Ewald Kraemer

Time-Resolved Heat Transfer Measurements on the Tip Wall of a Ribbed Channel Using a Novel Heat Flux Sensor—Part I: Sensor and Benchmarks

A novel heat flux sensor was tested that allows for time-resolved heat flux measurements in internal ribbed channels related to the study of passages in gas turbine blades. The working principle of the atomic layer thermopile (ALTP) sensor is based on a thermoelectric field created by a temperature gradient over an yttrium-barium-copper-oxide (YBCO) crystal (the transverse Seebeck effect). The sensors very fast frequency response allows for highly time-resolved heat flux measurements up to the 1 MHz range. This paper explains the design and working principle of the sensor, as well as the benchmark-ing of the sensor for several flow conditions. For internal cooling passages, this novel sensor allows for highly accurate, time-resolved measurements of heat transfer coefficients, leading to a greater understanding of the influence of fluctuations in temperature fields.

Transonic Tail Buffet Simulations for the Common Research Model

In advance of the ESWI RP measurement campaign in the European Transonic Wind Tunnel in 2014, numerical simulations for NASA’s Common Research Model were performed using the DLR TAU code. The presented investigations are focused on the interaction between the separated flow of the main wing with the horizontal tail plane at high Mach numbers. This phenomenon, called transonic tail buffet, poses a serious risk and therefore limits the flight envelope for commercial aircraft. A mesh for the CRM was built including a high resolved tetrahedral block to reduce dissipative effects in the wake. A validation was conducted which shows excellent agreement to the results of the Drag Prediction Workshops. Based on lift polar simulations the buffet onset regime was defined and an inflow condition beyond this point was selected to perform an unsteady simulation. It is shown that a corner separation leads to a movement of the complex shock system on the upper surface of the main wing. Large vortices are shed which propagate downstream and massively influence the flow around the HTP followed by a high frequency response which is found in all integral coefficients.

Trailing-Edge Noise Measurements Using a Hot-Wire Based Coherent Particle Velocity Method

This paper describes a hot-wire based method for two-dimensional trailing edge noise measurements mainly for use in wind tunnels with high background noise levels. The method is based on the cross correlation of two hot-wire signals, allowing to measure the Coherent Particle Velocity (CPV) of the emitted sound waves. The strong directional sensitivity of the hot-wires leads to a suppression of parasitic noise, which significantly improves the signal-to-noise ratio. Due to the small dimensions of the sensors together with the assured laminar flow condition they are well suited for inflow measurements. To obtain quantitative results in terms of sound pressure level, the sensitivity of the measurement setup is derived by simulation of the response of the hot-wires to a line source. For validation, trailing edge noise measurements were performed at 60m/s and a Reynolds number of 1.6×10 in the closed test section of the Laminar Wind Tunnel Stuttgart (LWT), using the CPV-method. Finally, the same airfoils were investigated in the open jet of the Aeroacoustic Wind Tunnel Braunschweig (AWB) using a phased microphone array. The quantitative comparison of the experimental results obtained in the two wind tunnels required the application of appropriate wind tunnel corrections. The obtained sound pressure frequency spectra are basically found to be parallel in the frequency range of sufficient measurement accuracy. The total sound pressure levels vs. lift coefficient show a more or less constant offset of about 2 dB between AWB and LWT. Given the totally different measurement principles this can be regarded as a good agreement. Finally results of a NACA 0012 airfoil are presented and compared to published data.

Numerical investigation of helicopter rotors in ground effect

This study intends to proof the capability of URANS computations to simulate the helicopter flight in the ground effect flow regime. Hover as well as forward flight cases are computed and compared to experimental data. For both flight conditions several parameter variations, such as ground distance, collective pitch and normalized advance ratio, were examined. Overall a good representation of the flow field structure could be obtained, as well as a satisfying approximation for the thrust coefficient. Furthermore, conclusions could be drawn regarding unsteady effects in the wake and their influence on the brown-out phenomenon. Especially the dynamic fluid movement around the Flow Separation Point is a considerable source of unsteadiness and energy introduced in the brown-out particle movement.

Hyperion UAV: An international collaboration

The Hyperion aircraft project was an international collaboration to develop an aerial vehicle to investigate new technologies with a focus on performance efficiencies. A delocalized international team of graduate and undergraduate students conceived, designed, implemented, and operated the aircraft. The project taught essential systems engineering skills through long-distance design and manufacturing collaborations with multidisciplinary teams of students located around the world. Project partners were the University of Colorado Boulder, USA, The University of Sydney, Australia, and the University of Stuttgart, Germany. The three teams are distributed eight hours apart; students can relay select work daily so that developments can “Follow-The-Sun”. Select components are manufactured and integrated both in Stuttgart and Colorado, giving the students an opportunity to learn multifaceted design tactics for manufacturing and interface control. Final flight testing was conducted by the global team in Colorado during the month of April 2011.

Evaluation of Measured Anisotropic Turbulent Two-point Correlation Data for the Accurate Prediction of the Turbulence Noise Sources

Noise emitted from 2D subsonic airfoil sections depends strongly on the specific properties of the turbulent boundary-layer passing the trailing edge. The intention of the present paper is three folds: (1) wind tunnel measurement of the two-point turbulent velocity correlations to analyze the trailing-edge near wake flow structure in detail, (2) assessment of the measurement data and post-processing of the turbulence properties in a way so that it can be comparable to numerical results, and (3) development of a relationship between isotropic theory and anisotropic measurement data to approximate quantities like isotropic longitudinal length scale Λf . Measurements of two-point turbulent velocity correlations were conducted in 2D turbulent boundary layer flows over two airfoils (NACA 0012 & NACA 643-418 ) at high Reynolds numbers (Re=1.5e6 and Re=2.5e6) in the Laminar Wind Tunnel (LWT) of the University of Stuttgart. Correlation tensor components were measured by two X-wire probes shortly downstream of the airfoil trailing edge with separation in vertical direction for different cases of boundary layer development (angles-of-attack, natural and fixed transition). Two different approaches to evaluate turbulence integral correlation length scales from the measured two-point correlation data are presented. The first methods provides anisotropic turbulence integral length scales estimated by fitting an exponential function to the measured two-point velocity correlation coefficient. The second method established a local relationship between the results from isotropic theory and anisotropic experimental data. This relationship permits efficient approximation of the isotropic longitudinal integral length scale (Λf), and enables the consideration of anisotropy effects in RANS predicted data. The outcome of both methods are been compared with RANS results and lead to significant improvement of the prediction of the turbulence noise sources. The benefit of these efforts will be further applied to a Turbulent Boundary-Layer Trailing-Edge (TBLTE) interaction noise prediction method (Rnoise) to analyze anisotropy effects of the airfoil trailing-edge near-wake flow.

Aeroacoustics of a High-Fidelity CFD Calculation of a Counter-Rotating Open Rotor in Take-Off Conditions

A very high-fidelity CFD calculation has been carried out for a 9x7 counter-rotating open rotor setup at take-off conditions. The total setup consisted of roughly 170 million grid cells and was used for a grid study as well as a subsequent acoustic evaluation. The analysis contained farfield aeroacoustics obtained with the acoustic analogy of Ffowcs Williams and Hawkings as well as nearfield acoustics extracted directly from the flow field data. The grid study showed a low grid dependency for integrated aerodynamic values. The acoustic evaluation in the farfield showed that the main noise characteristics can be resolved sufficiently with all three grid resolutions. Significant errors only occur in the low intensity high frequency range. A comparison of acoustic evaluation in the nearfield with the Ffowcs Williams and Hawkings approach and direct pressure output showed no benefit of the latter justifying the higher computational costs caused by the required grid resolution for the transport of the pressure disturbances. For both approaches the noise characteristics were similar with small discrepancies in the wake of the CROR and at higher frequencies.

Time-Resolved Heat Transfer Measurements on the Tip Wall of a Ribbed Channel Using a Novel Heat Flux Sensor—Part II: Heat Transfer Results

Measurements using a novel heat flux sensor were performed in an internal ribbed channel representing the internal cooling passages of a gas turbine blade. These measurements allowed for the characterization of heat transfer turbulence levels and unsteadiness not previously available for internal cooling channels. In the study of heat transfer, often the fluctuations can be equally as important as the mean values for understanding the heat loads in a system. In this study, comparisons are made between the time-averaged values obtained using this sensor and detailed surface measurements using the transient thermal liquid crystal technique. The time-averaged heat flux sensor and transient TLC results showed very good agreement, validating both methods. Time-resolved measurements were also corroborated with hot film measurements at the wall at the location of the sensor to better clarify the influence of unsteadiness in the velocity field at the wall on fluctuations in the heat flux. These measurements resulted in turbulence intensities of the velocity and heat flux of 20%. The velocity and heat flux integral length scales were about 60% and 35% of the channel width, respectively, resulting in a turbulent Prandtl number of 1.7 at the wall.

Time-Resolved Heat Transfer Measurements on the Tip Wall of a Ribbed Channel Using a Novel Heat Flux Sensor: Part I — Sensor and Benchmarks

A novel heat flux sensor was tested which allows for time-resolved heat flux measurements in internal ribbed channels related to the study of passages in gas turbine blades. The working principle of the Atomic Layer Thermopile (ALTP) sensor is based on a thermoelectric field created by a temperature gradient over an YBCO crystal (the transverse Seebeck effect). The sensors very fast frequency response allows for highly time-resolved heat flux measurements up to the 1 MHz range. This paper explains the design and working principle of the sensor, as well as the benchmarking of the sensor for several flow conditions. For internal cooling passages, this novel sensor allows for highly accurate, time-resolved measurements of heat transfer coefficients, leading to a greater understanding of the influence of fluctuations in temperature fields.Copyright

Aerodynamic and acoustic analysis of an extruded airfoil with a trailing edge device using Detached Eddy Simulation with a Discontinuous Galerkin method

In this paper the Discontinuous Galerkin (DG) method is applied for the simulation of an airfoil with a small trailing edge device (TED). The method discretizes the Reynoldsaveraged Navier-Stokes (RANS) equations including the Spalart-Allmaras turbulence equation, which can be extended easily to a Detached Eddy Simulation (DES) model. As the DG method is a high-order method, the turbulent length scale is adapted with the polynomial degree of the basis function to take the cell interior into account. This adaptation is validated on two examples, a backward facing step and a circular cylinder. The acoustic evaluation of the TED case is carried out in a post-processing step with a Ffowcs WilliamsHawkings (FW-H) method. A comparison to wind tunnel tests and a standard finite volume (FV) method for both the aerodynamic results and the acoustic results is presented.