Velocity Deficit Research Articles

Unsteady flows of a highly loaded turbine blade with flat endwall and contoured endwall

Strong secondary flow results in substantial aerodynamic loss for highly-loaded turbine. Accurate prediction of the complicated flow structures proposes challenges to the widely used Reynolds Average Navier-Stokes (RANS) approach. This work employs the hybrid RANS/Large Eddy Simulation (LES) method to study the unsteady flows for a highly-loaded turbine blade, with both flat endwall and optimized contoured endwall. Evolution of the unsteady flows in the endwall region is analyzed, with emphasis on the loss generation mechanism. Results show that for the flat endwall, the horseshoe vortex system is highly unsteady and is a significant source of unsteadiness in both the passage and the wake region. It contributes to the unsteady passage vortex, also the earlier breakdown of the trailing edge shedding vortex. The Probability Density Function (PDF) histogram of velocity in the wake region is bimodal, implying the perturbations from two mechanisms. For the contoured endwall, the unsteady evolution of the horseshoe vortex is blocked, which results in significantly reduced unsteadiness, also the merge between the horseshoe vortex with the passage vortex is prevented. Effect of the flow unsteadiness on the loss generation is assessed based on the entropy generation rates contributed by the time-averaged flow and the fluctuations. The effect of unsteadiness is two-fold: unsteady perturbations trigger the vortex breakdown into small-scale structures and thus weakened wake velocity deficit and loss generation; however, in both the passage and the wake region the entropy generation by the fluctuations are remarkable. For the contoured endwall, due to the reduced flow unsteadiness, loss generation contributed by the fluctuations are much smaller compared to the flat endwall. The results highlight the importance of including the loss generation by the fluctuations, also a possible mechanism to reduce the secondary loss by attenuating the flow unsteadiness with the contoured endwall.

Rotating detonative combustion in partially pre-vaporized dilute n-heptane sprays: Droplet size and equivalence ratio effects

Eulerian-Lagrangian simulations are conducted for two-dimensional Rotating Detonative Combustion (RDC) fueled by partially prevaporized n-heptane dilute sprays by using coarse mesh resolution. The air is used as the oxidizer in the present simulations. The influences of droplet diameter and total equivalence ratio on detonation combustion and droplet dynamics are studied. It is found that small n-heptane droplets (e.g. 5 µm) are completely vaporized around the detonation wave, while intermediate n-heptane droplets (e.g. 20 µm) are consumed in or behind the detonation wave, with the escaped ones being continuously evaporated and deflagrated. The droplet distributions in the rotating detonation combustor are significantly affected by the droplet evaporation behaviors. Both premixed and non-premixed combustion modes are seen in two-phase RDC. The detonated fuel fraction is high when the droplet diameters are small or large, reaching its minimal value with diameter being 20 µm. The detonation propagation speed decreases with increased droplet diameter and is almost constant when the diameter is larger (>30 µm). The velocity deficits are 2-18% compared to the respective gaseous cases. Moreover, the propagation speed increases as the total equivalence ratio increases for the same droplet diameter. It is also found that the detonation propagation speed and detonated fuel fraction are considerably affected by the pre-vaporized gas equivalence ratio. The specific impulse first decreases for cases with initial diameter<5 μm, then increases with droplet diameter between 5 μm and 20 μm, and finally decreases with droplet diameter>20 μm.

Modelling Yawed Wind Turbine Wakes: Extension of a Gaussian-Based Wake Model

Yaw-based wake steering control is a potential way to improve wind plant overall performance. For its engineering application, it is crucial to accurately predict the turbine wakes under various yawed conditions within a short time. In this work, a two-dimensional analytical model is proposed for far wake modeling under yawed conditions by taking the self-similarity assumption for the streamwise velocity deficit and skewness angle at hub height. The proposed model can be applied to predict the wake center trajectory, streamwise velocity, and transverse velocity in the far-wake region downstream of a yawed turbine. For validation purposes, predictions by the newly proposed model are compared to wind tunnel measurements and large-eddy simulation data. The results show that the proposed model has significantly high accuracy and outperforms other common wake models. More importantly, the equations of the new proposed model are simple, the wake growth rate is the only parameter to be specified, which makes the model easy to be used in practice.

Understanding of remora's \u201chitchhiking\u201d behaviour from a hydrodynamic point of view

Symbiotic relationships have developed through natural evolution. For example, that of the remora fish attached to the body of a shark. From the remora’s perspective, this could be associated to an increased hydrodynamic efficiency in swimming and this needs to be investigated. To understand the remora's swimming strategy in the attachment state, a systematic study has been conducted using the commercial Computational Fluid Dynamics (CFD) software, STAR-CCM + to analyse and compare the resistance characteristics of the remora in attached swimming conditions. Two fundamental questions are addressed: what is the effect of the developed boundary layer flow and the effect of the adverse pressure gradient on the remora’s hydrodynamic characteristics? According to the results, the resistance of the remora can generally be halved when attached. Besides, the results have also demonstrated that the drag reduction rate increases with the developed boundary layer thickness and can be estimated using the boundary layer thickness ratio and velocity deficit. The paper demonstrates that the most frequent attachment locations are also the areas that provide the maximum drag reduction rate.

The effect of sickle cell anemia on the linear growth of Nigerian children.

Despite the high prevalence of children with sickle cell anaemia (SCA) in West Africa, there is paucity of data on the height velocity and prevalence of growth failure in SCA patients. With advances in clinical care of SCA patients, could there be a spatial and secular trend in the growth pattern of these children? Hence, the compelling needs to embark on this study. The objectives of the study were to determine the prevalence of growth failure among patients with SCA and its correlation with age, gender and age at diagnosis. A Prospective longitudinal study of a cohort of sickle cell anaemic paediatric patients from Pediatrics SCA Clinic, University of Nigeria Teaching Hospital, Ituku Ozalla. Patients were enrolled over a period of two years using a non-parametric convenient sampling method. Their heights were measured at baseline, three months, six months and at 12 months intervals and subsequently plotted on a standard WHO growth chart. The height velocities at different monthly intervals were calculated and compared with the WHO standard normal linear growth rates) for children (used as control) to identify those with GF. (i.e. <10th percentile). The main outcome measures were the mean height velocities at different months' intervals calculated and compared using the repeated measurement analysis of variance (ANOVA) and the Wilcoxon signed test. A cohort of 316 children aged 1-18 years with SCA was evaluated with a male preponderance of 161 (57.4%). The mean age and age at diagnosis were 11.04±5.56 and 4.2±1.7 years, respectively. The prevalence of growth failure and short stature was 84.7%. The burden of GF was highest among post-pubertal participants (94.1%). The most important predictor of growth velocity deficit was age (R2=0.045, standard β coefficient=-0.22, t=-03.51, p=0.001). The study demonstrated high prevalence of growth failure in children and adolescents with SCA which intensified with advancement in age and older age at diagnosis.

Modelling Deep Green tidal power plant using large eddy simulations and the actuator line method

The Deep Green technique for tidal power generation is suitable for moderate flows which is attractive since larger areas for tidal energy generation hereby can be used. It operates typically at mid-depth and can be seen as a “flying” kite with a turbine and generator attached underneath. It moves in a lying figure-eight path almost perpendicular to the tidal flow.Large eddy simulations and an adaption of the actuator line method (in order to describe arbitrary paths) are used to study the turbulent flow with and without Deep Green for a specific site. This methodology can in later studies be used for e.g. array analysis that include Deep Green interaction. It is seen that Deep Green creates a unique wake composed of two velocity deficit zones with increased velocity in each wake core. The flow has a tendency to be directed downwards which results in locally increased bottom shear. The persistence of flow disturbances of Deep Green can be scaled with its horizontal path width, Dy, with a velocity deficit of 5% at approximately 8–10Dy downstream of the power plant. The turbulence intensity and power deficit are approximately two times the undisturbed value and 10%, respectively, at 10Dy.

A new wake‐merging method for wind‐farm power prediction in the presence of heterogeneous background velocity fields

AbstractThe difference in surface roughness between land and sea, and the terrain complexities, lead to spatially heterogeneous atmospheric conditions, and therefore affect the propagation and dynamics of wind‐turbine and wind‐farm wakes. Currently, these flow heterogeneities and their effects on plant aerodynamics are not modeled in the majority of engineering wake models. In this study, we address this issue by developing a new wake‐merging method capable of superimposing the waked flow on a heterogeneous background velocity field. We couple the proposed wake‐merging method with four different wake models, i.e. the Gaussian, super‐Gaussian, double‐Gaussian and Ishihara model, and we test its performance against LES results, dual‐Doppler radar measurements and SCADA data from the Horns Rev, London Array, and Westermost Rough farms. The standard Jensen model with quadratic superposition is also included. In homogeneous conditions, the new method predicts slightly higher velocity deficits than the linear superposition method. Overall, the distributions of the difference in power ratio between the two wake‐merging methods predictions and observations show a similar mean absolute error (MAE) and interquartile range (IQR) in such conditions. On the other hand, the new wake‐merging method predictions display a lower MAE with a similar IQR in case of a spatially varying background velocity, being overall more accurate than the ones obtained with linear superposition. The most accurate estimates are obtained when the wake‐merging methods are coupled with the double‐Gaussian and Gaussian single‐wake models. In contrast, the Jensen and super‐Gaussian wake models overestimate the velocity deficits for the majority of cases analyzed.

Investigation of the wake propagation behind wind turbines over hilly terrain with different slope gradients

In this study, a Reynolds-averaged Navier-Stokes (RANS) numerical simulation with wind turbine simulated through actuator disk model was conducted to quantify the characteristics of wind turbine wake over two-dimensional Gaussian hills with different slope gradients. The simulated flow characteristics in the wind turbine wake, such as velocity deficit, wake expansion and wake centerline, were compared with those given by the Jensen wake model to quantitatively evaluate the accuracy of the current engineering wake model for the optimal design of wind turbine layout over complex terrain. For the gentle slope hill, the topography behind the wind turbine has a significant effect on the wake characteristics, which is not taken into account in the Jensen wake model. A new method for calculating wake velocity is proposed based on the local speed-up factor and the simulated velocity in the wake of wind turbine sited on flat terrain that can more reasonably predict the wake velocity over complex terrain. For the steep slope hill, the propagation of wind turbine wake is not related to the hill shape in the region behind the hilltop. The assumption that the wake centerline follows the surface of the hill in the Jensen wake model is no longer applicable. The recovery of wind turbine wake in the far wake region was always faster for the steep slope hill compared with the gentle slope hill case. This acceleration of wake recovery is thought to be closely related to the separation flow at the lee side of the hill, which highly restricts the downward deflection and expansion of the wind turbine wake.

Experimental and Numerical Investigation of Gaseous Detonation in a Narrow Channel with Obstacles

Gaseous detonation propagation in a thin channel with regularly spaced cylindrical obstacles was investigated experimentally and numerically. The wave propagation with substantial velocity deficits is observed and the details of its propagation mechanism are described based on experimental measurements of the luminosity and pressure and on three-dimensional flow fields obtained by numerical simulations. Both experiments and simulations indicate a significant role of shock–shock and shock–obstacle interactions in providing high-temperature conditions necessary to sustain the reaction wave propagation.

Optimization of a wind farm by coupled actuator disk and mesoscale models to mitigate neighboring wind farm wake interference from repowering perspective

This study explores the effects of inter-farm wakes and proposes staggering schemes that are most suitable for optimization of existing wind farm arrays to mitigate the effects of compound wakes. The case study considers a total of 9 out of 33 most deteriorated wind turbine for a microscale numerical analysis using the steady-state actuator disk model coupled with the mesoscale boundary condition data. Furthermore, the convective atmospheric boundary layer has also been considered. For vertically staggered layouts, the effect of the inter-farm wakes appeared mild at 100 m, modest at 80 m, and high at 60 m; as the maximum velocity deficit observed under the influence of compound wakes is approximately 13.3%, 14.1%, and 15.2%, respectively. Onsite recorded power data has been used to validate the baseline predicted powers at 80 m hub height. Both vertical and horizontal staggering options have been assessed for partial repowering. By elevating the turbines to a 100 m hub height, the cumulative power generation from the 9 × turbines increased by approximately 13.5% while reducing the hub height to 60 m decreased the power output by approximately 11.5% of that of the baseline at 80 m hub height. Further increase in cumulative power of up to 23% compared to existing layout is achieved by applying a lateral repositioning of 3 × underperforming turbines now positioned at 100 m hub height. This paper hence presents an applied insight for partial repowering of onshore wind farms affected by inter-farm wakes.

Prediction of power generation of two 30 kW Horizontal Axis Wind Turbines with Gaussian model

In order to improve the total efficiency of power generation in a wind farm, this paper investigated the effect of different operating conditions of the upstream wind turbine on the power of the downstream wind turbine by Gaussian wake model. A three-bladed Horizontal Axis Wind Turbine with the generator capacity of 30 kW was used. Firstly, Gaussian model for evaluating the wake flow was developed, which fit well with experiment data. Then, the influence of the operating condition of the upstream turbine on the downstream turbine performance was estimated by calculating the energy amount resulted from the velocity deficit of the wake. Finally, the effect of the installation position of the downstream wind turbine was analyzed. In consequence, the total power coefficient Ptotal of the wind turbines was relatively higher when the upstream turbine was at tip speed ratio λ = 6.4 and pitch angle β = 2° among different test conditions. Meanwhile, the Ptotal showed the maximum value when the downstream turbine was installed at the streamwise location x/D = 2.0 and lateral offset location y/R = 1.5 when the inflow turbulence intensity is 0.22≤TIref ≤ 0.35. This paper provided a better guidance for optimizing the wind turbine layout and improving the total power output of the entire wind farm.

A case study of wind farm effects using two wake parameterizations in the Weather Research and Forecasting (WRF) model (V3.7.1) in the presence of low-level jets

Abstract. While the wind farm parameterization by Fitch et al. (2012) in the Weather Research and Forecasting (WRF) model has been used and evaluated frequently, the explicit wake parameterization (EWP) by Volker et al. (2015) is less well explored. The openly available high-frequency flight measurements from Bärfuss et al. (2019a) provide an opportunity to directly compare the simulation results from the EWP and Fitch scheme with in situ measurements. In doing so, this study aims to complement the recent study by Siedersleben et al. (2020) by (1) comparing the EWP and Fitch schemes in terms of turbulent kinetic energy (TKE) and velocity deficit, together with FINO 1 measurements and synthetic aperture radar (SAR) data, and (2) exploring the interactions of the wind farm with low-level jets (LLJs). This is done using a bug-fixed WRF version that includes the correct TKE advection, following Archer et al. (2020). Both the Fitch and the EWP schemes can capture the mean wind field in the presence of the wind farm consistently and well. TKE in the EWP scheme is significantly underestimated, suggesting that an explicit turbine-induced TKE source should be included in addition to the implicit source from shear. The value of the correction factor for turbine-induced TKE generation in the Fitch scheme has a significant impact on the simulation results. The position of the LLJ nose and the shear beneath the jet nose are modified by the presence of wind farms.

Numerical Analysis of Wind Wake Superposition and Boundary Layer Interaction

Understanding the wake impacts to a wind farm is critical for layout planning and turbine design. Wind Wake Models estimates wake development through a single turbine which is then aggregated with a wake variety of methods to calculate the combined wake profile for a wind farm. This study considered downstream wake interactions, and the energy content in the wake profile as the wake developed for three rows and five downstream turbines. This study also looks at the influence of different atmospheric stratifications to wind farm wake superposition. Our findings for the wake and boundary superimposition model determine that the streamwise velocity component range from 8.8 to 9.0 m s-1 (98 to 100 %) for the stable ABL ranges and 8.92 to 9.0 m s-1 (99 to 100 %) for the unstable ABL ranges. This study further determines that the minimum streamwise velocity for the stable and unstable ABL are 8.97 and 8.96 m s-1, and the mean normalized velocity deficit for the stable and unstable ABL are 0.972 and 0.975. The wake deficit for the superimposition of a coupled wake boundary model in the stable case surpasses that of the unstable case due to lower turbulence.

Turbulent flow mapping in a high-flow tidal channel using mobile acoustic Doppler current profilers

In this investigation, instrumented mobile platforms are used to spatially map the turbulent flows in Grand Passage, one of the Bay of Fundy's more energetic tidal channels in Nova Scotia, Canada. The aim is to characterize the flow around the PLAT-I floating tidal energy platform developed by Sustainable Marine Energy Canada (SMEC). GPS-tracked surface drifters equipped with fast-sampling acoustic Doppler current profilers (ADCPs) provide turbulence-resolving vertical profiles of velocity and turbulent kinetic energy dissipation rate along drifter trajectories, while vessel-mounted ADCP transects complement the mean flow velocity measurements. These data are used to construct tridimensional quasi-synoptic maps of mean velocities and turbulence parameters for several stages of the tide around PLAT-I's location including peak ebb and flood currents. The data set includes measurements under natural flow conditions, and while the turbines installed on PLAT-I were both not operational and operational. The measurement techniques and resulting maps successfully capture the spatial and temporal structure of the flow for unsteady conditions for various tidal conditions. The combined wake of the four 6.3 m operational PLAT-I turbines was measured for a single tidal stage. Closer to the turbines, the vertical extent of the wake is about 6 m (approximately one turbine diameter), increasing with distance downstream, while the maximum observed velocity deficit is 26% relative to not operating conditions, decreasing with distance downstream.

Experimental investigation of gradually-varied unsteady flow passed a circular pile

The flow field around a vertically mounted circular pile exposed to gradually-varied unsteady flow is experimentally investigated. Experiments were conducted in a 30 m-long and 1 m-wide recirculating flume equipped with a variable discharge pump. A circular cylinder with 9 cm diameter was used as the model pile. To understand the influence of accelerating and decelerating flow conditions, three unsteady cases with different unsteadiness degrees were tested as well as a reference steady flow case. The spatial and temporal variations of Reynolds-averaged velocity and turbulence characteristics around the pile, as well as undisturbed flow, were analyzed. Findings show that there are distinct differences between the tested gradually-varied unsteady flow cases and the reference steady flow case. The Reynolds-averaged velocity vs. TKE plots of undisturbed flow indicated a hysteresis effect, such that larger turbulence is generated during the falling stage of the flow compared to the rising stage. This hysteresis was considerably reduced in the pile wake, and even reversed hysteresis was seen at certain cases. The spatial variation of Reynolds-averaged velocity and turbulence in the peak instant of unsteady flow was qualitatively similar to that of steady flow, but quantitatively, turbulence, flow contraction, and velocity deficit in the near-wake region were smaller in the case of unsteady flow. Contrarily, the unsteady flow generated remarkably higher turbulence levels at further downstream in the pile wake. It is concluded that in the case of unsteady flow the pile behaves as if it has a more streamlined shape. The results were also interpreted from structure-bed interactions perspective, explaining the differences between the pile scour induced by steady and unsteady flow conditions.

Tidal turbine performance and near-wake characteristics in a sheared turbulent inflow

Turbulent non-homogenous flow environments at high-energy tidal sites lead to variation in power performance of tidal stream turbines. In addition, non-uniform loading due to combined effects of shear and elevated inflow turbulence affects device survivability. In the current study, an active-grid turbulence generator is used to mimic sheared-turbulent inflow representative of the flood (low shear) and ebb tides (high shear). The performance and near-wake characteristics of a turbine model measured in the sheared-turbulent inflow conditions are compared to conditions where the flow velocity is uniform with low and elevated turbulence intensities (Ti). The sheared-turbulent inflows were observed to result in a 5–15% drop in maximum power coefficient when compared to the low Ti case. An increase of ∼30% in torque- and ∼50% in thrust- fluctuations were observed for sheared inflows. Unlike the near axisymmetric wake profiles observed for homogeneous cases, inflow shear was observed to incite asymmetry in the near-wake with an immediate impact in wake turbulence and a more gradual effect on the velocity deficit and integral length scales. The data sets are deemed valuable in calibrating computational models to better predict turbine response and optimize turbine efficiency and the complementing power take-off system.

Probing the properties and mechanisms of failure waves in soda-lime glass

Soda-lime glass (SLG) and other silica glasses exhibit the failure wave phenomenon under shock compression. The mechanism responsible for this peculiar behavior of glasses is still unresolved. In this study, a series of plate impact experiments was performed at three different impact stresses of 6.4, 8.3, and 10.8 GPa to better understand the mechanisms underlying the failure wave phenomenon. Specifically, spall experiments were conducted to probe the speed and existence of failure waves at different stresses in SLG. A layered glass target was used to probe the possibility of a “renucleation” of the wave at the SLG–SLG interface. When it existed, the failure wave was inferred to propagate at a speed of 1.3 km/s. However, it was observed that the failure wave phenomenon ceases to exist for impact stresses higher than 10 GPa. In experiments with a 6.4 GPa impact stress, the peak free surface velocity was significantly less than what is predicted by stress-Hugoniot calculations. This velocity deficit and other important features of the measured free surface velocity profiles were simulated using finite element analysis by incorporating an abrupt densification of SLG at a critical stress in the equation of state. This densification feature is similar to what would be expected of a phase transition. Although unable to unambiguously reveal the mechanism causing the failure wave phenomenon, the results of the present work clearly indicate that the failure wave causes a secondary compression and densification in SLG.

Experimental study on the propagation characteristics of detonation wave in annular channels with convergent cross-sections

Methane (CH4) is an environmental-friendly fuel with a wide range of applications. The explosion and detonation hazards with respect to methane in tubes with different shapes should be paid in special attention. In this study, experiments were performed to investigate the propagation characteristics of detonation waves in annular channels with convergent cross-sections in stoichiometric methane-oxygen mixture at sub-atmospherical pressures (10 kPa-40 kPa). The annular channels with convergent cross-sections were assembled by inserting 0.5-m long cones into a 48-mm circular tube. Three conicities (dend/L) were employed, i.e., dend/L = 0.048, 0.072, 0.096, in which dend and L represent the bottom diameter and the length of the cone, respectively. The detonation velocity was obtained based on the time of arrival of the wave front. Smoked foils were used to register the detonation cellular evolution in the annular channels. The results show that the convergence of the channel has a great impact on the detonation velocity and cell sizes. Depending on the conicity, the detonation velocity and cellular structure exhibit different behaviors. For the smallest conicity, the continuous velocity deficit can be observed after detonation propagating into the annular tube. However, the characteristic of detonation velocity evolution shows distinct with the increase of the conicity, which is governed by the chemical scale (the detonation cell size) as well as the physical geometry (the channel width). The interaction between Mach reflection, boundary layer effect and the wall can be served as the dominating reason for the velocity behavior.

Comparison of large-eddy simulations of wakes with wind farm wake parametrizations using the Weather Research and Forecasting model

Wind farm parametrizations are nowadays commonly used to predict the output of wind farms in real-time numerical weather prediction mesoscale models. However, their accuracy both to reproduce the wind speed and turbulent kinetic energy fields behind turbines is a matter of debate. Here, and to the authors knowledge for the first time for a single turbine, the in-built wind farm parametrization of the Weather Research and Forecasting model is evaluated using detailed large-eddy simulations of the wake performed with a generalized actuator disk model that was implemented in the same modeling system. Thus, a fairer evaluation is achieved compared to previous works, as we try to set the simulations as similar as possible within the same modeling framework. We find that both types of simulations can be used to provide similar inflow conditions to the turbine. Most importantly, by comparing the detailed-wake results, within the area where the turbine is placed, with the mesoscale results at the analogous position, velocity deficits can differ up to 50%. However, within the same area, the vertical profile of turbine-generated TKE is nearly identical between the two types of simulations. Our findings demonstrate that the generalized actuator disk implementation can provide us with a solid foundation for evaluation of wind farm parametrizations within weather models.

On the noise reduction of a porous trailing edge applied to an airfoil at lifting condition

Numerical simulations, using a lattice-Boltzmann technique, have been carried out to study the effect of aerodynamic loading and Reynolds number on the aeroacoustics of a porous trailing-edge insert. The airfoil is a National Advisory Committee for Aeronautics 0018 with the last 20% of its chord being replaceable with porous insert based on a Ni-Cr-Al metal foam with a mean pore diameter of 0.8 mm. The porous insert is modeled as an equivalent fluid region governed by the Darcy's law. The angle-of-attack is set to 7.8°, and the freestream Reynolds numbers based on the airfoil chord are 2.7 × 105 and 5.4 × 105. The amount of noise reduction produced by the porous insert generally decreases as the angle-of-attack or Reynolds number is increased, although the far-field noise directivity remains similar to that of the solid insert case. Unlike for a solid insert, in which noise sources are concentrated at the trailing edge, those on the porous insert are distributed across the porous medium surface, and they promote phase interference effect that causes noise attenuation. This mechanism is realized by the pressure release process, which refers to the interaction between surface pressure fluctuations on both sides of the trailing edge through the porous medium. It is found that the pressure release process is strongly present at the last 25% of the porous insert extent, and thus the upstream segment plays a relatively limited role in noise attenuation. The porous insert also causes velocity deficit, enhanced Reynolds shear stress, and lower convection velocity in the turbulent boundary layer. Nevertheless, since only the flow field surrounding the porous insert is affected, the overall aerodynamic penalty is relatively minor. It has also been found that the effect of mean cross-flow inside the porous medium is almost negligible in the present investigation due to the small surface pressure difference between the two sides of the porous insert.