Effect Of Free-stream Turbulence Research Articles

Computational study on effect of free-stream turbulence on bio-inspired corrugated airfoil at different sections at low Reynolds number

Computational study on effect of free-stream turbulence on bio-inspired corrugated airfoil at different sections at low Reynolds number

Freestream turbulence effects on low Reynolds number NACA 0012 airfoil laminar separation bubble and lift generation

Laminar separation bubbles (LSB's) over the suction surface of a wing at low Reynolds number (O(104)−O(106) based on the airfoil chord length) can significantly affect the aerodynamic performance of the wing, and pose a unique challenge for the predictive capabilities of simulation tools due to their high sensitivity to flow environments and wing surface conditions. In this work a series of two-dimensional (2D) and three-dimensional (3D) low-order, and high-order accurate unstructured-grid-based numerical methods with varying model fidelity levels were used to study LSB physics over a NACA 0012 airfoil both in a clean freestream and in a turbulent freestream at a chord-based Reynolds number of 12,000. Lift production and time-averaged flow fields were compared with available experimental results. A major discovery is that in clean freestream flow a 3D high-order numerical scheme is necessary to capture LSB physics. This is due to the sensitivity of LSB-induced laminar-turbulent transition to flow conditions and boundary geometry at low Reynolds number. In freestream flows with moderate background turbulence (∼5%), 2D simulations failed to capture subtle 3D flow physics due to their intrinsic limitation, but can reasonably predict time-averaged airfoil performance. Similarity and distinction between freestream vortex-LSB interaction in 2D and eddy-LSB interaction in 3D were explained. The role of the Kelvin-Helmholtz instability and Klebanoff modes in the transition of 3D airfoils were shown to be critical for understanding laminar-turbulent transition and LSB formation on airfoils in clean and turbulent freestreams.

Numerical study of free stream turbulence effects on dynamic stall of pitching tubercled wings

The feasibility of a tubercled leading-edge in dynamic stall control has been validated, and the impacts of an upstream cylinder wake have already been discussed in the previous research. However, the influences of wake coherent characteristics on the dynamic stall process are inevitable. To this end, the synthetic turbulence method was adopted in the present study to generate incoming turbulence. The suppression of dynamic stall could be observed according to the shrink of hysteresis loop of aerodynamic lift. Then, the dynamic stall mechanisms have been analyzed with the help of mode decomposition method. The emergence and development of the leading-edge vortex indicate the increase in aerodynamic lift, while he detachment of dynamic stall vortex leads to the lift degradation during the onset of dynamic stall. Eventually, the influence of turbulence intensity (TI) has also been discussed through the comparison of TI = 5% and TI = 10% cases. It turns out that flow separation is more remarkable at both peak and trough sections in the case of TI = 5%, which is intrinsically caused by the suppression of resistance of adverse pressure gradient.

Separation-induced transition on a T106A blade under low and elevated free stream turbulence

The separation-induced transition on the suction surface of a T106A low pressure turbine blade is a complex phenomenon with implications for aerodynamic performance. In this numerical investigation, we explore an adverse pressure gradient-dominated flow subjected to varying levels of free stream excitation, as the underlying separation-induced transition is a critical factor in assessing blade profile loss. By comprehensively analyzing the effects of free stream turbulence (FST) on the transition process, we delve into the various mechanisms which govern the instabilities underlying bypass transition by studying the instantaneous enstrophy field. This involves solving the two-dimensional (2D) compressible Navier–Stokes equation through a series of numerical simulations, comparing a baseline flow to cases where FST with varying turbulent intensity (Tu=4% and 7%) is imposed at the inflow. Consistent with previous studies, the introduction of FST is observed to delay flow separation and trigger early transition. We explore the different stages of bypass transition, from the initial growth of disturbances (described by linear stability theory) to the emergence of unsteady separation bubbles that merge into turbulent spots (due to nonlinear interactions), by examining the vorticity dynamics. Utilizing the compressible enstrophy transport equation for the flow in a T106A blade passage, we highlight the various routes of bypass transition resulting from different levels of FST, emphasizing the relative contributions from baroclinicity, compressibility, and viscous terms.

Droplet supercooling in marine icing tests

Droplet supercooling in marine icing tests is studied theoretically using established correlations for convection and evaporation. The effect of freestream turbulence on heat and mass transfer is included through an empirical modification to the Nusselt and Sherwood numbers. Droplets are shown to cool following a modified exponential decay, with the trajectory length and shape determined by a thermal mixing length and a cooling exponent. The space needed for supercooling is shown to increase with (droplet diameter)1.8, and also depend on wind speed, injection speed and ambient temperature. Turbulence at realistic offshore levels shortens cooling distances by up to 50%. A reanalysis shows that previous marine icing tests have typically reached supercooling higher than 80% of cooling potential for small droplets up to 100–200 μm, and possibly up to 300–400 μm in long setups at low wind speeds if boosted by turbulence or a contracting tunnel; larger droplets have been significantly undercooled compared to field conditions.

A review and reassessment of the aerodynamics of cricket ball swing

This paper reviews 70 years of cricket ball swing literature and reassesses the results in light of new measurements. A comparison of ball tracking data with published experimental results shows that current understanding does not explain the behaviour observed in professional cricket matches. Descriptions of cricket ball boundary layer aerodynamics are updated with new results which show that the seam acts more like a series of vortex generators than a trip and that there is a laminar separation bubble on the seam side of the ball. Previous results for reverse swing are consolidated and compared with new data, showing that different magnitudes of swing occur depending on the condition of both sides of the ball. Variation in pressure and temperature should be included when the Reynolds number, a non-dimensional ball speed, is calculated, while humidity can be neglected. Studies on the effect of wind speed and direction are summarised and results considering the effect of free-stream turbulence are compared with new measurements. Throughout the paper, recommendations for future work are suggested. These include quantitative study of the effect of surface defects and roughness, an assessment of whether atmospheric turbulence can affect swing and investigation into the effect of backspin on swing.

Roughness induced transition delay in a swept-wing boundary layer in presence of freestream disturbances, Part 1: Turbulence effects

Results of experimental investigations on control of the laminar–turbulent transition location in a 45-degree swept-wing boundary layer by the distributed micro-sized roughness (MSR) elements in presence of freestream perturbations are discussed in this two-part work. Part 1 of this study (the present paper) is devoted to the freestream turbulence effects, while Part 2 (Borodulin et al., 2023) concentrates on consideration of the acoustic waves’ effects. The present paper (Part 1) contains: (i) a brief description of previous studies in the field, (ii) description of the experimental setup and techniques of the measurements (based on infrared thermography), (iii) the results of optimization of parameters of the MSR-elements, and (iv) results of studies of freestream turbulence effects on the MSR transition control. The MSR-elements are shown to excite in the boundary layer controlled stationary vortices, which are able to modify significantly both the base flow and its stability characteristics. Variation of the height and the period of the MSR-elements allowed us to find the most stabilizing effect on the boundary layer flow compared to the case without MSR as a reference one. A significant downstream shift in the position of the laminar–turbulent transition caused by the MSR has been detected and documented in detail. The robustness of this flow control method to freestream turbulence has been studied by increasing the turbulence level by means of turbulence generating grids. At enhanced freestream turbulence levels, the laminar–turbulent transition was found to move upstream, while the MSR-elements lost their effectiveness in the transition control.

Roughness induced transition delay in a swept-wing boundary layer in presence of freestream disturbances, Part 2: Acoustic stabilization

Results of experimental investigations on control of the laminar–turbulent transition location in a 45-degree swept-wing boundary layer by the distributed micro-sized roughness (MSR) elements in presence of freestream perturbations are discussed in this two-part work. Part 1 of this study Borodulin et al. (2023) is devoted to the freestream turbulence effects, while Part 2 (the present paper) concentrates on consideration of the acoustic waves’ effects. This paper contains: (i) a complementary description of the experimental setup and measurement techniques related to the acoustic excitation and (ii) the results of studies of the freestream acoustic-field effects on the MSR transition control. The robustness of the MSR transition-control mechanism has been studied by altering the wind-tunnel flow quality: by introducing a significant background sound and/or by simultaneous variation of both the freestream turbulence level and the level of the acoustic wave excitation. At enhanced freestream turbulence, the laminar–turbulent transition is found to move upstream regardless of the sound level and frequency, while the MSR-elements lost their effectiveness in transition control in all cases. At low freestream turbulence levels and in absence of the MSR, the acoustic waves did not affect basically the transition location. However, if the laminar–turbulent transition was delayed by the MSR, the sound of a certain frequency was found to be able to cause a significant additional transition delay.

A simplified CFD approach for bluff-body aerodynamics under small scale free stream turbulent flow

The study of free stream turbulence (FST) effects on bluff bodies by means of computational approaches has been addressed so far mainly applying scale resolving methods adopting a three-dimensional domain, which is cumbersome and complex. The development of an alternative more affordable approach capable of providing reliable data, at least qualitatively, about the impact of FST in aerodynamic responses of interest would be very beneficial for the penetration of CFD techniques in industrial applications. In this work, a 2D URANS approach is adopted to numerically replicate the well-known rod-induced small scale turbulent flow in wind tunnel testing. A square cylinder is selected as canonical application case to study the effects caused by ambient turbulence (0.7%), and 3.3% and 6.8% FST levels. The numerical results for the force coefficients, base pressure coefficient, mean and fluctuating pressure coefficient distributions, time-averaged flow structures, mean streamwise velocity and Reynolds stresses distributions are reported along with experimental data in the literature for exhaustive validation. The agreement between the proposed CFD approach and the wind tunnel data is remarkable as not only qualitative agreement has been reached, but in many cases consistency between numerical and experimental data has been obtained. The proposed approach, once its feasibility and accuracy has been satisfactorily assessed, may be applied for affordably study FST-dependent aerodynamics problems of interest in wind engineering.

Effects of freestream turbulence on the secondary instability of the roughness-induced crossflow vortex in swept flat plate boundary layers

In a three-dimensional boundary layer with spanwise constant flow and chordwise acceleration, which is unique to the vicinity of the leading edge of a swept wing, an inviscid instability induces the crossflow vortex (CFV), around which a secondary instability further occurs to yield a turbulent transition. It is well known that surface roughness induces stationary CFVs and that CFVs are receptive to freestream turbulence (FST), but the interference of both effects remains unclear. To determine the transition mechanisms in mixed roughness and FST environments, we performed direct numerical simulations of a Falkner–Skan–Cooke boundary layer under various conditions considering the presence of cylindrical roughness and the peak FST wavelength. Without roughness, the FST with a long wavelength comparable to the most unstable mode promotes CFV induction, resulting in an earlier transition. Conversely, the short-wavelength FST interacts with a wake vortex induced by the presence of roughness, yielding a hairpin vortex as a secondary instability. We discuss the main production terms of the disturbance energy associated with each of the type-1 and -2 secondary instabilities, which are accompanied by finger and hairpin vortices, respectively. The rapid turbulent transition via the type-1 secondary instability is due to the spanwise velocity gradient of the developed CFV, whereas that via the type-2 is due to a wall-normal gradient and is similar to the fully developed wall turbulence. The wavelength of FST played a key role in inducing a high-frequency secondary instability on the CFV or the CFV itself, and the receptivity to FST differed between the CFV and wake vortex.

Effects of Freestream Turbulence, Reynolds Number and Mach Number on the Boundary Layer in a Low Pressure Turbine

Effects of Freestream Turbulence, Reynolds Number and Mach Number on the Boundary Layer in a Low Pressure Turbine

Effects of incoming free-stream turbulence on the flow dynamics of a square finite wall-mounted cylinder

This study attempts to describe associated fluid dynamics of a square finite wall-mounted cylinder (FWMC) immersed within free-stream turbulent flow characterized by different turbulence intensities and integral length scales. An improved delayed detached eddy simulation method is adopted to numerically reproduce the fully developed turbulent flow fields. The results reveal that both the turbulence intensities and integral length scales have a significant effect on the separated shear layers, base pressure, and associated aerodynamic forces of the cylinder. Constrained streamlines along with critical point techniques are employed to further illustrate the influence of parameters of interest on a time-averaged flow pattern, including horseshoe vortex, surface flow, and wake topology. Distribution of second-order statistics within the wake region shows a shorter longitudinal length of the reversed flow region and enhanced vortex strength when background turbulence intensity increases. The time-dependent interaction between background turbulence and separated flow around the square FWMC is illustrated based on the phase difference between pressure of opposing side faces and the evolution of the reverse-flow region. In the end, the spectral proper orthogonal decomposition technique is employed to further investigate the effects of incoming flow turbulence on characteristics of the free-end shear flow and Von Kármán vortex shedding in the wake.

A Framework for Estimating Effects of Combustor Turbulence on Turbine Profile Loss Generation

Abstract This paper describes a framework to quantify the effect of freestream turbulence, generated by mixing processes in a combustor, on turbulent boundary layer loss generation in the high-pressure turbine downstream of the combustor. The regime of freestream turbulence common to gas turbine aero engines is identified and it is shown that the dissipation loss coefficient in this regime can be determined using existing measurements of the effect of freestream turbulence on skin friction. The paper shows that combustor-generated freestream turbulence can increase the profile loss coefficient of a typical high-pressure turbine blade by as much as 28%. A relation has been derived between a non-dimensional turbulence parameter, which characterizes the freestream turbulence, and the increase in turbine boundary layer dissipation, which quantifies the decrease in turbine efficiency. The relation provides guidelines for combustor turbulence modifications that lead to turbine performance benefits. The framework has been applied in example trade studies which show that increasing the size of dilution ports and increasing the length of the combustor can decrease high-pressure turbine profile loss generation to potentially increase stage efficiency up to 0.5%.

Influence of free-stream turbulence intensity on static and dynamic stall of a NACA 0018 aerofoil

In many engineering applications, aerofoils experience elevated free-stream turbulence levels. The present work experimentally investigates the effect of free-stream turbulence on the aerodynamic characteristics of a sinusoidally pitching NACA 0018 aerofoil at the transitional Reynolds number of 2.8×105. Wind tunnel tests are conducted in quasi-static and dynamic conditions at different turbulence intensities between 0.3% and 8.2%, considering reduced frequencies up to 0.1. The dynamic experiments investigate multiple angle-of-attack ranges in order to quantify the influence of free-stream turbulence in the attached-flow regime, in the stalled regime, and in-and-out of stall. The study demonstrates that high free-stream turbulence drastically changes the flow physics over the aerofoil for both static and dynamic conditions, producing large deviations in the lift and moment coefficients. The quasi-static experiment performed at low free-stream turbulence features a large stall hysteresis linked to the breakdown of the leading-edge laminar separation bubble, whereas static hysteresis is not present at higher turbulence levels. Moreover, dynamic experiments at high angles of attack show a strong dependency on the incoming turbulence intensity, which is found to delay flow separation during the upstroke and enhance reattachment during the downstroke. The work also reveals the intrinsic difficulty of predicting the dynamic stall behaviour under different turbulence conditions, and gives insight into why the existing empirical dynamic stall models are unlikely to succeed in accurately predicting the aerodynamic loads across different onset turbulence intensities.

LES of the T106 low-pressure turbine: Spectral proper orthogonal decomposition of the flow based on a fluctuating energy norm

This paper investigates the use of the spectral proper orthogonal decomposition (SPOD) on the flow around the T106 low-pressure turbine cascade. The chosen metric of the SPOD is based on the hydrodynamic flow disturbances for compressible flows and is used to characterize the flow structures holding large energy disturbances and synchronization processes with incoming wakes when considered. The input data for the SPOD is provided by large-eddy simulations of the T106 cascade. The Reynolds number and free-stream turbulence effects on the flow structures are studied using four operating points investigated in previous experiments and numerical simulations: Reynolds numbers of Re = 5.18 × 104 and Re = 1.48 × 105 without and with upstream wakes. The analysis of the SPOD indicates that the upstream wakes can amplify some modes of the suction side boundary layer (apparently a receptivity mechanism) and intensify the developing blade wake turbulent structures downstream of the trailing edge. This amplification happens over a reduced frequency bandwidth between Strouhal numbers equal to 7–10 for the low Reynolds number case and 20–24 for the high Reynolds number case.

Boundary layer transition induced by low-speed synthetic jets

The effect of low-speed, circular synthetic jets (SJ) on the turbulent transition of a laminar boundary layer is studied through direct numerical simulations. The SJ capability in fixating the streamwise location of transition onset is analyzed in terms of its operation parameters (reduced frequency F+ and momentum coefficient Cμ). The effect of free-stream turbulence (FST) on the near-wall vortical structures generated by the synthetic jet is analyzed as well, to mimic the actual operation of the control system. Velocity spectra, phase portraits, and dynamic mode decomposition allow us to investigate flow unsteadiness and transition to a chaotic state. In most of the investigated cases, SJs successfully promote transition, as the result of varicose-symmetric hairpin-like vortices generated at the jet exits. In particular, it is found that increasing the momentum coefficient always reduces the size of the laminar region; a non-monotonic behavior of the laminar fetch is noted as the reduced frequency is increased, suggesting the existence of an optimal frequency value. Combination of FST and SJ actuation results in spanwise-asymmetric vortical structures, with little difference in the location of the transition onset as compared to the previous case. The present analysis can be used to gather information on the practical implementation of low-speed SJ actuators as active turbulators.

Effects of free-stream turbulence on motion-induced aerodynamic forces of a long-span flexible flat roof using LES simulations

This study aims to investigate the effects of free-stream turbulence on motion-induced aerodynamic forces of a long-span flexible flat roof. Large-eddy simulations (LES) of both rigid and forced vibrating roofs under different free-stream turbulence conditions are performed to obtain the aerodynamic forces and flow field data simultaneously. Firstly, the LES simulations of the rigid and vibrating roofs are validated against the reference experiments. And then, characteristics of the distributed pressures on vibrating roofs under free-stream flow turbulence conditions with different turbulence intensities are elucidated. Moreover, the motion-induced forces acting on the vibrating roof under different turbulence conditions are identified and compared, in which the power spectra of wind forces and aerodynamic damping and stiffness are examined. Furthermore, the underlying flow mechanism of the turbulence effects on the motion-induced forces is investigated based on POD analysis of the flow around the vibrating roof. It is found that dual effects exist between the influences of free-stream turbulence and roof motion on the aerodynamic forces, the motion-induced aerodynamic forces on the roof would be greatly decreased with the increase of free-stream turbulence, and the motion-induced effects become less dominant under high free-stream turbulence. The outcomes of this study would facilitate our understanding of the role played by the motion-induced aerodynamic forces and improve the wind-resistant design of long-span roof structures for engineering practices.

Effect of free stream turbulence in critical Reynolds number regime (1.6 × 105−6.1 × 105) on flow around circular cylinder

The flow around a cylinder is a classical aerodynamic problem involving the effect of Reynolds number (Re). Thus far, the impact of turbulence has not been fully clarified despite its important practical value in engineering applications. This study mainly investigates the influence of turbulence in the critical Re regime on the smooth and turbulent flows around a cylinder. The foregoing is accomplished by conducting static pressure measurement model experiments in the Re range of 1.6 × 105–6.1 × 105 and turbulence intensity range (Iu) of 5%–13%. Consequently, a series of useful results for engineering wind resistance design is obtained. The effect of turbulence on each sub-flow regime varies. The structural response to turbulence is more dangerous under conditions such as when the wind pressure coefficients fluctuate strongly and non-Gaussianity is strong. The foregoing effects do not necessarily increase linearly with the turbulence intensity. In addition, the influence of turbulence evidently depends on the Re and corresponding flow regime. Therefore, turbulence must be cautiously managed in scaled model experiments and actual wind resistance design.

Freestream Turbulence Effects on the Aerodynamics of an Oscillating Square Cylinder at the Resonant Frequency

Flow past a bluff body in freestream turbulence can substantially change the flow behaviour compared to that in smooth inflow. This paper presents the study of wake flow and aerodynamics of an oscillating square cylinder at the resonant frequency in freestream turbulence, with the integral length not greater than the cylinder side and the turbulence intensity not greater than 10%. Large eddy simulations (LES) in the Cartesian grid using the Immersed Boundary Method (IBM) technique embedded in a FVM solver, together with an efficient synthetic turbulent inflow generator implemented in an in-house parallel FORTRAN code are used for the study. The results are compared with those for smooth inflow, and relevant data published in the literature. The key findings are: the freestream turbulence conditions evidently reduces the local turbulent scales and fluctuations in the shear layer compared to in smooth flow, as small scale freestream turbulence breaks down cylinder-generated larger scale eddies and weakens them; but does not evidently affect the vortex shedding frequency, or the length of the recirculation region behind the cylinder. This suggests negligible change of drag coefficient compared to in smooth inflow. Moreover, this is because the vortex shedding is dominated by the forced oscillation at the resonance frequency, and the turbulence intensity is small.

Effects of free-stream turbulence on the near wake flow and aerodynamic forces of a square cylinder

Despite a large volume of preceding studies on the effects of free-stream turbulence (FST) on the aerodynamic forces of a square cylinder, limited knowledge is available regarding the evolutionary characteristics of flow with the change of turbulence integral length scale and turbulence intensity. This paper presents a systematic experimental study of the near wake flow and aerodynamic forces of a two-dimensional (2D) square cylinder in grid-generated turbulent flows and smooth flow. Particular attention is devoted to exploring the respective influences of turbulence integral length scale and turbulence intensity. The effect of FST with a higher-level turbulence intensity than previous investigations is also investigated. The near wake flow and aerodynamic forces are studied by particle image velocimetry (PIV) and synchronous pressure measurements. The proper orthogonal decomposition (POD) analysis is also performed to further understand the effects of FST on the coherent structures of fluctuating pressures around the square cylinder. The results show that the FST with a larger turbulence integral length scale and higher turbulence intensity can cause an increase in the vortex formation length, weaker vortex shedding process and interactions between the oppositely signed vortex rows. These changes lead to significant reductions in Reynolds normal and shear stresses, turbulence kinetic energy and fluctuating lift forces. A completely different flow pattern featured with flow reattachment and reseparation is interestingly captured on the square cylinder in a highly turbulent flow with a turbulence intensity of 20%. This flow pattern results in an obvious mean pressure recovery on the side surfaces and much larger pressure fluctuations near the leading edges and fluctuating torsional moments compared to those in smooth flow and turbulent flows with lower turbulence intensities.