H. C. de Lange

On streak breakdown in bypass transition

Recent theoretical, numerical, and experimental investigations performed at the Department of Mechanics, KTH Stockholm, and the Department of Mechanical Engineering, Eindhoven University of Technology, are reviewed, and new material is presented to clarify the role of the boundary-layer streaks and their instability with respect to turbulent breakdown in bypass transition in a boundary layer subject to free-stream turbulence. The importance of the streak secondary-instability process for the generation of turbulent spots is clearly shown. The secondary instability manifests itself as a growing wave packet located on the low-speed streak, increasing in amplitude as it is dispersing in the streamwise direction. In particular, qualitative and quantitative data pertaining to temporal sinuous secondary instability of a steady streak, impulse responses both on a parallel and a spatially developing streak, a model problem of bypass transition, and full simulations and experiments of bypass transition itself are ...

Design optimization for fast heat-transfer gauges

Heat flux gauges at a flat plate are designed suited to measure the heat transfer in transition boundary layers. The measuring technique employed (cold thin films) is convenient for transient experimental facilities (such as a Ludwieg tube). The development of a well defined and optimized sensor design is discussed.

Modeling of a water vapor selective membrane unit to increase the energy efficiency of humidity harvesting

Air humidity is a promising source of clean and safe drinking water. However, in conventional systems a lot of energy is wasted on the production of cold air, rather than the condensation of water vapor. This study examines the possibility of using a hollow fiber membrane module to make this process more energy efficient, by separating the vapor from other gases, prior to the cooling process with the help of selective membranes. The water vapor concentration within a fiber has been modeled using a random walker approach, and the membrane permeability has been implemented as a re-bounce probability for simulation particles interacting with the membrane. Considering the additional work requirement for driving a feed flow through the membrane section and the computed water vapor permeation it could be shown that the energy demand per unit water is lowest for slow flow speeds and favors short and thin fibers. The total energy requirement was estimated to be less than half of the conventional one. Comparison with other CFD simulations and a real life module has shown a good level of agreement, indicating that a membrane section could improve the energy efficiency of humidity harvesting significantly.

The similarity of turbulent spots in subsonic boundary layers

The similarity of turbulent spots on the surface of a flat plate is shown by means of heat-flux measurements. The use of non-intrusive heatflux gauges makes it possible to track both the developement of single spots and to compare different spots in different experiments. The results proof that assumptions on the similarity of spots can be extended to these flows. Experiments are performed in a Ludwieg tube at intermediate Mach numbers (Ma = 0.33).

Effects of Compressibility and Turbulence Level on Bypass Transition

The influences of compressibility and turbulence level on boundary layer transition are studied using a Ludwieg tube set-up. Heat transfer measurements are performed on the flow over a flat plate. The Mach number is varied between 0.16 and 0.56 while the unit-Reynolds number is kept constant. Several turbulence generating grids are used giving turbulence levels between 1.2% and 4.4%. Increasing the Mach number results in a decreasing turbulence level. Besides, the transition start Reynolds number increases.The results indicate that besides the turbulence level another parameter is desired for the existing transition models. The dimensionless spot parameter is influenced by both the turbulence level and the Mach number. A tentative conclusion is that the start of transition depends on the inertial range minimum frequency in the energy spectrum while the shape of the transition curve, i.e. the intermittency, depends on the corresponding length scale.Copyright

Influence of Turbulence Intensity on Intermittency Model in By-Pass Transition

The influence of free stream turbulence intensity on boundary layer transition was studied for a weakly compressible flow along a flat plate. The test facility consisted of a Ludwieg tube in which values of Mach number, Reynolds number and free stream turbulence could be varied over the following ranges: 0.09 < M < 0.6, 5.105 < Reu/m < 1.107, 1.2% < Tu < 9%. In this paper the turbulence intensity was varied up to 4.0 %. Unsteady heat flux to the flat plate was measured using cold thin film gauges. From these measurements, the intermittency was computed using an integral technique. For turbulence intensities of 1.2 %, the intermittency distribution is somewhat below the Narasimha and Johnson model, whereas a good agreement is obtained for a free stream turbulence intensity of 4.0 %. The calculated dimensionless spot production rates is proved to agree very well with existing data sets from other experiments. For a Mach number equal to 0.36 the production rates seems a bit higher when compared to the incompressible data. However, the differences are small.Copyright

Response Time Optimisation of the Rankine Compression Gas Turbine (RCG)

The Rankine Compression Gas turbine (RCG) is a new type of combined cycle that delivers all power on one free power turbine. With its free power turbine the intended fields of application of the RCG are mechanical drives and ship propulsion. For the RCG to become successful in these fields of application a short response time from part-load to full-load is vital. Experiments with an experimental set-up at the Technische Universiteit Eindhoven showed that the response time would benefit from after-spray and supplementary firing. Therefore, these items were implemented in an overdrive controller that was designed to accelerate the RCG cycle more quickly. Simulations showed that the overdrive controller dramatically reduces the response time of the modeled RCG-cycle in a transient from 50% part-load to full-load from 20 minutes down to about 2 minutes. This is an impressive improvement of the response time and is believed to make the RCG suitable for mechanical drives and ship propulsion.Copyright

Development of Coherent Perturbations in a Laminar Boundary Layer

Bypass transition of a boundary layer takes place at high main stream turbulence levels, when main stream disturbances intrude directly into the viscous sublayer. This (non-linear) transition phenomenon is not yet understood. In this paper a numerical study is presented in which single coherent perturbations are introduced into a spatially developing laminar boundary layer. The (fully compressible) Navier-Stokes equations are solved using direct numerical simulation with a higher order finite difference method on a collocated grid. The results show that the initially smooth disturbance rapidly changes into an arrow-head structure with the characteristics of a turbulent spot.

Nonlinear Model Predictive Control Experiments on a Laboratory Gas Turbine Installation

Results on the feasibility and benefits of model based predictive control applied to a gas turbine are presented. For a laboratory gas turbine installation, the required dynamic simulation model and the real-time (nonlinear) Model Predictive Control (MPC) implementation are discussed. Results on both model validation and control performance are presented. We applied a nonlinear MPC configuration to control the laboratory gas turbine installation and succeeded in a real-time implementation. Although the available computation time for prediction and optimization of the model limits the sample time, the advantages of MPC, i.e. constraint handling, and anticipation to future (set-point) changes are fully reached, and the control performance is good. Special attention is paid to the performance of the applied filter that compensates for inevitable mismatches between model and process measurements. In general, the opportunities of model based control of turbomachinery are promising.Copyright