set ch2o to 0 for premelt reference visc calculation

docs/_pages/vbrmethods/visc/xfit_premelt.md

@@ -67,3 +67,7 @@ Output is stored in `VBR.out.viscous.xfit_premelt`. Unlike the other viscous met
   [2,1] = eta_meltfree
 }
 ```
+
+Note that `eta_meltfree` is both the melt-free **and** volatile-free viscosity: the pre-melting method 
+incorprates weakening from volatiles in the solidus depression. To incorporate effects of volatiles, 
+should calculate a volatile-dependent solidus. 

vbr/testing/test_vbrcore_010_premelt_volatiles.m

@@ -0,0 +1,73 @@
+function TestResult = test_vbrcore_010_premelt_volatiles()
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+% TestResult = test_vbrcore_010_premelt_volatiles()
+%
+% test for volatile behavior
+%
+% Parameters
+% ----------
+% none
+%
+% Output
+% ------
+% TestResult  struct with fields:
+%           .passed         True if passed, False otherwise.
+%           .fail_message   Message to display if false
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+
+    TestResult.passed=true;
+    TestResult.fail_message = '';
+
+    % the melt-free viscosity should match between the two cases (one with water,
+    % one without) because the solidus is not being changed between them. This is
+    % a math check more than a physical check.
+    VBR = get_VBR(0.0);
+    [VBR] = VBR_spine(VBR);
+    VBR_1 = get_VBR(1.0);
+    [VBR_1] = VBR_spine(VBR_1);
+
+    eta = VBR.out.viscous.xfit_premelt.diff.eta_meltfree;
+    eta_1 = VBR_1.out.viscous.xfit_premelt.diff.eta_meltfree;
+
+    if sum(abs(eta -eta_1)) > 0
+        TestResult.passed=false;
+        TestResult.fail_message = 'the viscosity values do not match!';
+    end
+
+    Q = VBR.out.anelastic.xfit_premelt.Q;
+    Q_1 = VBR_1.out.anelastic.xfit_premelt.Q;
+    if sum(abs(Q - Q_1)) > 0
+        TestResult.passed=false;
+        TestResult.fail_message = 'the Q values do not match!';
+    end
+
+    etaHK = VBR.out.viscous.HK2003.diff.eta;
+    etaHK_1 = VBR_1.out.viscous.HK2003.diff.eta;
+    if sum(etaHK > etaHK_1) > 0
+        % note: etaHK_1 (the case with water) should be < etaHK in all cases
+        TestResult.passed=false;
+        TestResult.fail_message = 'HK2003 eta with water should be weaker.';
+    end
+end
+
+function VBR = get_VBR(water_val)
+
+    VBR = struct();
+    VBR.in.elastic.methods_list={'anharmonic'};
+    VBR.in.viscous.methods_list={'xfit_premelt'; 'HK2003'};
+    VBR.in.anelastic.methods_list={'xfit_premelt'};
+
+    VBR.in.viscous.xfit_premelt = Params_Viscous('xfit_premelt');
+    VBR.in.viscous.xfit_premelt.eta_melt_free_method = 'HK2003';
+
+    sz = [5, 1];
+    VBR.in.SV.phi = full_nd(0.01, sz);
+    VBR.in.SV.Ch2o = full_nd(water_val, sz);
+    VBR.in.SV.T_K = full_nd(1350+273, sz);
+    VBR.in.SV.Tsolidus_K = full_nd(1300+273., sz);
+    VBR.in.SV.P_GPa = full_nd(2.5, sz); % pressure [GPa]
+    VBR.in.SV.dg_um=full_nd(0.01 * 1e6, sz); % grain size [um]
+    VBR.in.SV.rho = full_nd(3300, sz); % density [kg m^-3]
+    VBR.in.SV.sig_MPa = full_nd(.1, sz); % differential stress [MPa]
+    VBR.in.SV.f = 10.; % 1 Hz
+end

vbr/vbrCore/functions/visc_calc_xfit_premelt.m

@@ -56,6 +56,10 @@
     VBRtemp=VBR;
     VBRtemp.in.viscous.methods_list={visc_method}; % only use one method
     VBRtemp.in.SV.phi=0; % need melt-free viscosity
+    % additionally, water effects are accounted for by the homologous temperature
+    % scaling in the A_n factor, so **also** need to set the water content to 0
+    % for getting the reference viscosity. see https://github.com/vbr-calc/vbr/issues/96
+    VBRtemp.in.SV.Ch2o=0;
     VBRtemp=spineGeneralized(VBRtemp,'viscous');
     eta_meltfree = VBRtemp.out.viscous.(visc_method).diff.eta ;
   end