README.md

Moving body simulations in compressible flow

This repository contains the code for the implementation of a moving embedded boundary algorithm for compressible flows within the compressible Navier-Stokes (CNS) framework in AMReX. Currently only prescribed motion can be done. i.e. the simulations are not two-way coupled - the EB does not move in accordance with the force exerted by the fluid on it.

Simulations

Simulations are performed for a variety of inviscid and viscous test cases. Some of the simulations are shown below - shock-cylinder interaction, shock-wedge interaction, transonic buffet over a NACA0012 airfoil, shock-cone interaction, cylinder-oscillating piston system, and transversely oscillating cylinder in a crossflow.

Governing equations

The compressible Navier-Stokes equations for moving boundaries in the finite volume formulation are

$\frac{\partial\rho}{\partial t} = -\frac{1}{\alpha(t) V}\int\limits_{\partial\Omega(t)}\rho\mathbf{u}\cdot\mathbf{n} \ dA$
$\frac{\partial\rho\mathbf{u}}{\partial t} = \frac{1}{\alpha(t) V}\Bigg[-\int\limits_{\partial\Omega(t)} (p\mathbf{n} + \rho\mathbf{u} (\mathbf{u}\cdot\mathbf{n}))\ dA + \int\limits_{\partial\Omega(t)} \mathbf{\tau}\cdot\mathbf{n}\ dA\Bigg]$
$\frac{\partial\rho E}{\partial t} = \frac{1}{\alpha(t) V}\Bigg[-\int\limits_{\partial\Omega(t)} (p+\rho E) \mathbf{u}\cdot\mathbf{n}\ dA + \int\limits_{\partial\Omega(t)} \Bigg(\mathbf{u}\cdot\mathbf{\tau}\cdot\mathbf{n} + k\nabla T\cdot \mathbf{n}\Bigg)\ dA\Bigg]$
where the conservative variables $(\rho,\rho\mathbf{u},\rho E)$ are cell-averaged, $\partial\Omega(t)$ is the temporally varying surface of the control volume, $\mathbf{u}$ is the velocity of the fluid on the control surface, $p$ is the pressure, $\mathbf{n}$ is the outward unit normal to the control surface, $V=\Delta x\Delta y\Delta z$ is the cell volume, $\alpha(t)$ is the time-varying volume fraction of the fluid in the cell, $\tau$ is the viscous stress tensor, $k$ is the thermal conductivity of the fluid, $T$ is the temperature, and $A$ denotes the surface of the control volume.

How to run a case

To run a case, the prescribed motion of the EB and its velocity are to be specified. Follow the steps below.

1. The prescribed motion of the EB is specified as a case in one of the if loops in CNS_init_eb2.cpp. For eg., for a vertically oscillating cylinder with $y(t)=A \cos(2\pi ft)$, create an if loop for a geom_type of moving_cylinder, and this will be specified in the inputs file.

	if(geom_type == "moving_cylinder"){
	EB2::CylinderIF cf1(0.2, 10.0, 0, {0.0,0.2*0.4*cos(2.0*3.14159265359*24.375*1.0*time),7.0}, false);
	auto polys = EB2::makeUnion(cf1);
	auto gshop = EB2::makeShop(polys);
	EB2::Build(gshop, geom, max_coarsening_level, max_coarsening_level, 4,false);
	}

2. Specify the velocity of the EB in hydro/cns_eb_hyp_wall.f90. This routine calculates the invsicid fluxes. For the above vertically oscillating cylinder the vertical velocity is v = -2 pi f A sin(2 pi f t).

	ublade = 0.0d0
	vblade = -0.2d0*0.4d0*2.0d0*3.14159265359d0*24.375d0*1.0d0*sin(2.0d0*3.14159265359d0*24.375d0*1.0d0*time)
	wblade = 0.0d0

3. Specify the velocity of the EB (same as in 2) in diffusion/cns_eb_diff_wall.F90 as well. This routine calcuates the viscous fluxes.

	ublade = 0.0d0
	vblade = -0.2d0*0.4d0*2.0d0*3.14159265359d0*24.375d0*1.0d0*sin(2.0d0*3.14159265359d0*24.375d0*1.0d0*time)
	wblade = 0.0d0

4. The initial condition is specified in Exec/cns_prob.F90.
5. Modify inputs located in Exec/<casename> for domain size, number of cells, refinement, viscosity, and also specify the geom_type eb2.geom_type = moving_cylinder for the above example.

Notes

1. When there are parts of the EB which move and parts which are stationary, if loops are used to specify which part of the EB moves. For eg., the case of a moving piston in a stationary cylinder CNS_EBoft_ICE, the velocity in hydro/cns_eb_hyp_wall.f90 is

	if(x.le.0.075d0 .and. abs(anrmx).gt.1e-6)then
	ublade=-0.02d0*3000.0d0/8.0d0*sin(3000.0d0/8.0d0*time)
	vblade = 0.0d0
	wblade = 0.0d0
	else
	ublade = 0.0d0
	vblade = 0.0d0
	wblade = 0.0d0
	endif