface configurations

Turbulent drag reduction (DR) in an incompressible Taylor-Couette flow configuration using different patterns of “idealized” superhydrophobic surfaces (SHS) on rotating inner-wall is investigated using direct numerical simulations (DNS). Three dimensional DNS studies based on the finite difference method in cylindrical annuli of aspect ratio (Γ) = 6.0 and radius ratios (η) = 0.5 and 0.67 have been performed at Reynolds numbers (Re) 4000 and 5000. The SHS comprised of streamwise or azimuthal microgrooves (MG), spanwise or longitudinal MG, grooves inclined to the streamwise direction (spiral), and microposts. The SHS have been modeled as shearfree areas. We were able to achieve a maximum DR up to 34% for the streamwise aligned SHS, while we got drag enhancement of 4% for the spiral SHS at η = 0.67. The SHS cause slip at the wall as well as near-wall turbulence modification, both governing the DR. We have tried to understand the role of the effective slip and modified turbulence dynamics responsible for DR by analyzing the statistics of mean flow, velocity fluctuations, Reynolds stresses, turbulence kinetic energy (TKE), and near-wall streaks. Most of the results show enhanced production of near-wall streamwise velocity fluctuations and TKE resulting in near-wall turbulence enhancement, yet we observed DR for most of the cases, thereby implying slip to be the dominant contributor to DR in comparison to modified near-wall turbulence.