Surface In Laminar Flow Research Articles

Step-induced transition in compressible high Reynolds number flow

The effect of sharp forward-facing steps on boundary-layer transition is systematically investigated in this work in combination with the influence of variations in Mach number, Reynolds number and streamwise pressure gradient. Experiments have been conducted in a quasi-two-dimensional flow at Mach numbers up to 0.77 and chord Reynolds numbers up to 13 million in the Cryogenic Ludwieg-Tube Göttingen. The adopted experimental set-up allows an independent variation of the aforementioned parameters and enables a decoupling of their respective effects on the boundary-layer transition, which has been measured accurately and non-intrusively by means of a temperature-sensitive paint. The functional relations determined between a non-dimensional transition parameter and the non-dimensional step parameters allow the step effect on transition to be isolated from the influence of variations in Mach number, Reynolds number and pressure gradient. Criteria for acceptable heights of forward-facing steps on natural laminar flow surfaces for the examined test conditions are derived from the present functional relations. The measured transition locations are also correlated with the results of linear, local stability analysis for the smooth configuration, enabling the estimation of the step-induced increment of the amplification factor ΔN of Tollmien–Schlichting waves, which can be incorporated in the eN transition prediction method.

Numerical Investigation for the Laminar Flow Effects over Rough Surface

In this paper, the heat transfer effect of rough surface in laminar flow is introduced by fully developed fields for solving the Navier–Stokes equations of incompressible flow in assuming two-dimension using the direction splitting method. Firstly, the algorithm of the incompressible Navier–Stokes equations with pressure correct is carried out. Secondly, the effects of pressure drop and heat transfer and are investigated and discussed in different rough surface elements which are configured with triangular and rectangular element. The Reynolds number, roughness element spacing, and roughness height are also considered as the factors which affect the heat transfer. The results indicate that the parallel present method reaches the stage of basically stable state rapidly and accurately. Compared with the smooth surface, the global performance of heat transfer is improved by the roughness surface since the pressure drop is lost. The effects of triangular element roughness surface on laminar flow and heat transfer are much stronger than rectangular element roughness surface when the spacing of the rough is changed. The flow over the triangular element roughness surface becomes stronger, which contributes to enhancement heat transfer meanwhile increase the pressure drop when the roughness height is higher.

Experimental study of bump effects on boundary-layer transition in compressible high Reynolds number flow

The influence of surface bumps on boundary-layer transition was experimentally investigated in the present work. The experiments were conducted in a (quasi-) two-dimensional flow at low to high subsonic Mach numbers and chord Reynolds numbers up to 10 million in the low-turbulence Cryogenic Ludwieg-Tube Göttingen. Various streamwise pressure gradients relevant for natural laminar flow surfaces were examined. Quasi-two-dimensional bumps, with a sinusoidal shape in the streamwise direction, fixed length and three different heights, were installed on a two-dimensional flat-plate model. The model was equipped with temperature-sensitive paint for non-intrusive transition detection and with pressure taps for the measurement of the surface pressure distributions. Boundary-layer transition was shown to occur at a more upstream location with increasing bump height-to-length ratio. This was mainly due to the local adverse pressure gradient on the downstream side of the bump, which was particularly pronounced in the case of the bump with the largest height-to-length ratio, thereby inducing boundary-layer separation (as verified via oil-film visualizations). In the case of the bump with the smallest height-to-length ratio, bump-induced transition was found to be dependent on global pressure gradient, Mach number and Reynolds number; however, the influence of these parameters on transition induced by bumps with larger height-to-length ratios was significantly reduced. The sensitivity of boundary-layer transition to the effect of the bumps was shown to be more pronounced with stronger global flow acceleration and at smaller Mach numbers.

Solving the Two Objective Evolutionary Shape Optimization of a Natural Laminar Airfoil and Shock Control Bump with Game Strategies

In order to improve the performances of a civil aircraft at transonic regimes, it is critical to develop new computational optimization methods reducing friction drag. Natural laminar flow (NLF) airfoil/wing design remain efficient methods to reduce the turbulence skin friction. However, the existence of wide range of favorable pressure gradient on a laminar flow airfoil/wing surface leads to strong shock waves occurring at the neighborhood of the trailing edge of the airfoil/wing. Consequently, the reduction of the friction drag due to the extension of the laminar flow surface of the airfoil is compensated with an increase of the shock wave induced drag. In this paper, an evolutionary algorithm (EAs) hybridized with different games (cooperative Pareto game, competitive Nash game and hierarchical Stackelberg game) for comparison is implemented to optimize the airfoil shape with a larger laminar flow range and a weaker shock wave drag simultaneously due to a shock control bump (SCB) active device. Numerical experiments demonstrate that each game coupled to the EAs optimizer can easily capture either a Pareto front, a Nash equilibrium or a Stackelberg equilibrium of this two-objective shape optimization problem. From the analysis/synthesis of 2D results it is concluded that a variety of laminar flow airfoils with greener aerodynamic performances can be significantly improved due to optimal SCB shape and position when compared to the baseline airfoil geometry. This methodology illustrate the potentiality of such an approach to solve the challenging shape optimization of the NLF wings in industrial design environments.

Thermal Alternating Polymer Nanocomposite (TAPNC) Coating Designed to Prevent Aerodynamic Insect Fouling.

Insect residue adhesion to moving surfaces such as turbine blades and aircraft not only causes surface contamination problems but also increases drag on these surfaces. Insect fouling during takeoff, climb and landing can result in increased drag and fuel consumption for aircraft with laminar-flow surfaces. Hence, certain topographical and chemical features of non-wettable surfaces need to be designed properly for preventing insect residue accumulation on surfaces. In this work, we developed a superhydrophobic coating that is able to maintain negligible levels of insect residue after 100 high speed (50 m/s) insect impact events produced in a wind tunnel. The coating comprises alternating layers of a hydrophobic, perfluorinated acrylic copolymer and hydrophobic surface functional silicon dioxide nanoparticles that are infused into one another by successive thermal treatments. The design of this coating was achieved as a result of various experiments conducted in the wind tunnel by using a series of superhydrophobic surfaces made by the combination of the same polymer and nanoparticles in the form of nanocomposites with varying surface texture and self-cleaning hydrophobicity properties. Moreover, the coating demonstrated acceptable levels of wear abrasion and substrate adhesion resistance against pencil hardness, dry/wet scribed tape peel adhesion and 17.5 kPa Taber linear abraser tests.

Influence of hydrophobic and superhydrophobic surfaces on reducing aerodynamic insect residues

Insect fouling during takeoff, climb and landing can result in increased drag and fuel consumption for aircrafts with laminar-flow surfaces. This study investigates the effectiveness of various hydrophobic and superhydrophobic surfaces in reducing residue of insects on an aerodynamic surface at relatively high impact speeds (about 45m/s). An experimental setup consisting of a wind tunnel and a method to inject live flightless fruit flies was used to test the effectiveness of various surfaces against insect fouling. Insect fouling was analyzed based on residue area and height from multiple impacts. In general most of the residue area was due to the hemolymph spreading while most of the residue height was due to adhesion of exoskeleton parts. Hydrophobic and especially superhydrophobic surfaces performed better than a hydrophilic aluminum surface in terms of minimizing the residue area of various insect components (exoskeleton, hemolymph, and red fluid). Surfaces with reduced wettability and short lateral length scales tended to have the smallest residue area. Residue height was not as strongly influenced by surface wettability since even a single exoskeleton adhered to the surface upon impact was enough to produce a residue height of the order of one mm. In general, the results indicate that hemolymph spread needs to be avoided (e.g. by having reduced wettability and short lateral correlation lengths) in order to minimize the residue area, while exoskeleton adherence needs to be avoided (e.g. by having oleophobic properties and micro/nano roughness) in order to minimize the residue height. In particular, two of the superhydrophobic coatings produced substantial reduction in residue height and area, relative to the baseline surface of aluminum. However, the surfaces also showed poor mechanical durability on the high-speed insect impact location. This suggests that although low wettability materials show great insect anti-fouling behavior, their durability needs to be substantially improved in order to withstand harsh aerospace conditions.

Synthesis, Characterization and Evaluation of Urethane Co-Oligomers Containing Pendant Fluoroalkyl Ether Groups.

Coatings with the ability to minimize adhesion of insect residue and other debris are of great interest for future aircraft. These aircraft will exhibit increased fuel efficiency by maintaining natural laminar flow over greater wing chord distances. Successful coatings will mitigate the adhesion of debris on laminar flow surfaces that could cause a premature transition to turbulent flow. The use of surface modifying agents (SMA) that thermodynamically orient towards the air side of a coating can provide specific surface chemistry that may lead to a reduction of contaminate adhesion. Aluminum surfaces coated with urethane co-oligomers containing various amounts of pendant fluoroalky ether groups were prepared, characterized and tested for their abhesive properties. The coated surfaces were subjected to controlled impacts with wingless fruit flies (drosophila melanogaster) using both a benchtop wind tunnel and a larger-scaled wind tunnel test facility. Insect impacts were recorded and analyzed using high-speed digital photography and the remaining residues characterized using optical surface profilometry and compared to that of an aluminum control. It was determined that using fluorinated oligomers to chemically modify coating surfaces altered the adhesion properties relative to the adhesion of insect residues to the surface.

Experimental Investigation of the Effect of Forward-facing Steps on Boundary Layer Transition

The effect of geometric forward-facing steps on boundary layer transition was experimentally investigated at a high subsonic Mach number in a blow-down wind tunnel facility in Göttingen, Germany. Boundary layer transition was detected non- intrusively by means of the Temperature-Sensitive Paint (TSP) technique. Forward-facing steps of different height were installed on a spanwise invariant wind tunnel model. Streamwise pressure gradient and high chord Reynolds number were systematically varied and their effect on boundary layer transition was studied in the presence of forward-facing steps on the model surface. At all tested stability situations, the surface imperfections were shown to reduce the extent of the laminar region. Transition was observed to move gradually towards the step location with increasing step Reynolds numbers and increasing relative step height. For a given combination of step height and chord Reynolds number, more pronounced negative pressure gradients led to an increase in transition Reynolds number. The reduction in transition Reynolds number due to the effect of the surface imperfection was more marked at larger flow acceleration. The plots of the relative change in transition location as a function of the step Reynolds number and of the relative step height gave good correlation of the results. The correlations were found to be practically independent of the streamwise pressure gradient in the examined range. Criteria for the allowable tolerances on low- sweep Natural Laminar Flow surfaces can now be derived from the functional relations determined in this work.

The evaluation of hierarchical structured superhydrophobic coatings for the alleviation of insect residue to aircraft laminar flow surfaces

Surface contamination caused by insects on laminar flow wing surfaces causes a disruption of the flow, resulting in an increase in drag and fuel consumption. Consequently, the use of superhydrophobic coatings to mitigate insect residue adhesion was investigated. A range of hierarchical superhydrophobic coatings with different surface chemistry and topography was examined. Candidate coatings were characterized in terms of their morphology and hydrophobic properties by scanning electron microscopy (SEM) and static and dynamic contact angle measurements, respectively. Arithmetic mean surface roughness ( R a ) values were measured using profilometry. Only superhydrophobic coatings with a specific topography showed complete mitigation against insect residue adhesion. A surface which exhibited a specific microstructure ( R a = 5.26 μm) combined with a low sliding angle (SA = 7.6°) showed the best anti-contamination properties. The dynamics of an insect impact event and its influence on the wetting and adhesion mechanisms of insect residue to a surface were discussed.

Frost growth and densification on a flat surface in laminar flow with variable humidity

Experiments are performed concerning frost growth and densification in laminar flow over a flat surface under conditions of constant and variable humidity. The flat plate test specimen is made of aluminum-6031, and has dimensions of 0.3 m×0.3 m×6.35mm. Results for the first variable humidity case are obtained for a plate temperature of 255.4K, air velocity of 1.77m/s, air temperature of 295.1K, and a relative humidity continuously ranging from 81 to 54%. The second variable humidity test case corresponds to plate temperature of 255.4K, air velocity of 2.44m/s, air temperature of 291.8K, and a relative humidity ranging from 66 to 59%. Results for the constant humidity case are obtained for a plate temperature of 263.7K, air velocity of 1.7m/s, air temperature of 295K, and a relative humidity of 71.6%. Comparisons of the data with the author's frost model extended to accommodate variable humidity suggest satisfactory agreement between the theory and the data for both constant and variable humidity.

Biomimetic structures for fluid drag reduction in laminar and turbulent flows

Biomimetics allows one to mimic nature to develop materials and devices of commercialinterest for engineers. Drag reduction in fluid flow is one of the examples found in nature.In this study, nano, micro, and hierarchical structures found in lotus plant surfaces, as wellas shark skin replica and a rib patterned surface to simulate shark skin structure werefabricated. Drag reduction efficiency studies on the surfaces were systematically carried outusing water flow. An experimental flow channel was used to measure the pressure drop inlaminar and turbulent flows, and the trends were explained in terms of the measured andpredicted values by using fluid dynamics models. The slip length for various surfaces inlaminar flow was also investigated based on the measured pressure drop. Forcomparison, the pressure drop for various surfaces was also measured using airflow.

Numerical Predictions of Heat Transfer and Flow Characteristics of Heat Sinks with Ribbed and Dimpled Surfaces in Laminar Flow

Steady flow characteristics and surface Nusselt number distributions of heat sinks with smooth, ribbed, and dimpled surfaces are investigated in laminar flow using the numerical code FLUENT 6.2.16. Presented are local and spatially averaged surface Nusselt numbers, as well as distributions of numerically predicted flow characteristics within the heat sink passages. These include distributions of static pressure, velocity, and streamwise vorticity in flow cross-sectional planes. These results are valuable because of the design information provided to optimize heat sink thermal performance, without the use of costly and time-consuming experiments.

An investigation of surfactant and enzyme formulations for the alleviation of insect contamination on Hybrid Laminar Flow Control (HLFC) surfaces

Hybrid Laminar Flow Control (HLFC) is an active drag reduction technique that requires a small amount of air to be sucked through a porous skin surface, thus stabilising the boundary layer and permitting extended laminar flow along the wing surface. Contamination of the laminar flow surfaces by insects is a major concern for this technology. An overview of insect contamination and mitigating solutions is provided as background to research undertaken. The paper reports on an experimental investigation into the potential use of surfactants and enzymes as additives to contamination alleviation fluids. It was demonstrated that both approaches have merit. It is concluded that the use of fluorosurfactant containing solutions is preferable to enzyme borne solutions, partly due to the need to control the temperature of the enzyme solution.

Particle Deposition onto a Permeable Surface in Laminar Flow

A theoretical investigation of particle deposition onto a permeable surface in a parallel-plate channel is presented in this paper. The convective diffusion equation is rigorously formulated with the inclusion of lateral transport due to permeation drag and inertial lift, and transport due to gravitational, double layer, and van der Waals forces. A numerical procedure for solving the governing equation is also presented. The effects of particle size, permeation velocity, solution ionic strength, cross-flow velocity, and particle density on the initial rate of particle deposition are investigated. Results point out that the local and average particle deposition rates onto a permeable surface are determined by an interplay between several transport and interaction mechanisms, among which permeation drag, electric double layer repulsion, and inertial lift are most important.

Mass Transfer to a Continuous Moving Surface

Asymptotic correlations are presented for the rate of mass transfer to a continuous moving flat surface in laminar flow. A laboratory cell has been developed to test the theory for uniform surface concentrations. The limiting current measurement was carried out for the reduction of ferricyanide ion over the range of and . The experimental value agrees well with the theory for .

Heat Transfer in a Laminar Boundary Layer with Constant Fluid Properties and Constant Wall Temperature

A simple method is given for the calculation of the heat transfer from a laminar flow surface. Computation is by a quadrature. The method is essentially a simplification and extension of the Eckert(1) method, and is applicable both to two–dimensional and to axisymmetric flows.