Claus Christian Wolf

German Aerospace Center (DLR) | DLR · Department of Helicopters (HE)

In modern high-fidelity computational fluid dynamic simulations, the primary vortex system in hover often breaks down into secondary vortices. The sources of numerical error influencing the prediction of the vortex system were studied by performing high-fidelity simulations of the wake of a two-bladed rotor and comparing the predictions to stereosc...

An experimental aerodynamic study was conducted, analyzing the wake structure of a four-bladed model rotor with a forward-backward swept tip geometry inspired by the ONERA-DLR “Etude d´un Rotor Aéroacoustique Technologiquement Optimisé”-design. Similar tip designs are used on some modern helicopter main rotors. The experiments were conducted at the...

This article describes recent tests and developments of imaging and evaluation techniques for particle image velocimetry (PIV) that exploit the forward scattering of tracer particles by placing the camera in-line with the illuminating light source, such as a laser or a light emitting diode. These techniques have been in use for some time in microsc...

Constant temperature anemometry and background-oriented schlieren (BOS) measurements on a CH-53G helicopter (take off mass n ≈ 14t) are used to investigate the rotor wake including the vortex breakdown to statistical turbulence. Hovering flights at a range of hub heights 0:50 < h < 0:83 were investigated. The position of the rotorcraft was opticall...

The study presents the aeroelastic behavior of a double-swept rotor blade, that was investigated under climb conditions on a four-bladed rotor in the rotor test facility at the German Aerospace Center in Göttingen. The results of two measurement campaigns are combined to obtain a detailed insight in the coupling of blade deformation, integral loads...

A common phenomenon in modern high-fidelity CFD simulations is the breakdown of the primary vortex system in hover due to secondary vortex braids. In the current work, the development of secondary vortex structures in the wake of a two-bladed rotor was investigated by combining stereoscopic particle image velocimetry measurements in different measu...

The lift and drag forces of a Flettner rotor at a Reynolds number of about 800 are experimentally investigated, resulting in the first force measurements on Flettner rotors at Reynolds numbers being this low. Experiments comprise ratios of circumferential rotor velocity to freestream velocity of 0 ≤ α ≲ 4. The instrumentational facility consists of...

An experimental aerodynamic study was conducted, analyzing the wake structure of a four-bladed model rotor with a forward-backward swept tip geometry inspired by the ONERA-DLR "ERATO" design. Similar tip designs are used on some modern helicopter main rotors. The experiments were conducted at the rotor test stand Göttingen in hover-like conditions...

A method of taking background oriented schlieren technique (BOS) images using projected backgrounds is shown. The projected backgrounds can be scaled to the measurement requirements, providing more flexibility than printed backgrounds. The evaluation of the BOS image pairs using cross correlation or optical flow analysis was shown to produce equiva...

A method of taking background oriented schlieren technique (BOS) images using projected backgrounds is shown. The projected backgrounds can be scaled to the measurement requirements, providing more flexibility than printed backgrounds. The evaluation of the BOS image pairs using cross correlation or optical flow analysis was shown to produce equiva...

The blade tip vortex system is a crucial feature in the wake of helicopter rotors, and its correct prediction represents a major challenge in the numerical simulation of rotor flows. A common phenomenon in modern high-fidelity CFD simulations is the breakdown of the primary vortex system in hover due to secondary vortex braids. Since they are stron...

The main rotor wakes of the free-flying DLR test helicopters Airbus Bo105 and EC135 were investigated in ground effect during hover, vertical take-off, and forward flight. A high–speed schlieren system tracked the blade tip vortices at about 60 images per revolution. In addition, a constant temperature anemometry-system utilized arrays of fiber–fil...

The wake of its main rotor plays a key role in the overall performance and operational safety of a helicopter. The flow is governed by complex and unsteady features, such as the blade shear layers and blade tip vortices. In recent years, experimental measurement techniques have been developed and refined for this application, to keep up with an inc...

This talk will provide an overview of a well-defined reference data set for code calibration of boundary layer transition on a trimmed, scaled helicopter rotor in forward flight. The boundary layer transition was measured using differential infrared thermography (DIT) on the top (suction) side of the three-bladed NASA/Army "PSP rotor" at an advance...

An investigation into blade tip vortices of a sub-scale rotor under cyclic pitch conditions is carried out. Background oriented schlieren (BOS), particle image velocimetry (PIV), and computational fluid dynamics (CFD) are applied to the same test cases. This approach allows to combine the velocity data from PIV in a measurement plane, the density r...

Driven by the increasingly rapid progress of laser, camera and computer technology, several optical techniques used for optical flow diagnostics developed rapidly and did not only offered more potential for lab applications, but were more and more frequently applied in large industrial wind tunnel facilities and during full-scale flight tests. Part...

The blade tip vortex system is a crucial feature in the wake of helicopter rotors, and its correct prediction represents a major challenge in the numerical simulation of rotor flows. A common phenomenon in modern high-fidelity CFD simulations is the breakdown of the primary vortex system in hover due to secondary vortex braids. Since they are stron...

An investigation on blade tip vortices of a sub-scale rotor under cyclic pitch conditions is carried out. Background oriented schlieren (BOS), Particle image velocimetry (PIV), and computational fluid dynamics (CFD) are applied to the same test cases. This approach enables to combine the velocity data from PIV in a measurement plane, the density re...

A well-defined reference set of data for CFD and comprehensive code validation for a scaled helicopter main rotor with boundary layer transition in forward flight is presented. The boundary layer transition was measured using Differential Infrared Thermography (DIT) on the top (suction) side of the NASA/Army “PSP rotor” in the NASA Langley 14- by 2...

The main rotor wake of free–flying helicopters in ground effect was investigated during a measurement campaign with the DLR test rotorcraft Airbus Bo105 and EC135. An advanced measurement system combined high–speed schlieren imaging with point–wise constant temperature anemometry for a comprehensive analysis of the flow. In hover conditions, the wa...

A well-defined reference set of data for computational fluid dynamics and comprehensive code validation for a scaled helicopter main rotor with boundary layer transition in forward flight is presented. The boundary layer transition was measured using differential infrared thermography (DIT) on the top (suction) side of the NASA/Army “PSP rotor” in...

The main rotor wakes of the free-flying DLR test helicopters Airbus Bo105 and EC135 were investigated in ground effect during hover, vertical takeoff, and forward flight. A high-speed schlieren system tracked the blade tip vortices at about 60 images per revolution. In addition, a constant temperature anemometry system utilized arrays of fiber film...

Infrared thermography has been successfully adopted in the field of flow diagnostics over the last decades. Detecting the laminar{turbulent boundary layer transition through variations in the convective heat transfer is one of the primary applications due to its impact on the aerodynamic performance. Recent developments in fast{ response infrared c...

The vortex system of four rotating and pitching DSA-9A blades was examined numerically and experimentally. Numerical computations were performed using German Aerospace Center (DLR)’s finite-volume solver TAU and were validated against experimental data gathered using particle image velocimetry carried out at the rotor test facility (RTG) in Götting...

It is investigated whether a nonrotating lifting fan remaining uncovered during cruise flight, as opposed to being covered by a shutter system, can be realized with limited additional drag and loss of lift during cruise flight. A wind-tunnel study of a wing-embedded lifting fan has been conducted at the Side Wind Test Facility Göttingen of DLR, Ger...

Measurements in the wake of a free-flying full-scale helicopter in ground effect were performed for both quasi-steady and unsteady maneuvering flights using stereoscopic particle image velocimetry (PIV), a time-resolved background-oriented schlieren (BOS) setup, and an optical marker tracking technique. The systems were used in a complementary way...

In the published version, graphical abstract was missing. It is given below.

To characterize the vortex system of a helicopter rotor with cyclic pitch, an experimental investigation is carried out at the rotor test facility of the DLR, German Aerospace Center in Göttingen. A test case with attached flow conditions is chosen to study the blade tip vortex aerodynamics on a four-bladed rotor. The resulting azimuth-dependent vo...

The presented work tackles the lack of experimental investigations of unsteady laminar-turbulent boundary-layer transition on rotor blades at cyclic pitch actuation, which are important for accurate performance predictions of helicopters in forward flight. Unsteady transition positions were measured on the blade suction side of a four-bladed subsca...

A method of measuring density second derivative (shadowgraph) as a variant of background oriented schlieren (BOS) is presented. Pairs of images of a moving background with a small time difference are analysed using moving-window cross-correlation, giving the second derivative of density in the axis of background travel. The advantage over standard...

A new approach to measuring unsteady boundary layer transition in periodic processes with infrared thermography is introduced. The radiation from the heated suction surface of a pitching airfoil model is measured with an infrared camera. The extraction of the extrema of the measured radiation signal at fixed model locations yields instants of the m...

Boundary layer transition measurement was demonstrated using differential infrared thermography (DIT) on the top side of a helicopter rotor in forward flight, which detects the difference in the convective heat transfer at the boundary layer transition position. The tests used a FLIR X8500xc SLS long wave infrared camera to observe the DLR EC135 te...

The operation and aerodynamic performance of a helicopter rotor is strongly affected by the structure of its wake, in particular regarding vortex–vortex interactions of hovering rotors. Rotor simulations using modern computational methods have the potential to capture high levels of detail, which recently triggered discussions of secondary vortex b...

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...

The aerodynamic performance of a helicopter rotor is strongly affected by the structure of its wake. Rotor wake simulations using modern computational methods have the potential to capture a high level of detail, which recently triggered discussions of secondary vortex braids occurring in the shear layer stretching between the primary tip vortices....

The presented work tackles the lack of experimental investigations of unsteady laminar-turbulent boundary-layer transition on rotor blades at cyclic pitch actuation, which are important for accurate performance predictions of helicopters in forward flight. Unsteady transition positions were measured on the blade suction side of a four-bladed subsca...

The helicopter group at the DLR in Göttingen has been actively involved in the development of measurement techniques for unsteady flows, particularly as they apply to the problems found in unsteady rotor blade aerodynamics. This includes the development and validation of new techniques for the detection of dynamically moving boundary layer transiti...

An investigation was performed into the effect of positive and negative sweep angle on the boundary layer transition and dynamic stall behavior of a finite wing. The finite wing had a 6:1 aspect ratio, modern (SPP8) tip shape, and positive twist, moving the maximum load on the wing away from the wind tunnel wall. Experiments were performed with swe...

An investigation was performed into the effect of positive and negative sweep angle on the boundary layer transition and dynamic stall behaviour of a finite wing. The finite wing had a 6:1 aspect ratio, modern (SPP8) tip shape and positive twist, moving the maximum load on the wing away from the wind tunnel wall. Experiments were performed with swe...

Measurements in the wake of a free-flying full-scale helicopter in ground effect were performed for both quasi-steady and unsteady maneuvering flights using stereoscopic particle image velocimetry (PIV), a time-resolved backgroundoriented Schlieren (BOS) setup, and an optical marker tracking technique. The systems were used in a complimentary way t...

The vortex system of four rotating and pitching DSA-9A blades was examined numerically and experimentally. Numerical computations were performed using DLR's finite-volume solver TAU and were validated against experimental data gathered using particle image velocimetry (PIV) carried out at the rotor test facility in Gottingen (RTG). Algorithms deriv...

Boundary layer transition measurement was demonstrated using differential infrared thermography on the top side of a helicopter rotor in forward flight. The tests used a FLIR X8500xc SLS long wave infrared camera to observe the DLR EC135 test helicopter rotor. The boundary layer transition was detected for hover out of ground effect (150 ft) and fo...

The tip vortex system downstream of a four-bladed instrumented rotor was investigated experimentally through the application of stereoscopic particle image velocimetry (PIV). A dynamic stall test case was facilitated by a high cyclic pitch setting of the swashplate, with additional attached-flow and constant-pitch test cases for comparison reasons....

The main interest of today’s research in fluid mechanics is more and more directed to problems where unsteady and separated flows are predominant. For investigations of flow fields with pronounced spatial structures and/or rapid temporal or spatial changes (transition from laminar to turbulent flow, coherent structures, pitching airfoils in transon...

To characterize the vortex system of a rotor with collective and cyclic pitch settings, an experimental investigation was carried out at the rotor test facility (RTG) of the German Aerospace Center (DLR) in Gottingen. In addition to former studies based on velocity fields from particle image velocimetry (PIV) in the current measurement also a backg...

Differential infrared thermography (DIT) is a method of analyzing infrared images to measure the unsteady motion of the laminar–turbulent transition of a boundary layer. It uses the subtraction of two infrared images taken with a short-time delay. DIT is a new technique which already demonstrated its validity in applications related to the unsteady...

Boundary layer transition measurement was demonstrated using differential infrared thermography (DIT) on the top side of a helicopter rotor in forward flight, which detects the difference in the convective heat transfer at the boundary layer transition position. The tests used a FLIR X8500xc SLS long wave infrared camera to observe the DLR EC135 te...

A new approach to measuring unsteady boundary layer transition in periodic processes with infrared thermography is introduced. The radiation from the heated suction surface of a pitching airfoil model is measured with an infrared camera. The extraction of the extrema of the measured radiation signal at fixed model locations yields instants of the m...

This study describes time-resolved large-scale volumetric flow measurement of a helicopter model (1.55 m rotor-span, cord length = 61 mm) in close-to-ground condition (height = 0.5 D) at Reynolds number 4.1 x 10^5 (rotational frequency = 20.83 Hz) to examine CFD predicted secondary vortex structures (originating from primary rotor-tip vortices), as...

Differential infrared thermography (DIT) is a method of analyzing infrared images to measure the unsteady motion of the laminar-turbulent transition of a boundary layer. It uses the subtraction of two infrared images taken with a short time delay. DIT is a new technique which already demonstrated its validity in applications related to the unsteady...

An aerodynamic study on efficiency improvement of a wing embedded lifting fan with a diameter of 0.36 m remaining open and not closed by doors in cruise flight has been conducted. Two of these fans are used to enable an unconventional unmanned transport aircraft of 16 kg maximum take-off mass and almost 2 m wing span to take-off and land vertically...

The boundary layer transition detection technique by thermography has been extended to a wind tunnel model simulating helicopter forward flight conditions. The helicopter blade is a generic NASA RC series airfoil 66.75 in (1.7 m) span with a 5.45 in (13.8 cm) chord as a part of a three-blade system. Testing was done at the NASA Langley 14- x 22-foo...

The tip vortex–system downstream of a four–bladed instrumented rotor was investigated experimentally through the application of stereoscopic particle image velocimetry (PIV). A dynamic stall test case was facilitated by a high cyclic pitch setting of the swashplate, with additional attached–flow and constant–pitch test cases for comparison reasons....

The original version of this article unfortunately contained mistakes. Due to typesetting errors several expressions were not displayed correctly.

Numerical computations on a finite wing are carried out using DLR’s finite-volume solver TAU. The tip-vortex characteristics during static stall and deep dynamic stall are analyzed and compared to particle image velocimetry (PIV) measurements carried out in the side wind facility Göttingen. Computational fluid dynamics (CFD) and experiment are in g...

An investigation was performed into the effect of positive and negative sweep angle on the boundary layer transition and dynamic stall behaviour of a finite wing. The finite wing had a 6:1 aspect ratio, modern (SPP8) tip shape and positive twist, moving the maximum load on the wing away from the wind tunnel wall. Experiments were performed with swe...

A proof of concept test to measure the unsteady boundary layer transition locations on the lower surface of a Mach-scaled rotor in forward flight was performed during the Summer of 2017 in the NASA Langley 14- by 22-Foot Subsonic Tunnel. The transition locations were measured using high-speed infrared thermography with a rotating mirror assembly th...

The main interest of today’s research in fluid mechanics is more and more directed to problems where unsteady and separated flows are predominant. For investigations of flow fields with pronounced spatial structures and/or rapid temporal or spatial changes (transition from laminar to turbulent flow, coherent structures, pitching airfoils in transon...

Dynamic stall is a major concern for highly loaded helicopter rotors in fast forward flight. The potential of a back-flow flap for dynamic stall reduction is investigated. The flap assembly is mounted on the suction side of a helicopter main rotor-blade airfoil undergoing deep-stall pitch oscillations. Wind-tunnel experiments using high-speed parti...

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 th...

The background-oriented schlieren (BOS) imaging method has, for the first time, been applied in the investigation of the flow around a circular cylinder at low Mach numbers (\(M<0.1\)). The measurements were conducted in the high pressure wind tunnel, Göttingen, with static pressures ranging from 0.1 MPa to 6.0 MPa and covered a range of the Reynol...

Numerical computations on a finite wing are ca rried out using DLR's fin ite-volume solver TAU. The tip-vortex characteristics during static stall and deep dynamic stall are analyzed and compared to particle image velocimetry (PIV) measurements carried out in the side wind facility Göttingen. Computational flu id dynamics (CFD) and experiment are i...

A back-flow flap attached to the suction side of an airfoil is investigated in both passively and actively actuated modes for the control of dynamic stall. This method of dynamic stall control has low power requirements and no parasitic drag when not actuated. Experiments in a low-speed wind tunnel at 50 m/s were used to characterize the reduction...

A new method has been applied for the detection of dynamic stall on rotating blades with cyclic pitch variation. The method works robustly for carbon-fiber composite blades and does not require contact with the surface or any kind of surface treatment or on-blade instrumentation. It can produce maps of the rotor plane where areas of stalled and att...

An experimental investigation of static and dynamic stall on a rotor blade tip model with an aspect ratio of 6.2 at a chord Reynolds number of 900,000 and a Mach number of 0.16 is presented. The spatiotemporal behavior of the flow separation was analyzed using unsteady surface pressure measurements and high-speed particle image velocimetry. The max...

The main interest of today's research in fluid mechanics is more and more directed to problems where unsteady and separated flows are predominant. For investigations of flow fi elds with pronounced spatial structures and/or rapid temporal or spatial changes (transition from laminar to turbulent flow, coherent structures, pitching airfoils in transo...

A method for the reconstruction of 3D density distributions of blade tip vortices is presented. The reconstruction is performed using a filtered back-projection on 2D density gradient data from a high-speed LS-BOS (laser-speckle-illuminated background-oriented Schlieren) setup, captured during the STAR (smart-twisting active rotor) project. Density...

The potential of a back-flow flap for dynamic stall reduction is investigated. The flap assembly is mounted on the suction side of a helicopter airfoil undergoing deep-stall pitch oscillations. Wind-tunnel experiments using high-speed particle image velocimetry were conducted to identify the flow topology and to investigate the flap's method of ope...

A new method of detecting flow separation for static and pitching airfoils is described, with application to the generation of stall maps for helicopter rotors. An airfoil is heated using a lamp and a high-speed infrared camera monitors the surface temperature. Subtracting consecutive images and performing a spatial standard deviation over a region...

A new method of detecting flow separation for static and pitching airfoils is described, with application to the generation of stall maps for helicopter rotors. An airfoil is heated using a lamp, and a high-speed infrared camera monitors the surface temperature. Subtracting consecutive images and performing a spatial standard deviation over a regio...

The tip vortex of a finite rotor blade-tip model undergoing pitch oscillations is investigated by means of a wind-tunnel study. The motion and flow parameters as well as the model geometry were adapted to rotorcraft applications, including test cases with fully attached flow, light dynamic stall, and deep dynamic stall. High-speed particle image ve...

Flow separation plays an important role in the aerodynamics of propellers and helicopter rotors. Although the ideal is to retain attached flow under all flight conditions, the main rotor of a helicopter sees partial separation under highly loaded forward flight or for flight conditions which include the blades intersecting their own wakes. The pres...

Hot film anemometry, surface pressure analysis, and differential infrared thermography (DIT) were simultaneously applied for the detection of unsteady boundary layer transition on a pitching helicopter main rotor blade airfoil at M = 0.30 and Re = 1.8e6. Results are presented for test cases with periodic ramp motion and for cases with sinusoidal mo...

A back-flow flap attached to the suction side of an airfoil is investigated in both passively and actively actuated modes for the control of dynamic stall. Experiments in a low-speed wind tunnel at 50 m/s were used to characterise the reduction in dynamic stall using pressure measurements on the midline of an airfoil. The pitching moment peak is re...

Measurements on a pitching finite rotor blade tip are performed. Sectional forces obtained from surface pressure measurements show a significant difference in maximum loads and the extent of hystereses between a section near the parabolic blade tip and two sections further inboard. Different characteristics appear also in the flow topology over the...

The main interest of today’s research in fluid mechanics is more and more directed to problems where unsteady and separated flows are predominant. For investigations of flow fields with pronounced spatial structures and/or rapid temporal or spatial changes (transition from laminar to turbulent flow, coherent structures, pitching airfoils in transon...

Strömungsmesstechnik ist als Blockvorlesung für 7-11 März 2016 gesetzt worden. Termine am 7-9 März 2016 fanden in Clausthal statt (Zi 321), Termine am 10-11 März 2016 beim DLR in Göttingen. Inhalt: Montag 7.3.16 (TU Clausthal, ITM, Raum 321) 9:00-12:00: A.D. Gardner (Begrüßung, Grundlagen und Begriffe, Drucksonden, Druckmessgeräte) 13:30-15:00: A.D...

Hot film anemometry, surface pressure analysis, and differential infrared thermography(DIT) were simultaneously applied for the detection of unsteady boundary layer transition on a pitching helicopter main rotor blade airfoil at M = 0.30 and Re = 1.8E6. Results are presented for test cases with periodic ramp motion and for cases with sinusoidal mot...

The paper describes concept, design and realization of a wind tunnel model featuring an active back-flow flap for dynamic stall control. In order to investigate influence of a back-flow flap on the stall behavior of an airfoil with oscillating pitch angle, an existing wind tunnel model is retrofitted with a spoiler-type flap on the upper surface. T...

The unsteady boundary layer transition on the pitching helicopter main rotor blade airfoil DSA-9A was experimentally investigated by the use of hot film anemometry and unsteady pressure measurements. The unsteady flow characteristics on the upper and the lower side of the airfoil were analyzed for steady test cases and dynamic cases with sinusoidal...

An experimental investigation of static and dynamic stall on a rotor blade tip model with a parabolic tip geometry and aspect ratio 6.2 at a chord Reynolds number of 900,000 and a Mach number of 0.16 is presented. The resulting flow is analyzed based on unsteady surface pressure measurements and quantitative flow visualizations by high-speed particle...

The tip vortex of a finite wing undergoing pitch oscillations is investigated by means of a wind tunnel study. The motion and flow parameters as well as the model geometry were adapted to rotorcraft applications, including test cases with fully attached flow, light dynamic stall, and deep dynamic stall. High-speed Particle Image Velocimetry was use...

The main interest of today’s research in fluid mechanics is more and more directed to problems where unsteady and separated flows are predominant. For investigations of flow fields with pronounced spatial structures and/or rapid temporal or spatial changes (transition from laminar to turbulent flow, coherent structures, pitching airfoils in transon...

Noise and structural vibrations in rotorcraft are strongly influenced by interactions between blade–tip vortices and the structural components of a helicopter. As a result, knowing the three-dimensional location of vortices is highly desirable, especially for the case of full-scale helicopters under realistic flight conditions. In the current study...

Measurements on a pitching finite rotor blade tip are performed. Sectional forces obtained from surface pressure measurements show a significant difference in maximum loads and the extent of hystereses between a section near the parabolic blade tip and two sections further inboard. Different characteristics appear also in the flow topology over the...

A pulsed jet was investigated by means of the BOS techniques, using both standard and laser-speckle background patterns. It is observed that, in the current setup, the speckle technique tends toward higher background noise. For the future, a more detailed study on the pulse-to-pulse stability of the laser background illumination and its influence o...

Noise and structural vibrations in rotorcraft are strongly influenced by interactions between blade tip vortices and the structural components of a helicopter. As a result, the three-dimensional localization of vortices is highly desirable, especially for the case of full-scale helicopters under realistic flight conditions. In the current study, we p...

Interactions between blade tip vortices and the structural components of a helicopter play an important role in the generation of noise and structural vibrations. The detection of the exact vortex trajectories is highly desirable, therefore, especially for the case of full-scale helicopters under realistic flight conditions. The Background Oriented...