Development Of Turbulent Boundary Layer Research Articles

Equilibrium tides and magnetic activity in stars with close-by massive planets. The intriguing case of WASP-18

WASP-18 is an F6V star that hosts a planet with a mass of $ 10$ Jupiter masses and an orbital period of $ 0.94$ days. In spite of its relatively fast rotation and young age, the star remains undetected in X-rays, thus implying a very low level of magnetic activity. To account for such unexpected properties, we propose a mechanism that modifies the internal stratification and the photospheric magnetic activity of a late-type main sequence star with a close-by massive planet based on the action of the equilibrium tide. We speculate that the horizontal flow produced by the equilibrium tide may interact with the convective plumes in the overshoot layer below the stellar outer convective envelope. The interaction is characterised by a very high Reynolds number ($Re $), leading to the development of turbulent boundary layers at the surface of such structures, whereas turbulent wakes extend over most of the overshoot layer that they straddle. We propose that such a tidally induced turbulence can lead to a reduction of the filling factor of the downdrafts in the overshoot layer. As a consequence, the absolute value of the sub-adiabatic gradient increases in that layer hindering the emergence of magnetic flux tubes responsible for the formation of photospheric starspots. We conjecture that this process is occurring in WASP-18, thus providing a possible mechanism to account for the very low level of magnetic activity observed for such a planet host.

Characteristics of turbulent boundary layers generated by different tripping devices

In this study, we employ a high-fidelity flow visualisation technique to investigate the resemblance between a turbulent boundary layer generated in a lab environment with a limited development length by different tripping devices and a naturally developed canonical turbulent boundary layer. Furthermore, we aim to provide statistics of the wall-normal velocity fluctuation of the tripped boundary layers and analyse them compared with the canonical turbulent boundary layer. This is achieved by conducting two-dimensional particle image velocimetry measurements to acquire the velocity components in the stream-wise wall-normal plane downstream of the tripping location. Different criteria including integral parameters, diagnostic plot, stream-wise and wall-normal turbulence intensities, and mean velocity deficit profiles are employed to compare the boundary layer profiles generated by different trips with well-behaved turbulent boundary layers presented in the literature. The generation of Reynolds shear stress in tripped boundary layers is analysed using quadrant analysis, and suitably sized trips are shown to have contributions of sweeps and ejections in a similar manner to a well-behaved turbulent boundary layer. Furthermore, through the detection of uniform momentum zones, the presence of coherent structures in the logarithmic and outer layers of these well-behaved tripped boundary layers is shown. The findings of this study identify the blockage created by trips as the main factor affecting the turbulence statistics at a certain downstream distance. The results also suggest that the investigation of inner-scaled mean stream-wise velocity is recommended as a simple yet reliable and rapid assessment of the impact of a trip on the turbulent boundary layer development in short test sections.

Grey area in embedded wall-modelled LES on a transonic nacelle-aircraft configuration

A scale-resolving hybrid RANS–LES technique is applied to an aircraft-nacelle configuration under transonic flow conditions using the unstructured, compressible TAU solver. In this regard, a wall-modelled LES methodology is locally applied to the nacelle lower surface to examine shock-induced separation. To circumvent the grey-area issue of delayed turbulence onset, a Synthetic Turbulence Generator (STG) is used at the RANS–LES interface. Prior to the actual examinations, fundamental features of the simulation technique are validated by simulations of decaying isotropic turbulence as well as flat plate flows. For the aircraft-nacelle configuration at a Reynolds number of 3.3 million, a sophisticated mesh with 420 million points was designed which refines 32 % of the outer casing surface of the nacelle. The results show a development of a well-resolved turbulent boundary layer with a broad spectrum of turbulent scales which demonstrates the applicability of the mesh and method for aircraft configurations. Furthermore, the necessity of a low-dissipation low-dispersion scheme is demonstrated. However, a noticeable drop of the surface skin friction downstream of the STG motivates further research on the impact of the interface modelling on the shock–boundary layer interaction.

Trailing-edge noise reduction through finlet-induced turbulence

Biologically inspired finlet treatments have been shown to effectively reduce the trailing-edge noise of a flat plate and hence are a viable noise-suppression technology for engineering applications. The present work performs a thorough experimental investigation on the near-field dynamics of finlet surface treatments applied to a flat plate. To examine the underlying noise-reduction mechanism, the manipulated flow field is analysed using data from detailed static, unsteady wall-pressure as well as velocity measurements and their correlations. Specifically, the densely populated dynamic transducers allow for the tracking of the turbulent boundary-layer development from upstream to the wake of the finlet-treated area (see supplementary movies), which elucidates the formation of ‘finlet-induced turbulence’ through flow–finlet interaction. Associated turbulence structures are found to further develop within the treated area and structures shed from the top of the finlets are observed to mix and merge with the turbulence being channelled through the space between the finlets in the finlet wake. While the mixing process increases the spanwise turbulence length scale, it significantly attenuates the unsteady wall-pressure fluctuation at the trailing edge and thus leads to broadband reduction of the trailing-edge noise. Moreover, it corroborates the findings of earlier studies suggesting that there exists an optimal distance between finlets and trailing-edge where the mixing effects are most beneficial.

Characteristics of negative and undular surges in an impounded channel

ABSTRACT Physical experiments were conducted in an impounded channel to study the propagation of negative surges advancing in still water and evolution of undular surges against an instantaneously varied flow. The results demonstrated that the initial wave height of a negative surge could be accurately predicted using the simple wave solution of the Saint–Venant equations. Moreover, the wave height evolution during surge propagation could be satisfactorily described using the solution by Ippen and Kulin. The main characteristics of an undular surge were strongly correlated to the surge Froude number and turbulent boundary layer development at the toe of the surge. However, the occurrence of wave breaking at the first wave crest was independent of the inflow conditions and was close to the breaking criterion for solitary waves. Furthermore, the present results indicated the largest turbulent Reynolds stress components beneath the wave crests of an undular surge.

Influence of non-uniform thermal boundary on flow and heat transfer characteristics in rectangular channel

In this paper, the flow behavior and heat transfer characteristic in a rectangular channel are numerically investigated. The non-uniform thermal boundary condition is arranged along the streamwise direction at the bottom of the rectangular channel. Furthermore, based on the flow field parameters obtained with numerical simulation, the dynamic modal decomposition (DMD) is carried out for viscous layer, buffer layer, and logarithmic region, respectively. The numerical results show that the hot bands of non-uniform thermal boundary affect the interaction of the velocity streaks along the streamwise direction, which reduces the vorticity of the buffer layer and the fluctuation of the velocity gradient vector. In the terms of entropy analysis, it can be found that the hot bands of non-uniform thermal boundary play a similar role of “riblets” and block the self-sustainment of the turbulent coherent structures. Moreover, the results of DMD manifest that the hot bands of non-uniform thermal boundary can improve the stability of viscous layer and buffer layer. The development of turbulent boundary layer is delayed by affecting the fluid characteristics in buffer layer. Compared to the channel without non-uniform thermal boundary condition, the maximum drag reduction rate of 8.35% can be achieved in considered cases, while a reduction in heat transfer performance of 2.74% occurs. In addition, the comprehensive performance coefficient increases slightly to 1.0013.

Development Of A Turbulent Boundary Layer Over An Air Cavity

The turbulent boundary layer development over an air cavity is studied experimentally using planar Particle Image Velocimetry. An identification technique based on the gradient of correlation values was employed to detect the cavity interface which revealed a well-defined, asymmetric bump-like geometry at the spanwise-uniform region of the cavity. No separation was observed at the leeward side of the cavity, owing largely to the high boundary layer thickness to maximum cavity thickness ratio (δ/tmax ≈ 11.8). The shape of the cavity was found to dictate the turbulent boundary development by subjecting it to alternating streamwise pressure gradients: from an adverse to a favourable and back to an adverse, moving downstream along the cavity. Profiles o f mean s treamwise velocity and turbulent stresses show the alternate pressure gradients and air injection to be the dominant perturbations to the flow, with streamline curvature only having a minor influence, also based on previous observations.

Mechanism of development of turbulent boundary layer in a curved circular pipe under supersonic conditions

This study investigates the characteristics of a turbulent boundary layer in a curved circular pipe under an inflow of Ma = 3.0. The pipe consisted of a straight part and a curved part with a turning angle of 36° and a radius of curvature, γ = R/Rc, of γ = 0.0825. The nanoparticle-based planar laser scattering technique was applied to visualize the structure of the instantaneous flow field, and a large eddy simulation was conducted to uncover the physical aspects of development of the turbulent boundary layer. The distributions of density and vorticity, baroclinic pressure caused by the density and pressure gradients, and other physical quantities were analyzed. The asymmetry of the turbulent boundary layer in the curved part of the pipe, as influenced by curvature, secondary flow, and oblique shock, was obtained as the boundary layer on the inner wall gradually thickened while that on the outer wall gradually thinned. The secondary flow took the mainstream outer bend of the curved pipe and drew the low-energy fluid in the boundary layer to its inner bend, which led to the gradual thickening of the turbulent boundary layer from the outer to the inner bend along the circumferential direction. A low-speed region was formed in the downstream region of the inner part. The intersection between the oblique shock wave and the shear layer led to a high baroclinic pressure that promoted the generation of the vortex and forced the boundary layer to turn and inhibit the separation in the inner part.

Experimental and numerical studies of turbulent flows over two-dimensional and three-dimensional rough surfaces under an adverse pressure gradient

The effects of surface roughness and other surface topographical feature on turbulent boundary layer (TBL) development have been widely explored, especially for flow cases with a constant streamwise pressure gradient distribution. In the case where the TBL is exposed to the action of an adverse pressure gradient (APG), flow separation is expected to occur and the classical mechanisms responsible for energy transport between the inner and outer part of the TBL are invalid. The combined effect of roughness and APG on the TBL is not easy to predict, especially due to the lack of databases containing relevant research data. The main aims of the present study were to conduct experimental investigations and to perform large eddy simulations to determine the effects of different surface topographies (two-dimensional (2D) and three-dimensional (3D)) on turbulent flow development over a smooth wall under a strong APG downstream of the corrugation. The investigation was performed with a fixed value of the Reynolds number Reθ = 4900. In particular, the differences in the production and dynamics of near-wall vortical structures under different corrugation geometries were studied. It was found that the higher amplitude of 2D wavy surface the greater production of turbulence and the earlier separation of the TBL. The response of the TBL differed according to the 3D topography, where it was characterized by a greater streamwise velocity deficit, lower growth of all components of the velocity fluctuations, and a smaller increase in the normal velocity gradient relative to the 2D case with comparable amplitude. The results also demonstrated that the 3D case generated smaller and less energetic structures on the flat plate downstream of the corrugation compared with the 2D case.

Effect of uniform and non-uniform mesh on the development of turbulent boundary layer

For incompressible flow simulation, it is commonly accepted to use uniform meshes to solve the governing equation of turbulent boundary layer. It follows the laws of conservation stabilizing the flow field in the domain and preventing odd-even decoupling in the pressure field. In this study, Large Eddy Simulation (LES) has been conducted in a long channel. In order to calculate the turbulent boundary layer in the channel, the unsteady Navier-Stokes equations has been adopted at a Reynolds number =180, which is based on mean centerline velocity and the half-width of the channel. The mesh used in this study was based on both stretch and uniform mesh having grid points, which is corresponding to . Turbulence statistics were also calculated to compare to the existing results. In the results, the turbu lent boundary layer was fully developed at around . In addition, fully developed channel flow was achieved at the non-dimensional time of .

Numerical investigation of flows around an axisymmetric body of revolution by using Reynolds-stress model based hybrid Reynolds-averaged Navier–Stokes/large eddy simulation

Flows around an axisymmetric body of revolution at a zero yaw angle were studied using a hybrid Reynolds-averaged Navier–Stokes (RANS)/large eddy simulation (LES) approach, which employed a full Reynolds stress model (RSM) in the RANS branch with the aim of accounting for the Reynolds stress anisotropy, streamline curvature, and flow separations in the boundary layer. The SUBOFF model without appendages was applied to conduct the simulations, and the Reynolds number based on the free-stream velocity and the length of the body is ReL=1.2×106. The results, including time-averaged Cp, Cf, and velocity statistics, were compared with the experimental data and wall-resolved LES results available in the literature, and the overall agreement of the comparisons was satisfactory. To assess the performance of the RSM-based hybrid RANS/LES approach, we carried out shear-stress transport-based hybrid RANS/LES approach simulations under identical free-stream conditions for comparison. The sensitivity of the hybrid RANS/LES approach to the RANS models was observed for separated flow with surface curvature and adverse pressure gradient-induced separation. The RSM-based hybrid RANS/LES approach was found to provide a better prediction for the unsteady flows near the stern. That is because the effects of the streamline curvature and the strong interactions between individual stresses can be captured by the exact production terms in the RSM-based hybrid RANS/LES approach. These effects are important for predicting the development of turbulent boundary layers along the stern.

Impact of Lateral Groundwater Flow and Subsurface Lower Boundary Conditions on Atmospheric Boundary Layer Development over Complex Terrain

AbstractCredible soil moisture redistribution schemes are essential to meteorological models, as lower boundary moisture influences the balance of surface turbulent fluxes and atmospheric boundary layer (ABL) development. While land surface models (LSMs) have vastly improved in their hydrologic representation, several commonly held assumptions, such as free-draining lower boundary, one-dimensional moisture flux, and lack of groundwater representation, can bias the terrestrial water balance. This study explores the impact of LSM hydrology representation on ABL development in the Weather Research and Forecasting (WRF) meteorological model. The results of summertime WRF simulations with Noah LSM, characterized by 2-m-thick soil and one-dimensional flow, are shown for a domain in the Colorado Rocky Mountain headwaters region. A reference WRF simulation is compared to 1) the same model with soil moisture initialized by the hydrologic model ParFlow; 2) a deep, free-draining simulation; and 3) WRF coupled to ParFlow, a three-dimensional, integrated groundwater-surface water model. Results show that both lateral transport of groundwater and the rate of drainage from the lower soil layer can weaken or reverse the coupling strength between evaporative fraction and ABL over a 5-month summer period. The resulting shifts in low-level moist convection in river valleys and thermally driven airflows yield strengthened anabatic upslope winds and perturbations to regional precipitation.

An experimental investigation of the aerodynamic flows created by lorries travelling in a long platoon

The concept of autonomous road vehicles has recently gained a great deal of technical respectability. Expected advantages over normal driver-controlled vehicles are through increased safety, reliability and fuel efficiency. This paper presents a novel experimental study enabling for the first time a full understanding of the aerodynamic flow development of a long vehicle platoon. Moving model experiments were carried out at the University of Birmingham Transient Aerodynamic Investigation (TRAIN) rig facility on a 1/20th scale eight lorry platoon with three constant vehicle spacings. Slipstream velocity and pressures, as well as simultaneous on-board vehicle surface pressure measurements were made. Results indicated a highly turbulent boundary layer development, with slipstream pulse peaks near the front of each lorry; similar to previous findings on flows around container freight trains. The drag coefficient of an isolated lorry was in agreement with previous studies. There are substantial reductions in aerodynamic drag for the non-leading platoon vehicles. Drag results plateaued towards a constant value within the platoon. Vehicle spacing affected drag values, with decreases of 57% observed for the closest spacing of half a vehicle length, demonstrating the aerodynamic benefits of platooning.

Effect of Surface Wave on Development of Turbulent Boundary Layer Over a Train of Rib Roughness

Abstract In this paper, detailed experimental results are reported to study the effect of the surface wave of different frequencies on unidirectional current over the bed-mounted train of rib roughness. The model roughness used in this study is transverse square ribs that lengthened across the entire width of the recirculating wave channel. The center-to-center rib pitch (P) was constant during the experiments, thus generating a broad range of near-bed flow patterns for each of the three different surface wave frequencies studied here. The relative submergence associated with the roughness height (k) was 8, which fall in the category of large roughness. Velocity measurements were conducted using acoustic Doppler velocimeter (ADV), and a surface wave of different frequencies was generated using the plunger-type wavemaker. The measured velocity data were analyzed to determine the relative importance of mean flow over the train of rib roughness. Mean velocity profiles illustrate the well-known downward shift from the flat surface data of the semi-logarithmic portion of the law of the wall. The width of the turbulent boundary layer increases with the superposition of surface wave compared to that of the current-only flow. The results also show that the mean reattachment length decreases due to the superposition of surface wave on unidirectional current.

Large-eddy simulation of airfoil flow near stall condition at Reynolds number 2.1 × 106

This paper investigates an airfoil flow involving a turbulent transition and separations near the stall condition at a high Reynolds number Rec = 2.1 × 106 (based on the freestream velocity and the airfoil chord) and provides the wall-resolved large-eddy simulation (LES) database for near-wall models in LES. The present results are compared with the existing experimental and computational data. The wall-resolved LES with the finest mesh (Δξ+,Δη+,Δζ+: chordwise, wall normal, spanwise ≲25, 0.8, 13) and the widest spanwise extent (approximately 5% of the chord length) resolves the key phenomena of the flow (i.e., laminar separation, transition to turbulence, turbulent reattachment, turbulent boundary layer development, and turbulent separation) and well predicts turbulence statistics. The present LES also clarifies unsteady flow features associated with shear-layer instability: the high frequency unsteadiness of St ≃ 130 (based on the freestream velocity and the airfoil chord) at the laminar separation bubble near the leading edge and low frequency unsteadiness of St ≃ 2 at the turbulent separation near the trailing edge. The characteristic frequencies can be scaled to 0.035 and 0.033 by the local momentum thickness and the shear layer velocity which are similar to the natural frequency of the laminar and turbulent shear layer, respectively. With regard to the near-wall modeling in LES, the obtained database indicates that the pressure-gradient term in the mean streamwise-momentum equation is not negligible at the laminar and turbulent separated regions. This fact suggests that the widely used equilibrium wall model is not sufficient, and the inclusion of the pressure-gradient term is necessary for wall modeling in LES of such an airfoil flow. Additionally, influences of computational mesh resolution and spanwise extent on the computational results in wall-resolved LES are investigated.

Supercritical flows past a square cylinder with rounded corners

Large-eddy simulations were used to investigate the supercritical aerodynamics of a square cylinder with rounded corners in comparison with those in the subcritical regime. First, the numerical methods, especially the dynamic mixed model, were validated on the basis of their prediction of supercritical flows past a circular cylinder. Then, the supercritical flows past a rounded-corner square cylinder were simulated and systematically clarified. Strong Reynolds number (Re) effects existed in the forces and local pressures as Re increased from o(104) to o(106). Changeover of flow patterns occurred as Re increased. At the supercritical Re, the free stream overall flowed along the cross sections of the cylinder, separated from the leeward corners and generated Karman vortices behind the cylinder. This pattern resulted in a much smaller recirculation region behind the cylinder compared with the subcritical flow. At the micro level, the flow experienced laminar separation and flow reattachment near the frontal corners, followed by the spatial development of turbulent boundary layers (TBLs) on the side faces and turbulent separation near the leeward corners. The feedback by large-scale primary vortex shedding and the small-scale turbulent motions in the high-frequency region with a slope of −5/3 were detected in the TBL. Their interaction affected the spanwise correlations of wall pressure fluctuations. The TBL on the side face differed from the zero-pressure-gradient flat-plate one; it was subjected to pressure gradients varying in space and time.

Development of a turbulent boundary layer in a rotating square cross-section channel with relatively high local rotation parameter

The development of turbulent boundary layer in a rotating square cross-section straight channel were experimentally measured using hot wire in a new rotating facility. Measurements were conducted at four streamwise stations to study the development of turbulent boundary layer along the stream wise. The governing parameters of the boundary layer are local Reynolds number uτδ/ν and local rotation parameter Ων/uτ2 (uτ is the friction velocity, δ is the thickness of boundary layer, ν is the kinematic viscosity coefficient and Ω is the angular velocity of the channel, which is defined to be positive near the trailing side wall and negative near the leading side wall). The measurements are performed in the range of 199⩽uτδ/ν⩽367 and -0.0135⩽Ων/uτ2⩽0.0054 (these values correspond to u¯D/ν=19,000 and 0⩽|Ω|D/u¯⩽0.362, here, u¯ means the cross-section averaged streamwise velocity, and D means the hydraulic diameter of the channel). Due to the relatively high local rotation parameter |Ω|ν/uτ2 on the leading side (up to 0.0135), some new phenomena were discovered, such as the critical rotation number phenomenon revealed in the variations of wall friction coefficient (the critical rotation number ranges from 0.145 to 0.362), the so-called relaminarization process near the leading side wall. Moreover, in high local rotation parameter cases, the lower limit of the Coriolis force influenced region is too close to the wall to be recognized with the traditional method, hence a new method to evaluate the influence of Coriolis force was proposed in the present work.

Flow in the thin film created by a coherent turbulent water jet impinging on a vertical wall

When a liquid jet impinges on a vertical wall it forms a thin film which flows radially away from the point of impingement until a point where the outward momentum is balanced by surface tension and a film jump is formed. The model for the location for the film jump presented by Wilson et al. (Chem. Eng. Sci, 2012, vol. 68, pp 449–460) is revised to include the development of laminar and turbulent boundary layers in the thin film. The criterion for film jump formation is also revisited, and the analysis explains why the location is insensitive to the nature of the wall material at high flow rates. The model is compared with published data for velocity profiles in the thin film, the transition to turbulence, and new experimental data where the average velocity in the thin film was estimated from the initial growth of the radial flow pattern for flow rates of 1.95–4.01dm3s−1, corresponding to jet Reynolds numbers of 15,500–32,000. Very good agreement with the published and measured data is obtained, with no adjustable parameters, for jets impinging perpendicularly as well as at an oblique angle. The model shows that the parabolic velocity profile assumed by Wilson et al. gives a reasonable estimate of the average velocity, but it is not able to predict phenomena such as the observed transition to turbulence.

Development of turbulent boundary layers past a step change in wall roughness

In this study, the development of a boundary layer past a change in surface roughness (from rough to smooth, $\text{R}\rightarrow \text{S}$) is examined. Measurements of the flow were made by hot wires, whereas the friction velocity was estimated by Preston tube measurements. By means of a diagnostic plot of the turbulence intensity, it is shown that above the internal layer the flow exhibits characteristics of a rough, wall-bounded flow, whereas near the wall the turbulence intensity is similar to that of an isolated smooth wall. Similarly, viscous scaling of the mean streamwise velocity shows an excessive wake region downstream of the $\text{R}\rightarrow \text{S}$ wall surface change that diminishes with the fetch from the surface change. Above the internal layer a second peak in the streamwise Reynolds stress was associated with the upstream rough-wall flow. Examination of the turbulent spectra revealed the presence of large-scale motions within this region that gradually diminish in strength with increasing distance from the change in surface roughness. The magnitude of the near-wall peak failed to collapse to that of a comparable smooth-wall boundary layer under viscous scaling, however, the wall-normal location of the peak appears to be at $y^{+}\approx 15$ at all downstream distances. A new mixed scaling is proposed for the near-wall peak based on the corrected wake deficit and the friction velocity. This shows the importance of outer region to the growth of near-wall peak in this non-equilibrium boundary layer.

Aerodynamic Noise Computation of the Flow Field around NACA 0012 Airfoil Using Large Eddy Simulation and Acoustic Analogy

The current study presents the results of the aerodynamic noise prediction of the flow field around a NACA 0012 airfoil at a chord-based Reynolds number of 100,000 and at 8.4 degree angle of attack. An incompressible Large Eddy Simulation (LES) turbulence model is applied to obtain the instantaneous turbulent flow field. The noise prediction is performed by the Ffowcs Williams and Hawkings (FW-H) acoustic analogy. Both mean flow quantities and fluctuation statistics are studied. The behaviour of the turbulent vortical structures in the flow field from the perspective of the turbulent boundary layer development is visualized. Power spectral density of the lift coefficient is presented. The computed non-dimensional mean velocity profiles in the boundary layer compared reasonably well with the theoretical predictions. The boundary layer transition from a laminar state to a turbulent state is also brought into focus. The skin friction coefficient and the urms streamwise velocity fluctuations predicted a transition zone from x/c=0.23 to x/c=0.45. Then, the research focuses on the broadband noises of the turbulent boundary layers and the tonal noises that arise from the vortex shedding generated by the laminar boundary layers. The spectra computed from the acoustic pressure are compared with the experimental data. The effect of observer location on the overall sound pressure level (OASPL) is investigated and the results indicate that the OASPL varies logarithmically with the receiver distance, as was expected.