Boundary Layer Velocity Profiles Research Articles

Near-Wall Flow Measurements Using Frequency-Modulating Filtered Rayleigh Scattering (FM-FRS)

The frequency-modulating filtered Rayleigh scattering (FM-FRS) technique has been developed and applied for the boundary layer measurements. The FM-FRS technique effectively extracts Rayleigh scattering information from noisy low SNR signals caused by intense wall glare. The boundary layer velocity profiles measured by FM-FRS show excellent agreement with an independent pressure probe measurement and the law-of-the-wall, including approximately 100 μ m above the wall. This technique is desirable for the practical applications of the Rayleigh scattering technique for flow diagnostics to regions that were limited due to strong background and low signal-to-noise ratio.

Accurate Method for Estimating Wall-Friction Based on Analytical Wall-Law Model

A novel method is proposed for accurately determining the local wall friction through the near-wall measurement of time-average velocity profile in a Type-A turbulent boundary layer (TBL). The method is based on the newly established analytical wall-law in Type-A TBL. The direct numerical simulations (DNS) data of turbulence on a zero-pressure-gradient flat-plate (ZPGFP) is used to demonstrate the accuracy and the robustness of the approach. To verify the reliability and applicability of the method, a two-dimensional particle image velocimetry (PIV) measurement was performed in a ZPGFP TBL with a low-to-moderate Reynolds number (Re). Via utilizing the algorithm of single-pixel ensemble correlation (SPEC), the velocity profiles in the ZPGFP TBL were resolved at a significantly improved spatial resolution, which greatly enhanced the measurement accuracy and permitted us to accurately capture the near-wall velocity information. The accuracy of the approach is then quantitatively validated for the high Reynolds number turbulence using the ZPGFP TBL data. The research demonstrates that the current method can provide the precise estimation of wall friction with a mean error of less than 2%, which not only possesses the advantage of its insensitivity to the absolute wall-normal distance of the measuring point, but also its capability of providing an accurate prediction of wall shear stress based on fairly sparse experimental data on the velocity profile. The current study demonstrates that the wall shear stress can be accurately estimated by a velocity even at a single-point either measured or calculated in the near-wall region.

Control of linear instabilities in two-dimensional airfoil boundary layers

The present study deals with the control of linear, modal and non-modal growth of perturbations in a trapezoidal wing boundary layer. The asymmetric NACA 2515 airfoil is chosen for the analysis. The boundary layer velocity profiles, at different locations on the airfoil, are obtained using the Falkner–Skan equations. The non-modal linear stability theory is used to obtain the amplification of the initial perturbations, for which the state-space approach is adopted. The control is brought about using a Linear Quadratic Regulator (LQR) controller by the means of addition of fluid momentum in the wall-normal direction from the wing surface. Maximum transient energy growth is calculated across a range of wavenumber pairs, and control is applied to the pair with the highest energy growth. In this numerical analysis, we show that the transient growth is higher towards the trailing edge because of the adverse pressure gradient of the flow. Upon control actuation, the reduction in the transient energy growth of perturbations is achieved up to a maximum of 54.6%.

Study on the film thickness and surface wave velocity of the thin liquid film formed by a round jet obliquely impinging on a horizontal plate

For a thin liquid film (in a supercritical flow) prior to the formation of a non-circular hydraulic jump formed by a round jet obliquely impinging on a horizontal plate, the time-averaged film thickness and the surface wave velocity are extracted based on the measured transient film thickness. On the one hand, the effect of many factors, including the jet velocity, impingement angle, azimuthal angle, liquid viscosity, and surface tension, on the time-averaged film thickness and surface wave velocity are discussed. When the jet Reynolds number increases to about 1.4×104, the film thickness profile suddenly increases, and the transition of liquid flow from laminar to turbulent occurs. Meanwhile, a rapid increase is observed downstream of the turbulent film thickness profile. The influence of surface tension on the time-averaged film thickness and surface wave velocity is negligible for thin liquid films before non-circular hydraulic jumps. Nonetheless, the surface tension has a significant influence on the interface profile of non-circular hydraulic jumps. Furthermore, a “crescent” kink region upstream of the jump can be identified when the surface tension is lower than 40.6 mN/m. On the other hand, experimental results are used to verify the prediction accuracy of existing approximate solutions. The laminar approximate solution with a quadratic boundary layer velocity profile can accurately predict the film thickness distribution of most laminar thin liquid films, except downstream of the thin liquid films with a dynamic viscosity higher than 9.71 mPa s. The surface wave velocities are found to be close to the predicted surface velocities of the approximate solutions.

Stall control on the wind turbine airfoil via the single and dual-channel of combining bowing and suction technique

Based on the single-channel jet control strategy with a suction slot on the upper surface and injection slots near the trailing edge on the lower surface, proposed in this work is a novel flow control technique, known as the dual-channel jet, which amalgamates the beneficial aspects of leading-edge suction and a suction slot positioned downstream of the airfoil's maximum thickness position. To demonstrate the superior aerodynamic characteristics of the dual-channel over the single-channel, comprehensive comparisons of three distinct control strategies are made with two jet momentum coefficients employed on the S809 airfoils. Simulations using Reynolds-averaged Navier-Stokes with a transitional SST turbulence model are performed, showing that the position of a suction slot in the single-channel significantly impacts its ability to inhibit separation, while the dual-channel jet facilitates a fuller boundary layer velocity profile and a posterior shift in the stagnation point, outperforming the single-channel approach in removing low-momentum fluid and increasing the airfoil virtual camber. In the case with a jet strength of 0.004 and pitching oscillation, compared to the clean airfoil, the dual-channel achieves a remarkable 84.21 % increase in the lift coefficient at 18.1°, and the control efficiency can further be enhanced with the momentum coefficient increased.

Critical analysis of generalized Newtonian fluid flow past a non‐linearly stretched curved surface: A numerical solution for Carreau model

AbstractThe underlying intention of the present work is to elaborate the boundary layer flow of Carreau fluid over a non‐linear stretching curved surface. Firstly, we derived the equation of motion for a two‐dimensional curved surface using the Carreau constitutive relation. Employing the well‐known approximations of the boundary layer theory (order of magnitude analysis), the terms of higher and next order have been neglected. We developed an appropriate similarity transformation that reduced the considered partial differential equation into an ordinary differential equation (self‐similar formulation). The MATLAB built‐in function usually known as bvp5c is utilized to get the numerical solution of the considered problem. The impact of the power‐law index (n), Weissenberg number and curvature parameter (k) on velocity profile and skin friction are analyzed through several graphs and tables. The obtained results are also verified by employing the shooting method through Maple software. The results reveal that both boundary layer thickness and velocity profile increase by enlarging the dimensionless curvature parameter of the curved surface.

Assessing low Reynolds number airfoil performance in microburst environments for micro/unmanned aerial vehicles: A numerical study

This study aims to investigate the impact of microbursts, which are a common meteorological phenomenon, on aircraft performance. A multiphase computational fluid dynamics model has been developed to simulate the microburst-generated downdraft environment over an airfoil. The simulations were conducted at a 12° angle of attack for takeoff conditions with a Reynolds number of 2×105. The lift and drag coefficients were compared to experimental results and exhibited a strong correlation. Additionally, this study examined the volume of fluid and the coefficient of pressure distribution. It also examined the boundary-layer velocity profiles at five different chord positions, the location of rivulet formation on the upper surface of the airfoil, and water film height. The results revealed a maximum variation of 72.18% and 16.53% in lift and drag coefficients, respectively, between the numerical and experimental results. These findings provide valuable insight into the effects of microbursts on aircraft performance and have implications for the aviation industry.

On the development of a generalized atmospheric boundary layer velocity profile for offshore engineering applications considering wind–wave interaction

Within industrial standards, effects due to wind–wave interaction on the marine atmospheric boundary layer (ABL) may be included via the Charnock sea-surface roughness parameter for open-sea and near-coastal waters. This roughness parameter does not accommodate the wide variety of wave states possible, nor does it modify the ABL to include speed-up effects resulting from an undulating wave profile. In an attempt to provide an updated definition of the general ABL profile for offshore wind engineering applications, an experimental and numerical study is performed to assess the interaction of the ABL on a fixed wave geometry. By extracting the mean velocity field during the wind–wave interaction, bespoke values of the velocity profile power exponent and wind risk factor can be obtained. A range of wave heights and wave lengths are considered from which a Kriging surrogate model is trained to supply the relevant wind profile parameters depending on the wave state. These newly garnered parameters enable the practitioner to define a reference wind velocity, and then adjust the velocity profile characteristics to contain the influence of the wave. This approach provides a new inlet velocity condition that can be used within computational wind engineering investigations without the need to explicitly model the wave surface, as well as flexibility in specifying the underlying wave conditions. Application of the re-calibrated velocity profile shows that wind forces are significantly greater throughout an offshore wind turbine (OWT) swept blade area when large (H= 15 m) and small (H= 1.5 m) wave heights are compared.

Hydrodynamic Forces on a Near-Bottom Pipeline Subject to Wave-Induced Boundary Layer

Abstract Hydrodynamic forces on small diameter subsea pipelines and cables placed near seabed are important for their on-bottom stability design. In offshore environments, these pipelines are usually subjected to extreme wave conditions. The present study investigates hydrodynamic forces acting on a pipeline near a flat seabed subjected to a wave-induced boundary layer flow. The Keulegan–Carpenter numbers of the wave-induced boundary layer flow are 20, 140, and 200, defined based on the pipeline diameter (D), the maximum velocity of the undisturbed near-bed orbital velocity (Uw), and the period of the incoming oscillatory flow (Tw). Reynolds number is 1 × 104 based on Uw and D. A seabed roughness ratio ks/D (ks is the Nikuradse equivalent sand roughness) of up to 0.1 and different gap ratios of G/D = 0.05–0.5 between the pipeline and the seabed are considered. Numerical simulations have been carried out based on two-dimensional (2D) unsteady Reynolds-averaged Navier–Stokes equations combined with the k–ω shear stress transport turbulence model. A preliminary one-dimensional (1D) simulation is carried out to obtain a fully developed wave-induced boundary layer velocity profile, which is used as inlet flow for the 2D simulations. The numerical model is validated against the experimental data reported by Sumer et al. [1991, “Effect of a Plane Boundary on Oscillatory Flow Around a Circular Cylinder,” J. Fluid Mech., 225, pp. 271–300] at KC = 10. Influences of KC, ks/D, and G/D on the hydrodynamic forces and the surrounding flows are discussed in detail.

Effect of wall temperature in streamwise supersonic corner flow

The interaction between corner flow and heat transfer is very common in the internal and external flow of hypersonic vehicle surface, but it is not clear how the wall thermal boundary condition affects the supersonic corner flow boundary layer. In the present study, the Reynolds stress model is used to solve the Reynolds-averaged Navier–Stokes equations. The results show that when the symmetrical wall temperature Tw is less than or equal to the recovery temperature Taw, the mean velocity profile in the corner zone is similar to the plate zone, and the temperature is higher in the corner zone. At heating wall condition of Tw>Taw, both velocity and temperature profile of outer corner boundary layer are different from that in the plate zone. The near-wall temperature distribution increases relatively, and the temperature profile is no longer similar to the law of the wall when Tw rises. Along the spanwise wall, the deviation of the velocity–temperature quadratic curve and the generalized Reynolds analogy (GRA) relation is within 10%. In the supersonic corner flow, the velocity–temperature relation under non-adiabatic wall condition satisfies the GRA relation. The effect of wall temperature on corner boundary layer velocity and temperature is significant. It is necessary to consider wall thermal boundary condition in the supersonic corner flow boundary layer analysis.

Flow Characterization of the UTSA Hypersonic Ludwieg Tube

The characterization of a hypersonic impulse facility is performed using a variety of methods including Pitot probe scans, particle image velocimetry, and schlieren imaging to verify properties such as the velocity, Mach number, wall boundary layer thickness, and freestream turbulence intensity levels. The experimental results are compared to the numerical simulations of the facility performed with Ansys Fluent to compare the design and operational conditions. The presentation of results in this manuscript is prefaced by a description of the facility and its capabilities. The UTSA Ludwieg tube facility can produce a hypersonic freestream flow with a Mach number of 7.2 ± 0.2 and unit Reynolds numbers of up to 200 × 106 m−1. The Pitot probe profiles of the 203-mm-square test section indicate a 152 ± 10 mm square freestream core with turbulence intensity values ranging from 1% to 2%. Schlieren imaging of the oblique shockwaves on a 15° wedge model provided an alternate means of verifying the Mach number. Particle image velocimetry and previous molecular tagging velocimetry results showed a good agreement with the Pitot probe data and numerical simulations in the key parameters including freestream velocity, wall boundary layer velocity profiles, and wall boundary layer thickness.

COMPARATIVE ANALYSIS OF TURBULENCE MODELS FOR SIMULATING FLOW OVER A FLAT PLATE

This research paper presents a comparative analysis of turbulence models for simulating the flow over a flat plate. The accurate prediction of turbulence characteristics is crucial for various engineering applications, including aerodynamics and heat transfer. In this study, three commonly used turbulence models, namely the k-ε, k-ω, and Laminar, are evaluated and compared for their ability to capture the flow behaviour over a flat plate. The simulations are conducted using Ansys, a widely-used computational fluid dynamics (CFD) software package. The flow conditions and geometry are carefully defined to ensure a consistent and controlled setup across the turbulence models. The key parameters of interest include boundary layer thickness, velocity profiles, and flow separation characteristics. The results obtained from each turbulence model are compared to provide insights into their performance and limitations. The comparative analysis focuses on the accuracy, computational efficiency, and ability to capture complex flow phenomena exhibited by the turbulence models. The findings of this study contribute to the understanding of the turbulence modelling techniques for simulating flow over a flat plate. The outcomes can aid researchers and engineers in selecting the most appropriate turbulence model for similar flow scenarios in various engineering applications.

Separation control with elliptical air-jet vortex generators

The flow organisation of air-jet vortex generators (AJVGs) of elliptical cross sections and their control effectiveness on a 24^{circ }-compression-ramp-induced shock-wave/boundary-layer interaction was analysed on the basis of experiments at M_{infty } = 2.52 and Re_{theta _c} = 8225. We investigated a circular orifice and two elliptical orifices of aspect ratios 0.5 and 2; all characterised by the same hydraulic diameter. Measurements of separation lengths from oil-flow visualisation and PIV reveal that elliptical AJVGs achieve a 25% reduction in total separation length, which constitutes a strong improvement over the 17% reduction achieved with the commonly used circular AJVGs. The jet-induced structures from elliptical AJVGs penetrate on average 25% farther into the boundary layer. However, the lateral spread is limited to a maximum value equal to the inter-jet spacing in the control array, which highlights the onset of jet/jet interactions between adjacent jets in the array. A consequence of these interactions is better flow entrainment for the elliptical cases, as observed in the mean boundary-layer velocity profiles and an improved turbulent mixing (indicated by an increase in Reynolds-shear-stress magnitude).

Large-Eddy Simulation of Boundary-Layer Transition over a Zigzag Trip

Wall-resolved large-eddy simulations (WR-LES) are conducted to study transitional and turbulent boundary-layer properties and flow structures downstream of a zigzag trip on the low-Reynolds-number SD7032 airfoil at and and angle of attack . Without a zigzag trip, the airfoil features a prominent laminar separation bubble on the suction side. First, a verification and validation study of the LES method is conducted for the zigzag trip at a roughness-height-based Reynolds number . The boundary-layer velocity profiles and the integral parameters are compared with experimental data acquired using a long-distance -particle image velocimetry setup in a wind tunnel. In the LES, if the zigzag tape is thick enough to trigger transition just behind the tape, the transition to turbulence is generally quicker than what was observed in the experiment. Although the general phenomenology and the development of the turbulent boundary layer after transition are consistent with the experimental data, there remain some differences in the exact length of the transitional region and also in the associated airfoil drag. A zigzag tape with or thicker induces transition quickly behind the trip through a shedding process of the shear layer created by the tape. The boundary layer behind the zigzag tape features prominent three-dimensional vortical structures emanating from the zigzags that persist over long chordwise distances and well into the turbulent boundary layer. Nevertheless, in a spanwise-averaged sense, the integral thicknesses of the newly created turbulent boundary layer are actually quite similar to a two-dimensional boundary-layer theory solution, given that the tape has a suitable thickness. In contrast, an overly thick tape will generate a thicker turbulent boundary layer, where the increased displacement and momentum loss are sustained and also contribute to total drag.

Influence of melting heat transfer and chemical reaction on the flow of non-Newtonian nanofluid with Brownian motion: Advancement in mechanical engineering

The aim of the present study is to investigate the melting heat and mass transport characteristics on the stagnation point flow of Powell–Eyring nanofluid over a stretchable surface because melting is so important in many processes, such as Permafrost melting, magma solidification, and thawing of frozen grounds, are all examples of soil melting and freezing around the heat exchanger coils of a ground-based pump. The developing mathematical model under the boundary layer flow in terms of differential equations is solved through a numerical algorithm using a boundary value problem solver bvp4c/shooting technique with the help of MATLAB software. The impact of emerging parameters on the velocity profile, temperature profile, and concentration profile is elaborated graphically. The profile and boundary-layer width rate for the value stretching parameter less than one rises when A enhances while the thickness of boundary layer velocity profile for the value stretching parameter greater than one decreases as A. The velocity function shows a decrement response for M, while the opposite behavior is seen against the concentration field. Furthermore, the numeric data for the friction factor and Nusselt number are demonstrated in tabular form, and the result shows a remarkable agreement with the previously published data.

Control of separated flow transition over a highly loaded compressor blade via dynamic surface deformation

The laminar separation and transition process seriously deteriorate the aerodynamic performance of a compressor blade at a low Reynolds number (Re). To suppress the laminar separation without reattachment over a highly loaded compressor blade at Re=3.5 × 105, an active flow control strategy based on dynamic surface deformation (DSD) is numerically investigated by large eddy simulations (LESs). The control mechanism of DSD is analyzed, and the vortex dynamics in the transition process are resolved. Furthermore, the loss mechanism is clarified based on the turbulent kinetic energy (TKE) budget analysis. For the uncontrolled case, the rolling-up and breakdown of large-scale hairpin vortices accelerate the generation of turbulent fluctuations, which distinctly extend the region of high-level TKE production term and thus cause large total pressure loss. As the DSD is activated at the oscillation frequency f0 (f0 is the vortex shedding frequency in separated shear layers), periodic pressure fluctuation waves are induced with the appearance of small-scale vortices. The convection of these vortices enlarges the local vorticity and makes the boundary layer velocity profile fuller, which successfully eliminates the large-scale laminar separation on the compressor blade. As such, the region of high-level TKE production term shrinks distinctly due to the weakened vortex dynamics, and the total pressure loss is reduced by 78.9%. With increasing the oscillation frequency from f0 to 2f0 or 4f0, the loss is slightly enlarged due to the rapid growth of turbulent boundary layers, but it is still lower than that of the uncontrolled case. The results demonstrate that DSD is feasible in controlling large-scale laminar separation, and there exists an optimal oscillation frequency.

Influence of train roof boundary layer on the pantograph aerodynamic uplift: A proposal for a simplified evaluation method

The mean contact force between pantograph collectors and contact wire is affected by the aerodynamic uplift generated by aerodynamic forces acting on pantograph components. For a given pantograph geometry, orientation and working height, aerodynamic forces are strongly influenced by the position of the pantograph along the train roof, since an aerodynamic boundary layer grows along the train.This paper shows the experimental results of aerodynamic uplifts of full-scale pantographs located at four different positions along the roof of a high-speed train and adopts CFD simulations to examine the effect of the boundary layer velocity profile on the measured experimental forces. It is quantitatively demonstrated that the same pantograph located at different positions along the train roof can show relevant differences in the aerodynamic uplift, only due to the different flow characteristics.Moreover, a new simplified methodology is proposed to evaluate the aerodynamic uplifts at different positions of the pantograph along the train. Results of the proposed methodology are validated against full scale experimental results and full CFD simulations exploiting the complete model of the pantograph installed on the train roof.

Experimental study on flat plate skin friction control by porous media based on global fluorescent oil film measurement technology

Skin friction is a primary source of total aircraft drag. It is important, therefore, in science and engineering, to achieve drag reduction control in a boundary layer. In this paper, under the experimental conditions of Reθ = 5909 (x/c = 0.55) and with a zero-pressure gradient, the drag reduction control of a plate boundary layer in porous media is studied. The global skin friction of the plate is measured using fluorescent oil film test technology. The results show that, in contrast with the downstream frictional resistance coefficient of a flat plate that possesses a smooth surface, the coefficient for porous media reduced significantly. Also, the lower the pores per inch (PPI) of the porous media, the greater the drag reduction effect. Among the three porous media with different PPI, porous media with ten PPI has the best drag reduction effect. With increasing distance from the porous media, the drag reduction effect decreases gradually. Porous media significantly increase the slope of the logarithmic region of the velocity profile of the downstream turbulent boundary layer, the dimensionless wall velocity u+ moves upward, and the velocity pulsation in the logarithmic region increases so as to reduce skin friction.

Three-dimensional characteristics of crossing shock wave/turbulent boundary layer interaction in a double fin with and without micro-ramp control

The three-dimensional (3D) interactions between crossing shock waves and a turbulent boundary layer (CSWBLI) inside a symmetric double fin are experimentally studied using nanoparticle-based planar laser scattering, supersonic particle image velocimetry, and surface oil visualization. The possibility of controlling the separated flow generated by CSWBLI is considered by employing micro-ramp vortex generators. First, the fractal dimension, velocity profile, and logarithmic law of the incoming turbulent boundary layer at Mach number 2.8 are examined. Then, the flow structure and velocity distribution, which have seldom been presented in previous experiments, are measured in high resolution. The 3D behavior of the boundary layer after CSWBLI shows that the boundary layer becomes thicker behind the shock wave and converges toward the symmetry plane of the double fin. The converged effect contributes to the largest thickness of the boundary layer in the symmetry plane accompanied with a separation region near the wall. Introduction of seven equidistant micro-ramps upstream of the double fin is proved to suppress the separation region, where the arc-like vortices generated by the middle micro-ramps are found to be more sustainable along the streamwise direction. The micro-ramps can increase the momentum exchange between the boundary layer and the surrounding mainstream. At the same time, the momentum exchange induced by the micro-ramps decreases the flow velocity outside the converged region in comparison with the configuration without micro-ramps. The results obtained in this paper can provide an experimental insight into the 3D physical phenomena existing in the CSWBLI and its flow control.

Energy-Based Aerodynamic Analysis on the Blended-Wing-Body Aircraft with Boundary Layer Ingestion

This study is aimed at evaluating the energy saving benefit and its limitation of the boundary layer ingestion (BLI) applied to a transonic blended-wing-body (BWB) aircraft. The power balance method is adopted in the aerodynamic performance analysis of the aircraft. We further improve the one-dimensional analysis framework by adding the pressure volumetric work term in the power balance equation for the application in compressible flow with shock wave regions. A determining expression for the power-saving coefficient (PSC) of the BLI effect is also deduced, which is related to a BLI fraction, the airframe dissipation component ratios, and the specific thrust of the propulsor. This expression provides a way for the preliminary evaluation and comparison of BLI benefits applied in different aircraft configurations. The flow quantities of three different configurations are obtained by 3D Reynolds-averaged Navier–Stokes (RANS) numerical simulations for the calculation of aerodynamic dissipation components. The grid refinement and domain size study shows that the calculated force coefficients and total dissipation coefficient have a good convergence characteristic with the mesh refinement and are insensitive to the variation in domain size. The evaluated PSC of the BWB aircraft with a BLI propulsor is 5% using the previously developed determining expression. The maximum of PSC is 18.8%. The following comparison of dissipation components for the power-on and power-off cases shows that the benefit of BLI is slightly underestimated by the determining expression of PSC. The reason is that the variation in the boundary layer velocity profile at the trailing edge of the center body part is neglected, which is due to the installation of the propulsor. Furthermore, when the BLI propulsor is applied to a BWB aircraft, the variation in trailing edge vortex dissipation and volumetric pressure work term should be studied in detail.