Rolf Engler

Pressure sensitive paint systems for pressure distribution measurements in wind tunnels and turbomachines

We have used the pressure-sensitive paint (PSP) intensity and lifetime methods for basic research and PSP measurements in wind tunnels and turbomachines, to investigate and understand the qualitative and quantitative aerodynamic measurements mainly in transonic flow. We performed a number of investigations in different speed ranges from transonic to low-speed flow and compared them with conventional techniques like pressure taps and light sheets. The influence of errors was checked and a comparison with numerical methods performed. Various models were investigated, from the basic configuration of a double-delta-wing up to a complex Airbus A340 half model and oscillating turbine blades. Finally, two excellent PSP systems are now available to perform precise measurements and support the theory using these techniques.

High Reynolds number transition detection by means of temperature sensitive paint

Simulating true flight Reynolds numbers with scaled models requires the use of cryogenic wind tunnels. Transition detection in ‘warm’ wind tunnels can be realized using commercially available IR cameras. In cryogenic testing, IR imaging becomes more difficult because of the reduction in radiated energy and the shift to longer wavelengths. Therefore, the temperature sensitive paint method has become a promising alternative here. Transition detection measurements were carried out using different models in several cryogenic wind tunnels, namely the European Transonic Windtunnel, the pilot facility of the European Transonic Windtunnel, the cryogenic wind tunnel in cologne and the cryogenic Ludwieg Tube facility of in Gottingen. With the temperature sensitive paint method, similar information can be obtained as for IR imaging in warm wind tunnels, but with higher spatial resolution. This means, that one can determine the instability leading to transition, namely Tollmien-Schlichting or crossflow instabiltiy, observe laminar separation bubbles (and turbulent reattachment), detect flow separation and locate shocks as well as vortex signatures.

Application of Pressure-Sensitive Paint for Determination of Dynamic Surface Pressures on a Rotating 65° Delta Wing and an Oscillating 2D profile in Transonic Flow

Visualization and measurements of aerodynamic effects on a delta-wing model and a 2D-wing-profile model were conducted using an optical pressure measurement system, based on the pressure-sensitive paint (PSP) technique. The PSP technique can be used to obtain absolute pressure measurements on the surface of a model and in addition to evaluate quantitative aerodynamic flow phenomena by using scientific grade cameras and image processing techniques. The PSP technique has been used here for investigations of periodic and unsteady flows: first, a 65deg delta wing was tested in the transonic wind tunnel DNW-TWG in Gottingen. A specially designed roll apparatus enabled roll rates up to 10 Hz. The experiments were carried out at angles-of-attack up to alpha = 17deg at Ma = 0.8. Since the rotation of the delta wing is a periodic motion, the phase-locked unsteady PSP technique can be applied. In a second wind tunnel campaign in the DNW-TWG in collaboration with the DLR Institute of Aeroelasticity, a 2D-wing-profile model, which is pitch oscillating at up to 30 Hz, was investigated. The experiments were performed at angles-of-attack alpha = 1.12deg plusmn 0.6deg at Ma = 0.72. For these experiments pressure measurements were carried out in one wind tunnel entry by means of both phase-locked unsteady as well as unsteady PSP techniques.

Quantitative Wind Tunnel Studies Using Pressure- and Temperature Sensitive Paints

The pressure sensitive paint (PSP) intensity and lifetime system is an optical measurement technique to investigate absolute pressure fields on model surfaces for basic research in laboratories, industrial wind tunnels or high speed rotating turbo machines. Detailed qualitative and quantitative information and understanding of flow phenomena can be obtained in speed ranges from U∞=20 m/s up to Ma=5.0. A number of projects of industrial interest has been investigated in different wind tunnels covering low speed, transonic, trisonic and cryogenic facilities. The influence of the main error sources for the components of the PSP system have been checked. Comparison of experimental pressure fields obtained by means of PSP and the results of numerical calculations have been carried out. Different wind tunnel models ranging from basic configurations such as a cropped delta wing to a complex half model of a large propeller-driven transport aircraft with all flaps, rudders and shrouds, and rotating or oscillating models as well as Reynolds number effects on models have been investigated.

Appropriate Selection of Pressure-Sensitive Paint for Cryogenic Wind Tunnels

The appropriate combination of a luminophore and a binder for Pressure-Sensitive Paint (PSP) for a cryogenic wind tunnel has been investigated. The sensitivity of a PSP is governed by the luminescence lifetime of a luminophore, the oxygen permeability of a binder and the oxygen partial pressure in a test gas. Test conditions in cryogenic wind tunnels (oxygen concentrations: 0.1% or less, temperatures: 100 – 180 K) are greatly different from those in “warm” wind tunnels (oxygen concentrations: 21%, temperatures: 300-330 K). Therefore, PSPs for a cryogenic wind tunnel demand severe requirements for a luminophore and a binder. In this paper, we studied the necessary conditions of PSP for cryogenic wind tunnels and evaluated them experimentally.

Development of PSP Technique for Application on the VFE-2 65° Delta Wing Configuration

For the International Vortex Flow Experiment 2 (VFE-2) new wind tunnel tests on a NASA delta-wing model by using modern non-intrusive measurement techniques have been performed in the DNW-TWG (Gottingen) transonic wind tunnel. In order to obtain a good understanding of the flow around the wind tunnel model the PSP (Pressure Sensitive Paint) technique has been applied. The PSP technique can be used to realize absolute pressure measurements on the whole surface of a model using scientific grade cameras and advanced image processing techniques. By means of the planar pressure information of the flow topology can be investigated in more detail than would be possible with data of conventional pressure taps. Surface pressures on the entire surface of the model have been obtained using DLRs mobile PSP system, viewing from top and bottom by means of four CCD-cameras. Finally, the measured pressures have been integrated to calculate forces and moments of the full model.

PSP Measurements on a Linear Plug Nozzle: Aerodynamic Investigations on Linear Plug Nozzle Configurations

Pressure field measurements on a Linear Plug Nozzle model were conducted to evaluate the feasibility and accuracy of measurements using DLR’s Pressure Sensitive Paint (PSP) system in the high-speed wind tunnel at Merkers (HWK) of TU-Dresden. A part of the nozzle surface was coated with binary PSP paint. Results show that the optical pressure measurement system with binary paint can be used in the HWK facility. In addition, the paper gives a summary of the thrust vector calculations based on measured static pressure distributions on the plug surface using thrust vector control.

Visualization of Aerodynamic Effects on a Double-delta Wing Aircraft Model using Pressure Sensitive Paint (PSP) Technique

Visualization of aerodynamic effects on a three-dimensional double-delta wing aircraft model was conducted using an optical pressure measurement system, based on the Pressure Sensitive Paint (PSP) technique, and in addition a laser-light sheet method. The combination of PSP technology with the laser-light sheet method, provides a good understanding of the flow around the wind tunnel model. In recent years, this novel PSP-technology has attracted considerable attention in the aerospace community. The PSP technique can be used to realize absolute pressure measurements on a surface of a model and in addition to evaluate quantitative aerodynamic flow phenomena using a scientific grade camera and image processing techniques. The PSP system was tested in the Transonic Wind Tunnel of the German Aerospace Center in Göttingen (TWG) under real flow conditions. Instantaneous pressure distributions are recorded in almost real-time so that the recognition and analysis of the vortex dynamics on the model surface is possible. Even the vortex breakdown process, as well as the fine structured Kelvin-Helmholtz instabilities and secondary vortex structures, can be detected by this measurement technique.

Surface Pressure and acoustic Flow Field Measurements above a Delta Wing to determine Circulation

The observations of the travel-time changes of ultrasonic pulses passing through the vortices, generated by inclined wing profiles in wind tunnels have provided information on the particular flow configurations (e.g., separated flow and related recirculating flow regions) and data on turbulence and the circulation of vortices moving downstream [3] [4]. Based on the results of these experiments, an acoustical circulation meter has been created at the DLR Gottingen. This circulation meter used in the low-speed wind tunnel regime measured steady and periodic vortex fields in the wakes of various wings. Data obtained from downstream vorticity measurements were used to predict lift distributions. In order to evaluate the measured data, theoretical curves computed on the basis of a Hamel-Oseen vortex model [1] were optimally fitted to the primary travel-time curves. Thus the most significant parameters of the investigated vortices were obtained: vortex location, vortex core radius and circulation or the maximum circumferential velocity, respectively. For all these applications, the acoustical measuring technique provided the following capabilities:

Application of the direct PSP-lifetime method on pyren-based paint as a completion of the existing DLR PSP system

Pressure Sensitive Paint (PSP) provides a pressure map on the whole surface of a wind tunnel model without transducers and tubing. In principle there are two different ways to gather experimental data-by using the fluorescent decay time of excited luminophore molecules (lifetime method) or by detecting its integrated fluorescent emission (intensity method). During the last five years PSP measurements utilizing the intensity of the luminescence are performed at DLR with pyren-based paints. This paint is characterized by its low temperature sensitivity and high quantum efficiency-due to these qualities the lifetime technique is also developed for this paint type. PSP measurements utilizing the lifetime of the luminescence are independent of particles in the flow (dust, oil, condensed water, etc.), change in density, light source instabilities and light reflection and less sensitive to the model movements because there is no need for a reference image. The short luminescence decay time of pyren, of the order of 50 ns, gives the possibility to investigate fast flow processes (pressure measurements on helicopter blades, propellers, turbines, etc.).