Influence Of Free-stream Turbulence Research Articles

The influence of turbulence on cycling aerodynamics

Many previous studies have focused on characterising Reynolds number effects and skinsuit design to better understand and improve aerodynamic performance. However, the majority of this research has been conducted in wind tunnels with turbulence intensities of less than 2%, considerably lower than that experienced when competing outdoors or in the wakes of competitors. In this systematic experimental study using passive turbulence generation, the influence of freestream turbulence on the aerodynamics of a cyclist is determined. Force measurements, wake velocity surveys, total pressure measurements and surface flow visualisations are used to characterise and quantify the aerodynamic behaviour. It is found that the aerodynamic drag is strongly affected by turbulence with it monotonically decreasing with increasing turbulence level. Moderate levels of turbulence are shown to alter local flow separation, and consequently the drag, on parts of the cyclist’s body, which in turn is imprinted on the large-scale wake structures. The interaction of turbulence level and skinsuit roughness on the arms is also considered showing that these factors cannot be treated in isolation. Overall, this study highlights the importance of understanding the environment in which cyclists compete in order to optimise their performance.

Numerical study of the hydrodynamic stability of a wind-turbine airfoil with a laminar separation bubble under free-stream turbulence

The interaction of several instabilities and the influence of free-stream turbulence on laminar-turbulent transition on a 20% thick wind-turbine blade section with a laminar separation bubble (LSB) are investigated with wall-resolved large-eddy simulations (LES). Turbulence intensities (TI) of 0%, 2.2%, 4.5%, 8.6%, and 15.6% at chord Reynolds number 105 are considered. Linear receptivity occurs for the most energetic disturbances; high-frequency perturbations are excited via non-linear mechanisms for TI≥8.6%. Unstable Tollmien–Schlichting (TS) waves appear in the inflectional flow region for TI≤4.5%, shifting to inviscid Kelvin–Helmholtz (KH) modes upon separation and forming spanwise rolls. Sub-harmonic secondary instability occurs for TI=0%, with rolls intertwining before transition. Streaks spanwise modulate the rolls and increase their growth rates with TI for TI≤4.5%, reducing separation and shifting transition upstream. The TI=4.5% case presents the highest perturbations, leading to the smallest LSB and most upstream transition. Earlier inception of TS/KH modes occurs on low-speed streaks, inducing premature transition. However, for TI=8.6%, the effect of the streaks is to stabilize the attached mean flow and front part of the LSB. This occurs due to the near-wall momentum deficit alleviation, leading to the transition delay and larger LSB than TI=4.5%. This also suppresses separation and completely stabilizes TS/KH modes for TI=15.6%. Linear stability theory predicts well the modal evolution for TI≤8.6%. Optimal perturbation analysis accurately computes the streak development upstream of the inflectional flow region but indicates higher amplification than LES downstream due to the capture of low-frequency, oblique modal instabilities from the LSB. Only low-amplitude [O(1%)] streaks displayed exponential growth in the LES since non-linearity precludes the appearance of these modes.

The influence of free stream turbulence on the development of a wind turbine wake

The wake of an isolated model-scale wind turbine is analysed in a set of inflow conditions having free stream turbulence intensity between 3 % and 12 %, and integral time scales in the range of 0.1–10 times the convective time scale based on the turbine diameter. It is observed that the wake generated by the turbine evolves more rapidly, with the onset of the wake evolution being closer to the turbine, for high turbulence intensity and low integral time scale flows, in accordance with literature, while flows at higher integral time scales result in a slow wake evolution, akin to that generated by low-turbulence inflow conditions despite the highly turbulent ambient condition. The delayed onset of the wake evolution is connected to the stability of the shear layer enveloping the near-wake, which is favoured for low-turbulence or high-integral time scale flows, and to the stability of the helical vortex set surrounding the wake, as this favours interaction events and prevents momentum exchange at the wake boundary which hinder wake evolution. The rate at which the velocity in the wake recovers to undisturbed conditions is instead analytically shown to be a function of the Reynolds shear stress at the wake centreline, an observation that is confirmed by measurements. The rate of production of Reynolds shear stress in the wake is then connected to the power harvested by the turbine to explain the differences between flows at equivalent turbulence intensity and different integral time scales. The wake recovery rate, and by extension the behaviour of the turbine wake in high-integral time scale flows, is seen to be a linear function of the free stream turbulence intensity for flows with Kolmogorov-like turbulence spectrum, in accordance with literature. This relation is seen not to hold for flows with different free stream turbulence spectral distribution; however, this trend is recovered if the contributions of low frequency velocity components to the turbulence intensity are ignored or filtered out from the computation.

Influence of free-stream turbulence on the boundary layer stability of a wind turbine airfoil and near wake

Free-stream turbulence (FST) alters the boundary layer of wind turbine blades, changing the hydrodynamic stability and near wake. Large-eddy simulations (LES) of a blade section with a laminar separation bubble for several turbulence intensities (TI) and a Reynolds number of 105 are performed. The effects of boundary-layer streaks generated by FST on Tollmien-Schlichting (TS) and Kelvin-Helmholtz (KH) instabilities are analyzed with a model based on the parabolized stability equations (PSE). Two competing effects on flow stability are identified. The spanwise-averaged mean-flow distortion stabilizes primary TS/KH modes for increasing TI. However, this contribution seems dominant only for TI ≥ 8.6%. For lower TI, the spanwise-oscillating distortion caused by streaks destabilizes the flow, and the growth rates of secondary modal instabilities increase with the streak amplitude. The destabilization occurs mainly at spanwise locations with negative streaks since the inflection point shifts away from the wall, enhancing inviscid instabilities. Inflection points in the spanwise direction formed by the streaks also contribute to the destabilization. The modal structures from PSE and LES agree. Finally, the trailing-edge near-wake coherent structures are more energetic for TI ≥ 8.6% due to the partial stabilization of modal instabilities, delaying the turbulent breakdown.

Influence of freestream turbulence on the near-field growth of a turbulent cylinder wake: Turbulent entrainment and wake meandering

The influence of freestream turbulence (FT) on near-field development of turbulent wakes is assessed with regards to both the entrainment rate of background fluid into the wake and the extent of wake meandering. It is shown that FT increases the entrainment rate into the near wake, and thereby the rate at which the wake spreads. The influence of FT on wake meandering is more complex, but it is shown that the integral length scale is the dominant property affecting meandering. In a novel control experiment, large-scale coherent motion in the wake is suppressed, and the far-wake results are recovered: that is, turbulent entrainment is reduced via the suppression of small-scale nibbling.

Effect of free-stream turbulence on conical vortex dynamics

The influence of Free-Stream Turbulence (FST) on the space–time dynamics of a conical vortex developing along a A-pillar is studied experimentally. Measurements of unsteady wall pressure and velocity by High Speed-Stereo PIV highlight the important effects of turbulence on the mean and instantaneous properties of the vortex. Very significant increases in Reynolds stresses into the vortex region and in wall fluctuating pressure are observed in the presence of FST. In smooth flow, the frequency content of the pressure and velocity fields is very rich with low and high frequency contributions due to the meandering of the vortex and instabilities in the vortex core. Meandering shows, for the different integral length scales and intensities of turbulence tested, a great receptivity to the presence of a FST and we observe a global motion of the vortex structure at low frequency. This frequency is modulated by the value of the integral length scale of the FST. We show that the mean conical structure is a wave guide for the perturbations of the core but that, with FST, the spatio-temporal evolution of the envelope overwhelms the intrinsic instability of the vortex core observed in smooth flow.

Influence of free-stream turbulence on the aerodynamic performance of a three-dimensional airfoil

Influence of free-stream turbulence on flows past a static three-dimensional SD7003 airfoil is numerically investigated via wall-resolved Large-Eddy Simulation (wall-resolved LES). The Reynolds number and Mach number are 60 000 and 0.2, respectively, with the angle of attack (AoA) varying from 4° to 14°. The purpose of this investigation is to clarify the effects of intensities and integral length scales of turbulence in the approaching flow on the aerodynamic performance of the airfoil. To this end, a synthetic digital filtering technique and a specific source term concept are employed to generate turbulent fluctuations within the incoming flow. It turns out that turbulence with higher intensity in the approaching flow may cause the variations in effective AoAs, which not only postpone flow development but also promote the diversity of flow classifications from an instantaneous point of view. Furthermore, it is shown that the integral length scale has significant and systematic impacts on turbulent fluctuations in the boundary layer. Larger integral length scales tend to result in higher levels of turbulence intensities, which thus induce larger separations at all AoAs.

Influence of freestream turbulence on the flow over a wall roughness

We demonstrate that free-stream turbulence has a strong impact on the dynamics of flow past a cylindrical roughness element. Previous works have shown that, depending on operating conditions, the leading flow instability can either be varicose (symmetric) or sinuous (antisymmetric). In both cases, when the flow is excited by broadband frequency forcing, dynamic mode decomposition extracts only varicose coherent structures, even though optimal response analysis predicts strong amplification of sinuous disturbances with frequency near the marginally stable sinuous eigenmode. We show that this is due to the flow essentially being an amplifier of varicose perturbations rather than a resonator.

Adaptive detached eddy simulation of transition under the influence of free-stream turbulence and pressure gradient

Abstract

The influence of freestream turbulence on the temporal pressure distribution and lift of an airfoil

To gain insight on how freestream turbulence (FST) affects the aerodynamics of an airfoil in a systematic manner, the present study investigates a NREL S826 airfoil subjected to seven different incoming flows with varying degrees of FST. The Reynolds number was held at Rec ​= ​4.0 ​× ​105, while the turbulence intensity (Ti) was varied between 0.4% and 5.4%. An increase in Ti increases the maximum lift while having negligible effects on the stall angle. The lift slope in the linear region also generally increased with higher Ti. The latter observation contrasts with some earlier studies, and incoming flow homogeneity is a potential contributing factor to the differences seen here. Periodic pressure fluctuations are seen in the computed lift time-series signal when Ti is between 1% and 2% and the airfoil is operating in the linear region. These fluctuations arise from surface pressure oscillations that are likely excited by the relatively low incoming Ti. The overall effect is a reduction in the time-averaged lift under these operating conditions. At higher Ti, more energetic boundary layers develop over the airfoil’s suction side and the effect of these periodic pressure fluctuations is suppressed, leading to an increase in the produced lift.

Spatial characteristics of a zero-pressure-gradient turbulent boundary layer in the presence of free-stream turbulence

Particle image velocimetry measurements are performed to examine the spatial structure in boundary layers under the influence of free-stream turbulence (FST). A similarity of the structural organization inside the boundary layer is found between the present FST cases and the canonical flows.

Influence of the bluff body shear layers on the wake of a square prism in a turbulent flow

The influence of freestream turbulence on the transitional characteristics of a 2D square prism shear layer was considered experimentally. Bypass transition was observed to cause intermittent shear layer reattachment; this event has a cascading influence downstream, altering the formation of the von-K\'arm\'an street.

Numerical Simulation of Heat Transfer Coefficient on Turbine Blade using Intermittency Factor Equation

This paper deals with the numerical simulation of fluid flow and heat transfer coefficient on a gas turbine blade. The mathematical model of flow is based on the Reynolds Averaged Navier-Stokes (RANS) equations completed with the formulation of transition model by using an intermittency transport equation. An intermittency based transition model, which is critical for invoking transition onset according to Abu-Ghannam and Shaw correlation, is implemented into the proven Reynolds-Averaged N-S (RANS) solver. The intermittent behavior of the transitional flow is incorporated into the computation by modifying the eddy viscosity μt, obtained from a turbulence model. Wilcox [4] low Reynolds k−ω turbulence model is employed to calculate the eddy viscosity and others turbulent quantities. This model is designated for the prediction of flow transition under the influence of freestream turbulence (FST). The current model is applied to prediction of a modern high pressure turbine experiment, and detail comparisons of the computational results with the experimental data are presented. The model has been shown to be capable of predicting the high pressure turbine flow transition under a variety of Reynolds number and free stream turbulence conditions.

Flat-plate film cooling from a double jet holes: influence of free-stream turbulence and flow acceleration

Results of an experimental study of flat-plate film cooling effectiveness achieved with an inlet double jet scheme are reported. At low (m = 0.5) and medium (m = 1.0) blowing ratio the average film cooling effectiveness is about 20 % greater of the traditional two-row scheme of round holes data, while at higher m = 1.5 it is close to it. The free-stream turbulence (≈ 7 %) influences weekly on the average flat-plate film cooling effectiveness. The flow acceleration decreases the film cooling effectiveness down to 25 % when the pressure gradient parameter K is ranged from 0.5·10−6 to 3.5·10−6.

Effects of adverse and favorable pressure gradients on entropy generation in a transitional boundary layer region under the influence of freestream turbulence

Fundamental understanding of the entropy generation process in characteristic wall shear flows is developed. For entropy generated by fluid friction, the rates are predictable for developed turbulent flows and pure laminar flows. The main concern lies in prediction for flows undergoing so-called “bypass” transition from laminar to turbulent state. In this study, the entropy generation process in the bypass transition scenario is investigated for a flat plate boundary layer under effects of adverse and favorable pressure gradients. For the case studied, transition occurs prematurely due to the presence of strong levels of freestream turbulence. Reynolds-Averaged Navier–Stokes (RANS) models and direct numerical simulations (DNS) are implemented to study the local entropy generation in pre-transitional and transitional regions. Three of these RANS models are transitional models such as, traditional SST k–ω (2eq), SST k–ω (4eq) and k–kl–ω. They are used for prediction of the onset of transition and the results are compared with DNS. All the RANS models predicted transition onset prematurely and, consequently, over predict the integral entropy generation rate and the skin friction coefficient in the transition region. Four simulations have been performed for (1) zero, (2) favorable, (3) adverse and (4) strong-adverse pressure gradient cases. The numerical results show that the pressure gradient has a significant effect on the onset of transition and entropy generation. Transition is early and abrupt for the strong adverse pressure gradient but occurs further downstream and is longer with the increasingly favorable flows. Entropy generation rate at the wall increases as the pressure gradient grows.

Entropy generation in a transitional boundary layer region under the influence of freestream turbulence using transitional RANS models and DNS

In this study, the entropy generation process in the bypass transition scenario is investigated for a flat plate boundary layer. Here transition occurs prematurely due to the presence of strong levels of freestream turbulence. Reynolds-Averaged Navier–Stokes (RANS) models and Direct Numerical Simulations (DNS) are implemented to study the local entropy generation and energy dissipation in pre-transitional and transitional regions comprehensively. Two new transitional RANS models (SST k−ω(4eq) and k−kl−ω) were used for prediction of the onset of transition and the results are compared with DNS ones. Classical laminar theory underpredicts the observed entropy generated. In the pre-transitional boundary layer, the perturbations generated by the streaky structures modify the mean velocity profile and induce a quasi-turbulent contribution to indirect dissipation. In the transition region the pointwise entropy generation rate (S‴)+ initially increases near the wall and then decreases corresponding to the distribution predicted for a fully-turbulent boundary layer as the flow moves downstream. All the RANS models predicted transition onset prematurely and, consequently, overpredict the integral entropy generation rate and the skin friction coefficient in the transition region.

Direct numerical simulations of transition in a compressor cascade: the influence of free-stream turbulence

The flow through a compressor passage without and with incoming free-stream grid turbulence is simulated. At moderate Reynolds number, laminar-to-turbulence transition can take place on both sides of the aerofoil, but proceeds in distinctly different manners. The direct numerical simulations (DNS) of this flow reveal the mechanics of breakdown to turbulence on both surfaces of the blade. The pressure surface boundary layer undergoes laminar separation in the absence of free-stream disturbances. When exposed to free-stream forcing, the boundary layer remains attached due to transition to turbulence upstream of the laminar separation point. Three types of breakdowns are observed; they combine characteristics of natural and bypass transition. In particular, instability waves, which trace back to discrete modes of the base flow, can be observed, but their development is not independent of the Klebanoff distortions that are caused by free-stream turbulent forcing. At a higher turbulence intensity, the transition mechanism shifts to a purely bypass scenario. Unlike the pressure side, the suction surface boundary layer separates independent of the free-stream condition, be it laminar or a moderate free-stream turbulence of intensityTu~ 3%. Upstream of the separation, the amplification of the Klebanoff distortions is suppressed in the favourable pressure gradient (FPG) region. This suppression is in agreement with simulations of constant pressure gradient boundary layers. FPG is normally stabilizing with respect to bypass transition to turbulence, but is, thereby, unfavourable with respect to separation. Downstream of the FPG section, a strong adverse pressure gradient (APG) on the suction surface of the blade causes the laminar boundary layer to separate. The separation surface is modulated in the instantaneous fields of the Klebanoff distortion inside the shear layer, which consists of forward and backward jet-like perturbations. Separation is followed by breakdown to turbulence and reattachment. As the free-stream turbulence intensity is increased,Tu~ 6.5%, transitional turbulent patches are initiated, and interact with the downstream separated flow, causing local attachment. The calming effect, or delayed re-establishment of the boundary layer separation, is observed in the wake of the turbulent events.

Effects of Free-Stream Turbulence on Wing-Tip Vortex Formation and Near Field

The formation and near-field development of a wing-tip vortex under the influence of freestream turbulence were examined using flow visualization and hot-wire anemometry. A low turbulence freestream as well as two cases of grid turbulence with different intensities and length scales were considered. In all cases, the tip vortex was found to form from three smaller vortices, but the turbulence in its core was found to intensify with increasing freestream turbulence. The vortex trajectory was found to be unaffected by freestream turbulence, but the wing wake that was rolling up around the vortex was observed to have a curvature that decreased as freestream turbulence increased. The mean axial velocity distribution in the low-turbulence case was neither jetlike nor wakelike but had an annular shape. Time-averaged velocity profiles measured in the turbulent freestream cases were wakelike, and it was inferred that the instantaneous profiles would be significantly affected by vortex meandering. Mean circumferential velocity distributions in the vortex core displayed self-similar developments in all cases examined. Finally, it was found that the apparent diffusion in the shear layer shed from the wing increased with increasing freestream turbulence.

Computational Fluid Dynamics Simulation of the Solid Suspension in a Stirred Slurry Reactor

A comprehensive computational fluid dynamics CFD model was developed in the present study to gain insight into the solid suspension in a stirred slurry reactor. The preliminary simulations highlighted the need for the correct modeling of the interphase drag force. A two-dimensional model problem was then developed using CFD to understand the influence of free stream turbulence on the particle drag coefficient. The proposed correlation was then incorporated in a two-fluid model (Euler−Euler) along with the standard k−ε turbulence model with mixture properties to simulate the turbulent solid−liquid flow in a stirred reactor. A multiple reference frame approach was used to simulate the impeller rotation in a fully baffled reactor. A computational model was mapped on to a commercial CFD solver FLUENT6.2 (of Fluent Inc., USA). The model predictions were compared with the published experimental data of Yamazaki et al. [Powder Technol. 1986, 48, 205] and Godfrey and Zhu [AIChE Symp. Ser. 1994, 299, 181]. The predicted results show reasonably good agreement with the experimental data. The computational model and results discussed in this work would be useful for extending the applications of CFD models for simulating large stirred slurry reactors.

Sensitivity analysis of freestream turbulence parameters on stagnation region heat transfer using a neural network

A neural network has been used to predict stagnation region heat transfer in the presence of freestream turbulence. The neural network was trained using data from an experimental study to investigate the influence of freestream turbulence on stagnation region heat transfer. The integral length scale, Reynolds number, all three components of velocity fluctuations and the vorticity field were used to characterize the freestream turbulence. The neural network is able to predict 50% of the test data within ±1%, while the maximum error of any data point is under 3%. A sensitivity analysis of the freestream turbulence parameters on stagnation region heat transfer was performed using the trained neural network. The integral length scale is found to have the least influence on the stagnation line heat transfer, while the normal and spanwise turbulence intensities have the highest influence.