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NASA NTRS2018

Investigation of Turbulence Generated by Upwind Blockages at the Inlet of a 1/50th-Scale Model of the 80- by 120-Foot Wind Tunnel

A 1/50th scale model of the 80- by 120-Foot Wind Tunnel (hereafter referred to as the 80x120) was utilized to determine the magnitude of turbulence caused by buildings located upstream of the wind tunnel inlet. The 1/50th scale models of existing and proposed buildings were constructed to act as blockage for the test. Various inlet locations were sampled for turbulence intensity levels under a variety of blockage conditions including simple three-dimensional rectangular bodies creating quasi two-dimensional physics along the tunnel centerline, existing building structures in the vicinity of the full-scale wind tunnel inlet flow field, and proposed building structures that may someday be constructed at NASA Ames Research Center upwind of the inlet. Therefore, the testing performed and reported in this report can be considered representative of quiescent atmospheric conditions that exist when operating the full-scale 80x120 at night. At quiescent atmospheric conditions there is a measurable increase in turbulence intensity produced by upstream blockages. The blockages examined produced an average turbulence intensity level between 2% and 5% when measured at the inlet. Previous research has shown that the flow control of the 80x120 is capable of reducing this turbulence to less than 0.5% when measured in the test section. Additional research will need to be conducted to determine the influence of atmospheric wind on relative turbulence intensity at the inlet. These results show that for future buildings lying more than 1,000 ft upstream of the full-scale 80x120 inlet, these new buildings will have a negligible effect on the flow quality of the air entering the 80x120 test section under strictly quiescent atmospheric conditions. The Googleplex buildings modeled and tested in this experiment are located approximately 2,450 ft upstream and, as seen in this test campaign, have a negligible influence on the turbulence levels measured at the inlet under quiescent atmospheric conditions.

80- by 120-Foot Wind Tunnel↗

NASA NTRS1993

Application of laser Doppler velocimeter to chemical vapor laser system

A laser Doppler velocimeter (LDV) system was used to measure iodide vapor flow fields inside two different-sized tubes. Typical velocity profiles across the laser tubes were obtained with an estimated +/-1 percent bias and +/-0.3 to 0.5 percent random uncertainty in the mean values and +/-2.5 percent random uncertainty in the turbulence-intensity values. Centerline velocities and turbulence intensities for various longitudinal locations ranged from 13 to 17.5 m/sec and 6 to 20 percent, respectively. In view of these findings, the effects of turbulence should be considered for flow field modeling. The LDV system provided calibration data for pressure and mass flow systems used routinely to monitor the research laser gas flow velocity.

Gartrell, Luther R.↗

NASA NTRS1997

Unsteady Velocity Measurements Taken Behind a Model Helicopter Rotor Hub in Forward Flight

Drag caused by separated flow behind the hub of a helicopter has an adverse effect on aerodynamic performance of the aircraft. To determine the effect of separated flow on a configuration used extensively for helicopter aerodynamic investigations, an experiment was conducted using a laser velocimeter to measure velocities in the wake of a model helicopter hub operating at Mach-scaled conditions in forward flight. Velocity measurements were taken using a laser velocimeter with components in the vertical and downstream directions. Measurements were taken at 13 stations downstream from the rotor hub. At each station, measurements were taken in both a horizontal and vertical row of locations. These measurements were analyzed for harmonic content based on the rotor period of revolution. After accounting for these periodic velocities, the remaining unsteady velocities were treated as turbulence. Turbulence intensity distributions are presented. Average turbulent intensities ranged from approximately 2 percent of free stream to over 15 percent of free stream at specific locations and azimuths. The maximum average value of turbulence was located near the rear-facing region of the fuselage.

Berry, John D.↗

NASA NTRS1985

On helicopter rotor low frequency broadband noise

The effect of shear-layer-type inflow turbulence on the low-frequency broadband noise of a model helicopter rotor is experimentally studied. The measurements and the one-dimensional energy spectral density indicate that the upstream airfoil wake turbulence is nonisotropic, but approaches isotropy at high wavenumbers. Turbulence measurements also indicate that the wake turbulence is weak. The effect of the inflow turbulence intensity on the peak sound pressure level follows an intensity-velocity squared scaling law. A number of length scales and turbulence intensities exist which can be measured in the airfoil wake depending on the position at which the measurements are taken. Comparison of experimental and theoretical sound pressure power spectral densities indicates that the initial anisotropy of the inflow turbulence does not invalidate the isotropic turbulence assumption made in noise prediction models as long as measured turbulence intensities and length scales are used.

Williams, Morgan↗

NASA NTRS1976

Skin friction reduction by slot injection at Mach 0.8

Surface skin friction, boundary layer profiles and turbulent intensity due to axially symmetric tangential slot injection into a transonic boundary layer were measured. Effects of slot height, multiple slot injection, and injection mass flow rate on the surface skin friction downstream of the the slot have been investigated. Surface skin friction was a function of the injection mass flow rate for x/s 40. Large normal pressure gradient and relatively large turbulent intensity were found near the slot with small injection mass flow rate; the region the high turbulent intensity moved downstream with increasing injection mass flow rate. The results with two slot injections indicated that the distance between slots should be less than 30 slot heights in order to achieve some benefits from the first slot. Of significant importance in the present investigation is that the skin friction reduction obtained at transonic speed is of the same order as obtained in the hypersonic regime. Additional work is required in order to formulate a correlation between the turbulent intensity and the injection mass flow rate that may be used in future analysis.

Zakkay, V.↗

NASA NTRS1985

The influence of jet-grid turbulence on heat transfer from the stagnation region of a cylinder in crossflow

The effect of high-intensity turbulence on heat transfer from the stagnation region of a circular cylinder in crossflow was studied. The work was motivated by the desire to be able to more fully understand and predict the heat transfer to the leading edge of a turbine airfoil. In order to achieve high levels of turbulence with a reasonable degree of isotropy and homogeneity, a jet-injection turbulence grid was used. The jet grid provided turbulence intensities of 10 to 12 percent, measured at the test cylinder location, for downstream blowing with the blowing rate adjusted to an optimal value for flow uniformity. Heat transfer augmentation above the zeroturbulence case ranged from 37 to 53 percent for the test cylinder behind the jet grid for a cylinder Reynolds number range of 48,000 to 180,000, respectively. The level of heat transfer augmentation was found to be fairly uniform with respect to circumferential distance from the stagnation line. Stagnation point heat transfer results (expressed in terms of the Frossling number) were found to be somewhat low with respect to previous studies, when compared on the basis of equal values of the parameter Tu Re(1/2), indicating an additional Reynolds number effect as observed by previous investigators. Consequently, for a specified value of Tu Re(1/2), data obtained with a relatively high turbulence intensity will have a lower value of the Frossling number.

Obrien, J. E.↗

DOE OSTI2025

Freestream turbulence effects on unsteady wind turbine loads and wakes: An IDDES study

We investigate numerically the effects of freestream turbulence on the unsteady aerodynamics and wakes of the National Renewable Energy Laboratory Phase VI wind turbine rotor for increasing wind speed. Turbulence is modeled using the Improved Delayed Detached-Eddy Simulation (IDDES) method. As a first step, a detailed mesh resolution study is conducted with the decaying freestream turbulence model at turbulence intensity of 0.5%. Our blade-resolved IDDES simulations show that grid-independent average torque and thrust results can be achieved with relatively coarse meshes, whereas dramatically higher mesh resolution is required for grid-independent results for power spectral densities of thrust force, especially in the deep-stall regime. Comparing the loads with the Shear-Stress Transport model demonstrates the superiority of IDDES in predicting massively separated flows. The aerodynamic performance and wake predictions with the decaying freestream turbulence model are compared with the synthetic freestream turbulence model. Both models predict nearly the same loads, spectral energy content, and wake characteristics. The properties of both the near- and far-wake regions are then examined. Furthermore, we show that separated boundary layers accelerate turbulent mixing and entrainment of the external flow, which results in faster wake recovery. The effect of increasing turbulence intensity to 6% is investigated using the synthetic freestream turbulence model. In contrast with the fully attached boundary layer, higher freestream turbulence in deep stall does not significantly affect the loads and vortex-shedding characteristics. However, the turbulent mixing in the wake is enhanced, which further hastens the recovery of the self-similar velocity profile. In general, increasing the wind speed at high turbulence intensity shifts the recovery farther upstream and increases the wake width.

17 WIND ENERGY↗

DOE OSTI2024

Density fluctuation statistics and turbulence spreading at the edge of L–mode plasmas

Long-wavelength density fluctuations ($k{\rho _i}$ <1) are studied using beam emission spectroscopy (BES) at the edge of DIII-D L-mode plasmas (ρ = 0.88–1.1) in scenarios with electron cyclotron heating (ECH) power ramp (P ECH up to 1.5 MW), neutral beam injection (NBI) power ramp (P NBI up to 2.5 MW), and injected torque scan (-1 < T inj <0.6 Nm). We find that broadband turbulent density fluctuations (ƒ ~ 20–120 kHz) have a non-Gaussian distribution. The skewness of $\delta n/n$ changes sign from negative at ρ < 0.95–0.97 to positive at ρ > 0.97, indicating the prevalence of density 'voids' at inner radii and density 'blobs' at outer radii and outside of the separatrix. The turbulence intensity flux $\left\langle {{{\tilde v}_{\text{r}}}{{\tilde n}^2}} \right\rangle$ is calculated to characterize turbulence spreading at the plasma edge. During ECH/NBI power ramps and at counter-I p injected torque, $\left\langle {{{\tilde v}_{\text{r}}}{{\tilde n}^2}} \right\rangle$ is directed inward inside the separatrix, which is evidence of inward spreading of turbulence intensity from the edge gradient region caused by the inner propagation of density 'voids'. Significantly weaker $\left\langle {{{\tilde v}_{\text{r}}}{{\tilde n}^2}} \right\rangle$ is observed with co-I p torque. A correlation between co-I p torque, turbulence intensity $\delta n/n$ at ρ = 0.97, and increased srape-off layer (SOL) heat flux decay length ${\lambda _q}$ is found in the torque scan scenario, showing that edge turbulence plays a material role in determining the SOL conditions and heat flux width.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

NASA NTRS1973

Measurements of the distribution of sound source intensities in turbulent jets.

The spatial distribution of sound source intensities in jets is determined from the sound radiated into the acoustic far field by means of a concave mirror-microphone system. The mirror forms an image of the sound sources in a region far enough from the jet so that near field pressure fluctuations can be neglected. The sound intensity in the image is thus closely related to the strength of the actual sound sources in the jet. Results of measurements with jets of Mach numbers 0.7 to 1.9 emanating from circular nozzles and from slot nozzle-flap combinations demonstrate the influence of Mach number and of other parameters upon the sound source distribution.

Grosche, F.-R.↗

DOE OSTI2020

Design and analysis of a wake steering controller with wind direction variability

Wind farm control strategies are being developed to mitigate wake losses in wind farms, increasing energy production. Wake steering is a type of wind farm control in which a wind turbine's yaw position is misaligned from the wind direction, causing its wake to deflect away from downstream turbines. Current modeling tools used to optimize and estimate energy gains from wake steering are designed to represent wakes for fixed wind directions. However, wake steering controllers must operate in dynamic wind conditions and a turbine's yaw position cannot perfectly track changing wind directions. Research has been conducted on robust wake steering control optimized for variable wind directions. In this paper, the design and analysis of a wake steering controller with wind direction variability is presented for a two-turbine array using the FLOw Redirection and Induction in Steady State (FLORIS) control-oriented wake model. First, the authors propose a method for modeling the turbulent and low-frequency components of the wind direction, where the slowly varying wind direction serves as the relevant input to the wake model. Next, we explain a procedure for finding optimal yaw offsets for dynamic wind conditions considering both wind direction and yaw position uncertainty. We then performed simulations with the optimal yaw offsets applied using a realistic yaw offset controller in conjunction with a baseline yaw controller, showing good agreement with the predicted energy gain using the probabilistic model. Using the Gaussian wake model in FLORIS as an example, we compared the performance of yaw offset controllers optimized for static and dynamic wind conditions for different turbine spacings and turbulence intensity values, assuming uniformly distributed wind directions. For a spacing of five rotor diameters and a turbulence intensity of 10 %, robust yaw offsets optimized for variable wind directions yielded an energy gain equivalent to 3.24 % of wake losses recovered, compared to 1.42 % of wake losses recovered with yaw offsets optimized for static wind directions. In general, accounting for wind direction variability in the yaw offset optimization process was found to improve energy production more as the separation distance increased, whereas the relative improvement remained roughly the same for the range of turbulence intensity values considered.

17 WIND ENERGY↗

NASA NTRS1994

Measurements of the Influence of Integral Length Scale on Stagnation Region Heat Transfer

The purpose was twofold: first, to determine if a length scale existed that would cause the greatest augmentation in stagnation region heat transfer for a given turbulence intensity and second, to develop a prediction tool for stagnation heat transfer in the presence of free stream turbulence. Toward this end, a model with a circular leading edge was fabricated with heat transfer gages in the stagnation region. The model was qualified in a low turbulence wind tunnel by comparing measurements with Frossling's solution for stagnation region heat transfer in a laminar free stream. Five turbulence generating grids were fabricated; four were square mesh, biplane grids made from square bars. Each had identical mesh to bar width ratio but different bar widths. The fifth grid was an array of fine parallel wires that were perpendicular to the axis of the cylindrical leading edge. Turbulence intensity and integral length scale were measured as a function of distance from the grids. Stagnation region heat transfer was measured at various distances downstream of each grid. Data were taken at cylinder Reynolds numbers ranging from 42,000 to 193,000. Turbulence intensities were in the range 1.1 to 15.9 percent while the ratio of integral length scale to cylinder diameter ranged from 0.05 to 0.30. Stagnation region heat transfer augmentation increased with decreasing length scale. An optimum scale was not found. A correlation was developed that fit heat transfer data for the square bar grids to within +4 percent. The data from the array of wires were not predicted by the correlation; augmentation was higher for this case indicating that the degree of isotropy in the turbulent flow field has a large effect on stagnation heat transfer. The data of other researchers are also compared with the correlation.

Vanfossen, G. James↗

NASA NTRS1983

Investigation of the tip clearance flow inside and at the exit of a compressor rotor passage. II - Turbulence properties

The flow in the tip clearance region of a compressor rotor is highly turbulent due to the strong interaction of the leakage flow with the annulus wall boundary layer. This paper deals with the turbulence properties of the flow in the tip clearance region of a moderately loaded compressor rotor. The experimental results reported in this paper were obtained using a two-sensor hot-wire probe in combination with an ensemble averaging technique. Blade-to-blade distribution of the axial and tangential turbulence intensities at various radial locations and ten axial locations (four inside the blade passage and the remaining six outside the passage) were derived from this data. Isointensity contours in the clearance region at various radial locations were also obtained from the experimental data. A region of very high turbulence intensities was indicated at the half-chord location from these results. The turbulence intensity profiles also indicated that the leakage flow travels toward the midpassage before rolling up. The turbulence is almost isotropic beyond three-quarter chord downstream of the trailing edge.

Pandya, A.↗

NASA NTRS2002

Experimental Investigation of Separated and Transitional Boundary Layers Under Low-Pressure Turbine Airfoil Conditions

Modern low-pressure turbine airfoils are subject to increasingly stronger pressure gradients as designers impose higher loading in an effort to improve efficiency and to reduce part count. The adverse pressure gradients on the suction side of these airfoils can lead to boundary-layer separation, particularly under cruise conditions. Separation bubbles, notably those which fail to reattach, can result in a significant degradation of engine efficiency. Accurate prediction of separation and reattachment is hence crucial to improved turbine design. This requires an improved understanding of the transition flow physics. Transition may begin before or after separation, depending on the Reynolds number and other flow conditions, has a strong influence on subsequent reattachment, and may even eliminate separation. Further complicating the problem are the high free-stream turbulence levels in a real engine environment, the strong pressure gradients along the airfoils, the curvature of the airfoils, and the unsteadiness associated with wake passing from upstream stages. Because of the complicated flow situation, transition in these devices can take many paths that can coexist, vary in importance, and possibly also interact, at different locations and instances in time. The present work was carried out in an attempt to systematically sort out some of these issues. Detailed velocity measurements were made along a flat plate subject to the same nominal dimensionless pressure gradient as the suction side of a modern low-pressure turbine airfoil ('Pak-B'). The Reynolds number based on wetted plate length and nominal exit velocity, Re, was varied from 50;000 to 300; 000, covering cruise to takeoff conditions. Low, 0.2%, and high, 7%, inlet free-stream turbulence intensities were set using passive grids. These turbulence levels correspond to about 0.2% and 2.5% turbulence intensity in the test section when normalized with the exit velocity. The Reynolds number and free-stream turbulence level do not have a significant effect on the location of boundary-layer separation unless they are high enough to induce transition upstream of separation. The location and extent of the transition zone, in contrast, depend strongly on Re and TI. The beginning of reattachment closely follows the onset of transition. Under low free-stream turbulence conditions the boundary layer is laminar at separation and then begins to exhibit fluctuations in a finite frequency band in the shear layer over the separation bubble. These fluctuations are due to instability waves. The fluctuations grow in magnitude, higher harmonics are generated, and finally lead to a breakdown to turbulence. Transition begins in the shear layer, but quickly spreads to the near wall region and causes the boundary layer to reattach. The transition is rapid and the resulting turbulence contains a full range of high and low frequencies. Under high free-stream turbulence conditions, slowly growing low-frequency fluctuations are induced in the pretransitional boundary layer by the free-stream. The separation bubbles are considerably thinner than in the low TI cases, resulting in thinner boundary layers at the end of the test wall. At Re=50,000 and 100,000, the pre-transitional boundary layer separates at about the same location as in the low TI cases. Transition occurs through a bypass mode, begins upstream of the corresponding low-TI location, and proceeds in a manner similar to that of an attached boundary layer. Under high TI at Re=200,000 and 300,000, transition begins before separation. The boundary layer may separate, but if it does the separation bubble is very short and does not significantly affect the downstream development of the boundary layer. A comparison is made to previous work in a simulated cascade.

Hultgren, Lennart S.↗

NASA NTRS2001

Aero-thermal Calibration of the NASA Glenn Icing Research Tunnel (2000 Tests)

Aerothermal calibration measurements and flow quality surveys were made in the test section of the Icing Research Tunnel at the NASA Glenn Research Center. These surveys were made following major facility modifications including widening of the heat exchanger tunnel section, replacement of the heat exchanger, installation of new turning vanes, and installation of new fan exit guide vanes. Standard practice at NASA Glenn requires that test section calibration and flow quality surveys be performed following such major facility modifications. A single horizontally oriented rake was used to survey the flow field at several vertical positions within a single cross-sectional plane of the test section. These surveys provided a detailed mapping of the total and static pressure, total temperature, Mach number, velocity, flow angle and turbulence intensity. Data were acquired over the entire velocity and total temperature range of the facility. No icing conditions were tested; however, the effects of air sprayed through the water injecting spray bars were assessed. All data indicate good flow quality. Mach number standard deviations were less than 0.0017, flow angle standard deviations were between 0.3 deg and 0.8 deg, total temperature standard deviations were between 0.5 and 1.8 F for subfreezing conditions, axial turbulence intensities varied between 0.3 and 1.0 percent, and transverse turbulence intensities varied between 0.3 and 1.5 percent. Measurement uncertainties were also quantified.

Gonsalez, Jose C.↗

NASA NTRSDate not supplied

Effects of Trailing Edge Thickness and Blade Loading Distribution on the Aerodynamic Performance of Simulated CMC Turbine Blades

The aerodynamic performance of three blade sets that represent the geometric manufacturing constraints of Ceramic Matrix Composite (CMC) blades was measured experimentally in a large-scale transonic turbine blade cascade. The trailing edge thicknesses of CMC blades are anticipated to be significantly larger than those of current state-of-the-art metallic blades. The blades tested in the current study had trailing edge thicknesses of 5%, 7%, and 9% relative to the blade axial chord. The three blade sets were designed with matching throat dimensions, so the blade loading distributions were varied to retain similar overall loading levels. Data were acquired at four Reynolds numbers, covering a factor of six range. All data were acquired at the design isentropic exit Mach number of 0.74. Measurements include blade loading and five-hole probe surveys at two downstream stations. The effects of inlet turbulence intensity were also quantified. Total pressure loss data were integrated to determine overall loss levels for each of the three measured blade passages. Excellent periodicity was noted. For low inlet turbulence levels, losses were surprisingly lower for the thickest trailing edge at low Reynolds numbers, but were highest at the maximum Reynolds number. In general, losses were found to scale well with Reynolds number, although front loading was found to significantly reduce the sensitivity of loss to Reynolds number. For high inlet turbulence intensity, losses were found to scale with trailing edge thickness as expected, and the Reynolds number sensitivity was reduced for all three blade sets. Loss levels at the highest Reynolds number were comparable at low and high inlet turbulence intensity levels.

Paul W Giel↗

NASA NTRSDate not supplied

Effects of Trailing Edge Thickness and Blade Loading Distribution on the Aerodynamic Performance of Simulated CMC Turbine Blades

Turbine↗

NASA NTRS1991

Heat transfer measurements from a smooth NACA 0012 airfoil

Local convective heat transfer coefficients were measured from a smooth NACA 0012 airfoil having a chord length of 0.533 m. Flight data were taken for the smooth airfoil at Reynolds numbers based on chord in the range 1.24 to 2.50 million and at various angles of attack up to 4 deg. During these flight tests, the freestream velocity turbulence intensity was found to be very low. Wind tunnel data were acquired in the Reynolds number range 1.20 to 4.52 million and at angles of attack from -4 to +8 deg. The turbulence intensity in the IRT was 0.5-0.7 percent with the cloud-generating sprays off. A direct comparison between the results obtained in flight and in the IRT showed that the higher level of turbulence intensity in the IRT had little effect on the heat transfer for the lower Reynolds numbers but caused a moderate increase in heat transfer at the higher Reynolds numbers. Turning on the cloud-generating spray nozzle atomizing air in the IRT did not alter the heat transfer. The present data were compared with leading-edge cylinder and flat plate heat transfer correlations that are often used to estimate airfoil heat transfer in computer codes.

Poinsatte, Philip E.↗

NASA NTRS2007

Experimental Investigation of Transition to Turbulence as Affected by Passing Wakes: Effects of High FSTI and Increased Rod Spacing

Experimental results from a study of the effects of passing wakes upon laminar-to-turbulent transition in a low-pressure turbine passage are presented. The test section geometry is designed to simulate the effects of unsteady wakes resulting from rotor-stator interaction upon laminar-to-turbulent transition in turbine blade boundary layers and separated flow regions over suction surfaces. Single-wire, thermal anemometry techniques are used to measure time-resolved and phase-averaged, wall-normal profiles of velocity, turbulence intensity and intermittency at multiple streamwise locations over the turbine airfoil suction surface. The Reynolds number based on suction surface length and stage exit velocity is 50,000. This study compares a previously documented base case flow having an approach flow turbulence intensity of 2.5 percent and a wake passing Strouhal number of 0.792 to two additional cases: one having an increased rod spacing case having a wake passing Strouhal number of 0.396, and another having an elevated approach flow turbulence intensity of 10 percent. From these data, the effects of increased rod spacing and elevated FSTI upon transition and separation processes in the near-wall flow are documented. The results show that a decreased wake passing Strouhal number results in an earlier separation with a larger separation bubble, while the elevated FSTI results in earlier separation, but with a shorter, thinner, separation bubble. The data and animations are included in an accompanying CD ROM.

Kaszeta, Richard W.↗