Investigation of High-Speed Train Drag with Towing Tank Experiments and CFD

In order to assess the accuracy of drag prediction methods for high-speed trains, experimental and numerical investigations were performed. Besides the drag coefficient, skin friction and pressure distributions on and near the model have been measured for a 1:22 model of the ICE/V. For the experiments, a moving model rig called DIWA (Drag measurement in water) was realised in a 120 m long towing tank to allow for a realistic simulation of the flow around the train, even in the underbelly region. Numerical investigations were performed using Partially-averaged Navier-Stokes (PANS) simulations based on the k-ω-SST turbulence model. Both experimental and numerical methods can be considered as a novelty in the field of train aerodynamics. The results are compared with data from full-scale tests. It is shown, that the moving model rig DIWA allows for the measurement of drag coefficients of trains with high accuracy. Furthermore, the data acquired using the PANS approach compares well with the experimental data

Quantitative Tuft Flow Visualization on the Volvo S60 under realistic driving Conditions

The flow around a car is three-dimensional and turbulent. The main part of the dragis caused by pressure losses generated by various ow separations. In general a large anddistinct wake structure dominates the flow field at the base. Furthermore, diverse vertical structures are present at the backlight and the upper trunk region. The investigation ofthese owphenomena is important for the design and the optimization process of a car.Traditional ow visualization techniques, such as oil paint and tufts, are widely used andestablished for the study of surface friction lines. Aerodynamic investigations are usually conducted in sophisticated wind tunnels with a uniform freestream velocity and very lowturbulence intensities. However, this does not correspond to realistic road conditions. Inthis study, the applicability and the feasibility of a new tuft method is investigated. Theexperiments are conducted on a circuit test track and hence to realistic on-road conditions.A large amount of tufts are attached to the entire rear end section of a Volvo S60 passengercar. The movement of the tufts is continuously recorded as well as the freestream velocityusing a digital camera and a three-hole probe, respectively. Thus, every image can beassociated to a distinct inow conditions. A novel and Efficient image processing algorithmallows the extraction of the mean tuft angle afterwards. This data allows the calculationof the near wall streamlines and generates additionally statistical data of the directionalorientation of the tuft movement. The obtained time averaged surface traces indicate analmost symmetric flow field at the rear end of the car with head wind conditions. The effect of the streamwise vortices, which origins at the c-pillars, are evident on both sidesof the backlight. With side wind conditions the ow pattern at the backlight and trunkdeck shift slightly to the leeward side while the vorticalow structure at the base shiftsslightly windward. The analyses of the freestream velocity time series show Significantlyincreased turbulence intensities compared to wind tunnel tests. In summary, the new tuftow visualization and the usage of the three-hole probe in front of the car represent asimple and low cost tool for on-road test to study limiting streamline pattern

Detonation initiation by shock focusing at elevated pressure conditions in a pulse detonation combustor

This work contains experimental investigations on the correlation of the detonation initiation process via a shock-focusing device with various initial pressures and mass flow rates. A pulse detonation combustor is operated with stoichiometric hydrogen--air--oxygen mixtures in single cycle operation. A rotationally symmetric shock-focusing geometry evokes the onset of a detonation by the focusing of the reflected leading shock wave, while a blockage plate at the rear end of the test rig is applied to induce an elevated initial pressure. The results show that the reactivity has a major influence on the success rate of detonation initiation. However, measurements with different blockage plates suggest that the mass flow rate has to be considered as well when predicting the success rate. Three main statements can be drawn from the results. (1) An increase in the mean flow velocity induces higher velocity fluctuations which result in a stronger leading shock ahead of the accelerating deflagration front. (2) An increase in the initial static pressure reduces the critical shock strength that must be exceeded to ensure successful detonation initiation by shock focusing. (3) Since the initial pressure is directly linked to the mass flow rate, these contrary trends can cancel each other out, which could be observed for 40% vol. of oxygen in the oxidizer. High-speed images were taken, which confirm that the detonation is initiated in the center of the converging--diverging nozzle due to focusing of the leading shock.DFG, 414044773, Open Access Publizieren 2019 - 2020 / Technische Universität BerlinDFG, 200291049, SFB 1029: TurbIn - Signifikante Wirkungsgradsteigerung durch gezielte, interagierende Verbrennungs- und Strömungsinstationaritäten in Gasturbine

Experimental Investigation of the Near Wall Flow Downstream of a Passenger Car Wheel Arch

The flow around and downstream of the front wheels of passenger cars is highly complex and characterized by flow structure interactions between the external flow, fluid exiting through the wheelhouse, flow from the engine bay and the underbody. In the present paper the near wall flow downstream of the front wheel house is analyzed, combining two traditional methods. A tuft visualization method is used to obtain the limiting streamline pattern and information about the near wall flow direction. Additionally, time resolved surface pressure measurements are used to study the pressure distribution and the standard deviation. The propagation of the occurring flow structures is investigated by cross correlations of the pressure signal and a spectral analysis provides the characteristic frequencies of the investigated flow. It is found that two main flow phenomena can be observed: one originates from flow exiting the upper wheelhouse and a second one resulting from a separation on the lower wheel house edge. The frequency spectrum reveals a dominant Strouhal number of 0.2. As the observed flow structures are attributed to the wheel-wheelhouse interaction, a closed wheelhouse configuration is also investigated and the results confirm that the fluctuations and observed flow structures are created by the flow interaction between the wheel, wheelhouse and the rotation of the wheel