Showing papers by "Neil D. Sandham published in 2015"

Numerical study of oblique shock-wave/boundary-layer interaction considering sidewall effects

Abstract: Large-eddy simulations are conducted to uncover physical aspects of sidewall-induced three-dimensionality for a moderately separated oblique shock-wave/boundary-layer interaction (SWBLI) at M=2.7. Simulations are run for three different aspect ratios of the interaction zone. The swept SWBLI on the sidewalls and the corner flow behaviour are investigated, along with the main oblique SWBLI on the bottom wall. As the aspect ratio decreases to unity, the separation and reattachment points on the central plane are observed to move upstream simultaneously, while the bubble length initially increases and then stabilizes to a length 30 % larger than for the infinite-span quasi-two-dimensional case. A distorted incident shock and a three-dimensional (3D) bottom-wall separation pattern are observed, with a patch of attached flow between the central and corner separations. The 3D flow structure is found to be induced by the swept SWBLI formed on the sidewalls. The location of the termination point of the incident shock near the sidewall is limited by a sweepback effect, allowing the definition of a penetration Mach number Mp that is shown to correlate well with the spanwise extent of the core flow. The structure and strength of the incident shock are modified significantly by the swept SWBLI on the sidewalls, along with a compression wave upstream and a secondary sidewall shock downstream, leading to a highly 3D pressure field in the main flow above the main SWBLI on the bottom wall. The reflection of the swept SWBLI from the bottom wall leads to a corner compression wave and strong transverse flow close to the bottom wall. A physical model based on the quasi-conical structure of the swept SWBLI on the sidewall is proposed to estimate the 3D SWBLI pattern on the bottom wall, in which the swept SWBLI features and the aspect ratio of the interaction zone are considered to be the critical factors

Direct Numerical Simulations of a High-Pressure Turbine Vane

Abstract: In this paper we establish a benchmark data set of a generic high-pressure turbine vane generated by direct numerical simulation (DNS) to resolve fully the flow. The test conditions for this case are a Reynolds number of 0.57 million and an exit Mach number of 0.9, which is representative of a modern transonic high-pressure turbine vane. In this study we first compare the simulation results with previously published experimental data. We then investigate how turbulence affects the surface flow physics and heat transfer. An analysis of the development of loss through the vane passage is also performed. The results indicate that free-stream turbulence tends to induce streaks within the near wall flow, which augment the surface heat transfer. Turbulent breakdown is observed over the late suction surface, and this occurs via the growth of two-dimensional Kelvin-Helmholtz spanwise roll-ups, which then develop into lambda vortices creating large local peaks in the surface heat transfer. Turbulent dissipation is found to significantly increase losses within the trailing-edge region of the vane.

Influence of boundary-layer disturbances on the instability of a roughness wake in a high-speed boundary layer

Abstract: The excitation of instability modes in the wake generated behind a discrete roughness element in a boundary layer at Mach 6 is analysed through numerical simulations of the compressible Navier–Stokes equations. Recent experimental observations show that transition to turbulence in high-speed boundary layers during re-entry flight is dominated by wall roughness effects. Therefore, understanding the roughness-induced transition to turbulence in this flow regime is of primary importance. Our results show that a discrete roughness element with a height of about half the local boundary-layer thickness generates an unstable wake able to sustain the growth of a number of modes. The most unstable of these modes are a sinuous mode (mode SL) and two varicose modes (modes VL and VC). The varicose modes grow approximately 17% faster than the most unstable Mack mode and their growth persists over a longer streamwise distance, thereby leading to a notable acceleration of the laminar–turbulent transition process. Two main mechanisms are identified for the excitation of wake modes: the first is based on the interaction between the external disturbances and the reverse flow regions induced by the roughness element and the second is due to the interaction between the boundary-layer modes (first modes and Mack modes) and the non-parallel roughness wake. An important finding of the present study is that, while being less unstable, mode SL is the preferred instability for the first of the above excitation mechanisms, which drives the wake modes excitation in the absence of boundary-layer modes. Modes VL and VC are excited through the second mechanism and, hence, become important when first modes and Mack modes come into interaction with the roughness wake. The new mode VC presents similarities with the Mack mode instability, including the tuning between its most unstable wavelength and the local boundary-layer thickness, and it is believed to play a fundamental role in the roughness-induced transition of high-speed boundary layers. In contrast to the smooth-wall case, wall cooling is stabilising for all the roughness-wake modes.

Parametric geometry models for hypersonic aircraft components: blunt leading edges

Abstract: In this paper we report the results of investigations into the efficient parameterization of blunt leading edge shapes for hypersonic aircraft geometries. The investigations mostly revolve around waverider geometries generated with inverse design techniques, such as the osculating cones waverider forebody design method. The shapes presented however, can be utilized to introduce bluntness to any wedge-like geometry with sharp leading edges. Initially, we present detailed descriptions of three different variations of the rational Bezier curve based parameterization that was developed, and the variety of shapes that can be obtained is demonstrated. Afterwards their performance is evaluated utilizing 2D CFD analysis. In our simulations, the rational Bezier curve leading edges outperform circular ones when it comes to minimizing both drag and peak heating rates or peak temperatures. Additionally, with higher order rational Bezier leading edge shapes, higher levels of geometric continuity can be achieved at the interface between the blunt part and the original wedge-like geometry, resulting in a smoother transition. Preliminary results indicate that this can potentially affect the receptivity and hence transition mechanisms. Finally, the 2D geometry formulations are extended to full 3D waverider forebody geometries.

Instability Study of the Wake Behind a Discrete Roughness Element in a Hypersonic Boundary-Layer

Abstract: The linear instability induced by an isolated roughness element in a boundary-layer at Mach 6 has been analysed through spatial BiGlobal and three-dimensional parabolised (PSE-3D) stability analyses. It is important to understand transition in this flow regime since the process can be slower than in incompressible flow and is critical to prediction of local heat loads on next-generation flight vehicles. The results show that the roughness element, with a height of the order of the boundary-layer displacement thickness, generates an convectively unstable wake where different instability modes develop. Furthermore, at this high Mach number, boundary-layer modes develop at high frequencies and are also covered here. Important discrepancies are observed between BiGlobal and PSE-3D predictions, mainly for the roughness-induced wake modes. Results are in qualitative agreement with a full Navier-Stokes receptivity study of the same flow.

Leading-Edge Receptivity to Acoustic Waves for High-Speed Flows over a Blunt Wedge

Abstract: The leading-edge receptivity to acoustic disturbances of supersonic/hypersonic boundary layers on a cylinder-wedge of 20◦ half-wedge angle and 0.1 mm nose radius is numerically investigated for a set of six different cases with Mach number ranging from 3.0 to 7.3, through direct numerical simulation (DNS) of the two-dimensional (2D) Navier-Stokes equations. Two angles of attack (0◦, 10◦), and two inclination angles of the acoustic waves (0◦, 10◦) are considered among the different numerical cases. For the Mach 3.0 case both fast and slow planar acoustic waves with multiple frequencies are inserted into the flowfield of the steady state solution in order to carry out unsteady computations, while for the remaining cases the unsteady computations are performed only for fast waves in the freestream. The results show that the response along the wall is stable in the nose region, up to 400 nose radii downstream, and that at Mach 3.0 there is a higher amplitude for the fast mode than for the slow mode. The wall pressure and heat flux perturbation spectra show that the receptivity is higher at the higher Mach numbers and at the higher frequencies, while for the lower Mach numbers (Mach 6.0 and 3.0) a frequency-dependent oscillatory behaviour is shown by the pressure perturbation distribution along the wall. The angle of incidence of the acoustic waves seems to slightly increase the amplitude of the response along the wall on the top (lee) side of the body at the higher frequencies, and to produce a flatter response on the same side for the lower frequencies. Including an angle of attack decreases the receptivity along the top side, as the shock is weaker here, and amplifies the response on the windward side. These results serve to quantify the relationship between the disturbances measured by sensors near the leading-edge of a measurement probe and the freestream disturbances in hypersonic wind tunnels.

DNS database of roughness-induced instability and transition at Mach 6

Abstract: Raw data generated by direct numerical simulations of the flow behind isolated roughness elements at Mach 6. More information and details about the numerical set-up and the resulting data can be found in:Jeroen Van den Eynde, "Stability and Transition of the Flow behind Isolated Roughness Elements in Hypersonic Boundary Layers", PhD thesis (University of Southampton), 2015To request access go to http://library.soton.ac.uk/datarequest