Boundary Layer Transition Measurements Research Articles

Measurement of boundary layer transition on oscillating airfoil using cntTSP in low-speed wind tunnel

cntTSP is a flow visualization technique that combines temperature-sensitive paint (TSP) and carbon nanotube (CNT). TSP is a sprayable temperature sensor, and CNT is used as a thin inner electric heater for the TSP. The cntTSP technique allows the visualization of the movement of the boundary layer transition on a moving airfoil. In this study, cntTSP was applied to a pitch-oscillating airfoil in a low-speed wind tunnel. Three methods were compared as methods for detecting the boundary layer transition position. The transition position was successfully evaluated using a heat transfer coefficient that was calculated by the surface temperature. Moreover, at a high-reduced frequency, the detected transition positions were different between the pitching-up and pitching-down procedure. Moreover, we observed a hysteresis loop of the transition position.

Final Design of the Boundary Layer Transition (BOLT) Flight Experiment

To improve scientific knowledge of boundary-layer transition physics on increasingly complex geometries, the Air Force Research Laboratory/Air Force Office of Scientific Research has sponsored the Boundary Layer Transition (BOLT) Experiment to obtain measurements of hypersonic boundary-layer transition in flight on a low-curvature concave surface with highly swept leading edges. This paper will summarize the completed design of the BOLT flight experiment. The final selection and layout of the flight scientific instrumentation was driven by extensive ground testing and numerical analyses. The complex flight experiment hardware was designed with over 344 scientific measurements to characterize transition in flight, and to provide a new validation dataset for the development of next-generation transition prediction methods.

Boundary Layer Transition Measurement Flight Test Based on Infrared Image Technique

According to the requirements of boundary layer transition detection measurement flight test, the study on the detection technique of infrared thermal image in real flight conditions was carried out. In this paper, the basic principle of infrared measurement technique was introduced firstly. Through the analysis of the main factors, such as emissivity, radiation incidence angle and temperature difference between the measured object and environment, the test means were determined, which includes surface treatment, infrared light path adjustment, solar radiation and internal heating of tested parts. Then the whole flight test method of boundary layer transition measurement was designed, including selecting test object and test instrument, determining test procedure and image processing procedure, and giving transition location criterion. Finally, the validity of this flight test method was demonstrated through testing results, the research of which would be helpful and meaningful in boundary layer transition measurement.

Unsteady boundary-layer transition measurements and computations on a rotating blade under cyclic pitch conditions

The presented work tackles the lack of experimental investigations of unsteady laminar-turbulent boundary-layer transition on rotor blades at cyclic pitch actuation, which are important for accurate performance predictions of helicopters in forward flight. Unsteady transition positions were measured on the blade suction side of a four-bladed subscale rotor by means of non-intrusive differential infrared thermography (DIT). Experiments were conducted at different rotation rates corresponding to Mach and Reynolds numbers at 75% rotor radius of up to {M}_{75} = 0.21 and {mathrm {Re}}_{75} = 3.3 times 10^5 and with varying cyclic blade pitch settings. The setup allowed transition to be measured across the outer 54% of the rotor radius. For comparison, transition was also measured using conventional infrared thermography for steady cases with collective pitch settings only. The study is complemented by numerical simulations including boundary-layer transition modeling based on semi-empirical criteria. DIT results reveal the upstream and downstream motion of boundary-layer transition during upstroke and downstroke, a reasonable comparison to experimental results obtained using the already established sigma c_p method, and noticeable agreement with numerical simulations. The result is the first systematic study of unsteady boundary-layer transition on a rotor suction side by means of DIT including a comparison to numerical computations.

PTV Measurement of Boundary Layer Transition Due to Free-Stream Turbulence Using LED

PTV Measurement of Boundary Layer Transition Due to Free-Stream Turbulence Using LED

Helicopter Rotor Boundary Layer Transition Measurement in Forward Flight Using an Infrared Camera

Boundary layer transition measurement was demonstrated using differential infrared thermography (DIT) on the top side of a helicopter rotor in forward flight, which detects the difference in the convective heat transfer at the boundary layer transition position. The tests used a FLIR X8500xc SLS long wave infrared camera to observe the DLR EC135 test helicopter rotor. The boundary layer transition was detected for hover out of ground effect (150 ft) and for forward flight at 80 kt (1700 ft). The measured boundary layer transition positions are consistent with previous measurements of the EC135 hovering in ground effect, and with predicted boundary layer transition positions. A method for the analysis of DIT images for a rotor in forward flight is shown, based on computational analysis of a pitching airfoil with varying inflow.

HIFiRE-5 Boundary-Layer Transition Measured in a Mach-6 Quiet Tunnel with Infrared Thermography

The principal goal of the Hypersonic International Flight Research Experimentation (HIFiRE) flight 5 is to measure hypersonic boundary-layer transition on a three-dimensional body. This paper presents measurements of heat flux and boundary-layer transition in the Boeing/U.S. Air Force Office of Scientific Research Mach-6 Quiet Tunnel. This facility has been developed to provide quiet flow at high Reynolds number, with low noise levels comparable to flight. Previously, the global heat flux and location of the transition front were measured with temperature-sensitive paint (TSP). A new HIFiRE-5 model was built with a polyether ether ketone shell, which is suitable for infrared–thermographic heat-flux measurements. Quiet-flow tests at Reynolds numbers of and zero angle of attack indicate a centerline transition location within 4% of the earlier TSP results, on the order of the uncertainty of the two techniques. This good agreement reinforces confidence in the earlier measurements and the new infrared-based technique. Examination of the off-centerline transition front indicates that the wavelength of stationary crossflow vortices is the same for both models and instrumentation techniques, although the streaks do not coincide precisely.

Optimization of differential infrared thermography for unsteady boundary layer transition measurement

Differential infrared thermography (DIT) is a method of analyzing infrared images to measure the unsteady motion of the laminar–turbulent transition of a boundary layer. It uses the subtraction of two infrared images taken with a short-time delay. DIT is a new technique which already demonstrated its validity in applications related to the unsteady aerodynamics of helicopter rotors in forward flight. The current study investigates a pitch-oscillating airfoil and proposes several optimizations of the original concept. These include the extension of DIT to steady test cases, a temperature compensation for long-term measurements, and a discussion of the proper infrared image separation distance. The current results also provide a deeper insight into the working principles of the technique. The results compare well to reference data acquired by unsteady pressure transducers, but at least for the current setup DIT results in an additional measurement-related lag for relevant pitching frequencies.

Boundary-layer transition measurements on Mach-scaled helicopter rotor blades in climb

In this work, laminar-turbulent boundary-layer transition is investigated on the suction side of Mach-scaled helicopter rotor blades in climb. The phenomenon is assessed by means of temperature-sensitive paint (TSP). Results are compared to a data sample acquired by infrared (IR) thermography and accompanied by integral thrust- and local surface pressure measurements at two radial blade sections. Spatially, high-resolved data allow for precise detection of boundary-layer transition along the outer 60% of the blade span. Results obtained via TSP and IR show remarkable agreement with minor deviations due to different surface qualities of the respective blades tested. TSP data are obtained at various collective pitch angles and three different rotating speeds corresponding to chord Reynolds and Mach numbers based on blade tip speed of $$Re_{\rm tip} = 4.8 - 9.3\times 10^5$$ and $$M_{\rm tip} = 0.29 - 0.57$$ , respectively. The transition position is detected with an accuracy of better than 1% chord and the findings show overall coherence as blade loading and tip chord Reynolds number are varied. Experimental findings are shown to be consistent with two-dimensional simulations using the $$e^N$$ -envelope method for transition prediction. Based on quantitative agreement between measured and calculated surface pressures, a comparison of the corresponding transition results suggests a critical amplification factor of $$N_{\rm cr.} = 5.5$$ best suited for transition prediction in the rotating test facility of the DLR Gottingen.

Hypersonic Wind-Tunnel Measurements of Boundary-Layer Transition on a Slender Cone

Boundary-layer transition was studied on a sharp 7 deg cone in two hypersonic wind tunnels at Mach numbers of 5, 6, 8, and 14 over a range of freestream Reynolds numbers between 3.3 and . High-speed schlieren measurements visualized the intermittent formation of instabilities and turbulent spots within the transitional boundary layer. Surface pressure and heat-transfer measurements revealed how the intermittent behavior of the boundary layer produces the mean character of these quantities. Transition at Mach 5 appeared to be initiated by a combination of first- and second-mode instabilities. These disturbances were isolated and surrounded by an otherwise smooth boundary layer. At higher Mach numbers, the boundary layer was dominated by second-mode instabilities, which covered most of the model before breakdown into turbulent spots. The spots remain surrounded by second-mode waves throughout the transitional region. These differences alter the pressure fluctuations and heat transfer profiles during transition. Higher frequency pressure measurements peaked upstream of the onset of transition because of the growth of second-mode instabilities. Lower frequency pressure fluctuations and the surface heat transfer did not rise significantly until further downstream where turbulent regions developed.

Robust, automated processing of IR thermography for quantitative boundary-layer transition measurements

A technique for automated, quantitative, global boundary-layer transition detection using IR thermography is developed. Transition data are rigorously mapped onto model coordinates in an automated fashion on moving targets. Statistical analysis of transition data that is robust to environmental contamination is presented.

Boundary layer transition determination for periodic and static flows using phase-averaged pressure data

A method of boundary layer transition measurement is presented for wind tunnel models instrumented with surface pressure taps. The measurement relies on taking a number of theoretically identical measurements at different times and then analysing the standard deviation of the pressures. Due to the slight unsteady movement of the transition position, a peak in the standard deviation of pressure σCPpeak is found at the transition position, and this is correlated with measurements of the transition position with an infrared camera and hot film anemometers. In contrast to microphone measurements, it is shown that the transition detection works for data which has been low-pass filtered with a cutoff of 1 Hz. The application to static and dynamic transition measurements on static and periodically pitching helicopter rotor blade airfoils at Mach 0.3-0.5 is demonstrated.

Quiet Tunnel Measurements of HIFiRE-5 Boundary-Layer Transition

The Hypersonic International Flight Research Experimentation (HIFiRE) program is a hypersonic flight test program executed by the Air Force Research Laboratory and Australian Defence Science and Technology Organisation. The principal goal of HIFiRE flight five is to measure hypersonic boundary-layer transition on a three-dimensional body. The HIFiRE flight tests are supported by a ground test campaign; this paper presents measurements of heat flux and boundary-layer transition in the Boeing/AFOSR Mach-6 Quiet Tunnel. This facility has been developed to provide quiet flow at high Reynolds number, with low noise levels comparable to flight. This tunnel’s good optical access enabled global measurement of the heat flux by means of temperature-sensitive paint. Two modes of transition were observed: transition along the centerline, suspected to arise from the amplification of second-mode waves in the boundary layer, and transition roughly halfway between the centerline and leading edges, probably due to the breakdown of crossflow vortices. Reducing the noise level from conventional to quiet substantially delayed both transition modes. Increasing the angle of attack from 0 to 4 deg delayed the crossflow transition mode on the windward side. Transition moved forward as freestream unit Reynolds number increased from to .

Boundary-layer transition measurements on hovering helicopter rotors by infrared thermography

High-speed infrared thermography was applied for boundary-layer transition measurements on the upper side of helicopter rotors. The transition detection is based on the analysis of a single instantaneous thermal image of the rotating blade and allows the determination of both the locations of the onset and of the end of the transition region. Measurements were performed on a Mach-scaled BO105 model rotor for different rotation frequencies. The transition characteristics are presented and compared to two-dimensional numerical simulations, and the measurement scatter is discussed. Additional transition measurements were performed on the main rotors of the DLR research helicopters Eurocopter BO105 and EC135. The transition behavior of the EC135 rotor blade is presented for different cases, and the effect of the contamination of the rotor blade leading edge on the laminar flow is discussed.

Measurement of ground effect and boundary-layer transition by towing wind tunnel

A newly constructed towing wind tunnel facility is introduced. In this wind tunnel, unlike a conventional wind tunnel, a testing model moves in still air on a testing track. This towing wind tunnel facility can therefore simulate highly complex flow between steady ground and a moving model without using a moving belt system. The performance of this facility is first explained. Some results of wings in ground effect experiments and boundary-layer transition experiments obtained using this facility are then given. From these results, we conclude that this facility has enough performance for complex flow testing.

Boundary layer pressure fluctuation measurements in transitional and turbulent flow.

Pressure fluctuations were measured for external flows over an axisymmetric surface. Measurements were made in laminar, transitional, and turbulent flow using piezoelectric pressure transducers. The entire surface (including the pressure transducers) was coated with a 1/16‐in. thick elastomer, in order to ensure a smooth surface. All measurements of boundary layer transition were made in an adverse pressure gradient. Results showed that root‐mean‐square pressure fluctuation levels during transition were as much as 15 dB higher than for fully developed turbulent flow. The transitional spectra were dominated by Tollmien‐Schlichting waves, and associated nonlinearities. The spectra of Tollmien‐Schlichting waves during amplification and subsequent transformation to turbulence are shown nondimensionally in comparison with Bakewell’s turbulence spectral density.

Wavelet Cross-Spectrum Analysis of Multi-Scale Disturbance Instability and Transition on Sharp Cone Hypersonic Boundary Layer

Experimental measurement of hypersonic boundary layer stability and transition on a sharp cone with a half angle of 5° is carried out at free-coming stream Mach number 6 in a hypersonic wind tunnel. Mean and fluctuation surface-thermal-flux characteristics of the hypersonic boundary layer flow are measured by Pt-thin-film thermocouple temperature sensors installed at 28 stations on the cone surface along longitudinal direction. At hypersonic speeds, the dominant flow instabilities demonstrate that the growth rate of the second mode tends to exceed that of the low-frequency mode. Wavelet-based cross-spectrum technique is introduced to obtain the multi-scale cross-spectral characteristics of the fluctuating signals in the frequency range of the second mode. Nonlinear interactions both of the second mode disturbance and the first mode disturbance are demonstrated to be dominant instabilities in the initial stage of laminar-turbulence transition for hypersonic shear flow.

超精密加工臨界ノズルに発生する境界層遷移の精密測定

超精密加工臨界ノズルに発生する境界層遷移の精密測定

Boundary-layer transition measurements in hypervelocity flows in a shock tunnel

Boundary-layer transition measurements in hypervelocity flows in a shock tunnel

Periodic Unsteady Flow Aerodynamics and Heat Transfer: Studies on a Curved Surface, Combined Part I and II

Aerodynamic and heat transfer investigations were done on a constant curvature curved plate in a subsonic wind tunnel facility for various wake passing frequencies and zero pressure gradient conditions. Steady and unsteady boundary layer transition measurements were taken on the concave surface of the curved plate at different wake passing frequencies where a rotating squirrel‐cage generated the unsteady wake flow. The data were analyzed using timeaveraged and ensemble‐averaged techniques to provide insight into the growth of the boundary layer and transition. Ensemble‐averaged turbulence intensity contours in the temporal spatial domain showed that transition was induced for increasing wake passing frequency and structure. The local heat transfer coefficient distribution for the concave and convex surface was determined at those wake passing frequencies using a liquid crystal heat transfer measurement technique. Detailed aerodynamic and heat transfer investigations showed that higher wake passing frequency caused transition to occur earlier on the concave surface. Local Stanton numbers were also calculated on the concave surface and compared with Stanton numbers predicted using a differential boundary layer and heat transfer calculation method. On the convex side, no effect of wake passing frequency on heat transfer was observed due to a separation bubble that induced transition.