## Abstract

To simulate incompressible Navier–Stokes equation, a temporal splitting scheme in time and high-order symmetric interior penalty Galerkin (SIPG) method in space discretization are employed, while the local Lax-Friedrichs flux is applied in the discretization of the nonlinear term. Under a constraint of the Courant–Friedrichs–Lewy (CFL) condition, two benchmark problems in 2D are simulated by the fully discrete SIPG method. One is a lid-driven cavity flow and the other is a circular cylinder flow. For the former, we compute velocity field, pressure contour and vorticity contour. In the latter, while the von Kármán vortex street appears with Reynolds number 50≤Re≤400, we simulate different dynamical behavior of circular cylinder flows, and numerically estimate the Strouhal numbers comparable to the existing experimental results. The calculations on vortex dominated flows are carried out to investigate the potential application of the SIPG method.

### Keywords

- Navier–stokes equations
- von Kármán vortex street
- discontinuous Galerkin method
- interior penalty

## 1. Introduction

The Navier–Stokes equations are a concise physics model of low Knudsen number (i.e. non-rarefied) fluid dynamics. Phenomena described with the Navier–Stokes equations include boundary layers, shocks, flow separation, turbulence, and vortices, as well as integrated effects such as lift and drag. Analytical solutions of real flow problems including complex geometries are not available, therefore numerical solutions are necessary. The Navier–Stokes equation has been investigated by many scientists conducting research on numerical schemes for approximation solutions (see [1, 2, 3, 4, 5, 6, 7, 8]).

There exist two ways to provide reference data for such problems: One consists in the measurement of quantities of interest in physical experiments and the other is to perform careful numerical studies with highly accurate discretizations. With the prevalence and development of high-performance computers, advanced numerical algorithms are able to be tested for the validation of approaches and codes and for high-order convergence behavior of delicate discretizations.

Among discontinuous Galerkin (DG) methods, primal schemes and mixed methods are distinguished. The former depend on appropriate penalty terms of the discontinuous shape functions, while the latter rely on the mixed methods as the original second-order or higher-order partial differential equations are written as a system of first order partial differential equations with designed suitable numerical fluxes. Interior penalty discontinuous Galerkin methods (IPDG) are known as the representative of primal schemes whereas local discontinuous Galerkin methods (LDG) belong to the class of mixed methods [9]. About IPDG, symmetric interior penalty Galerkin (SIPG) and non-symmetric interior penalty Galerkin (NIPG) methods were first introduced originally for elliptic problems by Wheeler [10] and Rivière et al. [11]. Then, we presented some numerical analysis and simulations on nonlinear parabolic problems [12]. Recently, some work based on the SIPG and NIPG methods has been successfully applied to the steady-state and transient Navier–Stokes Equations [2, 13, 14], with careful analysis being conducted on optimal error estimates for the velocity.

The physics of Navier–Stokes flows are non-dimensionalized by Mach number *Re*,

where

The related concept of circulation

The concept of a vortex is that of vorticity concentrated along a path [15].

Lid driven cavity flows are geometrically simple boundary conditions testing the convective and viscous portions of the Navier Stokes equation in an enclosed unsteady environment. The cavity flow is characterized by a quiescent flow with the driven upper lid providing energy transfer into the cavity through viscous stresses. Boundary layers along the side and lower surfaces develop as the Reynolds number increases, which tends to shift the vorticity center of rotation towards the center. A presence of the sharp corner at the downstream upper corner increasingly generates small scale flow features as the Reynolds number increases. Full cavity flows remain a strong research topic for acoustics and sensor deployment technologies.

For non-streamlined blunt bodies with a cross-flow, an adverse pressure gradient in the aft body tends to promote flow separation and an unsteady flow field. The velocity field develops into an oscillating separation line on the upper and lower surfaces. This manifests as a series of shed vortices forming and then convecting downstream with the mean flow. The von Kármán vortex street is named after the engineer and fluid dynamicist Theodore Kármán (1963; 1994). Vortex streets are ubiquitous in nature and are visibly seen in river currents downstream of obstacles, atmospheric phenomena, and the clouds of Jupiter (e.g. The Great Red Spot). Shed vortices are also the primary driver for the zig-zag motion of bubbles in carbonated drinks. The bubble rising through the drink creates a wake of shed vorticity which impacts the integrated pressures causing side forces and thus side accelerations. The physics of sound generation with an Aeoleans harp operates by alternating vortices creating harmonic surfaces pressure variations leading to radiated acoustic tones. Tones generated by vortex shedding are the so-called Strouhal friction tones. If the diameter of the string, or cylinder immersed in the flow is

where

Let

subject to the boundary conditions on

Here the parameter

Using the divergence free constraint, problem (6)–(8) can be rewritten in the following conservative flux form [16]:

with the flux

and

A locally conservative DG discretization will be employed for the Navier–Stokes Eq. (9)–(11). We denote by

We define the spaces of discontinuous functions

The jump and average of a function

Further, let

where

Let

We mainly cite the content of [17], in which was motivated by the work of Girault, Rivière and Wheeler in a series of papers [2, 14]. Some projection methods [6, 18] have been developed to overcome the incompressibility constraints

The chapter is organized following [17]. In Section 2, a temporal discretization for the Navier–Stokes equation is listed with operator-splitting techniques, and subsequently, the nonlinear term is linearized. Both pressure and velocity field can be solved successively from linear elliptic and Helmholtz-type problems, respectively. In Section 3, a local numerical flux will be given for the nonlinear convection term and an SIPG scheme will be used in spacial discretization for those linear elliptic and Helmholtz-type problems with appropriate boundary conditions, and in Section 4, simulation results are presented for a lid-driven cavity flow up to

## 2. Temporal splitting scheme

We consider here a third-order time-accurate discretization method at each time step by using the previous known velocity vectors. Let

which has a timestep constraint based on the CFL condition (see [19]):

where

The first stage

When

with the following coefficients for the subsequent time levels (

Especially, by using the Euler forward discretization at the first time step (

and

which adopts the following coefficients to construct a second-order difference scheme for the time level (

Note that the coefficients in (15) are adjustable, but high-order time discrete schemes need to be verified with stability analysis.

The second stage

The pressure projection is as follows

To seek

with a Neumann boundary condition being implemented on the boundaries as

One can compute the vorticity

The third stage is completed by solving

which can be written as a Helmholtz equation for the velocity

From the three stages given above, we notice that (15) in the semi-discrete systems is presented in a linearized and explicit process, moreover, (17) and (18) are obviously a type of elliptic and Helmholtz problems at each time step as

## 3. The spatial discretizations

For spacial approximations, assume that piecewise polynomials of order

where the term

with

For the pressure correction step (17) and the viscous correction step (18), we use the SIPG method to approximate the correction steps. Choosing the orthonormal Legendre basis and the Legendre-Gauss-Lobatto quadrature points gives a well-conditioned Vandermonde matrix and the resulting interpolation well behaved, which greatly simplifies the formulas. The

where

In general,

where

and

where

## 4. Numerical results

We present a lid-driven flow problem to verify the efficiency and robustness of the interior penalty discontinuous Galerkin method, and then investigate a flow past a cylinder with walls or without a wall, as well as the relationship between the Strouhal number and the Reynold number. Throughout the section, time steps

*Example 1.* The lid-driven boundary conditions are given by:

Here the mesh size of the initial coarse grid is 0.2 and then it is uniformly refined three times with piecewise discontinuous elements being applied into the fully discrete SIPG approach.

The boundary condition at the vertex is a jump from zero velocity on the edge to a unit velocity on the upper edge. Nature prevents this singularity with a boundary layer forming along all walls, making the vertex velocity zero. It is reasonable to adopt adaptive meshes for solving those singularity problems. Here, we apply the semi-implicit SIPG method with approximation polynomials of order

*Example 2.* We simulate a channel flow past a circular cylinder with a radius

Cylinder flow contains the fundamentals of unsteady fluid dynamics in a simplified geometry. The flow properties and unsteadiness are well defined through years of experimental measurements across a wide range of Reynolds numbers [25], making the cylinder an ideal validation testcase for unsteady numerical fluid dynamics simulations.

Verification in a numerical domain requires insights from physics for a proper comparison to experimental and theoretical data. In Figure 7, we observe that the boundary layers forms along the upper and lower walls. From continuity of mass, the presence of a boundary layer decreasing the flow velocity near the wall requires an increase in the centerline average flow velocity. The cylinders wake provides a similar increase in centerline velocities. This implies a non-intuitive reality that drag can increase velocities within constrained domains. This effect is compensated for in wind tunnel test [26] environments topologically similar to Figure 7 with a constant mass flow rate and no-slip walls. Drela [15] develops an analysis for 2D wind tunnels resulting in an effective coefficient of drag of

and an effective Reynolds number of

where the

Alternatively, the flow without walls in Figure 8 has no interference of the boundary layers along the channel on the up and bottom boundaries, thus the pressure contours expend after flow passing through the cylinder. We also compared the components of the velocity profiles along the

We localize the domain around the cylinder and refine the mesh, then show the vorticity startup behavior in Figure 11 as well as the pressure Figure 12. Upon startup, two vortices of opposite direction are formed on the upper and lower aft portion of the cylinder. Given a low total simulation time, the flow field resembles the symmetrical low Reynolds number steady flow. As time progresses however, instabilities are magnified and the upper-lower symmetry increases. Given a total time of beyond

To reduce the effect of the boundary layers along the walls, the coefficients of drag and lift as well as the difference of pressure between the leading edge and the trailing edge on the cylinder shall be computed in a larger domain. Then in a domain

We use the definition of

where

with different values

which comes from the classical definition (5). From the evolution of

## 5. Conclusions

A SIPG solver is developed for the incompressible Navier Stokes equations of fluid flow. Two testcases are presented: a lid-driven cavity and a cylinder flow. The DG method produces stable discretizations of the convective operator for high order discretizations on unstructured meshes of simplices, as a requirement for real-world complex geometries. There are still some open problems, such as how the strouhal number of the von Kármán vortex street changes against the Reynolds number after oscillations or noises are added to the incident flow.

## Acknowledgments

The author would like to thank INTECHOPEN for their fully sponsor the publication of this chapter and waive the Open Access Publication Fee completely.

## Conflict of interest

The author declares no conflict of interest.

## Abbreviations

IPDG | Interior penalty discontinuous Galerkin methods |

SIPG | Symmetric interior penalty Galerkin method |

NIPG | Non-symmetric interior penalty Galerkin method |

CFL | Courant–Friedrichs–Lewy |