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/* This file is part of Cloudy and is copyright (C)1978-2008 by Gary J. Ferland and
 * others.  For conditions of distribution and use see copyright notice in license.txt */
/*RT_recom_effic generate escape probability function for continua, */
#include "cddefines.h"
#include "physconst.h"
#include "rfield.h"
#include "phycon.h"
#include "opacity.h"
#include "rt.h"

double RT_recom_effic(long int ip)
{
        long int i;
        double dEner, 
          denom, 
          escin, 
          escout, 
          hnukt, 
          receff_v, 
          sum, 
          tin, 
          tout;

        DEBUG_ENTRY( "RT_recom_effic()" );

        /* escape probability function for continua,
         * formally correct for photoelectric absorption only */

        ASSERT( ip > 0 && ip <= rfield.nupper );

        if( ip > rfield.nflux )
        {
                /* >>chng 01 dec 18, return had been zero, but this did not
                 * work for case where gas much hotter than continuum, as in a
                 * coronal plasma.  change to return of unity */
                receff_v = 1.;
                return( receff_v );
        }

        /* bug in following statement unocvered June 93 S. Schaefer */
        hnukt = TE1RYD*rfield.anu[ip-1]/phycon.te;

        /* rfield.chDffTrns = "OU2" by default */
        /* inward optical depth and escape prob */
        if( strcmp(rfield.chDffTrns,"OSS") == 0 )
        {
                /* which side of Lyman limit is this? */
                if( rfield.anu[ip] > 0.99 )
                {
                        /* this is a simple OTS, turned on with DIFFUSE OTS SIMPLE */
                        receff_v = SMALLFLOAT;
                }
                else
                {
                        receff_v = 1.;
                }
        }
        else if( strcmp(rfield.chDffTrns,"OTS") == 0 )
        {
                tin = opac.TauAbsGeo[0][ip-1];
                if( tin < 5. )
                {
                        escin = esccon(tin,hnukt);
                }
                else
                {
                        escin = 1e-4;
                }

                /* outward optical depth */
                tout = opac.TauAbsGeo[1][ip-1] - tin;

                if( iteration > 1 )
                {
                        /* check whether we have overrun the optical depth scale */
                        if( tout > 0. )
                        {
                                /* good optical depths in both directions, take mean */
                                if( tout < 5. )
                                {
                                        escout = esccon(tout,hnukt);
                                }
                                else
                                {
                                        escout = 1e-4;
                                }
                                receff_v = 0.5*(escin + escout);
                        }
                        else
                        {
                                /* >>chng 91 apr add logic to prevent big change in
                                 * esc prob, resulting in terminal oscillations, when optical
                                 * depth scale overrun
                                 * tau was negative, use 5% of inward optical depth */
                                escout = esccon(tin*0.05,hnukt);
                                receff_v = 0.5*(escin + escout);
                        }
                }
                else
                {
                        receff_v = escin;
                }
        }
        else if( strcmp(rfield.chDffTrns,"OU1") == 0 )
        {
                receff_v = opac.ExpZone[ip+1];
        }
        else if( strcmp(rfield.chDffTrns,"OU2") == 0 )
        {
                /* this is the default rt method, as set in zero
                 * E2TauAbsFace is optical depth to illuminated face */
                receff_v = opac.E2TauAbsFace[ip+1];
        }
        else if( strcmp(rfield.chDffTrns,"OU3") == 0 )
        {
                receff_v = 1.;
        }
        else if( strcmp(rfield.chDffTrns,"OU4") == 0 )
        {
                /* this cannot happen, was the former outward treat
                 * optical depth for this zone */
                if( rfield.ContBoltz[ip-1] > 0. )
                {
                        i = ip;
                        dEner = phycon.te/TE1RYD*0.5;
                        sum = 0.;
                        denom = 0.;
                        while( rfield.ContBoltz[i-1] > 0. &&
                           rfield.anu[i-1]-rfield.anu[ip-1] < (realnum)dEner &&
                           i <= rfield.nflux )
                        {
                                sum += rfield.ContBoltz[i-1]*opac.tmn[i-1];
                                denom += rfield.ContBoltz[i-1];
                                i += 1;
                        }
                        receff_v = sum/denom;
                }
                else
                {
                        receff_v = opac.tmn[ip-1];
                }
        }
        else
        {
                fprintf( ioQQQ, " RECEFF does not understand the transfer method=%3.3s\n", 
                  rfield.chDffTrns );
                cdEXIT(EXIT_FAILURE);
        }

        receff_v = MAX2((double)opac.otsmin,receff_v);
        /* can get epsilon above unity on cray */
        receff_v = MIN2(1.,receff_v);
        return( receff_v );
}

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