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docs:classes:flowfield

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FlowField

The FlowField class represents vector-valued fields of the form


{\bf u}({\bf x}) &= \sum_{k_x,k_y,k_z,j}  \hat{u}_{k_x k_y k_z j}\bar{T}_{k_y}(y) \; e^{2 \pi i (k_x x/L_x + k_z z/L_z)} {\bf e}_j

and also scalar and tensor fields with appropriate changes in the dimensionality of the coefficients. The barred T function is a Chebyshev polynomial scaled to fit the domain y ∈ [a,b]. 1) The spatial domain of a FlowField is Ω = [0,Lx] x [a,b] x [0,Lz], with periodicity in x and z.

In channelflow programming, fields such as velocity, pressure, stress tensors, vorticity, etc. are stored as variables of type FlowField. The main functionality of the FlowField class is

  • algebraic and differential operations, +/-, +=, ∇, ∇2, norms, inner products, etc.
  • transforming back and forth between spectral coefficients  \hat{u}_{k_x k_y k_z j} and gridpoint values u_j (x_{n_x}, y_{n_y}, z_{n_z})
  • serving as input to DNS algorithms, which map velocity fields forward in time: u(x,t) → u(x, t+Δt)
  • setting and accessing scpetral coefficients and gridpoint values
  • reading and writing to disk

Constructors / Initialization

FlowFields are initialized with gridsize and cellsize parameters, read from disk, or assigned from computations. Examples:

   FlowField f;                                   // null value, 0-d field on 0x0x0 grid 
   FlowField u(Nx, Ny, Nz, Nd, Lx, Lz, a, b);     // Nd-dim field on Nx x Ny x Nz grid, [0,Lx]x[a,b]x[0,Lz]
   FlowField g(Nx, Ny, Nz, Nd, 2, Lx, Lz, a, b);  // Nd-dim 2-tensor  
   FlowField h("h");                              // read from file "h.ff"
   FlowField omega = curl(u);

Algebraic and differential operators

Assume f,g,h etc. are FlowField variables with compatible cell and grid sizes. Examples of possible operations

   f += g;                  // f = f + g
   f = curl(g); 
   f = lapl(g);
   f = div(g);
   f = diff(g, j, n);       // f_i  = d^n g_i /dx_j
   f = grad(g);             // f_ij = dg_i / dx_j
   f = cross(g,h);      
   f *= 2.7;                // f = 2.7*f
  
   
   Real c = L2IP(f,g);      // L2 inner product of f,g
   Real n = L2Norm(u);
   Real D = dissipation(u);
   Real E = energy(u); 
   Real I = wallshear(u);

The latter functions are defined as

 $ \begin{align*}
L2IP(f,g) &= \frac{1}{L_x L_y L_z} \int_{\Omega} {\bf f} \cdot {\bf g} \,\, d{\bf x} \\
L2Norm(u) &= \left(\frac{1}{L_x L_y L_z} \int_{\Omega} \|{\bf u}\|^2\,\, d{\bf x} \right)^{1/2}\\
     E(u) &= \frac{1}{2 L_x L_y L_z} \int_{\Omega} \|{\bf u}\|^2\,\, d{\bf x}  \\
     D(u) &= \frac{1}{L_x L_y L_z} \int_{\Omega} \|\nabla \times {\bf u}\|^2 \,\, d{\bf x} \\
     I(u) &= \frac{1}{L_x L_z} \int_{y=a,b} \frac{\partial u}{\partial y} \, dx dz
\end{align*} $

Transforms and data access

FlowField transforms are a complicated subject –there are transforms in x,y, and z and implicit symmetries in complex spectral coefficients due to the real-valuedness of the field, for instance. This section outlines the bare essentials of transforms and data access methods. For further details see the {docs:chflowguide.pdf|Channelflow User Guide}}.

docs/classes/flowfield.1234813466.txt.gz · Last modified: 2009/02/16 11:44 by gibson