Freestream Turbulence Intensity Research Articles

The influence of background turbulence on Ahmed-body wake bistability

The discovery of wake bistability has generated an upsurge in experimental investigations into the wakes of simplified vehicle geometries. Particular focus has centred on the probabilistic switching between two asymmetrical bistable wake states of a square-back Ahmed body; however, the majority of this research has been undertaken in wind tunnels with turbulence intensities of less than$1\,\%$, considerably lower than typical atmospheric levels. To better simulate bistability under on-road conditions, in which turbulence intensities can easily reach levels of$10\,\%$or more, this experimental study investigates the effects of free-stream turbulence on the bistability characteristics of the square-back Ahmed body. Through passive generation and quantification of the free-stream turbulent conditions, a monotonic correlation was found between the switching rate and free-stream turbulence intensity.

Statistical characterization of free-stream turbulence induced transition under variable Reynolds number, free-stream turbulence, and pressure gradient

In this work, the free-stream turbulence (FST) induced transition of a flat plate boundary layer is studied using particle image velocimetry (PIV) under variable Reynolds number (Re), FST intensity, and adverse pressure gradient (APG). Overall, 10 different flow conditions were tested concerning the variation of these parameters. The streak spacing and the probability density function (PDF) of turbulent spot nucleation are computed for all cases. The streak spacing is shown to be constant in the transition region once scaled with the turbulent displacement and momentum thickness, with resulting values of around 3 and 5, respectively. Nucleation events are shown to occur near the position where the dimensionless streak spacing reaches such constant values. The streamwise position where most turbulent spots are formed is strongly influenced by the FST intensity level. Additionally, the PDF of spot nucleation becomes narrower with increase in the APG, while FST has the opposite effect. A common distribution of all the PDFs is provided as a function of a similarity variable accounting for the streak spacing, the shape factor of the boundary layer, and the FST intensity.

Influence of turbulence intensity on vortex pattern for a rigid cylinder with turbulence stimulation in flow induced oscillations

The effect of ambient turbulence intensity on the flow induced oscillations of rigid circular cylinders with symmetric and asymmetric local turbulence stimulation is studied. Cylinders oscillate in one degree of freedom transversely to a steady uniform flow. Two-dimensional unsteady Reynolds-Average Navier-Stokes equations with the Spalart-Allmaras turbulence model are used to solve the problem numerically. Three freestream turbulence intensity values (0.2%, 1%, 5%) are used to study the influence of turbulence intensity on vortex pattern. Simulation results are compared with experimental data measured in the Marine Renewable Energy Laboratory of the University of Michigan in the range of 30,000 ≤ Re ≤ 110,000. The amplitude ratio, lift coefficient, vortex modes, and the interactions between vortices and cylinders are observed and discussed. Major conclusions are: (a) Vortex patterns strongly depend on the freestream ambient turbulence intensity. (b) Low turbulence intensity can generate multi-vortex patterns at high Re. (c) High turbulence intensity results in fewer separations of a shed vortex into multiple vortices inducing emergence of elongated vortex patterns.

Identification of a Pressure–Strain Correlation-Based Bypass Transition Onset Marker

Abstract Temporally developing direct numerical simulations (T-DNS) are performed for freestream turbulence induced bypass transition flow with zero-pressure gradient over a flat-plate boundary layer for a range of freestream turbulence intensities (Tu) of 1.4% to 6%. The objective is to understand the role of pressure–strain terms on bypass transition onset and to propose and validate a phenomenological hypothesis for the identification of a robust transition onset marker for use in transition-sensitive Reynolds-averaged Navier–Stokes (RANS) simulations. Results show that transition initiates at a location where the slow pressure–strain term becomes more dominant than the rapid term in the pretransitional boundary layer region. A simple transition onset marker based on one-point statistical quantities is derived from the scaling of the ratio of the slow and rapid pressure fluctuation source terms. The critical value of the marker is found to vary within a narrow range (±2.8%) and satisfies previously identified criteria for a robust transition onset marker.

Free-Stream Turbulence-Induced Boundary-Layer Transition in Low-Pressure Turbines

Abstract The aerodynamic efficiency of turbomachinery blades is profoundly affected by the occurrence of laminar-turbulent transition in the boundary layer since skin friction and losses rise for the turbulent state. Depending on the free-stream turbulence level, we can identify different paths toward a turbulent state. The present study uses direct numerical simulation as the primary tool to investigate the flow behavior of the low-pressure turbine blade. In the simulations, the flow past only one blade is computed, with periodic boundary conditions in the cross-flow directions to account for the cascade. Isotropic homogeneous free-stream turbulence is prescribed at the inlet. The free-stream turbulence is prescribed as a superposition of Fourier modes with a random phase shift. Two levels of the free-stream turbulence intensity were simulated (Tu=0.19% and 5.2%), with the integral length scale being 0.167c, at the leading edge. We observed that in the case of low free-stream turbulence on the suction side, the Kelvin–Helmholz instability dominated the transition process and full-span vortices were shed from the separation bubble. Transition on the suction side proceeded more rapidly in the high-turbulence case, where streaks broke down into turbulent spots and caused bypass transition. On the pressure side, we have identified the appearance of longitudinal vortical structures, where increasing the turbulence level gives rise to more longitudinal structures. We note that these vortical structures are not produced by Görtler instability.

Large Eddy Simulation of the By-pass Transition Process under Different Inlet Turbulence Conditions

The transition process of the boundary layer developing over a flat plate with elevated inlet Free Stream Turbulence Intensity (FSTI) has been studied by means of Large Eddy Simulation (LES). To this purpose, four cases with different inflow disturbances have been tested varying the magnitude and the length scale of turbulence. LES has been performed by using the finite-volume ANSYS Fluent code. The computational domain, which was constituted by a rectangular domain with a zero thickness plate, was based on an ERCOFTAC test case in order to provide a validation with a well-known set of data by comparing the boundary layer integral parameters and mean and fluctuating streamwise velocity profiles.The four cases were discussed within the paper by looking at classical statistical properties as well as advanced post-processing tools. It was shown that the decrease in the free stream turbulence level postpones the transition location, whereas the variation of the integral length scale has a very low influence on the distribution of the time-mean flow properties. Proper Orthogonal Decomposition (POD) has been applied to the instantaneous LES flow fields in order to provide a statistical representation of the structures responsible for transition and their response to free-stream turbulence intensity and length scale. The presence of vortical filaments parallel to the wall, typically referred as boundary layer streaks, is clearly identified; their characteristic dimensions and how they change as a function of FSTI properties were analyzed within the paper.

Experimental investigation of the effects of length-to-diameter ratio of hot-wire sensor on the dynamic response to velocity fluctuations

This paper reconsidered the criterion on the length-to-diameter ratio (l/d) of the hot-wire sensor, $$l/d \ge 200$$ , for end-conduction effects on the dynamic response of constant-temperature hot-wire anemometer (CTA) to be neglected. Experiments were first conducted using a low-noise compact CTA with hot-wire sensors of l/d = 80, 120, 150 and 200 in a two-dimensional (2-D) circular cylinder wake at a Reynolds number Re = 77, at which a 2-D von Karman vortex street developed, allowing us to exclude spatial filtering and tangential velocity effects. The damping coefficient in the CTA bridge was tuned to 0.70 for all sensors to reduce possible influences of over- or under-damping on measurements in a frequency range up to 6 kHz. The stubs and prongs of probes consisted of identical materials and dimensions so that the thermal properties were considered to be identical for all probes. Careful comparisons of the amplitudes of the fundamental (1.9 kHz) and higher harmonic components of time-periodic wake vortices showed that the attenuation due to the end-conduction effect in the absence of the spatial filtering and damping coefficient effects was at most 2.5% in terms of the rms value of velocity fluctuations even for l/d = 80. The similar weak attenuation (about 2.6% for l/d = 80) was also observed in the measurements of low-frequency free-stream turbulence. In addition, effects of l/d on the yaw angle response of the sensor were examined in the free stream, revealing that the yaw angle ( $$\psi $$ ) variation of the measured velocity (both the mean and fluctuations) obeyed the cosine law for $$\psi \le 60^\circ $$ . The Champagne constant k was 0.12 for l/d = 200 and 0.2 even for l/d = 80 in the mean velocity measurement using the present probe with long stubs. On the other hand, the intensity of free stream turbulence measured using a $$45^\circ $$ -yawed sensor was significantly affected by the sensor’s l/d; the rms value of velocity fluctuation was attenuated by about 7.5% for l/d = 80.

Inspection of structures interaction in laminar separation bubbles with extended proper orthogonal decomposition applied to multi-plane particle image velocimetry data

This work reports the application of an extended proper orthogonal decomposition (E-POD) procedure to multi-plane particle image velocimetry (PIV) measurements describing the evolution of laminar separation bubbles (LSBs). Measurements were performed over a flat plate installed between adjustable end-walls providing a prescribed adverse pressure gradient for two Reynolds numbers (Re = 70 000, 150 000) and free-stream turbulence intensity levels (Tu = 1.5%, 2.5%). A wall-normal and two wall-parallel measuring planes located at different distance from the wall were considered. POD was applied to the entire PIV planes as well as on their sub-domains, showing the main flow features occurring in the different regions of the LSB. Then, the application of E-POD on different plane partitions revealed the existing correlation between the main dynamics observed in the forward part of the bubble and the breakup events occurring in the reattachment region. The E-POD modes computed in the breakup region resemble streaky structures when PIV snapshots are projected onto the POD eigenvectors of the near wall plane. Otherwise, Kelvin–Helmholtz rolls dominate the E-POD modes obtained by projection of the snapshot matrices on the basis computed in the plane located far from the wall. The main scales of the coherent structures highlighted by the E-POD modes were also characterized by means of the streamwise and spanwise autocorrelation functions of E-POD filtered fields. Data in this work clearly highlight the similarity properties of the main flow features observed in LSBs once scaled with the momentum thickness of the boundary layer at the separation position.

Uniform-momentum zones in a turbulent boundary layer subjected to freestream turbulence

Abstract

Mechanisms underlying flat surface forced convection enhancement by rectangular flexible strips

Employing turbulence generators, such as rectangular flexible strips, is a simple and effective approach to enhancing forced convection. In the current study, 12.7 mm wide rectangular flexible strips (with thickness of 0.1 mm, 0.2 mm and 0.25 mm, height of 25.4 mm, 38.1 mm and 50.8 mm, transversal spacing of 12.7 mm, 25.4 mm and 38.1 mm) were investigated at Reynolds number (Re) of 8500 and 6000, and freestream turbulence intensity (Tu) of 0.4%, 7%, 9%, and 11%. The convective heat transfer augmentation was analyzed in terms of the normalized Nusselt number (Nu/Nu0). The relationships between the averaged Nu/Nu0 and the strip thickness, height, spacing, Re and Tu were determined using multiple linear regression. The correlations between the local Nu/Nu0 and turbulent flow parameters, including turbulent kinetic energy, downwash velocity and near surface streamwise velocity were also illustrated. The freestream turbulence intensity (Tu) had the most significant effect in augmenting the averaged Nu/Nu0, and the local Nu/Nu0 correlated best with the local ke. The maximal averaged Nu/Nu0 over 23 W downstream, within ± 1 and ± 4 strip widths cross-stream was found for Tu = 7% case and Tu = 11% case, respectively.

Numerical Study on the Aerodynamic Characteristics of the NACA 0018 Airfoil at Low Reynolds Number for Darrieus Wind Turbines Using the Transition SST Model

A symmetrical NACA 0018 airfoil is often used in such applications as small-to-medium scale vertical-axis wind turbines and aerial vehicles. A review of the literature indicates a large gap in experimental studies of this airfoil at low and moderate Reynolds numbers in the previous century. This gap has limited the potential development of classical turbulence models, which in this range of Reynolds numbers predict the lift coefficients with insufficiently accurate results in comparison to contemporary experimental studies. Therefore, this paper validates the aerodynamic performance of the NACA 0018 airfoil and the characteristics of the laminar separation bubble formed on its suction side using the standard uncalibrated four-equation Transition SST turbulence model and the unsteady Reynolds-averaged Navier-Stokes (URANS) equations. A numerical study was conducted for the chord Reynolds number of 160,000, angles of attack between 0 and 11 degrees, as well as for the free-stream turbulence intensity of 0.05%. The calculated lift and drag coefficients, aerodynamic derivatives, as well as the location and length of the laminar bubble quite well agree with the results of experimental measurements taken from the literature for validation. A sensitivity study of the numerical model was performed in this paper to examine the effects of the time-step size, geometrical parameters and mesh distribution around the airfoil on the simulation results. The airfoil data sets obtained in this work using the Transition SST and the k-ω SST turbulence models were used in the improved double multiple streamtube (IDMS) to calculate aerodynamic blade loads of a vertical-axis wind turbine. The characteristics of the normal component of the aerodynamic blade load obtained by the Transition SST approach are much better suited to the experimental data compared to the k-ω SST turbulence model.

The effects of free-stream turbulence on the performance of a model wind turbine

Free-stream turbulence characteristics play an important role in the mechanisms of power harvesting for wind turbines. Acquisitions of power and thrust from a model wind turbine of diameter 0.18 m have been carried out in a wind tunnel for a wide range of turbulent base flows, with varying free-stream turbulence intensity in the range between 3% and 16% and integral timescale spanning from 0.1 to 10 times the turbine rotation period. The results demonstrate that power is significantly affected by both the inflow turbulence scales and its intensity, while thrust is scarcely affected by free-stream turbulence. Fluctuations in the generated torques are also measured, with their behavior dominated by the free-stream turbulence scale, and only moderately affected by turbulence intensity. The frequency response of thrust fluctuations has been measured for a selected subset of operating conditions, demonstrating that the turbine thrust is unaffected by high-frequency components in the inflow. Conclusions are drawn on the necessity to match both turbulence intensity and base flow frequency content in wind tunnel studies if realistic results are to be obtained from small-scale studies.

Investigation of Laminar Separation Bubble on Flat Plate with Adverse Pressure Gradient: Time-Averaged Flow Field Analysis

The performance of turbomachinery blade profiles, at low Reynolds numbers, is influenced by laminar separation bubbles (LSBs). Such a bubble is caused by a strong adverse pressure gradient (APG), and it makes the laminar boundary layer to separate from the curved profile surface, before it becomes turbulent. The paper consists on a joint experimental and numerical investigation on a flat plate with adverse pressure gradient. The experiment provides detailed results including distribution of wall pressure coefficient and boundary layer velocity and turbulence profiles for several values of typical influencing parameters on the behavior of the flow phenomena: Reynolds number, free stream turbulence intensity, and end-wall opening angle, which determines the adverse pressure gradient intensity. The numerical work consists on carrying out a systematic analysis, with Reynolds Average Navier-Stokes (RANS) simulations. The results of the numerical simulations are critically investigated and compared with the experimental ones in order to understand the effect of the main physical parameters on the LSB behavior. For RANS simulations, different turbulence and transition models are compared at first to identify the adaptability to the flow phenomena; then, the influence of the three aforementioned parameters on the LSB behavior is investigated under a typical aggressive adverse pressure gradient. Boundary layer integral parameters are discussed for the different cases in order to understand the flow phenomena in terms of flow time-mean properties.

Film Cooling Performance of a Fully Cooled Vane at High Subsonic Conditions

This work focuses on the experimental study of the heat transfer and film cooling characteristics of a fully cooled vane under high subsonic conditions; the transient heat transfer measurements with thermocouples were employed. The inlet Reynolds numbers of the cascade ranged from to and the exit Mach number was 0.8. Two freestream turbulence intensities (1.3% and 14.7%) and three mass flow ratios (5.5%, 8.4%, and 12.5%) were tested. The averaged film cooling effectiveness of the current vane with shaped holes was found to be increased by 15%–43% more than that of the vane only with cylindrical holes, and elevated turbulence intensity reduced the averaged film cooling effectiveness by 3%–26%. The heat transfer augmentation of the current vane was lower than that of the vane only with cylindrical holes. With the mass flow ratio increasing, the heat transfer augmentation gradually increased on the pressure side, whereas it was nearly kept invariable on the suction side. The net heat flux reduction results showed that the vane with shaped holes significantly improved the cooling performance, whereas elevated turbulence intensity significantly deteriorated the cooling performance.

Steady flow around a square cylinder near a plane boundary

Steady flow around a square cylinder placed near a plane wall boundary is investigated experimentally in the present work covering the range of Reynolds number of Re = 7.34 × 104–4.12 × 105, gap ratios of G/D = 0–3 with two different freestream turbulent intensity Iu = 1% and 9%. The influence of Reynolds number, gap ratios and turbulent intensity on the hydrodynamic characteristics have been studied systematically. Two-dimensional Large-eddy simulation (LES) were employed for flow visualizations. It is found that the hydrodynamic forces on a square cylinder are independent of Re even for the very small gap ratio cases. For the influence of the low turbulent intensity (Iu = 1%), the hydrodynamic features and vortex shedding are insensitive to the gap effect as G/D > 1. With the decrease of G/D, the increase of the base pressure Cpb results in the decrease of the drag coefficient CD. The amplitude of the periodic fluctuations of the vortex shedding process decreases with the vortex shedding process continuing. As G/D ≤ 0.3, the vortex shedding is observed to be completely suppressed. The blockage effect leads to an entrainment of the flow passing through the gap and a low-speed recirculation is formed at the plane boundary in the region of x/D < 7.75. High turbulence tends to increase the base pressure and reduce the drag coefficient of the square cylinder for large gap ratio. For G/D ≤ 0.8, the high turbulence triggers the growth of separated shear layer and inherent instabilities for the near-wall cylinder, which results in the higher fluctuating drag and lift coefficient. The critical gap ratio for the vortex shedding suppression is reduced to G/D = 0.2.

Three-Dimensional Development of Coherent Structures in a Two-Dimensional Laminar Separation Bubble

Three-dimensional flow development is experimentally assessed in a convectively unstable laminar separation bubble formed over a NACA 0018 airfoil at a chord Reynolds number of 125,000, an angle of attack of 4 deg, and a freestream turbulence intensity of 0.07%. The flow is weakly excited in a spanwise uniform manner at a frequency matching that of the most unstable disturbances in the natural separation bubble, leaving the base flow unmodified while enabling three-dimensional reconstructions of the dominant coherent structures using phase-locked planar particle image velocimetry measurements. Time-averaged flowfield reconstructions show a strongly two-dimensional topology of the separation bubble for both the natural and weakly excited cases. Analysis of the flow development demonstrates that, for both the natural and excited flows, spanwise-oriented and strongly two-dimensional shear-layer vortices form in the separation bubble upstream of the mean maximum height location. Spanwise undulations develop in the vortex filaments that continually intensify with downstream convection as a result of the streamwise forward sections of the filaments lifting away from the surface. This motion reorients the vorticity of the primary structures from the spanwise direction into the streamwise and wall-normal directions, forming hairpinlike structures above the vortex core region. These findings offer new quantitative insight into the vortex dynamics and breakdown process of the shear-layer vortices in two-dimensional, convectively unstable laminar separation bubbles subject to low freestream turbulence levels.

The Effect of Step Misalignment on Purge Flow Cooling of Nozzle Guide Vane at Transonic Conditions

Abstract This study describes a detailed investigation on the effects that upstream step misalignment and upstream purge film cooling have on the endwall heat transfer for first stage nozzle guide vanes (NGVs) in a gas turbine at transonic conditions. Endwall Nusselt number and adiabatic film-cooling effectiveness distributions were experimentally measured and compared with flow visualization. Tests were conducted in a transonic linear cascade blowdown facility at an inlet freestream turbulence intensity of 16%, an exit Mach number of 0.85, and an exit Re = 1.5 × 106 based on axial chord. Varied upstream purge blowing ratios (BRs) and a no-blowing case were tested for three different upstream step geometries, the baseline (no misalignment), a span-wise upstream step of +4.86% span, and a step of −4.86% span. Experimentation shows that compared with no-blowing case, the addition of upstream purge film cooling increases the Nusselt number at injection upward of 50% but lowers it in the passage throat by approximately 20%. The backward facing step induces more turbulent mixing between the coolant and mainstream flows, thus reducing film effectiveness coverage and increasing Nusselt number by nearly 40% in the passage throat. In contrast, the presence of a forward step creates a more stable boundary layer for the coolant flow aiding to help keep the film attached to the endwall. Increasing the blowing ratio increases film-cooling effectiveness and endwall coverage up to a certain point, beyond which, the high momentum of the coolant results in poor cooling performance due to jet liftoff.

Free-stream turbulence interaction with a wing-tip vortex

The interaction of a wing-tip vortex of a rectangular, square-tipped wing having a NACA 0012 airfoil section with a grid-generated turbulent flow was investigated in this paper. The experiments were conducted in the near and mid-wake regions at three free stream turbulence (FST) intensities of 0.5%, 3% and 6%, and at two Reynolds numbers, Re, based on the wing chord length, cw, of 2×105 and 3×105. Stereoscopic Particle Image Velocimetry (SPIV) and hot-wire measurements were carried out at four downstream positions, namely x/cw=1.25,3.25,6.25 and 7.75. Streamwise velocity contours showed that the wing-tip vortex decayed with increased FST and downstream distance. In the vortex core region, the streamwise velocity decelerated while the vortex adopted a wake-like profile. FST was found to decrease the vortex circulation and to increase the vortex radius and vortex meandering amplitude. When increasing Reynolds number, the grid cases showed little variation of the vortex radius and peak vorticity levels, particularly at larger downstream positions, suggesting that the effects of FST and Re number on the vortex development are nearly independent. The measured total turbulent kinetic energy (TKE) was found to be mostly due to vortex meandering. In that, total TKE levels devoid of meandering showed a virtually turbulence-free vortex core.

A two-equation local-correlation-based laminar-turbulent transition modeling scheme for external aerodynamics

A local-correlation-based laminar-turbulent transition model has been developed for external aerodynamic problems and coupled with the Spalart-Allmaras (S−A) turbulence model. This laminar-turbulent transition modeling scheme is then termed as the γ−SA model. It has been validated against a series of two- and three-dimensional test cases with a relatively wide range of Reynolds numbers and free-stream turbulence intensity levels. Results show that the γ−SA model can give accurate transition-onset-location predictions, as well as more accurate predictions on the force coefficients than the underlying S−A turbulence model at various angles of attack. Comparison with the classic SST−γ−Reθ transition model shows that the γ−SA model can achieve comparable results, with the expect of less time or memory consumption, as the γ−SA model has two equations fewer than the former one. With a newly proposed local-farfield mixed turbulence intensity strategy, the turbulence intensity on the inlet or farfield boundary can be conveniently set directly with the value tested for the wind tunnel or the value as expected, while achieving local turbulence intensity variation. Moreover, the γ−SA model employs only local variables, which makes it totally modern-CFD-compatible and very appropriate for large-scale parallel running using unstructured grids. In the future, it can be modified into a robust and efficient DES-type model with the consideration of transition process.

Controlled Synthetic Freestream Turbulence Intensity Introduced by a Local Volume Force

Generating freestream turbulence within the computational domain instead of applying it as a boundary condition requires a method to introduce the turbulent fluctuations at a specific location. A method based on applying local volume forces has been adapted and supplemented with a control loop in order to compensate for alterations of the turbulence structure resulting from the numerical treatment and physical reasons. The criteria for the tuning of the controller have been developed and the performance of the approach has been assessed. The capabilities of the method are demonstrated for the flow around an airfoil at high angle of attack and with massive flow separation.