Turbulent Boundary Layer Separation Research Articles

Numerical analysis of novel wavy wall based control of turbulent boundary layer separation

In this work, a comprehensive insight into the new passive flow control method, i.e. a two-dimensional streamwise waviness (recently proposed by Dróżdż et al. [1]) is provided using Large Eddy Simulations. This approach was demonstrated to be effective in postponing turbulent boundary layer separation at high Re, which is out of reach for other commonly known passive flow control strategies. Although the effectiveness of the wavy wall in delaying flow separation has been confirmed, the physical mechanisms standing behind the skin friction enhancement are not fully understood which opens a space for future work. Of particular interest is an insight into the fluid flow in the near-wall region just above the waviness and this paper is aimed at doing so. To separate the effect of the global adverse pressure gradient, the results were confronted with the ones obtained under zero pressure gradient conditions for the identical wavy wall geometry and the same Reτ=2500. Also additional simulations of the turbulent boundary layer building up on the flat wall were conducted to have an additional reference for the flow fields obtained above the wavy wall under zero and adverse pressure gradients respectively. A detailed inspection of the streamwise velocity and its fluctuation profiles indicated that the contribution of the global adverse pressure gradient to the flow statistics begins when the Rotta-Clauser pressure gradient parameter β≥3. The paper confirms an enhancement of the wall shear stress τw downstream the corrugation, which is the necessary condition to postpone the separation. Interestingly, the effect of increased τw was not observed for the case with the same waviness and without pressure gradient.

Extraction of geometric features and analysis of flow mechanism of high loaded compressor airfoils at low Reynolds number

When the aircraft cruising at high altitude, the aerodynamic performance of the compressor sharply decreases due to the enhanced boundary layer separation loss. Therefore, it is of great significance to design the high-performance airfoils suitable for low Reynolds numbers (Re) to improve the aerodynamic performance of compressor. In this paper, numerical simulations were carried out on a high-loaded subsonic compressor cascade V103 at low Re. Meanwhile, a multi-objective optimization of blade profile was performed to get optimal solutions, and data mining techniques were applied to extract valuable design knowledge from the optimal database. The results show that the Pareto-optimal airfoils achieve a maximum reduction of 27.32% in total pressure loss coefficient under design condition and 36.71% under near stall condition. Compared with the original airfoil, the curvature distribution law of the camber line with larger values at both ends and smaller values in the middle is advantageous for the performance of the compressor cascade under design condition. This distribution pattern causes a forward shift for the loading distribution of airfoils, accompanied by an earlier transition onset. Additionally, it reduces adverse pressure gradient at the rear part of suction surface, which suppresses the development and growth of laminar separation bubble and delays the separation of turbulent boundary layer. Under near stall condition, maintaining larger leading edge angle and uniform flow diffusion is beneficial for improving the performance of compressor cascade. This is because the larger leading edge angle reduces incidence angle, while the uniform flow diffusion inhibits a great flow separation.

Application of Hybrid RANS/LES Methods for the Simulation of Shock-Induced Turbulent Boundary Layer Separation

Application of Hybrid RANS/LES Methods for the Simulation of Shock-Induced Turbulent Boundary Layer Separation

Influence of posture during gliding flight in the flying lizard Draco volans

The agamid lizards of the genus Draco are undoubtedly the most renown reptilian gliders, using their rib-supported patagial wings as lifting surfaces while airborne. Recent investigations into these reptiles highlighted the role of body posture during gliding, however, the aerodynamics of postural changes in Draco remain unclear. Here, we examine the aerodynamics and gliding performances of Draco volans using a numerical approach focusing on three postural changes: wing expansion, body camber, and limb positioning. To this aim, we conducted 70 three-dimensional steady-state computational fluid dynamics simulations of gliding flight and 240 two-dimensional glide trajectory calculations. Our results demonstrate that while airborne, D. volans generates a separated turbulent boundary layer over its wings characterized by a large recirculation cell that is kept attached to the wing surface by interaction with wing–tip vortices, increasing lift generation. This lift generating mechanism may be controlled by changing wing expansion and shape to modulate the generation of aerodynamic force. Furthermore, our trajectory simulations highlight the influence of body camber and orientation on glide range. This sheds light on how D. volans controls its gliding performance, and conforms to the observation that these animals plan their glide paths prior to take off. Lastly, D. volans is mostly neutral in pitch and highly maneuverable, similar to other vertebrate gliders. The numerical study presented here thus provides a better understanding of the lift generating mechanism and the influence of postural changes in flight in this emblematic animal and will facilitate the study of gliding flight in analogous gliding reptiles for which direct observations are unavailable.

The Influence of Spurious Periodicity on the Flow Characteristics of a Separated Turbulent Boundary Layer with and Without Active Flow Control

The Influence of Spurious Periodicity on the Flow Characteristics of a Separated Turbulent Boundary Layer with and Without Active Flow Control

Turbulent Boundary Layer Separation Control Using Magnetohydrodynamic Plasma Actuator

The pulse electric arc discharge in an external magnetic field is studied as a vortex generator in the subsonic boundary layer. A pulsed Ampere force induces a hairpin vortex near the wall; its structure depends on the relative direction of arc propagation and external flow velocity. The data presented in this article were obtained from parametric studies of vortex characteristics and their effects on the boundary layer profile at various actuator momentum coefficients (Cμ=1−30) and vortex sizes relative to the boundary layer thickness (D/δ=0.5−1.2). Also, the control of turbulent boundary layer separation on a bump at a flow velocity up to 50 m/s was attempted. An average shift of the separation line by 15% of the bump height was obtained at a flow velocity of 50 m/s and a total momentum coefficient of 0.6%.

Experimental Characterization of an Axisymmetric Transonic Separated Flow for Computational Fluid Dynamics Validation

An experimental characterization of transonic, turbulent, separated flow generated by an axisymmetric model is presented, with intent for use as a computational validation case. The model is a scaled version of a geometry inspired by Bachalo and Johnson (“Transonic, Turbulent Boundary-Layer Separation Generated on an Axisymmetric Flow Model,” AIAA Journal, Vol. 24, No. 3, 1986, pp. 437–442), consisting of a circular bump on a constant-diameter cylinder aligned with the flow. The Mach 0.875 flow is turbulent approaching the bump and becomes locally supersonic at the apex. This leads to a shock-wave/boundary-layer interaction, a separation bubble, and unsteady flow reattachment downstream. Tunnel boundary conditions are characterized, and mean surface pressure, mean skin friction, and both mean and fluctuating velocity fields are measured throughout the interaction region. Uncertainty estimates are provided for all measurements. The degree of flow axisymmetry and evidence of relaminarization are discussed, and a comparison is made to historical data.

Effect of MHD Control on Turbulent Boundary Layer Separation Flow in Scramjet Inlet

In order to study the effect of MHD control on the separation flow of turbulent boundary layer in the scramjet inlet based on the assumption of low magnetic Reynolds number, considering the source term of turbulent electromagnetic dissipation, the numerical simulation method is developed for electromagnetic field coupled with flow field. A Mach 4 plate is adopted as the basic configuration to assess the accuracy of the solver’s source term of turbulent electromagnetic dissipation. On this basis, numerical simulation of MHD control on the separation flow of boundary layer in scramjet inlet is carried out to study the influence of different electric field strengths and magnetic induction intensity on it. The results show that applying MHD local control alleviates the boundary layer separation in the scramjet inlet. Under different electric field strengths and magnetic induction intensity, the separation vortex’s scale significantly changes. The scale of the separation zone continues to decrease when the electric field strength or magnetic induction intensity increases, and the effect of MHD control is more significant.

Wall-Modeled Large Eddy Simulation and Detached Eddy Simulation of Wall-Mounted Separated Flow via OpenFOAM

Considering grid requirements of high Reynolds flow, wall-modeled large eddy simulation (WMLES) and detached eddy simulation (DES) have become the main methods to deal with near-wall turbulence. However, the flow separation phenomenon is a challenge. Three typical separated flows, including flow over a cylinder at ReD = 3900 based on the cylinder diameter, flow over a wall-mounted hump at Rec = 9.36 × 105 based on the hump length, and transonic flow over an axisymmetric bump with shock-induced separation at Rec = 2.763 × 106 based on the bump length, are used to verify WMLES, shear stress transport k-ω DES (SST-DES), and Spalart–Allmaras DES (SA-DES) methods in OpenFOAM. The three flows are increasingly challenging, namely laminar boundary layer separation, turbulent boundary layer separation, and turbulent boundary layer separation under shock interference. The results show that WMLES, SST-DES, and SA-DES methods in OpenFOAM can easily predict the separation position and wake characteristics in the flow around the cylinder, but they rely on the grid scale and turbulent inflow to accurately simulate the latter two flows. The grid requirements of Larsson et al. (δ/Δx,δ/Δy,δ/Δz≈(12,50,20)) are the basis for simulating turbulent boundary layers upstream of flow separation. A finer mesh (δ/Δx,δ/Δy,δ/Δz≈(40,75,40)) is required to accurately predict the separation and reattachment. The WMLES method is more sensitive to grid scales than the SA-DES method and fails to obtain flow separation under a coarser grid, while SST-DES method can only describe the vortices generated by the separating shear layer, but not within the turbulent boundary layer, and overestimates the separation-reattachment zone based on the grid system in this paper.

Artificial neural network-based wall-modeled large-eddy simulations of turbulent channel and separated boundary layer flows

Wall-models in a large-eddy simulation (LES) are essential to alleviate the large near-wall resolution requirements for high-Reynolds-number turbulent flow simulations. Among the existing wall-models for a LES, an equilibrium wall-stress model has the highest computational efficiency. Because this model has limitations, such as a lack of non-equilibrium effects and the assumption of a particular law of the wall in the mean velocity, we propose artificial neural network-based wall-stress models (AWMs). The input variables for the AWMs are extracted from the decomposition of the skin-friction coefficient proposed by Fukagata et al. [1], and the AWMs are shown to be able to predict the wall-shear stress in complex flows accurately. The performance of the AWMs is tested for two types of flows, a fully developed turbulent channel flow and a separated turbulent boundary layer flow. A direct comparison of the turbulence statistics with those obtained by previous wall-models (i.e., a log-law-based wall-stress model and a non-equilibrium wall-stress model) shows that better predictions are achieved using the AWMs for both flows, even with untrained Reynolds numbers. When using a coarse grid along the wall-normal direction in wall-modeled LESs (WMLESs) with the AWMs, an upward shift of the mean velocity profile (positive log-layer mismatch, LLM) compared to direct numerical simulation data is found, consistent with previous studies. However, this LLM problem can be overcome by imposing a filtered wall-normal velocity at the wall that is dynamically determined based on the continuity equation and the Taylor series expansion within wall-adjacent cells.

Numerical analysis of the influence of wall roughness on the turbulent boundary layer separation

Passive control of turbulent boundary layer (TBL) separation in a channel with adverse pressure gradient (APG) condition towards the outlet is investigated using Reynolds-averaged Navier-Stokes (RANS) methods and Large Eddy simulations (LES) with the help of commercial ANSYS software. Isothermal and incompressible flow in the near-wall area of the flat plate as well as various configurations of wavy wall is studied. It is shown that the amplitude and period of the waviness significantly influence the near wall region and point of separation.

Experimental characterization of smooth body flow separation topography and topology on a two-dimensional geometry of finite span

A smooth body, adverse pressure gradient (APG), turbulent boundary layer (TBL) separation is experimentally studied. The geometry features canonical TBL development prior to encountering a smooth, two-dimensional convex ramp geometry of finite span onto which a streamwise APG that is fully adjustable is imposed. Both large- and small-scale separations are studied, and all data are archived on the NASA Turbulence Modeling Resource website. This paper describes the large-scale separation case with focus on the surface topography and topology of both separation and reattachment. Despite the spanwise uniform approach TBL and ramp geometry, the separation is highly three-dimensional but the reattachment is spanwise uniform. The surface flow topology is characterized by the ‘owl-face pattern of the fourth kind’ – found to be highly repeatable over multiple experiments. This ubiquitous topology has been reported for a variety of flows including inclined bodies of revolution. It is demonstrated that the APG and the secondary flow associated with the sidewall–ramp juncture is responsible for the formation of the surface separation patterns.

Comparative study on single-incident and dual-incident shock wave/turbulent boundary layer interactions with identical total deflection angle

Interactions between the boundary layer and two successive incident shock waves (ISWs) often occur in the supersonic mixed-compression inlets. However, the flow mechanism involved in such interactions has been studied rarely. In this study, we investigate experimentally and analytically the turbulent boundary layer separation flow induced by the single ISW and dual ISWs at the identical total deflection angles in a Mach 2.73 flow. Schlieren photography, wall pressure measurement and surface oil-flow visualisation are employed to diagnose the flow field. Experiments with the impingement points of the two ISWs intersecting on the bottom wall exhibit a separated flow with a triangle-like separation bubble, namely the first kind of dual-ISW/turbulent boundary layer interaction (ISWTBLI). Comparative studies show that various flow features of this kind of dual-ISWTBLI, including the extent of the separation region, pressure distribution and surface-flow topological structures, are nearly identical to those of the single-ISWTBLI with an identical total deflection angle. As the distance between the two ISWs increases, the shape of the separation region in the dual-ISWTBLI changes from triangle-like to quadrilateral-like, and the height of the separation region decreases accordingly, forming the second kind of dual-ISWTBLI. Furthermore, an inviscid model is developed for the dual-ISWTBLI to describe the complex shock wave system and elucidate the cause of a quadrilateral-like separation bubble in the second kind of dual-ISWTBLI. Moreover, based on a previous work by Souverein et al. (J. Fluid Mech., vol. 714, 2013, pp. 505–535) on the single-ISWTBLI, a modified scaling method is established for the first kind of dual-ISWTBLI.

Explicit Algebraic Reynolds-Stress Modeling of Pressure-Induced Separating Flows in the Presence of Sidewalls

AbstractReynolds-averaged Navier–Stokes (RANS) approach is used to simulate the steady-state of a family of pressure-induced turbulent separation bubbles in the presence of sidewalls. Different turbulence models are employed with a specific emphasis on the baseline explicit algebraic Reynolds stress model (BSL-EARSM) and the simulations are compared with experimental data. The separation and reattachment of a flat-plate turbulent boundary layer are generated through a combination of adverse and favorable pressure gradients (APG-FPG) by numerically reproducing the geometry of the wind-tunnel test section used for the experiments. Three cases are considered a large (LB) and a medium (MB) bubble presenting mean backflow, and a small bubble (SB) without mean-flow reversal. This is achieved by varying the streamwise position of the APG/FPG transition. Good agreement between the BSL-EARSM-computed solutions and the experimental results are obtained for wall-pressure and skin-friction distributions on the centerline plane of the test section as well as for the overall three-dimensional flow topology. However, both detachment and reattachment are predicted too early and the bubble length is slightly overestimated for cases LB and MB. For case LB, the streamwise Reynolds stress is estimated fairly well but its production is somewhat delayed. Normal and shear stresses are in good agreement with the experiments in the upstream part of the bubble but are significantly over-estimated in the reattachment region. The k−ω shear-stress transport (SST) model with the so-called reattachment modification performs better than the other tested linear-eddy-viscosity models but it is still unable to reproduce accurately the three-dimensional flow topology even for the “simplest” case SB. Overall, the results suggest that BSL-EARSM is the most suitable turbulence model for this flow configuration.

Passive separation control of shortfin mako shark skin in a turbulent boundary layer

This experimental investigation discusses the effectiveness of shortfin mako shark skin to passively control turbulent boundary layer separation. Experiments were conducted in a water tunnel facility with the shark skin specimens mounted on a smooth flat plate subjected to an adverse pressure gradient. The two skin specimens have shark scales with different sizes, shapes, and bristling angles. The flank region scales (B2) are slender, tall, and can bristle at 50°, while the scales in the region between flank and dorsal fin (B1) are wide, short, and can bristle at 30°. The adverse pressure gradient on the plate was generated by a rotating cylinder. DPIV measurements indicate that unpainted scales B2 can fully control or delay boundary layer separation and reduce the fraction of time the flow is reversed. The bristling of the scales caused fluctuation in the skin friction curve and the single spike intensity of the normal stress very close to the wall. These capabilities are directly related to the scale bristling angle. In contrast, the low bristling angle of the scales from region B1 did not impede the reversing flow, and therefore separation was enhanced. The increase in Reynold’s stress fluctuations away from the wall indicated that the separation region increased in size due to vortex structures further from the wall. Once the bristling mechanism of the B2 scales was eliminated by painting over the skin, the scales functioned as a rough surface but still promoted separation delay. In addition, the streamwise Reynolds stress profile for the flow over the painted scales measured after separation occurred, along with the high intensity of the normal stress near the surface are both characteristics of a flow over a rough surface. In summary, the shark skin surface showed an ability to both delay and increase flow separation depending on the capabilities of the scale bristling to inhibit reversing flow.

Kinematics of the Turbulent and Nonturbulent Interfaces in a Subsonic Airfoil Flow

The subject of turbulent and nonturbulent interfaces (TNTIs) has been extensively studied using idealized free-shear flows and zero-pressure-gradient flat-plate boundary layers. However, it remains to be addressed whether a TNTI can be quantitively identified in complex aerodynamic flows where separation bubbles, transition, turbulent boundary-layer separation and an asymmetric wake coexist in a complex spatially developing fashion. Here, we report a direct numerical simulation study at and at a low Reynolds number past a NACA-0012 airfoil at a angle of attack. The threshold-free fuzzy cluster method is used for TNTI identification, and it is corroborated by a joint probability density function-based method. The TNTIs detected are confirmed to be physical a posteriori by the distinctive quasi-step jump behavior in conditionally averaged statistics along traverses normal to the interfaces. The possible connection between the TNTI curvature and local entrainment is also investigated. Airfoil TNTI curvature parameters are found to be noticeably affected by the transitional state of the flow; at the same time, there are only minor differences between the TNTIs in the boundary-layer region and in the wake region. Conditionally sampled results suggest that there is little propensity for local entrainment to occur on either the leading or trailing edge of the TNTIs. Downstream of transition, local entrainment is more pronounced on relatively flat TNTI surfaces for both the airfoil wake and boundary layer.

Effective use of the streamwise waviness in the control of turbulent separation

It is commonly known that for a sufficiently high Reynolds number, for flow over a rough surface and other previously examined wall topologies, turbulent boundary layer separation generally occurs earlier, and the separation bubble is noticeably larger due to a greater momentum deficit caused by wall irregularities. The recently discovered amplitude modulation effect has provided an improved understanding of the momentum transport that occurs from the outer to the inner part of the boundary layer and its subsequent effect on skin friction. At sufficiently large Reynolds numbers, large-scale motion first causes an increase and then decrease in the flow velocity near the wall. This results in an increased net convection velocity of small structures in the streamwise direction, and consequently an increase in wall shear stress. In this work, we demonstrate that a wavy wall, with a streamwise waviness with carefully selected amplitude and period, can effectively enhance the amplitude modulation effect and ensure an increase in the wall shear stress, thereby postponing turbulent separation. The experimental results are presented in normalized general form using suitable flow scaling. The research goal is to find values of the amplitude, period, and length (always with the total number of periods) of the wavy wall for which the highest rise in skin friction is gained at a fixed point downstream of the wavy wall. The most effective wavy wall geometry investigated ensured a 13% increase in skin friction (relative to the value at zero-pressure-gradient inlet flow) at the location where flow separation occurred on an unmodified surface (i.e. on a flat plate).

A review on hydrodynamics of horseshoe vortex at a vertical cylinder mounted on a flat bed and its implication to scour at a cylinder

A comprehensive review of the local scour due to vortical flow around a cylindrical bridge pier under steady current is presented in this paper. The mechanism of the formation of vortices, the size, velocity and strength of horseshoe vortex (HSV), formation of the HSV by the separation of laminar and turbulent boundary layer and the scour around a cylindrical pier due to vortices have been presented. The complexity involved in the scour-related calculations, and the scope for future research are discussed in the last section.

Numerical Simulation of Vortex Street Around the Two-Dimensional Cylinder Based on LES

In order to study the characteristics of the Karman vortex street generated in the flow around the bluff body, this paper uses the numerical model of the large eddy simulation to numerically simulate the flow around the two-dimensional cylinder. Combined with the simulation data, this paper validates and analyzes the Karman vortex phenomenon, and studies the properties of the vortex shedding when the boundary layer is separated in the flow around the cylinder. The simulation results show that the inflow around the cylinder will produce two different characteristics of the vortex. The high velocity vortex is generated outside the boundary layer, and the small velocity vortex is generated inside the boundary layer, and the two alternately fall off to form the vortex street; When the size of the flow-through cylinder is constant, the falling vortex increases with the increase of the flow velocity, and when the flow velocity increases to a certain value, the vortex size will remain unchanged; When kept constant, the vortex is positively correlated with the size of the cylinder. The results obtained in the paper describe the Carmen vortex street generated in the flow around the two-dimensional cylinder in a relatively detailed way. Finally, this paper provides a reference for studying turbulent energy dissipation and boundary layer separation.

Time-resolved topology of turbulent boundary layer separation over the trailing edge of an airfoil

The unsteady organization of a separated turbulent boundary layer was investigated upstream from the trailing edge of a NACA 4418 airfoil. The angle of attack was $9^{\circ }$ in the pre-stall regime. Two particle image velocimetry fields of view were of interest: a streamwise–wall-normal plane at midspan of the airfoil and a streamwise–spanwise plane parallel to and near the surface of the airfoil. In the near-surface streamwise–spanwise plane, the mean velocity field revealed a saddle point near midspan and a pair of counter-rotating foci at the sides. This pattern is reminiscent of a stall cell, which has been traditionally associated with flow separation on thick airfoils at and slightly beyond the angle of attack of maximum lift. Isolating the low frequencies showed that the instantaneous separation front consisted of several smaller structures that also resembled a stall cell pattern, but they were an order of magnitude smaller than the one found in the mean pattern. These instantaneous stall cells were of two types: forward and backward. The forward stall cells were formed by strong high-speed streaks from upstream, while backward stall cells formed as a result of strong backflow just downstream from the separation front, resulting in a foci pair and a saddle point on their upstream side. In both cases, the foci pairs acted to mobilize high-speed momentum of the associated streak into a rotational motion, causing these streaks to dissipate. Finally, proper orthogonal decomposition revealed that low-order modes were associated with the movement and distortion of the separation front.