Streamlined body

How to define geometry in case of flow around a cylinder?
If you set the geometry the way I do it, then the time step becomes nan.

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def __init__(self):
  O.periodic = True
  O.cell.setBox(0.4,0.4,0.4)

  numspheres=1000
  young = 5e6
  density = 1000

  mat1 = FrictMat(young=young,poisson=0.5,frictionAngle=radians(15),density=density,label='spheremat')
  O.materials.append(mat1)
  mat2 = FrictMat(young=young,poisson=0.5,frictionAngle=0,density=0,label='wallmat')
  O.materials.append(mat2)

  epsilon = 1e-08
  minval = 0 + epsilon
  maxval = 0.4 - epsilon
  #wall coords, use facets for wall BC:
  v0 = Vector3(minval, minval, minval)
  v1 = Vector3(minval,minval,maxval)
  v2 = Vector3(maxval,minval,minval)
  v3 = Vector3(maxval,minval,maxval)

  v4 = Vector3(minval,maxval,minval)
  v5 = Vector3(minval,maxval,maxval)
  v6 = Vector3(maxval,maxval,minval)
  v7 = Vector3(maxval, maxval, maxval)

  lf0 = facet(vertices=[v0,v1,v2], material='wallmat')
  O.bodies.append(lf0)
  lf1 = facet(vertices=[v0,v2,v3], material='wallmat')
  O.bodies.append(lf1)

  uf0 = facet(vertices=[v4,v5,v6], material='wallmat')
  O.bodies.append(uf0)
  uf1 = facet(vertices=[v4,v6,v7], material='wallmat')
  O.bodies.append(uf1)

  ff0 = facet(vertices=[v1,v2,v6], material='wallmat')
  O.bodies.append(ff0)
  ff1 = facet(vertices=[v1,v6,v5], material='wallmat')
  O.bodies.append(ff1)

  bf0 = facet(vertices=[v0,v3,v7], material='wallmat')
  O.bodies.append(bf0)
  bf1 = facet(vertices=[v0,v7,v4], material='wallmat')
  O.bodies.append(bf1)

  ######cylinder######
  cyl = Cylinder(segment=[0,0,1],radius=0.05)
  b = Body()
  b.shape = cyl
  b.mat = O.materials['wallmat']
  O.bodies.append(b)

  #spheres
  mn, mx= Vector3(minval + epsilon, minval + epsilon, minval + epsilon), Vector3(-0.06, maxval - epsilon, maxval - epsilon)
  sp = pack.SpherePack();
  sp.makeCloud(mn,mx,rMean=0.00075,rRelFuzz=0.10, num=numspheres)
  O.bodies.append([sphere(center,rad,material='spheremat') for center,rad in sp])
  sphereIDs = [b.id for b in O.bodies if type(b.shape)==Sphere]