Experimental investigation of re-entry aerodynamic phenomena - Development of non-intrusive ?ow diagnostics in a Ludwieg tube

To further improve the design of hypersonic vehicles so that fully reusable systems become a reality, the detailed understanding of the ?ow ?eld that the vehicle generates is of paramount importance. The thesis deals with the aspects of experimental ?ow research which includes also the ?ow facility and the measurement techniques. First a description is given of the investigation and assessment of the Hypersonic Test Facility Delft (HTFD), which is the facility used in the experiments. Thereafter two measurement techniques, particle image velocimetry (PIV) and quantitative infrared thermography (QIRT) are introduced that enable to obtain high quality measurement data of the hypersonic ?ow ?eld generated by the HTFD. In the remainder of the thesis two important ?ow phenomena, shock wave boundary layer interaction and boundary layer transition, have been investigated using PIV and QIRT that are of great importance for re-entry vehicles.AerodynamicsAerospace Engineerin

Effect of predictor-corrector filtering on the stability and spatial resolution of iterative PIV interrogation

The stability and resolution of iterative PIV image analysis methods is investigated. The study focuses on the effects of stabilization by means of spatial filtering when implemented into the iterative process. Two filtering approaches are studied: predictor and corrector filtering respectively. A family of convolution filters is proposed, which allows to vary the filtering strength in a systematic way and primarily affects the system stability and to a smaller extent its spatial response. A critical value for the filter parameter is identified which guarantees the stability of the iterative process. A theoretical analysis is provided that determines the asymptotic properties of the iterative method with varying filter parameters. The study is completed with a numerical assessment and concludes with an application to real experiments, showing the consequence of an incorrect implementation of the iterative scheme under experimental conditions.Aerospace Engineerin

Experimental investigation of a supersonic backward facing step flow

Aerospace Engineerin

On Reynolds Number Dependence of Micro-ramp-Induced Transition

The transitional flow features past a micro-ramp are investigated in the incompressible flow regime at critical and supercritical roughness-height based Reynolds number (Reh). Tomographic PIV is adopted as the diagnostic technique to identify the instantaneous three-dimensional flow behaviour in the domain of 73 ramp height streamwise. The instantaneous and time-averaged flow topology are characterized in detail focusing on the influence of Reynolds number. A main difference is the longer streamwise presence range of primary vortex pair in the time-averaged flow topology at critical Reh compared with supercritical condition, resulting in different type of modification to the mean flow. The transition process is significantly delayed when decreasing Reh. By performing Proper Orthogonal Decomposition (POD) analysis, two major types of eigenmodes with symmetric and asymmetric spatial distribution of velocity fluctuations in the near wake are observed, corresponding to the presence vortex shedding and sinuous wiggling motion respectively. The correlation between the former POD modes and the transition process strongly depends on Reh.AerodynamicsAerodynamics, Wind Energy & Propulsio

Infrared thermography investigation of transitional flow over isolated roughness at high speed

The transitional flow features over and downstream of isolated roughness elements in hypersonic flow are investigated by means of infrared thermography. The local heat flux distribution in the wake of the roughness element reveals the footprint of multiple streamwise counter-rotating vortex pairs. The formation of a turbulent wedge with the spanwise spreading of the wake in the downstream region indicates the early stage of transition and breakdown to turbulence. The onset location of transition is predicted by detecting the origin of the turbulent wedge. Strong dependence of the transition process on the roughness geometry is observed, with significant variations of the lateral spreading rate in the turbulent wedge. Instead, for the present flow conditions negligible variations are associated to changes in RehAerodynamicsAerodynamics, Wind Energy & Propulsio

On Reynolds number dependence of micro-ramp-induced transition

The variation of transitional flow features past a micro-ramp is investigated when the Reynolds number is decreased approaching the critical regime. Experiments are conducted in the incompressible flow spanning from supercritical to subcritical roughness-height-based Reynolds number ( , 730, 460 and 320) with tomographic particle image velocimetry. The effect of on three-dimensional flow behaviour is analysed in a domain encompassing 73 ramp heights in the streamwise direction. Above the critical , the primary vortex pair and induced central low-speed region in the mean flow field are active over longer range when decreasing. In the instantaneous flow, at <![CDATA[Reh, the hairpin vortices induced by Kelvin-Helmholtz (K-H) instability progress gradually from close to the micro-ramp into the region where the overall shear layer is destabilized, indicating the correlation between the K-H instability and the onset of transition. The breakdown of K-H vortices as observed at , does not occur at lower. Decreasing , the secondary vortex structures make their first appearance significantly downstream, postponing the formation of sideward disturbances, which destabilize the local shear layer by ejection events. Two major types of eigenmodes with symmetric and asymmetric spatial distribution of velocity fluctuations in the near wake are clearly identified by proper orthogonal decomposition. The symmetric and asymmetric modes correspond to the presence of vortex shedding and a sinuous wiggling motion respectively. It is found that is the key factor determining the importance of the symmetric mode. At , the disturbance energy of the symmetric mode decays before the onset of transition, suggesting that it is relatively insignificant in the process. However, decreasing to 730 and 460, the symmetric mode produces continuous growth of high level disturbance energy, leading to transition.Wind EnergyAerodynamicsAerodynamics, Wind Energy & Propulsio

A parametric study of laminar and transitional oblique shock wave reflections

High-resolution particle image velocimetry measurements were performed on laminar and transitional oblique shock wave reflections for a range of Mach numbers (M D 1:6-2:3), Reynolds numbers (Rexsh D 1.4×106-3.5×106) and flow deflection angles (θ 1°-5° or p3=p1 D 1.11-1.64). The laminar interactions revealed a long, flat and triangular shaped separation bubble. For relatively strong interactions (p3=p1 > 1.2), the bubble grows linearly in the upstream direction with increasing shock strength. Under these conditions, the boundary layer keeps an on average laminar velocity profile up to the shock impingement location, followed by a quick transition and subsequent reattachment of the boundary layer. For weaker interactions (p3=p1 < 1.2), the boundary layer is able to remain laminar further downstream of the bubble, which consequently results in a later reattachment of the boundary layer. The pressure distribution at the interaction onset for all laminar cases shows excellent agreement with the free-interaction theory, therefore supporting its validity even for incipiently separated laminar oblique shock wave reflections.greenAerodynamic

High-resolution PIV measurements of a transitional shock wave–boundary layer interaction

This study investigates the effects of boundary layer transition on an oblique shock wave reflection. The Mach number was 1.7, the unit Reynolds number was 35 × 106 m?1, and the pressure ratio over the interaction was 1.35. Particle image velocimetry is used as the main flow diagnostics tool, supported by oil-flow and Schlieren visualizations. At these conditions, the thickness of the laminar boundary layer is only 0.2 mm, and seeding proved to be problematic as practically no seeding was recorded in the lower 40 % of the boundary layer. The top 60 % could, however, still be resolved with good accuracy and is found to be in good agreement with the compressible Blasius solution. Due to the effects of turbulent mixing, the near-wall seeding deficiency disappears when the boundary layer transitions to a turbulent state. This allowed the seeding distribution to be used as an indicator for the state of the boundary layer, permitting to obtain an approximate intermittency distribution for the boundary layer transition region. This knowledge was then used for positioning the oblique shock wave in the laminar, transitional (50 % intermittency) or turbulent region of the boundary layer. Separation is only recorded for the laminar and transitional interactions. For the laminar interaction, a large separation bubble is found, with a streamwise length of 96 ??i,0 . The incoming boundary layer is lifted over the separation bubble and remains in a laminar state up to the impingement point of the shock wave. After the shock, transition starts and a turbulent profile is reached approximately 80–90 ??i,0 downstream of the shock. Under the same shock conditions, the transitional interaction displays a smaller separation bubble (43 ??i,0 ), and transition is found to be accelerated over the separation bubble.Aerodynamics, Wind Energy & PropulsionAerospace Engineerin

Investigation of supersonic, large wall roughness elements with QIRT and PIV

The aim of the present study is to quantify the influence of large, ribbed wall roughness elements on the mean flow and heat transfer properties of a turbulent, supersonic boundary layer (M = 2.0). Fifteen test geometries, including one smooth and fourteen rough surfaces are tested using Schlieren, PIV and QIRT. Heat transfer measurements were obtained by the heated-thin-foil method, providing a constant heat flux boundary condition. The QIRT setup was designed to yield an accurate mapping of the surface temperature. It was observed that the geometries followed supersonic, single-cavity type flow classifications with clear, distinguishable flow features.Aerodynamic

The interaction between a sonic jet and an oblique shock wave in a supersonic crossflow

The current study considers the configuration of a transverse jet in a supersonic crossflow that is interacting with an impinging oblique shock wave. The relevance of this configuration is associated to high-speed air-breathing propulsion systems, where a shock may be induced in the combustion chamber to enhance mixing. To investigate the behaviour of this system, experiments were conducted in a supersonic test facility at a freestream Mach number of 2, analyzing the effect of three main control variables: the jet momentum flux ratio, the shock strength and the impingement position of the shock on the jet plume. Measurements were acquired using surface oil-flow visualization, schlieren photography and Particle Image Velocimetry. Results reveal that near-field momentum-driven mixing remains largely unaffected after the introduction of the impinging shock wave, while mid-to-far-field mixing mechanisms do change. An increase in jet plume elevation was observed, as well as the formation of a strong shear layer downstream of the jet, which acts as a source of vorticity that promotes entrainment towards the jet mid-field. A stronger shock wave was found to be more beneficial for mixing performance. This effect was seen to decrease with weakening shock strength or by shifting the strong shock further downstream.Aerodynamic