Freestream Turbulence Conditions Research Articles

Separation Control on a Very High Lift Low Pressure Turbine Airfoil Using Pulsed Vortex Generator Jets

Boundary layer separation control has been studied using vortex generator jets (VGJs) on a very high lift, low-pressure turbine airfoil. Experiments were done under high (4%) freestream turbulence conditions on a linear cascade in a low speed wind tunnel. Pressure surveys on the airfoil surface and downstream total pressure loss surveys were documented. Instantaneous velocity profile measurements were acquired in the suction surface boundary layer. Cases were considered at Reynolds numbers (based on the suction surface length and the nominal exit velocity from the cascade) of 25,000 and 50,000. Jet pulsing frequency, duty cycle, and blowing ratio were all varied. Computational results from a large eddy simulation of one case showed reattachment in agreement with the experiment. In cases without flow control, the boundary layer separated and did not reattach. With the VGJs, separation control was possible even at the lowest Reynolds number. Pulsed VGJs were more effective than steady jets. At sufficiently high pulsing frequencies, separation control was possible even with low jet velocities and low duty cycles. At lower frequencies, higher jet velocity was required, particularly at low Reynolds numbers. Effective separation control resulted in an increase in lift and a reduction in total pressure losses. Phase averaged velocity profiles and wavelet spectra of the velocity show the VGJ disturbance causes the boundary layer to reattach, but that it can reseparate between disturbances. When the disturbances occur at high enough frequency, the time available for separation is reduced, and the separation bubble remains closed at all times.

Separated Flow Measurements on a Highly Loaded Low-Pressure Turbine Airfoil

Boundary layer separation, transition, and reattachment have been studied on a new, very high lift, low-pressure turbine airfoil. Experiments were done under low freestream turbulence conditions on a linear cascade in a low speed wind tunnel. Pressure surveys on the airfoil surface and downstream total pressure loss surveys were documented. Velocity profiles were acquired in the suction side boundary layer at several streamwise locations using hot-wire anemometry. Cases were considered at Reynolds numbers (based on the suction surface length and the nominal exit velocity from the cascade) ranging from 25,000 to 330,000. In all cases, the boundary layer separated, but at high Reynolds number the separation bubble remained very thin and quickly reattached after transition to turbulence. In the low Reynolds number cases, the boundary layer separated and did not reattach, even when transition occurred. This behavior contrasts with previous research on other airfoils, in which transition, if it occurred, always induced reattachment, regardless of Reynolds number.

A Three-Equation Eddy-Viscosity Model for Reynolds-Averaged Navier–Stokes Simulations of Transitional Flow

An eddy-viscosity turbulence model employing three additional transport equations is presented and applied to a number of transitional flow test cases. The model is based on the k-ω framework and represents a substantial refinement to a transition-sensitive model that has been previously documented in the open literature. The third transport equation is included to predict the magnitude of low-frequency velocity fluctuations in the pretransitional boundary layer that have been identified as the precursors to transition. The closure of model terms is based on a phenomenological (i.e., physics-based) rather than a purely empirical approach and the rationale for the forms of these terms is discussed. The model has been implemented into a commercial computational fluid dynamics code and applied to a number of relevant test cases, including flat plate boundary layers with and without applied pressure gradients, as well as a variety of airfoil test cases with different geometries, Reynolds numbers, freestream turbulence conditions, and angles of attack. The test cases demonstrate the ability of the model to successfully reproduce transitional flow behavior with a reasonable degree of accuracy, particularly in comparison with commonly used models that exhibit no capability of predicting laminar-to-turbulent boundary layer development. While it is impossible to resolve all of the complex features of transitional and turbulent flows with a relatively simple Reynolds-averaged modeling approach, the results shown here demonstrate that the new model can provide a useful and practical tool for engineers addressing the simulation and prediction of transitional flow behavior in fluid systems.

A Comparison of Cylindrical and Fan-Shaped Film-Cooling Holes on a Vane Endwall at Low and High Freestream Turbulence Levels

Fan-shaped film-cooling holes have been shown to provide superior cooling performance to cylindrical holes along flat plates and turbine airfoils over a large range of different conditions. Benefits of fan-shaped holes include less required cooling air for the same performance, increased part lifetime, and fewer required holes. The major drawback, however, is increased manufacturing cost and manufacturing difficulty, particularly for the vane platform region. To this point, there have only been extremely limited comparisons between cylindrical and shaped holes on a turbine endwall at either low or high freestream turbulence conditions. This study presents film-cooling effectiveness measurements on an endwall surface in a large-scale, low-speed, two-passage, linear vane cascade. Results showed that film-cooling effectiveness decreased with increasing blowing rate for the cylindrical holes, indicating jet liftoff. However, the fan-shaped passage showed increased film-cooling effectiveness with increasing blowing ratio. Overall, fan-shaped holes increased film-cooling effectiveness by an average of 75% over cylindrical holes for constant cooling flow.

Simulation of the effect of high-intensity turbulence flow parameters on unsteady boundary layers with streamwise pressure gradients

The dynamic and thermal characteristics of steady and unsteady wall boundary layers are investigated on the basis of the two-parameter turbulence model under conditions of high-intensity free-stream turbulence and exposure to a negative streamwise pressure gradient. Flows from the laminar to the completely developed turbulent regimes, including the transition regime, are calculated. The possibility of the theoretical description of the transition and the developed turbulent regime by means of a quasi-steady-state turbulence model when the free-stream velocity distributions are time-periodic is demonstrated. The properties of the calculated characteristics of the dynamic and thermal boundary layers are analyzed for various oscillation parameters and a moderate steamwise pressure gradient. The numerical results are compared with experimental and theoretical data.

Predicting Worker Exposure—The Effect of Ventilation Velocity, Free-Stream Turbulence and Thermal Condition

Three-dimensional computational fluid dynamics (CFD) simulations were used to predict the flow field and resulting worker exposures when toxic airborne contaminants were released into the wake region of a mannequin that had its back to the airflow while holding the source of airborne contaminants. The effects of ventilation velocity, free-stream turbulence, and various thermal conditions on fluid flow and exposure levels were evaluated. The results showed good agreement between predicted and experimental concentrations at the mouth at a broad range of airflow velocities when the mannequin was both heated and unheated. When the mannequin was unheated, the exposure level decreased as the ventilation velocity increased. The expectation that buoyancy provided by the heat from the mannequin would be most important at very low velocities and decreasingly important at high velocities was proved true for both the predicted and observed exposures. The result was that when the mannequin was heated to normal human body temperatures, exposure levels had an inverted V relationship with velocity. These findings are important, since they call into question the common practice of modeling human exposures with mannequins at ambient temperatures. In addition, free-stream turbulence could be used to reduce worker exposure to airborne pollutants as suggested by the simulations. CFD enabled a detailed investigation of the effect of particular factors for exposure predictions in a cost-effective way.

Flow over a system of several ribs under conditions of high free-stream turbulence

Results of an experimental hydrodynamic and heat-transfer study of the turbulent separated flow developing over a system of several cross-flow ribs are reported. Conditions with low and high free-stream turbulence are considered. Visualization and heat-transfer data are presented, and a comparison for two turbulence levels is given. In the system of three or more ribs, a very unstable flow in the second inter-rib cell was observed under low-turbulence conditions. Under a high level of free-stream turbulence, the flow in the first inter-rib cell is unstable; this observation is supported by measured pressure distributions. Addition of each next rib makes the separation flow region behind the last rib and the pressure recovery region less extended, and decreases the coordinate at which the rate of heat transfer attains its maximum. In the high-turbulent flow, the heat-transfer intensification in the second inter-rib cell amounts to 30 %.

Film-Cooling Efficiency in a Laval Nozzle Under Conditions of High Freestream Turbulence

In the present work we experimentally examined the effect of enhanced freestream turbulence on the film-cooling efficiency in an axisymmetric supersonic nozzle. A considerable reduction in the film-cooling efficiency was observed with increasing level of flow turbulence, both in the subsonic and supersonic parts of the Laval nozzle. For instance, an increase in the freestream turbulence number from 0.2% to 15% resulted in more than twofold deterioration of film-cooling efficiency. A similar decrease of film-cooling efficiency was also observed under off-design flow conditions. At the same time, the increase in the freestream turbulence number had almost no effect on the recovery factor and on the distribution of static pressure over the length of the nozzle. The Kutateladze-Leont'ev asymptotic theory of gas cooling films was used to generalize the experimental data for nozzle flows with allowance for flow nonisothermality, compressibility, longitudinal pressure gradient, and high freestream turbulence number.

Effect of Free-Stream Turbulence on the Reduction of Surface Friction in a Turbulent Boundary Layer Behind LEBU-Devices

The effect of increased free-stream turbulence on the reduction of the surface friction coefficient cf in a turbulent boundary layer behind large-eddy break-up (LEBU) devices is investigated using a gravimetric method. The turbulence level was e ≈ 1.9–4.9 % and the turbulence scale Le ≈ 40–110 mm. The boundary layer Reynolds number Re** was varied from 2300 to 7500, with the boundary layer thickness being varied on the range δ = 33–44 mm. It is shown that an increase in the turbulence level e has almost no impact on the relative reduction of friction behind LEBU-devices, whereas, under similar conditions of elevated free-stream turbulence, for another method, namely, the use of surface riblets, the friction reduction may be more strongly expressed.

Large-Eddy Simulation of Transition in a Separation Bubble

In this paper, large-eddy simulation of the transition process in a separation bubble is compared to experimental results. The measurements and simulations are conducted under low freestream turbulence conditions over a flat plate with a streamwise pressure distribution typical of those encountered on the suction side of turbine airfoils. The computational grid is refined to the extent that the simulation qualifies as a “coarse” direct numerical simulation. The simulations are shown to accurately capture the transition process in the separated shear layer. The results of these simulations are used to gain further insight into the breakdown mechanisms in transitioning separation bubbles.

Effect of Elevated Free-Stream Turbulence on Transitional Flow Heat Transfer Over Dual-Scaled Rough Surfaces

The surface roughness over a serviced turbine airfoil is usually multiscaled with varying features that are difficult to be universally characterized. However, it was previously discovered in low free-stream turbulence conditions that the height of larger roughness produces separation and vortex shedding, which trigger early transition and exert a dominant effect on flow pattern and heat transfer. The geometry of the roughness and smaller roughness scales played secondary roles. This paper extends the previous study to elevated turbulence conditions with free-stream turbulence intensity ranging from 0.2% to 6.0%. A simplified test condition on a flat plate is conducted with two discrete regions having different surface roughness. The leading-edge roughness is comprised of a sandpaper strip or a single cylinder. The downstream surface is either smooth or covered with sandpaper of grit sizes ranging from 100 to 40 Ra=37-119 μm. Hot wire measurements are conducted in the boundary layer to study the flow structure. The results of this study verify that the height of the largest-scale roughness triggers an earlier transition even under elevated turbulence conditions and exerts a more dominant effect on flow and heat transfer than does the geometry of the roughness. Heat transfer enhancements of about 30–40%-over the entire test surface are observed. The vortical motion, generated by the backward facing step at the joint of two roughness regions, is believed to significantly increase momentum transport across the boundary layer and bring the elevated turbulence from the freestream towards the wall. No such long-lasting heat transfer phenomenon is observed in low free-stream turbulence cases even though vortex shedding also exists in the low turbulence cases. The heat transfer enhancement decreases, instead of increases, as the downstream roughness height increases.

An investigation of the scales in transitional boundary layers under high free-stream turbulence conditions

The scales in a transitional boundary layer subject to high (initially 8%) free-stream turbulence and strong acceleration (K as high as 9×10−6) were investigated using wavelet spectral analysis and conditional sampling of experimental data. The boundary layer shows considerable evolution through transition, with a general shift from the lower frequencies induced by the free-stream unsteadiness to higher frequencies associated with near-wall-generated turbulence. Within the non-turbulent zone of the intermittent flow, there is considerable self-similarity in the spectra from the beginning of transition to the end, with the dominant frequencies in the boundary layer remaining constant at about the dominant frequency of the free-stream. The frequencies of the energy-containing scales in the turbulent zone change with streamwise location and are significantly higher than in the non-turbulent zone. When normalized on the local viscous length scale and velocity, however, the turbulent zone spectra also show good self-similarity throughout transition. Turbulence dissipation occurs almost exclusively in the turbulent zone. The velocity fluctuations associated with dissipation are isotropic, and their normalized spectra at upstream and downstream stations are nearly identical. The distinct differences between the turbulent and non-turbulent zones suggest the potential utility of intermittency based transition models in which these zones are treated separately. The self-similarity noted in both energy containing and dissipation scales in both zones suggests possibilities for simplifying the modeling for each zone.

Boundary-Layer Transition Affected by Surface Roughness and Free-Stream Turbulence

This paper presents experimental results documenting the effects of surface roughness and free-stream turbulence on boundary-layer transition. The experiments were conducted on a flat surface, upon which a pressure distribution similar to those prevailing on the suction side of low-pressure turbine blades was imposed. The test matrix consists of five variations in the roughness conditions, at each of three free-stream turbulence intensities (approximately 0.5%, 2.5%, and 4.5%), and two flow Reynolds numbers of 350,000 and 470,000. The ranges of these parameters considered in the study, which are typical of low-pressure turbines, resulted in both attached-flow and separation-bubble transition. The focus of the paper is on separation-bubble transition, but the few attached-flow test cases that occurred under high roughness and free-stream turbulence conditions are also presented for completeness of the test matrix. Based on the experimental results, the effects of surface roughness on the location of transition onset and the rate of transition are quantified, and the sensitivity of these effects to free-stream turbulence is established. The Tollmien–Schlichting instability mechanism is shown to be responsible for transition in each of the test cases presented. The root-mean-square height of the surface roughness elements, their planform size and spacing, and the skewness (bias towards depression or protrusion roughness) of the roughness distribution are shown to be relevant to quantifying the effects of roughness on the transition process.

Experiments With Three-Dimensional Passive Flow Control Devices on Low-Pressure Turbine Airfoils

The effectiveness of three-dimensional passive devices for flow control on low pressure turbine airfoils was investigated experimentally. A row of small cylinders was placed at the pressure minimum on the suction side of a typical airfoil. Cases with Reynolds numbers ranging from 25,000 to 300,000 (based on suction surface length and exit velocity) were considered under low freestream turbulence conditions. Streamwise pressure profiles and velocity profiles near the trailing edge were documented. Without flow control a separation bubble was present, and at the lower Reynolds numbers the bubble did not close. Cylinders with two different heights and a wide range of spanwise spacings were considered. Reattachment moved upstream as the cylinder height was increased or the spacing was decreased. If the spanwise spacing was sufficiently small, the flow at the trailing edge was essentially uniform across the span. The cylinder size and spacing could be optimized to minimize losses at a given Reynolds number, but cylinders optimized for low Reynolds number conditions caused increased losses at high Reynolds numbers. The effectiveness of two-dimensional bars had been studied previously under the same flow conditions. The cylinders were not as effective for maintaining low losses over a range of Reynolds numbers as the bars.

Convective Heat Transfer Near One-Dimensional and Two-Dimensional Wall Temperature Steps

Steady-state experiments with one-dimensional and two-dimensional calorimeters were used to study the convective heat transfer near sharp steps in wall temperature in a turbulent boundary layer. Data acquired under low and high freestream turbulence conditions indicated that spanwise turbulent diffusion is not a significant heat transport mechanism for a two-dimensional temperature step. The one-dimensional calorimeter heat transfer data were predicted within ±5 percent using the STAN7 boundary layer code for situations with an abrupt wall temperature step. The conventional correlation with an unheated starting length correction, in contrast, greatly under-predicts the heat transfer for the same experimental cases. A new correlation was developed that is in good agreement with near and far-field semi-analytical solutions and predicts the calorimeter heat transfer data to within ±2 percent for temperature step boundary condition cases.

Passive Flow Control on Low-Pressure Turbine Airfoils

Two-dimensional rectangular bars have been used in an experimental study to control boundary layer transition and reattachment under low-pressure turbine conditions. Cases with Reynolds numbers (Re) ranging from 25,000 to 300,000 (based on suction surface length and exit velocity) have been considered at low (0.5%) and high (8.5% inlet) free-stream turbulence levels. Three different bars were considered, with heights ranging from 0.2% to 0.7% of suction surface length. Mean and fluctuating velocity and intermittency profiles are presented and compared to results of baseline cases from a previous study. Bar performance depends on the bar height and the location of the bar trailing edge. Bars located near the suction surface velocity maximum are most effective. Large bars trip the boundary layer to turbulent and prevent separation, but create unnecessarily high losses. Somewhat smaller bars had no immediate detectable effect on the boundary layer, but introduced small disturbances that caused transition and reattachment to move upstream from their locations in the corresponding baseline case. The smaller bars were effective under both high and low free-stream turbulence conditions, indicating that the high free-stream turbulence transition is not simply a bypass transition induced by the free stream. Losses appear to be minimized when a small separation bubble is present, so long as reattachment begins far enough upstream for the boundary layer to recover from the separation. Correlations for determining optimal bar height are presented. The bars appear to provide a simple and effective means of passive flow control. Bars that are large enough to induce reattachment at low Re, however, cause higher losses at the highest Re. Some compromise would, therefore, be needed when choosing a bar height for best overall performance.

Conditional Sampling in a Transitional Boundary Layer Under High Freestream Turbulence Conditions

Conditional sampling has been performed on data from a transitional boundary layer subject to high (initially 9%) freestream turbulence and strong (K=ν/U∞2dU∞/dx as high as 9×10−6) acceleration. Methods for separating the turbulent and nonturbulent zone data based on the instantaneous streamwise velocity and the turbulent shear stress were tested and found to agree. Mean velocity profiles were clearly different in the turbulent and nonturbulent zones, and skin friction coefficients were as much as 70% higher in the turbulent zone. The streamwise fluctuating velocity, in contrast, was only about 10% higher in the turbulent zone. Turbulent shear stress differed by an order of magnitude, and eddy viscosity was three to four times higher in the turbulent zone. Eddy transport in the nonturbulent zone was still significant, however, and the nonturbulent zone did not behave like a laminar boundary layer. Within each of the two zones there was considerable self-similarity from the beginning to the end of transition. This may prove useful for future modeling efforts.

Effects of Concave Curvature on Boundary Layer Transition Under High Freestream Turbulence Conditions

An experimental investigation has been carried out on a transitional boundary layer subject to high (initially 9%) freestream turbulence, strong acceleration (K=ν/Uw2dUw/dx as high as 9×10−6), and strong concave curvature (boundary layer thickness between 2% and 5% of the wall radius of curvature). Mean and fluctuating velocity as well as turbulent shear stress are documented and compared to results from equivalent cases on a flat wall and a wall with milder concave curvature. The data show that curvature does have a significant effect, moving the transition location upstream, increasing turbulent transport, and causing skin friction to rise by as much as 40%. Conditional sampling results are presented which show that the curvature effect is present in both the turbulent and nonturbulent zones of the transitional flow.

Predictions of Transitional Flows in Low-Pressure Turbines Using Intermittency Transport Equation

A transport equation for the intermittency factor is employed to predict the transitional e ows in low-pressure turbine applications. The intermittent behavior of the transitional e ows is taken into account and incorporated into computations by modifying the eddy viscosity πt with the intermittency factor °. Turbulence quantities are predicted by using Menter’ s shear stress transport two-equation turbulence model, and the intermittency factor is obtained from the solution of a recently developed transport equation model. The transport equation model not only can reproduce the experimentally observed streamwise variation of the intermittency in the transition zone, but it also provides a realistic cross-stream variation of the intermittency proe le. The current model is applied to predictions of a modern low-pressure turbine experiment, and detailed comparisons of the computational results with the experimental data are presented. The model has been shown to be capable of predicting the low-pressure turbine e ow transition under a variety of Reynolds number and freestream turbulence conditions.

Detailed Boundary Layer Measurements on a Turbine Stator Vane at Elevated Freestream Turbulence Levels

High freestream turbulence levels have been shown to greatly augment the heat transfer on a gas turbine airfoil. To better understand these effects, this study has examined the effects elevated freestream turbulence levels have on the boundary layer development along a stator vane airfoil. Low freestream turbulence measurements (0.6 percent) were performed as a baseline for comparison to measurements at combustor simulated turbulence levels (19.5 percent). A two-component LDV system was used for detailed boundary layer measurements of both the mean and fluctuating velocities on the pressure and suction surfaces. Although the mean velocity profiles appeared to be more consistent with laminar profiles, large velocity fluctuations were measured in the boundary layer along the pressure side at the high freestream turbulence conditions. Along the suction side, transition occurred further upstream due to freestream turbulence.