Assessment of high enthalpy flow conditions for re-entry aerothermodynamics in the plasma wind tunnel facilities at IRS

This article presents the full operational experimental capabilities of the plasma wind tunnel facilities at the Institute of Space Systems at the University of Stuttgart. The simulation of the aerothermodynamic environment experienced by vehicles entering the atmosphere of Earth is attempted using three different facilities. Utilizing the three different facilities, the recent improvements enable a unique range of flow conditions in relation to other known facilities. Recent performance optimisations are highlighted in this article. Based on the experimental conditions demonstrated a corresponding flight scenario is derived using a ground-to-flight extrapolation approach based on local mass-specific enthalpy, total pressure and boundary layer edge velocity gradient. This shows that the three facilities cover the challenging parts of the aerothermodynamics along the entry trajectory from Low Earth Orbit. Furthermore, the more challenging conditions arising during interplanetary return at altitudes above 70 km are as well covered.

Development of a boundary layer parameters identification method for transition prediction with complex grids

Predicting boundary layer transition accurately is important to thermal protection and drag reduction of flight vehicles. Up to now, there has been many transition prediction methods. However, most of those methods need boundary layer parameters, which are difficult to obtain in massively parallel execution since some parameters are nonlocal variables, thus greatly limiting the application of those methods. A grid-reorder method is developed to obtain the boundary layer parameters, which is suitable for parallel computing in this paper. With the grid-reorder method the wall normal grid cells can be easily found, and two criteria are used to determine the boundary layer edge in the wall normal direction, then the boundary layer parameters such as boundary layer thickness, boundary layer momentum thickness, boundary layer edge velocity, cross-flow velocity, and so on, can be obtained accurately and efficiently. The method has been coupled to three transition prediction methods, the γ-Reθ model, the k-ω-γ model, and the transition correlations, to validate its effectiveness. For the γ-Reθ model, the cross-flow velocity is obtained with the grid-reorder method, then a cross-flow intermittency factor is developed and introduced into the model, and the inclined prolate spheroid case is used to test the performance of the model. For the k-ω-γ model, the grid-reorder method is applied to obtain the boundary layer edge velocity and the inflection point velocity which are of vital importance to form the second-mode timescale for hypersonic transition prediction. For the transition correlations, Reθ/ Me is obtained effectively with the grid-reorder method. The X-51 forebody is selected to test the effectiveness of Reθ/Me for complex geometries and the results show a good correspondence with the experiment results. The successful application in three transition prediction methods demonstrates that the grid-reorder method has an excellent performance in obtaining the boundary layer parameters and can broaden the application of the existing transition prediction method in engineering.

Experimental investigation into the routes to bypass transition and the shear-sheltering phenomenon

The objective of this investigation is to give experimental support to recent direct numerical simulation (DNS) results which demonstrated that in bypass transition the flow first breaks down to turbulence on the low-speed streaks (or so-called negative jets) that are lifted up towards the boundary-layer edge region. In order to do this, wall-normal profiles of the streamwise fluctuation velocity are presented in terms of maximum positive and negative values over a range of turbulence intensities (1.3–6%) and Reynolds numbers for zero pressure gradient flow upstream of, and including, transition onset. For all turbulence intensities considered, it was found that the peak negative fluctuation velocity increased in magnitude above the peak positive fluctuations and their positions relative to the wall shifted as transition onset approached; the peak negative value moved towards the boundary-layer edge and the peak positive value moved toward the wall. An experimental measure of the penetration depth (PD) of free-stream disturbances into the boundary layer has been gained through the use of the skewness function. The penetration depth (measured from the boundary-layer edge) scales as PD ∝ (ω Rexτw)−0.3), where ω is the frequency of the largest eddies in the free stream, Rex is the Reynolds number of the flow based on the streamwise distance from the leading edge and τw is the wall shear stress. The parameter dependence demonstrated by this scaling compares favourably with recent solutions to the Orr–Sommerfeld equation on the penetration depth of disturbances into the boundary layer. The findings illustrate the importance of negative fluctuation velocities in the transition process, giving experimental support to suggestions from recent DNS predictions that the breakdown to turbulence is initiated on the low-speed regions of the flow in the upper portion of the boundary layer. The representation of pre-transitional disturbances in time-averaged form is shown to be inadequate in elucidating which disturbances grow to cause the breakdown to turbulence.

An Investigation Using Wavelet Analysis Into Velocity Perturbations Under the Influence of Elevated Freestream Turbulence at Transition Onset

The development of streamwise-orientated disturbances at transition onset for zero-pressure gradient boundary layer flow under the influence of 1.3% freestream turbulence intensity is presented. The analysis concentrates on the development of turbulent spots and other coherent structures with the use of wavelet analysis. The turbulent spot structure is shown to change dramatically in shape, sign of perturbation velocity and energy content from the near wall region to the boundary layer edge. An increased number of trubulent structures are observed near the boundary layer edge, all with negative perturbation velocities, compared to those of positive perturbation velocity in the near wall region. The wavelet maps demonstrate some interesting features of turbulent spot development including regions of high frequency disturbance growth prior to the spot passing the sensor. Distributions of peak negative, peak positive and averaged perturbation velocities were obtained at three streamwise positions prior to transition onset. As transition onset approached the magnitude of the negative value far exceeded the positive and their relative positions within the boundary layer changed considerably. The results presented in this report give further insight into the physics of pre-transitional flow illustrating the influence of negative perturbation velocity in the transition process. Furthermore, the importance of peak instantaneous perturbations compared to averaged values is also demonstrated, a feature of the flow that computational techniques will have to model in order to accurately predict transition phenomena.

An Entropic Minimization Technique for Turbine Blade Profiles

The optimization of the boundary layer edge velocity distribution may hold the key to the minimization of entropy generation in the boundary layers of turbomachinery blades. A preliminary optimization analysis in the laminar region of a non film cooled turbine blade is presented, which demonstrates the concept of how the entropy generation rate may be reduced by varying the boundary layer edge velocity distribution along the suction surface, whilst holding the work done by the blade constant. In the laminar region the analytical technique developed by Pohlhausen and others to predict the boundary layer momentum thickness in the presence of pressure gradients has been adopted to predict the entropy generated as described in other papers by the same authors. The result gives an expression for the entropy generation rate in terms of the boundary layer edge velocity distribution for incompressible flows. The boundary layer edge velocity distribution may then be represented as a polynomial with undefined variables. This allows a minimization technique to be used to minimize the entropy generation rate on these variables. Constraints are included to keep the work output constant and the diffusion low to avoid separation. In this analysis it is only the laminar region that is considered for minimization, thus it is necessary to ensure that the modified boundary layer edge velocity distribution does not undergo transition earlier than the baseline boundary layer edge velocity distribution. This is accomplished by considering transition and separation criteria available in the literature. The result of this analysis indicates that the entropy generation rate may be reduced in the laminar boundary layers by using this technique.

Measurements of the Turbulence Structure in the Vicinity of a 3-D Separation

The flow in the cross-flow separation region of a 6:1 prolate spheroid at 10 deg angle of attack, ReL = 4.20 × 106, was investigated using a novel, miniature, 3-D, fiber-optic Laser Doppler Velocimeter (LDV). The probe was used to measure three simultaneous, orthogonal velocity components from within the model, from approximately y+ = 7 out to the boundary layer edge. Velocity, Reynolds stress, and velocity triple product measurements are presented. These measurements are used to calculate the skin friction and to examine the convection, production, and diffusion of turbulent kinetic energy (TKE) about the three-dimensional separation. Comparisons of the measured production and diffusion of TKE in the cross-flow separation region—as well as in nonseparated regions of the flow—to the production and diffusion predicted by several models for these terms are shown.

The Effect of a Turbulent Wake on the Stagnation Point: Part I — Skin Friction Results

The response of a boundary layer in the stagnation region of a two-dimensional body to fluctuations in the freestream is examined. The analysis is restricted to laminar incompressible flow. The assumed form of the velocity distribution at the edge of the boundary layer represents both a pulsation of the incoming flow, and an oscillation of the stagnation point streamline. Both features are essential in accurately representing the effect which freestream spatial and temporal nonuniformities have upon the unsteady boundary layer. Finally, a simple model is proposed which relates the characteristic parameters in a turbulent wake to the unsteady boundary-layer edge velocity. Numerical results are presented for both an arbitrary two-dimensional geometry and a circular cylinder.