Spanwise Correlation Research Articles

Experimental investigation of aerodynamic forces and flow structures of bionic cylinders based on harbor seal vibrissa

In the present study, we investigated a new type of non-uniform surface cylinder based on a harbor seal vibrissa to reduce aerodynamic forces and suppress vortex shedding. Three vibrissa-based bionic cylinder models (i.e., models #1, #2, and #3) with different undulation wavelengths were manufactured and tested in a wind tunnel. The wind tunnel experiments were conducted at a Reynolds number of Re ≈ 50,000 based on the hydrodynamic diameter (Dh) and incoming airflow speed. The control effects on the aerodynamic forces and flow structures around the bionic cylinders were studied and compared with those of a baseline circular cylinder. In addition to measuring the surface pressure distributions using an array of digital pressure transducers, a high frequency force balance was used to determine the aerodynamic forces acting on the test models. Moreover, a digital particle image velocimetry (PIV) system was utilized to quantify the stream-wise flow structure and span-wise flow field to assess the effectiveness of the bionic cylinder models. The results revealed that a bionic non-uniform surface distribution could reduce the mean drag and suppress the fluctuating lift to a certain extent. Bionic cylinder model #2, which had a wavelength of two minor axes, worked best among the three models, and it was found to achieve a drag reduction of 15% and a maximum fluctuating lift suppression of 58% when the angle of the incoming airflow was 0°. The PIV results indicated that the non-uniform surface structure would disrupt the consistency of span-wise flow and decrease the span-wise correlation in the near wake of the test models, which could decrease the turbulent kinetic energy, mean drag, and fluctuating lift of the test models.

Spatial modulations of kinetic energy in the roughness sublayer

High-Reynolds-number experiments are conducted in the roughness sublayer of a turbulent boundary layer developing over a cubical canopy. Stereoscopic particle image velocimetry is performed in a wall-parallel plane to evidence a high degree of spatial modulation of the small-scale turbulence around the footprint of large-scale motions, despite the suppression of the inner layer by the high roughness elements. Both Fourier and wavelets analyses show that the near-wall cycle observed in smooth-wall-bounded flows is severely disrupted by the canopy, whose wake in the roughness sublayer generates a new range of scales, closer to that of the outer-layer large-scale motions. This restricts significantly scale separation, hence a diagnostic method is developed to divide carefully and rationally the fluctuating velocity fields into large- and small-scale components. Our analysis across all turbulent kinetic energy terms sheds light on the spatial imprint of the modulation mechanism, revealing a very different signature on each velocity component. The roughness sublayer shows a preferential arrangement of the modulated scales similar to what is observed in the outer layer of smooth-wall-bounded flows – small-scale turbulence is enhanced near the front of high momentum regions and damped at the front of low momentum regions. More importantly, accessing spanwise correlations reveals that modulation intensifies the most along the flanks of the large-scale motions.

Noise reduction mechanisms of sawtooth and combed-sawtooth trailing-edge serrations

Trailing-edge serrations are add ons retrofitted to wind-turbine blades to mitigate turbulent boundary-layer trailing-edge noise. This manuscript studies the physical mechanisms behind the noise reduction by investigating the far-field noise and the hydrodynamic flow field. A conventional sawtooth and a combed-sawtooth trailing-edge serration are studied. Combed-sawtooth serrations are obtained by filling the empty space between the teeth with combs (i.e. solid filaments). Both serration geometries are retrofitted to a NACA 0018 aerofoil at zero degree angle of attack. Computations are carried out by solving the explicit, transient, compressible lattice Boltzmann equation, while the acoustic far field is obtained by means of the Ffowcs Williams and Hawkings analogy. The numerical results are validated against experiments. It is confirmed that the combed-sawtooth serrations reduce noise more than the conventional sawtooth ones for the low- and mid-frequency range. It is found that the presence of combs affects the intensity of the scattered noise but not the frequency range of noise reduction. For both configurations, the intensity of the surface pressure fluctuations decreases from the root to the tip, and noise sources are mainly located at the serrations root for the low- and mid-frequency range. The presence of the filaments generates a more uniform distribution of the noise sources along the edges with respect to the conventional serration. The installation of combs mitigates the interaction between the two sides of the aerofoil at the trailing edge and the generation of a turbulent wake in the empty space between teeth. As a result, the inward (i.e. from the serration edge to the centreline) and outward (i.e. from the serration centreline to the edge) flow motions, due to the presence of the teeth, are mitigated. It is found that the installation of serrations affects the surface pressure fluctuations integral parameters. Both the spanwise correlation length and convective velocity of the surface pressure fluctuations increase with respect to the baseline straight configuration. When both quantities are similar to the one obtained for the straight trailing edge, the effect of the slanted edge is negligible, thus corresponding to no noise reduction. It is concluded that the changes in sound radiation are mainly caused by destructive interference of the radiated sound waves for which a larger spanwise correlation length is beneficial. Finally, the difference between measurements and the literature is caused by an incorrect modelling of the spanwise correlation length, which shows a different decay rate with respect to the one obtained for a straight trailing edge.

Experimental and Computational Investigation of Blunt Trailing-Edge Airfoils with Splitter Plates

Experimental and computational examinations of the effect of splitter plate length on the aerodynamic performance and vortex shedding behavior of a blunt trailing-edge airfoil are presented along with an evaluation of spanwise shedding coherence. The FB-3500-1750 airfoil, which has a maximum thickness of 35% chord and trailing-edge thickness of 17.5% chord, was tested in the University of California, Davis, Aeronautical Wind Tunnel at a chord Reynolds number of 666,000. The flow behavior was also examined computationally using OVERFLOW, a Reynolds-averaged Navier–Stokes solver. Splitter plates with lengths ranging from 50 to 150% of the trailing-edge thickness were applied to the airfoil model. Drag reductions of at least 27% from the baseline were observed with the inclusion of a splitter plate with length 50% of the trailing-edge thickness, accompanied by a 5% loss in maximum lift. An increasing splitter plate length was shown to decrease the base drag, maximum lift, and stall angle. The Strouhal number also increased with an increasing splitter plate length over the range examined. The spanwise correlation length of the vortex shedding was determined to be four to five times the trailing-edge thickness for both fixed and free transition at two different angles of attack.

Large eddy simulation of flow over inclined wavy cylinders

The wavy cylinders have been proven effective in suppressing the Kármán vortex shedding and mitigating the aerodynamic forces. As yet their performance in the inclined incoming flow has not been well understood. In the current work, large eddy simulations are carried out to unveil the aerodynamics of the flow around inclined wavy cylinders at the Reynolds number of 5000. Three inclination angles, 0°, 30° and 45°, together with 2 × 2 combinations of shape parameters, namely λ∕Dm=2 and 6, a∕Dm=0.1 and 0.15, have been taken into consideration. Firstly, the aerodynamic force coefficients in terms of both the span-wise averaged and sectional values are discussed at length. It is revealed that the growing inclination angle invites not only a surge in their span-wise averaged values, but also an enlargement in the sectional difference of the force coefficients. The span-wise correlation of the lift force is found to be enhanced for the wavy cylinders with the increase of inclination angle, while the opposite is true for the normal cylinders. By examining the mean wake properties, it is disclosed that the increase in the span-wise averaged drag force coefficients is closely related to the shrinkage in the vortex formation length and the regression of the base pressure, whereas the sectional distribution of the drag coefficients is largely affected by the stagnation pressure. The mean wall shear stress fields are exploited to shed light on the flow topology of the cylinder surfaces. It is discovered that the wavy cylinders, especially the short-wavelength one, exhibit significant span-wise variation in the separation structure in the inclined flow. Besides, the chaotic surface flow in the rear side of the non-inclined cylinders could be regulated to maintain symmetry by the secondary axial flow in the near wake of the inclined cylinders. The instantaneous three-dimensional vortical structures are visualized by the Q isosurfaces at last to collaborate the previous discussions. A preliminary mechanism for the cessation of flow control efficacy for the wavy cylinders in inclined flow is also presented.

Numerical analysis of the impact of permeability on trailing-edge noise

The impact of porous surfaces on the near-wall turbulent structures and the generated trailing-edge noise is analyzed for several trailing-edge shapes of finite thickness using a high resolution large-eddy simulation (LES)/computational aeroacoustics (CAA) method. The porous surface of the trailing edge is defined by the porosity and the viscous permeability determined by the solution of a turbulent flat plate boundary layer at a Reynolds number 1280 based on the displacement thickness in the inflow cross section. The volume-averaged approach for the homogeneous porous medium shows that the porous impedance scales linearly with the porosity and exponentially with the mean structure size of a porous medium. The drag induced by the porous surface changes the friction velocity and the permeability Reynolds number ReK which determines the porous impedance Rs scaled by ReK−2/3. The trailing-edge noise is analyzed for three solid and three porous trailing edges. The effect of a finite span is investigated by the spanwise correlation model based on the measured coherence distribution. The acoustic prediction shows a good agreement with measurements of the broadband spectrum and the strong tone generated by a finite trailing-edge thickness. The pressure gradient inside the porous media is redistributed by the Darcy drag defined by the viscous permeability and the porosity. The mean pressure increases in the upstream direction inside the porous medium such that the flow acceleration involved in the acoustic generation is reduced inside the porous medium. The noise reduction by a porous medium reaches 11 dB for the trailing-edge shape which possesses a sharp corner for the solid surface. The porous surface applied to a semi-circular trailing edge achieves a 4 dB noise reduction. The directivity pattern for individual components of the acoustic spectrum shows that the massive noise reduction is determined at the tone. Enhanced wave diffraction by the thick flat plate changes the directivity pattern in the high frequency range.

Aerodynamic admittance of truss girder in homogenous turbulence

An approach to directly identify truss girder's aerodynamic lift admittance by buffeting force measurement is put forward. Based on an assumption that a truss girder is equivalent to a continuous cross-section girder, three forms of aerodynamic lift admittance are deduced: complete 2-dimensional, 3-dimensional and 2-wavenumber. Span-wise lift coherence and correlation functions of the equivalent truss cross-section are derived as well. A large-span truss girder bridge is adopted as prototype to validate the approach. Numerical solutions of above-mentioned items are obtained through wind tunnel tests. The configuration of truss girder lift admittance is proved different from traditional known admittance feature. Energy transferring tendency is revealed by 2-wavenumber admittance. Complete 2-dimensional lift admittance is found possible to overestimate truss girder's buffeting response at the most concerned wavenumber range. Span-wise lift correlation and coherence of the equivalent cross-section are proved slightly lower than that of truss segment. In addition, a brief study on influence of slight varied incident angle is carried out, in which aerodynamic lift span-wise coherence is proved obviously affected at low wavenumber range.

Identification and application of six-component aerodynamic admittance functions of a closed-box bridge deck

A colligated residue least square method of auto and cross spectra (CRLSMACS) is presented for identifying six-component aerodynamic admittance functions (AAFs) base on common force and pressure measurement tests in the passive grid-generated turbulent flow. The incomplete spanwise correlation of the buffeting force on the sectional model in the turbulent flow is corrected. The six-component AAFs of a flat closed-box deck of a single-tower cable-stayed bridge are identified with the presented method for the service states without/with wind barriers and the construction state under 0° wind attack angle, respectively. The buffeting responses of the bridge are then calculated for both the service and typical construction sates by using a finite element approach based on the quasi-steady buffeting theory with the tested six-component AAFs to consider the unsteady effect of buffeting forces. The calculated buffeting responses are finally compared with and found to agree well with those obtained via full bridge aeroelastic model tests, verifying the feasibility of the proposed CRLSMACS as well as the reliability of the identified six-component AAFs and the calculated buffeting responses.

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.

Transient Growth and Receptivity of Steady Disturbances to Irregular Rough Walls

Direct numerical simulations (DNS) are performed to investigate the transient growth of a steady disturbance induced by a numerically generated Gaussian rough wall in a laminar boundary layer. In the calculation of the interaction between the rough wall and the fluid, the multiple direct force and immersed boundary method (MDF/IBM) are adopted. The evolution of the streak structures and the energy of the disturbances generated by the rough wall are presented. A similar evolution into an almost sinusoidal modulation for the cylindrical roughness element is found for the current irregular rough wall, and the disturbance energy also undergoes the classical transient growth mode. Moreover, the influences of the skewness, kurtosis, and correlation length on the evolution of spanwise harmonics are also analyzed. The results show that the effects of skewness and kurtosis are on the distribution of energy among the wavelengths and the subsequent growth processes, while the wavelengths of the harmonics are linked to both the streamwise and spanwise correlation lengths of the rough wall.

Experimental investigation of correction factor for VIV amplitude of flexible bridges from an aeroelastic model and its 1:1 section model

Evaluation of the maximum amplitudes of vortex-induced vibrations (VIV) in suspension bridges is mainly relied upon section model wind tunnel tests of bridge decks. Extrapolation to prototype responses requires addition correction to take into account the differences both in structural mode shape and in spanwise correlation of excitation forces between the section model and the prototype. The paper presents the results of VIV amplitudes measured on an aeroelastic model and on its section model with the same size of cross-section. The aeroelastic model is a new, elastically multi-supported one which is capable of reproducing the closely-spaced vertical modes of a suspension bridge. The vortex-induced vibrations for a number of vertical modes were successfully measured from the aeroelastic model test in smooth flow. The spring-mounted section model having the same size of cross-section as the aeroelastic model was also tested in smooth flow at the same mass, damping ratio and vibration frequency. It is shown that both the VIV onset wind velocity and lock-in range for each mode of vibration are in good agreement between the aeroelastic model and the section model. The aeroelastic model consistently produces a roughly 30% higher maximum amplitude than the section model for the observed modes of vibration. It is confirmed that the combined three-dimensional (3D) effects due to mode shape and imperfect correlation of excitation forces amplify the VIV amplitudes and disregarding these effects may underestimate the VIV amplitudes for section model tests. A correction factor of 1.3 for the VIV amplitude of vertical modes in suspension bridges may be adopted for practical use when extrapolating the section-model results to their corresponding full-scale values.

Development of streak instability modes excited by turbulent fluctuations

Development of streak instability modes excited by disturbances whose frequency-spectrum was similar to that of wall turbulence was examined experimentally for a single screen-generated low-speed streak. The disturbances were introduced into the low-speed streak through two small holes made on both the sides of the low-speed streak. The result showed that even when the low-speed streak was forced by turbulent fluctuations with zero spanwise correlation, sinuous instability modes were excited with distinct frequency selectivity around the most unstable frequency of the linear streak instability and dominated streak breakdown in the downstream region. The magnitude of the sinuous modes excited was compared to the case when the anti-symmetric forcing with the same turbulence spectrum was applied.

Investigation on characteristics and span-wise correlation of vortex-induced forces on a twin-box deck using newly-developed wind-tunnel test technique

Vortex-induced vibration (VIV) is a typical type of wind-induced vibration of long-span bridges, which has the potential to cause large-amplitude oscillation at low wind speeds. The span-wise correlation of vortex-induced forces (VIFs) should be considered in the prediction of vortex-induced responses. At present, forced-motion tests with pressure measurements are often used to investigate the span-wise correlation of VIFs, but these tests may not be able to truly represent the nonlinear wind-structure interaction in VIV. This paper uses a newly-developed wind tunnel test technique to measure vortex-induced responses and forces at different span-wise locations. This study investigates the frequency-domain characteristics and the span-wise correlation of the VIFs on a twin-box deck under both smooth and turbulent flow fields. The results show that the fundamental- and triple-frequency components of the VIFs play major roles in the VIV. The fundamental-frequency component of the VIFs can be seen as perfectly correlated within the lock-in range under smooth wind flow, even when the structural damping ratio increases from 0.35% to 0.8%. Under turbulent flow field, the correlation decays mildly with the increase of turbulence intensity, but within the lock-in range, the VIF is still well correlated even with a turbulence intensity of 8%.

Numerical study on reduction of aerodynamic noise around an airfoil with biomimetic structures

A biomimetic airfoil featuring leading edge waves, trailing edge serrations and surface ridges is proposed in this study, based on flow control with each section meeting the NACA 0012 airfoil profile. Numerical simulations have been conducted to compare aerodynamic and acoustic performances between the NACA 0012 and biomimetic airfoils. These simulations utilize the large eddy simulation (LES) method and aeroacoustic analogy at an angle of attack of 0° and a Reynolds number of 1.0×105, based on using the airfoil chord as the characteristic length. The simulation results reveal the overall sound pressure levels (OASPLs) for all frequencies and at the seven observer points around the biomimetic airfoil, and a decrease of 13.1–13.9dB is observed, whereas the drag coefficient is almost unchanged. The biomimetic structures can transform the shedding vortices in laminar mode for the NACA 0012 airfoil to regular horseshoe-type vortices in the wake, and reduce the spanwise correlation of the large-scale vortices, thereby restrain the vortex shedding noise around the biomimetic airfoil.

Wake of a Lifting Wing with Cut-In Sinusoidal Trailing Edges

The wake behind a NACA0012 wing at incidence with cut-in sinusoidal trailing edges is experimentally investigated. A wing model with interchangeable trailing edges is used to study their impact on the wake properties. Both vertical and spanwise traverses of hot wires are done at different downstream positions to obtain the downstream evolution of statistical properties and to perform spectral analysis. Stereoscopic particle image velocimetry is used to study the flow structure in a spanwise/cross-stream plane. Spanwise inhomogeneity of the velocity deficit and of the wake width is observed and explained by the presence of a spanwise/cross-stream flow induced by the cut-in modifications. Spectral analysis shows a decrease of shedding intensity with a shorter trailing-edge wavelength, with a reduction of up to 57% when compared to a straight blunt wing. Blunt sinusoidal trailing edges exhibit a reduction of spanwise correlation compared to a blunt straight one. A sharp cut-in design is also studied, which exhibits a more broadband shedding spike at a lower frequency.

Span-wise correlation of wind-induced fluctuating forces on a motionless flat-box bridge deck

A detailed wind tunnel test has been carried out on a motionless flat-box bridge deck model with three special grid-generated wind fields using an electronically scanned pressure transducer system, enabling almost instantaneous capture of 372 pressure tap signals from 6 sections of 62 different spacing taps at each section. A very complete analysis has been performed on the pressure signals, and fluctuating forces through weighted integrations of pressure at each section were formed. The integral scales of fluctuating force were found to be dependent mainly on the integral scale of turbulence and the size of deck. The root coherence of buffeting force relied chiefly on integral scales of force and the separation of two sections, and the span-wise correlation of the fluctuating forces were larger than those of wind turbulence. An empirical model of root coherence for the fluctuating lift for a flat-box deck has been developed and proved to be feasible and effective for fitting the test data. The fitted parameters can be defined by simple expressions related to the oncoming turbulence and the size of deck, and thus it has practical applications. The proposed model can fully explain the character of low frequency curves in the root coherence.

Large-eddy simulations of flow past a square cylinder using structured and unstructured grids

The flow past a square cylinder at Re=2.2×104 is analyzed by large-eddy simulation (LES) using the fine grids in order to represent details of near-cylinder flows. The accuracy of LES on structured and unstructured grids is assessed from the engineering viewpoint, compared with previous studies. The finite differencing method code with 4th order central scheme for the convective term is used for structured LES, while the open-source finite volume method code (OpenFOAM 2.3.0) with 1st–2nd order schemes is applied for unstructured LES. Typical schemes in OpenFOAM are tested, i.e., “LUST” (blending of linear and linear upwind schemes), “limitedLinear” (TVD) and “linearUpwind” (linear upwind). In this study, three effects are emphasized: numerical schemes for convective terms, meshing strategies, and spanwise resolution and length. On the whole, OpenFOAM can obtain fairly accurate prediction of the time-averaged and r.m.s quantities no matter which numerical scheme is used. “LUST” and “linearUpwind” are suggested. Meshing refinement in wake can be a solution to improve the far-wake velocity distribution and to overcome the earlier energy decay of turbulent motions in the inertial subrange caused by artificial dissipation. Different degrees of instantaneous flow reattachment near the trailing edges are found in the results obtained by OpenFOAM. Flow reattachment is closely associated with the roll-up of shear layer, and the flow topology between the shear layer and the side wall featured by the frontal and leeward secondary vortex. In this regard, the refinement of hexahedra cells near cylinder gives a best solution among all present cases tested compared with the previous DNS result. Generally the Kelvin–Helmholtz instability can be accurately predicted by OpenFOAM under the present numerical conditions. In addition, the spanwise resolution seems no big effect in predicted results when less than 0.05D. By contrast, the increase in spanwise length to 14D from 4D plays an important role in obtaining reasonable spanwise correlation of pressure and consequently the overall fluctuating lifts.

Spanwise correlation of aerodynamic forces on oscillating rectangular cylinder

The aerodynamic forces (lift and moment) on oscillatory rectangular cylinder with side ratio B/D=5 are investigated by synchronized surface pressure measurement and forced motion technique. The effects of amplitude of vibration, incident angle and Reynolds number on the spanwise correlation of aerodynamic loading are studied. Results show that aerodynamic forces along the span are not fully correlated, which indicates that the application of the strip assumption may overestimate the overall aerodynamic loading on large-span structures, and consequently may result in a large error margin in evaluation of flutter and aeroelastic response. It is also shown that the pattern of vibration and its amplitude play a significant role in dektermining the spanwise correlation of the aerodynamic forces, including the motion-induced force and the vortex-induced force. Furthermore, the effects of the incident angle and Reynolds number are related with the vibrating pattern, and should not be negligible in some cases.

Numerical study on the effect of shape modification to the flow around circular cylinders

Shape modification has been one of the effective ways of manipulating the flow past bluff bodies. Aiming at exploration of the flow control mechanisms of shape modified circular cylinders, a series of numerical simulations are conducted at the Reynolds number of 5000. The modifications adopted in the current paper could be divided into two categories, the 2-dimensional cross-sectional modifications (polygonal and ridged) and 3-dimensional span-wise modifications (linear wavy, sinusoidal wavy and O-ringed). By exploiting the numerical data obtained from the Large Eddy Simulations, several aspects, including the wake flow properties, the aerodynamic forces, the flow instabilities and span-wise correlations are compared in detail. The two wavy cylinders are found to modify the flow wake significantly, leading to considerable reduction in aerodynamic forces. The polygonal and ridged cylinders, however, are of no such effect in aerodynamic force mitigation. The O-rings only have slight effect on the flow wake and the forces. As for the flow instabilities, the modifications adopted in this paper barely affects the Kármán vortex shedding frequency. The shear layer frequency, however, differs from case to case. Particularly, in the cases of the two wavy cylinders, disparity exists in the shear layer frequency at the node and saddle. This is explained by the boundary layer properties at separation. While recognizing the insufficiency in the span-wise length of the cylinders, the span-wise correlations of the aerodynamic forces are inspected. It is found that the lift forces of the 3-dimensional cylinders are generally less correlated than that of the 2-dimensional ones.

On the Formation Mechanisms of Artificially Generated High Reynolds Number Turbulent Boundary Layers

We investigate the evolution of an artificially thick turbulent boundary layer generated by two families of small obstacles (divided into uniform and non-uniform wall normal distributions of blockage). One- and two-point velocity measurements using constant temperature anemometry show that the canonical behaviour of a boundary layer is recovered after an adaptation region downstream of the trips presenting $$150~\%$$ higher momentum thickness (or equivalently, Reynolds number) than the natural case for the same downstream distance ( $$x\approx 3\,$$ m). The effect of the degree of immersion of the trips for $$h/\delta \gtrsim 1$$ is shown to play a secondary role. The one-point diagnostic quantities used to assess the degree of recovery of the canonical properties are the friction coefficient (representative of the inner motions), the shape factor and wake parameter (representative of the wake regions); they provide a severe test to be applied to artificially generated boundary layers. Simultaneous two-point velocity measurements of both spanwise and wall-normal correlations and the modulation of inner velocity by the outer structures show that there are two different formation mechanisms for the boundary layer. The trips with high aspect ratio and uniform distributed blockage leave the inner motions of the boundary layer relatively undisturbed, which subsequently drive the mixing of the obstacles’ wake with the wall-bounded flow (wall-driven). In contrast, the low aspect-ratio trips with non-uniform blockage destroy the inner structures, which are then re-formed further downstream under the influence of the wake of the trips (wake-driven).