Boundary-layer Transition Detection Research Articles

A simple boundary-layer transition detection method using mean static pressure measurements

A simple boundary-layer transition detection method using mean static pressure measurements

An experimental study on boundary layer transition detection over a pitching supercritical airfoil using hot-film sensors

In the present work, experimental tests are conducted to study boundary layer transition over a supercritical airfoil undergoing pitch oscillations using hot-film sensors. Tests have been undertaken at an incompressible flow. Three reduced frequencies of oscillations and two mean angles of attack are studied and the influences of those parameters on transition location are discussed. Different algorithms are examined on the hot-film signals to detect the transition point. Results show the formation of a laminar separation bubble near the leading edge and at relatively higher angles of attack which leads to the transition of the boundary layer. However, at lower angles of attack, the amplification of the peaks in voltage signal indicate the emergence of the vortical structures within the boundary layer, introducing a different transition mechanism. Moreover, an increase in reduced frequency leads to a delay in transition onset, postponing it to a higher angle of attack, which widens the hysteresis between the upstroke and downstroke motions. Rising the reduced frequency yields in weakening or omission of vortical disturbances ensuing the removal of spikes in the signals. Of the other important results observed, is faster movement of the relaminarization point in the higher mean angle of attack. Finally, a time–frequency analysis of the hot-film signals is performed to investigate evolution of spectral features of the transition due to the pitching motion. An asymmetry is clearly observed in frequency pattern of the signals far from the bubble zone towards the trailing edge; this may reflect the difference between the transition and relaminarization physics. Also, various ranges of frequency were obtained for different transition mechanisms.

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.

Advanced infrared thermography data analysis for unsteady boundary layer transition detection

Advanced data processing methods for detecting unsteady boundary layer transition in periodic aerodynamic processes by means of infrared thermography measurements are presented. The thermal radiation emitted from the heated suction surface of a pitching airfoil model in subsonic flow is measured with an infrared camera. The unsteady boundary layer transition location is detected by analyzing the difference in the infrared radiation signal over short periods of time with differential infrared thermography (DIT). The DIT method is optimized and automated in the present study, which facilitates the extension of the part of the motion period where valid DIT transition measurements are produced. Additionally, a new infrared thermography data processing method is introduced in this study. The extraction of the extrema of the measured radiation signal at fixed locations on the model surface yields instants of the motion period that relate to the occurrence of boundary layer transition. The local infrared thermography (LIT) approach can be extended to measuring the two-dimensional unsteady boundary layer transition front.

Boundary-layer transition detection by thermography and numerical method around bionic train model in wind tunnel test

Methods of diagnosing aerodynamic characteristics are constantly developing in order to conduct the precise and energy efficient wind tunnel testing of transport vehicles in the prototype design early stages. This is of a special importance when facing the time/cost consumption problems of detection of the transition zone over the simplified design of the high-speed train. Herein the applied thermodynamics found a very significant role in the field of experimental aerodynamics. With the intention of detecting the boundary-layer transition zone the following measurements were applied: the infrared thermography, flow visualization, and drag force measurements. In addition, the CFD was applied to predict the flow behaviour and transition zone, solving PDE consisting of the Reynolds-averaged Navier-Stokes equations, energy equation, and the equation of state for an ideal gas employing density-based solver. The thermal imaging defined the transition zone by simple application, and fast recognition, while the transition bounds were defined in the analysis. The flow visualization confirmed thermography results and the method itself as favourable, especially in the most expensive early phases of redesigning for aerodynamically optimized and energy efficient solutions. The numerical method was confirmed by the experiments, resulting in acceptable differences in the definition of the transition zone. For a better understanding of the phenomenon, the overlapped implementation of the presented methods focused on forced convection showed as the best solution. Based on the experiences of this research, development of the additional equipment and adjustments will be introduced in the future experiments.

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.

Analysis of differential infrared thermography for boundary layer transition detection

This paper presents an analysis of the differential infrared thermography (DIT) technique, a contactless method of measuring the unsteady movement of the boundary layer transition position on an unprepared surface. DIT has been shown to measure boundary layer transition positions which correlate well with those from other measurement methods. In this paper unsteady aerodynamics from a 2D URANS solution are used and the resulting wall temperatures computed. It is shown that the peak of the temperature difference signal correlates well with the boundary layer transition position, but that the start and end of boundary layer transition cannot be extracted. A small systematic time-lag cannot be reduced by using different surface materials, but the signal strength can be improved by reducing the heat capacity and heat transfer of the surface layer, for example by using a thin plastic coating. Reducing the image time separation used to produce the difference images reduces the time-lag and also the signal level, thus the optimum is when the signal to noise ratio is at the minimum which can be evaluated.

XFOIL vs CFD performance predictions for high lift low Reynolds number airfoils

Blade Element Momentum (BEM) theory is an extensively used technique for calculation of propeller aerodynamic performance. With this method, the airfoil data needs to be as accurate as possible. At the same time, Computational Fluid Dynamics (CFD) is becoming increasingly popular in the design and optimization of devices that depend on aerodynamics. For fixed and rotary wing applications, the airfoil lift over drag coefficient is the dominant airfoil performance parameter. Selecting a suitable computational tool is crucial for the successful design and optimization of this ratio. The XFOIL code, the Shear Stress Transport k−ω turbulence model and a refurbished version of k−kl−ω transition model were used to predict the airfoil aerodynamic performance at low Reynolds numbers (around 2.0×105). It has been shown that the XFOIL code gives the overall best prediction results. Also, it is not clear that CFD turbulence models, even with boundary layer transition detection capability, can compute better airfoil performance predictions data.

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.

Differential infrared thermography for boundary layer transition detection on pitching rotor blade models

Differential infrared thermography (DIT) was investigated and applied for the detection of unsteady boundary layer transition locations on a pitching airfoil and on a rotating blade under cyclic pitch. DIT is based on image intensity differences between two successively recorded infrared images. The images were recorded with a high framing rate infrared camera. A pitching NACA0012 airfoil served as the first test object. The recorded images were used in order to investigate and to further improve evaluation strategies for periodically moving boundary layer transition lines. The measurement results are compared with the results of unsteady CFD simulations based on the DLR-TAU code. DIT was then used for the first time for the optical measurement of unsteady transition locations on helicopter rotor blade models under cyclic pitch and rotation. Image de-rotation for tracking the blade was employed using a rotating mirror to increase exposure time without causing motion blur. The paper describes the challenges that occurred during the recording and evaluation of the data in detail. However, the results were found to be encouraging to further improve the method toward the measurement of unsteady boundary layer transition lines on helicopter rotor models in forward flight.

Closed-Loop Control Validation of a Morphing Wing Using Wind Tunnel Tests

In this paper a rectangular finite aspect ratio wing, having a wing trailing edge airfoil reference airfoil cross section, was considered. The wing upper surface was made of a flexible composite material and instrumented with Kulite pressure sensors and two smart memory alloys actuators. Unsteady pressure signals were recorded and visualized in real time while the morphing wing was being deformed to reproduce various airfoil shapes by controlling the two actuators displacements. The controlling procedure was performed using two methods which are described in the paper. Several wind-tunnel test runs were performed for various angles of attack and Reynolds numbers in the 6 × 9 foot wind tunnel at the Institute for Aerospace Research at the National Research Council Canada. The Mach number was varied from 0.2 to 0.3, the Reynolds numbers varied between 2.29 and 3.36 x 10 6 , and the angle-of-attack range was within -1 to 2 degrees. Wind-tunnel measurements are presented for airflow boundary layer transition detection using high sampling rate pressure sensors.

New device and method for measuring thermal conductivity of thin-films

Abstract Thermal sensitive paints (TSPs) are used for global nonintrusive detection of boundary layer transition in flow over the surface of wind tunnel research models. Since the transition is a transient process, the TSP should have a fast response characteristic. A low paint thermal conductivity is required for fast response. A thin-film thermal conductivity meter (TFTCM) was designed and built to measure thermal conductivity of the TSPs, which are typically between 50 and 150/μm thick. In this paper, the design and operating features of the TFTCM are described. Measurement of the thermal conductivity with this TFTCM of three standard thin-film low conductivity specimens, Kapton, Teflon, and Borofloat glass, showed good agreement with the manufacturer quoted values, thus validating the instrument and the procedure. Consistently repeatable values for thermal conductivity ( k=0.41±0.02 W/mK) were also obtained for the TSP specimen (TSB-B, 75 /μm) tested.

B305 大型低温風洞で用いる感温塗料の開発

A new Temperature-Sensitive Paint (TSP) technique for boundary-layer transition detection in a production-type large cryogenic wind tunnel is present. The formulation of Ruthenium complex TSP system has been optimized in terms of luminescence intensity and robustness. The optimum dye-binder-solvent combination has been determined through systematic sample tests. A new binder has been introduced and the resulting coating was found free from cracking at cryogenic temperatures. This is contrary to the silicone-based cryogenic TSPs that are subject to micro cracks at reduced temperatures. The new TSP can meet the root-mean-square roughness requirement less than 0.15 ?m. We applied this TSP in the JAXA 0.1-m Transonic Cryogenic Wind Tunnel, and then European Transonic Windtunnel (ETW). A variety of transition patterns have been successfully visualized.

低温風洞における境界層遷移の感温塗料による可視化

Temperature-sensitive paint (TSP) has been applied to boundary-layer transition detection in a cryogenic wind tunnel. Three TSP dyes, Ru (bpy), Ru (trpy) and Ru (VH127), were evaluated for a temperature range from 100 to 298 Kelvin using a LN2-cooled cryostat and a spectrofluorometer. It was found that TSP based on Ru (trpy) has the highest sensitivity among these three. We used epoxy-based sprayable paint as insulating material for a test model. No cracks appeared at cryogenic temperatures as long as the total thickness is kept less than 80 microns. To demonstrate the capability of this TSP/insulating-layer combination, we conducted a 2-D airfoil model test in NAL 0.1-m Cryogenic Transonic Wind Tunnel. Natural and forced transition patterns were successfully visualized using TSP at temperatures from 100 to 150 Kelvin.

Unified Model Deformation and Flow Transition Measurements

The number of optical techniques that may potentially be used during a given wind tunnel test is continually growing. These include parameter sensitive paints that are sensitive to temperature or pressure, several different types of off-body and on-body flow visualization techniques, optical angle-of-attack (AoA), optical measurement of model deformation, optical techniques for determining density or velocity, and spectroscopic techniques for determining various flow field parameters. Often in the past the various optical techniques were developed independently of each other, with little or no consideration for other techniques that might also be used during a given test. Recently two optical techniques have been increasingly requested for production measurements in NASA wind tunnels. These are the video photogrammetric (or videogrammetric) technique for measuring model deformation known as the video model deformation (VMD) technique, and the parameter sensitive paints for making global pressure and temperature measurements. Considerations for, and initial attempts at, simultaneous measurements with the pressure sensitive paint (PSP) and the videogrammetric techniques have been implemented. Temperature sensitive paint (TSP) has been found to be useful for boundary-layer transition detection since turbulent boundary layers convect heat at higher rates than laminar boundary layers of comparable thickness. Transition is marked by a characteristic surface temperature change wherever there is a difference between model and flow temperatures. Recently, additional capabilities have been implemented in the target-tracking videogrammetric measurement system. These capabilities have permitted practical simultaneous measurements using parameter sensitive paint and video model deformation measurements that led to the first successful unified test with TSP for transition detection in a large production wind tunnel.

Shear-stress sensitive liquid crystals for hypersonic boundary-layer transition detection

Covers advancements in spacecraft and tactical and strategic missile systems, including subsystem design and application, mission design and analysis, materials and structures, developments in space sciences, space processing and manufacturing, space operations, and applications of space technologies to other fields.

Boundary-layer transition detection with infrared imaging emphasizing cryogenic applications

This paper reviews the technique of boundary-layer transition detection using infrared (IR) imaging, emphasizing cryogenic wind-tunnel testing. With the exception of the low-temperature effects on the IR radiation, the discussion is relevant to conventional wind-tunnel and flight testing as well. At low temperatures, IR imaging encounters a reduction in the radiated energy throughout the IR spectrum, combined with a shift to longer wavelengths of the bulk of the radiation. This radiation behavior affects the minimum resolvable temperature difference (MRTD) of the IR imaging system because of its fixed wave band sensitivity. In the absence of commercial long wavelength IR imaging systems, operating at wavelengths longer than 13 micron, some measures can be taken to alleviate the problem caused by the MRTD limitation. The thermal signature of transition can be enhanced by allowing a small and controlled temperature increase of the wind-tunnel flow that induces a transient heat transfer to the model. This action temporarily reveals the model area under the turbulent regime through its higher heating rate compared with the laminar regime. The contrast between the areas exposed to the two regimes can be enhanced by subtraction of thermograms (the equilibrium thermogram from the transient thermogram). Further visual improvement can be obtained through shade stretching or binary shading.

Boundary-layer transition-detection in a cryogenic wind tunnel usinginfrared imaging

Cryogenic wind tunnels are currently the only option available to test models of medium to large transport airplanes at high Reynolds numbers identical to those encountered in flight. This matching in Reynolds numbers is required for reproducing the flow viscous effects, most notably the laminar to turbulent boundary-layer transition. Infrared imaging is particularly attractive for transition detection in cryogenic wind tunnels because it is nonintrusive, fast, and provides a global view of the boundary-layer status on the model. An experimental method using a commercial 8- to 12-p infrared imaging system was developed for transition detection in cryogenic wind tunnels. So far, transition has been detected at temperatures down to 170 K. At this temperature, tunnels with a pressurization capability of only a few atmospheres can generate transonic flows at Reynolds numbers beyond 3 X108 per meter. Nomenclature M = Mach number Pt = total pressure R/m = Reynolds number per meter Rc = chord Reynolds number Tt = total temperature a = angle of attack ATt = total temperature step set in the wind-tunnel flow

Comparisons of boundary-layer transition measurement techniques at supersonic Mach numbers

Four different boundary-layer transition detection techniques—very thin hot films, liquid crystals, infrared imaging, and an optical interferometer—were compared using the same flat-plate model for the same tunnel conditions. The comparisons, conducted at the NASA Langley Research Center, involved not only their sensitivity to transition but also their ease of use. The thin films, as expected, gave excellent quantitative information and were used as the standard for evaluating the other techniques. Both the liquid crystals and infrared imaging were able to detect transition before the boundary-layer intermittency factor had reached 50%. The optical interferometer was unsuccessful. Conditions sampled included a range of Mach numbers from 1.5 to 2.5 and unit Reynolds numbers from 3.3 to 13.1 x 10 6/m.

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