Implements a getter function for the strain rate tensor

src/lbmpy/__init__.py

@@ -12,6 +12,7 @@ from .lbstep import LatticeBoltzmannStep
 from .macroscopic_value_kernels import (
     pdf_initialization_assignments,
     macroscopic_values_getter,
+    strain_rate_tensor_getter,
     compile_macroscopic_values_getter,
     compile_macroscopic_values_setter,
     create_advanced_velocity_setter_collision_rule,
@@ -42,6 +43,7 @@ __all__ = [
     "LatticeBoltzmannStep",
     "pdf_initialization_assignments",
     "macroscopic_values_getter",
+    "strain_rate_tensor_getter",
     "compile_macroscopic_values_getter",
     "compile_macroscopic_values_setter",
     "create_advanced_velocity_setter_collision_rule",

src/lbmpy/macroscopic_value_kernels.py

 import functools
+import sympy as sp
 from copy import deepcopy
 from lbmpy.simplificationfactory import create_simplification_strategy
-from pystencils import create_kernel, CreateKernelConfig
+from pystencils import create_kernel, CreateKernelConfig, Assignment
 from pystencils.field import Field, get_layout_of_array
 from pystencils.enums import Target
 
 from lbmpy.advanced_streaming.utility import get_accessor, Timestep
+from lbmpy.relaxationrates import get_shear_relaxation_rate
+from lbmpy.utils import second_order_moment_tensor
 
 
-def pdf_initialization_assignments(lb_method, density, velocity, pdfs,
-                                   streaming_pattern='pull', previous_timestep=Timestep.BOTH,
-                                   set_pre_collision_pdfs=False):
-    """Assignments to initialize the pdf field with equilibrium"""
+def get_field_accesses(lb_method, pdfs, streaming_pattern, previous_timestep, pre_collision_pdfs):
     if isinstance(pdfs, Field):
         accessor = get_accessor(streaming_pattern, previous_timestep)
-        if set_pre_collision_pdfs:
+        if pre_collision_pdfs:
             field_accesses = accessor.read(pdfs, lb_method.stencil)
         else:
             field_accesses = accessor.write(pdfs, lb_method.stencil)
-    elif streaming_pattern == 'pull' and not set_pre_collision_pdfs:
+    elif streaming_pattern == 'pull' and not pre_collision_pdfs:
         field_accesses = pdfs
     else:
         raise ValueError("Invalid value of pdfs: A PDF field reference is required to derive "
                          + f"initialization assignments for streaming pattern {streaming_pattern}.")
+    return field_accesses
+
+
+def pdf_initialization_assignments(lb_method, density, velocity, pdfs,
+                                   streaming_pattern='pull', previous_timestep=Timestep.BOTH,
+                                   set_pre_collision_pdfs=False):
+    """Assignments to initialize the pdf field with equilibrium"""
+    field_accesses = get_field_accesses(lb_method, pdfs, streaming_pattern, previous_timestep, set_pre_collision_pdfs)
 
     if isinstance(density, Field):
         density = density.center
-
     if isinstance(velocity, Field):
         velocity = velocity.center_vector
 
@@ -41,17 +48,9 @@ def pdf_initialization_assignments(lb_method, density, velocity, pdfs,
 def macroscopic_values_getter(lb_method, density, velocity, pdfs,
                               streaming_pattern='pull', previous_timestep=Timestep.BOTH,
                               use_pre_collision_pdfs=False):
-    if isinstance(pdfs, Field):
-        accessor = get_accessor(streaming_pattern, previous_timestep)
-        if use_pre_collision_pdfs:
-            field_accesses = accessor.read(pdfs, lb_method.stencil)
-        else:
-            field_accesses = accessor.write(pdfs, lb_method.stencil)
-    elif streaming_pattern == 'pull' and not use_pre_collision_pdfs:
-        field_accesses = pdfs
-    else:
-        raise ValueError("Invalid value of pdfs: A PDF field reference is required to derive "
-                         + f"getter assignments for streaming pattern {streaming_pattern}.")
+
+    field_accesses = get_field_accesses(lb_method, pdfs, streaming_pattern, previous_timestep, use_pre_collision_pdfs)
+
     cqc = lb_method.conserved_quantity_computation
     assert not (velocity is None and density is None)
     output_spec = {}
@@ -65,6 +64,27 @@ def macroscopic_values_getter(lb_method, density, velocity, pdfs,
 macroscopic_values_setter = pdf_initialization_assignments
 
 
+def strain_rate_tensor_getter(lb_method, strain_rate_tensor, pdfs, streaming_pattern='pull',
+                              previous_timestep=Timestep.BOTH, use_pre_collision_pdfs=False):
+
+    field_accesses = get_field_accesses(lb_method, pdfs, streaming_pattern, previous_timestep, use_pre_collision_pdfs)
+
+    if isinstance(strain_rate_tensor, Field):
+        strain_rate_tensor = strain_rate_tensor.center_vector
+
+    omega_s = get_shear_relaxation_rate(lb_method)
+    equilibrium = lb_method.equilibrium_distribution
+    rho = equilibrium.density if equilibrium.compressible else equilibrium.background_density
+
+    f_neq = sp.Matrix([field_accesses[i] for i in range(lb_method.stencil.Q)]) - lb_method.get_equilibrium_terms()
+    pi = second_order_moment_tensor(f_neq, lb_method.stencil)
+    strain_rate_tensor_equ = - 1.5 * (omega_s / rho) * pi
+
+    assignments = [Assignment(strain_rate_tensor[i * lb_method.stencil.D + j], strain_rate_tensor_equ[i, j])
+                   for i in range(lb_method.stencil.D) for j in range(lb_method.stencil.D)]
+    return assignments
+
+
 def compile_macroscopic_values_getter(lb_method, output_quantities, pdf_arr=None,
                                       ghost_layers=1, iteration_slice=None,
                                       field_layout='numpy', target=Target.CPU,

tests/test_macroscopic_value_kernels.py

 import pytest
 import numpy as np
+import sympy as sp
 
 from lbmpy.creationfunctions import create_lb_method, LBMConfig
 from lbmpy.enums import Stencil, ForceModel, Method
 from lbmpy.macroscopic_value_kernels import (
-    compile_macroscopic_values_getter, compile_macroscopic_values_setter)
+    compile_macroscopic_values_getter, compile_macroscopic_values_setter, strain_rate_tensor_getter)
 from lbmpy.advanced_streaming.utility import streaming_patterns, Timestep
 from lbmpy.stencils import LBStencil
 
@@ -83,3 +84,18 @@ def test_set_get_constant_velocity(stencil, force_model, compressible):
         computed_velocity = velocity_output_field[0, 0, 0, :]
 
     np.testing.assert_almost_equal(np.array(ref_velocity[:method.dim]), computed_velocity)
+
+
+@pytest.mark.parametrize('stencil', [Stencil.D2Q9, Stencil.D3Q19])
+@pytest.mark.parametrize("method_enum", [Method.SRT, Method.TRT, Method.MRT, Method.CUMULANT])
+def test_get_strain_rate_tensor(stencil, method_enum):
+    stencil = LBStencil(stencil)
+    lbm_config = LBMConfig(stencil=stencil, method=method_enum, compressible=True)
+    method = create_lb_method(lbm_config=lbm_config)
+
+    pdfs = sp.symbols(f"f_:{stencil.Q}")
+    strain_rate_tensor = sp.symbols(f"strain_rate_tensor_:{method.dim * method.dim}")
+
+    getter_assignments = strain_rate_tensor_getter(method, strain_rate_tensor, pdfs)
+
+    assert len(getter_assignments) == method.dim * method.dim