Large-eddy Breakup Research Articles

On the enhancement and suppression of turbulent wall pressure by the Large Eddy BreakUp devices

An experimental study is presented on the application of Large Eddy BreakUp (LEBU) on a flat plate as an aeroacoustics noise source-targeting device to perturb the wall pressure fluctuations of a turbulent boundary layer. When interacting with a LEBU wake, the wall pressure spectra can establish a self-similar behavior against s′, which is a normalized separation distance between the LEBU's trailing edge and the targeted location for aeroacoustics noise source mitigation. It is found that s′>3 is needed to achieve an overall reduction in the wall pressure fluctuations. The fundamental mechanism by which the LEBU can reduce the wall pressure fluctuation is investigated by studying the spatio-temporal evolution of turbulent spots. When the emanated LEBU wake is interacted with the outer part of turbulent spot, the shielding effect can always inhibit the turbulent fluids ejection from the spot's leading edge. However, incursion of the high momentum fluids wall-sweeping event at the spot's trailing edge can only be effectively prevented when s′>3.

Large-scale and small-scale contribution to the skin friction reduction in a modified turbulent boundary layer by a large-eddy break-up device

In this work, we assess the contributions of large-scale and small-scale Reynolds shear stress events to the skin friction reduction of a turbulent boundary layer modified by a large-eddy break-up device, based on the quadrant analysis of Reynolds shear stress, the Fourier mode decomposition, and an extension of a skin friction decomposition scheme.

The skin-friction coefficient of a turbulent boundary layer modified by a large-eddy break-up device

A computational study based on well-resolved large-eddy simulations is performed to study the skin friction modification by a large-eddy breakup device (LEBU) in a zero-pressure-gradient turbulent boundary layer. The LEBU was modeled using an immersed boundary method. It is observed that the presence of the device leads to the generation of wake vortices, which propagate downstream from the LEBU and toward the wall. A skin friction decomposition procedure is utilized to study different physical mechanisms of the observed skin friction reduction. From the skin friction decomposition, it is found that the skin friction reduction can be characterized by three universal regions of different changes for the skin friction contributions. The first region is predominantly associated with the formation of the wake vortices and the reduction of Reynolds shear stress. In the second region, the mean streamwise velocity fields show that a region of velocity deficit formed downstream of the LEBU propagates toward the wall and leads to turbulence reduction due to wake wall interactions, which also induces a local maximum skin friction reduction. In the third region, the dissipation of wake vortices leads to the regeneration of Reynolds shear stress. A quadrant analysis of the Reynolds shear stress contribution reveals that the LEBU increases the Q2 and Q4 contributions and attenuates the Q1 and Q3 contributions in the first region, followed by an onset of Reynolds shear stress further downstream.

Influence of turbulence structure with different scale on aero-optics induced by supersonic turbulent boundary layer

Studying the effects of turbulent structure with different scale on aero-optics is of great significance to improve the accuracy of wavefront measurement and simulation, and to guide the suppression of aero-optics through large eddy breakup. Based on the high-spatial resolution (11.4 μm/pixel) density field of the supersonic (M∞ = 3.0) turbulent boundary layer (SPTBL) obtained by Nano-tracer-based planar laser scattering (NPLS) technique, the curvelet transformation is used to extract the density field at a specific turbulence scale. The statistical results of the propagation of Gaussian plane light wave in the corresponding density field at different turbulence scales are calculated. The smaller the scale of turbulence structure is, the less contribution it makes to aero-optical distortion. The fully developed SPTBL has less effect on the beam center offset (x¯), but more obvious effect on the beam spread about the center (x'2¯). x¯ is mainly caused by the turbulent structure with relatively large scale (turbulence characteristic length scale Lj>δ), while the contribution of large-scale structure to the x'2¯ is not significant. The x'2¯ is mainly caused by the turbulent structure with relatively small size (Lj≈0.13δ) in the flow field. The smaller scale (Lj<0.94θ) turbulence structure has less influence on the x¯ and x'2¯. θ can be also used as a validation index of aero-optical effective scale.

Mathematical modeling of turbulent boundary layers, modified by wall-localized drag reduction techniques

Skin friction drag reduction via turbulence manipulating implies a purposeful influence on a very complicated mechanism of vortical structure dynamics with wide range of scales and the disturbances localized in the wall vicinity. The goal of this research is to propose the complex unified approach for semi-empirical modeling of turbulent viscosity with possibility to take into account both simultaneous and separate influence of such factors as: streamlined surface relief (from macro ribs to microgrooves), uniform and intermitted microblowing, polymer additives injection and Large Eddy BreakUp devices. There are three principal advantages of the developed model: 1) It has hybrid structure, joining algebraic (in the wall region) and differential (in the wake region) modeling descriptions, that makes them very flexible in various cases of different turbulent flows prediction; 2) It is built on the base of common principles and its potential generalization to the case of heat conductivity modeling doesn't require the use of Reynolds analogy (incorrect in the vicinity of the rough surface) in the wall region of turbulent boundary layer; 3) The proposed approach is not limited by only RANS level of flow modeling, it can be effectively adapted for improving the near-wall subgrid turbulence model in frames of LES/DES techniques. The elaborated model due to its ability to take into account the mentioned above factors both independently and in different combinations demonstrate universal approach that allows to simulate simultaneous acting the different flow control methods, investigate their non-linear interaction and optimize geometric characteristics and operating conditions for searching the synergistic modes of functioning. The applicability of the developed model is proved by qualitative agreement of wide range of mentioned above kinds of turbulent flows predictions and the experimental results, obtained by different researchers. The proposed modeling approach can be recommended for its application in different research and engineering fields, associated with turbulence vortical structure manipulating.

Large-Eddy BreakUp Devices \u2013 a 40 Years Perspective from a Stockholm Horizon

In the beginning of the 1980’s Large Eddy BreakUp (LEBU) devices, thin plates or airfoils mounted in the outer part of turbulent boundary layers, were shown to be able to change the turbulent structure and intermittency as well as reduce turbulent skin friction. In some wind-tunnel studies it was also claimed that a net drag reduction was obtained, i.e. the reduction in skin-friction drag was larger than the drag on the devices. However, towing-tank experiments with a flat plate at high Reynolds numbers as well as with an axisymmetric body showed no net reduction, but instead an increase in total drag. Recent large-eddy simulations have explored the effect of LEBUs on the turbulent boundary layer and evaluations of the total drag show similar results as in the towing tank experiments. Despite these negative results in terms of net drag reduction, LEBUs manipulate the boundary layer in an interesting way which explains why they still attract some interest. The reason for the positive results in the wind-tunnel studies as compared to drag measurements are discussed here, although no definite answer for the differences can be given.

Influence of a Large-Eddy-Breakup-Device on the Turbulent Interface of Boundary Layers

The effects of implementing a large-eddy break-up device (LEBU) in a turbulent boundary layer on the interaction with the boundary layer is investigated with particular emphasis on the turbulent/non-turbulent interface (TNTI). The simulation data is taken from a recent well-resolved large eddy simulation (Chin et al. Flow Turb. Combust. 98, 445–460 2017), where the LEBU was implemented at a wall-normal distance of 0.8 δ (local boundary layer thickness) from the wall. A comparison of the TNTI statistics is performed between a zero-pressure-gradient boundary layer with and without the LEBU. The LEBU is found to delay the growth of the turbulent boundary layer and also attenuates the fluctuations of the TNTI. The LEBU appears to alter the structure size at the interface, resulting in a narrower and shorter dominant structure (in an average sense). Further analysis beneath the TNTI using two-point correlations shows that the LEBU affects the turbulent structures in excess of 100 δ downstream of the LEBU.

Influence of a large-eddy breakup device on the frictional drag in a turbulent boundary layer

A direct numerical simulation of a spatially developing turbulent boundary layer with a large-eddy breakup (LEBU) device was performed to investigate the influence of the LEBU device on the near-wall turbulence and frictional drag. The LEBU device, which is thin and rectangular in shape, was located at 80% of the boundary layer thickness (δ). The LEBU device reduced the skin-friction coefficient (Cf) up to 17%. The breakdown of the outer structures passing through the LEBU device reduced the energy of the long wavelength motions (λz+&amp;gt;200) along the wall-normal direction. The reduction of Cf mainly arose from the contribution of the Reynolds shear stress by the difference in the spatial coherence of the outer high- and low-speed structures. We investigated the relationship between the large-scale motions and the velocity–vorticity correlations (vωz and −wωy), which directly contribute to Cf. The contributions of vωz and −wωy accounted for 80% of the total Cf reduction. The amount of the Cf reduction induced by vortex stretching arose from the contributions of the intense positive uL (uL+≈2) and the relatively weak negative uL (uL+≈−0.7). Owing to the negative contribution of the advective vorticity transport, the total Cf reduction was obtained under the negative uL even though drag reduction was achieved by the suppression of vortex stretching under the intense positive uL.

Simulation of a Large-Eddy-Break-up Device (LEBU) in a Moderate Reynolds Number Turbulent Boundary Layer

A well-resolved large eddy simulation (LES) of a large-eddy break-up (LEBU) device in a spatially evolving turbulent boundary layer is performed with, Reynolds number, based on free-stream velocity and momentum-loss thickness, of R e θ ≈ 4300. The implementation of the LEBU is via an immersed boundary method. The LEBU is positioned at a wall-normal distance of 0.8 δ (δ denoting the local boundary layer thickness at the location of the LEBU) from the wall. The LEBU acts to delay the growth of the turbulent boundary layer and produces global skin friction reduction beyond 180δ downstream of the LEBU, with a peak local skin friction reduction of approximately 12 %. However, no net drag reduction is found when accounting for the device drag of the LEBU in accordance with the towing tank experiments by Sahlin et al. (Phys. Fluids 31, 2814, 1988). Further investigation is performed on the interactions of high and low momentum bulges with the LEBU and the corresponding output is analysed, showing a ‘break-up’ of these large momentum bulges downstream of the LEBU. In addition, results from the spanwise energy spectra show consistent reduction in energy at spanwise length scales for $\lambda _{z}^{+} > 1000$ independent of streamwise and wall-normal location when compared to the corresponding turbulent boundary layer without LEBU.

Freeman Scholar Review: Passive and Active Skin-Friction Drag Reduction in Turbulent Boundary Layers

A variety of skin-friction drag reduction (FDR) methods for turbulent boundary layer (TBL) flows are reviewed. Both passive and active methods of drag reduction are discussed, along with a review of the fundamental processes responsible for friction drag and FDR. Particular emphasis is given to methods that are applicable to external hydrodynamic flows where additives are diluted by boundary layer entrainment. The methods reviewed include those based on engineered surfaces (riblets, large eddy breakup devices (LEBUs), and superhydrophobic surfaces (SHS)), those based on additives (polymer injection and gas injection), and those based on morphological alterations in the boundary layer flow (air layers and partial cavity formation). A common theme for all methods is their disruption of one or more of the underlying physical processes responsible for the production of skin-friction drag in a TBL. Opportunities and challenges for practical implementation of FDR techniques are also discussed.

0818 LEBUを用いた粗面乱流境界層の制御に関する実験的研究

0818 LEBUを用いた粗面乱流境界層の制御に関する実験的研究

Modelling of turbulent boundary layers on the body of revolution in the presence of large eddy breakup devices

A novel approach was proposed to developing a combined algebraic-differential model of turbulent viscosity, which was then used to modify the traditional method for calculating turbulent boundary layers on bodies of revolution immersed in an incompressible flow. Ample experimental data were invoked to test the developed model through prediction of features inherent in turbulent boundary layers on the body of revolution in the presence of Large Eddy BreakUp (LEBU) devices installed in the vicinity of the streamlined surface. Within the developed approach, we showed it possible to calculate the evolution of local governing characteristics of turbulent flow in a broad range of LEBU parameters.

Effect of vertical large eddy breakup devices on the drag of a flat plate

Experiments are performed to study the possibility of decreasing the net drag of a flat plate with the use of streamwise-oriented vertical elements mounted normal to the surface in an incompressible equilibrium turbulent boundary layer. The Reynolds number based on the boundary-layer momentum thickness in the section where the vertical elements are placed is 820. It is demonstrated that vertical large eddy breakup elements with the geometry used do not reduce the drag of a flat plate in the major part of the range of Reynolds numbers Rex examined. It is only at extremely low values of Rex that a certain gain in the net drag is reached, as compared with the value for a non-modified flow.

On the vertical large eddy breakup device capability to decrease the turbulent drag

The possibility of decreasing the turbulent friction with the use of streamwise-aligned vertical large eddy breakup devices installed normal to the surface of a flat plate in an incompressible equilibrium turbulent boundary layer with a nominally gradientless flow past this plate is studied experimentally. The Reynolds number based on the boundary-layer momentum thickness is 1099 in the cross section where these vertical devices are mounted. It is shown that elements of this geometry are effective tools for modification of the turbulent boundary layer from the viewpoint of both the gain in friction drag and the suppression of turbulent fluctuations of velocity and, hence, can be considered as one of the most important factors of controlling the structure of the near-wall turbulence in the flow past a flat surface.

Management of a Plane Turbulent Wall Jet by the Large-Eddy Break-up Device

A Large Eddy Break-up (LEBU) device was applied to the flow management of a plane turbulent wall jet. The LEBU device was positioned at three different distances from the wall and the mean velocity, Reynolds shear stress profiles and wall shear stress were measured. The experimental data show that the LEBU device reduces the thickness of the shear layer and the magnitude of the Reynolds shear stress. The wall shear stress tends to be reduced even if the LEBU device is placed in the outer layer. These effects are most significant when the LEBU device is placed at the height at which the turbulent production term reaches its maximum. The length scale of the large eddies is reduced in manipulated flows, particularly in the spanwise direction.

Effect of Free-Stream Turbulence on the Reduction of Surface Friction in a Turbulent Boundary Layer Behind LEBU-Devices

The effect of increased free-stream turbulence on the reduction of the surface friction coefficient cf in a turbulent boundary layer behind large-eddy break-up (LEBU) devices is investigated using a gravimetric method. The turbulence level was e ≈ 1.9–4.9 % and the turbulence scale Le ≈ 40–110 mm. The boundary layer Reynolds number Re** was varied from 2300 to 7500, with the boundary layer thickness being varied on the range δ = 33–44 mm. It is shown that an increase in the turbulence level e has almost no impact on the relative reduction of friction behind LEBU-devices, whereas, under similar conditions of elevated free-stream turbulence, for another method, namely, the use of surface riblets, the friction reduction may be more strongly expressed.

MANAGEMENT OF A PLANE TURBULENT WALL JET BY THE LARGE-EDDY BREAK-UP DEVICE(Wall Jet and Wall Flow)

Large Eddy Break-up Device (LEBU) was applied to flow management of a plane turbulent wall jet. LEBU was located at three different distances from the wall and the mean velocity, Reynolds shear stress profiles and the wall shear stress were measured. The experimental data show that LEBU reduces development of thickness of the shear layer and magnitude of Reynolds shear stress. The wall shear stress tends to be reduced even if LEBU is placed in the outer layer. These effects are largest when LEBU is placed at the height where the turbulent production term reaches its maximum. Length scale of the large-eddies is reduced in manipulated flows, especially, in the spanwise direction.

Heat transfer enhancement of turbulent duct flow by vortex generators attached to a large eddy break-up manipulator

An experimental study was conducted to investigate the characteristics of wall heat transfer and momentum loss for turbulent duct flow disturbed by insertion of a complicated body composed of a Large Eddy Break-Up (LEBU) plate and winglet-type vortex generators. It was found that the LEBU plate reduces the wall heat transfer in the region downstream of the insertion position and that this suppression of heat transfer could be recovered by attaching vortex generators to the LEBU plate, i.e., conspicuous heat transfer enhancement was achieved over a large streamwise distance. The spatial distribution of the heat transfer coefficient obtained shows the same features as that observed in a previous study of a flat plate turbulent boundary layer. Therefore, the flow and thermal field structure of the turbulent duct flow downstream of the inserted body should be basically the same as those in the same region of the turbulent boundary layer. The effect of a notch, open in the LEBU plate behind the vortex generator, on heat transfer and pressure drop was also examined. The notch simulates the hole of the LEBU plate to be produced in a practical application when a vortex generator is produced by punching from the original plain LEBU plate. It was found that a vortex generator with an open notch works best in augmenting the wall heat transfer and also in suppressing the increase of momentum loss. © 1999 Scripta Technica, Heat Trans Asian Res, 28(3): 189–200, 1999

Heat transfer enhancement of turbulent duct flow by vortex generators attached to a large eddy break‐up manipulator

An experimental study was conducted to investigate the characteristics of wall heat transfer and momentum loss for turbulent duct flow disturbed by insertion of a complicated body composed of a Large Eddy Break-Up (LEBU) plate and winglet-type vortex generators. It was found that the LEBU plate reduces the wall heat transfer in the region downstream of the insertion position and that this suppression of heat transfer could be recovered by attaching vortex generators to the LEBU plate, i.e., conspicuous heat transfer enhancement was achieved over a large streamwise distance. The spatial distribution of the heat transfer coefficient obtained shows the same features as that observed in a previous study of a flat plate turbulent boundary layer. Therefore, the flow and thermal field structure of the turbulent duct flow downstream of the inserted body should be basically the same as those in the same region of the turbulent boundary layer. The effect of a notch, open in the LEBU plate behind the vortex generator, on heat transfer and pressure drop was also examined. The notch simulates the hole of the LEBU plate to be produced in a practical application when a vortex generator is produced by punching from the original plain LEBU plate. It was found that a vortex generator with an open notch works best in augmenting the wall heat transfer and also in suppressing the increase of momentum loss. © 1999 Scripta Technica, Heat Trans Asian Res, 28(3): 189–200, 1999

Heat transfer enhancement of turbulent duct flow by vortex generators attached to a large eddy break-up manipulator

Heat transfer enhancement of turbulent duct flow by vortex generators attached to a large eddy break-up manipulator