filaments.h

Curved vortex filaments
Representation of vortex filaments in Basilisk
References

Curved vortex filaments

This section borrows from the development for a general curved vortex presented by Callegari and Ting (1978) and notes from the book by Maurice Rossi, Stéphane Le Dizès, and others.

Geometrical elements for a curved line (a) in the osculating plane (b) in the plane orthogonal to the vortex axis

Let us define a curvilinear orthogonal basis associated with a reference space-curve \mathcal{C}(s,t). Here s is the arc-length along the initial curve measured from a given reference point.

Each point along \mathcal{C}(s,t) is characterized by its position vector \boldsymbol{x}_c, local curvature \kappa(s,t), local torsion \tau(s,t), and the corresponding Frenet–Serret frame composed of three vectors (\boldsymbol{\hat{t}}, \boldsymbol{\hat{n}}, \boldsymbol{\hat{b}}) \begin{aligned} \boldsymbol{\hat{t}}(s) \equiv \frac{d\boldsymbol{x}_c}{ds}, \quad \boldsymbol{\hat{n}}(s) \equiv \frac{1}{\kappa}\frac{d\boldsymbol{\hat{T}}}{ds}, \quad \boldsymbol{\hat{b}}(s) \equiv \boldsymbol{\hat{T}}\times\boldsymbol{\hat{N}} \end{aligned} which are related to one another as: \begin{aligned} \frac{d}{ds} \begin{bmatrix} \boldsymbol{\hat{t}} \\ \boldsymbol{\hat{n}} \\ \boldsymbol{\hat{b}} \end{bmatrix} = \begin{bmatrix} 0 & \kappa(s) & 0 \\ -\kappa(s) & 0 & \tau(s) \\ 0 & -\tau(s) & 0 \end{bmatrix} \begin{bmatrix} \boldsymbol{\hat{t}} \\ \boldsymbol{\hat{n}} \\ \boldsymbol{\hat{b}} \end{bmatrix} \end{aligned}

The position of a given point M can be expressed as two coordininates in the plane \mathcal{A}(s,t) locally perpendicular to \mathcal{C}(s,t) and one coordinate s along \mathcal{C}(s,t). This way, we may introduce a local radial and angular coordinates (\rho,\varphi) and write the position vector of a point M as: \begin{aligned} \boldsymbol{x} = \boldsymbol{x}_c (s) + \rho\boldsymbol{e}_\rho(s,\varphi) \end{aligned} where the unit vectors \boldsymbol{e}_\rho and \boldsymbol{e}_\varphi are defined as \begin{aligned} \boldsymbol{\hat{e}}_\rho &=& \cos\varphi\boldsymbol{\hat{n}} + \sin\varphi\boldsymbol{\hat{b}} \\ \boldsymbol{\hat{e}}_\varphi &=& -\sin\varphi\boldsymbol{\hat{n}} + \cos\varphi\boldsymbol{\hat{b}} \end{aligned}

Note that (\rho,\varphi,s) are curvilinear coordinates but they are not orthogonal in general. However, if we define an angle \theta \begin{aligned} \theta = \varphi - \theta_0(s), \quad \text{ where } \quad \frac{d\theta_0}{ds} = -\tau(s) \end{aligned} then (\rho,\theta,s) form a set of curvilinear orthogonal coordinates. In this system, the infinitesimal increment reads as \begin{aligned} d\boldsymbol{x} &=& d\rho ~\boldsymbol{\hat{e}}_\rho(s,\varphi) + \rho d\varphi ~\boldsymbol{\hat{e}}_\varphi(s,\varphi) + h_s ds ~\boldsymbol{\hat{T}}(s) \end{aligned} where \begin{aligned} h_s = (1-\kappa(s)\rho\cos(\theta+\theta_0(s))) \end{aligned}

Representation of vortex filaments in Basilisk

vortex_filament: defines the properties of a vortex filament

This structure represents a vortex filament. It includes properties such as the number of segments, a scalar quantity, and various vectors representing the geometry and orientation of the filament.

nseg: number of segments
t: arbitrary of parameter, usually \xi_n
C: Cartesian coordinates describing the curve, \boldsymbol{x}_c \in \mathcal{C}(s(\xi_n))
Tvec: tangent unit vector, \boldsymbol{\hat{T}}(s(\xi_n))
Nvec: normal unit vector, \boldsymbol{\hat{N}}(s(\xi_n))
Bvec: binormal unit vector, \boldsymbol{\hat{B}}(s(\xi_n))
s: arclenght coordinate, s(\xi_n) or \ell(\xi_n)
pcar: placeholder for a Cartesian coordinate
sigma: local stretching factor, \sigma(s(\xi_n)) (optional)
kappa: local curvature, \kappa(s(\xi_n)) (optional)
tau: local torsion, \tau(s(\xi_n)) (optional)
theta0: cumulative torsion, \varphi_0(s(\xi_n)) (optional)

#include "PointTriangle.h"
struct vortex_filament{
  int     nseg;     // number of segments
  double* phi;    // arbitrary parametrisation of C  
  coord*  C;        // cartesian coords of filament
  coord*  Tvec;     // unit tangent vector
  coord*  Nvec;     // unit normal vector
  coord*  Bvec;     // unit binormal vector
  double* s;        // arc-length coordinate
  coord   pcar;     // current point in Cartesian coordinates  
  double* sigma;    // Stretching factor
  double* kappa;    // Curvature
  double* tau;      // Torsion
  double* theta0;  // Cumulative torsion
  double* a;        // Core size
  coord*  dC;       // 
  coord*  d2C;      // 
  coord*  d3C;      // 
  coord*  Ulocal;   //
  coord*  Uauto;    // 
  coord*  Umutual;  //
  coord*  U;        // 
  coord*  Uprev;    // 
  double* vol;      // Initial volume 
  
};

// Function to allocate memory for the *members* of a vortex_filament struct
// Assumes the struct itself has already been created (e.g., on the stack or
// heap)
void allocate_vortex_filament_members(struct vortex_filament* filament, int nseg) {
  
  if (filament == NULL) {
    perror("Failed to allocate memory for filament");
    return ;
  }

  filament->nseg  = nseg;
  filament->pcar  = (coord) {0.0, 0.0, 0.0};

  // Allocate memory for the double arrays
  filament->phi = malloc(nseg * sizeof(double));  
  filament->s     = malloc(nseg * sizeof(double));
  filament->sigma = malloc(nseg * sizeof(double));
  filament->kappa = malloc(nseg * sizeof(double));
  filament->tau   = malloc(nseg * sizeof(double));  
  filament->a     = malloc(nseg * sizeof(double));
  filament->vol   = malloc(nseg * sizeof(double));
  filament->theta0 = malloc(nseg * sizeof(double));
  
  // Allocate memory for the coord arrays
  filament->C     = malloc(nseg * sizeof(coord));
  filament->dC    = malloc(nseg * sizeof(coord));
  filament->d2C   = malloc(nseg * sizeof(coord));
  filament->d3C   = malloc(nseg * sizeof(coord));  
  filament->Tvec  = malloc(nseg * sizeof(coord));
  filament->Nvec  = malloc(nseg * sizeof(coord));
  filament->Bvec  = malloc(nseg * sizeof(coord));

  filament->Ulocal  = malloc(nseg * sizeof(coord));
  filament->Uauto   = malloc(nseg * sizeof(coord));
  filament->Umutual = malloc(nseg * sizeof(coord));
  filament->U       = malloc(nseg * sizeof(coord));
  filament->Uprev   = malloc(nseg * sizeof(coord));
}

// Function to free memory for the *members* of a vortex_filament struct
// Assumes the struct itself will NOT be freed by this function
void free_vortex_filament_members(struct vortex_filament* filament) {
  if (filament == NULL) {
    // Nothing to free
    return;
  }

  // Free the double arrays
  free(filament->phi);      filament->phi = NULL; // Set to NULL after freeing
  free(filament->s);        filament->s = NULL;
  free(filament->sigma);    filament->sigma = NULL;
  free(filament->kappa);    filament->kappa = NULL;
  free(filament->tau);      filament->tau = NULL;
  free(filament->a);        filament->a = NULL;
  free(filament->vol);      filament->vol = NULL;
  free(filament->theta0);   filament->theta0 = NULL;

  // Free the coord arrays
  free(filament->C);        filament->C = NULL;
  free(filament->dC);       filament->dC = NULL;
  free(filament->d2C);      filament->d2C = NULL;
  free(filament->d3C);      filament->d3C = NULL;
  free(filament->Tvec);     filament->Tvec = NULL;
  free(filament->Nvec);     filament->Nvec = NULL;
  free(filament->Bvec);     filament->Bvec = NULL;

  free(filament->Ulocal);   filament->Ulocal = NULL;
  free(filament->Uauto);    filament->Uauto = NULL;
  free(filament->Umutual);  filament->Umutual = NULL;
  free(filament->U);        filament->U = NULL;
  free(filament->Uprev);    filament->Uprev = NULL;
}

struct local_filament{  
  bool near;       // flag based on distance from C
  double phi;      // arbitrary parametrisation of C  
  coord  C;        // cartesian coords of filament
  coord  Tvec;     // unit tangent vector
  coord  Nvec;     // unit normal vector
  coord  Bvec;     // unit binormal vector
  double s;        // arc-length coordinate
  coord  pcar;     // current point in Cartesian coordinates  
  double sigma;    // Stretching factor
  double kappa;    // Curvature
  double tau;      // Torsion
  double theta0;   // Cumulative torsion
  double a;        // Core size
  coord Mcar;      //
  coord Mrad;      //
  double rho;      //
  double theta;    //
};

Interpolate Values Using Cubic Splines

gsl_interp1d_vector(): performs 1D interpolation on a vector using cubic splines

This function performs 1D interpolation on a vector using cubic splines from the GNU Scientific Library (GSL). It takes discretized coord values V0 at given spatial coordinate phi0 and interpolates to find the corresponding value at a coordinate phiq.

The arguments and their descriptions are:

nseg: number of segments
phi0: array with the coordinates \phi_0
V0: array with the coord values V_0
phiq: target value at which to interpolate V_q

#pragma autolink -lgsl -lgslcblas
#include <gsl/gsl_spline.h>
coord gsl_interp1d_vector( int nseg, double* phi0, coord * V0, double phiq){
  coord Vq;
  gsl_interp_accel *acc = gsl_interp_accel_alloc();

  foreach_dimension(){
    double *V_x;
    V_x = malloc(sizeof(double)*nseg);
    for (int i = 0; i < nseg; i++)
      V_x[i] = V0[i].x;

    gsl_spline *spline_x = gsl_spline_alloc(gsl_interp_cspline, nseg);
    gsl_spline_init(spline_x, phi0, V_x, nseg);
    Vq.x = gsl_spline_eval (spline_x, phiq, acc);
    gsl_spline_free (spline_x);
    free(V_x);
  }
  gsl_interp_accel_free (acc);
  return Vq;
}

gsl_interp1d_scalar(): performs 1D interpolation on a scalar using cubic splines

This function performs 1D interpolation on a scalar using cubic splines from the GNU Scientific Library (GSL). It takes discretized scalar values P0 at given spatial coordinate phi0 and interpolates to find the corresponding value at a coordinate phiq.

The arguments and their descriptions are:

nseg: number of segments
phi0: array with the coordinates \phi_0
P0: array with the scalar values P_0
phiq: target value at which to interpolate P_q

double gsl_interp1d_scalar( int nseg, double* phi0, double * P0, double phiq){
  double Pq;
  gsl_interp_accel *acc = gsl_interp_accel_alloc();
  gsl_spline *spline_x = gsl_spline_alloc(gsl_interp_cspline, nseg);
  gsl_spline_init(spline_x, phi0, P0, nseg);
  Pq = gsl_spline_eval (spline_x, phiq, acc);
  gsl_spline_free (spline_x);
  gsl_interp_accel_free (acc);
  return Pq;
}

Project a Point onto the Frenet Frame of a Vortex Filament

frenet_projection(): projects a point onto the Frenet frame

This function calculates the projection of a point M onto the Frenet-Serret frame of a vortex filament at a specified position s(\xi_q). The Frenet frame (\boldsymbol{\hat{T}}, \boldsymbol{\hat{N}}, \boldsymbol{\hat{B}}) is interpolated at this position.

The function returns the dot product of the vector from the origin \boldsymbol{O}(s(\xi_q)) to the point \boldsymbol{M} with the tangent vector \boldsymbol{\hat{T}}(s(\xi_q)):

(\boldsymbol{M} - \boldsymbol{O}(s(\xi_q))) \cdot \boldsymbol{\hat{T}}(s(\xi_q)).

If the point \boldsymbol{M} lies within the plane \mathcal{A}(s(\xi_q)), the output of this function will be zero. This property will be utilized as the objective function to minimize when projecting points onto the Frenet-Serret frame.

The arguments and their descriptions are:

tq: double representing the position along the vortex filament where the projection is to be computed.
params: pointer to a struct vortex_filament, which contains the properties of the vortex filament, including the number of segments, coordinates, and Frenet frame vectors.

double frenet_projection (double phiq, void *params){
  struct vortex_filament *p = (struct vortex_filament *) params;

  coord ccar, frenet[3];
  ccar = gsl_interp1d_vector( p->nseg, p->phi, p->C, phiq);

  frenet[0] = gsl_interp1d_vector( p->nseg, p->phi, p->Tvec, phiq);
  frenet[1] = gsl_interp1d_vector( p->nseg, p->phi, p->Nvec, phiq);
  frenet[2] = gsl_interp1d_vector( p->nseg, p->phi, p->Bvec, phiq);

  return vecdot(vecdiff(p->pcar, ccar), frenet[0]); 
}

frenet_projection_min(): find the position with the minimum projection on the Frenet frame

This function solves a minimization problem to find the position along a vortex filament where the projection of a point pcar onto the tangent vector of the Frenet-Serret frame is minimized. In other words, we search for \xi_q such that : (\boldsymbol{M} - \boldsymbol{O}(s(\xi_q))) \cdot \boldsymbol{\hat{T}}(s(\xi_q)) = 0

It uses the Brent method from the GNU Scientific Library (GSL) to find the root of the projection function within a specified interval.

The arguments and their descriptions are:

params: pointer to a struct vortex_filament, which contains the properties of the vortex filament, including the number of segments, coordinates, and Frenet frame vectors.
r: an initial guess for \xi_q

#include <gsl/gsl_roots.h>
#include <gsl/gsl_errno.h>
double frenet_projection_min(struct vortex_filament params, double r) {

  int status, verbose = 0;
  int iter = 0, max_iter = 100;
  const gsl_root_fsolver_type *T;
  gsl_root_fsolver *s;

  double x_lo = r - 0.25, x_hi = r + 0.25;
  gsl_function F;

  F.function = &frenet_projection;
  F.params = &params;

  T = gsl_root_fsolver_brent;
  s = gsl_root_fsolver_alloc (T);
  gsl_set_error_handler_off();
  gsl_root_fsolver_set (s, &F, x_lo, x_hi);

  if (verbose == 1) {
    printf ("using %s method\n", gsl_root_fsolver_name (s));
    printf ("%5s [%9s, %9s] %9s %10s %9s\n", "iter", "lower", "upper", "root", "err", "err(est)");
  }

  do {
    iter++;
    status = gsl_root_fsolver_iterate (s);
    r = gsl_root_fsolver_root (s);
    x_lo = gsl_root_fsolver_x_lower (s);
    x_hi = gsl_root_fsolver_x_upper (s);
    status = gsl_root_test_interval (x_lo, x_hi, 0, 1e-8);

    if ((status == GSL_SUCCESS) && (verbose == 1)){
      printf ("Converged:\n");
      printf ("%5d [%.7f, %.7f] %.7f %.7f\n", iter, x_lo, x_hi, r, x_hi - x_lo);
    }
  }
  while (status == GSL_CONTINUE && iter < max_iter);
  gsl_root_fsolver_free (s);

  return r;
}

get_local_coordinates(): computes the coordinates in the local frame

This function computes the local coordinates required for the vorticity field. Each point M is projected into the local Frenet-Serret frame to obtain a set of local coordinates, such that:

\begin{aligned} \textit{i)} \quad\quad & (\boldsymbol{M} - \boldsymbol{O}(s(\xi_q))) \cdot \boldsymbol{\hat{T}}(s(\xi_q)) = 0 \\ \textit{ii)} \quad\quad & (\boldsymbol{M} - \boldsymbol{O}(s(\xi_q))) \cdot \boldsymbol{\hat{N}}(s(\xi_q)) = x_N \\ \textit{iii)} \quad\quad & (\boldsymbol{M} - \boldsymbol{O}(s(\xi_q))) \cdot \boldsymbol{\hat{B}}(s(\xi_q)) = x_B \end{aligned}

This requires finding the value of s(\xi_q) along each space curve that verifies i) through a minization process.

Then, we use the local coordinates (x_N, x_B) to define a local radial and angular coordinates: \begin{aligned} x_\rho = \sqrt{x_N^2 + x_B^2}, && x_\phi = \arctan(x_B,x_N) \end{aligned}

struct local_filament get_local_coordinates(int spatial_period, double max_distance=L0, struct vortex_filament *vortex) {
  struct local_filament local_coordinates = {0}; 

  coord cart_coord, radial_coord; 
  double min_distance = 1e30, min_phi = 0;
  double local_radial_coord, local_angular_coord;
  
  // Iterate through each segment to find the segment closest to the point
  // this should give us a good starting point
  for (int i = 0; i < vortex->nseg; i++) {
    double current_distance = vecdist2(vortex->pcar, vortex->C[i]);
    if (current_distance < min_distance) {
      min_distance = current_distance;
      min_phi = vortex->phi[i];
    }
  }

  // Adjust the initial phi guess if the curve has periodicity
  if (spatial_period != 0)
    min_phi = fmod(min_phi + spatial_period * 2 * pi, spatial_period * 2 * pi);

  
  // If the point is close enough to the vortex, refine the initial guess
  if (min_distance < max_distance) {
    
    // Find the value of xi
    double phi = frenet_projection_min(*vortex, min_phi);

    // Find the Cartesian coordinates of point O(xi)
    coord Ocar = gsl_interp1d_vector(vortex->nseg, vortex->phi, vortex->C, phi);

    // Compute the Frenet-Serret frame vectors at the projected phi
    coord Tvec, Nvec, Bvec;
    Tvec = gsl_interp1d_vector(vortex->nseg, vortex->phi, vortex->Tvec, phi);
    Nvec = gsl_interp1d_vector(vortex->nseg, vortex->phi, vortex->Nvec, phi);
    Bvec = gsl_interp1d_vector(vortex->nseg, vortex->phi, vortex->Bvec, phi);
    
    // Compute local coordinates in the Frenet-Serret frame
    cart_coord.x = vecdot(vecdiff(vortex->pcar, Ocar), Tvec); // x_T (must be zero) 
    cart_coord.y = vecdot(vecdiff(vortex->pcar, Ocar), Nvec); // x_N 
    cart_coord.z = vecdot(vecdiff(vortex->pcar, Ocar), Bvec); // x_B

    // Convert local coordinates to radial coordinates
    local_radial_coord = sqrt(vecdot(cart_coord, cart_coord));
    local_angular_coord = atan2(cart_coord.z, cart_coord.y);
    
    // Compute the torsion angle
    double torsion_angle = gsl_interp1d_scalar(vortex->nseg, vortex->phi, vortex->theta0, phi);

    // Set radial coordinates
    radial_coord.x = cart_coord.x;
    radial_coord.y = local_radial_coord;
    radial_coord.z = local_angular_coord - torsion_angle;

    // Then, we compute the other properties
    double s       = gsl_interp1d_scalar( vortex->nseg, vortex->phi, vortex->s,       phi);
    double sigma   = gsl_interp1d_scalar( vortex->nseg, vortex->phi, vortex->sigma,   phi);
    double kappa   = gsl_interp1d_scalar( vortex->nseg, vortex->phi, vortex->kappa,   phi);
    double tau     = gsl_interp1d_scalar( vortex->nseg, vortex->phi, vortex->tau,     phi);    
    double a       = gsl_interp1d_scalar( vortex->nseg, vortex->phi, vortex->a,       phi);    

    local_coordinates = (struct local_filament){1, phi, Ocar, Tvec, Nvec, Bvec, 
      s, vortex->pcar, sigma, kappa, tau, torsion_angle, a, cart_coord, radial_coord, 
      local_radial_coord, local_angular_coord - torsion_angle}; 
  }
  return local_coordinates;
}

double get_min_distance(int spatial_period, double max_distance=L0, struct vortex_filament *vortex) {
    
  double min_distance = 1e30;
  
  // Iterate through each segment to find the segment closest to the point
  // this should give us a good starting point
  for (int i = 0; i < vortex->nseg; i++) {
    double current_distance = vecdist2(vortex->pcar, vortex->C[i]);
    if (current_distance < min_distance) {
      min_distance = current_distance;      
    }
  }
  return min_distance;
}

Compute the Finite Difference Derivative of a Coordinate Array

fd_derivative(): calculates the first derivative of a coordinate array using finite differences

This function computes the first derivative of a coordinate array X using a central finite difference scheme. It handles the interior points using a standard central difference formula and applies a periodic boundary condition with an optional shift to the endpoints.

The arguments and their descriptions are:

n: integer representing the number of points in the coordinate array.
dphi: double representing the spacing between consecutive points in the array, used as the step size in the finite difference calculation.
shift: a coord structure representing an optional shift applied to the periodic boundary condition.
X: pointer to a coord array representing the input coordinate values.
dX: pointer to a coord array where the computed derivatives will be stored.

void fd_derivative( int n, double dphi, coord shift, coord *X, coord *dX){
  for (int i = 1; i < n-1; i++){
    foreach_dimension(){
      dX[i].x = (X[i+1].x - X[i-1].x)/(2*dphi);
    }
  }
  foreach_dimension(){
    dX[0].x   = (X[1].x - X[n-2].x + shift.x)/(2*dphi);
    dX[n-1].x = (X[1].x - X[n-2].x + shift.x)/(2*dphi);
  }
}

Finally, we create a macro so we can initialize the vortex filaments more easily

macro initialize_filaments (struct vortex_filament filament, int nseg, double dphi, double* phi, double* a, coord* C, coord xshift, coord dxshift)
{
  
  for (int i = 0; i < nseg; i++){   
    filament.a[i] = a[i]; 
    filament.phi[i] = phi[i]; 
    foreach_dimension(){            
      filament.C[i].x =  C[i].x;  
    }
  }  

  // Find the 1st, 2nd, and 3rd derivatives of C  
  fd_derivative(nseg, dphi,  xshift,   filament.C,  filament.dC);
  fd_derivative(nseg, dphi, dxshift,  filament.dC, filament.d2C);
  fd_derivative(nseg, dphi, dxshift, filament.d2C, filament.d3C);

  // Compute the Frenet-Serret frame, curvature, and torsion
  for (int i = 0; i < nseg; i++){   
    foreach_dimension(){                      
      filament.Tvec[i].x =  filament.dC[i].x/sqrt(vecdot( filament.dC[i],  filament.dC[i]));      
      filament.Nvec[i].x = filament.d2C[i].x/sqrt(vecdot(filament.d2C[i], filament.d2C[i]));      
    }
    filament.sigma[i] = sqrt(vecdot( filament.dC[i],  filament.dC[i]));
    filament.Bvec[i] = vecdotproduct(filament.Tvec[i], filament.Nvec[i]);   
      
    filament.kappa[i] = sqrt(vecdot(filament.d2C[i], filament.d2C[i]))/sq(filament.sigma[i]);
    coord var1 = vecdotproduct(filament.dC[i], filament.d2C[i]);    
    filament.tau[i] = vecdot(var1, filament.d3C[i])/vecdot(var1,var1);        
  }

  // Compute the arc-lenght coordinate and cumulative torsion  
  memset (filament.s,      0, nseg*sizeof (double));
  memset (filament.theta0, 0, nseg*sizeof (double));  
  for (int i = 0; i < nseg-1; i++){
    filament.s[i+1] = filament.s[i] + filament.sigma[i+1]*dphi;
    filament.theta0[i+1] = filament.theta0[i] - filament.sigma[i+1]*filament.tau[i+1]*dphi;
  }
}

Compute the curvature terms at next to leading order for a Batchelor vortex

At the next to leading order, corrections due to curvature are obtained by solving a second- order differential equation \mathbf{L} \psi = R

In the functions below, we solve numerically \psi and compute the corrections at this order.

#pragma autolink -lgsl -lgslcblas
#include <gsl/gsl_integration.h>
#include <gsl/gsl_errno.h>

Struct to pass parameters (a, U_c) to the GSL integration functions

typedef struct {
  double a;
  double U_c;
} integration_params;

Struct to hold the results: \psi(\rho) and its first two derivatives

typedef struct {
  double A;    // Psi(rho)
  double A_p;  // First derivative Psi'(rho)
  double A_pp; // Second derivative Psi''(rho)
} integration_results;

f(\rho,a) = \exp(-\rho^2/a^2)

double gauss(double s, const integration_params *params) {
  return exp(-s * s / (params->a * params->a));
}

R = u^{(0)}_\theta + 2\rho\omega^{(0)}_\xi - 2\rho\omega^{(0)}_\theta u^{(0)}_\xi/u^{(0)}_\theta Here, \boldsymbol{u}^{(0)} and \boldsymbol{\omega}^{(0)} are the leading order solutions. Subscripts (\rho,\theta,\xi) indicate the local radial, azimuthal, and axial components, respectively.

double right_hand_side(double s, void *p) {
  integration_params *params = (integration_params *)p;
  const double a = params->a;
  const double U_c = params->U_c;
  const double tol = 1e-18;

  if (fabs(s) < tol) return 0.0;

  double gs = gauss(s, params);
  double u_th = (1.0 / s) * (1.0 - gs);
  double u_xi = U_c * gs;
  double w_th = U_c * (2.0 * s / sq(a)) * gs;
  double w_xi = (2.0 / sq(a)) * gs;
  double R = 2.0 * s * w_xi + u_th - 2.0 * s * w_th * u_xi / (u_th + tol);
  return R;
}

The solution of \psi is obtained using the method of variation of constants \psi = y_1 \int_0^\rho G(s)/(s y_1^2(s)) ds where the integrand G(s) reads G(\rho) = \int_0^\rho s y_1(s) R(s) ds and y_1 is the fundamental solution y_1 = u^{(0)}_\theta = \frac{1}{\rho} (1 - f(\rho,a))

double integrand_G(double s, void *p) {
  integration_params *params = (integration_params *)p;
  return (1.0 - gauss(s, params)) * right_hand_side(s, p);
}

double integrand_Psi(double s, void *p) {
  integration_params *params = (integration_params *)p;

  if (fabs(s) < 1e-9) return 0.0;

  gsl_integration_workspace *w = gsl_integration_workspace_alloc(1000);
  double G_s, error;
  
  gsl_function F_G;
  F_G.function = &integrand_G;
  F_G.params = p;

  gsl_integration_qag(&F_G, 0, s, 0, 1e-7, 1000, GSL_INTEG_GAUSS61, w, &G_s, &error);
  gsl_integration_workspace_free(w);

  double gs = gauss(s, params);
  double denominator = sq(1.0 - gs);
  
  if (fabs(denominator) < 1e-18) return 0.0;
  
  return (s * G_s) / denominator;
}

We perform the numerical integration using GLS and evaluate the first and second derivatives with respect to \rho

void compute_A_with_derivatives(double rho, double a, double U_c, integration_results *results) {
  const double tol = 1e-18;
  if (rho < tol){
    results->A = 0.;
    results->A_p = 0.;
    results->A_pp = 0.; 
    return;
  }

  integration_params params = {a, U_c};

  // --- Step 1: Compute the necessary integrals ---
  gsl_integration_workspace *w = gsl_integration_workspace_alloc(1000);
  double G_rho, I_psi_rho, error;
  
  gsl_function F_G, F_psi;
  F_G.function = &integrand_G;
  F_G.params = &params;
  F_psi.function = &integrand_Psi;
  F_psi.params = &params;
  
  // Temporarily turn off default GSL error handler
  gsl_error_handler_t *old_handler = gsl_set_error_handler_off();

  // Integral G(rho)
  gsl_integration_qag(&F_G, 0, rho, 0, 1e-7, 1000, GSL_INTEG_GAUSS61, w, &G_rho, &error);
  
  // Integral for A(rho), which we call I_psi(rho)
  gsl_integration_qag(&F_psi, 0, rho, 0, 1e-7, 1000, GSL_INTEG_GAUSS61, w, &I_psi_rho, &error);
  
  gsl_set_error_handler(old_handler); // Restore handler
  gsl_integration_workspace_free(w);

  // --- Step 2: Compute intermediate values at rho ---
  double g_rho = gauss(rho, &params);
  double R_rho = right_hand_side(rho, &params);

  // --- Step 3: Calculate A, A', and A'' using analytical formulas ---
  results->A = (1.0 / rho) * (1.0 - g_rho) * I_psi_rho;

  // First derivative A'(rho)
  double C1 = -(1.0 - g_rho) / sq(rho) + (2.0 * g_rho) / sq(a);
  results->A_p = C1 * I_psi_rho + G_rho / (1.0 - g_rho);

  // Second derivative A''(rho)
  double one_minus_g_sq = sq(1.0 - g_rho);
  double Ipsip_rho = (rho * G_rho) / one_minus_g_sq; // This is I_psi'(rho)
  
  double C1p = 2.0 * (1.0 - g_rho) / cube(rho) 
         - (2.0 * g_rho) / (sq(a) * rho) 
         - (4.0 * rho * g_rho) / (sq(a) * sq(a));
         
  double C2p = R_rho - (2.0 * rho * g_rho * G_rho) / (sq(a) * one_minus_g_sq);
  
  results->A_pp = C1p * I_psi_rho + C1 * Ipsip_rho + C2p;
}

Having solved \psi we may compute the velocity and vorticity terms

// Struct to hold the final calculated output values.
typedef struct {
  double u0_r;  // Velocity at leading order
  double u0_th; 
  double u0_xi;
  double w0_r;  // Vorticity at leading order
  double w0_th;
  double w0_xi;
  double u1_r;  // Velocity at the following order
  double u1_th;
  double u1_xi;
  double w1_r;  // Vorticity at the following order
  double w1_th;
  double w1_xi;
} batchelor_vortex;

At leading-order, the velocity reads u^{(0)}_\rho = 0, \quad u^{(0)}_\theta = \frac{1}{\rho}(1 - f(\rho,a)), \quad u^{(0)}_\xi = U_c f(\rho,a) while the vorticity reads \omega^{(0)}_\rho = 0, \quad \omega^{(0)}_\theta = (2 U_c \rho / a^2) f(\rho,a), \quad \omega^{(0)}_\xi = (2/a^2) f(\rho,a)

while the next-to-leading-order terms are functions of \psi, \kappa, \varphi and the leading order terms.

batchelor_vortex calculate_vortex_flow(struct local_filament* vortex, double U_c, 
  const integration_results* corr) {
  
  batchelor_vortex results;

  // Extracting values for readability
  double rho = vortex->rho; 
  double a = vortex->a; 
  double kappa = vortex->kappa; 
  double phi = vortex->theta + vortex->theta0;

  double A = corr->A;
  double dA = corr->A_p;
  double d2A = corr->A_pp;
  

  // Intermediate calculations
  double g_rho = exp(-sq(rho / a));
  double rho2 = sq(rho);
  double a2 = sq(a);

  // --- Zeroth-Order Base Flow (u0, w0) ---
  results.u0_r = 0.0;
  results.u0_th = (1.0 / rho) * (1.0 - g_rho);
  results.u0_xi = U_c * g_rho;

  results.w0_r = 0.0;
  results.w0_th = U_c * (2.0 * rho / a2) * g_rho;
  results.w0_xi = (2.0 / a2) * g_rho;

  // --- Intermediate terms needed for first-order terms ---
  // --- Swirl number at leading order
  double S0 = rho * results.w0_th / results.u0_th; 
  double denom = 1.0 - g_rho;
  double dS0_dr = (3.0 * rho2 - 2.0 * sq(rho2) / a2) * g_rho / denom
                - (2.0 * sq(rho2) / a2) * g_rho / (denom * denom);
  dS0_dr *= 2.0 * U_c / a2;

  // --- First-Order Perturbations (u1, w1) ---
  double cos_phi = cos(phi);
  double sin_phi = sin(phi);

  results.u1_r  = -A/rho2;
  results.u1_th = results.u0_th - dA/rho;
  results.u1_xi = results.u0_xi + S0*A/rho2;

  results.w1_r  = -S0*A/cube(rho);
  results.w1_th = results.w0_th + S0*A/cube(rho) - S0*dA/rho2 - dS0_dr*A/rho2;
  results.w1_xi = results.u0_th/rho + results.w0_xi - d2A/rho - dA/rho2 + A/cube(rho);
  
  results.u1_r  *= kappa * rho * sin_phi;
  results.u1_th *= kappa * rho * cos_phi; 
  results.u1_xi *= kappa * rho * cos_phi;
  results.w1_r  *= kappa * rho * sin_phi; 
  results.w1_th *= kappa * rho * cos_phi;
  results.w1_xi *= kappa * rho * cos_phi;

  return results;
}

References

[blanco2015internal]	Francisco J Blanco-Rodríguez, Stéphane Le Dizès, Can Selçuk, Ivan Delbende, and Maurice Rossi. Internal structure of vortex rings and helical vortices. Journal of Fluid Mechanics, 785:219–247, 2015.
[callegari1978]	AJ Callegari and Lu Ting. Motion of a curved vortex filament with decaying vortical core and axial velocity. SIAM Journal on Applied Mathematics, 35(1):148–175, 1978.