Free-stream Turbulence Research Articles

Heat Transfer Coefficient and Adiabatic Effectiveness Measurements on a Nozzle Guide Vane With a Single Row of Cylindrical Holes

Abstract Film-cooling injection significantly affects the thermal behavior of turbine vane surfaces. In addition to the beneficial effect of the film shielding the vane from the hot gas flow, alteration of the thermal boundary layer should also be taken into account. The aim of the present work is to detail the film-cooling performance in terms of adiabatic effectiveness and external heat transfer coefficient on a 2D nozzle guide vane. A single row of cylindrical holes was tested on both pressure and suction sides of a literature vane, the VKI LS89, in a linear cascade. The employed measurement technique is a transient thermal method based on infrared thermography, which was thoroughly described and validated in a previous work. The influence of inlet freestream turbulence and blowing ratio was evaluated, and two different injection angles were considered for both pressure and suction sides. Spatially resolved distributions of adiabatic effectiveness and heat transfer coefficient (HTC) on the vane surface allow us to precisely quantify the above-mentioned aspects and highlight qualitative differences between pressure side and suction side behavior. Details regarding the generated non-uniformities in the measured parameters could be also provided, to emphasize how average quantities are not always sufficient to characterize such complex phenomena. The impact of different reference conditions to scale HTC results was also investigated. Such effect was found not negligible on the overall performance of the film-cooling system, especially on the suction side where transition plays a critical role. Ultimately, the collected results constitute a wide and detailed experimental database for numerical modeling validation in a well-studied environment as the LS89 configuration.

An Experimental Database for the Analysis of Bursting of a Laminar Separation Bubble

The bursting phenomenon consists in the switch of a laminar separation bubble from a short to a long configuration. In the former case, reduced effects on profile pressure distribution are typically observed with respect to the attached condition. On the contrary, long bubbles provoke significant variations in the loading coefficient upstream of the separation position, with increased risk of stall of the lifting surfaces. The present work presents an experimental database describing separated boundary layers evolving under different Reynolds numbers, adverse pressure gradients and free-stream turbulence levels. Overall, more than 80 flow conditions were tested concerning short and long bubbles for the characterization of separated flows under turbine-like conditions. Measurements were performed on a flat plate geometry using a fast-response Particle Image Velocimetry (PIV) system. For each flow case, two sets of 6000 flow records were acquired with an acquisition frequency equal to 300 and 1000 Hz. Based on existing criteria for the identification of the bursting phenomenon, the flow cases were clustered in terms of short and long bubble states. Additionally, the kind of instability (i.e., convective or absolute) developing into the separated boundary layer was identified based on flow statistics. The present data captures the existing link between the bursting of a laminar separation bubble and the onset of the absolute instability of the separated shear layer, with stationary vortices forming in the dead air region.

Comparison Study of the k − kL − ω and γ − Reθ Transition Model in the Open-Water Performance Prediction of a Rim-Driven Thruster

The present work examines the capabilities of two transition models implemented in ANSYS Fluent in the open-water performance prediction of a rim-driven thruster (RDT). The adopted models are the three-equation k−kL−ω and the four-equation γ−Reθ models. Both of them are firstly tested on a ducted propeller. The numerical results are compared with available experimental data, and a good correlation is found for both models. The simulations employing two transition models are then carried out on a four-bladed rim-driven thruster model and the results are compared with the SST k−ω turbulence model. It is observed that the streamline patterns on the blade surface are significantly different between the transition and fully turbulent models. The transition models can reveal the laminar region on the blade while the fully turbulent model assumes the boundary layer is entirely turbulent, resulting in a considerable difference in torque prediction. It is noted that unlike the fully turbulent model, the transition models are quite sensitive to the free-stream turbulence quantities such as turbulent intensity and turbulent viscosity ratio, as these quantities determine the onset of the transition process. The open-water performance of the studied RDT and resolved flow field are also presented and discussed.

Experimental investigation of instantaneous lift on NACA 0018 airfoil section due to a non-harmonic pitching motion

In this study, we explore the problem of an airfoil in a harmonic and arbitrary periodic pitching motion and seek to understand how theoretical estimation with first-order potential theory match experiments. We adopt Jones’ approximation for the Wagner function for lift computations in the time domain and Theodorsen’s model in the frequency domain. We experimentally investigate a two-dimensional symmetric NACA 0018 airfoil in pitching motion under attached flow with reduced frequencies up to 0.25 and Reynolds number varying from 1.5×105 to 3×105. To eliminate the influence of the laminar separation bubble, free-stream turbulence intensity was elevated above the baseline level by placing turbulence generating grid, thus, obtaining better compatibility between the flow conditions in the experiments and the theoretical assumptions. Time-resolved sectional lift is determined by simultaneous static pressure measurement with miniature pressure transducers. The transient response is captured with accuracy in the time domain by quantifying the non-circulatory contribution. Excellent agreement is achieved in the time and frequency domains due to the highly accurate measurement of the instantaneous magnitude and phase of the time-resolved surface pressures. We further explore the applicability of the theory for high-pitching amplitudes, which results in a temporary deviation of the measured lift from the theoretical predictions. As the frequency domain unsteady theory is more widely studied than its time domain counterpart, this study provides a unique opportunity to highlight the significance of time-domain analysis in estimating instantaneous lift in non-harmonic motion.

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.

Experimental investigation of flame morphology and instabilities in turbulent wind-driven flames

This paper investigates the effect of freestream turbulence on flame profile parameters and streak behavior for boundary layer flames stabilized over a liquid fuel-soaked wick. In contrast to conventional gaseous burners, the study utilized a condensed fuel surface to better emulate realistic burning scenarios, thereby increasing the reliability of the obtained results. This study explores the impact of crossflow velocity (ranging from 1 m/s to 2 m/s) and turbulent intensity (varying between 1.8 % and 16 %) on the dynamic evolution of flame geometry and streak profile. For a given flow velocity, the results indicate a reduction in flame length and flame attachment length with turbulence intensity; in contrast, an increase in tilt angle was observed with increasing turbulence intensity. However, the rate of their increase or decrease was found to vary across different ranges of freestream turbulence intensity. Also, the flame exhibits a curling action in higher turbulence intensity scenarios that influence the global flame structure. The average mass burning rate was also found to be positively related to the flow turbulence in all considered cases. Freestream turbulence induces significant disturbances that amplify and destabilize coherent structures, accelerating the transition to turbulence in the overall flow and resulting in a shorter flame attachment length. Streak spacing experiences accelerated growth with higher turbulence levels, with a sharp surge in the initial region due to the blowing effect caused by the condensed fuel surface. The influence of flow turbulence was also observed by plotting lognormal distribution curves representing streak spacing data. The impact of elevated freestream turbulence on the dispersion of the distribution curve was found to be more pronounced than that of the flow velocity, emphasizing its crucial role in governing the streak dynamics.

Wake suppression of a cylinder immersed in turbulence using rotating rods

In this study, we conducted three-dimensional direct numerical simulations to investigate the control performance of the free-stream turbulence past a circular cylinder with two small rotating rods at Re = 3900. In the case A and case B, two control rods with different rotating ratio α are positioned symmetrically at the circumferential angle θ=±45° and θ=±120° from the forward stagnation point of the main cylinder, respectively. Here, α is the ratio of the tangential velocity of the control rod surface ur to the inflow velocity U∞. Compared to the bare cylinder immersed in turbulence, both installation positions of the rotating rods at α = 2 result in a significant suppression on the wake flow. Specifically, the time-averaged drag coefficient Cd¯ was reduced by 25%, and the root mean square value of the lift coefficient measured to reduce by up to 30%. Statistical analysis is then performed, in terms of the Reynolds stresses, mean field, and the turbulent wake visualization to show variations in the flow dynamics. As expected, the front-mounted control rods inject kinetic energy into the boundary layer, effectively suppressing the turbulence fluctuations on both sides of the main cylinder. The work done by viscous forces around the rotating control rods and the suppression of turbulent fluctuations contribute to the pressure recovery observed in the rear-mounted control rod case. Proper orthogonal decomposition method is further employed to analyze the key features of the controlled wake of two cases with α = 2.

Impact of large-scale free-stream turbulence on a pitching airfoil

This paper investigates the impact of large-scale turbulence on the aerodynamic characteristics of a pitching wind turbine blade at Reynolds number 135 000, whose cross-section is a NACA0012 airfoil with constant chord length. Large-eddy simulations at reduced frequencies, $k_{red} = 0.05$ and 0.1, were validated against reference data from the literature. An efficient method capable of generating synthetic large-scale turbulence at the inlet was applied by using two streamwise integral length scales $L_{x} = 1c$ and 1.5 $c$ , which represent energetic turbulence eddies at the height where the wind turbine operates. For $k_{red}= 0.1$ , the change in the maximum lift coefficient at the dynamic stall angle near the maximum angle of attack is on average 20 % lower and during the downstroke it is on average 22 % lower, compared with the smooth inflow. A higher reduced frequency ( $k_{red} = 0.2$ ) apparently does not further change the lift, drag and moment coefficients, and the inflow turbulence disordered leading-edge vortices. The turbulent shear stress and the phase-averaged dispersive shear stress in the wake are of the same magnitude, but with negative and positive signs, respectively, suggesting that the large-scale phase-averaged fluctuations transfer momentum in the opposite direction compared with the turbulent fluctuations, reducing the drag on the suction-side flow, and subsequently increasing the averaged lift coefficient. This demonstrates the critical importance of the large-scale non-turbulent unsteadiness in the wake of a pitching wing.

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.

Evaluation of Wavy-Trailing-Edge Film Cooling Versus Pressure Side Cutback Cooling

This study measures the film cooling effectiveness along the trailing edge (TE) of a turbine blade in a linear cascade. The film cooling effectiveness is measured and analyzed using the pressure-sensitive paint (PSP) technique. Two different TE designs are investigated: the standard pressure side cutback and the new alternating discharge design (referred to as a wavy-TE design). The alternating discharge design is a new design with a wavy structure between the pressure and suction surfaces at the TE. The new wavy structure allows the coolant to discharge from the TE so that it alternates between the pressure and suction surfaces. Testing is carried out in a five-blade, linear, steady-state cascade with inlet and exit Mach numbers of 0.20 and 0.30, respectively. The freestream turbulence intensity is measured to be 10.5% upstream of the blade leading edge. Coolant-to-mainstream mass flow ratios (MFRs) vary from 0.30 to 1.25% and coolant-to-mainstream density ratios (DRs) of 1.0, 1.5, and 2.0 are examined. A positive MFR and DR effect is witnessed. The results demonstrate the potential of the alternating discharge as a promising TE design and provide gas turbine designers with an improved TE cooling scheme.

Effects of Riblet Dimensions on the Transitional Boundary Layers Over High-Lift Turbine Blades

Abstract Substantial research exists in the literature on reducing the profile loss of transitional boundary layers over low-pressure turbine (LPT) blades via different mechanisms such as freestream turbulence, upstream wakes, and surface roughness. These mechanisms have proven to be beneficial in mitigating the separation bubble-related losses in ultra-high-lift blade designs, despite an increase in the loss due to increased turbulent wetted area (TWA). In this work, we adopt a strategy of employing surface roughness in the transitional regime to minimize the separation bubble-related losses and flush-mounted riblets downstream to further mitigate the skin-friction drag and boundary layer losses due to an increase in the TWA. Several high-fidelity scale-resolving simulations are performed on this “rough-ribbed blade surface” to discern the effect of varying the riblet spacing (s+) and height (h+). The streamwise evolution of skin-friction coefficient, boundary layer integral parameters, and shape factor are compared and contrasted among riblets of different dimensions. The instantaneous flow features and second-order statistics such as the Reynolds stress, turbulent kinetic energy, and its production are analyzed for different test cases to determine the impact of riblets on these quantities. When compared to the roughness alone configuration, the scalloped shape riblets with s+ = 17 and h+ = 22 reduced the net skin-friction drag by 7.3% and the trailing edge momentum thickness by 14.5%, thereby demonstrating the efficacy of riblets in reducing the mixing losses under adverse pressure gradients. Through an analysis of flow blockage introduced by the application of riblets, the deleterious effects of increasing the riblet height along with the necessity of optimizing the riblet ramp are highlighted.

Particle Image Velocimetry Measurements in a High-Speed Low-Reynolds Low-Pressure Turbine Cascade

Abstract Particle image velocimetry (PIV) measurements in the blade-to-blade (B2B) plane and cascade outlet plane (COP) of a high-speed low-pressure turbine (LPT) cascade were performed at engine-representative outlet Mach number (0.70–0.95), and Reynolds number (70–120 k) under steady flow conditions. The freestream turbulence characteristics were imposed by means of a passive turbulence grid. The PIV results on the B2B plane were compared against five-hole probe (5HP) and Reynolds-averaged Navier–Stokes (RANS) computations to assess the validity of measurement and simulation techniques in the engine-relevant LPT cascade flows. The PIV captured the wake depth and width measured by the 5HP whereas the RANS displayed an overprediction of the wake Mach number deficit. The 5HP was found to impose sinewave fluctuations of the measured flow angle downstream, around three times higher than PIV. Additionally, PIV estimated turbulence intensity (TI) in the cascade, showing TI decay along a streamline. At the highest Mach number, a peak TI occurred past a shock wave. Measurements of the outlet flow field highlighted a high TI in the secondary flow region whereas high degree of anisotropy (DA) was registered in the boundary of the secondary flow and freestream regions. The contribution of the streamwise fluctuation component was found to be less than the crosswise and radial components in the freestream region. Increasing the cascade outlet Mach number, the contribution of streamwise fluctuation to the DA was observed to decrease.

A data-driven analysis of short and long laminar separation bubbles

This work investigates the statistical response of short and long laminar separation bubbles to external flow parameters, such as Reynolds number, free-stream turbulence intensity and streamwise pressure gradient, known to govern bubble formation and characteristics. A parametric experimental campaign has been performed using particle image velocimetry on a flat plate to provide a comprehensive database for the characterization of separation-induced transition in both short and long separation bubbles. The proper orthogonal decomposition (POD) was applied to the data set of all dividing streamlines commonly used to identify a laminar separation bubble. This provides an optimal state-space basis for the data-driven classification of the state of a laminar separation bubble, with the leading modes capturing the change in length and height of the laminar separation bubble in response to changes in the flow parameters. When projected onto the POD subspace constituted by the first three leading modes, the normalized data from the present study and the results from prior investigations not used in the modal analysis collapse on the same trajectory in the low-dimensional space. The present POD basis can be therefore adopted for the description of the general response of the time-mean shape of a laminar separation bubble to changes in the main influencing parameters. A well-defined pattern was observed in the case of short laminar separation bubbles in the reduced-order space defined by the first three POD coefficients, whereas a higher dispersion in the long-bubble regime indicates an increased sensitivity of long bubbles to the external flow characteristics.

Airfoil Tonal Noise Prediction Using Urans

Abstract To examine the feasibility of the laminar boundary layer (LBL), vortex shedding (VS) tonal noise modelling using unsteady Reynolds-averaged Navier–Stokes (URANS) was investigated for the non-symmetric S834 airfoil. A transition SST turbulence model was used to model the laminar-turbulent transition and its vital influence on the laminar bubble and hydrodynamic instabilities generation. The influence of turbulence on the unsteady vortex patterns was investigated. Hence, the hybrid aeroacoustic analysis with Lighthill analogy was conducted to obtain the acoustic pressure field. The approach allowed us to model hydrodynamic instabilities and the resulting VS tonal noise. The frequency of VS matched the experimental data, giving the same 1/3 octave tonal peak only for a limited freestream turbulence regime. The simplification of the present method did not allow us to model the aeroacoustic feedback loop, and resulted in lack of instabilities for higher freestream turbulence.

Features of laminar separation bubble subjected to varying adverse pressure gradients

This article describes the spatial development of a laminar separation bubble (LSB), its transition, and eventual breakdown under the influence of adverse pressure gradients (APGs) similar to those experienced by low-pressure turbine blades. The investigation combines a comprehensive experimental approach with a well-resolved large eddy simulation (LES). The streamwise pressure gradients were varied by manipulating the upper wall within the test section. The Reynolds number (Re), based on the plate length and inlet velocity, was 0.2 × 106 with a freestream turbulence intensity of 1.02%. The particle image velocimetry (PIV) and hotwire data were used to illustrate the vortex dynamics, growth of perturbations, and intermittency. The onset and end of transition progressively shift upstream, resulting in a reduction of the laminar shear layer length and bubble length with increasing APG. Interestingly, the flow features exhibit self-similarity in velocity profiles and the growth rate of velocity fluctuations when normalized against the bubble length. The formation of two-dimensional Kelvin–Helmholtz (K–H) rolls is apparent in the beginning, resulting in the selective amplification of frequency and exponential growth of fluctuations. Linear stability theory explains the most amplified frequency and phase speed of convective vortices, apart from the growth of disturbances. Analysis of LES data reveals intricate inviscid–viscous interactions that trigger shear layer breakdown. In brief, evolving perturbations within the braid region of vortices in the latter half interact with the advecting K–H rolls, culminating in the breakdown and the onset of turbulent flow downstream.

Increase of trailing-edge noise due to inflow turbulence

Altough the regulatory imposition of low-noise limits, airframe noise currently limits the design space of several applications, such as, wind energy, marine, and aeronautics. The noise of those applications is significantly increased by the presence of inflow turbulence. It has been believed that only the leading-edge component is increased. However, recent results show that the trailing-edge noise is also increased because of the free-stream turbulence penetration in the airfoil boundary layer. This research measures the increase of trailing-edge noise due to inflow turbulence for NACA 63018 and NACA 0008 airfoils. Noise measurements were conducted for velocities ranging from 15 to 40 m/s, with and without inflow turbulence. A rod installed upstream of the airfoils generated an inflow turbulence of ≈ 12%. The wall-pressure fluctuations were measured close to the trailing edge to analyze the near-field phenomenon. The inflow turbulence increases the wall-pressure spectrum level in the entire frequency range and the spanwise correlation length in the low-frequency range. The trailing-edge noise increases due to the inflow turbulence in the entire frequency range at least 2 db up to more than 15 dB for all the cases. The contribution of leading- and trailing-edge noise to the total noise varies with the airfoil geometry, with the trailing-edge noise dominating in a larger frequency range for the thickest airfoil.

Study of the influence of multiple factors on the boundary layer of a high-lift LPT with the RBF-GA method

In a high-altitude cruising state, boundary layer separation exists in high-lift low-pressure turbines, and inflow conditions corresponding to different blade designs can directly affect the working efficiency of low-pressure turbines. In particular, the reduced frequency of wake and free-stream turbulence intensity in an inlet flow can greatly influence boundary layer separation and transition development. In this paper, the influence of different inflow turbulence intensities and reduced wake frequencies on the development of suction surface boundary layers in high-lift low-pressure turbines under the influence of upstream wakes is studied by numerical simulations and experiments. Due to the combination of inflow free-stream turbulence intensity and reduced wake frequency, many inflow conditions can be chosen in the design process, and the unsteady influence of upstream wakes complicates the boundary layer flow. In this paper, an RBF (radial basis function)-GA (genetic algorithm) machine learning method is used to explore the optimal inlet conditions corresponding to the minimum profile loss of the Pak-B profile. The search region of the free-stream turbulence intensity is 2%–4%, and the reduced frequency of the wake is changed by changing the flow coefficient, whose variation range is 0.7–1.3. It is found that the RBF-GA machine learning method can attain an inflow condition with a lower profile loss while using the same amount of computation and effort.

Leading-Edge Effects on Freestream Turbulence Induced Transition of an Organic Vapor

Leading-Edge Effects on Freestream Turbulence Induced Transition of an Organic Vapor

Large Eddy Simulation of a Low-Pressure Turbine Cascade with Turbulent End Wall Boundary Layers

We present results of implicit large eddy simulation (LES) and different Reynolds-averaged Navier–Stokes (RANS) models of the MTU 161 low pressure turbine at an exit Reynolds number of 90000\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$90\\,000$$\\end{document} and exit Mach number of 0.6. The LES results are based on a high-order discontinuous Galerkin method and the RANS is computed using a classical finite-volume approach. The paper discusses the steps taken to create realistic inflow boundary conditions in terms of end wall boundary layer thickness and freestream turbulence intensity. This is achieved by tailoring the input distribution of total pressure and temperature, Reynolds stresses and turbulence length scale to a Fourier series based synthetic turbulence generator. With this procedure, excellent agreement with the experiment can be achieved in terms of blade loading at midspan and wake total pressure losses at midspan and over the channel height. Based on the validated setup, we focus on the discussion of secondary flow structures emerging due to the interaction of the incoming boundary layer and the turbine blade and compare the LES to two commonly used RANS models. Since we are able to create consistent setups for both LES and RANS, all discrepancies can be directly attributed to physical modelling problems. We show that both a linear eddy viscosity model and a differential Reynolds stress model coupled with a state-of-the-art correlation-based transition model fail, in this case, to predict the separation induced transition process around midspan. Moreover, their prediction of secondary flow losses leaves room for improvement as shown by a detailed discussion of turbulence kinetic energy and anisotropy fields.

Effects of integral length scale variations on the stall characteristics of a wing at high free-stream turbulence conditions

The effect of variations in the integral length scale of incoming free-stream turbulence on a NACA0012 wing is investigated with the use of force, moment and particle image velocimetry measurements. At a chord-based Reynolds number ( $Re = U_\infty c/\nu$ where c is the chord length, $U_\infty$ is the free-stream velocity and $\nu$ is the kinematic viscosity) of $2\times 10^5$ , an active grid generates turbulence intensities of 15 % at normalised integral length scales ranging from 0.5 $c$ to 1 $c$ . The introduction of turbulence improves the time-averaged performance characteristics of the wing by delaying stall and increasing the peak lift coefficient. It is found that for half-chord integral length scales, the magnitude of the fluctuations in forces and moments is larger than that of full-chord integral length scales, as the former amplifies the naturally occurring unsteadiness in the flow (when there is no free-stream turbulence). The increase in magnitude is ascribed to a larger density of smaller-scale vortices within the separated flow and wake region of the wing.