Hot-film Array Research Articles

Characterization of Low-Frequency Oscillations in the Flowfield About an Iced Airfoil

Wind-tunnel measurements were used to identify and characterize the low-frequency oscillation present in the flowfield about an NACA 0012 airfoil with a horn-ice shape. This low-frequency mode was identified in the unsteady content of the airfoil lift coefficient and the corresponding Strouhal numbers compared well with those reported in the literature. To study the relationship between the ice-induced separation bubble dynamics and the airfoil circulation, the unsteady shear-layer reattachment location was determined using measurements from a surface-mounted hot-film array. At the low-frequency mode, the lift coefficient led the reattachment location by a phase of . Additional characterization of the relationship between the unsteady shear-layer reattachment location and the unsteady airfoil pressure distribution was also completed, along with an analysis of the convective qualities of the low-frequency oscillation throughout the airfoil pressure distribution.

An experimental study of airfoil instability tonal noise with trailing edge serrations

This paper presents an experimental study of the effect of trailing edge serrations on airfoil instability noise. Detailed aeroacoustic measurements are presented of the noise radiated by an NACA-0012 airfoil with trailing edge serrations in a low to moderate speed flow under acoustical free field conditions. The existence of a separated boundary layer near the trailing edge of the airfoil at an angle of attack of 4.2 degree has been experimentally identified by a surface mounted hot-film arrays technique. Hot-wire results have shown that the saw-tooth surface can trigger a bypass transition and prevent the boundary layer from becoming separated. Without the separated boundary layer to act as an amplifier for the incoming Tollmien–Schlichting waves, the intensity and spectral characteristic of the radiated tonal noise can be affected depending upon the serration geometry. Particle Imaging Velocimetry (PIV) measurements of the airfoil wakes for a straight and serrated trailing edge are also reported in this paper. These measurements show that localized normal-component velocity fluctuations that are present in a small region of the wake from the laminar airfoil become weakened once serrations are introduced. Owing to the above unique characteristics of the serrated trailing edges, we are able to further investigate the mechanisms of airfoil instability tonal noise with special emphasis on the assessment of the wake and non-wake based aeroacoustic feedback models. It has been shown that the instability tonal noise generated at an angle of attack below approximately one degree could involve several complex mechanisms. On the other hand, the non-wake based aeroacoustic feedback mechanism alone is sufficient to predict all discrete tone frequencies accurately when the airfoil is at a moderate angle of attack.

Coated Hot-Film Sensors for Transition Detection in Cruise Flight

In this paper results of wind-tunnel experiments and flight tests that demonstrate the effectiveness of coated multisensor hot-film arrays using constant current anemometry to determine boundary-layer transition under cruise conditions are presented. The study focuses on the analysis of several coating procedures, materials, and coating thicknesses to improve mechanical durability and signal quality. After several numerical simulations and environmental resistance tests, for example, in climate chambers and low-speed flight experiments, wind-tunnel experiments and flight tests at transonic speeds are carried out. The numerous experimental and numerical investigations and flight tests in cooperation with the Deutsche Airbus GmbH on a Beluga airplane (300-600ST) show a 1-μm parylene layer covering the extremely sensitive hot-film sensors and conducting paths to be sufficiently robust to stand the environmental conditions in cruise flight for several months. Furthermore, flight tests at transonic speeds on the Mystere Falcon 20 E5 at an altitude up to 30,000 ft and Mach numbers up to 0.79 are conducted in a joint project with DLR Oberpfaffenhofen. Modified wing models are mounted under both wings of the Falcon. The surfaces are instrumented with coated hot-film arrays using 128 sensors. The tests on the Falcon prove parylene coated hot-film sensors to still possess the sensitivity to locate transition under real flight conditions.

The Influence of Turbulence on Wake Dispersion and Blade Row Interaction in an Axial Compressor

The influence of free-stream turbulence on wake dispersion and boundary layer transition processes has been studied in a 1.5-stage axial compressor. An inlet grid was used to produce turbulence characteristics typical of an embedded stage in a multistage machine. The grid turbulence strongly enhanced the dispersion of inlet guide vane (IGV) wakes. This modified the interaction of IGV and rotor wakes, leading to a significant decrease in periodic unsteadiness experienced by the downstream stator. These observations have important implications for the prediction of clocking effects in multistage machines. Boundary layer transition characteristics on the outlet stator were studied with a surface hot-film array. Observations with grid turbulence were compared with those for the natural low turbulence inflow to the machine. The transition behavior under low turbulence inflow conditions with the stator blade element immersed in the dispersed IGV wakes closely resembled the behavior with elevated grid turbulence. It is concluded that with appropriate alignment, the blade element behavior in a 1.5-stage axial machine can reliably indicate the blade element behavior of an embedded row in a multistage machine.

Unsteady Boundary Layer Development Due to Wake Passing Effects on a Highly Loaded Linear Compressor Cascade

The effects of wake passing on boundary layer development on a highly loaded linear compressor cascade were investigated in detail on the suction side of a compressor blade. The experiments were performed in the High Speed Cascade Wind Tunnel of the Institut fuer Strahlantriebe at Mach and Reynolds numbers representative for real turbomachinery conditions. The experimental data were acquired using different measurement techniques, such as fast-response Kulite sensors, hot-film array and hot-wire measurements. The incoming wakes clearly influence the unsteady boundary layer development. Early forced transition in the boundary layer is followed in time by calmed regions. Large pressure fluctuations detectable in the ensemble averaged Kulite data reveal the existence of coherent structures in the boundary layer. Distinct velocity variations inside the boundary layer are amplified when approaching the blade surface. The time–mean momentum thickness values are reduced compared to the steady ones and therefore clarify the potential for a loss reduction due to wake passing effects.

Quantitative detection of turbulent reattachment using a surface mounted hot-film array

A robust method to detect the mean turbulent reattachment location with a flush surface-mounted array of hot-film sensors is presented. The method has the advantages of requiring no sensor calibration, no dependence on the presence of a dominant frequency or oscillation period and it requires no qualitative interpretation of sensor time-series signals. The method is developed by investigating the flow downstream of a backward-facing step. Through computation of the “time of flight” of convected flow disturbances over adjacent sensor pairs, the method offers a quantitative resolution of the mean location of reattachment for turbulent flows.

Boundary-layer investigations on a model of the ELAC 1 configuration at high Reynolds numbers in the DNW

Results of boundary-layer investigations on the leeward side of a 1: 12 scale model of the ELAC 1 configuration of a space transportation system are presented. The configuration has the shape of a thick delta wing with a rounded leading edge. The model length is 6 m; the experiments were carried out in the 8×6 m 2 low-speed German-Dutch-Windtunnel at Reynolds numbers up to Re=40·10 6 . In a first series of experiments mean velocity profiles were determined in the turbulent boundary layer on the leeward side of the model, with a single hot-wire probe in the plane of symmetry at four positions. Comparison calculations with a numerical solution of the boundary-layer equations showed good agreement up to angles of attack α=10° . In a second series of tests the laminar-turbulent transition of the flow and its separation near the rounded leading edge were investigated at three positions with multi-sensor hot-film arrays with 40, 56, and 96 elements. These measurements demonstrated that the flow near the rounded leading edge is markedly influenced by the nose radius.

Natural Transition Phenomena on an Axial Compressor Blade

Data from a surface hot-film array on the outlet stator of a 1.5-stage axial compressor are analyzed to look for direct evidence of natural transition phenomena. An algorithm is developed to identify instability waves within the Tollmien–Schlichting (T–S) frequency range. The algorithm is combined with a turbulent intermittency detection routine to produce space-time diagrams showing the probability of instability wave occurrence prior to regions of turbulent flow. The paper compares these plots for a range of blade loading, with free-stream conditions corresponding to the maximum and minimum inflow disturbance periodicity produced by inlet guide vane clocking. Extensive regions of amplifying instability waves are identified in nearly all cases. The implications for transition prediction in decelerating flow regions on axial turbomachine blades are discussed.

Periodic Transition on an Axial Compressor Stator: Incidence and Clocking Effects: Part I—Experimental Data

Periodic wake-induced transition on the outlet stator of a 1.5-stage axial compressor is examined using hot-film arrays on both the suction and pressure surfaces. The time-mean surface pressure distribution is varied by changing the blade incidence, while the free-stream disturbance field is altered by clocking of the stator relative to an inlet guide vane row. Ensemble-averaged plots of turbulent intermittency and relaxation factor (extent of calmed flow following the passage of a turbulent spot) are presented. These show the strength of periodic wake-induced transition phenomena to be significantly influenced by both incidence and clocking effects. The nature and extent of transition by other modes (natural, bypass, and separated flow transition) are altered accordingly. Leading edge and midchord separation bubbles are affected in a characteristically different manner by changing free-stream periodicity. There are noticeable differences between suction and pressure surface transition behavior, particularly as regards the strength and extent of calming. In Part II of this paper, the transition onset observations from the compressor stator are used to evaluate the quasi-steady application of conventional transition correlations to predict unsteady transition onset on the blading of an embedded axial compressor stage.

Calibration of single-surface hot films and in-line hot-film arrays in laminar or turbulent flows

Experimental and numerical investigations were performed concerning the heat balance and calibration of single-surface hot films and surface hot-film arrays in laminar or turbulent flows. Emphasis is placed on the influence of the thermal interaction among sensor, fluid and the wall and among different sensors from an in-line hot-film array on the calibration of the sensors. Experimental and numerical investigations of the coupled flow and structure temperature field in the vicinity of an in-line surface hot-film array are presented. The mechanism of thermal interactions is demonstrated, namely the strong effect of an increasing thermal boundary layer thickness on the convective and conductive heat losses of each single sensor of the array due to an increasing number of upstram sensors being `operated'. The numerical simulations were performed both for laminar and the turbulent flow as well as for a different number of sensors. The results are compared with experiments demonstrating that, due to the thermal interaction, the downstream decrease in the total heat losses of a sensor from an in-line array has a great effect on the calibration.