Johannes Bosbach

Application of particle image velocimetry under cryogenic conditions

Cryogenic investigations of the trailing vortices of large future transport aircrafts have been performed on an aircraft half-model with different wing tip devices. The model has been installed in the cryogenic wind tunnel (KKK) of the German-Dutch wind tunnels (DNW) in Cologne. During the tests, PIV flow field measurements have been performed in addition to conventional force, moment, and pressure distribution measurements. The measurements of the velocity field gave an insight into important aerodynamic effects as, for example, the dependents of vortex strength with changing angles of incidence as well as the differences in the spatial vorticity distribution in the wake. The PIV technique offers special advantages for the investigation of those wakes at very low temperatures, but also yields to special experimental difficulties, which are also described in this article.

Simultaneous Measurement of Temperature and Velocity Fields in Convective Air Flows

Thermal convective air flows are of great relevance in fundamental studies and technical applications such as heat exchangers or indoor ventilation. Since these kinds of flow are driven by temperature gradients, simultaneous measurements of instantaneous velocity and temperature fields are highly desirable. A possible solution is the combination of particle image velocimetry (PIV) and particle image thermography (PIT) using thermochromic liquid crystals (TLCs) as tracer particles. While combined PIV and PIT is already state of the art for measurements in liquids, this is not yet the case for gas flows. In this study we address the adaptation of the measuring technique to gaseous fluids with respect to the generation of the tracer particles, the particle illumination and the image filtering process. Results of the simultaneous PIV/PIT stemming from application to a fluid system with continuous air exchange are presented. The measurements were conducted in a cuboidal convection sample with air in- and outlet at a Rayleigh number Ra ? 9.0 ? 107. They prove the feasibility of the method by providing absolute and relative temperature accuracies of ?T = 0.19?K and ??T = 0.06?K, respectively. Further open issues that have to be addressed in order to mature the technique are identified.

Flow Structure Formation of Turbulent Mixed Convection in a Closed Rectangular Cavity

An experimental investigation of flow structure formation in turbulent mixed convection in a closed rectangular cavity with an aspect ratio of 1:1:5 and air as working fluid is presented. Mixed convection at Re = 1.1·104 and Ra = 3.0·108 is studied under well-defined conditions by combination of forced and thermal convection. The resulting flow structures strongly depend on the ratio of inertia and buoyancy forces. A 2D mean wind, which can be approximated by a solid body rotation, is found at pure forced convection. With increasing Archimedes number (Ar), realized by a temperature gradient between bottom and ceiling of the convection cell, this structure becomes instable. Leading to four convection rolls for Ar = 3.4, which are oriented in longitudinal direction of the cell, are observed.

Development of combined particle image velocimetry and particle image thermography for air flows

Simultaneous measurements of instantaneous velocity and temperature fields of air flows by means of Particle Image Velocimetry (PIV) and Particle Image Thermography (PIT) enables highly demanded studies on thermal plumes, their dynamics and the resulting heat transfer for Pr ≈ 0.7. Thereby, small particles of thermochromic liquid crystals (TLCs), which reveal temperature depending reflection properties are used as tracer particles for combined PIT and PIV. The feasibility of the method is demonstrated in a Rayleigh-Benard convection experiment in a cubical enclosure. Furthermore, a new particle generator being able to produce continuously very small monodisperse droplets of TLCs has been designed. The improvement of the developmental process for mixed and Rayleig-Benard convection studies is discussed. Thereby, special focus is laid on the production process of small TLCs, the generation of monodisperse acetone-TLC droplets and the temperature depending colour play of the produced particles.Copyright

Flight Testing of Alternative Ventilation Systems for Aircraft Cabins

Cabin displacement ventilation (CDV) has been evaluated for the first time in a real passenger aircraft cabin under flight conditions. Two ventilation systems, i.e. pure CDV and a hybrid ventilation (HV) system, which provides 30 % of the total volume flow rate through the lateral mixing ventilation outlets, were tested in an A320 aircraft. While pure CDV was found to provide low air velocities, high heat removal efficiencies and a very good dynamic performance regarding control of the air temperatures, the hybrid system, still allowing for comfortable flow velocities and good heat removal efficiencies, was shown to significantly improve the cooling and heating rates at the cabin surfaces. With both systems, the local temperature in the passenger zone is distributed very homogeneously among the investigated seat and aisle positions.

Oscillations of Large-Scale Structures in turbulent Mixed Convection in a rectangular enclosure

The term mixed convection (MC) is used to describe the process of heat transfer in fluids where forced convection (FC) and thermal convection (TC) coexist. Mixed convection is an often occurring flow condition e.g. in the oceans, atmosphere, indoor climatisation or industrial processes and applications [1]. In many flow situations convection is the prevalent transport mechanism of heat whereas the heat transfer strongly depends on the dynamics of the largescale structures. In this study we investigate the formation of large-scale circulation (LSC) in mixed convection and the influence of the dynamics of the LSC, also known as mean wind, on the heat transfer.

Transient temperature fields of turbulent mixed convection in an aircraft cabin caused by a local heat source

The impact of a localized, transient head load on the temperature fields in aircraft cabin air flows was investigated in the Do 728 test bed of the German Aerospace Center. Surface and fluid temperatures were analyzed using more than 340 local sensors as well as a grid of polystyrene spheres, which was imaged by an infrared camera. To simulate the impact of the real passengers, thermal passenger dummies were used. The spatial spreading of the heat released by the local source could be characterized for the state of the art mixing ventilation system (MV—mixing ventilation) and a novel ventilation system (CCDV—ceiling based cabin displacement ventilation), which provides fresh air at low momentum through the cabin ceiling. As expected, CCDV provides lower cabin temperatures due to its higher heat removal efficiency. While MV tends to accumulate the heat released by the local source in the passenger zone, warm air is able to ascend to the ceiling more easy at CCDV due to the lower inertia forces. The different amount of inertia forces in the flow between MV and CCDV has a measureable impact on the temporal behavior of the resulting flow patterns during operation of the heat source, which is well reflected in the fluctuations of the temperature elevations.

Temperature Oscillations in Turbulent Mixed Convective Air Flows

We report on measurements of temperature oscillations observed in turbulent mixed convection in a rectangular enclosure. They are correlated to fluctuations and spontaneous reorientations of the large-scale circulations in the flow. Stable temperature configurations, periodic reconfigurations as well as spontaneous reversals and reorientations were detected. We ascribe the observed dynamics of the large-scale flow to the interaction between the superimposed shear forces of the forced convective flow and the buoyancy driven flow. Comparison with smoke visualisations allowed to assign the observed temperature distributions to a certain number and configuration of convection rolls. A frequency analysis of the oscillations as a function of the characteristic numbers is conducted revealing a sophisticated dependency of the dynamics of the large-scale flow on Ar, Re or Ra.

Large-scale Coherent Structures in Turbulent Mixed Convective Air Flow

We report on an experimental study of mixed convective air flow in a cuboidal container with an inlet and outlet slot at \(\mathcal {A} r = 3.3\), \(\mathcal {R} e= 1.0 \times 10^4\), \(\mathcal {R} a= 2.4 \times 10^8\) at a fluid pressure of \(P = 11.6\,{\mathsf {bar}}\). Particle image velocimetry were performed with the objective to identify the dynamics of the large-scale circulations. In addition, we applied temperature measurements to determine the heat transport. Due to the unsteady nature of mixed convection in the present configuration, the velocity vector fields were subjected to a proper orthogonal decomposition in order to extract the predominant coherent structures. The analysis uncovers two structures consisting of three and four thermally induced large-scale circulations, which are arranged in longitudinal direction. Further, we found that the dynamics and the topology of the convection rolls strongly influences the heat transport between the container‘s inlet and outlet.

In-Flight Infrared Thermography for Studies of Aircraft Cabin Ventilation

To improve the convenience of aircraft cabins, Cabin Displacement Ventilation (CDV) was investigated during flight tests in an Airbus A320 with an automatically rotatable infrared camera. For this purpose a programmable, step motor driven infrared camera setup was developed, allowing for time resolved acquisition of the temperatures on the interior cabin surfaces during the whole flights. From the temporal development of the surface temperatures the cooling and heating performance of pure CDV as well as a hybrid system (HV), where 30 % of the fresh air was supplied through the original lateral outlets, was analyzed and characterized. Static measurements reveal a characteristic, yet homogeneous temperature distribution in the cabin for both scenarios, pinpointing to a homogeneous cooling of the heat loads in the whole cabin. In order to study the performance of CDV and HV, dedicated “pull-up” and “pull-down” scenarios, those are abrupt changes of the inflow temperature with the objective to study the heating and cooling dynamics, were conducted during the flight tests. Analysis of time resolved measurements discloses that the cooling performance of the cabin is limited by thermal diffusion of heat inside of the interior cabin materials.