Browsing by Author "Townsend, Jamie F."

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    A 3D CFD analysis of flow past a hipped roof with comparison to industrial building standards
    (Techno Press, 2022-06-25) Khalil, Khalid; Khan, Huzafa; Chahar, Divyansh; Townsend, Jamie F.; Rana, Zeeshan A.
    Three-dimensional (3D) computational fluid dynamics (CFD) analysis of flow around a hipped-roof building representative of UK inland conditions are conducted. Unsteady simulations are performed using three variations of the k-ϵ RANS turbulence model namely, the Standard, Realizable, and RNG models, and their predictive capability is measured against current European building standards. External pressure coefficients and wind loading are found through the BS 6399-2:1997 standard (obsolete) and the current European standards (BS EN 1991-1-4:2005 and A1:20101). The current European standard provides a more conservative wind loading estimate compared to its predecessor and the k-ϵ RNG model falls within 15% of the value predicted by the current standard. Surface shear stream-traces and Q-criterion were used to analyze the flow physics for each model. The RNG model predicts immediate flow separation leading to the creation of vortical structures on the hipped-roof along with a larger separation region. It is observed that the Realizable model predicts the side vortex to be a result of both the horseshoe vortex and the flow deflected off it. These model-specific aerodynamic features present the most disparity between building standards at leeward roof locations. Finally, pedestrian comfort and safety criteria are studied where the k-ϵ Standard model predicts the most ideal pedestrian conditions and the Realizable model yields the most conservative levels.
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    Advanced numerical methods for dissipative and non-dissipative relativistic hydrodynamics
    (Cranfield University, 2020-05) Townsend, Jamie F.; Könözsy, László Z. ; Jenkins, Karl W.
    High-energy physical phenomena such as astrophysical events and heavy-ion collisions contain a hydrodynamic aspect in which a branch of fluid dynamics called relativistic hydrodynamics (RHD) is required for its mathematical description. The resulting equations must be, more often than not, solved numerically for scientists to ascertain useful information regarding the fluid system in question. This thesis describes and presents a twodimensional computational fluid dynamics (CFD) solver for dissipative and non-dissipative relativistic hydrodynamics, i.e. in the presence and absence of physically resolved viscosity and heat conduction. The solver is based on a finite volume, Godunov-type, HighResolution Shock-Capturing (HRSC) framework, containing a plethora of numerical implementations such as high-order Weighted-Essentially Non-Oscillatory (WENO) spatial reconstruction, approximate Riemann solvers and a third-order Total Variation Diminishing (TVD) Runge–Kutta method. The base numerical solver for the solution of non-dissipative RHD is extensively tested using a series of one-dimensional test cases, namely, a smooth flow problem and shock-tube configurations as well as the two-dimensional vortex sheet and Riemann problem test cases. For the case of non-dissipative relativistic hydrodynamics the relativistic CFD solver is found to perform well in terms of the orders of accuracy achieved and its ability to resolve shock wave patterns. Numerical pathologies have been identified when the relativistic HLLC Riemann solver is used in multi-dimensions for problems exhibiting strong shock waves. This is attributed to the so-called Carbuncle problem which is shown to occur because of pressure differencing within the process of restoring the missing contact discontinuity of its predecessor, the HLL Riemann solver. To avoid this numerical pathology and improve the robustness of numerical solutions that make use of the HLLC Riemann solver, the development of a rotated-hybrid Riemann solver arising from the hybridisation of the HLL and HLLC (or Rusanov and HLLC) approximate Riemann solvers is presented. A standalone application of the HLLC Riemann solver can produce spurious numerical artefacts when it is employed in conjunction with Godunov-type high-order methods in the presence of discontinuities. It has been found that a rotated-hybrid Riemann solver with the proposed HLL/HLLC (Rusanov/HLLC) scheme could overcome the difficulty of the spurious numerical artefacts and presents a robust solution for the Carbuncle problem. The proposed rotated-hybrid Riemann solver provides sufficient numerical dissipation to capture the behaviour of strong shock waves for relativistic hydrodynamics. Therefore, focus is placed on two benchmark test cases (odd-even decoupling and double-Mach reflection problems) and the investigation of two astrophysical phenomena, the relativistic Richtmyer– Meshkov instability and the propagation of a relativistic jet. In all presented test cases, the Carbuncle problem is shown to be eliminated by employing the proposed rotated-hybrid Riemann solver. This strategy is problem-independent, straightforward to implement and provides a consistent robust numerical solution when combined with Godunov-type highorder schemes for relativistic hydrodynamics...[cont.]
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    Experimental and numerical aerodynamic analysis of an elevated beachfront house
    (Elsevier, 2022-11-23) Townsend, Jamie F.; Teschner, Tom-Robin; Xu, Guoji; Zou, Lianghao; Han, Yan; Cai, C. S.
    Elevating coastal houses enables residential communities to reduce the risk of flooding due to tropical cyclones. However, wind-induced damage during such events requires an understanding of the inherent wind forces to improve damage mitigation techniques and assessment of climate-related risk in insurance models. In this study, wind-tunnel experiments and computational fluid dynamics (CFD) simulations are conducted for a typical elevated 1:25 scale beachfront house, possessing a 5:12 pitched gable roof with overhanging eave. An atmospheric boundary layer (ABL) wind field is generated in a low-speed wind-tunnel to replicate conditions experienced during tropical cyclones. Testing is performed for a range of incident wind angles to understand the full aerodynamic consequences of strong winds. Measured pressure coefficient (Cp) distributions are compared with CFD simulations using steady-state and transient Delayed Detached-Eddy Simulation (DDES) within ANSYS Fluent 2021 R1. Net Cp values surrounding the overhanging eave are considered to evaluate the role of this typical geometrical feature. It was found that larger uplift suction occurred at incident wind angles of 45°and above, after which the suction remained stable. The roof panels are subjected to the greatest upward suction, where critical regions occur at the roof ridge. The size of the low-pressure regions is determined by the incident wind angle and ensuing flow separation wherein DDES is found to reproduce additional aerodynamic features arising from unsteady turbulent flow. DDES offers improved predictive capability when mean pressure forces are considered but falls short as an accurate means to efficiently evaluate peak distributions.
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    On high-order numerical schemes for viscous relativistic hydrodynamics through the Kelvin–Helmholtz instability
    (Oxford University Press, 2022-06-24) Townsend, Jamie F.; Inutsuka, Shu-ichiro; Könözsy, László Z.; Jenkins, Karl W.
    This work assesses the dissipative properties of high-order numerical methods for relativistic hydrodynamics. A causal theory of physical dissipation is included within a finite volume high-resolution shock-capturing framework based on the Israel–Stewart theory to study high-order WENO (weighted-essentially non-oscillatory) schemes for simulating the relativistic Kelvin–Helmholtz instability. We provide an estimation of the numerical dissipation of high-order schemes based on results obtained both with and without physically resolved dissipation and determine an empirical relationship between the numerical dissipation and the grid resolution. We consider the appearance of secondary flow features within the evolution of the Kelvin–Helmholtz instability and determine that they are numerical artifacts — this is partly based on arguments presented in terms of a frame-dependent form of the relativistic Reynolds number. There is a potential advantage of using high-order schemes in terms of their accuracy and computational cost on coarser grid resolutions when directly compared to low-order schemes on a fine grid in the presence of physical viscosity. It is possible to find reasonable agreement between numerical results that employ lower-order schemes using a finer grid resolution and results that employ higher order schemes at a coarser grid resolution when sufficient viscosity is present. Overall, the present analysis gives an insight into the numerical dissipation of high-order shock-wave capturing schemes which can be relevant to computational studies of astrophysical phenomena in the relativistic regime. The results presented herein are problem and scheme-dependent and serve to highlight the different roles of numerical and physical dissipation.
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    On the development of a rotated-hybrid HLL/HLLC approximate Riemann solver for relativistic hydrodynamics
    (Oxford University Press, 2020-06-13) Townsend, Jamie F.; Könözsy, László Z.; Jenkins, Karl W.
    This work presents the development of a rotated-hybrid Riemann solver for solving relativistic hydrodynamics (RHD) problems with the hybridization of the HLL and HLLC (or Rusanov and HLLC) approximate Riemann solvers. A standalone application of the HLLC Riemann solver can produce spurious numerical artefacts when it is employed in conjunction with Godunov-type high-order methods in the presence of discontinuities. It has been found that a rotated-hybrid Riemann solver with the proposed HLL/HLLC (Rusanov/HLLC) scheme could overcome the difficulty of the spurious numerical artefacts and presents a robust solution for the Carbuncle problem. The proposed rotated-hybrid Riemann solver provides sufficient numerical dissipation to capture the behaviour of strong shock waves for RHD. Therefore, in this work, we focus on two benchmark test cases (odd–even decoupling and double-Mach reflection problems) and investigate two astrophysical phenomena, the relativistic Richtmyer–Meshkov instability and the propagation of a relativistic jet. In all presented test cases, the Carbuncle problem is shown to be eliminated by employing the proposed rotated-hybrid Riemann solver. This strategy is problem-independent, straightforward to implement and provides a consistent robust numerical solution when combined with Godunov-type high-order schemes for RHD