Large Eddy Simulation with wall functions of Ahmed body

Large Eddy Simulation with Wall Function (WFLES) is known to be a cheap alternative to classical LES methods for simulation of flow where large and complex computational meshes are typically required. This makes it attractive for engineering applications. However experience of applying such methods to complex turbulent flows with flow separation and reattachment is still little-known in literature. In this work WFLES of flow around simplified car body with slant angle equal to 25 degrees and ReL = 2.8 · 106 is carried out on Octree mesh to demonstrate the capabilities and limitations of the method in such type of the flow. The results on a series of meshes show that even though the general flow topology is well captured, the critical part of the flow on the slant is hardly predicted even on 100 mln mesh. It is concluded that the prediction of separation above the slant requires significant mesh refinement even in the frame of WFLES.

Skin-Friction-Based Identification of the Critical Lines in a Transonic, High Reynolds Number Flow via Temperature-Sensitive Paint

In this contribution, three methodologies based on temperature-sensitive paint (TSP) data were further developed and applied for the optical determination of the critical locations of flow separation and reattachment in compressible, high Reynolds number flows. The methodologies rely on skin-friction extraction approaches developed for low-speed flows, which were adapted in this work to study flow separation and reattachment in the presence of shockwave/boundary-layer interaction. In a first approach, skin-friction topological maps were obtained from time-averaged surface temperature distributions, thus enabling the identification of the critical lines as converging and diverging skin-friction lines. In the other two approaches, the critical lines were identified from the maps of the propagation celerity of temperature perturbations, which were determined from time-resolved TSP data. The experiments were conducted at a freestream Mach number of 0.72 and a chord Reynolds number of 9.7 million in the Transonic Wind Tunnel Göttingen on a VA-2 supercritical airfoil model, which was equipped with two exchangeable TSP modules specifically designed for transonic, high Reynolds number tests. The separation and reattachment lines identified via the three different TSP-based approaches were shown to be in mutual agreement, and were also found to be in agreement with reference experimental and numerical data.

A computational investigation of laminar shock/wave boundary layer interactions

Hypersonic laminar flow past a compression corner has been numerically investigated using time-accurate computational fluid dynamics (CFD) approach. Two flow conditions were considered relevant to high and low enthalpy conditions with a total specific enthalpy of 19MJ/kg and 2·8MJ/kg. The Mach number and unit Reynolds number per metre were 7·5, 9·1 and 3·10 × 105and 32·2 × 105respectively. These free stream conditions provided attached, incipiently separated and fully separated flows for ramp angles between θw= 5° to 24°. A grid independence study has been carried out to estimate the sensitivity of heat flux and skin friction in the strong interaction regions of the flow. The investigation was carried out assuming the flow to be laminar throughout and high temperature effects such as thermal and chemical nonequilibrium are studied using Park’s two temperature model with finite rate chemistry. A critical comparison has been made with existing steady state computational and experimental data and the study has highlighted the importance of high temperature effects on the flow separation and reattachment.

Spatio-Temporal Characteristics of Heat Transfer in Separated and Reattaching Flows

Spatio-temporal characteristic of heat transfer accompanied by the flow separation and reattachment was investigated using a high-speed infrared thermograph that recorded the temperature fluctuation on a heated thin-foil. In this study, the heat transfers behind a backward-facing step, behind a forward-facing step, and on a blunt plate were investigated. In all configurations, the heat transfer in the flow reattaching region had spot-like characteristics, which spread with time and overlapped with others to form a complex feature in terms of spatio-temporal characteristics of the heat transfer. The mean Nusselt number distribution behind the reattaching region was approximately proportional to 2/3 power of Reynolds number, as indicated in the previous researches. The time-space distribution of the heat transfer had a typical spanwise wavelength and fluctuating frequency in the reattaching region, which was likely to be related to the dimension of the separation region (characteristic length H or the separation length xR). This suggests that the origin of these periodicities is not the instability upstream of the flow separation, but due to some instability, which is accompanied by the flow separation and reattachment.

Numerical Simulation and Structure Optimization of the Main-Control Reversing Valve of Electrical-Hydraulic Hammer

Based on viscosity hydrodynamics and hydro-finite element theory, flow field of the main-control reversing valve of electrical-hydraulic hammer was simulated. The N-S equations were discretized by the finite volume method. Hydraulic valve’s pressure and velocity distribution were analyzed in the working process. Relationship between the structure of the flow field (velocity, pressure, flow separation and reattachment, appearing and disappearing of vortices) and the energy loss and component performance was proposed. The design of the hydraulic valve structure was optimized which turns out to be high-efficiency and energy-saving

Resistance and Fluctuating Pressures of a Large Elbow in High Reynolds Numbers

For the Japan Atomic Energy Agency sodium-cooled fast reactor, an experimental study on the fluctuating pressure of the hot legs was carried out with tests in a 1/3-scale model. The total resistance coefficient is consistent with published data, and, additionally, our research has given data up to the Reynolds number of 8.0×106. The flow visualization and velocity measurement confirmed the independence of the flow on the Reynolds number. Pressures on the pipe wall were statistically examined to predict the characteristics of fluctuating pressures of the hot legs. It reveals that generation of fluctuating pressure is dominant on the boundary of flow separation and reattachment.

Near-Wall Turbulence Modeling Without Wall Distance

A low-Reynolds number k-ε model is proposed in which the anisotropic production in near-wall regions is accounted for substantially by modifying the model constants Cε(1,2) and adding cross-diffusion terms in the ε equation. Hence, it reduces the kinetic energy and length scale magnitudes to improve prediction of adverse pressure gradient flows, involving flow separation and reattachment. Unlike the conventional k-ε model, it requires no wall function/distance parameter that bridges the near-wall integration. The model coefficients/functions depend nonlinearly on both the strain-rate and vorticity invariants. The model is validated against a few flow cases, yielding predictions in good agreement with the direct numerical simulation (DNS) and experimental data.