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Knobling, Rafael 
Document type:
Dissertation / Thesis 
Development and qualification of microstructured hotfilm sensors for the analysis of coherent structures in turbulent flows 
Kähler, Christian J., Prof. Dr. 
Kähler, Christian J., Prof. Dr.; Seidel, Helmut, Prof. Dr. 
Date oral examination:
Publication date:
Turbulente Strömung ; Kohärenz ; Strömungsgeschwindigkeit ; Strömungsmesstechnik ; Sensor ; Heißluftanemometrie ; Particle-Image-Velocimetry ; Hochschulschrift 
hotfilm, backward facing step, turbulent flow, PIV 
The objective of this thesis is to identify and characterize the footprint of coherent flow structures at the wall by means of electric wall-bounded sensor arrays. Microstructured hotfilm sensors are ideally suited for this due to their sensitivity, ampling rate and design flexibility. As much as 64 sensors with 1.5 mm pitch can be arranged in a linear array. In a second novel sensor design, a tightly packed array of 63 sensors in 7 spanwise rows with 9 sensors with 4 mm pitch in each direction was used to capture 2D projections of 3D flow phenomena. The potential of the sensor array, which dynamic signal did not require extensive calibration, was demonstrated by analysing the rich diversity of well-known flow structures present in a backward-facing step flow with step-height h. In a first experiment the linear hotfilm array was integrated within the backward-facing step splitter plate and then investigated at a free-stream velocity of u0 = 7.1 m/s in a free-stream wind-tunnel. Simultaneously, particle image velocimetry was used to investigate the flow field. The examination of the dynamical structures with image-processing routines yielded consistent results to the integral methods for both hotfilm anemometry and particle image velocimetry measurements. Besides the reattachment-length of xr ⇡ 8.2, the Group velocity was determined to lie in in the range of ug = 0.4 − 0.5 u0. Both the vortex shedding frequency with Sr = 0.16 and the flapping frequency with Sr = 0.01 were determined. In a second experiment conducted in the trisonic windtunnel of the Universität der Bundeswehr München, the backward facing step model was investigated at Mach 0.8 and the corresponding reattachment length and group velocity were measured. Under such flow conditions, flow structures were erratic due to the turbulent flow state and no noticeable Peak in the power spectrum occurred. This flow condition also persisted when applying rectangular lobes to affect the separation line with an induced additional geometric turbulence. However, the reattachment location moved further upstream to xh = 1.5−3 and the dynamic intensity was suppressed overall in positions xh < 6. Here the largest suppression was observed for the insert with amplitude h. The circular 2D sensor array showed the structure size at location xh to be approximately 2 h streamwise and approximately 0.8 h spanwise, without separation line alteration. Both structure dimensions were found to increase for the rectangular separation line alteration. This thesis demonstrates that image processing techniques applied to the output feed of sensor arrays are proficient to determine flow-parameters associated with coherent structures. Both the linear and the 2D sensor array were found to be well suited to retrieve timedependent information from coherent structures under high-speed flow-conditions. These results highlight the great potential of sensor system arrays as analysis and diagnostic tool for future active flow control schemes. 
DDC notation:
Fakultät für Luft- und Raumfahrttechnik 
LRT 7 - Institut für Strömungsmechanik und Aerodynamik 
Kähler, Christian J. 
Open Access yes or no?:
Ja / Yes