Effects of direction decoupling in flux calculation in finite volume solvers

In a finite volume CFD method for unsteady flow, fluxes of mass, momentum and energy are exchanged between cells over a series of small time steps. The conventional approach, which we will refer to as direction decoupling, is to estimate fluxes across interfaces in a regular array of cells by using a one-dimensional flux expression based on the component of flow velocity normal to the interface between cells. This means that fluxes cannot be exchanged between diagonally adjacent cells since they share no cell interface, even if the local flow conditions dictate that the fluxes should flow diagonally. The direction decoupling imposed by the numerical method requires that the fluxes reach a diagonally adjacent cell in two time-steps. In order to evaluate the e®ects of this direction decoupling, we examine two numerical methods which differ only in that one uses direction decoupling while the other does not. We examine a generalized form of Pullin's Equilibrium Flux Method (EFM) [J. Comput. Physics, v34, 1980, pp 231-244] which we have called the True Direction Equilibrium Flux Method (TDEFM). The TDEFM fluxes, derived from kinetic theory, flow not only between cells sharing an interface, but ultimately to any cell in the grid. TDEFM is used here to simulate a blast wave and an imploding flow problem on a structured rectangular mesh and is compared with results from direction decoupled EFM. Since both EFM and TDEFM are identical in the low CFL number limit, differences between the results demonstrate the detrimental e®ect of direction decoupling. Differences resulting from direction decoupling are also shown in the simulation of hypersonic flow over a rectangular body. The computational cost of allowing the EFM fluxes to flow in the correct directions on the grid is minimal

Does the pinching force dissipate the rower's energy?

Regardless of the angle the oar makes to the forward direction during a rowing stroke, there is negligible loss of energy between the oar and the gate. However, it does not follow from this that there is no decrease in oar efficiency at the extreme oar angles necessary for a very long catch. The loss of energy from the oar occurs, not at the gate, but at the blade. The water which is moved by the blade, against the water reaction force on the blade, absorbs mechanical energy from the oar. Calculations, experimental data and “on-water” measurements of rowing show that the energy efficiency of the oar is generally lowest for oar angles less than 55 degrees to the boat-forward direction (i.e. in the early part of the stroke)

μ-DSMC: A general viscosity method for rarefied flow

A modified DSMC method for rarefied flows is described, by which any viscosity law mu = mu(T) may be simulated, including experimental data directly. The collision cross-section of a simple collision model is made to vary from cell to cell, based on the time-averaged cell temperature and the required viscosity at that temperature. The new method is tested in two different flows: high speed Couette flow and a plane 1D shock. For Couette flow, the shear stress and heat transfer, calculated from the velocity distribution, agree with the theoretical values calculated from the flow gradients and the theoretical transport coefficients. For the 1D shock, the new method is compared with the generalized hard sphere (GHS) model. The new method produces profiles of density and temperature which are generally indistinguishable from the GHS results except for a deviation in the Tx temperature component in a small region ahead of the shock. For the worst case the deviation is 4.6%, but it can be reduced by basing the imposed viscosity on the maximum component of kinetic temperature rather than the mean kinetic temperature. The new method is shown to be insensitive to the number of simulator particles used in each cell. Three translational degrees of freedom are considered. However, because mu-DSMC is based on a hard sphere or VHS cross-section, it is compatible with the most commonly used Borgnakke-Larsen model for translational-rotational energy exchange

Mechanical and Space Engineering, a degree for undergraduates

Australia is a country which makes significant use of space based services and technology, but is dependent primarily on overseas suppliers. This paper discusses a new degree offered at The University of Queensland, aimed at preparing graduates for the integration of space related activities into the Engineering workplace. The degree is based on the current Mechanical Engineering degree offered at the University, and contains extra material relevant to pursuing Engineering practice in space. The structure of the degree is such that a mixture of stand alone space subjects are offered, along with space related ‘add ons’ whereby a traditional Mechanical subject is interpreted in a space related context. The basic premise which lead to the new degree is that if a substantial space engineering Industry is to develop in Australia, then it must do so from the existing Industrial base, and Engineers with the appropriate background will be required. Mechanical Engineering being an important discipline to space based projects, it is sensible, and educationally efficient, to use it as a structure for a space engineering degree. The paper details the structure of the course, discusses the methodology behind its formulation and its perceived role in Australia’s future

Simulation of a complete reflected shock tunnel showing a vortex mechanism for flow contamination

Simulations of a complete reflected shock tunnel facility have been performed with the aim of providing a better understanding of the flow through these facilities. In particular, the analysis is focused on the premature contamination of the test flow with the driver gas. The axisymmetric simulations model the full geometry of the shock tunnel and incorporate an iris-based model of the primary diaphragm rupture mechanics, an ideal secondary diaphragm and account for turbulence in the shock tube boundary layer with the Baldwin-Lomax eddy viscosity model. Two operating conditions were examined: one resulting in an overtailored mode of operation and the other resulting in approximately tailored operation. The accuracy of the simulations is assessed through comparison with experimental measurements of static pressure, pitot pressure and stagnation temperature. It is shown that the widely-accepted driver gas contamination mechanism in which driver gas jets along the walls through action of the bifurcated foot of the reflected shock, does not directly transport the driver gas to the nozzle at these conditions. Instead, driver gas laden vortices are generated by the bifurcated reflected shock. These vortices prevent jetting of the driver gas along the walls and convect driver gas away from the shock tube wall and downstream into the nozzle. Additional vorticity generated by the interaction of the reflected shock and the contact surface enhances the process in the over-tailored case. However, the basic mechanism appears to operate in a similar way for both the over-tailored and the approximately tailored conditions

Rarefied hypersonic flow about a flat-ended circular cylinder

LD:4360.17(IC-Aero--81-02). / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo

Axisymmetric shock wave interaction with a cone: a benchmark test

Results of the benchmark test are presented of comparing numerical schemes solving shock wave of M-s = 2.38 in nitrogen and argon interacting with a 43 degrees semi-apex angle cone and corresponding experiments. The benchmark test was announced in Shock Waves Vol. 12, No. 4, in which we tried to clarify the effects of viscosity and heat conductivity on shock reflection in conical flows. This paper summarizes results of ten numerical and two experimental applications. State of the art in studies regarding the shock/cone interaction is clarified