Showing papers on "Boundary layer published in 2023"

Numerical study on aerodynamic resistance reduction of high-speed train using vortex generator

Abstract: Vortex generator (VG) is one of the potential technical tools to reduce the aerodynamic resistance of high-speed trains. The passive control resistance reduction research of a high-speed train is carried out by using VG. Three typical installation locations, including flow separation point, boundary layer mutation and streamline transition location, and several nearby locations were selected to study the effect of VG location on the train aerodynamic characteristics. The results show that the tail car drag is significantly reduced when the VG is arranged at the boundary layer mutation, the tail car’s resistance can be reduced by 15.42%. The tail car reduces resistance by 3.87% when VG is set at the flow separation point. By analyzing the flow field structure, we found that the VG arranged in front of the separation point triggers the flow separation, which destroys the balance between the separation and longitudinal vortex. It effectively reduces the strength of the separation vortex, thereby reducing the tail car’s aerodynamic drag. The research provides a new thought for the aerodynamic resistance reduction of high-speed trains, and is of great significance to break the limitations of traditional aerodynamic drag reduction technology of trains.

Direct numerical simulation of supersonic internal flow in a model scramjet combustor under a non-reactive condition

Abstract: A Mach 1.5 non-reactive flow in a cavity-stabilized combustor of a model scramjet is studied via a direct-numerical simulation approach, and the analysis is focused on the interaction among boundary layer, free shear-layer above the cavity and shock wave. It is found that the impingement of the free shear-layer on the aft wall of the cavity leads to strong turbulence kinetic energy, high local pressure, and a fan of compression waves. The compression waves evolve into an oblique shock, which reflects between the upper and lower walls and interacts with the boundary layers attached to the two walls. The analysis of the turbulence production reveals that the amplification of turbulence in the core of the shear-layer and around the reattachment point is mainly due to the shear production, but the deceleration production mechanism presents a significant impact in the regions above the aft wall of the cavity and around the shock interaction points. The very low frequency commonly observed in shock wave/boundary layer interactions is not observed in the present research, which might be due to the low Reynolds number of the studied case.

Magnetohydrodynamics Streamwise and Cross Flow of Hybrid Nanofluid along the Viscous Dissipation Effect: Duality and Stability

Abstract: One of the most pressing issues in contemporary applied mathematics is the regulation of energy transfer via the application of external forces. The processes of heat transfer are affected by magnetic force, which has many practical uses in industry, engineering, and medicine. This research explores the magnetohydrodynamics (MHD) three-dimensional stable axisymmetric boundary layer over a permeable moving plate, which consist of water as a base liquid and binary distinct nanoparticles to generate a hybrid nanofluid. In all of these, flow beyond the boundary layer area might be calculated by a small crosswise velocity. As a result of its high thermal conductivity, a pair of distinct kinds of nanoparticles have been considered, namely alumina and copper, which are integrated into the base water. The mathematical model is built within a boundary of specified geometry and then converted into a set of ordinary differential equations (ODEs). Resultant ODEs are solved numerically using the technique of three-stage Lobatto IIIa in bvp4c solver in 2017, MATLAB software. Results revealed that two branches exist in certain ranges of moving parameter. Impacts of an increasing physical parameter on profiles of velocities and temperature with skin friction as well as with heat transfer rate are represented in graphs. The effect of viscous dissipation on the temperature profile in the -direction has the same rising results as observed in the -direction. According to the results of the temporal stability analysis, the upper branch is realizable and stable.

Heat and Mass Transfer Analysis of Single Walled Carbon Nanotubes-Water and Multi Wall Carbon Nanotubes-Water Based Maxwell Nanofluid Flow Over Stretchable Rotating Disks

Abstract: Influence of thermal radiation and magnetic field on heat transfer and flow analysis of water-CNTs type nanofluid between two stretchable revolving disks with heat generation/absorption and convective boundary condition is numerically examined in this analysis. The most extensively validated Finite element technique is employed to solve the reduced non-linear ordinary differential equations together with boundary conditions. Velocity and temperature distributions are calculated and are displayed through graphs for various values of pertinent parameters entered into the problem. Furthermore, the values of rates of change of velocity and temperature are examined in detail and are portrayed in tabular form. The values of skin friction co-efficient at both upper and lower disks elevates in the boundary layer regime with rising values of Deborah number in both nanofluids and this augmentation is higher in MWCNTs-water than SWCNTs-water based Maxwell nanofluid. Temperature of the fluid in both nanofluids deteriorates as the values of nanoparticle volume fraction parameter upsurges and this deterioration in temperature distributions is higher in MWCNTs-water than the SWCNTs-water based Maxwell nanofluid.

On low-frequency unsteadiness in swept shock wave–boundary layer interactions

Abstract: Abstract We derive a scaling law for the characteristic frequencies of wall pressure fluctuations in swept shock wave/turbulent boundary layer interactions in the presence of cylindrical symmetry, based on analysis of a direct numerical simulations database. Direct numerical simulations in large domains show evidence of spanwise rippling of the separation line, with typical wavelength proportional to separation bubble size. Pressure disturbances around the separation line are shown to be convected at a phase speed proportional to the cross-flow velocity. This information is leveraged to derive a simple model for low-frequency unsteadiness, which extends previous two-dimensional models (Piponniau et al., J. Fluid Mech., vol. 629, 2009, pp. 87–108), and which correctly predicts growth of the typical frequency with the sweep angle. Inferences regarding the typical frequencies in more general swept shock wave/turbulent boundary layer interactions are also discussed.

Boundary Layer Turbulence Flight Experiment in Memory of Mike Holden: Side A Flight Data

Abstract: The AFOSR Boundary Layer Turbulence Flight Experiment in Memory of Mike Holden was successfully flown on March 21, 2022. The objective of the flight experiment was to quantify natural and forced transition and turbulence during flight for three-dimensionally strained hypersonic boundary layers. Two flight experimental test windows were sampled: ascent and descent. On ascent, Mach numbers were greater than 5.0 for ReL = 0.3 - 17.5 million. On descent, the hypersonic test window was extended to ReL = 45 million. The flight vehicle was instrumented with 301 sensors including heat flux, skin friction, surface (and back face) temperature, pressure, pressure fluctuations, pressure power spectra, and thin film turbulent burst sensors. Texas A&M University and the Calspan-University of Buffalo Research Center (CUBRC), along with the Air Force Research Laboratory (AFRL) and NASA, were the science mission performers for the natural transition experiments (Side A). A description of the Side A data is presented in this paper. In summary, transition, turbulence, thermal, and mechanical loading were tracked on both ascent and descent, as were boundary layer instability modes. These data, along with the extensive ground test and simulations, provided a new benchmark for aerothermodynamic prediction and extrapolation to hypersonic flight.

Implementation of Self-Aligned Focusing Schlieren for Hypersonic Boundary Layer Measurements

Abstract: A self-aligned focusing schlieren (SAFS) apparatus based on a recently published design is implemented for the purpose of making near-planar density gradient measurements of hypersonic boundary layers. At a working distance of one meter, the current SAFS setup is shown to be capable of depths of focus on the order of 1-10 mm while also achieving framerates in the 100 kHz to 1+ MHz range at exposures as short as 20 ns. These qualities are achieved while remaining resistant to tunnel vibrations due to the nature of the retroreflected beam path. The design space of the apparatus is also investigated and reveals the influence of key components/parameters on the overall system performance and capabilities. The design is found to be extensible to a variety of scales that may prove applicable to a wide range of research facilities. Finally, a series of experiments were executed in the Air Force Research Laboratory Mach-6 Ludwieg Tube over a variety of axisymmetric and non-axisymmetric geometries using the SAFS setup to capture images of the developing boundary layer and passing flow disturbances. Comparisons between SAFS and traditional, line-of-sight schlieren are presented alongside discussions on the overall performance compared to the predicted design space.

Multiple exact solutions for micropolar slip flow and heat transfer of a bidirectional moving plate

Abstract: The analysis of micropolar fluid flow over a deforming permeable surface is carried out taking into account velocity slip with constant and linearly growing temperature field conditions. Upon using the boundary layer approximation and the method of similarity transformation, the constitutive equations are remodeled into coupled, nonlinear differential equations which are then solved for the exact solutions of fluid flow and heat transport under different physically acceptable parametric values. In particular, the physical domains of mass transfer and micropolar parameters in determining the existence, singleness and multipleness of exact solutions play a leading role. The examination of critical values for the mass transfer parameter exhibits the borderline for the existence and nonexistence of solutions. Unique solution is detected for the stretching sheet, whereas the shrinking sheet demonstrates twofold solutions. Exact fluid flow and heat transfer solutions under special parametric effects are also considered. The need to examine the prominent physical features of the flow system, closed form formulas for velocity, angular velocity, heat transfer, skin friction and reduced Nusselt number are derived, which are for analysis purpose presented graphically.

Summary of a Mach 2.5 Shock Wave Turbulent Boundary Layer Interaction Experiment in a Circular Test Section

Abstract: A series of experiments were performed at Mach 2.5 in a 17 cm diameter circular test section to characterize an impinging/reflected shock wave turbulent boundary layer interaction generated by a cone-cylinder centerbody. The cone-cylinder centerbody generates a conical shock wave that interacts with the naturally occurring boundary layer developing on the test section wall. Three different cone angles were used in the experiment to study unseparated, incipiently separated, and separated interactions. When the cone-cylinder centerbody is positioned on the centerline, a flowfield which is two-dimensional in the mean is generated. Three dimensional interactions were also created by offsetting the cone-cylinder centerbody from the test section centerline. The results are intended to provide benchmark quality datasets for computational fluid dynamics (CFD) validation without the pitfalls inherent in rectangular configurations where corner effects prohibit a truly two-dimensional flow in the mean. The experimental measurements included surface flow visualization, wall static pressure, flowfield Pitot tube pressure, constant-voltage anemometry (CVA) normal hot-wire, and particle image velocimetry (PIV) measurements. The hot-wire measurements were used to calculate mean mass flux and total temperature profiles, mass flux and total temperature turbulence intensities, and the mass flux-total temperature correlation. The PIV measurements provide three-dimensional mean velocity measurements. Agreement between the pressure, hot-wire, and PIV measurements is established in the undisturbed upstream flowfield.

Multiple exact solutions of second degree nanofluid slip flow and heat transport in porous medium

Abstract: The influence of second degree velocity slip and thermal jump conditions on water-based nanofluid flow and heat transfer over a permeable bidirectional moving surface under a porous medium environment are investigated. The intention is to secure closed form analytical solutions, locate domains for existence and also the nonexistence of dual or triple solutions, and analyze the behavior of governing parameters such as the mass transfer, stretching or shrinking sheet, nanofluid volume fraction, first and second degree velocity slips, and thermal jump on the water nanofluid mixture flow and heat transfer. It is observed in the process of boundary layer that the velocity profile decreases when both the suction or injection increases. The presence of nanoparticles slows down linear velocity as well as the temperature distribution. Moreover, the first velocity slip, as it should, depletes axial velocity, whereas the effect of second slip is on the opposite. Thermal jump as can be noticed from the formula of temperature distribution minimizes the role of temperature on the wall. The parametric effects on skin friction and temperature gradient are analyzed and shown graphically. Finally, we consider special parametric values and asymptotic situations that produces special exact solutions.

Reduced Order Model Predictions for Spherically Blunted Circular Cones

Abstract: Reduced Order Models (ROM)s for boundary and entropy layer height were created for a 7 degree half angle spherically blunted circular cone with varying nose radii. These ROMs were created to aid in the understanding of an experimentally observed wisp-like transition structure. A design space covering the range of nose radii and tunnel operating conditions was defined, and 80 training points selected using Latin Hypercube Sampling (LHS) with incremental refinement. An additional 20 test points were selected based on experimentally run conditions, and used to evaluate the ROMs predictive performance. The boundary and entropy layer thickness from the ROMs were overlayed on top of Spectral Proper Orthogonal Decompositions (SPOD)s from Schlieren images, along with over a preliminary Implicit Large Eddy Simulation (ILES) for one of the test points. For the blunt nose conditions, the observed wisp-like structures appear to exist outside the boundary and entropy layers. Additionally, results from the preliminary ILES show no noticeable disturbance in the surface heat transfer or surface pressure in the vicinity of a wisp-like transition structure.

Double-diffusive stagnation point flow over a vertical surface with thermal radiation: Assisting and opposing flows

Abstract: In numerous industrial procedures, the main concern of design engineers is ensuring adequate heat and mass transfer, such as in the heating and cooling practices of solar water heaters, geothermal systems, extrusion of metal, insulation of buildings, electronics, turbines, aerodynamics, electronics, paper manufacturing, and glass fiber production. The unsteady double-diffusive mixed convection flow of boundary layer nanofluids above a vertical region near stagnation point flow is developed and examined here. The Brownian motion and thermophoresis effects are incorporated by using Buongiorno's model. In the thermal energy equations, diffusion of regular and cross types is also used. By the use of the local similarity method along with suitable similarity transformations, nonlinear unsteady partial differential equations are converted to nonlinear ordinary differential equations and are numerically solved by the Keller–Box method. The investigation expresses that these profiles of solute concentration and nanoparticle concentration, temperature, and velocity in their boundary layers, respectively, depending on several parameters. A graphic analysis of all these parameters' possessions on nature's boundary layers is depicted. The highest rate of heat transfer is obtained with negligible thermophoresis effect. Furthermore, it is perceived that an increase in Nc and Nt results in a reduction in the reduced Sherwood number of nanoparticles, whereas addition results in an increase in the Nb number. There is a reverse effect on the temperature field and layer thickness for heat generation. In the wake of the above-mentioned potential applications, the current study of fluid flow has been found to be very interesting and innovative in the analysis of the influence of Brownian motion and thermophoresis effects near stagnation point flow, which will further make revolutions in industrial fields. Moreover, Buongiorno's model predicts the characteristics of double-diffusive fluids in enhancing heat transfers. This investigation has been established as a result of the numerous industrial applications mentioned above.

Hypersonic Stability and Breakdown Measurements on the AFRL/AFOSR BOLT II Geometry

Abstract: The Air Force Research Laboratory/Air Force Office of Scientific Research (AFRL/AFOSR) introduced the Boundary Layer Turbulence (BOLT II) flight experiment to further the understanding and modeling of a hypersonic turbulent boundary layer on a geometry that features concave surfaces with swept leading edges. In support of the flight experiment, this paper identifies and documents the transition instabilities and quantifies the breakdown to turbulence on a 25% scale BOLT II model in hypersonic flow. Experiments were conducted in the conventional ACE wind tunnel facility and the M6QT located at the Texas A&M University National Aerothermochemistry and Hypersonics Laboratory. Global surface heating was viewed using IR thermography with on-surface measurements made using high-frequency Kulite and PCB surface pressure transducers. Modal growth was observed amongst the sensors both upstream and downstream of the model. Off-surface measurements were made in the mixed-mode region utilizing constant temperature hot-film anemometry in the M6QT. Comparisons were made to the DNS results from the University of Minnesota. Amplified content was observed between f = 53-75 kHz in the RMS voltage fluctuations of the hot-film measurements.

Turbulent structure of the Arctic boundary layer in early summer driven by stability, wind shear and cloud-top radiative cooling: ACLOUD airborne observations

Abstract: Abstract. Clouds are assumed to play an important role in the Arctic amplification process. This motivated a detailed investigation of cloud processes, including radiative and turbulent fluxes. Data from the aircraft campaign ACLOUD were analyzed with a focus on the mean and turbulent structure of the cloudy boundary layer over the Fram Strait marginal sea ice zone in late spring and early summer 2017. Vertical profiles of turbulence moments are presented from contrasting atmospheric boundary layers (ABLs) from 4 d. They differ by the magnitude of wind speed, boundary-layer height, stability, the strength of the cloud-top radiative cooling and the number of cloud layers. Turbulence statistics up to third-order moments are presented, which were obtained from horizontal-level flights and from slanted profiles. It is shown that both of these flight patterns complement each other and form a data set that resolves the vertical structure of the ABL turbulence well. The comparison of the 4 d shows that especially during weak wind, even in shallow Arctic ABLs with mixing ratios below 3 g kg−1, cloud-top cooling can serve as a main source of turbulent kinetic energy (TKE). Well-mixed ABLs are generated where TKE is increased and vertical velocity variance shows pronounced maxima in the cloud layer. Negative vertical velocity skewness points then to upside-down convection. Turbulent heat fluxes are directed upward in the cloud layer as a result of cold downdrafts. In two cases with single-layer stratocumulus, turbulent transport of heat flux and of temperature variance are both negative in the cloud layer, suggesting an important role of large eddies. In contrast, in a case with weak cloud-top cooling, these quantities are positive in the ABL due to the heating from the surface. Based on observations and results of a mixed-layer model it is shown that the maxima of turbulent fluxes are, however, smaller than the jump of the net terrestrial radiation flux across the upper part of a cloud due to the (i) shallowness of the mixed layer and (ii) the presence of a downward entrainment heat flux. The mixed-layer model also shows that the buoyancy production of TKE is substantially smaller in stratocumulus over the Arctic sea ice compared to subtropics due to a smaller surface moisture flux and smaller decrease in specific humidity (or even humidity inversions) right above the cloud top. In a case of strong wind, wind shear shapes the ABL turbulent structure, especially over rough sea ice, despite the presence of a strong cloud-top cooling. In the presence of mid-level clouds, cloud-top radiative cooling and thus also TKE in the lowermost cloud layer are strongly reduced, and the ABL turbulent structure becomes governed by stability, i.e., by the surface–air temperature difference and wind speed. A comparison of slightly unstable and weakly stable cases shows a strong reduction of TKE due to increased stability even though the absolute value of wind speed was similar. In summary, the presented study documents vertical profiles of the ABL turbulence with a high resolution in a wide range of conditions. It can serve as a basis for turbulence closure evaluation and process studies in Arctic clouds.

Radiative transport of MHD stagnation point flow of chemically reacting Carreau nanofluid due to radially stretched sheet

Abstract: The flow of Carreau nanofluid with generation/absorption and magnetic field can be valuable for modifying solar energy production. In this work, we extended the study of the MHD boundary layer flow of Carreau nanofluid with heat generation/absorption close to a stagnation point over the radially extending plate. Likewise, the features of radiation and magnetic field with convective boundary conditions are considered. Further, keeping in view the importance of chemical reactions, their effect is also incorporated during the modelling process. For motivation, the impact of thermophoresis and Brownian motion has been taken into account. Using appropriate similarity transformation, we converted nonlinear governing partial differential equations of Carreau nanofluid into a couple of nonlinear ODEs. Using a recognized shooting technique and the MATLAB bvp4c solver and Mathematica ND-solve built-in command, we were able to get numerical results for these modeled ODEs. Through graphs and tables, the effects of different physical parameters like magnetic, Weissenberg number, Brownian motion, thermal radiation, thermophoresis, Prandtl number, chemical reaction, and rate of heat generation/absorption on non-dimensional velocity, temperature, and concentration profile are discussed.

Computation of Sakiadis flow of an Eyring-Powell rheological fluid from a moving porous surface with a non-Fourier heat flux model

Abstract: This article examines theoretically and numerically the effect of non-Fourier heat flux on non-Newtonian (Eyring-Powell) Sakiadis convective flow from a moving permeable surface accompanied by a parallel free-stream velocity, as a simulation of polymeric coating processes. The Cattaneo-Christov model is deployed which features thermal relaxation effects as these are important in thermal polymer processing. The physical flow problem is modeled in a Cartesian coordinate system and the governing conservation differential equations and associated boundary conditions are rendered dimensionless by applying suitable transformations. Liquid velocity and thermal distributions are computed considering numerical procedure namely, a shooting method in conjunction with the 5th order Runge–Kutta algorithm (R-K5) executed in a symbolic software. Validation with the three-stage Lobatto IIIA algorithm in MATLAB is included. The impact of key parameters on streamline distributions is also computed. Velocity is increased with increment in Eyring-Powell first parameter for the Sakiadis case whereas it is reduced with Eyring-Powell second parameter for the case where sheet and liquid are inspiring in the similar direction. The special case of Blasius flow is also examined (stationary sheet). For higher injection, there is a solid dampening in the boundary-layer flow for both Sakiadis and Blasius scenarios.

Eyring-Powell nano liquid flow through permeable elongated sheet conveying inclined magnetic field subject to constructive chemical reaction and multiple slip effects

Abstract: ABSTRACT The present study examines the multiple slip effects and constructive chemical reaction on Eyring-Powell nanoliquid through a permeable elongated sheet on an inclined magnetic field. The energy balance equation includes Heat and Joule dissipation terms. The renowned Buongiorno nanofluid model is employed extensively to explore the thermophoresis and Brownian motion phenomena. The primary partial differential equations (PDEs) of governing flow phenomena are remodeled to non-linear ordinary differential equations (ODEs) by adopting proper similarity invariants. Runge-Kutta 4th order quadrature employing shooting technique is utilized to transform boundary value problem to initial value problem to seek numerical results via graphs and tables on physically interesting parameters. Current outcomes are validated through the comparison with previously published studies. An applied magnetic field slows down the fluid and raises both thermal and solutal boundary layers. It is significant to notice that the thermal boundary layer rises, as the Brownian and Thermophoresis are amplified.

Two-scale solution for tripped turbulent boundary layer

Abstract: Abstract Recent findings on the Reynolds-number-dependent behaviour of near-wall turbulence in terms of the ‘foot-printing’ of outer large-scale structures call for a new modelling development. A two-scale framework was proposed to couple a local fine-mesh solution with a global coarse-mesh solution by He (Intl J. Numer. Meth. Fluids, vol. 86, 2018, pp. 655–677). The methodology was implemented and demonstrated by Chen & He (J. Fluid Mech, vol. 933, 2022, p. A47) for a canonical turbulent channel flow, where the mesh-count scaling with Reynolds number is potentially reduced from $O(R{e^2})$ for a conventional wall-resolved large-eddy simulation (WRLES) to $O(R{e^1})$. The present work extends the two-scale method to turbulent boundary layers. A two-dimensional roughness element is used to trip a turbulent boundary layer. It is observed that large-scale disturbances originating at the trip have a much shorter lifetime and weaker foot-printing signatures on near-wall flow compared to those long streaky coherent structures in well-developed wall-bounded turbulent flows. Modal analyses show that the impact of trip-induced large scales can be adequately captured by a locally embedded fine-mesh block. For the tripped turbulent boundary layer, a Chebyshev block-spectral mapping is adopted to propagate source terms from the local fine-mesh blocks to the global coarse-mesh domain, driving to a target solution for the upscaled equations. The computed mean statistics and energy spectra are in good agreement with corresponding experimental data, WRLES and direct numerical simulation (DNS) results. The overall mesh count–$Re$ scaling is estimated to reduce from $O(R{e^{1.8}})$ for the full wall-resolved LES to $O(R{e^{0.9}})$ for the present two-scale solution.

Laminar/Transitional Fin-induced Shock Wave Boundary-Layer Interactions at Mach 5

Abstract: An experimental investigation on Shockwave/Boundary Layer Interaction (SBLI) induced by an unswept fin is performed at Mach 5 with both laminar and transitional boundary layers. The unswept fin geometry is the scaled-down model (3:8) of the fin from the DLR STORT flight test program. A flat plate fin-induced SBLI is investigated at Rex = 2.50×10^6 (laminar) and Rex = 3.24×10^6 (transitional) in an indraft wind tunnel. Oil flow visualization, mean pressure, and unsteady pressure measurements are used to characterize the SBLI. Oil flow visualization and mean pressure measurements show strong similarities with turbulent interactions. The footprint of laminar/transitional SBLIs shows quasi-conical features and dependence on interaction strength. Mean pressure measurements show the two-dimensional free interaction theory also applies to the scaling of fin-induced SBLI examined here. Surface pressure fluctuations in the laminar SBLI show low frequency unsteadiness while the transitional case shows both low-frequency and Mack mode (second mode) fluctuations.

Disturbance growth in a laminar separation bubble subjected to free-stream turbulence

Abstract: Abstract Experiments were conducted to study the transition and flow development in a laminar separation bubble (LSB) formed on an aerofoil. The effects of a wide range of free-stream turbulence intensity ($0.15\,\%< Tu<6.26\,\%$) and streamwise integral length scale ($4.6\ {\rm mm}<\varLambda _{u}<17.2\ {\rm mm}$) are considered. The co-existence of modal instability due to the LSB and non-modal instability caused by streaks generated by free-stream turbulence is observed. The flow field is measured using hot-wire anemometry, which showed that the presence of streaks in the boundary layer modifies the mean-flow topology of the bubble. These changes in the mean flow field result in the modification of the convective disturbance growth, where an increase in turbulence intensity is found to dampen the growth of the modal instability. For a relatively fixed level of $Tu$, the variation of $\varLambda _{u}$ has modest effects. However, a slight advancement of the nonlinear growth of disturbances and eventual breakdown with the decrease in $\varLambda _{u}$ is observed. The data show that the streamwise growth of the disturbance energy is exponential for the lowest levels of free-stream turbulence and gradually becomes algebraic as the level of free-stream turbulence increases. Once a critical turbulence intensity is reached, there is enough energy in the boundary layer to suppress the laminar separation bubble, resulting in the non-modal instability taking over the transition process. Linear stability analysis is conducted in the fore position of the LSB. It accurately models incipient disturbance growth, unstable frequencies and eigenfunctions for configurations subjected to turbulence intensity levels up to 3 %, showing that the mean-flow modification due to the non-modal instability dampens the modal instability.